Abstract
Substitution of methanol in [Zn(quin)2(CH3OH)2] (quin− denotes an anionic form of quinoline-2-carboxylic acid, also known as quinaldinic acid) with pyridine (Py) or its substituted derivatives, 3,5-lutidine (3,5-Lut), nicotinamide (Nia), 3-hydroxypyridine (3-Py-OH), 3-hydroxymethylpyridine (3-Hmpy), 4-hydroxypyridine (4-Py-OH) and 4-hydroxymethylpyridine (4-Hmpy), afforded a series of novel heteroleptic complexes with compositions [Zn(quin)2(Py)2] (1), [Zn(quin)2(3,5-Lut)2] (2), [Zn(quin)2(Nia)2]·2CH3CN (3), [Zn(quin)2(3-Py-OH)2] (4), [Zn(quin)2(3-Hmpy)2] (5), [Zn(quin)2(4-Pyridone)] (6) (4-Pyridone = a keto tautomer of 4-hydroxypyridine), and [Zn(quin)2(4-Hmpy)2] (7). In all reactions, the {Zn(quin)2} structural fragment with quinaldinate ions bound in a bidentate chelating manner retained its structural integrity. With the exception of [Zn(quin)2(4-Pyridone)] (6), all complexes feature a six-numbered coordination environment of metal ion that may be described as a distorted octahedron. The arrangement of ligands is trans. The coordination sphere of zinc(II) in the 4-pyridone complex consists of only three ligands, two quinaldinates, and one secondary ligand. The metal ion thereby attains a five-numbered coordination environment that is best described as a distorted square-pyramid (τ parameter equals 0.39). The influence of substituents on the pyridine-based ligand over intermolecular interactions in the solid state is investigated. Since pyridine and 3,5-lutidine are not able to form hydrogen-bonding interactions, the solid state structures of their complexes, [Zn(quin)2(Py)2] (1) and [Zn(quin)2(3,5-Lut)2] (2), are governed by π···π stacking, C–H∙∙∙π, and C–H∙∙∙O intermolecular interactions. With other pyridine ligands possessing amide or hydroxyl functional groups, the connectivity patterns in the crystal structures of their complexes are governed by hydrogen bonding interactions. Thermal decomposition studies of novel complexes have shown the formation of zinc oxide as the end product.
Highlights
The main interest in coordination chemistry of zinc finds its origin in its biological importance.In 1940, carbonic anhydrase II was discovered by Keilin and Mann as the first zinc-containing metalloenzyme [1]
A straightforward synthesis of heteroleptic Zn(II) complexes with quinaldinate and a secondary pyridine ligand was based on a facile substitution of labile methanol ligands in [Zn(quin)2 (CH3 OH)2 ]
Hydroxypyridines are prone to enol-ketonic tautomerism which strongly affects their nature, including their ability to function as ligands [44,45]
Summary
The main interest in coordination chemistry of zinc finds its origin in its biological importance. In 1940, carbonic anhydrase II was discovered by Keilin and Mann as the first zinc-containing metalloenzyme [1]. The enzyme plays a key role in the transformation of carbon dioxide into bicarbonate in blood or in the reverse reaction in lungs. Over 1000 zinc metalloenzymes, covering all classes of enzymes, have been discovered [2,3,4,5,6]. The prevalent use of zinc in biological systems is due to its unique properties [3,5,7,8,9,10,11,12,13,14]. Zinc(II) ion is characterized by a filled d subshell
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