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

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 (4A-4C, where R = H, Me, OMe para to N on the pyridine ring) and performed RCM on 4B, followed by hydrogenation, to form a novel macrocyclic ruthenium catalyst, 6B. These four catalysts were tested for the photocatalytic reduction of 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 6B showed a TON of 1185 for CO over 72 hours compared to a TON of 775 for CO for the acyclic catalyst 4B. The remote substituents at the para-position of the central pyridine ring significantly influence catalyst activity with R = OMe > H = Me.