378
Acta Chim. Slov. 2015, 62, 378-384
DOI: 10.17344/acsi.2015.1424
Scientific paper
Hydroxylammonium Tetrafluoridooxidovanadate(V) - (NH3OH)[VOF4]
Matic Lozin{ek*
Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
* Corresponding author: E-mail: matic.lozinsek@gmail.com
Received: 12-03-2015 Dedicated to the memory of Prof. Dr. Jurij V. Brenčič.
Abstract
Hydroxylammonium tetrafluoridooxidovanadate(V) has been prepared by reaction of (NH3OH)Cl and VOF3 in anhydrous HF solvent and characterised in the solid state by single-crystal X-ray diffraction and Raman spectroscopy. Colourless (NH3OH)[VOF4] crystallises in the monoclinic space group P21/n with a = 7.2349(2) À, b = 5.0351(2) À, c = 12.8322(4) À, ß = 94.843(3)°, V = 465.79(3) À3 and Z = 4 at 150 K. The crystal structure of the title compound is comprised of polymeric chain anions, [VOF4]nn-, interconnected by hydrogen bonds with hydroxylammonium cations, (NH3OH)+.
Keywords: Fluoridooxidovanadate; Vanadium oxyfluoride; Hydroxylamine; Hydroxylammonium; X-ray crystallography; Anhydrous hydrogen fluoride
1. Introduction
Several hydroxylammonium fluoridometallate salts of the type (NH3OH)x[MFn]y have been synthesised and structurally characterised. The solid state structures display a variety of anionic types ranging from isolated, 0-dimensional anions (monomers: [SiF6]2-,1 [ScF6]3-,2 [TiF6]2-,3 [CrF6]3-,4 [FeF6]3-,5 [GaF6]3-,6 [InF6]3-,3 and [VOF5]3-,7; dimeric anions: [Zr2F14]6-,8 [Hf2F14]6-)9 through 1-dimensional chain species ([AlF5]2-,10 [ZrF6]2-)8, to 2-dimensional anionic layers ([MnF4]2-,2 [CoF/-, [CuF/-,11 [ZnFJ2-).12
A successful synthetic procedure which leads to these salts was developed in aqueous hydrogen fluoride media, employing a reaction between hydroxylammo-nium fluoride,13 (NH3OH)F, and desired metal fluoride. The latter reagent is usually prepared in situ by reacting the selected metal or metal oxide with hydrofluoric acid.
However, not all metal oxidation states, and hence anionic species are stable under the foregoing conditions and accessible by the aforementioned synthetic route. This is reflected in the formation of (NH3OH)2[MnF4],2 (NH3OH)2[CoF4],11 and (NH4)(NH3OH)2[VOF5]7 salts with Mn(II), Co(II), and V(IV) fluorido or fluoridooxido anions and not, for instance, Mn(III), Co(III), or V(V)
products, respectively. Likewise, this method is not suitable for routine preparation of hydroxylammonium fluoridooxidometallates or respective salts with moisture-sensitive high-oxidation-state anions.
These shortcomings can be overcome by devising a different synthetic procedure, namely the Lewis acid-base reaction of the corresponding metal fluoride or oxyfluori-de with (NH3OH)F in anhydrous HF (aHF). Moreover, hydroxylammonium fluoride could be prepared in situ by solvolysis of readily available hydroxylammonium chloride, (NH3OH)Cl, in aHF solvent.14
This approach is demonstrated by the synthesis of hydroxylammonium tetrafluoridooxidovanadate(V), (NH3OH)+[VOF4]-, and its characterisation in the solid state by single-crystal X-ray diffraction and Raman spectroscopy reported herein.
2. Experimental
2. 1. General Experimental Procedures
A vacuum line constructed of PTFE (polytetrafluo-roethylene), FEP (perfluorinated ethylene propylene), nickel, and equipped with a Monel Helicoid pressure gauge, a soda lime scrubber, liquid nitrogen cooled traps, and a
two-stage rotary vane pump was used for the solvent transfer. Other involatile reagents were stored and handled in an argon atmosphere in a glovebox having a maximum water content less than 0.5 ppm (M. Braun, Garching, Germany). A PTFE valve equipped a T-shaped FEP crystallisation vessel which consisted of a larger diameter "reaction" arm constructed from 16-mm i.d. (19-mm o.d.) FEP tubing, to which a narrower "crystallisation" arm constructed from 6-mm i.d. (8-mm o.d.) FEP tube was attached at right angles, was used for synthesis and crystal growth.15 The solution was stirred in the wider FEP arm with the aid of a PTFE-coated magnetic stir bar. All FEP crystallisation vessels were passivated with F2 prior to use.
