Acta Chim. Slov. 2002, 49, 243-250. 243 SYNTHESIS AND STRUCTURE OF HYDROXYLAMMONIUM FLUOROSILICATESt Matjaž Kristl, Miha Drofenik Faculty of Chemistry and Chemical Technology, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia Ljubo Golič Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, SI-1000 Ljubljana, Slovenia tThis paper is dedicated to the memory of Professor Karel Lutar Received 01-10-2001 Abstract A new hydroxylammonium compound with the formula (NH3OH)2SiF6 has been synthesized by reaction of solid NH3OHF and H2SiF6. By recrystallization from aqueous HF, single-crystals of (NH3OH)2SiF6.2H2O were isolated. Both compounds were characterized by elemental analysis and thermal analysis, the structure of (NH3OH)2SiF6.2H2O has been determined by single crystal x-ray analysis. The compound crystallizes monoclinic with cell parameters: a = 10.772(5) A, b = 7.336(6) A, c = 10.447(3) A, ß = 102.09(3)°. Introduction Fluorosilicates of ammonium and hydrazinium are well known and there are several papers reporting their synthesis and structure.1-6 On the other hand, there are only a few reports about hydroxylammonium fluorometallates. The first hydroxylammonium fluorosilicate with the formula “(NH3O)2.H2SiF6 + 2H2O” was reported in 1908,7 but the compound was characterized only by the chemical analysis and the fluorine ion was determined indirectly. Several hydroxylammonium fluorometallates of main group III and side group IV metals were synthesized in the last decade8-12 whereas no recent reports about silicone compounds could be found in literature. So we decided to study reactions in the systems NH2OH - H2SiF6 and NH3OHF - H2SiF6 and to investigate the product by modern methods. By using solid NH3OHF instead of the aqueous solution of NH2OH, used in the original work, a new compound with the formula (NH3OH)2SiF6 was obtained,13 but no single-crystals, suitable for structure determination, could be isolated. In this paper we report the M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates 244 Acta Chim. Slov. 2002, 49, 243-250. synthesis and structure of the compound with the formula (NH3OH)2SiF62H2O, obtained by recristallization of (NH3OH)2SiF6 from aqueous solution. Results and discussion Chemical analysis of (NH3OH)2SiF6 gave the following results: 31.9% NH3OH+ (calc. 32.4%), 13.4% Si (calc. 13.4%) and 54.2% F- (calc. 54.2%). The compound was characterized using X-ray powder diffraction analysis. Indexing of X-ray powder diffraction data for (NH3OH)2SiF6 gave the best matching (FOM) in the triclinic system with unit cell parameters: a = 6.167(10) A, b = 10.386(20) A, c = 6.054(11) A; a = 98.58(11)°, ß= 108.56(11)°, y= 97.27(13)0, V= 357.22 A3. (NH3OH)2SiF62H2O has also been identified by elemental analysis: 27.4% NH3OH+ (calc. 27.6%), 11.3% Si (calc. 11.4%) and 46.1% F- (calc. 46.3%). The compound crystallizes monoclinic, / 2/a, with four formula units in the unit cell. The stereoscopic picture is shown in Fig. 1. Figure 1. Stereoscopic picture of the (NH3OH)2SiF6.2H2O unit cell The structure consists of isolated SiF62- octahedra, which are connected with hydrogen bridges with NH3OH+ ions and H2O molecules. The atomic coordinates and equivalent displacement parameters are given in Table 1, some selected bond lengths and angles can be found in Table 2. M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates Acta Chim. Slov. 2002, 49, 243-250. 