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 (H(2)L(Et)) was structurally characterised. Potentiometric titrations revealed rather low pK(a) values for the deprotonation of the first amide group of H(2)L(Et) (14.2) and N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H(2)L(Me), 13.1). Two tetranuclear copper(ii) square complexes of H(2)L(Et) with a paddle-wheel appearance, in which each ligand strand acts as a linear N(3)-NO hybrid terdentate-bidentate chelate, have been isolated and structurally characterised. Complex [Cu(II)(4)(H(2)L(Et))(2)(HL(Et))(2)](BF(4))(6).3MeCN.0.5H(2)O (.3MeCN.0.5H(2)O), with two nondeprotonated zwitterionic ligand strands and two monodeprotonated ligand strands, is formed in the 1 : 1 reaction of H(2)L(Et) and Cu(BF(4))(2).4H(2)O. It has a polymeric chain structure of tetranuclear subunits connected by N-H[dot dot dot]N hydrogen bonds. The same reaction carried out with one equivalent of base gives the related compound [Cu(II)(4)(HL(Et))(4)](BF(4))(4) (), with all four ligand strands monodeprotonated. It consists of isolated tetranuclear units. In both .3MeCN.0.5 H(2)O and the copper(ii) ions are in five-coordinate N(4)O environments but the degree of trigonality (tau) differs [.3MeCN.0.5H(2)O 0.14 </=tau</= 0.26; tau = 0.45]. Under the same reaction conditions as for but using Ni(BF(4))(2).6H(2)O a tetranuclear [2 x 2] grid-type complex, [Ni(II)(4)(HL(Et))(4)](BF(4)).10H(2)O (.10H(2)O), is formed. The structure determination showed that the nickel(ii) ions have N(6) distorted octahedral coordination spheres and all four ligand strands are monodeprotonated and act as N(3)-N(3) bis-terdentate chelates. Magnetic susceptibility data show that the complexes .4H(2)O, and .10H(2)O exhibit very weak antiferromagnetic spin coupling. The energies and multiplicities of the spin states of .4H(2)O and 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 .4H(2)O and , 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 are consistent with the expected d(x(2)-y(2)) ground state for five-coordinate copper(ii) in an approximately square pyramidal configuration.