Diprotonation of Guanidine in Superacidic Solutions

Guanidinium chloride reacts with the superacidic solutions HF/MF5 (M=As, Sb) at a molar ratio of 1:2 under formation of the diprotonated guanidinium salts [C(NH2)(2)(NH3)][AsF6](2) and [C(NH2)(2)(NH3)][SbF6](2). The compounds were characterized by using infrared and Raman spectroscopy. Furthermore, single-crystal X-ray structure analysis of the guanidinium(2+) salts [C(NH2)(2)(NH3)][SbF6](2)center dot HF, [C(NH2)(2)(NH3)](2)[Ge3F16]center dot HF, and [C(NH2)(2)(NH3)](2)[Ge3F16]center dot 2 HF and the guanidinium(1+) salt [C(NH2)(3)][SbF6] is reported. The discussion of the experimental data is supported by quantum-chemical calculations of the [C(NH2)(2)(NH3)](2+) and [C(NH2)(3)](+) ions to investigate the modification of the resonance stabilization during the protonation process at the PBE1PBE/6-311G++(3df,3pd) level of theory. The planar CN3 skeleton of the guanidinium(2+) ion has two carbon-nitrogen bonds in the range 1.286(4)-1.293(4) angstrom and one carbon-nitrogen bond of 1.453(4) angstrom, which can be explained with a decreased resonance stabilization relative to the guanidinium(1+) ion.