Anion packing, hole filling, and HF solvation in A2(HF) nB12F12 and K2(HF)TiF6 (A = K, Cs)

Article abstract of DOI:10.1016/j.jfluchem.2012.10.009

The crystal structures of three new HF solvates of fluoroanion salts of alkali metal ions are reported, K₂(HF)TiF₆, K ₂(HF)₃B₁₂F₁₂, and Cs ₂(HF)B₁₂F₁₂. The anion packing in K ₂(HF)TiF₆ (P2₁/m) is distorted cubic close-packed with Ti?Ti distances that range from 5.717(1) to 7.394(1) (average 6.18). Half of the K⁺ ions are in T_d holes and half are in O_h holes (i.e., this is a distorted version of the Cs₂S structure). Each HF molecule is bonded to a K⁺ ion in the O_h holes (KF(H) = 2.679(5)) and also weakly interacts with two other K⁺ ions in adjacent O_h holes (K?F(H) = 3.238(2)). The anion packing in K₂(HF)₃B ₁₂F₁₂ (Fm3?m) is simple cubic. The (B₁₂ centroid)?(B₁₂ centroid) distance (??? distance) is 7.242, and disordered K₂(μ-HF)₃ ²⁺ cations occupy each cube. The anion packing in Cs ₂(HF)B₁₂F₁₂ (P2₁/c) is distorted hexagonal close-packed with ??? distances that range from 7.217 to 9.408 (average 8.304). The HF molecule bridges Cs⁺ ions in adjacent O_h holes, forming infinite Cs⁺(μ-HF)Cs ⁺(μ-HF) chains. The other half of the Cs⁺ ions are in T_d holes, displaced nearly 1 from the center of those holes. This structure is similar to the distorted Ni₂In structure exhibited by Cs₂(H₂O)B₁₂F₁₂. The new results are used to compare and contrast the strength of M-F(H) interactions with M-F interactions involving F atoms from fluoroanions as well as the solid-state packing of icosahedral B₁₂F₁₂^2- anions and octahedral MF₆^2- anions in alkali-metal salts, both with and without the inclusion of weakly-basic HF solvent molecules.

Full text of DOI:10.1016/j.jfluchem.2012.10.009

Journal of Fluorine Chemistry 145 (2013) 118–127
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Anion packing, hole filling, and HF solvation in A2(HF)nB12F12 and
K2(HF)TiF6 (A = K, Cs)
Dmitry V. Peryshkov a, Roland Friedemann b, Evgeny Goreshnik c, Zoran Mazej c,
,
*
a,***
b,**
Konrad Seppelt
, Steven H. Strauss
a Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
b Institute of Inorganic and Analytical Chemistry, Free University Berlin, D-14195 Berlin, Germany
c
ˇ
Jozef Stefan Institute, 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 crystal structures of three new HF solvates of fluoroanion salts of alkali metal ions are reported,
Received 4 October 2012
K2(HF)TiF6, K2(HF)3B12F12, and Cs2(HF)B12F12. The anion packing in K2(HF)TiF6 (P21/m) is distorted cubic
+
Received in revised form 11 October 2012
Accepted 17 October 2012
Available online 29 October 2012
˚
˚
close-packed with Tiꢂ ꢂ ꢂTi distances that range from 5.717(1) to 7.394(1) A (average 6.18 A). Half of the K
ions are in Td holes and half are in Oh holes (i.e., this is a distorted version of the Cs2S structure). Each HF
molecule is bonded to a K+ ion in the Oh holes (K–F(H) = 2.679(5) A) and also weakly interacts with two
˚
other K+ ions in adjacent Oh holes (Kꢂ ꢂ ꢂF(H) = 3.238(2) A). The anion packing in K2(HF)3B12F12 (Fm3m) is
˚
¯
Keywords:
˚
simple cubic. The (B12 centroid)ꢂ ꢂ ꢂ(B12 centroid) distance (ꢁꢂ ꢂ ꢂꢁ distance) is 7.242 A, and disordered
Fluoroanion
K2(m
-HF)32+ cations occupy each cube. The anion packing in Cs2(HF)B12F12 (P21/c) is distorted hexagonal
Hydrogen fluoride complex
Potassium
˚
˚
close-packed with ꢁꢂ ꢂ ꢂꢁ distances that range from 7.217 to 9.408 A (average 8.304 A). The HF molecule
bridges Cs+ ions in adjacent Oh holes, forming infinite Cs+–(
m
-HF)–Cs+–(
m
-HF)– chains. The other half of
Polyhedral borane
+
˚
the Cs ions are in Td holes, displaced nearly 1 A from the center of those holes. This structure is similar to
the distorted Ni2In structure exhibited by Cs2(H2O)B12F12. The new results are used to compare and
contrast the strength of M–F(H) interactions with M–F interactions involving F atoms from fluoroanions
as well as the solid-state packing of icosahedral B12F122ꢀ anions and octahedral MF62ꢀ anions in alkali-
metal salts, both with and without the inclusion of weakly-basic HF solvent molecules.
ß 2012 Elsevier B.V. All rights reserved.
