铬族金属氢化物中M-H键键能的从头计算

Ab Initio Calculation of M—H Bond Dissociation Energies of Cr-Group Metal Hydrides

铬族金属氢化物M—H键的均裂是在自由基环化反应中的关键过程,它直接影响反应的催化效率和选择性.使用理论方法精确预测铬族金属氢化物M—H键键能不仅有助于了解这类催化剂的结构与性能关系,而且对进一步发展高效的自由基环化反应催化剂具有重要的指导意义.为达此目的,我们使用不同的理论方法计算14个有可靠实验值的铬族金属氢化物M—H键键能,通过比较发现B3P86/lanl2dz+p方法表现最佳(计算精度为1.6 kcal/mol).之后我们运用这个理论方法系统地研究了铬族金属氢化物M—H键键能的构效关系.就周期性规律而言,在铬族金属氢化物中,金属对于M—H键能的影响大于配体的影响.M—H键键裂解能的大小顺序为:铬氢络合物<钼氢络合物<钨氢络合物.此外,我们发现含双茂配体的铬氢化合物中Cr—H键键能与伸缩振动频率呈良好的线性相关.有趣的是,在含双茂配体的铬族金属氢化物中,M—H键能与键长呈现出良好的正线性相关性.即"键长越小,键能越弱".通过自然成键轨道分析,这可能是因为杂化缺失造成原子半径收缩程度比成键变弱引起的扩张程度大.

The metal-hydrogen M—H bond homolysis of the Cr-group metal hydrides is a key process in the radical cyclization reactions mediated by these compounds,which directly affects the catalytic efficiency and selectivity of the reactions.Accurate prediction of M—H BDEs（bond dissociation energies） using theoretical methods not only improves our understanding about the structures and properties of the Cr-group metal hydride based catalysts,but also provides important insights into design of new generations of catalysts for radical cyclization reactions.For this purpose,we have calculated the M—H BDEs with different density functional theory methods（including MPW1K,MPW1b95,MPW1PW91,PBE1PBE,B3P86,O3LYP,TPSSH,MPW1KCIS,and TPSS） and compared the theoretical predictions with 14 reliable experimental M—H BDE values recently reported for the Cr-group metal hydrides.It is found that the B3P86/lanl2dz＋p method could accurately predict the M—H BDEs with a precision of 1.6 kcal/mol.Using the B3P86/lanl2dz＋p method,we next studied the structure-property relationship for the M—H BDEs in Cr-group metal hydrides.As to the periodical trends,we found that the effects of the metals on the M—H BDEs are greater than the effects of the ligands.The M-H BDEs increase in the order： first row metal〈second row metal〈third row metal.The Cr—H,Mo—H and W—H BDEs are in the ranges of 40～65,65～75,and 70～80 kcal/mol,respectively.Moreover,only for the chromium hydrides a good correlation is found between the M—H BDEs and the M—H vibration frequencies.Interestingly,the M—H bond strengths and lengths show a good positive linear correlation for the chromium hydrides.That is,weaker bonds are also shorter bonds.With the aid of natural bond orbital analysis of the Cr—H bonds,the reason may be attributed to that the atom size contraction degree caused by hybridization defects is more than the expansion degree caused by the weak bonding.