Assessment of the Electronic Factors Determining the Thermodynamics of “Oxidative Addition” of C–H and N–H Bonds to Ir(I) Complexes

A study of electronic factors governing the thermodynamics of C–H and N–H bond addition to Ir­(I) complexes was conducted. DFT calculations were performed on an extensive series of <i>trans</i>-(PH₃)₂IrXL complexes (L = NH₃ and CO; X = various monodentate ligands) to parametrize the relative σ- and π-donating/withdrawing properties of the various ligands, X. Computed energies of oxidative addition of methane to a series of three- and four-coordinate Ir­(I) complexes bearing an ancillary ligand, X, were correlated with the resulting (σ_X, π_X) parameter set. Regression analysis indicates that the thermodynamics of addition of methane to <i>trans</i>-(PH₃)₂IrX are generally strongly <i>disfavored</i> by increased σ-donation from the ligand X, in contradiction to widely held views on oxidative addition. The trend for oxidative addition of methane to four-coordinate Ir­(I) was closely related to that observed for the three-coordinate complexes, albeit slightly more complicated. The computational analysis was found to be consistent with the rates of reductive elimination of benzene from a series of isoelectronic Ir­(III) phenyl hydride complexes, measured experimentally in this work and previously reported. Extending the analysis of ancillary ligand energetic effects to the oxidative addition of ammonia to three-coordinate Ir­(I) complexes leads to the conclusion that increasing σ-donation by X also <i>disfavors</i> oxidative addition of N–H bonds to <i>trans</i>-(PH₃)₂IrX. However, coordination of NH₃ to the Ir­(I) center is disfavored even more strongly by increasing σ-donation by X, which explains why the few documented examples of H–NH₂ oxidative addition to transition metals involve complexes with strongly σ-donating ligands situated trans to the site of addition. An orbital-based rationale for the observed results is presented