Influence of noncovalent interactions on the structures of metal-organic hybrids based on a (VO2(2,6-pydc)) tecton with cations of imidazole, pyridine and its derivatives†

Seven different dioxido(pyridine-2,6-dicarboxylato)vanadate(V) compounds with pyridinium (Hpy+) (1·2H2O and 1), 2-hydroxypyridinium (H2pyon+) (2·H2O), 4-aminopyridinium (H4apy+) (3·H2O and 3), 4-(dimethylamino)pyridinium (Hdmap+) (4·H2O) and imidazolium (Himd+) (5) cations have been prepared via different pathways starting either from pyridine-2,6-dicarboxylic acid or its esters, and were structurally characterized by single-crystal X-ray diffraction. The vanadium metal center in dioxido(pyridine-2,6-dicarboxylato)vanadate(V) anion is pentacoordinated in all of the compounds: having two oxido oxygen atoms in a mutual cis position and a tridentate pyridine-2,6-dicarboxylic ligand. Study of hydrogen bonds and weak interactions in the compounds revealed the relationship between the type of cation and the hydrogen bonding network in the compounds. While in 1·2H2O, 2·H2O and 4·H2O a one-dimensional (band, pillar or chain) hydrogen bonding network via N/O–H⋯O bonds is preferred, anhydrous 3 and 3·H2O favor a two-dimensional hydrogen-bonded framework, and the Himd+ cation facilitates a three-dimensional hydrogen bonding in 5. The unique vanadium coordination environment with two easily accessible oxido oxygen atoms of the VO2+ unit is suitable for the construction of non-covalent metal–organic hybrids. In 2·H2O, 3·H2O, 4·H2O and 5 both oxido oxygen atoms of the VO2+ unit participate as acceptors, however, in 1·2H2O and 3 only one oxido oxygen atom is involved in classical hydrogen bonding. Besides N/O–H⋯O hydrogen bonding, also other weak non-covalent interactions, such as C–H⋯O, π⋯π and C–H⋯π interactions, play an important role in stabilizing the crystal lattices.


