METHANE ACTIVATION BY GROUP-IVB IMIDO COMPLEXES

An ab initio study of methane activation by group IVB imido complexes, when coupled with available experimental data, reveals an interesting picture of this important reaction. Initial interaction of methane and (H)2M=NH leads to the formation of alkane complexes bound by almost-equal-to 9 kcal mol-1. Experiment indicates that the polarity of the metal-ligand bond upon which the C-H is activated plays an important role in facilitating subsequent scission. Calculations support this hypothesis and suggest that formation of the alkane complex acts to increase C(delta-H(delta+) polarization, setting the stage for C-H cleavage. Calculated methane elimination barriers for (H)2M(CH3)(NH2) (M = Ti, Zr, Hf) are in good agreement with experimental models in terms of absolute numbers and trends as a function of metal. Calculated methane activation barriers follow the order Ti > Zr > Hf, in line with calculated exothermicities. Calculated geometries indicate a late transition state for methane elimination, in agreement with experimentally determined activation parameters. The TSs have a kite-shaped geometry with an obtuse angle about the H of the C-H bond being activated (H(t)) and a short MH(t) distance, 1-2% greater than normal. The short MH(t) distance suggests a stabilizing interaction, supported by a positive bond overlap population. Calculation of the intrinsic reaction coordinate demonstrates the importance of agostic interactions between N-H and M along the reaction coordinate.