Chemical and Electrochemical Properties of [Cp*Rh] Complexes Supported by a Hybrid Phosphine-Imine Ligand

Abstract

A series of [Cp*Rh] complexes (Cp* = η5-pentamethylcyclopentadienyl) bearing the κ2-[P,N]-8-(diphenylphosphino)­quinoline (PQN) ligand have been prepared and characterized. Chemical or electrochemical reduction of the rhodium­(III) form generates an isolable rhodium­(I) complex; this rhodium­(I) complex reacts with a range of organic acids to yield a rhodium­(III) hydride bearing [Cp*] in the η5 mode and [PQN] in the expected κ2 mode. Solid-state structures of these three compounds from X-ray diffraction studies reveal only small changes in the intraligand bond distances across the series, suggesting the redox events associated with interconversion of these compounds are primarily metal centered. Cyclic voltammetry data show that the rhodium­(III) chloride complex undergoes a two-electron reduction at −1.19 V vs ferrocenium/ferrocene, whereas the analogous solvento rhodium­(III) acetonitrile complex undergoes two sequential one-electron reductions. The rhodium­(III) hydride undergoes an irreversible, ligand-centered reduction near −1.75 V vs ferrocenium/ferrocene. Carrying out this reduction alone or in the presence of added of triethylammonium as a source of protons results in only modest yields of H2, as shown by bulk electrolyses and chemical reduction experiments. These results are discussed in the context of recent work with [Cp*Rh] complexes bearing more symmetric 2,2′-bipyridyl and diphosphine ligands.

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

“…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 ].…”

“…Disentangling the nature of the elementary steps in this chemistry is of high interest, as the resulting 2e − -reduced complexes often undergo subsequent reactivity with protons. Notably, our recent work examining the case of a [Cp*Rh] complex bearing the hybrid 8-(diphenylphosphino)quinoline (PQN) ligand suggests that the first reduction of [Cp*Rh III (PQN)Cl]PF 6 is rhodium-centered, and leads to ejection of the chloride ligand at the Rh II oxidation state [ 26 ]. The electrochemical behavior of the [Cp*Rh] complex supported by the dimethyldipyridylmethane (Me 2 dpma) ligand is also consistent with initial metal-centered reduction [ 40 ].…”

“…Metal-centered reduction of [Cp*Rh III ] complexes is associated with ejection of monodentate ligands such as chloride from the first coordination sphere. This phenomenon is driven by formation of transient, 19e − Rh II intermediates upon reduction that release the monodentate ligand to form more stable 17e − species [ 26 ]. The assignment of ligand-centered reduction of 3 thus prompted us to explore whether analogous reactivity takes place here.…”

“…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 ].…”

Unraveling the Light‐Activated Reaction Mechanism in a Catalytically Competent Key Intermediate of a Multifunctional Molecular Catalyst for Artificial Photosynthesis

“…In as econd step,t he catalytically active species ( Figure 1A,c enter) was generated by chemical reduction with CoCp 2 and tested for its reactivity ( Figure 1C,D and Figure S18 in the Supporting Information). [10] Thereduction induces aloss of the chloro ligand, altering the original piano-stool geometry of the Rh III (Cp*)Cl center in Ru(tpphz)RhCp*,t hat is,t he plane of the cyclopentadienyl ligand is now oriented perpendicular to the tpphz ligand ( Figure 1A,c enter). Finally,t he intermediate was prepared in an electrochemical approach and characterized by UV/Vis,r R-SEC,T A-SEC, and quantum-chemical simulations.T he electrochemical characterization reveals that the first and second reduction in Ru(tpphz)RhCp* is located at the Rh ion:The irreversible reduction at À0.6 Vvs.…”