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.