Crystal Chemistry of Zinc Quinaldinate Complexes with Pyridine-Based Ligands

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.


Introduction
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.Since then, 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.Because of the latter, it lacks any redox activity which makes it an ideal metal cofactor for reactions that require a redox-stable center.The function of the ion is structural or that of a Lewis acid-type catalyst [15].Since it does not act consistently either as a soft or as a hard Lewis acid, it presents a borderline case with no strong preference for coordination of either oxygen-, nitrogen-or sulfur-donor ligands [16].Another important property, which is again a direct consequence of the filled d subshell, is its ligand-field stabilization energy which is zero in all coordination environments [17].Although no geometry is inherently more stable than another, the structurally characterized zinc enzymes display in most cases a distorted tetrahedral coordination environment of the metal ion [18,19].Studies of synthetic analogues of enzymes, i.e., model compounds, could help in understanding how the immediate environment of the metal ion in an enzyme modulates its chemistry [20][21][22].
Quinaldinic acid (IUPAC name: quinoline-2-carboxylic acid), shown in Scheme 1, is a structural analogue of pyridine-2-carboxylic acid (known also as picolinic acid) which has found a wide use in the coordination chemistry of many transition metals.Upon giving off their protons, both acids form anions which possess two functional groups, a quinoline/pyridine nitrogen and a carboxylate, that can bind to transition metal cations.Although the recent literature abounds with structural data on picolinate complexes [23][24][25][26][27], the quinaldinate complexes remain relatively rare [28][29][30][31][32].The quinaldinate ion can coordinate to metal ions in several ways (Scheme 2).A recent survey of the structurally characterized zinc(II) complexes with quinaldinate produced altogether six hits [33].In all, the quinaldinate ion adopts the N,O-chelating binding mode through nitrogen and carboxylate oxygen, depicted as a in Scheme 2 and labelled as 1.101 in Harris notation [34].One of the complexes, [Zn(quin) 2 (CH 3 OH) 2 ] [35], was chosen for its labile methanol ligands to serve as an entry into the coordination chemistry of heteroleptic complexes that would contain apart from quinaldinate also a pyridine-based ligand.The pyridine ligands, used in this study (shown in Scheme 3), differ in the nature of their substituents and their relative positions on the ring.The ability of chosen ligands to participate in hydrogen-bonding is markedly different.The pyridine ligand is thus expected to have a crucial impact over strength and directionality of intermolecular interactions among complex molecules in solid state structures.Herein, we present the syntheses and the crystal structures of a series of zinc(II) complexes with quinaldinate and pyridine-based ligands.Reactions of methanol complex with pyridine-based ligands afforded [Zn(quin) 2 (Py) 2 ] (1), [Zn(quin) 2 (

General
All manipulations and procedures were conducted in air.With the exception of acetonitrile, chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received.Acetonitrile was dried over molecular sieves, following the published procedure [36].The IR spectra were measured on solid samples using a Perkin Elmer Spectrum 100 series FT-IR spectrometer (PerkinElmer, Shelton, CT, USA), equipped with ATR.Elemental analyses were performed by the Chemistry Department service at the University of Ljubljana. 1 H NMR spectra of DMSO-d 6 solutions were recorded on Bruker Avance DPX 300 MHz and/or Bruker Avance III 500 MHz NMR instruments (Bruker BioSpin GmbH, Rheinstetten, Germany).The chemical shifts (δ) were referenced to tetramethylsilane.The choice of dimethyl sulfoxide (DMSO) as a solvent was governed by a meagre solubility of complexes in other solvents.Upon dissolving, a substitution of pyridine-based ligands with DMSO molecules in the zinc(II) coordination sphere took place in all cases.From the solutions in NMR tubes, a complex of zinc(II) with DMSO crystallized as large, block-like crystals of a light yellow color.Their cell dimensions were the same as those of the known compound, [Zn(quin) 2 (DMSO) 2 ]•2DMSO [37].The NMR spectra of compounds are thus the spectra of the DMSO complex and free pyridine-based ligands.Two representative spectra are given in Supplementary Materials.Thermal analyses were performed on a Mettler Toledo TG/DSC 1 instrument (Mettler Toledo, Schwerzenbach, Switzerland) in argon or air atmosphere at a 50 mL/min gas flow.Masses of the samples were in the 4-32 mg range.Samples in platinum crucibles were heated from room temperature to 800 • C with a heating rate of 5 K/min.In each case, the baseline was subtracted.The solid residue, a mixture of grey amorphous material and zinc oxide (zincite modification), was identified by X-ray powder diffraction (XRPD).XRPD data were collected with a PANalytical X'Pert PRO MD diffractometer (PANALYTICAL, Almelo, The Netherlands) by using Cu-Kα radiation (λ = 1.5406Å).

