Structural and magnetic characterization of three tetranuclear Cu(II) complexes with face-sharing-dicubane/double-open-cubane like core framework

Abstract

Three novel tetranuclear copper(II) complexes namely [Cu4(L1)4]∙(dmf) (1), [Cu4(L1)4] (2) and [Cu4(L2)2(HL2)2(H2O)2]∙2(ClO4)∙6(H2O) (3) (H2L1, (E)-2-((1-hydroxybutan-2-ylimino)methyl)phenol; H2L2, (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol)) were synthesised from the self-assembly of copper(II) perchlorate and the tridentate Schiff base ligands. The structural determination reveals that complex 1 crystallizes in the monoclinic system with space group C2/c, whereas both the complexes 2 and 3 crystallize in the triclinic system with space group P-1. Complexes 1 and 2 possess face-sharing dicubane core, on the other hand complex 3 has double open cubane core structure. The copper(II) ions in the cubane core are in distorted square planar geometries, and weak π¿π and C-H¿π interactions lead to formation of a 2D supramolecular architecture for complexes 1 and 2. At room temperature complexes 1, 2 and 3, exhibit fluorescence with a quantum yield (Φs) of 0.47, 0.49 and 0.38, respectively. Variable temperature magnetic susceptibility measurements in the range 2-300 K indicate an overall weak antiferromagnetic exchange coupling in all complexes. The PHI program was used to study their magnetic behaviour. In agreement with their face-sharing dicubane structure, a Hamiltonian of the type H = - J1(S1S2+S1S2'+S1'S2+S1'S2') - J2S1S1', where S1 = S1' = S2 = S2' = SCu = 1/2, was used for studying complexes 1 and 2. Simulations performed suggest magnetic exchange constants with values close to J1 = -20 cm-1 and J2 = 0 cm-1 for these complexes. On the other hand, the spin Hamiltonian H = - J1(S1S4+S2S3) - J2(S1S3+S2S4) - J3S1S2, where S1 = S2 = S3 = S4 = SCu = 1/2, was used to study the magnetic behaviour of the double open cubane core of complex 3 and a good agreement between the experimental and simulated results was found by using the parameters g1 = g2 = 2.20, g3 = g4 = 2.18, J1 = -36 cm-1, J2 = -44 cm-1 and J3 = 0 cm-1.