Acta Chim. Slov. 2004, 51, 769-778. 769 Short Communication CALCIUM HEXAFLUOROARSENATES WITH METAL COORDINATED HF MOLECULES Melita Tramšek* and Primož Benkič Jožef Štefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 21-06-2004 Abstract Two calcium hexafluoroarsenates(V) with metal coordinated HF were structurally characterised: Ca(HF)(AsF6)2 and Ca(HF)6(AsF6)2. Ca(HF)(AsF6)2 ciystallizes in a monoclinic crystal system, space group P2fc, with a =962.4(6) pm, b = 926.4(5) pm and c = 1059.2(7) pm, ß = 104.161(13)°, V = 0.9157(10) nm3 and Z = 4. Ca(HF)6(AsF6)2 ciystallizes in a monoclinic crystal system, space group Flxlc, with a =1004.9(5) pm, b = 1498.7(7) pm and c = 984.0(5) pm, ß = 109.56(11)°, V = 1.3965(12) nm3 and Z = 4. Key words: calcium, hexafluoroarsenates, hydrogen fluorid Introduction Metal fluorides dissolve in anhydrous hydrogen fluoride (aHF) acidifted with Lewis acids (AsF5, SbF5) yielding solutions of solvated cations and relatively weakly coordinated corresponding anions.1 Several years ago the first structure with HF coordinated directly to the metal center was determined: La(HF)(AsF6)3.2 Since then several examples of metal hexafluoroarsenates and antimonates with coordinated HF were found: Pb(HF)(AsF6)2,3 Ca(HF)2(SbF6)2,4 Mg(HF)2(SbF6)2,4 ([(Os03F)(HF)2]-(AsF6))2,5 Au(HF)2(SbF6)2,6 and Cd(HF)(AsF6)2.7 Ca(HF)(AsF6)2 and Ca(HF)6(AsF6)2 present new examples of metal HF coordinated compounds. Ca(HF)(AsF6)2, which is isostructural with Cd(HF)(AsF6)27 has been briefly mentioned in a review paper.8 Details of structure determination for both compounds are given here for the first tirne. Results and discussion Ca(AsF6)2 can be prepared by the reaction between CaF2 and AsF5 in aHF solvent as described in the literature.9 The lattice energy of Ca(AsF6)2 is rather low as a consequence of the relatively large volume of AsF6" (110 A3 10). The AsF6" anion is a M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules 770 Acta Chim. Slov. 2004, 51, 769-778. weak Lewis base, so even poor Lewis base solvents, such as aHF, can provide sufficient solvation energy to dissolve the Ca(AsF6)2 salt to form solvated [Ca(HF)n]2+ cations and AsF6" anions. Molecules of the solvent, in this čase HF, remain coordinated to the metal center during the process of crystallisation and help to stabilise crystal packing. This seems to be important in the cases of the compounds with low lattice energy, which cannot fully compensate the energy needed for the desolvation of the cation. In the type of compounds M(L)(AF6)2 (L = HF, XeF2) which were recently synthesised in our laboratory calcium compounds shows the largest variety of the structural types found so far. In the system Ca2+/XeF2/AsF6- three compounds were found: Ca2(XeF2)9(AsF6)2,u [Ca(XeF2)4](AsF6)2 and [Ca(XeF2)25](AsF6)2.12 In the first two compounds layer structure arrangement was found, while for the latter 3D structure type was determined. In these cases, the mole ratio between metal center Ca2+ and ligand molecule XeF2 was important in the reaction and crystallisation process. Similarly, parameters that influence stability of the Ca(HF)n2+ coordination sphere must play important role in the reaction system of Ca2+/HF/AsF6". Crystals with compositions Ca(HF)(AsF6)2 were found in the crystallization vessel after crystallisation of the [Ca(XeF2)4](AsF6)2 (see details in the experimental section), while after crystallization from saturated aHF solution of the Ca(AsF6)2, crystals with composition Ca(HF)6(AsF6)2 