Introduction
The self-assembly of tectons into extended network structures is the core of supramolecular chemistry and crystal engineering.Metal-organic hybrids and especially metal-organic frameworks are currently an extremely important topic and an active area of research because of their intriguing architectures and topologies, 1 as well as due to their potential application in catalysis, 2 chemical separation processes, 3 gas storage, 4 magnetism 5 and as sensors. 6etal-organic hybrids can be assembled by a covalent approach using bridging ligands or by a non-covalent approach using hydrogen bonding and other weak interactions.The covalent approach is primarily based on strong coordinate bonds connecting metal cations and organic ligands into robust polymeric structures.Different kinds of these materials have been designed with special attention dedicated to the geometry of the metal ions as well as flexibility, bridging potential and coordination preferences of different organic linkers. 1 In the non-covalent approach much weaker forces, such as hydrogen bonding, C-HÁ Á Áp/F interactions, pÁ Á Áp stacking, and halogen bonding, are employed.Although weak by nature multiple noncovalent forces can adjust the dimensionality and enable new topologies to arise.Therefore, the desired functions of supramolecular assemblies can be achieved. 7-10Among non-covalent interactions the hydrogen bonding is a particularly powerful building motif used in crystal engineering since it provides unique directionality and can be easily introduced into structures.There exists a great variety of hydrogen bonding donors-acceptors and their numbers can be varied through simple design, thus making them a particularly good choice for the construction of selfassemblies.With this approach a wide variety of mononuclear, dinuclear and polynuclear coordination compounds/ions can be assembled into desirable motifs.Multiple weak non-covalent interactions can even control the topology of metal-organic frameworks 11 as well as the coordination geometry. 12lthough great efforts have been made toward the understanding of the assembly process, rational control in the construction of supramolecular structures of complexes is still a challenging task.For the construction of a desired framework and functionality, it is important to control and understand the factors, such as counterions, solvents, temperature and pH value, that tend to influence the structural prediction on the assembly of the final coordination frameworks and govern the crystal growth and the stability of the overall crystals.1b,13 Among the above factors, it has been demonstrated that counterions have a significant effect on the formation of the product.Different coordination abilities, sizes, and geometries of anions have a great influence on the structural assembly and importantly influence the prediction of the overall supramolecular architectures of coordination compounds.1b,14 Cations can also have a significant influence on the crystal architecture.''Naked'' alkali cations are useful tectons due to their different sizes and polarisability, while in the case of NH 4 + and hydrated cations, besides their size and charge, the hydrogen bonding capacity can enable the formation of high-dimensional frameworks. 15Diverse effects can be introduced by organic cations, most commonly protonated cationic amines and pyridines.Organic cations can be varied through simple design and diverse functionalities can be combined.Coulombic interactions are a principal force for cation-anion arrangements in supramolecular structures. 16However, protonated organic cations can also act as multi-hydrogen bond donors as well as acceptors and thus easily adjust the topologies via additional non-covalent interactions.Charge-assisted or ionic hydrogen bonds are in general stronger hydrogen bonds since ionic charge on a donor or an acceptor enhances the hydrogen bond strength. 17Furthermore, employing organic cations enables introduction of additional weak noncovalent interactions, such as pÁ Á Áp stacking, C-HÁ Á Áp interactions and halogen bonding. 18ecently, the use of bioactive framework materials (BioMOFs) has gained considerable attention in biology and medicine.This has stimulated the search for new types of bioactive organic linkers capable of ligating biorelevant metal ions for the design of functional materials. 19,20Multidentate pyridinedicarboxylato ligands (pydc 2À ) have been widely used in recent years for the construction of organic-inorganic hybrid materials.Because they possess diverse coordination abilities, flexibilities and various bridging modes, supramolecular networks of high structural stability have been assembled either via coordination bonds, hydrogen bonds and/or aromatic interactions. 21Some biological activities of multidentate pyridinedicarboxylate have already been demonstrated, such as antimicrobial activity and DNA cleavage. 22he field of vanadium metal-organic hybrids started to grow considerably since the discovery of MIL-26 and MIL-47 in the early period of the last decade. 23We are especially interested in extending the knowledge of self-assembly of vanadium compounds because of their potential therapeutic application such as insulin-enhancing agents.An important advantage of vanadium compounds in the treatment of diabetes mellitus compared to insulin is the possibility of oral administration. 24 this study new supramolecular networks built by pyridine-2,6-dicarboxylic acid (2,6-H 2 pydc, dipicolinic acid) were generated containing various organic cations in order to evaluate the role of cations and anions in crystal engineering.Compared with other transition metal cations, vanadium ions possess distinctly different properties and coordination modes, which play key roles in the formation of both coordination structures and packing structures of complexes.For example, vanadium(V) compounds usually contain a typical cis-VO 2 + moiety with an overall five-or six-coordination.The VO 2 + as well as VO 2+ group can also participate in weak VQOÁ Á ÁC interactions as pointed out recently. 25In the studied [VO 2 (2,6-pydc)] À systems the VO 2 + moiety is additionally coordinated by one tridentate 2,6-pydc 2À ligand forming pentacoordinated geometry.While oxygen atoms being part of the anion allow the formation of numerous hydrogen bonds, the aromatic pyridine ring of 2,6-pydc 2À ligand enables additional pÁ Á Áp stacking interactions.

Materials and physical measurements
Reagents and chemicals were purchased as reagent grade from commercial sources and were used without any further purification.Diethyl pyridine-2,6-dicarboxylate ester was prepared according to a published procedure. 26Infrared (IR) spectra (4000-600 cm À1 ) of the samples were recorded using a Perkin-Elmer Spectrum 100, equipped with a Specac Golden Gate Diamond ATR as a solid sample support.Elemental (C, H, N) analysis was performed using a Perkin-Elmer 2400 Series II CHNS/O Elemental Analyzer.Powder X-ray diffraction patterns were collected on a Siemens D-5000 diffractometer with y-2y Bragg-Brentano geometry operating using Cu-K a radiation.

