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

作者：Tanaka Saya; Masaoka Shigeyuki; Yamauchi Kosei; Annaka Masahiko; Sakai Ken^*

来源：Dalton Transactions, 2010, 39(46): 11218-11226.

DOI：10.1039/c0dt00741b

A series of dinuclear Rh(II) complexes, [Rh-2(mu-OAc)(4)(H2O)(2)] (HOAc = acetic acid) (1), [Rh-2(mu-gly)(4)(H2O)(2)] (Hgly = glycolic acid) (2), [Rh-2(mu-CF3CO2)(4)(acetone)(2)] (3), and [Rh-2(bpy)(2)(mu-OAc)(2)(OAc)(2)] (4), were found to serve as H-2-evolving catalysts in a three-component system consisting of tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)(3)(2+)), methylviologen (MV2+), and ethylenediaminetetraacetic acid disodium salt (EDTA). It was also confirmed that thermal reduction of water into H-2 byMV(+.), in situ generated by the bulk electrolysis of MV2+, is effectively promoted by 1 as a H-2-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 by products of MV+. is dramatically retarded in the presence of the Rh(II) 2 catalysts, for the H-2 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, H-1 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 Rh-2 core is responsible for the hydrogenic activation. The quenching of Ru*(bpy)(3)(2+) by 1 was also investigated by luminescence spectroscopy. The rate of H-2 evolution was found to decrease upon increasing the concentration of 1, indicating that the quenching of Ru*(bpy)(3)(2+) 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-2(II)(mu-OAc)(4)(H2O)] + H+ and [Rh-2(II)(mu-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 H-2 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.

• 出版日期2010