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Suboxides with Complex Anions The Suboxoindate Cs9InO4.

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DOI: 10.1002/anie.200805736
Complex Suboxides
Suboxides with Complex Anions: The Suboxoindate Cs9InO4**
Constantin Hoch,* Johannes Bender, and Arndt Simon
The crystal structures of rubidium and cesium suboxides
exhibit a peculiar spatial separation of ionic and metallic
bonding.[1] This results in interesting physical properties, such
as very low values of work function and photoemission in the
infrared spectral range.[2] In recent years, numerous subnitrides of alkaline earth metals have been discovered that
follow this general structural principle, and they frequently
contain additional alkali metals.[3]
To date, only such compounds with discrete O2 and N3
anions were known. Herein we report on the synthesis and
characterization of a cesium suboxoindate, Cs9InO4, the first
suboxide containing a complex anion.
Cs9InO4 was first found in attempts to reduce Cs2O and
In2O3 at 250 8C with cesium metal. The aim was the formation
of indide?indates as a special case of mixed-valence compounds that contain indium in both negative and positive
oxidation states, and should exhibit structural elements of
intermetallic indides as well as oxoindates in the sense of
intergrowth structures. Corresponding compounds are known
from systematic studies in the system alkali metal?tetrel?
Finally we have succeeded in preparing pure Cs9InO4 in
gram amounts from stoichiometric mixtures of Cs, Cs2O, and
In2O3. It forms large violet-bronze tetragonal prisms that are
extremely sensitive towards moisture. Its crystal structure and
chemical bonding can best be described by comparison with
the subnitrides NaxBa3N. Their crystal structures contain
Ba6N octahedra that are linked to Ba N chains via opposite
faces. The compound Ba3N is built from those chains only,[3a]
with ionic bonding within and metallic bonding between the
chains, according to (Ba2+)3N3 �e . In NaBa3N[3b] and
Na5Ba3N,[3c] the metallic part of the structure is expanded by
insertion of sodium atoms. Projections of the structures of
NaBa3N and Cs9InO4 along [001] are compared in Figure 1.
Substituting the N3 ions by the much larger InO45 ions
results in a larger coordination polyhedron, which has the
shape of an elongated four-capped Cs12 cube (or strongly
distorted cuboctahedron). Such [Cs12InO4] entities, shown in
Figure 2 a, are linked by the non-capped faces to form
[*] Dr. C. Hoch, Prof. Dr. A. Simon
Max-Planck-Institut fr Festkrperforschung
Heisenbergstrasse 1, 70569 Stuttgart (Germany)
Fax: (+ 49) 711-689-1091
J. Bender
Institut fr Anorganische Chemie, Universitt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
[**] We thank R. Eger for differential thermoanalyses and F. Kgel for
synthesizing cesium oxide.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 2415 ?2417
Figure 1. Projections of the crystal structures along [001] of a) NaBa3N
and b) Cs9InO4. Cs atoms: light and dark blue; N atoms (a) and
In atoms (b): green; O atoms (b): red; hexagonal and tetragonal unit
cells, respectively, are outlined.
[Cs Cs4InO4] columns (Figure 3). As the charge balance
(Cs+)8(InO4)5 �e indicates, the columns are linked by
metallic bonding, giving space for an additional cesium
atom per formula unit, which has no contact to the anions.
The shape of the orthoindate(III) anion deviates only
slightly from ideal tetrahedral geometry (105.9(3)8 a(OIn-O) 111.3(1)8), with In O bond lengths d(In-O) =
206.8(5) pm, corresponding to those in known orthoindates(III) (d(In-O) = 205.6?207.5 pm in a- and 205.5?211.3 pm in
b-Na5InO4, 204.4?209.0 pm in K5InO4, and 206.3?209.9 pm in
Cs2Na3InO4).[6] As in many other [MO4]n -containing compounds,[7] the oxygen atoms are coordinated in a distorted
octahedral environment by five cesium and one indium atom.
Both Cs Cs and Cs O distances within the [Cs8InO4]
columns indicate mainly ionic bonding in this region
(d(Cs-Cs) = 405.5(1)?424.1(1) pm,
d(Cs-O) = 281.7(5)?
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. a) The InO45 ion in Cs9InO4 (gray tetrahedron; In: green),
with coordination of the oxygen atoms (red) by 12 cesium atoms
(blue). b) Cesium atom Cs(4) (dark blue), in a distorted cuboctahedral
environment formed by 12 cesium atoms (light blue). Ellipsoids are
set at 80 % probability level.
Figure 3. The crystal structure of Cs9InO4. The [Cs12InO4] units from
Figure 2 a are connected to columns (green) along the c direction.
They alternate with columns of distorted cuboctahedra (blue) surrounding Cs(4) that differ from those in Figure 2 b by 908 rotation.
