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

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

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

The synthesis of active and efficient catalysts for solar fuel generation is nowadays of high relevance for the scientific community, but at the same time poses great challenges. Critical requirements are mainly associated with the kinetic barriers due to the multi-proton and multi-electron nature of the hydrogen evolution reaction (HER) and the CO2 reduction reaction (CO2RR) as well as to selectivity issues. In this regard, natural enzymes can be a source of inspiration for the design of effective and selective catalysts to target such fundamental reactions. In this Feature Article we review some recent works on molecular catalysts for both the HER and the CO2RR performed in our labs and other research teams which mainly address (i) the role of redox non-innocent ligands, to lower the overpotential for catalysis and control the selectivity, and (ii) the role of internal relays, to assist formation of catalytic intermediates via intramolecular routes. The selected exemplars have been chosen to emphasize that, although the molecular structures and the synthetic motifs are different from those of the active sites of natural enzymes, many affinities in terms of catalytic mechanism and functionality are instead present, which account for the observed remarkable performances under operative conditions. The data discussed herein thus demonstrate the great potential and the privileged role of molecular catalysts towards the design and construction of hybrid photochemical systems for solar energy conversion into fuels.

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

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 of assisting proton transfer processes via intramolecular routes involving detached pyridine units. With the aim of gaining insights into such catalytic routes, three new proton reduction catalysts based on amino-polypyridyl ligands are reported, focusing on substitution of the pyridine ortho-position. Specifically, a carboxylate (C2) and two hydroxyl substituted pyridyl moieties (C3, C4) are introduced with the aim of promoting intramolecular proton transfer which possibly enhances the efficiency of the catalysts. Foot-of-the-wave and catalytic Tafel plot analyses have been utilized to benchmark the catalytic performances under electrochemical conditions in acetonitrile using trifluoroacetic acid as the proton source. In this respect, the cobalt complex C3 turns out to be the fastest catalyst in the series, with a maximum turnover frequency (TOF) of 1.6 (±0.5) × 105 s-1, but at the expense of large overpotentials. Mechanistic investigations by means of Density Functional Theory (DFT) suggest a typical ECEC mechanism (i.e. a sequence of reduction - E - and protonation - C - events) for all the catalysts, as previously envisioned for the parent unsubstituted complex C1. Interestingly, in the case of complex C2, the catalytic route is triggered by initial protonation of the carboxylate group resulting in a less common (C)ECEC mechanism. The pivotal role of the hexadentate chelating ligand in providing internal proton relays to assist hydrogen elimination is further confirmed within this novel class of molecular catalysts, thus highlighting the relevance of a flexible polypyridine ligand in the design of efficient cobalt complexes for the HER. Photochemical studies in aqueous solution using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the sacrificial electron donor support the superior performance of C3.

Modulable Photocatalyzed Strategies for the Synthesis of α-C-Glycosyl Alanine Analogues via the Giese Reaction with Dehydroalanine Derivates.

Herein, we present the α-selective Giese reaction between pyranosyl/furanosyl bromides and dehydroalanine analogues, which provides access to a library of highly valuable α-C-glycosyl alanines. The key C-glycosyl radical is generated through photocatalysis by either the new generation copper(I) complex [(DPEPhos)(bcp)Cu]PF6 or [Ru(bpy)3](BF4)2. The reactions proceed smoothly, affording the desired α-C-glycosyl alanines in up to 99% yield when diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate [Hantzsch ester (HE)] is used as an additive. N,N-Diisopropylethylamine (DIPEA) has been selected as a reductant in both protocols. A mechanistic study by means of transient absorption spectroscopy unveils a halogen-atom transfer (XAT) process in C-glycosyl radical formation.

Air- and Water-Stable Heteroleptic Copper (I) Complexes Bearing Bis(indazol-1-yl)methane Ligands: Synthesis, Characterisation, and Computational Studies.

A series of four novel heteroleptic Cu(I) complexes, bearing bis(1H-indazol-1-yl)methane analogues as N,N ligands and DPEPhos as the P,P ligand, were synthesised in high yields under mild conditions and characterised by spectroscopic and spectrometric techniques. In addition, the position of the carboxymethyl substituent in the complexes and its effect on the electrochemical and photophysical behaviour was evaluated. As expected, the homoleptic copper (I) complexes with the N,N ligands showed air instability. In contrast, the obtained heteroleptic complexes were air- and water-stable in solid and solution. All complexes displayed green-yellow luminescence in CH2Cl2 at room temperature due to ligand-centred (LC) phosphorescence in the case of the Cu(I) complex with an unsubstituted N,N ligand and metal-to-ligand charge transfer (MLCT) phosphorescence for the carboxymethyl-substituted complexes. Interestingly, proper substitution of the bis(1H-indazol-1-yl)methane ligand enabled the achievement of a remarkable luminescent yield (2.5%) in solution, showcasing the great potential of this novel class of copper(I) complexes for potential applications in luminescent devices and/or photocatalysis.

Rethinking Electronic Effects in Photochemical Hydrogen Evolution Using CuInS2@ZnS Quantum Dots Sensitizers.

Molecular catalysts based on coordination complexes for the generation of hydrogen via photochemical water splitting exhibit a large versatility and tunability of the catalytic properties through chemical functionalization. In the present work, we report on light-driven hydrogen production in an aqueous solution using a series of cobalt polypyridine complexes as hydrogen evolving catalysts (HECs) in combination with CuInS2@ZnS quantum dots (QDs) as sensitizers, and ascorbate as the electron donor. A peculiar trend in activity has been observed depending on the substituents present on the polypyridine ligand. This trend markedly differs from that previously recorded using [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and can be ascribed to different kinetically limiting pathways in the photochemical reaction (viz. protonation kinetics with the ruthenium chromophore, catalyst activation via electron transfer from the QDs in the present system). Hence, this work shows how the electronic effects on light-triggered molecular catalysis are not exclusive features of the catalyst unit but depend on the whole photochemical system.

Recent findings and future directions in photosynthetic hydrogen evolution using polypyridine cobalt complexes.

The production of hydrogen gas using water as the molecular substrate currently represents one of the most challenging and appealing reaction schemes in the field of artificial photosynthesis (AP), i.e., the conversion of solar energy into fuels. In order to be efficient, this process requires a suitable combination of a light-harvesting sensitizer, an electron donor, and a hydrogen-evolving catalyst (HEC). In the last few years, cobalt polypyridine complexes have been discovered to be competent molecular catalysts for the hydrogen evolution reaction (HER), showing enhanced efficiency and stability with respect to previously reported molecular species. This perspective collects information about all relevant cobalt polypyridine complexes employed for the HER in aqueous solution under light-driven conditions in the presence of Ru(bpy)32+ (where bpy = 2,2'-bipyridine) as the photosensitizer and ascorbate as the electron donor, trying to highlight promising chemical motifs and aiming towards efficient catalytic activity in order to stimulate further efforts to design molecular catalysts for hydrogen generation and allow their profitable implementation in devices. As a final step, a few suggestions for the benchmarking of HECs employed under light-driven conditions are introduced.