Reports on the reactions of vanadium halides with liquid ammonia, especially of the chloride, have lead to discussions about the compositions of the obtained products. The first insight into the reaction of VCl3 with gaseous ammonia forming presumably vanadium nitrides at room temperature and at low temperature (–78 °C) was published by Meyer and Backa in the early 1920s . In the same work the authors reported the compounds [V(NH3)6]X (X = Cl, Br), which were obtained by the reaction of the respective vanadium halide with liquid ammonia. The work was questioned by Ephraim and Ammann  and the group of Jantsch et al.  based on vapor pressure measurements (tensieudiometric measurements). They observed that the reaction of VCl3 with ammonia leads to the formation of VCl3/NH3 adducts with varying ammonia content: VCl3 · 12 NH3 was observed at low temperatures, VCl3 · 7 NH3 at room temperature, and VCl3 · 3 NH3 at 242 °C. Some years later Remy and May described the reaction of VCl3 and VBr3 with gaseous ammonia with eq. (1) :
In a second step, occurring at low temperature, the vanadium amide halides add ammonia as a ligand or as ammonia of crystallization. In the case of VCl3 the compound V(NH2)Cl2 · 7 NH3 and in the case of VBr3 the compound V(NH2)Br2 · 8 NH3 are formed. As expected, the usage of V in higher oxidation states promotes the deprotonation of ammine ligands. The reaction of VCl4 with liquid ammonia leads to the formation of ammonium chloride and the ammonobasic vanadium(IV) chloride, VCl(NH2)3 . The first ammine complex of a vanadium(III) halide was isolated in pure form by Meyer and coworkers from the reaction of ammonium bromide with vanadium metal . They reported an octahedrally coordinated vanadium(III) atom with one molecule of ammonia and five bromine atoms [V(NH3)Br5]2–. Due to the acidic conditions, a deprotonation of the ammine ligands does not occur. From the reaction of VI2 with liquid ammonia, Jacobs and coworkers obtained [V(NH3)6]I2 .
In order to complement these previous studies on the reactions of vanadium(III) halides with ammonia (liquid and gaseous), we now report our results on the reaction of VF3 with liquid ammonia at –40 °C and at ambient temperature. Under both conditions lilac plate-shaped crystals of mer-triammine trifluorido vanadium(III), mer-[VF3(NH3)3] (1), were obtained, of which the composition was elucidated using low-temperature single-crystal X-ray diffraction.
2 Results and discussion
The green vanadium trifluoride reacts with liquid ammonia under the formation of lilac crystals of mer-triammine trifluorido vanadium(III), mer-[VF3(NH3)3] (1). The title compound crystallizes in the monoclinic space group type P21/c (for further crystallographic data, see Table 1). The vanadium atom V(1) is coordinated by three symmetry-inequivalent fluorine atoms F(1) to F(3) and three symmetry-inequivalent ammine ligands with nitrogen atoms N(1) to N(3). All atoms occupy general positions (Wyckoff position 4e). The ammine ligands and the fluorine atoms form a coordination polyhedron best described as an octahedron. As expected, the mer-isomer (Fig. 1) is obtained. The V–N distances are observed in the narrow range from 2.140(8) to 2.161(9) Å, and the V–F distances range from 1.896(6) to 1.901(6) Å. The V–F distance is shortened in comparison to that in VF3 (MF6 octahedra) where a V–F distance of 1.95 Å is observed [8, 9]. The coordination polyhedron of the [V(NH3)Br5]2– anion of the compound (NH4)2[V(NH3)Br5] was also described octahedron-like, and the V–N distance observed at 2.14 Å  agrees with that observed for compound 1.
Selected crystallographic data for compound 1.
|Color and appearance||lilac plates|
|Molecular mass, gmol–1||159.04|
|Space group (no.)||P21/c (No. 14)|
|R(F) (I ≥ 2σ(I)/all data)||0.025/0.036|
|wR(F2) (I ≥ 2σ(I)/all data)||0.056/0.060|
|S (all data)||0.98|
mer-[FeF3(NH3)3] crystallizes isotypically to the title compound . In comparison, it shows very similar interatomic distances of Fe–F (1.8981(15) to 1.908(16) Å) and Fe–N (2.118(3) to 2.139(3) Å) . This can be attributed to the similar ionic radii of the trivalent metal ions (V3+ 0.780 Å, Fe3+(hs) 0.785 Å, both for C. N. 6) . The observed F–M–F angles are very close to 90° and 180° (N(1)–V(1)–N(3) 179.38(3)°, F(3)–V(1)–N(1) 90.28°), which shows the very minor distortions of the coordination polyhedron.
A projection of the unit cell on the ac plane shows that the vanadium atoms are arranged in channels along the b axis. The unit cell is shown in Fig. 2. By hydrogen bonding, the molecules of compound 1 are interconnected to form a three-dimensional network. Every fluorine atom, as an acceptor, is coordinated to three different ammine ligands of three neighboring molecules of [VF3(NH3)3]. In turn, the ammine ligands are coordinated by three different fluorine atoms each (Fig. 3). The shortest contact between a fluorine and a hydrogen atom is 2.025(6) Å long.
