Catalytic fixation of atmospheric carbon dioxide by copper(ii) complexes of bidentate ligands

New copper(ii) complexes, [Cu(L1)₂(H₂O)](ClO₄)₂, 1 [L1 = 2-pyridin-2-yl-quinoline], [Cu(L2)₂(H₂O)](ClO₄)₂, 2 [L2 = 2-pyridin-2-yl-quinoxaline], [Cu(L3)₂(H₂O)](ClO₄)₂, 3 [L3 = 6,7-dimethyl-2-pyridin-2-yl-quinoxaline], [Cu(L4)₂(H₂O)](ClO₄)₂, 4 [L4 = 4-phenyl-2-pyridin-2-yl-quinoline] and [Cu(L5)₂(H₂O)](ClO₄)₂, 5 [L5 = 4-phenyl-2-pyridin-2-yl-quinazoline], were synthesized and characterized as catalysts for selective fixation of atmospheric CO₂. The molecular structure of 2 was determined by single-crystal X-ray studies and shown to have an unusual trigonal bipyramid geometry (τ, 0.936) around the copper(ii) center, with the coordination of two ligand units and a water molecule. The Cu-N_quin (2.040, 2.048 Å) bonds are slightly longer than the Cu-N_pyr (1.987 Å) bonds but shorter than the Cu-O_water bond (2.117 Å). Well-defined Cu(ii)/Cu(i) redox potentials of around 0.352 to 0.401 V were observed for 1-5 in acetonitrile. The electronic absorption spectra of 1-5 showed ligand-based transitions at around 208-286 nm with a visible shoulder at around 342-370 nm. The d-d transitions appeared at around 750-800 and 930-955 nm in acetonitrile. The rhombic EPR spectra of 1-5 exhibited three different g values g_x, 2.27-2.34; g_y, 2.06-2.09; and g_z, 1.95-1.98 at 70 K. Atmospheric CO₂ was successfully fixed by 1-5 using Et₃N as a sacrificial reducing agent, resulting in CO₃^2--bound complexes of type [Cu(L)CO₃(H₂O)] that display an absorption band at around 614-673 nm and a ν_st at 1647 cm^-1. This CO₃^2--bound complex of 1 was crystallized from the reaction mixture and it displayed a distorted square pyramidal geometry (τ, 0.369) around the copper(ii) center via the coordination of only one ligand unit, a carbonate group, and water molecules. Furthermore, treatment of the carbonate-bound Cu(ii) complexes with one equivalent of H⁺ under N₂ atmosphere resulted in the liberation of bicarbonate (HCO₃^-) and regenerated the parent complexes. These regenerated catalysts were active enough to fix CO₂ in eight repeating cycles without any change in efficiency. The fixation of CO₂ possibly occurs via the formation of Cu(i)-species, which is accompanied by the formation of an MLCT band at around 450-500 nm. The rates of Cu(i)-species formation, k_obs, were determined and found to be 5.41-10.31 × 10^-3 s^-1 in the presence of Et₃N in acetonitrile at 25 °C. Interestingly, the copper(i)-species of 3 has been successfully crystallized and displayed a distorted tetrahedral geometry through the coordination of two units of ligand L3.