1. Introduction
or a truncated icosahedron such as C60. More commonly, Td, Oh, or
Ih species have bonds that radiate from a central atom (e.g., CH4,
The packing of regular convex polyhedra in three-dimensional
space is a longstanding problem in mathematics, physics, and
chemistry [1–3]. Although chemical species with Td, Oh, or Ih
symmetry abound, only rarely are such species congruent with a
tetrahedron (i.e., a species having just four atoms (vertexes) joined
by six bonds (edges), as in P4), an octahedron (a species having just
six atoms joined by 12 bonds, as in the Ga6 clusters in Ba5Ga6 [4] or
BF4ꢀ, Zn(CN)42ꢀ, SF6, AsF6ꢀ, CrF62ꢀ [14], Mo(CO)6, Fe(CN)63ꢀ/4ꢀ) or
from the vertexes of a tetrahedral, octahedral, cubic, dodecahedral,
or icosahedral core (e.g., B4Cl4 [15], B6H62ꢀ [16], C8H8 [17], C20H20
[18], or B12H122ꢀ [19], respectively). When the polyhedral species
is an anion, the three-dimensional packing includes counter-
cations. These can be as simple as atomic metal cations but can also
include cationic polynuclear metal clusters, metal cations with a
complement of ligands, or non-metal cations.
2ꢀ
the B6 clusters in CaB6 [5] and BaB6 [6]), a cube (8 atoms, 12
2ꢀ
bonds), a dodecahedron (20 atoms, 30 bonds, as in the hypothetical
phosphorus allotrope P20 [7–9]), an icosahedron (12 atoms, 20
bonds, as in the B12 units in several allotropes of boron [10] or an
icosahedral species that also has a 13th atom at the center, as in the
Ag13 cluster in Ag13OsO6 [11] or the Zn13 cluster in NaZn13 [12,13]),
Salts of icosahedral B12X12 anions have exhibited, and no
doubt will continue to exhibit, unanticipated as well as unprece-
dented structures from the standpoint of anion packing and cation
hole filling (X = H, Cl, F). For example, K2B12H12 and Li2B12H12 both
2ꢀ
have CCP arrays of B12H12 anions (in this paper we equate the
2ꢀ
2ꢀ
packing arrangement of B12X12
and MF6
anions with the
packing arrangement of their B12 centroids (abbreviated ꢁ) or
central M atoms, respectively, not with the arrays of X or F atoms).
However, the cations in K2B12H12 are in tetrahedral holes (the
antifluorite structure) [20] while the cations in Li2B12H12 are nearly
centered in trigonal holes in the close-packed layers (they are
* Corresponding author. Tel.: +386 1 477 3301.
** Corresponding author. Tel.: +49 30 838 54289; fax: +49 30 838 53310.
*** Corresponding author. Tel.: +1 970 491 5104; fax: +1 970 491 1801.
E-mail addresses: zoran.mazej@ijs.si (Z. Mazej), seppelt@chemie.fu-berlin.de
(K. Seppelt), steven.strauss@colostate.edu (S.H. Strauss).
˚
˚
0.44 A out of the plane of the trigonal hole and 2.32 A from the
0022-1139/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2012.10.009

D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
119
center of an Oh hole (i.e., pairs of Li+ cations share the Oh holes in
Table 1
Crystallographic and data-collection parameters.a
this structure)) [21]. In another example, we recently reported the
unusual Ni2In structure of K2B12F12, a structure that is not
Compound
K2(HF)TiF6
K2(HF)3B12F12
Cs2(HF)B12F12
2ꢀ
exhibited by any other alkali-metal MF6 salt despite the fact
that K2MF6 salts do exhibit a wide variety of structure types [22].
We also recently reported the syntheses and structures of the
ternary salts K3(AsF6)B12F12 and Cs3(AsF6)B12F12 in Ref. [23]. It was
found that the formula-unit volumes of these salts are slightly
larger than the sum of the formula-unit volumes of the
corresponding MAsF6 and M2B12F12 salts (by 5% and 2%,
respectively), showing that, at least in these cases, ternary salts
with fluoroanions of different shapes and charges do not pack as
efficiently as the component binary salts. At the other end of the
Empirical formula
Formula weight
Habit, color
HF7KTi
260.06
H3B12F15K2
495.93
HB12Cs2F13
643.55
Plate,
Prism,
Prism,
colorless
light-yellow
colorless
Crystal system
Space group
Monoclinic
P21/m
Cubic
Monoclinic
P21/c
¯
Fm3c
2ꢀ
2ꢀ
spectrum of cations, a series of B12H12 and B12F12 salts of
heterocyclic cations such as imidazolium(1+) and triazolium(1+)
have been reported, in which weak (N)Hꢂ ꢂ ꢂH(B) and (N)Hꢂ ꢂ ꢂF(B)
hydrogen bonding was observed [24–26]. Finally, it has recently
˚
a (A)
6.187(3)
5.986(2)
8.895(4)
99.330(4)
325.1(2)
2
14.4841(17)
14.4841(17)
14.4841(17)
–
8.3463(8)
16.1160(17)
11.5031(9)
98.059(7)
1532.0(3)
4
˚
b (A)
˚
c (A)
2ꢀ
been shown that B12F12 is the least basic of the four anions
B12X122ꢀ (X = F, Cl, Br, I) [27]; it is almost certainly less basic than
b
(8)
B
12H122ꢀ, and DFT calculations predicted that the fluoroanion is
3
˚
significantly less polarizable than B12H122ꢀ [28]. For these reasons,
we expect that diffraction-quality single crystals of solvent-free
V (A )
3038.6(6)
8
Z
2ꢀ
salts of B12F12 will be rare, and to date the only example is
T (K)
200(2)
2.657
133(2)
133(2)
K2B12F12 [22].
