Scientific paper
Synthesis and Crystal Structures of Lanthanoid(III) Hexafluoroarsenates with AsF3 Ligands
Melita Tramsek,* Evgeny Goreshnik, Gasper Tavcar and Zoran Mazej
Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail: melita.tramsek@ijs.si Received: 04-07-2012
Dedicated to Prof. Dr. Boris @emva on the occasion of receiving the Zois Award for lifetime achievements.
Abstract
Lanthanoid(III) hexafluoroarsenates with AsF3 as a ligand were prepared with the reactions of solutions of Ln(AsF6)3 in anhydrous hydrogen fluoride and AsF3. Solid products with composition Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) were isolated at 233 K. The attempt to prepare corresponding Yb and Lu compounds failed. Single crystals of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr) were prepared by the reaction of LnF3 (Ln = Ce, Pr) with AsF5 and aHF under sol-vothermal conditions above critical temperature of AsF5. During the crystallization the reduction of some AsF5 occurred and AsF3 was formed. Compounds crystallize in a hexagonal crystal system, space group P 6 2c (a = 10.6656(7) A (Ce); 10.6383(7) A (Pr); c = 10.9113(9) A (Ce), 10.878(2) A (Pr); V = 1074.9(1) A3 (Ce), 1066.2(2) A3 (Pr); Z = 2). Ln atoms are coordinated by nine fluorine atoms in the shape of the tri-capped trigonal prism and are further connected in three-dimensional framework via trans bridging AsF6 units. Three fluorine atoms are provided by AsF3 (capped positions) and six by AsF6 units. X-ray powder analysis of Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) show that they are isostructural with corresponding Ce and Pr compounds.
Keywords: Lanthanoid metals, hexafluoroarsenates, arsenic trifluoride
1. Introduction
Lanthanoid trifluorides are insoluble in aHF, but are readily dissolved in aHF acidified with AsF5. LnF3 dissolves when mole ratio of Ln to AsF5 is 1:3 or higher. If isolation of lanthanoid(III) fluoroarsenates(V) from aHF/As-F5 is performed at ambient temperature, products, with the exception of La(AsF6)3, readily lose AsF5. Compounds with compositions LnF(AsF6)2 (Ln = Ce-Er) and Ln2F3(AsF6)3 (Ln = Tm-Lu) were isolated.1 This behaviour perfectly reflects the increased fluoroacidity of lantha-noid centers along the series. Lanthanoid hexafluoroarse-nates are convenient precursors for the preparation of many coordination compounds with various ligands such as HF2, SO23, OPF3, NSF3,4 XeF25 and also AsF3. The Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr) compounds and their structures have been first mentioned briefly more than a decade ago,6 but details of their syntheses and crystal structures are given here for the first time. The paper is also dealing with the selected examples of other elements of the lanthanoid series in order to study the stability of the Ln(AsF3)3(AsF6)3 compounds along the series.
AsF3 can act as a ligand to metal ions in two different ways: either by coordination through its fluorine li-gands or by coordination through its electron lone pair on arsenic. Examples of AsF3 coordination to a metal centre through fluorine atom are: M(AsF3)2(AsF6)2 (M = Fe, Co, Ni),7-8 (H(O)4La2F(AsF()2(AsF6)9,9 S^AsF^^y0 and Ca(AsF3)(AsF6)2.11 Interesting cases of coordination of the AsF3 via electron lone pair on arsenic were found in the F3As-AuSbF6 and [(F^AuXeHSb-jFJ.12-13
2. Experimental 2. 1. General Experimental Procedure and Reagents
Volatile materials (anhydrous HF, AsF5, AsF3, F2) were handled in an all PTFE vacuum line equipped with PTFE (polytetrafluoroethylene) valves. The manipulations of the non-volatile materials were carried out in a dry-box (M. Braun). The residual water in the atmosphere within the dry-box never exceeded 1 ppm. The reactions
were carried out in FEP (tetrafluoroethylene-hexafluoro-propylene; Polytetra GmbH, Germany) reaction vessels (height 250-300 mm with inner diameter 15.5 mm and outer diameter 18.75 mm) equipped with PTFE valves and PTFE coated stirring bars. Prior to their use all reaction vessels were passivated with elemental fluorine. Fluorine was used as supplied (Solvay Fluor and Derivate GmbH, Germany). Anhydrous HF (Linde, 99.995 %) was treated with K2NiF6 (Advance Research Chemicals, Inc.) for several hours prior to use. Arsenic pentafluoride was prepared by high-pressure fluorination of As2O3.14 Arsenic trifluoride was prepared according to a modified literature procedure by the reaction of As2O3 with aHF in a
The volume of low-temperature-isolated solid products of the heavier lanthanoid elements (Tm, Yb, Lu) expanded immediately after warming to ambient temperature. This effect was less visible in the case of Tm and the most visible in the case of Lu. This indicates the release of gases (AsF3, AsF5) and partial (Tm) or complete decomposition (Yb, Lu) of solids isolated at 233 K. Thulium compound still gave the X-ray pattern which was in agreement with Ln(AsF3)3(AsF6)3, meanwhile attempts to get X-ray powder patterns of Yb and Lu compounds failed.
