doi.org/10.26434/chemrxiv.14338907.v1

Record High Magnetic Anisotropy in Three Coordinate MnIII and CrIIComplexes: A Theoretical PerspectiveArup Sarkar, Reshma Jose, Harshit Ghosh, Rajaraman Gopalan

Submitted date: 30/03/2021 • Posted date: 31/03/2021Licence: CC BY-NC-ND 4.0Citation information: Sarkar, Arup; Jose, Reshma; Ghosh, Harshit; Gopalan, Rajaraman (2021): Record HighMagnetic Anisotropy in Three Coordinate MnIII and CrII Complexes: A Theoretical Perspective. ChemRxiv.Preprint. https://doi.org/10.26434/chemrxiv.14338907.v1

Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe3)2}3] (1) and[K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (2) reveal record-high magnetic anisotropy with the D values -64cm-1 and -15 cm-1 respectively, enlisting d4 ion back in the race for single-ion magnets. For the first time, adetailed spin-vibrational analysis was performed in 1 and 2 that suggest a dominant under barrier relaxationdue to flexible coordination sphere around the metal ion offering design clues for low coordinate transitionmetal SIMs.

File list (4)

download fileview on ChemRxivManuscript_GR_final_Arxiv2.pdf (914.22 KiB)

download fileview on ChemRxivSupporting_Information.pdf (1.06 MiB)

download fileview on ChemRxivManuscript_GR_final_Arxiv2.docx (4.55 MiB)

download fileview on ChemRxivSupporting_Information.docx (4.97 MiB)

1

Record High Magnetic Anisotropy in Three

Coordinate MnIII and CrII Complexes: A Theoretical

Perspective

Arup Sarkar, Reshma Jose, Harshit Ghosh and Gopalan Rajaraman*

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076,

Maharashtra, India

Keywords: Low-coordinate, Ab initio, NEVPT2, Zero-field Splitting, Spin-vibronic coupling

ABSTRACT: Ab initio calculations performed in two three-coordinate complexes

[Mn{N(SiMe3)2}3] (1) and [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (2) reveal record-high

magnetic anisotropy with the D values -64 cm-1 and -15 cm-1 respectively, enlisting d4 ion back in

the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was

performed in 1 and 2 that suggests a dominant under barrier relaxation due to flexible coordination

sphere around the metal ion offering design clues for low coordinate transition metal SIMs.

2

Introduction

Single-Molecule Magnets (SMMs) have become a fascinating research area as this class of

molecules exhibit magnetization just like permanent magnets below a critical temperature defined

as blocking temperature TB.1 An important parameter associated with the blocking temperature is

the barrier height for magnetization reversal (Ueff) which is correlated to the magnetic moment of

different microstate and the nature of anisotropy. While in lanthanide complexes, the first-order

spin-orbit coupling (SOC) is strong enough to produce large barrier heights,2 in transition metal

(TM) systems spin-orbit coupling is generally weak, leading to relatively smaller anisotropy which

is reflected in the axial zero-field splitting parameter (D) which can be tuned at will using ligand

field.3

There are several challenges in enhancing the blocking temperature TB in SMMs as several

relaxation mechanisms other than the Orbach process spoil the direct correlation of TB to Ueff

values. Among others, quantum tunneling of magnetization (QTM) and spin-phonon/vibrational-

mode relaxation mechanisms are a prominent source of relaxation, as shown in recent years by

various groups.4 Earlier research in the SMM area was focused on increasing the total spin (S) of

the complexes by increasing the number of metal centers. After the discovery of a very small Ueff

barrier in {Mn19} cluster possessing record high ground state S value, it becomes clear that

increasing the number of metal centers or S value diminishes the axial anisotropy (D term) as

evident from the equation proposed originally by Abragam and Bleaney5 and adapted in ab initio

calculations later on. For this reason, mononuclear TM complexes gained significant attention

leading to the birth of several single-ion magnets (SIMs) based on the low coordination number

such as Fe(II/I), Ni(II), and Co(II), exhibiting very large Ueff values.6

3

In the early years of SMMs, the focus has been on transition metal cluster particularly that of

Mn(III) ions, as this offer an easy source of negative D parameter for the chemists and unearthed

numerous SMMs albeit with smaller Ueff/TB values. 1a, 7 The Mn(III) ions are very robust and can

be easily incorporated in cluster aggregation, and are relatively redox stable6a but exhibit only small

D values of the order of ~|5| cm-1. 8 While several low-coordinate transition metal ions were

pursued recently for potential SMMs, Mn(III) has not been studied in detail, perhaps due to the

perception that the expected D values are rather small.8 Apart from Ueff values, the blocking

temperature is an important criterion which is often very small. This suggests that apart from the

QTM effect, spin-vibrational relaxations are at play in such systems.4 How these effects manifest

in these complexes are not fully understood.

To ascertain complexes that exhibit large negative D values and also to correlate the relaxation

mechanism to spin-vibrational coupling, we undertake theoretical studies based on multi-

configurational ab initio calculations SA-CASSCF/NEVPT2 using the ORCA suite.9 Here, we

have studied in detail two three-coordinate d4 systems Mn(III) and Cr(II): [Mn{N(SiMe3)2}3]10 (1)

and [K(18-crown-6)(Et2O)2] [Cr{N(SiMe3)2}3]11 (2) using their reported X-ray structure. Our

NEVPT2 calculations yield a record axial D value of -64 cm-1 and -15 cm-1 for 1 and 2,

respectively, with a negligible E/D value. The D value computed for both complexes is larger than

any examples reported to-date and suggests a potential SMMs characteristic for these robust

building block metal ions.

Computational Details

All the ab initio single point calculations have been performed using ORCA 4.0.0 program.9 DKH

(Douglas-Kroll-Hess) Hamiltonian was used to account for the scalar relativistic effect. DKH

4

contracted versions of the basis sets were used during the calculations- DKH-def2-TZVP for Mn,

Cr, Si; DKH-def2-TZVP(-f) for N and DKH-def2-SVP for the rest of the atoms. During the orbital

optimization step in SA-CASSCF (state-averaged complete active space self-consistent field)

method, 4 metal electrons in 5 metal d-orbitals were taken into consideration and optimized with

5 quintet and 35 triplet roots for Mn(III) and Cr(II) metal centers. Additional calculations have

also been carried out with 5 quintets, 45 triplet roots, and 5 quintets, 35 triplet, and 22 singlet roots

to check the effect of high-lying excited states on the Spin-Hamiltonian (SH) parameters. The

addition of extra 10 triplet roots and 22 singlet roots marginally affect the SH parameters (see

Table S1 in ESI). NEVPT2 (N-electron valence perturbation theory second-order) calculation has

also been performed on the top of converged SA-CASSCF wavefunction to include the dynamic

electron correlation. Spin-orbit interaction was accounted with quasi-degenerate perturbation

theory (QDPT) approach using SOMF (spin-orbit mean field) operator. Only spin-orbit

contributions towards zero-field splitting were computed. Final Spin-Hamiltonian parameters were

determined with effective Hamiltonian approach (EHA) formalism.12 Ab initio ligand field theory

(AILFT) calculations have also been performed to obtain the d-orbital energies.13

Geometry optimization and single point frequency calculations have been carried out in Gaussian

09 (Rev. D.01) program.14 Hybrid unrestricted B3LYP-D2 functional was used for the DFT

calculations along with Ahlrich’s triple- valence polarized (TZVP) basis set for Mn, Cr, Si, N

and Ahlrich’s spilt valence polarized (SVP) basis set for rest of the atoms.15

5

Result and Discussions

Complex 1 possesses a perfect D3h symmetry as the three N-Mn-N bond angles are 120.02,

120.02 and 119.95 and the three Mn-N bond lengths are 1.89, 1889, and 1.89 (in Å unit). It was

also noticed that the {MnN3} core was planar, and the bulky trimethylsilyl groups surrounded the

central moiety stabilizes the low coordinate molecule from further coordination via steric

arrangements (see Figure 1). The NEVPT2-QDPT calculated major anisotropy axes, i.e., Dzz and

gzz axes, were found to be exactly perpendicular to the Mn-N3 plane, i.e., exactly collinear with the

C3 axis, which describes the axial nature of anisotropy present in the molecule.

Figure 1: NEVPT2 computed Dzz axis of the molecule plotted on the X-Ray structure (left) and

three Mn-N bond lengths (in Å) and N-Mn-N angles () shown on the molecule (right). Colour

code: Mn: pink, N: blue, Si: light green, C: dark grey. Hydrogens are omitted due to clarity.

6

A record axial zero-field splitting (ZFS) was found for this complex, showing a D value of -64 cm-

1 with E/D estimated to be 0.0003, indicating strong easy-axis type anisotropy (see Table 1) for

complex 1. A very similar geometry was observed in the Cr(II) analog, and the N-Cr-N bond

angles (123.9, 115.81 and 120.29) are not exactly similar and significantly deviated from D3h

symmetry. The D value for complex 2 is less than complex 1 due to these structural deviations and

also smaller spin-orbit coupling constant () values of Cr(II) than Mn(III). For complex 2, the E/D

value is estimated to be 0.003, which is ten times larger than complex 1 (see Figure S1 and Table

S1 in ESI).

The ground state electronic configuration of complex 1 is dz21dyz

1dxz1dxy

1dx2

-y20, and this comprises

77% of the overall wavefunction. The major contribution (-62 cm-1) towards the negative D value

arises from the first excited state, which consists of dxy dx2-y

2 (same ML valued) electronic

excitation and this excited state contribute ~97% of the overall D value (see Figure 2 and Table S2

in ESI). Other electronic transitions were found to contribute negligibly to the overall D value. A

very close analysis of the NEVPT2 states reveals that the first excited quintet state is only 19 cm-

1 apart from the ground state and consequently results in a very strong second-order spin-orbit

coupling. While the first excited state is the spin-allowed quintet, the second, third and fourth

excited states arise from the spin-flipped triplet transitions. These three excited states consist of a

mixture of dxy dxz/dyz and dxy dz2 transitions (see Table S2 in ESI). The computed gx, gy and

gz values are 1.67, 1.67 and 1.14 respectively for true spin S =2 and 0.00 0.00 and 5.14, respectively

for pseudospin �� =1/2 manifold. In the case of complex 2, the first excited state contributes 96%

(-14.4 cm-1) towards the overall D value. Again, the D value is negative due to the coupling with

the prominent first excited state involving the same ML level dxy dx2

-y2 electronic excitation (see

Figure S2 in ESI). Here one major difference of complex 2 from complex 1 is that due to

7

significant distortion from D3h and lower ligand field of Cr(II), the first excited state is 756 cm-1

apart and the next three excited states are quintets (see Table S3). The computed gx, gy and gz

values are 1.97, 1.97, and 1.58 respectively for true spin S=2 and 0.00 0.00 and 6.351, respectively

for pseudospin �� =1/2 manifold.

In the case of non-Kramers ions like in these two cases studied, the tunnel-splitting is generally

larger, leading to faster relaxation via the QTM process. The tunnel-splitting strongly depends on

the local symmetry and ligand field environment around the metal ion. The high symmetry present

in complexes 1 and 2 leads to smaller tunnel splitting (see Figure S3 and Table S4 in ESI). The

first excited pseudo-KDs is separated by 154 cm-1 in case of complex 1 and 45 cm-1 in case of

Figure 2: NEVPT2-LFT d-orbital diagram of complex 1. The orange arrow indicates the first

excited spin-allowed transition.

8

complex 2. The multi-determinant nature of the ground state leads to mixing of the |+2 and |-2

states, and this is very prominent in complex 1 compared to 2 (see Table S4).

The static electronic picture is insufficient to describe the relaxation mechanism or the spin

dynamics of the system. Recent reports of spin-vibronic coupling that describe the role of

vibrational frequencies of a single molecule or of the surrounding lattice are very important to

elucidate the dynamic scenario of SMMs.4 In this regard, we have attempted to investigate the role

of molecular vibrations occurring at low temperatures on the spin-orbit or Ms levels in the two

complexes. Therefore, we have performed frequency calculations (normal modes) on the X-ray

structures of 1 and 2 using Density Functional Theory (DFT) methods (B3LYP-D2/TZVP, a

similar vibrational pattern also found for the optimized geometries, see Table S5 in ESI).

