Methane C–H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study

This paper reports
a density functional theory study of 3d transition-metal
methoxide complexes with potentially redox-noninnocent pincer supporting
ligands for methane C–H bond activation to form methanol (LnM-OMe + CH4 → LnM–Me + CH3OH). The three types
of tridentate pincer ligands [terpyridine (NNN), bis­(2-pyridyl)­phenyl-C,N,N′ (NCN), and
2,6-bis­(2-phenyl)­pyridine-N,C,C′ (CNC)] and different first-row transition metals
(M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the
reaction mechanism as well as the effect of the metal identity on
the thermodynamics and kinetics of a methane activation reaction.
Spin-density analysis indicates that some of these systems, the NNN
and NCN ligands, have redox-noninnocent character. A four-centered,
kite-shaped transition state, σ-bond metathesis, or oxidative
hydrogen migration has been found for methane activation for the complexes
studied. Calculations suggest that the d electron count is a more
significant factor than the metal formal charge in controlling the
thermodynamics and kinetics of C–H activation and late 3d metal
methoxides, with high d counts preferred. Notably, early-to-middle
metals tend toward oxidative hydrogen migration and late metals undergo
a pathway that is more akin to σ-bond metathesis, suggesting
that metal methoxide complexes that favor σ-bond metathesis
pathways for methane activation will yield lower barriers for C–H
activation.