5d Metal(IV) Imide Complexes. The Impact (or Lack Thereof) of d-Orbital Occupation on Methane Activation and Functionalization.

Abstract

This research evaluates 5d metal imide complexes, (OH)2M═NMe (M = W, Re, or Os), and their reactions with methane to form dimethylamine. Each is calculated to follow a consistent reaction pathway regardless of the metal's d-orbital occupation, whereby the methane C-H bond undergoes oxidative addition (OA) to the metal and then the methyl migrates from the metal to the nitrogen to form an amide. Finally, hydrogen migrates to the nitrogen before dissociating to form amine products. While homolytic M-imide, M-amide M-H, and M-CH3 bond dissociation free energies (BDFEs) were analyzed, the BDFEs of neither hexavalent nor tetravalent metal moieties reflect the oxidative addition kinetics. Instead, a strain theory approach, supported by electron density analysis for the OA transition state, is found to be explanatory. Notably, the rate-determining step, the hydrogen migration transition state, has a consistent jump in free energy versus the preceding intermediate, (OH)2MIV(H)-N(CH3)Me, for all metals evaluated. Thus, the height of the RDS is largely reflective of the stability of the MIV-amide intermediate, suggesting a strategy for viable catalysis.