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Searched For and Found Analogies between Reduced Oxomolybdates and Cluster Compounds of Rare Earth Metals.

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Cluster Compounds
DOI: 10.1002/anie.200603169
Searched For and Found: Analogies between
Reduced Oxomolybdates and Cluster Compounds
of Rare Earth Metals**
Lorenz Kienle, Hansjrgen Mattausch, and
Arndt Simon*
The concept of cluster condensation[1] has proved valuable as
an ordering scheme for structures of metal-rich compounds,
and it has frequently opened new areas.[2] Reduced oxomolybdates illustrate the value of the concept particularly well.
In their structures discrete cluster anions [Mo4n+2O6n+4](n+3)
occur which are formed from n {Mo6O12} clusters linked
through opposite edges of the Mo6 octahedra. So far,
monomeric and oligomeric entities with n = 1–6[3] are
known as well as the polymeric Mo4O6 chain with n = 1.[4]
Oligomeric clusters occur in crystal structures which are
composed of a single type of cluster only, as well as of
different types of clusters in ordered[5] or disordered arrays.[6]
With rare earth metals (Ln), compounds containing
discrete [Ln6ZX12][7] and [Ln10(Z)2X18][8] clusters (X =
halide) have been known for a long time. These compounds
correspond to the initial members (n = 1 and 2, respectively)
of the molybdenum cluster series. Owing to the electron
deficiency of the rare earth metals, additional endohedral
atoms or groups of atoms (Z) are incorporated in the cluster
centers which (over)compensate for the lack of metal–metal
bonding by strong LnZ bonding.[2a, 9] The polymeric chain
Ln4ZX6 was also found repeatedly.[10] Recently, we characterized Ln14(C2)3I20 (Ln = La, Ce) as the first representatives
for n = 3.[11] The details of preparation of the ordered phases
are quite critical. Minute deviations from the optimized
reaction temperatures[12] lead to heterogeneous products
which exhibit characteristic disorder phenomena in the
crystals. The disorder is seen in single-crystal X-ray analyses,
and its nature becomes clear from investigations of the real
structure in the electron microscope.
The heterogeneous character of a sample of La14(C2)3I20
prepared at 900 8C shows up in the X-ray as well as the
electron diffraction pattern of a selected single crystal. The
observed reflections in the latter are shown and interpreted
(Figure 1) in terms of coherently intergrown domains of
La14(C2)3I20 and La10(C2)2I18 clusters that are arranged layerwise. Real structures of this type[13] have not been detected
before in cluster phases of rare earth metals; however, they
[*] Dr. L. Kienle, Dr. H. Mattausch, Prof. Dr. A. Simon
Max-Planck-Institut f6r Festk8rperforschung
Heisenbergstr. 1, 70569 Stuttgart (Germany)
Fax: (+ 49) 711-689-1642
[**] Thanks are due to Viola Duppel for electron microscopic investigations and to Roland Eger for numerous sample preparations.
Figure 1. Left: Electron diffraction pattern, zone axis [1̄10] referring to
n = 3; right: scheme of an intergrowth of La14(C2)3I20 (n = 3) and
La10(C2)2I15 (n = 2).
are reminiscent of a similar intergrowth with oxomolybdates,
for which larger clusters (n > 3) also frequently occur.[6]
A systematic search for such larger clusters with rare earth
metals finally was successful through concentration on
crystals which attracted attention owing to diffuse streaks in
their electron diffraction patterns. In Figure 2 simulated
contrasts of a model structure are shown which is composed
of cluster layers with n = 2, 3, and 4. There is a clear
correlation between the respective length of the cluster and
the corresponding contrasts at varying focus values. The
different unit cells for each layer are evident. In the Scherzer
focus[14] the distances between the elongated light spots
(parallelogram corners) are a measure of the cluster length.
Figure 3 presents the experimentally observed contrasts in a
disordered sample of mean composition Ce14(C2)3I20. These
contrasts can be associated with well-defined cluster sizes.
Whereas Figure 3 a reproduces layered domains of clusters
with n = 1, 2, and 3 which are well-known, though observed
for the first time in compounds of cerium, Figure 3 b contains
the images of significantly larger clusters (n = 8, 10).
Extended investigations of numerous crystals verify the
existence of an entire spectrum of clusters with n > 3.
The results described above establish surprising similarities in the structural chemistry of reduced oxomolybdates
and that of carbide halides of rare earth metals. However,
significant differences also exist. On the one hand, the cluster
sizes that are observed with the compounds of rare earth
metals exceed those of oxomolybdates, which does not seem
to be a principal difference but rather hints at experimental
deficiencies to be investigated further. On the other hand,
there is an essential difference. In the oxomolybdates the
clusters are anions and have to be interconnected in a definite
way to provide space for the cations, for example, in
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8036 –8038
Figure 2. Structure model with n = 2, 3, and 4; top: projection, zone
axis [1̄10] referring to n = 3; bottom: simulated micrograph
(Df = 55 nm, thickness: 3.5 nm).
