University of Groningen Mechanisms in Ruthenium(II) … · 2017. 2. 10. · Mechanism of an...
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University of Groningen
Mechanisms in Ruthenium(II) photochemistry and Iron(III) catalyzed oxidationsUnjaroen, Duenpen
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CHAPTER 6
Mechanism of the oxidation of benzyl alcohols catalyzed by a µ-oxo-diiron(III)bis-phenolato polypyridyl complex The µ‐oxo‐diiron(III) polypyridyl complex [(L)Fe(µ‐O)Fe(L)](ClO4)2 (1) (where HL is 2‐(((di(pyridin‐2‐
yl)methyl)(pyridin‐2‐ylmethyl)amino)methyl)phenol), in which each of the Fe(III) centers is
coordinated to a phenolato ligand, was applied over a decade ago to the selective oxidation of
benzyl alcohols to aldehydes with H2O2. However, in this chapter shows clearly that under catalytic
conditions with excess H2O2, rapid oxidation (< 5 s) and loss of the phenolato moiety occurs
resulting the formation of an N4 ligated Fe(III) complex. This complex forms a Fe(III)‐OOH species
initially before conversion to a relatively stable oxo‐bridged dinuclear Fe(III) complex that does not
bear a phenol moiety. The data show that although the phenolato moiety imparts interesting redox
properties on complex 1 it plays no apparent role in oxidation catalysis with H2O2.
D. Unjaroen, A. Draksharapu, S. K. Padamati, M. Swart, W. R. Browne, manuscript in preparation
Chapter 6
64
6.1 Introduction The redox chemistry of transition metal complexes is a central aspect in mechanisms for many
homogeneous and enzymatic catalytic reactions, with progress in both areas accelerated by the
focus on understanding and modelling the active site of metalloenzymes.1 In addition to the redox
chemistry of the metal centers in metalloenzymes, redox non‐innocent active organic moieties play
an important role, not least the phenolato moiety, such as in the enzyme galactose oxidase (GO).2
GO is a mononuclear copper enzyme that catalyzes the two‐electron oxidation of primary alcohols
to their corresponding aldehydes, with subsequent reduction of dioxygen to H2O2. The structure of
the catalytic site in the inactive form of GO shows that it contains two imidazole rings (histidine
residues) and two phenol moieties provided by tyrosine residues, as well as an acetato ligand bound
to the copper(II) ion.5a,3
The formal oxidation state of the active form of GO has been assigned as copper(III), however,
it is better described as a copper(II)‐phenoxyl radical considering its spectroscopic properties.6
Phenolato containing enzymes have received, as a consequence, considerable attention in recent
years in the context of non‐redox‐innocent ligands,4 as one‐electron oxidation of metal‐phenolato
complexes can result in the formation of either a higher valent metal species (Mn+1L‐) or ligand
radicals (MnL), which can be viewed as electronic isomers.5 Wieghardt et al.6 reported a stable
Fe(III)‐phenoxyl radical complex as early as 1993 and the number of small molecule metal‐phenoxyl
complexes have grown to include a variety of metals and ligand frameworks.7 Indeed, in 1998,
Wieghardt and Stack8 reported a functional model for GO that could oxidize alcohols to aldehydes
with high yield and selectivity.
Diiron complexes have been studied extensively towards modeling the activation of oxygen by
nonheme iron enzymes, such as methane monooxygenase (MMO), ribonucleotide reductase, and
fatty acid desaturases.9 In addition, several µ‐oxo‐diiron(III) complexes have also been reported as
catalysts for the oxidation of organic substrates. Palaniandavar et al.,10 reported the oxidation of
alkanes to alcohols with m‐chloroperbenzoic acid (m‐CPBA) catalyzed by a µ‐oxo‐bridged non‐
heme bis‐phenolate‐diiron(III) complex and Ligtenbarg et al.11 reported the synthesis and structural
characterization of a µ‐oxo‐diiron(III)bis‐phenolato polypyridyl complex (1), as well as
demonstrating its activity in the oxidation of benzylic alcohols to aldehydes with H2O2 as terminal
oxidant.
This latter system (1) showed a notable time dependence for the oxidation of benzyl alcohol and
in particular, the duration of an initial lag period was found to correspond to the loss of visible
absorption (assigned to a phenolate to iron(III) LMCT‐ligand to metal charge transfer band).
Furthermore, addition of a strong acid, i.e. CF3SO3H, resulted in a red shift in visible absorption and
a complete elimination of the lag period ascribed to breaking up of the dinuclear complex to a
catalytically active mononuclear species.11 These data it was proposed that break‐up of the
dinuclear complex to a mono‐nuclear complex occurred upon addition of acid with dissociation of
the phenolato moiety occurring prior to the onset of catalytic activity.
Although phenolato based ligands impart additional flexibility to metal complexes in terms of
redox chemistry (i.e. non‐innocent redox active ligands), these data raise pertinent questions as to
the role and indeed usefulness of such ligand fragments in oxidation catalysis.
In this chapter, we explore the time dependent spectroscopies of complex 1 under the reaction
conditions employed in benzyl alcohol oxidation. We demonstrate that the addition of acid does
not, in fact, lead to break‐up of the dinuclear structure but instead facilitates the reversible
oxidation of 1 by H2O2 to form formally Fe(III)Fe(IV) and Fe(IV)Fe(IV) species in which the phenolato
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
65
moieties are better described as phenoxyl radicals. A side reaction is observed in which phenol‐
phenol C−C coupling (at the para‐positions) occurs leading to the formation of Fe(III)‐µ‐(4,4’‐
biphenol)‐Fe(III) species, manifested in the transient appearance of intense near‐UV and vis
absorption bands. UV‐vis and resonance Raman confirm that these species are identical to those
observed to form upon electrochemical oxidation, which was reported recently by our group.12
Figure 1. Structures of complexes 1 and 2.
Furthermore, we show that the lag phase observed earlier during the oxidation of alcohol is due to
the time is taken for partial degradation of the ligand to occur, specifically the removal of the
phenolato moiety following its oxidation to a phenoxyl radical, in the presence of excess H2O2 and
water. Furthermore, phenol dimerization through radical C−C coupling is blocked by tertbutyl
groups in the analogous complex 2, addition of excess H2O2 and water results in essentially the
same spectroscopic changes. These data lead to the conclusion that in fact, the catalytically active
species is generated as a result of ligand degradation to yield an N4 ligand based complex similar
to ([Fe(MeN3Py)(CH3CN)]2+ (where MeN3Py = 1,1‐di(pyridin‐2‐yl)‐N‐methyl‐N‐(pyridin‐2‐
ylmethyl)methanamine).13 Indeed, time resolved spectroscopy shows that such species13b form
rapidly under reaction conditions.
The data hold considerable implications for the use of phenoxyl moieties in ligand design
strategies in homogeneous oxidation catalysis and highlight the challenge in disentangling the
complex spectroscopy of such systems to enable elucidation of the precise nature of the species
responsible for catalytic oxidation.
6.2 Results The dinuclear phenolato complexes 1 and 2 were available from chapter 5,12 and the single crystal
X‐ray structure of 1 was reported earlier by Ligtenbarg et al.11 The complexes were characterized
earlier in the solid state by FTIR, Raman spectroscopy and in solution by 1H NMR, and UV‐vis
absorption spectroscopy, ESI mass, cyclic voltammetry, UV‐vis absorption, Raman, and
spectroelectrochemistry.12 Briefly, the dinuclear structures of complexes 1 and 2 in the solid state
are retained upon dissolution in dichloromethane, acetone, and acetonitrile. The µ‐oxo bridge
between the Fe(III) centers is stabilized by strong antiferromagnetic coupling, which is manifested
in broadened 1H NMR signals in the range of ‐5 to 40 ppm in CD3CN and in EPR silence (X‐band 77
K).12 Both complexes show strong visible absorption (nm4.1×103 M‐1 cm‐1) and more intense
near‐UV absorption bands assigned to ligand to metal charge transfer (LMCT) transitions involving
both phenolato to Fe(III)14 as well as pyridyl to Fe(III) charge transfer, based on the resonance
Raman spectra at exc 532 and 355 nm, and TDDFT.12 The absorption band at 343 nm is assigned to
a mixture of phenolato‐to‐Fe(III) and oxo‐to‐Fe(III) charge‐transfer transitions.15 Notably, the
absorption spectra in acetonitrile are not concentration dependent (between 3.9 µM and 1 mM),
Chapter 6
66
in agreement with ESI mass data, which indicates that dissociation to a mononuclear species does
not occur to a significant extent.12
6.2.1 Protonation of 1 and 2.
The UV‐vis absorption spectra of 1 and 2, both in acetonitrile and in acetone, undergo a substantial
bathochromic shift upon addition of TfOH (Figure 2). Earlier these changes were interpreted as
evidence for the formation of mononuclear complexes through protonation of the ‐oxo bridge.11 However, several observations confirm that this is not the case. The absorption bands of 1 shift to
347 nm, 410 nm, 605 nm (shoulder) and 804 nm (Figure 2a, red line) upon addition of TfOH,
however, the extent of the shift is less pronounced with weaker acids such as acetic and
trichloroacetic acid (Figure 2c).
Figure 2. UV‐vis absorption spectrum of (a) 1 in acetonitrile (black line), after addition of 1 equiv. of
TfOH, (red line) and subsequent addition of H2O (0.36.6 vol%). (b) 2 in acetonitrile (0.2 mM),
addition of 1 equiv. of TfOH, and subsequently addition of water. (c) 1 in acetonitrile 0.25 mM with
various acids (45 equiv. of triflic acid, 40 equiv. of trichloroacetic acid and 70 equiv. of acetic acid).
Furthermore, successive addition of acid equivalents leads to changes in the absorption spectrum
without maintenance of isosbestic points, indicative of multiple protonation steps. Indeed the
resonance Raman spectrum of 1 under acidic conditions at 1064 nm is different to that at 785 nm
in regard to the Raman shift of the phenol C−O stretch band, confirming the presence of several
distinct but structurally similar species which exhibit different degrees of red shift of the lowest
absorption bands.
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
67
OFeFe
N
NO
N
NNO
N
NN
2+
1
OFeFe
N
NOH
N
NNO
N
NN
3+
H1+
+H+O
FeFe
N
NOH
N
NNOH
N
NN
4+
H212+
+H+
Scheme 1. Propose the protonation forms of 1.
DFT calculationsi indicate that sequential protonation of phenolato oxygens is thermodynamically
more favorable than protonation of the µ‐oxo bridge as well as the dissociation and protonation of
a pyridyl moiety. Comparison of resonantly enhanced Raman spectra (at exc 785 nm and 1064 nm)
of 1 with TfOH with the Raman spectrum of 1 at 1064 nm shows that the major differences are to
the Fe‐O(phenol) and C−O(phenol) stretching modes in the 1400 cm‐1 and 600 cm‐1 regions, with
most other (i.e. pyridyl) bands remaining relatively unchanged by addition of acid (Figure 3b).
