Dendritic Pyridine-Functionalized Polyesters and Their Polycationic

Dendritic Pyridine-Functionalized Polyesters and Their Polycationic Hydrogen Bonded Picrates: Synthesis and X-ray Structural Study of Weak Hydrogen Bonding Kalle I. Na¨ttinen and Kari Rissanen*

CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 3 339-353

Department of Chemistry, University of Jyva¨ skyla¨ , P. O. Box 35, FIN-40351 Jyva¨ skyla¨ , Finland Received February 21, 2003

ABSTRACT: Nicotinato and isonicotinato functionalized pentaerythritol and dipentaerythritol dendritic polyester compounds were synthesized. The compounds were crystallized, and the single-crystal structures were determined. Protonation by picric acid produced charged dendritic polyesters. Analysis of the role of weak hydrogen bonding CH‚‚‚π and π‚‚‚π interactions in the solid state was performed and compared to the corresponding benzoxy analogues which were also synthesized. The tetranicotinate 3b and tetrabenzoate 4 were found to have analogous structures, which in turn differ from the structure of the tetraisonicotinate 3c. The difference is attributed to the crucial role of CH‚‚‚O and CH‚‚‚N hydrogen bonding networks between the layers in the crystal lattice. The layers are held together by intermolecular CH‚‚‚π and π‚‚‚π interactions. The compound 3c formed an intertwined, tetrahedral network due to the acceptor ability of the aromatic nitrogen in the para position of the compound. Compound 9c exhibited an incomplete protonation by picric acid. Of six available aromatic nitrogens, only five were protonated. Introduction The structural studies of the dendrimers and dendrimer precursors is based mainly on the information collected by methods such as NMR, matrix-assisted laser desorption ionization (MALDI), and electrospray ionization (ESI) mass spectrometry, because the dendritic molecules have a tendency to remain liquid, or at best solid and noncrystalline.1 The noncrystalline nature of dendrimers is caused by the flexibility of the dendrons and furthermore, in the higher generations, the globular shape of the molecule.2 However, X-ray crystallography would be the perfect tool for investigating the arrangement of the dendrons, the inter- and intramolecular interaction of the dendrons as well as the packing patterns in the crystal lattice. Some examples of crystal structures of dendrimers are available, the most complex of them being second generation.3 Probably because of the reputation of dendrimers being “anticrystallization agents”, no attempts to systematic study of the crystallinity of these compounds by single-crystal X-ray crystallography have yet been made. However, some publications on the studies of crystallinity of dendrimers by other methods do exist.4 The aim of this work is to take the first step on this path, through a synthesis of a family of pentaerythritoland dipentaerythritol-based dendritic polyester molecules and their crystal structure analysis. To obtain solid and crystalline material, two fundamental requirements have to be fulfilled. Attractive interactions between molecules are needed to assemble the individual molecules to form the supramolecular entity of the crystal.5 These interactions can be ionic, hydrogen bonds, weak hydrogen bonds, or based on van der Waals forces. The molecules also need to be able to form self-complementary structures. Most molecules are * To whom correspondence should be addressed. E-mail: [email protected]). Phone: +358 14 260 2672. Fax: +358 14 260 2501.

not self-complementary, which places more emphasis on the van der Waals, hydrogen, and weak hydrogen bonding. Another possibility is to apply an auxiliary molecule to allow cocrystallization.6 The demand of selfcomplementarity is basically an extension of the ageold wisdom of “nature abhors vacuum” by Aristotle.7 Utilization of the strong O-H‚‚‚OdC type hydrogen bonds might lead to more easily predictable structures. While the strong hydrogen bonds are well-known and thoroughly studied,8 the weak hydrogen bond is still a target of continuous interest, especially in the field of supramolecular chemistry.9,10 Thus, we concentrated on the role of weak hydrogen bonds in inducing crystal growth. One of the first definitions of the hydrogen bond defines it as a phenomenom where “...there is evidence of a bond and that there is evidence that this bond sterically involves a hydrogen atom already bonded to another atom” (Pimentel, McClellan11). After this, the essential attribute of directionality has been added to the definition. The special types of weak hydrogen bonds are the C-H‚‚‚π and π‚‚‚π interactions,12 although it can be debated whether the π‚‚‚π bond actually belongs to this group since there is no hydrogen atom involved. These interactions are even weaker than the C-H‚‚‚O type of nonclassical hydrogen bonds. Nevertheless, in the absence of stronger bonding forces, these forces determine the nature of the packing or at least participate in it.13,14 In the current paper, this philosophy is utilized by investigating compounds with no strong hydrogen bonding donors, but a multitude of weak ones: aromatic and methylene hydrogens as well as π bonds (of the pyridine and benzyl rings). The idea of the weak hydrogen bond was not generally accepted before 1982. However, after several studies in addition to the well-known Taylor & Kennard paper,15 its existence is now widely accepted. Data have been collected from numerous statistical analyses of Cam-

10.1021/cg0340291 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/03/2003

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bridge Structural Database (CSD),16 showing the correlation between the strength of the acceptor and the directionality of CdX‚‚‚H angle, the reduction of thermal vibrations caused by hydrogen bonding,17 the dependence of the C-H‚‚‚O distance of the acidity of the C-H group, and the need to consider hydrogen bonds to explain a sterically unfavored, eclipsed, conformation of methyl hydrogens.18,19 The directionality requirements for the weak hydrogen bond are diffuse: generally a bond is assumed if the D-H‚‚‚A angle θ is larger than 120 degrees. However, smaller angles down to 90 degrees are occasionally accepted as bonding, depending on the distance of the donor from the acceptor.20 The hydrogen bonding is physically understood as an intermediate between the covalent bonding and the pure electrostatic forces.20,21 The term “crystal engineering” was first used in 1971 by Schmidt and redefined by Desiraju in 1989 to mean the deliberate utilization of the knowledge of intermolecular forces to form a crystal with the desired chemical and physical properties.22,23 With all the research on the subject of weak hydrogen bonding, and the forces present in the crystal state,24 the outcome of crystallizations still remains uncertain, as stated by Maddox already in 1988.25 Attempts to predict the crystal structures of selected compounds have been made. Desiraju and Gavezotti investigated the crystal structures of aromatic hydrocarbons and the way their packing depended on the properties of these compounds. The analyses were based on homologues, i.e., compounds with equal size and shape were generally found to pack similarly.26 To be able to accurately analyze the hydrogen-bonding pattern, the hydrogen bonds should be normalized to values obtained by neutron diffraction experiments. The bond lengths determined from X-ray analysis appear significantly shorter than the actual distance of the proton from the atom it is bonded to, because hydrogen only has one electron. The X-rays scatter from the electrons and hence, the electron density of the one and only electron being dispositioned, the bond seems shorter. The dislocation is partly caused by the larger electron withdrawing force of the more electronegative atoms. This results in measured bond lengths up to 0.2 Å shorter than they are in reality. However, it is somewhat unclear whether the position of the hydrogen should be considered to be the center of gravity of the nucleus or that of the electron.27,28 Therefore, in detailed structures of hydrogen bonded structures, hydrogencarbon atom bond lengths have been normalized to 1.08 Å and the hydrogen-nitrogen and hydrogen-oxygen bonds respectively normalized to 1.00 Å.29 Other values have also been proposed.10 The compounds in this work do not contain strong hydrogen bond donors (besides the protonated pyridine rings in the picrate series), and therefore the forces determining the crystal structure are weak C-H‚‚‚OdC and C-H‚‚‚N interactions.30 The ester function and especially the carbonyl group, one of the best C-H‚‚‚O hydrogen bond acceptors,31 were chosen as acceptors in the inner shell of the molecules, and the pyridine group was chosen to provide an acceptor in the terminus of the arms. Besides the good acceptor capabilities of nitrogen in the pyridine ring, the aromatic ring itself

