Mechanistic Elucidation of Dimer Formation and Strategies for Its Suppression in Electrochemical Reduction of Fac‐Mn(bpy)(CO)₃Br

Abstract

Development of new catalytic approaches for reduction of small molecules is an aspiring technology to alleviate increasing energy demands without use of fossil fuels. One of the most promising molecular catalysts for reduction of CO 2 is fac-Mn(bpy)(CO) 3 Br (bpy = 2,2'-bipyridine). In this work, the mechanism of electrochemical reduction of this complex is elucidated using cyclic voltammetry along with digital simulations. It is revealed that the dimer complex, Mn 2 (bpy) 2 (CO) 6 , is not, as often assumed, formed by dimerization of the singly reduced manganese complex, [Mn(bpy)(CO) 3 ] * , but instead from its reduction to [Mn(bpy)(CO) 3 ] À , followed by further reaction with Mn(bpy)(CO) 3 Br (rate constant = 1.75 × 10 5 M À 1 s À 1 ). On the basis of cyclic voltammetry, infrared-spectroelectrochemistry, and 1 H NMR spectroscopy, we establish that this parent-child reaction involving Mn(bpy)(CO) 3 Br as electrophile can be diverted by adding either acids or iodomethane as substituting electrophiles. This demonstrates that [Mn(bpy)(CO) 3 ] À , which is the central catalyst for CO 2 reduction, can be formed at less extreme potentials than previously believed by suppressing the parent-child reaction.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives

“…However, in the presence of Et 3 N/iPrOH/CO 2 , competing protonation of the two-electron product, Mn − , to generate MnH becomes viable. 27 Thus, formation of MnH is made possible by the enhanced electrolyte acidity that CO 2 purging is causing (vide infra). It even occurs at a potential as low as −1.62 V vs Fc + / Fc, before the second peak.…”

“…4 Et 3 NHBF 4 was prepared as described previously. 27 Cyclic Voltammetry. Cyclic voltammetry measurements were recorded using a standard three-electrode setup.…”

“…The crucial point is to form a manganese hydride intermediate, Mn(bpy)-(CO) 3 H, 18,19 from Mn(bpy)(CO) 3 Br, preferentially by implementing a low-overpotential strategy to generate [Mn-(bpy)(CO) 3 ] − initially as recently reported by our group. 27 In the best-case scenario, HCOOH is generated with FE HCOOH = 59 ± 1% at as low overpotential (η) as 260 mV, although FE HCOOH can be further raised to 71 ± 3% at η = 560 mV. Notably, significant decrease of FE HCOOH occurs if Et 3 N is replaced by Et 2 NH, owing to diethylcarbamic acid formation.…”

“…In this study, we show that the prototypical Mn­(bpy)­(CO) 3 Br complex used in combination with externally added tertiary amine (Et 3 N) and proton source ( i PrOH) at a glassy carbon (GC) electrode in MeCN can provide significantly higher HCOOH selectivity than previously seen. The crucial point is to form a manganese hydride intermediate, Mn­(bpy)­(CO) 3 H, , from Mn­(bpy)­(CO) 3 Br, preferentially by implementing a low-overpotential strategy to generate [Mn­(bpy)­(CO) 3 ] − initially as recently reported by our group . In the best-case scenario, HCOOH is generated with FE HCOOH = 59 ± 1% at as low overpotential (η) as 260 mV, although FE HCOOH can be further raised to 71 ± 3% at η = 560 mV.…”

Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis

“…Mn −I (L)(CO) 3 ] − is obtained after reductive cleavage of the [Mn 0 (L)(CO) 3 ] 2 dimer formed via a parent–child reaction on the first reduction wave. 14 The catalytic wave is only slightly sensitive to the addition of water ( Fig. 4 ).…”