The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)₃ Complexes on the CO₂ Reduction Catalysis

Abstract

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn­(CO)3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H2 formation forming [Mn2(H–1L1)­(CO)6Br] and [Mn­(H–1L3)­(CO)3], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn–H species. 2 dimerizes during reduction, forming [Mn2(L2)­(CO)6]2, but 1 and 3 do not. Reduction of 1, 2, and 3 is accompanied by bromide loss, and the final species represent [Mn2(H–1L1)­(CO)6]3–, [Mn2(L2)­(CO)6]2–, and [Mn­(H–1L3)­(CO)3]2–, respectively. 1 and 2 are active catalysts in the electrochemical CO2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H+/2e– reduced products of CO2 in dmf/water mixtures. For 2, CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1. Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn–H species and subsequent CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s–1 are higher with 1 than with 2, that is, the OH group is beneficial for catalysis due to the internal proton transfer.

Chemoselective Electrochemical Hydrogenation of Ketones and Aldehydes with a Well‐Defined Base‐Metal Catalyst

“…IR spectroelectrochemical experiments (IR‐SEC) were conducted to get structural information on the in situ formed reduced Mn(CO) 3 species during catalysis (Figure 3). [26] The redox chemistry of 1 in DMF has been investigated in detail previously by various methods, and the species formed in THF upon reduction are essentially the same (Figure S46) [14] . Initial reduction of 1 in the absence of any substrate leads to the formation of A (blue trace).…”

“…B exhibits a characteristic CO vibration at a frequency of 1982 cm −1 . The shift of the A ’ CO vibrational mode of B with regard to the one of 1 is characteristic for a 1 ē reduction at each α‐diimine unit and a five‐coordinated metal centre [14, 28] . Thus, B has been assigned previously to [Mn 2 (H −1 L)(CO) 6 ] − [14] .…”

“…Complex 1 was synthesised and characterised as described previously [14] . It exhibits several reduction processes, between −1.9 V and −2.9 V, which are coupled by chemical reactions as described previously, and the potentials depend on the solvent (Figure 2, Table 1, Figure S14, Figure S15) [14] . All potentials in this study are referenced versus the Fc +|0 redox couple and a GC working electrode was used for all CV measurements.…”

“…Initial reduction of 1 in the absence of any substrate leads to the formation of A (blue trace). The CO stretching frequencies of A at 2017, 1923, and 1904 cm −1 are characteristic for a dinculear Mn species in which both Mn I ions are coordinated by one X‐type ligand, three facial carbonyl ligands, and the α‐diimine units of L. Thus, A was previously assigned to [Mn 2 (H −1 L)(CO) 3 Br] (H −1 L denotes the ligand deprotonated at the phenol unit forming a phenolate) [14] . That is, initial reduction of 1 induces homolytic O−H bond cleavage, loss of one bromide ligand and coordination of the resulting phenolate to the metal ion.…”

“…We had reasonable evidence that the binuclear manganese complex 1 may serve as a catalyst in the electrochemically‐driven hydrogenation reaction of C=O bonds (Figure 1). We knew from a previous study on 1 that (i) the proton relay in 1 facilitates the formation of Mn−H species upon electrochemical reduction and protonation, (ii) the Mn−H species reacts with CO 2 forming formate, and (iii) it shows only very low reaction rate for the H 2 evolution [14] . Furthermore, 1 exhibits several structural features, which proved to be beneficial in manganese hydrogenation catalysts.…”