(N2H6)3TaF8TaF7HF2and Na7TaF8(TaF7)2: Novel fluoride tantalates with different isolated anionic polyhedra

Article abstract of DOI:10.1016/j.jfluchem.2017.01.006

The reaction of hydrazinium difluoride and TaF₅in 66% hydrofluoric acid water solution resulted in the formation of rather unusual (N₂H₆)₃TaF₈TaF₇HF₂compound. Similar procedure started from a TaF₅[sbnd]NaF mixture performed in 35% HF water solution resulted in the formation of a Na₇TaF₈(TaF₇)₂compound. Synthesis, crystal structures and Raman spectra of these new compounds are reported in this communication. The main feature of both compounds is the presence of discrete TaF₈^3?and TaF₇^2?anions in their crystal structures, which was also confirmed by Raman spectroscopy.

Full text of DOI:10.1016/j.jfluchem.2017.01.006

Journal of Fluorine Chemistry 195 (2017) 13–17
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short Communication
(N2H6)3TaF8TaF7HF2 and Na7TaF8(TaF7)2: Novel fluoride tantalates
with different isolated anionic polyhedra
E. Goreshnik
Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
A R T I C L E I N F O
A B S T R A C T
Article history:
The reaction of hydrazinium difluoride and TaF5 in 66% hydrofluoric acid water solution resulted in the
formation of rather unusual (N2H6)3TaF8TaF7HF2 compound. Similar procedure started from a TaF5ꢀꢀNaF
mixture performed in 35% HF water solution resulted in the formation of a Na7TaF8(TaF7)2 compound.
Synthesis, crystal structures and Raman spectra of these new compounds are reported in this
communication. The main feature of both compounds is the presence of discrete TaF8 and TaF7
anions in their crystal structures, which was also confirmed by Raman spectroscopy.
Received 14 December 2016
Received in revised form 4 January 2017
Accepted 6 January 2017
Available online 13 January 2017
3ꢀ
2ꢀ
Keywords:
Heptafluorotantalate
Octafluorotantalate
Crystal structure
Raman spectra
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
structural study reveals that the salt with K3TaF8 chemical formula
is a double salt K2TaF7ꢁKF [20], similarly to that in the case of
(trenH3)TaF7ꢁF [13]. Synthesis and characterization of hydrazinium
fluoridotantalates(V) were reported many years ago [21]. The
compound crystallized with N2H6F2: TaF5 ratio of 1.5:1 from 45%
hydrofluoric acid water solution was identified, basing on
vibrational spectroscopy and mass balance, as (N2H6)3(TaF8)2H2O.
As authors [21] concluded: “The investigation of the formation
conditions of hydrazinium fluorotantalates posed more questions
than were answered mainly because the ionic equilibria in the
solutions containing tantalum, hydrofluoric acid and hydrazine are
very complex”. Furthermore, structural data of the isolated
hydrazinium fluorotantalates were absent, their X-ray powder
diffraction patterns were complicated and indexing of them was
not attempted. Reaction of hydrazinium difluoride and TaF5 in 66%
HF water solution resulted in the formation of rather unusual
(N2H6)3TaF8TaF7HF2 compound. An intention to grow Na3TaF8
crystals for comparison of its Raman spectrum and the spectrum of
(N2H6)3TaF8TaF7HF2 resulted in the formation of Na7TaF8(TaF7)2
compound. Synthesis, crystal structures and Raman spectra of
these new compounds are reported in this communication.
Tantalum pentahalides because of its Lewis acidity and partial
coordination unsaturation form various adducts with O-donors
[1,2], N-heterocyclic carbenes [3], diphosphine and diarsine [4] etc.
Tantalum pentafluoride can also accept fluorine anions, resulting
in the formation of [TaF6]ꢀ, [TaF7]2–, and [TaF8]3– complex anions.
