The homogeneous CO2 reduction activity of several nickel cyclam complexes was examined by cyclic voltammetry and controlled potential electrolysis. CO production with high efficiency from ...unsubstituted Ni(cyclam) was verified, while the activity was found to be attenuated with methyl substitution of the amines on the cyclam ring. Reactivity with CO2 was also probed using density functional theory (DFT) calculations. The relative CO2 binding energies to the NiI state obtained from DFT were found to match well with the experimental results and shed light on the possible importance of the isomeric form of Ni(cyclam) in determining the catalytic activity.
Five Re(bipy)(CO)3Cl complexes were prepared and studied where bipy was 4,4′-dicarboxyl-2,2′-bipyridine (1), 2,2′-bipyridine (2), 4,4′-dimethyl-2,2′-bipyridine (3), 4,4′-di-tert-butyl-2,2′-bipyridine ...(4), and 4,4′-dimethoxy-2,2′-bipyridine (5). From this group, a significantly improved catalyst, Re(bipy-tBu)(CO)3Cl (4), for the reduction of carbon dioxide to carbon monoxide was found. The complex shows two one-electron reductions under argon, one reversible at −1445 mV (vs SCE), and one irreversible at −1830 mV. Under CO2 the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich−Koutecky method (1.1 × 10−5 cm2/s for the neutral complex and 8.1 × 10−6 cm2/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO2 of 1000 M−1 s−1 was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, ηCO = 99 ± 2% in acetonitrile.
Re(bpy)(CO)3− is a well-established homogeneous electrocatalyst for the reduction of CO2 to CO. Recently, substitution of the more abundant transition metal Mn for Re yielded a similarly active ...electrocatalyst, Mn(bpy)(CO)3−. Compared to the Re catalyst, this Mn catalyst operates at a lower applied reduction potential but requires the presence of a weak acid in the solution for catalytic activity. In this study, we employ quantum chemistry combined with continuum solvation and microkinetics to examine the mechanism of CO2 reduction by each catalyst. We use cyclic voltammetry experiments to determine the turnover frequencies of the Mn catalyst with phenol as the added weak acid. The computed turnover frequencies for both catalysts agree to within one order of magnitude of the experimental ones. The different operating potentials for these catalysts indicate that different reduction pathways may be favored during catalysis. We model two different pathways for both catalysts and find that, at their respective operating potentials, the Mn catalyst indeed is predicted to take a different reaction route than the Re catalyst. The Mn catalyst can access both catalytic pathways, depending on the applied potential, while the Re catalyst does not show this flexibility. Our microkinetics analysis predicts which intermediates should be observable during catalysis. These intermediates for the two catalyzed reactions have qualitatively different electronic configurations, depending on the applied potential. The observable intermediate at higher applied potentials possesses an unpaired electron and therefore should be EPR-active; however, the observable intermediate at lower applied potentials, accessible only for the Mn catalyst, is diamagnetic and therefore should be EPR-silent. The differences between both catalysts are rationalized on the basis of their electronic structure and different ligand binding affinities.
We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of ...amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TONCO = 100 ± 4) and quantum yield (ΦCO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TONCO = 28 ± 4 and ΦCO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzodimidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO2 reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.
Realization of heterogeneous electrochemical CO2-to-fuel conversion via molecular catalysis under high-flux conditions requires the assembly of large quantities of reactant-accessible catalysts on ...conductive surfaces. As a proof of principle, we demonstrate that electrophoretic deposition of thin films of an appropriately chosen metal–organic framework (MOF) material is an effective method for immobilizing the needed quantity of catalyst. For electrocatalytic CO2 reduction, we used a material that contains functionalized Fe-porphyrins as catalytically competent, redox-conductive linkers. The approach yields a high effective surface coverage of electrochemically addressable catalytic sites (∼1015 sites/cm2). The chemical products of the reduction, obtained with ∼100% Faradaic efficiency, are mixtures of CO and H2. These results validate the strategy of using MOF chemistry to obtain porous, electrode-immobilized, networks of molecular catalysts having competency for energy-relevant electrochemical reactions.
