Synthesis, spectroscopic characterization and electrochemical behaviour of nickel(II) complexes with...

Transition Met. Chem., 18, 523-527 (1993)

spectroscopic of nickel(II)

Synthesis, behaviour hexamethylcyclotetradecane of {Ni(Me6[14]aneN4)}I 2

Rafael Ruiz and Miguel Julve*

Nickel(II) macrocyclic complexes 523

characterization and electrochemical complexes with C-meso-5,5,7,12,12,14-

(Me6[14]aneN4). Crystal structure

Departament de Quimica Inorggmica, Facultat de Quimica de la Universitat de Valdncia, Dr. Moliner 50, 46100 Burjassot ( V alkncia ), Spain

Juan Cano, Juan Soto, Ram6n Martinez-Mfifiez, Carmen Mufioz ~ and Jorgel Pay/l �82 Departamento de Quimiea, I.Departamento de Fisica Aplicada, �82 de lngemieria de la Construcci6n, Universidad Politkcnica de Valkncia, Camino de Vera s/n, 46071, Valkncia, Spain

Summary

A series of complexes of formula {Ni(Me6[14]aneN4)X2} (Me6[14]aneN 4 = C-meso-5,5,7,12,12,14-hexamethyl-l,4,- 8,11-tetraazacyclotetradecane and X = N 3 (1), NCO (2), NCS (3), AcO (4) or I (5) has been synthesized. The crystal structure of (5) has been determined by X-ray diffraction methods. The nickel atom is surrounded by a square-planar array of nitrogen atoms, giving a low-spin complex. The macrocyclic ligand shows the most stable conformation, the six-membered rings exhibiting the chair form while the gauche conformation is adopted by the five-membered rings. Spectroscopic and magnetic data for (1)-(4) are consistent with octahedral coordination around the nickel atom, which is achieved by four N atoms of the macrocyclic ligand in a single plane and by two azido-N (1), cyanato-N (2), thiocyanato-N (3) and carboxylato-O (4) groups occupying the axial positions. An electrochemical study of the oxidation of ( I ) - (5) has been carried out in nonaqueous solvents and the prospects of synthesis of the corresponding nickel(III) species are discussed in the light of the electrochemical data.

Introduction

The ligand 5,5,7,12,12,14-hexamethyl- 1,4,8,11 -tetraazacyclotetradecane (Me6114]aneN4, 1) belongs to the family of saturated tetraaza macrocycles that can be prepared by direct synthesis without the aid of template or other ligand reactions m. In fact, the free cyclic tetraamine as well as its metal complexes exhibit isomerism which gives rise to C-meso and C-racemic forms (2). Although both isomers can coordinate to transition-metal ions as tetradentate ligands through the amine nitrogen atoms in a coplanar arrangement to yield square planar (3-6) or trans disubstituted octahedral (4'v's) complexes, most octahedral derivatives of the C-rac amine have the macrocycle in a folded conformation, often with a chelate occupying the two additional cis coordination sites ~9-14). Coplanar arrangement of the amine nitrogen atoms of C-meso- Me6[14]aneN4 in its metal complexes is the usual situation, the folded conformation being rare for this isomer (15). The differences in the preferred stereochemistry of these iso- meric tetraaza macrocycles have been attributed to non- bonded interactions involving the methyl substituents (16).

* Author to whom :ill correspondence should be directed.

NH HN

NH HN ~

Scheme 1

The diamagnetic square planar {Ni(C-meso-Me6[14]- aneN4)} 2+ complex is potentially a very interesting pre- cursor for S = 1 one-dimensional systems of formula [Ni(C-meso-Me6[14]aneNg)X2], X being a suitable co- ordinating bridging ligand. These compounds can be model systems for antiferromagnetic coupling, to check the Haldane conjecture about the presence of a quantum gap in S = 1, quasi-one-dimensional Heisenberg anti- ferromagnets ~17'1s). On the other hand, the fact that nickel(III) complexes of this macrocycle have been prepared, suggests the possibility of designing X-bridged one- dimensional nickel(III) nickel(II) mixed-valence compounds.

