Supramolecular architectures and magnetic properties of self-assembled windmill-like dinuclear copper(II) complexes with purine ligands

Abstract

Three new CuII compounds, [Cu2(μ-ade) 4(H2O)2]·(bpa)·8H2O (1), [Cu2(μ-Hade)4(H2O)2]NO 3)4[ICu(pic)2H2O)]·6H 2O (2) and [Cu2(μ-6Clpur)4(H 2O)2]·∼6H2O (3) [in which ade = aden-inate anion, Hade = 7H-adenine, 6Clpur = 6-chloropurinate anion, bpa = 1,2-bis(4-pyridyl)ethane, pic = 2-pyridinecar-boxylate anion], are reported. Their crystal structures consist of dinuclear copper(II) units, in which four ligands arranged in a windmill-like fashion bridge the metal ions through a nonlinear NCN group. Supramolecular crystal building of compound 1 is essentially maintained by an extensive network of hydrogen-bonding interactions that involves direct contacts between the nucleobases. In contrast, in compound 2 there is no evidence for direct interactions between the nucleobases, but the hydrogen bonds among the nucleobases and nitrate counteranions are especially relevant. Compound 3 is mainly built up from T-shaped noncovalent Cl···N interactions and hydrogen-bonding interactions involving the crystallization solvent molecules. Magnetic susceptibility measurements indicate the presence of strong intradimeric antif erromagnetic interactions with large singlet-triplet splittings of -222 (for 1), -288 (for 2) and -255 (for 3) cm-1. DFT calculations were carried out to evaluate the dependence of the magnetic coupling value as a function of some key structural parameters: the Cu-N basal plane distance, the axial Cu-Ow distance and the Cu···Cu distance. DFT calculations also revealed that the nature of the bridging ligand is a crucial factor. In this way, the strength to transmit the magnetic interaction of the different bridging ligands is explained on the basis of the number of electron lone pairs. © Wiley-VCH Verlag GmbH & Co. KGaA.