In this work, we report the structures of three new HF solvates,
K2(HF)TiF6, K2(HF)3B12F12, and Cs2(HF)B12F12. The new results are
used to compare and contrast the strength of M–F(H) interactions
with M–F interactions involving F atoms from fluoroanions as well
as the solid-state packing of icosahedral B12F12
rcalc (g cmꢀ3
)
2.168
2.790
R(F) (I > 2
s
(I))a
0.0582
0.1468
ꢀ0.89, 0.97
P
0.0283
0.0760
ꢀ0.17, 0.23
P
0.0204
wR(F2) [all data]a
0.0495
2ꢀ
anions and
2ꢀ
octahedral MF6
anions in alkali-metal salts, both with and
ꢀ3
˚
Min., max. Dr (e A
)
ꢀ0.57, 0.75
without the inclusion of weakly-basic HF solvent molecules. The
results also reveal the reasons that the transformations
K2(HF)TiF6(s) ? K2TiF6(s) + HF(g)
K2(HF)3B12F12(s) ? K2B12F12(s) + 3 HF(g) can be rapid at 25 8C
even though the cation/anion packing in K2(HF)TiF6 and K2TiF6, or
in K2(HF)3B12F12 and K2B12F12, are not the same (i.e., even though
solvation and desolvation by gaseous HF requires lattice recon-
struction).
P
P
a
2
R1
=
jjFoj ꢀ jFcjj/ jFoj; wR2 = [ [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]]1/2
.
and
both K2(HF)CrF6 [34] and K2(HF)2CrF6 [14] are known), but there is
no evidence for this at the present time. Crystallographic and data-
collection parameters are listed in Table 1, and selected
interatomic distances and angles are listed in Table 2.
2. Results and discussion
2.2. Structure of K2(HF)TiF6
2.1. General Information
Drawings of the structure that emphasize the packing of anions
and cations and the corrugated ribbons of edge-sharing [K2(HF)2]2+
rhombs are shown in Figs. 1–4. The structure consists of an
idealized cubic close-packed array of TiF62ꢀ anions with half of the
K+ cations in idealized Td holes (K1) and half in idealized Oh holes
(K2), as shown in Figs. 1–3. The Ti–F distances and F–Ti–F angles
are normal [35,36]. The Tiꢂ ꢂ ꢂTi distances range from 5.717(1) to
There are a growing number of structurally-characterized
compounds with HF molecules coordinated to metal ions from all
regions of the periodic table [14,29,30] (these are distinctly
different from compounds with HF2ꢀ anions coordinated to metal
ions [31], which we will not discuss in this paper). The 29
compounds that have been structurally characterized by single-
crystal X-ray diffraction, including the three new compounds
reported here, are listed in supplementary data Table S-1. In the
remainder of this paper, frequent reference will be made to
˚
7.394(1) A. The K1 cations are nearly centered in the Td holes, with
˚
four K1ꢂ ꢂ ꢂTi distances that range from 3.561(1) to 3.633(1) A. In
contrast, the six K2ꢂ ꢂ ꢂTi distances span a wider range, from
˚
4.242(1) to 4.777(1) A.
individual K–F and Cs–F bond valence (BV) and sums of bond
The distribution of K1 cations in half of the Td holes in the CCP
P
valence
(
BV) values for the title compounds and related
lattice of TiF62ꢀ anions is such that they form nearly-square-planar
˚
compounds. The bond-valence parameters and bond-valence
equation used in this work are standard [32] and are listed in
the supplementary data along with tables of K–F and Cs–F bond
valences for the compounds discussed below (Tables S-2–S-8).
The title compounds were crystallized either by cooling or by
evaporating anhydrous HF solutions of K2TiF6 [33], K2B12F12 [28],
and Cs2B12F12 [28], (see Section 3). The lability of the HF ligands
and the corrosive nature of HF precluded characterization of the
title compounds by elemental analysis or spectroscopic methods. It
arrays, with K1ꢂ ꢂ ꢂK1 distances of 4.235(1) and 4.377(1) A. There
are also four K2 nearest neighbors for each K1 cation, at 4.125(1)
˚
(ꢃ2), 4.259(1), and 4.277(1) A. The K2 cations are linked by triply-
bridging HF ligands, with K2–F(H) distances of 2.679(5)
˚
(BV = 0.156) and 3.238(2) A (ꢃ2; BV = 0.035). The K2 cations form
rigorously-planar rectangular arrays, with K2ꢂ ꢂ ꢂK2 distances of
˚
5.986(1) and 6.187(1) A, that are parallel to the K1 arrays.
