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From Oxides to Nitrides Recent Developments in the Structural Chemistry of AlkaliAlkaline Earth Metal Sub-Compounds.

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by incorporation of the new amino acid at the C-terminus of the
peptide chain, the peptide synthesis by the metal-based "artificial ribosome" occurs in the reverse direction.
German version: A n g i w Chem. 1996, 108. 1287- 1289
Keywords: bioorganornetallic chemistry = complexes with carbon hgands . peptides ruthenium compounds - sandwich
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1991. 30, 931 ~ 9 4 1 .
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and 1983.22.879-907; H. Brunner, B. Reiter, G. Riepl, C/ic.m Ber 1984. 117.
Chem. Cormnlm.
1330&1354;P. Kvintovics. B. R. James, B. Heil, J. Chem. SIC
1986, 1810-1811; G. Suss-Fink, T. Jenke, H. Heitz, M. A. Pellinghelli, A
Trippicchio, J Orgunornet. Clzem. 1989, 379, 31 1-323.
[5] R. M. M0riarty.Y.-Y Ku, U. S. Gill. J. Chem So1 Chiw Comnzrui. 1987,
[6] W S. Sheldrick, A. Gleichmann. J. Orgunonzer. ChiJm. 1994, 470. 183
171 A. J Gleichmann, J. M. Wolff, W. S. Sheldrick, J. Chiw SIIC.,Ddron Truns.
1995. 1549-1554.
181 J. M. Wolff, A. J Gleichmann. C. Schmidt, W. S. Sheldrick. J. inorg. Biochem.
1995, 59. 219.
[9] W. H. Soine. C. E. Guyer, F. F. Knapp, Jr.. J Med. Chrm. 1984. 27, 803
[lo] A. J. Pearson, K. Lee, J Org. Chem. 1994, 59, 2304-2313
[ l l ] Review: M. F. Semmelhack in Comprehensiw Organic $vnt/iiws, Vui. 4. (Eds.:
B M Trost, I . Fleming). Pergamon, Oxford. 1991. pp. 517-549.
[12] J. W. Janetka, D. H. Rich, J. Am. Chem. Soc. 1995. 117. 10585 - 10886.
[13] R . Krimer. M. Maurus. R Bergs, K. Polborn, K. Siinkel. B. Wagner. W. Beck.
Chmi. Ber. 1993. 126. 1969-1980.
[14] W Beck, R. Krimer, A n g m . Chem. 1991. 103, 1492-1493. .4ngw. Chem. i n / .
Ed. Engi. 1991. 30. 1467- 1468.
[18] M . Maurus, Dissertation, Universitit Munchen. 1994.
From Oxides to Nitrides: Recent Developments in the Structural Chemistry of
Alkali/Alkaline Earth Metal Sub-Compounds
Caroline Rohr*
Almost a hundred years after the discovery of the structurally
remarkable rubidium and cesium suboxides and over twenty
years since the elucidation of their structures and their thermoanalytical characterization by Simon,"] this structural chemistry has been extended to the corresponding ternary sodium barium nitrides in the last few years. The compound
Nai,Bai,CaN,[21 recently described by Simon and Steinbrenner forms a link between the new subnitrides and the classic
suboxides. With a degree of face-sharing of coordination octahedra about O / N never achieved before, the quaternary phase is
more than just a new final member in the series of "inverse
clusters'. of the sub-compounds; it is an important extension of
the structural chemistry of this class of compounds, since in this
case cations are also present in the saltlike interior of a cluster.
The chemical bonding and the characteristic building units of
the structures are similar in the Rb/Cs suboxides and the NaBa subnitrides. A remarkable and at the same time special feature of these compounds are clusters comprising face-shared,
X-centered octahedra (X = 0 , N ) of the electropositive partners
(A) rubidium and cesium, and barium, respectively. The bonds
within the octahedra are ionic, whereas between the clusters
coordination numbers and interatomic distances occur such as
those in metals. Rubidium and cesium as well as barium form
the "cluster skin" and act as a link between the ionic and the
metallic region. The metallic matrix and thus the composition of
the phases can be varied over a wide range in all systems. That
compounds with this kind of remarkable bonding also show
unusual physical properties (color, low electronic work function, etc.) is as unsurprising as the fact that many of the clusters
were detected, for example, also in molten metals.