Single crystals of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr) were prepared with the same procedure as used for preparation of single crystals (H3O)4La2F(AsF3)2(AsF6)9 9 and
nickel reaction vessel at 493 K. The purity of AsF3 was La(HF)(AsF6)3.2 Approximately, 1 mmol of LnF3 (Ln
checked by IR spectroscopy. Lanthanoid trifluorides purchased from Johnson Matthey GmbH, Alfa Products, 99.9% (REO) or Wako Chemicals (99.5%) were used as supplied. Caution: aHF, AsF5 and AsF3 must be handled in a well-ventilated fume hood, and protective clothing must be worn at all times.
2. 2. Synthesis and Characterization Procedures
The Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) compounds were prepared by the following procedure. LnF3 (from 200 to 250 mg) was weighted into reaction vessel. Anhydrous hydrogen fluoride (ap-prox. 2 to 3 ml) and AsF5 (molar ratio between LnF3 and AsF5 must be higher than 1:3) were condensed onto the reaction vessel with LnF3 at 77 K and the reaction vessel was warmed to ambient temperature. After one day of intense stirring, clear solutions of Ln(AsF6)3 in aHF/AsF5 were cooled in liquid nitrogen and AsF3 was added (the molar ratio between Ln and AsF3 was higher than 1:3). The reaction vessel was warmed again to ambient temperature and left 24 hours. The products were isolated by pumping away the solvent and excessive As-F5 and AsF3 at 233 K. The processes of isolation at such low temperature lasted at least several weeks (from 3 to 14 weeks).
Products were characterized by X-ray powder diffraction. Experimental X-ray powder patterns of Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) matched the X-ray powder patterns calculated from the crystal structures of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr). Tb(AsF3)3(AsF6)3 was characterized also by chemical analysis (Calcd. for Tb(AsF3)3(AsF6)3: Tb, 14.2; As, 40.1; F, 45.8; AsF6-, 50.5; Found: Tb,14.8; As, 39.2; Ft, 45.0; AsF6, 49.2). The content of terbium was determined by complexometric titration.16 The content of fluorine (F) was determined by direct potentiometry using fluoride ion selective electrode after preceding total decomposition with NaKCO3.17-19 The total arsenic content was determined after preceding fusion with NaKCO3 with iodometric titration and of AsF6- gravimetrically.20
Ce, Pr) was loaded in the dry-box into a stainless steel autoclave with a Teflon liner. AsF5 (approx. 15 mmol) and a-HF (approx. 3 mmol) were added at 77 K. The autoclave was heated at 398 K for several months. During the crystallization the reduction of some AsF5 occurred and AsF3 was formed. The most probable way for the formation of AsF3 is reduction of some AsF5 on the wall of protective metal vessel on the Teflon liner due to permanent diffusion of AsF5 through the Teflon. After all volatiles were pumped away, only few single crystals were found between powdered materials. Selected ones were mounted in quartz glass capillaries (0.3 mm). Preparations of single crystals from saturated aHF solutions failed.
