Crystal structure of the 1,3,6,8-tetraazatricyclo[4.3.1.13,8]undecane (TATU)–4-nitrophenol (1/2) adduct: the role of anomeric effect in the formation of a second hydrogen-bond interaction

The components of the ternary cocrystalline adduct are linked by intermolecular O—H⋯N hydrogen bonds This is the first example of a 1:2 adduct containing two 4-nitrophenol molecules and one azaadamantane structure.

In the title ternary co-crystalline adduct, C 7 H 14 N 4 Á2C 6 H 5 NO 3 , molecules are linked by two intermolecular O-HÁ Á ÁN hydrogen bonds, forming a tricomponent aggregates in the asymmetric unit. The hydrogen-bond formation to one of the N atoms is enough to induce structural stereoelectronic effects in the normal donor!acceptor direction. In the title adduct, the two independent nitrophenol molecules are essentially planar, with maximum deviations of 0.0157 (13) and 0.0039 (13) Å . The dihedral angles between the planes of the nitro group and the attached benzene rings are 4.04 (17) and 5.79 (17) . In the crystal, aggregates are connected by C-HÁ Á ÁO hydrogen bonds, forming a supramolecular dimer enclosing an R 6 6 (32) ring motif. Additional C-HÁ Á ÁO intermolecular hydrogenbonding interactions form a second supramolecular inversion dimer with an R 2 2 (10) motif. These units are linked via C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds, forming a three-dimensional network.

Chemical context
The chemistry of the aminoalkylation of aromatic substrates by the Mannich reaction is of great interest and chemical importance (Tramontini et al., 1988). Some modern variants of Mannich reactions have been developed using preformed aminals or hemiaminals as Mannich electrophiles for aminomethylation reactions (Katritzky et al., 2005). The use of these preformed aminomethylating reagents, particularly those derived from common amines, is becoming more frequent (Tramontini & Angiolini, 1990), a mechanism involving initial formation of a hydrogen-bonded complex between a Mannich preformed reagent and the phenolic substrate has been documented (Burckhalter & Leib, 1961). Electron density at the free ortho position of the phenol and the reactivity of the phenolic hydroxy group played a crucial role and the reaction does not occur in the absence of the hydroxy group (Deng et al. 2014). The phenol-N complex has also served as a good model for the investigation of proton and electron-transfer processes occurring in living matter, it being generally assumed that this interaction consists solely of the attraction between the lone pair of the amine N atom and the phenolic hydroxy proton (Lu et al. 2006). In addition to the typical features of intermolecular hydrogen bonding, these systems have an extra advantage over many other complexes because they play an important role in probing the anomeric effect in N-C-N (aminal) systems even though the anomeric effect is ISSN 2056-9890 well recognized as an important factor in defining the predominant conformational state of many cyclic heteroatomcontaining compounds (Dabbagh et al. 2002). There is little evidence in the literature for bond shortening and lengthening in cyclic aminals (Takahashi et al. 2007). We have undertaken a long-term project designed to systematically investigate the structures, chemical properties and reactivity of macrocyclic aminals as preformed electrophilic reagents for the synthesis of phenolic Mannich bases through simple and efficient methodologies. As part of this investigation we have recently reported the synthesis of 1, 3,6,8-tetraazatricyclo[4.3.1.1 3,8 ]undecane (TATU), (II) (Rivera et al. 2004). TATU which has two non-equivalent hydrogen-bond acceptor N-atom sites is a good model for the investigation of intermolecular hydrogen bonding with phenols and for studying the nature of the anomeric effect in the N-C-N molecular segment. One interesting feature of the structure of TATU is that two of the N atoms are similar to those in 1,3,5,7-tetraazatricyclo[3.3.1.1 3,7 ]decane [(III), also known as urotropine, and hexamethylenetetramine, HMTA] and the other two are similar to those in 1, 3,6,8-tetraazatricyclo[4.4.1.1 3,8 ]dodecane [(IV), TATD]. We have previously studied the structure of the 1:1 complex produced by the reaction of TATU with hydroquinone (Rivera et al. 2007). In that work, we found that the preference for a particular hydrogen-bond interaction site depends strongly upon the lone-pair orbital hybridization of the N atom. We also demonstrated that a greater degree of sp 3 character favours the NÁ Á ÁH-O interactions. Later (Rivera et al. 2011), we reported the preparation and structure of an acid-base adduct assembled from TATU and pentachlorophenol (PCP). X-ray diffraction analysis of this salt confirmed that the -OH group of the PCP transfers a proton to the N atoms of the aminal moiety. The main consequence of this protonation is the distortion of the cage structure which was attributed to the anomeric effect that governs the aminal group. Studies of phenol complexes with tertiary amines in the solid state show that the proton transfer depends not only on the ÁK a (pK a amine À pK a acid) value, but also on steric and packing effects (Majerz & Sawka-Dobrowolska, 1996). Because of the acidity of the phenolic group, the proton potential of the hydrogen-bonded system can be fundamentally changed by substituents in the 4-position of the phenol ring (Xiong et al., 2002). Because of this and as a logical extension of our previous efforts to understand the hydrogenbonding interactions between TATU and phenols, we investigated the reaction of TATU with 4-nitrophenol. It is worth noting that, contrary to our initial expectation, the substitution of hydroquinone by 4-nitrophenol afforded the title compound, (I). Its crystal structure shows a 1:2 stoichiometry. The aminal moiety is linked to two 4-nitrophenol molecules via O-HÁ Á ÁN hydrogen bonds where one interaction is to a more basic site (N3) and the other one to an N atom adjacent to the ethylene bridge (N1).

