Computational and experimental test of steric influence on agostic interactions: A homologous series for Ir(III)

Chloride abstraction (using NaBAr'4, Ar' = 3,5-(CF3)2C6H3) from Ir(H)2Cl(P(t)Bu2Ph)2 gives cis, trans-Ir(H)2(P(t)Bu2Ph)2+, which has two agostic interactions with methyl C-H groups on different 1Bu groups. The molecule exists as diastereomers, due to stereochemistry at P. Chloride can be similarly abstracted from orthometalated IrH(η2- C6H4P(t)Bu2)Cl(P(t)Bu2Ph) to give square-pyramidal IrH(η2- C6H4P(t)Bu2)(P(t)Bu2Ph)+, which has only one agostic interaction, involving a (t)BuC - H bond; steric constraints on each phosphine leave no more C-H bonds available to donate to the remaining empty Ir(III) orbital. The smaller ligand PCy2Ph yields only the tris-phosphine complex Ir(H)2(PCy2Ph)3+, and this is shown to have a square-pyramidal structure with one agostic cyclohexyl group and large Pax-Ir-P angles (104-106°). The analogous Ir(H)2(P(i)Pr2Ph)3+ has similar inter-phosphorus angles, but no agostic interaction. Geometrical optimization of IrH2L3+ (PCy2Ph, P(i)Pr2Ph) with the hybrid quantum mechanics/molecular mechanics (QM/MM) method (IMOMM) at the IMOMM (B3LYP:MM3) and IMOMM (MP2:MM3) levels permits a more detailed understanding of the influence of steric factors on the occurrence of an agostic bond. The MP2/MM3 method gives the results in closer agreement with experiment. Steric factors place the agostic bond in the vicinity of the metal center but at a distance that is too long to be considered as bonding. The electron-donating ability of the C-H bond and the electron accepting capacity of the metal center, which are introduced only at the QM level, bring the two partners in a bonding situation.