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

Abstract

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 H 2 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 H 2 generation. These results are discussed in the context of catalytic H 2 generation, and development of design rules for improved catalysts bearing the [Cp*] ligand.

Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand

“…The observation of four one-electron reduction events with complex 3 contrasts with the single two-electron, ECE-type reduction [ 15 ] events measured for most other diimine and diphosphine complexes of [Cp*Rh] [ 8 , 9 , 10 , 24 , 25 , 26 ]. Results from our laboratory suggest that the first reduction is rhodium metal-centered in most of these cases [ 26 , 40 ].…”

“…The electrochemical response of complex 3 sharply contrasts with the behavior commonly encountered for other [Cp*Rh III ] complexes. Most other chloride-bound complexes in this family containing other diimine [ 8 , 10 , 24 , 40 ], diphosphine [ 9 , 25 ], or hybrid phosphine-monoimine ligands [ 26 ] undergo a net two-electron reduction that appears as a single redox process in cyclic voltammetry experiments. As described in the Introduction, this ECE-type electrochemical response implicates that a chemical reaction follows the initial reduction of the metal complex and leads to formation of a species that undergoes immediate transfer of a second electron [ 7 , 15 ].…”

“…The 1 H-NMR data reveal that several (≥3) diamagnetic dnbpy containing species are generated, based on the presence of multiple sets of dnbpy-like resonances in the aromatic region (see SI, Figure S19 ). However, no metal hydride signals were observable in the upfield region near −10 ppm ( Figure S20 ) [ 25 , 26 ], nor were the characteristic signals corresponding to formation of [Cp*H] (e.g., a doublet near 0.5 ppm) detected ( Figure S19b,c in SI ) [ 24 ]. Thus, we conclude that decomposition accompanies reduction and protonation of 3 , resulting in formation of multiple products but no H 2 .…”

“…Cp*Rh(bpy) undergoes reaction with protons to form [Cp*H] species that are active intermediates in catalytic H 2 production [ 23 , 24 ]. Therefore, the observation of virtually no H 2 -generating reactivity of reduced forms of 3 with protons is interesting from the perspective of understanding the structure and bonding features that engender catalysis involving H-atom transfer [ 24 , 25 , 26 ] from [Cp*Rh] complexes. In the chemistry of most [Cp*Rh] complexes, apparently metal-centered reduction is followed by reactivity with protons to form either [Cp*H] complexes [ 24 ] or stable rhodium(III) hydride species [ 25 , 26 ].…”

“…Our group recently showed that installation of electron-withdrawing trifluoromethyl groups at the 4 and 4′ positions of the bpy ligand (as in 2 ) results in a previously unobserved catalytic pathway involving reduction of the 4,4′-bis(trifluoromethyl)-2,2′-bipyridyl ligand on [(Cp*H)Rh I ] species, followed by H 2 evolution [ 10 ]. We have also found that [Cp*Rh] complexes bearing bidentate diphosphine [ 25 ] or hybrid phosphine-quinoline ligands [ 26 ] are not capable of similar catalysis. Thus, although the role of supporting bidentate ligand structure in formation of [Cp*H] intermediates is not yet clear, it is of high interest considering the new reactivity manifolds that may be accessible with [Cp*H] complexes [ 27 , 28 ].…”