Exploration of Pynaphthyridine and Binaphthyridine Manganese(I) Tricarbonyl Complexes: Influence on Carbon Dioxide Reduction Electrocatalysis

For CO2 electrocatalysts of the form fac-[Mn(diimine)(CO)3Br], we previously reported that expanding the π-system of the chelating ligand from 2,2′-bipyridine (bpy) to 2-(2-pyridyl)quinoline (pq) to 2,2′-biquinoline (bqn) introduced a shift in the mechanism for CO2 reduction to CO. Here, we have synthesized pynaphthyridine and binaphthyridine ligands which incorporate extra nitrogen atoms that can act as H-bond acceptors. Though these ligands redshift the MLCT bands, which has previously been associated with a decrease in electrocatalytic performance for CO2 reduction, MnBr(1,8-pynap) in acetonitrile (MeCN) displays a smaller overpotential for CO2 reduction than MnBr(pq). This effect was not observed in N,N-dimethylformamide (DMF), suggesting that H-bonding plays a major role in increasing the catalytic activity. While MnBr(1,8-bn) also had a higher Faradaic efficiency for CO formation than MnBr(bqn), it was not more effective than MnBr(1,8-pynap) in DMF. To demonstrate the importance of the nitrogen ring positions, MnBr(1,6-pynap) was also synthesized, where the aryl nitrogen is facing away from the Mn center. The MnBr(1,6-pynap) was a poor electrocatalyst for CO2 reduction, suggesting that the nonligated N atom in the 1,8-complexes stabilizes the Mn–C(O)OH intermediate.