˚
However, the shortest K2ꢂ ꢂ ꢂK2 distance is 4.575(1) A. The K1
nearly-planar and K2 planar arrays are tilted ca. 498 from the
idealized close-packed Ti planes.
is possible that phases of K2(HF)nTiF6, K2(HF)nB12F12
, and
Cs2(HF)nB12F12 with n values other than those observed for the
title compounds could be isolated using different conditions (e.g.,
The K1 and K2 cations form bonds, albeit with signficant
electrostatic contributions, with 12 and 9 F atoms, respectively, as

120
D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
Table 2
˚
Selected distances (A) and angles (8).
a
a
b
Parameter K2(HF)TiF6
K2(HF)3B12F12
Cs2(HF)B12F12
Mꢂ ꢂ ꢂF(Ti) or Mꢂ ꢂ ꢂF(B) range 2.784(3)–3.317(4) (K1); 2.666(3)–2.932(4) (K2)
Mꢂ ꢂ ꢂF–Ti or Mꢂ ꢂ ꢂF–B range
2.710(1) ꢃ 4
2.981(2)–3.706(2)
99.5(2)–164.2(2)
134.71(5)
Mꢂ ꢂ ꢂF(H)
2.679(5), 3.238(2) ꢃ 2
2.62(1), 2.68(1), 2.76(1) 3.05(1), 3.44(1)
Mꢂ ꢂ ꢂF(H)ꢂ ꢂ ꢂM
100.3(3) ꢃ 2, 100.8(3)
130.0(3)
MFn coordination sphere
K(1)F12; K(2)F9
KF7
Cs(1)F12; Cs(2)F10
P
(bond valences to F(Ti))
P
1.07 (K1); 0.85 (K2)
(bond valences to F(B))
0.58
1.04
0.96 (Cs1); 0.99 (Cs2)
0.96 (Cs1); 1.18 (Cs2)
P
(all bond valences)
1.07 (K1); 1.07 (K2)
Mꢂ ꢂ ꢂM
4.235(1), 4.377(1) (K1ꢂ ꢂ ꢂK1); 5.986(1), 6.187(1) (K2ꢂ ꢂ ꢂK2); 4.125 (K1ꢂ ꢂ ꢂK2) 4.129(1)
5.787(3) (Cs1ꢂ ꢂ ꢂCs1);
5.155(3), 5.616(3) (Cs1ꢂ ꢂ ꢂCs2);
5.890(3) (Cs2ꢂ ꢂ ꢂCs2)
c
Mꢂ ꢂ ꢂTi or Mꢂ ꢂ ꢂꢁ
3.561(1)–3.633(1) (K1); 4.242(1)–4.777(1) (K2)
5.352 ꢃ 4
5.122–6.073 (Cs1);
5.249–6.012 (Cs2)
c
Tiꢂ ꢂ ꢂTi or ꢁꢂ ꢂ ꢂꢁ
5.714(1)–6.187(1)
1.842(3)–1.884(4)
7.242 ꢃ 6
7.217–9.408
Ti–F or B–F range
B–B range
1.387(2) ꢃ 12
1.794(2)–1.796(2)
1.371(4)–1.392(4)
1.772(4)–1.804(5)
a
M = K.
b
M = Cs.
c
ꢁ = B12 centroid.
P
shown in Fig. 4. The BV values are 1.07 for both K1 and K2. The
KF12 coordination sphere for K1 is an idealized CCP array of F
atoms. The KF9 coordination sphere for K2 is a tri-capped trigonal
prism. The three bonds between K2 and the HF molecules, which
bridge three or four K+ ions: F1 and F4 atoms, which account for
four of the six F atoms, bridge three cations each, F3 bridges four
cations, and F2 is bonded to only one K+ cation).
Crystals of K2(HF)TiF6 rapidly lost HF in an air-free environment
at 25 8C. Powder X-ray diffraction demonstrated that the trigonal
represent one-third of the nine F–K2 interactions, account for only
21% of the BV value for K2. Nevertheless, the shortest K2–F(H)
distance, at 2.679 A, is shorter than all but two of the six K2–F(Ti)
distances (see Table S-3). Another structural feature relevant to the
discussion below is that five of the six F atoms of the TiF62ꢀ anions
P
˚
Fig. 1. The packing of K+ cations and Ti atoms in K2(HF)TiF6 (F and H atoms omitted
for clarity. The CCP arrangement of TiF62ꢀ anions is indicated by the Tiꢂ ꢂ ꢂTi vectors.
The nearly-planar layers of K1 cations and the rigorously-planar layers of K2 cations
are both tilted ca. 498 from the idealized close-packed planes of Ti atoms.
Fig. 2. The disposition of the K1 cations in Td holes of the idealized CCP lattice of
TiF62ꢀ anions in the structure of K2(HF)TiF6 (50% probability ellipsoids). The 12 K1–
˚
F distances range from 2.784(3) to 3.317(4) A.