Increasing the acidity of the solution promotes the formation of
hexafluoridotantalate-anion, whereas solution dilution or adding
alkali metal fluorides shifts the equilibriums towards [TaF7]2– and
[TaF8]3– [5]. The lower solubility of MTaF7 compounds (in
comparison with M(TaF6)2 ones) is an important reason of the
crystallization of alkaline earth metals heptafluoridotantalates(V)
even from anhydrous HF [6]. The most widely studied are salts of
inorganic cations and protonated organic amines containing
octahedral complex [TaF6]ꢀanion. Heptafluoridotantalates(V) ap-
pear much rarely. Industrially important K2TaF7 salt is well studied
and structurally characterized [7,8,]. The crystal structures of
Rb2TaF7 [9], MTaF7 (M = Ca, Sr, Ba, Pb) [6], (enH2)TaF7 [10],
(C4H10N2H2)TaF7 [11], (trenH4)(TaF7)2 and (trenH4)(TaF7)2ꢁH2O
[12], (trenH3)TaF7ꢁF [13], (C2H5N4)2TaF7, (C2H6N4)TaF7ꢁH2O [14]
have been reported. The primary crystallographic information, i.e.
crystal symmetry and unit cell parameters, for the Na2TaF7,
Na2TaF7ꢁH2O [15], Li2TaF7ꢁ2H2O, M2TaF7 (M = , Cs, NH4) [16]
MTaF7ꢁ6H2O (M = Mn, Co, Ni, Zn, and Cd) compounds was also
published [17]. Octafluoridotantalate(V) anions were found in only
two compounds, Na3TaF8 [18] and Ba5F(TaF8)(TaF6O)2 [19]. X-ray
2. Results and discussion
2.1. Crystal structure
The crystal structure of (N2H6)3TaF8TaF7HF2 compound consists
2+
of discrete TaF83ꢀ, TaF72ꢀ and HF2ꢀ anions, bound with the N2H6
cations via NꢀꢀHꢁ ꢁ ꢁF hydrogen bonds (Fig. 1). The presence of both
seven- and eight-coordinated tantalum centrum was reported
E-mail address: evgeny.goreshnik@ijs.si (E. Goreshnik).
http://dx.doi.org/10.1016/j.jfluchem.2017.01.006
0022-1139/© 2017 Elsevier B.V. All rights reserved.

14
E. Goreshnik / Journal of Fluorine Chemistry 195 (2017) 13–17
in all other structurally characterized metal heptafluoridotanta-
lates the TaF7 polyhedron may be described as a mono-capped
trigonal prism, whereas in hybrid salts with organic cations mono-
2ꢀ
capped trigonal prismatic and pentagonal bi-pyramidal TaF7
geometry appears nearly evenly (Table 1.) The flexibility of seven-
coordination in general [23] and in particular for tantalates is
generally well known. As K.O. Christe [24] has noted, “heptacoor-
dinated structures exhibit fluxionality and are readily deformed
from the idealized 1/3/3, 1/5/1, or 1/4/2 arrangements by the
presence of polyatomic ligands, solid-state and packing effects, or
the influence of counterions”. One may also add, that in metal salts
cation – anion interactions are probably more susceptible to
packing, whereas in the case of hybrid organic-inorganic
compounds the geometry of organic cations influences on
hydrogen bonds direction and, in turn, could be responsible for
2ꢀ
the formatiꢀon of definite shape of TaF7 polyhedron.
Fig. 1. Crystallographically independent part of the structure of (N2H6)3TaF8-
TaF7HF2 salt. Thermal ellipsoids are drawn at 50% probability.
The HF2 anion appears to be a little bit asymmetric, with the
FꢀꢀH distances of 1.08(8)–1.18(8) Å. The Fꢁ ꢁ ꢁF distance of 2.264(5) Å
is practically the same length as those in structures of [Cu(HF2)
(pyz)2]SbF6 (2.286(2) Å), [Cu2F(HF)(HF2)(pyz)4](SbF6)2 (2.281(4) Å)
at 150 K [25], a little bit longer than that in the crystal structure of
[N(CH3)4][HF2] (2.213(4) Å) [26], but shorter than those in
Ba4F4HF2(PF6)3 (2.28(2) Å), Pb2F2(HF2)(PF6) (2.32(4) Å), Ca(HF2)2
(2.283(2) Å) [27] and KHF2 (2.277(6) Å) [28]. Each F atom forms
rather strong hydrogen bonds with two cations.
