Ligand radical localization in a nonsymmetric one-electron oxidized Ni II bis-phenoxide complex

Article abstract of DOI:10.1002/chem.201001401

Catching the radical: Characterization of an oxidized nonsymmetric NiII-Sala-len complex 2⁺ has demonstrated that the ligand radical is localized on the more electron-rich aminophenolate, and can be described as a Class II intervalence complex. This is in con-trast to the oxidized symmetric salen analogue 1⁺, in which the radical is fully delocalized across the ligand framework, a Class III species.

Full text of DOI:10.1002/chem.201001401

DOI: 10.1002/chem.201001401
Ligand Radical Localization in a Nonsymmetric One-Electron Oxidized
NiII Bis-phenoxide Complex
Tim Storr,[a, b] Pratik Verma,[a] Yuichi Shimazaki,*[c] Erik C. Wasinger,[d] and
T. Daniel P. Stack*[a]
The interplay of electronic structure and reactivity in tran-
sition-metal complexes is an area of considerable research
effort.[1,2] The cooperative effect of redox-active ligands and
metal sites in enzymatic systems,[3] and more recently in syn-
thetic systems,[4] adds significant flexibility to catalyst func-
tion. Depending on the relative energies of the redox-active
orbitals, metal complexes with proradical ligands can exist
in a limiting description as a metal–ligand radical (Mn+(L ))
C
or a high-valent metal complex (M(n+1)+(Lꢀ)). In certain
cases, subtle changes to the system through variation of the
ligand field, or temperature is sufficient to shift the oxida-
tion locus.[5,6] Recent work in this area has focused on bis-
(salicylidene)diamine complexes 1–3 (Scheme 1).[6–11] The
one-electron oxidized Ni derivatives exist in the ligand radi-
Cꢀ
cal form NiII
ACHTUNGTRENNUNG(L ) in solution and the solid state, however
Cꢀ
the addition of exogenous ligands to NiII
ACTHNUTRGNE(NUG L ) in solution re-
ACTHNUTRGNEUNG
sults in a shift in the oxidation locus to the NiIII(L2ꢀ)
form.[7–10] The oxidized Cu derivative of 1 exists as the high-
valent metal complex in the solid state. In solution this com-
plex exhibits temperature-dependent valence tautomerism
between the ligand radical and high-valent metal forms,
Scheme 1. Nickel bis-phenoxide complexes.
demonstrating the nearly isoenergetic nature of these two
species.[6]
Oxidation studies to date have centered on symmetric
bis(salicylidene)diamine complexes, resulting in full delocali-
zation of the radical over the ligand framework. Interesting-
ly, recent work has shown that the oxidized Pd analogue of
1 exhibits partial radical localization on one of the two phe-
nolates as this metal ion limits coupling between the redox-
active ligands.[12] Intrigued by this example, we have synthe-
sized the Ni analogue of a Salalen ligand 4,[13] a nonsymmet-
ric variant of 1.[14] We hypothesized that the reduced amino-
phenolate of 4 would undergo a one-electron oxidation at a
lower potential than the iminophenolate, resulting in a local-
ized ligand radical complex. Preferential redox tuning of
phenolate ligands has been demonstrated previously in a
functional model of galactose oxidase.[15]
[a] Prof. T. Storr, P. Verma, Prof. T. D. P. Stack
Department of Chemistry, Stanford University
Stanford, CA 94305 (USA)
Fax : (+1)650-725-0259
E-mail: stack@stanford.edu
[b] Prof. T. Storr
Present address: Department of Chemistry
Simon Fraser University
Burnaby, BC, V5A-1S6 (Canada)
[c] Prof. Y. Shimazaki
College of Science, Ibaraki University
Bunkyo, Mito, 310-8512 (Japan)
Fax : (+81)29-228-8403
E-mail: yshima@mx.ibaraki.ac.jp
[d] Prof. E. C. Wasinger
California State University–Chico
Chico, CA 95929 (USA)
Compound 4 exhibits two reversible redox couples by
cyclic voltammetry (E1/21 =0.13 V and E1/22 =0.56 V vs. Fc+/
Fc; Fc: ferrocene; see Figure S1 in the Supporting Informa-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001401.
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Chem. Eur. J. 2010, 16, 8980 – 8983

