Light-Driven Proton Reduction in Aqueous Medium Catalyzed by a Family of Cobalt Complexes with Tetradentate Polypyridine-Type Ligands

Abstract

A series of tetradentate 2,2':6',2″:6″,2‴-quaterpyridine-type ligands related to ppq (ppq = 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) have been synthesized. One ligand replaces the 1,10-phenanthroline (phen) moiety of ppq with 2,2'-bipyridine and the other two ligands have a 3,3'-polymethylene subunit bridging the quinoline and pyridine. The structural result is that both the planarity and flexibility of the ligand are modified. Co(II) complexes are prepared and characterized by ultraviolet-visible light (UV-vis) and mass spectroscopy, cyclic voltammetry, and X-ray analysis. The light-driven H2-evolving activity of these Co complexes was evaluated under homogeneous aqueous conditions using [Ru(bpy)3](2+) as the photosensitizer, ascorbic acid as a sacrificial electron donor, and a blue light-emitting diode (LED) as the light source. At pH 4.5, all three complexes plus [Co(ppq)Cl2] showed the fastest rate, with the dimethylene-bridged system giving the highest turnover frequency (2125 h(-1)). Cyclic voltammograms showed a significant catalytic current for H2 production in both aqueous buffer and H2O/DMF medium. Combined experimental and theoretical study suggest a formal Co(II)-hydride species as a key intermediate that triggers H2 generation. Spin density analysis shows involvement of the tetradentate ligand in the redox sequence from the initial Co(II) state to the Co(II)-hydride intermediate. How the ligand scaffold influences the catalytic activity and stability of catalysts is discussed, in terms of the rigidity and differences in conjugation for this series of ligands.

Structure–Activity and Stability Relationships for Cobalt Polypyridyl‐Based Hydrogen‐Evolving Catalysts in Water

“…Electrochemical analysis of cobalt‐based HECs in water is challenging for several reasons: i) mercury electrodes are needed, as typical cobalt polypyridyl catalysts display overpotentials of 400–700 mV; ii) labile coordination sites are fundamental for catalysis, but can lead to mixtures of active compounds in aqueous electrolytes; iii) proton reduction is intrinsically an irreversible process and identification of half‐wave potentials requires stringent search in the scan rate domain; iv) reduction can lead to lower charge density or neutral compounds, which may deposit on the electrode; v) most importantly, proton reduction is pH dependent and experimental variation in the pH domain is basic for understanding the complex pH dependencies of the hydrogen evolution reaction (HER). Ott and co‐workers showed that examination of a HEC in a Britton Robinson buffer (BRB; an equimolar mixture of H 3 PO 3 , AcOH, and B(OH) 3 ) at various pH values gave valuable insights into the properties of a water reduction catalyst .…”

“…Ott and co‐workers showed that examination of a HEC in a Britton Robinson buffer (BRB; an equimolar mixture of H 3 PO 3 , AcOH, and B(OH) 3 ) at various pH values gave valuable insights into the properties of a water reduction catalyst . Thummel and co‐workers investigated a series of tetradentate, planar cobalt polypyridyl catalysts in buffered aqueous solution, and found two pH‐independent reductions, followed by a catalytic current enhancement . Similar studies have been performed by Savéant et al.…”

“…Research on artificial photosynthesis and the production of solar fuels has resulted in numerous molecular catalysts for both water oxidation and proton reduction reactions in recent years . For proton reduction, a multitude of hydrogen‐evolving catalysts (HECs) based on earth‐abundant metals like cobalt, nickel, molybdenum, and iron have been reported. The performance of catalysts is described in organic solvents or in buffered aqueous solution for electro‐ or photocatalytic H 2 production.…”

“…There are, from a spectroscopic point of view, only few studies focusing on the follow‐up reactions of the reduced cobalt species in water or organic solvents . The picture is however complemented by some very careful electrochemical studies on proton and CO 2 reduction by cobalt and other catalysts, in organic media and in water, or mixtures . The qualities of HECs are characterized by the following properties: i) high complex stability and turnover numbers (TONs; H 2 HEC −1 ); ii) high catalytic activity and turnover frequencies (TOFs; H 2 HEC −1 h −1 ); iii) low overpotentials ( η ), that is, catalytic proton reduction potentials close to the thermodynamic limit .…”

Tracking the Structural and Electronic Configurations of a Cobalt Proton Reduction Catalyst in Water

“…In this context, the use of molecular cobalt-based catalysts, stabilized by nitrogen donor ligands, has proven to be a good alternative given the high turnover numbers and stability achieved under catalytic conditions. [4][5][6][7][8][9] The catalyst precursor [LCo III Cl 2 ] + (L = macrocyclic ligand) in Scheme 1 belongs to this family of HEC and has been used in electrochemical as well as photochemical catalysis showing excellent results. [10][11][12][13][14] In contrast to many other molecular HEC that are active only in organic solvents, complex [LCo III Cl 2 ] + works in pure aqueous conditions showing remarkable stability over a period of several hours.…”