earlier for a Ba5F(TaF8)(TaF6O)2 [19] compound. The Ta1 atom from
TaF83ꢀ anion possesses a square-antiprismatic surrounding. Such a
geometry agrees well with a Raman spectrum (see below). Eight
TaꢀꢀF bonds lengths vary from 1.943(3) to 2.111(3) Å. Similar
tantalum atom coordination sphere with the TaꢀꢀF distances of
1.981(3) ꢀ 2.439(3) Å (153 K) was observed in recently re-
determined structure of Na3TaF8 salt [22]. In above mentioned
Ba5F(TaF8)(TaF6O)2 corresponding values were 1.941–2.040 Å [19].
Two shortest Ta1–F distances, namely 1.943(3) and 1.951(3) Å,
correspond to terminal fluorine atoms, whereas other six
participate in a formation of rather strong NꢀꢀHꢁ ꢁ ꢁF hydrogen
There are four crystallographically independent hydrazinium
cations in structure discussed. Two of them are built from two pairs
of symmetrically generated (axis 2) atoms. The NꢀꢀN distances
2+
form a narrow range of 1.422(9)–1.441(9) Å. Each of N2H6
moieties demonstrate a unique anionic surrounding despite a
formation of equal (six) amount of NꢀꢀHꢁ ꢁ ꢁF hydrogen bonds. The
N1ꢀꢀN1 molecule is bound to four TaF83ꢀ and two HF2ꢀ units. The
3ꢀ
2+
bonds, connecting TaF8 anion with eight N2H6 cations.
The TaF7 anion adopts a shape of distorted pentagonal bi-
pyramide, with two closer located axial fluorine ligands (Ta2–Fax
1.888(4)–1.894(4) Å) and five slightly removed from the central
atom equatorial F centers (Ta2–Fax 1.930(4)–2.002(3) Å). Four
equatorial fluorine atoms form hydrogen bonds with six cations.
Similar surrounding of the Ta ion was found earlier in CaTaF7 with
the 1.878(14)–2.044(13) Å TaꢀꢀF bonds lengths. One may note that
3ꢀ
2ꢀ
N2ꢀꢀN2 is surrounded by four TaF8 and two TaF7 anions. The
N3ꢀꢀN4 is bonded to three pairs of HF2ꢀ, TaF83ꢀ and TaF72ꢀ anions
respectively. The last one, N5ꢀꢀN6 cation is linked to one HF2ꢀ, two
3ꢀ
2ꢀ
TaF8 and three TaF7 units. Mentioned above hydrogen bonds
connect anions and cations into three-dimensional network.
Hydrogen bonds in this structure appear to be rather strong, with
Table 1
Observed geometry in structurally characterized heptafluoridotantalates.
Compound
K2TaF7
Polyhedron
T, K
TaꢀꢀF distances
1.918–1.975
Ref.
CTP* dist
CTP dist
CTP
CTP
BP dist
CTP
CTP
CTP
BP
BP
CTP dist
CTP dist
CTP dist
BP
CTP dist
CTP
RT
RT
RT
123
200
200
200
200
100
RT
RT
RT
RT
RT
RT
RT
RT
[7,8]
[19]
[9]
[9]
[6]
[6]
[6]
[6]
[10]
[11]
[12]
[12]
[12]
[12]
[13]
[14]
[14]
K2TaF7
Rb2TaF7
ꢁ
KF
1.9643(1)–1.9953(1)
1.940(5)–1.945(2), 2.013(3)
1.933(3)–1.941(4), 1.997(3)
1.878(14)–1.894(12), 1.940(16)–2.044(13)
1.908(16)–2.019(12)
1.916(5)–2.004(5)
1.868(3)–1.982(3)
1.900(1), 1.961(5)–2.005(2)
1.906(2), 1.949(2)–1.996(2)
1.894(7)–1.969(7), 2.019(7)
1.914(7)–1.988(5), 1.990(5)
1.86(2)–1.96(2), 2.00(2)
1.89(2)–1.90(2), 1.93(1)–2.01(1)
1.932(7)–1.966(6)
1.859(2)–1.999(1), 2.024(2)
1.890(3)–1.895(3),
1.914(7)–2.001(9)
Rb2TaF7
CaTaF7
SrTaF7
BaTaF7
PbTaF7
(enH2)TaF7
(pipH2)TaF7
[H4tren](TaF7)2
[H4tren](TaF7)2
[H4tren](TaF7)2
[H4tren](TaF7)2
ꢁ
ꢁ
2H2O
2H2O
[H3tren][TaF7]F
(C2H5N4)2TaF7
(C2H6N4)TaF7ꢁH2O
(N2H6)3(TaF8)(TaF7)HF2
Na7TaF8(TaF7)2
BP dist
BP dist
150
150
1.888(4)–1.894(4),
1.930(4)–2.002(3)
1.926(5)–1.961(4),
This work
This work
CTP dist
2.011(4)–2.018(4)
*
CTP – mono-capped trigonal prism, BP – pentagonal bi-pyramide, dist – distorted geometry, for BP geometry the shortest distances to F atoms at apical positions are typed
first and separated by commas, for CTP distances to the “capped” fluorine atoms are printed at the end and separated by commas (if distinguishable). For [H4tren](TaF7)2 2H2O
ꢁ
and [H4tren](TaF7)2 parameters for each Ta center are shown in separate row.