were obtained. Calcium atom in the Ca(HF)(AsF6)2 is coordinated to nine fluorine atoms in the form of tricapped trigonal prism (Figure la). Six of the fluorine atoms in the apexes of the trigonal prism arise from six AsF6 units with the Ca-F distances in the range of 231.5-246.2 pm. Additionally, two AsF6 units act as bidentate ligands interacting with calcium atoms via second fluorine atom placed over the rectangular faces (F23 and F13) of the trigonal prism at distances 267.8 pm and 277.3 pm. Fluorine atom from HF molecule is placed over the third rectangular face of the trigonal prism (Cal-Fl is 239.6 pm). AsF6 units are distorted due to the interactions with calcium atoms as is clearly seen from the range of distances and angles (see Table 1). Calcium atoms are further connected via two crystalographically different AsF6 units, forming square rings with composition [Ca2(AsF6)2]2+. The square rings are alternately connected by trans bridges of AsF6 units forming zigzag ladders (Figure lb). Described ladders runs in the [1,1,1] direction. Each Ca atom from the [1,1,1] ladder is M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules Acta Chim. Slov. 2004, 51, 769-778. 771 part of an identical ladder running in the [-1,-1,-1] direction. Calcium atoms connect both types of ladders in a 3D network, where channels are formed along a axis. b) Figure 1. a) Coordination of Ca and b) Rings in the structure of Ca(HF)(AsF6)2; Symmetiy operations: B '-x, y + Vi, -z + Vi ; c '-x, -y, -z' ; D 'x, -y - Vi, z - Vi. M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules 772 Acta Chim. Slov. 2004, 51, 769-778. HF molecules coordinated via fluorine atom directly to calcium atom are placed in these channels. Distances between Fl atom and surrounding atoms that could be involved in hydrogen boding of HF molecule are given in Table 2. Distances Fl-F for atoms Fll, F13, F14, F21, F26 are short, but these atoms are involved in the bridging interactions As-F-Ca and are regarded to similar structures [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)6 4 usually not involved in hydrogen bonding. Terminal fluorine atoms on the AsF6" (F25, F16’, F25, F16, F26, F15) units seem to be more appropriate for hydrogen bonds. Lengthening of the As-F bond in the čase of hydrogen bond can be expected, which gives us the most likely candidates: F1-H1-F16 and F1-H1-F16'. The Fl -F16 distance is shorter that F-F16' but according to small angle Cal-Fl-F16 (90.7 °C) with respect to the angle Cal-Fl-F16' (111.0°), hydrogen atom was placed between F1-F16'. Nevertheless disorder of the HF molecule cannot be excluded. Table 1. Selected distances and angles in Ca(HF)(AsF6)2 and Ca(HF)6(AsF6)2. Ca(HF)(AsF6)2 Ca(HF)6(AsF6)2 Distance [pm] Angle [°] Distance [pm] Angle [°] Cal-Fl 239.6(9) Cal-Fl 245(3) Cal-Fll 234.7(10) Cal-F2 227(3) Cal-F12 238.2(9) Cal-F3 253(2) Cal-Fl 3 277.3(11) Cal-F4 242(3) Cal-F14 246.2(10) Cal-F5 239(2) Cal-F21 236.4(8) Cal-F6 236(3) Cal-F22 238.5(8) Cal-Fll 250(2) Cal-F23 267.8(11) Cal-F23 259(3) Cal-F24 231.5(9) Asl-Fll 176.8(9) Fll-Asl-F12 176.0(5) Asl-Fll 173.8(19) Fll-Asl-F13 89.1(11) Asl-F12 172.3(8) F12-Asl-F14 87.0(5) Asl-F12 171(2) F12-Asl-Fll 178.8(11) Asl-F13 172.7(9) F13-Asl-Fll 89.3(4) Asl-F13 172(2) F13-Asl-F15 178.1(10) Asl-F14 172.5(9) F14-Asl-F16 176.2(5) Asl-F14 170(2) F14-Asl-Fll 90.0(13) Asl-F15 168.6(8) F15-Asl-F13 173.4(5) Asl-F15 173(2) F15-Asl-F12 91.9(12) Asl-F16 171.6(8) F16-Asl-F12 95.4(4) Asl-F16 169(2) F16-Asl-F13 90.0(12) As2-F21 173.7(7) F21-As2-F24 90.7(4) As2-F21 172(2) F21-As2-F22 178.4(14) As2-F22 172.1(8) F22-As2-F21 177.5(5) As2-F22 169(3) F22-As2-F25 89.3(15) As2-F23 172.7(9) F23-As2-F24 82.6(5) As2-F23 171(2) F23-As2-F25 179.5(11) As2-F24 175.4(8) F24-As2-F22 88.4(4) As2-F24 170(2) F24-As2-F23 90.2(14) As2-F25 167.4(10) F25-As2-F23 174.2(5) As2-F25 172(2) F25-As2-F21 90.6(14) As2-F26 166.5(10) F26-As2-F24 176.2(5) As2-F26 171.0(18) F26-As2-F23 90.2(13) M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules Acta Chim. Slov. 2004, 51, 769-778. 