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

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posted on 02.07.2012, 00:00 by Peter Klaus Sawinski, Richard Dronskowski
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.