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

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 x 2] tetranuclear M-4 (M = Cu(II) and Ni(II)) square grids structures [Cu-4(PyPzOAP)(4)](NO3)(4) (1), [Cu-4(PzOAP)(4)](ClO4)(4) (2) and [Ni-4(PyPzOAP)(4)](NO3)(4)center dot 8H(2)O center dot 2CH(3)CN (3). While the ligand 2-PzCAP forms a dicopper(II) complex [Cu-2(2-PzCAP)(OH)(NO3)(H2O)](NO3)center dot 2H(2)O (4). The complex 1 is a perfect square grid (a = 4.201 angstrom), 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 Cu-4 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)).