A tetranuclear complex from the employment of pyridine-2,6-dimethanol in copper(II) nitrate chemistry: Synthetic, structural and magnetic studies

The use of pyridine-2,6-dimethanol (pdmH(2)) in copper(II) nitrate chemistry is reported. The reaction of Cu(NO3)(2)center dot 3H(2)O with one equivalent of pdmH(2) in MeCN affords the known mononuclear complex [Cu(pdmH(2))(2)](NO3)(2) (1) in high-yield. The reaction of I and NaOMe in an 1: 1 ratio, as well as the reaction between Cu(NO3)(2)center dot 3H(2)O, pdmH(2) and NaOMe in an 1:1:1 ratio, in MeOH gives the tetranuclear complex [Cu-4(NO3)(2)(pdmH)(4)(H2O)(MeOH)](NO3)(2) (2) in moderate yields. The cation of 2 possesses a slightly distorted tetrahedral Cu-4 topology with a [Cu-4(mu(2)-OR)(4)](4+) core. The pdmH(-) ions behave as eta(1):eta(1):eta(2):mu(2) ligands. Strong intramolecular hydrogen bonds and pi-pi stacking interactions provide thermodynamic stability on compound 2. Variable-temperature, solid-state dc magnetic studies were carried out on complex 2 in the 2.0-300 K range. The data indicate predominant antiferromagnetic exchange interactions and a resulting S = 0 ground state, which is expected for a solely, mu(2)-alkoxide-bridged system with obtuse Cu-O-Cu bond angles that magnetically behaves as a Cu-4 ring. A simplified 1 - J model was found to be adequate to describe the variable-temperature dc susceptibility data. The data were fitted to the appropriate equation derived from the Hamiltonian H = J(1)(S-1 center dot S-2 + S-2 center dot S-4 + S-3 center dot S-4 + S-1 center dot S-3), giving the parameters J(1) = -99.5 cm(-1) and g = 2.11(4). The combined work demonstrates the ligating flexibility of the pdmH(2) chelate and its usefulness in the synthesis of oligo- and polynuclear Cu-x(II) clusters with interesting structural and magnetic properties, without requiring the co-presence of carboxylate ligands. (C) 2009 Elsevier Ltd. All rights reserved.