Examining the Modular Synthesis of [Cp*Rh] Monohydrides Supported by Chelating Diphosphine Ligands

[Cp*Rh] hydride complexes are invoked as intermediates in certain catalytic cycles, but few of these species have been successfully prepared and isolated, contributing to a relative shortage of information on the properties of such species. Here, the synthesis, isolation, and characterization of two [Cp*Rh] hydrides are reported; the hydrides are supported by the chelating diphosphine ligands bis(diphenylphosphino)methane (dppm) and 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos). In both systems, reduction of the precursor Rh( III) chloride complexes with Na(Hg) results in the clean formation of isolable, formally 18e(-) Rh(I) species, and subsequent protonation by addition of near-stoichiometric quantities of anilinium triflate to the Rh(I) species returns high yields of the desired monohydride complexes. Single-crystal X-ray diffraction data for these compounds provide evidence of direct Rh-H interactions, confirmed by complementary infrared spectra showing Rh-H stretching frequencies at 1982 cm(-1) (for the dppm-supported hydride) and 1936 cm-1 (for the Xantphos-supported hydride). Findings from comprehensive multinuclear NMR experiments reveal the properties of the unique and especially rich spin systems for the dppm-supported hydride; multifrequency NMR studies in concert with spectral simulations enabled a full characterization of splitting patterns attributable to couplings involving heterotopic methylene protons for this complex. When they are taken together with prior reports of related monohydrides, the results show that the reduction/protonation reaction sequence is modular for the preparation of [Cp*Rh] monohydrides supported by diverse diphosphine ligands spanning from four- to eight-membered rhodacycles.

Automated summary

[Cp*Rh] hydride complexes have been successfully synthesized and isolated using chelating diphosphine ligands, showing direct Rh-H interactions with characteristic infrared spectra frequencies. Comprehensive NMR experiments revealed unique spin systems and coupling patterns for these complexes, demonstrating a modular reduction/protonation reaction sequence for the preparation of diverse [Cp*Rh] monohydrides.