Preparation, Characterization, and Electrochemical Activation of a Model [Cp*Rh] Hydride

Monomeric half-sandwich rhodium hydride
complexes are often proposed
as intermediates in catalytic cycles, but relatively few such compounds
have been isolated and studied, limiting understanding of their properties.
Here, we report preparation and isolation of a monomeric rhodium­(III)
hydride complex bearing the pentamethylcyclopentadienyl (Cp*) and
bis­(diphenylphosphino)­benzene (dppb) ligands. The hydride complex
is formed rapidly upon addition of weak acid to a reduced precursor
complex, Cp*Rh­(dppb). Single-crystal X-ray diffraction data for the
[Cp*Rh] hydride, which were previously unavailable for this class
of compounds, provide evidence of the direct Rh–H interaction.
Complementary infrared spectra show the Rh–H stretching frequency
at 1986 cm–1. In contrast to results with other
[Cp*Rh] complexes bearing diimine ligands, treatment of the isolated
hydride with strong acid does not result in H2 evolution.
Electrochemical studies reveal that the hydride complex can be reduced
only at very negative potentials (ca. −2.5 V vs ferrocenium/ferrocene),
resulting in Rh–H bond cleavage and H2 generation.
These results are discussed in the context of catalytic H2 generation, and development of design rules for improved catalysts
bearing the [Cp*] ligand.