773 Table 2. Distances F(H)···F [pm] in Ca(HF)(AsF6)2. F1-F14 293.1 F1-F23 320.6 F1-F24 281.1 F1-F13 320.4 F1-F21 330.1 F1-F25' 337.2 F1-F11 299.2 F1-F15' 329.7 F1-F26 311.1 F1-F16' 337.3 F1-F16 298.3 F1-F26' 334.9 Calcium in the Ca(HF)6(AsF6)2 is coordinated to eight fluorine atoms in the form of Arhimedian antiprism. Six of fluorine atoms arise from HF molecules and two from two bridging AsF6 units (Figure 2). Two calcium atoms are connected via AsF6 units (cis) into the ring (dimer) with composition [Ca2(As(1)F6)2]2+. Distances Ca-F(H) are in the range of 227 pm to 253 pm, while the Ca-F(As) distances are 250 pm and 259 pm. Charge of the calcium is compensated by free As(2)F6 units. Figure 2. Ring [Ca2(As(l)F6)]2+ and two As(2)F6 units in the structure of the Ca(HF)6(AsF6)2 Symmetiy operations: B '-x, y + Vi, -z + V2' ; c '-x, -y, -z' ; D 'x, -y - Vi, z - Vi Symmetry of the space group (Fljc) defines the position of the dimers in the crystal packing, so layers of the rings ([Ca2(As(l)F6)2]2+), perpendicular to the b axis, are formed (Figure 3). Those layers sandwich the layer of free AsF6 units, which interconnect dimers with electrostatic interactions and hydrogen bonds. Distances F(H)-F, which can be involved in the hydrogen bonds are given in Table 3. On the basis of surrounding F atoms and slight elongation of the As-F distances hydrogen atoms can be placed between F6 and F21, F3 and F25 and F4 and F15. The other possible hydrogen bonds, which involve interactions of HF molecules with surrounding fluorine atoms, were tentative defined also on the similarity basis with known structures of the poly-hydrogen fluoride anions. In the crystal structure of M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules 774 Acta Chim. Slov. 2004, 51, 769-778. 13 anhydrous hydrogen fluoride the angle F-F-F" is 120° 1J and an average angle of H2F3" anion in KF-2HF is 1350.14 Angles F'-F-F" in the proposed hydrogen bond interactions are in the range from 101.9 to 128.7° (smaller angles in the range from 62° to 93.5° were considered as less likely), so it is reasonable to expect that they form stabile structural moieties. Figure 3. Packing in the structure of Ca(HF)6(AsF6)2. Table 3. Distances F(H)-F [pm] in Ca(HF)6(AsF6)2. F1-F5 301.1 F2-F23 323.9 F4-F5 303.7 F5-F26 296.4 F1-F25 300.1 F2-F26 327.1 F4-F22 280.1 F5-F14 323.3 F1-F21 305.3 F3-F2 298.7 F4-F16 283.6 F6-F21 283.0 F2-F3 298.7 F3-F23 278.4 F4-F15 298.4 F6-F24 289.0 F2-F21 307.9 F3-F21 286.0 F5-F1 301.0 F6-F12 303.0 F2-F14 321.7 F3-F25 300.0 F5-F4 303.7 M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules Acta Chim. Slov. 2004, 51, 769-778. 