Dipotassium tetrachromate(VI), K2Cr4O13

Article abstract of DOI:10.1107/S0108270105035833

The structure of dipotassium tetrachromium(VI) tridecaoxide, K ₂Cr₄O₁₃, has been determined from single-crystal X-ray data collected at 173 (2) K on a racemically twinned crystal with monoclinic Pc space-group symmetry. The structure is composed of discrete [Cr₄O₁₃]^2- zigzag chains held together by the charge-balancing potassium ions. The conformations adopted by the tetrachromate anion in alkali metal salts and Cr₈O₂₁ are different and can be divided into three categories.

Full text of DOI:10.1107/S0108270105035833

inorganic compounds
of [Cr3O10]2 anions, together with inorganic cations (Mattes
& Meschede, 1973; Kolitsch, 2003; Blum et al., 1979; Blum &
Acta Crystallographica Section C
Crystal Structure
Communications
È
Guitel, 1980; Lofgren, 1974) or organic cations (Ding et al.,
Â
2004; SteÎpien & Grabowski, 1977; Garrison et al., 2001; Luis et
ISSN 0108-2701
2
Â
al., 1995; Fosse et al., 2001). In these structures, the [Cr3O10]
units adopt different conformations as a result of the diversity
in packing (Casari & Langer, 2006).
Four compounds containing the tetrachromate unit have
been structurally characterized before now, including three
alkali metal salts and one mixed-valence binary oxide. The
chemical analogues K2Cr4O13 (Golovachev et al., 1970;
Dipotassium tetrachromate(VI),
K2Cr4O13
Barbara M. Casaria* and Vratislav Langerb
aDepartment of Chemistry, Inorganic Chemistry, Goteborg University, SE-41296
È
È
Kuz'min et al., 1972), Rb2Cr4O13 (Lofgren, 1971, 1973) and
Goteborg, Sweden, and bDepartment of Chemical and Biological Engineering,
Cs2Cr4O13 (Kolitsch, 2004) all belong to the monoclinic
system, and have space groups Pc, P21/c and P21/n, respec-
tively. Blum & Tran Qui (1979) reported indexed powder
diffraction data on (NH4)2Cr4O13 and assigned the space
È
Division of Materials and Surface Chemistry, Subdivision of Inorganic Environmental
Chemistry, Chalmers University of Technology, SE-41296 Goteborg, Sweden
È
Correspondence e-mail: casari@chem.gu.se
È
group to be P21/c, as determined for Rb2Cr4O13 (Lofgren,
1973) and unpublished work on K2Cr4O13 by Lofgren. Doubts
Received 30 August 2005
Accepted 1 November 2005
Online 30 November 2005
È
about both the space-group assignment and the unit-cell
parameters of the K2Cr4O13 structure were expressed by
Kolitsch (2004). Furthermore, neither s.u. values nor aniso-
tropic displacement parameters are given in the papers
(Golovachev et al., 1970; Kuz'min et al., 1972), and an R value
of 0.108 was reported, based on ®lm data. We present here a
redetermination of the structure of K2Cr4O13.
The K2Cr4O13 structure belongs to the non-centrosym-
metric space group Pc, while the other three chemical
analogues belong to space group P21/c, with unit cells doubled
The structure of dipotassium tetrachromium(VI) tridecaoxide,
K2Cr4O13, has been determined from single-crystal X-ray data
collected at 173 (2) K on a racemically twinned crystal with
monoclinic Pc space-group symmetry. The structure is
composed of discrete [Cr4O13]2 zigzag chains held together
by the charge-balancing potassium ions. The conformations
adopted by the tetrachromate anion in alkali metal salts and
Cr8O21 are different and can be divided into three categories.
È
in the a direction (Lofgren, 1973; Kolitsch, 2004; Blum & Tran
Qui, 1979). The reciprocal space was searched carefully for
Comment
weak extra re¯ections, especially for h = (2n
1)/2, but
CrO3 derivatives are a ®eld of current interest for surface
®nishing, which proceeds in systems containing CrO3 dissolved
in water. The structure of the predominant species in chro-
mium electroplating baths has been investigated extensively
without success. The crystal under investigation was twinned
by an inversion operation and the twin volume ratio was
re®ned to 0.64 (3)/0.36 (3).
There are one discrete chromate tetramer and two non-
equivalent potassium ions in the asymmetric unit of K2Cr4O13
(Fig. 1). The tetrachromate ion, [Cr4O13]2 , is composed of a
chain of four CrO4 tetrahedra, joined by shared corners. The
CrÐO bridging distances, can be divided into two groups
(Table 1). The two terminal CrO4 units have longer CrÐO
Æ
(Radnai & Dorgai, 1992; Sarmaitis et al., 1996; CË engeloglu et
al., 2003). These compounds are also continuing to attract
interest (Carlsen et al., 1995; Islam et al., 2005; Karunakaran &
Suresh, 2004) as they represent the most widely used group of
oxidizing agents in organic chemistry, able to oxidize almost
every organic functional group (Cainelli & Cardillo, 1984).
Chromic acid is a common reagent but the active species in the
oxidizing solutions may vary depending on the reaction
conditions. The polymerization of the [CrO4]2 units into
corner-sharing dimers, trimers or longer chains is pH and
Ê
bridging distances [mean 1.834 (4) A] than the two inner units
Ê
[mean 1.74 (4) A]. The non-bridging CrÐO bonding distances
Æ
concentration dependent (Sarmaitis et al., 1996). The defor-
mation of the CrO4 tetrahedra increases with the length of the
chains (Gili & Lorenzo-Louis, 1999). Pressprich et al. (1988)
studied and compared the CrÐO bond lengths within anions
2
of the formula [CrnO3n+1
]
. They found that, with increasing
polymerization, the average bridging bond length increases,
while the average non-bridging bond length decreases. The
Æ
Â
structure of CrO3 (Hanic & Stempelova, 1960; Stephens &
Cruickshank, 1970) consists of in®nite chains of corner-sharing
Ê
CrO4 tetrahedra, with the bridging CrÐO distances 0.15 A
longer than the mean terminal distance, resulting in deformed
CrO4 units. The structures of trichromate compounds consist
Figure 1
The asymmetric unit of K2Cr4O13, with atomic displacement ellipsoids
drawn at the 50% probability level.
Acta Cryst. (2005). C61, i117±i119
DOI: 10.1107/S0108270105035833
# 2005 International Union of Crystallography i117

