Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology CSIR, Trivandrum 695 019, India.
Inorg Chem. 2010 May 17;49(10):4665-9. doi: 10.1021/ic1004243.
Topographical analysis of the molecular electrostatic potential (MESP) has been carried out for a variety of N-heterocyclic carbenes at the B3LYP, BP86, and M05 levels of density functional theory (DFT) using a 6-311++G** basis set. The electron rich character of the carbene carbon is assessed in terms of the absolute minimum of the MESP at the carbene lone pair region (V(min)), as well as the MESP at the carbene nucleus (V(C)). A linear relationship is established between the V(C) and Tolman electronic parameter (TEP) which suggested the use of the former as a simple and efficient descriptor for the electron donating power of N-heterocyclic carbene ligands toward metal coordination. The V(min) of the carbene also showed good correlation with TEP. However, the deviation from linearity was higher than V(C)-TEP correlation, and the reason for this was attributed to the steric effect of N-substituents at the lone pair region. The greater coordinating power of N-heterocyclic carbenes over phosphines is explained on the basis of deeper V(min) values obtained for the former, and in fact even the V(min) of the least electron rich N-heterocyclic carbene is comparable to the highly electron rich phosphine ligands. Thus the MESP topographical approach presented herein offers quantification of the inherent electron donating power of a free N-heterocyclic carbene ligand.
已在密度泛函理论（DFT）的 B3LYP、BP86 和 M05 水平上，使用 6-311++G**基组，对各种 N-杂环卡宾的分子静电势（MESP）进行了拓扑分析。通过卡宾孤对区域（V（min））和卡宾核（V（C））处 MESP 的绝对最小值来评估卡宾碳的富电子特性。V（C）和托耳曼电子参数（TEP）之间建立了线性关系，这表明可以将前者用作 N-杂环卡宾配体向金属配位供电子能力的简单有效描述符。卡宾的 V（min）也与 TEP 很好地相关。然而，与 V（C）-TEP 相关性相比，这种线性关系的偏差更高，原因归因于孤对区域 N-取代基的空间效应。N-杂环卡宾比膦具有更强的配位能力，这可以根据前者获得的更深的 V（min）值来解释，实际上，即使是 N-杂环卡宾中电子最不丰富的 V（min），也可与高度富电子的膦配体相媲美。因此，本文提出的 MESP 拓扑方法提供了对游离 N-杂环卡宾配体固有供电子能力的定量描述。
