title: CO(2) Fixation by Novel Copper(II) and Zinc(II) Macrocyclic Complexes. A Solution and Solid State Study.
abstract: Solutions containing Zn(II) and Cu(II) complexes with [15]aneN(3)O(2) rapidly adsorb atmospheric CO(2) to give {[ZnL](3)(&mgr;(3)-CO(3))}.(ClO(4))(4) (2) and {[CuL](3)(&mgr;(3)-CO(3))}.(ClO(4))(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) Å(3), Z = 6, R = 0.0666, R(w)(2) = 0.1719; for 4, space group R3c, a, b = 22.292(7) Å, c = 10.096(8) Å, V = 7788(5) Å(3), Z = 6, R = 0.0598, R(w)(2) = 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 {[CuZn(2)L(3)](&mgr;(3)-CO(3))}.(ClO(4))(4) has been synthesized. X-ray analysis (space group R3c, a, b = 22.323(7) Å, c = 17.989(9) Å, V = 7763(5) Å(3), Z = 6, R = 0.0477, R(w)(2) = 0.1371) and EPR measurements are in accord with a &mgr;(3)-carbonate bridging one Cu(II) and two Zn(II) ions in {[CuZn(2)L(3)](&mgr;(3)-CO(3))}(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)/Lhave been studied by means of potentiometric measurements in 0.15 mol dm(-)(1) NaCl and in 0.1 mol dm(-)(3) NaClO(4) 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 CO(2) fixation is due to the presence of such hydroxo-species, which can act as nucleophiles toward CO(2). 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 HCO(3)(-) and CO(3)(2)(-) to the [ZnL](2+) complex have been potentiometrically determined. Only [ML(HCO(3))](+) and [ML(CO(3))] species are found in aqueous solution. A mechanism for the formation of {[ML](3)(&mgr;(3)-CO(3))}.(ClO(4))(4) is suggested.
authors.affiliation: Department of Chemistry and Department of Energetic, University of Florence, Via Maragliano 75/77, 50144 Florence, Italy, Research and Development Division, Bracco SpA, Via Folli 50, 20134 Milano, Italy, Institute of Chemical Sciences, University of Urbino, Piazza Rinascimento 6, 61029 Urbino, Italy, and Department of Chemistry, University of Calabria, Arcavata di Rende, Italy.
authors.forename: Carla
authors.initials: C
authors.lastname: Bazzicalupi
authors.forename: Alessandro
authors.initials: A
authors.lastname: Bencini
authors.forename: Andrea
authors.initials: A
authors.lastname: Bencini
authors.forename: Antonio
authors.initials: A
authors.lastname: Bianchi
authors.forename: Federica
authors.initials: F
authors.lastname: Corana
authors.forename: Vieri
authors.initials: V
authors.lastname: Fusi
authors.forename: Claudia
authors.initials: C
authors.lastname: Giorgi
authors.forename: Paola
authors.initials: P
authors.lastname: Paoli
authors.forename: Piero
authors.initials: P
authors.lastname: Paoletti
authors.forename: Barbara
authors.initials: B
authors.lastname: Valtancoli
authors.forename: Claudia
authors.initials: C
authors.lastname: Zanchini