Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Abstract

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H 2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P 2 Ph N2 Ph )2(CH3CN)](BF4)2, 3a (where P 2 Ph N2 Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H 2 production rate to be determined. The turnover frequency of 350 s ؊1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P 2 Cy N2 Bz )2](BF4)2, 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (K eq ‫؍‬ 190 atm ؊1 at 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, which have been identified by a combination of one-and two-dimensional 1 H, 31 P, and 15 N NMR spectroscopies as Ni (0) catalysis ͉ hydrogen oxidation ͉ hydrogen production

The roles of the first and second coordination spheres in the design of molecular catalysts for H2production and oxidation

“…The structure of 12 has been established by extensive variable-temperature 1 H, 2 D, 31 P, and 15 N NMR studies of isotopically labeled species. 47 Our spectroscopic studies of the reaction of H 2 with [Ni(P Cy 2 N Bz 2 ) 2 ] 2+ , 10c, provided no direct evidence for a dihydrogen intermediate, and dihydrogen complexes of nickel(II) are very rare. However DFT calculations suggest that a dihydrogen ligand is stabilized by interaction with the positioned nitrogen atoms as shown in intermediate 11.…”

“…36,46,47 Kinetic studies of the reaction of 10a established that the catalytic rate is first order in catalyst and second order in acid at low acid concentrations, and it becomes independent of acid at high concentrations. In the high [acid] regime (where protonated dimethylformamide is the acid in acetonitrile), a turnover frequency for H 2 production of 350 s À1 at 22 1C has been observed with an overpotential of approximately 0.35 V. 47 This catalytic rate is similar to that reported for NiFe-H 2 ase (700 s À1 at 30 1C), 48 although the overpotential is larger. In contrast when 10b is the electrocatalyst, the rate of hydrogen formation decreases by about two orders of magnitude, with a turnover frequency of 5 s À1 and an overpotential of ca.…”

“…However DFT calculations suggest that a dihydrogen ligand is stabilized by interaction with the positioned nitrogen atoms as shown in intermediate 11. 47 The heterolytic cleavage of hydrogen in this system appears to involve a novel mechanism that relies on the presence of two positioned pendant bases. Polarization of the H 2 molecule is proposed to occur by the removal of two electrons by nickel from a symmetric transition state in which both of the hydrogen atoms gradually develop a more positive charge.…”

Molecular mechanisms of cobalt-catalyzed hydrogen evolution

“…Synthetic complexes of nickel (12)(13)(14)(15), cobalt (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), iron (27)(28)(29), and molybdenum (30)(31)(32) have been developed recently as electrocatalysts for the production of hydrogen. Co-diglyoxime complexes have been shown to generate hydrogen from protic solutions at relatively modest overpotentials.…”

Fabrication of Carbon-Platinum Interdigitated Array Electrodes and Their Application for Investigating Homogeneous Hydrogen Evolution Catalysis

“…36,37 The catalytic reduction is initiated with sequential one-electron reductions of PhPh at potentials of −0.84 V and −1.02 V vs FeCp 2 +/0 . Each reduction step is followed by a protonation step eventually releasing H 2 to complete the catalytic cycle ( Figure 2).…”