Deidra L. Gerlach

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features and for metal–ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1–5. In general, NHC-pyOR complexes 1–3 were modest precatalysts for both reactions. NHC complexes 1–3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5–8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.

Crystal structures of bis- and hexakis[(6,6'-di-hydroxy-bipyridine)copper(II)] nitrate coordination complexes.

Two copper(II) complexes, a dinuclear and a hexanuclear complex with bridging hydroxyl and nitrate ligands, were obtained from reaction of copper nitrate with dihydroxybipyridine at neutral and slightly acidic pH. Formation of multi-nuclear complexes contrasts with the equivalent sulfate compounds which formed discrete mononuclear complexes. The complexes feature intramolecular and intermolecular hydrogen bonding.

Crystal structure of 4′-bromo-2′,5′-dimeth­oxy-2,5-dioxo-[1,1′-biphen­yl]-3,4-dicarbo­nitrile [BrHBQ(CN)2] benzene hemisolvate

The geometry of the title hemibiquinone is different from previous examples and may be correlated with the weak interactions in the crystal.

Crystal structure of (perchlorato-κO)(1,4,7,10-tetra­aza­cyclo­dodecane-κ4N)copper(II) perchlorate

The crystal structure of (perchlorato-κO)(1,4,7,10-tetraazacyclododecane-κ4 N)copper(II) perchlorate is reported. The crystal was grown from a solution of methanol at ambient temperature which resulted in no co-crystallization of solvent.