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]aneN(3)O(2) rapidly adsorb atmospheric CO2 to give {[ZnL](3)(mu(3)-CO3)}.(ClO4)(4) (2) and {[CuL](3)(mu(3)-CO3)}.(ClO4)(4) (4) complexes. The crystal st ructures of both complexes have been solved (for 2, space group R3c, a , b = 22.300(5) Angstrom, c = 17.980(8) Angstrom, V = 7743(4) Angstrom (3), Z = 6, R = 0.0666, R(w)(2) = 0.1719; for 4, space group R3c, a, b = 22.292(7) Angstrom, c = 10.096(8) Angstrom, V = 7788(5) Angstrom(3) , Z = 6, R = 0.0598, R(w)(2) = 0.1611), and the spectromagnetic behavi or of 4 has been studied. In both compounds a carbonate anion triply b ridges 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 compou nds are isomorphous, they are not isostructural, because the coordinat ion geometries of Zn(II) in 2 and Cu(II) in 4 are different. The mixed complex {[CuZn(2)L(3)](mu(3)-CO3)}.(ClO4)(4) has been synthesized. X- ray analysis (space group R3c, a, b = 22.323(7) Angstrom, c = 17.989(9 ) Angstrom, V = 7763(5) Angstrom(3), Z = 6, R = 0.0477, R(w)(2) = 0.13 71) and EPR measurements are in accord with a mu(3)-carbonate bridging one Cu(II) and two Zn(II) ions in {[CuZn(2)L(3)](mu(3)-CO3)}(4+). Bot h 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 sys tems Zn(II)/L and Cu(II)/L have been studied by means of potentiometri c measurements in 0.15 mol dm(-1) NaCl and in 0.1 mol dm(-3) NaClO4 aq ueous solutions, the species present in solution and their stability c onstants have been determined. In both systems [ML](2+) species and hy droxo 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 hydroly sis of p-nitrophenyl acetate has been studied. The [ZnLOH](+) complex promotes such a reaction, where the Zn(II)-bound OH- acts as a nucleop hile to the carbonyl carbon. The equilibrium constants for the additio n of HCO3- and CO32- to the [ZnL](2+) complex have been potentiometric ally determined. Only [ML(HCO3)](+) and [ML(CO3)] species are found in aqueous solution. A mechanism for the formation of {[ML](3)(mu(3)-CO3 )}.(ClO4)(4) is suggested.