Mixed dicationic and monocationic benzidine species in the proton-transfer compound of benzidine with 3,5-dinitrosalicylic acid.

The crystal structure of the proton-transfer compound of 1,1'-biphenyl-4,4'-diamine (benzidine) with 3,5-dinitrosalicylic acid, viz. 1,1'-biphenyl-4,4'-diaminium bis(4'-amino-1,1'-biphenyl-4-aminium) tetrakis(2-carboxy-4,6-dinitrophenolate) ethanol disolvate, C(12)H(14)N(2)(2+)x2C(12)H(13)N(2)(+)x4C(7)H(3)N(2)O(7)(-)x2C(2)H(6)O, shows the presence of both diprotonated and monoprotonated benzidine cations. The diprotonated species lie across crystallographic inversion centres in the unit cell, while the monoprotonated species occupy general sites. All amine H atoms participate in hydrogen bonding with carboxyl, phenolate and nitro O-atom acceptors of the salicylate anions, which also participate in hydrogen bonding with the disordered ethanol solvent molecules. Significant inter-ring anion-anion and anion-monocation pi-pi interactions are also present, giving a three-dimensional framework structure.