E. Goreshnik / Journal of Fluorine Chemistry 195 (2017) 13–17
15
Fig. 2. Crystallographically independent part of the structure of Na7TaF8(TaF7)2 salt.
Thermal ellipsoids are drawn at 50% probability. NaꢀꢀF bonds are omitted for
clarity.
Hꢁ ꢁ ꢁF distances of 1.77–2.05 Å and NꢀꢀHꢁ ꢁ ꢁF angles varying from
141.4 to 173.7ꢂ.
The structure of compound II is comprised from two pairs of
2ꢀ
3ꢀ
crystallographically independent TaF7 and TaF8
anions and
3ꢀ
eight independent Na cations (Fig. 2). Both TaF8
anions
demonstrate, similarly to that in compound I, square-antiprismatic
geometry. The Ta1, F11 and F12 atoms from Ta(1)F8 unit occupy
special positions at mirror plane. Similarly, the Ta2, F22 and F24
atoms are located at mirror plane in the case of Ta(2)F8 moiety. The
TaꢀꢀF distances vary from 1.962(7) to 2.021(5) Å similarly to those
2ꢀ
in (N2H6)3TaF8TaF7HF2. Both TaF7 anions possess mono-capped
trigonal prismatic shape with TaꢀꢀF bonds lengths of 1.926(5)–
Fig. 3. Raman spectra of (N2H6)3TaF8TaF7HF2 and Na7TaF8(TaF7)2 compounds.
Raman spectrum of Na3TaF8 has shown for comparison. Marked by asterisks two
peaks at 500 and 710 cmꢀ1 in spectrum of (N2H6)3TaF8TaF7HF2 belong to silicon
grease, used for crystal mounting on a glass capillary for previously performed XRD
experiment.
1.961(4)
Å within trigonal prism and noticeably elongated
distances to capped fluorine center of 2.011(4)–2.018(4) Å. Such
a shape differs from that in (N2H6)3TaF8TaF7HF2 and much more
similar to the coordination polyhedron of tantalum atom in
mentioned above BaTaF7 [6].
Sodium ions in Na7TaF8(TaF7)2 possess unusually high coordi-
nation numbers. The Na1, Na2, Na3 and Na6 centers possess
coordination number 8, whereas coordination environment of
Na4, Na5, Na7 and Na8 ions comprise from 7 fluorine atoms. The
NaꢀꢀF distances are in the range of 2.163(8)–2.979(6) Å.
