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

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journal contribution
posted on 1996-09-11, 00:00 authored by Carla Bazzicalupi, Alessandro Bencini, Andrea Bencini, Antonio Bianchi, Federica Corana, Vieri Fusi, Claudia Giorgi, Paola Paoli, Piero Paoletti, Barbara Valtancoli, Claudia Zanchini
Solutions containing Zn(II) and Cu(II) complexes with [15]aneN3O2 rapidly adsorb atmospheric CO2 to give {[ZnL]33-CO3)}·(ClO4)4 (2) and {[CuL]33-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) Å3, Z = 6, R = 0.0666, Rw2 = 0.1719; for 4, space group R3c, a, b = 22.292(7) Å, c = 10.096(8) Å, V = 7788(5) Å3, Z = 6, R = 0.0598, Rw2 = 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) Å3, Z = 6, R = 0.0477, Rw2 = 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 CO32- 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]33-CO3)}·(ClO4)4 is suggested.