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.

Seven ruthenium catalysts with the general formula [(CNC)Ru(CH 3 CN) 2 Cl]OTf have been used to understand structure function relationships in the sensitized photocatalytic CO 2 reduction reaction. Herein, CNC is a pincer ligand containing imidazole-based N-heterocyclic carbenes (NHCs) attached to a central pyridyl ring with R groups at the 3-or 4-position. Two new complexes (R = 3-OMe, 4-NPh 2) have been fully characterized by analytical and spectroscopic methods and single-crystal X-ray diffraction. Furthermore, three previously synthesized complexes (R = 4-Me, 4-NMe 2 , and 4-OH) are used for photoca-[a

A new method to synthesize complexes of the type [(CNC)Ru II -(NN)L] n + has been introduced, where CNC is a tridentate pincer composed of two (benz)imidazole derived NHC rings and a pyridyl ring, NN is a bidentate aromatic diimine ligand, L = bromide or acetonitrile, and n = 1 or 2. Following this new method a series of six new complexes has been synthesized and characterized by spectroscopic, analytic, crystallographic, and computational methods. Their electrochemical properties have been studied via cyclic voltammetry under both N 2 and CO 2 atmospheres. Photocatalytic reduction of CO 2 to CO was performed using these complexes both in the presence (sensitized) and absence (self-sensitized) of an external photosensitizer. This study evaluates the effect of different CNC, NN, and L ligands in sensitized and self-sensitized photocatalysis. Catalysts bearing the benzimidazole derived CNC pincer show much better activity for both sensitized and self-sensitized photocatalysis as compared to catalysts bearing the imidazole derived CNC pincer. Furthermore, self-sensitized photocatalysis requires a diimine ligand for CO 2 reduction with catalyst 2 ACN being the most active catalyst in this series with TON = 85 and TOF = 22 h À 1 with an electron donating 4,4'-dimethyl-2,2'-bipyridyl (dmb) ligand and a benzimidazole derived CNC pincer.

Increasing the rate of productive interfacial electron transfer reactions in dye-sensitized solar cells is critically important toward improving device performances. Preorganized electron transfer systems at a metal oxide interface are an interesting approach toward favoring fast electron transfer reactions. This study focuses on facilitating electron transfer reactions from a redox shuttle to an oxidized dye at a TiO 2 surface via a transient redox shuttle− dye coordination complex. By design, the cobalt redox shuttle is supported by a pentadentate polypyridyl ligand with a remaining labile coordination site on the metal. The organic dye is designed with pyridyl groups on the donor region for coordinating to the open site on the redox shuttle to preorganize the redox shuttle−dye pair via a Lewis acid−Lewis base interaction. DSC devices fabricated with this dye−redox shuttle pair are studied via current−voltage curves, incident photon-to-current conversion efficiencies (IPCEs), photocurrent dynamics, electrochemical impedance spectroscopy, and transient absorption spectroscopy. Results show that dye binding to the redox shuttle increases the rate of dye regeneration, even in the complex electrolyte environment where coordinating species such as tertbutylpyridine and multiple oxidation states of the redox shuttle are present, leading to a dramatically higher performance of the DSC device under fluorescent lighting (13.0% for [Co(PY5Me 2 )(MeCN)] 3+/2+ versus 5.6% PCE for [Co(bpy) 3 ] 3+/2+ ).

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