Polynuclear complexes with bridging pyrophosphate ligands: synthesis and characterisation of {[(bipy)Cu(H2O)(μ-P2O7)Na2(H2O)6]·4H2O}, {[(bipy)Zn-(H2O)(μ-P2O7)Zn(bipy)]2·14H2O} and {[(bipy)(VO)2]2(μ-P2O7)]·5H2O}

Abstract

The reaction in water of M(II) ions (M = Cu, 1; Zn, 2; VO, 3) with 2,2′-bipyridine (bipy) followed by Na₄P₂O₇ leads to the formation of three new complexes which feature the pyrophosphate anion, P₂O₇⁴⁻, as a bridging ligand. Single crystal X-ray diffraction revealed 1 to be {[(bipy)Cu(H₂O)(μ-P₂O₇)Na₂(H₂O)₆]·4H₂O}, and 2 as a tetranuclear Zn(II) complex, {[(bipy)Zn(H₂O)(μ-P₂O₇)Zn(bipy)]₂·14H₂O}. The structure of 1 consists of a mononuclear [(bipy)Cu(H₂O)(P₂O₇)]²⁻ unit that links via a pyrophosphate bridge to two Na atoms. The hydrated six-coordinate Na atoms themselves join together through bridging water molecules to generate a 2D Na–water sheet. The structure of 2 consists of a tetranuclear Zn(II) cluster (dimer-of-dimers) with two pyrophosphate ligands bridging between four metal centres. Adjacent clusters interact through face-to-face π–π interactions via the bipy ligands to yield a 2D sheet. Adjacent sheets pack in register to create channels, which are filled by the water molecules of crystallisation. An intricate 2D H-bonded water network separates adjacent sheets and encapsulates the tetranuclear clusters. Aspects of the pyrophosphate coordination modes in 1 and 2 are of structural relevance to those found within the inorganic pyrophosphatases. Compound 3, {[(bipy)(VO)₂]₂(μ-P₂O₇)]·5H₂O}, was isolated as an insoluble lime-green powder. Its dinuclear structure was elucidated from elemental and thermal analysis, magnetic susceptibility measurement and IR spectroscopy. The latter displayed characteristic bridging pyrophosphate and signature VO stretches, which were corroborated by contrast to the IR spectra of 1 and 2 and through comparison with those found in the structurally characterised dinuclear complex, {[(bipy)Cu(H₂O)]₂(μ-P₂O₇)·7H₂O}, 4.