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 (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.