2. 2. Reagents
Hydroxylamine hydrochloride, (NH3OH)Cl, (Alfa Aesar, 99%) and vanadium oxytrifluoride, VOF3, (Aldrich, 99%) were used as received. The solvent, aHF, was prepared by treating commercial anhydrous HF (Linde, Fluorwasserstoff 3.5) with K2NiF6 (Ozark-Mahoning, 99%) for at least 12 h prior to use. Caution: Anhydrous HF must be handled with great care in a well-ventilated fume hood and protective gear must be worn at all times.
2. 3. Synthesis and Crystallisation of
(NH3OH)[VOF4]
In a typical experiment, (NH3OH)Cl (70 mg, 1.01 mmol) and VOF3 (126 mg, 1.02 mmol), were weighed into the larger diameter arm of a FEP-crystallisation vessel, and aHF solvent (2.0 mL) was condensed at liquid nitrogen temperature into the same arm. Upon warming of the frozen reaction mixture, vigorous evolution of HCl gas occurred, which was removed on a vacuum line. A clear, pale yellow solution resulted which was decanted from the larger diameter reaction arm of the T-shaped crystallisation vessel into the narrower crystallisation arm. The latter was left at room temperature while the former was slowly cooled. Due to the steadily increasing temperature gradient (up to 35-45 °C), aHF evaporated from the crystallisation arm into the reaction arm in the course of 3-4 weeks. The resulting white crystalline material was transferred to a glovebox where a portion of the crystals were poured from the FEP tube onto a watch glass and covered with a protective layer of perfluorinated oil (ABCR, AB 102850, perfluorodecalin, 98%, cis and trans). Suitable plate-like colourless crystals were selected under a polarizing microscope outside the glovebox, mounted on a glass capillary pin with the aid of Baysilone-paste, and quickly transferred onto the goniometer head with magnetic base, where it was cooled by a cold nitrogen stream. The product decomposes rather slowly in air, but undergoes a rapid reaction and effervesces upon contact with water.
2. 4. Single-crystal X-ray Structure Determination
X-ray diffraction data were collected with the Crys-Alis PRO16 software package on an Agilent New Gemini, Dual, diffractometer equipped with an Atlas CCD area detector using graphite-monochromatised Mo Ka radiation at 150 K. An analytical absorption correction was applied to the data set. Using Olex2,17 the structure was solved by the charge-flipping algorithm using the structure solution program Superflipls and refined with SHELXL (v. 2014/7)19 software by least squares minimisation. Hydrogen atoms were located in a difference Fourier map and isotropically refined. While atoms H1-H3 were well behaved, the H4-atom displayed a short H4-N1 distance (0.73 À), therefore, a distance restraint (DFIX) of 0.91 À with a sigma value of 0.02 À was employed. Small residual electron density minima and maxima are located 0.78 À from V1 and 0.73 À from O1, respectively. The figures were prepared using DIAMOND program.20 Details of the data collection, data processing, and refinement are given below (Table 1). The supplementary crystallographic data (CIF file) may be obtained from the Fachinformationszentrum Karlsruhe (FIZ), D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the deposition number CSD-429634 for (NH3OH)[VOF4].
2. 5. Raman Spectroscopy
Raman spectra were obtained from a crystalline sample that was powdered and transferred into a quartz capillary under an inert atmosphere and flame-sealed. Spectra were measured at room temperature in the range 50-3900 cm-1 on a Horiba Jobin-Yvon LabRAM HR Raman microscope using He-Ne laser excitation (632.8 nm, 25 mW).