Na¨ttinen and Rissanen

provides the necessary negative charge for weak interactions. More information of the weak hydrogen bonding and packing of target molecules was sought by using a simple benzene ring instead of a picoline, nicotine, or isonicotine ring. This offers a chance to evaluate the effect on the whole packing of removing one good hydrogen bond acceptor per arm. The use of different isomers of the pyridines as terminal groups provides information on the flexibility of the arms and the flexibility of the hydrogen bonding as well. The compounds with different isomers of pyridine in the periphery were expected to have similar packing patterns due to the soft, electrostatic nature of the weak C-H‚‚‚N bonds. Finally, protonation by addition of picric acid was expected to result in a totally different behavior, due to the salt-type structure and the changed overall shape of the molecule. The objective of the protonation is to study whether this will enhance or prevent the lattice formation, or whether the structures will be able to form a required level of hydrogen bond networks to enable the formation of a crystal. Discussion Synthesis: Acid Chlorides. The synthesis of acid chlorides from corresponding pyridinecarboxylic acids by thionyl chloride is a well-known procedure.32 However, the procedure will lead to the formation of pyridinecarboxylic acid chloride hydrochlorides (1d-f), with the Cl anions hydrogen bonded to the protonated nitrogen on the pyridine ring. Additionally, if the process is applied to picolinic acid, the synthesis will lead to a heavily chromatic solid mixture of compounds, probably a mixture of a dimer, a cyclic hexamer, and a polymer. Applying oxalyl chloride instead of thionyl chloride to a suspension of the pyridinecarboxylic acid at 0 °C resulted in evaporation of the hydrochloric acid and formation of the nonprotonated acid chloride (1a-c). This yielded the picolinic acid chloride 1a without the formation of the polymers (Scheme 1). However, when applied to the synthesis of the pentaerythritol and dipentaerythritol derivatives, the reaction conditions seemed to promote the formation of the previously mentioned dimer, hexamer, and polymer, resulting in a failure to produce the target compounds. The nicotinic and isonicotinic acid chloride hydrochloride were used in the synthesis of the corresponding pentaerythritol and dipentaerythritol derivatives. Crystals suitable for X-ray diffraction study of pyridinecarboxylic acid chloride hydrochlorides 1d-f were obtained; however, the X-ray structures of 1d-f will be reported elsewhere. Pyridinates. The synthesis of pyridinates was accomplished by the reaction of the corresponding acid chloride hydrochloride and pentaerythritol (2), or dipentaerythritol (5), respectively (Scheme 2 and Scheme 3). Pyridine was used in the reactions to form a reactive acetylpyridinium ion intermediate33 and triethylamine to trap the formed hydrochloric acid as triethylamine hydrochloric acid. In the synthesis of 6b, 4-(dimethylamino)pyridine (DMAP) was utilized as an additional catalyst to provide solely the fully substituted product.34 A reaction without DMAP gave a mixture of penta- and hexasubstituted products.

Dendritic Pyridine-Functionalized Polyesters Scheme 1. Synthesis of the Pyridine Acid Chlorides and Acid Chloride Hydrochloridesa

Crystal Growth & Design, Vol. 3, No. 3, 2003 341 Scheme 3. Synthesis of Dipentaerythritol-Based Pyridinates with the Crystallographic Numbering of the Atoms of the Products

a Asterisk: with Ar ) o-pyridine, the reaction yielded a mixture of polymeric products of which crystals of 1d were obtained.

Scheme 2. Synthesis of Pentaerythritol-Based Pyridinates with the Crystallographic Numbering of the Atoms of the Products

No picolinates were produced because the reactions that were supposed to give the correct product instead yielded complex mixtures of probably polymeric products judging from NMR and ESI. Attempts to grow crystals from the mixtures failed as well.

Benzoxates. The synthesis of benzoxates 4 and 7 was performed as described in Schemes 2 and 3. The reaction of benzoyl chloride with pentaerythritol produced an equimolar mixture of the tri- and tetrasubstituted products. Dissolving the mixture in chloroform and allowing it to evaporate produced perfect prismatic crystals which were picked out mechanically for the characterization and X-ray measurements. The reaction of benzoyl chloride with dipentaerythritol produced solely the fully substituted product. The compound 4, a derivative of pentaerythritol, is a known compound. However, its crystal structure was until now unpresented. Picrates. The synthesis was accomplished by dissolving the corresponding pyridinate (3b, 3c, 6b, or 6c) in acetone and adding an equimolar solution of picric acid in acetone (Schemes 4 and 5). The reaction of 6b with the picric acid yielded a solid product, but no X-ray quality crystals were obtained from it. Due to the explosivity of the picric acid, it is stored moistscontaining 50% of water. This is why water was “used” in the crystallization of picrates even though it is not favorable to the quality of the crystals. Due to the possible explosivity of the picrate derivatives, they were not dried under vacuum, but a certain amount of water (reported in the NMR spectra as “incl. Protons”) was allowed to remain within the product. X-ray quality crystals of pentapicrate 9c were obtained, but due to the water and excess picrate used in the synthesis, the elemental analysis of macroscopic amounts of the compound gave an incoherent result. Therefore, these data are not presented for pentapicrate 9c.

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Scheme 4. Synthesis of Pentaerythritol-Based Picrates with the Crystallographic Numbering of the Atoms of the Products

Crystal Structures. The compounds were slowly allowed to crystallize from various solvents by the spontaneous evaporation of the solvent. The crystal lattice of the molecules was studied extensively to find an explanation for the crystallization of the molecules and the packing they show. The lack of traditional hydrogen bonds presented no significant difficulty for the crystallization, except in the attempts to crystallize the hexapicrate of the dipentaerythritol hexanicotinate, 9b. The strongest weak hydrogen bonds in the pyridinate, benzoxate, and picrate structures were found to be the ones between the carbonyl group oxygens and the methylene hydrogens of the pentaerythritol and dipentaerythritol core and the aromatic hydrogens in the ortho position of the pyridine rings. Other important interactions essential for the formation of the structures were found to be the π‚‚‚π and CH‚‚‚π bonds. The picrates are hydrogen bonded to the protonated nitrogen atoms via their deprotonated phenolic oxygens. Various criteria were utilized in selecting the actual bonds among the van der Waals interactions. Due to the variable nature of especially the π bonds, it is difficult

to determine a perfect cutoff distance. The applied cutoff distances are presented in the Table 1. The cutoff distance for the D‚‚‚A distance of the π‚‚‚π bonds is not reported because of the varying bond types. If the D‚‚‚A would be understood to represent the centroid-centroid distance, the cutoff would be totally different for the stacked and edge-on-edge bonding. The collected and combined data of the bond lengths and angles of the weak hydrogen bonds of the measured and analyzed compounds are presented in the Tables 2-5. Due to the large number of the bonds, they are available in the Supporting Information. Four-Armed Polyester Structures. The conformation of the four arms of the tetrabenzoate 4 is planar. Despite the structural similarity of the tetranicotinate 3b and tetraisonicotinate 3c, their respective conformations differ significantly (Figure 1). The conformation of the arms and the packing of the tetranicotinate 3b is similar to that of the tetrabenzoate 4, while the conformation and packing of the tetraisonicotinate 3c are clearly different. The conformation of the structure of the tetraisonicotinate is tetrahedral, resulting in a completely

Dendritic Pyridine-Functionalized Polyesters

Crystal Growth & Design, Vol. 3, No. 3, 2003 343

Scheme 5. Synthesis of the Dipentaerythritol-Based Picrates with the Crystallographic Numbering of the Atoms of the Products

