Synthesis, Characterization, and Catalytic CO2 Reduction Reactivity of Ruthenium CNC Pincer Complexes Containing Macrocyclic or Long Chain Wingtips

Abstract

Ruthenium CNC pincer complexes, comprised of N-heterocyclic carbenes (NHCs) and a pyridyl ring, are highly active catalysts for carbon dioxide reduction. We hypothesized that the addition of long chain aliphatic groups with an olefin terminus as wingtips on these CNC pincers could be used to form macrocyclic catalysts by ring closing metathesis (RCM). We have synthesized three new ruthenium pincer catalysts, [(CNC)Ru(CH₃CN)₂Cl]OTf, containing a long chain olefin wingtip (where the substituent para to N on the pyridine ring is H, Me, or OMe) and performed RCM on one compound with R = Me, followed by hydrogenation, to form a novel macrocyclic ruthenium catalyst. These four catalysts were tested for the photocatalytic reduction of carbon dioxide (CO₂) in the presence (sensitized) and absence (self-sensitized) of an external photosensitizer. With a photosensitizer, these catalysts produced mostly CO (775 to 1210 TON) with smaller amounts of H₂ also formed. The methyl substituted macrocyclic catalyst showed a TON of 1185 for CO over 72 h compared to a TON of 775 for CO for the analogous nonmacrocyclic catalyst. The remote substituents at the para-position of the central pyridine ring significantly influence catalyst activity with R = OMe > H ≈ Me.