311.9(4) pm). In contrast, the cesium contacts between two
adjacent [Cs8InO4] columns (500.1(1)?556.1(1) pm) lie in a
range corresponding to elemental cesium (d(Cs-Cs) 525 pm at
78 K).[8] The longest Cs Cs distances, ranging from 531.3(1)
to 578.4(1) pm, are found for the coordination of Cs(4), which
is surrounded by only cesium atoms. Very similar distances
are observed for atoms in the metallic regions of cesium
suboxides: for example, 545.0(6) to 613.3(6) pm for the
corresponding contacts in Cs7O.[9b]
Therefore, the bonding in the [Cs8InO4] columns must be
described as ionic inside and metallic outside. As expected,
Cs9InO4 exhibits metallic conductivity (s = 8.1 104 W 1 cm 1
at 273 K and 6.0 105 W 1 cm 1 at 7 K), which is of the same
order of magnitude as for metallic cesium (s0 = 5.3 104 W 1 cm 1)[9a] and cesium suboxides, such as Cs7O (s =
2.0 104 W 1 cm 1).[9b]
Orthometallates of small and highly charged anions
together with large cations of low charge (for example
cesium orthogermanate(IV) or cesium orthoindate(III)) are
scarce, and their existence is rather uncommon in terms of
Paulings second rule for ionic crystals. However, such
structures can be stabilized by the insertion of substructures.
In the case of Cs10(GeO4)2O,[11] it is a discrete oxide ion
linking [Cs12GeO4] clusters. Herein, the stabilization is
realized by a metallic substructure.
Further systematic investigations showed that, in a synthesis starting from a 1:1 ratio of cesium and rubidium, only
Cs(4), the cesium atom without any contact to the anion, can
be substituted by rubidium. The observed maximal degree of
substitution ranges from x = 0.87 in (RbxCs1 x)Cs8InO4 to x =
0.57 in the isotypic compound (RbxCs1 x)Cs8GaO4.[12] Thus,
these phases mimic the behavior of Cs4O, which can be
formulated as Cs[Cs11O3]. The single cesium atom can be
substituted by rubidium, however, changing the structure for
the ternary suboxide RbCs11O3.[13] The formation of Rb9InO4
has not been observed to date.
Suboxides containing a complex anion are not confined to
substitutions in the alkali metal sublattice. In fact, there is also
the possibility of varying the oxometallate anions. The
Cs9InO4 structure type reported herein enables numerous
substitutions of this type, which is demonstrated by the
syntheses of compounds Cs9MO4 with M = Al, Ga, Fe, Sc and
the mixed crystals Cs9M1xM21 xO4 with M1 = In, M2 = Sc (x =
0.67) and with M1 = In, M2 = Al (x = 0.50).[12] The latter two
phases contain statistical mixtures of two different oxometallate anions. Further investigations are needed to show
whether the [Cs12GeO4] clusters in the above-mentioned
compound Cs10(GeO4)2O allow stabilization not only by
oxide anions but also by additional metallic bonding.
The binary subnitride Ba3N can be obtained by distilling
off the metallically bound sodium from NaBa3N in vacuum.[3a]
First attempts to transform Cs9InO4 into a hypothetical
suboxoindate with lower alkaline metal content, such as
Cs8InO4, only resulted in the formation of the oxoindates
Cs6In2O6 and Cs8In2O7,[12] together with Cs2O. Experiments to
enlarge the metallic substructure in analogy to Na5Ba3N[3c] by
reaction with excess cesium also have failed. However,
considering the large number of suboxides and subnitrides
of the Group 1 and 2 metals, it can be assumed that there is a
good chance to extend the chemistry of these novel compounds.
Experimental Section
Pure-phase Cs9InO4 is obtained by heating stoichiometric mixtures of
In2O3, Cs2O, and Cs; e.g. 107.9 mg (0.3886 mmol) In2O3, 547.8 mg
2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 2415 ?2417
(1.9438 mmol) Cs2O, and 413.4 mg (3.1104 mmol) Cs, in tantalum
crucibles under argon to 250 8C. EDX analyses on several crystals
result in a ratio In/Cs = 1:8.9(2). According to DTA analyses, Cs9InO4
melts congruently at 223(4) 8C. The room-temperature lattice constants given in Ref. [5] result from powder diffractometry and
Rietveld refinement. Details of the synthesis, data collection,
structure solution, and refinement along with tables of bond lengths
and angles are contained in the Supporting Information.
Received: November 25, 2008
Published online: February 13, 2009
Keywords: indates � metal?metal bonding �
solid-state structures � suboxides
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[5] Crystal structure data for Cs9InO4 at T = 233.0(2) K: tetragonal,
space group I4/mcm (No. 140), a = 15.517(2) , c = 12.900(2) ,
V = 3106.0(7) 3 , lattice constants at room temperature: a =
15.4968(9) , c = 12.8754(5) (Rietveld refinement).[15] Z = 4,
1calc = 2.940 g cm 3 ; diffractometer: STOE IPDS 1 (AgKa radiation, graphite monochromator); m(AgKa) = 5.897; 2#max = 458;
Angew. Chem. Int. Ed. 2009, 48, 2415 ?2417
11 744 observed intensities, 1086 unique; Lorentz, polarization,
and numerical absorption correction;[15] least-squares refinement (all atoms anisotropic) with SHELXL-97;[15] 25 free
variables; GOF = 1.139; R values (I 2s(I)): R1 = 0.0287,
wR2 = 0.0741; max./min. residual electron density: 0.716/
0.939 e 3. Further details of the crystal structure investigations
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76344 Eggenstein-Leopoldshafen, Germany (fax: (+ 49) 7247808-666; e-mail:, on quoting the
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suboxoindate, complex, anion, suboxide, cs9ino4
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