Computational Determination of pKa(C–H) in 3d Transition Metal-Methyl Complexes

A DFT study of 76 complexes comprised of 3d transition metals, Ti through Ni, bonded to at least one methyl group was undertaken to compute the Brønsted acidity of methyl C–H bonds. Example complexes were gathered from determined coordinates of experimental crystal structures from the Cambridge Structural Database (CSD). The level of theory was BMK/6-31+G­(d), and the SMD solvation model used DMSO as the continuum solvent. Deprotonation of the metal-methyl complex by an equivalent of DMSO resulted in the formation of the conjugate acid of DMSO and the conjugate base (an anionic metal-methylidene complex). From the free energy of this reaction, the pKa(C–H) of the methyl group was calculated to show the effects of metal identity, ligands, oxidation state, spin state, and conjugate base stabilization upon its acidity. In general, the acidity of the C–H bonds of transition metal-methyl complexes decreases from left to right across the 3d row with some anomalies. Factors affecting the range in pKa for each metal are discussed.