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

Abstract

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)₂·6H₂O to form a self-assembled, ferromagnetically coupled, alkoxide bridged tetranuclear homoleptic Cu(II) square grid-complex [Cu₄(PzOAP)₄(NO₃)₂] (NO₃)₂·4H₂O (1) with a central Cu₄[µ-O₄] core, involving four ligand molecules. In the Cu₄[µ-O₄] 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°) and Cu–Cu distances are short (4.043–4.131 Å), 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_x²−y²) lying almost orthogonally between the metal centers. The exchange pathways parameters have been evaluated from density functional calculations.