Effect of Ancillary Ligands on Oxidative Addition of CH4 to Ta(III) Complexes Ta(OC2H4)3A (A = B, Al, CH, SiH, N, P): A Density Functional Theory Study

A DFT study of oxidative addition of methane to Ta­(OC2H4)3A (where A may act as ancillary ligand) was conducted to understand how A may affect the propensity of the complex to undergo oxidative addition. Among the A groups studied, they can be a Lewis acid (B or Al), a saturated, electron-precise moiety (CH or SiH), a σ-donor (N), or a σ-donor/π-acid (P). By varying A, we seek to understand how changing the electronic properties of A can affect the kinetics and thermodynamics of methane C–H activation by these complexes. For every reaction two transition states (H or CH3 trans to A) leading to two corresponding products were identified. For all A, the TS with H trans to A is favored kinetically; except for SiH and CH, the kinetically favored product is not thermodynamically favored. For the kinetic products, the ΔG⧧ values for A = B, Al are highest among the 2p and 3p elements, respectively. Upon moving from electron-deficient to electron-rich moieties (P and N) the computed C–H activation barrier for the kinetic product decreases significantly. Thus, changing A greatly influences the barrier for methane C–H oxidative addition by these complexes.