Preparation
Procedure A. General procedure for the synthesis of V(V) complexes 1Á2H 2 O and 2ÁH 2 O starting from pyridine-2,6dicarboxylic acid.To a mixture of sodium metavanadate (0.50 mmol, 97 mg) and pyridine-2,6-dicarboxylic acid (0.50 mmol, 84 mg) water (3 mL) was added and stirred for 10 min at room temperature to yield a yellow solution.Separately, py or 2hypy (0.50 mmol) was dissolved in a mixture of water (2 mL) and hydrochloric acid (2 M, 0.25 mL) and then slowly added to the vanadium solution.The mixture was stirred at 70 1C for 15 min, filtered and stored under room conditions.The solution was allowed to evaporate slowly at room temperature for a week, and crystals suitable for single crystal X-ray diffraction analysis were obtained.
Procedure B. General procedure for the synthesis of V(V) complexes 3ÁH 2 O, 4ÁH 2 O and 5 starting from pyridine-2,6dicarboxylic acid.Ammonium metavanadate (0.50 mmol, 59 mg) and pyridine-2,6-dicarboxylic acid (0.50 mmol, 84 mg) were dissolved in water (3 mL).To the yellow solution a solid nitrogen base (0.50 mmol) and methanol (3 mL) were added.The mixture was stirred at 70 1C for 30 min, filtered and stored under room conditions.The solution was allowed to evaporate slowly at room temperature for a few days, and crystals suitable for single crystal X-ray diffraction analysis were obtained.
Procedure C. General procedure for the synthesis of V(V) complexes 1, 3, 4ÁH 2 O and 5 starting from diethyl pyridine-2,6dicarboxylate ester.A mixture of diethyl pyridine-2-dicarboxylate (112 mg, 0.50 mmol) and sodium hydroxide (20 mg, 0.50 mmol)  in ethanol (6 mL) was added dropwise to an ethanol (2 mL) solution of VOSO 4 Á5H 2 O (63 mg, 0.25 mmol).After addition of a nitrogen base (0.25 mmol) and toluene (1 mL), the green mixture was stirred at 50 1C for 15 min.The solution was cooled to room temperature and filtered.It was allowed to evaporate slowly at room temperature for a few days, and crystals suitable for single crystal X-ray diffraction analysis were obtained.
Procedure D. Procedure for the synthesis of V(V) complex 2Á H 2 O starting from 6-((pyridin-2-yloxy)carbonyl)pyridine-2-carboxylic acid.A mixture of 6-((pyridin-2-yloxy)carbonyl)pyridine-2-carboxylic acid (49 mg, 0.20 mmol) and sodium acetate trihydrate (27 mg,  0.20 mmol) in methanol (3 mL) was added dropwise to a methanol (2 mL) solution of VOSO 4 Á5H 2 O (25 mg, 0.10 mmol).The resulting yellow to green mixture was stirred at 50 1C for 20 min, and then filtered and stored under room conditions.The solution was allowed to evaporate slowly at room temperature for a few days, and crystals suitable for single crystal X-ray diffraction analysis were obtained.

Crystallography
Single-crystal X-ray diffraction data were collected at room temperature (1, 3, 4ÁH 2 O, 5) or at 150 K (1Á2H 2 O, 2ÁH 2 O, 3Á H 2 O) either on a Nonius Kappa CCD Diffractometer or an Agilent Technologies SuperNova Dual diffractometer with an Atlas detector using monochromated Mo-K a radiation (l = 0 The data were processed using DENZO 28 or CrysAlis Pro. 29 The structures were solved by direct methods using the program SHELXS-97 30 32 (3ÁH 2 O), and refined on F 2 using full-matrix least-squares procedures (SHELXL-97). 30ll non-hydrogen atoms were refined anisotropically, except atoms of a minor part (34%) of disordered 4-aminopyridinium cation in 3ÁH 2 O.In 4ÁH 2 O the water molecule (O6) and the hydrogen atoms in the methyl groups of the Hdmap cation are disordered over the mirror plane in 0.50 : 0.50 ratio.Hydrogen atoms on aromatic rings, methyl, amino and hydroxyl groups were treated as riding atoms in geometrically idealized positions.Hydrogen atoms on water molecules were located from difference Fourier maps and refined by fixing the bond lengths and isotropic temperature factors as U iso (H) = 1.5U eq (O).Crystallographic data are summarized in Table 1.