Structural commentary
In the ternary cocrystal of TATU with two molecules of 4nitrophenol, (I) (Fig. 1), the asymmetric unit was chosen such that the two nitrophenol molecules are linked by two intermolecular O-HÁ Á ÁN hydrogen bonds (Table 1). This arrangement contrasts with related structures (Ng, 2008;Ng et al. 2001), where the urotropine (HMTA) moiety uses only one of its four N atoms to link to a 4-nitrophenol molecule via a hydrogen bond. It is possible, however, that the presence of a solvent water molecule may influence this observation.
In the three-component aggregates observed here, the O1Á Á ÁN3 distance [2.6551 (13) Å ], is similar to those observed previously in hydrogen-bond adducts between HMTA and 4nitrophenol (Ng, 2008;Ng et al. 2001). However, this is shorter than the O21Á Á ÁN1 distance in the second contact [2.7377 (14) Å ]. This is a consequence of the relative proton affinities of the N atoms in TATU (Rivera et al., 2007). Though this polyamine has four potential protonation sites it is evident that atoms N1 and N3 are not equivalent to one another. In particular, they differ in terms of their pyramidal character which can be estimated from the sum of the bond angles around each N atom, AE(CNC) (339.5 for N1 and 328.3 for N3). The greater sp 3 character of the N atom contributes to the increase in proton affinity for N3. The main questions concerning the structure of this ternary cocrystal concerns the site of the formation of the second hydrogen bond. If all N atoms were equivalent, as in HMTA, the second hydrogen- The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level. Hydrogen bonds are drawn as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). bond interaction would be possible with any of the amino groups. However, in TATU, as expected, the first hydrogen bond involves more basic site (N3) rather than forming at a site than to adjacent to the ethylene bridge (N1 or N2). For the second hydrogen-bond contact, there are three potential alternative sites. First, a symmetric structure using the more basic N4 site. Alternatively, one of the less basic N1 or N2 atoms linked by the ethylene bridge could be used, resulting in a less symmetric cocrystal.