245 Table 1. Fractional coordinates and Equivalent Displacement Parameters (A2) for (NH3OH)2SiF6.2H2O. *U = Ueq = 1/3 ?i?jUijai*aj*aiaj x/a y/b z/c Ueq Si 3/4 0.13605(4) 1/2 * 0.01463(9) F(1) 0.74629(7) -0.02392(9) 0.61412(7) * 0.03432(19) F(2) 0.59069(5) 0.13435(9) 0.45962(6) * 0.02772(17) F(3) 0.74587(6) 0.29835(8) 0.61299(6) * 0.02769(16) O(1) 0.54483(9) 0.84021(10) 0.84156(8) * 0.03118(22) O(2) 0.63563(7) 0.64076(10) 0.59620(7) * 0.02724(19) N(1) 0.49066(7) 0.67277(10) 0.79569(7) * 0.02207(19) H(1) 0.5210(21) 0.649(3) 0.7297(23) 0.036(4) H(2) 0.4154(23) 0.683(3) 0.7713(22) 0.038(4) H(3) 0.5122(23) 0.592(4) 0.8494(25) 0.043(5) H(4) 0.5588(24) 0.831(4) 0.921(3) 0.047(5) H(21) 0.6817(24) 0.553(4) 0.6101(24) 0.045(5) H(22) 0.6812(25) 0.725(4) 0.6098(24) 0.047(5) The bond lengths and angles for the SiF62- octahedra are in good agreement with literature data1, 3, 5 for similar fluorosilicates, where bond lengths from 1.668 A to 1.688 A and angles from 89.08° to 90.92° are reported. The O-N lengths are also in good agreement with values, reported in papers8-12 for other fluorometallates of hydroxylammine. The hydrogen bonds (N-H…F and O-H…F) in the structure of (NH3OH)2SiF6.2H2O (described in Table 3) are rather long, compared to most of the reported hydroxylammonium fluorometallates, where bond lengths N-H…F varied from 2.666 A to 2.738 A and the reported O-H…F lengths were in the range 2.503 A – 2.661 A. The only known hydroxylammonium fluorometallate with similar hydrogen bond lengths is the hexafluorotitanate-dihydrate,10 (NH3OH)2TiF6.2H2O, with N-H…F lengths 2.856 A – 2.903 A and O-H…F lengths 2.755 A – 2.758 A. M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates 246 Acta Chim. Slov. 2002, 49, 243-250. Table 2. Bond lengths (A) and angles (°) of (NH3OH)2SiF6.2H2O Si - F(1) 1.6793(10) N(1) - H(1) 0.840(25) Si - F(2) 1.6793(9) N(1) - H(2) 0.801(24) Si - F(3) 1.6837(10) N(1) - H(3) 0.817(25) O(1) - N(1) 1.4006(14) O(2) - H(21) 0.808(25) O(1) - H(4) 0.82(3) O(2) - H(22) 0.78(3) F(1) - Si - F(2) 89.89(4) H(4) - O(1) - N(1) 104.3(19) F(1) - Si - F(3) 89.34(6) H(1) - N(1) - O(1) 104.7(15) F(2) - Si - F(3) 90.32(3) H(2) - N(1) - O(1) 110.2(17) H(3) - N(1) - O(1) 111.4(17) H(21) - O(2) - H(22) 105(3) Table 3. Hydrogen bond lengths (A) and angles (°) Distance Distance Distance Angle Symmetry A-H…B A-H B-H A-B A-H…B operation of B N(1)-H(1) …O(2) 0.840(25) 2.048(25) 2.8637(14) 163.5(20) x, y, z N(1)-H(2)…F(3) 0.801(24) 2.196(21) 2.9211(13) 150.7(23) -1+x, 1-y, z N(1)-H(3)…F(2) 0.817(25) 2.093(25) 2.8978(17) 168.0(25) x, 1-y, 1+ z O(1)-H(4)…O(2) 0.82(3) 1.855(25) 2.6387(13) 161(3) x, 3/2-y, 1+z O(2)-H(21)…F(3) 0.808(25) 1.99(3) 2.7683(21) 162(3) x, y, z O(2)-H(22)…F(1) 0.78(3) 1.97(3) 2.7228(21) 162(3) x, 1+y, z Thermal behavior of both compounds showed similar results. (NH3OH)2SiF6 and (NH3OH)2SiF62H2O are thermally stable up to 70 °C. The decomposition of (NH3OH)2SiF6 starts at 75 °C, becomes significant above 125 0C, reaches the peak temperature at 231.3 °C and is finished at 265 °C. In the case of (NH3OH)2SiF62H2O, the corresponding temperatures are lower: decomposition starts at 70 °C, becomes faster at 120 °C, the peak temperature is 228.7 °C and the decomposition is finished at 260 °C.13 The measured mass loss is 100% for both compounds. During the thermal analysis of (NH3OH)2SiF62H2O, only one decomposition step could be observed. Even by lowering the heating rate down to 1K/min, no separated decomposition peaks could be observed. According to the results, only the total decomposition equation can be assumed: (NH3OH)2SiF6 2H2O -> 2NH2OH + 2HF + SiF4 + 2H2O M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates Acta Chim. Slov. 2002, 49, 243-250. 247 The results show some significant differences, compared to the thermal decomposition of hydroxylammonium fluorometallates of side group 4.8-10 Hydroxylammonium fluorosilicates decompose at lower temperatures than hydroxylammonium compounds of zirconium and hafnium, a fact, that can be explained with weaker hydrogen bonds (O-H … F and N-H … F) in silicone compounds. Hydroxylammine decomposes to NH3 and N2 at elevated temperatures and ammonium fluorometallates were always found as intermediates in the thermal decomposition of hydroxylammonium fluorotitanates, -zirconates and