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]∙2(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 synthesized 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 2–300K 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 =−20cm−1 and J2 =0cm-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 =−36cm-1, J2 =−44cm-1 and J3 =0cm-1.