Hydrogen Bonding Networks and Solid-State Conversions in Benzamidinium Salts

Ten benzamidinium salts of carboxylic acids, amides, and sulfonamides have been crystallized from solution. Single-crystal X-ray analyses revealed various hydrogen bonding motifs which are discussed in terms of supramolecular synthons and graph sets. Benzamidinium hydrogen maleate (5a) crystallizes as large needles of up to >3 cm in length. Attempts to influence the crystal habit and size through a change of solvent and the presence of additives yielded a second polymorph (5b). The formation of the benzamidinium salts by mechanochemical reaction was also investigated. Grinding of benzamidine with nicotinic acid, salicylic acid, p-aminobenzoic acid, cyanuric acid, pimelic acid, saccharin, and sulfathiazole with mortar and pestle or using a ball-mill gave compounds identical to those obtained by crystallization from solution. Time-dependent X-ray powder patterns of a stoichiometric benzamidine/cyanuric acid mixture suggested that the mechanochemical salt formation occurred via the amorphous state. Ball-milling of benzamidine with sulfamerazine generated amorphous benzamidinium sulfamerazinate that was stable toward crystallization for at least 2 weeks, when stored at 25% relative humidity.