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 (1Co‑ ) or OMe (2Co‑ ) 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 1PMe3 , 1PPh3 , and 2PPh3 complexes with the structure [(CNC)Co(CO)(PR′3)]+[BArF24]−. In 1DPPP , 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, 1PMe3 , 1PPh3 , 1DPPP , 2, 2PPh3 , and Na[Co(CO)4] are PCO2RR catalysts with a methoxy substituent deactivating and a phosphine ligand activating. With monodentate phosphines, catalyst 1PPh3 (1 μM) had the highest turnover frequency (TOFM = 3.9 h–1) and turnover number (TON = 199). The dinuclear 1DPPP complex was the most active and robust catalyst with TON = 278 and TOF = 21.1 h–1 at 1 μM loading. Under dilute conditions (1 nM), 1PPh3 produced up to 36,000 TON with TOF = ∼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.
Integration of polycyclic aromatic hydrocarbon (PAH) units into semi-fluorinated polymers affords high thermal stability and excellent processability for potential applications in optoelectronic, gas-separation, and advanced composites. Base-promoted step-growth polycondensation...
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