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

Abstract

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.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron–Porphyrin Catalyst

“…In such a CO 2 reduction system, the catalyst is reduced after gaining electrons from the oxidative/reductive quenching process of the excited photosensitizer to generate the catalytically active species. Apart from the noble metal complexes (i.e., Re, − Ru, − Rh, and Ir complexes − ), many non-noble transition metal complexes, such as Mn, − Fe, − Co, − Ni, − and Cu complexes, − have also been used in photochemical CO 2 reduction. Modifying the ligands from neutral to charged groups can stabilize the critical M–CO 2 and M–CO intermediates through noncovalent electrostatic interactions, thus promoting the CO 2 RR toward target products.…”

Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO2 Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

“…NHCs and multidentate ligands containing NHCs are emerging as a powerful class of ligands in organometallics and homogeneous catalysis due to a strong σ donor ability, straightforward synthesis, and several possible modifications. − For example, NHC ligand donor strengths can be altered by saturation of the heterocycle, benzannulation, varying the N -substituents, or introducing a third heteroatom in the heterocyclic ring. − The combination of pyridine rings and NHCs have been used to make bidentate, tridentate, and tetradentate ligated metal complexes, which have found application in catalytic CO 2 reduction. ,,,− Unlike NHCs, pyridine is a weaker σ donor but can serve as a π acceptor, and substitutions on the pyridyl ring can provide another handle to further tune the electronics at the metal center. Tridentate CNC pincers consisting of a pyridyl ring and two NHC rings have often been used along with bipyridine in ruthenium chemistry (Figure ).…”

Synthesis and catalytic activity of well-defined Co(i) complexes based on NHC–phosphane pincer ligands

“…1 H NMR (CD 3 CN, 400 MHz): δ 7.85-7.11 (m, 10H, CH Ar + 2H, CH im ), 4.29-4.12 (m, 4H, NCH 2 ), 3.12-2.96 (m, 4H, PCH 2 ). 13…”

“…The FT-IR spectrum of 1 shows two strong bands at 1991 and 1935 cm −1 , which are consistent with the presence of two CO ligands, 28 and an intense band at 1882 cm −1 that can be attributed to the [Co(CO) 4 ] − anion. 13 In contrast, complex 2 shows only two strong bands at 1989 and 1930 cm −1 that correspond to the two carbonyl ligands of the Co(I) cation (Fig. 3).…”

“…Co( i ) complexes have often been proposed as the active species in cobalt-catalysed processes; 12,11 b however, convenient synthetic methods for the preparation of well-defined Co( i ) complexes are scarce. 13 In particular, the synthesis of NHC–Co( i ) complexes (NHC = N-heterocyclic carbene) has been described by the reaction of the free NHC ligand with [Co(PPh 3 ) 3 Cl], 14 by the reduction of NHC–Co( ii ) complexes, 15 or by the oxidation of NHC–Co(0) complexes with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). 16 The limitations of these synthetic methodologies have hampered the preparation of NHC–Co( i ) organometallic complexes, which are promising homogeneous catalysts owing to the unique properties of NHCs and their success story as ancillary ligands in catalysis.…”