Our first attempts to prepare complexes of [Ni(C-meso- Me6[14]aneN4)] 2+ with X led to the preparation of complexes of formula [Ni(C-meso-Me6[14]aneN4)X2], where X = N 3 (1), NCO (2), NCS (3), AcO (4) or I (5). In this work we focus on their preparation and structural characterization. The redox behaviour of the nickel(III)/ nickel(II) couple has been investigated in nonaqueous solvents and is discussed as a function of the nature of the axial ligand X.

Experimental

Regents and solvents

C-meso-Me6[14]aneN 4 was prepared as previously described (1). NAN3, NaOCN, NH4SCN, Ni(OAc)2"4H20, NiI 2 and Ni(C104)2.6H20 were Aldrich reagents. [Ni- (Me6[14]aneN4)](C104) 2 was obtained as a yellow solid by reaction of stoichiometric amounts of the nickel(II) salt and the macrocycle in MeOH at room temperature. The salt [NBu]][PF6] was recrystallized from EtOH and dried at 80 ~ C under vacuum. Tetrahydrofuran (THF) and MeCN were purified by standard procedures.

0340-4285 �9 1993 Chapman & Hall

524 Ruiz et al.

Preparation of complexes ( I ) - (5)

Compounds (1) and (2) were prepared as follows: an excess of NaN 3 or NaOCN dissolved in a minimum volume of water was added to an aqueous solution of [Ni(Me 6 [ 14] aneN4)] (CI04) 2. The resulting yellow solution was extracted with CHCI 3. The products were obtained as violet powders by evaporation of the solutions. Re- crystallization from H20 yielded crystals of both complexes (Found: C, 44.9; H, 8.3; N, 32.7. C16H36NiNlo (1) Calcd: C, 45.0; H, 8.4; N, 32.8~. Found: C, 50.3; H, 8.3; N, 19.5. C18H36NiN60 2 (2). Calcd: C, 50.6; H, 8.4; N, 19.7~o).

Compound (3) was obtained as very small pink crystals from aqueous solutions containing stoichiometric amounts of [Ni(Me6[14]aneN4)](CIO4) 2 and NH4SCN. All our attempts to grow single crystals of (3) were unsuccessful (Found: C, 47.0; H, 7.6; N, 18.1. C18H36NiN6S 2 (3). Calcd: C, 47.1; H, 7.8; N, 18.3~o).

Compound (4) was obtained as violet crystals by slow evaporation of MeOH solutions containing stoichiometric amounts of (Me6[ 14-]aneN4) and Ni(OAc)2.4H20. All our attempts to grow single crystals of (4) were unsuccessful (Found: C, 52.0; H, 8.9; N, 12.0. C2oH42NiN404 (4). Calcd: C, 52.1; H, 9.1; N, 12.2~o).

Compound (5) was obtained as red single crystals by slow evaporation of MeOH solutions containing stoichiometric amounts of (Me 6[ 14]aneN4) and NiI 2 (Found: C, 32.0; H, 5.9; N, 9.2. C16H36NiN4I 2 (5). Calcd: C, 32.2; H, 6.0; N, 9.4~o).

Physical techniques

I.r. spectra were recorded on a Perkin-Elmer 1750 FTIR spectrometer as KBr pellets in the 4000-250 cm- 1 region. Magnetic susceptibility measurements were carried out at 300 77K with a previously described pendulum-type apparatus (19) equipped with a nitrogen cryostat. The apparatus was calibrated with Hg[Co(NCS)4 ]. The electrochemical experiments were carried out in a three-electrode cell under an inert atmosphere. MeCN and THF were used as solvents and 0.1moldm -3 [NBu]][PF6] was used as background electrolyte. The working and reference electrodes were graphite and saturated calomel, respect-

Transition Met. Chem., 18, 523-527 (1993)

ively. The latter was electrically connected to the nonaqueous solution by a salt bridge containing the nonaqueous solvent and supporting electrolyte. The aux- iliary electrode was of platinum mesh. Cyclic voltammograms were obtained with a programmable function generator Tacussel IMT-1, connected to a Tacusel PJT 120-1 potentiostat. The system was calibrated against [Fe(r/-CsHs)2].

