Computational Study of 3d Metals and Their Influence on the Acidity of Methane C-H Bonds

CCSD(T) methods in conjunction with correlation consistent basis sets are used to predict the pK _a for the deprotonation of methane in a 3d metal ion adduct, [M···CH₄]⁺ (M = Sc-Cu), in dimethyl sulfoxide solvent, which is modeled by the SMD continuum solvent model. Results show that the coordination of methane to different M⁺ ions has a substantial difference of ∼27 pK _a units, from most to least acidic, and increases the acidity of the methane C-H bond from ∼8 to 36 pK _a units. Furthermore, even with the omission of the more expensive quadruple and quintuple zeta basis sets in the prediction process, similar trends in pK _a(C-H) as a function of 3d metal ions are exhibited. This research serves to illustrate the substantial effect that metal ion identity has on the acidity of a coordinated hydrocarbon and the utility that correlation consistent basis sets have in lowering the computational cost of modeling larger systems.