COMMUNICATION
tion). The first oxidation for 4 occurs at a potential 0.3 V
lower than that for 1, and compares well with the electro-
chemistry of a Cu tetrahydrosalen complex.[16] This is attrib-
uted to oxidation of the more electron-rich aminophenolate.
Treatment of 4 in CH2Cl2 with one equivalent of the oxi-
of the contraction expected for a phenoxyl radical with
semi-quinone character.[17,20]
The EPR spectum of 4+ in CH2Cl2 at 77 K exhibits a
broad (S=1/2) signal with a giso =2.018 (Figure 2a), indicat-
ing that the unpaired electron is ligand-based. The measured
dants AgSbF6 (E1/2 =+0.65 V vs. Fc+/Fc), thianthrenyl radi-
ꢀ
cal [thianthrene]+ SbF6 (E1/2 =+0.89 V vs. Fc+/Fc), or
C
(NH4)2CeACHTUNGTRENNUNG(NO3)6 results in an immediate color change from
red-brown to green, signifying formation of 4+. AgSbF6 as
ꢀ
the chemical oxidant provides crystals of 4+ SbF6 suitable
for X-ray analysis (Figure 1).[13]
ꢀ
Figure 1. Molecular structure of 4+ SbF6 (50% probability ellipsoids).
ꢀ
ꢀ
Selected interatomic distances [ꢁ] and angles [8]: Ni1 N1 1.872(5), Ni1
ꢀ
ꢀ
ꢀ
ꢀ
N2 1.824(5), Ni1 O1 1.883(4), Ni1 O2 1.823(3), C1 O1 1.365(6), C20
ꢀ
ꢀ
O2 1.329(6), N1 C7 1.472(2), N2 C14 1.283(7); N1-Ni1-N2 86.7(2), O1-
Ni1-O2 84.2(1), N1-Ni1-O1 93.3(2), N2-Ni1-O2 95.3(2).
Figure 2. X-band EPR spectra of 4+ recorded in frozen CH2Cl2 at 77 K
(experimental spectra: solid lines; simulations: dashed lines): a) 1 mm 4+,
b) 1 mm 4+ + 20 equiv pyridine. Inset: Calculated spin density plot for
4+, showing localization of the unpaired electron on the more electron-
rich aminophenolate.
The structures of 4 (see Figure S2 in the Supporting Infor-
mation) and 4+ possess a slightly distorted square-planar ge-
ometry about the Ni center, similar to 1 and 1+. While the
coordination sphere of 1 contracts symmetrically upon oxi-
dation to 1+,[10] the Ni coordination sphere of 4+ is nonsym-
metric (Table 1), with considerable lengthening of the ami-
g value is lower than that for 1+ (giso =2.045)[10] indicating a
smaller contribution of metal d orbitals (mostly dyz) to the
SOMO of 4+, and greater ligand-radical character. This
result corresponds well with the lower calculated Ni spin
density (Figure 2 inset) for 4+ (5%), in comparison to that
for 1+ (10%).[21] These results suggest that the electronic
coupling between the two redox-active phenolates, mediat-
ed, presumably through a hole transfer superexchange
mechanism involving the Ni dyz orbital,[2,10,12] is reduced sub-
stantially in 4+ as compared to that in 1+ (vide infra).
Addition of 20 equivalents of pyridine to 4+ at 233 K and
subsequent cooling to 77 K results in an anisotropic EPR
pattern (Figure 2b) that is consistent with formation of a
NiIII species [4(py)2]+ (gx =gy =2.230, gz =2.032, Az =19ꢂ
10ꢀ4 cmꢀ1, gav =2.16). This result is consistent with modula-
tion of the ligand field upon axial binding of pyridine, and a
consequent shift in the locus of oxidation to form a NiIII
complex.[8–10]
Table 1. Experimental and calculated (in parentheses) coordination
sphere metrical parameters for the complexes in ꢁ.
ꢀ
Ni O1
ꢀ
Ni O2
ꢀ
Ni N1
ꢀ
Ni N2
4
1.846 (1.876)
1.883 (1.892)
1.841 (1.861)
1.823 (1.836)
1.845 (1.945)
1.872 (1.951)
1.845 (1.877)
1.824 (1.854)
4+
ꢀ
nophenolate Ni O1 bond by 0.04 ꢁ in comparison to 4. The
ꢀ
lengthening of the Ni O1 bond is consistent with a decrease
in electron-donating ability of a phenoxyl ligand relative to
that of phenolate,[17,18] indicating localized oxidation of the
aminophenolate. A similar nonsymmetric coordination envi-
ronment was observed in the X-ray structure of the Pd ana-
logue of 1+.[12] DFT calculations[19] of 4 and 4+ reproduce
the observed coordination sphere asymmetry in the oxidized
Ni K-edge X-ray absorption spectroscopy (XAS) was
used to further probe the metal oxidation state and structure
of frozen solutions of 4, 4+, and [4(py)2]+ (Figure S3 in the
ꢀ
form (Table 1). Interestingly, an elongation of the C O
bond lengths is observed upon oxidation of 4 to 4+, instead
Chem. Eur. J. 2010, 16, 8980 – 8983
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8981