Preparation Procedures
Preparation of [Zn(quin) 2 (CH 3 OH) 2 ].A teflon container was loaded with zinc(II) acetate dihydrate (110 mg, 0.50 mmol) and quinaldinic acid (173 mg, 1.00 mmol).Methanol (15 mL) was added.The container was closed and inserted into a steel autoclave.The autoclave was heated for 24 h at 105 • C. The reaction vessel was then allowed to cool slowly to room temperature.Large colorless crystals of [Zn(quin) 2 (CH 3 OH) 2 ] were collected by filtration.The mass of the dried product was 100 mg.Yield: 0.21 mmol, 42%.Note.The crystals were found to lose their lustre after a prolonged period.
Preparation of [Zn(quin) 2 (Py) 2 ] (1).A teflon container was loaded with [Zn(quin) 2 (CH 3 OH) 2 ] (50 mg, 0.11 mmol), pyridine (2 mL), and dried acetonitrile (7.5 mL).The container was closed and inserted into a steel autoclave.The autoclave was heated for 24 h at 105 • C. The reaction vessel was then allowed to cool slowly to room temperature.Large colorless crystals of 1 were collected by filtration.The mass of the dried product was 41 mg.Yield: 0.072 mmol, 66% Preparation of [Zn(quin) 2 (3-Py-OH) 2 ] (4).A teflon container was loaded with [Zn(quin) 2 (CH 3 OH) 2 ] (50 mg, 0.11 mmol), 3-hydroxypyridine (150 mg, 1.58 mmol), and dried acetonitrile (10 mL).The container was closed and inserted into a steel autoclave.The autoclave was heated for 24 h at 105 • C. The reaction vessel was then allowed to cool slowly to room temperature.The solid consisted mostly of the unreacted ligand (large orange-colored crystals), [Zn(quin) 2 (H 2 O)] (colorless, crystalline solid) and [Zn(quin) 2 (3-Py-OH) 2 ] (4) (a few block-like colorless crystals).The identity of the orange crystalline material was determined by its infrared spectrum and 1 H NMR which were identical to the spectra of pure ligand.The aqua complex was identified by its infrared spectrum.Repeated attempts to improve the yield were not met with success.

X-ray Structure Determinations
Each crystal was mounted on the tip of a glass fiber with a small amount of silicon grease and transferred to a goniometer head.Data were collected on an Agilent SuperNova diffractometer (Agilent Technologies XRD Products, Oxfordshire, UK), using graphite-monochromatized Mo-Kα radiation at 150 K. Data reduction and integration were performed with the software package CrysAlis PRO [38].Corrections for the absorption (multi-scan) were made in all cases.The coordinates of the majority of non-hydrogen atoms were found via direct methods using the structure solution program SHELXS [39].The positions of the remaining non-hydrogen atoms were located by use of a combination of least-squares refinement and difference Fourier maps in the SHELXL-97 program [40].The positions of NH, NH 2 , and OH hydrogen atoms in 3-7 were unambiguously located from the residual electron density maps.All other hydrogen atoms were placed in geometrically calculated positions and refined using a riding model.A summary of crystal data and refinement parameters for 1-7 is given in Table 1.Figures depicting the structures were prepared by ORTEP3 [41], Mercury [42], and CrystalMaker [43].