A model for the assessment of the strength of hydrogen bonding is the shortening of the hydrogen···acceptor distance (H···A), compared to the sum of the van-der-Waals radii [11, 12], and the deviation of the D–H···A angles (D = donor) from 180° . The shortest N–H···F hydrogen bond with 24 % shortening and a deviation of 3.2° from linearity is found for N(1)–H(1A)···F3#1 with a D···A distance of 2.942(1) Å. A quite similar shortening is observed for N(2)–H(2C)···F(2)#2 with 21 % and a deviation of 1.6° from linearity (D···A distance: 2.965(1) Å). The weakest hydrogen bond is observed for N(2)–H(2B)···F(2)#5 with a D···A distance of 3.267(1) and a deviation of 12.6° from 180°. The hydrogen bond shortening plotted versus the deviation from linearity can be found in Fig. 4. The strongest hydrogen bonds are found in the upper left and the weakest ones in the lower right region of the plot. The N–H···F values are in good agreement with values found in the compounds [M(NH3)4F4] NH3 (M = Zr, Hf)  or in the isotypic compound mer-[FeF3(NH3)3] . The latter compound shows H···A shortenings of 19 to 33 % and deviations of 5 to 19° from linearity. Due to these observations, the N–H···F hydrogen bonds of mer-[VF3(NH3)3] may be seen as strong ones, as expected. A comparison with the hydrogen bonding of the isotypic iron compound is shown in Fig. 4.
In summary, the reaction of vanadium trifluoride with liquid ammonia at –40 °C leads to plate-shaped lilac crystals of triammine trifluoride vanadium(III), mer-[VF3(NH3)3] (1), which crystallizes in the monoclinic space group type P21/c (No. 14). Similar to the isotypic compound mer-[FeF3(NH3)3] , synthesized by the reaction of FeF3 in liquid ammonia, the title compound also shows extensive N–H···F hydrogen bonding.
3 Experimental part
All work was carried out excluding humidity and air in an atmosphere of dried and purified argon (Westfalen AG, Münster, Germany) using high-vacuum glass lines or a glove box (MBraun, Garching, Germany). Liquid ammonia (Westfalen AG) was dried and stored over sodium (VWR, Darmstadt, Germany) in a special high-vacuum glass line. All reaction vessels for handling liquid ammonia were made out of borosilicate glass and were flame-dried before use. Hydrogen fluoride (Solvay Fluor, Solvay, Bad Wimpfen, Germany, 99.5 %) was handled in a prepassivated stainless steel line with an adapted resistance furnace. The magnesium boat was passivated in a stream of 10 % fluorine and 90 % argon. The purity of vanadium(III)-oxide (Sigma-Aldrich, Taufkirchen, Germany, 99.99 %) was checked by powder X-Ray diffractometry, and the sample could be used without further purification.
VF3 was prepared by the reaction of black vanadium(III)-oxide V2O3 with gaseous anhydrous HF (aHF). A magnesium boat was charged with 150 mg (1 mmol) V2O3 and placed into a furnace. Before the vanadium(III)-oxide was fluorinated by aHF, the furnace was purged for 10 min with argon. To start the fluorination, the argon was replaced by aHF gas, and the oven was heated with a rate of 2 °C min–1 to 400 °C. This temperature was held for 4 h, and afterwards the temperature was lowered to 50 °C with a rate of 2 °C min–1. At this temperature, the flow of hydrogen fluoride was replaced by argon. The resulting green powder was characterized by its powder X-ray diffraction pattern and IR spectrum after additional annealing at 550 °C for 3 days in a sealed nickel ampule.
For the synthesis of compound 1, a reaction vessel was charged with 120 mg (10 mmol) VF3 under argon atmosphere. After cooling to –78 °C, dry liquid ammonia (approximately 10 mL) was condensed onto the green powder leading to a colorless solution and an undissolved green residue. The reaction vessel was stored at –40 °C until lilac crystals of a size suitable for the X-ray diffraction experiment were obtained.
In a second synthesis of compound 1, a glass tube was charged with 13 mg (0.12 mmol) VF3. Dry liquid ammonia (0.5 mL) was condensed at –78 °C on the top of the green powder leading to a dark green-black undissolved residue and a colorless solution. After flame sealing, the glass tube was stored at +40 °C. Under these conditions, the reaction as well as the crystallization is significantly faster.
The crystals were separated manually at –40 °C under perfluoroether oil (Galden PFPE, Solvay Solexis) under a nitrogen atmosphere and mounted on the diffractometer by using MicroLoops (MiTeGen).
The X-Ray structure analysis of single crystals of compound 1 was carried out using an Oxford XCalibur3 diffractometer with monochromated molybdenum radiation (MoKα, λ = 0.71073 Å) and a CCD-type detector. The crystals were mounted using the MiTeGen MicroLoop system. The evaluation of the diffraction data was carried out with the crysalis red  software. An empirical absorption correction was applied with spherical harmonics within the scale3 abspack software . The structure was solved using Direct Methods (shelxs-97 [16, 17]) and refined against F2 (shelxl-97 [18, 19]). Non-hydrogen atoms were located by difference Fourier syntheses and refined anisotropically. Hydrogen atoms were located from the difference Fourier syntheses and refined isotropically. The residual electron density is located close to the vanadium atom.
Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: firstname.lastname@example.org, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-428849.
F. K. would like to thank the Deutsche Forschungsgemeinschaft for his Heisenberg professorship. P. W. would like to thank the Deutsche Forschungsgemeinschaft for financial support. We thank Solvay for the donation of fluorine and anhydrous hydrogen fluoride.
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