Table 1. Cutoff Distances of Different Types of Weak Hydrogen Bonds type of bonds

DH‚‚‚A cutoff (Å)

D‚‚‚A cutoff (Å)

CH‚‚‚OdC CH‚‚‚N CH‚‚‚π π‚‚‚π

2.8 3.0 3.3 3.9

3.7 3.9 4.1

different packing pattern. The layers that can be found from the crystal lattices of 4 and 3b (Figure 2) are not present in the crystal lattice of 3c, due its threedimensionally intertwined packing (Figure 3). The compounds 4 and 3b both form a four-connected two-dimensional intertwined square 2-D network of molecules. The three-dimensional packing is formed

when these layers are stacked. Intermolecular CH‚‚‚π as well as π‚‚‚π bonds provide the necessary interactions for the formation of the layers. The chloroform molecule fits in a cavity of 4 formed in the layer. Since a cavity of 40 Å3 can also be observed in the structure of 3b, it is obvious that the mode of packing is determined by the shape and functionality of the molecules. The chloroform molecule only fills the hole that would otherwise have been left empty in the structure. The bonds between different layers in the crystal lattice of both 4 and 3b were found between the carbonyl groups and the methylene hydrogens of the pentaerythritol core as well as the hydrogens in the ortho-position of the benzene ring. However, presumably due to the aromatic

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Table 2. Intermolecular CH‚‚‚OdC Bonds of Compounds 4-9c compound

4

7

3b

6b

3c

6c

8b

8c

9c

no. of CH‚‚‚OdC bonds min distance (Å) D-H‚‚‚A avg distance (Å) D-H‚‚‚A min distance (Å) D‚‚‚A avg distance (Å) D‚‚‚A max angle (deg) D-H‚‚‚A avg angle (deg) D-H‚‚‚A

9 2.3 2.46 3.29 3.42 164 148.3

8 2.37 2.48 3.18 3.39 166.9 145.4

3 2.31 2.45 3.32 3.40 155.3 147.6

13 2.28 2.48 3.15 3.41 163.6 145.6

7 2.29 2.46 2.88 3.25 170.2 134.7

14 2.22 2.41 2.74 3.21 166.6 133.9

37 2.27 2.53 2.72 3.29 168.1 130.5

49 2.15 2.47 2.85 3.28 174.1 135.3

32 2.13 2.44 2.86 3.28 175.2 137.9

Table 3. Intermolecular CH‚‚‚N (Aromatic) Bonds of Compounds 3b-9c compound

3b

6b

3c

6c

8b

8c

9c

no. of CH‚‚‚N bonds min distance (Å) D-H‚‚‚A avg distance (Å) D-H‚‚‚A min distance (Å) D‚‚‚A avg. distance (Å) D‚‚‚A max angle (deg) D-H‚‚‚A avg angle (deg) D-H‚‚‚A

2 2.49 2.54 3.41 3.52 159.5 151.0

4 2.45 2.55 3.38 3.47 159.3 144.1

4 2.59 2.74 3.30 3.45 129.0 122.9

5 2.44 2.59 3.27 3.56 173.8 154.4

3 2.22 2.32 2.85 2.92 124.9 118.3

1 2.62

2 2.38 2.39 3.29 3.35 156.6 149.1

3.46 134.3

Table 4. Intermolecular π‚‚‚π Bonds of Compounds 4-9c compound

4

7

3b

6b

3c

6c

8b

8c

9c

no. of π‚‚‚π bonds min D (atom)‚‚‚A (centroid) distance/Å average D (atom)‚‚‚A (centroid) distance/Å min D (centroid)‚‚‚A (centroid) distance/Å average D (centroid)‚‚‚A (centroid) distance/Å average angle (deg) between planes

2 3.70 3.74 3.91 4.24 24.6

2 3.60 3.73 4.04 4.39 6.2

2 3.35 3.47 3.78 3.79 2.5

1 3.71

3 3.40 3.53 3.92 4.21 0.0

2 3.26 3.30 3.50 3.74 7.3

4 3.40 3.53 3.52 3.87 3.9

8 3.31 3.41 3.47 3.82 8.5

4 3.10 3.36 3.58 3.93 20.4

3.43 1.6

Table 5. Intermolecular CH‚‚‚π Bonds of Compounds 4-9c compound

4

7

no. of CH‚‚‚π bonds min distance (Å) D-H‚‚‚A avg distance (Å) D-H‚‚‚A min distance (Å) D‚‚‚A avg distance (Å) D‚‚‚A max angle (deg) D-H‚‚‚A avg angle (deg) D-H‚‚‚A

2 2.67 2.69 3.50 3.58 145.5 142.6

4 2.74 2.90 3.59 3.69 141.8 131.0

3b

nitrogen present in the structure of 3b, the successive layers of molecules are in 45 degree angle with respect to each other, whereas the successive layers of 4 are only slightly transpositioned, not rotated. The fact that the layers of 4 are not “rotated”, leads to a formation of channels with the chloroform molecules in them (Figure 4). No channels exist is 3b, since the angle between the successive layers causes the channels to be blocked by the arms of the molecules. A possible explanation for the difference between the conformation and packing of the molecules of tetrabenzoate 4, tetranicotinate 3b, and tetraisonicotinate 3c, respectively, is the difference in the bonding capacity of the “sides” of the arms. The tetraisonicotinate 3c has the hydrogen bond accepting nitrogen in the para position. In a packing with distinct layers such as in the lattice of 4 and 3b, the hydrogen bonding capacity of the nitrogen could not be used for interactions between layers. However, with tetrahedral coordination, relatively strong interactions between molecules in three dimensions are possible. Six-Armed Polyester Structures. The structures of the hexabenzoate 7, hexanicotinate 6b, and hexaisonicotinate 6c all have their arms in a (nearly) planar conformation. Another common feature with the structures is the trans conformation of the carbonyl groups of the two terminal arms with respect to the central, adjacent arms, with the exception of another one of the terminal arms of the and one of the adjacent, central arms of 6c. In the structure of 6b, also the

6b

3c

6c

3 2.73 2.87 3.64 3.76 155.9 141.9

2 3.04 3.11 3.71 3.74 120.6 117.4

3 2.69 2.92 3.69 3.80 154.0 141.1

8b

8c

9c

2 2.98 3.11 3.84 3.96 136.8 135.9

aromatic nitrogens assume a trans conformation with each carbonyl group in each arm. All the aromatic rings of 6b and 7 are perpendicular to the plane of the molecule, whereas in 6c, a pyridyl ring of another one of the terminal arms and one of the adjacent, central arms are coplanar with the plane of the molecule (Figure 5). This twist from the regular perpendicular conformation allows a possibility for terminal overlap of the opposing arms, resulting in more tight packing and additional CH‚‚‚N (aromatic) interactions. The central coplanar and the perpendicular terminal ring are also involved in the resulting triangular set of CH‚‚‚N, CH‚‚‚π, and π‚‚‚π interactions. This packing leaves an empty disc-shaped space between the terminal pyridine rings of every pair of molecules. The distance separating the rings is approximately 5.5 Å. This space could probably host especially benzene. Unlike 6c, the molecules of hexanicotinate 6b and hexabenzoate 7 both form a two-dimensional intertwined network where the molecules are bound together by π‚‚‚π and CH‚‚‚π interactions. These layers that are formed resemble the layers formed by molecules of tetrabenzoate 4 and tetranicotinate 3b, except that the 2-fold symmetry instead of 4-fold symmetry does not lead to a fourconnected network, but instead causes far more intermolecular connections. The skewed conformation of the arms of 7 leads to a skewing of the network. The packing of the molecules of 6b, 6c, and 7 is presented in Figure 6.


