Methane inversion on transition metal ions: A possible mechanism for stereochemical scrambling in metal-catalyzed alkane hydroxylations

The configurational inversion of both free methane and methane bound to first-row transition-metal ions is discussed using the density functional theory (DFT) calculations at the B3LYP level of theory. Computed transition states for the inversion of methane on the M⁺(CH₄) complexes have C_s structures in which one pair of C-H bonds is about 1.2 Å in length and the other pair is about 1.1 Å, where M is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The barrier height for the methane inversion decreases significantly from 109.4 kcalmol^-1 for free methane to 17-23 kcalmol^-1 for the late transition-metal complexes, Fe⁺(CH₄), Co⁺(CH₄), Ni⁺(CH₄), and Cu⁺(CH₄). The computational results suggest that the inversion can occur under ambient conditions through a thermally accessible transition state, and it may lead to an inversion of stereochemistry at a carbon atom of substrate if an alkane-complex is formed as a reaction intermediate in C-H bond activation reactions. We propose that a radical mechanism based on a planar carbon species may not be the sole source of the observed loss of stereochemistry in transition-metal catalyzed alkane hydroxylation reactions and other related reactions.