D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
121
Fig. 3. Left. The disposition of the K2 cations in Oh holes of the idealized CCP lattice TiF62ꢀ anions in the structure of K2(HF)TiF6 (50% probability ellipsoids except for the three
H atoms, which are shown as spheres of arbitrary size). The three F5 atoms, from the HF ligands, are highlighted for clarity. The 12 K2–F distances range from 2.666(3) to
3.328(2) A. Right. Fragment of the corrugated infinite ribbon of edge-sharing [K2(HF)2]2+ rhombs involving the K2 cations and triply-bridging HF ligands in K2(HF)TiF6. Each
˚
˚
rhomb shares K–F(H) = 2.679(5) A edges with two nearest-neighbor rhombs.
polymorph of K2TiF6 (which exhibits the K2GeF6 structure) was
formed [35,36]. In this structure, the stacking of TiF6 anions is
1.07 for K1 and K2 in K2(HF)TiF6 and 1.02 in trigonal K2TiF6. Finally,
the perpendicular distances between planes of Ti atoms are
2ꢀ
AAAꢂ ꢂ ꢂ and the K+ ions are in trigonal-prismatic holes defined by
the Ti atoms and are each bonded to 12 F atoms (see Figure S-1;
interestingly, the F atoms form an idealized CCP array around K1 in
K2(HF)TiF6 and form an idealized HCP array around the K atoms in
trigonal K2TiF6). Fig. 5 depicts the transformations of the idealized
CCP lattice and the actual CCP-like lattice of K2(HF)TiF6 to solvent-
free trigonal K2TiF6 (only two layers of Ti atoms are shown; the
idealized lattice was constructed by moving one layer of Ti atoms
so that they were centered over trigonal holes in the adjacent
layer). For the two K+ ions depicted in these drawings, the Kꢂ ꢂ ꢂK
virtually the same, 4.69 A in K2(HF)TiF6 and 4.66 A in trigonal
K2TiF6.
˚
˚
2.3. Structure of K2(HF)3B12F12
Drawings of the structure are shown in Figs. 6–9. The distances
2ꢀ
and angles in the B12F12
anions are normal [22,37]. The B12
centroids form a rigorous cubic array with a ꢁꢂ ꢂ ꢂꢁ edge length of
˚
7.242(2) A, as shown in Fig. 6. The anions are oriented 908 relative
to each other, as shown in Fig. 7. The high symmetry of the unit cell
requires that (i) there is only one unique B–F moiety in the
asymmetric unit and (ii) the K+ ions are disordered over six
positions, with the three Kꢂ ꢂ ꢂK vectors coincident with the C4 axes
of the cube. In this model, the three HF ligands, two of which are
symmetry-related, are disordered over 21 positions. The best
model, after analysis of K–F(B), K–F(H), and (H)Fꢂ ꢂ ꢂF(B) contacts, is
˚
˚
distance is 7.40 A in K2(HF)TiF6 and 6.86 A in trigonal TiF6
˚
(
D
(Kꢂ ꢂ ꢂK) = 0.54 A; note that these are not the closest Kꢂ ꢂ ꢂK
˚
distances in these structures). There is a ca. 3.3 A shift of one close-
packed array of Ti atoms on going from the CCP-like packing in
K2(HF)TiF6 to the AAAꢂ ꢂ ꢂ packing in trigonal K2TiF6. Nevertheless,
P
the desolvation can apparently occur with little change in BV
2+
during the transformation, and hence can occur relatively rapidly
that K2(HF)3 units, with idealized trigonal-bipyramidal K2F3
cores having axial K+ ions, are centered in each B12-centroid cube,
P
(this is depicted in more detail in Figure S-2). The BV values are
Fig. 4. The idealized CCP array of F atoms around K1 (left) and the tri-capped trigonal prismatic array of F atoms around K2 in K2(HF)TiF6 (right; 50% probability ellipsoids
except for the three H atoms, which are shown as spheres of arbitrary size). The three F5 atoms, from the HF ligands, are highlighted for clarity. The separation of the cations is
not to scale.

122
D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
Fig. 5. Depictions of the transformation of K2(HF)TiF6 to solvent-free trigonal K2TiF6 (F and H atoms omitted for clarity). The red and light blue spheres are central Ti atoms of
TiF62ꢀ anions in two adjacent close-packed or pseudo-close-packed layers. The short arrows depict the approximate directions (but not the distances) of the motions of the Ti
atoms in the upper layer during the desolvation process. The two labeled Ti atoms are the same before and after desolvation.
as shown in Figs. 6 and 8. The choice of the two K+ positions was
straightforward. If one K+ ion is arbitrarily selected, then the
choice of the F(H) atom positions was similarly straightforward. All
+
˚
F atom positions less than 2.2 A from the chosen pair of K ions and/
˚
˚
second one can only be in the opposite ‘‘octahedral’’ position in
or less than 2.4 A from other F atoms were excluded (cf. the 2.66 A
K–F distance in KF).
