Electrochemical generation of rhodium porphyrin hydrides. Catalysis of hydrogen evolution

In polar solvents, Rh(III) porphyrins are directly reduced in Rh(I) complexes which react readily with Bronsted acids to give Rh(III) hydrides. They then undergo, at a more negative potential, an additional electron uptake to yield the corresponding Rh(II) hydrides. The electrogenerated rhodium(II) complex is the key intermediate of catalytic hydrogen evolution according to a mechanism which heavily depends on the solvent and on axial ligands. In DMSO, hydride transfer from Rh(II)H- to the acid, yielding H-2, competes with hydride transfer reduction of the solvent by both Rh(III)H and Rh(LH)H-. In a less-complexing solvent, such as butyronitrile, hydrogen evolution occurs both by hydride transfer to the acid and H-atom abstraction to the solvent. The latter pathway is shut off by the addition of strong and soft Ligands such as tertiary phosphines. With PEt3, a particularly strong electron-donating ligand, not only Rh(II)H- but also Rh(III)H triggers H-2 evolution. The various changes of the hydrogen evolution mechanism as well as the stability of the catalyst can be rationalized by the variation of the electron density distribution brought about by the presence or the absence of the axial ligand.