A special position among the subnitrides is held by the alkaline earth metal nitrides A,N (A = Ca, Sr, Ba), which crystallize
in the anti-CdC1, type, and thus are the only sub-compounds to
exhibit N-centered alkaline earth metal octahedra that are connected not through faces but through six of the twelve edges of
the octahedron. All the other suboxides and subnitrides can be
arranged according to the increasing linkage of the A octahedra
(A = Rb, Cs, Ba, Ca) in the inverse cluster around the anions X
(X = 0'-, N3-) and to the corresponding decreasing ratio A / X
( A = number of corners of the octahedra, X = number of centers of octahedra; Table 1, Fig. 1). The series begins at the
maximum ratio A / X = 6 for discrete [NBa,] octahedra in
Na,,Ba,N,[31 which are arranged body-centered cubic and are
Dr. Ing. C. Rohr
Edu,ird-Zintl-ln\titut der Technische Hochschule, Anorganische Chemie I1
Hochschulstrdsse 10. D-64289 Darmstadt (Germany)
Fax: In1 code +(6151)166029
e-mail c;ii-oline ( t
It11 E d Engl. 1996, 35, N o . 11
Table 1. Summary o f the structural elements in suboxides and nitrides (A = Ba,
Rh, Cs; A' = Ca; X = 0. N : *: additional Na, Rb. or Cs atoms in the metallic
Structural element
Fig 1
discrete octahedra [XA,]
P[X,A,,] cluster
strand of octahedra :[XA,
,O[X,AA,,] cluster
from six octahedra [XAA,]
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mbH, 0.69451
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0570-0833/96/3511-1199$ I5.00+ 2 5 0
Fig. 2. Section of the crystal structure of the subnitride Na,,Ba,,CaN, (octahedra:
[NCaBa,], spheres: Na). which shows the cubic face-centered arrangement of the
clusters [Ba,,CaN,].
Fig 1. The clusters in the crystal structures of the suboxides and subnitrides
Na,,Ba,N (a). Rb,O, (b), Cs, ,O, ( c ) , Cs,O, NaBa,N, and Na,Ba,N (d) as well as
Na,,Ba,,CaN, (e). A : number of corners of octahedra, X:number of centers of
separated from one another by a large number of sodium atoms.
All R b suboxides show clusters [X,A,] comprising two facesharing octahedra, which in Rb,0,[4] are discrete and in
Rb,Oc5I are present together with “metallic” rubidium. Units
comprising three octahedra condensed through three common
faces with a common edge are prevalent in the Cs oxides. These
clusters [X3All] form the saltlike part in the crystals of
CS,O,[~]CS.,O,[~](in the latter two again together
with additional Cs that does not participate in the construction
of the octahedra). The linkage of octahedra each through two
opposite faces to give infinite strands occurs both in CS,O[’~]
(which has not been fully characterized by X-ray crystallography) and in NaBa,N[’] and Na,Ba,N.[l’lThe Na atoms in these
two compounds are located between the strands of octahedra.
With the synthesis of the quaternary nitride Na,,Ba,,CaN,
Simon and Steinbrenner have achieved the preparation of a
“heterocluster” containing both barium (at the cluster periphery) and calcium (in the center of the cluster) for the first time:[21
six octahedrally arranged octahedra [NBaBa,,,Ca,,,] are Paceshared in such a way that the calcium ion forms the common
corner of all six octahedra and thus in total eight common edges
between three octahedra as well as 24 common faces are present
per cluster-a cubic close packing with (distorted) octahedra!
Alternatively and directly related to the Rb/Cs suboxides, the
[Ba,,CaN,] cluster can also be derived from three [X,A,] o r
from two [X,A, ,] building units, which clearly shows that this
cluster is a new, (provisionally) final member in the series
of sub-clusters. As in the other Na-Ba subnitrides, in
Na,,Ba,,CaN, the clusters, which are arranged face-centered
cubic in the crystal, are separated from one another by Na
atoms (Fig. 2). If one considers the large variability of the contents A and the crystal structures, for example, of Cs suboxides
containing the same cluster, and the possibility of incorporating
different amounts of Na between the rigid one-dimensional infinite strands of octahedra in NaBa,N and Na,Ba,N, it appears
Verlugsgesellschafi mbH, 0-69451 Weinlieim, 1996
correct to assume the existence of further stoichiometric phases
of the composition Na,Ba,,CaN, (n =17. 21, 22).
