Electro- and photochemical H₂ generation by Co(ii) polypyridyl-based catalysts bearing ortho-substituted pyridines

Abstract

Cobalt(II) complexes featuring hexadentate amino-pyridyl ligands have been recently discovered as highly active catalysts for the Hydrogen Evolution Reaction (HER), whose high performance arises from the possibility to assist proton...

Bioinspired motifs in proton and CO2 reduction with 3d-metal polypyridine complexes

“…4) based on the hexadentate DBPy-PyA ligand (DBPy-PyA = 1-([2,2′-bipyridin]-6-yl)- N -([2,2′-bipyridin]-6-ylmethyl)- N -(pyridin-2-ylmethyl)methanamine). 44–48 These complexes show indeed a strong ligand contribution upon one- and two-electron reduction. This property was identified as one of the main reasons for the outstanding performances shown by these complexes (Table 1) both in electrochemical (FE in the order of 75–92%) 45,48 and photochemical proton reduction ( Φ up to 11.3% in aqueous conditions at pH = 4 in the presence of [Ru(bpy) 3 ] 2+ and ascorbic acid).…”

“…44–48 These complexes show indeed a strong ligand contribution upon one- and two-electron reduction. This property was identified as one of the main reasons for the outstanding performances shown by these complexes (Table 1) both in electrochemical (FE in the order of 75–92%) 45,48 and photochemical proton reduction ( Φ up to 11.3% in aqueous conditions at pH = 4 in the presence of [Ru(bpy) 3 ] 2+ and ascorbic acid). 46 From the analysis of this series of compounds, it can be speculated that the splitting of the d orbitals induced by the geometry might play a role in the redox activity or inactivity of the bipyridyl unit.…”

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement

“…The cobalt complex 1, available from previous studies, [63][64][65][66][67] was prepared by treatment of the hexadentate DBPy-PyA ligand with Co(BF 4 ) 2 • 6H 2 O in a methanol solution and subsequent precipitation with diethyl ether. The absorption spectrum in acetonitrile solution (Figure S1) shows two bands in the visible region (attenuation coefficients of ɛ~40 L mol À 1 cm À 1 ) with maxima at 450 and 505 nm and a shoulder at longer wavelengths, assigned to d-d transitions involving the cobalt (II) center.…”

“…[55][56][57][58][59][60][61][62] In particular, we have reported on the hydrogen evolution activity by cobalt polypyridine complexes featuring the redox-active, hexadentate DBPy-PyA ligand, where DBPy-PyA = (1-([2,2'-bipyridin]-6-yl)-N-([2,2'-bipyridin]-6-ylmethyl)-N-(pyridin-2-ylmethyl) methanamine, and synthetic variations thereof. [63][64][65][66][67] These complexes display a peculiar heptacoordinated structure in the solid state which is rather uncommon for firstrow transition metal complexes. Despite the absence of a free coordination site, the complexes showed high activity towards the HER, particularly when catalysis was driven by photochemical means in acidic aqueous solutions in the presence of [Ru(bpy) 3 ] 2 + (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor.…”

“…Despite the absence of a free coordination site, the complexes showed high activity towards the HER, particularly when catalysis was driven by photochemical means in acidic aqueous solutions in the presence of [Ru(bpy) 3 ] 2 + (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor. [63,65,67] As a matter of fact, the ligand is substantially flexible to suitably accommodate the different oxidation states of the cobalt during the HER catalysis and the external pyridine ligand(s) can detach from the metal center under turnover conditions to provide internal proton relays enabling proton transfer steps via intramolecular routes. [64] The presence of biomimetic acid-base functionalities in the catalyst platform has been indeed demonstrated to be a fundamental prerequisite to promote fast and efficient catalysis for both proton and CO 2 reduction.…”