Here we have carefully analyzed five lower energy vibrational modes below 80 cm-1, and these

are 1 45.1 (40.1), 2 45.8(46.2), 3 58.4(50.3), 4 70.3(58.8) and 5 72.9 (69.9) for complex 1 (2)

(see Figure S4 in ESI). Out of the five vibrations mentioned, the 4 and 5 vibrations were found

to be IR active and also break the D3h symmetry (see Figure S4). Here 4 corresponds to N-M-

N bond angle bending and correlates to Jahn-Teller active vibration (E irreducible representation

in D3h symmetry). The 5 corresponds to out-of-plane (M-N-N-N) bending vibration of the metal

ion and associate with A2” irreducible representation. Several displacement points in 4 and 5

vibrational surfaces were considered for CASSCF/NEVPT2 calculations. The maximum

displacement scale of a particular vibration j, denoted by xj, was fixed at 2.0 for both the complexes

as suggested earlier (see Table S6 and S7 in ESI).16

An angular distortion parameter Q was introduced, which is a sum of deviation from 120 from

each of the equatorial N-M-N angles (denoted as ) (see Table S6 and S7).6e Here, in order to

9

find out the spin-vibronic coupling, the variation of D and E/D have been computed with respect

to the displacement of nuclear coordinates (x) using the following Hamiltonian:

��𝑠−𝑣𝑖𝑏 = (𝜕𝐷

𝜕𝑥)𝑥 [𝑆𝑍

2 −𝑆(𝑆 + 1)

3] + (

𝜕𝐸

𝜕𝑥)𝑥(𝑆𝑥

2 − 𝑆𝑦 2) … 𝑒𝑞𝑛. 1)

In Figure 3, we plot computed D values with respect to Q and xj, and these plots show that as the

Q diverges from zero, the magnitude of D decreases for 4 vibrations (see Figure S5 in ESI). This

is because an increase in Q breaks the D3h symmetry and, consequently, increases the gap between

the dxy and dx2-y

2 orbitals (see Fig. S6). In complex 1, the X-Ray structure shows the highest

negative D value and minimum E/D value (see Fig. S5) at equilibrium geometry or zero

displacement point, but for complex 2, the X-ray structure is significantly deviated from the ideal

D3h symmetry and therefore do not have the largest negative D or the lowest E/D at zero

Figure 3: Variation of D values in complexes 1 (left) and 2 (right) with respect to the distortion

parameter Q and displacement factor xj for 4 vibrational mode.

10

displacement point. In complex 2, at xj = 0.8 (see Fig. 3), the Q parameter shows a minimum and

predicts a D value as high as -46 cm-1.

Furthermore, we have developed a three-dimensional magneto-structural correlation to see the

effect of angle change on the D values for complex 1 and 2 (see Figure 4 and S7 in ESI). It is very

clear that the D is maximum when all the three equatorial angles are 120. For 1, the variation in

D values is found to be relatively smaller for 5 vibrations compared to 4 mode (see Table S8-S9

in ESI). For 2 no spin-vibrational coupling is detected as a much smaller change in D is noted. To

rationalize this observation, the AILFT computed d-orbitals are plotted, and this reveals that the

dxy-dx2-y

2 orbital energy gap is altered only slightly in 1 and negligibly in 2 (see Fig. S8-S9). This

suggests that 4 vibrational mode is dominant in controlling the magnetic anisotropy in trigonal

planar d4 systems, and this vibration likely offers a smaller barrier height for relaxation at lower

temperatures. Between complex 1 and 2, the spin-vibronic coupling is found to be stronger in the

former.

At the equilibrium geometries, neglecting other effects, the computed Ucal values for complexes

1 and 2 are 153.8 and 44.7 cm-1 for complexes 1 and 2, respectively. Considering the vibrational

relaxation 4 modes at the displacement scale of xj=±2, the Ueff value is expected to be diminished

to 19 cm-1 and 15 cm-1 for complexes 1 and 2, respectively (neglecting the QTM effects). This is

substantially smaller than the barrier height estimated from the Orbach process and suggests a

dominant spin-vibrational relaxation role in the magnetization relaxation in these complexes. This

may be attributed to the fact that the N-M-N bond angle bending vibration is very subtle and

does not require significant energy for structural distortion and is strongly correlated to the dxy and

dx2

-y2 gap altering the magnetic anisotropy. This advocates a design principle that a rigid structure

11

with a robust N-Mn-N angle could block such relaxation, and this is possible if a chelate type or

macrocyclic type ligands are employed.

Conclusion

To the end, we have successfully employed an accurate ab initio method to explore the zero-field

splitting and ligand field parameters in two MnIII and CrII high spin complexes. A record-high D

value of -64 cm-1 and -15 cm-1 was found for the X-ray structures of 1 and 2, respectively. These

two values are higher than any other reported D values for any mononuclear d4 systems (see Table

S10 in ESI). While a significant barrier for magnetization relaxation is found for both the

complexes, our detailed analysis revealed a strong spin-vibration coupling both the complexes that

are likely to yield smaller blocking temperatures.

Figure 4. Three-dimensional magneto-structural correlation of D obtained from 4 mode for complex 1.

12

ASSOCIATED CONTENT

The following files are available free of charge.

AUTHOR INFORMATION

Corresponding Author

Gopalan Rajaraman. Email: rajaraman@chem.iitb.ac.in

Present Addresses

†Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076,

Maharashtra, India

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval

to the final version of the manuscript.

Funding Sources

This work was funded by DST and SERB (CRG/2018/000430; DST/SJF/CSA-03/2018-10;

SB/SJF/2019-20/12), UGC-UKIERI (184-1/2018(IC)) and SUPRA (SPR/2019/001145).

ACKNOWLEDGMENT

RJ thanks DST-INSPIRE and AS thanks IIT Bombay for IPDF funding.

13

REFERENCES

1. (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M., Magnetic bistability in a metal-ion

cluster. Nature 1993, 365 (6442), 141-143; (b) Gatteschi, D.; Sessoli, R.; Villain, J., Molecular

nanomagnets. Oxford University Press on Demand: 2006; Vol. 5.

2. Woodruff, D. N.; Winpenny, R. E.; Layfield, R. A., Lanthanide single-molecule magnets.

Chem. Rev. 2013, 113 (7), 5110-5148.

3. Gomez-Coca, S.; Cremades, E.; Aliaga-Alcalde, N.; Ruiz, E., Mononuclear single-

molecule magnets: tailoring the magnetic anisotropy of first-row transition-metal complexes. J.

Am. Chem. Soc. 2013, 135 (18), 7010-8.

4. (a) Gomez-Coca, S.; Urtizberea, A.; Cremades, E.; Alonso, P. J.; Camon, A.; Ruiz, E.;

Luis, F., Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy. Nat.

Commun. 2014, 5, 4300; (b) Rechkemmer, Y.; Breitgoff, F. D.; van der Meer, M.; Atanasov, M.;

Hakl, M.; Orlita, M.; Neugebauer, P.; Neese, F.; Sarkar, B.; van Slageren, J., A four-coordinate

cobalt(II) single-ion magnet with coercivity and a very high energy barrier. Nat. Commun. 2016,

7, 10467; (c) Escalera-Moreno, L.; Suaud, N.; Gaita-Arino, A.; Coronado, E., Determining Key

Local Vibrations in the Relaxation of Molecular Spin Qubits and Single-Molecule Magnets. J.

Phys. Chem. Lett. 2017, 8 (7), 1695-1700; (d) Lunghi, A.; Totti, F.; Sessoli, R.; Sanvito, S., The

role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets. Nat.

Commun. 2017, 8, 14620; (e) Moseley, D. H.; Stavretis, S. E.; Thirunavukkuarasu, K.; Ozerov,

M.; Cheng, Y.; Daemen, L. L.; Ludwig, J.; Lu, Z.; Smirnov, D.; Brown, C. M.; Pandey, A.;

Ramirez-Cuesta, A. J.; Lamb, A. C.; Atanasov, M.; Bill, E.; Neese, F.; Xue, Z. L., Spin-phonon

couplings in transition metal complexes with slow magnetic relaxation. Nat. Commun. 2018, 9 (1),

2572.

5. Abragam, A.; Bleaney, B., Electron paramagnetic resonance of transition ions. OUP

Oxford: 2012.

6. (a) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.; Long,

J. R., Magnetic blocking in a linear iron (I) complex. Nat. Chem. 2013, 5 (7), 577; (b) Zadrozny,

J. M.; Atanasov, M.; Bryan, A. M.; Lin, C.-Y.; Rekken, B. D.; Power, P. P.; Neese, F.; Long, J.

R., Slow magnetization dynamics in a series of two-coordinate iron (II) complexes. Chem. Sci.

2013, 4 (1), 125-138; (c) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang, M.-W.; Jiang,

S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W., Two-coordinate Co (II) imido

complexes as outstanding single-molecule magnets. J. Am. Chem. Soc. 2016, 139 (1), 373-380; (d)

Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita, M.;

Overgaard, J.; Van Slageren, J.; Neese, F.; Long, J. R., A linear cobalt (II) complex with maximal

orbital angular momentum from a non-Aufbau ground state. Science 2018, 362 (6421), eaat7319;

(e) Craig, G. A.; Sarkar, A.; Woodall, C. H.; Hay, M. A.; Marriott, K. E. R.; Kamenev, K. V.;

14

Moggach, S. A.; Brechin, E. K.; Parsons, S.; Rajaraman, G.; Murrie, M., Probing the origin of the

giant magnetic anisotropy in trigonal bipyramidal Ni(ii) under high pressure. Chem. Sci. 2018, 9

(6), 1551-1559; (f) Sarkar, A.; Dey, S.; Rajaraman, G., Role of coordination number and geometry

in controlling the magnetic anisotropy in FeII, CoII, and NiII single-ion magnets. Chem. Eur. J

2020, 26, 14036-14058.

7. Ako, A. M.; Hewitt, I. J.; Mereacre, V.; Clérac, R.; Wernsdorfer, W.; Anson, C. E.; Powell,

A. K., A ferromagnetically coupled Mn19 aggregate with a record S= 83/2 ground spin state.

Angewandte Chemie 2006, 118 (30), 5048-5051.

8. (a) Maurice, R.; de Graaf, C.; Guihery, N., Magnetostructural relations from a combined

ab initio and ligand field analysis for the nonintuitive zero-field splitting in Mn(III) complexes.

The Journal of chemical physics 2010, 133 (8), 084307; (b) Ishikawa, R.; Miyamoto, R.; Nojiri,

H.; Breedlove, B. K.; Yamashita, M., Slow relaxation of the magnetization of an Mn(III) single

ion. Inorg. Chem. 2013, 52 (15), 8300-2; (c) Vallejo, J.; Pascual-Alvarez, A.; Cano, J.; Castro, I.;

Julve, M.; Lloret, F.; Krzystek, J.; De Munno, G.; Armentano, D.; Wernsdorfer, W.; Ruiz-Garcia,

R.; Pardo, E., Field-induced hysteresis and quantum tunneling of the magnetization in a

mononuclear manganese(III) complex. Angewandte Chemie 2013, 52 (52), 14075-9; (d) Chen, L.;

Wang, J.; Liu, Y.-Z.; Song, Y.; Chen, X.-T.; Zhang, Y.-Q.; Xue, Z.-L., Slow Magnetic Relaxation

in Mononuclear Octahedral Manganese(III) Complexes with Dibenzoylmethanide Ligands. Eur.

J. Inorg. Chem. 2015, 2015 (2), 271-278; (e) Craig, G. A.; Marbey, J. J.; Hill, S.; Roubeau, O.;

Parsons, S.; Murrie, M., Field-induced slow relaxation in a monometallic manganese(III) single-

molecule magnet. Inorg. Chem. 2015, 54 (1), 13-5; (f) Pascual-Alvarez, A.; Vallejo, J.; Pardo, E.;

Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.; Wernsdorfer, W.; Cano, J., Field-Induced Slow

Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex. Chemistry 2015, 21

(48), 17299-307; (g) Duboc, C., Determination and prediction of the magnetic anisotropy of Mn

ions. Chem. Soc. Rev. 2016, 45 (21), 5834-5847; (h) Realista, S.; Fitzpatrick, A. J.; Santos, G.;

Ferreira, L. P.; Barroso, S.; Pereira, L. C.; Bandeira, N. A.; Neugebauer, P.; Hruby, J.; Morgan, G.

G.; van Slageren, J.; Calhorda, M. J.; Martinho, P. N., A Mn(iii) single ion magnet with tridentate

Schiff-base ligands. Dalton Trans. 2016, 45 (31), 12301-7; (i) Sanakis, Y.; Krzystek, J.; Maganas,

D.; Grigoropoulos, A.; Ferentinos, E.; Kostakis, M. G.; Petroulea, V.; Pissas, M.;

Thirunavukkuarasu, K.; Wernsdorfer, W., Magnetic Properties and Electronic Structure of the S=

2 Complex [MnIII {(OPPh2) 2N} 3] Showing Field-Induced Slow Magnetization Relaxation.

Inorg. Chem. 2020, 59 (18), 13281-13294.

9. Neese, F., Software update: the ORCA program system, version 4.0. WIRES: Comp. Mol.

Sci. 2018, 8 (1), e1327.

10. Ellison, J. J.; Power, P. P.; Shoner, S. C., First examples of three-coordinate manganese

(III) and cobalt (III): synthesis and characterization of the complexes M [N (SiMe3) 2] 3 (M= Mn

or Co). J. Am. Chem. Soc. 1989, 111 (20), 8044-8046.

