STRUCTURE OF AMINOGUANIDINE HEMIOXALATE - IMPLICATIONS FOR THE SYNTHESIS OF AMIDINOHYDRAZONES

The crystal and molecular structure of aminoguanidine hemioxalate, a s alt in which aminoguanidine exists in the monocation form, was determi ned 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 Angstrom, beta = 92.57 degrees, and Z = 4. The st ructure contains one oxalate ion for every two CN4H7+ ions, the latter being practically planar. The structure of the monocation is largely similar to those of aminoguanidine dications except that the monocatio n is devoid of one of the protons attached to the terminal hydrazine n itrogen. This result is of interest considering the synthesis of amidi nohydrazones, indicating that the concentration of the active nucleoph ile is nearly maximal even when aminoguanidine exists in the monocatio n form. Therefore, the synthesis of amidinohydrazones should be perfor med in the pH range in which aminoguanidine exists mainly in the monoc ation 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 aminogu anidine exists as the free base.