M. Koskinen et al., STRUCTURE OF AMINOGUANIDINE HEMIOXALATE - IMPLICATIONS FOR THE SYNTHESIS OF AMIDINOHYDRAZONES, Monatshefte fuer Chemie, 128(8-9), 1997, pp. 767-775
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.