The crystallographic parameters and summaries of data collection for both compounds are presented in Table 1. Single-crystal data were collected on a Rigaku AFC7 diffractometer using graphite monochromatized MoKa radiation at room temperature from crystals sealed in glass capillary. An empirical absorption correction based on azimuthal scans of several reflections was applied. Both structures were solved by direct methods using SIR-92 21 and SHELXS-97 programs (teXan crystallo-graphic software package of Molecular Structure Corpo-ration22) and refined with SHELXL-9723 software, implemented in program package WinGX24. Full-matrix least-squares refinements based on F2 were carried out for the positional and thermal parameters for all non-hydrogen atoms. Determined crystal structure demonstrated satisfactory geometry but show huge thermal ellipsoids of terminal fluorine atoms belonging to AsF3 group. In order to improve the structural model, the position of F4 terminal fluorine atom was split on two positions (F41, F42) resulting in lower R-value and slightly smaller thermal ellipsoids but in some inadequate As-F bond lengths. Attempts to find better models in space group with lower symmetry were unsuccessful. To resolve these problems crystal of Ce(AsF6)3(AsF3)3 was re-measured at 200 K and at 100 K. Since no real improvement was achieved only unit cell dimensions, determined at these temperatures, are listed in Table 1. The figures were prepared using DIAMOND 3.1 software.25 Further details of the crystal-structure investigation may be obtained from the Fachin-
Table 1. Crystal data and structure refinement for Ce(AsF3)3(AsF6)3 (I) and Pr(AsF3)3(AsF6)3 (II) compounds.
	I	II
Empirical formula	CeAs6F27	PrAs6F27
Formula weight	1102.64	1103.43
Wavelength	0.71069 Ä	0.71069 Ä
Crystal system, space group	hexagonal, P 6 2 c	hexagonal, P 6 2 c
Temperature, K	293(2)	293(2)
Unit cell dimensions, A		
a, A	10.6656(7)	10.6383(7)
	10.6495(7)a	
	10.6329(7)b	
c, A	10.9113(9)	10.878(2)
	10.8971(9)a	
	10.8677(9)b	
V, A3	1074.9(1)	1066.2(2)
	1070.3(1)a	
	1064.1(1)b	
Z	2	2
Calculated density, g/cm3	3.407	3.437
Absorption coeff., mm-1	11.5	11.745
F(000)	998	1000
Crystal size, mm	0.11 x 0.06 x 0.06	0.03 x 0.03 x 0.02
Colour	colourless	green
Theta range for data collection, deg	2.2 - 29.97	2.21 - 29.97
Limiting indices	0 <h <12, 0 <k <12, 0 <l <15	0 <h <14, -14 <k <12, -15 <l <15
Refinement method	Full-matrix least-squares on F2	Full-matrix least-squares on F2
Measured reflections	630	1233
Used in refinement	630	797
Free parameters	65	66
Goodness-of-fit on F2	0.865	0.834
R indices	R1 = 0.0364, wR1 = 0.088	R1 = 0.0386, wR1 = 0.0839
Largest diff. peak and hole	0.961 and -1.248 e Ä-3	0.942 and -1.153 e Ä-3
a at 200 K, b at 100 K.
formationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number: CSD-424853 (Ce(AsF3)3(AsF6)3), CSD-424852 (Pr(As-
F3)3(AsF6)3).
3. Results and Discussion
The Ln(AsF3)3(AsF6)3 compounds can be prepared by the reaction between corresponding aHF solutions of Ln(AsF6)3 and AsF3. Isolated solids are less stable in dynamic vacuum at room temperature as similar compounds with XeF2 as a ligand.26 Several experiments with the isolation of the desired compounds (Ln = Nd, Sm, Gd) at room temperature were done. The final masses of the products isolated at room temperature were significantly lower than the calculated for Ln(AsF3)3(AsF6)3. The products become both AsF3 and AsF5 deficient. Additionally, it was found that the stability of Ln(AsF3)3(AsF6)3 compounds decreases from La to Tm. For that reason, Ln(AsF3)3(AsF6)3 prepared in aHF should be isolated at low temperature. When the Ln(AsF3)3(AsF6)3 compounds are completely dry, they are stable at room temperature
however, they should be kept in the inert atmosphere since they readily react with moisture. The results of chemical analysis in the case of Tb confirmed the composition Tb(AsF3)3(AsF6)3. Additionally, the X-ray powder patterns obtained from the bulk reaction products (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) are identical to the simulated X-ray powder patterns calculated from the single crystal structure of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr). X-ray powder patterns of solid products, which were isolated at room temperature, also show the presence of Ln(AsF3)3(AsF6)3 phases. Yb and Lu products, which were isolated at low temperature, decomposed immediately after warming to ambient temperature.
