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Carbon dioxide hydrogenation with base to generate formate salts can provide a means of storing hydrogen in an energy dense solid. However, this application requires catalytic CO2 hydrogenation, which would ideally use an earth abundant metal catalyst. In this article, six new (CNC)CoIL2 pincer complexes were synthesized and fully characterized, including single crystal X-Ray diffraction analysis on four new complexes. These complexes contain an imidazole-based (1R) N-heterocyclic carbene (NHC) ring or a benzimidazole based NHC ring (2R) in the CNC pincer. The R group is para to N on the pyridine ring and been varied from electron withdrawing (CF3) to donating (Me, OMe) substituents. The L type ligands have included CO and phosphine ligands (in PPh32 and PMe32). Thus, two known Co complexes (1, 1OMe) and six new complexes (1Me, 1CF3, 2, 2OMe, PPh32, PMe32) were studied for the CO2 hydrogenation reaction. In general, the unsubstituted CNC pincer complexes bearing two carbonyl ligands led to the highest activity. The best catalyst, 2, remains active for over 16 h and produces a turnover number of 39,800 with 20 bars of 1:1 CO2 / H2 mixture at 60 °C. A computational study of the mechanism of CO2 hydrogenation is also reported. 

Robust earth-abundant transition metal-based photocatalysts are needed for photocatalytic CO2 reduction. A series of six Ni(II) complexes have been synthesized with a tridentate CNC pincer ligand composed of two imidazole or benzimidazole derived N-heterocyclic carbene (NHC) rings and a pyridyl ring with different R substituents (R = OMe, Me, H) para to N of the pyridine ring. These complexes have been characterized using spectroscopic, analytic, and crystallographic methods. The electrochemical properties of all complexes were studied by cyclic voltammetry under N2 and CO2 atmospheres. Photocatalytic reduction of CO2 to CO and HCO2– was analyzed using all the complexes in the presence and absence of an external photosensitizer (PS). All of these complexes are active as photocatalysts for CO2 reduction with and without the presence of an external PS with appreciable turnover numbers (TON) for formate (HCO2–) production and typically lower amounts of CO. Notably, all Ni(II) CNC-pincer complexes in this series are also active as self-sensitized photocatalysts. Complex 4Me with a benzimidazole derived CNC pincer ligand was found to be the most active self-sensitized photocatalyst. Ultrafast transient absorption spectroscopy (TAS) experiments and computational studies were performed to understand the mechanism of these catalysts. Whereas sensitized catalysis involves halide loss to produce more active complexes, self-sensitized catalysis requires some halide to remain coordinated to allow for a favorable electron transfer between the excited nickel complex and the sacrificial electron donor. This then allows the nickel complex to undergo CO2 reduction catalysis via NiI or Ni0 catalytic cycles. The two active species (NiI¬ and Ni0) demonstrate distinct reactivity and selectivity which influences the formation of CO vs. formate as the product. 

Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6′-dhbp) or distal (4,4′-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)2Ru(n,n′-dhbp)]Cl2, or as the doubly deprotonated (O− bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7A, [(dop)2Ru(4,4′-dhbp)]Cl2, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(Do/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1–4 bearing the 6,6′-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5–8 bearing the 4,4′-dhbp ligand, the deprotonated Ru complexes (5B–8B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the 3LLCT excited state and charge transfer from the [O2-bpy]2− ligand to the N,N spectator ligand. The protonated OH bearing 4,4′-dhbp Ru complexes (5A–8A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with 3O2 to yield 1O2.