Crystal data

C16Ha6NiN4I 2 (5 ) ,F w = 597.0. Monoclinic, a = 7.837(2)~ b = 9.106(2), c = 16.264(4)/~, r = 101.67(2), V = 1144.0(5))~ (the unit cell parameters were determined by least-squares refinement of 25 reflections in the 0 range 20-28~ space group P21/n, Z = 2, D x = 1.72 g cm- 3, red prismatic crystals, crystal size 0.40 x 0.30 x 0.20 mm, #(MoK~) = 35.1 cm-1, F(000) -- 588, T = 293 K.

Diffraction data were collected with an Enraf-Nonius CAD-4 diffractometer using graphite-monochromated MoK~ radiation (2 = 0.710 69 /~). Examination of three standard reflections monitored throughout the data collection showed no significant decay. Intensity data were collected by the 09-20 scan technique in the 20 range 2-50 ~ with scan width 0.8 + 0.35 tan 0 and scan speed 5.47 ~ min- 1 Data collection showed systematic absences (hOl, h + l = 2n + 1; OkO, k = 2n + 1) which uniquely define the monoclinic space group P21/n. The data were corrected for Lorentz and polarization effects. An empirical (q) scan) absorption correction was also applied. Of the 2943 measured reflections, 1479 were unique with I ~> 3.03(I) and were used for the structure refinement.

The structure was solved by direct methods using the M U L T A N 84 (20) set of computer programs and refined by full-matrix least-squares with the SHELX 76 program (21). All non-H atoms were refined anisotropically. The H atoms were not found. They were introduced in geometrical positions and refined with an overall isotropic thermal parameter. The function minimized was Zw 11Fo[ - I Fc [I 2 with unit weights. Scattering factors and anomalous dispersion corrections for neutral atoms were taken from (22). The final value of the discrepancy indices R

Table 1. Bond distances (,~) and angles (degrees) for non-hydrogen atoms of complex (5) a.

Distances Ni--N(1) 1.995(6) N(4)--C(5) 1.479(11) Ni--N(4) 1.997(7) C(5)--C(5A) 1.558(13) N(1)--C(7) ~ 1.416(11) C(5)--C(5B) 1.532(12) N(1)--C(2) 1.477(11) C(5)--C(6) 1.513(13) C(2)--C(3) 1.483(10) C(6)--C(7) 1.519(13) C(3)--N(4) 1.479(11) C(7)--C(7A) 1.563(13)

Angles N(1)--Ni--N(4) i 92.2(3) N(1)--Ni--N(4) 87.8(3) Ni--N(1)--C(2) 104.6(5) Ni--N(1)--C(7) i 119.9(5) C(7)~--N(1)--C(2) 113.2(7) N(1)--C(2)--C(3) 107.6(5) C(2)--C(3)--N(4) 109.0(5) C(3)--N(4)--Ni 104.6(5) Ni--N(4)--C(5) 124.6(5) C(3)--N(4)- C(5) 115,7(6)

N(4)--C(5)--C(5A) N(4)--C(5)--C(5B) N(4)--C(5)--C(6) C(5A)--C(5)--C(5B) C(5A)--C(5)--C(6) C(5B)--C(5)--C(6) C(5)--C(6)--C(7) C(6)--C(7)--C(7A) C(6)--C(7)--N(1) i C(7A)--C(7)--N(1) i

lO8.8(8) 111.9(7) 109.9(7) 109.6(8) 106.2(8) 110.4(7) 115.6(8) 108.6(8) 111.2(7) 111.5(7)

aRoman numeral superscript refers to the following equivalent position relative to x, y, z: i = -x, 1 -y , -z.

