Crystal and Molecular Structure of Aminoguanidine Sulphate, an Important Enzyme Inhibitor and Starting Material of Drug Syntheses

Abstract

Aminoguanidine is not only an agent with a variety of pharmacological effects but also an important starting material of am idinohydrazone-type drugs and enzyme inhibitors. There­ fore, we have now synthesized am inoguanidine sulphate CN4H8 2+.SO4 2- and determined its structure by single-crystal X-ray diffraction. The doubly protonated (dication) form of amino­ guanidine that is present in the sulphate could, in principle, exist in the form of several different tautom ers. The crystal studied consisted exclusively of one tautomer: one of the nitrogens of the hydrazine moiety bears three hydrogen atoms while the other one (the one bound to the carbon) bears one hydrogen. The other two nitrogens are bound to two hydrogens each. The predominance of this tautomer can be explained by the very strong resonance in it. The dication of aminoguanidine is remarkably planar. The hydrogens of the hydrazine moeity are, however, clearly out of the plane of the other atoms. There is a strong hydrogen bond between the proton of the m onoprotonated nitrogen and one sulphate oxygen. This bond obviously causes the deviation of the hydrogen from the plane. The bonds between the carbon atom and the adjecnt nitrogens are essentially equally long, indicating that each bond has approximately the same amount of double bond character. One of the positive charges of the dication is thus delocalized, being shared by all of the atoms of the CN3 moiety. In this respect, the structure is similar to that of all bis(amidinohydrazones) whose structures have been determined. The other positive charge of aminoguanidine dication is localized at the nitrogen bearing three hydrogens.