In the crystal structure of Ln(AsF3)3(AsF6)3 the lant-hanoid atom is coordinated to nine fluorine atoms in the form of tri-capped trigonal prism (Figure 1). Six fluorine atoms provided from six AsF6- anions are located at the corners of the trigonal prism and three additional fluorine atoms belong to three AsF3 units. The Ln-F(AsF6) distances are shorter than Ln-F(AsF3) ones (2.421(7) versus 2.47(1) A in the case of Ce, 2.4-15(6) versus 2.435(7) A for Pr, respectively). The lanthanoid atom is located at the special 2d crystallographic position (at -), therefore all six
Ln- F(AsF6) distances are equal, as well as all three Ln-F(AsF3) distances.
Figure 2. Twenty-four member ring of Ln (Ln = Ce, Pr) atoms, As atoms and F atoms from AsF6 units, forming channels along c-axis. All fluorine atoms are shown in isotropic mode for clarity.
Figure 1. Nine-fold coordination of Ln (Ln = Ce, Pr) atom in the crystal structure of Ln(AsF3)3(AsF6)3. Fluorine atoms of AsF3 units are presented in non-split isotropic mode for clarity.
AsF6 units connect Ln atoms via trans-bridges. The As-Fbridge distances are longer than As-F^^ (1.765(6) vs 1.67(1) A in Ce compound, 1.761(5) vs 1.665(9) A in Pr derivative). The As-F distances are listed in Table 2. Twenty-four member zigzag rings of Ln atoms, As atoms and F atoms from AsF6 units are found. Every second Ln atom of this 24-member ring is in the same plane. AsF3 moieties are located inside these channels of approx. 4 A radii without any further interactions (Figure 2). Due to the trans-bridging role of AsF6 units, Ln centres with attached AsF3 units are interconnected into three-dimensional framework. Two of bridging AsF6 units are involved in a formation of above mentioned rings, next two associate these rings into infinite channels (bonding along 001 crystallo-graphic direction), and the last two AsF6 units are responsible for a bonding in 100 and 010 directions. (Figure 3).
In the case of AsF6 units, shape and orientation of the thermal ellipsoids describe librational motions of terminal fluorine atoms around Fb-As-Fb axis. The AsF3 units are bound to the Ln centres via only one fluorine bridge and, therefore, possess higher "degree of freedom". To overcome the problem with librational (roughly around Ln-Fb-As axis) disorder, positions of both terminal fluorine atoms in AsF3 moiety were split into two positions (Figure 4). Unfortunately, even such approach results in large thermal parameters for each terminal F atom and inadequate As-F bond lengths. In the crystal structure of Ce(AsF3)3(AsF6)3 the length of the terminal As2-F42 bond is practically the same as bridging As2-F2 distance, whereas the As2-F41 distance is shorter. The data obtained by a low-temperature single-crystal X-ray diffraction has led to a practically identical model as at room temperature. Although the shapes of thermal ellipsoids are smaller at 100 K, splitting of two fluorine positions was still observed. The situation with As-F bond lengths became even worse, because at 100 K the terminal As2-F42 di-
Table 2. Interatomic distances (Â) of AsF6 and AsF3 moieties in Ce(AsF3)3(AsF6)3 (I) and Pr(AsF3)3(AsF6)3 (II)
I
As1-F1, As1-F1a	1.765(6)	As2-F2	1.78(1)
As1-F3, As1-F3n	1.67(1)	As2-F41, As2-F41i	1.69(2) a)
As1-F5, As1-F5n	1.68(1)	As2-F42, As2-F42i	1.767(2) a)
II			
As1-F1, As1-F1n	1.761(5)	As2-F2	1.800(8)
As1-F3, As1-F3n	1.652(8)	As2-F41, As2-F41i	1.76(2) a)
As1-F5, As1-F5n	1.665(9)	As2-F42, As2-F42i	1.67(2) a)
a) Due to the huge thermal ellipsoids the positions of terminal F4 atoms belonging to AsF3 were split on two sites (F41, F42). Symmetry codes: (i) x, y, -z+3/2; (ii) y, x, -z+1.
