Crystal structure of a G-1 dendrimer of aminoisophtalic acid

The G-1 dendrimer 4,4',4'',4'''-methanetetrayltetrakis(N-(3,5-dicarboxyphenyl)benzamide) has been obtained from 4,4',4'',4'''-methanetetrayltetrabenzoic acid and isophtalic acid. The compound was recrystallized from methanol and its structure resolved. The crystal belongs to the tetragonal crystal system space group I-4 2d, the cell lengths being a=b=18.9585(16) Å, and c=23.703(2) Å. The crystal structure evidences the formation of cavities. Only one type of hydrogen bond is observed implying the nitrogen atom (acting as donor) and the oxygen atom of one carboxy group of the aminoisophtalic residue (as acceptor), while the other carboxy group does not participate in the network. The donor-acceptor distance is 3.024(2) Å.


Introduction
Crystals are described by translation of the unit cell into all three directions of space, but by considering them as supramolecular entities, they may be analyzed them in terms of networks.This way of analyzing molecular crystals is called molecular tectonics. 1Consequently, molecular tectonics is a supramolecular construction using tectonic subunits 2 and, according to Wang et al, 2 a tecton is a molecule whose interactions are dominated by specific attractive forces that induce the assembly of aggregates with controlled geometries.Therefore, tectons are active building units bearing recognition information and thus capable of recognizing each other, 1 and consist of multiple peripheral sticky sites linked to a core that holds them in a suitable orientation. 3traphenylmethane may be considered as a reference molecule in designing tetrapodal tectons.Its crystal structure was first reported by Sumsion and McLachlan in 1950, 4 and latter refined by Robbins et al. 5 The X-ray diffraction studies showed that the crystal is tetragonal, space group P4 2 1 c, with unequal central valence angles, not far from the tetrahedral ones.Several tectons were designed by linking functional groups to the aromatic rings.Examples are hydroxy, 6 halogens, 7 carboxy, 8 ethynylpyridinone, 9 and acetamido and aminobenzamido 10 groups, whose crystal structures have been resolved.The resulting crystal structures, where hydrogen bonding, coordination to metals or weak interactions play a decisive role, usually have large chambers. 8e central carbon atom has been substituted by Si, Sn and Pb, 2,6,[11][12] and other central nuclei or cores may be used as well.Adamantyl tetrapodal core, 13-14 2,2',6,6'tetracarboxybiphenyl, 15 and the pentaerythrityl-tetraphenil ether 3,16 are nice examples.
Alternative residues to the phenyl moities which constitute the four branches 17 have also been used for, as well as other cores with different podal degrees. 18ndrimers differ from classical polymers by their symmetry, starburst branching and terminal functionality density. 190][21] The divergent method consists on the sequential addition of repeating units to a starting core (normally a small molecule or ion), thus forming shells or generations.Dendrimers are of great interest as carriers of functional groups, 22 the number of which depends on the number of branches at the core (core multiplicity); the number of branches on each monomer repeating unit, and the number of generations.Different cores, particularly with different number of functionalities, have been used. 23e crystal structure of tetrakis(4-carboxyphenyl)methane have been reported by Malek et al 8 as well as the structure of the silane analogous. 11To this core, four aminoisophalic groups were attached forming a G-1 dendrimer which exhibits eight carboxy groups in the surface.In this communication, the crystal structure of 4,4',4'',4'''methanetetrayltetrakis(N-(3,5-dicarboxy-phenyl)benzamide), in which four aminoisophtalic units are linked to the core tetrakis(4-carboxyphenyl)methane is presented.

Experimental
Thyonil chloride was used as received from Aldrich.1,4-dioxane was from Panreac and dried over sodium/benzophenone.5] The final compound was obtained according to the scheme of Figure 1.Synthesis of 4,4',4'',4'''-methanetetrayltetrabenzoyl chloride: A mixture of tetrakis(4carboxyphenyl)methane (0.34g, 0.68 mmol) and 15 mL of thionyl chloride was heated to and held at a gentle reflux until all the solids were dissolved.Then, excess of thionyl chloride was removed under reduced pressure.The product obtained was used without further purification.
1 H and 13 C NMR spectra were recorded on a Brucker AC-300 MHz.

Results and discussion
The tetrahedral geometry of the molecule is shown in Figure 2, together with all hydrogen bonds (dot red lines) in which it participates.The central atom of the molecule is a tetrahedral carbon atom and consequently a diamondoid crystal structure results. 26The C-C-C angles are either 113.4 or 101.9 o , far from the tetrahedral value (Figure 2, right).The aromatic rings directly bonded to the central atom are quasi-planar, the maximum distance from any carbon atom to the plane being 0.005 Å.On the other hand, these distances are higher in the isophtalic rings, now reaching a maximum distance value of 0.019 Å.The angle between interior aromatic rings planes are either 88.9º or 60.8º.The angle between the two aromatic rings planes belonging to the same branch is 70.9 o .Figure 3 shows the crystal packing along the b and c crystallographic axes.The packing along the a axis is equal to the packing along b but rotated 90º.The distance between the central carbon atoms of molecules connected through hydrogen bonds is 17.893 Å.

Figure 3
Figure 3 evidences the formation of channels along the a and b axes.Figure4

Figure 4 shows
Figure 3 evidences the formation of channels along the a and b axes.Figure 4 shows an enlarged view of the cross section along b (or equivalently along a).The cross section is an almost perfect elipse with radius values of 6.1 Å and 3.7 Å, giving an area of 71 Å 2 .A wide range of cross sections values have been observed for closely related tectons.Table2resumes some results.

Figure 4 .
Figure 4. Cross section of channels formed along the a and b axes.
the number of possible donor/acceptor hydrogen bond sites, each branch of the molecule is only bonded to other two molecules through N-HO hydrogen bonds.It is to say, only one type of hydrogen bond is observed implying the nitrogen atom (acting as donor) and the oxygen atom of one carboxy group of the aminoisophtalic residue (as acceptor), while the other carboxy group does not participate in the network.The geometric parameters of the hydrogen bond are shown in
(http://www.ccdc.cam.ac.uk/prods/mercury).A summary of the crystal data and experimental details are listed in Table1.

Table 2 .
Published cross sections for several tecton crystals.

Table 3 .Table 3 .
Geometric parameters of the N-HO hydrogen bond.