Moreover, in Ca(HF2)2 we did not find (possibly because of the
weakness) corresponding to HF2 anion Raman mode. One may
ꢀ
conclude, that observed in Raman spectrum of (N2H6)3TaF8TaF7HF2
at 614.7 cmꢀ1 peak belongs to TaF8 unit, and peak from HF2ꢀ anion
is invisible (or appears as a very small shoulder on the lower
frequency side of mentioned peak). The Raman spectrum of
Na7TaF8(TaF7)2 appears, as could be expected, much simpler and
easier for assignment than that for (N2H6)3TaF8TaF7HF2. Peaks at
2.2. Raman spectroscopy
285, 343, and 651 cmꢀ1 basing on similarity with the Raman
Raman spectrum of (N2H6)3TaF8TaF7HF2 appears to be rather
complicated, as one may expect regarding the presence of four
spectroscopically distinguishable moieties (Fig. 3). Strong split
peak at 1060 cmꢀ1 could be attributed to NꢀꢀN vibration [29]. A
pair of strong peaks at 2905 and 2964 cmꢀ1 represent sym and
asym modes of NꢀꢀH vibrations. The peak at 656.6 cmꢀ1 appears
exactly as the same position as it was found in Raman spectrum of
BaTaF7 [6] and, consequently, belongs to TaF72ꢀ anion. Also peak at
,
2ꢀ
spectra of MTaF7 (M = Ca, Sr, Pb, Ba) [6], originate from TaF7
anion. Weak doublet at 382/394 cmꢀ1, observed in the Raman
spectrum of K2TaF7 [8] at 377/390 cmꢀ1, also belongs therefore to
2ꢀ
TaF7 unit. The strongest peak from TaF8 moiety, found in the
Raman spectrum of Na3TaF8 at 616 cmꢀ1, appears in the spectrum
of Na7TaF8(TaF7)2 at 620 cmꢀ1. Basing on rather low intensity of
this band comparatively to those from TaF72ꢀ, one may assume that
weak peaks, visible in the spectrum of Na3TaF8 at 372 and 410 cmꢀ1
respectively, are invisible because too low intensity.
2ꢀ
387.7 could be attributed to TaF7 moiety, correspondingly to
similar position in BaTaF7.
Peak at 614.7 cmꢀ1 could be assigned to the anion HF2ꢀ. In [N
(CH3)4]HF2 the peak at 576 cmꢀ1 [26], in NaHF2 the peak at
630 cmꢀ1 [30], in Sr2(H2F3)(HF2)2(AsF6) the peak at 620 cmꢀ1 [31]
and the peak at 592ꢀcmꢀ1 in Ba4F4(HF2)(PF6)3 [27] were assigned to
the symmetric HF2 anion.
3. Conclusions
3ꢀ
2ꢀ
First time compounds containing both TaF8
and TaF7
discrete anions were obtained and characterized. One may
emphasize, that the only two compounds (namely Na3TaF8 and
Ba5F(TaF8)(TaF6O)2) containing TaF83ꢀ anion were earlier structur-
ally characterized. The appearance of both – pentagonal bi-
pyramidal and mono-capped trigonal prismatic – TaF72ꢀ polyhedra
On the other hand, mentioned peaks observed in Sr2(H2F3)
(HF2)2(AsF6) and Ba4F4(HF2)(PF6)3 were relatively weak, whereas a
peak found at 620 cmꢀ1 in Raman spectrum of Na3TaF8 and
3ꢀ
assigned to TaF8 vibration, was reported as a very strong [32].

16
E. Goreshnik / Journal of Fluorine Chemistry 195 (2017) 13–17
Table 2
Crystallographic data for (N2H6)3TaF8TaF7 (HF2) and Na7TaF8(TaF7)2 compounds.
Empirical formula
Formula weight
Crystal system
Space group
F17H19N6Ta2
788.06
orthorhombic
Pbcm (#60)
150
F22Na7Ta3
1121.78
monoclinic
P21/m (#11)
150
T, K
a, Å
b, Å
c, Å
a
11.4019(4)
11.6967(4)
22.1807(7)
90
7.66154(16)
26.4269(6)
7.83676(19)
90
b
g
V, Å3
90
90
92.757(2)
90
92.757(2)
4
4.701
1976
MoKa
21.086
1.124
0.034
0.0637
2958.09(18)
8
3.539
2880
MoK
14.983
1.280
0.0263
0.0600
Z
Dcalc., g/cm3
F(000)
Radiation
a
m
, mmꢀ1
Goodness-of-fit on F2
Ra
wRb
P
P
a
R ¼ ðjjF0j ꢀ jFcjjÞ= jF0j.