Furthermore, the resonance Raman spectra (exc 785 nm, Figure 3a) of Hn1n+ are essentially
independent of the acid used and the solvent (acetone vs. acetonitrile) with minor differences
centered around the phenolato C−O stretch and in the low (ca. 600 cm‐1) wavenumber region (785
nm is resonant with the absorption spectra of several of the species formed).
These data indicate that neither the conjugate base of the acids nor solvent, coordinate to the
Fe(III) centers under acidic conditions. Analysis by Evan’s NMR method indicated that solutions of
1 (µeff = 2.7 B.M.) undergo an increase in paramagnetic character in the presence of TfOH (µeff = 6.3
B.M). However, the formation of mononuclear species is unlikely since, as for 1, the UV‐vis
absorption spectra of Hn1n+ show negligible dependence on concentration between 4 µM and 1
mM, and although addition of acid results in broadening of the signals in the 1H NMR spectrum of
1 in acetonitrile‐d3 and in acetone‐d6 between ‐10 to 120 ppm (Appendix A 3.4), only a weak signal
at g = 4.3 is observed by X‐band EPR spectroscopy at 77 K. These data indicate that the
antiferromagnetic coupling between the Fe(III) centers, and especially the spin state, is affected by
protonation, but that dissociation to mononuclear iron(III) complexes to any significant extent can
be excluded.
i The DFT calculation was carried out by Prof. Marcel Swart, Universitat de Girona
Chapter 6
68
Figure 3. Raman spectra of 1 in acetonitrile with 1 equiv. of TfOH (a) exc 785 nm; 1 (9 mM, gray
line) and with TfOH (blue line). (b) exc 1064 nm; saturated solution (black line) and 1 (1 mM) with
TfOH (red line). *distortion due to imperfect subtraction of solvent spectral feature
Although, addition of 1 equiv. of TfOH to 1 in acetonitrile results in a large red‐shift of the longest
wavelength absorption band from 540 nm to ca. 808 nm, subsequent addition of increasing
amounts of water results in a progressive blue shift of the visible absorption bands of 1 (Figure 2a).
The absence of an isosbestic point as the spectrum returns towards its original state confirms that
several species are formed upon protonation and is consistent with the difference observed
between the resonance Raman spectra recorded at 785 and 1064 nm (vide supra). The recovery of
1 upon addition of water (vide supra), due to the levelling effect, is confirmed by the recovery of
the resonantly enhanced Raman (exc 355 nm) bands of 1 at 411 cm‐1 and 431 cm‐1 which
disappeared upon addition of acid, and reappeared upon addition of (6.6 vol%) water (Figure 4).
The loss of these bands upon addition of acid despite the minimal change in absorbance at 355 nm
indicates that protonation affects the moieties associated with these transitions, i.e. the phenolato
groups.
The base peak in ESI mass spectrum of 1 in CH3CN is at m/z 445 ([(L1)Fe(µ‐O)Fe(L1)]2+‐(ClO4)2).12
Addition of TfOH results in the observation of only mononuclear species, with a base peak at m/z
454 ([L1FeIII‐OH]+) and additional signals at m/z 227, 218, 586 assigned to [LFe(III)‐OH2]2+, [LFe]2+,
and [LFe‐OTf]+, respectively. The observation of [LFe]2+ is consistent with the presence of an
unstable dinuclear complex in solution and hence the observation of ions consistent with
mononuclear complexes does not, a priori, confirm their presence in solution as the major species.
Notably a significant signal consistent with either low or high spin mononuclear Fe(III) complex is
absent in the EPR spectrum of 1 upon addition of acid and/or water. As with rR and UV‐vis
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
69
absorption spectroscopy, subsequent addition of 3 vol% water results in the reappearance of the
signal at m/z 445.
Figure 4. rRaman spectra of 1 in acetonitrile (0.3 mM, black line) at exc 355 nm, with 1 equiv. of
TfOH (red line) and after subsequent addition of water (6.6 vol%, gray line).
6.2.2 Reaction of 1 with near stoichiometric H2O2 in the absence and presence of H2O and acid.
Addition of between 0.5‐3.0 equiv. H2O2 (0.66 vol% in water) to 1 in CH3CN does not affect its UV‐
vis absorption spectrum, however, in the presence of TfOH (2 equiv.) the formation and subsequent
decay of the intense absorption bands at 460 nm and ca. 850 nm is observed (Figure 5).
Figure 5. (a) UV‐vis absorption spectra of 1 in acetonitrile (black line) after addition of 2 equiv. of
TfOH (red line) and 2 equiv. of H2O2 (in H2O, blue line), (b) time dependence of absorbance at 460
nm and 850 nm.
Addition of 0.5‐3.0 equiv. H2O2 (in CH3CN), resulted in similar changes, however, the band at 460
nm appeared immediately and with a much higher maximum absorbance, followed by its decay
concomitant with the appearance of new bands at 780 nm and 850 nm (which decayed more
slowly). Addition of water together with H2O2 is necessary to observe the intense absorption band
at 460 nm. However, if water is added in sufficient excess the equilibrium shifts from H1+ to 1 (c.f.
leveling effect, Figure 6) and hence the rapid oxidation of 1 does not proceed.
Chapter 6
70
Figure 6. UV‐vis absorption spectra of 1 in acetonitrile (a) after addition of 2 equiv. of TfOH and 2
equiv. of H2O2 (in CH3CN), (c) addition of water 10 µL after TfOH, (b) and (d) time dependence of
absorption at selected wavelengths at 460 nm and 850 nm.
The transient concentration of the intermediate species responsible for the bands at 460 nm and
850 nm was maximized with 2 equiv. of TfOH, 0.3 vol% of water and 2 equiv. of H2O2 (Figure 7a).
The decay of the band at 460 nm proceeds concomitant with the increase in absorbance at 780 nm
and 850 nm (Figure 7b) with an isosbestic point maintained at 585 nm. The molar absorptivity of
the band at 460 nm, assuming 40‐50% conversion (vide infra), is comparable to that expected for
a phenoxyl radical. Importantly, however, the initial spectrum of 1 recovers subsequently upon
addition of 6‐10 vol% water together with the reappearance of a base peak at m/z 445 in the ESI
mass spectrum of the reaction mixture. The band at 460 nm reappears upon subsequent addition
of a 2nd and 3rd equivalent of H2O2, although the maximum absorbance reached is less each time.
Furthermore, the recovery of the absorption spectrum of 1 upon subsequent addition of water is
incomplete.
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
71
Figure 7. (a) UV‐vis absorption spectra of 1 in acetonitrile (0.25 mM, black line) after addition of 2
equiv. of TfOH (red line), 5 µl of H2O (blue line) and 2 equiv. of H2O2. (b) Time dependence of
absorbance at 460 nm and 850 nm following addition of 2 equiv. of H2O2 (at 2 s) to 1 in CH3CN (0.25
mM), (b) Time dependence of Raman band intensities at 1617 cm‐1 (at exc 473 nm) and 1608 cm‐1
(at exc 785 nm).
Despite the appearance of absorption bands typical of phenoxyl radicals, the only signal observed
by X‐band EPR spectroscopy (77 K) at any point (i.e. with samples flash frozen at specific time
points) was a weak band at g = 4.3, which had appeared already upon addition of TfOH to 1 and
was essentially unaffected by addition of water and H2O2 in CH3CN. Hence, all major species present
are EPR silent at 77 K indicating that the phenoxyl radicals are spin coupled with the 3d‐electrons
of the Fe(III) center (vide infra).
The resonance Raman spectra of the phenoxyl radicals reported earlier show characteristic bands
at 1500 cm‐1 and 1600 cm‐1, assigned to the 7a (C−O stretching) and 8a (C−C stretching) mode
of the phenoxyl radical, respectively.16 The species formed upon reaction of 1 with H2O2 were
characterized by time‐dependent resonance Raman spectroscopy at 473 and 785 nm. At exc 473
nm, the Raman spectra show the appearance and subsequent disappearance of strongly resonance
enhanced bands at 1481 cm‐1, 1555 cm‐1, and 1617 cm‐1 and at exc 785 nm, at 1264 cm‐1, 1344 cm‐
1, 1396 cm‐1, 1522 cm‐1 and 1608 cm‐1 (Figure 9).
Chapter 6
72
Figure 8. Raman spectra of 1 in acetonitrile (0.25 mM) at exc 1064 nm with 1 equiv. of TfOH (dark
blue) and after 1 equiv. of H2O2. * Solvent bands.
The time dependence of the change in intensity of the Raman bands at exc 473 nm and exc 785
nm (w.r.t. solvent bands) tracks well the increase and decrease in absorbance at 460 nm and 850
nm, respectively (Figure 7c). Notable, however, the time‐dependent Raman spectra show that the
species absorbing at 460 nm reaches a maximum concentration immediately followed by a slow
decay, whereas the species absorbing at 850 nm reaches a maximum more slowly and then decays.
The time dependence of the formation and decay of this latter species is less apparent from the
absorbance at 850 nm due to overlap with the absorbance of H1+ which decreases concomitant
with the increase in absorbance at 850 nm, and vice versa.
Excitation at λexc 1064 nm is preresonant with both H1+ and the species generated by addition
of H2O2 which absorbs at 850 nm (Figure 8). Bands characteristic of phenoxyl radical species are
not observed but the band at 1330 cm‐1 corresponds to the addition and loss of H2O2. The changes
observed in the Raman spectrum of 1 with TfOH and H2O2 indicate that at most 50% conversion is
observed and that 1 recovers partially after the H2O2 is consumed.
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
73
OFeFe
N
NO
N
NNO
N
NN
2+
1
OFeFe
N
NOH
N
NNO
N
NN
3+
H1+
+H+
O
Fe
Fe
N
NO
NN N
O
N NN
O
C-C coupled bis phenol complex
OFeFe
N
NOH
N
NNOH
N
NN
4+
H212+
+H+
+ H2O2
+ H2O
OFeFe
N
NO
N
NNO
N
NNH
3+
OOH
H
Scheme 2. Protonation of 1 and subsequent oxidation and C−C coupling of the phenolato moieties.
6.2.3 Identification of species formed upon reaction of 1 with stoichiometric H2O2.
Comparison of the Raman spectra at exc 473 nm and exc 785 nm during the reaction of 1 with a
few equiv. of H2O2 with the products of electrochemical and chemical {[(NH4)2Ce(NO3)6] (CAN)}
oxidation of the same complex, reported earlier,12 allow for definitive assignment of the species
formed. Oxidation of 1 with CAN, i.e. without TfOH or water present, results in the immediate
appearance of an absorption band at 465 nm, which decays over time concomitant with the
appearance of bands at 780 nm and 850 nm. The changes in the absorption spectrum of 1 are
essentially the same as those observed upon addition of near stoichiometric H2O2 under acidic
conditions and, conclusively, the Raman spectra recorded at exc 473 nm and 785 nm are identical
to those obtained with TfOH/H2O/H2O2 (Figure 9).
Chapter 6
74
Figure 9. Comparison the Raman spectra at (a) exc 437 nm and (b) exc 785 nm of 1 in acetonitrile
after addition of 1 equiv. of TfOH and 1 equiv. of H2O2 (black line) to the spectra obtained12 upon
addition of 1 equiv. of CAN to 1 in acetonitrile (red line).
