Heptacoordinate Co^II Complex: A New Architecture for Photochemical Hydrogen Production

Abstract

The first heptacoordinate cobalt catalyst for light‐driven hydrogen production in water has been synthesized and characterized. Photochemical experiments using [Ru(bpy)3]2+ as photosensitizer gave a turnover number (TON) of 16300 mol H2 (mol cat.)−1 achieved in 2 hours of irradiation with visible (475 nm) light. This promising result provides a path forward in the development of new structures to improve the efficiency of the catalysis.

Rationalizing Photo‐Triggered Hydrogen Evolution Using Polypyridine Cobalt Complexes: Substituent Effects on Hexadentate Chelating Ligands

“…Indeed, this observation is corroborated by the analysis of UV/Vis absorption spectra in water (see the Supporting Information, Figure S1), which are very similar, except for the obvious differences in the UV region ascribable to the different bipyridine ligands. All complexes showed paramagnetic properties, and the magnetic moments calculated with the Evans method gave values between 3.8 and 4.3 BM, as expected for a cobalt metal center in a high‐spin configuration (Figure S2) [13,15] …”

“…Photochemical H 2 production experiments were carried out in acetate buffer (1.0 m ) at pH 4.0 in the presence of the catalyst (1 μ m ), Ru(bpy) 3 2+ (0.5 m m ), and ascorbic acid (0.1 m ) at 20 °C upon irradiation with a LED light at 475 nm. The experimental conditions were chosen according to previously optimized data for the parent complex C0 [13] . Hydrogen evolution quantum yields were measured by using a Ru(bpy) 3 2+ /9,10‐diphenylanthracene actinometer [21] .…”

“…Following the strategy of using a single polydentate ligand (that ensures a unique design of the stereochemistry of the metal complex, avoiding the presence of different isomers), we have recently reported the first heptacoordinate cobalt complexes ( C0 and C0’ ) [13] characterized by a surprisingly high activity towards photochemical hydrogen production in aqueous media (Scheme 2). Herein, we study a new series of heptacoordinate cobalt polypyridyl complexes based on the modification of the DBPy‐PyA {DBPy‐PyA: 1‐([2,2′‐bipyridin]‐6‐yl)‐ N ‐([2,2′‐bipyridin]‐6‐ylmethyl)‐ N ‐(pyridin‐2‐ylmethyl)methanamine} ligand scaffold bearing −CF 3 or −OCH 3 in the para ‐position of the pyridine or of the bipyridine moieties (Scheme 2).…”

A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution

“…In the same study, the pentapyridyl complex [Co II ( PY5Me 2 )(H 2 O) 2 ] 2+ shows a slightly enhanced turnover number (TON Co ) of 300 compared with the tetrapyridyl analogue [Co II ( PY4Me 2 H )(MeCN)(OTf)] + , which has a TON Co of 250 under identical photocatalytic conditions (Figure ), perhaps due to greater catalyst stability from higher denticity . In accordance with this, very recently, a heptacoordinate Co II complex featuring a hexadentate polypyridyl ligand, plus coordinated chloride or water, has achieved an optimised TON Co of 16 300 in aqueous solution . Finally, increasing the electron‐donating ability of polypyridyl ligands through peripheral substituents has been shown, in some cases, to increase, and in other cases to decrease, the activity of the cobalt centre towards photocatalytic HER.…”

Electronic structure and optical properties of isolated and TiO2‐grafted free base porphyrins for water oxidation: A challenging test case for DFT and TD‐DFT

“…At the base of the efficient dye‐sensitized photoelectrodes, there is the optimal integration of the three main components: the dye sensitizer, the semiconductor, and the catalyst . While the development of molecular components (catalysts and sensitizers) for the proton reduction, half reaction has reached important advances over the last years and the main limiting factor on the photocathode side is represented by the nonoptimal characteristics of the p ‐type semiconductor (NiO), the four‐electron water oxidation reaction, requiring the accumulation of four oxidative equivalents at a catalyst site in competition with back electron transfer of injected electrons to the oxidized dye‐catalyst assembly, is the main bottleneck toward the design of efficient and stable water splitting devices. To assure effective light harvesting and interfacial charge (electron/hole) separation, the dye sensitizer should possess a wide and intense optical absorption spectrum, extending to the red and near infra‐red regions, a long‐lived charge‐separated excited state, possibly strongly electronically coupled to the oxide CB states, and ground and excited state oxidation potentials (ESOPs), which properly match the redox potential of the catalyst and the semiconductor CB/VB, Figure .…”