Syntheses, crystal structures and Hirshfeld surface analysis of three salts of 1-(4-nitro­phenyl)­piperazine

The structures and Hirshfeld surface analysis of three salts of 1-(4-nitro­phenyl)­piperazine are discussed. In 4-(4-nitro­phen­yl)piperazin-1-ium salicylate (C(10)H(14)N(3)O(2) (+)·C(7)H(5)O(3) (−)), there are strong hydrogen bonds between cation and anion and the 4-nitro­phenyl substituent occupies an equatorial position in the piperazinium ring. The cation and anion are linked together by supra­molecular inter­actions [graph-set notation of hydrogen bonding [Image: see text] (6) propagating in the a-axis direction]. Additionally, there is π–π stacking involving the salicylate anion and the piperazinium cation in adjacent asymmetric units as well as a C—H⋯π inter­action between a hydrogen atom on the piperazine ring and the phenyl ring within the salicyclate anion. In bis­[4-(4-nitro­phen­yl)piperazin-1-ium] bis­(4-fluoro­benzoate) trihydrate (2C(10)H(14)N(3)O(2) (+)·2C(7)H(4)FO(2) (−)·3H(2)O), there are two cations, two anions, and three water mol­ecules of solvation in the asymmetric unit, all linked by hydrogen bonds [graph-set notation of hydrogen bonding R (2) (2)(20) between adjacent cations and R (3) (3)(9) between a cation and its adjacent anion]. In the anion, the 4-nitro­phenyl ring occupies an axial substitution position in the piperazinium ring, which is relatively rare. Within the asymmetric unit, the phenyl groups in the cations show an offset π–π inter­action. Additionally, there is a C—H⋯π inter­action between a hydrogen atom on the phenyl ring within a cation and the phenyl ring within an anion. In 4-(4-nitro­phen­yl)piperazin-1-ium 3,5-di­nitro­benzoate (C(10)H(14)N(3)O(2) (+)·C(7)H(4)N(2)O(6) (−)), there is a strong N—H⋯O hydrogen bond linking the cation and anion and the 4-nitro­phenyl ring occupies an axial substitution position in the piperazinium ring, as seen in the previous structure. In the crystal, the cation and the anion form a complex three-dimensional hydrogen-bonded array involving R (2) (2)(8), R (4) (4)(12) and R (4) (4)(20) rings propogating in the a-axis direction. The nitro­phenyl group is disordered with occupancies of 0.806 (10) and 0.194 (10).