Figure 1. The structures of tetrakis-nicotinoxymethylmethane, 3b, tetrakis-isonicotinoxymethyl-methane, 3c, and tetrakis-benzoxymethyl-methane, 4. The solvent molecule (chloroform) of 4 is excluded.

The layers of molecules presented in Figure 6 have a variety of weak interactions (CH(arom)‚‚‚OdC and CH2‚‚‚OdC) between them. A tendency toward tetrahedral conformation and intertwined packing of layers can be observed in the conformational twist of the molecules of 6c. Four-Armed Picrates. Despite the relative complexity of the structures, crystal structures were obtained from two of the tetrapicrates, namely, from tetranicotinato tetrapicrate 8b and tetraisonicotinato tetrapicrate 8c. The tetranicotinato tetrapicrate 8b crystallized neatly from acetonitrile giving a structure with a good R-value of 5.43%. The structure of com-


pound 8b includes four deprotonated picric acid molecules, a tetraprotonated form of the compound 3b, and three solvent molecules: an acetonitrile molecule and two disordered acetonitrile molecules. In the first nondefined acetonitrile molecule, the disorder is located around the methyl group. The other acetonitrile molecule is so severely disordered that only two of its atoms could be located. The nitro groups of two picrate anions are also slightly disordered. The tetraisonicotinato tetrapicrate 8c crystallized from a mixture of acetonitrile and water with two molecules in the asymmetric unit. Besides the two tetraprotonated tetraisonicotinates and eight deprotonated picric acids, there are two water molecules and four acetonitrile molecules in the crystal lattice. One of the picrate molecules (O90-O105) is completely disordered. A model for refinement was built by splitting the whole picrate molecule in two different spatial positions giving each part population parameter 0.5. This model accurately describes the disorder. There was also some disorder in one of the nitro groups (N82O84) of an another picrate. Thermal ellipsoid (50% probability) representations of 8b and 8c are presented in Figure 7. The two different molecules of the asymmetric unit of 8c are presented as two separate images (8c (1) and 8c (2)) for clarity. The aromatic moieties, especially neutral ones, are known to interact with one another by π-stacking. There is also evidence that the π‚‚‚π interactions can act as a mechanism for association of like-charged, in this case negative, species.35 These forces no doubt act in accordance with the simple ionic forces in the process of deprotonation and the formation of the structures of the picrates. As a consequence of this, the picrate anions of structures 8b, 8c, and 9c are arranged in stacks, due to the π‚‚‚π interactions. The deprotonated picric acids are strongly hydrogen bonded via their phenolic oxygen atoms to the protonated nitrogens. The distances from the oxygen to the hydrogen vary from 1.62 to 1.93 Å. The shortness of the distances is due to a strong interaction which is ionic by nature. The molecules form intertwined layers with the picrates tied in the structure with the ionic interaction and π‚‚‚π, CH‚‚‚O, CH‚‚‚O2N-, and NH‚‚‚O2N- interactions. The molecules of tetrapicrate 8b form cages of six tetranicotinates in which two picrates are included. A space filling presentation of a cage without the tetranicotinate in front is displayed in Figure 8. The conformation of the arms of tetranicotinate 3b is planar and that of the tetraisonicotinate 3c is respectively tetrahedral. However, with the weak hydrogen bond accepting pyridyl nitrogens blocked by picrates, the difference in the position of the nitrogen is not visible with regard to the packing. The conformations and packing patterns of the tetrapicrates are reversed with respect to the structures of the tetrapyridyl compounds. The tetranicotinate tetrapicrate 8b forms a complex intertwined pattern, while the crystal lattice of the tetraisonicotinate tetrapicrate 8c is composed of skewed piles of the core tetraisonicotinates and the picrates are positioned between these piles. The packing of the molecules of the tetraisonicotinate tetrapicrate 8c is presented in Figure 9. The picrates block the protonated aromatic nitrogens from interactions other than those to the picrate.

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Figure 2. Four molecules of the tetraisonicotinate 3b and tetrabenzoate 4. Intermolecular π‚‚‚π and CH‚‚‚π interactions and the solvent inclusion of 4 are illustrated.

Figure 3. The packing of the molecules of tetraisonicotinate 3c.

However, there are quite short contacts from the nitro groups to the hydrogen of the protonated nitrogen. The ionic hydrogen bonding interaction is the primary reason for the position of the picrates. Therefore, the question arises whether the interactions between the protonated nitrogens and the nitro group oxygens adjacent to the deprotonated phenolic oxygens are actual hydrogen bonds or only due to very strong hydrogen bond between the phenoxide and the ammonium nitrogen. The short distances and the large angles (all except one are >120 degrees) suggest that they are weak secondary interactions between the nitro group and the ammonium hydrogen with the partial positive charge. Six-Armed Picrates. Despite various methods applied to the crystallization of the compound 9b, no crystals with sufficient size for X-ray measurement were obtained. However, crystallization of the hexaisonicotinato hexapicrate 9c from acetonitrile solution yielded an interesting result. The structure, surprisingly, contains only five picrate molecules, even though an excess of picric acid was used in the synthesis. This is most probably due to the more favorable packing of

Figure 4. Perspective view of channels in the structure of tetranicotinate 3b; chloroform molecules are removed from the channel on the left.

the pentaprotonated core molecule. Near the position where the sixth picrate molecule would have been are two acetonitrile molecules, another one of them with population parameter 0.25. Two nitro groups of the picrate anions and one carbonyl group of the dipentaerythritol core are disordered. There is also one water molecule (delocalized in four different positions, each with a poulation parameter 0.25) in the structure. The structure without solvent molecules and the disorder removed is presented in Figure 10. The packing of the structure 9c follows the way of the previous dipentaerythritol pyridinate derivatives: The arms of the molecules assume a planar conformation, with the aromatic rings perpendicular to the plane of the molecule. The picrates are situated in rows on top of the core of the pentapicrate hexaisonicotinate of the neighboring layer of molecules and between the central arms of a pair of core molecules in a same layer.



Figure 5. The structures of hexanicotinate 6b, hexaisonicotinate 6c, and hexabenzoate 7.

There is a space of approximately 6 Å between the terminal hydrogens of the central arms of neighboring molecules. This space is due to the picrate anions. The picrates of neighboring molecules are neatly intertwined, creating a similar packing as 6b and 6c. The molecules in the neighboring layers are positioned so that the picrates are covered with the pentapicrate hexaisonicotinate core of the neighboring layer. The pyridine rings of the terminal arms of the adjacent molecules in the same layer are superpositioned, although not quite coplanar (Figure 11). There is a bond from the aromatic hydrogen H104 (which belongs to one of the picrate molecules) to the unprotonated aromatic nitrogen N21 in the terminal arm. This bond is interesting since the donor molecule (picrate) sterically blocks the pyridine ring, making it difficult for the sixth picric acid molecule to approach the pyridine ring especially when considering the presence of the two acetonitrile molecules in close proximity.

Figure 6. The packing of molecules of 6b, 6b, and 7 with intermolecular CH‚‚‚π and π‚‚‚π interactions presented.

Conclusions Numerous dendritic structures containing a large variety of weak hydrogen bonds and a few classical ones were studied. The absence of classical hydrogen bond donor sites was not found to affect the crystallization negatively. Instead, good quality crystals were obtained from nearly all of the compounds studied. In general, small changes in the structure of the molecules had no effect on the packing, unless the change concerned a factor critical for the formation of the intermolecular (weak) hydrogen bonding network. The crystal structures of both tetrabenzoxate 4 and tetranicotinate 3b contain a cavity that is filled by solvent (chloroform) in the structure of 4. The packing patterns of the tetranicotinate 3b and tetrabenzoate 4 are similar but they

348



Figure 8. The space-filling model representation of the cage formed by the tetranicotinates of the compound 8b with the picrates, solvent molecules, and the tetranicotinate in front excluded.

