Cooperative Effects of Heterodinuclear Ir-M Complexes on Catalytic H Evolution from Formic Acid Dehydrogenation in Water.

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