Novel pyrazolate-based copper(II) [2×2] grid complexes: Synthesis, structure and properties

5-Acetyl-substituted pyrazole-3-carboxylic acid (H2L) forms [2×2] grid-like tetranuclear Cu(II) complexes with four five-coordinated copper(II) ions bridged by pyrazolate groups. Despite a significant dissociation of [Cu4L4(H2O)4]·4H2O (1) in aqueous solution1, it was possible to substitute the coordinated water molecules by pyridine ligands or azide anions. The resulting tetranuclear complexes [Cu4L4Py4]·2H2O (2) and Na4[Cu4L4(N3)4]·7MeOH (3) were isolated and studied by X-ray diffraction analysis. In 2 and 3 the azide anions or pyridine molecules complete the distorted square-pyramidal coordination of each copper(II) center. Magnetic susceptibilities of the obtained compounds have been measured by SQUID techniques. Simulation of the data using a Heisenberg spin Hamiltonian approach showed that the bridges between the metals mediate weak intramolecular antiferromagnetic coupling (J in the range −13.3 to −17.1 cm−1) and lead to a singlet ground state in all cases.