hafnates. In the case of hydroxylammonium fluorosilicates, the hydrogen bonds cleave at low temperatures Table 4. Crystal data, data collection and refinement summary (NH3OH)2SiF62H2O Mr = 246.18 Monoclinic, I 2/a, No.: 15 a = 10.772(5) A b = 7.336(6) A c = 10.447(3) A ß= 102.09(3)° V = 807.2(8) A3 F(000) = 503.92 Enraf-Nonius CAD4 diffractom co-20 scan No absorption correction 9515 measured reflections 2533 unique reflections 1127 reflections with I>2.5cj(I) Refinement on F R (on Fobs) = 0.025 wR (on Fobs) = 0.028 S = 0.973 85 parameters H-atoms refined with isotropic displacement parameters Z = 4 Dx = 2.026 Mg m-3 Mo Ka radiation Cell parameters from 25 reflections 6 = 8.59-15.25° //= 0.3943 mm-1 T = 293(1) K Irregular form colorless 0.46 x 0.57 x 0.84 mm eter Rint = 0.027 0 range = 3.38 - 29.97° h = -15-> 15 k = -10-> 10 1 = -14-> 14 Empirical weighting scheme Zachariasen's extinct. coeff =3.8 103 (A/a)max = 0.00068 (A/o)ave = 0.00011 ^Pmax = 0.30 eA-3 ^Pmin = -0.36 eA-3 M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates 248 Acta Chim. Slov. 2002, 49, 243-250. (even below 80 °C), which are insufficient for disproportionation of hydroxylammine. Mass spectrometry of (NH3OH)2SiF6 confirmed13 the presence of NH2OH as a decomposition product at 80 °C. Conclusions The use solid NH3OHF as a source of hydroxylammine has led to sucessful synthesis of two hydroxylammonium compounds. The new compound with the formula (NH3OH)2SiF6 has been characterized by elemental analysis, thermal analysis, mass spectra and X-ray powder diffraction.13 The second compound, (NH3OH)2SiF6.2H2O, mentioned in the early work by Ebler and Schott,7 was prepared by a new method. Besides the characterization by elemental and thermal analysis, the structure has been determined by single crystal x-ray analysis. The compound crystallizes monoclinic with cell parameters: a = 10.772(5) A, b = 7.336(6) A, c = 10.447(3) A, ß = 102.09(3)°. Experimental Synthesis. In the first step, hydroxylammine was isolated in ethanolic solution by the reaction of solid hydroxylammonium chloride with sodium ethylate14. Crystals of NH3OHF were obtained by adding 40% HF dropwise to the ethanolic solution at 0 °C.15 Various amounts of NH3OHF were dissolved in the H2SiF6 solution in a platinum beaker. Evaporation at room temperature yielded colourless crystals. The product with the formula (NH3OH)2SiF6 was filtered, dried and characterized by chemical and thermal analysis and X-ray powder diffraction,12 but the crystals were of poor quality and a determination of the structure was impossible. In an attempt to solve this problem, the product was dissolved again in 40% HF and recrystallized by slow evaporation of the solvent, introducing some crystals of the primary product as crystallization seeds. After some days, colourless crystals of (NH3OH)2SiF6.2H2O were obtained and characterized by chemical and thermal analysis, the structure was determined by single-crystal X-ray analysis. Methods. Hydroxylammonium was determined by titration with KMnO416 and fluorine by direct calibration with a combination fluoride electrode17. Silicon was M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates Acta Chim. Slov. 2002, 49, 243-250. 249 determined by gravimetric methods7. Thermal analysis (TG and DSC) was carried out for both compounds in a nitrogen stream with a heating rate of 10 K/min, using platinum (TG) and gold crucibles (DSC). X-ray powder diffraction data was collected with an AXS-Bruker/Siemens/D5005 diffractometer using CuKa radiation at 293(1) K. The samples were finely ground, placed on a silicone single-crystal holder and measured from 47 to 52 hours in the range 50 < 20 < 65° with a step of 0.01° and a scanning speed of 1 s/step. The values for divergence and antiscattering slit were fixed at 0.2 mm. The measurements were corrected for the