Facile synthesis of anion dependent versatile Cu^I and mixed-valent porous Cu^I/Cu^II frameworks

Abstract

CuX₂ (X = ClO₄^-, NO₃^-, 0.5 SO₄²⁻, Cl^-) and 2,3-dihydroxyfumaric acid (dhfa) undergo redox reaction in slightly basic aqueous solution forming Cu^IX and 2,3-diketosuccinic acid. Pyrazine (pyz) and 1,2-bis(4-pyridyl)ethylene (bpee) linkers have been used for stabilizing and isolating the Cu^I state in this reaction. The three different Cu^I-frameworks, {Cu(pyz)_1.5(ClO₄)}_n (1), u(pyz)(NO₃)}_n (2), and {[Cu(bpee)Cl]·2H₂O}_n (3) with different anions have been synthesized and structurally characterized. The framework topology and dimensionality depends upon the anions. This is a one step synthesis of Cu^I coordination frameworks with 1:2 stoichiometry in relation to dhfa and Cu^II. By controlling the concentration of dhfa, a mixed-valent porous Cu^I/Cu^II framework, [Cu^IICu^I₂(2,4-pyrdc)₂(pyz)₃]·2H₂O}_n (4) has been synthesized and characterized by XPS and magnetic studies. Dehydrated frameworks of 3 and 4 exhibit interesting sorption properties with different solvent molecules. In a similar reaction CuCl₂ and pyz without dhfa produced the corresponding Cu^II-compound, {Cu(pyz)Cl₂}_n (5), suggesting the reducing role of dhfa. Therefore by this simple reaction, it is possible to control the oxidation state as well as the coordination number and geometry of Cu ions. This is a novel synthetic methodology for the stabilization of the Cu +1 state in aqueous solution and open atmosphere.