Synthetic Aspects
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 (CH 3 OH) 2 ].The {Zn(quin) 2 } core with quinaldinates bound in a chelating manner was not expected to undergo any significant structural changes.With the exception of the hydroxypyridine ligands, the proposed strategy resulted in novel complexes in reasonable yields.An excess of pyridine ligand was required to avoid the coordination of water to the zinc ion.The amount of water remaining in the solvent after drying was sufficient for the formation of an aqua complex.The reaction outcomes of the 3or 4-hydroxypyridine systems were not as anticipated.Hydroxypyridines are prone to enol-ketonic tautomerism which strongly affects their nature, including their ability to function as ligands [44,45].For 4-hydroxypyridine, the keto form is a dominant one in the solid state.The product isolated from the 4-Py-OH reaction mixture is [Zn(quin) 2 (4-Pyridone)] ( 6), a complex with pyridone.It is to be noted that for its isolation the reaction mixture had to contain a stoichiometric amount of triethylamine.In the absence of base, no complex of zinc(II) with any form of 4-Py-OH was obtained.Following literature examples [46,47], the amine should assist in deprotonation of hydroxypyridine with the resulting anionic species engaging both donor sites in coordination.In the case of 3-Py-OH, the ligand in the solid state is known to consist of a mixture of both the neutral and the bipolar forms connected by strong intermolecular hydrogen bonds [48].The formation of zwitterionic species is associated mainly with the acid-base properties of 3-Py-OH.Upon the protonation of nitrogen, the ligand's tendency towards ligation of metal ions is highly diminished.A negligible yield of [Zn(quin) 2 (3-Py-OH) 2 ] (4) speaks in favor of the zwitterion being a major contributor also in the reaction system described in this paper.

Description of Structures
The title zinc(II) quinaldinate complexes with pyridine ligands are neutral.The first coordination sphere of zinc(II) ion consists in all but a 4-pyridone complex 6 of two quinaldinates and two pyridine-based ligands in an all trans arrangement.The N 4 O 2 donor set renders a distorted octahedral environment around the zinc(II) ion.Because of the similarity of complexes, their molecular structures will be described together.They crystallize in centrosymmetric space groups with zinc ions located on the inversion centers.The coordination molecules possess an inherent crystallographic symmetry which places them in C i point group.In each case, the asymmetric unit consists of a half of a complex molecule.The Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawings of [Zn(quin) 2 (Py     Its non-planarity may be described by a torsion angle involving four of its atoms.The largest value, i.e., 13.6(3) • , calculated for 3-hydroxypyridine complex [Zn(quin) 2 (3-Py-OH) 2 ] (4), corresponds to the least planar ring.As a consequence, the carboxylate functional group is not coplanar with the quinoline plane.Its non-coordinated or free oxygen atom points out from the aromatic plane.The disruption of planarity can be measured by a dihedral angle formed between the quinoline plane and the carboxylate plane.The largest value was observed again in the 3-Py-OH complex 4, whereas the 4-Hmpy complex 7 shows the smallest deviation from planarity.A twist of carboxylate with respect to the quinoline ring was noted previously for a related compound, [Zn(quin) 2 (1-methylimidazole) 2 ] [49].The distortion was attributed to the involvement of free carboxylate oxygen in intermolecular interactions.As will be shown presently, the free carboxylate oxygen atoms in compounds 1-5 and 7 also participate in intermolecular interactions.For each complex in the homologous series, the extent of distortion was compared to the strength of intermolecular interactions.It turns out that obvious correlation exists only for [Zn(quin) 2 (3-Py-OH) 2 ] (4) where the greatest deviation corresponds to the shortest and thus the strongest interaction, i.All known zinc(II) quinaldinate complexes possess quinaldinato ligands coordinated in the N,O-chelating fashion.
Not only the composition but also the arrangement of ligands in three of these compounds is similar to compounds 1-5 and 7: the mutual arrangement of ligands in [Zn(quin) 2 (1-methylimidazole) 2 ] [49], [Zn(quin) 2 (CH 3 OH) 2 ] [35], and [Zn(quin) 2 (DMSO) 2 ]•2DMSO [37] is trans.In contrast to the above examples, the arrangement of ligands in [Zn(quin) 2 (Him) 2 ], a complex with imidazole, is cis [52].As can be seen from Tables 2 and 3 6) with the τ value of 0.39.An appropriate description of the coordination polyhedron of the 4-pyridone complex is thus that of a distorted square pyramid with pyridone oxygen located at the apical site.In 6, the zinc(II) ion lies ca.0.5 Å above the basal plane, defined by the quinaldinate donor atoms.The similarity of bonding parameters of 4-pyridone complex 6 and aqua complex is obvious.Both display significantly shorter bonds than complexes with six donor atoms.This adds validity to the rule that with a smaller number of ligands, the bonds in the immediate vicinity of the metal ion are shorter. 1The complex has no inherent crystallographic symmetry.Two sets of zinc-to-quinaldinate bond distances are given thereby. 2 Not relevant to this study.
