Computational study of the impact of ancillary ligands upon a tungsten (IV) imide complex for catalytic methane functionalization

Abstract

A DFT study is reported of the effects of various ancillary ligands upon a tungsten imide complex (W(OSiH3)2(NMe)), as it reacts with a methane CH bond. The ancillary ligands (NMe3, PMe3, BMe3) are used to modulate steric and electronic effects for catalytic alkane functionalization. Oxidative addition (OA) was the preferred mechanism of CH activation via the Lewis bases NMe3 and PMe3, leading to formation of octahedral WVI products. However, the Lewis acid BMe3 reacted differently, as it experienced a coordination change from tungsten to nitrogen during methane activation, following an alternative CH activation pathway: [2 + 2] addition across the WB bond. NMe3 has the most favorable activation free energy among the three ancillary ligands studied (NMe3 = 33.2 kcal/mol, PMe3 = 39.9 kcal/mol, BMe3 = 40.0 kcal/mol). However, in contrast to a recent study of tethered ancillary ligands, the absence of an ancillary ligand proved to be even more energy favored as it was calculated to have a lower energy barrier (31.8 kcal/mol) than NMe3. This difference is interesting and implies too strong metal-ancillary ligand interactions will quench reactivity even for Lewis bases that donate more electron density to the metal.