Structure of aminoguanidine hemioxalate. Implications for the synthesis of amidinohydrazones

Summary The crystal and molecular structure of aminoguanidine hemioxalate, a salt in which aminoguanidine exists in the monocation form, was determined by single crystal X-ray diffraction. The salt crystallizes in the monoclinic space group P2(1)/n with unit cell dimensions of a =4.95, b =10.46, c =10.40 Å, β=92.57°, and Z =4. The structure contains one oxalate ion for every two CN 4 H 7 + ions, the latter being practically planar. The structure of the monocation is largely similar to those of aminoguanidine dications except that the monocation is devoid of one of the protons attached to the terminal hydrazine nitrogen. This result is of interest considering the synthesis of amidinohydrazones, indicating that the concentration of the active nucleophile is nearly maximal even when aminoguanidine exists in the monocation form. Therefore, the synthesis of amidinohydrazones should be performed in the pH range in which aminoguanidine exists mainly in the monocation form, i.e. at a pH higher than 2. There is, however, no need to elevate the pH to values at which a considerable proportion of aminoguanidine exists as the free base.