775 As already mentioned calcium and cadmium compounds with one HF coordinated to the metal are isostructural and 3D network crystal packing is found as in the several other cases: Ca(HF)2(SbF6)2, Mg(HF)2(SbF6)2,4 La(HF)2(AsF6)3.2 In the crystal packing of Ca(HF)6(AsF6)2, formation of the dimer is interesting. This means, that first solvatation sphere of Ca2+ cation is almost preserved in the crystal packing and that under certain condition of crystallisation, formation of Ca(HF)6(AsF6)2 is favorite enough without additional cation-anion contacts in the solid. Nevertheless, those are required to raise free energy of crystal formation in the čase of Ca(HF)(AsF6)2, where calcium solvatation sphere have to be almost completely removed. Conclusions Two calcium hexafluoroarsenates(V) with metal HF coordinated molecules were structurally characterised: Ca(HF)(AsF6)2 and Ca(HF)6(AsF6)2 The structural diversity of compounds with same cation, ligand and anion is illustrated. Experimental A nickel vacuum line and Teflon vacuum system were used as previously described.15 Non-volatile materials, which were very sensitive to traces of moisture, were handled in the dry argon atmosphere in a glove box with maximum content of 0.1 ppm of water vapour (MBraun, Garching, Germany). CaF2 was used as purchased (Merck, Suprapur), anhydrous HF (aHF) (Praxair, 99.9%) was treated with K2NiF6 (Ozark-Mahoning, 99%) for several days prior to use. AsF5 was prepared by high-pressure fluorination of As203, using a procedure previously described for PFy16 Ca(AsF6)2 was prepared by the reaction of CaF2 (1.004 g, 12.89 mmol) and excessive AsF5 (6.01 Ig, 35.38 mmol) in aHF as previously described.9 AsF5 and aHF were pumped off on the vacuum line and Ca(AsF6)2 was isolated (5.379 g, 12.87 mmol) and characterized by X ray powder diffraction data and chemical analysis: Calcd for Ca(AsF6)2: Ca 9.6, As 35.9, F 54.6, AsF6" 90.4. Found: Ca 9.6, As 35.7, F 53.6, AsF6" 90.0. Crystals were prepared in the T shaped reaction vessel constructed from large (16 mm i.d.) and smaller (4 mm i.d.) FEP tubes joined at right angle and equipped with a Teflon valve and were grown from aHF solution by the procedure previously M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules 776 Acta Chim. Slov. 2004, 51, 769-778. described.12 In both cases crystals were immersed in perfluorinated oil in the dry box and were selected under the microscope and transferred into the cold nitrogen stream at the diffractometer. Crystals of Ca(HF)(AsF6)2 were found in the same crystallization batch as crystals of [Ca(XeF2)4](AsF6)2, only several months later, after they were isolated and stored in the dry-box. Crystals with composition Ca(HF)6(AsF6)2 were grown from the saturated aHF solution of Ca(AsF6)2 (aprox.: 0.080 g of Ca(AsF6)2 and aprox. 4 mL aHF). In this čase crystals were isolated by a quick removal of aHF on the vacuum line and they were transferred in the diffractometer within the one hour. Table 4. Crystal data and Structure refinement for Ca(HF)(AsF6)2 and Ca(HF)6(AsF6)2. Parameter Ca(HF)(AsF6)2 Ca(HF)6(AsF6)2 fw 437.93 537.97 temp (K) 200(2) 200(2) space group P21/c P21/c a (pm) 962.4(6) 1004.9(5) L(pm) 926.4(5) 1498.7(7) c (pm) 1059.2(7) 984.0(5) PO 104.161(13) 109.56(11) V (nm3) 0.9157(10) 1.3965(12) Z 4 4 Dcalcd (g/cm3) 3.177 2.559 2(pm) 71.069 71.069 //(mm"1) 8.038 5.345 Data measured/unique 6648/2269 10435/3380 Rmt 0.064 0.083 R\; wR2 [F2>2o(F2)] 0.0825; 0.1410 0.0750; 0.1840 R1; wR2 (all data) 0.1381; 0.1619 0.1222; 02029 Refined Parameters 149 190 Both single crystal data sets were collected using a Mercury CCD area detector coupled to a Rigaku AFC7 diffractometer with graphite monochromated Mo-Koc radiation. The data were corrected for Lorentz and polarization effects. A multi-scan absorption correction was applied to both data sets. Ali calculations during the data processing were performed using the CrystalClear software suite.17 Structures were solved using direct methods18 and expanded using Fourier techniques. Full-matrix least squares refmement of F2 against ali reflections was performed using the SHELX 97 M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules Acta Chim. Slov. 2004, 51, 769-778. 