Isolating substituent effects in Re(I)-phenanthroline electrocatalysts for CO2 reduction

Abstract

The development of highly-active molecular electrocatalysts for reducing CO2 to value-added products requires an intimate knowledge of the structural and electronic features surrounding the active site. We previously illustrated how electronic modifications to fac-[ReI(R2phen)(CO)3Cl] electrocatalysts (R2phen = 2,9-disubstituted-1,10-phenanthrolines) strongly dictate CO2 reduction; more specifically, introducing methoxy substituents at both the ortho/para positions of a phenyl ring attached to phenanthroline generated high catalytic activity. In the current work, we have prepared four structurally-related Re(I) electrocatalysts to isolate the electronic effects associated with each methoxy group’s positioning around the phenyl ring (i.e. none, ortho, meta, or para). The diimine ligands 2,9-diphenyl-1,10-phenanthroline (Ph2phen), 2,9-bis(2,6-dimethoxyphenyl)-1,10-phenanthroline ((2,6-dmp)2phen), 2,9-bis(3,5-dimethoxyphenyl)-1,10-phenanthroline ((3,5-dmp)2phen), and 2,9-bis(4-methoxyphenyl)-1,10-phenanthroline ((4-mp)2phen) were prepared, and the subsequent fac-[ReI(R2phen)(CO)3Cl] complexes were synthesized, characterized, and studied for electrocatalytic CO2 reduction. Following 90 min electrolysis at Eapp = −2.55 V vs. Fc+/0 in DMF solvent, the ortho methoxy-substituted analogue (labeled Re((2,6-dmp)2phen)) produced 35 μmol CO, corresponding to a turnover number = 14 (TON = mol CO per mol Re catalyst) and a Faradaic efficiency = 84% (F.E. = mol CO per mol electrons). The remaining Re electrocatalysts produced significantly lower levels of CO (Re(Ph2phen): 1.8 μmol CO; Re((3,5-dmp)2phen): 2.4 μmol; Re((4-mp)2phen): 4.2 μmol CO), emphasizing the important electronic contribution that ortho substituted methoxy groups provide towards enhancing electron density at the Re active site to rapidly, and selectively, convert CO2 substrate to CO product.