Manganese as a substitute for rhenium in CO2 reduction catalysts: the importance of acids.

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(bpy-tBu)(CO)3Br and [Mn(bpy-tBu)(CO)3(MeCN)](OTf) are reported. Addition of Brönsted acids to CO2-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO2. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpy-tBu)(CO)3Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at -2.2 V vs SCE, a TOF of 340 s(-1) was calculated for Mn(bpy-tBu)(CO)3Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO2, with no observable production of H2. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn(bpy-tBu)(CO)3](-), shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn(bpy-tBu)(CO)3Br. These were of the parent Mn(bpy-tBu)(CO)3Br complex, the dimer [Mn(bpy-tBu)(CO)3]2, and the [Mn(bpy-tBu)(CO)3](-) anion.

Kinetic and structural studies, origins of selectivity, and interfacial charge transfer in the artificial photosynthesis of CO.

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy-tBu)(CO)(3)(L) catalyst for the reduction of CO(2) to CO. A remarkable selectivity for CO(2) over H(+) was observed by stopped-flow UV-vis spectroscopy of [Re(bipy-tBu)(CO)(3)](-1). The reaction with CO(2) is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied d(z)(2), which is believed to determine selectivity by favoring CO(2) (σ + π) over H(+) (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re -CO(2) is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.

Tunable, light-assisted co-generation of CO and H2 from CO2 and H2O by Re(bipy-tbu)(CO)3Cl and p-Si in non-aqueous medium.

The light-assisted co-generation of carbon monoxide and hydrogen from carbon dioxide and water is reported. The combination of a homogeneous CO-evolving electrocatalyst and a heterogeneous H(2)-evolving photoelectrode surface provides for tunability of the H(2)/CO ratio. A total Faradaic efficiency of 102 ± 5% and a H(2)/CO ratio of 2:1 were achieved at a low homogeneous catalyst concentration (0.5 mM) in acetonitrile/water mixtures.

Re(bipy-tBu)(CO)3Cl-improved catalytic activity for reduction of carbon dioxide: IR-spectroelectrochemical and mechanistic studies.

Five Re(bipy)(CO)(3)Cl 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)(3)Cl (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 CO(2) the second irreversible wave displays a large catalytic enhancement in current. Diffusion coefficients were determined using the Levich-Koutecky method (1.1 × 10(-5) cm(2)/s for the neutral complex and 8.1 × 10(-6) cm(2)/s for the singly reduced species), and a second order rate constant for the electrochemical reduction with CO(2) of 1000 M(-1) s(-1) was measured. Bulk electrolysis studies were performed to measure Faradaic efficiencies for the primary gaseous products, η(CO) = 99 ± 2% in acetonitrile.

Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels.

Research in the field of catalytic reduction of carbon dioxide to liquid fuels has grown rapidly in the past few decades. This is due to the increasing amount of carbon dioxide in the atmosphere and a steady climb in global fuel demand. This tutorial review will present much of the significant work that has been done in the field of electrocatalytic and homogeneous reduction of carbon dioxide over the past three decades. It will then extend the discussion to the important conclusions from previous work and recommendations for future directions to develop a catalytic system that will convert carbon dioxide to liquid fuels with high efficiencies.

Isostructuralism among bridge-flipped' isomeric benzylideneanilines and phenylhydrazones.

Bridge-flipped' isomers may be defined as pairs of molecules related by a reversal of a bridge of atoms connecting two major parts of the individual molecules. This kind of isomerism is commonly found among benzylideneanilines and phenylhydrazones. Isostructural pairs might be suitable for co-crystallization and are thus useful in the preparation of new solid materials. Although most of the examples of bridge-flipped isomeric benzylideneanilines and phenylhydrazones in the crystallographic literature are not isostructural, a small number of isostructural pairs have been reported by previous workers. This paper describes the molecular and crystal structures of four pairs of bridge-flipped isomers: two isostructural phenylhydrazones, (E)-2-bromobenzaldehyde 4-cyanophenylhydrazone (I) and (E)-4-cyanobenzaldehyde 2-bromophenylhydrazone (II); two pairs of isostructural benzylideneanilines, N-(2-trifluoromethylbenzylidene)-2-methylaniline (III) and N-(2-methylbenzylidene)-2-trifluoromethylaniline (IV), and N-(2-bromobenzylidene)-2-methylaniline (V) and N-(2-methylbenzylidene)-2-bromoaniline (VI); and a pair of benzylideneanilines with closely similar unit-cell dimensions but different packing arrangements, N-(4-methylbenzylidene)-4-cyanoaniline (VII) and N-(4-cyanobenzylidene)-4-methylaniline (VIII). The structure of (V) is disordered. The packing arrangement of (VIII) resembles that of the chloro-/methyl-substituted benzylideneanilines MBZCLA/MBZCLB [N-(4-methylbenzylidene)-4-chloroaniline and N-(4-chlorobenzylidene)-4-methylaniline]. Although intermolecular hydrogen bonding plays a part in the isostructuralism of the two phenylhydrazones, the other examples of isostructuralism occur in the absence of similar, relatively strong intermolecular interactions.