Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

A bioinspired cobalt catalyst based on a tripodal imidazole/pyridine platform capable of water reduction and oxidation

The prototypical drug carrier [Co^II(L¹)Cl]PF₆ (1), where L¹ is a tripodal amine bound to pyridine and methyl-imidazoles, had its electrocatalytic water splitting activity studied under different pH conditions. This species contains a high-spin 3d⁷ Co^II metal center, and is capable of generating both H₂ from water reduction and O₂ from water oxidation_. Turnover numbers reach 390 after 3 h for water reduction. Initial water oxidation activity is molecular, with TONs of 71 at pH 7 and 103 at pH 11.5. The results reveal that species 1 can undergo several redox transformations, including reduction to the 3d⁸ Co^I species that precedes a ^LS3d⁶ hydride for water reduction, as well as nominal Co^IVO and Co^III-OOH species required for water oxidation. Post-catalytic analyses confirm the molecular nature of reduction and support initial molecular activity for oxidation.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO₂ ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO₂ and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO₂ . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO₂ .

Macrocycle-Induced Modulation of Internuclear Interactions in Homobimetallic Complexes

A synthetic route has been developed for a series of 3d homobimetallic complexes of Mn, Fe, Co, Ni, and Cu using three different pyridyldiimine and pyridyldialdimine macrocyclic ligands with ring sizes of 18, 20, and 22 atoms. Crystallographic analyses indicate that while the distances between the metals can be modulated by the size of the macrocycle pocket, the flexibility in the alkyl linkers used to construct the macrocycles enables the ligand to adjust the orientation of the PD(A)I fragments in response to the geometry of the [M₂(μ-Cl)₂]²⁺ core, particularly with respect to Jahn-Teller distortions. Analyses by UV-vis spectroscopy and SQUID magnetometry revealed deviations in the properties [M₂(μ-Cl)₂]²⁺-containing complexes bound by standard mononucleating ligands, highlighting the ability of macrocycles to use ring size to control the magnetic interactions of pseudo-octahedral, high-spin metal centers.

Electrocatalytic proton reduction by dinuclear cobalt complexes in a nonaqueous electrolyte

Two dinuclear CoII complexes 1 and 2 have been synthesized and characterized using various spectroscopic methods. Both the complexes were employed for H+ reduction in organic media. Faradaic efficiency of 82–90% was obtained for the H2 evolution.

Electrocatalytic H2 evolution using binuclear cobalt complexes as catalysts

Two molecular catalysts based on dinuclear cobalt complexes show an attractive catalytic activity for H2 evolution in organic medium.

Electrocatalytic hydrogen production by dinuclear cobalt(ii) compounds containing redox-active diamidate ligands: a combined experimental and theoretical study

The chiral dicobalt(ii) complex [Co^II₂(μ₂-L)₂] (1) (H₂L = N²,N⁶-di(quinolin-8-yl)pyridine-2,6-dicarboxamide) and its tert-butyl analogue [Co^II₂(μ₂-LBu)₂] (2) were structurally characterized and their catalytic evolution of H₂ was investigated.

Influence of nitro substituents on the redox, electronic, and proton reduction catalytic behavior of phenolate-based [N2O3]-type cobalt(iii) complexes

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt(iii) complexes, namely [CoIII(L1)MeOH] (1) and [CoIII(L2)MeOH] (2). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences.