Poly[perfluorotitanate(IV)] salts of [H3O]+, Cs +1, [Me4N]+, and [Ph4P]+ and about the existence of an isolated [Ti2F9]- anion in the solid state

Article abstract of DOI:10.1021/ic9009338

The increase In the size of monocatlons (A⁺) does not favor the formation of [Ti₂F₉]^- against [Ti ₄F₁₈]^2- salts (with isolated [Ti ₂F₉]^- or [Ti

Full text of DOI:10.1021/ic9009338

6918 Inorg. Chem. 2009, 48, 6918–6923
DOI: 10.1021/ic9009338
Poly[perfluorotitanate(IV)] Salts of [H3O]þ, Csþ, [Me4N]þ, and [Ph4P]þ and about
the Existence of an Isolated [Ti2F9]- Anion in the Solid State
Zoran Mazej* and Evgeny Goreshnik
Department of Inorganic Chemistry and Technology, Jozꢀef Stefan Institute, Jamova 39, SI-1000 Ljubljana,
Slovenia
Received May 13, 2009
The increase in the size of monocations (Aþ) does not favor the formation of [Ti2F9]- against [Ti4F18]2- salts (with
isolated [Ti2F9]- or [Ti4F18]2- anions, respectively) as previously proposed (Passmore, J.; et al. Angew. Chem., Int. Ed.
2005, 44, 7958-7961). The crystal structure determination of [Me4N]þ and [Ph4P]þ salts showed that both compounds
are [Ti4F18]2- salts; i.e., [Me4N]2[Ti4F18] and [Ph4P]2[Ti4F18] were obtained instead of [Me4N][Ti2F9] and [Ph4P][Ti2F9].
The product of the reaction of CsF with 2TiF4 could be formulated as CsTi2F9; however, instead of isolated [Ti2F9]-
anions, infinite ([Ti2F9]-)n double chains are present. In the case of H3OTi2F9, a similar result was obtained. On the basis
of the similarities of vibrational spectra of CsTi2F9 and NF4Ti2F9, it is also unlikely that the latter consists of isolated
[Ti2F9]- ions.
Introduction
titanium(IV) salts were grown. Results of their crystal struc-
ture determination are given in the present paper.
The reaction between TiF4 and an excess of [15]crown-5 in
CH3CN yielded [TiF2([15]crown-5)][Ti4F18] 0.5CH3CN.1 Its
3
crystal structure consists of discrete [TiF2([15]crown-5)]2þ
cations and [Ti4F18]2- anions. The estimation of how thesizes
of mono- and dications promote the formation of [Ti2F9]-
versus [Ti4F18]2- salts, and inversely, was also given in the
same paper.1 On the basis of the estimates of the correspond-
ing energetics, applying the “volume-based” thermodynamic
(VBT) approach, it was proposed that all monocations (Aþ),
with the exception of smaller ones [V(Aþ)<0.019 nm3], favor
the formation of [Ti2F9]- against [Ti4F18]2- salts. It was
suggested that NF4Ti2F9 [V([NF4]þ)=0.06 nm3]2 probably
contains isolated [Ti2F9]- anions;3 however, for a slightly
smaller Csþ ion [V(Csþ)=0.01882 nm3],2 it is still an open
question of which salt is formed.1 To verify those predictions (i.e.,
would the reaction between AF and 2TiF4 yield [Ti2F9]- or
[Ti4F18]2- salt), selected single-charged cations (Aþ) of vari-
ous sizes were chosen and single crystals of their corresponding
Experimental Section
Caution! Anhydrous HF and some fluorides are highly toxic
andmustbehandledusingappropriateapparatusandprotective
gear.
Apparatus and Reagents. Volatile materials (anhydrous HF
and F2) were handled in an all-Teflon vacuum line equipped
with Teflon valves. The manipulation of the nonvolatile materi-
als was done in a drybox (M. Braun). The residual water in the
atmosphere within the drybox never exceeded 2 ppm. The
reactions were carried out in tetrafluoroethylene-hexafluo-
ropropylene (FEP; Polytetra GmbH, Monchengladbach,
::
Germany) reaction vessels (length, 250-300 mm; i.d., 15.5 mm;
o.d., 18.75 mm) equipped with Teflon valves and Teflon-coated
stirring bars. Prior to use, all reaction vessels were passivated with
elemental fluorine. Fluorine (Solvay Fluor und Derivate GmbH,
Hannover, Germany), CsF (Aldrich, 99.9%), Me4NF (Aldrich,
97%), and Ph4PCl (Alfa Aesar, 98%) were used as supplied.
Anhydrous HF (Fluka, Purum) was treated with K2NiF6 (Ozark
Mahoning) for several hours prior to use. The compound TiF4
was synthesized by the reaction between TiCl3 (Aldrich, 99.999%)
and elemental fluorine in aHF. The powder X-ray diffraction
pattern and Raman spectrum of isolated TiF4 were in agreement
with the literature data.4,5
*To whom correspondence should be addressed. E-mail: zoran.
mazej@ijs.si.
(1) Decken, A.; Jenkins, H. D. B.; Knapp, C.; Nikiforov, G. B.; Passmore,
J.; Rautiainen, J. M. Angew. Chem., Int. Ed. 2005, 44, 7958–7961.
(2) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg.
Chem. 1998, 38, 3609–3620.
(3) Here we have in mind the geometry where two Ti atoms are connected
via three bridging F atoms and coordination of each Ti is completed by three
terminal F atoms. In that way, the CN of each Ti in the [Ti2F9]- anion is
equal to 6. Another possibility is the case where two Ti atoms are connected
via one bridging F atom and coordination of each Ti is completed by four
terminal F atoms. In that way, CN of each Ti is equal to 5. CN=5 is clearly
energetically less favorable than CN=6.
Raman Spectroscopy. Raman spectra with a resolution of
2 cm-1 were recorded (10-20 scans) on a Renishaw Raman
Imaging Microscope System 1000, using the 632.8 nm excitation
line of a He-Ne laser.
(4) Christe, K. O.; Schack, C. J. Inorg. Chem. 1977, 16, 353–359.
::
::
(5) Bialowons, H.; Muller, M.; Muller, B. G. Z. Anorg. Allg. Chem. 1995,
621, 1227–1231.
r
pubs.acs.org/IC
Published on Web 06/22/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6919
Table 1. Crystal Data and Structure Refinement for H3OTi2F9, CsTi2F9, [Me4N]2[Ti4F18], and [Ph4P]2[Ti4F18
]
chemical formula
space group
a (pm)
b (pm)
c (pm)
R (deg)
β (deg)
H3OTi2F9
Pnma
898.8(4)
545.1(2)
1474.8(6)
90
90
90
0.7226(5)
4
285.76
2.627
100
2.335
0.0467
0.1081
1.257
CsTi2F9
C2/c
1136.3(3)
1471.1(3)
533.18(14)
90
116.41(2)
90
0.7982(4)
4
[Me4N]2[Ti4F18
Pnma
1327.76(10)
1049.35(6)
1744.84(13)
90
90
90
2.4311(3)
4
]
[Ph4P]2[Ti4F18
P1
]
1011.720(10)
1300.11(3)
2091.32(12)
83.880(9)
80.335(8)
69.988(6)
2.54444(19)
2
1212.12
1.582
200
0.767
0.0753
γ (deg)
V (nm3)
Z
fw (g mol-1
)
399.71
3.325
200
681.77
1.863
200
Dcalcd (g cm-3
T (K)
)
μ (mm-1
)
6.596
1.401
R1a
0.0617
0.1391
1.149
0.0607
0.1455
1.167
wR2 [I > 2.00 σ(I)]b
GOF
0.1871
1.094
P
P
P
P
P
a R1= ||Fo| - |Fc||/ |Fo|. b wR2=[ (w(Fo2-Fc2)2/ (w(Fo2)2]1/2, GOF=[ w(Fo2 - Fc2)2)/No-Np)]1/2, where No=no. of reflections and Np=no. of
refined parameters.
Powder X-ray Diffraction Patterns. Powder X-ray diffraction
patterns were obtained using the Debye-Scherrer technique
with Ni-filtered Cu KR radiation. Samples were loaded into
quartz capillaries (0.3 mm) in a drybox. Intensities were esti-
mated visually.
program (program package TeXsan) and refined with
SHELXL-977 software, implemented in the program package
WinGX.8 The figures were prepared using DIAMOND 3.19 and
Balls & Sticks, freely available software.10 The crystal data and
details of the structure refinement are given in Table 1.
Crystal Growth of A2[Ti4F18] (A=[Me4N]þ and [Ph4P]þ) and
ATi2F9 (A=[H3O]þ and Csþ). In a general procedure, single
crystal growth was carried out in a double T-shaped apparatus
consisting of two FEP tubes (19 and 6 mm. o.d.). Mixtures of
AF/2TiF4 (AF = 0.25 mmol of CsF and 0.28 mmol of MeN4F)
and Ph4PCl/2TiF4 (0.17 mmol of Ph4PCl) were loaded into the
wider arm of the crystallization vessel in a drybox. aHF (∼4 mL
for MeN4 and Ph4þ and 10 mL for Csþ salt) was then condensed
onto the starting material at 77 K. The crystallization mixtures
were brought up to ambient temperature, and the clear solutions
that developed were decanted into the narrower arm. The
evaporation of the solvent from these solutions was carried
out by first maintaining a temperature gradient correspond-
ing to about 10 K between both tubes for 2 ([Ph4P]þ salt), 4
([Me4N]þ salt), or 12 (Csþ salt) weeks, respectively. In the case of
[Me4N]þ salt, the temperature gradient had been later increased
to about 10 K for an additional 1 week. The effect of this
treatment was to enable aHF to be slowly evaporated from a
narrower tube into a wider tube, leaving the colorless crystals of
A2[Ti4F18] (A=[Me4N]þ and [Ph4P]þ) and CsTi2F9. Single crys-
tals of H3OTi2F9 were obtained unexpectedly as one of the
phases obtained after crystallization of the products obtained by
the reaction between TiO and AsF5 in aHF. A detailed char-
acterization of the products obtained in the TiO/AsF5/aHF
system is beyond the scope of this study and will be published
elsewhere. Selected single crystals of A2[Ti4F18] (A=[Me4N]þ
and [Ph4P]þ) and ATi2F9 (A=Csþ and H3Oþ) were placed inside
0.3 mm quartz capillaries in a drybox and their Raman spectra
recorded.
Results and Discussion
Previously, the estimations of the ΔH298(s) (≈ΔG298
-
(s)) values of the reaction 2[cation]þ[Ti2F9]-(s) f [ca-
tion]2þ[Ti4F18]2-(s) (eq 1) by using the equation of Jenkins
et al. and applying the VBT approach were reported.1 Accord-
ing to the results, the monocations with V(Aþ)>0.019 nm3
(i.e., [NF4]þ, [Me4N]þ, [Ph4As]þ, etc.) favor the formation of
[cation][Ti2F9] with isolated [Ti2F9]- anions [because ΔH(s)
(≈ΔG(s)) > 0]. The Csþ cation with V(Csþ)=0.01882 nm3 2
and ΔG(eq 1)=-3 ((10) kJ mol-1 represents the borderline
case where it was in question as to which anion is present. For
those, we decided to prepare single crystals of poly[perfluoro-
titanate(IV)] salts of Csþ (V(Csþ)=0.01882 nm3) as well as of
larger cations such as [Me4N]þ (V([Me4N]þ) = 0.113 nm3)2
and [Ph4P]þ (V[Ph4P]þ ∼ V[Ph4As]þ=0.456 nm3).1 Acciden-
tally, single crystals of [H3O]þ poly[perfluorotitanate(IV)] salt
were also obtained (V([H3O]þ) ∼ V(K)þ = 0.00986 nm3).2
On the basis of previous estimations,1 the following salts,
with isolated [Ti2F9]- or [Ti4F18]2- anions, were expected:
[H3O]2[Ti4F18], [Me4N][Ti2F9], [Ph4P][Ti2F9], and Cs2[Ti4F18]
or CsTi2F9. At first, larger cations such as [Me4N]þ and
[Ph4P]þ were inspected. Single crystals of [Me4N]þ salt were
grown from an aHF solution of Me4NF and TiF4 in a 1:2
molar ratio. Because Ph4PF has not been commercially
available, the corresponding chloride was used instead. In
the presence of aHF, it was converted to fluoride salt, which
further reacted with TiF4. The crystal structure determination
of grown crystals was not in accordance with the theoretically
expected ones.1 Isolated products were [Ti4F18]2- and not
[Ti2F9]- salts (Figures 1 and 2). Important bond lengths and
bond angles for [Ti4F18]2- units are listed in Table 2.
Crystal Structure Determination. Products of crystallization
were immersed in a perfluorinated oil (ABCR, FO5960, melting
point 263 K) in a drybox. Single crystals were then selected from
the crystallization products under the microscope (at tempera-
tures between 265 and 273 K) outside the drybox and then
transferred into the cold nitrogen stream of the Oxford Instru-
ments cooling system, installed on Rigaku AFC7 diffractometer
(Mercury CCD area detector, with graphite-monochromated
Mo KR radiation). Data were collected at 100 or 200 K. The
data were corrected for Lorentz and polarization effects. A
multiscan absorption correction was applied to all data sets.
Both structures were solved by direct methods using the SIR-926
In all three compounds, the corresponding Ti-F bond
lengths are in the same range and, as expected, Ti-Ft bond
(7) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(8) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
(9) DIAMOND v3.1; Crystal Impact GbR: Bonn, Germany, 2004-2005
(10) Ozawa, T. C.; Kang, S. J. J. Appl. Crystallogr. 2004, 37, 679; Balls &
Sticks: Easy-to-Use Structure Visualization and Animation Creating Program
(http://www.softbug.com/toycrate/bs/).
(6) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343–350.

6920 Inorganic Chemistry, Vol. 48, No. 14, 2009
Mazej and Goreshnik
Table 2. Selected Bond Lengths (pm), Nonbonding Contacts (pm), and Bond
2-
Angles (deg) in [Ti4F18
]
Salts of [TiF2([15]crown-5)]2þ, [Me4N]þ, and [Ph4P]þ
[TiF2([15]-
crown-5)]2þ a
[Me4N]þ b)
[Ph4P]þ b
Ti-Ftc
Ti-Fb
175.4(3)-177.7(3) 175.8(4)-176.8(4) 175.2(4)-178.3(4)
196.5(3)-201.0(3) 197.5(4)-200.3(4) 195.9(4)-201.9(4)
Ti Ti
Ft Ft
Fb Fb
382.9-387.5
260.9-267.1
259.6-264.1
384.0-385.9
262.1-263.7
261.5-263.2
383.3-386.5
264.2-272.0
260.8-264.4
3 3 3
3 3 3
3 3 3
Ti Ti
3 3 3 3 3 3
∼60
∼60
∼60
Ti
Ft-Ti-Ft 94.9(1)-98.0(1)
Fb-Ti-Fb 81.9(1)-83.4(1)
95.9(3)-97.4(2)
81.6(2)-83.4(2)
97.0(2)-98.9(2)
82.1(2)-83.2(2)
a 198 K, ref 1. b 200 K, this work. c Ft=terminal F atom (Ti-Ft); Fb=
bridging F atom (Ti-Fb-Ti).
Figure 1. Part of the crystal structure of [Me4N]2[Ti4F18].
Figure 3. Infinite ([Ti2F9]-)n double chains in the crystal structure of
CsTi2F9.
Figure 2. Part of the crystal structure of [Ph4P]2[Ti4F18].
distances [175.2(4)-178.3(4) pm] are shorter than Ti-Fb
ones [195.9(4)-201.9(4) pm]. Both sets of distances are in
agreement with previously observed ones in various titanium-
(IV) fluoride complexes.11,12 In TiF4, the corresponding
distances are slightly shorter. The Ti-Ft bond distances are
found in the 170.4-172.5 pm range and Ti-Fb in the 193.1
-197.8 pm range.5 The basic unit of the crystal structure of
TiF4 consists of three [TiF6] octahedra sharing joint apexes.
So-formed [Ti3F15] rings are further connected via trans-
bridging F atoms into infinite columns.5
Figure 4. Packing of infinite ([Ti2F9]-)n double chains in the crystal
structure of CsTi2F9.
and Fb-Ti-Fb angles, where the latter is smaller than the
former. As was already mentioned in ref 1, this is expected
from VSPER13 and a ligand-ligand repulsion model.14,15
From the similarity of the corresponding bonding and
corresponding nonbonding distances of [Ti4F18]2- anions
in all three salts (Table 2), it is obvious that the geometry
of the [Ti4F18]2- anion is not afected by the size of the cation.
The Csþ cation (V(Csþ)=0.01882 nm3)2 is much smaller
than [Me4N]þ (V([Me4N]þ) =0.113 nm3)2 and [Ph4P]þ
The corresponding Ti Ti, Ft Ft, and Fb Fb non-
3 3 3
3 3 3
3 3 3
bonding distances in [Ti4F18]2- of all three salts are also very
similar (Table 2). It is valid also for corresponding Ft-Ti-Ft
(11) Roesky, H. W.; Haiduc, I. J. Chem. Soc., Dalton Trans. 1999, 2249–
2264.
(12) Murphy, E. F.; Murugavel, R.; Roesky, H. W. Chem. Rev. 1997, 97,
3425–3468.
(13) Gillespie, R. J.; Hargittai, I. The VSPER Model of Molecular
Geometry; Allyn and Bacon: Boston, 1991.
(14) Robinson, E. A.; Gillespie, R. J. Inorg. Chem. 2003, 42, 3865–38752.
(15) Gillespie, R. J.; Robinson, E. A. Chem. Soc. Rev. 2005, 34, 396–407.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6921
Table 3. Selected Bond Lengths (pm) and Bond Angles (deg) in H3OTiF5, H3OTi2F9, CsTi2F9, and TiF4
[H3O][TiF5]a
[H3O][Ti2F9]b
CsTi2F9
TiF4
c
a
Ti-Ftd
Ti-Fb
176.4(4)-183.5(5)
196.1(3)-202.1(3)
175.4(2)-180.3(3)
196.6(3)-200.4(2)
175.8(7)-179.0(7)
195.5(3)-198.8(7)
170.4-172.5
193.1-197.8
Ti-Fb-Ti
146.6
143.7(2)/166.4(2) e
149.3(6)/156.3(4) e
159.2/169.9 f
a References 5 and 16. b 100 K. c 200 K. d Ft=terminal F atom (Ti-Ft); Fb=bridging F atom (Ti-Fb-Ti). e See Figure 5. f Inside and between [Ti3F15
rings.
]
Figure 7. Raman spectra of [H3O][Ti2F9] and CsTi2F9.
Figure 5. Ti-Fb-Ti angles in the crystal structure of [H3O]Ti2F9 [(a)
143.7(2)°; (b) 166.4(2)°] and CsTi2F9 [(a) 149.3(6)°; (b) 156.3(4)°].
Table 4. Raman Spectra of [H3O][Ti2F9] and CsTi2F9 together with the Pre-
viously Reported Data for CsTi2F9 and NF4Ti2F9
a
a
b
b
H3OTi2F9
756(10)
CsTi2F9
CsTi2F9
752(10)
NF4Ti2F9
751(10)
752(10)
721(0.7)
702(0.7)
667(0.5)
635(0.5)
397(1.5)
701(0.1)
670(0.4)
645(þ0)
702(0.7)
670(0.5)
645(þ0)
397(1)
293(1)
388(0.9)
327(0.3)
290(0.8)
247(2.4)
238(2.4)
225(sh)
389(0.9)
326(0.3)
290(0.9)
247(2.4)
237(2.4)
225(sh)
294(0.9)
253(2)
241(1.9)
229(1.6)
211(0.7)
245(1.2)
229(1)
Figure 6. Formation of infinite ([Ti2F9]-)n double chains in the crystal
198(sh)
192(0.5)
162(0.4)
192(0.5)
162(0.4)
structures of CsTi2F9 (a), [H3O]Ti2F9 (b), and O2Mn2F9 (c).18
170(0.5)
a This work. b Reference 4.
(V[Ph4P]þ∼V[Ph4As]þ=0.456 nm3).1 According to the ther-
modynamic calculations, it was not clear if it would yield salt
with isolated [Ti4F18]2- or [Ti2F9]- anions.1 The crystal struc-
ture determination of the product between CsF and 2TiF4
showed that obtained product could be formulated as
CsTi2F9; however, instead of isolated [Ti2F9]- anions, infinite
([Ti2F9]-)n double chains are present (Figures 3 and 4). Only
vibrational spectra of CsTi2F9 have been previously re-
ported.4 In the case of [H3O]þ, which is even smaller than
the Csþ cation, a similar result was obtained; i.e., H3OTi2F9
has been formed. Its crystal structure also consists of infinite
([Ti2F9]-)n double chains. Previously, only H3OTiF5 has
been known.16 The attempts to prepare H3OTi2F9 by the
same method as that of H3OTiF5 were unsuccessful.16
Important bond lengths and bond angles of CsTi2F9
and H3OTi2F9 are listed in Table 3. For comparison, some
bond lengths and bond angles of H3OTiF516 and TiF45 have
been added.
In all four compounds, the Ti4þ ions are found in an
octahedral coordination of six F atoms. The [TiF6] units are
sharing joint apexes, forming in that way ([TiF5]-)n single
chains, i.e., [H3O][TiF5], or ([Ti2F9]-)n double chains, i.e.,
[H3O][Ti2F9] and CsTi2F9, of cis-connected octahedra.
Some of the Ti-Ft bond distances are slightly elongated
because of hydrogen bonding. The shortest F (H)
O
3 3 3
3 3 3
distances in H3OTiF5 and H3OTi2F9 are all in the limits of
250-320 pm. Hydrogen bonds falling in this range are
classified as moderately strong hydrogen bonds, which could
be described as mostly electrostatic.17
The Ti-Fb-Ti angles within the individual single zigzag
chains are kinked with an angle equal to 166.4(2)° for [H3O]þ
salt and 156.3(4)° for Cs salt (Figure 6). The Ti-Fb-Ti
angles, where Ti atoms belong to two neighboring chains, are
143.7(2)° for [H3O]þ salt and 149.3(6)° for Cs salt (Figure 5).
(16) Cohen, S.; Selig, H. J. Fluorine Chem. 1982, 20, 349–356.
(17) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48–76.

6922 Inorganic Chemistry, Vol. 48, No. 14, 2009
Mazej and Goreshnik
Table 5. Raman Spectra of [Me4N]2[Ti4F18] and [Ph4P]2[Ti4F18] together with the
Calculated Modes for the [Ti4F18]
2- Anion
a
a
2- b
[Me4N]2[Ti4F18
]
[Ph4P]2[Ti4F18
]
calcd [Ti4F18]
745(10)
720(0.7)
697(0.5)
681(0.3)
372(2)
319(0.6)
280(1.2)
219(sh)
204(3)
747(10)
738(10)
712(0.7)
688(0.3)
c
*
695(0.5)
*
368(1)
318(0.5)
364(0.9)
320(0.1)
279(0.2)
215(0.1)
197(0.5)
162(0.1)
*
*
169(0.1)
2-
a This work. b Reference 1. c The asterisk (*) denotes [Ti4F18
vibrational bands that overlap with [Ph4P]þ.
]
Figure 8. Raman spectra of [Me4N]2[Ti4F18] and [Me4N]F. The asterisk
(*) denotes bands that could be assigned to [Me4N]þ.
Table 6. List of Symmetric In-Phase Terminal Ti-F Stretching Modes for
Various Complex Ti Anions
cation
anion
ν(Ti-F)
ref
Csþ
[TiF6]2-
[TiF6]2-
599
601
703
724
745
747
751/752
756
770
4
4
[NF4]þ
Csþ
2-
[Ti2F10
]
4
16
this work
this work
4 and this work
this work
4
4
4
[H3O]þ
[Me4N]þ
[Ph4P]þ
Csþ, [NF4]þ
[H3O]þ
[NF4]þ
[NF4]þ
TiF4
([TiF5]-)n
2-
[Ti4F18
]
]
2-
[Ti4F18
([Ti2F9]-)n
([Ti2F9]-)n
-
[Ti3F13
[Ti6F25
[TiF4]n
]
-
]
784
807
Although some of the vibrational bands of [Ti4F18]2-
anions overlapped with those of the cations, the [Ti4-
F18]2- anion modes could be readily assigned with the
help of Raman spectra of starting [Me4N]F or Ph4PCl,
respectively. Experimental data are in good agreement
with calculated ones for the [Ti4F18]2- anion.
Because of their polymeric ([H3O][Ti2F9] and CsTi2F9)
or oligomeric structures ([Me4N]2[Ti4F18] and [Ph4-
P]2[Ti4F18]), all of the compounds reported in this work
give very complex spectra. As reported, the most useful
Raman band for identification of complex Ti anions is
symmetric in-phase terminal Ti-F stretching mode, where
the frequency of the band increases with an increase in the
content of TiF4 and a decrease in the charge of the anion.4
The Raman data of [H3O][Ti2F9], [Me4N]2[Ti4F18], and
[Ph4P]2[Ti4F18] fit well in that correlation (Table 6).