6.2.4 Reaction of 1 with excess H2O2 in the absence and presence of water and acid.
The reaction of 1 in acetonitrile, with and without H2O, with 100 equiv. of H2O2 was monitored over
time by UV‐vis absorption spectroscopy. A decrease in the absorbance of 1 at 535 nm was observed
only after a lag phase of ca. 60 min, which is consistent with the observation made earlier that the
onset of catalytic activity occurred concomitant with the change in color of the reaction mixture
(after ca. 1.5 h) from purple to yellow. The disappearance of the visible absorption bands indicates
loss of the phenolato moiety from the complex. In the present study, the lag phase was found to
eliminated by addition of water (Figure 10), however, the rate of loss of absorbance was reduced
also.
Figure 10. Time dependence of absorption at selected wavelengths at 535 nm after addition of 100
equiv. of H2O2 into 1 in acetonitrile with (red line, 5 vol%) and without H2O (black line).
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
75
Closer examination of the initial changes that occur upon addition of 100 equiv. of H2O2 to 1 in
acetonitrile with 1 equiv. of TfOH shows a transient increase in absorbance at 460 nm and 730 nm,
both appearing and decaying within 4 s. Subsequently, a band at 556 nm appears reaching a
maximum at 6 s and decaying again over 1.5 min to give a yellow solution (Figure 11a/b). A band at
420 nm appeared over 1 to 2 h (vide infra). The intermediate species formed, which absorbs at 556
nm, was characterized by Raman spectroscopy at λexc 532 nm. Addition of 100 equiv. of H2O2 to 1
in acetonitrile with TfOH (1 equiv.) resulted in the appearance of bands at 801, and 624 cm−1 (Figure
11d). The band at 802 cm−1 was assigned, tentatively, to an O−O stretch and the band at 624 cm−1
to the Fe−O stretch of a [(R‐N3Py)Fe(III)(X)(OOH)]2+ species (where X = solvent, R = H).13b
Figure 11. UV‐vis absorption spectra of 1 in acetonitrile (0.25 mM) with 3 equiv. of TfOH (red line)
(a) upon addition of 100 equiv. of H2O2, (b) before (black) and after (red) addition of TfOH and after
addition of 100 equiv. of H2O2; 2 min (gray), 1 h (green) and 2 h (yellow). (c) Time dependence of
absorption at selected wavelengths at 420 and 556 nm after addition of 100 equiv. of H2O2. (d)
rRaman spectra of 1 in acetonitrile at exc 532 nm with TfOH (black), after addition of 100 equiv. of
H2O2; 4 s (blue), 8s (red) and 12s (yellow).
Additionally, the yellow species that forms after decay of the band at 556 nm showed resonance
enhanced Raman bands at exc 355 nm showed bands at 1612, 1569, 1275, 1158, 1057 and 1032
cm‐1 that are typical of pyridyl modes (Figure 12). The bands at 1291, 1275 and 1210 cm‐1 are
assigned to the alkyl amine backbone of the ligand and the band at 1004 cm‐1 is assigned to a phenyl
ring mode by comparison to the resonance Raman spectrum of [FeII(N4Py)(CH3CN)](ClO4)2.17
Chapter 6
76
Figure 12. Raman spectrum of the yellow solution at exc 355 nm obtained 5 min after addition of
100 equiv. of H2O2 to 1 with 3 equiv. TfOH in acetonitrile. *Distortion due to imperfect solvent
subtraction.
Raman spectra recorded with a time resolution of 200 ms (Figure 13) enabled identification of the
species responsible for the transient absorption at 730 nm. The spectrum of the intermediate is
similar to those obtained with 1‐3 equiv. H2O2, i.e. consistent with a phenoxyl radical species, but
there are clear differences that confirm that it is not the same species (vide supra).
Figure 13. (a) rRaman spectra of 1 in acetonitrile at exc 785 nm with 3 equiv. TfOH (a) before, and
0.5 s (blue) and 2 s (red) after addition of 1000 equiv. of H2O2 and (b) time dependence of signal at
1598 cm‐1 (intermediate, red) and at 870 cm‐1 (H2O2, black).
6.2.5 Spectroscopy of 1 during the catalyzed oxidation of 1‐phenylethanol.
Although the formation of phenxoyl radicals from 1 upon reaction with near stoichiometric H2O2 is
evident, the relevance of such species under conditions used,11 i.e. with up to 100 equiv. of H2O2,
in the oxidation of benzyl alcohols catalyzed by 1 is, a priori, uncertain. The catalytic oxidation of 1‐
phenylethanol in acetonitrile under the conditions reported earlier by Ligtenbarg et al.11 was
monitored by UV‐vis absorption and Raman spectroscopy (Figure 14). Addition of 100 equiv. of
H2O2 to 1 (0.88 mM) and 1‐phenylethanol (1000 equiv.) results in a decay of the absorption band
at 540 nm within 1 h following a lag phase, the duration of which was increased compared to that
observed in the absence of substrate (vide supra). Raman spectroscopy shows that the
consumption of substrate and H2O2 commences only after the end of the lag period. 1H NMR
spectra of the crude reaction in acetonitrile‐d3 showed acetophenone in ~4% yield (40% efficiency
in oxidant).
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
77
Figure 14. (a) UV‐vis absorption of 1 in acetonitrile (0.88 mM) during catalyzed the oxidation of 1‐
phenylethanol (880 mM) with H2O2 (88 mM). (b) Time dependence of the absorbance at 540 nm.
(c) Raman spectra (exc 785 nm) before and after 160 min and (d) intensity of the bands due to
phenyl‐1‐ethanol, acetophenone and H2O2 over time (with 1,2‐dichlorobenzene as internal
reference).
The presence of 1 equiv of TfOH, prior to addition of H2O2 resulted in dramatic differences in the
time dependence of the changes in both UV‐vis absorption and Raman spectra with rapid loss of
the absorbance of H1+ at ca. 800 nm and the immediate appearance over 10 s of an intermediate
(i.e. FeIII−OOH), which absorbs at 560 nm, followed by its subsequent decay more slowly that in the
absence of substrate (Figure 15a/b). The X‐band EPR spectra (at 77 K) of 1 in acetonitrile with TfOH
and 1‐phenylethanol after addition of 100 equiv. of H2O2 (i.e. at the point a maximum absorbance
at 560 nm was reached) showed a relatively strong signal at g = 4.25, characteristic for a high‐spin
iron(III) species together with a signals at g = 2.12 and 1.96 that are characteristic of a low‐spin (S
= ½) FeIII−OOH species (Figure 15c). After warming the intermediate FeIII−OOH species to room
temperature and subsequently freezing to 77 K again, the signals of the S = ½ species disappeared
together with the decrease in the intensity of the signal of high‐spin iron(III) species. The X‐band
EPR spectrum of the yellow species that formed after the absorption band at 560 nm had
disappeared exhibited only a weak signal at g = 4.25, indicating that mononuclear high‐spin iron(III)
species are intermediates also leading ultimately to formation of anti‐ferromagnetically coupled
dinuclear complexes.13b
Chapter 6
78
Figure 15. (a) UV‐vis absorption of 1 in acetonitrile (0.88 mM) with 1000 equiv. 1‐phenylethanol
and 1 equiv. of TfOH before (thick red line) and after addition of 100 equiv. H2O2. (b) Time
dependence of absorbance at 560 nm. (c) X‐band EPR spectra at 77 K of 1 in acetonitrile with TfOH
and 1‐phenylethanol; 14 s after addition of 100 equiv. of H2O2 (blue), after warm sample to room
temperature (black), 5 min after addition of 100 equiv. of H2O2 (gray).
6.2.6 Reaction of 2 with H2O2 and catalytic oxidation of 1‐phenylethanol.
The structure of 2, and specifically the tert‐butyl groups, precludes oxidative C−C coupling of the
phenols. Addition of 2 equiv. H2O2 has little effect on the UV‐vis absorption spectrum of 2, even
with TfOH, when excess water (0.33 vol%) was present (vide supra), however, with 100 equiv. H2O2
a steady decrease in visible absorbance is observed. Under more acidic conditions (i.e. with less
water present) addition of 100 equiv. of H2O2 to 2 with 1 equiv of TfOH resulted in a rapid decrease
in NIR absorbance within 8 s with a weaker absorption at ca. 550 nm appearing and persisting for
ca. 20 s (Figure 16a/b).
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
79
Figure 16. (a) UV‐vis absorption spectrum of 2 (0.88 mM) in acetonitrile with 1 equiv. of TfOH (black)
followed by addition of 100 equiv. of H2O2 (spectra are at 2 s intervals). (b) Absorbance at 750 and
550 nm over time. (c) EPR spectra (77 K, X‐band) flash frozen at 0 s (black), 10 s (red), 40 s (blue)
and 74 s (green). (d) rRaman spectra at 0 s (red), 10 s (orange), 20 s (black), 30 s (blue) and 40 s
(green) after addition of H2O2 (100 equiv.).
EPR and Raman spectroscopy confirms that H2+ undergoes rapid oxidation and loss of the
phenolato moiety (the bands at 837, 602 and 568 cm‐1 disappear within 10 s) followed by the
transient formation of an Fe(III)‐OOH species, manifested in the characteristic bands at 801 and
622 cm‐1 and a low spin (S = ½) EPR signal (Figure 16c/d). These changes coincide with the loss of
H2O2 (867 cm‐1) over 1 min. The presence of substrate (1‐phenylethanol) in excess results in an
increase in the maximum concentration of the Fe(III)‐OOH species (max 550 nm) and its persistence
over 15 min.
The loss of the phenolato moiety in 2 is consistent with spectral changes observed for 1 under
catalytic conditions. Resonance Raman spectroscopy at 785 nm confirms that in the presence of
excess H2O2 and water, oxidation phenoxyl radical is followed by a rapid loss, presumably by a
nucleophillic attack of water and subsequent disociation of the phenolato unit, to leave an N3Py
type ligand. Importantly, the C‐C coupling reaction observed with 1 upon (electro)chemical
oxidation and upon addition of stoichiometric amounts of H2O2, is not observed with excess H2O2.
The complexes formed by loss of the phenoxyl units from 1 and 2 are therefore identical and, given
that it forms within a few seconds of addition of H2O2 addition, the catalytic activity of both
complexes should be, and indeed appears to be, the same.
Chapter 6
80
Scheme 3. Summary of processes observed for 1 in solution upon oxidation with stoichiometric
H2O2 and with excess H2O2.
