Solvothermal Synthesis, Crystal Growth, and Structure Determination of Sodium and Potassium Guanidinate

Phase-pure NaCN3H4 and KCN3H4 were synthesized from molecular guanidine and
elemental metal
in liquid ammonia at room temperature and elevated pressure close
to 10 atm. The crystal structures were determined at 100 K using single-crystal
X-ray diffraction. Both compounds crystallize in the monoclinic system
(P21/c, No. 14) but are
far from being isotypical. NaCN3H4 (a = 7.9496(12) Å, b = 5.0328(8) Å, c = 9.3591(15) Å, β = 110.797(3)°, Z = 4) contains a tetrahedrally N-coordinated sodium cation
while KCN3H4 (a = 7.1200(9)
Å, b = 6.9385(9) Å, c =
30.404(4) Å, β = 94.626(2)°, Z =
16) features a very large c axis and a rather complicated
packing of irregularly N-coordinated potassium cations. In the crystal
structures, the guanidinate anions resemble the motif known from RbCN3H4, that is, with one elongated C–(amino)N single bond and two shorter C–(imino)N bonds
(bond order = 1.5) although the orientation of one N–H bond
differs in the guanidinate anion of NaCN3H4.
Both crystal structures and infrared spectroscopy evidence the presence
of hydrogen-bridging bonds, and the vibrational properties were analyzed
by ab initio phonon calculations.