A common feature of the suboxides and subnitrides are regions with ionic partial structures in a metal matrix, in which the
atoms of the “cluster skin” (for the nitrides to date only Ba and
for the oxides only Cs and Rb) play a “mediating role”, corresponding approximately to oxidation states between Bao and
Ba2+.The bonding becomes clear based on the A-A distances:
The edges of the octahedra within the cluster are short and the
A- A distances resemble those in ele~trovalent[*~
binary oxides
and nitrides; the A-A distances between the clusters are large
(interatomic distances as in the metals), and the lengths of the
edges of the [XA,] octahedra on the cluster skin vary with the
number of octahedra in which the respective edge participates.
That despite the severe face-sharing of octahedra the N - N
distances in Na,,Ba,,CaN, are even a little longer than those in
the ternary Na-Ba subnitrides is attributed to the displacement
of the X atoms from the centers of the A octahedra (away from
the common faces of the octahedra towards the cluster periphery) known for suboxide clusters. Consequently, the N - Ca distances are large and the N-Ba distances are small in comparison to the typical distances observed for ionic bonds.
The distribution of the electropositive bonding partners in the
crystal structure is interesting, particularly with regard to the
quaternary compound. All the results obtained so Par indicate
that only the large “soft” Ba can form the cluster skin. Although
Na and Ca have similar ionic and metallic radii as well as ionization energies, the distribution of elements determined follows
the formation of compounds in the binary phases: C a nitrides
are known, Na,N is unknown. However, binary intermetallic
phases are known in the Na-Ba system, but not for Ca-Ba. In
the [Ba,,CaN,] cluster Ca occupies the position (the common
corner of six coordination octahedra) with the highest positive
charge. A similar situation occurs in the mixed Rb-Cs suboxide
Rb,Cs,O, ,[l in whose structure the lighter homologue prefer[*] Compounds are referred to a s electrovalent when the valence electrons. in accord with common assumptions. can he attributed unequivocally to specific
$ /5.00+ .25/0
A i i p r . (%mi.
hi.E d Eiigl. 1996, 35. No.
ably substitutes the Cs atoms of the common edge of all three
[OCs,] octahedra of the [Os,Cs,,] cluster.
The nature of the structural units formed in each of the subcompounds is evidently primarily determined by types of atoms
that make up the cluster: [03Cs,,] predominates in the Cs oxides, dimeric [X,A,] units are known only in the R b - 0 system,
and infinite chains of octahedra ;[NBa,,,] are the characteristic
building unit in the Na-Ba nitrides. Na14Ba14CaN6represents
a remarkable subnitride of the Na-Ca-Ba-N system: It contains
a new cluster, in which the calcium ion plays an important dual
role with respect to the stability both for geometrical reasons
(distortion of the octahedra for the extreme face-sharing) and
for electronic reasons (first example of a “real” cation (Ca2+)in
the saltlike interior of a sub-cluster).
German version: Angeir. Chem 1996, 108, 1289-1291
Keywords: clusters
- subnitrides . suboxides
[l] A. Simon. Sfrucf.Bonding (Berlin) 1979, 36, 81.
121 U. Steinbrenner. A. Simon, Angew. Chem. 1996,108, 595-597; Angeir. Chem
I n f . Ed. Engl. 1996, 35, 552-554.
[3] G. J. Snyder, A. Simon, A n g e w Chem. 1994,106.713-715. .Ingen. Chem. In[.
Ed. Engl 1994, 33, 689-691.
[4] A. Simon. Nufurwissenschaften 1971, 58. 623.
[5] A. Simon. Nufurn.rssensch&n 1971, 58, 623.
[6] A. Simon, Nufurwissenschaften 1971, 58. 622-623.
[7] A. Simon, E Westerbeck, Angew. Chem. 1972,84, 1190-1 191; Angen.. Chem.
I n f . Ed. Engl. 1972, 11, 1105-1106.
[8] H. Deiseroth, E. Westerbeck. B. Hillenkoetter, Z . Anorg Allg. Chem. 1976,
423. 203-211.
[9] P. E. Rauch, A. Simon, Angew. Chem. 1992, 104, 1505-1506; Angew. Chem.
I n [ . Ed Engl. 1992, 31, 1519-1521.
[lo] K. R. Tsdi. P. M. Harris, E. N. Lassettre, J. Phys. Chem. 1956, 60, 345341.
[ l l ] G. J. Snyder, A. Simon, J. Am. Chem. Soc. 1995, 117, 1996 -1999.
[12] H. J. Deiseroth, A. Simon. Rev. Chim. Miner. 1983, 20, 475-487.
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development, chemistry, recen, structure, oxide, compounds, metali, alkalialkaline, nitride, sub, earth
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