Synthesis and coordination chemistry of the tetradentate ligands 6,6′-bis(3-pyrazolyl)-2,2′-bipyridine and 6,6′-bis(2-hydroxyphenyl)-2,2′-bipyridine: intramolecular hydrogen-bonding in complexes of Cu(II), and a dinuclear double helicate with Ag(I)

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Simple syntheses of the potentially tetradentate chelating ligands 6,6′-bis(3-pyrazolyl)-2,2′-bipyridine ( H 2 L 1), and 6,6′-bis(2-hydroxyphenyl)-2,2′-bipyridine (H 2L 2) are described: H 2L 1 is a new ligand, whereas H 2L 2 is known, but investigation of its coordination chemistry has been hampered by the lack of a simple synthesis. Complexes of both have been structurally characterised and reveal many interesting features. [Cu(HL 1)(H 2O)][PF 6] is square pyramidal with an axial H 2O ligand, but, in the solid state, forms a hydrogen-bonded dimer in which the peripheral pyrazolyl groups of HL 1 (one protonated, one deprotonated) in one complex unit form a two-point ‘chelating’ hydrogen-bonding interaction with the axial water ligand of the second, and vice versa. In contrast, [Ag 2(L 1) 2][BF 4] 2 is a dinuclear double helicate because of the preference of Ag(I) ions for a pseudo-tetrahedral geometry. [Cu(L 2)] has a typical near-planar geometry with a N 2O 2 donor set, and monomeric units are associated into centrosymmetric dimers in the crystal via weak axial Cu⋯O(phenolate) interactions to give an asymmetric [Cu 2(μ-phenolate) 2] core. In {[Cu(L 2)] 2H}(PF 6), the two monomeric [Cu(L 2)] units are also associated via axial phenolate interactions to give a dimer with a [Cu 2(μ-phenolate) 2] core, but, in addition, the extra proton per dimer unit is located at the centre of a short, strong O⋯H⋯O hydrogen-bond that links a phenolate group from each of the two monomer units. The geometry of dimer formation is changed in order to allow the phenolate groups to approach one another closely enough for this hydrogen-bond to form.