Electrochemical Capture and Release of Carbon Dioxide Using a Disulfide–Thiocarbonate Redox Cycle

We describe a new electrochemical cycle that enables capture and release of carbon dioxide. The capture agent is benzylthiolate (RS^–), generated electrochemically by reduction of benzyldisulfide (RSSR). Reaction of RS^– with CO₂ produces a terminal, sulfur-bound monothiocarbonate, RSCO₂^–, which acts as the CO₂ carrier species, much the same as a carbamate serves as the CO₂ carrier for amine-based capture strategies. Oxidation of the thiocarbonate releases CO₂ and regenerates RSSR. The newly reported <i>S</i>-benzylthiocarbonate (IUPAC name benzylsulfanylformate) is characterized by ¹H and ¹³C NMR, FTIR, and electrochemical analysis. The capture–release cycle is studied in the ionic liquid 1-butyl-1-methylpyrrolidinium bis­(trifluoro­methyl­sulfonyl)­imide (BMP TFSI) and dimethylformamide. Quantum chemical calculations give a binding energy of CO₂ to benzyl thiolate of −66.3 kJ mol^–1, consistent with the experimental observation of formation of a stable CO₂ adduct. The data described here represent the first report of electrochemical behavior of a sulfur-bound terminal thiocarbonate

Probing the Nature of Charge Transfer at Nano–Bio Interfaces: Peptides on Metal Oxide Nanoparticles

Characterizing the nano–bio interface has been a long-standing endeavor in the quest for novel biosensors, biophotovoltaics, and biocompatible electronic devices. In this context, the present computational work on the interaction of two peptides, A6K (Ac-AAAA­AAK-NH₂) and A7 (Ac-AAAA­AAA-NH₂) with semiconducting TiO₂ nanoparticles is an effort to understand the peptide–metal oxide nanointerface. These investigations were spurred by recent experimental observations that nanostructured semiconducting metal oxides templated with A6K peptides not only stabilize large proteins like photosystem-I (PS-I) but also exhibit enhanced charge-transfer characteristics. Our results indicate that α-helical structures of A6K are not only energetically more stabilized on TiO₂ nanoparticles, but the resulting hybrids also exhibit enhanced electron transfer characteristics. This enhancement can be attributed to substantial changes in the electronic characteristics at the peptide-TiO₂ interface. Apart from understanding the mechanism of electron transfer (ET) in peptide-stabilized PS-I on metal oxide nanoparticles, the current work also has implications in the development of novel solar cells and photocatalysts

A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst

Here we report the electrocatalytic reduction of protons to hydrogen by a novel S₂P₂ coordinated nickel complex, [Ni­(bdt)­(dppf)] (bdt = 1,2-benzene­dithiolate, dppf = 1,1′-bis­(diphenyl­phos­phino)­ferro­cene). The catalysis is fast and efficient with a turnover frequency of 1240 s^–1 and an overpotential of only 265 mV for half activity at low acid concentrations. Furthermore, catalysis is possible using a weak acid, and the complex is stable for at least 4 h in acidic solution. Calculations of the system carried out at the density functional level of theory (DFT) are consistent with a mechanism for catalysis in which both protonations take place at the nickel center

A Nickel Phosphine Complex as a Fast and Efficient Hydrogen Production Catalyst

Here we report the electrocatalytic reduction of protons to hydrogen by a novel S₂P₂ coordinated nickel complex, [Ni­(bdt)­(dppf)] (bdt = 1,2-benzene­dithiolate, dppf = 1,1′-bis­(diphenyl­phos­phino)­ferro­cene). The catalysis is fast and efficient with a turnover frequency of 1240 s^–1 and an overpotential of only 265 mV for half activity at low acid concentrations. Furthermore, catalysis is possible using a weak acid, and the complex is stable for at least 4 h in acidic solution. Calculations of the system carried out at the density functional level of theory (DFT) are consistent with a mechanism for catalysis in which both protonations take place at the nickel center

Catalytic Hydrogen Evolution by Fe(II) Carbonyls Featuring a Dithiolate and a Chelating Phosphine

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)­Fe­(CO)­P₂ [bdt = benzene-1,2-dithiolate; P₂ = 1,1′-diphenylphosphinoferrocene (<b>1</b>) or methyl-2-{bis­(diphenylphosphinomethyl)­amino}­acetate (<b>2</b>)] were prepared as functional, biomimetic models for the distal iron (Fe_d) of the active site of [FeFe]-hydrogenase. X-ray crystal structures of the complexes reveal that, despite similar ν­(CO) stretching band frequencies, the two complexes have different coordination geometries. In X-ray crystal structures, the iron center of <b>1</b> is in a distorted trigonal bipyramidal arrangement, and that of <b>2</b> is in a distorted square pyramidal geometry. Electrochemical investigation shows that both complexes catalyze electrochemical proton reduction from acetic acid at mild overpotential, 0.17 and 0.38 V for <b>1</b> and <b>2</b>, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity toward CO (1 bar). However, ligand centered protonation by the strong acid, HBF₄·OEt₂, triggers quantitative CO uptake by <b>1</b> to form a dicarbonyl analogue <b>[1­(H)-CO]⁺</b> that can be reversibly converted back to <b>1</b> by deprotonation using NEt₃. Both crystallographically determined distances within the bdt ligand and density functional theory calculations suggest that the iron centers in both <b>1</b> and <b>2</b> are partially reduced at the expense of partial oxidation of the bdt ligand. Ligand protonation interrupts this extensive electronic delocalization between the Fe and bdt making <b>1­(H)⁺</b> susceptible to external CO binding

Non-exponential Length Dependence of Conductance in Iodide-Terminated Oligothiophene Single-Molecule Tunneling Junctions

An exponential decrease of molecular conductance with length has been observed in most molecular systems reported to date, and has been taken as a signature of non-resonant tunneling as the conduction mechanism. Surprisingly, the conductance of iodide-terminated oligothiophene molecules presented herein does not follow the simple exponential length dependence. The lack of temperature dependence in the conductance indicates that tunneling still dominates the conduction mechanism in the molecules. Transition voltage spectroscopy shows that the tunneling barrier of the oligothiophene decreases with length, but the decrease is insufficient to explain the non-exponential length dependence. X-ray photoelectron spectroscopy, stretching length measurement, and theoretical calculations show that the non-exponential length dependence is due to a transition in the binding geometry of the molecule to the electrodes in the molecular junctions as the length increases

CO₂ Preactivation in Photoinduced Reduction via Surface Functionalization of TiO₂ Nanoparticles

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO₂ using a combination of ultraviolet–visible spectroscopy (UV–vis), surface enhanced Raman scattering (SERS), ¹³C NMR, and electron paramagnetic resonance (EPR). We find CO₂ binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO₂, suggesting successful charge transfer from the sensitizer to the neighboring site of CO₂. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO₂ on semiconducting surfaces and proposes a set of tools and experiments to do so