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

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journal contribution
posted on 2011-03-21, 00:00 authored by Kevin D. Hesp, Felix O. Kannemann, Matthew A. Rankin, Robert McDonald, Michael J. Ferguson, Mark Stradiotto
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 KOtBu 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 RuII 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.