Surprisingly, in spite of the commercial availability of pyridine-based ligands, their zinc complexes with the exception of pyridine itself and nicotinamide are extremely rare.The scarce zinc(II) complexes with the enol form of 3-hydroxypyridine serving as a ligand invariably display coordination through a nitrogen atom.A single 3-Py-OH ligand in a penta-coordinated zinc(II) complex with the [Zn(C 4 H 8 NOS 2 ) 2 ( 4) is yet another manifestation of the influence of the number of ligands over the bonding pattern (see above).The keto form of 3-hydroxypyridine can also coordinate to zinc(II).In [Zn 2 (N 3 ) 4 (3-Pyridone)]•2H 2 O, an azide complex with a two-dimensional layer structure, 3-pyridone is coordinated via an oxygen atom to two metal centers with Zn-O distances of 2.106(4) Å [57].Furthermore, the literature reports on a zinc complex with the anionic form of 3-Py-OH, i.e., a 3-pyridinolato ion.3-pyridinolate whose negative charge is localized on oxygen links a zinc porphyrin moiety with a boron subphthalocyanine [46].The anion is coordinated to zinc via its pyridine nitrogen at a distance of 2.155(3) Å.Similar compounds are known also for 4-pyridinolato ion [46,47] 15) Å [59].It is pertinent to note that no complex of zinc(II) with the enol form of 4-Py-OH has been reported so far [34].When binding to zinc(II), 3-hydroxymethylpyridine employs two coordination modes.A monodentate coordination via pyridine nitrogen was observed in a heptametallic zinc cluster with Zn-N distances of 2.042(3) Å [60], whereas a polymeric zinc complex with benzoate features a bidentate bridging coordination via both functional groups with Zn-N distance of 2.141(1) Å and Zn-O distance of 2.204(1) Å [61].
In contrast to other pyridine ligands used in this study, pyridine itself and 3,5-lutidine possess no functional groups that could participate in hydrogen-bonding interactions.Therefore only weak intermolecular interactions may be observed in the solid state structures of [Zn(quin) 2 (Py) 2 ] (1) and [Zn(quin) 2 (3,5-Lut) 2 ] (2).In the pyridine complex, C-H•••O contacts, i.e., C(21 y + 0.5, −z + 0.5] = 3.106(5) Å, which occur between pyridine carbon atoms and a non-coordinated or free carboxylate oxygen link molecules into layers that are coplanar with the bc plane.The arrangement of complex molecules within layers is such that a π•••π stacking interaction occurs between pairs of quinaldinate rings: a pyridine part of one quinaldinate makes a close approach to an arene part of another.With quinaldinate ligands two other types of π•••π stacking interactions are possible: an interaction of two arene moieties or an interaction of two pyridine moieties.According to Janiak [63], there is a prevalent occurrence of pyridine•••arene interactions.The relevant parameters of this interaction in 1 are: centroid-centroid distance = 3.593( 2 S2).
The solid state structures of other compounds are governed by stronger interactions.Following the general rule, all good hydrogen-bond donors and acceptors are engaged in hydrogen-bonding [64].Hydrogen-bonding interactions are summed up in Table 4.For a complete list of weaker intermolecular interactions see Tables S1-S7.A well-known synthon [65] in supramolecular chemistry may be recognized in the structure of [Zn(quin) 2 (Nia) 2 ]•2CH 3 CN (3), i.e., an amide•••amide homosynthon with the N-H•••O(carbonyl group) distance of 2.975(3) Å (Figure 5).

Compound Functions, Engaged in an Interaction
1 If not stated otherwise, COO − refers to a non-coordinated carboxylate oxygen atom. 2 For atom labels, see respective figures. 3The distances may be compared to the corresponding sums of the van der Waals radii, 3.07 Å for N+O, and 3.04 Å for O+O [66]. 4 A short contact between a coordinated oxygen of one COO − and a non-coordinated oxygen from another.See text.In all structures, the hydroxyl moiety is engaged in a strong hydrogen bond with a non-coordinated carboxylate oxygen with the shortest bond occurring in 4, a complex with 3-hydroxypyridine (Table 4).In 4, each molecule forms four O-H•••O interactions with four neighboring molecules.As a result, layers that are coplanar with the [1 0 −1] plane are formed (Figure 7).