777 program.19 Hydrogen atoms could not be located from the difference Fourier map. The most probable hydrogen positions have been suggested on the basis of the acceptor atoms around HF molecules to which they (HF) could donate hydrogen bond. Those positions were not included in the refmement of the structural parameters. More details on the data collection and structure determinations are given in Table 4. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49) 7247-808-666); e-mail: crysdata@fiz-karlsruhe.de) on quoting the depository numbers: Ca(HF)(AsF6)2: CSD-414129, Ca(HF)6(AsF6)2: CSD-414130. Acknowledgements The Ministry of Education, Science and Šport of the Republic of Slovenia within the Research Program Pl-0045 Inorganic Chemistry and Technology supported this work. We would like to thank Prof. Boris Žemva for careful reading of the manuscript and helpful suggestions. References 1. T. A. O'Donnell, Superacids and Acidic melts as Inorganic chemical Reaction Media, V.C.H., Wienheim, Germany, 1993, Chapter 6. 2. Z. Mazej, H. Borrmann, K. Lutar, B. Žemva, Inorg. Chem. 1998, 37, 5912–5914. 3. M. Tramšek, E. Lork, R. Mews, B. Žemva, J. Fluorine. Chem. 2001, 110, 123–131. 4. A. Turičnik, P. Benkič, B. Žemva, J. Fluorine. Chem. 2003, 121, 245–251. 5. M. Gerken, D. A. Dixon, G. J. Schrobilgen, Inorg. Chem. 2002, 41, 259–277. 6. I. C. Hwang, K. Seppelt, Z. Anorg. Allg. Chem. 2002, 628, 765–769. 7. G. Tavčar, P. Benkič, B. Žemva, Inorg. Chem. 2004, 43, 1452–1457. 8. M. Tramšek, P. Benkič, A. Turičnik, G. Tavčar, B. Žemva, J. Fluorine. Chem. 2002, 114, 143–148. 9. B. Frlec, D. Gantar, J. H. Holloway, J. Fluorine. Chem. 1982, 19, 485–500. 10. H. D. B. Jenkins, H. K. Roobottom, J. Passmore, L. Glasser, Inorg. Chem. 1999, 38, 3609–3620. 11. M. Tramšek, P. Benkič, B. Žemva, Angewandte Chem. Int. Ed. 2004, 43, 3456–3458. 12. P. Benkič, M. Tramšek, B. Žemva, Solid State Sci. 2002, 4, 1425–1434. 13. M. Atoji, W. N. Lipscomb, Acta Cryst. 1954, 7, 173–175. 14. B. A. Coyle, L. W. Schroeder, J. A. Ibers, J. Solid State Chem. 1970, 1, 386–393. 15. Z. Mazej, P. Benkič, K. Lutar, B. Žemva, J. Fluorine Chem. 2001, 112, 173–183. 16. A. Jesih, B. Žemva, Vestn. Slov. Kem. Druš. 1986, 33, 25–28. 17. CrystalClear: Rigaku Corporation, 1999, Woodlands, Texas, USA. 18. A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, J. Appl. Crystallogr. 1993, 26, 343–350. 19. G. M. Sheldrick, SHELX97-2 (1997), University of Göttingen, Germany, 1997. M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules 778 Acta Chim. Slov. 2004, 51, 769-778. Povzetek Kalcijeva heksafluoroarzenata(V): Ca(HF)(AsF6)2 in Ca(HF)6(AsF6)2 sta bila karakterizirana z rentgensko strukturno analizo. Ca(HF)(AsF6)2 kristalizira v monoklinskem kristalnem sistemu, prostorska skupina P21/c, a =962.4(6) pm, b = 926.4(5) pm in c = 1059.2(7) pm, ß = 104.161(13)°, V = 0.9157(10) nm3 in Z = 4. Ca(HF)6(AsF6)2 kristalizira v monoklinskem kristalnem sistemu, prostorska skupina P21/c, a =1004.9(5) pm, b = 1498.7(7) pm in c = 984.0(5) pm, ß = 109.59(11)°, V = 1.3965(12) nm3 in Z = 4. M. Tramšek, P. Benkič: Calcium Hexafluoroarsenates with Metal Coordinated HF Molecules