Photochemical and thermal hydrogen production from water catalyzed by carboxylate-bridged dirhodium(ii) complexes

Abstract

A series of dinuclear Rh(II) complexes, [Rh₂(μ-OAc)₄(H₂O)₂] (HOAc = acetic acid) (1), [Rh₂(μ-gly)₄(H₂O)₂] (Hgly = glycolic acid) (2), [Rh₂(μ-CF₃CO₂)₄(acetone)₂] (3), and [Rh₂(bpy)₂(μ-OAc)₂(OAc)₂] (4), were found to serve as H₂-evolving catalysts in a three-component system consisting of tris(2,2′-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺), methylviologen (MV²⁺), and ethylenediaminetetraacetic acid disodium salt (EDTA). It was also confirmed that thermal reduction of water into H₂ by MV⁺˙, in situ generated by the bulk electrolysis of MV²⁺, is effectively promoted by 1 as a H₂-evolving catalyst. The absorption spectra of the photolysis solution during the photocatalysis were monitored up to 6 h to reveal that the formation of photochemical or thermal byproducts of MV⁺˙ is dramatically retarded in the presence of the Rh(II)₂ catalysts, for the H₂ formation rather than the decomposition of MV⁺˙ becomes predominant in the presence of the Rh(II)₂ catalysts. The stability of the Rh(II)₂ dimers was confirmed by absorption spectroscopy, ¹H NMR, and ESI-TOF mass spectroscopy. The results indicated that neither elimination nor replacement of the equatorial ligands take place during the photolysis, revealing that one of the axial sites of the Rh₂ core is responsible for the hydrogenic activation. The quenching of Ru*(bpy)₃²⁺ by 1 was also investigated by luminescence spectroscopy. The rate of H₂ evolution was found to decrease upon increasing the concentration of 1, indicating that the quenching of Ru*(bpy)₃²⁺ by the Rh(II)₂ species rather than by MV²⁺ becomes predominant at the higher concentrations of 1. The DFT calculations were carried out for several possible reaction paths proposed (e.g., [Rh^II₂(μ-OAc)₄(H₂O)] + H⁺ and [Rh^II₂(μ-OAc)₄(H₂O)] + H⁺ + e⁻). It is suggested that the initial step is a proton-coupled electron transfer (PCET) to the Rh(II)₂ dimer leading to the formation of a Rh(II)Rh(III)–H intermediate. The H₂ evolution step is suggested to proceed either via the transfer of another set of H⁺ and e⁻ to the Rh(II)Rh(III)–H intermediate or via the homolytic radical coupling through the interaction of two Rh(II)Rh(III)–H intermediates.