Inn+1Mo4n+2O6n+4.[3–5] In contrast, the carbide halide clusters
are neutral entities, allowing for variable patterns of interconnection and resulting in three homologous series
(Ln4n+2ZnX6n+4[15, 10] , Ln4n+2ZnX6n+6,[8] and Ln4n+2ZnX5n+5[15]),
which are characterized by different kinds of bridging through
X atoms.[11] Representatives of all these series are known
particularly for n = 2 and as final members with n = 1. With
these findings, a wide-open field of systematic preparations
lies ahead.
Experimental Section
Heavily disordered real crystals of La14(C2)3I20 are obtained by
weighing La/LaI3/C in a ratio of 11:10:9 and annealing at 900 8C
(12 days).
Strongly disordered crystals of Ce14(C2)3I20 are obtained by
weighting Ce/CeI3/C in a ratio of 9:10:3 (900 8C, 21 days) with
Ce5C2I9 and Ce6C2I10 as by-products.[11]
Electron microscopy: HRTEM investigations were performed
with a Philips CM30 ST electron microscope (300 kV, LaB6 cathode).
Crystals of the moisture-sensitive compounds were ground under dry
argon and transferred to the microscope by using a self-constructed
tool (P. O. Jeitschko, A. Simon, R. Ramlau, H. Mattausch, Eur.
Microsc. Anal. 1997, 2, 21). For SAED investigations, a diaphragm
limited the electron diffraction to a selected circular area (diameter
100 nm). All high-resolution micrographs were filtered after Fourier
transformation by applying a suitable mask (software: Digital Micrograph 3.6.1, Gatan). The EMS program package (P. A. Stadelmann,
Ultramicroscopy 1987, 21, 131) served for the simulation of highAngew. Chem. Int. Ed. 2006, 45, 8036 –8038
Figure 3. High-resolution micrograph and structure schemes for intergrowth of layers with a) n = 1, 2, 3 and b) n = 3 and n = 8, 10 (zone
axis: [1̄10] referring to n = 3).
resolution micrographs (multislice procedure Cs = 1.15 mm, D =
7 nm, a = 1.2 mrad).
X-ray diffraction: Powders of the reaction products were
analyzed by X-ray diffraction (CuKa1, l = 1.54056 G, internal standard
Si with a = 5.43035 G, Fujifilm BAS-5000 image-plate system) by
means of a modified Guinier technique (A. Simon, J. Appl.
Crystallogr. 1970, 3, 11) to characterize the phases under investigation. Precession photographs were recorded for checking the quality
of single crystals.
Received: August 4, 2006
Published online: October 30, 2006
Keywords: cerium · cluster compounds · electron microscopy ·
lanthanum · rare earths
[1] A. Simon, Angew. Chem. 1981, 93, 23; Angew. Chem. Int. Ed.
Engl. 1981, 20, 1; A. Simon, Angew. Chem. 1988, 100, 163;
Angew. Chem. Int. Ed. Engl. 1988, 27, 159.
[2] a) A. Simon, H. Mattausch, G. J. Miller, W. Bauhofer, R. K.
Kremer in Handbook on the Physics and Chemistry of Rare
Earths, Vol. 15 (Eds.: K. A. Gschneidner, L. Eyring), Elsevier
Science, Amsterdam, 1991, p. 191; b) J. D. Corbett, J. Chem. Soc.
Dalton Trans. 1996, 575; c) G. Meyer, Chem. Rev. 1998, 98, 3295;
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
d) A. Simon, H. Mattausch, M. Ryazanov, R. K. Kremer, Z.
Anorg. Allg. Chem. 2006, 632, 919.
a) n = 1: S. J. Hibble, A. K. Cheetham, A. R. L. Bogle, H. R.
Wakerley, D. E. Cox, J. Am. Chem. Soc. 1988, 110, 3295; b) n = 2:
R. Dronskowski, A. Simon, Angew. Chem. 1989, 101, 775;
Angew. Chem. Int. Ed. Engl. 1989, 28, 758; c) n = 3: G. L.
Schimek, D. E. Nagaki, R. E. McCarley, Inorg. Chem. 1994, 33,
1259; d) n = 4: E. Fais, H. Borrmann, H. Mattausch, A. Simon, Z.
Anorg. Allg. Chem. 1995, 621, 1178; e) n = 5: R. Dronskowski,
H. Mattausch, A. Simon, Z. Anorg. Allg. Chem. 1993, 619, 1397.
C. C. Torardi, R. E. McCarley, J. Am. Chem. Soc. 1979, 101, 3963.
H. Mattausch, A. Simon, E.-M. Peters, Inorg. Chem. 1986, 25,
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491, 323.
S. Satpathy, O. K. Andersen, Inorg. Chem. 1985, 24, 2604.
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1986, 64, 331; b) for Gd4SiBr6 : H. Mattausch, A. Simon, Z.
Kristallogr. New Cryst. Struct. 2005, 220, 1.
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Kremer, Z. Anorg. Allg. Chem. 2006, 632, 1661.
Ordered crystals were only obtained in long-term-annealed
samples (T = 930 8C).
A. Magneli, Microsc. Microanal. Microstruct. 1990, 5/6, 299.
O. Scherzer, J. Appl. Phys. 1949, 20, 20.
H. Mattausch, E. Warkentin, O. Oeckler, A. Simon, Z. Anorg.
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2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 8036 –8038
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clusters, compounds, found, metali, searches, oxomolybdates, rare, reduced, analogies, earth
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