[Cu(H2btec)(bipy)]∞: a novel metal organic framework (MOF) as heterogeneous catalyst for the oxidation of olefins

Abstract

A new extended metal-organic framework [Cu(H₂btec)(bipy)]_∞ (1) (H₄btec= 1,2,4,5-benzenetetracarboxylic acid; bipy= 2,2′-bipyridine) has been hydrothermally synthesized. Violet crystals are formed in a monoclinic system with a space group C2/c; a = 10.1810(18) Å, b = 14.4360(18) Å, c= 12.894(3) Å, β= 112.94(3)°. In the title compound 1 each Cu(II) centre has a distorted square planar environment, completed by two N atoms from one bipy ligand and two O atoms belonging to two dihydrogen benzene-1,2,4,5-tetracarboxylate anions (H₂btec²⁻). The {Cu(bipy)}²⁺ moieties are bridged by H₂btec²⁻ anions to form an infinite one-dimensional coordination polymer with a zig-zag chain structure along the c axis. A double-chain structure is formed by hydrogen bonds between adjacent zig-zag chains. There are also π–π stacking interactions between the bipy ligands, with an average distance of 3.62 Å resulting in a two-dimensional network structure. Compound 1 was tested as a catalyst for the oxidation of cyclohexene and styrene, with tert-butyl hydroperoxide (TBHP) as oxidant. The catalytic activity (24 h and 75 °C) found for [Cu(H₂btec)(bipy)]_∞ shows a high value for the conversion of cyclohexene (64.5%), and a lower one for styrene (23.7%). High turnover frequency (TOF) values for the epoxide products were observed, indicating that the catalyst synthesized in this work, not only has a high activity and selectivity for epoxidation reactions but is also very efficient.