Group 6 Metal Complexes as Electrocatalysts of CO₂ Reduction: Strong Substituent Control of the Reduction Path of [Mo(η³‑allyl)(CO)₂(x,x′‑dimethyl-2,2′-bipyridine)(NCS)] (x = 4–6)

A series of complexes [Mo­(η³-allyl)­(CO)₂)­(x,x′-dmbipy)­(NCS)] (dmbipy = dimethyl-2,2′-bipyridine; x = 4–6) have been synthesized and their electrochemical reduction investigated using combined cyclic voltammetry (CV) and variable-temperature spectroelectrochemistry (IR/UV-vis SEC) in tetrahydrofuran (THF) and butyronitrile (PrCN), at gold and platinum electrodes. The experimental results, strongly supported by density functional theory (DFT) calculations, indicate that the general cathodic path of these Group 6 organometallic complexes is closely related to that of the intensively studied class of Mn tricarbonyl α-diimine complexes, which, themselves, have recently been identified as important smart materials for catalytic CO₂ reduction. The dimethyl substitution on the 2,2′-bipyridine ligand backbone has presented new insights into this emerging class of catalysts. For the first time, the 2e^– reduced 5-coordinate anions [Mo­(η³-allyl)­(CO)₂)­(x,x′-dmbipy)]⁻ were directly observed with infrared spectroelectrochemistry (IR SEC). The role of steric and electronic effects in determining the reduction-induced reactivity was also investigated. For the 6,6′-dmbipy, the primary 1e^– reduced radical anions exert unusual stability, radically changing the follow-up cathodic path. The 5-coordinate anion [Mo­(η³-allyl)­(CO)₂)­(6,6′-dmbipy)]⁻ remains stable at low temperature in strongly coordinating butyronitrile and does not undergo dimerization at elevated temperature, in sharp contrast to reactive [Mo­(η³-allyl)­(CO)₂)­(4,4′-dmbipy)]⁻ that tends to dimerize in a reaction with the parent complex. The complex with the 5,5′-dmbipy ligand combines both types of reactivity. Under aprotic conditions, the different properties of [Mo­(η³-allyl)­(CO)₂)­(x,x′-dmbipy)]⁻ are also reflected in their reactivity toward CO₂. Preliminary CV and IR SEC results reveal differences in the strength of CO₂ coordination at the free axial position. Catalytic waves attributed to the generation of the 5-coordinate anions were observed using CV, but only a modest catalytic performance toward the production of formate was demonstrated by IR SEC. For 6,6′-dmbipy, a stronger catalytic effect was observed for the Au cathode, compared to Pt.