Table 1. Crystal data and structure refinement summary
Compound
(NH3OH)[VOF4]
Fw T (K)
Crystal system Space group a (À) b (À) c (À)
ß(°) У (À3) Z
Pcalc (g/cm3)
A(À) ^ (mm-1)
Reflns./indep./with [I > 2o(I)] R R
int' sigma
Data/restraints/parameters S
Rj, wR2 [I > 2o(I)] Rj, wR2 [all data]
P , APmax (eÀ-3)
176.98 150
monoclinic P21/n 7.2349(2) 5.03513(16) 12.8322(4) 94.843(3) 465.79(3) 4
2.524
0.71073
2.141
3232/1139/1066 0.0180, 0.0218 1139/1/90 1.063
0.0180, 0.0436 0.0197, 0.0444 -0.327, 0.273
3. Results and Discussion
3. 1. Synthesis
The only previously reported and structurally characterised hydroxylammonium fluoridovanadate is the vanadium^) double salt, (NH4)(NH3OH)2[VOF5],7 which formed as a product of partial decomposition of (NH3OH)+ ion and aerial mono-oxygenation of vanadium in aqueous HF. An altogether different synthetic procedure was therefore employed for the synthesis of hydroxy-lammonium vanadium(V) oxyfluoride, (NH3OH)[VOF4]. The latter was synthesised by a Lewis acid-base reaction in which the Lewis acid, VOF3, accepts a fluoride ion from the base, (NH3OH)F, which is formed in situ by sol-volysis of (NH3OH)Cl in aHF. This reaction produces hydrogen chloride gas which is sparingly soluble in anhydrous hydrogen fluoride.21
aHF
(NH3OH)Cl + VOF3 + HF ^ (NH3OH)[VOF4] + HCl T
Figure 1. The asymmetric unit is composed of a (NH3OH)+ cation and a VOF4-unit which has been expanded to reveal a zigzag [VOF4]n- chain anion comprised of cis-corner sharing [VOF3F2/2] octahedra. Thermal ellipsoids are drawn at the 70% probability level and hydrogen atoms are depicted as small spheres of arbitrary radius.
3. 2. Crystal Structure
The title compound crystallises in the monoclinic space group P21/n with all atoms of the asymmetric unit (V1, F1-F4, O1, O2, N1, and H1-H4) occupying general positions (Wyckoff 4e) (Figure 1). The (NH3OH)[VOF4] salt has a 1-dimensional polymeric anion and this group of hydroxylammonium fluoridometallate salts is, along with the family possessing oligomeric anions, the least populated. The ^([VOF4D anion is a zigzag cis-corner sharing chain, whereas essentially linear chains of transcorner sharing AlF6-octahedra and trans-perpendicular-edge sharing ZrF8-units have been observed in the crystal structures of (NH3OH)2[AlF5]10 and (NH3OH)2[ZrF6],8 respectively. It is also noteworthy that the described compound represents a rare example of a structurally characterised hydroxylammonium fluoridometallate having a 1:1 cation-anion stoichiometry, whereas 3:1 and 2:1 salt-types prevail.
The central vanadium(V) cation of the [VOF4]- anion is coordinated by five fluorine and one oxygen atom to form a distorted [VOF3F2/2] octahedron with a short V=O (1.5662(10) À) and a long bridging trans-V-Fb (2.3306(7) À) bonds. Equatorial bridging cis-V-Fb distance (1.8540(8) À) is, as expected, elongated in comparison with the three terminal V-Ft bonds (1.8094(7)-1.8315(7) À). The bond valence sum calculation on vanadium gives 5.01, in agreement with the expected value of 5 valence units.22 The "roof effect" or the repulsion of the axial oxido ligand exerted on the equatorial fluorido ligands is manifested by the deviation of cis-O-V-F angles (99.20(4)-101.07(4)°) from 90° (Table 2).
The consequence of the described arrangement of oxido and fluorido ligands is the axial displacement of va-
nadium ion (by 0.3157(5) À) from the least-square plane of four equatorial fluorido ligands of the [VOF5] octahedral unit towards the O-atom. This so-called intrinsic primary C4 [001] out-of-centre distortion23 is typical of early d0 transition metal fluoridomonooxido anions.24 Thus, the polar i([VOF4]-) zigzag cis-corner sharing chains are oriented antiparallel with respect to each other along the b-crystallographic axis and, as a result, a centrosymmetric material is formed (Figure 2).