Figure 9. Raman spectra of [Ph4P]2[Ti4F18] and [Ph4P]Cl.
In the case of Csþ salt, the [TiF6] octahedra of the one
individual zigzag chain are slightly rotated in view of the
second chain, resulting in the lower symmetry of Csþ
(Figure 6a) in comparison to [H3O]þ salt (Figure 6b). In the
literature, only one additional example of the compound with
the general formula AM2F9 (A=Aþ and M=M4þ) could be
found. This is the unique dioxygenyl compound O2Mn2F9.18
In the latter, the linking of octahedra does not result in infinite
straight chains but in crenellated strings (Figure 6c).
Vibrational Spectroscopy. Raman spectra of ([Ti2F9]-)n
salts are shown in Figure 7, with additional details given in
Table 4. The ([Ti2F9]-)n compounds have complex Raman
spectra. The anion consists of infinite double chains
([Ti2F9]-)n of [TiF6] octahedra that share cis vertices. In the
crystal structure of CsTi2F9, there is additional tilting of
[TiF6] octahedra, resulting in lower symmetry and in larger
numbers of vibrational bands than those in [H3O][Ti2F9]. The
Raman spectrum of CsTi2F9 reported in this work is in
agreement with previously reported data.4 The similarity of
Raman data of Csþ with [NF4]þ salt (Table 4) and their com-
parison with calculated vibrational data for an isolated
[Ti2F9]- anion1 clearly indicate that it is unlikely that NF4Ti2F9
consists of isolated [Ti2F9]- anions as previously proposed.1
Attempts to record the Raman spectrum of [TiF2([15]-
Conclusions
Unlike the expectation on the basis of the VBT approach,1
it seems that cations larger than Csþ [V(Csþ) = 0.01882 nm3]
always favor the formation of [Ti4F18]2- salts, where the
geometry of the anion is not affected by the different size of
the cation. In the case of Csþ and smaller cations, chain-type
([Ti2F9]-)n salts were obtained. The assumption that only the
size of the cation influenced the formation of [Ti2F9]- or
[Ti4F18]2- salts (containing isolated anions) was too simplied,
not considering possible aggregation of [Ti2F9] units into
polymeric ([Ti2F9]-)n chains. The presence of isolated
[Ti2F9]- anions has been confirmed so far only by 19F
NMR spectroscopy in solutions of di-n-propylammonium
hexafluorotitanate(IV) in liquid SO2.19 In aHF as the solvent,
the rapid exchange between the solvent and the anion
crown-5)][Ti4F18] 0.5CH3CN were not successful.1 Only
3
the IR spectrum has been reported.1 The Raman spectra
of [Me4N]2[Ti4F18] and [Ph4P]2[Ti4F18] are shown in
Figures8and9. TheobservedfrequenciesarelistedinTable4.
::
(18) Muller, B. G. J. Fluorine Chem. 1981, 17, 409–421.
(19) Dean, P. A. W. Can. J. Chem. 1973, 51, 4024–4030.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6923
prevents the observation of well-resolved spectra. In liquid
SO2, besides [Ti2F9]-, also [Ti2F10]2- and [Ti3F13]3- were
observed. On the basis of the composition of isolated pro-
ducts from AF/2TiF4/HF (A=[H3O]þ and Csþ) mixtures, we
can assume that the main species in aHF solutions are
isolated [Ti2F9]- anions. When the solvent is removed, it is
energetically more preferable that isolated [Ti2F9]- anions
polymerize into ([Ti2F9]-)n chains than staying isolated. In
the case of larger cations, the formation of dimeric [Ti4F18]2-
anions is more preferable because it allows better packing
with larger cations than if the interstices in the lattice
of the infinite ([Ti2F9]-)n chains are filled by the large
cations.
The search for the isolated triply fluorine-bridged [Ti2F9]-
anion in the solid state remains open. The closest approxima-
tion could be found in [(C5Me5)2Ti2F7] units found in various
titanium(IV) cyclopentadienyl compounds,20 where two ad-
jacent Ti atoms are connected via three bridging F atoms.
The coordination of each Ti atom is completed by two
terminal F atoms and one [C5Me4] group.
ꢀ
ꢀ
(20) Pevec, A.; Demsar, A.; Gramlich, V.; Petricek, S.; Roesky, H. W.
J. Chem. Soc., Dalton Trans. 1997, 2215–2216.