Abstract
The coordination properties of four hydroxypyridinecarboxylates, designed for the treatment of iron-overloading conditions as bidentate O,O'-donor ligands, have been studied with ZnII in the solid state. The coordination compounds [Zn(A1)2(H2O)2] (1), [Zn(A2)2(H2O)] (2), [Zn(A3)2(H2O)]·2H2O (3) and [Zn2(B1)4(H2O)2]·4H2O (4), where the ligands are 1-methyl-4-oxidopyridinium-3-carboxylate (A1, C7H6NO3), 1,6-dimethyl-4-oxidopyridinium-3-carboxylate (A2, C8H8NO3), 1,5-dimethyl-4-oxido-pyridinium-3-carboxylate (A3, C8H8NO3) and 1-methyl-3-oxidopyridinium-4-carboxylate (B1, C7H6NO3), 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 ZnII complexes. Ligand B1 shows a dimeric structure, where the two Zn2+ dications have slightly distorted octahedral geometry and the pyridinolate O atom of the neighbouring complex bridges them. The coordination spheres of the Zn2+ 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 CuII analogues. The study of the individual complexes is complemented with a comprehensive analysis of ZnII complexes with oxygen donor ligands with data from the Cambridge Structural Database.
Highlights
Hydroxypyridinecarboxylic acid (HPC) derivatives have been considered (Di Marco et al, 2002; Crisponi et al, 2013; Sija et al, 2014; Dean et al, 2018) as potential chelating agents for the treatment of iron-overloading conditions
Solution equilibrium studies and density functional theory (DFT) calculations revealed a significant difference in the electronegativity of the donor carboxylate and hydroxy O atoms
Solution speciation of ligand A3 (DQ715, 1,5dimethyl-4-oxidopyridinium-3-carboxylate) with CuII and ZnII has been reported previously (Sija et al, 2014) and it was found that A3 forms only mononuclear complexes with ZnII, i.e. [ZnL] and [ZnL2], and the stabilities of the formed complexes are lower compared to their CuII analogues
Summary
Hydroxypyridinecarboxylic acid (HPC) derivatives have been considered (Di Marco et al, 2002; Crisponi et al, 2013; Sija et al, 2014; Dean et al, 2018) as potential chelating agents for the treatment of iron-overloading conditions. With these divalent metal ions the stability of the obtained complex is significantly lower than the stability with FeIII or AlIII, which makes HPCs good candidates as FeIII or AlIII chelators Following on from this previous work, we report here our solid-state studies on the complexation to ZnII of four HPC ligands, namely, 1-methyl-4-oxidopyridinium-3-carboxylate (A1), 1,6-dimethyl-4-oxidopyridinium-3-carboxylate (A2), 1,5-dimethyl-4-oxido-pyridinium-3-carboxylate (A3) and 1-methyl3-oxidopyridinium-4-carboxylate (B1) (Fig. 1), by singlecrystal X-ray diffraction. Our structural comparison of CuII and ZnII complexes containing the same and related ligands revealed the structural features originating from (i) the steric and electronic effects of the ligands themselves, (ii) the geometrical preferences of the metal ions and (iii) the intermolecular forces between the molecules in the crystals These are important aspects of the goals of inorganic crystal engineering (ICE), where coordination bonds connect metal ions and organic building blocks to each other. ICE aims at a better understanding of structure-directing effects in order to find strategies to control molecular self-assembly (Biradha et al, 2011; Desiraju, 2003)
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