Three different anions with the empirical formula VOF4- have been described in the literature, namely, the edge-sharing dimer25,15 - [V2O2F8]2-, the corner-sharing cyclic trimer26 - [V3O3F12]3-, and the polymeric zigzag cis-chain27-29 - [VOF4]n-. The other three examples of structurally characterised i([VOF4]-) anions display bond lengths comparable to those observed in (NH3OH)[VOF4] (Table 2): Cs[VOF4]:27 V=O 1.529(12) À, V-Ft 1.783(10) À and 1.788(6) À, V-Fb 1.793(10) and 2.312(10) À; K[VOF4]:28 V=O 1.572(8) À, V-Ft 1.786(7)-1.804(7) À, V-Fb 1.875(7) and 2.333(7) À; Ag[VOF4]:29 V=O 1.571(3) À, V-Ft 1.798(2)-1.822(2) À, V-Fb 1.857(2) and
2.287(2) À. t	b
It is interesting to note that the ^([VOF4]-) anion apparently possess sufficient degree of conformational flexibility to the extent that it can accommodate and efficiently pack into a crystal lattice with monovalent cations of various sizes (Ag+, K+, Rb+, Cs+) as well as different shapes (H3N+-OH). However, in the aforementioned salts, the zigzag cis-chain [VOF4]- anions adopt different solidstate conformations,29 and in the present hydroxylammo-nium salt, a "helical" geometry is observed, which is identical to that of the conformer described in the Ag[VOF4] crystal structure. This is reflected in the good agreement between geometric parameters of ^([VOF4D
Table 2. Selected Interatomic distances and angles.
Distances (À)
Angles (°)
V1-O1	1.5662(10)	O1	-V1	-F1	101.07(4)	V1-F4-	-V111	150.89(4)
V1-F1	1.8094(7)	O1	-V1	-F2	99.83(5)	F1-V1-	••V1ii-F1ii	-157.19(3)
V1-F2	1.8100(8)	O1	-V1	-F3	99.72(4)	N1-O2-	-H1	98.7(16)
V1-F3	1.8315(7)	O1	-V1	-F4	99.20(4)	O2-N1-	-H2	105.0(14)
V1-F4	1.8540(8)	O1	-V1	-F4i	178.84(4)	O2-N1-	-H3	109.7(12)
V1-F4i	2.3306(7)	F1-	V1-	-F3	159.09(4)	O2-N1-	-H4	108.9(17)
N1-O2	1.4111(14)	F2-	V1-	-F4	160.96 (4)	H2-N1-	-H3	112(2)
V1-(F4)-V1ii	4.0521(4)	F2-	V1-	-F4i	79.03(3)	H2-N1-	-H4	113(2)
V1-V1vi	5.03513(16)	F4-	V1-	-F4i	81.932(14)	H3-N1-	-H4	108(2)
Figure 2. Packing in the unit cell of (NH3OH)[VOF4] viewed along b- and a-crystallographic axes, respectively. Note the resemblance of [VOF4] anion depicted in [010] direction to the dimeric and centrosymmetric edge-sharing [V2O2F8]2- dianion,2515 as well as the packing similarities between the crystal structures of (NH3OH)[VOF4] and Ag[VOF4].29
anion described herein (Table 2) and those reported for the Ag+ salt (V—(Fb)—V 4.0159(8) À; V-V 5.0071(3) À; V-Fb-V 151.3(1)°).29
The hydroxylammonium cation, (NH3OH)+, which is isoelectronic with methanol, has an N-O distance (1.4111(14) À) that is in excellent agreement with the bond length (1.414(2) À) reported in the crystal structure of hydroxylammonium 4-(ethyloxycarbonyl)-5-oxo-2,5-dihydroisoxazolide, (NH3OH)+(C6H6NO4)-, determined at 150 K,30 and with the N-O bond length (1.416(5) À) found in the crystal structure of bis(hydroxyl-ammonium) hexachloridoplatinate(IV)-18-crown-6 (1/2), (NH3OH)2[PtCl6]2C12H24O6, measured at 100 K.31 The N-O bond length in (NH3OH)[VOF4] also falls well within the range of reported N-O bond distances (1.375(2)-1.416(2) À; average of 24 values: 1.406(2) À) for other known hydroxylammonium fluoridometallates that were structurally characterised at room temperatu-re.1-8' 10-12 Despite the lack of available high-quality, low-temperature X-ray diffraction data on hydroxylamine or its cocrystals, it is evident that the N-O bond in the N-pro-tonated form or hydroxylammonium cation is significantly shortened in comparison with the neutral hydroxyl-
amine molecule.32 For the latter, the N-O bond distance value in the gas phase is available from the microwave spectroscopy (1.453(2) À; room temperature)33 and in the solid state from a single-crystal X-ray diffraction determination which unfortunately suffers from the omission of hydrogen atoms in the refinement which, in turn, is reflected in a large estimated probable error for the N-O bond length (1.476(30) À; 213 K).34 The hydroxylammonium cation typically adopts a staggered conformation, however, the energetically soft parameters, namely the H-O-N-H torsion angles, are likely to be to some extent affected by hydrogen bonding interactions. In the present crystal structure, the conformation of the cation is almost midway between staggered (60°) and eclipsed (0°), as demonstrated by the values of the H1-O2-N1-H3 (-80(2)°) and H1-O2-N1-H4 (38(2)°) torsion angles.