11. Wagner, C. L.; Phan, N. A.; Fettinger, J. C.; Berben, L. A.; Power, P. P., New

Characterization of V{N(SiMe3)2}3: Reductions of Tris[bis(trimethylsilyl)amido]vanadium(III)

15

and -chromium(III) To Afford the Reduced Metal(II) Anions [M{N(SiMe3)2}3](-) (M = V and

Cr). Inorg. Chem. 2019, 58 (9), 6095-6101.

12. Maurice, R.; Bastardis, R.; Graaf, C. d.; Suaud, N.; Mallah, T.; Guihery, N., Universal

theoretical approach to extract anisotropic spin Hamiltonians. J. Chem. Theory Comput. 2009, 5

(11), 2977-2984.

13. Singh, S. K.; Eng, J.; Atanasov, M.; Neese, F., Covalency and chemical bonding in

transition metal complexes: An ab initio based ligand field perspective. Coord. Chem. Rev. 2017,

344, 2-25.

14. Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;

Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G., gaussian 09, Revision d. 01, Gaussian.

Inc., Wallingford CT 2009, 201.

15. (a) Schäfer, A.; Horn, H.; Ahlrichs, R., Fully optimized contracted Gaussian basis sets for

atoms Li to Kr. J. Chem. Phys. 1992, 97 (4), 2571-2577; (b) Schäfer, A.; Huber, C.; Ahlrichs, R.,

Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr. J.

Chem. Phys. 1994, 100 (8), 5829-5835.

16. Goodwin, C. A.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P., Molecular magnetic

hysteresis at 60 kelvin in dysprosocenium. Nature 2017, 548 (7668), 439.

16

TOC: Using Ab initio NEVPT2 calculations, we offer a design principle to enhance ZFS in high-

spin d4 complexes of first-row transition elements.

download fileview on ChemRxivManuscript_GR_final_Arxiv2.pdf (914.22 KiB)

Supporting Information

Record High Magnetic Anisotropy in Three Coordinate MnIII and CrII

Complexes: A Theoretical Perspective

Arup Sarkar, Reshma Jose, Harshit Ghosh, and Gopalan Rajaraman*

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076, India

Email: rajaraman@chem.iitb.ac.in

Table S1: Comparison of the ZFS parameters using different triplet and singlet roots during

the SA-CASSCF/NEVPT2 calculations.

Sets Quintet roots Triplet roots Singlet roots D (cm-1) E/D

Complex 1 5 35 - -64.5 0.0002

Complex 1 5 45 - -64.3 0.0003

Complex 1 5 35 22 -63.5 0.0003

Complex 2 5 35 - -15.2 0.003

Complex 2 5 45 - -15.1 0.003

Complex 2 5 45 22 -14.8 0.003

Figure S1: Major anisotropic axes plotted on complex 2 from NEVPT2 level of theory

(left). Equatorial N-Cr-N bond angles and Cr-N bond lengths shown on X-Ray

structure of complex 2 (right). Colour code: Cr: green, N: blue, Si: bluish green, C: dark

grey. Hydrogens are omitted for clarity.

Table S2: NEVPT2 vertical excitation spectrum, CASSCF electronic configuration and their

respective contribution towards D and E values for ground state and six lower lying excited

states for complex 1.

Table S3: NEVPT2 vertical excitation spectrum, CASSCF electronic configuration and their

respective contribution towards D and E values for ground state and four lower lying excited

states for complex 2 (The irreducible representations are assigned according to C2v symmetry).

States Major CASSCF electronic

configuration

NEVPT2

Energy (cm-

1)

Contribution to D

(cm-1)

Contribution to

E (cm-1)

5E dz

21dyz1dxz

1dxy1dx

2-y

20 (77%)

dz21dyz

1dxz1dxy

0 dx2-y

21 (76%)

0.0

18.7

0.0

-62.16

0.0

0.00

3E

dz21dyz

1dxz2dxy

0 dx2-y

20 (37%)

dz21dyz

2dxz1dxy

0 dx2-y

20 (12%) 13593.2 -0.38 0.38

dz21dyz

2dxz1dxy

0 dx2-y

20 (38%)

dz21dyz

1dxz2dxy

0 dx2-y

20 (12%) 13597.6 -0.23 -0.38

3A2

dz22dyz

1dxz1dxy

0 dx2-y

20 (42%)

dz22dyz

1dxz0dxy

1 dx2-y

20 (15%) 13720.0 -0.09 0.09

5E dz

21dyz1dxz

0dxy1 dx

2-y

21 (74%)

dz21dyz

0dxz1dxy

1 dx2-y

21 (74%)

18929.8

18949.3

0.18

0.13

-0.18

0.18

States Major CASSCF electronic

configuration

NEVPT2

Energy (cm-

1)

Contribution to D

(cm-1)

Contribution to

E (cm-1)

5A2 dz21dyz

1dxz1dxy

1 dx2-y

20 (91%) 0.0 0.0 0.0 5A2 dz

21dyz1dxz

1dxy0 dx

2-y

21 (91%) 755.8 -14.43 0.00 5B1

dz21dyz

1dxz0dxy

1 dx2-y

21 (94%) 10368.3 0.16 -0.12 5B2

dz21dyz

0dxz1dxy

1 dx2-y

21 (94%) 10808.9 0.13 0.10

5A1 dz

20dyz1dxz

1dxy1 dx

2-y

21

(100%) 12854.9 0.00 -0.00

Table S4: Wavefunction decomposition spin-orbit coupled states (Ms) of S =2 manifold for

complex 1 and 2.

Figure S2: NEVPT2-LFT d-orbital splitting diagram for complex 2. Green arrows

indicate the first excited state electronic transition. The green arrow indicates 1st excited

state electronic excitation.

Figure S3: Energy profile diagram of the lowest five spin-orbit states of complex 1

(left) and complex 2 (right) obtained from NEVPT2/QDPT/EHA method.

Table S5: Selected structural parameters of the X-Ray structure and optimised geometries of

complex 1 and 2.

Complex 1

Spin-orbit

State S MS

Quintet Weightage

Energy

(cm-1)

1

2 2 GS 26%

0.00 2 2 1st ES 24%

2 -2 GS 26%

2 -2 1st ES 24%

2

2 2 GS 26%

0.04 2 2 1st ES 24%

2 -2 GS 26%

2 -2 1st ES 24%

3

2 1 GS 27%

153.81 2 1 1st ES 23%

2 -1 GS 27%

2 -1 1st ES 23%

4

2 1 GS 27%

153.83 2 1 1st ES 23%

2 -1 GS 27%

2 -1 1st ES 23%

5 2 0 GS 100% 296.17

Complex 2

1

2 2 GS 47%

0.00 2 2 1st ES 3%

2 -2 GS 47%

2 -2 1st ES 3%

2

2 2 GS 47%

0.01 2 2 1st ES 3%

2 -2 GS 47%

2 -2 1st ES 3%

3 2 1 GS 50%

44.69 2 -1 GS 50%

4 2 1 GS 50%

44.99 2 -1 GS 50%

5 2 0 GS 100% 60.90

Complex

N1-

Mn-N2

()

N2-

Mn-

N3()

N3-

Mn-N1

()

Mn-

N1

(Å)

Mn-

N2

(Å)

Mn1-

N3

(Å)

4 and 5

(cm-1)

Table S6: Variation of D and E/D values with respect to Q parameter along with their energies

of the first excited state for complex 1. The frequency points (P’s) are taken from 4 vibration.

*n =1 for positive xj and -1 for negative xj.

(Angle

A)

(Angle

B)

(Angle

C)

1 (X-Ray) 120.02 119.95 120.02 1.889 1.890 1.890 70.3, 72.9

1 (optimised

in toluene) 113.93 113.87 132.20 1.911 1.902 1.911 68.6, 72.6

N1-Cr-

N2 ()

(Angle

A)

N2-Cr-

N3 ()

(Angle

B)

N3-Cr-

N1 ()

(Angle

C)

Cr-N1

(Å)

Cr-N2

(Å)

Cr-N3

(Å)

2 (X-Ray) 115.81 120.29 123.90 2.021 2.027 2.033 58.7, 69.9

2 (optimised

in n-hexane) 113.28 113.08 133.65 2.051 2.038 2.050 61.0, 73.5

Figure S4: DFT computed IR spectrum of the two complexes at far-IR frequencies of

complex 1 (left) and complex 2 (right). The black parenthesis and the black arrows

indicate the N-M-N angle bending and out-of-plane bending vibrations.

4 & 5

4 &

5

Table S7: Variation of D and E/D values with respect to Q parameter along with their energies

of the first excited state for complex 2. The frequency points (P’s) are taken from the fourth

frequency point.

Bond Angle ( =

A/B/C)

D (cm-

1) |E/D|

Energy of

the 1st

excited

state (cm-

1)

Q =

n|120

- |*

D Displacement

scale (xj)

P1

A=116.50

C=111.07

B=131.89

-6.47 0.019 4445.8 -24.32 57.81 -2.0

P2

A=117.70

C=114.07

B=128.00

-8.86 0.012 3007.0 -16.23 55.42 -1.3

P3

A=118.84

C=116.98

B=124.12

-15.33 0.005 1567.1 -8.30 48.95 -0.7

P4

A=119.21

C=117.91

B=122.85

-20.63 0.003 1094.8 -5.73 43.65 -0.4

P5

(X-Ray

Structure)

A=120.02

C=120.02

B=119.95

-64.28 0.0003 18.7 0.09 0 0.0

P6

A=120.56

C=121.42

B=118.01

-27.50 0.001 750.2 3.97 36.78 0.3

P7

A=121.01

C=122.62

B=116.33

-17.05 0.002 1374.5 7.3 47.23 0.6

P8

A=122.05

C=125.42

B=112.33

-9.22 0.006 2841.3 15.14 55.06 1.2

P9

A=123.23

C=128.73

B=107.51

-6.29 0.008 4565.2 24.45 57.99 2.0

*n =1 for positive xj and -1 for negative xj.

Bond Angle ( =

A/B/C)

D

(cm-1) |E/D|

Energy of

the 1st

excited

state (cm-1)

Q =

n|120

- |*

D Displacement

scale (xj)

P1

A=106.15

B=119.75

C=133.79

-5.00 0.022 2639.6 -27.89 10.10 -2.0

P2

A=109.41

B=119.97

C=130.48

-6.37 0.014 2014.7 -21.10 8.73 -1.4

P3

A=112.18

B=120.12

C=127.64

-8.45 0.009 1471.6 -15.58 6.65 -0.8

P4

(X-Ray

Structu

re)

A=115.81

B=120.29

C=123.90

-15.10 0.003 755.8 -8.38 0 0.0

P5

A=119.69

B=120.43

C=119.84

-45.87 0.000 22.6 0.90 30.77 0.8

P6

A=122.67

B=120.49

C=116.69

-21.08 0.001 575.0 6.47 5.98 1.4

P7

A=125.29

B=120.52

C=113.88

-11.57 0.003 1069.2 11.93 3.53 2.0

Figure S5: Variation of E/D with respect to Q parameter and xj for complex 1 (left)

and for complex 2 (right) in 4 vibration.

-32 -24 -16 -8 0 8 16

0.000

0.005

0.010

0.015

0.020

0.025-2.4 -1.6 -0.8 2.40.8 1.6

E/D

Q

xj

0.0

-30 -20 -10 0 10 20 30

-2.4 -1.6 -0.8 0.0 0.8 1.6 2.4

0.000

0.005

0.010

0.015

0.020

xj

E/D

Q

Figure S6: Variation of LFT d-orbital energies with respect to Q parameter for complex

1 (left) and for complex 2 (right) in 4 vibration.

-24 -18 -12 -6 0 6 12 18 24

0

3000

6000

9000

12000

15000

18000

dxy

dx

2-y

2

En

erg

y (c

m-1

)

Angular Distortion parameter (Q)

dxy

dx

2-y

2

dz

2

dxz

dyz

dyz

dxz

dz

2

-30 -24 -18 -12 -6 0 6 12

0

2000

4000

6000

8000

10000

12000

dz

2

dxz

dyzE

ner

gy

(cm

-1)

Angular distortion parameter (Q)

dx

2-y

2

dxy

dyz

dxz

dz

2

dx

2-y

2

dxy

Figure S7. Three dimensional magneto-structural correlation of D parameter obtained

from 4 vibrational points in case of complex 2.

Table S8: Variation of D, E/D, 1st excited state NEVPT2 energy values with respect to the

displacement of Mn along z-direction for complex 1. The frequency points (P’s) are taken from

the 5 vibrations.

