Probing Mesitylborane and Mesitylborate Ligation Within the Coordination Sphere of Cp*Ru(PⁱPr₃)⁺: A Combined Synthetic, X-ray Crystallographic, and Computational Study

The reaction of Cp*Ru(PⁱPr₃)Cl (1) with MesBH₂ (Mes = 2,4,6-trimethylphenyl) afforded the mesitylborate complex Cp*Ru(PⁱPr₃)(BH₂MesCl) (2, 66%). Exposure of 2 to the chloride abstracting agent LiB(C₆F₅)₄·2.5OEt₂ provided [Cp*Ru(PⁱPr₃)(BH₂Mes)]⁺B(C₆F₅)₄⁻ (3, 54%), which features an unusual η²-B-H monoborane ligand. The related borate complex Cp*Ru(PⁱPr₃)(BH₃Mes) (5, 65%) was prepared from 1 and LiH₃BMes. Attempts to effect the insertion of unsaturated organic substrates into the B−H bonds of 3 were unsuccessful, and efforts to dehydrohalogenate 2 using KO^tBu instead afforded the mesitylborate complex Cp*(PⁱPr₃)Ru(BH₂MesOH) (6, 48%). Treatment of 1 with benzyl potassium generated an intermediate hydridoruthenium complex (7) resulting from dehydrogenation of a PⁱPr fragment, which in turn was observed to react with MesBH₂ to afford the mesitylborate complex Cp*(P(ⁱPr)₂(CH₃CCH₂))Ru(BH₃Mes) (8, 47%). Crystallographic characterization data are provided for 2, 3, 5, 6, and 8. A combined X-ray crystallographic and density functional theory (DFT) investigation of 3 and 5, using Natural Bond Orbital (NBO) and Atoms in Molecules (AIM) analysis, revealed that 3 and 5 are best described as donor−acceptor complexes between a Cp*(PⁱPr₃)Ru⁺ fragment and a bis(η²-B-H) coordinating mesitylborane(borate) ligand. Significant σ-donation from the B−H bonds into the Ru^II center exists as evidenced by the NBO populations, bond orders, and AIM delocalization indices. In the case of 3, the vacant p orbital on boron is stabilized by Ru→B π back-donation as well as by resonance with the mesityl group.