Synthesis and spectroscopic properties
Seven structures of dioxido(pyridine-2,6-dicarboxylato)vanadate(V) salts with pyridinium (Hpy + ), 2-hydroxypyridinium (H2pyon + ), 4-aminopyridinium (H4apy + ), 4-(dimethylamino)pyridinium (Hdmap + ), and imidazolium (Himd + ) counterions were determined.Colourless to pale yellow vanadium(V) pyridine-2,6dicarboxylic complexes were isolated from aqueous or alcoholic mixtures of the ammonium or sodium metavanadate, pyridine-2,6-dicarboxylic acid (dipicolinic acid) and imidazole or pyridine analogues stirred at 70 1C for 15-30 minutes.After slow evaporation of solvents, crystals of 1Á2H 2 O, 2ÁH 2 O, 3ÁH 2 O, 4ÁH 2 O and 5 were grown from the solutions.We have also attempted to prepare vanadium(IV) complexes with the pyridine-2,6-dicarboxylato ligand.However, when combining vanadyl sulphate with esters of pyridine-2,6-dicarboxylic acid and the corresponding nitrogen base only vanadium(V) complexes were obtained in rather low yields.In all these cases vanadium(IV) turned into vanadium(V) and the same structures, i.e. 2ÁH 2 O and 4ÁH 2 O, 5 were obtained, except in the case of 4apy and py where the anhydrous 1 and 3 were formed.We have observed that monohydrate 3ÁH 2 O and dihydrate 1Á2H 2 O are unstable outside the solution.When exposed to air, crystals of 3ÁH 2 O decompose, lose the crystal water, and, with the respect to the elemental analysis and IR spectra, transform into anhydrous compound 3. Crystals of dihydrate 1Á2H 2 O also decompose, but lose only one equivalent of crystal water, as confirmed by elemental analysis.
The symmetric and asymmetric stretching vibrations of the VO 2 + moiety are observed in the range 951-907 cm À1 and have a shape of shoulder or are split into two or three bands.The position of these bands is similar to those in the NH 4 [VO 2 (2,6pydc)] complex. 33Strong characteristic bands of the carboxyl groups are observed in the range 1695-1656 cm À1 for the asymmetric vibrations (n as ) and 1342-1333 cm À1 for the symmetric vibrations (n s ).The difference between asymmetric and symmetric stretching vibrations (D = n as À n s ) of the carboxylate groups between 319 and 352 cm À1 is in accordance with monodentate coordination to the VO 2 + moiety. 34The vibra- through the pyridyl nitrogen atom and two carboxylate oxygen atoms (Scheme 1).
Selected bond distances and angles for individual complexes are summarized in Tables S1 and S2 (in ESI †), respectively.The bond distances between vanadium and pyridyl nitrogen atoms for all complexes are in the range 2.0824(19)-2.0993(13)Å, the distances between vanadium and carboxylate oxygen atoms are slightly shorter and are in the range 1.9746(17)-2.0109(17)Å.Double bonds between vanadium and oxido oxygen atoms of 1.6078( 14)-1.6311(14) Å are as expected the shortest bonds in complexes.
Distortion of the pentacoordinated structure due to the chelation of the pyridine-2,6-dicarboxylato ligand is even more evident from the observed bond angles around vanadium.Angles between carboxylate oxygen atoms and pyridyl nitrogen atom are 73.86(5)-75.02(4)1,and angles between carboxylate oxygen atoms are 147.97(5)-150.04(7)1.Angles between carboxylate oxygen atoms and oxido oxygen atoms are 97.92(15)-100.15(15)1, and angles between two oxido oxygen atoms are in the range of 107.92(7)-110.49(13)1.These bond distances and angles are in the same range as previously reported for ammonium, guanidinium 33 and 2,9-dimethyl-1,10-phenanthrolinium 35 compounds.The distortion of a square-pyramid can be best described by the structural parameter t (0 for an ideal square pyramid and 1 for an ideal trigonal bipyramid), 36 which in these cases is in the range 0.38-0.42,except for 3 and 1Á2H 2 O where it is 0.16 and 0.35, respectively.This difference will be discussed later.
The Anhydrous pyridinium salt 1 also crystallizes in the triclinic space group P% 1.One asymmetric unit of anhydrous 1 contains two complex anions and two cations (Fig. S2, ESI †).Hydrogen bonds and weak C-HÁ Á ÁO interactions found in anhydrous 1 are listed in Table S3 (in ESI †).Due to the absence of crystal water, only two hydrogen bonds could be formed in one asymmetric unit of the anhydrous compound 1.Furthermore, both cations in the asymmetric unit are connected to the same anion via N-HÁ Á ÁO hydrogen bonds with the D 1 1 (2) graph set motifs.The first pyridinium cation is hydrogen-bonded to carboxylate O7 atom (d DÁ Á ÁA = B3.13Å), and the second cation is hydrogenbonded to the oxido O11 atom (d DÁ Á ÁA = B2.88Å).The second vanadium anion is not involved in the hydrogen bonding (Fig. 2).
Interestingly, due to the lack of strong non-covalent intermolecular interactions such as hydrogen bonds anions are involved to a larger extent in weak interactions.For instance, weak VQOÁ Á ÁC interactions previously described by Stilinovic ét al. 25 have been observed between the adjacent V1 and V2 complex anions of the asymmetric unit.The VQOÁ Á ÁC interactions are formed between oxido groups and carboxyl carbon atoms with OÁ Á ÁC distances of 2.968( 5) and 3.039(5) Å and VQOÁ Á ÁC angles of 139.1(2) and 141.0(2)1.Ions of different asymmetric units are further connected only by weak C-HÁ Á ÁO and pÁ Á Áp stacking interactions.pÁ Á Áp stacking interactions were observed along the a axis between pairs of cations that belong to the same asymmetric unit with a centroid-to-centroid distance of 3.878(4) Å and an inter-ring dihedral angle of 6.3(3)1, and between cations of the adjacent asymmetric units with a centroid-to-centroid distance of 4.230(4) Å (Fig. 2, Table S8, ESI †).Supramolecular architecture is therefore controlled by additional weak hydrogen bonds extending in all three dimensions between aromatic CH groups of Hpy + and V2 anions, oxido and carbonyl oxygen atoms of V1 anions, and oxido and carboxyl oxygen atoms of V2 anions (Fig. 2).moiety.Hence, the pillar structure is a consequence of strong hydrogen bonding interactions between the polar hydrophilic regions and leads to the exposure of the hydrophobic region to the exterior.Such organization and separation of hydrophobic and hydrophilic regions has an important influence on supramolecular arrangement. 38he three-dimensional framework is achieved by connecting the pillars to each other by several C-HÁ Á ÁO interactions (d DÁ Á ÁA = 3.14-3.48Å) (Fig. 3).However, no significant pÁ Á Áp interactions have been observed.View of the packing along the b-axis reveals formation of canals between anions and cations that are parallel to the b-axis and are occupied by water molecules (Fig. 4).
4-Aminopyridinium dioxido(pyridine-2,6-dicarboxylato-N,O,O 0 )vanadate(V) (3) and monohydrate (3ÁH 2 O).4-Aminopyridinium salt crystallizes as a monohydrate 3ÁH 2 O or in its anhydrous form 3. 3Á H 2 O crystallizes in the monoclinic space group P2 1 /m.Cation, complex anion and a water molecule lie on a mirror plane.One asymmetric unit of 3ÁH 2 O therefore contains one half of each  (Fig. 6).Such two-dimensional frameworks are packed into crystal structure; however, no significant C-HÁ Á ÁO or pÁ Á Áp interactions have been observed among them.View of the packing along the b-axis reveals a similar structure to that in 2ÁH 2 O. Between anions and cations canals parallel to the b-axis filled with water are formed (Fig. 7).Anhydrous 4-aminopyridinium salt 3 crystallizes in the monoclinic space group P2 1 /c.One asymmetric unit of anhydrous 3 contains one complex anion and one H4apy + cation (Fig. S5, ESI †).Similar to the 3ÁH 2 O structure the amino group and the protonated pyridine group in H4apy + act as hydrogen bond donors, but only one oxido and two carboxyl oxygen atoms act as hydrogen bond acceptors.Hydrogen bonds found in anhydrous 3 are listed in Table S5 (ESI †).
As mentioned earlier the structural parameter t describing the distortion of a square-pyramid is noticeably smaller for anhydrous 3 (t = 0.16) than for all the other complex anions (t = 0.35-0.42).The parameter t is based on the difference between the largest two X-M-X angles a and b in the complex according to the equation t = (a À b)/601. 33We have investigated the reasons for such unexpected difference in t values in the same type of complexes and observed that the largest angle a is O carboxyl -V-O carboxyl , and has similar values in all compounds (B1481); however, the second largest angle b is N-V-O oxido and is close to 1251 in all cases, except in the anhydrous 3 being 138.42( 7 S8) with a dihedral angle of 0.34(10)1 and a centroid-tocentroid distance of 3.6787( 13) Å (Fig. 11).Weak C-HÁ Á ÁO interactions spreading in all three dimensions are formed between carboxyl oxygen and aromatic hydrogen atoms of the

