Synthesis,\nCharacterization, and Catalytic CO<sub>2</sub> Reduction Reactivity\nof Ruthenium CNC Pincer Complexes Containing\nMacrocyclic or Long Chain Wingtips

Ruthenium\nCNC pincer complexes, comprised of <i>N</i>-heterocyclic\ncarbenes (NHCs) and a pyridyl ring, are highly active\ncatalysts for carbon dioxide reduction. We hypothesized that the addition\nof long chain aliphatic groups with an olefin terminus as wingtips\non these CNC pincers could be used to form macrocyclic catalysts by\nring closing metathesis (RCM). We have synthesized three new ruthenium\npincer catalysts, [(CNC)Ru(CH<sub>3</sub>CN)<sub>2</sub>Cl]OTf, containing\na long chain olefin wingtip (where the substituent <i>para</i> to N on the pyridine ring is H, Me, or OMe) and performed RCM on\none compound with R = Me, followed by hydrogenation, to form a novel\nmacrocyclic ruthenium catalyst. These four catalysts were tested for\nthe photocatalytic reduction of carbon dioxide (CO<sub>2</sub>) in\nthe presence (sensitized) and absence (self-sensitized) of an external\nphotosensitizer. With a photosensitizer, these catalysts produced\nmostly CO (775 to 1210 TON) with smaller amounts of H<sub>2</sub> also\nformed. The methyl substituted macrocyclic catalyst showed a TON of\n1185 for CO over 72 h compared to a TON of 775 for CO for the analogous\nnonmacrocyclic catalyst. The remote substituents at the <i>para</i>-position of the central pyridine ring significantly influence catalyst\nactivity with R = OMe > H ≈ Me.