Figure 9. The space-filling presentation of the packing of 8c seen from the top of the skewed piles formed by the tetraisonicotinates. Four asymmetric units are shown, and the picrates and solvent molecules are excluded.

Figure 7. The structures of 8b, 8c (1), and 8c (2), with the solvent molecules excluded and disorder removed. The ionic interactions between the phenolic oxygens and the protonated nitrogens are also displayed.

differ from that of the tetraisonicotinate 3c. Opposite to what was expected, the structurally very similar tetranicotinate 3b and tetraisonicotinate 3c did not form

isomorphous structures. This was attributed to the dominating CH‚‚‚O and C‚‚‚N interactions between the layers of molecules and the absence of them in compound 3c, if its arms assume a planar conformation. A tetrahedral structure combined with intertwined packing allows stronger intermolecular interactions for 3c. This is due to the para position of the nitrogen in the aromatic ring of 3c. Bonds are formed between the hydrogens of the pyridine ring and the pyridine nitrogen in the next molecule, in addition to four CH‚‚‚O bonds between arms of adjacent molecules. This leads to formation of dimers of 3c which then form a fourconnected tetrahedral network. It has been proposed that formation of single, not interpenetrating, tetrahedral networks would probably create a structure with a high percentage of void space and therefore a too low density for stability.36 Instead


Figure 10. The crystal structure of hexaisonicotinate pentapicrate, 9c. The disorder is removed and solvent molecules are excluded.

Figure 11. The packing of the molecules of pentapicrate 9c inside a layer (green) with the picrates between central arms (red). The disorder is removed; water and acetonitrile molecules are excluded.

of an “edge-to-edge” connected packing, the molecules of compound 3b pack with “face-to-face” connections.37 This creates a dense structure since the space that would be left between adjacent molecules is filled by the molecules themselves. This “face-to-face” packing is further supported by the ability of this type of molecule to create π‚‚‚π bonds between the arms. The explanation for the planar conformation of 4 seems to be that in the abscence of aromatic nitrogen, the CH‚‚‚O interactions between layers, and CH‚‚‚π and π‚‚‚π interactions within each layer, become the factor that determines the structure. There are no interactions between the terminal ends of the arms that would cause the orientation of the arms to be tetrahedral and result in a tetrahedral packing. It was rather surprising that instead of a tetrahedral shape, the planar cross-shaped form was favored in the solid state. Of the five four-arm structures investigated, three were clearly planar (3b, 4, 8c), one was an intermediate (8b), and only one (3c) was clearly tetrahedral. All the six-arm structures (6b, 6c, 9c) were planar (with the exception of the noticeable but small twist toward tetrahedral conformation in the structure of 9c). The six-arm compounds are by themselves not


capable of forming a tetrahedral network due to the lack of a four-fold symmetry. In the planar six-arm structures, π‚‚‚π interactions are mainly responsible for the intramolecular organization of the arms of the molecules. The CH‚‚‚O interactions, besides some π‚‚‚π and CH‚‚‚π hydrogen bonds, constitute the intermolecular weak hydrogen bonding defining the form of the packing. It is deduced that despite the intramolecular repulsion of the arms, the molecules assume a shape that is determined by weak hydrogen bonds alone, contrary to the sterical optimization. Small changes such as the difference between para and meta isomers result into completely different packing of the molecules. This adds emphasis on the importance of the careful considerations necerssary in the selection of molecular tectons when building supramolecular structures. Notwithstanding the admittedly flexible nature of the compounds studied, this also further testifies to the importance of the weak hydrogen bonding in determining the final outcome of the structure.17-19 The fact that the smaller tetrapyridinate molecules tend to crystallize without any solvent molecules suggests an efficient packing without significant voids. The picrate derivatives, expectedly, could not have as compact and regular packing pattern since the picrate anions reserve a certain amount of space. The need for additional space leads to bulkier structures and some difficulties in crystallization. This also forces the empty space to be filled by inclusion of solvents (acetonitrile and water) in the structure. However, the large number of hydrogen bond donors availablesthe nitro groups and π bonds of the picratessallowed sufficient amount of interactions for the crystals to grow from all compounds except the hexanicotinate hexapicrate 9b. One of the future objectives of this research is to find out whether the difficulties in the crystallization of the picrates is due to the bulkiness introduced to the structure by the picrate anions, or whether the flexibility of the structure prevents the crystallization. In addition to the study of the weak hydrogen bonds and packing, the synthesized dendritic polyesters can act as ligands for a variety of metal cations.38 Several interesting transition metal complexes with polymeric networks have already been obtained and the results will be published elsewhere. Experimental Section General Methods. The pyridinates and compounds 1a-f were produced under nitrogen atmosphere except 3c, for which argon was used instead. For benzoxates and picrates, no protective gas was utilized. The pyridine was distilled from KOH. Toluene was sodium dried. Acetone and acetonitrile were used without drying. Picolinic, nicotinic and isonicotinic acid, pentaeythritol, dipentaerythritol, oxalyl chloride, benzoyl chloride, and triethylamine were commercial products of sufficient purity. 1 H and 13C NMR spectra were recorded on a Bruker Avance 250 MHz spectrometer unless otherwise noted. CDCl3 or DMSO-d6 were used as a solvent unless otherwise stated. Chemical shifts are reported as δ (ppm). The purity of products was confirmed by elemental analysis. The mass spectrometry was done with a Micromass ESITOF spectrometer with variable solvents MeCN/H2O/HCOOH 50:50:0.2 mixture.

350


Melting points were measured with a Mettler Toledo FP62 apparatus. The IR measurements were performed with a Perkin-Elmer System 2000 FT-IR. The elemental analysis was done at the University of Joensuu with CE-Instruments EA1110.

