Low-Valent Cobalt(I) CNC Pincer Complexes as Catalysts for Light-Driven Carbon Dioxide Reduction

Durable catalysts based on abundant metals are needed for the photocatalytic CO2 reduction reaction (PCO2RR). Thus, we synthesized a series of low-valent cobalt(I) complexes, [(CNC)Co(CO)(2)](+)[Co(CO)(4)](-), with H (1(Co-)) or OMe (2(Co-)) in the 4-position of the pyridyl N donor group (where CNC = L1 and L2 from double deprotonation of the [CNC](2+) preligands L1(HOTf)(2) = 1,1'-(pyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium) ditriflate and L2-(HOTf)(2) = 1,1'-(4-methoxypyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium) ditriflate). Anion exchange for [BArF24]- (tetrakis(3,5-trifluoromethyl)phenyl)-borate) produced 1 and 2 and phosphine substitution produced 1(PMe3), 1(PPh3), and 2(PPh3) complexes with the structure [(CNC)Co(CO)(PR'(3))](+)[BArF24](-). In 1(DPPP), the DPPP ligand bridges two Co(I) centers (DPPP = 1,3-bis(diphenylphosphino)propane). All complexes were fully characterized, and electrochemical measurements suggest that for most of the phosphine complexes, CO2 binding by the complex occurs prior to reduction due to a vacant coordination site. Intriguingly, the introduction of a phosphine ligand resulted in a geometry change from trigonal bipyramidal to square pyramidal which correlates to preassociation of CO2 to the complex and higher reactivity in the PCO2RR. Complexes 1, 1(PMe3), 1(PPh3), 1(DPPP), 2, 2(PPh3), and Na[Co(CO)(4)] are PCO2RR catalysts with a methoxy substituent deactivating and a phosphine ligand activating. With monodentate phosphines, catalyst 1(PPh3) (1 mu M) had the highest turnover frequency (TOFM = 3.9 h(-1)) and turnover number (TON = 199). The dinuclear 1(DPPP) complex was the most active and robust catalyst with TON = 278 and TOF = 21.1 h(-1) at 1 mu M loading. Under dilute conditions (1 nM), 1(PPh3) produced up to 36,000 TON with TOF = similar to 800 h(-1) over 6 days, which shows that this is a durable molecular catalyst acting with fast rates in the PCO2RR. Thus, stabilizing low-valent cobalt can offer a unique entry point to highly active PCO2RR catalysts. While cobalt(I) has been proposed as a catalytic species, catalysts that start from Co(I) have not been made previously and the use of phosphine co-ligands has allowed these catalysts to achieve high activity.