Cooperative Effects of Heterodinuclear Ir^III-M^II Complexes on Catalytic H₂ Evolution from Formic Acid Dehydrogenation...

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 H₂ evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H₂ evolution in comparison with the corresponding mononuclear Ir^III and M^II complexes as the components of the Ir^III-M^II complexes. The H₂ 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 < Ir^III-Co^II complex < Ir^III-Ni^II complex . The Ir^III-H intermediates of the Ir^III-M^II complexes were successfully detected by ultraviolet-visible, ¹H nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H₂ 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 H₂ 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 H₂ 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.