Steric and Electronic Analyses of Ligand Effects on the Stability of σ‑Methane Coordination Complexes: A DFT Study

Developing efficient catalysts for methane functionalization
is
a longstanding goal in inorganic chemistry. Here, we present theoretical
calculations to support efforts to synthesize σ-methane complexes
that can be studied by NMR spectroscopy. The systems studied are osmium
complexes of stoichiometry (C5R5)Os(diphosphine)(CH3)(H)+: when both cyclopentadienyl and diphosphine
are relatively strong electron donors, the methyl/hydride structure
is in rapid equilibrium with its σ-methane tautomer at low temperatures,
as shown experimentally some years ago. Here, using density functional
theory, we examine how changing the steric and electronic properties
of the ancillary cyclopentadienyl and diphosphine ligands affects
the relative energies of the two tautomers, with the goal of identifying
a ligand set for which the σ-methane structure, rather than
the methyl/hydride form, is the predominant species in equilibrium.
We also examine how varying the ancillary ligands affects the barrier
for methane dissociation. The calculations suggest that osmium complexes
bearing weakly donating and sterically undemanding ligands stabilize
the σ-methane structure both relative to its methyl/hydride
tautomer and toward dissociation of the methane ligand. More specifically,
osmium σ-methane complexes of fluorinated diphosphines (CF3)2PCH2P(CF3)2 and
(CF3)2PCF2P(CF3)2 are predicted to be stable enough to be observed by variable-temperature
NMR spectroscopy.