Synthesis, Characterization, and Photocatalytic H₂-Evolving Activity of a Family of [Co(N4Py)(X)]ⁿ⁺ Complexes in Aqueous Solution

Abstract

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Heptacoordinate CoII Complex: A New Architecture for Photochemical Hydrogen Production

“…[13] Moreover, photochemical experiments conducted in the presence of 1 mL Hg(0) did not show any change in the activity, as expected for a homogeneous catalysis. [14] However, according to the results it is not possible to exclude with certainty the formation of nanoparticles smaller than 2 nm.…”

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

“…Blank experiments assess interference of experimental outcomes by background activities, especially at very low HEC concentrations. Photocatalytic experiments without HEC reveal H 2 evolution by the PS only, and catalysis with equal amounts of cobalt salts instead of the complexes are relevant since the Co II cations might also form active species in the catalytic environment (donor, electrolyte, decomposition products). Both blank tests must be performed to substantiate the performance of the supposed catalysts.…”

“…For instance, quantities are strongly influenced even by subtle variations of the different components (e.g., SED, PS, pH, concentrations/ratios, and light source) . There is a great deal of reported information on the initial steps of photocatalysis in homogeneous solutions, that is, the study of the processes that follow excitation of the photosensitizer (reductive/oxidative quenching, first electron transfer to cobalt, and follow‐up reactions of oxidized electron donors), both in organic and aqueous media, as well as in mixtures . 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 .…”

From non-innocent to guilty: on the role of redox-active ligands in the electro-assisted reduction of CO2 mediated by a cobalt(ii)-polypyridyl complex

“…2, process II) couples. 49,61 At a more negative potential of À1.99 V vs. Fc + /Fc, a last irreversible process, attributed to ligand-based reduction centered on the bipyridine moiety, is observed ( Fig. 2, process III).…”