Dicobalt II−II, II−III, and III−III Complexes as Spectroscopic Models for Dicobalt Enzyme Active Sites

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• 作者：Frank B. Johansson ; Andrew D. Bond ; Ulla Gro Nielsen ; Boujemaa Moubaraki ; Keith S. Murray ; Kevin J. Berry ; James A. Larrabee ; Christine J. McKenzie
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：June 16, 2008
• 年：2008
• 卷：47
• 期：12
• 页码：5079 - 5092
• 全文大小：1055K
• 年卷期：v.47,no.12(June 16, 2008)
• ISSN：1520-510X

文摘

A matched set of dinuclear cobalt complexes with II−II, II−III, and III−III oxidation states have been prepared and structurally characterized. In [(bpbp)Co₂(O₂P(OPh)₂)₂]ⁿ⁺ (n = 1, 2, or 3; bpbp⁻ = 2,6-bis((N,N′-bis-(2-picolyl)amino)-methyl)-4-tertbutylphenolato), the nonbonded Co···Co separations are within the range 3.5906(17) to 3.7081(11) Å, and the metal ions are triply bridged by the phenolate oxygen atom of the heptadentate dinucleating ligand and by two diphenylphosphate groups. The overall structures and geometries of the complexes are very similar, with minor variations in metal−ligand bond distances consistent with oxidation state assignments. The Co^IICo^III compound is a valence-trapped Robin−Day class II complex. Solid state ³¹P NMR spectra of the diamagnetic Co^IIICo^III (3) and paramagnetic Co^IICo^III (2) and Co^IICo^II (1) complexes show that ³¹P isotropic shifts broaden and move downfield by about 3000 ppm for each increment in oxidation state. Cyclic voltammetry corroborates the existence of the Co^IICo^II, Co^IICo^III, and Co^IIICo^III species in solution. The redox changes are not reversible in the applied scanning timescales, indicating that chemical changes are associated with oxidation and reduction of the cobalt centers. An investigation of the spectroscopic properties of this series has been carried out for its potential usefulness in analyses of the related spectroscopic properties of the dicobalt metallohydrolases. Principally, magnetic circular dichroism (MCD) has been used to determine the strength of the magnetic exchange coupling in the Co^IICo^II complex by analysis of the variable-temperature variable-field (VTVH) intensity behavior of the MCD signal. The series is ideal for the spectroscopic determination of magnetic coupling since it can occur only in the Co^IICo^II complex. The Co^IICo^III complex contains a nearly isostructural Co^II ion, but since Co^III is diamagnetic, the magnetic coupling is switched off, while the spectral features of the Co^II ion remain. Analysis of the MCD data from the Co^IICo^III complex has been undertaken in the theoretical context of a ⁴T_1g ground-state of the Co^II ion, initially in an octahedral ligand field that is split by both geometric distortion and zero-field splitting to form an isolated doublet ground state. The MCD data for the Co^IICo^II pair in the [(bpbp)Co₂(O₂P(OPh)₂)₂]⁺ complex were fitted to a model based on weak antiferromagnetic coupling with J = −1.6 cm⁻¹. The interpretation is confirmed by solid state magnetic susceptibility measurements.