The N-H—F type hydrogen bonding usually dominates intermolecular interactions in the solid-state structures of nitrogen base-templated hybrid organic-inorganic fluoride compounds.35-38 Similarly, the anions in the crystal structure of (NH3OH)[VOF4] interact with the cations through a myriad of hydrogen bonds (Table 3). All four hydrogen atoms of the cation are involved in hydrogen
bonding interactions, with those residing on nitrogen being bifurcated donors, which is reflected in the deviation of the N-H—F/O hydrogen bonds from linearity. Conversely, in the VOF4-unit of the anion, just two F-atoms act as a bifurcated acceptors (F1, F3), and all possible acceptors, including oxygen, are utilised, with the exception of the bridging fluorine (F4). The effect of partially charged donors (NH3+) and acceptors (VOF4) is demonstrated by the shorter N—F hydrogen bonding distances (2.747(2)-2.952(1) À) observed in the crystal structure of (NH3OH)[VOF4] salt as compared, for instance, to those reported between neutral molecules of either mer-[VF3(NH3)3] (2.922(1)-3.267(1) À)39 or [TaF5(NH3)3] (2.848(6)-3.519(6) À).40
Table 3. Hydrogen-bond geometry.a
D-H-A D-H (À) H-A (À) D-A (À) D-H-A(°)
O2-	-H1-F3iii	0.83(2)	1.77(2)	2.5884(13)	167(2)
N1-	-H2-F1i	0.90(2)	2.03(2)	2.7471(15)	135(2)
N1-	-H2—F3	0.90(2)	2.28(2)	2.8973(15)	125.5(19)
N1-	-H3-F2iv	0.87(2)	1.98(2)	2.7665(15)	148.7(18)
N1-	-H3-O1v	0.87(2)	2.50(2)	3.1360(16)	130.9(16)
N1-	-H4-F1ii	0.86(2)	2.38(2)	2.9516(14)	125(2)
N1	-H4-O2lli	0.86(2)	2.20(2)	2.9433(16)	145(3)
ding interactions, the cation-cation N-H-O hydrogen bonding is observed which gives rise to hydrogen-bonded zigzag chains oriented along the ft-crystallographic direction. Each aforementioned cationic chain is surrounded by four parallel i([VOF4]-) chain anions, and vice versa (Figure 2). In this fashion, conjoined hydrogen bonded ring patterns with the graph-set notations42 R2(8) and R4(12) connect the C3 cationic chain with two neighbouring chain anions in the ftc-plane (Figure 3), and ^2(6), R33(6), and R2(8) rings to the adjoining anions above and below in the direction of the a-crystallographic axis.