inorganic compounds
Ê
are signi®cantly shorter [mean 1.605 (6) A] (Fig. 1 and
Table 1). Despite the variation of bond lengths, the mean CrÐ
O distance within the individual tetrahedra remains constant
2
[1.66 (5) A]. The [Cr4O13] anions form isolated zigzag chains
Ê
in the c direction, whose charge is counterbalanced by the
intercalating potassium ions (Fig. 2).
The two cations, K1 and K2, are irregularly coordinated
Ê
(within 3.28 A) by 11 and 10 O atoms, respectively (Table 1),
Ê
but have similar mean KÐO bond lengths [2.96 (3) A]. The O
atoms neighbouring each potassium ion belong to six different
tetrachromate chains. The relatively high potassium coordi-
nation, compared with the ninefold coordination in K2Cr3O10
(Blum et al., 1979), may be attributed to the high oxygen/
È
potassium ratio (Lofgren, 1973) or to the packing features, as
Figure 2
The packing in K2Cr4O13.
in the case of the 11-coordinate ammonium ions in
ꢀ-(NH4)2Cr3O10 (Casari & Langer, 2006).
Comparing the structural arrangement in the tetrachromate
analogues, it is evident that Cs2Cr4O13 (Fig. 3a) differs from
Rb2Cr4O13 and K2Cr4O13 (Figs. 3b and 3c). The latter are quite
similar but contain different cation±cation distances for the
Rb1±Rb1, Rb2±Rb2, K1±K1 and K2±K2 pairs (Figs. 3b and
Ê
3c). These distances vary by 0.111 (3) A in Rb2Cr4O13 and
Ê
0.038 (4) A in K2Cr4O13. Furthermore, every other tetra-
chromate group is rotated in Rb2Cr4O13 compared with
K2Cr4O13, but the shapes of the tetrachromate chains are
almost the same (Fig. 4a). In Cs2Cr4O13, the packing of the
[Cr4O13]2 anions and cations is different, as is the shape of
the chromate chain (Fig. 4b). Structural data have so far been
determined for only one other compound containing the
[Cr4O13]2 unit, i.e. the mixed-valence Cr8O21 oxide, which is
better described as Cr2III(CrVIO4)2(CrV4 IO13) (Norby et al.,
1991). In this compound, the tetrachromate chain adopts a
third conformation (Fig. 4c), resembling a section of the one-
Æ
Â
dimensional chains in CrO3 (Hanic & Stempelova, 1960),
except for the CrÐOÐCr angle of 180.0 (7)ꢀ.
Values of the Cr1ÐCr2ÐCr3ÐCr4 torsion angles, and the
Cr1ÐCr2ÐCr3 and Cr2ÐCr3ÐCr4 angles in tetrachromate
structures, are presented in Table 2. In the nearly planar
tetrachromate units, the Cr1ÐCr2ÐCr3 and Cr2ÐCr3ÐCr4
angles seem to occur in pairs of a small and a large angle. A
combination of two extreme values [86.48 (2) and 127.29 (2)ꢀ]
is encountered in the Cs2Cr4O13 structure (Kolitsch, 2004).
The Cr1ÐCr2ÐCr3 angles have also been examined in
trichromates (Casari & Langer, 2006), which interestingly
showed a range of angles between 86.85 (2) and 127.73 (4)ꢀ.
In summary, the structure of K2Cr4O13 has been rede-
termined from a racemic twinned crystal and it has been
shown that, even if the structure shows similarities with
Figure 3
A comparison of the packing in (a) Cs2Cr4O13 (Kolitsch, 2004), (b)
È
Rb2Cr4O13 (Lofgren, 1973) and (c) K2Cr4O13 (this work). The numbers 1
and 2 denote M1±M1 and M2±M2 cation pairs in (b) and (c).
È
Rb2Cr4O13 (Lofgren, 1973), the compounds are different
enough to lead to a different unit cell and space group.
Experimental
Crystals of K2Cr4O13 were formed during an attempt to prepare
Ce(CrO4)2ÁH2O and/or Ce(CrO4)2Á2H2O. Ce(SO4)2Á4H2O (1.50 g,
3.75 mmol) was dissolved in water (10 ml) and Ce(OH)4 was preci-
pitated with 15 M ammonia. Ce(OH)4 (0.12 g 0.76 mmol) was added
to a saturated solution of K2CrO4 (1.5 ml), and then concentrated
Figure 4
The con®guration of [Cr4O13]2 chains in (a) K2Cr4O13 (this paper), (b)
Cs2Cr4O13 (Kolitsch, 2004) and (c) Cr8O21 (Norby et al., 1991).
ꢁ
i118 Casari and Langer
K2Cr4O13
Acta Cryst. (2005). C61, i117±i119

