Pyrazolate-based copper(II) and nickel(II) [2 x 2] grid complexes: protonation-dependent self-assembly, structures and properties

Abstract

The pyrazole-based diamide ligand N,N'-bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide (H3L) has been structurally characterised and successfully employed in the preparation of [2 x 2] grid-type complexes. Thus, the reaction of H3L with Cu(ClO4)(2)center dot 6H(2)O or Ni(ClO4)(2)center dot 6H(2)O in the presence of added base (NaOH) affords the tetranuclear complexes [M-4(HL4)]center dot 8H(2)O (1: M = Cu, 2: M = Ni). Employment of a mixture of the two metal salts under otherwise identical reaction conditions leads to the formation of the mixed-metal species [CuxNi4-x(HL)(4)]center dot 8H(2)O (x <= 4; for x = 2: 3). Complexes 1-3 have been structurally characterised and found to be isomorphous, with each ligand strand acting as a hybrid N-3-NO chelator. The copper ions in 1 are in a distorted square- pyramidal N4O coordination environment with rather long M-O-apical distances. The coordination sphere about the nickel ions in 2 is roughly the same, but with even longer M center dot center dot center dot O distances, and it is therefore best described as N-4 square-planar with low-spin nickel(II) ions. The single crystal X-ray data obtained for the mixed-metal complex 3 gave the best results assuming a statistical distribution of copper and nickel ions. X-Band EPR spectra of 1 and 2 indicate magnetically coupled copper(II) ions and low-spin nickel(II), respectively. EPR spectra of a powdered sample of a complex with the general formulation [CuxNi4-x(HL)(4)]center dot 8H(2)O with a large excess of Ni2+ (95%) was shown to be characteristic for individual copper(II) ions in the tetranuclear grid system. Magnetic susceptibility data of 1 indicate weak antiferromagnetic spin coupling between the copper ions (J = -8.2 +/- 0.4 cm(-1)), which is explained by the particular spatial arrangement of the magnetic orbitals.