Transition Met. Chem., 18, 523-527 (1993) Nickel(II) macrocyclic complexes 525

(•[AF]/Z[Fo]) and R w ({Zw[AF]2/~,w[Fo]2} 1/2) was 0.062 for all observed reflections. The final difference Fourier maps showed residual maxima and minima of 1.03 and -1.15 e/~ -3, located around the iodine atom. All calculations were performed with an IBM 3090 computer. The molecular illustration was produced with SCHAKAL (23). Interatomic bond distances and angles for complex (5) are listed in Table 1. Tables of atomic positional and thermal parameters and observed and calculated structure factors have been deposited.

Results and discussion

Description of the structure of complex (5)

The structure is made up of cationic square-planar [Ni- (Me6114]aneN4)] 2 + units and uncoordinated iodide anions linked by ionic interactions. The structure of the cation has a crystallographically imposed-1 symmetry. A perspective view of complex (5) with the atomic numbering scheme is depicted in Figure 1.

The nickel atom is surrounded by the four nitrogen atoms of the macrocyclic ligand to give a square-planar diamagnetic nickel(II) complex. The average Ni N- (macrocycle) bond distance is 1.996/k, a value that is in the normal range for low-spin nickel(II) complexes with four planar N donors (4-6'24'25). A point of particular interest in this regard is the correspondence of the N i - - N bond lengths with the spin states of the nickel(II) ion. The average Ni--N(macrocycle) bond distance lies in the range of 1.954-1.996 A for the diamagnetic [Ni(Me6114 ]- aneN,~] 2 + species, whereas it is significantly longer (2.08- 2.10 A) for the parent octahedral complexes'(4, 7).; the shorter N i - - N bond distances in the former group reflect the stronger in-plane ligand field strength of the macrocycle for this family, which also produce the spin-state variation in these nickel(II) complexes containing the same macrocyclic ligand. The shortest Ni - - I distance in (5) [3.539(1)/~ for Ni--I(8)] is still longer than those observed (2.55- 2.90/~) in iodonickelato(II) complexes (26), and the nickel atom can only be considered as having two very distant iodide ligands in the axial positions. The angles subtended

Figure 1. Perspective view of complex (5) showing the atom labelling scheme. Hydrogen atoms are omitted for the sake of clarity.

at the metal ion exhibit significant deviations from the ideal value of 90 ~ [92.2(3) and 87.8(3) ~ for N(1) - -Ni - - N(4) i and N(1)--Ni--N(4), respectively].

The macrocyclic ligand adopts the most stable conformation: the six-membered rings take the chair form with the C(7)--Me and C(14)--Me bonds in equatorial positions, whereas the five-membered rings are in the gauche conformation. Significant deviations from the tetrahedral angles are observed for Ni--N(4)--N(5) [124.6(5)~ Ni--N(1)--C(7) i [119.9(5) ~ and C(5)--C(6)-- C(7) [115.6(8)~ these distortions being common to six- membered chelate rings of metal complexes with the Me6114]aneN 4 ligand in planar conformation.

Infrared and magnetic data

The most interesting feature of the i.r. spectra of the complexes Ni(Me6[ 14]aneNg)X 2 is the possibility of deter- mining the modes of coordination of the X groups. For complex (1), the occurrence of three peaks centred at 2040vs (2020sh), 1310w and 610wcm -1 which are as- signable to vas(Na), vs(N3) and ~(N3) , respectively, is consistent with terminally bound azide (27). The Va~(NCO), v~(NCO) and 6(NCO) bonds of complex (2) appear at 2200vs (2170sh), 1305m, and 610w (620w)cm-1, respectively. The value of Va~(NCO ), which is greater than the corresponding value for free NCO- (2170 cm-1), is consistent with either terminally N-bonded or bridging cyanate (2s'29). However, the 6 splitting of only a few wavenumbers (10 cm - 1) in this complex supports the first possibility (3~ For complex (3), Vas(NCS), vs(NCS) and 6(NCS) are located at 2080vs, 790m and 475were-i, respectively. The small change in the position of Va~ from the free-ion value (2053 cm-1) and the occurrence of v~ and 6 vibrational modes in the regions near 800 and 480 cm- 1 support the terminal N-bonding of thiocyanate in (3) (29'31). Finally, for complex (4), Va~(COO ) and v~(COO) are centred at 1580s and 1400s era-1, giving a value of A Ivan(COO ) -v~(COO)] of 180cm-1. Taking into account that the A value for ionic acetate is 164 cm- 1 and that generally A/> 200cm -1 is found in systems where structural studies show unidentate acetate coordination (32), one might conclude that acetate is uncoordinated in complex (4). However, the fact that the KBr pellet of (4) readily becomes yellow ([Ni{Me6114 ]- aneN4}Br2].2H20 is yellow) (s) throws some doubt on this conclusion. In fact, the pink colour of (4) and its magnetic moment (see below) are consistent with an octahedral or tetragonal nickel(II) species.

