Formaldehyde-aminoguanidine condensation and aminoguanidine self-condensation products: syntheses, crystal structures and characterization

Guanidine is the functional group on the side chain of arginine, one of the fundamental building blocks of life. In recent years, a number of compounds based on the aminoguanidine (AG) moiety have been described as presenting high anticancer activities. The product of condensation between two molecules of AG and one molecule of formaldehyde was isolated in the protonated form as the dinitrate salt (systematic name: 2,8-diamino-1,3,4,6,7,9-hexaazanona-1,8-diene-1,9-diium dinitrate), C3H14N82+center dot 2NO(3)(-), (I). The cation lacks crystallographically imposed symmetry and comprises two terminal planar guanidinium groups, which share an N-C-N unit. Each cation in (I) builds 14 N-H center dot center dot center dot O hydrogen bonds and is separated from adjacent cations by seven nitrate anions. The AG self-condensation reaction in the presence of copper(II) chloride and chloride anions led to the formation of the organic-inorganic hybrid 1,2-bis(diaminomethylidene) hydrazine-1,2-diium tetrachloridocuprate(II), (C2H10N6)[CuCl4], (II). Its asymmetric unit is composed of half a diprotonated 1,2-bis(diaminomethylidene) hydrazine-1,2-diium dication and half a tetrachloridocuprate(II) dianion, with the Cu-II atom situated on a twofold rotation axis. The planar guanidinium fragments in (II) have their planes twisted by approximately 77.64 (5)degrees with respect to each other. The tetrahedral [CuCl4](2-) anion is severely distorted and its pronounced 'planarity' must originate from its involvement in multiple N-H center dot center dot center dot Cl hydrogen bonds. It was reported that [CuCl4](2-) anions, with a trans-Cl-Cu-Cl angle (Theta) of similar to 140 degrees, are yellow-green at room temperature, with the colour shifting to a deeper green as Theta increases and toward orange as Theta decreases. Brown salt (II), with a Theta value of 142.059 (8)degrees, does not fit the trend, which emphasizes the need to take other structural factors into consideration. In the crystal of salt (II), layers of cations and anions alternate along the b axis, with the minimum Cu center dot center dot center dot Cu distance being 7.5408 (3) angstrom inside a layer. The structures of salts (I) and (II) were substantiated via spectroscopic data. The endothermic reaction involved in the thermal decomposition of (I) requires additional oxygen. The title salts may be useful for the screening of new substances with biological activity.