Heterogeneous Aqueous CO2 Reduction Using a Pyrene-Modified Rhenium(I) Diimine Complex.

The development of molecular catalysts and materials that can convert carbon dioxide (CO2) into a value-added product is a great chemical challenge. Molecular catalysts set benchmarks in catalyst investigation and design, but their incorporation into solid-state materials, and optimization of the electrochemical operating conditions, is still needed. For example, rhenium(I) diimine catalysts show almost quantitative selectivity for the conversion of CO2 to carbon monoxide (CO) in acetonitrile (MeCN), but the modification of diimine backbones can be challenging if the goal is to incorporate such molecules into materials. Presented here is a rhenium(I) complex with a 2-(2'-quinolyl)benzimidazole (QuBIm-H) ligand, where N-alkylation with a pyrene derivative allows access to a catalyst that can be adsorbed onto electrodes for aqueous CO2 reduction chemistry. The rhenium(I) catalysts are inactive for homogeneous CO2 reduction in MeCN. However, when adsorbed on edge-plane graphite, the same complexes show good activity for heterogeneous aqueous CO2 reduction, with 90% selectivity for CO. Comparative electrochemical studies between covalent and noncovalent modification of the graphite surfaces were also carried out for related rhenium(I) tricarbonyl complexes.

Powerful Bispyridinylidene Organic Reducing Agents with Iminophosphorano π-Donor Substituents.

Four members of a new family of powerful bispyridinylidene organic reducing agents have been prepared, which exploit iminophosphorano (-N=PR3; R = Ph, Cy) π-donor substituents. Electrochemical studies show exceptionally high oxidation potentials, ranging from 1.30 to 1.51 V versus SCE. These new reductants were shown to effectively convert 1-bromonaphthalene to naphthalene under mild reaction conditions. From the redox potentials, substituent constants (σp(+)) for the iminophosphorano groups Ph3P=N- (-1.82) and Cy3P=N- (-2.21) were determined, demonstrating their superior π-donating properties compared to traditional amino substituents.

Pushing the limits of neutral organic electron donors: a tetra(iminophosphorano)-substituted bispyridinylidene.

A new ground-state organic electron donor has been prepared that features four strongly π-donating iminophosphorano substituents on a bispyridinylidene skeleton. Cyclic voltammetry reveals a record redox potential of -1.70 V vs. saturated calomel electrode (SCE) for the couple involving the neutral organic donor and its dication. This highly reducing organic compound can be isolated (44 %) or more conveniently generated in situ by a deprotonation reaction involving its readily prepared pyridinium ion precursor. This donor is able to reduce a variety of aryl halides, and, owing to its redox potential, was found to be the first organic donor to be effective in the thermally induced reductive SN bond cleavage of N,N-dialkylsulfonamides, and reductive hydrodecyanation of malonitriles.