Mechanism of an diiron(III)bis‐phenolato complex catalyzed oxidation
81
6.3 Summary The oxidative chemistry of 1 and 2 is summarized in Scheme 3. Initially, oxiation either
electrochemical or chemical (e.g., Ce(IV), R‐OOH, H2O2 etc.) results in the generation of phenoxyl
radicals which undergo essentially instantaneous C−C coupling in the case of 1 and, in the case of
both 1 and 2, nucelophillic attack by water. The C−C coupled product (3) undergoes oxidation even
more rapidly that 1, due to its lower oxidation potential to form 32+, and equivalent oligomeric
species. In the presence of excess H2O2, however, rapid cleavage of the phenoxyl unit from the
Fe(III) bond L1 takes place to form an N3Py‐Fe(III) complex, analogous to catalytically active
complexes reported earlier. Indeed under catalytic conditions, this series of reactions occurs so
rapidly (with the mixing time) that for 1, C−C coupling is not observed. The role of acid in the
chemistry of 1 was suggested earlier11 to break up the dinuclear structure of the complex, however,
both spectroscopic and DFT studies indicate that this is not the case and instead its role is to
facilitate oxidation with H2O2. It should be noted, however, that acid is not essential and only
accelerates an already spontaneous reaction, allowing intermediates to be observed. Complex 2
bears tert‐butyl groups that are effective in preventing C−C coupling and reducing the rate of
oxidation of the phenolato moieties by H2O2, however, ultimately, phenolato centred oxidation
occurs followed by loss of the phenolato moiety.
6.4 Conclusions The design and synthesis of bio‐inspired homogeneous catalysts benefit from the breadth of
reactivity presented by nature’s enzymes and its remarkable ability to couple the redox chemistry
of metals with that of organic ligands – so called redox non‐innocent ligands. The porphyrin and
phenolato based enzymes stand out in this regard, however, efforts to mimic such redox versatility
are hampered by the lack of the protective protein environment of the enzyme in synthetic
systems. In the present contribution, we have focused on the redox chemistry and spectroscopy of
a phenolato based catalyst applied earlier by Feringa and coworkers in benzyl‐alcohol oxidation
with H2O2. We have shown that although the complexes generate spectroscopically interesting
species upon oxidation, and that the initial oxidation is essentially phenolato centered and only
formally an Fe(III)/Fe(IV) redox couple. These species are at most side reactions or intermediates
on the way towards loss of the phenolato unit and formation of the catalytically active species. In
a broader context, the present study highlights the challenge faced in studying bioinspired catalysts
bearing phenolato moieties and especially where these moieties are not only ‘redox non‐innocent’
in the classic sense but in fact the primary redox active groups. Furthermore, the use of tert‐butyl
groups, although effective in preventing C−C coupling reactions, does not prevent reaction of the
phenolato unit with small nucleophiles and oxidatively induced cleavage of the benzylic C−N bond.
Hence, future efforts towards ligand design should take these aspects into account in order to
reliably harness the potential the redox active phenol units present in oxidation catalysis.
6.5 Acknowledgements Prof. M. Swart (Universitat de Girona) is acknowledged for provision of DFT data.
Chapter 6
82
6.6 References and notes
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Am. Chem. Soc. 1997, 119, 8217−8227.
(4) Butin, K. P.; Beloglazkina, E. K.; Zyk, N. V. Russ. Chem. Rev. 2005, 74, 531−553.
(5) (a) Shimazaki, Y. Adv. Mater. Phys. and Chem. 2013, 3, 60−71; (b) Lyons, C. T.; Stack, T. D., Coord. Chem.
Rev. 2013, 257, 528−540; (c) Shimazaki, Y.; Yamauchi, O., Inorg. Phys. Theor. and Anal. 2011, 50,
383−394.
(6) Hockertz, J.; Steenken, S.; Wieghardt, K.; Hildebrandt, P. J. Am. Chem. Soc. 1993, 115, 11222−11230.
(7) Chaudhuri, P.; Wieghardt, K., Phenoxyl radical complexes. Progress in Inorganic Chemistry 2001, 50,
151−216.
(8) (a) Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T. Science 1998, 279, 537−540; (b)
Chaudhuri, P.; Hess, M.; Flörke, U.; Wieghardt, K. Angew. Chem. Int. Ed. 1998, 37, 2217−2220.
(9) (a) Stoian, S. A.; Xue, G.; Bominaar, E. L.; Que, L., Jr.; Munck, E. J. Am. Chem. Soc. 2014, 136, 1545−1558;
(b) Xue, G.; Geng, C.; Ye, S.; Fiedler, A. T.; Neese, F.; Que, L., Jr. Inorg. Chem. 2013, 52, 3976‐3984; (c)
Do, L. H.; Xue, G.; Que, L., Jr.; Lippard, S. J. Inorg. Chem. 2012, 51, 2393−2402; (d) Cranswick, M. A.;
Meier, K. K.; Shan, X.; Stubna, A.; Kaizer, J.; Mehn, M. P.; Munck, E.; Que, L., Jr. Inorg. Chem. 2012, 51,
10417−10426; (e) De Hont, R. F.; Xue, G.; Hendrich, M. P.; Que, L., Jr.; Bominaar, E. L.; Munck, E. Inorg.
Chem. 2010, 49, 8310−8322; (f) Stubna, A.; Jo, D.‐H.; Costas, M.; Brenessel, W. W.; Andres, H.; Bominaar,
E. L.; Münck, E.; Que, L. Inorg. chem. 2004, 43, 3067−3079.
(10) Mayilmurugan, R.; Stoeckli‐Evans, H.; Suresh, E.; Palaniandavar, M., Dalton trans. 2009, 26, 5101−5114.
(11) Ligtenbarg, A. G. J.; Oosting, P.; Roelfes, G.; Crois, R. M. L.; Hage, R.; Feringa, B. L.; Lutz, M.; Spek, A. L.,
Chem. Commum. 2001, 4, 385−386.
(12) Unjaroen, D.; Swart, M.; Browne. W. R. Inorg. Chem. 2017, 56, 470−479. (13) (a) Klopstra, M.; Roelfes, G.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur J Inorg Chem. 2004, 846−856. (b)
Padamati, S. K.; Draksharapu, A.; Unjaroen, D.; Browne, W. R., Inorg. Chem. 2016, 55, 4211−4222.
(14) Ito, S.; Suzuki, M.; Kobayashi, T.; Itoh, H.; Harada, A.; Ohba, S.; Nishida, Y. J. Chem. Soc., Dalton Trans.
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(15) Yan, S.; Que, L.; Taylor, L. F.; Anderson, O. P. J. Am. Chem. Soc. 1988, 110, 5222−5224.
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2001, 123, 2165−2175. (c) Lyons, C. T.; Stack, T. D. Coord. Chem. Rev. 2013, 257, 528−540.
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Appendix A
Synthesis and Characterization of Ligands and Complexes
Appendix A
84
Commercially available chemicals were used as received without further purification, unless stated
otherwise.
Caution. Perchlorate salts of metal complexes incorporating organic ligands are potentially
explosive. These compounds should be prepared in small quantities and handled with suitable
protective safeguards.
A. 1 Synthesis and characterization of Fe(II) complexes (chapter 2)
Scheme A1. Synthesis of dppz ligand.
1,10‐phenanthroline‐5,6‐dione (4). Ice‐cold mixtures of a concentrate of H2SO4 (30 mL) and HNO3
(15 mL) were added to 1,10‐phenantroline (3.07 g, 17.05 mmol) and KBr (3.04 g, 25.59 mmol). The
mixture was heated at reflux for 3 h. The hot yellow solution was poured over 500 mL of ice water
and neutralized carefully with NaOH solution. The separated precipitated of 1,10‐phenanthroline‐
5,6‐dione was filtrated off, washed with distilled water and dried. The rest of the product was
extracted with CH2Cl2 (3×10 mL) from the water phase, and then the solvent was evaporated to
give the yellow solid (1.50 g, 42%). 1H NMR (400 MHz, CDCl3): 9.07 (dd, J = 4.7 Hz, 1.8 Hz, 2H), 8.46 (dd, J = 7.9 Hz, 1.8 Hz, 2H), 7.56 (dd, J = 7.8 Hz, 4.7Hz, 2H). 13C NMR (125 MHz, CDCl3): 178.7, 156.4, 152.9, 137.3, 128.1, 125.6.
Dipyrido[3,2‐a:2',3'‐c]phenazine (5). A mixture of 1,10‐phenanthroline‐5,6‐dione (1.32 g, 6.27
mmol) and 1,2‐phenylenediamine (0.81 g, 7.53 mmol) in ethanol (250 mL) were stirred at 50 °C for
2 h and then at room temperature overnight. The solvent was evaporated to obtain the cream
solid. The crude product was left to stand for 8 h, methanol‐water (10:90) was then added, and the
product was filtered and recrystallized from methanol to give a cream solid (1.27 g, 72%). 1H NMR
(400 MHz, CDCl3): 9.64 (dd, J = 8.1 Hz, 1.6 Hz, 2H), 9.27 (d, J = 3.2 Hz, 2H), 8.34 (dd, J = 6.5 Hz, 3.4 Hz, 2H) 7.92 (dd, J = 6.6 Hz, 3.4 Hz, 2H), 7.79 (q, J = 8.0 Hz, 4.4 Hz, 2H). 13C NMR (125 MHz, CDCl3):
152.5, 148.3, 142.5, 141.1, 133.8, 130.7, 129.5, 127.6, 124.2.
[Fe(py)4(NCS)2]. Pyridine (10 mL), a solution of hydrated ferrous perchlorate (9.0 g, 35.33 mmol) in
water (250 mL) and a pinch of ascorbic acid were added to a solution of potassium thiocyanate (4.0
g, 41.16 mmol) in water (400 mL) to precipitate the yellow form. The yellow precipitate was filtered
and washes with a mixture consisting of 65% ethanol, 25% water, and 10% pyridine and dried in
vacuo for 30 min.
[Fe(L)2(NCS)2]. The solution of Ligand (L) (0.5 mmol) in hot pyridine (20 mL) was added to a solution
of [Fe(py)4(NCS)2] (0.5 mmol) in hot pyridine (80 mL). After filtration, the filtrate was allowed to
Synthesis and characterization of ligands and complexes
85
stand for one month at room temperature to produce the complex as a dark violet solid. All
operations were conducted under a nitrogen atmosphere.
[Fe(phen)2(NCS)2] (1). Anal. Calc. for C26H16FeN6S2: C, 58.65; H, 3.03; N, 15.78% Found: C, 57.71; H,
2.98; N, 15.51.
[Fe(dppz)2(NCS)2]∙py (2). Anal. Calc. for C43H25FeN11S2: C, 63.31; H, 3.09; N, 18.98% Found: C, 63.01;
H, 3.02; N, 18.76.
A. 2 Synthesis and characterization of Ru(II) complexes (chapter 3 and 4)
Scheme A2. Synthesis of [Ru(CH3CN)(MeN4Py)]2+ and [Ru(CH3CN)(N4Py)]2+
[Ru(Cl)(MeN4Py)]+. A mixture of MeN4Py; 1,1‐di(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐
amine (100.1 mg, 0.26 mmol), ruthenium(III) chloride hydrate (RuCl3xH2O) (122.4 mg, 0.59 mmol),
and L‐ascorbic acid (92.4 mg, 0.52 mmol) were heated at reflux overnight in EtOH/H2O (10/15 mL)
and then cooled to room temperature. The solvent was removed in vacuum and the crude product
purified by the column chromatography on neutral alumina, eluting with CH3CN to yield
[Ru(Cl)(MeN4Py)](Cl). Saturated aqueous sodium tetrafluoroborate (NaBF4) was added to
[Ru(Cl)(MeN4Py)](Cl) in water resulting in precipitation of [Ru(Cl)(MeN4Py)](BF4) as a red solid
(107.5 mg, 74%).1H NMR (400 MHz, CD3CN)9.35 (d, J = 5.4 Hz, 2H), 9.04 (d, J = 5.4 Hz, 2H), 7.88 (t, J = 7.8 Hz, 2H), 7.64 (d, J = 8.0 Hz, 2H), 7.54 (t, J = 7.7 Hz, 2H), 7.29 (t, J = 6.4 Hz, 2H), 7.24 (t, J =
6.4 Hz, 2H), 6.98 (d, J = 8.0 Hz, 2H), 4.15 (qAB, J = 18.6 Hz, 4H), 2.25 (s, 3H). ESI mass: m/z 518 [M‐
BF4]+.
