Wrapping Rhodium in a Borane Canopy: Implications for Hydride Formation and Transfer

The unique reactivity of metal hydrides has encouraged considerable advances in both synthesis and catalysis. In this arena, ligand optimization has played a significant role, wherein metal-ligand cooperativity can be employed to guide reactivity. Of the organometallic hydride donors, rhodium(I) bis(diphosphine) compounds are known to be among the strongest with hydricity (AGH-) values near that of conventionally used reducing agents: e.g., trialkyl-substituted borohydrides (Delta G(H)- approximate to 26 kcal mol(-1) for [HBEt3](-)). Herein, we assess the thermodynamic site of hydride transfer using a rhodium(I) compound bearing a boron-rich diphosphine ligand: [Rh-I(P2B4Cy)MBPh4 (P2B4Cy = 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane). The divergent reactivity of [Rh-I(P2B4Cy)(2)]BPh4 and its all-alkyl relative [Rh-I(dnppe)(2)]BPh4 (dnppe = 1,2-bis(di-n-propylphosphino)-ethane) toward hydride donors is underscored, where for the former the preferred site of hydride transfer is the ligand scaffold and not rhodium. This result is contrasted by facile generation of [Rh-I(dnppe)(2)(H)]-a species having a value of Gil- ostensibly similar to that of [Rh-I(P2B4Cy)(2)(H)] (which is not observed), underscoring a noninnocent and cooperative role for the boron-rich secondary coordination sphere of the P2B4Cy scaffold. The use of the ligand to serve as a hydride source was accordingly examined.

Automated summary

The unique reactivity of metal hydrides has led to significant advances in synthesis and catalysis. Ligand optimization is crucial in guiding reactivity, with metal-ligand cooperativity playing a significant role. Rhodium(I) bis(diphosphine) compounds are among the strongest organometallic hydride donors, and the use of a boron-rich diphosphine ligand in a rhodium(I) compound reveals the noninnocent and cooperative role of the secondary coordination sphere in hydride transfer reactions.