Structural comparison
The vanadium center in the dioxido(pyridine-2,6-dicarboxylato)vanadate(V) anion has a distorted square-pyramidal structure.Distortion also depends on the presence of hydrogen bonding.In most cases both oxido oxygen atoms are involved in the hydrogen bonding and both VQO bond lengths are of the same range.The value of trigonality parameter t in these compounds is usually 0.38-0.42.However, in 1Á2H 2 O and 3 only one oxido oxygen atom is involved in the hydrogen bonding and this VQO distance is elongated compared to the others.Interestingly, in these two compounds the distortion of the vanadium coordination sphere is the least pronounced, as can be seen by the t values (in 1Á2H 2 O is 0.35 and in 3 is 0.16).
In the imidazolium salt, in which each of the two NH groups binds to two acceptors, three-dimensional hydrogen bonding is possible.In 4-aminopyridinium salts with NH and NH 2 donating groups two-dimensional systems were observed due to the classical hydrogen bonding.On the other hand, in pyridinium salts with only one donating NH group only isolated or one-dimensional hydrogen bonded systems could be formed and additional packing is stabilized by weak C-HÁ Á ÁO and pÁ Á Áp interactions.We have also noticed that the water molecules in the crystal lattice acting as hydrogen bond donors and acceptors additionally increase the number and diversity of hydrogen bonds, but do not necessarily increase the stability of the structures.Powder X-ray diffraction (PXRD) experiments were carried out in order to confirm the phase purity of the bulk materials.We have observed that 1Á2H 2 O is unstable outside the solution.When exposed to air, crystals of 1Á2H 2 O decompose and lose one equivalent of crystal water, as confirmed by elemental analysis.Due to this partial dehydration the experimental PXRD pattern of monohydrate 1ÁH 2 O does not correspond with the one computer-simulated from the single crystal data of dihydrate 1Á2H 2 O (Fig. S8 in ESI †).We have also observed that 3ÁH 2 O is not stable outside the solution.When exposed to air, crystals of 3ÁH 2 O decompose, lose the crystal water, and, with respect to the elemental analysis and IR spectra, transform into anhydrous 3 compound.The experimental PXRD patterns of 2ÁH 2 O, 3, 4ÁH 2 O and 5 correspond well with the ones computer-simulated from the single crystal data, indicating the high purity of the synthesized samples (Fig. S9 in ESI †).The differences in reflection intensities between the simulated and the experimental pattern are due to the variation in the preferred orientation of the powder samples as well as due to the fact that crystal 2ÁH 2 O was measured at 150 K while all PXRD data were collected at room temperature.
It has to be stressed that vanadium possesses a unique coordination environment in comparison to the other first row transition metals.In addition to the carboxylic group of the 2,6-pydc 2À ligand, the [VO 2 (2,6-pydc)] À moiety has two Thus, the rational choice of the cation may be an effective way to construct novel metal-organic hybrids with desired structures and properties.This research can be of great help in the field of emerging vanadium hybrid materials particularly from the viewpoint of being aware of the existing building blocks and their binding potentials according to the concepts of crystal engineering.
2-Hydroxypyridinium dioxido(pyridine-2,6-dicarboxylato-N,O,O 0 )vanadate(V) monohydrate (2ÁH 2 O).2-Hydroxypyridinium salt 2ÁH 2 O crystallizes in the monoclinic space group P2 1 /m.One asymmetric unit of 2ÁH 2 O contains one half of the complex anion, one half of the cation and one half of the water molecule, which lie on the mirror plane parallel to the ac plane (Fig. S3, ESI †).Hydrogen bonds and weak C-HÁ Á ÁO interactions found in 2ÁH 2 O are listed in Table S4 (in ESI †).The ionic pair on the symmetry plane is connected via an O-HÁ Á ÁO hydrogen bond (d DÁ Á ÁA = B2.58Å) involving the hydroxyl moiety of H2pyon + and the carbonyl oxygen atom of the 2,6-pydc 2À ligand.An infinite square pillar is formed along the b axis due to the connection of ionic pairs through water molecules forming an R 6 6 (24) motif via charge-assisted N-HÁ Á ÁO (d DÁ Á ÁA = B2.67Å) and O-HÁ Á ÁO (d DÁ Á ÁA = B2.81Å) hydrogen bonding connecting NH moieties of H2pyon through water molecules to oxido atoms of the VO 2 +

