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The one electron electrochemical reduction of vanadium(v) substituted-Keggin type polyoxometalate anions [XVVM11O40]4−(XVVM11) where X = P, As; M = Mo or W has been investigated in CH3CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(iv), (XVIVM11), species. In the absence of acid or in the presence of a large concentration excess, the V(v/iv) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(v/iv) processes are observed in the presence of a moderate concentration of acid in the case of XVVMo11but only two with XVVW11. NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(v) and V(iv) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(v) and V(iv) redox chemistry but that the reduced V(iv) state is much more basic than the V(v) one and that (XVIVMo11) is more basic than (XVIVW11). Digital simulations of voltammograms for reduction of XVVMo11and XVVW11have been undertaken in CH3CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(v/iv) process and the equilibrium constants that accompany the coupled acid–base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available. [ABSTRACT FROM AUTHOR]

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 dx2−y2 ground state for five-coordinate copper(ii) in an approximately square pyramidal configuration. [ABSTRACT FROM AUTHOR]