D. P. Stack, Y. Shimazaki et al.
Supporting Information). The Ni K-edge 1s ! 3d transition,
or pre-edge, is a successful indicator of Ni oxidation state,[22]
and the similar energies of the pre-edge feature for 4
(8332.0 eV) and 4+ (8331.9 eV) are consistent with a NiII ox-
idation state for both complexes. The shift of the pre-edge
to higher energy for [4(py)2]+ (8332.4 eV) signifies oxidation
to NiIII, matching the EPR results (vide supra).
The UV-Vis-NIR spectra of 4 is typical of a low-spin d8
square-planar metal complex, and changes substantially
upon oxidation to 4+ (Figure 3). The spectrum of 4+ exhib-
Figure 4. Resonance Raman (rR) spectra of 4 (solid line) and 4+ (dashed
*
.
line) in CH2Cl2 at 213 K (lex =413 nm). Solvent=
tional modes resonant with both phenoxyl p ! p* and phe-
nolate-NiII LMCT transitions are observable in the spectrum
of 4+. The features at 1501 and 1581 cmꢀ1 are attributed to
ꢀ
characteristic phenoxyl radical C O stretching, n7a, and
Cortho Cmeta stretching, n8a, modes respectively.[25] The rR in-
ꢀ
tensity ratio, I
A
(n7a) ꢁ 1, is often used as a spectral
marker for metal-coordinated phenoxyl radicals with p-me-
thoxy substituents.[24] However, the tert-butyl substituents
employed in this work may minimize the semi-quinoid char-
ꢀ
acter of phenoxyl radicals, and the elongated Ni O1 bond
length could further influence the enhancement of the n8a
mode.[24,26] A combination of these factors presumably leads
to the reduced intensity of n8a relative to n7a observed in 4+;
a similar intensity pattern is reported for a NiII–phenoxyl
radical complex with the same tert-butyl substitution pat-
tern.[27]
Figure 3. Electronic absorption spectra of 0.08 mm solutions of 4 (solid
line) and 4+ (dashed line) in CH2Cl2 at 298 K. Calculated transitions
shown as vertical lines. Inset: TD-DFT assignment (b-HOMO ! b-
LUMO) of the lowest energy NIR transition (5500 cmꢀ1) for 4+.[13]
The phenolate/phenoxyl radical n8a mode in 1+[9] is red-
shifted by 22 cmꢀ1 (1625 cmꢀ1 to 1605 cmꢀ1) in comparison
to the phenolate n8a mode in 1,[28] consistent with delocaliza-
tion of the ligand radical for 1+ on the Raman timescale.
The feature at 1615 cmꢀ1 in 4+, assigned to the phenolate n8a
mode, is only red-shifted by 4 cmꢀ1 in comparison to the cor-
responding feature in 4.[28] These rR results further support
a localized ligand radical description for 4+.
In summary, we have characterized the electronic struc-
ture of a nonsymmetric one-electron oxidized NiII bis-phen-
oxide complex 4+. While the symmetric derivative 1+ is a
Class III mixed-valence species, 4+ is best described as a
Class II mixed valence species due to localization of the
ligand radical on the more electron rich amino-phenolate.
its a new intense band at 25000 cmꢀ1 (9300mꢀ1 cmꢀ1), and
low-energy bands at 13000 cmꢀ1 (3000mꢀ1 cmꢀ1), 11300 cmꢀ1
(shoulder; 2700mꢀ1 cmꢀ1), and 6500 cmꢀ1 (2000mꢀ1 cmꢀ1).
The absence of low-energy transitions (<12000 cmꢀ1) in 4
and [4(py)2]+ (see Figure S4 in the Supporting Information)
indicates that the low-energy bands are associated with the
ligand radical. The lowest energy NIR band is predicted by
time-dependent density functional theory (TD-DFT) to be a
phenolate to phenoxyl intervalence charge transfer (IVCT)
transition (Figure 3 inset), and is of much weaker intensity
in comparison to the NIR band for the Class III[23] delocal-
ized
mixed-valence
complex
1+
(4700 cmꢀ1,
21500mꢀ1 cmꢀ1).[10] This attenuation in NIR band intensity
reflects the limited electronic coupling between the two
redox-active phenolates in 4+. The intense band at
25000 cmꢀ1 is assigned to a phenoxyl radical p ! p* transi-
tion.[24] This band is obscured by other intense LMCT transi-
tions in the absorption spectrum of 1+.[9,11]
Experimental Section
Synthesis of 4+ SbF6ꢀ: Compound 4 (0.097 g, 0.16 mmol) was dissolved in
CH2Cl2 (3 mL), and solid AgSbF6 (0.055 g, 0.16 mmol) was added. A
bright green suspension formed immediately. After 1 h the mixture was
filtered through celite and the solvent was removed in vacuo to afford a
The resonance Raman (rR) spectra of 4 and 4+ (Figure 4)
exhibit key differences upon oxidation, consistent with local-
ization of the ligand radical on the Raman timescale. Vibra-
green solid. The material was recrystallized from CH2Cl2/pentane to
ꢀ
afford
4
+ SbF6 as green block-like crystals. Magnetic susceptibility;
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Chem. Eur. J. 2010, 16, 8980 – 8983

