Self assembled tetranuclear Cu4(ii), Ni4(ii) [2 × 2] square grids and a dicopper(ii) complex of heterocycle based polytopic ligands - Magnetic studies

Abstract

The ditopic ligand PyPzOAP (N-[(Z)-amino(pyridin-2-yl)methylidene]-5-methyl-1-(pyridin-2-yl)-1H-pyrazole-3-carbohydrazonic acid) and the polytopic ligand 2-PzCAP (N′³,N′⁵-bis[(1E)-1-(pyridin-2-yl)ethylidene]-1H-pyrazole-3,5-dicarbohydrazide) were synthesized in situ by condensation of methyl imino picolinate with 5-methyl-1-(2-pyridyl) pyrazole-3-carbohydrazide and 2-acetyl pyridine with pyrazole-3,5-dicarbohydrazide respectively. The ligands PyPzOAP and PzOAP (reported earlier, Dalton Trans., 2007, 1229) self-assemble to form homoleptic [2 × 2] tetranuclear M₄ (M = Cu(II) and Ni(II)) square grids structures [Cu₄(PyPzOAP)₄](NO₃)₄ (1), [Cu₄(PzOAP)₄](ClO₄)₄ (2) and [Ni₄(PyPzOAP)₄](NO₃)₄·8H₂O·2CH₃CN (3). While the ligand 2-PzCAP forms a dicopper(II) complex [Cu₂(2-PzCAP)(OH)(NO₃)(H₂O)](NO₃)·2H₂O (4). The complex 1 is a perfect square grid (a = 4.201 Å), whereas, 2 and 3 are almost square grids. All these compounds have been characterized by X-ray structural analyses and variable temperature magnetic susceptibility measurements. EPR studies have also been carried out for complexes 1, 2 and 4. In the Cu₄ grid (1), all the Cu(II) centers are in a distorted octahedral environment with N₄O₂ chromophore, while, in complex 2, all four Cu(II) centers have a square pyramidal environment with N₃O₂ chromophore. In complex 3, all four Ni(II) centers have distorted octahedral geometry with N₄O₂ chromophore. In compound 4, the Cu(II) centers are in square pyramidal environment with N₃O₂ chromophore. The magnetic properties of compounds 1 and 2 show the presence of intramolecular ferromagnetic exchange interaction (J = 5.88 cm⁻¹ for 1 and 4.78 cm⁻¹ for 2). The complex 3 shows weak intramolecular antiferromagnetic interaction (J = −4.02 cm⁻¹). While, complex 4, shows strong antiferromagnetic behavior (J = −443 cm⁻¹).