ꢀ
ꢀ
ꢁ
ꢁ
1
h
ꢀ
ꢁ
i
ꢀ
ꢁ
=
P
P
2
2
2
2
2
v
R ¼
v
jF0j ꢀ jFcj
=
vjF0j
; v
¼ 1= s2 F20 þ ð0:0898PÞ þ 0:7472P ; whereP ¼ F02 þ 2F2c =3.
b
confirms well known flexibility of seven-coordinated tantalum
coordination environment. The formation of these new compound
emphasizes once more how complicated are ionic equilibria in the
solutions containing tantalum and hydrofluoric acid.
4.2. Crystallography
Single-crystal data for both compound were collected on a
Gemini A diffractometer equipped with an Atlas CCD detector,
using graphite monochromated MoKaradiation. The data were
4. Experimental part
treated using the CrysAlisPro software suite program package [34].
Analytical absorption correction has been applied to all data sets.
Structures were solved with charge-flipping method using the
Superflip program (Olex crystallographic software [35]). The
electron density map, obtained by Superflip software, was
analysed by EDMA program [36], which gave initial models of
structures. Structure refinement has been performed with the
SHELXL-2014 [37] software, implemented in the program package
WinGX [38] (Table 2). Figures were prepared using Diamond 3.2
software [39]. Further details of the crystal structure investigations
may be obtained from the Fachinformationszentrum Karlsruhe,
76344 Eggenstein-Leopoldshafen, Germany (Fax: +49 7247 808
666; E-Mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.
de/request for deposited data.html) on quoting the depository
numbers CSD-432011 ((N2H6)3TaF8TaF7HF2) and -432010
(Na7TaF8(TaF7)2).
4.1. Synthesis
4.1.1. Starting materials
HF (Riedek-de Haen AG, 66%), N2H6F2 (Alfa Aesar, 99%), NaF
(Merck, 99%) were used as supplied. TaF5 was prepared by
fluorination of tantalum powder under high-pressure as previous-
ly described [33].
4.1.2. (N2H6)3TaF8TaF7HF2
A mixture of hydrazinium hydrofluoride and TaF5 (molar ratio
1:1) was dissolved in 66% hydrofluoric acid water solution. After
short heating in a water base and slow cooling crystals of
(N2H6)3TaF8TaF7HF2 compound were found. Crystals of unreacted
(N2H6)F2 and TaF5 (identified by single crystal X-ray diffraction)
were also observed.
4.3. Raman spectroscopy
4.1.3. Na7TaF8(TaF7)2
First this compound was obtained occasionally during an
attempt to grow crystals of Na3TaF8. Unattended heating of the NaF
and TaF5 mixture (3 to 1 molar ratio) dissolved in 5% water solution
of hydrofluoric acid has led to full solvent evaporation and
appearance of Na7TaF8(TaF7)2 crystals. The rest of the poly-
crystallinic substance was not analysed. For intentional prepara-
tion a mixture of NaF and TaF5 in a 7:3 molar ratio has been
recrystallized from 35% water solution of HF (such a concentration
has been chosen as a concentration of azeotropic HFꢀꢀH2O
mixture). Crystals of desired compound, contaminated by crystals
of NaHF2 by-product (identified by single crystal X-ray diffraction),
were grown. Additional crystallizations of the same NaFꢀꢀTaF5
mixture from 5% and from 66% water solution of HF were
performed. Plate-like crystals of Na3TaF8 in the first case and cubic
crystals of NaTaF6 in the last case were found.
A Raman spectrum was measured with a Horiba Jobin–Yvon
LabRAM HR spectrometer using the 632.81-nm excitation line of a
He–Ne laser with a power of 17 mW. To avoid decomposition of the
sample, a density filter was applied to reduce the power of the laser
to 1.7 mW. An Olympus ꢃ50 long-distance lens was used. The
spectra were obtained by accumulating 50 scans with an
integration time of 5 s directly in the air at room temperature.
Prior to recording, the spectrometer was calibrated using a Si
polycrystalline plate as a standard with a characteristic band at
520.6 cmꢀ1
.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
jfluchem.2017.01.006.

E. Goreshnik / Journal of Fluorine Chemistry 195 (2017) 13–17
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