Samantha I. Johnson

Proton–hydride tautomerism in hydrogen evolution catalysis

Significance The discovery of efficient hydrogen evolution catalysts for solar fuels production continues to be an active research field. Catalyst optimization depends on detailed knowledge of the elementary chemical reaction steps involved in catalysis. Isolation of intermediates in catalytic processes is uncommon owing to their necessarily low stability. By using weak acids, we have isolated and characterized an intermediate in the 2e− + 2H+ → H2 reaction catalyzed by η5-pentamethylcyclopentadienyl (Cp*) Rh(κ2-2,2′-bipyridyl) [Rh(bpy)]. We find that the preferred site of Cp*Rh(bpy) protonation is not the metal center but is the Cp* ligand. Despite the reputation of Cp* as a stable ligand in organometallic chemistry, these results suggest an important role for close metal–ligand cooperation in promoting hydrogen–evolution catalysis. Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e– + 2H+ → H2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, η5-pentamethylcyclopentadienyl; bpy, κ2-2,2′-bipyridyl) with acid, is not a hydride species but rather, bears [η4-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H2 and reformation of η5-Cp* bound to rhodium(III). With suitable choices of acids and bases, the Cp*Rh(bpy) complex catalyzes facile and reversible interconversion of H+ and H2.

Reactivity of a Series of Isostructural Cobalt Pincer Complexes with CO2, CO, and H+

The preparation and characterization of a series of isostructural cobalt complexes [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (Py = pyridine, E = CH2, NH, O, and X = BF4 (1a-c)) and the corresponding one-electron reduced analogues [Co(t-Bu)2P(E)Py(E)P(t-Bu)2(CH3CN)2][BF4]2 (2a-c) are reported. The reactivity of the reduced cobalt complexes with CO2, CO, and H(+) to generate intermediates in a CO2 to CO and H2O reduction cycle are described. The reduction of 1a-c and subsequent reactivity with CO2 was investigated by cyclic voltammetry, and for 1a also by infrared spectroelectrochemistry. The corresponding CO complexes of (2a-c) were prepared, and the Co-CO bond strengths were characterized by IR spectroscopy. Quantum mechanical methods (B3LYP-d3 with solvation) were used to characterize the competitive reactivity of the reduced cobalt centers with H(+) versus CO2. By investigating a series of isostructural complexes, correlations in reactivity with ligand electron withdrawing effects are made.