Supramolecular Architectures and Magnetic Properties of Self‐Assembled Windmill‐Like Dinuclear Copper(II) Complexes with Purine Ligands

Abstract

AbstractThree new CuII compounds, [Cu2(μ‐ade)4(H2O)2]·(bpa)·8H2O (1), [Cu2(μ‐Hade)4(H2O)2](NO3)4·2[Cu(pic)2(H2O)]·6H2O (2) and [Cu2(μ‐6Clpur)4(H2O)2]·~6H2O (3) [in which ade = adeninate anion, Hade = 7H‐adenine, 6Clpur = 6‐chloropurinate anion, bpa = 1,2‐bis(4‐pyridyl)ethane, pic = 2‐pyridinecarboxylate 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 antiferromagnetic 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, 69451 Weinheim, Germany, 2009)