Ligand Radical Localization in Nickel Complexes
COMMUNICATION
(Evanꢃs Method) meff =1.6 BM. Elemental analysis (%) calcd for
C36H54N2O2NiSbF6: C 51.40, H 6.47, N 3.33; found: C 51.56, H 6.18, N
3.28.
[11] O. Rotthaus, O. Jarjayes, C. P. Del Valle, C. Philouze, F. Thomas,
Chem. Commun. 2007, 4462.
[12] Y. Shimazaki, T. D. P. Stack, T. Storr, Inorg. Chem. 2009, 48, 8383.
[13] See the Supporting Information for details.
[14] a) K. Nakano, M. Nakamura, K. Nozaki, Macromolecules 2009, 42,
6972; b) A. Berkessel, M. Brandenburg, E. Leitterstorf, J. Frey, J.
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Pierre, Angew. Chem. 2002, 114, 3173; Angew. Chem. Int. Ed. 2002,
41, 3047.
ꢀ
X-ray details for 4 and 4+ SbF6 are available in the Supporting Informa-
tion. CCDC-770780 and CCDC-770781 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif Synthetic procedures, X-ray data,
computations, electrochemistry, UV/Vis data, X-ray absorption spectros-
copy, and Raman analysis are available as Supporting Information.
[16] R. C. Pratt, T. D. P. Stack, J. Am. Chem. Soc. 2003, 125, 8716.
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Acknowledgements
[18] L. Benisvy, A. J. Blake, D. Collison, E. S. Davies, C. D. Garner,
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This work was supported by NIH grant GM-50730 (T.D.P.S.), the CSUC
College of Natural Science (E.C.W.), an NSERC post-doctoral scholar-
ship (T.S.), and a Grant-in-Aid for Scientific Research No. 17750055
(Y.S.). Dr. Allen Oliver is thanked for X-ray analysis. SSRL operations
are funded by the DOE, Office of Basic Energy Services. Prof. Fumito
Tani, Kushu University is acknowledged for Raman analysis. Mithi Adhi-
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Keywords: intervalence complexes
ligands · nickel · oxidation locus
· localization · N,O
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[28] Selected Raman peaks [cmꢀ1
] (lex =413 nm) 4: n˜ =1350, 1530,
+
1619 cmꢀ1 (n8a); 4 : 918, 1350, 1370, 1501 (PhO n7a), 1530, 1581
C
ꢀ1
(PhO n8a), 1615 cm (n8a); 1: 1351, 1533, 1627 cmꢀ1 (n8a); 1+: 1370,
C
ꢀ1
C
C
1504 (PhO n7a), 1605 cm (PhO n8a).
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Received: April 7, 2010
Published online: July 19, 2010
Chem. Eur. J. 2010, 16, 8980 – 8983
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8983