Crystal structures of zinc(II) complexes with β-hydroxypyridinecarboxylate ligands: examples of structure-directing effects used in inorganic crystal engineering.

The coordination properties of four hydroxypyridinecarboxylates, designed for the treatment of iron-overloading conditions as bidentate O,O'-donor ligands, have been studied with Zn in the solid state. The coordination compounds [Zn(A1)(HO)] (1), [Zn(A2)(HO)] (2), [Zn(A3)(HO)]·2HO (3) and [Zn(B1)(HO)]·4HO (4), where the ligands are 1-methyl-4-oxidopyridinium-3-carboxylate (A1, CHNO), 1,6-dimethyl-4-oxidopyridinium-3-carboxylate (A2, CHNO), 1,5-dimethyl-4-oxido-pyridinium-3-carboxylate (A3, CHNO) and 1-methyl-3-oxidopyridinium-4-carboxylate (B1, CHNO), have been synthesized and analysed by single-crystal X-ray diffraction. The ligands were chosen to probe (i) the electronic effects of inverting the positions of the O-atom donor groups (i.e. A1 versus B1) and (ii) the electronic and steric effects of the addition of a second methyl group in different positions on the pyridine ring. Two axially coordinated water molecules resulting in a six-coordinated symmetrical octahedron complement the bis-ligand complex of A1. Ligands A2 and A3 form five-coordinated trigonal bipyramidal complexes with one additional water molecule in the coordination sphere, which is a rarely reported geometry for Zn complexes. Ligand B1 shows a dimeric structure, where the two Zn dications have slightly distorted octahedral geometry and the pyridinolate O atom of the neighbouring complex bridges them. The coordination spheres of the Zn dications and the supramolecular structures are discussed in detail. The packing arrangements of 1-3 are similar, having alternating hydrophilic and hydrophobic layers, however the similarity is broken in 4. The obtained coordination geometries are compared with their previously determined Cu analogues. The study of the individual complexes is complemented with a comprehensive analysis of Zn complexes with oxygen donor ligands with data from the Cambridge Structural Database.