sample height and the Ka2 radiation was stripped off. The X- ray diffraction pattern was indexed with the help of the automatic indexing program TREOR18 and ITO19. X-ray structure analysis. Diffraction data for (NH3OH)2SiF62H2O were collected on an Enraf Nonius CAD-4 diffractometer at room temperature with MoKa radiation and graphite monochromator. The structure was solved by direct methods using SIR92 program.20 A full-matrix least-squares refinement on F was employed with anisotropic displacement parameters for all non-hydrogen atoms and isotropic for hydrogen atoms, using the weighting function: w = 9.5 x Wf x Ws, where Wf ( | Fo | < 2.9) = (|Fo|/2.9), Wf (|Fo|> 19.0) = (19.0/|Fo|)1.5, Wf (2.9<|Fo|<19.0) = 1 and Ws (sin9 < 0.6) = (sine/0.6)3, Ws (sin6 > 0.7) = (0.7/ sin9)4, Ws (0.6 < sin9 < 0.7) = 1. Xtal 3.4 system21 of programs was used for the correlation and reduction of data, structure refinement and interpretation.Graphics were produced using ORTEPII program22. Further details of crystal data, data collection and refinement can be found in Table 4. Detailed crystallographic data have been deposited with the Fachinformationszentrum Karlsruhe (FIZ), D-76344 Eggenstein-Leopoldshafen, with the deposition number CSD 412185. References 1. E. O. Schlemper, W. C. Hamilton, J. J. Rush, Jour. Chem. Physics 1966, 44, 2499-2505. 2. E. O. Schlemper, W. C. Hamilton, Jour. Chem. Physics 1966, 45, 408-409. 3. B. Frlec, D. Gantar, L. Golič, I. Leban, Acta Cryst. B 1980, 36, 1917-1918. 4. J. L. Hoard, M. B. Williams, Jour. Amer. Chem. Soc. 1942, 64, 633-637. M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates 250 Acta Chim. Slov. 2002, 49, 243-250. 5. A. I. Popov, M. D. Val’kovskii, V. F. Sukhoverkhov, N. A. Chumaevskii, A. V. Sahkarov, V. O. Gel’mbol’dt, A. A. Ennan, Zh. Neorgan. Khimii, 1991, 36, 375-380. 6. T.E. Jenkins, J. Phys. C: Solid State 1986, 19, 1065-1069. 7. E. Ebler, E. Schott, J. Pr. Chem. 1908, 78, 338-340. 8. I. Bajc, S. Milićev, B. Volavšek, Monatsh. Chem. 1992, 123, 321-324. 9. I. Ban, L. Golič, S. Milićev, B. Volavšek, Monatsh. Chem. 1995, 126, 1279-1289. 10. I. Ban, M. Kristl, B. Volavšek, L. Golič, Monatsh. Chem. 1999, 130, 401-408. 11. M. Pintarič, S. Milićev, B. Volavšek, Monatsh. Chem. 1990, 121, 357-360. 12. M. Kristl, B. Volavšek, L. Golič, Monatsh. Chem. 1996, 127, 581-586. 13. M. Kristl, I. Ban, M. Drofenik, A. Popovič, J. Fluor. Chem. 2001, 109, 209-212. 14. G. Brauer, Handbuch der Präparativen Anorganischen Chemie, Bd. 1; Enke, Stuttgart, 1975, pp. 450. 15. M. Kristl, L. Golič, B. Volavšek, Monatsh. Chem. 1994, 125, 1207-1213. 16. N. H. Furman, Standard methods of chemical analysis, vol. 1, 6th edn.; Van Nostrand, Princeton, NJ, 1962, pp. 196. 17. ORION, Combination Fluoride Instruction Manual 1991. 18. P.E. Werner, L. Eriksson, J. Westdahl, J. Appl. Cryst. 1985, 18, 367-377. 19. J. W. Visser, J. Appl. Cryst. 1969, 2, 89-98. 20. A. Altomare, M. C. Burla, M. Camalli, G. Dascarano, C. Giacovazzo, A. Guagliardi, G. Polidori, J. Appl. Cryst. 1994, 27, 435. 21. S. R. Hall, G. S. D. King, J. M. Stewart, The Xtal3.4 User’s Manual, University of Western Australia: Lamb, Perth 1995. 22. C. K. Johnson, ORTEPII. Report ORNL-5138. Oak Ridge National Laboratory, Tennesee, USA, 1976. Povzetek Z reakcijo med trdnim NH3OHF in raztopino H2SiF6 smo sintetizirali novo spojino hidroksilamina s formulo (NH3OH)2SiF6. Pri rekristalizaciji te spojine iz vodne raztopine HF smo izločili monokristale s sestavo (NH3OH)2SiF6.2H2O. Obe spojini smo karakterizirali s kemijsko in termično analizo, strukturo (NH3OH)2SiF6.2H2O smo določili z rentgensko strukturno analiza na osnovi monokristala. Spojina kristalizira monoklinsko s parametri osnovne celice: a = 10.772(5) A, b = 7.336(6) A, c = 10.447(3) A, ß = 102.09(3)°. M. Kristl, M. Drofenik, L. Golič: Investigations of Hydroxylammonium Fluorosilicates