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

Abstract

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 CO₃²⁻-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 CO₃²⁻-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.