Synthesis General Procedure for the Synthesis of Compounds 1a-c. In a round-bottom two-neck 250 mL flask equipped with a magnetic stirrer, a condenser, a CaCl2 tube, and N2 atmosphere, 20 g (162 mmol) of the corresponding pyridinoic acid was suspended in 150 mL of dry toluene at 0 °C. A total of 28.26 mL (324 mmol) of oxalyl chloride was added slowly from a dropping funnel. After 1 h, the reaction was left to warm to room temperature at which it was held overnight. Filtering the solid after washing it with 120 mL of toluene and drying in vacuo at RT resulted in the acid chloride with a quantitative yield. (a) Picolinic Acid Chloride 1a. 1H NMR (DMSO-d6): 8.9 (d, 1H, J ) 5.0 Hz), 8.55 (td, 1H, J ) 7.7, 1.3 Hz), 8.35 (d, 1H, J ) 7.7), 8.1 ppm (qd, 1H, J ) 7.7, 5.0, 1.3 Hz). 13C NMR (DMSO-d6): 162.2, 145.3, 144.8, 143.1, 129.7, 126.8 ppm. (b) Nicotinic Acid Chloride 1b. 1H NMR (DMSO-d6): 9.2 (d, 1H, J ) 1.7 Hz), 9.0 (dd, 1H, J ) 5.5, 1.4 Hz), 8.75 (dt, 1H, J ) 8.0 Hz, 1.7), 8.0 ppm (dd, 1H, J ) 8.0, 5.5 Hz). 13C NMR (DMSO-d6): 164.1, 147.0, 144.6, 143.6, 129.3, 126.6 ppm. (c) Isonicotinic Acid Chloride 1c. 1H NMR (DMSO-d6): 9.0 (dd, 2H, J ) 6.5, 3.5 Hz), 8.22 (dd, 2H, J ) 6.5, 3.5 Hz), 13 C NMR (DMSO-d6): 164.4, 145.1, 126.9, 125.5 ppm. General Procedure for the Synthesis of Compounds 1d-f. In a round-bottom two-neck 250 mL flask equipped with a magnetic stirrer, a condenser, a CaCl2 tube, and N2 atmosphere, 24.5 mL (336 mmol) of SOCl2 was added slowly from a dropping funnel on 20 g (162 mmol) of the corresponding pyridine carboxylic acid. The reaction was refluxed for 4 h and evaporated to dryness under reduced pressure at RT to yield the corresponding acid chloride hydrochloride in a quantitative yield. (a) Picolinic Acid Chloride Hydrochloride 1d. The reaction yielded a black, chromatic solid the NMR analysis of which suggested a polymer of a sort. The X-ray analysis of crystals found sublimated on the brim of the flask after evaporation of SOCl2, however, yielded a correct structure. (b) Nicotinic Acid Chloride Hydrochloride 1e. 1H NMR (DMSO-d6): 9.95 (b, 1H), 9.4 (d, 1H, J ) 0.7 Hz), 9.0 (dd, 1H, J ) 2.3, 0.7 Hz), 8.65 (dt, 1H, J ) 4.1, 0.7), 7.75 (dd, 1H, J ) 4.1, 2.3), 13C NMR (DMSO-d6): 164.1, 147.2, 144.8, 143.1, 129.1, 126.6 ppm. (c) Isonicotinic Acid Chloride Hydrochloride 1f. 1H NMR (DMSO-d6): 9.9 (b, 1H), 9.1 (d, 2H, J ) 4.5 Hz), 8.28 (d, 2H, J ) 4.5 Hz), 13C NMR (DMSO-d6): 164.9, 145.6, 128.8, 126.6 ppm. Tetrakis-nicotinoxymethyl-methane (3b). In a roundbottom two-neck 250 mL flask equipped with a magnetic stirrer, a condenser, a CaCl2 tube, and N2 atmosphere, 1.37 g (7.7 mmol) of nicotinic acid chloride hydrochloride 1e was stirred at 0 °C in 30 mL of pyridine. A total of 0.26 g (1.9 mmol) of pentaerythritol in 50 mL of pyridine was added through a dropping funnel. After 1 h 2.14 mL (15.4 mmol) of triethylamine was added and the mixture was heated to reflux for 2 h. Filtrate of the cooled reaction was evaporated to dryness under reduced pressure, recrystallized from ethanol/water 10: 90 mixture, and dried in vacuo. Yield 58%. 1H NMR (CDCl3): 9.2 (dd, 4H, J ) 2.0, 0.7 Hz), 8.8 (dd, 4H, J ) 5.0, 1.7 Hz), 8.25 (dt, 4H, J ) 8.0, 2.0 Hz), 7.35 (ddd, 4H, J ) 8.0, 5.0, 0.7 Hz), 4.72 ppm (s, 8H). 13C NMR (DMSO-d6): 164.4, 153.8, 150.1, 136.9, 125.2, 123.8, 63.5, 42.7 ppm. IR (KBr, cm-1) ν 3068 (CH2), 1725 (aryl CdO), 1591, 1580, 1479, 1425 (2-subst. pyridine), 1280, 1114 (ester C-O bond). Mp: 175 °C. MS (ESITOF N2): m/z 557 (M + 1, 100%). Anal. (C29 H24 N4 O8), Calculated: C(62.59) H(4.35) N(10.07), Found: C(62.18) H(4.37) N(9.85). Tetrakis-isonicotinoxymethyl-methane (3c). In a roundbottom two-neck 250 mL flask equipped with a magnetic

Na¨ttinen and Rissanen stirrer, a condenser, a CaCl2 tube, and argon-atmosphere, 4.00 g (22.5 mmol) of isonicotinic acid chloride hydrochloride 1f was stirred at 0 °C in 75 mL of pyridine. A total of 0.76 g (5.6 mmol) of pentaerythritol in 75 mL of pyridine was added through a dropping funnel. After 1 h 6.23 mL (45 mmol) of triethylamine was added and the mixture was heated to reflux for 4 h. Filtrate of the cooled reaction was evaporated to dryness under reduced pressure, recrystallized from ethanol/water 10:90 mixture, and dried in vacuo. Yield 55%. 1H NMR (CDCl3): 8.8 (dd, 8H, J ) 6.1, 2.7 Hz), 7.8 (dd, 8H, J ) 6.1, 2.7 Hz), 4.71 ppm (s, 8H). 13C NMR (CDCl3): 165.5, 151.9, 137.1, 123.6, 64.5, 44.4 ppm. IR (KBr, cm-1) ν 3054 (CH2), 1730 (aryl CdO), 1598, 1562, 1466, 1408 (3-subst. pyridine), 1275, 1122 (ester C-O bond). Mp: 170.9 °C. MS (ESI-TOF N2): m/z 557 (M + 1, 100%). Anal. (C29 H24 N4 O8), Calculated: C(62.59) H(4.35) N(10.07), Found: C(62.35) H(4.44) N(9.76). Tetrakis-benzoxymethyl-methane (4). In a roundbottom 250 mL flask equipped with a magnetic stirrer, a condenser, and a CaCl2 tube, 3.00 g (22.0 mmol) of pentaerythritol was refluxed in 20.34 mL (176.2 mmol) of benzoyl chloride for 4 h and evaporated to dryness under reduced pressure. The resulting 12.5 g was a mixture of di-, tri- and tetrasubstituted products and a trace of BzCl, with a ratio of 1:4:8. This mixture was recrystallized from CHCl3, yielding large perfect crystals of the tetrasubstituted product which were mechanically picked from the syrupy liquid, washed with CHCl3, and dried in air. 1H NMR (CDCl3): 8.0 (d, 8H, J ) 7.9, 1.5 Hz), 7.55 (d, 4H, J ) 7.9, 1.5 Hz), 7.4 (t, 8H, J ) 7.9 Hz), 4.71 ppm (s, 8H). 13C NMR (DMSO-d6): 166.3, 133.6, 129.9, 129.6, 128.7, 63.9, 43.3 ppm. IR (KBr, cm-1) ν 3064 (CH2), 2960 (arom CdC), 1721 (aryl CdO) 1267, 1110 (ester C - O bond). Mp: 51.9 C. MS (ESI-TOF N2): m/z 575 (M + Na, 100%), Anal. (C33 H28 O8 + CHCl3), Calculated: C(71.73) H(5.11) N(0), Found: C(66.51) H(4.79) N(0). 2,2,2-Tris-(nicotinoxy)-ethoxymethyl-tris-nicotinoxymethyl-methane (6b). In a round-bottom two-neck 250 mL flask equipped with a magnetic stirrer, a condenser, a CaCl2 tube, and N2 atmosphere, 3.00 g (21.2 mmol) of nicotinic acid chloride 1e was refluxed in 100 mL of pyridine with 1.301 g (10.65 mmol) of 4-(dimethylamino)pyridine (DMAP). A total of 0.449 g (1.75 mmol) of dipentaerythritol suspended 50 mL of pyridine was added through a dropping funnel in an hour and the mixture was refluxed overnight. The cooled reaction product was evaporated to dryness under reduced pressure, stirred with 100 mL of H2O, filtered, and the solid washed with 150 mL of H2O and dried in vacuo for 6 h to give the product with 99% yield. 1H NMR (DMSO-d6): 9.0 (d, 6H, J ) 2.0 Hz), 8.7 (dd, 6H, J ) 4.9, 1.5 Hz), 8.2 (dt, 6H, J ) 8.0, 2.0 Hz), 7.4 (dd, 6H, J ) 8.0, 4.9 Hz), 4.61 (s, 12H), 3.86 ppm (s, 4H). 13C NMR (DMSO-d6): 163.8, 152.9, 149.3, 135.9, 124.8, 122.9, 69.4, 63.3, 43.1 ppm. IR (KBr, cm-1) ν 3066 (CH2), 1728 (aryl Cd O), 1592, 1580, 1476, 1423 (2-subst. pyridine), 1280, 1119 (ester C-O bond). Mp: 257.7 °C. MS (ESI-TOF N2): m/z 885 (M + 1, 100%), Anal. (C46 H40 N6 O13), Calculated: C(62.44) H(4.56) N(9.50), Found: C(62.31) H(4.62) N(9.51). 2,2,2-Tris-(isonicotinoxy)-ethoxymethyl-tris-isonicotinoxymethyl-methane (6c). In a round-bottom two-neck 250 mL flask equipped with a magnetic stirrer, a condenser, a CaCl2 tube, and N2 atmosphere, 6.00 g (33.7 mmol) of isonicotinic acid chloride hydrochloride 1f was stirred at 0 °C in 50 mL of pyridine. A total of 1.43 g (5.6 mmol) of dipentaerythritol in 50 mL of pyridine was added through a dropping funnel. After 1 h, 6.8 mL (67.4 mmol) of triethylamine was added and the mixture was heated to reflux for 4 h. Filtrate of the cooled reaction product was evaporated to dryness under reduced pressure, dissolved in DMSO, precipitated with water, and dried in vacuo. Yield 27%. 1H NMR (DMSO-d6): 8.7 (dd, 12H, J ) 5.9, 2.7 Hz), 7.75 (dd, 12H, J ) 5.9, 2.7 Hz), 4.57 (s, 12H), 3.82 ppm (s, 4H). 13C NMR (DMSO-d6): 164.1, 150.4, 136.3, 122.4, 68.8, 63.8, 43.2 ppm. IR (KBr, cm-1) ν 3034 (CH2), 1733 (CH2), 1597, 1562, 1460, 1409 (3-subst. pyridine), 1275, 1124 (ester C-O bond). Mp: 166.1 °C. MS (ESI-TOF N2): m/z 885 (M + 1, 100%), Anal. (C46 H40 N6 O13), Calculated: C(62.44) H(4.56) N(9.50), Found: C(61.09) H(4.67) N(9.18).



