The homogeneous reduction of CO2 by [Ni(cyclam)]⁺: increased catalytic rates with the addition of a CO scavenger.

The homogeneous electrochemical reduction of CO2 by the molecular catalyst [Ni(cyclam)](2+) is studied by electrochemistry and infrared spectroelectrochemistry. The electrochemical kinetics are probed by varying CO2 substrate and proton concentrations. Products of CO2 reduction are observed in infrared spectra obtained from spectroelectrochemical experiments. The two major species observed are a Ni(I) carbonyl, [Ni(cyclam)(CO)](+), and a Ni(II) coordinated bicarbonate, [Ni(cyclam)(CO2OH)](+). The rate-limiting step during electrocatalysis is determined to be CO loss from the deactivated species, [Ni(cyclam)(CO)](+), to produce the active catalyst, [Ni(cyclam)](+). Another macrocyclic complex, [Ni(TMC)](+), is deployed as a CO scavenger in order to inhibit the deactivation of [Ni(cyclam)](+) by CO. Addition of the CO scavenger is shown to dramatically increase the catalytic current observed for CO2 reduction. Evidence for the [Ni(TMC)](+) acting as a CO scavenger includes the observation of [Ni(TMC)(CO)](+) by IR. Density functional theory (DFT) calculations probing the optimized geometry of the [Ni(cyclam)(CO)](+) species are also presented.

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.

Homogeneous CO2 reduction by Ni(cyclam) at a glassy carbon electrode.

The homogeneous CO(2) 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 CO(2) was also probed using density functional theory (DFT) calculations. The relative CO(2) binding energies to the Ni(I) 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.