[Ru(Cl)(N4Py)]+. Following the procedure as for [Ru(Cl)(MeN4Py)]+ complex, N4Py; 1,1‐di(pyridin‐2‐
yl)‐N,N‐bis(pyridin‐2‐ylmethyl)methanamine (296 mg, 0.80 mmol), RuCl3xH2O (376 mg, 1.81
mmol), and L‐ascorbic acid (284 mg, 1.61 mmol) in EtOH/H2O (20/30 mL) yield [Ru(Cl)(N4Py)](Cl)
which was saturated aqueous potassium hexafluorophosphate (KPF6) resulting in precipitation of
[Ru(Cl)(N4Py)](PF6) as a red solid (164 mg, 31%). 1H NMR (400 MHz, CD3CN):9.38 (d, J = 1.4 Hz, 2H), 9.00 (d, J = 1.4 Hz, 2H), 7.82‐7.89 (m, J = 7.7 Hz, 4H), 7.56 (d, J = 7.8 Hz, 1.6 Hz, 2H), 7.32 (t, J =
Appendix A
86
6.3 Hz, 2H), 7.26 (t, J = 6.4 Hz, 2H,), 7.01 (t, J = 7.9 Hz, 2H), 6.39 (s, 1H), 4.30 (qAB, JAB = 17.6 Hz, 4H),
(*4% of [Ru(CH3CN)(N4Py)](PF6) is observed by 1H NMR spectroscopy).
[Ru(CH3CN)(MeN4Py)](BF4)2. Addition of 1 mL of water into the solution of [Ru(Cl)(MeN4Py)](BF4)
(65 mg, 0.11 mmol) in CH3CN (9 mL) and the mixture was stirred overnight at 55 °C. The reaction
was cooled to room temperature and the solvent reduced in vacuum. A few drops of saturated
aqueous NaBF4 was subsequently added to a product in water yielded [Ru(CH3CN)(MeN4Py)](BF4)2
as a yellow solid (69.0 mg, 92%). 1H NMR (400 MHz, CD3CN) 8.94 (d, J = 5.4 Hz, 2H), 8.84 (d, J = 5.4 Hz, 2H), 7.97 (t, J = 7.7 Hz, 2H), 7.74 (d, J = 8.0 Hz, 2H), 7.64 (t, J = 7.8 Hz, 2H), 7.37 (t, J = 6.6 Hz,
2H), 7.29 (t, J = 6.6 Hz, 2H), 7.07 (d, J = 8.0 Hz, 2H), 4.36 (s, 4H), 2.73 (s, 3H), 2.32(s, 3H). ESI mass:
m/z 262 [M‐(BF4)2]2+. Anal. Calc. for RuC26H26B2F8N6: C, 44.79; N, 12.06; H, 3.75. Found: C, 43.31; N,
11.75; H, 3.76%.
[Ru(CH3CN)(N4Py)](PF6)2. Following the procedure as for [Ru(CH3CN)(MeN4Py)]2+. [RuCl(N4Py)](PF6)
(152 mg, 0.23 mmol) in CH3CN (30 mL) and water (3 mL) were stirred overnight at 60 °C. A few
drops of saturated aqueous KPF6 was subsequently added to a product in water yielded
[Ru(CH3CN)(N4Py)](PF6)2 as a yellow solid (138 mg, 74%). 1H NMR (400 MHz, CD3CN): 8.95 (d, J = 5.6 Hz, 2H), 8.82 (d, J = 5.4 Hz, 2H), 7.89‐7.97 (m, 2H), 7.64 (td, J = 7.8 Hz, 1.5Hz, 2H), 7.38 (t, J = 6.4
Hz, 2H), 7.29 (t, J = 6.4 Hz, 2H), 7.09 (d, J = 7.9 Hz, 2H), 6.49 (s, 1H), 4.49 (s, 4H), 2.72(s, 3H). 13C
NMR (101 MHz, CD3CN,): 163.3, 159.6, 156.2, 152.6, 138.5, 137.39, 125.4, 124.9, 124.5, 121.6,
78.1, 64.8, 4.2. ESI mass: m/z 255 [M‐(PF6)2]2+. Anal. Calc. for C25H24F12N6P2Ru: C, 37.55; N, 10.51;
H, 3.03. Found: C, 36.87; N, 10.67; H, 2.98%.
A. 2.1 Characterization of photoproduct [Ru(CD3CN)(CH3CN)(MeN4Py)]2+
Scheme A3. 2D NMR Characterization of [Ru(CD3CN)(CH3CN)(MeN4Py)]2+
[Ru(CD3CN)(CH3CN)(MeN4Py)]2+. 1H NMR (400 MHz, CD3CN) 9.02 (d, J = 5.8 Hz, 1H), 8.83 (d, J = 5.6 Hz, 1H), 8.80 (d, J = 5.4 Hz, 1H), 8.75 (d, J = 4.4 Hz, 1H), 8.01 (t, J = 7.8 Hz, 1H), 7.88 (t, J = 7.8 Hz,
1H), 7.74, (t, J = 7.7 Hz, 1H), 7.64 (d, J = 8.0 Hz, 1H), 7.59 (m, 2H), 7.53 (d, J = 7.8 Hz, 1H), 7.44 (t, J
= 6.8 Hz, 1H), 7.34 (t, J = 6.9 Hz, 1H), 7.26 (t, J = 6.6 Hz, 1H), 7.14 (d, J = 8.2 Hz, 1H), 6.79 (d, J = 7.6
Hz, 1H), 5.42 (d, J = 14.7 Hz, 1H), 5.21 (d, J = 15.0 Hz, 1H), 3.71 (d, J = 17.5 Hz, 1H), 3.31 (d, J = 17.7
Hz, 1H), 2.78 (s, 3H), 2.36 (s, 3H). 13C NMR (101 MHz, CD3CN): 170.5, 163.2, 161.1, 156.6, 153.9,
Synthesis and characterization of ligands and complexes
87
152.3, 151.8, 149.8, 138.0, 137.9, 137.4, 136.7, 126.5, 124.51, 124.9, 124.8, 124.3, 124.3, 124.0,
123.5, 120.5, 117.3, 81.8, 64.7, 63.0, 28.6, 2.8.
Figure A1. DQF‐COSY NMR spectrum of [Ru(CD3CN)(CH3CN)(MeN4Py)]2+ complex in
acetonitrile‐d3.
A. 2.2 Characterization of photoproduct [Ru(CD3CN)(CH3CN)(N4Py)]2+ A. 2.2.1 2D NMR Spectroscopy
Scheme A4. 2D NMR characterization of [Ru(CD3CN)(CH3CN)(N4Py)]2+ 2a and 2b.
2a, 1H NMR (400 MHz, CD3CN) 9.02 (d, J = 5.5 Hz, 1H), 8.85 (d, J = 4.7 Hz, 1H), 8.69 (d, J = 5.5, 1H), 8.13 (td, J = 7.7 Hz, 1.8 Hz, 1H), 7.93 (d, J = 7.2 Hz, 1H), 7.77‐7.74 (m, 2H), 7.67 (m, 1H), 7.58
(t, J = 7.8 Hz, 1H), 7.39 (t, J = 6.6 Hz, 1H), 7.35 (d, J = 7.8 Hz, 1H), 7.33‐7.27 (m, 1H), 7.25 (t, J = 6.7
Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 7.00 (d, J = 7.8 Hz, 1H), 6.19 (s, 1H), 5.56 (d, J =18.8 Hz, 1H), 5.24
(d, J = 15.3 Hz, 1H), 4.54 (d, J = 15.3 Hz, 1H), 3.89 (d, J = 18.8 Hz, 1H), 2.77 (s, 3H). 13C NMR (101
MHz, CD3CN) 165.6, 165.4, 164.7, 156.5, 155.7, 154.6, 153.5, 153.0, 140.8, 140.5, 140.0, 139.9, 130.8, 128.7, 127.9, 127.8, 127.5, 126.7, 125.6, 122.8, 82.2, 71.3, 71.3, 65.4, 65.4, 6.8.
Appendix A
88
2b, 1H NMR (400 MHz, CD3CN) 9.13 (d, J = 5.6 Hz, 1H), 8.80, (d, J = 5.5 Hz, 1H), 8.69 (d, J = 5.5 Hz, 1H), 8.62 (d, br, 1H), 7.96 (m, 1H), 7.91 (m, 1H), 7.72 (m, 1H), 7.67 (m, 1H), 7.55 (m, 1H), 7.52 (m),
7.44 (t, J = 6.6 Hz, 1H), 7.33‐7.27 (m, 2H), 7.08 (d, J = 12.0 Hz, 1H), 6.50 (d, br, 1H), 5.68 (s, 1H), 5.18
(d, J = 15.4 Hz, 1H), 4.95 (d, J = 15.4 Hz, 1H), 4.64, (d, J = 16.0 Hz), 4.18 (d, J = 16.0 Hz, 1H), 2.76 (s,
3H). 13C NMR (101 MHz, CD3CN) 166.5, 166.0, 165.3, 156.5, 155.0, 154.6, 153.3, 140.8, 140.7, 140.2, 139.4, 129.3, 128.0, 127.9, 127.9, 127.2, 127.0,126.3, 126.2, 125.1, 81.9, 72.1, 72.1, 66.6,
66.6, 6.8.
The 1H COSY NMR spectra indicate a correlation between coupled protons of the photoproducts,
with blue lines referring to 2a (major product) and green lines to 2b (minor product).
Figure A2. 1H COSY NMR spectra of 2 in acetonitrile‐d3 after irradiation at 457 nm (region between
3.3‐6.1 ppm).
Synthesis and characterization of ligands and complexes
89
Figure A3. 1H COSY NMR spectra of 2 in acetonitrile‐d3 after irradiation at 457 nm (region between
6.8‐9.3 ppm).
In the 1H NOESY spectrum shows cross peaks with a yellow line indicating correlation of protons of
the photoproduct 2a. Due to the low concentration of the minor product 2b cross peaks are weak.
Figure A4. 1H‐1H NOESY NMR spectra of 2 in acetonitrile‐d3 after irradiation at 457 nm.
Appendix A
90
Figure A5. HSQC NMR spectra of 2 in acetonitrile‐d3 after irradiation at 457 nm.
