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.