+
˚
Fm3c (the other four possible K positions are only 2.2 A away). The
In addition to the three HF molecules, the K+ ions are
coordinated to four F(B) atoms, one from each of four B12F12
anions that form one face of the cube. Each K+ ion is located inside
2ꢀ
˚
the B12-centroid cube, 1.554 A along a perpendicular from the
center of the closest face of cube (this face is coplanar with the four
F1 atoms coordinated to K+ and the four B1 atoms to which these F1
atoms are attached). Even though the disorder of the HF molecules
made it impossible to locate the H atoms with any precision, it is
clear that there is significant hydrogen bonding between the HF
˚
molecules and F(B) atoms. The (H)F2ꢂ ꢂ ꢂF(B) distance is 2.58(2) A,
˚
similar to the (H)Fꢂ ꢂ ꢂF(H) distances of 2.51–2.53 A observed in
crystalline HF (DF) and gas-phase DF oligomers [38,39]. Two
˚
(H)F3ꢂ ꢂ ꢂF(B) contacts of 2.72(1) A suggests that this HF molecule is
involved in a bifurcated pair of hydrogen bonds.
Only seven (7) F atoms are coordinated to each K+ ion in
K2(HF)3B12F12, as shown in Fig. 9, fewer than in K2B12F12 (8 and 10
for the two different K+ ions), fewer than in K2(HF)TiF6 (9 and 12 for
the two different K+ ions), and far fewer than in any K2MF6 or
K2MF4 salt (e.g., 12 for trigonal K2TiF6 [35,36]). The K–F distances,
and hence the corresponding BV values, are essentially the same
for the K–F(B) and K–F(H) bonds. The four BV values for the
equivalent K–F(B) bonds are 0.144, and the three BV values for
2+
Fig. 6. The K2(HF)3 unit and one of the eight equivalent B12F12 anions that
2ꢀ
˚
surround it at the corners of a cube with ꢁꢂ ꢂ ꢂꢁ distances of 7.242 A (50% probability
ellipsoids; ꢁ = B12 centroid). Four F1 atoms from four corners coordinate to each K+
cation. The F2 and F3 atoms belong to the HF molecules. The H atoms were not
located.
the K–F(H) bonds are 0.183, 0.156, and 0.125 (average 0.155). The
P
K–F(H) BV values account for 45% of the BV, which is 1.04 (note
that 3/7 is equivalent to 43%). It appears that the B12F122ꢀ anion is

D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
123
Fig. 7. One face of the cube of B12F122ꢀ anions in the structure of K2(HF)3B12F12. The ꢁꢂ ꢂ ꢂꢁ distances are 7.242 A. The anions are oriented 908 relative to their nearest neighbors
˚
˚
to minimize Fꢂ ꢂ ꢂF repulsions. The closest Fꢂ ꢂ ꢂF contacts are 3.066(1) A.
so weakly coordinating that a neutral HF molecule can compete
with it for coordination to K+: two of the three K–F(H) BV values are
larger than the K–F(B) BV values. This was also the case for
K2(HF)TiF6 (see Table S-3). However, a significant difference
between the structures of K2(HF)3B12F12 and K2(HF)TiF6 is that
none of the F atoms in the former structure bridge K+ ions and five
of the six F atoms in the latter structure bridge either three or four
K+ ions. In fact, no B12F122ꢀ salt reported to date has an F atom that
bridges more than two metal ions.
depicts the transformations of the idealized simple-cubic lattice
of K2(HF)3B12F12 and the pseudo-HCP lattice of K2(HF)3B12F12
.
Even though these are entirely different lattices, the transfor-
mations caused by desolvation/resolvation can occur with only
small changes in the relative positions of the cations and anions
(as was the case with the transformation of K2(HF)TiF6 to
solvent-free trigonal K2TiF6). The ꢁꢂ ꢂ ꢂꢁꢂ ꢂ ꢂꢁ angles change from
908 in K2(HF)3B12F12 to ca. 608 in K2B12F12 (54.38, 56.48, and
˚
60.18), the ꢁꢂ ꢂ ꢂꢁ distances change from 7.242 A in K2(HF)3B12F12
˚
Crystals of K2(HF)3B12F12 also rapidly lost HF in an air-free
environment at 25 8C, forming solvent-free K2B12F12 [22]. Fig. 10
to 7.517, 8.207, and 8.236 A in K2B12F12, and the Kꢂ ꢂ ꢂꢁ distances
˚
˚
change from 5.352 A in K2(HF)3B12F12 to 4.719 (ꢃ2) and 4.810 A
˚
for K1 and 5.445 (ꢃ2), 5.445 (ꢃ2), and 5.445 A (ꢃ2) for K2 in
˚
K2B12F12
.
The Kꢂ ꢂ ꢂK distance changes from 4.129(1) A in
˚
K2(HF)3B12F12 to 5.629(1) A in K2B12F12 (the latter is the
Fig. 9. The KF7 coordination sphere in the structure of K2(HF)3B12F12 (50%
probability ellipsoids). The F2 and F3 atoms belong to the three HF molecules that
bridge two K+ ions. The H atoms belonging to these HF molecules were not located.