Infrared Spectroscopy
The most relevant features of the infrared spectra of the title compounds concern the carbon-oxygen frequencies of the quinaldinato ligand and the characteristic absorptions of the pyridine-based ligands.A pronounced similarity in the behavior of pyridine, 3,5-lutidine, 3-and 4-hydroxymethylpyridine complexes upon heating to 800 • C in argon was observed.In all, two well-resolved regions of major mass loss may be observed.Considering the higher volatility of pyridine-based ligands, the first step is due to their liberation from the zinc(II) coordination sphere, whereas the second one is governed by the pyrolysis of quinaldinate with the subsequent formation of zinc oxide.It is to be noted that the thermal stability of pyridine and 3,5-lutidine complexes is very similar.As will be shown, their decomposition starts at a 20-30 • C lower temperature than in other complexes.A narrower thermal stability window of the pair can be traced to the strength of intermolecular interactions in their solid state structures.The thermogravimetric (TG) curve of the pyridine complex, [Zn(quin) 2 (Py) 2 ] (1), is shown in Figure 13, the TG curves of other compounds are given in Supplementary Materials.The first decomposition process of 1 starts at 155 • C and continues up to 230 • C. The accompanying mass loss is in excellent agreement with the release of pyridine ligands, calcd./found:27.86%/27.90%.The residue {Zn(quin) 2 } is stable until about 360 • C which marks the onset of the second decomposition process.The latter lasts up to ca. 570 • C and brings about a 49.12% decrease in mass.As shown by the differential scanning calorimetry (DSC) curve, both processes are endothermic.The final residual represents 28.97% of the initial mass.As confirmed by the powder X-ray diffraction, the resulting grey solid is a mixture of zinc oxide (calcd.14.33%) with an amorphous material.The liberation of 3,5-lutidine ligands starts at 165 • C and continues up to 260 • C, calcd./found:34.34%/34.31%.The second decomposition process occurs in the 330-500 • C interval with the 36.97%loss.The decomposition of the 4-Hmpy complex 7 starts at a slightly higher temperature, ca.195 • C, and lasts up to 295 • C with the observed loss differing from the theoretical value for the release of 4-Hmpy molecules, calcd./found:34.73%/29.16%.The first decomposition stage is immediately followed by the second which is completed at ca. 570 • C and is accompanied by the 39.91% mass loss.The TG curve of 3-Hmpy complex 5 reveals a small, yet a non-negligible reduction of mass, ca.1-2%, in the 25-125 • C interval that is probably due to the solvent associated with the sample.The first major decomposition process in the 185-225 • C interval is followed by the second in the 230-290 • C interval with mass losses of 18.49% and 17.27%, respectively.Their sum is consistent with the liberation of 3-Hmpy molecules, calcd./found:34.76%/35.76%.The final decomposition commences at 350 • C and lasts up to 570 • C. The inspection of thermal properties of [Zn(quin) 2 (3-Hmpy) 2 ] (5) in the oxidizing atmosphere revealed a similar behavior with two major differences: (i) the onset of all processes is at lower temperatures, and (ii) the final decomposition stage is highly exothermic.A mass loss of 1.45% may be observed in the 80-125 • C interval, followed by a two-step dissociation of 3-Hmpy ligands in the 165-275 • C interval with the total loss amounting to 34.20%.The overall decomposition with the formation of zinc oxide starts at 290 • C and is completed by 490 • C, calcd./found:12.96%/12.38%.
The TG curve of [Zn(quin) 2 (4-Pyridone)] ( 6), obtained in argon atmosphere, differs from other studied compounds.It shows three distinct regions of mass loss.The first one is completed by 140 • C and accounts for ca.7.5% of the initial mass.It may be ascribed to the solvent content, present in the sample.The latter could not be verified from the analytical data.The second one in the 215-335 • C interval is due to the 4-pyridone elimination, calcd./found:18.84%/19.37%.The last one commences at 340 • C. The degradation of the compound continues at higher temperature without completion.
