Rhodium Disulfur and Dioxygen Complexes: Examination of Boron Secondary Coordination Sphere Effects

A series of diphosphine-ligated rhodium(III) h2-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(h2-Ch2)]BPh4 (Ch = O or S) were characterized. For the octaboranyl compound, [RhI(P2BCy4)2]BPh4 (P2BCy4 = 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(P2BCy4)2(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 were found to suggest kinetic and thermodynamic feasibility of a B—O interaction with DG(B—O) = —2.5 kcal mol-1; a stationary point for the related B—S system was not located.