Figure A6. HMBC NMR spectra of 2 in acetonitrile‐d3 after irradiation at 457 nm
Synthesis and characterization of ligands and complexes
91
A. 2.2.2 X−ray Crystallography of Photoproducts
Figure A7. Molecular structure of compound 2a, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
A single crystal of compound 2a was mounted on top of a cryoloop and transferred into the
cold nitrogen stream (150 K) of a Bruker‐AXS D8 Venture diffractometer. Data collection and
reduction was done using the Bruker software suite APEX2.1 The final unit cell was obtained from
the xyz centroids of 9479 reflections after integration. A multiscan absorption correction was
applied, based on the intensities of symmetry‐related reflections measured at different angular
settings (SADABS). The structures were solved by direct methods using SHELXT2 and refinement of
the structure was performed using SHELXL.3 Refinement of the initial solution indicated that one of
the PF6 counter ions was disordered, with the electron density of four of the six F atoms smeared
out. This disorder was initially described by two components, but subsequent refinement of that
model indicated that the two components were each also disordered (and might be further split).
To constrain the atomic displacement parameters to reasonable values, ultimately DELU/SIMU
instructions were applied to these F atoms. The sof of the major fraction of the two‐site occupancy
model was refined to 0.85. The hydrogen atoms were generated by geometrical considerations,
constrained to idealized geometries and allowed to ride on their carrier atoms with an isotropic
displacement parameter related to the equivalent displacement parameter of their carrier atoms.
Crystal data and details on data collection and refinement are presented in Table A1.
Table A1. Crystallographic data for 2a.
chem formula C27 H27 F12 N7 P2 Ru
Mr 840.56
cryst syst orthorhombic
color, habit yellow, platelet
size (mm) 0.33 x 0.27 x 0.04
space group Pbca
a (Å) 15.206(4)
b (Å) 17.787(4)
c (Å) 24.112(7)
V (Å3) 6522(3)
Z 8
Appendix A
92
calc, g.cm‐3 1.712
µ(Mo K ), cm‐1 0.680
F(000) 3360
temp (K) 150(2)
range (deg) 2.785 – 27.174
data collected (h,k,l) ‐19:19, ‐22:19, ‐30:30
no. of rflns collected 86605
no. of indpndt reflns 7202
observed reflns 6027 (Fo 2 (Fo)) R(F) (%) 3.99
wR(F2) (%) 9.62
GooF 1.82
Weighting a,b 0.0239, 13.7413
params refined 481
restraints 54
min, max resid dens ‐0.756, 1.025
Figure A8. Molecular structure of compound 2b, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
A single crystal of compound 2b was mounted on top of a cryoloop and transferred into the
cold nitrogen stream (150 K) of a Bruker‐AXS D8 Venture diffractometer. Data collection and
reduction was done using the Bruker software suite APEX2.1 It was clear from the initial unit cell
determination of several crystals that the quality of the batch of crystals was poor, and all crystals
were rather small. Ultimately, the ‘best’ crystal was measured up to a resolution of 0.95 Å using Cu
K radiation. A dataset was measured to a completeness of ca. 90%. Due to the poor crystal quality
the structure determination only serves the purpose to establish atom connectivity. The final unit
cell was obtained from the xyz centroids of 3789 reflections after integration. A multiscan
absorption correction was applied, based on the intensities of symmetry‐related reflections
measured at different angular settings (SADABS). The structures were solved by direct methods
using SHELXT2 and refinement of the structure was performed using SHELXL.3 The hydrogen atoms
were generated by geometrical considerations, constrained to idealized geometries and allowed to
ride on their carrier atoms with an isotropic displacement parameter related to the equivalent
displacement parameter of their carrier atoms. Crystal data and details on data collection and
refinement are presented in Table A2.
Synthesis and characterization of ligands and complexes
93
Table A 2. Crystallographic data for 2b.
chem formula C27 H27 F12 N7 P2 Ru
Mr 840.56
cryst syst triclinic
color, habit yellow, platelet
size (mm) 0.13 x 0.17 x 0.04
space group P‐1
a (Å) 10.1549(4)
b (Å) 10.3638(4)
c (Å) 16.5618(8)
, deg 76.291(3)
, deg 80.761(3)
, deg 75.251(3)
V (Å3) 1628.23(12)
Z 2
calc, g.cm‐3 1.714
µ(Cu K ), cm‐1 5.760
F(000) 840
temp (K) 150(2)
range (deg) 4.508 – 54.223
data collected (h,k,l) ‐10:9, ‐10:10, ‐16:16
no. of rflns collected 7502
no. of indpndt reflns 3533
observed reflns 2492(Fo 2 (Fo)) R(F) (%) 6.33
wR(F2) (%) 17.83
GooF 1.12
Weighting a,b 0.0454, 18.6676
params refined 444
restraints 287
min, max resid dens ‐1.097, 1.233
Appendix A
94
A. 3 Synthesis and characterization of dimeric Fe(III) complexes (chapter 5 and 6) A. 3.1 Synthesis of ligand HL1
Scheme A5. Synthesis of ligand HL1.
Di(pyridin‐2‐yl)methanone oxime (4). Di(pyridin‐2‐yl)methanone (4.19 g, 22.80 mmol) was
combined with hydroxylamine hydrochloride (3.16 g, 45.60 mmol) and pyridine (25 mL). The
reaction mixture was heated at reflux for 2 h. After stirring overnight at room temperature the
mixture was poured onto ice water (100 mL) yielding a precipitate, which was recovered by vacuum
filtration and washed with cold water to yield 4 (4.24 g, 93%) as a white solid which was used
without further purification. 1H NMR (400 MHz, CDCl3) 8.63 (m, 2H), 7.85 (m, 3H), 7.67 (d, J = 8.0
Hz, 1H), 7.45 (m, 1H), 7.35 (td, J = 6.7 Hz, 2.2 Hz, 1H).
Di(pyridin‐2‐yl)methanamine (5). Zinc powder (3.12 g, 47.76 mmol) was added slowly at room
temperature to a mixture of ammonium acetate (1.02 g, 13.26 mmol) and di(pyridin‐2‐
yl)methanone oxime (2.64 g, 13.26 mmol) in 25% NH3/H2O (20 mL) and ethanol (15 mL). The
mixture was heated at reflux for 2.5 h. After stirring at room temperature overnight, the solution
was filtered over Celite, which was subsequently washed with ethanol. After evaporation of the
solvent in vacuo, 2 M NaOH (30 mL) was added to the residue and the product was extracted with
CH2Cl2 (3×30 mL). The combined CH2Cl2 layers were washed with brine (20 mL), dried (over
anhydrous Na2SO4) and the solvent removed in vacuum to yield 5 (1.93 g, 79%) as a yellow oil. 1H
NMR (400 MHz, CDCl3) 8.54 (d, J = 4.8 Hz, 2H), 7.60 (t, J = 7.6 Hz, 2H), 7.37 (d, J = 8.0 Hz, 1H), 7.11 (t, J = 6.4 Hz, 2H).
Synthesis and characterization of ligands and complexes
95
1,1‐Bis(pyridin‐2‐yl)‐N‐(pyridin‐2‐ylmethyl)methanamine (7). Pyridine‐2‐carboxaldehyde (1.12 g,
10.44 mmol) was added to di(pyridin‐2‐yl)methanamine (1.93 g, 10.44 mmol) was added. A cream
solid was obtained after stirring for 10 min, which was washed with cyclohexane to remove traces
of unreacted starting material, yielding 6. Subsequently, 6 was dissolved in MeOH (30 mL) and
NaBH4 (0.79 g, 20.88 mmol) added in small portions. After stirring at room temperature for 2 h, HCl
(aq) was added to bring the solution to pH < 2. After stirring for 30 min 5 M NaOH (aq) was added
to raise the pH to > 9. The methanol was removed in vacuum and the aqueous layer extracted with
ethyl acetate (3×30 mL). The combined organic layers were washed with brine (30 mL) and dried
over anhydrous Na2SO4. Removal of solvent in vacuum provided 7 (2.63 g, 92% over 2 steps) as a
yellow oil. 1H NMR (400 MHz, CDCl3) 8.54 (m, 3H), 7.60 (m, 3H), 7.46 (d, J = 8.0 Hz, 2H), 7.34 (d, J
= 8.0 Hz, 1H), 7.12 (m, 3H), 5.16 (s, 1H), 3.93, (s, 2H).
o‐Tolyl acetate (9). Acetyl chloride (4.20 g, 30.10 mmol) was added to a solution of o‐cresol (1.93 g,
17.89 mmol) in 1% TfOH/CH3CN (65 mL). After stirring overnight at room temperature the solution
was poured onto cold water and extracted with ethyl acetate (3×20 mL). The combined organic
layers were washed with 1 M HCl (30 mL), sat. NaHCO3 (30 mL), brine (30 mL), and dried over
anhydrous Na2SO4. Removal of solvent in vacuum yielded 9 (2.59 g, 96%) as a clear oil. 1H NMR (400
MHz, CDCl3) 7.24 (m, 2H), 7.16, (td, J = 7.2 Hz, 1.2 Hz, 1H), 7.03 (d, J = 8.0 Hz, 1H), 2.33 (s, 3H),
2.21 (s, 1H).
2‐(Bromomethyl)phenyl acetate (10). Azobisisobutyronitrile (1.36 g, 8.32 mmol) and N‐
bromosuccinimide (3.00 g, 16.65 mmol) were added to a solution of o‐tolyl acetate (2.50 g, 16.65
mmol) in benzene (40 mL). After heating at reflux for 6 h the solution mixture was cooled to room
temperature and extracted with CH2Cl2 (2×30 mL). The combined organic layers were washed with
sat. Na2CO3 (30 mL), brine (30 mL) and dried over anhydrous Na2SO4. Evaporation of solvent in
vacuum followed by column chromatography on silica (10% EtOAc/Pentane) yielded 10 (3.12 g,
79%) as a yellow oil. 1H NMR (200 MHz, CDCl3) 7.55 (m, 2H), 7.36 (m, 2H), 4.61 (s, 2H), 2.55 (s,
3H).
2‐(((Di(pyridin‐2‐yl)methyl)(pyridin‐2‐ylmethyl)amino)methyl)phenol (HL1). Diisopropylethylamine
(0.86 g, 6.62 mmol) was added to a mixture of amine 6 (1.20 g, 4.42 mmol) and 2‐
(bromomethyl)phenyl acetate 9 (1.31g, 5.17 mmol) in ethyl acetate (30 mL). After heating at reflux
overnight, the solvent was removed under vacuum and the residue purified by column
chromatography on silica (EtOAc/Pentane /Et3N = 7:2:1). A mixture of acylated 10 and deacylated
amine HL1 was obtained (1.03 g). The mixture (1.03 g) was dissolved in MeOH/ H2O (35:5 mL) and
sat NaHCO3 (20 mL) added. After stirring overnight, water was added (20 mL) and the mixture
extracted with CH2Cl2 (2×20 mL). The combined organic layers were washed with brined (30 mL)
and dried over anhydrous Na2SO4. Evaporation of the solvent in vacuum yielded HL1 (0.81 g, 48%
over 2 steps) as a yellow oil. 1H NMR (400 MHz, CDCl3) 8.65 (d, J = 5.6 Hz, 2H), 8.49 (d, J = 5.6 Hz, 1.2 Hz, 1H), 7.63 (td, J = 7.6 Hz, 1.6 Hz, 2H), 7.55 (td, J = 7.6 Hz, 1.6 Hz, 1H), 7.44 (d, J = 8.0 Hz, 1H),
7.22 (m, 4H), 7.14 (td, J = 7.6 Hz, 1.6 Hz, 1H), 7.08 (m, 1H), 6.94 (m, 2H), 6.70 (td, J = 7.2 Hz, 1.2 Hz,
1H), 5.27 (s, 1H), 3.92 (s, 2H), 3.77 (s, 2H).
