A pyrazole-based orthogonal ferromagnetically coupled [2 × 2] homoleptic square Cu 4 grid: Magnetostructural correlations

Abstract

The synthesis and structure of a pyrazole-based orthogonal ferromagnetically coupled tetracopper(II) 2 × 2 homoleptic grid complex [Cu 4(PzOAPyz) 4(ClO 4) 2](ClO 4) 2 · 6H 2O ( 1), formed by the reaction between the ditopic ligand PzOAPyz and Cu(ClO 4) 2 · 6H 2O, are described. The ligand contains terminal pyrazole and pyrazine residues bound to a central flexible diazine subunit (N–N) as well as one potentially bridging alkoxo group. The two adjacent metal centers are linked by an alkoxo oxygen forming essentially a square Cu 4(μ-O 4) cluster. In the Cu 4(μ-O 4) core, out of the 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 one of the oxygen atoms of a coordinated perchlorate ion. Complex 1 has been characterized structurally and magnetically. Although the large Cu–O–Cu bridge angles (137–138°) and short Cu–Cu distances (3.964–3.970 Å) are suitable for the transmission of the expected antiferromagnetic coupling, the square-based Cu 4(μ-O 4) cluster exhibits an intramolecular ferromagnetic exchange ( J = 7.47 cm −1) between the metal centers with an S = 2 magnetic ground state associated with the quasi orthogonal arrangement of the magnetic orbitals ( d x 2 - y 2 ). The exchange pathway parameters have been evaluated from density functional calculations.