Magnetic susceptibility measurements of (1)-(5) in the temperature range 300-77K show that complexes (1)-(4) have triplet ground states (magnetic moments ranging from 3.16 to 3.22BM), whereas complex (5) is diamagnetic. Hence the spectroscopic and magnetic data support octahedral or tetragonal nickel(II) geometries for (1)-(4) and a planar nickel(II) geometry for (5), confirmed by the X-ray structure.

Electrochemical study

The electrochemical behaviour of the complexes [Ni(Me 6- [14]aneN4)X2] IX = N 3 (1), NCO (2), NCS (3), AcO (4), I (5), C104 (6), C1 (7) or Br (8)] has been investigated by cyclic voltammetry using a rotating disk electrode (RDE) in tetrahydrofuran and acetonitrile. The electrochemical data are summarized in Table 2.

526 Ruiz et al. Transition Met. Chem., 18, 523-527 (1993)

Table2. Electrochemical data "'b for Ni(Me6[14]aneN4)X 2 complexes in THF and MeCN at 20~ and 0.1moldm -3 [n-Bu4N] FPF6].

X THF MeCN Ep(A) A Ep ip( A )/ iv( A ') Ep(A) AEp iv( A )/ip( A' )

N 3 (I) 0.80 - - 0.88 - NCO (2) 0.83 148" 0.75* 0.73 157" 0.69* NCS (3) 0.96 121" 0.66* 0.89 127" 0.65* OAc (4) 0.62 78 0.90 0.49 75 0.98 I (5) 0.61 - - 0.60 - - ClO 4 (6) ~ 1.40 - - 1.31 76 0.90 Cl (7) b 0.76 68 0.99 0.71 69 0.99 Br (8) b 0.82 70 0.98 0.79 80 0.98

"All values at scan rate 0.2 V s-1 except those marked with an asterisk which are given at 2 V s-1; referred to s.c.e, using a graphite electrode. In the same conditions ferrocene gives AEp = 68 and 70 mV in THF and MeCN, respectively. UThese complexes were prepared in situ by mixing stoichiometric amounts of Ni(Me6[14]aneN4)(C104) 2 and the corresponding n-Bu~NX in nonaqueous solvent.

Linear sweep voltammetry of complexes (1)-(8) in both solvents shows one oxidation peak (A). If the sweep is reversed after peak A, the corresponding reduction peak (A') is detected only for complexes (4) (Figure 2a), (6), (7) and (8) at a scan rate of 0.2Vs -1. Although no well-defined oxidation wave was found for (6) in T H F because the oxidation of the solvent occurs near the oxidation of the complex, an approached value of 1.40 can be given. By increasing the scan rate, the nickel(II)/ nickel(III) oxidation process in complexes (2) and (3) (Figure 2b) becomes more reversible, as shown by the values of ip(A)/4(A') (ca. 0.7 at 2Vs -1) (see Table 2). However, no reverse peak is detected for (1) and (5) even when working at 10 V s- 1.

