Structural and Kinetic Study of Reversible CO2 Fixation by Dicopper Macrocyclic Complexes. From Intramolecular Binding to Self-Assembly of Molecular Boxes

A study of the reversible CO₂ fixation by a series of macrocyclic dicopper complexes is described. The dicopper macrocyclic complexes [Cu₂(OH)₂(Me2p)](CF₃SO₃)₂, 1(CF₃SO₃)₂, and [Cu₂(nu-OH)₂(Me2m)](CF₃SO₃)₂, 2(CF₃SO₃)₂, (Scheme 1) containing terminally bound and bridging hydroxide ligands, respectively, promote reversible inter- and intramolecular CO₂ fixation that results in the formation of the carbonate complexes [{Cu₂(Me2p)}₂(nu-CO₃)₂](CF₃SO₃)₄, 4(CF₃SO₃)₄, and [Cu₂(nu-CO₃)(Me2m)](CF₃SO₃)₂, 5(CF₃SO₃)₂. Under a N₂ atmosphere the complexes evolve CO₂ and revert to the starting hydroxo complexes 1(CF₃SO₃)₂ and 2(CF₃SO₃)₂, a reaction the rate of which linearly depends on [H₂O]. In the presence of water, attempts to crystallize 5(CF₃SO₃)₂ afford [{Cu₂(Me2m)(H₂O)}₂(nu-CO₃)₂](CF₃SO₃)₄, 6(CF₃SO₃)₄, which appears to rapidly convert to 5(CF₃SO₃)₂ in acetonitrile solution. [Cu₂(OH)₂(H3m)]²⁺, 7, which contains a larger macrocyclic ligand, irreversibly reacts with atmospheric CO₂ to generate cagelike [{Cu₂(H3m)}₂(nu-CO₃)₂](ClO₄)₄, 8(ClO₄)₄. However, addition of 1 equiv of HClO₄ per Cu generates [Cu₂(H3m)(CH₃CN)₄]⁴⁺ (3), and subsequent addition of Et₃N under air reassembles 8. The carbonate complexes 4(CF₃SO₃)₄, 5(CF₃SO₃)₂, 6(CF₃SO₃)₄, and 8(ClO₄)₄ have been characterized in the solid state by X-ray crystallography. This analysis reveals that 4(CF₃SO₃)₄, 6(CF₃SO₃)₄, and 8(ClO₄)₄ consist of self-assembled molecular boxes containing two macrocyclic dicopper complexes, bridged by CO₃^2- ligands. The bridging mode of the carbonate ligand is anti-anti–1:1 in 4(CF₃SO₃)₄, anti-anti–2:1 in 6(CF₃SO₃)₄ and anti-anti–2:2 in 5(CF₃SO₃)₂ and 8(ClO₄)₄. Magnetic susceptibility measurements on 4(CF₃SO₃)₄, 6(CF₃SO₃)₄, and 8(ClO₄)₄ indicate that the carbonate ligands mediate antiferromagnetic coupling between each pair of bridged CuII ions (J = -23.1, -108.3, and -163.4 cm^-1, respectively, H = –JS₁S₂). Detailed kinetic analyses of the reaction between carbon dioxide and the macrocyclic complexes 1(CF₃SO₃)₂ and 2(CF₃SO₃)₂ suggest that it is actually hydrogen carbonate formed in aqueous solution on dissolving CO₂ that is responsible for the observed formation of the different carbonate complexes controlled by the binding mode of the hydroxy ligands. This study shows that CO₂ fixation can be used as an on/off switch for the reversible self-assembly of supramolecular structures based on macrocyclic dicopper complexes.