Coordinates of Mn ion in (x y z)

format (Å)

D (cm-1) E/D

Energy of 1st

Excited State

(cm-1) D

Displacement

scale (xj)

P1 (-0.040 -0.019 0.360) -16.2 0.004 1407.4 48.1 2.0

P2 (-0.027 -0.013 0.248) -21.5 0.002 1016.3 42.8 1.4

P3 (-0.013 -0.006 0.120) -34.8 0.001 523.6 29.5 0.7

P4

(X-Ray

Structure)

(0.000 0.001 0.000) -64.3 0.000 18.7 0.0 0.0

P5

(0.014 0.008 -0.123)

-32.2 0.002 593.1 32.1 -0.7

P6 (0.029 0.015 -0.254)

-18.3 0.006 1232.1 46.0 -1.4

P7 (0.040 0.021 -0.359) -13.4 0.011 1751.9 50.9 -2.0

-2 -1 0 1 2

-28

-24

-20

-16

-12

-8

-4

0

D (

cm-1)

Displacement scale (xj)

complex 2

-2 -1 0 1 2-70

-60

-50

-40

-30

-20

-10

0

D (

cm-1)

Displacement scale (xj)

complex 1

Figure S8: Variation of D parameter of complex 1 (left) and complex 2 (right) with respect

to 5 vibration.

Table S9: Variation of D, E/D, 1st excited state NEVPT2 energy values with respect to the

displacement of Cr along z-direction for complex 2. The frequency points (P’s) are taken from

the 5 vibration.

Coordinates of Cr ion in (x y z)

format (Å)

D (cm-1) E/D

Energy of

1st Excited

State (cm-1) D

Displacement

scale (xj)

P1 (-0.092 -0.106 0.312) -11.1 0.005 1077.6 4.0 2.0

P2 (-0.080 -0.101 0.218) -12.1 0.004 983.9 3.0 1.4

P3 (-0.066 -0.093 0.103) -13.6 0.004 868.2 1.4 0.7

P4

(X-Ray

Structure)

(-0.05 -0.086 -0.008) -15.1 0.003 755.8 0.0 0.0

P5

(-0.040 -0.080 -0.102)

-16.7 0.003 677.4 1.6 -0.6

P6 (-0.025 -0.073 -0.221)

-18.7 0.002 584.0 3.6 -1.4

P7 (-0.012 -0.067 -0.327) -20.3 0.002 515.1 5.2 -2.0

Figure S9: Variation of ab initio LFT d-orbital splitting of complex 1 (left) and complex

2 (right) with respect to 5 vibration.

-2 -1 0 1 2

0

3000

6000

9000

12000

15000

18000

dz

2

dyz

dxy

dx

2-y

2

dxz

dyz

En

ergy

(cm

-1)

Displacement scale (xj)

dxy

dx

2-y

2

dxz

dz

2

-2 -1 0 1 2

0

2000

4000

6000

8000

10000

12000

dz

2

dxz

dyzE

ner

gy

(cm

-1)

Displacement scale (xj)

dx

2-y

2

dxy

dyz

dxz

dz

2

dx

2-y

2

dxy

Table S10: Literature survey of all reported mononuclear high-spin Mn(III) and Cr(II)

complexes.

Sr. No Complex Dexp

(cm-1) |E/D|exp

Dcal

(cm-1) |E/D|cal YearRef.

1 trans-[Mn(cyclam)I2]I +0.60 0.05 - - 20021

2 [Mn(dbm)3] -4.35 0.06 -4.55 0.06 19972

3 [MnTPPCl] -2.29 0.00 - - 19993

4 [(tpfc)Mn(OPPh3)] -2.69 0.06 - - 20004

5 [(terpy)Mn(N3)3] -3.29 0.15 -3.29 0.16 20015

6 CsMn(SO4)2‚12D2O -4.52 0.06 - - 20016

7 [Mn(cyclam)Br2]Br -1.67 0.01 - - 20017

8 [Mn(OH2)6]3+ -4.49 0.06 - - 20038

9 [Mn(acac)3] - - -4.21 0.10 20069

10 [Mn(bpia)(OAc)(OCH3)]

(PF6) +3.53 0.16 +3.24 0.16 200810

11

[MnIII(5-TMAM(R)-

salmen)(H2O)-

CoIII(CN)6]·7H2O·MeCN

-3.30 - - - 201311

12 Ph4P[Mn(opbaCl2)(py)2] -3.42 0.04 -3.47 0.01 201312

13 [Mn(dbm)2(DMSO)2]

(ClO4) -3.42 0.22 -3.64 0.00 201513

14 [Mn(dbm)2(py)2]

(ClO4) -4.46 0.21 -3.95 0.00 201513

15 Na5[Mn(L-tart)2]·12H2O -3.23 0.01 - - 201514

16 [Mn(TPP)(3,5-

Me2pyNO)2]ClO4 -3.82 0.04 -3.94 0.01 201515

17 [Mn(3-OEt-salme)2]BPh4 -4.60 0.32 -3.81 0.27 201616

18 MnL(NCS)·0.4H2O +2.53 0.18 +2.66 0.20 201717

19

Mn(tpfc) (tpfc = 5,10,15-

tris(pentafluorophenyl)corrole

trianion)

-2.67 0.01 - - 202018

20 [Mn{(OPPh2)2N}3] -3.92 0.00 -3.52 0.01 202019

21 [CrII(N(TMS)2)2(py)2] -1.80 0.01 -1.50 0.04 201520

22 [CrII(N(TMS)2)2(THF)2] -2.00 0.01 -1.66 0.05 201520

23 Cr(CO)3(η

6,η6-C6H5C6H5)

Cr(Al2(OC(CF3)2H)4) -2.15 0.00 - - 201721

24

{[1-N-3,5-tBu2dp)4Cr][5-

(N,N,C,C,P))2K(1-O-

THF)2[2}

-1.86 0.01 - - 202022

25 [Cr(iPrNC(CH3)NiPr)2] -1.74 0.04 -1.50 0.02 202123

26 [Cr(CyNC(CH3)NCy)2] -1.82 0.05 -1.47 0.02 202123

27 [Cr(DippNC(CH3)NDipp)2] -1.71 0.04 -1.48 0.02 202123

28 [Cr(tBuNC(CH3)NtBu)2] -1.94 0.02 -1.68 0.00 202123

29 [Cr{N(SiMe2Ph)2}2] -2.70 0.07 -2.70 0.07 202124

References:

1. Mossin, S.; Weihe, H.; Barra, A.-L., Is the axial zero-field splitting parameter of tetragonally

elongated high-spin manganese (III) complexes always negative? J. Am. Chem. Soc. 2002, 124 (30),

8764-8765.

2. Barra, A. L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.; Fabretti, A. C.; Uytterhoeven,

M. G., Electronic Structure of Manganese (III) Compounds from High‐Frequency EPR Spectra.

Angewandte Chemie International Edition in English 1997, 36 (21), 2329-2331.

3. Krzystek, J.; Telser, J.; Pardi, L. A.; Goldberg, D. P.; Hoffman, B. M.; Brunel, L.-C., High-

frequency and-field electron paramagnetic resonance of high-spin manganese (III) in porphyrinic

complexes. Inorg. Chem. 1999, 38 (26), 6121-6129.

4. Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z., High-field (high-frequency) EPR

spectroscopy and structural characterization of a novel manganese (III) corrole. Chemical

Communications 2000, (19), 1957-1958.

5. Limburg, J.; Vrettos, J. S.; Crabtree, R. H.; Brudvig, G. W.; de Paula, J. C.; Hassan, A.; Barra,

A.-L.; Duboc-Toia, C.; Collomb, M.-N., High-Frequency EPR Study of a New Mononuclear

Manganese (III) Complex:[(terpy) Mn (N3) 3](terpy= 2, 2 ‘: 6 ‘, 2 ‘‘-Terpyridine). Inorg. Chem. 2001,

40 (7), 1698-1703.

6. Basler, R.; Tregenna-Piggott, P. L.; Andres, H.; Dobe, C.; Güdel, H.-U.; Janssen, S.; McIntyre,

G. J., Magnetic Excitations of CsMn (SO4) 2⊙ 12D2O, Measured by Inelastic Neutron Scattering. J.

Am. Chem. Soc. 2001, 123 (14), 3377-3378.

7. Mossin, S.; Stefan, M.; ter Heerdt, P.; Bouwen, A.; Goovaerts, E.; Weihe, H., Fourth-order

zero-field splitting parameters of [Mn (cyclam) Br 2] Br determined by single-crystal W-band EPR.

Applied Magnetic Resonance 2001, 21 (3-4), 587-596.

8. Tregenna-Piggott, P. L.; Weihe, H.; Barra, A.-L., High-field, multifrequency EPR study of the

[Mn (OH2) 6] 3+ cation: influence of π-bonding on the ground state zero-field-splitting parameters.

Inorg. Chem. 2003, 42 (25), 8504-8508.

9. Neese, F., Importance of direct spin− spin coupling and spin-flip excitations for the zero-field

splittings of transition metal complexes: A case study. J. Am. Chem. Soc. 2006, 128 (31), 10213-10222.

10. Scheifele, Q.; Riplinger, C.; Neese, F.; Weihe, H.; Barra, A.-L.; Juranyi, F.; Podlesnyak, A.;

Tregenna-Piggott, P. L., Spectroscopic and theoretical study of a mononuclear manganese (III) complex

exhibiting a tetragonally compressed geometry. Inorg. Chem. 2008, 47 (2), 439-447.

11. Ishikawa, R.; Miyamoto, R.; Nojiri, H.; Breedlove, B. K.; Yamashita, M., Slow relaxation of

the magnetization of an MnIII single ion. Inorg. Chem. 2013, 52 (15), 8300-8302.

12. Vallejo, J.; Pascual‐Álvarez, A.; Cano, J.; Castro, I.; Julve, M.; Lloret, F.; Krzystek, J.; De

Munno, G.; Armentano, D.; Wernsdorfer, W., Field‐induced hysteresis and quantum tunneling of the

magnetization in a mononuclear manganese (III) complex. Angewandte Chemie 2013, 125 (52), 14325-

14329.

13. Chen, L.; Wang, J.; Liu, Y. Z.; Song, Y.; Chen, X. T.; Zhang, Y. Q.; Xue, Z. L., Slow magnetic

relaxation in mononuclear octahedral manganese (III) complexes with dibenzoylmethanide ligands.

Eur. J. Inorg. Chem. 2015, 2015 (2), 271-278.

14. Craig, G. A.; Marbey, J. J.; Hill, S.; Roubeau, O.; Parsons, S.; Murrie, M., Field-induced slow

relaxation in a monometallic manganese (III) single-molecule magnet. Inorg. Chem. 2015, 54 (1), 13-

15.

15. Pascual‐Álvarez, A.; Vallejo, J.; Pardo, E.; Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.;

Wernsdorfer, W.; Cano, J., Field‐Induced Slow Magnetic Relaxation in a Mononuclear Manganese

(III)–Porphyrin Complex. Chemistry–A European Journal 2015, 21 (48), 17299-17307.

16. Realista, S.; Fitzpatrick, A. J.; Santos, G.; Ferreira, L. P.; Barroso, S.; Pereira, L. C.; Bandeira,

N. A.; Neugebauer, P.; Hruby, J.; Morgan, G. G.; van Slageren, J.; Calhorda, M. J.; Martinho, P. N., A

Mn(iii) single ion magnet with tridentate Schiff-base ligands. Dalton Trans. 2016, 45 (31), 12301-7.

17. Shova, S.; Vlad, A.; Cazacu, M.; Krzystek, J.; Bucinsky, L.; Breza, M.; Darvasiová, D.; Rapta,

P.; Cano, J.; Telser, J., A five-coordinate manganese (III) complex of a salen type ligand with a positive

axial anisotropy parameter D. Dalton Trans. 2017, 46 (35), 11817-11829.

18. Krzystek, J.; Schnegg, A.; Aliabadi, A.; Holldack, K.; Stoian, S. A.; Ozarowski, A.; Hicks, S.

D.; Abu-Omar, M. M.; Thomas, K. E.; Ghosh, A., Advanced Paramagnetic Resonance Studies on

Manganese and Iron Corroles with a Formal d4 Electron Count. Inorg. Chem. 2020, 59 (2), 1075-1090.

19. Sanakis, Y.; Krzystek, J.; Maganas, D.; Grigoropoulos, A.; Ferentinos, E.; Kostakis, M. G.;

Petroulea, V.; Pissas, M.; Thirunavukkuarasu, K.; Wernsdorfer, W.; Neese, F.; Kyritsis, P., Magnetic

Properties and Electronic Structure of the S = 2 Complex [Mn(III){(OPPh2)2N}3] Showing Field-

Induced Slow Magnetization Relaxation. Inorg. Chem. 2020, 59 (18), 13281-13294.

20. Deng, Y. F.; Han, T.; Wang, Z.; Ouyang, Z.; Yin, B.; Zheng, Z.; Krzystek, J.; Zheng, Y. Z.,

Uniaxial magnetic anisotropy of square-planar chromium(II) complexes revealed by magnetic and HF-

EPR studies. Chem Commun (Camb) 2015, 51 (100), 17688-91.

