CO₂ Fixation by Novel Copper(II) and Zinc(II) Macrocyclic Complexes. A Solution and Solid State Study

Solutions containing Zn(II) and Cu(II) complexes with [15]aneN₃O₂ rapidly adsorb atmospheric CO₂ to give {[ZnL]₃(μ₃-CO₃)}·(ClO₄)₄ (2) and {[CuL]₃(μ₃-CO₃)}·(ClO₄)₄ (4) complexes. The crystal structures of both complexes have been solved (for 2, space group R3c, a, b = 22.300(5) Å, c = 17.980(8) Å, V = 7743(4) ų, Z = 6, R = 0.0666, R_w² = 0.1719; for 4, space group R3c, a, b = 22.292(7) Å, c = 10.096(8) Å, V = 7788(5) ų, Z = 6, R = 0.0598, R_w² = 0.1611), and the spectromagnetic behavior of 4 has been studied. In both compounds a carbonate anion triply bridges three metal cations. Each metal is coordinated by one oxygen of the carbonate, three nitrogens, and an oxygen of the macrocycle; the latter donor weakly interacts with the metals. Although the two compounds are isomorphous, they are not isostructural, because the coordination geometries of Zn(II) in 2 and Cu(II) in 4 are different. The mixed complex {[CuZn₂L₃](μ₃-CO₃)}·(ClO₄)₄ has been synthesized. X-ray analysis (space group R3c, a, b = 22.323(7) Å, c = 17.989(9) Å, V = 7763(5) ų, Z = 6, R = 0.0477, R_w² = 0.1371) and EPR measurements are in accord with a μ₃-carbonate bridging one Cu(II) and two Zn(II) ions in {[CuZn₂L₃](μ₃-CO₃)}⁴⁺. Both the Zn(II) and Cu(II) cations exhibit the same coordination sphere, almost equal to that found in the trinuclear Zn(II) complex 2. The systems Zn(II)/L and Cu(II)/L have been studied by means of potentiometric measurements in 0.15 mol dm^-1 NaCl and in 0.1 mol dm^-3 NaClO₄ aqueous solutions; the species present in solution and their stability constants have been determined. In both systems [ML]²⁺ species and hydroxo complexes [M(II)LOH]⁺ (M = Zn, Cu) are present in solution. In the case of Cu(II), a [CuL(OH)₂] complex is also found. The process of CO₂ fixation is due to the presence of such hydroxo-species, which can act as nucleophiles toward CO₂. In order to test the nucleophilic ability of the Zn(II) complexes, the kinetics of the promoted hydrolysis of p-nitrophenyl acetate has been studied. The [ZnLOH]⁺ complex promotes such a reaction, where the Zn(II)-bound OH^- acts as a nucleophile to the carbonyl carbon. The equilibrium constants for the addition of HCO₃^- and CO₃^2- to the [ZnL]²⁺ complex have been potentiometrically determined. Only [ML(HCO₃)]⁺ and [ML(CO₃)] species are found in aqueous solution. A mechanism for the formation of {[ML]₃(μ₃-CO₃)}·(ClO₄)₄ is suggested.