Fig. 8. Part of the structure of K2(HF)3B12F12 (50% probability ellipsoids). The F2 and
F3 atoms belong to the three HF molecules. The H atoms were not located, but are
probably positioned between their respective F2 or F3 atoms and the F1 atoms to
˚
Selected interatomic distances (A) and angles (8): K–F1, 2.710(1); K–F2, 2.76(1); K–
˚
which they are closest: F2ꢂ ꢂ ꢂF1, 2.58(1); F3ꢂ ꢂ ꢂF1, 2.72(1) A.
F20, 2.62(1); K–F3, 2.68(1); F1ꢂ ꢂ ꢂF10, 3.136(2); F2ꢂ ꢂ ꢂF20, 3.40(2); F2ꢂ ꢂ ꢂF3, 2.84(2).

124
D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
Fig. 10. The transformation of the simple-cubic array of B12 centroids in K2(HF)3B12F12 (left) to the HCP array of B12 centroids in solvent-free K2B12F12 (right).
distance between K1 and K2 in Fig. 10; the shortest K1ꢂ ꢂ ꢂK2
2.4. Structure of Cs2(HF)B12F12
˚
distance in K2B12F12 is actually 5.421(1) A). Finally, Fig. 11
shows the change from four K–B12F122ꢀ interactions with a total
Drawings of the structure are shown in Figs. 13 and 14. The
distances and angles in the B12F12 anions are normal [22,37].
2ꢀ
2ꢀ
of four K–F bonds in K2(HF)3B12F12 to three K–B12F12
interactions in K2B12F12 with the same set of anions but with
a total of eight K–F bonds.
The structure of K2(HF)3B12F12 more closely resembles the
structure of K2(HF)2CrF6 [14] as shown in Fig. 12, than it does
the structure of K2(HF)TiF6. There is an elongated pseudo-cubic
The B12 centroids form a distorted HCP array ꢁꢂ ꢂ ꢂꢁ distances that
˚
˚
range from 7.217 to 9.408 A (average 8.305 A), as shown in Fig. 12.
This is essentially the same structure as the monosolvate
Cs2(H2O)B12F12 [22] and these two structures are shown together
for comparison in Figure S-4. It is also the same structure type as
exhibited by K2B12F12, viz. the Ni2In structure. The shortest Fꢂ ꢂ ꢂF
array of CrF6 anions with K2(HF)2 moieties inside; each K+
ion is also engaged in K–F(H)–F bridging to another K+ ion in a
neighboring cube. The result is that each K+ ion is bonded to
three HF molecules, as in K2(HF)3B12F12. The Kꢂ ꢂ ꢂK distance
2ꢀ
2+
˚
contact between anions in Cs2(HF)B12F12 is 2.776(2) A. The HF
molecule is disordered over two sites (see Section 3); only the
major F atom will be used in the discussion that follows.
The Cs+ ions close to the trigonal holes in the close-packed
layers of anions (Cs1) are bonded to 12 F atoms. The Cs1–F
˚
˚
inside each cube is 4.773(1) A(cf. 4.129(1) A in K2(HF)3B12F12).
However, the shorter Crꢂ ꢂ ꢂCr distances in K2(HF)2CrF6, 5.235(1)
˚
˚
and 5.837 (ꢃ2) A, relative to the ꢁꢂ ꢂ ꢂꢁ distances of 7.242 A in
K2(HF)3B12F12, allow K+ ions in neighboring cubes in K2(HF)2CrF6
to engage in HF-bridge bonding; the Kꢂ ꢂ ꢂK distances between
distances range from 3.105(2) (BV = 0.123) to 3.520(2)
P
(BV = 0.038), with a BV value of 0.96. The Cs+ ions in Oh holes
(Cs2) are bonded to 10 F atoms. The Cs2–F distances range from
P
˚
cubes in K2(HF)2CrF6 are 4.136(1) and 4.353(1) A, whereas the
3.021(2) (BV = 0.154) to 3.443(2) (BV = 0.049), with a BV value of
1.18. The corresponding CsF12 and CsF10 coordination spheres are
shown in Fig. 13. The H atom of the HF molecule is hydrogen
bonded to an anion F atom, with (H)Fꢂ ꢂ ꢂF(B) and F–Hꢂ ꢂ ꢂF(B) values
of 2.67(1)8 and 157(2)8, respectively.
˚
Kꢂ ꢂ ꢂK distances between cubes in K2(HF)3B12F12 are 5.574 A, too
long for HF-bridging.
Fig. 11. Perpendicular views of the transformation of K2(HF)3B12F12 (left; HF ligands omitted) to solvent-free K2B12F12 (right; the K+ ion shown is K1). The K+ ions move from
being 1.554 A below the plane of the B12 centroids in K2B12F12 to being co-planar with the B12 centroids. In K2(HF)3B12F12, the three HF ligands complete the KF7 coordination
2ꢀ
sphere; in K2(HF)3B12F12, two B12F12 anions above and below the plane (not shown) each donate one F atom to complete the KF10 coordination sphere.