21. Wang, W.; Li, J.; Yin, L.; Zhao, Y.; Ouyang, Z.; Wang, X.; Wang, Z.; Song, Y.; Power, P. P.,

Half-sandwich metal carbonyl complexes as precursors to functional materials: from a near-infrared-

absorbing dye to a single-molecule magnet. J. Am. Chem. Soc. 2017, 139 (34), 12069-12075.

22. Su, J.; Yin, L.; Ouyang, Z.; Wang, Z.; Zheng, W., 1,2-Diaza-4-phospholide complexes of

chromium(ii): dipotassium organochromates behaving as single-molecule magnets. Dalton Trans.

2020, 49 (21), 6945-6949.

23. Zhai, Y. Q.; Ge, N.; Li, Z. H.; Chen, W. P.; Han, T.; Ouyang, Z. W.; Wang, Z.; Zheng, Y. Z.,

Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes. Inorg.

Chem. 2021, 60 (3), 1344-1351.

24. Freitag, K.; Stennett, C. R.; Mansikkamaki, A.; Fischer, R. A.; Power, P. P., Two-Coordinate,

Nonlinear Vanadium (II) and Chromium (II) Complexes of the Silylamide Ligand–N (SiMePh2) 2:

Characterization and Confirmation of Orbitally Quenched Magnetic Moments in Complexes with Sub-

d5 Electron Configurations. Inorg. Chem. 2021.

download fileview on ChemRxivSupporting_Information.pdf (1.06 MiB)

Record High Magnetic Anisotropy in Three

Coordinate MnIII and CrII Complexes: A Theoretical

Perspective

Arup Sarkar, Reshma Jose, Harshit Ghosh and Gopalan Rajaraman*

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076,Maharashtra, India

Keywords: Low-coordinate, Ab initio, NEVPT2, Zero-field Splitting, Spin-vibronic coupling

ABSTRACT: Ab initio calculations performed in two three-coordinate complexes

[Mn{N(SiMe3)2}3] (1) and [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (2) reveal record-high

magnetic anisotropy with the D values -64 cm-1 and -15 cm-1 respectively, enlisting d4 ion back in

the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was

performed in 1 and 2 that suggests a dominant under barrier relaxation due to flexible

coordination sphere around the metal ion offering design clues for low coordinate transition

metal SIMs.

1

Introduction

Single-Molecule Magnets (SMMs) have become a fascinating research area as this class of

molecules exhibit magnetization just like permanent magnets below a critical temperature

defined as blocking temperature TB.1 An important parameter associated with the blocking

temperature is the barrier height for magnetization reversal (Ueff) which is correlated to the

magnetic moment of different microstate and the nature of anisotropy. While in lanthanide

complexes, the first-order spin-orbit coupling (SOC) is strong enough to produce large barrier

heights,2 in transition metal (TM) systems spin-orbit coupling is generally weak, leading to

relatively smaller anisotropy which is reflected in the axial zero-field splitting parameter (D)

which can be tuned at will using ligand field.3

There are several challenges in enhancing the blocking temperature TB in SMMs as several

relaxation mechanisms other than the Orbach process spoil the direct correlation of TB to Ueff

values. Among others, quantum tunneling of magnetization (QTM) and

spin-phonon/vibrational-mode relaxation mechanisms are a prominent source of relaxation, as

shown in recent years by various groups.4 Earlier research in the SMM area was focused on

increasing the total spin (S) of the complexes by increasing the number of metal centers. After

2

the discovery of a very small Ueff barrier in {Mn19} cluster possessing record high ground state S

value, it becomes clear that increasing the number of metal centers or S value diminishes the

axial anisotropy (D term) as evident from the equation proposed originally by Abragam and

Bleaney5 and adapted in ab initio calculations later on. For this reason, mononuclear TM

complexes gained significant attention leading to the birth of several single-ion magnets (SIMs)

based on the low coordination number such as Fe(II/I), Ni(II), and Co(II), exhibiting very large

Ueff values.6

In the early years of SMMs, the focus has been on transition metal cluster particularly that of

Mn(III) ions, as this offer an easy source of negative D parameter for the chemists and unearthed

numerous SMMs albeit with smaller Ueff/TB values. 1a, 7 The Mn(III) ions are very robust and can

be easily incorporated in cluster aggregation, and are relatively redox stable6a but exhibit only

small D values of the order of ~|5| cm-1. 8 While several low-coordinate transition metal ions were

pursued recently for potential SMMs, Mn(III) has not been studied in detail, perhaps due to the

perception that the expected D values are rather small.8 Apart from Ueff values, the blocking

temperature is an important criterion which is often very small. This suggests that apart from the

QTM effect, spin-vibrational relaxations are at play in such systems.4 How these effects manifest

in these complexes are not fully understood.

To ascertain complexes that exhibit large negative D values and also to correlate the relaxation

mechanism to spin-vibrational coupling, we undertake theoretical studies based on multi-

3

configurational ab initio calculations SA-CASSCF/NEVPT2 using the ORCA suite.9 Here, we

have studied in detail two three-coordinate d4 systems Mn(III) and Cr(II): [Mn{N(SiMe3)2}3]10

(1) and [K(18-crown-6)(Et2O)2] [Cr{N(SiMe3)2}3]11 (2) using their reported X-ray structure.

Our NEVPT2 calculations yield a record axial D value of -64 cm-1 and -15 cm-1 for 1 and 2,

respectively, with a negligible E/D value. The D value computed for both complexes is larger

than any examples reported to-date and suggests a potential SMMs characteristic for these robust

building block metal ions.

Computational Details

All the ab initio single point calculations have been performed using ORCA 4.0.0 program.9

DKH (Douglas-Kroll-Hess) Hamiltonian was used to account for the scalar relativistic effect.

DKH contracted versions of the basis sets were used during the calculations- DKH-def2-TZVP

for Mn, Cr, Si; DKH-def2-TZVP(-f) for N and DKH-def2-SVP for the rest of the atoms. During

the orbital optimization step in SA-CASSCF (state-averaged complete active space self-

consistent field) method, 4 metal electrons in 5 metal d-orbitals were taken into consideration

and optimized with 5 quintet and 35 triplet roots for Mn(III) and Cr(II) metal centers. Additional

calculations have also been carried out with 5 quintets, 45 triplet roots, and 5 quintets, 35 triplet,

and 22 singlet roots to check the effect of high-lying excited states on the Spin-Hamiltonian (SH)

parameters. The addition of extra 10 triplet roots and 22 singlet roots marginally affect the SH

parameters (see Table S1 in ESI). NEVPT2 (N-electron valence perturbation theory second-

4

order) calculation has also been performed on the top of converged SA-CASSCF wavefunction

to include the dynamic electron correlation. Spin-orbit interaction was accounted with quasi-

degenerate perturbation theory (QDPT) approach using SOMF (spin-orbit mean field) operator.

Only spin-orbit contributions towards zero-field splitting were computed. Final Spin-

Hamiltonian parameters were determined with effective Hamiltonian approach (EHA)

formalism.12 Ab initio ligand field theory (AILFT) calculations have also been performed to

obtain the d-orbital energies.13

Geometry optimization and single point frequency calculations have been carried out in Gaussian

09 (Rev. D.01) program.14 Hybrid unrestricted B3LYP-D2 functional was used for the DFT

calculations along with Ahlrich’s triple- valence polarized (TZVP) basis set for Mn, Cr, Si, N

and Ahlrich’s spilt valence polarized (SVP) basis set for rest of the atoms.15

Result and Discussions

Complex 1 possesses a perfect D3h symmetry as the three N-Mn-N bond angles are 120.02,

120.02 and 119.95 and the three Mn-N bond lengths are 1.89, 1889, and 1.89 (in Å unit). It was

5

Figure 1: NEVPT2 computed Dzz axis of the molecule plotted on the X-Ray structure (left) and three Mn-N bond lengths (in Å) and N-Mn-N angles () shown on the molecule (right). Colour code: Mn: pink, N: blue, Si: light green, C: dark grey. Hydrogens are omitted due to clarity.

also noticed that the {MnN3} core was planar, and the bulky trimethylsilyl groups surrounded the

central moiety stabilizes the low coordinate molecule from further coordination via steric

arrangements (see Figure 1). The NEVPT2-QDPT calculated major anisotropy axes, i.e., Dzz and

gzz axes, were found to be exactly perpendicular to the Mn-N3 plane, i.e., exactly collinear with

the C3 axis, which describes the axial nature of anisotropy present in the molecule.

A record axial zero-field splitting (ZFS) was found for this complex, showing a D value of -64

cm-1 with E/D estimated to be 0.0003, indicating strong easy-axis type anisotropy (see Table 1)

for complex 1. A very similar geometry was observed in the Cr(II) analog, and the N-Cr-N

6

bond angles (123.9, 115.81 and 120.29) are not exactly similar and significantly deviated from

D3h symmetry. The D value for complex 2 is less than complex 1 due to these structural

deviations and also smaller spin-orbit coupling constant () values of Cr(II) than Mn(III). For

complex 2, the E/D value is estimated to be 0.003, which is ten times larger than complex 1 (see

Figure S1 and Table S1 in ESI).

The ground state electronic configuration of complex 1 is dz21dyz

1dxz1dxy

1dx2-y

20, and this comprises

77% of the overall wavefunction. The major contribution (-62 cm-1) towards the negative D value

arises from the first excited state, which consists of dxy dx2-y

2 (same ML valued) electronic

excitation and this excited state contribute ~97% of the overall D value (see Figure 2 and Table

S2 in ESI). Other electronic transitions were found to contribute negligibly to the overall D

value. A very close analysis of the NEVPT2 states reveals that the first excited quintet state is

only 19 cm-1 apart from the ground state and consequently results in a very strong second-order

spin-orbit coupling. While the first excited state is the spin-allowed quintet, the second, third and

fourth excited states arise from the spin-flipped triplet transitions. These three excited states

consist of a mixture of dxy dxz/dyz and dxy dz2 transitions (see Table S2 in ESI). The computed

gx, gy and gz values are 1.67, 1.67 and 1.14 respectively for true spin S =2 and 0.00 0.00 and 5.14,

respectively for pseudospin S =1/2 manifold. In the case of complex 2, the first excited state

contributes 96% (-14.4 cm-1) towards the overall D value. Again, the D value is negative due to

the coupling with the prominent first excited state involving the same ML level dxy dx2-y

2

7

Figure 2: NEVPT2-LFT d-orbital diagram of complex 1. The orange arrow indicates the first excited spin-allowed transition.

electronic excitation (see Figure S2 in ESI). Here one major difference of complex 2 from

complex 1 is that due to significant distortion from D3h and lower ligand field of Cr(II), the first

excited state is 756 cm-1 apart and the next three excited states are quintets (see Table S3). The

computed gx, gy and gz values are 1.97, 1.97, and 1.58 respectively for true spin S=2 and 0.00

0.00 and 6.351, respectively for pseudospin S =1/2 manifold.

In the case of non-Kramers ions like in these two cases studied, the tunnel-splitting is generally

larger, leading to faster relaxation via the QTM process. The tunnel-splitting strongly depends on

the local symmetry and ligand field environment around the metal ion. The high symmetry

8

present in complexes 1 and 2 leads to smaller tunnel splitting (see Figure S3 and Table S4 in

ESI). The first excited pseudo-KDs is separated by 154 cm-1 in case of complex 1 and 45 cm-1 in

case of complex 2. The multi-determinant nature of the ground state leads to mixing of the |+2

and |-2 states, and this is very prominent in complex 1 compared to 2 (see Table S4).

The static electronic picture is insufficient to describe the relaxation mechanism or the spin

dynamics of the system. Recent reports of spin-vibronic coupling that describe the role of

vibrational frequencies of a single molecule or of the surrounding lattice are very important to

elucidate the dynamic scenario of SMMs.4 In this regard, we have attempted to investigate the

role of molecular vibrations occurring at low temperatures on the spin-orbit or Ms levels in the

two complexes. Therefore, we have performed frequency calculations (normal modes) on the X-

ray structures of 1 and 2 using Density Functional Theory (DFT) methods (B3LYP-D2/TZVP, a

similar vibrational pattern also found for the optimized geometries, see Table S5 in ESI).

Here we have carefully analyzed five lower energy vibrational modes below 80 cm-1, and these

are 1 45.1 (40.1), 2 45.8(46.2), 3 58.4(50.3), 4 70.3(58.8) and 5 72.9 (69.9) for complex 1 (2)

(see Figure S4 in ESI). Out of the five vibrations mentioned, the 4 and 5 vibrations were found

to be IR active and also break the D3h symmetry (see Figure S4). Here 4 corresponds to N-M-

N bond angle bending and correlates to Jahn-Teller active vibration (E irreducible

representation in D3h symmetry). The 5 corresponds to out-of-plane (M-N-N-N) bending

vibration of the metal ion and associate with A2” irreducible representation. Several displacement

9

Figure 3: Variation of D values in complexes 1 (left) and 2 (right) with respect to the distortion parameter Q and displacement factor xj for 4 vibrational mode.

points in 4 and 5 vibrational surfaces were considered for CASSCF/NEVPT2 calculations. The

maximum displacement scale of a particular vibration j, denoted by xj, was fixed at 2.0 for both

the complexes as suggested earlier (see Table S6 and S7 in ESI).16

An angular distortion parameter Q was introduced, which is a sum of deviation from 120 from

each of the equatorial N-M-N angles (denoted as ) (see Table S6 and S7).6e Here, in order to

find out the spin-vibronic coupling, the variation of D and E/D have been computed with respect

to the displacement of nuclear coordinates (x) using the following Hamiltonian:

H s−vib=(∂ D∂x

) x [SZ2−S (S+1 )

3 ]+(∂E∂ x

)x (Sx2−Sy

2)…eqn.1¿

10

In Figure 3, we plot computed D values with respect to Q and xj, and these plots show that as

the Q diverges from zero, the magnitude of D decreases for 4 vibrations (see Figure S5 in ESI).

