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

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 Ci-H, Mo-H and W-H BDEs are in the ranges of 40 similar to 65, 65 similar to 75, and 70 similar to 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 Ci-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.