3. 3. Raman Spectroscopy
Hydroxylammonium fluoride-salts that have been studied by Raman spectroscopy include (NH3OH)F,14 (NH3OH)[UF6],43 and (NH3OH)2[TiF6].44 Their respective N-O stretching bands of the hydroxylammonium cation were observed at 1017, 1015, and 1014 cm-1. Characteristic V=O and V-F stretches appear in Raman spectra of tetrafluoridooxidovanadate(V) salts: Cs[VOF4] (V=O 1031 cm-1; V-F 620, 585 cm-1),45 K[VOF4] (V=O 1019 cm-1; V-F 643, 618, 577 cm-1),28 and Ag[VOF4] (V=O 1008 cm-1; V-F 621, 595, 570 cm-1).29
a Symmetry transformations for the generation of equivalent atoms: (i) -x+1/2, y-1/2, -z+1/2; (ii) -x+1/2, y+1/2, -z+1/2; (iii) -x+3/2, y+1/2, -z+1/2; (iv) x+1/2, -y+1/2, z+1/2; (v) x+1/2, -y+3/2, z+1/2; (vi) x, y+1, z.
In hydroxylammonium fluoridometallates, as well as in other hydroxylammonium salts,32,41 the shortest, most linear, and hence presumably the strongest hydrogen bond typically involves the hydroxyl group of the cation as a donor and an anion as an acceptor (O—F 2.588(1) À). In addition to the described cation-anion hydrogen bon-
Figure 3. Hydrogen-bonded zigzag cationic chain flanked by two zigzag ^([VOFJ-) anions in the èc-crystallographic plane. The hydrogen bonding network is depicted with dashed lines and graph-set notation.42
Figure 4. The Raman spectrum of (NH3OH)[VOF4] with values of Raman band frequencies and relative intensities (given in parentheses) denoted. The regions (1750-2900 and 3500-3900 cm-1) where no signals were observed are omitted.
The Raman frequencies observed in the case of (NH3OH)[VOF4] for both the cation (N-O 1017 cm-1) and the anion (V=O 1028 cm-1; V-F 629, 585 cm-1) are similar to the aforementioned reported values (Figure 4). Weak bands are observed in the O-H and N-H stretching regions, 3200-3000 cm-1, as well as in the H-N-H bending and NH3+ rocking or O-H bending mode ranges, 1600-1500 cm-1 and 1300-1100 cm-1, respectively.
4. Conclusion
The novel hydroxylammonium salt, hydroxylammo-nium tetrafluoridooxidovanadate(V), has been synthesi-sed and its crystal structure elucidated at low temperature. The solid-state structure which consists of (NH3OH)+ cations interconnected by hydrogen bonds with a helical conformation exhibiting [VOF4]- chain anions is discussed in detail. In addition, the reported Raman spectrum is in good agreement with the published vibrational spectra of similar compounds. Hydroxylammonium salts with fluoride or oxide fluoride anions of metals in oxidation states that cannot be isolated from aqueous HF solutions, can conveniently be prepared from corresponding Lewis acidic metal fluorides or oxyfluorides and hydroxylam-monium chloride in anhydrous HF, as exemplified by the reported synthesis of (NH3OH)[VOF4]. Furthermore, this procedure bypasses the synthesis and isolation of hydroxylamine and hydroxylammonium fluoride, by formation of the latter Lewis base fluoride in situ by the sol-volysis of the chloride in aHF.14 The described synthetic approach could thus be utilised to expand the library of known hydroxylammonium fluoridometallates.
5. Acknowledgements
This work was supported by the Slovenian Research Agency (ARRS) within the research program P1-0045 Inorganic Chemistry and Technology. Useful suggestions and stimulating discussions as well as continuous support by Prof. Gary J. Schrobilgen, Prof. Boris Zemva, and Dr. Mirela Dragomir is greatly appreciated by the author, who also acknowledges the support of his research by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme.
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Povzetek
Spojina hidroksilamonijev tetrafluoridooksidovanadat(V) je bila pripravljena z reakcijo med (NH3OH)Cl in VOF3 v brezvodnem HF in okarakterizirana v trdnem stanju z rentgensko difrakcijo na monokristalu in Ramansko spektroskopijo. Brezbarvni kristali (NH3OH)[VOF4] kristalizirajo v monoklinski prostorski skupini P21/n z dimenzijami osnovne celice a = 7.2349(2) A, b = 5.0351(2) A, c = 12.8322(4) A, ß = 94.843(3)°, V = 465.79(3) A3 in Z = 4 pri 150 K. Kristalno strukturo sestavljajo polimerni verižni anioni, [VOFJJJ-, ki se z vodikovimi vezmi povezujejo s hidroksilamonijevimi kationi, (NH3OH)+.