Crystal structure and spectroscopic, magnetic, and electrical properties of a copper(II) dimer, melaminium hexachlorodicuprate(II), exhibiting a new stacking interaction

The (C 3N 6H 8)Cu 2Cl 6 compound (C 3N 6H 8 = melaminium dication, hereafter abbreviated as melH 2) has been synthesized and its crystal structure determined. A number of its physical properties were examined, including its electronic and vibrational spectra, as well as magnetic and electrical behavior. The crystals are monoclinic, space group P2 1/c, with a = 6.6600 (1) \uc5, b = 10.0598 (2) \uc5, c = 19.2691 (12) \uc5, \u3b2 = 96.011 (3)\ub0, Z = 4, d calcd = 2.42 g\ub7cm -3, and d measd = 2.45 g\ub7cm -3. The structure, solved by direct methods and refined by least-squares calculations to R = 3.9%, consists of melaminium dications and Cu 2Cl 6 2- dimers stacked to form infinite chains in a manner not previously observed. The coordination of the copper atoms may be described as distorted square pyramidal. Electronic and vibrational results are discussed on the basis of the crystal structure. The compound exhibits predominantly antiferromagnetic exchange interactions. An intradimer singlet-triplet energy splitting of \u394E/k = 54 K is deduced, with weaker ferromagnetic coupling between dimers. A comparison with the magnetic behavior of other Cu 2Cl 6 2- dimers is given. Electrical conductivity measurements classify the compound as an insulating materia