Crystal structures of three anhydrous salts of the Lewis base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with the ring-substituted benzoic acid analogues 4-aminobenzoic acid, 3,5-dinitrobenzoic acid and 3,5-dinitrosalicylic acid

The anhydrous morpholinium salts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with 4-aminobenzoic acid, 3,5-dinitrobenzoic acid and 3,5-dinitrosalicylic acid, provide one example of a three-dimensional hydrogen-bonded network polymer and two of weakly inter-associated hydrogen-bonded cation–anion units.


Chemical context and database survey
The Lewis base 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU) is an alkaloid isolated from the sponge Niphates digitalis (Regalado et al., 2010) but is commonly synthesized. It finds use as a curing agent for epoxy resins, as a catalyst in organic syntheses, and as a counter-cation in metal complex chemistry, e.g. with the pentabromo(triphenylphosphane)platinum(IV) monoanion (Motevalli et al., 1989). It has also found use in binding organic liquids (BOLs), which usually comprise a mixture of amidines or guanidine and alcohol, and are used to reversibly capture and release gases such as CO 2 , CS 2 , SO 2 or COS (Shannon et al., 2015;Pé rez et al., 2004;Heldebrant et al., 2009). The structure of one of these formed from the absorption of CO 2 is the bicarbonate (Pé rez et al., 2004).
No reported crystal structures of salts with simple substituted benzoic acids are found, so in order to examine the hydrogen-bonding in crystals of the DBU salts with some common ring-substituted benzoic acids, a number of these were prepared. Suitable crystals were obtained with 4aminobenzoic acid (PABA), (3,5-dinitrobenzoic acid (DNBA) and (3,5-dinitrosalicylic acid (DNSA), giving the anhydrous salts, C 9 H 17 N 2 + C 7 H 6 NO 2 À (I), C 9 H 17 N 2 + C 7 H 3 N 2 O 6 À (II) and C 9 H 17 N 2 + C 7 H 3 N 2 O 7 À (III), respectively and their structures and hydrogen-bonding modes are reported herein.

Figure 2
The atom-numbering scheme and the molecular conformation of the DBU cation (A) and the DNBA anion (B) in (II) with displacement ellipsoids drawn at the 40% probability level. The bonds in the minor disordered section of the six-membered ring of the cation and the cationanion hydrogen bonds are shown as dashed lines.

Figure 3
The atom-numbering scheme and the molecular conformation of the DBU cation (A) and the DNSA anion (B) in (III) with displacement ellipsoids drawn at the 40% probability level. The bonds in the minor disordered section of the six-membered ring of the cation are shown as dashed lines.

Figure 5
The packing of the hydrogen-bonded cation-anion pairs in the unit cell of (II), viewed along a. The minor-component disordered atoms and the non-associative H atoms have been omitted.

Synthesis and crystallization
The title compounds (I)-(III) were prepared by first dissolving 100 mg of either PABA, DNBA, or DNSA in 5 mL of warm  ethanol followed by the addition, with stirring, of 111 mg (I), 72 mg (II) or 67 mg (III) of BDU, respectively. Slow evaporation at room temperature gave colourless needles of (I), colourless prisms of (II), and fine yellow needles of (III), from which specimens were cleaved for the X-ray analyses.

Refinement details
Crystal data, data collection and structure refinement details are given in Table 4. Hydrogen atoms were placed in calculated positions [C-H aromatic = 0.95 Å or C-H methylene = 0.99 Å ] and were allowed to ride in the refinements, with U iso (H) = 1.2U eq (C). The amine and aminium H-atoms were located in difference-Fourier analyses and were allowed to refine with distance restraints [N-H = 0.90 (2) Å ] and with U iso (H) = 1.2U eq (N). Disorder involving atoms C9A and C10A of the six-membered ring systems of both (II) and (III) gave refined minor occupancy sites C12A and C13A, with site occupancy factors of 0.735 (3)/0.265 (3) and 0.686 (4)/ 0.314 (4), respectively. Also in (III), the phenol group of the DNSA anion was found to be disordered with the minor occupancy site (O21B) having a SOF = 0.28, which was fixed in the final cycles of refinement. In the structure of (I), although of no relevance in the achiral molecule, the Flack parameter (Flack, 1983) was determined as À0.1 (13) for 1668 Friedel pairs, which serves to indicate the lack of any usable anomalous scattering signal, as expected for an all-light-atom structure determined with Mo K X-rays.

sup-1
Acta Cryst.  H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0438P) 2 + 0.0476P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.20 e Å −3 Δρ min = −0.25 e Å −3 Absolute structure: Flack (1983), 1668 Friedel pairs Absolute structure parameter: −0.1 (13) Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 > 2sigma(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.