Fig. 2
Fig. 2 Hydrogen bonding network in anhydrous 1.(a) Hydrogen bonds and pÁ Á Áp stacking interaction in the asymmetric unit of anhydrous 1.(b) Packing diagram along the a axis facilitated by weak interactions.(c) Space filled representation of packing along the [110] direction.(d) Packing diagram emphasizing the pÁ Á Áp stacking interactions.
)1.This larger angle b (N1-V1-O6) in anhydrous 3 causes a small tilt of a VO 2 + group, as can be observed also by a smaller N1-V1-O5 angle (113.63(7)1).The distortion of the VO 2 + moiety probably happens due to the involvement of only one oxido oxygen atom of the VO 2 + moiety in the hydrogen bonding.This has also an effect on the difference in VQO bond lengths of the VO 2 + moiety (1.6311(14) vs. 1.6078(14)Å), which is the largest difference in all compounds.Coulomb interactions between H4apy + cations and [VO 2 (2,6pydc)] À anions supported by charge-assisted hydrogen bonding between them organize the packing of cations and anions.Cations and anions are connected via N-HÁ Á ÁO hydrogen bonds (d DÁ Á ÁA = 2.78-3.00Å) into infinite chains with the C 6 6 (30) graph set motif involving NH and NH 2 moieties of H4apy and one oxido oxygen of the VO 2 + moiety as well as the carboxyl and carbonyl oxygen atoms of the 2,6-pydc 2À ligand.Chains are further connected into infinite double layers perpendicular to the c axis by R 4 4 (16) and R 8 8 (44) graph set motifs.Further stabilization of the crystal lattice is enabled by weak C-HÁ Á ÁO interactions between cations and anions (d DÁ Á ÁA = 3.08-3.23Å) or between adjacent anions (d DÁ Á ÁA = B3.48Å) that help to connect two-dimensional layers into a three-dimensional framework (Fig. 8).No significant pÁ Á Áp stacking has been observed in this crystal structure.4-(Dimethylamino)pyridinium dioxido(pyridine-2,6-dicarboxylato-N,O,O 0 )vanadate(V) monohydrate (4ÁH 2 O).4-Dimethylaminopyridinium salt 4ÁH 2 O crystallizes in the orthorhombic

