Crystal structures of three anhydrous salts of the Lewis base 1,8-di-aza-bicyclo-[5.4.0]undec-7-ene (DBU) with the ring-substituted benzoic acid analogues 4-amino-benzoic acid, 3,5-di-nitro-benzoic acid and 3,5-di-nitro-salicylic acid.

The anhydrous salts of the Lewis base 1,8-di-aza-bicyclo-[5.4.0]undec-7-ene (DBU) with 4-amino-benzoic acid [1-aza-8-azoniabi-cyclo-[5.4.0]undec-7-ene 4-amino-benzoate, C9H17N2 (+)·C7H6NO2 (-) (I)], 3,5-di-nitro-benzoic acid [1-aza-8-azoniabi-cyclo-[5.4.0]undec-7-ene 3,5-di-nitro-benzoate, C9H17N2 (+)·C7H3N2O6 (-), (II)] and 3,5-di-nitro-salicylic acid (DNSA) [1-aza-8-azoniabi-cyclo-[5.4.0]undec-7-ene 2-hy-droxy-3,5-di-nitro-benzoate, C9H17N2 (+)·C7H3N2O7 (-), (III)] have been determined and their hydrogen-bonded structures are described. In both (II) and (III), the DBU cations have a common disorder in three of the C atoms of the six-membered ring moieties [site-occupancy factors (SOF) = 0.735 (3)/0.265 (3) and 0.686 (4)/0.314 (4), respectively], while in (III), there is additional rotational disorder in the DNSA anion, giving two sites (SOF = 0.72/0.28, values fixed) for the phenol group. In the crystals of (I) and (III), the cation-anion pairs are linked through a primary N-H⋯Ocarbox-yl hydrogen bond [2.665 (2) and 2.869 (3) Å, respectively]. In (II), the ion pairs are linked through an asymmetric three-centre R 1 (2)(4), N-H⋯O,O' chelate association. In (I), structure extension is through amine N-H⋯Ocarbox-yl hydrogen bonds between the PABA anions, giving a three-dimensional structure. The crystal structures of (II) and (III) are very similar, the cation-anion pairs being associated only through weak C-H⋯O hydrogen bonds, giving in both overall two-dimensional layered structures lying parallel to (001). No π-π ring associations are present in any of the structures.