D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
125
2ꢀ
2.5. Comments on the crystal packing of B12F12 anions
2ꢀ
A crude estimation of the radius of the icosahedral B12F12
anion, if it is considered to be spherical, is half of the apical Fꢂ ꢂ ꢂF
˚
distance, ca. 3.08 A, plus the van der Waals radius of fluorine,
˚
˚
1.47 A, the sum of which is 4.55 A. However, ꢁꢂ ꢂ ꢂꢁ distances in
2ꢀ
˚
lattices are considerably shorter than 9.10 A (i.e.,
B12F12
2 ꢃ 4.55 A), even when solvent molecules are present (e.g.,
˚
˚
˚
7.242 A in K2(HF)3B12F12, 7.276–7.295 A in K2(H2O)2B12F12 [40],
˚
˚
6.915–8.598 A in K2(H2O)4B12F12 [40], and 7.204–8.236 A in
solvent-free K2B12F12 [22]. Therefore, the effective radius of
B
12F122ꢀ, due to the dovetail-like interpenetration of adjacent
˚
anions shown in Fig. 7, is ca. 3.5–3.6 A.
It appears that the symmetry of B12F122ꢀ allows it to pack more
densely in a planar square array than in a planar close-packed
hexagonal array, at least for the structures determined to date. The
2
˚
area per anion in square-planar arrays in K2(HF)3B12F12 is 52.5 A ,
whereas the area per anion in the hexagonal arrays in K2B12F12
˚
(ꢁꢂ ꢂ ꢂꢁ = 8.207 and 8.236 (ꢃ2) A within the close-packed planes of
2
˚
anions) is 58.4 A . If the structure of Li2B12F12 can be determined,
and if it has the antifluorite structure, it is possible that the anions
will pack more densely, in the close-packed planes, than in
K2(HF)3B12F12
.
3. Experimental
All manipulations were carried out under rigorously anhydrous
conditions. Anhydrous HF (aHF; Linde) and F2 (Solvay) were
handled using a nickel–Teflon vacuum line; solids were handled in
an argon-filled glovebox. The compounds KF and TiCl4 (Sigma–
Aldrich) were used as received. The compound TiF4 was prepared
by treating TiCl4 with F2 in a HF. Formation of TiF4 was confirmed
by Raman spectroscopy and chemical analysis. The compounds
K2B12F12 and Cs2B12F12 were prepared as previously described[28].
Single crystals of K2(HF)TiF6 were prepared in an F2-passivated
T-shaped apparatus consisting of a 15.5 mm ID FEP tube and a 4 mm
ID FEP tube, both equipped with Teflon valves. Anhydrous HF (5 mL)
was condensed at 77 K in the wider FEP tube containing KF
(0.82 mmol), TiF4 (0.41 mmol), and a Teflon-coated stir bar. This
mixture was warmed to 23(2) 8C and, upon stirring, formed a clear,
colorless solution, which was decanted into the 4 mm ID tube. Slow
condensation of HF vapor in the 15.5 mm ID tube was accomplished
by maintaining a ca. 10 8C temperature gradient between the two
tubes, resulting in the formation of crystals of K2(HF)TiF6 (and
K2TiF6). The crystals, immersed in perfluorodecalin (ABCR, 90%),
Fig. 12. The structure of K2(HF)2CrF6. See text for relevant interatomic distances.
The compound Cs2B12F12 crystallizes from anhydrous HF or
from H2O with only one HF or H2O ligand, respectively, while
K2B12F12 crystallizes from these solvents with three HF ligands
and either two or four H2O ligands [40]. Note that this is similar
to the situation observed when the alkali-metal salts of
B
12Cl12
were crystallized from SO2: the K+ salt crystallized
2ꢀ
with eight SO2 molecules per formula unit, with the formation of
six K–O bonds, while the Cs+ salt crystallized with only one SO2
molecule per formula unit, and with only one, very weak, Cs–O
bond [27].
The compound Cs2(HF)4CrF6 has an entirely different structure
than Cs2(HF)B12F12 [14] and it is not fruitful to compare its
structure to Cs2(HF)B12F12. Each Cs+ ion is bonded to six HF
molecules, each of which bridges three Cs+ ions. Two drawings of
this structure are shown in Figures S-5 and S-6.
Fig. 13. The structure of Cs2(HF)B12F12
.

126
D.V. Peryshkov et al. / Journal of Fluorine Chemistry 145 (2013) 118–127
Fig. 14. The CsF12 and CsF10 coordination units of Cs1 and Cs2, respectively, in Cs2(HF)B12F12 (50% probability ellipsoids except for the two H atoms, which are shown as
spheres of arbitrary size).
Appendix A. Supplementary data
were examined with a microscope in the glovebox and a suitable
crystal was mounted on a glass fiber and attached to a goniometer in
a stream of cold (ꢀ73 8C) N2 gas. The crystals rapidly lost HF at room
temperature in the glovebox. For this reason, it was not possible to
obtain a ‘‘bulk’’ sample of pure K2(HF)TiF6 for spectroscopic or
compositional analysis. However, PXRD analysis of a bulk sample of
crystals that had been crushed to a powder inside the glovebox
showed that the dry powder was identical to a bonafide sample of
K2TiF6.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jfluchem.
2012.10.009.
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equipped with a Mercury CCD area detector. The data were
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˚
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˚
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