Figure 3. Part of the three-dimensional framework in the crystal structure of Ln(AsF3)3(AsF6)3. All fluorine atoms are shown in isotropic mode for clarity.
stance is longer than the bridging As2-F2 one. Careful examination of thermal ellipsoids of As2 and its fluorine neighbours shows that fluorine ellipsoids become noticeably smaller with decrease of temperature, whereas the ellipsoid of As2 is large even at 100 K. Based on that observations, and taking into account the direction of the main axis of the As2 ellipsoid (Figure 4) it could be concluded that at 293 K there is a combination of librational motion of terminal fluorine atoms in the AsF3 group and the disordering of the whole AsF3 unit in both structures. Lowering the temperature (200 K and 100 K) decreases thermal motion of atoms, but does not influence the disorder of atoms. X-ray diffraction experiments give us an average (in manifold unit cells) atomic coordinates and, consequently, average interatomic distances. Because of that, the observed As-Fterminal bond lengths of disordered AsF3 units are not realistic. It could be concluded that a weak fixation of the AsF3 group and rather low (71%) packing index of the structure promotes disorder of AsF3 groups.
Figure 4. Large thermal ellipsoids of fluorine atoms in AsF3 unit at 293 K (left), model with split fluorine positions at 293 K (centre), model with split fluorine positions at 100 K (right).
4. Conclusions
Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) compounds can be prepared by the reaction of LnF3, AsF5 and AsF3 in aHF as solvent. Isolation of the products from corresponding solutions must be performed at low temperature. Compounds, with exception of Tm, are stable at room temperature when dry, but they are moisture sensitive. Attempts to prepare corresponding Yb and Lu compounds failed.
Isolated solids give similar X-ray powder diffraction patterns, which match the X-ray powder diffraction data calculated from the crystal structures of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr). On the basis of this data it could be concluded that all Ln(AsF3)3(AsF6)3 compounds are isostructural. In the crystal structure of Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr) compounds Ln atoms are nine-fold coordinated by fluorine atoms in the shape of the tri-capped trigonal prism. Three fluorine atoms are provided by three AsF3 groups (capped positions) and six fluorine atoms are provided by six AsF6 units. The latter further connect Ln atoms in three-dimensional framework. At 293 K there is a combination of librational motions of terminal fluorine atoms of AsF3 group and the disordering of the whole As-F3 units. Lowering the temperature (200 and 100 K) decreases thermal motions of atoms, but does not influence the disordering of atoms.
5. Acknowledgement
The authors gratefully acknowledge the Slovenian Research Agency (ARRS) for financial support of Research Program P1-0045 (Inorganic Chemistry and Technology). We thank Dr. Maja Ponikvar-Svet for the chemical analyses and Dr. Primož Benkič for his help in single crystal analysis and helpful comments.
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Povzetek
Z reakcijami med lantanoidnimi(III) heksafluoroarzenati(V) in AsF3 v brezvodnem HF kot topilu smo sintetizirali spojine s sestavo Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm). Spojineje potrebno izolirati pri 233 K. Poskusi priprave spojin Yb in Lu niso bili uspešni. Monokristale Ln(AsF3)3(AsF6)3 (Ln = Ce, Pr) smo pripravili z reakcijo LnF3 (Ln = Ce, Pr) in AsF5in HF pri 393 K, nad kritično temperaturo AsF5. Med procesom kristalizacije se je del AsF5 redu-ciralv AsF3. Spojine kristalizirajo v heksagonalnem kristalnem sistemu, prostorski skupini P 6 2c (a = 10.6656(7) A (Ce); 10.6383(7) A (Pr); c = 10.9113(9) A (Ce), 10.878(2) A (Pr); V = 1074.9(1) A3 (Ce), 1066.2(2) A3 (Pr); Z = 2). Atomi Ln so koordinirani z devetimi fluorovimi atomi. Sest fluorovih atomov, ki jih prispevajo AsF6 enote, tvori pravilno trigonalno prizmo okoli atoma Ln. Nad stranskimi ploskvami prizme so trije fluorovi atomi, ki pripadajo AsF3 molekulam. Atomi Ln so preko AsF6 enot povezani v tridimenzionalno kristalno mrežo. Podatki rentgenske praškovne analize spojin Ln(AsF3)3(AsF6)3 (Ln = La, Nd, Sm, Eu, Gd, Tb, Er, Tm) kažejo, da so spojine izostrukturne s spojinama Ce in Pr.