D-HÁ
In principle, it might be expected that the supramolecular structure of the title compound could be considered to be similar to that of the cocrystal formed between hydroquinone and TATU (Rivera et al., 2007). In that case, however, the asymmetric unit contains only one half of hydroquinone molecule and the aminal cage structure, and self-assembly into a symmetric supramolecular structure via hydrogen bonds join them into a zigzag chain extending along the crystal b axis (Rivera et al., 2007). Certainly each aminal unit links to two hydroquinone molecules via O-HÁ Á ÁN hydrogen bonds once the symmetry operation is applied, but, in the present case, the asymmetric unit comprises a pair of 4-nitrophenol molecules and one complete molecule of the aminal cage not related by symmetry elements. Account should also be taken of the effect that the phenol pK a and the extent of polarization of the N-CH 2 -N bonds upon hydrogen-bond formation. In fact, although the values of the bond lengths and angles in the aminal cage of the title compound are within expected values, there are notable differences when compared to the related TUTU/hydroquinone system (Rivera et al., 2007).
Comparison of the C-N bond lengths in the title compound with respect to the mean value of 1.469 Å (Allen et al., 1987) points out that the presence of strong hydrogen bonds in the title compound affect the length of the CH 2 -N single bonds in the heterocyclic cage system. While in the related structure, the formation of a hydrogen bond with hydroquinone does not effect the CH 2 -N single-bond lengths significantly [mean values 1.469 (2) Å ] (Rivera et al., 2007), for the title compound, the C5-N3 bond is lengthened [1.4815 (14) Å ], while C5-N4 is shortened to 1.4639 (15) Å . In addition, the C4-N3 bond in the title compound is longer than in the related structure by 0.016 Å . The shortest C-N bond within the aminal cage ring in the title compound is the N2-C4 bond at 1.4517 (15)  The hydrogen-bonded inversion dimer displaying an R 4 4 (32) motif in the crystal of the title compound, with hydrogen bonds drawn as dashed lines.

Figure 3
The hydrogen-bonded inversion dimer displaying an R 2 2 (10) motif in the crystal of the title compound, with hydrogen bonds drawn as dashed lines.

Figure 4
The overall packing of (I). H atoms not involved in hydrogen bonds have been omitted.
connected to presence of the very strong O-HÁ Á ÁN hydrogen bonds, between the N atoms of the aminal cage structure and the 4-nitrophenol molecules. Thus, in the title compound, the aminal cage structure acquires a more pronounced anomeric effect due to these hydrogen-bond interactions (Alder et al., 1999). It noteworthy that in the title compound the lengthening of C5-N3 facilitates the interaction of the nonbonding molecular orbitals of N4 and N2 with the *C5-N3 antibonding orbital and thus these N atoms are less likely to form the second hydrogen bond and results in the observed second hydrogen-bond interaction between the N1 atom from the aminal cage and the O21-H21 group of the second 4-nitrophenol molecule.
The two independent nitrophenol molecules are essentially planar, with maximum deviations of 0.0157 (13) and 0.0039 (13) Å . The nitro groups are almost coplanar with the aromatic ring plane; the dihedral angles between the planes of the nitro group and the attached benzene rings are 4.04 (17) and 5.79 (17) . The coplanarity of the nitro groups with the aromatic rings is stabilized by weak C-HÁ Á ÁO hydrogen bonds between the nitro O atoms and the H atoms of neighbouring structures (Table 1). In addition, the two of the hydroxy substituents C-O bonds are similar in length, but are somewhat longer than the normal value for a OH group bound to an aromatic ring (1.362Å ; Allen et al., 1987).

Supramolecular features
In the crystal structure of (I), two 1:2 adducts are linked to one another by C26-H26Á Á ÁO13 i bond pairs (Table 1) so that an inversion dimer is formed (Table 1 and Fig. 2), which displays an R 6 6 (32) motif (Bernstein et al., 1995). The hydrogen bond with atom C6 as the donor firms an inversion dimer, generating forming a zigzag chain running parallel to [111] through a second C6-H6AÁ Á ÁO23 iv hydrogen bond (Table 1 and Fig. 4). Additional C5-H5AÁ Á ÁO11 ii intermolecular hydrogen-bonding interactions form a second supramolecular inversion dimer with an R 2 2 (10) motif (Fig. 3). Both dimers are further linked by a weak intermolecular C25-H25Á Á ÁN2 v hydrogen bond (Table 1). These contacts combine to generate a three-dimensional network structure.

Synthesis and crystallization
Solvent-free direct preparation of the title compound from 1,3,6,8-tetraazatricyclo[4.3.1.1 3,8 ]dodecane (TATU) (0.15 g, 1.00 mmol) and 4-nitrophenol (0.21 g, 1.5 mmol) was carried out by a mechanochemical interaction in a mortar at room temperature. 30 min were required to complete the reaction. The mixture was then dissolved in a minimum amount of methanol and left to crystallize at room temperature. Subsequent recrystallization with MeOH gave the title compound as colourless crystals in 70% yield (m.p. 388-389 K).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.