Table 6. Crystallographic Parameters for Tetranicotinate 3b, Tetraisonicotine 3c, and Tetrabenzoate 4 compound

3b

3c

4

empirical formula formula wt crystal color, shape crystal dimensions (mm) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) calc density (mg/m3) temp of collection (K) Z Robs Rall GooF θ range for data collection (°) scan type refl collected/unique/cell refinement refined parameters res. electron density e/Å3

C29H24N4O8 556.52 colorless, prism 0.01 × 0.05 × 0.08 orthorhombic Pcan - no. 60 11.3366(4) 11.2741(4) 21.3129(11) 90 90 90 2724.0(2) 1.357 173 4 R1 ) 0.0675, wR2 ) 0.1311 R1 ) 0.1424, wR2 ) 0.1615 1.067 3.19 to 24.97 Phi/Omega, 1° 19475/9323/7682 186 0.214/-0.212

C29H24N4O8 556.52 colorless, prism 0.05 × 0.1 × 0.2 triclinic P-1 - no. 2 10.4715(3) 10.7168(3) 13.2433(3) 66.187(2) 88.791(2) 89.5740(10) 1359.35(6) 1.360 173 2 R1 ) 0.0536, wR2 ) 0.1097 R1 ) 0.1063, wR2 ) 0.1318 1.018 3.75 to 25.03 Phi/Omega, 1° 12732/4775/4371 371 0.205/-0.207

C33H28O8 + CHCl3 552.55 + 119.36 colorless, prism 0.3 × 0.4 × 0.8 triclinic P-1 - no. 2 11.4099(4) 12.1715(5) 12.8714(6) 112.245(5) 104.518(5) 90.025(5) 1592.6(11) 1.401 123 2 R1 ) 0.0501, wR2 ) 0.1192 R1 ) 0.0749, wR2 ) 0.1329 1.034 3.00 to 25.03 Phi/Omega, 2° 14753/5592/5159 406 0.234/-0.521

Table 7. Crystallographic Parameters for Hexanicotinate 6b, Hexaisonicotine 6c, and Hexabenzoate 7 compound

6b

6c

7

empirical formula formula wt crystal color, shape crystal dimensions (mm) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) calc density (mg/m3) temp of collection (K) Z Robs Rall GooF θ range for data collection (°) scan type refl collected/unique/cell refinement refined parameters res. electron density e/Å3



C52H46O13 878.89 colorless, block 0.15 × 0.2 × 0.2 triclinic P-1 - no. 2 10.8473(1) 14.6090(1) 14.8989(2) 74.0096(4) 88.5161(4) 79.4782(4) 2230.74(4) 1.308 173 2 R1 ) 0.0403, wR2 ) 0.1017 R1 ) 0.0476, wR2 ) 0.1073 1.036 2.91 to 25.68 Phi/Omega, 1° 21360/8238/7264 586 0.219/-0.190

2,2,2-Tris-(benzoxy)-ethoxymethyl-tris-benzoxymethylmethane (7). In a round-bottom 250 mL flask equipped with a magnetic stirrer, a condenser, and a CaCl2 tube, 8.00 g (31.5 mmol) of dipentaerythritol was refluxed with 21.75 mL (188.8 mmol) of benzoyl chloride and 26.2 mL (188.8 mmol) of trietylamine in acetonitrile overnight, evaporated to dryness under reduced pressure, recrystallized from CHCl3 and dried in vacuo. Yield 24.2 g (87%) 1H NMR (CDCl3): 7.9 (dd, 12H, J ) 8.0, 1.4 Hz), 7.6 (tt, 6H, J ) 7.3, 1.4 Hz), 7.4 (t, 12H, J ) 7.5 Hz), 4.53 (s, 12H), 3.8 ppm (s, 4H). 13C NMR (DMSO-d6): 165.3, 133.2, 129.2, 129.1, 128.5, 69.5, 63.4, 43.3 ppm. IR (KBr, cm-1) ν 3064 (CH2), 2960 (arom CdC), 1721 (aryl CdO) 1268, 1111 (ester C-O bond). Mp: 182.6 °C. MS (ESI-TOF N2): m/z 901 (M + Na, 100%), Anal. (C52 H46 O13), Calculated: C(71.06) H(5.28) N(0), Found: C(70.83) H(5.32) N(0.15). Tetrakis-nicotinoxymethyl-methane-tetrapicrate (8b). In a round-bottom 100 mL flask equipped with a magnetic stirrer, 0.20 g (0.36 mmol) of compound 3b dissolved in 40 mL of acetone was mixed with 0.825 g (1.8 mmol) of picric acid (incl 50% H2O) dissolved in 40 mL of acetone. The precipitate (0.43 g) was collected and air-dried. The mother liquid was also evaporated and found to contain a part of the product and