Appendix A
96
A. 3.2 Synthesis of ligand HL2
Scheme A6. Synthesis of Ligand HL2.
2,4‐Di‐tert‐butyl‐6‐(hydroxymethyl)phenol (12). Addition of suspensions of paraformaldehyde (1.50
g, 149 mmol) in MeOH (40 mL) to the solution mixture of 2,4‐di‐tert‐butylphenol (30 g, 145 mmol)
and LiOH (0.28 g, 11.63 mmol) in MeOH (40 mL). The reaction mixture was heated at reflux for 16
h, the solvent then was removed under reduced pressure to give a cream solid residue. Dissolve
the residue with diethyl ether and then washed organic solvent with water and dried over
anhydrous Mg2SO4. Removing the solvent to obtained a white powder which purified by
recrystallization from hexane to yield 12 (17.05 g, 50%) as white solid. 1H NMR (400 MHz, CDCl3) δ
7.55 (s, 1H), 7.30 (s, 1H), 6.91 (s, 1H), 4.84 (s, 2H), 2.13 (br, 1H), 1.45 (s, 9H), 1.30 (s, 9H). 13C NMR
(101 MHz, CDCl3) δ 155.8, 144.3, 139.2, 126.7, 126.6, 125.3, 68.5, 37.6, 36.9, 34.3, 32.4.
2‐(Bromomethyl)‐4,6‐di‐tert‐butylphenol (13). A solution of PBr3 (5.77 g, 21.32 mmol) in CHCl3 (60
mL) was added drop‐wise to a solution of 12 (10.08 g, 42.65 mmol) in CHCl3. The reaction mixture
was stirred at room temperature for overnight. Addition of water drop‐wise to quench the reaction
results a white solution. Organic layer was separated and aqueous was extracted with
dichloromethane (2×30 mL). The organic layer was combined, dried over anhydrous Mg2SO4, and
solvent were removed under pressure to give 13 (12.12 g, 95%) as white solid. 1H NMR (400 MHz,
CDCl3) δ 7.33 (d, J = 2.5 Hz, 1H), 7.10 (d, J = 2.4 Hz, 1H), 4.59 (s, 2H), 2.18 (s, 1H), 1.43 (s, 9H), 1.30
(s, 9H). 13C NMR (101 MHz, CDCl3) δ 154.3, 145.6, 139.8, 128.3, 127.3, 125.9, 37.6, 37.0, 35.3, 34.2,
32.5.
3,5‐di‐tert‐butyl‐2‐(((di(pyridin‐2‐yl)methyl)(pyridin‐2‐ylmethyl)amino)methyl)phenol (HL2).
Addition of 2‐(bromomethyl)‐4,6‐di‐tert‐butylphenol (13) (2.32 g, 7.76 mmol) into the solution
mixture of 1,1‐bis(pyridin‐2‐yl)‐N‐(pyridin‐2‐ylmethyl)methanamine (7) (2.12 g, 7.76 mmol) and
KOH (0.7 g, 12.41 mmol) in dry toluene (60 mL). The reaction mixture was heat at 70 °C under N2
for overnight. Filtered cool solution and solvent was then removed under reduce pressure. Addition
of ethyl acetate (50 mL) to crude product then washed with sat. NaHCO3, brine and dried over
anhydrous Na2SO4. Evaporation of solvent in vacuum followed by column chromatography over
silica (Pentane/EtOAc/Et3N = 2:1:0.5) yielded HL2 (2.18 g, 57 %) as a yellow oil. 1H NMR (400 MHz,
Synthesis and characterization of ligands and complexes
97
CDCl3) δ 11.23 (s, 1H), 8.66 (d, J = 4.6 Hz, 2H), 8.48 (d, J = 4.7 Hz, 1H), 7.62 (t, J = 7.6 Hz, 2H), 7.53
(t, J = 7.6, Hz, 1H), 7.46 (d, J = 7.6 Hz, 1H), 7.31 (d, J = 7.8 Hz, 2H), 7.15–7.23 (m, 3H), 7.06 (t, J = 6.9
Hz, 1H), 6.77 (d, J = 2.4 Hz, 1H), 5.29 (s, 1H), 3.95 (s, 2H), 3.80 (s, 2H), 1.49 (s, 9H), 1.23 (s, 9H). 13C
NMR (101 MHz, CDCl3) δ 162.5, 161.6, 156.8, 151.6, 151.1, 142.3, 139.1, 139.0, 138.1, 128.3, 127.2,
126.2, 125.7, 124.9, 124.9, 124.4, 73.0, 58.9, 57.9, 37.7, 36.7, 34.3, 33.6, 32.3.
A. 3.3 Synthesis of ligand HL2
Scheme A7. Synthesis of Complex 1 and 2.
[(L1)Fe(µ‐O)Fe(L1)](ClO4)2 (1). Fe(ClO4)2∙6H2O (0.16 g, 0.44 mmol) in MeOH (1 mL) was added to a
solution of 2‐(((di(pyridin‐2‐yl)methyl)(pyridin‐2‐ylmethyl)amino)methyl)phenol (HL1) (0.14 g, 0.37
mmol) in MeOH (5 mL). Subsequently, Et3N (52 µL, 0.37 mmol) was added dropwise. After stirring
for 15 min the reaction mixture was filtered over cotton wool and placed in an EtOAc bath. Dark
micro‐crystalline solid formed over several days, which were collected and washed with EtOAc and
recrystallized from MeOH, yielding 1 as a purple crystalline solid. (70 mg, 17 %). 1H NMR (400 MHz,
acetonitrile‐d3) 40 (br), 34 (br), 26 (br), 20.3, 18.6, 17.7, 17.1, 16.0, 14.7, 13.7, 10.9, 8.0, 6.7, 3.1, 1.2, ‐2.8. ESI mass: m/z 445 {M‐2ClO4‐}; Anal. Calc. For C48H42N8O11Fe2Cl2∙H2O: C 52.1; H 4.01; N
10.13%, found: C 52.1; H 3.98; N 9.97%.
[(L2)Fe(µ‐O)Fe(L2)](ClO4)2 (2). Fe(ClO4)2∙6H2O (0.12 g, 0.48 mmol) in MeOH (1 mL) was added to the
solution of 3,5‐di‐tert‐butyl‐2‐(((di(pyridin‐2‐yl)methyl)(pyridin‐2‐ylmethyl)amino)methyl)phenol
(HL2) (0.22 g, 0.44 mmol) in methanol (3 mL) Subsequently, Et3N (62 µL, 0.44 mmol) was added
dropwise. After stirring for 30 min the reaction mixture was filtered over cotton wool and placed in
an EtOAc bath. Dark micro‐crystalline solid formed over several days, which were collected and
washed with EtOAc and recrystallized from MeOH, yielding 2 as a purple crystalline solid. (145 mg,
25 %). 1H NMR (400 MHz, acetonitrile‐d3) δ 29.9 (br), 21.9 (br), 18.1, 17.4, 15.35, 14.27, 12.11, 8.14,
8.13, 7.88, 7.20, 4.06, 3.42, 1.76, 1.55, 1.20. (ESI mass: m/z 557 {M‐2ClO4‐}; Anal. Calc. For
C64H72Cl2Fe2N8O11H2O: C 57.71; H 5.75; N 8.41%, found: C 57.69; H 5.73; N 8.27%.
Appendix A
98
A. 3.4 Extra information
Figure A7. 1H NMR spectra of 1 with 1 equiv. of TfOH in acetone‐d6. The sharp spikes at ca. 23 and
42 ppm are spectral artefacts.
Figure A8. 1H NMR spectra of oxidation reaction of 1‐phenylethanol catalyzed by 1 in acetonitrile
after 2h (a) without acid, (b) with 1 equiv. of TfOH.
Synthesis and characterization of ligands and complexes
99
Figure A9. 1H NMR spectra of oxidation reaction of 1‐phenylethanol catalyzed by 2 in acetonitrile
(a) with 1 equiv. of TfOH (b) without acid.
A. 3.5 References
(1) Bruker, (2012). APEX2 (v2012.4‐3), SAINT (Version 8.18C) and SADABS (Version 2012/1). Bruker AXS Inc.,
Madison, Wisconsin, USA.
(2) Sheldrick, G. M. SHELXT – Integrated Space‐Group and Crystal‐Structure Determination. Acta Cryst.
2015, A71, 3−8.
(3) Sheldrick, G. M. A Short History of SHELX. Acta Cryst. 2008, A64, 112−122.
Appendix B
General Description of Techniques and Measurements
Appendix B
102
Solvents for electrochemical and spectroscopic measurements were of UVASOL (Merck) grade or
better.
B. 1 Physical methods UV‐vis absorption spectroscopy. UV‐vis absorption spectrum is recorded with a Specord600
(AnalytikJena) spectrophotometer in 10 mm path length quartz cuvettes.
Nuclear magnetic resonance spectroscopy. 1H NMR spectra (400, 500 and 600 MHz) and 13C NMR
spectra were recorded on a Varian Mercury Plus and Varian Inova, respectively. Chemical shifts are
denoted relative to the residual solvent peak (1H NMR spectra CD3CN, 1.94 ppm; CDCl3, 7.26 ppm).
Dichloromethane was used as an internal reference with Evan’s method for determination of
magnetic susceptibility. 2D NMR; COSY, NOESY, HSQC and HMBC spectra (400 MHz) were recorded
on a Varian Mercury Plus. 1H NMR spectroscopy with in situ irradiation at 420 nm (LED, M420F2,
10 mW at source, Thorlabs) and 365 nm (LED, M365FP1, 10 mW at source, Thorlabs) with light
delivered via a 5 m (400 micron diameter) optical fiber in which the last 3 cm of cladding was
removed and the bare fiber was lightly sanded to give an approximately uniform emission. The bare
fiber end was inserted in the inner tube of a 5 mm Evan’s NMR tube with the sample solution held
in the outer compartment of the tube. NMR spectra were recorded during irradiation.
Elemental analysis. CHN analysis were performed with a Foss‐Heraeus CHN Rapid or a EuroVector
Euro EA elemental analyzer.
Electrospray ionization analysis. ESI mass spectra of complexes were recorded on a Triple
Quadrupole LC/MS/MS mass spectrometer (API 3000, Perkin‐Elmer Sciex Instruments). High
resolution mass spectra (HRMS) were recorded on Bruker MicroOTOF‐Q Instrument at Serveis
Tecnic of University of Girona.
