Rhodium disulfur and dioxygen complexes: examination of boron secondary coordination sphere effects

Abstract

A series of diphosphine-ligated rhodium(III) η 2-peroxo and -persulfido compounds are targeted with one subset containing a boron-rich secondary coordination sphere (SCS). A systematic investigation using both experimental and theoretical methods has been performed to assess whether peripheral boranes can be used to engage with rhodium(III)-bound chalcogenides. For the model compound, [RhI(dnppe)2]BPh4 (dnppe = 1,2-bis(di-n-propylphosphino)ethane), adducts of the form [RhI(dnppe)2(η 2-Ch2)]BPh4 (Ch = O or S) were characterized. For the octaboranyl compound, [RhI(P2BCy 4)2]BPh4 (P2BCy 4 = 1,2-bis(di(3-dicyclohexylboranyl)propylphosphino)ethane), however, treatment with O2 resulted in SCS decomposition via B–O bond formation, while reaction with 0.25 equivs. S8 provided [RhI(P2BCy 4)2(η 2-S2)]BPh4, which, based on variable-temperature 31P NMR spectroscopic measurements, does not exhibit a B–S interaction. Using a compound with a single pendant borane as a model, potential energy surface (PES) scans suggest kinetic and thermodynamic feasibility of a B–O interaction with ΔG(B–O) = −2.5 kcal mol−1; a stationary point for the related B–S system was not located.