Cooperative Effects of Heterodinuclear IrIII-MII Complexes on Catalytic H2 Evolution from Formic Acid Dehydrogenation in Water

Abstract

Novel heterodinuclear Ir-III-M-II complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and employed as catalysts for H-2 evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H-2 evolution in comparison with the corresponding mononuclear Ir-III and M-II complexes as the components of the Ir-III-M-II complexes. The H-2 evolution rate for the Ir-III-M-II complexes was at most 350-fold higher than that of the mononuclear Ir-III complex. The catalytic activity increased in the following order: Ir-III-Cu-II complex < (IrCoII)-Co-III complex < Ir-III-Ni-II complex . The Ir-III-H intermediates of the Ir-III-M-II complexes were successfully detected by ultravioletvisible, H-1 nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H-2 evolution by the Ir-III-M-II complexes indicates that the Ir-III-H species formed in the Ir-III moiety act as reactive species and the M-II moieties act as acceleration sites by the electronic effect from the M-II center to the Ir-III center through the bridging bpp ligand. The Ir-III-M-II complexes may also activate H2O at the 3d M-II centers as a proton source to facilitate H-2 evolution. In addition, the affinity of formate for the Ir-III-M-II complexes was investigated on the basis of Michaelis-Menten plots; the Ir-III-Co-II and Ir-III-Ni-II complexes exhibited affinities that were relatively higher than that of the Ir-III-Cu-II complex. The catalytic mechanism of H-2 evolution by the Ir-III-M-II complexes was revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.