Wai Yip Fan

Metal-organic frameworks (MOFs) are typically assembled by bridging metal centers with organic linkers for various applications, including providing robust support for heterogeneous catalysts for CO₂ reduction. In this study, we have demonstrated the solubilization of a MOF tethered to a CO₂-reducing electrocatalyst and studied its fundamental electrochemistry in THF solvent using infrared spectroelectrochemistry (IR-SEC). The fundamental electrochemical properties of this immobilized catalyst were compared to that of its homogeneous counterpart. This approach provides a foundation for future experimental studies to bridge the gap between homogeneous and heterogeneous electrocatalysis.

A series of mononuclear first-row transition metal pyrithione M(pyr) _n complexes (M = Ni(II), Mn(II), n = 2; M = Co(III), Fe(III), n = 3) have been prepared from the reaction of the corresponding metal salt with the sodium salt of pyrithione. Using cyclic voltammetry, the complexes have been shown to behave as proton reduction electrocatalysts albeit with varying efficiencies in the presence of acetic acid as the proton source in acetonitrile. The nickel complex displays the optimal overall catalytic performance with an overpotential of 0.44 V. An ECEC mechanism is suggested for the nickel-catalyzed system based on the experimental data and supported by density functional theory calculations.

Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO)₃] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO)₃(MeCN)]⁺ cationic complex prepared via debromination of [MnBr(pybz)(CO)₃] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO)₃(MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO)₃] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO)₃] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.

Radical trifluoromethylation of α,β-unsaturated carbonyl compounds has been achieved using proflavine hemisulfate as a metal-free photoredox catalyst under visible light irradiation. With sodium triflinate (CF3SO­2Na) and trifluoroacetic acid as the CF3 and H-atom sources in oxygen-free acetonitrile, hydrotrifluoromethylation addition takes place across the double bond of substrates such as maleimide, enone and acrylamide. In contrast, CF3-substitution at the double bond is preferred for coumarin, uracil and (hetero)arenes while both addition and substitution are observed for thymine. A mechanism has been proposed where the excited-state proflavine hemisulfate performs a dual role as an electron acceptor to generate the CF3 radical and as a H-atom abstraction agent for the CF3-substitution reaction. A proflavine-H radical species is suggested to be the H-atom donor for hydrotrifluoromethylation.

A series of Schiff-base nickel(II) complexes were prepared from the reaction of nickel(II) acetate with N-salicylcysteamine [HO-C₆H₄-CH═N(CH₂)₂SH] ligands. These complexes were analyzed to be dimeric nickel complexes containing two bridging thiolato ligands. Using cyclic voltammetry, they were found to be efficient homogeneous proton reduction electrocatalysts when acetic acid was used as the proton source in acetonitrile. Catalysis was triggered upon electrochemical reduction of the nickel complex. In particular, rate constants (k_obs) in the range of 10⁴ s^–1 at moderate overpotentials of 0.5–0.6 V were achieved when chloro- or bromo-containing nickel complexes were used. Combined with the experimental data, density functional theory calculations lent support to an ECEC mechanism, with the first electrochemical reduction step contributing significantly to the rate-determining step.

Four copper(I) complexes [Cu(MeCN)₄]PF₆ (1), [Cu(dppe)₂]PF₆ (2), [Cu(bpy)₂]PF₆ (3) and [Cu(bpy)(dppe)]PF₆ (4) have been used as electrocatalysts in the generation of dihydrogen from proton reduction. Low overpotentials of 0.4 V have been measured using cyclic voltammetry when 1 and 3 were used as the catalysts in the presence of acetic acid as the proton source in acetonitrile. In contrast, a higher overpotential is observed for 4 while 2 is unable to catalyse the process. A mechanism for proton reduction has been proposed.

This paper describes the catalytic activity of air stable and easy to handle manganese complexes towards the hydrosilylation of aldehydes. These catalysts incorporate a bulky diazabutadiene ligand and exhibit good functional group tolerance and chemoselectivity in the hydrosilylation of aldehydes, utilizing primary silanes as the reducing agent. The reactions proceed with turnover frequencies approaching 150 h⁻¹ in some instances, similar to those observed for other manganese-based catalysts. The conversion of aromatic aldehydes to the corresponding alcohols was found to be more efficient than that for the analogous aliphatic systems.