This is because an increase in Q breaks the D3h symmetry and, consequently, increases the gap

between the dxy and dx2-y

2 orbitals (see Fig. S6). In complex 1, the X-Ray structure shows the

highest negative D value and minimum E/D value (see Fig. S5) at equilibrium geometry or zero

displacement point, but for complex 2, the X-ray structure is significantly deviated from the ideal

D3h symmetry and therefore do not have the largest negative D or the lowest E/D at zero

displacement point. In complex 2, at xj = 0.8 (see Fig. 3), the Q parameter shows a minimum and

predicts a D value as high as -46 cm-1.

Furthermore, we have developed a three-dimensional magneto-structural correlation to see the

effect of angle change on the D values for complex 1 and 2 (see Figure 4 and S7 in ESI). It is

very clear that the D is maximum when all the three equatorial angles are 120. For 1, the

variation in D values is found to be relatively smaller for 5 vibrations compared to 4 mode (see

Table S8-S9 in ESI). For 2 no spin-vibrational coupling is detected as a much smaller change in

D is noted. To rationalize this observation, the AILFT computed d-orbitals are plotted, and this

reveals that the dxy-dx2-y

2 orbital energy gap is altered only slightly in 1 and negligibly in 2 (see

Fig. S8-S9). This suggests that 4 vibrational mode is dominant in controlling the magnetic

anisotropy in trigonal planar d4 systems, and this vibration likely offers a smaller barrier height

11

for relaxation at lower temperatures. Between complex 1 and 2, the spin-vibronic coupling is

found to be stronger in the former.

At the equilibrium geometries, neglecting other effects, the computed Ucal values for complexes

1 and 2 are 153.8 and 44.7 cm-1 for complexes 1 and 2, respectively. Considering the vibrational

relaxation 4 modes at the displacement scale of xj=±2, the Ueff value is expected to be diminished

to 19 cm-1 and 15 cm-1 for complexes 1 and 2, respectively (neglecting the QTM effects). This is

substantially smaller than the barrier height estimated from the Orbach process and suggests a

dominant spin-vibrational relaxation role in the magnetization relaxation in these complexes.

This may be attributed to the fact that the N-M-N bond angle bending vibration is very subtle

and does not require significant energy for structural distortion and is strongly correlated to the

dxy and dx2-y

2 gap altering the magnetic anisotropy. This advocates a design principle that a rigid

structure with a robust N-Mn-N angle could block such relaxation, and this is possible if a

chelate type or macrocyclic type ligands are employed.

12

Figure 4. Three-dimensional magneto-structural correlation of D obtained from 4 mode for complex 1.

Conclusion

To the end, we have successfully employed an accurate ab initio method to explore the zero-field

splitting and ligand field parameters in two MnIII and CrII high spin complexes. A record-high D

value of -64 cm-1 and -15 cm-1 was found for the X-ray structures of 1 and 2, respectively. These

two values are higher than any other reported D values for any mononuclear d4 systems (see

Table S10 in ESI). While a significant barrier for magnetization relaxation is found for both the

complexes, our detailed analysis revealed a strong spin-vibration coupling both the complexes

that are likely to yield smaller blocking temperatures.

13

ASSOCIATED CONTENT

The following files are available free of charge.

AUTHOR INFORMATION

Corresponding Author

Gopalan Rajaraman. Email: rajaraman@chem.iitb.ac.in

Present Addresses

†Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076,

Maharashtra, India

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval

to the final version of the manuscript.

Funding Sources

This work was funded by DST and SERB (CRG/2018/000430; DST/SJF/CSA-03/2018-10;

SB/SJF/2019-20/12), UGC-UKIERI (184-1/2018(IC)) and SUPRA (SPR/2019/001145).

ACKNOWLEDGMENT

14

RJ thanks DST-INSPIRE and AS thanks IIT Bombay for IPDF funding.

REFERENCES

1. (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M., Magnetic bistability in a metal-ion cluster. Nature 1993, 365 (6442), 141-143; (b) Gatteschi, D.; Sessoli, R.; Villain, J.,Molecular nanomagnets. Oxford University Press on Demand: 2006; Vol. 5.2. Woodruff, D. N.; Winpenny, R. E.; Layfield, R. A., Lanthanide single-molecule magnets.Chem. Rev. 2013, 113 (7), 5110-5148.3. Gomez-Coca, S.; Cremades, E.; Aliaga-Alcalde, N.; Ruiz, E., Mononuclear single-molecule magnets: tailoring the magnetic anisotropy of first-row transition-metal complexes. J.Am. Chem. Soc. 2013, 135 (18), 7010-8.4. (a) Gomez-Coca, S.; Urtizberea, A.; Cremades, E.; Alonso, P. J.; Camon, A.; Ruiz, E.;Luis, F., Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy. Nat.Commun. 2014, 5, 4300; (b) Rechkemmer, Y.; Breitgoff, F. D.; van der Meer, M.; Atanasov, M.;Hakl, M.; Orlita, M.; Neugebauer, P.; Neese, F.; Sarkar, B.; van Slageren, J., A four-coordinatecobalt(II) single-ion magnet with coercivity and a very high energy barrier. Nat. Commun. 2016,7, 10467; (c) Escalera-Moreno, L.; Suaud, N.; Gaita-Arino, A.; Coronado, E., Determining KeyLocal Vibrations in the Relaxation of Molecular Spin Qubits and Single-Molecule Magnets. J.Phys. Chem. Lett. 2017, 8 (7), 1695-1700; (d) Lunghi, A.; Totti, F.; Sessoli, R.; Sanvito, S., Therole of anharmonic phonons in under-barrier spin relaxation of single molecule magnets. Nat.Commun. 2017, 8, 14620; (e) Moseley, D. H.; Stavretis, S. E.; Thirunavukkuarasu, K.; Ozerov,M.; Cheng, Y.; Daemen, L. L.; Ludwig, J.; Lu, Z.; Smirnov, D.; Brown, C. M.; Pandey, A.;Ramirez-Cuesta, A. J.; Lamb, A. C.; Atanasov, M.; Bill, E.; Neese, F.; Xue, Z. L., Spin-phononcouplings in transition metal complexes with slow magnetic relaxation. Nat. Commun. 2018, 9(1), 2572.5. Abragam, A.; Bleaney, B., Electron paramagnetic resonance of transition ions. OUPOxford: 2012.6. (a) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.;Long, J. R., Magnetic blocking in a linear iron (I) complex. Nat. Chem. 2013, 5 (7), 577; (b)

15

Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C.-Y.; Rekken, B. D.; Power, P. P.; Neese, F.;Long, J. R., Slow magnetization dynamics in a series of two-coordinate iron (II) complexes.Chem. Sci. 2013, 4 (1), 125-138; (c) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang,M.-W.; Jiang, S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W., Two-coordinate Co(II) imido complexes as outstanding single-molecule magnets. J. Am. Chem. Soc. 2016, 139 (1),373-380; (d) Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita,M.; Overgaard, J.; Van Slageren, J.; Neese, F.; Long, J. R., A linear cobalt (II) complex withmaximal orbital angular momentum from a non-Aufbau ground state. Science 2018, 362 (6421),eaat7319; (e) Craig, G. A.; Sarkar, A.; Woodall, C. H.; Hay, M. A.; Marriott, K. E. R.; Kamenev,K. V.; Moggach, S. A.; Brechin, E. K.; Parsons, S.; Rajaraman, G.; Murrie, M., Probing theorigin of the giant magnetic anisotropy in trigonal bipyramidal Ni(ii) under high pressure. Chem.Sci. 2018, 9 (6), 1551-1559; (f) Sarkar, A.; Dey, S.; Rajaraman, G., Role of coordination numberand geometry in controlling the magnetic anisotropy in FeII, CoII, and NiII single-ion magnets.Chem. Eur. J 2020, 26, 14036-14058.7. Ako, A. M.; Hewitt, I. J.; Mereacre, V.; Clérac, R.; Wernsdorfer, W.; Anson, C. E.;Powell, A. K., A ferromagnetically coupled Mn19 aggregate with a record S= 83/2 ground spinstate. Angewandte Chemie 2006, 118 (30), 5048-5051.8. (a) Maurice, R.; de Graaf, C.; Guihery, N., Magnetostructural relations from a combinedab initio and ligand field analysis for the nonintuitive zero-field splitting in Mn(III) complexes.The Journal of chemical physics 2010, 133 (8), 084307; (b) Ishikawa, R.; Miyamoto, R.; Nojiri,H.; Breedlove, B. K.; Yamashita, M., Slow relaxation of the magnetization of an Mn(III) singleion. Inorg. Chem. 2013, 52 (15), 8300-2; (c) Vallejo, J.; Pascual-Alvarez, A.; Cano, J.; Castro, I.;Julve, M.; Lloret, F.; Krzystek, J.; De Munno, G.; Armentano, D.; Wernsdorfer, W.; Ruiz-Garcia, R.; Pardo, E., Field-induced hysteresis and quantum tunneling of the magnetization in amononuclear manganese(III) complex. Angewandte Chemie 2013, 52 (52), 14075-9; (d) Chen,L.; Wang, J.; Liu, Y.-Z.; Song, Y.; Chen, X.-T.; Zhang, Y.-Q.; Xue, Z.-L., Slow MagneticRelaxation in Mononuclear Octahedral Manganese(III) Complexes with DibenzoylmethanideLigands. Eur. J. Inorg. Chem. 2015, 2015 (2), 271-278; (e) Craig, G. A.; Marbey, J. J.; Hill, S.;Roubeau, O.; Parsons, S.; Murrie, M., Field-induced slow relaxation in a monometallicmanganese(III) single-molecule magnet. Inorg. Chem. 2015, 54 (1), 13-5; (f) Pascual-Alvarez,A.; Vallejo, J.; Pardo, E.; Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.; Wernsdorfer, W.;Cano, J., Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-PorphyrinComplex. Chemistry 2015, 21 (48), 17299-307; (g) Duboc, C., Determination and prediction of

the magnetic anisotropy of Mn ions. Chem. Soc. Rev. 2016, 45 (21), 5834-5847; (h) Realista, S.;

16

Fitzpatrick, A. J.; Santos, G.; Ferreira, L. P.; Barroso, S.; Pereira, L. C.; Bandeira, N. A.;Neugebauer, P.; Hruby, J.; Morgan, G. G.; van Slageren, J.; Calhorda, M. J.; Martinho, P. N., AMn(iii) single ion magnet with tridentate Schiff-base ligands. Dalton Trans. 2016, 45 (31),12301-7; (i) Sanakis, Y.; Krzystek, J.; Maganas, D.; Grigoropoulos, A.; Ferentinos, E.; Kostakis,M. G.; Petroulea, V.; Pissas, M.; Thirunavukkuarasu, K.; Wernsdorfer, W., Magnetic Propertiesand Electronic Structure of the S= 2 Complex [MnIII {(OPPh2) 2N} 3] Showing Field-InducedSlow Magnetization Relaxation. Inorg. Chem. 2020, 59 (18), 13281-13294.9. Neese, F., Software update: the ORCA program system, version 4.0. WIRES: Comp. Mol.Sci. 2018, 8 (1), e1327.10. Ellison, J. J.; Power, P. P.; Shoner, S. C., First examples of three-coordinate manganese(III) and cobalt (III): synthesis and characterization of the complexes M [N (SiMe3) 2] 3 (M=Mn or Co). J. Am. Chem. Soc. 1989, 111 (20), 8044-8046.11. Wagner, C. L.; Phan, N. A.; Fettinger, J. C.; Berben, L. A.; Power, P. P., NewCharacterization of V{N(SiMe3)2}3: Reductions of Tris[bis(trimethylsilyl)amido]vanadium(III)and -chromium(III) To Afford the Reduced Metal(II) Anions [M{N(SiMe3)2}3](-) (M = V andCr). Inorg. Chem. 2019, 58 (9), 6095-6101.12. Maurice, R.; Bastardis, R.; Graaf, C. d.; Suaud, N.; Mallah, T.; Guihery, N., Universaltheoretical approach to extract anisotropic spin Hamiltonians. J. Chem. Theory Comput. 2009, 5(11), 2977-2984.13. Singh, S. K.; Eng, J.; Atanasov, M.; Neese, F., Covalency and chemical bonding intransition metal complexes: An ab initio based ligand field perspective. Coord. Chem. Rev. 2017,344, 2-25.14. Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G., gaussian 09, Revision d. 01, Gaussian.Inc., Wallingford CT 2009, 201.15. (a) Schäfer, A.; Horn, H.; Ahlrichs, R., Fully optimized contracted Gaussian basis sets foratoms Li to Kr. J. Chem. Phys. 1992, 97 (4), 2571-2577; (b) Schäfer, A.; Huber, C.; Ahlrichs, R.,Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr.J. Chem. Phys. 1994, 100 (8), 5829-5835.16. Goodwin, C. A.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P., Molecular magnetichysteresis at 60 kelvin in dysprosocenium. Nature 2017, 548 (7668), 439.

17

TOC: Using Ab initio NEVPT2 calculations, we offer a design principle to enhance ZFS inhigh-spin d4 complexes of first-row transition elements.

18

download fileview on ChemRxivManuscript_GR_final_Arxiv2.docx (4.55 MiB)

Supporting Information

Record High Magnetic Anisotropy in Three Coordinate MnIII and CrII

Complexes: A Theoretical Perspective

Arup Sarkar, Reshma Jose, Harshit Ghosh, and Gopalan Rajaraman*

Department of Chemistry, Indian Institute of Technology Bombay, Mumbai- 400076, India

Email: rajaraman@chem.iitb.ac.in

Figure S1: Major anisotropic axes plotted on complex 2 from NEVPT2 level of theory (left). Equatorial N-Cr-N bond angles and Cr-N bond lengths shown on X-Ray structure of complex 2 (right). Colour code: Cr: green, N: blue, Si: bluish green, C: dark grey. Hydrogens are omitted for clarity.

Table S1: Comparison of the ZFS parameters using different triplet and singlet roots during the SA-CASSCF/NEVPT2 calculations.

Sets Quintet roots Triplet roots Singlet roots D (cm-1) E/D

Complex 1 5 35 - -64.5 0.0002

Complex 1 5 45 - -64.3 0.0003

Complex 1 5 35 22 -63.5 0.0003

Complex 2 5 35 - -15.2 0.003

Complex 2 5 45 - -15.1 0.003

Complex 2 5 45 22 -14.8 0.003

Table S2: NEVPT2 vertical excitation spectrum, CASSCF electronic configuration and theirrespective contribution towards D and E values for ground state and six lower lying excitedstates for complex 1.

StatesMajor CASSCF electronic

configuration

NEVPT2Energy (cm-

1)

Contribution to D(cm-1)

Contribution toE (cm-1)

5Edz

21dyz1dxz

1dxy1dx

2-y

20 (77%)dz

21dyz1dxz

1dxy0 dx

2-y

21 (76%)0.018.7

0.0-62.16

0.00.00

3E

dz21dyz

1dxz2dxy

0 dx2-y

20 (37%)dz

21dyz2dxz

1dxy0 dx

2-y

20 (12%)13593.2 -0.38 0.38

dz21dyz

2dxz1dxy

0 dx2-y

20 (38%)dz

21dyz1dxz

2dxy0 dx

2-y

20 (12%)13597.6 -0.23 -0.38

3A2dz

22dyz1dxz

1dxy0 dx

2-y

20 (42%)dz

22dyz1dxz

0dxy1 dx

2-y

20 (15%)13720.0 -0.09 0.09

5Edz

21dyz1dxz

0dxy1 dx

2-y

21 (74%)dz

21dyz0dxz

1dxy1 dx

2-y

21 (74%)18929.818949.3

0.180.13

-0.180.18

Table S3: NEVPT2 vertical excitation spectrum, CASSCF electronic configuration and theirrespective contribution towards D and E values for ground state and four lower lying excitedstates for complex 2 (The irreducible representations are assigned according to C2v

symmetry).

StatesMajor CASSCF electronic

configuration

NEVPT2Energy (cm-

1)

Contribution to D(cm-1)

Contribution toE (cm-1)

5A2 dz21dyz

1dxz1dxy

1 dx2-y

20 (91%) 0.0 0.0 0.05A2 dz

21dyz1dxz

1dxy0 dx

2-y

21 (91%) 755.8 -14.43 0.005B1 dz

21dyz1dxz

0dxy1 dx

2-y

21 (94%) 10368.3 0.16 -0.125B2 dz

21dyz0dxz

1dxy1 dx

2-y

21 (94%) 10808.9 0.13 0.105A1 dz

20dyz1dxz

1dxy1 dx

2-y

21 (100%) 12854.9 0.00 -0.00

Figure S2: NEVPT2-LFT d-orbital splitting diagram for complex 2. Green arrows indicate the first excited state electronic transition. The green arrow indicates 1st excited state electronic excitation.

Figure S3: Energy profile diagram of the lowest five spin-orbit states of complex 1 (left) and complex 2 (right) obtained from NEVPT2/QDPT/EHA method.

Complex 1Spin-orbit

StateS MS

QuintetWeightage

Energy (cm-

1)

1

2 2 GS 26%

0.002 2 1st ES 24%2 -2 GS 26%2 -2 1st ES 24%

2 2 2 GS 26% 0.04

2 2 1st ES 24%2 -2 GS 26%2 -2 1st ES 24%

3

2 1 GS 27%

153.812 1 1st ES 23%2 -1 GS 27%2 -1 1st ES 23%

4

2 1 GS 27%

153.832 1 1st ES 23%2 -1 GS 27%2 -1 1st ES 23%

5 2 0 GS 100% 296.17Complex 2

1

2 2 GS 47%

0.002 2 1st ES 3%2 -2 GS 47%2 -2 1st ES 3%

2

2 2 GS 47%

0.012 2 1st ES 3%2 -2 GS 47%2 -2 1st ES 3%

32 1 GS 50%

44.692 -1 GS 50%

42 1 GS 50%

44.992 -1 GS 50%

5 2 0 GS 100% 60.90Table S4: Wavefunction decomposition spin-orbit coupled states (Ms) of S =2 manifold for complex 1 and 2.

Table S5: Selected structural parameters of the X-Ray structure and optimised geometries of complex 1 and 2.

Complex N1-Mn-N2

()(Angle

N2-Mn-N3()

(AngleB)

N3-Mn-N1

()(Angle

Mn-N1(Å)

Mn-N2(Å)

Mn1-N3(Å)

4 and 5 (cm-

1)

Figure S4: DFT computed IR spectrum of the two complexes at far-IR frequencies of complex 1 (left) and complex 2 (right). The black parenthesis and the black arrows indicate the N-M-N angle bending and out-of-plane bending vibrations.

4 & 5

A) C)1 (X-Ray) 120.02 119.95 120.02 1.889 1.890 1.890 70.3, 72.9

1 (optimisedin toluene)

113.93 113.87 132.20 1.911 1.902 1.911 68.6, 72.6

N1-Cr-N2 ()(Angle

A)

N2-Cr-N3 ()(Angle

B)

N3-Cr-N1 ()(Angle

C)

Cr-N1(Å)

Cr-N2(Å)

Cr-N3(Å)

2 (X-Ray) 115.81 120.29 123.90 2.021 2.027 2.033 58.7, 69.92 (optimisedin n-hexane)

113.28 113.08 133.65 2.051 2.038 2.050 61.0, 73.5

Table S6: Variation of D and E/D values with respect to Q parameter along with theirenergies of the first excited state for complex 1. The frequency points (P’s) are taken from 4

vibration.

*n =1 for positive xj and -1 for negative xj.

Bond Angle ( = A/B/C)

D (cm-

1)|E/D|

Energy ofthe 1st

excitedstate (cm-

1)

Q =n|120- |*

DDisplacement

scale (xj)

P1A=116.50C=111.07B=131.89

-6.47 0.019 4445.8 -24.32 57.81 -2.0

P2A=117.70C=114.07B=128.00

-8.86 0.012 3007.0 -16.23 55.42 -1.3

P3A=118.84C=116.98B=124.12

-15.33 0.005 1567.1 -8.30 48.95 -0.7

P4A=119.21C=117.91B=122.85

-20.63 0.003 1094.8 -5.73 43.65 -0.4

P5(X-Ray

Structure)

A=120.02C=120.02B=119.95

-64.28 0.0003 18.7 0.09 0 0.0

P6A=120.56C=121.42B=118.01

-27.50 0.001 750.2 3.97 36.78 0.3

P7A=121.01C=122.62B=116.33

-17.05 0.002 1374.5 7.3 47.23 0.6

P8A=122.05C=125.42B=112.33

-9.22 0.006 2841.3 15.14 55.06 1.2

P9A=123.23C=128.73B=107.51

-6.29 0.008 4565.2 24.45 57.99 2.0

Table S7: Variation of D and E/D values with respect to Q parameter along with theirenergies of the first excited state for complex 2. The frequency points (P’s) are taken from thefourth frequency point.

*n =1 for positive xj and -1 for negative xj.

Bond Angle ( = A/B/C)

D(cm-1)

|E/D|

Energy ofthe 1st

excitedstate (cm-1)

Q = n|120 - |

*D

Displacementscale (xj)

P1A=106.15B=119.75C=133.79

-5.00 0.022 2639.6 -27.89 10.10 -2.0

P2A=109.41B=119.97C=130.48

-6.37 0.014 2014.7 -21.10 8.73 -1.4

P3A=112.18B=120.12C=127.64

-8.45 0.009 1471.6 -15.58 6.65 -0.8

P4(X-RayStructu

re)

A=115.81B=120.29C=123.90

-15.10 0.003 755.8 -8.38 0 0.0

P5A=119.69B=120.43C=119.84

-45.87 0.000 22.6 0.90 30.77 0.8

P6A=122.67B=120.49C=116.69

-21.08 0.001 575.0 6.47 5.98 1.4

P7A=125.29B=120.52C=113.88

-11.57 0.003 1069.2 11.93 3.53 2.0

Figure S5: Variation of E/D with respect to Q parameter and xj for complex 1 (left) and for complex 2 (right) in 4 vibration.

-32 -24 -16 -8 0 8 16

0.000

0.005

0.010

0.015

0.020

0.025-2.4 -1.6 -0.8 2.40.8 1.6

E/D

Q

xj

0.0

-30 -20 -10 0 10 20 30

-2.4 -1.6 -0.8 0.0 0.8 1.6 2.4

0.000

0.005

0.010

0.015

0.020

xj

E/D

Q

Figure S6: Variation of LFT d-orbital energies with respect to Q parameter for complex 1 (left) and for complex 2 (right) in 4 vibration.

-24 -18 -12 -6 0 6 12 18 24

0

3000

6000

9000

12000

15000

18000

dxy

dx2-y2

En

ergy

(cm

-1)

Angular Distortion parameter (Q)

dxy

dx2-y2

dz2

dxz

dyz

dyz

dxz

dz2

-30 -24 -18 -12 -6 0 6 12

0

2000

4000

6000

8000

10000

12000

dz2

dxz

dyzE

nerg

y (c

m-1

)

Angular distortion parameter (Q)

dx2-y2

dxy

dyz

dxz

dz2

dx2-y2

dxy

Figure S7. Three dimensional magneto-structural correlation of D parameter obtained from 4 vibrational points in case of complex 2.

-2 -1 0 1 2

-28

-24

-20

-16

-12

-8

-4

0

D (

cm-1)

Displacement scale (xj)

complex 2

-2 -1 0 1 2-70

-60

-50

-40

-30

-20

-10

0

D (

cm-1)

Displacement scale (xj)

complex 1

Figure S8: Variation of D parameter of complex 1 (left) and complex 2 (right) with respect to 5 vibration.

Table S8: Variation of D, E/D, 1st excited state NEVPT2 energy values with respect to thedisplacement of Mn along z-direction for complex 1. The frequency points (P’s) are takenfrom the 5 vibrations.

Coordinates of Mn ion in (x y z)format (Å) D (cm-1) E/D

Energy of 1st

Excited State(cm-1)

DDisplacemen

t scale (xj)

P1 (-0.040 -0.019 0.360) -16.2 0.004 1407.4 48.1 2.0

P2 (-0.027 -0.013 0.248) -21.5 0.002 1016.3 42.8 1.4

P3 (-0.013 -0.006 0.120) -34.8 0.001 523.6 29.5 0.7

P4(X-Ray

Structure)(0.000 0.001 0.000) -64.3 0.000 18.7 0.0 0.0

P5 (0.014 0.008 -0.123)-32.2 0.002 593.1 32.1 -0.7

P6(0.029 0.015 -0.254)

-18.3 0.006 1232.1 46.0 -1.4

P7 (0.040 0.021 -0.359) -13.4 0.011 1751.9 50.9 -2.0

Figure S9: Variation of ab initio LFT d-orbital splitting of complex 1 (left) and complex 2 (right) with respect to 5 vibration.

