This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

The tetrahedral copper(I) diimine complex [Cu(pq)₂]BF₄ displays high photocatalytic activity for the H₂ evolution reaction with a turnover number of 3564, thus representing the first type of a Cu(I) quinoxaline complex capable of catalyzing proton reduction. Electrochemical experiments indicate that molecular mechanisms prevail and DFT calculations provide in-depth insight into the catalytic pathway, suggesting that the coordinating nitrogens play crucial roles in proton exchange and hydrogen formation.

Two heteroleptic nickel oxothiolate complexes, namely [Ni(bpy)(mp)] (1) and [Ni(dmbpy)(mp)] (2), where mp = 2-hydroxythiophenol, bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine were synthesized and characterized with various physical and spectroscopic methods. Complex 2 was further characterized by single crystal X-ray diffraction data. The complex crystallizes in the monoclinic P 2_1/c system and in its neutral form. The catalytic properties of both complexes for proton reduction were evaluated with photochemical and electrochemical studies. Two different in their nature photosensitizers, namely fluorescein and CdTe-TGA-coated quantum dots, were tested under various conditions. The role of the electron donating character of the methyl substituents was revealed in the light of the studies. Thus, catalyst 2 performs better than 1, reaching 39.1 TONs vs. 4.63 TONs in 3 h, respectively, in electrochemical experiments. In contrast, complex 1 is more photocatalytically active than 2, achieving a TON of over 6700 in 120 h of irradiation. This observed reverse catalytic activity suggests that HER mechanism follows different pathways in electrocatalysis and photocatalysis.

The prominence of binuclear, bimetallic catalysts underlines the need for the design and development of diverse bifunctional ligand frameworks that exhibit tunable electronic and structural properties. Such strategies enable metal‐metal and ligand‐metal cooperation towards catalytic applications, improve catalytic activity, and are essential for advancing multi‐electron transfers for catalytic application. In this work we present the synthesis, crystal structure, and photocatalytic properties of a binuclear Ni(II) complex, [Ni₂(1,10‐phenanthroline)₂(2‐sulfidophenolate)₂] (1). Complex 1 crystallizes in the centrosymmetric triclinic system (P‐1) showing extensive intra‐ and inter‐ non‐coordinated interactions. 1 is employed as a catalyst for light driven hydrogen evolution. Its catalytic efficiency in a noble‐metal‐free photo‐driven system using fluorescein as photosensitizer and triethanolamine as the electron donor, reaches TON 2900, threefold the efficiency of the corresponding homoleptic mononuclear complex [Ni(2‐sulfidophenolate)₂]. Efficiency rises up to 9000 TONs when thioglycolic‐coated CdTe quantum dots are used as photosensitizers in the presence of ascorbic acid at pH 4.5. UV‐Vis spectroscopy, dynamic light scattering techniques, and Hg‐poisoning measurements reveal that 1 maintains its molecular structure during catalysis. Electrochemical studies in DMF with TFA as the proton source were also performed for the elucidation of the mechanism of its catalytic action and its stability, suggesting that the proximity of two nickel ions plays a part in the increased catalytic activity, facilitating hydrogen evolution.

This study aims to advance the field of green chemistry and catalysis by exploring alternatives to conventional non-renewable energy sources. Emphasis is placed on hydrogen as a potential fuel, with a focus on the catalytic properties of Ni(II) complexes when coordinated with o-phenylenediamine and diimine ligands. We report the synthesis and comprehensive characterization, with various physical and spectroscopic techniques, of three heteroleptic Ni(II) complexes: [Ni(1,10-phenanthroline)(o-phenylene diamine)] (1), [Ni(2,2-dimethyl-2,2-bipyridine)(o-phenylene diamine)] (2), and [Ni(5,5-dimethyl-2,2-bipyridine)(o-phenylene diamine)] (3). The catalytic activity of these complexes for hydrogen evolution was assessed through photochemical studies utilizing visible light irradiation. Two distinct photosensitizers, fluorescein and quantum dots, were examined under diverse conditions. Additionally, their electrocatalytic behavior was investigated to elucidate the hydrogen evolution reaction (HER) mechanism, revealing a combined proton-coupled electron transfer (PCET)/electron-coupled proton transfer (ECPT) mechanism attributed to the chemical nature of the diamine ligand. The influence of ligand substituent position, ligand chemical nature, and photosensitizer type on catalytic performance was systematically studied. Among the complexes investigated, complex 2 demonstrated superior catalytic performance, achieving a turnover number (TON) of 3357 in photochemical experiments using fluorescein as a photosensitizer. Conversely, complex 1 exhibited the highest TON of 30,066 for HER when quantum dots were employed as the photosensitizer.