Cini Renzo, Moore Scott J., Marzilli Luigi G.
Department of Chemical and Biosystem Sciences and Technologies, University of Siena, Pian dei Mantellini 44, I-53100 Siena, Italy, and Department of Chemistry, Emory University, Atlanta, Georgia 30322.
Inorg Chem. 1998 Dec 28;37(26):6890-6897. doi: 10.1021/ic980666i.
The CH(2)OCH(3) ligand has a large trans influence comparable to that of bulky alkyl groups but lacks the complication of marked steric effects. However, crystals of model complexes with this ligand have proved difficult to obtain. The crystal structures of the cobaloximes reported here, Me(3)BzmCo(DH)(2)CH(2)OCH(3).0.6CH(3)OH (1) and 4-MepyCo(DH)(2)CH(2)OCH(3) (2) [DH = monoanion of dimethylglyoxime, Me(3)Bzm = 1,5,6-trimethylbenzimidazole, and 4-Mepy = 4-methylpyridine], triple the number of cobaloxime structures with CH(2)OCH(3). Also, these are the first structures in this class of models with an N-donor planar heterocyclic axial donor ligand, L. The planes of the Me(3)Bzm and 4-Mepy ligands are almost perpendicular to the respective planes of the four equatorial DH nitrogen donors and bisect the (O(-))N-Co-N(OH) angles. The Co-N(L) bond distances average 2.096(2) Å, confirming the strong trans influence of CH(2)OCH(3). Geometry optimization via molecular mechanics using MacroModel 5.0 and an AMBER-type force field was applied to both cobaloxime-type and imine/oxime-type B(12) models. In our initial work with some imine/oxime models, the calculated structures did not compare well to the solid-state structures. Therefore, adjustments to the force field were evaluated. The major adjustment that improved the fit of the computed and experimental structures was an approximately 10% reduction of the van der Waals (vdw) parameters for both the N donors and the C(sp(2)) atoms linked to the N donors; this adjustment may reflect the electron-withdrawing effect of the metal center. Analysis of (1)H-(13)C coupling constants of Me(3)Bzm lends support to the concept that the atoms in the ligands were modified, but only slightly, by the metal center. To reproduce the dependence of geometric features on the trans influence, different force field parameters for L-Co bonds were used for compounds with weak (Cl) and those with strong (CH(2)OCH(3)) trans influence ligands. These small modifications allowed us to model the structural features of both classes of models well.
CH₂OCH₃配体具有与庞大烷基相当的大的反位影响，但没有明显空间效应带来的复杂性。然而，已证明难以获得含有该配体的模型配合物的晶体。本文报道的钴胺肟的晶体结构，Me₃BzmCo(DH)₂CH₂OCH₃·0.6CH₃OH (1) 和4-MepyCo(DH)₂CH₂OCH₃ (2) [DH = 二甲基乙二肟的单阴离子，Me₃Bzm = 1,5,6-三甲基苯并咪唑，4-Mepy = 4-甲基吡啶]，使含有CH₂OCH₃的钴胺肟结构数量增加了两倍。此外，这些是这类模型中首个具有N供体平面杂环轴向供体配体L的结构。Me₃Bzm和4-Mepy配体的平面几乎垂直于四个赤道面的DH氮供体的各自平面，并平分(O⁻)N-Co-N(OH)角。Co-N(L)键长平均为2.096(2) Å，证实了CH₂OCH₃的强反位影响。使用MacroModel 5.0和AMBER型力场通过分子力学对钴胺肟型和亚胺/肟型B₁₂模型进行了几何优化。在我们对一些亚胺/肟模型的初步工作中，计算得到的结构与固态结构的对比效果不佳。因此，对力场进行了评估。改善计算结构与实验结构拟合度的主要调整是将与N供体相连的N供体和C(sp²)原子的范德华（vdw）参数降低约10%；这种调整可能反映了金属中心的吸电子效应。对Me₃Bzm的¹H-¹³C耦合常数的分析支持了这样的概念，即配体中的原子仅被金属中心轻微修饰。为了重现几何特征对反位影响的依赖性，对于具有弱反位影响配体（Cl）和强反位影响配体（CH₂OCH₃）的化合物，使用了不同的L-Co键力场参数。这些小的修改使我们能够很好地模拟这两类模型的结构特征。