inorganic compounds
sulfuric acid was added until the cerium hydroxide was completely
dissolved. This particular sample was left covered and unguarded
and dark-orange±red crystals of K2Cr4O13, suitable for single-crystal
X-ray analysis, were obtained after nine months.
Table 2
A comparison of intrapolyhedral angles (ꢀ) for the different conforma-
tions of the [Cr4O13]2 units.
a
b
c
d
Angle
K2Cr4O13
Rb2Cr4O13
Cs2Cr4O13
Cr8O21
Crystal data
Cr1ÐCr2ÐCr3ÐCr4
Cr1ÐCr2ÐCr3
Cr2ÐCr3ÐCr4
Cr2ÐOÐCr3
172.99 (3)
94.06 (3)
121.45 (3)
138.0 (3)
172.30 (5)
96.05 (5)
122.33 (5)
139.3 (4)
177.58 (2)
86.48 (2)
127.29 (2)
131.6 (2)
180.0 (9)
117.5 (5)
117.5 (5)
180.0 (7)
K2Cr4O13
Mo Kꢀ radiation
Cell parameters from 3954
re¯ections
Mr = 494.20
Monoclinic, Pc
a = 8.6165 (2) A
ꢂ = 2.4±33.0ꢀ
Ê
Ê
Notes: (a) this work; (b) LoÈfgren (1973); (c) Kolitsch (2004); (d) Norby et al. (1991).
1
b = 7.4725 (1) A
ꢃ = 4.30 mm
T = 173 (2) K
Ê
c = 9.2811 (3) A
ꢁ = 92.746 (2)ꢀ
V = 596.89 (3) A
Rhomb, orange±red
0.06 Â 0.06 Â 0.04 mm
The authors thank Uwe Kolitsch for a constructive review
of an early draft of this paper.
3
Ê
Z = 2
Dx = 2.750 Mg m
3
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: BC1081). Services for accessing these data are
described at the back of the journal.
Data collection
Siemens SMART 1K CCD area-
detector diffractometer
! scans
Absorption correction: multi-scan
(SADABS; Sheldrick, 2002)
Tmin = 0.782, Tmax = 0.847
7800 measured re¯ections
4076 independent re¯ections
3268 re¯ections with I > 2ꢄ(I)
Rint = 0.045
ꢂmax = 33.0ꢀ
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Æ
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Acta Cryst. (2005). C61, i117±i119
Casari and Langer K2Cr4O13 i119