In the light of these data, three electrochemical patterns are clearly observed: (i) reversible systems with an ip(A)/ ip(A') ratio greater than 0.90 and AEp values close to

1.0

(b)

~~ 1:0 (o)

E (V)

Figure 2. Cyclic voltammograms in THF for complexes (4) (a) and (3) (b) at scan rates of 0.2 and 2 V s- 1, respectively.

that obtained for ferrocene under the same conditions, indicating the monoelectronic reversible oxidation from nickel(II) to nickel(III) in (4), (6), (7) and (8); (ii) partially reversible processes, only at high scan rates, for (2) and (3); (iii) a totally irreversible oxidation process for (1) and (5).

In general, higher oxidation potential peaks are observed in T H F than in acetonitrile, probably due to the different soloatation properties. In fact, the ferrocene standard exhibits an oxidation potential ca. 70 mV greater in T H F than in acetonitrile. Complex (6) has a higher oxidation potential than the other compounds and the value is in good agreement with those reported for similar macrocyclic complexes of nickel(II) (33'34). Conductimetric measurements in both T H F and acetonitrile show that complexes (5) and (6) are dissociated in solution, whereas in both solvents the X groups of complexes (1)-(4), (7) and (8) remain bound to the metal. The donor effect of these axial ligands causes a decrease in the oxidation potential, and moreover, the cationic nature of the square-planar species compared to the neutrality of the octahedral complexes reinforces this trend. The first oxidation potential of complex (5) is clearly attributed to the oxidation of the iodine counterion, and a second oxidation peak is detected after peak A that corresponds to the oxidation of the [Ni(Me6[14qaneN4)] 2+ species.

A second oxidation peak is also detected for the octahedral complexes (1), (2) and (3) at more anodic potentials after peak A, at the same value observed in complex (6), indicating that the square-planar nickel(II) species is generated after the first oxidation process. On the other hand, the coulometry of these complexes gave in all cases no well-defined processes but always more than 1 F mol-1 was consumed. These features suggest that final irreversible oxidation of the X ligand occurs.

A reversible step is found for (4), (7) and (8) and only the first monoelectronic oxidation process occurs. A chemical reaction follows the oxidation for the remaining octahedral complexes. This reaction may consist of electron transfer from the metal to the axial ligands to give the corresponding Xox form, leading finally to the square-planar nickel(II) species, which oxidizes further at higher potentials. At first sight, the nature of this chemical reaction is not obvious, the key point being whether the electron-transfer process follow an inner- or outer-sphere mechanism. Studies dealing with this subject are scarce, but inner- sphere mechanisms have been proposed (3s). However,

Transition Met . Chem., 18, 523-527 (1993) Nickel(II) macrocyclic complexes 527

from a synthetic point of view the important question is whether the intermediate nickel(III) species is able to oxidize the X ligand, since, if not, the latter would be stable and the oxidation could be proposed as a synthetic method for such complexes. In fact, some studies of the rate of formation of monohalo complexes of nickel(III) with macrocyclic ligands have shown that, whereas direct complex formation is observed using chloride and bromide, the reaction with iodide results in oxidation of the halide (noninnocent behaviour of the X ligand) (35'36). When using Me6[14]dieneN 4 as the ligand, the strong oxidizing power of the corresponding nickel(III) complex causes the oxidation even of bromide to bromine ~37). In the present family, only the chloro- and bromo-nickel(III) complexes are stable. The structures of the former (s) and of bromo- nickel(III) complexes with tetraaza macrocycles (3s) were reported quite recently. The knowledge of these oxidation potentials is very useful in attempting use of the mono- nuclear nickel(III) complexes as building blocks for poly- nuclear mixed-valence compounds or nickel(III)-radical species(33,3s).

Acknowledgements

We thank the Comisi6n Interministerial de Ciencia y Tecnologia (Project PB91-0807) and the Universidad Polit6cnica de Val6ncia (Proyecto Precompetitivo) (Spain) for financial support. Thanks are also extended to the Servicio de Espectroscopia and the Centro de CfiJculo from the University of Val6ncia for instrumental facilities. One of us (R. R.) acknowledges the Conselleria de Cultura, Educaci6 i Ci6ncia de la Generalitat Valenciana for a doctoral grant.

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(Received 12 August 1992) TMC 2852

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