Co-ordination chemistry of 6-(2-hydroxyphenyl)pyridine-2-carboxylic acid: a terdentate ligand with a mixed phenolate/pyridyl/carboxylate donor set†

Abstract

The new ligand 6-(2-hydroxyphenyl)pyridine-2-carboxylic acid (H₂L) having a terdentate phenolate–pyridyl–carboxylate (O,N,O) donor set has been found to co-ordinate to transition-metal and lanthanide(III) ions as a dianionic terdentate chelate. The complexes K[M^IIIL₂] (M = Cr or Fe) are both octahedral with a trans-N₄O₂ donor set, and according to X-ray analysis have the K⁺ ion associated with the complex anion via interactions with phenolate and carboxylate oxygen atoms of the ligands. The iron(III) complex is high spin according to EPR and UV/VIS spectroscopy, and is structurally similar to iron(III) complexes of natural siderophores such as desferriferrithiocin. Two dinuclear copper(II) complexes were crystallographically characterised: [Cu₂L₂(MeOH)₂] has a planar {Cu₂L₂} core with two phenolate ligands bridging the copper(II) centres, and an axial MeOH ligand on each Cu atom, one directed to either side of the {Cu₂L₂} core; in [Cu₂L₂(MeOH)(H₂O)] in contrast the axial solvent molecules, one H₂O and one MeOH, are both on the same face of the {Cu₂L₂} core which induces a substantial ‘bowing’ of the core to minimise steric interference between them. The terbium(III) complex K[TbL₂(H₂O)₂]·2H₂O was also prepared, and has eight-co-ordination; there is an extensive hydrogen-bonding network involving the K⁺ ions and water molecules. Luminescence spectroscopic studies in MeOH and MeOD gave a value of 4.9 for the number of co-ordinated solvent molecules (q), consistent with an additional contribution to quenching from hydrogen-bonded solvent molecules, and/or partial dissociation of the L^2– moieties.