Pyrazolate-based copper(ii) and nickel(ii) [2 × 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 × 2] grid-type complexes. Thus, the reaction of H₃L with Cu(ClO₄)₂·6H₂O or Ni(ClO₄)₂·6H₂O in the presence of added base (NaOH) affords the tetranuclear complexes [M₄(HL₄)]·8H₂O (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 [Cu_xNi_4−x(HL)₄]·8H₂O (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 N₃–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⋯O distances, and it is therefore best described as N₄ 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 [Cu_xNi_4−x(HL)₄]·8H₂O 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.