Dicobalt II-II, II-III, and III-III complexes as spectroscopic models for dicobalt enzyme active sites
Dicobalt II-II, II-III, and III-III complexes as spectroscopic models for dicobalt enzyme active sites
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DOI：
10.1021/ic7020534
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发表时间：
2008-06-16
影响因子：
4.6
通讯作者：
McKenzie, Christine J.
中科院分区：
文献类型：
--
作者：
Johansson, Frank B.;Bond, Andrew D.;McKenzie, Christine J.
一组匹配的双核钴配合物与II-II，II-III，和III-III氧化态的制备和结构表征。In [（bpbp）CO2（O2P（OPh）（2））（2）]n（+）（n = 1、2或3; bpbp（-）= 2，6-二（（N，N ′-双-（2-吡啶甲基）氨基）-甲基）-4-叔丁基苯酚根合），未键合的Co中心点Co间距在3.5906（17）至3.7081（11）埃的范围内，并且金属离子通过七齿双核配体的酚氧原子和两个磷酸二苯酯基团三重桥接。配合物的整体结构和几何形状非常相似，金属-配体键距的微小变化与氧化态分配一致。（CoCoIII）-Co-II化合物是价态捕获的Robin-Day II类络合物。抗磁性（CoCoIII）-Co-III（3）和顺磁性（CoCoIII）-Co-II（2）和（CoCoII）-Co-II（1）络合物的固态P-31 NMR光谱显示，对于氧化态的每个增量，P-31各向同性位移加宽并向低场移动约3000 ppm。循环伏安法证实溶液中存在（CoCoII）-Co-II、（CoCoII）-Co-II和（CoCoIII）-Co-III物质。氧化还原的变化是不可逆的，在所施加的扫描时间尺度，表明化学变化与氧化和还原的钴中心。对该系列化合物的光谱性质进行了研究，以探讨其在分析二钴金属水解酶的相关光谱性质中的潜在用途。原则上，磁性圆二色性（MCD）已被用来确定的（CoCoII）-Co-II复合物中的磁交换耦合的强度，通过分析的可变温度可变场（VTVH）的MCD信号的强度行为。该系列是磁耦合的光谱测定的理想选择，因为它只能发生在（CoCoII）-Co-II复合物中。（CoCoIII）-Co-II复合物含有几乎同构的Co-II离子，但由于Co-III是抗磁性的，因此磁性耦合被关闭，而Co-II离子的光谱特征保留。从（CoCoIII）-Co-II复合物的MCD数据的分析已进行的理论背景下的T-1（1g）基态的Co-II离子，最初在八面体配体场分裂的几何畸变和zoro场分裂，形成一个孤立的双重基态。[（bpbp）Co-2（O2 P（OPh）（2））]（+）络合物中（CoCoII）-Co-II对的MCD数据拟合为基于弱反铁磁耦合的模型，J = -1.6 cm（-1）。固态磁化率测量证实了解释。
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 center dot center dot center dot Co separations are within the range 3.5906(17) to 3.7081 (11) angstrom, 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 (CoCoIII)-Co-II compound is a valence-trapped Robin-Day class II complex. Solid state P-31 NMR spectra of the diamagnetic (CoCoIII)-Co-III (3) and paramagnetic (CoCoIII)-Co-II (2) and (CoCoII)-Co-II (1) complexes show that P-31 isotropic shifts broaden and move downfield by about 3000 ppm for each increment in oxidation state. Cyclic voltammetry corroborates the existence of the (CoCoII)-Co-II, (CoCoII)-Co-II, and (CoCoIII)-Co-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 (CoCoII)-Co-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 (CoCoII)-Co-II complex. The (CoCoIII)-Co-II 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 (CoCoIII)-Co-II complex has been undertaken in the theoretical context of a T-1(1g) ground-state of the Co-II ion, initially in an octahedral ligand field that is split by both geometric distortion and zoro-field splitting to form an isolated doublet ground state. The MCD data for the (CoCoII)-Co-II pair in the [(bpbp)Co-2(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.