We report here the efficient reduction of CO2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the azacalix[4](2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the Co(I/0) couple, therefore decreasing the overpotential for CO2 reduction.
N-(2,6-Diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Diso) reacts with the (cyclooctadiene)iridium chloride dimer to form a monoiminoxolene complex, (Diso)Ir(cod)Cl. Reaction of 2 equiv of the iminoquinone with chlorobis(cyclooctene)iridium dimer affords the bis-iminoxolene (Diso) 2 IrCl. This five-coordinate complex adopts a distorted square pyramidal structure with an apical chloride ligand and undergoes halide exchange to form an air-stable iodide complex. (Diso) 2 IrCl can be reduced by one electron to form neutral, square planar (Diso) 2 Ir, while oxidation with PhICl 2 gives octahedral trans-(Diso) 2 IrCl 2 . The cis isomer can be prepared by air oxidation of (Diso) 2 IrCl; cis/ trans isomerization is not observed even on prolonged heating. Structural and spectroscopic features of the complexes are consistent with the presence of strong, covalent π bonding between the metal and the iminoxolene ligands, with structural data suggesting between 45 and 60% iridium character in the π bonding orbitals, depending on the ancillary ligands. The spectroscopic similarity of (Diso) 2 Ir and (Diso) 2 IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.
A xanthenediamine-based bis(iminocatecholate) occupies trans sites in square pyramidal silicon adducts and binds univalent ions in a lower N2O3 pocket.
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