茂基铱络合物中Ir–H键解离能的理论预测及其形变-结合能分析

Prediction of Ir–H bond cleavage energies of cyclopentadienyl iridium complexes by DFT methods and their distortion-interaction analyses

铱氢络合物是诸多铱催化反应中的催化剂和关键中间体,Ir–H键的断裂与生成在这些反应中往往起到至关重要的作用.本文利用密度泛函方法(DFT)对多种茂基配位的铱氢络合物的铱氢键均裂能和异裂能(负氢解离)进行了系统研究.计算发现,铱中心贫电子有利于Ir–H键的均裂,而吸电子基团以及强p电子受体有利于减弱Ir中心电子密度.刚性螯环以及大位阻取代基可以通过释放空间位阻进一步减小其键能.而如2-苯基吡啶、联吡啶配体等配位的铱氢化合物,由于Ir中心d轨道可以与这些配体的芳香体系形成大p键,因此降低了这些物种的H^-解离焓.烯烃配体有利于均裂过程,却不利于异裂的H^-解离.膦配体对均裂过程影响不大,但是可以大大减小H^-解离焓.贫电子的B原子,有利于均裂,因会形成Ir–H–B的氢桥键,不利于解离H^-.Si原子相对富电子,对这两个过程均不利.

Iridium hydride complexes are ubiquitous in Ir-catalyzed reactions as catalysts or critical intermediates and the cleavage or formation of Ir–H bonds are usually involved in these transformations. In this work the bond dissociation enthalpies and the hydride-generating heterolytic dissociation enthalpies of the Ir–H bonds in various cyclopentadienyl iridium complexes were studied with the aid of density functional theory（DFT）. Computational results show that the deficient-electron iridium center benefits homolytic cleavage of Ir–H bonds while electron withdrawing groups as well as strong ？ electron acceptors result in weakened electron density of central iridium. Rigid chelate rings and large steric substituents could reduce bond energies by releasing steric hindrance. Such as 2-phenylpyridine bipyridyl coordinated iridium hydride compounds could use d orbitals to form delocalization ？bonds with the aromatic system of the ligands thereby decreases the hydride-generating heterogeneous cleavage enthalpies of these species. Olefin ligands promoted homolysis reactions and restrained heterolysis reactions. Phosphine ligands have little influence on homolysis while significantly reduce the hydride ion cleavage enthalpies. Deficient-electron boron atoms favor the homolysis and disfavor the heterolysis because the later reaction would form Ir–H–B bridge bonds. Silicon atoms are relatively electron-rich which result in higher enthalpies of both processes.