Electron paramagnetic resonance spectroscopy. EPR spectra (X‐band, 9.46 GHz) were recorded on
a Bruker ECS106 spectrometer, equipped with a Bruker ECS 041 XK microwave bridge and a Bruker
ECS 080 magnet in liquid nitrogen (77 K) and Bruker EMXnano spectrometer version 001DRAFT
(110 K).
Fourier transform infrared spectroscopy. FTIR spectra were recorded using a UATR (ZnSe) with a
Perkin Elmer Spectrum400, equipped with a liquid N2 cooled MCT detector.
Raman spectroscopy. Raman spectra were recorded at 785 nm using a Perkin Elmer Raman Station
at room temperature. Raman spectra were recorded at 1064 nm in ordinary glass 1 cm cuvettes,
using a idus‐InGaAs‐512 diode array coupled to a Shamrock 163 spectrograph with a 600 l/mm
grating blazed at 1200 nm and a 500 mW (200 at sample) 1064 nm laser (Rumba, Cobalt) combined
using an Inphotonics Raman probe. Raman spectra recorded at 355 (10 mW Cobalt lasers), 473
(100 mW Cobolt Lasers) and 561 nm (100 mW, Cobolt Lasers) used a home built system in which
the laser was focused on the sample in a 180° backscattering arrangement and Raman scattering
was collected collimated and subsequently refocused via a pair of 2.5 cm diameter plano‐convex
lens (f = 10 cm). The collected light was filtered by an appropriate long pass edge filter (Semrock)
and dispersed by a Shamrock300i spectrograph (slit width 80 micron, Andor Technology) with a
General description of techniques and measurements
103
1200 L/mm grating blazed at 500 nm. Raman spectra at 532 nm (200 mW, Cobolt Lasers) was
recorded in a 90° backscattering arrangement. Raman spectra recorded at 488 nm used a Nikon
TE‐eclipse inverted microscope and 60x confocal objective. Excitation was provided by a Laser
combiner (Andor technology) equipped with an AOTF to control laser power. Raman scattering was
collected and feed into a Shamrock 300 spectrograph using a 1200 l/mm grating blazed at 500 nm
and a iVac CCD detector (Andor Technology). Data were recorded and processed using Andor Solis
(Andor Technology) with spectral calibration performed using the Raman spectrum of
acetonitrile/toluene 50:50 (v:v).1 A multipoint baseline correction was performed for all spectra.
The concentrations used for resonance Raman studies were 0.25‐0.30 mM in acetonitrile and
dichloromethane and samples were held in quartz 1 cm path length cuvettes (3 mL volume).
Irradiation with UV‐vis absorption detection was carried out at 355 nm (7 mW, Cobolt lasers) 405
nm (ONDAX lasers) and 457 nm (50 mW, Cobolt lasers).
Electrochemistry. Electrochemical measurements were carried out on a model CHI760B, CHI600C
or CHI1200C Electrochemical Workstation (CH Instruments). A Teflon‐shrouded glassy carbon (3
mm diameter), indium tin oxide (ITO) on glass slides (1 cm × 1.4 cm), gold and platinum macro
electrode were employed as working electrode (CH Instruments). A Pt wire was used as an auxiliary
electrode, and Ag/AgCl reference electrode or a saturated calomel electrode (SCE) was used as
reference electrode. Cyclic voltammograms were obtained at a sweep rate of 100 mV s‐1 in
anhydrous dichloromethane or acetonitrile containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) with analyte concentrations of 0.5 to 1 mM unless stated
otherwise. Potentials are quoted with respect to SCE unless stated otherwise. Redox potential
(Ep,a, anodic peak potential; Ep,c, cathodic peak potential; E1/2 = (Ep,a + Ep,c)/2) values are ± 10
mV.
Spectroelectrochemistry. Spectroelectrochemical experiments in solution were carried out using an
OTTLE cell (a liquid IR cell modified with Infrasil windows and a platinum mesh working and counter
electrode and a Ag/AgCl reference electrode) mounted in a Specord600 UV‐vis spectrometer with
potential controlled by a CHI600C potentiostat. In situ UV‐vis absorption spectroelectrochemistry
of poly‐1 was carried out by initial modification of an ITO electrode by cyclic voltammetry follow by
transfer to 2 mm quartz cuvette as an electrochemical cell.
B. 2 Quantum yield determinations Absolute quantum yields were determined with reference to the actinometer potassium
ferrioxalate.2 Laser flux was determined by the method of total absorption using 2 mL of 0.15M
potassium ferrioxalate in a 1 cm pathlength cuvette with stirring. The actinometer was irradiated
for 40 s (at 457 nm) and a reference cuvette was held apart from the excitation source. After
irradiation, 1 mL of each solution was added to buffered aqueous phenanthroline (0.1 M, 2 mL),
and diluted 10 fold with water and left to stand in the dark for at least 30 min and the absorbance
determined at 510 nm. The photon flux was calculated using equation (1).
∆Ф
20 1
Appendix B
104
Where L is the pathlength of the cuvette, ε is the molar absorptivity of iron(II) tris‐phenanthroline
(11100 L mol‐1 cm‐1 at λmax 510 nm), Ф is quantum yield of the actinometer at 457 nm, i.e. 0.85,3 t
is the irradiation time (40 s), F is the fraction of the light the actinometer absorbed; as the
absorbance of the ferrioxalate solution was above 2, F was taken to be 100%.
2 mL of a solution of Ru(II) complexes 1 (or 2) (concentration was determined by the absorbance
at 457 nm, which should be is close to 2) was irradiated under identical conditions with the
absorbance at 457 nm monitored by UV‐vis absorption spectroscopy. The change in absorbance
with time was used to determine the photochemical quantum yield using equation (2).
Ф /
2
∆ / /
/ ∗
Where ∆ is the change in absorbance at 457 nm over the irradiation time, is the molar
absorbtivity of complexes at 457 nm, the irradiation time was dependent on the change in
absorbance and was less than 0.2, allowing for the approximation that the number of moles of
photons absorbed per unit time is essentially unchanged, i.e. ca. 0% transmittance and hence equal
to Nhv/t.
B. 3 Spin crossover occurrence The spin crossover effect is observed for complexes of the transition metal ions particularly those
of the first transition series with configurations d4 to d7, which splitting of the energy of the d
orbitals into the t2g and eg sets. The strength of ligand filed about the metal ion can exist in either
the high‐spin (HS) or low‐spin (LS) state that a weak field and strong field stabilized the HS and LS
state, respectively (Figure B1). The spin transition (ST) or spin crossover (SCO) exhibit a switching
phenomenon by external perturbation upon change of temperature, pressure, irradiation with light
or in a magnetic field.4 The population of LSHS transition involves of anti‐bonding orbital which
accompanies a lengthening and weakening of Fe‐L bond lengths (vide infra).5 The change in spin
state is involvement the change in electronic structure of central ion which make the physical and
chemical properties of the whole molecule change.
Figure B1. Electronic distribution between HS and LS states of an octahedral for a d6 Fe(II)
coordination compound exhibited spin crossover (top). Potential energy curve for spin crossover
system, r = change in bond length (bottom).
General description of techniques and measurements
105
B. 4 Light-induced spin transition Light‐induced spin transition in solution is first observed by McGarvey et al.6 who reported using
laser excitation within an MLCT absorption band of Fe(II) spin crossover complexes to increase the
population of the high spin state. In particular, inducing a spin transition of [Fe(ptz)6](BF4)2 complex
in a solid state at 10 K from LS to HS by irradiation with light, know as light induced excited spin
state trapping (LIESST) effect, has attracted much attention since Decurtins and co‐workers7
discovered in 1984.
Figure B2. Schematic showing LIESST and reverse‐LIESST. Light induced excitation is shown by full
arrows, dotted arrows denote non‐radiative internal conversion or formally spin‐forbidden
intersystem crossing.
The photophysical process involved in LIESST phenomenon are represented in the Jablonski
diagram (Figure B2). Upon irradiation of the LS isomer with green light results in a spin‐allowed
transition to a singlet state followed by rapid internal conversion to the lowest 1T state (1T1).
Intersystem crossing to a low‐lying 3T state (weakly allowed through spin‐orbit coupling) which can
decay back to the LS state or to the HS state where it is “frozen” (vide supra). Light‐induced back
conversion (reverse‐LIESST) of metastable HS state is effected by irradiation with red light,
whereby the HS in 5T2 state is excited to 5E state and relaxes to the low spin 1A1 configuration.
B. 5 Resonance Raman scattering Raman spectroscopy involves excitation of the molecule with monochromatic light from initial to
virtual state and then molecule relaxes to the same or different vibrational level of incident light.
When the wavelength of incident light coincides with an electronic transition of the molecule thus
the Raman scattering observed might be enhanced by 104‐106 times under dilute solution.
However, the enhancement is not in general observed for all vibrational modes, rather certain
modes of nature electronic transitions become a selective enhancement.8
Appendix B
106
Figure B3. Schematic diagram depicting resonance Raman scattering.
B. 6 References
(1) Calibration reference for Raman measurement
(2) Hatchard, C.; Parker, C. A., A new sensitive chemical actinometer. II. Potassium Ferrioxalate as a
Standard Chemical Actinometer. Proceedings of the Royal Society of London. Series A. Mathematical and
Physical Sciences 1956, 235, 518−536.
(3) Demas, J.; Bowman, W.; Zalewski, E.; Velapoldi, R., Determination of the Quantum Yield of the
Ferrioxalate Actinometer with Electrically Calibrated Radiometers. J. Phys Chem. 1981, 85, 2766−2771.
(4) (a) Gütlich, P.; Hauser, A. Spiering. H. Thermal and Optical Switching of Iron(II) Complexes. Angew.
Chem. Int. Ed. Engl. 1994, 33, 2024−2054. (b) P. Gütlich, Spin Crossover – Quo Vadis?, Eur. J. Inorg.
Chem. 2013, 5, 518−591.
(5) Browne, R.W.; McGarvey J. J.; Raman Scattering and Photophysics in Spin‐State‐Labile d6 Metal
Complexes Coord. Chem. Rev. 2006, 250, 1696−1709.
(6) McGravey, J. J.; Lawthers, I. Photochemically‐induced Perturbation of the 1A⇌5T Equilibrium in
FeII Complexes by Pulsed Laser Irradiation in the Metal‐to‐Ligand Charge‐Transfer Absorption Band. J.
Chem. Soc., Chem. Commun. 1982, 906−907.
(7) Decurtins, S.; Gütlich, P.; Köhler, C.P.; Spiering, H. Light‐Induced Excited Spin State Trapping in a
Transition‐Metal Complex: The Hexa‐1‐propyltetrazole‐Iron(II) Tetrafluoroborate Spin‐Crossover
System. Chem. Phy. Lett. 1984, 105,1−4.
(8) Browne, W. R.; McGarvey,. J. J. The Raman Effect and its Application to Electronic Spectroscopies in
Metal‐Centered Species: Techniques and Investigations in Ground and Excited States. Coord. Chem. Rev.
2007, 251, 454−473.