Crystal structures of 1,4-diazabicyclo[2.2.2]octan-1-ium 4-nitrobenzoate dihydrate and 1,4-diazabicyclo[2.2.2]octane-1,4-diium bis(4-nitrobenzoate): the influence of solvent upon the stoichiometry of the formed salt

Solvent-dependent outcomes are noted in co-crystallization experiments between DABCO and 4-nitrobenzoic acid with mono- and diprotonated forms of DABCO are isolated.


Chemical context
The formation of co-crystals or salts is dependent on the difference in pK a of the interacting species (Childs et al., 2007). Thus, when the Á(pK a ) [= pK a (base) À pK a (acid)] value is greater than three, a salt is anticipated. In this context, it is not surprising that a search of the Cambridge Structural Database (CSD, version 53.5, last update November 2013;Allen, 2002) showed that nearly 90% of the 57 multi-component crystals, containing species derived from highly basic 1,4-diazabicyclo[2.2.2]octane (DABCO) and a carboxylic acid, contained at least a mono-protonated form of DABCO. It was in the context of on-going studies of co-crystallization experiments (Broker & Tiekink, 2007;Arman & Tiekink, 2013;Arman et al., 2014) between nitrogen-containing molecules and carboxylic acids, that the title salts were isolated. The co-crystallization experiments yielding the title salts produced unexpected outcomes in that while (1) formed as a 1:1 salt dihydrate from the 1:1 co-crystallization of DABCO and 4-nitrobenzoic acid in ethanol/water (3/1) solution, a 1:2 salt (2) was isolated from the 1:1 co-crystallization of DABCO and 4-nitrobenzoic acid in methanol solution. The molecular and crystal structures of (1) and (2) are described herein. ISSN 1600-5368

Figure 2
The molecular structures of the three independent constituents of (2), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines (see Table 2 for details).
Thus, links between layers are of the type water-O1W-HÁ Á ÁN3 and water-OW2-HÁ Á ÁO2(carboxylate). Additional stability to the supramolecular assembly is afforded by methylene-C-HÁ Á ÁO2(carboxylate) and O2W(water) interactions; it is noteworthy that both of the former interactions involve hydrogen atoms derived from the same methylene-C12 atom (Table 1). A methylene-C-HÁ Á ÁO3(nitro) interaction is also formed; the nitro-O4 atom does not form a significant interaction in this scenario. Although there is an alignment of benzene rings, the closestcontact is 3.7376 (7) Å , occurring between centrosymmetrically related rings [symmetry operation: 2 À x, 1 À y, 1 À z].
In (2), the di-cation is linked to two anions via strong N-HÁ Á ÁO hydrogen bonds ( Fig. 4 and Table 2). Globally, the three ion aggregates assemble into layers in the ab plane that stack along the c axis. A large number of C-HÁ Á ÁO interactions occur, remarkably featuring a narrow range of HÁ Á ÁO separations, i.e. 2.41-2.42 Å ( Table 2). All interactions involve methylene-H atoms as donors. The carboxylate-O2 and O4 atoms and all nitro but O3 atoms are acceptors; both methylene-H atoms of methylene-C17 and C20 participate in C-HÁ Á ÁO interactions. The result of these interactions is the formation of a three-dimensional architecture (Fig. 4). Additional stability to the supramolecular assembly is afforded by interactions between inversion-related rings, i.e. intercentroid distances = 3.5644 (16) Å for interactions between the C2-C8 rings (symmetry code: Àx + 1, Ày + 2, Àz + 2) and 3.6527 (16) Å between C9-C14 rings (symmetry code: Àx + 2, Ày + 1, Àz). An alternate description of the global crystal packing is based on alternating of layers of cations and layers of anions along the c axis (Fig. 5).

Database survey
As mentioned in the Chemical context, there are 57 species in the crystallographic literature containing DABCO or its mono-or diprotonated forms and a carboxylic acid or carboxylate anion. In fact, co-crystals are rare, being around 10% of all structures. Co-crystals are formed with several dicarboxylic acids where the functional groups are separated by long chains of over four carbon atoms (Braga et al., 2003;Moon & Park, 2012), with phosphonoacetic acid (Bowes et al., 2003) and with isophthalic acid (Marivel et al., 2010). While the majority of the remaining structures contain species derived from a dicarboxylic acid, there are 13 examples of structures containing species derived from a mono-carboxylic acid which are more directly suitable for comparison with (1) and (2). Further, in each case the original carboxylic acid was connected to an aromatic ring. Of the sub-set of 13 structures, Unit-cell contents shown in projection down the a axis for (1). The O-HÁ Á ÁO, O-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds are shown as orange, blue and green dashed lines, respectively (see Table 1 for details).

Figure 4
Unit-cell contents shown in projection down the a axis for (2). The N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds are shown as blue and green dashed lines, respectively (see Table 2 for details).

Figure 5
Unit-cell contents shown in projection down the b axis for (2). The N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds are shown as blue and green dashed lines, respectively (see Table 2 for details).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Carbon-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) = 1.2U eq (C). The N-bound H-atoms were located in a difference Fourier map but were refined with a distance restraint: N-H = 0.88 (1) Å with U iso (H) = 1.2U eq (N). For (1), the water-bound H atoms were refined with distance restraints: O-H = 0.84 (1) and HÁ Á ÁH = 1.39 (2) Å with U iso (H) =1.5U eq (O). For (2), the maximum and minimum residual electron density peaks of 0.60 and 0.58 e Å À3 , respectively, were located 0.81 and 0.10 Å from atoms O5 and H4N, respectively. In order to confirm the location of the Nbound H atoms, in a separate refinement these were refined  Table 3 Experimental details.
( H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0467P) 2 + 0.8811P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.40 e Å −3 Δρ min = −0.30 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0076 (6) Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.63866 (13)    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.60 e Å −3 Δρ min = −0.58 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.