-2 -1 0 1 2

0

3000

6000

9000

12000

15000

18000

dz2

dyz

dxy

dx

2-y

2

dxz

dyz

Ene

rgy

(cm

-1)

Displacement scale (xj)

dxy

dx2-y2

dxz

dz2

-2 -1 0 1 2

0

2000

4000

6000

8000

10000

12000

dz2

dxz

dyzE

nerg

y (c

m-1)

Displacement scale (xj)

dx2-y2

dxy

dyz

dxz

dz2

dx2-y2

dxy

Table S9: Variation of D, E/D, 1st excited state NEVPT2 energy values with respect to thedisplacement of Cr along z-direction for complex 2. The frequency points (P’s) are takenfrom the 5 vibration.

Coordinates of Cr ion in (x y z)format (Å) D (cm-1) E/D

Energy of1st ExcitedState (cm-1)

DDisplacement

scale (xj)

P1 (-0.092 -0.106 0.312) -11.1 0.005 1077.6 4.0 2.0

P2 (-0.080 -0.101 0.218) -12.1 0.004 983.9 3.0 1.4

P3 (-0.066 -0.093 0.103) -13.6 0.004 868.2 1.4 0.7

P4(X-Ray

Structure)(-0.05 -0.086 -0.008) -15.1 0.003 755.8 0.0 0.0

P5 (-0.040 -0.080 -0.102)-16.7 0.003 677.4 1.6 -0.6

P6(-0.025 -0.073 -0.221)

-18.7 0.002 584.0 3.6 -1.4

P7 (-0.012 -0.067 -0.327) -20.3 0.002 515.1 5.2 -2.0

Table S10: Literature survey of all reported mononuclear high-spin Mn(III) and Cr(II)complexes.

Sr. No ComplexDexp

(cm-1)|E/D|exp

Dcal

(cm-1)|E/D|cal YearRef.

1 trans-[Mn(cyclam)I2]I +0.60 0.05 - - 20021 2 [Mn(dbm)3] -4.35 0.06 -4.55 0.06 19972

3 [MnTPPCl] -2.29 0.00 - - 19993

4 [(tpfc)Mn(OPPh3)] -2.69 0.06 - - 20004

5 [(terpy)Mn(N3)3] -3.29 0.15 -3.29 0.16 20015 6 CsMn(SO4)2‚12D2O -4.52 0.06 - - 20016 7 [Mn(cyclam)Br2]Br -1.67 0.01 - - 20017

8 [Mn(OH2)6]3+ -4.49 0.06 - - 20038

9 [Mn(acac)3] - - -4.21 0.10 20069

10[Mn(bpia)(OAc)(OCH3)]

(PF6)+3.53 0.16 +3.24 0.16 200810

11[MnIII(5-TMAM(R)-salmen)

(H2O)-CoIII(CN)6]·7H2O·MeCN

-3.30 - - - 201311

12 Ph4P[Mn(opbaCl2)(py)2] -3.42 0.04 -3.47 0.01 201312

13[Mn(dbm)2(DMSO)2]

(ClO4)-3.42 0.22 -3.64 0.00 201513

14[Mn(dbm)2(py)2]

(ClO4)-4.46 0.21 -3.95 0.00 201513

15 Na5[Mn(L-tart)2]·12H2O -3.23 0.01 - - 201514

16[Mn(TPP)(3,5-

Me2pyNO)2]ClO4-3.82 0.04 -3.94 0.01 201515

17 [Mn(3-OEt-salme)2]BPh4 -4.60 0.32 -3.81 0.27 201616 18 MnL(NCS)·0.4H2O +2.53 0.18 +2.66 0.20 201717

19Mn(tpfc) (tpfc = 5,10,15-

tris(pentafluorophenyl)corroletrianion)

-2.67 0.01 - - 202018

20 [Mn{(OPPh2)2N}3] -3.92 0.00 -3.52 0.01 202019

21 [CrII(N(TMS)2)2(py)2] -1.80 0.01 -1.50 0.04 201520

22 [CrII(N(TMS)2)2(THF)2] -2.00 0.01 -1.66 0.05 201520

23Cr(CO)3(η6,η6-C6H5C6H5)

Cr(Al2(OC(CF3)2H)4)-2.15 0.00 - - 201721

24{[1-N-3,5-tBu2dp)4Cr][5-

(N,N,C,C,P))2K(1-O-THF)2[2}

-1.86 0.01 - - 202022

25 [Cr(iPrNC(CH3)NiPr)2] -1.74 0.04 -1.50 0.02 202123

26 [Cr(CyNC(CH3)NCy)2] -1.82 0.05 -1.47 0.02 202123

27 [Cr(DippNC(CH3)NDipp)2] -1.71 0.04 -1.48 0.02 202123

28 [Cr(tBuNC(CH3)NtBu)2] -1.94 0.02 -1.68 0.00 202123

29 [Cr{N(SiMe2Ph)2}2] -2.70 0.07 -2.70 0.07 202124

References:

1. Mossin, S.; Weihe, H.; Barra, A.-L., Is the axial zero-field splitting parameter of tetragonallyelongated high-spin manganese (III) complexes always negative? J. Am. Chem. Soc. 2002, 124 (30),8764-8765.2. Barra, A. L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.; Fabretti, A. C.;Uytterhoeven, M. G., Electronic Structure of Manganese (III) Compounds from High‐Frequency EPRSpectra. Angewandte Chemie International Edition in English 1997, 36 (21), 2329-2331.3. Krzystek, J.; Telser, J.; Pardi, L. A.; Goldberg, D. P.; Hoffman, B. M.; Brunel, L.-C., High-frequency and-field electron paramagnetic resonance of high-spin manganese (III) in porphyriniccomplexes. Inorg. Chem. 1999, 38 (26), 6121-6129.4. Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z., High-field (high-frequency) EPRspectroscopy and structural characterization of a novel manganese (III) corrole. ChemicalCommunications 2000, (19), 1957-1958.5. Limburg, J.; Vrettos, J. S.; Crabtree, R. H.; Brudvig, G. W.; de Paula, J. C.; Hassan, A.;Barra, A.-L.; Duboc-Toia, C.; Collomb, M.-N., High-Frequency EPR Study of a New MononuclearManganese (III) Complex:[(terpy) Mn (N3) 3](terpy= 2, 2 ‘: 6 ‘, 2 ‘‘-Terpyridine). Inorg. Chem.2001, 40 (7), 1698-1703.6. Basler, R.; Tregenna-Piggott, P. L.; Andres, H.; Dobe, C.; Güdel, H.-U.; Janssen, S.;McIntyre, G. J., Magnetic Excitations of CsMn (SO4) 2⊙ 12D2O, Measured by Inelastic NeutronScattering. J. Am. Chem. Soc. 2001, 123 (14), 3377-3378.7. Mossin, S.; Stefan, M.; ter Heerdt, P.; Bouwen, A.; Goovaerts, E.; Weihe, H., Fourth-orderzero-field splitting parameters of [Mn (cyclam) Br 2] Br determined by single-crystal W-band EPR.Applied Magnetic Resonance 2001, 21 (3-4), 587-596.8. Tregenna-Piggott, P. L.; Weihe, H.; Barra, A.-L., High-field, multifrequency EPR study ofthe [Mn (OH2) 6] 3+ cation: influence of π-bonding on the ground state zero-field-splittingparameters. Inorg. Chem. 2003, 42 (25), 8504-8508.9. Neese, F., Importance of direct spin− spin coupling and spin-flip excitations for the zero-fieldsplittings of transition metal complexes: A case study. J. Am. Chem. Soc. 2006, 128 (31), 10213-10222.10. Scheifele, Q.; Riplinger, C.; Neese, F.; Weihe, H.; Barra, A.-L.; Juranyi, F.; Podlesnyak, A.;Tregenna-Piggott, P. L., Spectroscopic and theoretical study of a mononuclear manganese (III)complex exhibiting a tetragonally compressed geometry. Inorg. Chem. 2008, 47 (2), 439-447.11. Ishikawa, R.; Miyamoto, R.; Nojiri, H.; Breedlove, B. K.; Yamashita, M., Slow relaxation ofthe magnetization of an MnIII single ion. Inorg. Chem. 2013, 52 (15), 8300-8302.12. Vallejo, J.; Pascual‐Álvarez, A.; Cano, J.; Castro, I.; Julve, M.; Lloret, F.; Krzystek, J.; DeMunno, G.; Armentano, D.; Wernsdorfer, W., Field‐induced hysteresis and quantum tunneling of themagnetization in a mononuclear manganese (III) complex. Angewandte Chemie 2013, 125 (52),14325-14329.13. Chen, L.; Wang, J.; Liu, Y. Z.; Song, Y.; Chen, X. T.; Zhang, Y. Q.; Xue, Z. L., Slowmagnetic relaxation in mononuclear octahedral manganese (III) complexes with dibenzoylmethanideligands. Eur. J. Inorg. Chem. 2015, 2015 (2), 271-278.14. Craig, G. A.; Marbey, J. J.; Hill, S.; Roubeau, O.; Parsons, S.; Murrie, M., Field-induced slowrelaxation in a monometallic manganese (III) single-molecule magnet. Inorg. Chem. 2015, 54 (1), 13-15.15. Pascual‐Álvarez, A.; Vallejo, J.; Pardo, E.; Julve, M.; Lloret, F.; Krzystek, J.; Armentano, D.;Wernsdorfer, W.; Cano, J., Field‐Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)–Porphyrin Complex. Chemistry–A European Journal 2015, 21 (48), 17299-17307.16. Realista, S.; Fitzpatrick, A. J.; Santos, G.; Ferreira, L. P.; Barroso, S.; Pereira, L. C.;Bandeira, N. A.; Neugebauer, P.; Hruby, J.; Morgan, G. G.; van Slageren, J.; Calhorda, M. J.;Martinho, P. N., A Mn(iii) single ion magnet with tridentate Schiff-base ligands. Dalton Trans. 2016,45 (31), 12301-7.

17. Shova, S.; Vlad, A.; Cazacu, M.; Krzystek, J.; Bucinsky, L.; Breza, M.; Darvasiová, D.;Rapta, P.; Cano, J.; Telser, J., A five-coordinate manganese (III) complex of a salen type ligand with apositive axial anisotropy parameter D. Dalton Trans. 2017, 46 (35), 11817-11829.18. Krzystek, J.; Schnegg, A.; Aliabadi, A.; Holldack, K.; Stoian, S. A.; Ozarowski, A.; Hicks, S.D.; Abu-Omar, M. M.; Thomas, K. E.; Ghosh, A., Advanced Paramagnetic Resonance Studies onManganese and Iron Corroles with a Formal d4 Electron Count. Inorg. Chem. 2020, 59 (2), 1075-1090.19. Sanakis, Y.; Krzystek, J.; Maganas, D.; Grigoropoulos, A.; Ferentinos, E.; Kostakis, M. G.;Petroulea, V.; Pissas, M.; Thirunavukkuarasu, K.; Wernsdorfer, W.; Neese, F.; Kyritsis, P., MagneticProperties and Electronic Structure of the S = 2 Complex [Mn(III){(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation. Inorg. Chem. 2020, 59 (18), 13281-13294.20. Deng, Y. F.; Han, T.; Wang, Z.; Ouyang, Z.; Yin, B.; Zheng, Z.; Krzystek, J.; Zheng, Y. Z.,Uniaxial magnetic anisotropy of square-planar chromium(II) complexes revealed by magnetic andHF-EPR studies. Chem Commun (Camb) 2015, 51 (100), 17688-91.21. Wang, W.; Li, J.; Yin, L.; Zhao, Y.; Ouyang, Z.; Wang, X.; Wang, Z.; Song, Y.; Power, P. P.,Half-sandwich metal carbonyl complexes as precursors to functional materials: from a near-infrared-absorbing dye to a single-molecule magnet. J. Am. Chem. Soc. 2017, 139 (34), 12069-12075.22. Su, J.; Yin, L.; Ouyang, Z.; Wang, Z.; Zheng, W., 1,2-Diaza-4-phospholide complexes ofchromium(ii): dipotassium organochromates behaving as single-molecule magnets. Dalton Trans.2020, 49 (21), 6945-6949.23. Zhai, Y. Q.; Ge, N.; Li, Z. H.; Chen, W. P.; Han, T.; Ouyang, Z. W.; Wang, Z.; Zheng, Y. Z.,Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes.Inorg. Chem. 2021, 60 (3), 1344-1351.24. Freitag, K.; Stennett, C. R.; Mansikkamaki, A.; Fischer, R. A.; Power, P. P., Two-Coordinate,Nonlinear Vanadium (II) and Chromium (II) Complexes of the Silylamide Ligand–N (SiMePh2) 2:Characterization and Confirmation of Orbitally Quenched Magnetic Moments in Complexes withSub-d5 Electron Configurations. Inorg. Chem. 2021.

download fileview on ChemRxivSupporting_Information.docx (4.97 MiB)