Charged Macromolecular Rhenium Bipyridine Catalysts with Tunable CO2 Reduction Potentials.

A series of polymeric frameworks with functional assemblies were designed to alter the catalytic activity of covalently bound ReI electrocatalysts. Norbornenyl polymers containing positively charged quaternary ammonium salts, neutral phenyl, or negatively charged trifluoroborate groups were end-labelled with a ReI fac-tricarbonyl bipyridine electrocatalyst via cross metathesis. Electrochemical studies in acetonitrile under an inert atmosphere and with saturated CO2 indicate that the quaternary ammonium polymers exhibit a significantly lower potential for CO2 reduction to CO (ca. 300 mV), while neutral polymers behave consistently with what has been reported for the free, molecular catalyst. In contrast, the trifluoroborate polymers displayed a negative shift in potential and catalytic activity was not observed. It is demonstrated that a single catalytically active complex can be installed onto a charged polymeric framework, thereby achieving environmentally tuned reduction potentials for CO2 reduction. These materials may be useful as polymer-based precursors for preparing catalytic and highly ordered structures such as thin films, porous catalytic membranes, or catalytic nanoparticles.

Tuning Electron Delocalization and Transfer Rates in Mixed-Valent Ru3O Complexes through "Push-Pull" Effects.

Electron transfer rates in a series of oxo-centered triruthenium clusters featuring an extended aromatic ancillary ligand of the type [Ru3(OAc)6(µ3-O)(CO)(L)(pep)], where L = 4-cyanopyridine (cpy), pyridine (py), or 4-(dimethylamino)pyridine (dmap) and pep = 4-(phenylethynyl)pyridine were investigated. The electron self-exchange rate constants for the 0/- couple were determined by (1)H NMR line broadening experiments and found to range from 4.3 to 9.2 (× 10(7) M(-1) s(-1)) in deuterated acetonitrile (ACN-d3). Relative rates of self-exchange can be rationalized on the basis of increased contact area between self-exchanging pairs, and a push-pull modulation of electron density between the pep vs ancillary pyridine ligands. Faster self-exchange was observed with increasing electron-donating character of the ancillary pyridine ligand substituent (dmap > py > cpy), and this was attributed to increased orbital overlap between self-exchanging pairs. These results are supported by trends observed in (1)H NMR contact shifts of the pep ligand that were found to depend on the electron-donating or -withdrawing nature of the ancillary pyridine ligand.

Improving the Efficiency and Activity of Electrocatalysts for the Reduction of CO2 through Supramolecular Assembly with Amino Acid-Modified Ligands.

The use of hydrogen-bonding interactions to direct the noncovalent assembly of a Re-based bimetallic supramolecular electrocatalyst containing either tyrosine or phenylalanine residues is reported. Computational modeling and spectroelectrochemical characterization indicate that under catalytic conditions the phenol residues of tyrosine can act both as pendant proton sources and participate in the structural assembly of the bimetallic active species. As a result, an increased rate of catalysis is observed experimentally for the reductive disproportionation of CO2 to CO and CO3(2-) by a tyrosine-modified complex in comparison to a control complex containing phenylalanine residues. These findings demonstrate that noncovalent assembly is a powerful method for generating new bimetallic electrocatalyst systems where the choice of substituent can be used to both control structural assembly and introduce cocatalytic moieties.

Supramolecular assembly promotes the electrocatalytic reduction of carbon dioxide by Re(I) bipyridine catalysts at a lower overpotential.

The addition of methyl acetamidomethyl groups at the 4,4'-positions of a 2,2'-bipyridyl ligand is found to enhance the rate of a bimolecular reduction mechanism of CO2 by Re(I) fac-tricarbonyl chloride complexes. Electrochemical studies, spectroelectrochemical measurements, and molecular dynamics simulations indicate that these methyl acetamidomethyl groups promote the formation of a hydrogen-bonded dimer. This supramolecular complex catalyzes the reductive disproportionation of CO2 to CO and CO3(2-) at a lower overpotential (ca. 250 mV) than the corresponding single-site 2 e(-) reduction of CO2 to CO and H2O catalyzed by the corresponding model complex with a 4,4'-dimethyl-2,2'-bipyridyl ligand. These findings demonstrate that noncovalent self-assembly can modulate the catalytic properties of metal complexes by favoring alternate catalytic pathways.