e., a hydrogen bond of the O-H•••COO − type.Contrary to expectations, [Zn(quin) 2 (Py) 2 ] (1), which lacks strong intermolecular interactions, still displays a non-negligible distortion.The 4-Pyridone complex [Zn(quin) 2 (4-Pyridone)] (6) differs from other members of the series.Its ORTEP drawing is presented in Figure 4.The first coordination sphere of zinc(II) consists of two quinaldinates and a single 4-pyridone ligand, rendering the N 2 O 3 donor set.The pyridone ligand has coordinated to zinc(II) through the phenoxide oxygen atom.The molecule possesses no intrinsic crystallographic symmetry.The pair of quinaldinate ions, bound in the usual N,O-chelating manner with bite angles of 79.23(5) and 79.85(6) • , is no longer nearly coplanar as in the case of the other six complexes.In the absence of the fourth ligand, the quinaldinates bend away from the equatorial plane.The value of a dihedral angle between the quinaldinates amounts to 52.65(4) • .Furthermore, the zinc-to-quinaldinate bonds in 6 are significantly shorter than in the other six complexes.The orientation of pyridone is skewed with respect to the Zn-O vector, an angle of 136.40(6)  • is formed between the latter and the O-N pyridone axis.The H•••Zn distance is just below the 3.2 Å cut-off, the limit in considering the pseudo agostic H•••M (M = transition metal cation) bonds[50,51].Nevertheless, in complex 6 it is more the ligand structure that holds the C-H bond close to the metal than the attraction of C-H to the metal ion.
The former are often used to diagnose the carboxylate binding mode [70,71].In the case of a monodentate binding mode of the carboxylate, as in title compounds, a large difference in the positions of the ν asym (COO) and ν sym (COO) bands, known also as the splitting value ∆, may be observed.The title complexes display the splitting values ∆ in the range 261-299 cm −1 .It is to be noted that values closer to 299 cm −1 were determined for [Zn(quin) 2 (Py) 2 ] (1) and [Zn(quin) 2 (3,5-Lut) 2 ] (2).Although in both compounds the free carboxylate oxygen participates in weak C-H•••O interactions, the pair lacks stronger intermolecular interactions, i.e., hydrogen bonds.On the other hand, the spectra of the nicotinamide, 3-hydroxylmethylpyridine, 4-pyridone and 4-hydroxymethylpyridine complexes, where the carboxylate oxygen is engaged in hydrogen bonds, show values that are closer to 260 cm −1 .Our results are in line with the known influence of the strength of intermolecular interactions over the carboxylate splitting value ∆ [52].The positions of the ν asym (COO) and ν sym (COO) bands, i.e., 1654-1624 and 1368-1351 cm −1 , respectively, agree well with the data observed for related complexes, [Zn(quin) 2 (1-methylimidazole) 2 ] [1642 and 1366 cm −1 ] and [Zn(quin) 2 (H 2 O)] [1630 and 1390 cm −1 ] [49,54].Pyridine-based ligands, i.e., nicotinamide, 3-hydroxylmethylpyridine, and 4-hydroxymethy lpyridine, have functional groups that can be unambiguously identified by infrared spectroscopy [72].A strong band at 1692 cm −1 in the spectrum of [Zn(quin) 2 (Nia) 2 ]•2CH 3 CN (3) may be assigned to the stretching vibration of the C=O moiety of the amide, whereas the absorptions due to the asymmetric and symmetric stretching vibrations of the NH 2 moiety occur as broad bands at 3292 and 3147 cm −1 .Absorptions of the alcohol C-O stretching vibrations appear in the spectra of [Zn(quin) 2 (3-Hmpy) 2 ] (5) and [Zn(quin) 2 (4-Hmpy) 2 ] (7) in the 1095-1010 cm −1 region.Two or three very intense bands may be observed.Their positions are like the ones found in the spectra of pure ligands.The ν(O-H) absorption of the hydroxymethylpyridine ligands may be seen in the spectra of [Zn(quin) 2 (3-Hmpy) 2 ] (5) and [Zn(quin) 2 (4-Hmpy) 2 ] (7) as a sharp, medium intensity band at 3367 or 3287 cm −1 , respectively.In the spectra of pure hydroxymethylpyridines this band is broad.A change of the shape of the ν(O-H) band in the spectra of complexes, as compared to that of pure ligands, results from a more organized hydrogen-bonding pattern involving the OH moiety in the solid state structures of complexes 5 and 7, i.e., hydrogen bonds of the OH•••COO − type and of moderate strength (Table 4).The absence of the ν(O-H) absorption in the spectrum of [Zn(quin) 2 (4-Pyridone)] (6) confirms the keto form of the ligand.
• C. The reaction vessel was then allowed to cool slowly to room temperature.Large block-like crystals of 5 of a gold yellow color were collected by filtration.Mass of the dried product was 43 mg.