New copper(II) tetramer with arylhydrazone of barbituric acid and its catalytic activity in the oxidation of cyclic C5-C8 alkanes

2-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)terephthalic acid (H5L) was synthesized by azocoupling of diazonium salts of 2-aminoterephthalic acid with barbituric acid and characterized by IR, H-1 and C-13 NMR spectroscopies and ESI-MS analysis. In solution and solid state, H5L exists in the hydrazone form. The reaction of H5L with copper(II) nitrate hydrate in methanol affords the tetramer species [Cu-2(mu(2)-1 kappa NOO':2 kappa O'O-H3L)(2)](2) (1) where carboxylate groups bridge two Cu(II) centers in a bridging chelate mu(2)-(O,O',O') fashion, giving rise to a cyclic Cu4O4 core. A rare and intense intermolecular non-covalent interaction of the type O-carboxylic(lone pair)center dot center dot center dot mu(pyrimidine), with O center dot center dot center dot centroid distance of 2.705 angstrom, was found in 1. In addition, medium intense N-H center dot center dot center dot O and O-H center dot center dot center dot O contacts expand the structure to a 3D network. Compound 1 behaves as efficient catalyst in the mild oxidation, by aqueous H2O2 in acidic MeCN/H2O media, of different cyclic C-5-C-8 alkanes to produce the corresponding alcohols and ketones. Overall product yields up to ca. 25% (based on alkane) were achieved. (C) 2016 Elsevier Ltd. All rights reserved.