An orthogonal ferromagnetically coupled tetracopper(II) 2 x 2 homoleptic grid supported by μ-O4 bridges and its DFT study

A pyrazole based ditopic ligand (PzOAP), prepared by the reaction between 5-methylpyrazole-3- carbohydrazide and methyl ester of imino picolinic acid, reacts with Cu(NO3)(2) center dot 6H(2)O to form a self-assembled, ferromagnetically coupled, alkoxide bridged tetranuclear homoleptic Cu(II) square grid-complex [Cu-4(PzOAP)(4)(NO3)(2)] (NO3)(2) center dot 4H(2)O (1) with a central Cu-4[mu-O-4] core, involving four ligand molecules. In the Cu-4[mu-O-4] core, out of four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by the nitrate ion. The complex 1 has been characterized structurally and magnetically. Although Cu - O - Cu bridge angles are too large (138 - 141 degrees) and Cu - Cu distances are short (4.043 - 4.131 angstrom), suitable for propagation of expected antiferromagnetic exchange interactions within the grid, yet intramolecular ferromagnetic exchange ( J = 5.38 cm(-1)) is present with S = 4/2 magnetic ground state. This ferromagnetic interaction is quite obvious from the bridging connections (d(x)2(-y)2) lying almost orthogonally between the metal centers. The exchange pathways parameters have been evaluated from density functional calculations.