the excess picric acid. The analysis was performed on the precipitate alone. The crude yield calculated from the precipitate was 81%. 1H NMR (DMSO-d6): 9.2 (d, 4H, J ) 2.0 Hz), 8.9 (dd, 4H, J ) 5.2, 1.6 Hz), 8.58 (s, 8H), 8.5 (dt, 4H, J ) 8.0, 1.8 Hz), 7.7 (ddd, 4H, J ) 8.0, 5.0, 0.7 Hz), 4.79 (s, 8H), 4.49 (s, 4H + incl. H2O protons). 13C NMR (DMSO-d6): 163.7, 160.7, 151.6, 148.3, 141.8, 139.5, 126.1, 125.1, 124.8 124.3, 63.5, 42.7 ppm. IR (KBr, cm-1) ν 3464 (H2O), 3076 (O-H), 1742 (aryl CdO), 1631 (R-NO2), 1605, 1568, 1475, 1430 (2-subst. pyridine), 1364, 1319 (R-NO2), 1275, 1113 (ester C-O bond). Mp: 148.0 °C, Anal. (C53 H36 N16O36 + 3 x C2H3N), Calculated: C(44.84) H(2.74) N(15.96), Found: C(43.47) H(2.75) N(14.42). Tetrakis-isonicotinoxymethyl-methane-tetrapicrate (8c). In a round-bottom 100 mL flask equipped with a magnetic stirrer, 0.80 g (1.44 mmol) of compound 3c dissolved in 40 mL of acetone was mixed with 3.3 g (7.2 mmol) of picric acid (incl 50% H2O) dissolved in 40 mL of acetone. The precipitate was collected and air-dried. The mother liquid was also evaporated and found to contain a part of the product and the excess picric acid. The analysis was performed on the precipitate alone. The crude yield calculated from the mass of

352



Table 8. Crystallographic Parameters for Picrates 8b-9c compound empirical formula formula wt crystal color, shape crystal dimensions (mm) crystal system space group a (Å) b (Å) c (Å) R (deg.) β (deg.) γ (deg.) V (Å3) calculated density (mg/m3) temp of collection (K) Z Robs Rall GooF θ range for data collection (°) scan type refl collected/unique/cell refinement refined parameters res. electron density e/Å3 a

8b

8c

9c

C53H36N16O36 + 2 × C2H3N + C2a 1472.98 + 2 × 41.05 + 24.02 yellow, prism 0.25 × 0.25 × 0.4 monoclinic P21/n - no. 14 20.7953(2) 13.4913(1) 24.8136(3) 90 105.4780(4) 90 6709.12(11) 1.563 173 4 R1 ) 0.0543, wR2 ) 0.1302 R1 ) 0.0850, wR2 ) 0.1475 1.020 2.91 to 25.68 Phi/Omega, 1° 51061/13101/12403 1026 0.433/-0.396

C53H36N16O36 + 2 × C2H3N + O 1472.98 + 2 × 41.05 + 16.00 yellow, block 0.2 × 0.2 × 0.3 triclinic P-1, no. 2 12.5258(3) 17.5879(4) 31.6436(8) 84.771(1) 85.442(2) 71.810(2) 6585.6(3) 1.585 123 4 R1 ) 0.0929, wR2 ) 0.2251 R1 ) 0.1867, wR2 ) 0.2755 1.015 2.91 to 25.03 Phi/Omega, 1° 56844/22776/19544 1987 0.880/-0.495

C76H55N21O48 + 1.25 × C2H3N + (4 × 0.25 × O) + O 2030.41 + 1.25 × 41.05 + 2 × 16.00 yellow, block 0.2 × 0.3 × 0.4 monoclinic P21/c, no. 14 13.6746(3) 22.2874(4) 30.3715(8) 90 91.246(1) 90 9254.2(4) 1.517 173 4 R1 ) 0.0848, wR2 ) 0.2191 R1 ) 0.1410, wR2 ) 0.2522 1.055 2.91 to 24.71 Phi/Omega, 1° 63045/15959/15199 1396 0.871/-0.484

Disordered acetonitrile molecule. Table 9. Solvents Used in the Crystallizations compound

solvent(s)

3b 3c 4 6b 6c 7 8b 8c 9c

EtOH MeCN CHCl3 MeOH/DCM EtOH DMSO-d6 MeCN H2O/MeCN MeCN

solvent ratio

50:50

50:50

the precipitate was 66%. 1H NMR (DMSO-d6): 8.8 (dd, 4H, J ) 6.1, 3.1 Hz), 8.58 (s, 8H), 7.9 (dd, 4H, J ) 6.1, 3.1 Hz), 6.18 (br, 4H + incl. H2O protons), 4.78 ppm (s, 8H). 13C NMR (DMSO-D6): 165.5, 162.2, 151.2, 143.3, 139.0, 126.6, 125.5, 124.6, 65.4, 44.1 ppm. IR (KBr, cm-1) ν 3450 (H2O), 3102 (OH), 1742 (aryl CdO), 1635 (R-NO2), 1608, 1550, 1475, 1432 (3-subst. pyridine), 1342, 1317 (R-NO2), 1270, 1123 (ester C-O bond). Mp: 157.0 °C. 2,2,2-Tris-(isonicotinoxy)-ethoxymethyl-tris-isonicotinoxymethyl-methane pentapicrate (9c). In a roundbottom 100 mL flask equipped with a magnetic stirrer, 0.20 g (0.22 mmol) of compound 5b dissolved in 20 mL of acetonitrile was mixed with 3.3 g (7.2 mmol) of picric acid (incl 50% H2O) dissolved in 20 mL of acetone. The precipitate was filtered and air-dried. The mother liquid was also evaporated and found to contain a part of the product and the excess picric acid. The analysis was performed on the precipitate alone. The crude yield calculated from the mass of the precipitate was 85%. 1H NMR (DMSO-d6): 8.8 (dd, 12H, J ) 6.3, 3.1 Hz), 8.58 (s, 10H), 8.0 (dd, 12H, J ) 6.3, 3.1 Hz), 5.88 ppm (b, 4H + incl. H2O protons), 4.61 (s, 12H), 3.83 (s, 4H) ppm. 13C NMR (DMSOd6): 163.4, 160.7, 148.3, 148.0, 141.8, 138.8, 125.1, 124.3, 123.7, 68.7, 64.0, 43.2 ppm. IR (KBr, cm-1) ν 3450 (H2O), 3079 (OH), 1741 (aryl CdO), 1637 (R-NO2), 1606, 1568, 1516, 1499, 1432 (3-subst. pyridine), 1365, 1318 (R-NO2), 1272, 1122 (ester C-O bond). Mp: 149.0 °C, Anal. (C76 H55 N21O48 + 1.25 x C2H3N + 1.25 x H2O), C(44.80) H(2.93) N(14.81), Found: C(44.82) H(2.79) N(14.59). X-ray. The crystallographic parameters for the following compounds are presented in Tables 6-8. The single-crystal X-ray diffraction was performed with a Nonius KappaCCD diffractometer with graphite monochromatized MoKR(λ ) 0.71073 Å) radiation. Collect software was

used in the measurement and DENZO-SMN in the processing of the data. The structures were solved and refined by fullmatrix least-squares on F2 with WinGX-software package39 utilizing SHELXS9740 and SHELXL9741 modules. Hydrogen atoms were refined by a riding model. No absorption correction was performed for any of the compounds. The graphic presentations of the structures were created with the software Diamond, Siemens XP, POV-Ray, and ViewerLite.42-45 The crystals were obtained by slow evaporation of solvent. The solvents used in the crystallization of the compounds are presented in the following table (Table 9).

Acknowledgment. We thank the Finnish Academy for the financial support. Supporting Information Available: CCDC 200075200083 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21 EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]). Additional data (inter- and intramolecular distances) on the compounds 3b-9c are available free of charge via the Internet at http://pubs.acs.org.

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CG0340291

Dendritic Pyridine-Functionalized Polyesters and Their Polycationic

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