CO₂ 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.

Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction

“…A second feature is the apparently fast reaction rate, given that CO 2 comprises only 0.039 vol % of the atmosphere. Although fixation of atmospheric CO 2 by metal hydroxide species has been previously observed (4,5,7,10), including with Ni II (7,(11)(12)(13), no quantitation of reaction rates and analysis of reaction mechanism are available. The fixation of carbon dioxide, whereby this greenhouse gas is rendered in chemically combined forms, is of current environmental and biological interest.…”

Trinuclear zinc complexes for biologically relevant μ3-oxoanion binding and carbon dioxide fixation

“…The solution stability of carbonate-bridged multinuclear zinc complexes is dependent on the coordination modes of their carbonate ligands. For carbonatebridged trinuclear zinc complexes, the typical monodentate coordination of the m 3 -carbonate to each Zn(II) centre is sensitive to hydrolysis 8,28,36,40,41 .…”

“…In the zinc area, triply bridging carbonate units are often found in the solid structures of multinuclear zinc complexes and coordination polymers. Although self-assembled trinuclear m 3 -carbonate species that have adequate stability in solution are known for other metal systems 39 , those based on zinc are intrinsically less stable and thus more difficult to analytically intercept in solution 8,28,36,40,41 . To gain more insight into the coordination dynamics of carbonate and zinc, especially those with triply bridging carbonate units, it is necessary to develop m 3 -carbonate trinuclear zinc species that have sufficient and controllable stability and that are amenable to solution studies.…”

A theoretical thermodynamic investigation of cascade reactions in dinuclear octa-azacryptates involving carbon dioxide

“…As a result of potentially alarming reports which link global warming to the steadily increasing concentration of anthropogenic CO 2 in the atmosphere, there is currently much effort underway to 'mimic' CO 2 fixation artificially [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Many studies refer to the importance of host cavities in transition metal complexes as catalysts in taking up and activating CO 2 from air [3][4][5][6][7][8][9][10][11][12][13][14]. One such strategy of cryptand complexation was developed by Lehn and co-workers over 30 years ago [17], the central idea of which lies in the replacement of the first solvation shell of a cation by a surrogate shell of a three-dimensional caged ligand-like "cryptand" to enhance both the thermodynamic and kinetic stability of the product (the so-called "cryptate effect") [17].…”