A [2 × 2] nickel(ii) grid and a copper(ii) square result from differing binding modes of a pyrazine-based diamide ligand

Abstract

The potentially bis-terdentate diamide ligand N,N′-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-dicarboxamide (H₂L^Et) was structurally characterised. Potentiometric titrations revealed rather low pK_a values for the deprotonation of the first amide group of H₂L^Et (14.2) and N,N′-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H₂L^Me, 13.1). Two tetranuclear copper(II) square complexes of H₂L^Et with a paddle-wheel appearance, in which each ligand strand acts as a linear N₃–NO hybrid terdentate–bidentate chelate, have been isolated and structurally characterised. Complex [Cu^II₄(H₂L^Et)₂(HL^Et)₂](BF₄)₆·3MeCN·0.5H₂O (3·3MeCN·0.5H₂O), with two nondeprotonated zwitterionic ligand strands and two monodeprotonated ligand strands, is formed in the 1 : 1 reaction of H₂L^Et and Cu(BF₄)₂·4H₂O. It has a polymeric chain structure of tetranuclear subunits connected by N–H⋯N hydrogen bonds. The same reaction carried out with one equivalent of base gives the related compound [Cu^II₄(HL^Et)₄](BF₄)₄ (4), with all four ligand strands monodeprotonated. It consists of isolated tetranuclear units. In both 3·3MeCN·0.5 H₂O and 4 the copper(II) ions are in five-coordinate N₄O environments but the degree of trigonality (τ) differs [3·3MeCN·0.5H₂O 0.14 ≤ τ ≤ 0.26; 4τ = 0.45]. Under the same reaction conditions as for 4 but using Ni(BF₄)₂·6H₂O a tetranuclear [2 × 2] grid-type complex, [Ni^II₄(HL^Et)₄](BF₄)·10H₂O (5·10H₂O), is formed. The structure determination showed that the nickel(II) ions have N₆ distorted octahedral coordination spheres and all four ligand strands are monodeprotonated and act as N₃–N₃ bis-terdentate chelates. Magnetic susceptibility data show that the complexes 3·4H₂O, 4 and 5·10H₂O exhibit very weak antiferromagnetic spin coupling. The energies and multiplicities of the spin states of 3·4H₂O and 4 were determined from the temperature dependence of the magnetic susceptibility and indicate that a singlet state is lowest and the quintet state highest. This is consistent with the X-band EPR spectra of polycrystalline powders of 3·4H₂O and 4, measured down to 2.3 K, where the resonances observed at the lowest temperature are due to a triplet state. The g-values of the individual ions of 4 are consistent with the expected d_x²−y² ground state for five-coordinate copper(II) in an approximately square pyramidal configuration.