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 (ΔGH–) values near that of conventionally used reducing
agents: e.g., trialkyl-substituted borohydrides (ΔGH– ≈ 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: [RhI(P2BCy4)2]­BPh4 (P2BCy4 = 1,2-bis­(di­(3-dicyclohexylboraneyl)­propylphosphino)­ethane).
The divergent reactivity of [RhI(P2BCy4)2]­BPh4 and its all-alkyl relative
[RhI(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 [RhI(dnppe)2(H)]a species
having a value of ΔGH– ostensibly similar to that of [RhI(P2BCy4)2(H)] (which is not observed),
underscoring a noninnocent and cooperative role for the boron-rich
secondary coordination sphere of the P2BCy4 scaffold. The use of the ligand to serve as a hydride source
was accordingly examined.