铬族金属氢化物中M-H 键键能的从头计算

The metal-hydrogen M—H bond homolysis of the Cr-group metal hydrides is a key process in the radical cycli- zation 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 under- standing about the structures and properties of the Cr-group metal hydride based catalysts, but also provides important in- sights 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 metalchromium 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 posi- tive 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 hybridi- zation defects is more than the expansion degree caused by the weak bonding. Keywords transition-metal hydride; bond dissociation energy; ab initio calculation; hybridization defects