In order to help develop robust and deployable molecular electrocatalysts for the reduction of CO2 to CO, we must understand the effects of tuning their structure and catalytic conditions. To this ...end, we quantify how modifications to the catalyst fac-Re(4,4′-R-bpy)(CO)3X (bpy = 2,2′-bipyridine, R = OCH3, CH3, tBu, H, CN, CF3; X = Cl, Br, py(OTf), or CH3CN(OTf)) with and without an added proton source (phenol, acetic acid, 2,2,2-trifluoroethanol) affect the catalyst stability, activity, and overpotential. Through cyclic voltammetry experiments, we found that the substituents and proton source had a large effect on both overpotential and activity. Substituents with moderate electron-donating ability (tBu and CH3) increased activity and overpotential in comparison to the unsubstituted complex Re(bpy)(CO)3Cl. In contrast, substituents resulting in too much electron density distributed over the bpy ligand, either from too-strong electron-donating ability (OCH3) or from the requirement of a third reduction to activate the complex (CN and CF3), destabilized the catalyst. An added proton source both increased the activity and decreased the overpotential by 200 mV for all catalyst derivatives, shifting the catalytic mechanism from an electron-first pathway to a proton-first pathway. We used binding energies calculated via density functional theory to help understand the substituent effect on the catalyst affinity for CO2 and other intermediates relevant to the catalytic mechanism. Catalyst activity was quantified using intrinsic rate constants determined through the utilization of catalytic plateau currents, as well as the application of a foot of the wave analysis, which yielded incongruent values. Of those complexes tested, Re(4,4′-tBu-bpy)(CO)3Cl with an added 1 M phenol yielded the most active catalytic system (k cat = 6206 s–1) at an overpotential of 0.67 V.
The use of a bulky bipyridine ligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy), to enable the reduction of carbon dioxide by a Ru-based molecular electrocatalyst is reported. Under catalytic ...conditions, this compound exhibits turnover frequencies of 1300 s–1 and 95% Faradaic efficiency for the production of CO and H2O from CO2 in the presence of Brønsted acids. Mechanistic electrochemical and spectroelectrochemical studies, supplemented by the direct synthesis of relevant intermediates, indicate that this behavior is the result of the cooperative redox response of the bipyridine ligand and Ru metal center at negative potentials, as well as the inhibition of Ru–Ru bond formation through steric interactions.
In electrochemical processes, an oxidation half-reaction is always paired with a reduction half-reaction. Although systems for reactions such as the reduction of CO2 can be coupled to water oxidation ...to produce O2 at the anode, large-scale O2 production is of limited value. One may replace a low-value half-reaction with a compatible half-reaction that can produce a valuable chemical compound and operate at a lower potential. In doing so, both the anodic and cathodic half-reactions yield desirable products with a decreased energy demand. Here we demonstrate a paired electrolysis in the case of the oxidative condensation of syringaldehyde and o-phenylenediamine to give 2-(3,5-dimethoxy-4-hydroxyphenyl)benzimidazole coupled with the reduction of CO2 to CO mediated by molecular electrocatalysts. We also present general principles for evaluating current–voltage characteristics and power demands in paired electrolyzers.
The hydricity ΔG°H– of a metal hydride is an important parameter for describing the reactivity of such complexes. Here, we compile a comprehensive data set consisting of 51 transition-metal hydride ...complexes M-H(n−1)+ with known ΔG°H– values in acetonitrile for which the one-electron reduction of the parent complex M n+ is reversible. Plotting the hydricity as a function of respective E 1/2(M n+/(n–1)+) yields a robust linear correlation. While this correlation has been previously noted for limited data sets, our analysis demonstrates that this trend extends over a wide range of metal identities, ligand architectures, structural geometries, and overall charges of the metal hydride. This correlation is modeled using established thermochemical cycles relating the hydricity and homolytic bond free energy of the metal–hydride bond. The linear trend of the model enables the estimation of hydricity simply on the basis of the reduction potential of the parent complex and thus provides a guide for the rational design and tuning of metal hydride catalysts for small-molecule reduction, such as CO2 to formic acid.
The use of infrared spectroelectrochemistry (IR-SEC) as a characterization method for molecular electrocatalysts allows researchers to identify key intermediates and products during the course of an ...electrochemical reaction. Since such reactions are driven by the application of potential, properly designed cells allow for the examination of the stepwise formation of active species and their subsequent reactivity with substrate. These results can be compared to and independently verified by the concomitant generation of similar species by chemical means. Recently we have used such an approach to characterize the catalytically relevant species and products for the reduction of CO2 by 2,2′-bipyridyl-based ReI and MnI fac-tricarbonyl electrocatalysts. This tutorial review summarizes the requirements of IR-SEC cells and details some examples of mechanistic questions addressed in our laboratory using these methods. IR-SEC is presented as a general method that could be adapted by other laboratories to answer questions of interest in the spectroscopic characterization of the full complement of redox states of organometallic complexes and their reactivities.
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