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(II) in the solid state. The coordination com­pounds [Zn(A1)(2)(H(2)O)(2)] (1), [Zn(A2)(2)(H(2)O)] (2), [Zn(A3)(2)(H(2)O)]·2H(2)O (3) and [Zn(2)(B1)(4)(H(2)O)(2)]·4H(2)O (4), where the ligands are 1-methyl-4-oxidopyridinium-3-­carboxyl­ate (A1, C(7)H(6)NO(3)), 1,6-di­methyl-4-oxidopyridinium-3-carboxyl­ate (A2, C(8)H(8)NO(3)), 1,5-di­methyl-4-oxido-pyridinium-3-carboxyl­ate (A3, C(8)H(8)NO(3)) and 1-methyl-3-oxidopyridinium-4-carboxyl­ate (B1, C(7)H(6)NO(3)), 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(II) com­plexes. Ligand B1 shows a dimeric structure, where the two Zn(2+) dications have slightly distorted octahedral geometry and the pyridino­late O atom of the neighbouring com­plex bridges them. The coordination spheres of the Zn(2+) 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(II) analogues. The study of the individual com­plexes is com­plemented with a com­prehensive analysis of Zn(II) com­plexes with oxygen donor ligands with data from the Cambridge Structural Database.