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Issue 6, 2007
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A [2 × 2] nickel(ii) grid and a copper(ii) square result from differing binding modes of a pyrazine-based diamide ligand

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The potentially bis-terdentate diamide ligand N,N′-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-dicarboxamide (H2LEt) was structurally characterised. Potentiometric titrations revealed rather low pKa values for the deprotonation of the first amide group of H2LEt (14.2) and N,N′-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H2LMe, 13.1). Two tetranuclear copper(II) square complexes of H2LEt 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 [CuII4(H2LEt)2(HLEt)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 H2LEt 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 [CuII4(HLEt)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, [NiII4(HLEt)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 dx2y2 ground state for five-coordinate copper(II) in an approximately square pyramidal configuration.

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

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The article was received on 11 Oct 2006, accepted on 28 Nov 2006 and first published on 14 Dec 2006

Article type: Paper
DOI: 10.1039/B614796H
Dalton Trans., 2007, 633-645

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

    J. Klingele (née Hausmann), J. F. Boas, J. R. Pilbrow, B. Moubaraki, K. S. Murray, K. J. Berry, K. A. Hunter, G. B. Jameson, P. D. W. Boyd and S. Brooker, Dalton Trans., 2007, 633
    DOI: 10.1039/B614796H

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