Fig. 3
Fig. 3 Hydrogen bonding network in 2ÁH 2 O. (a) A fragment of the crystal structure of 2ÁH 2 O emphasizing the connection of ionic pairs through a water molecule via the R 6 6 (24) graph set motif.(b) Square pillar formation due to the hydrogen bonding.(c) Space filled representation of the packing.(d) Space filled representation of the packing along the b-axis.

Fig. 4
Fig. 4 Packing of 2ÁH 2 O along the b-axis showing the canals filled with water molecules.Water molecules are shown as space-filling models for clarity.

Fig. 6
Fig. 6 Hydrogen bonding network in 3ÁH 2 O for the major part of a disordered 4-aminopyridinium cation.(a) A fragment of the crystal structure of 3ÁH 2 O emphasizing the connection of adjacent chains through a water molecule via the R 6 6 (26) graph set motif.(b) Chains connected into a two-dimensional framework.(c) Packing diagram along the b-axis.(d) Space filled representation of the packing along the c-axis.

Fig. 7
Fig. 7 Packing of 3ÁH 2 O along the b-axis showing the canals filled with water molecules.Water molecules are shown as space-filling models for clarity.

Fig. 8
Fig. 8 Hydrogen bonding network in anhydrous 3. (a) A fragment of the crystal structure of 3 emphasizing the connection of cations and anions via the R 4 4 (16) graph set motif.(b) View along the c axis on the two-dimensional framework emphasizing the R 8 8 (44) motif.(c) Space filled representation of the packing of layers facilitated by weak C-HÁ Á ÁO interactions.(d) Packing diagram along the b axis.

Fig. 9
Fig. 9 Hydrogen bonding network in 4ÁH 2 O. (a) A hydrogen-bonded chain parallel to the c axis with the C 4 4 (12) motif.(b) pÁ Á Áp stacking interactions between parallel anions and cations.(c) Space filled representation and packing diagram along the b axis indicating pÁ Á Áp stacking interactions.(d) Packing diagram along the c axis (facilitated by weak interactions).

Fig. 10
Fig. 10 Packing of 4ÁH 2 O along the c-axis showing the canals filled with water molecules.Water molecules are shown as space-filling models for clarity.

Table 1
Crystal data and structure refinement details for compounds 1-5