Polynuclear complexes with bridging pyrophosphate ligands: Synthesis and characterisation of {[(bipy)Cu(H₂O)

The reaction in water of M(II) ions (M = Cu, 1; Zn, 2; VO, 3) with 2,2′-bipyridine (bipy) followed by Na₄P₂O ₇ leads to the formation of three new complexes which feature the pyrophosphate anion, P₂O₇^4-, as a bridging ligand. Single crystal X-ray diffraction revealed 1 to be {[(bipy)Cu(H ₂O)(μ-P₂O₇)Na₂(H ₂O)₆]·4H₂O}, and 2 as a tetranuclear Zn(II) complex, {[(bipy)Zn(H₂O)(μ-P₂O ₇)Zn(bipy)]₂·14H₂O}. The structure of 1 consists of a mononuclear [(bipy)Cu(H₂O)(P₂O ₇)]^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-P₂O ₇)]·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 V=O 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-P₂O ₇)·7H₂O}, 4.