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LiCaN and Li4SrN2 Derivatives of the Fluorite and Lithium Nitride Structures.

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LiCaN and Li,SrN,, Derivatives of the
Fluorite and Lithium Nitride Structures**
By Gerhard Cordier, Axel Gudat, Riidiger Kniep,
and Albrecht Rabenau *
Dedicated to Robert Juza on the occasion of his 85th birthday
Ternary nitrides"] preferentially crystallize in the fluorite
structure when the ionic radius ratio('] r(Li@)/r(M"@)2 1.
The cations occupy the tetrahedral holes of the approximately cubic close-packed matrix of nitride ions either statistically (LiMgN13]) or fully ordered (L~ZIIN,'~]
Li3GaN,,14] Li,GeN, ,Is1Li,TiN,,lsJ Li,VN,,[61 and
Li,MnN,"'). The recently determined crystal structures of
Li,FeN,, Li,,Cr,N,, and Li,(M)N, (M =Cr, Mo, W)"' are
also consistent with this generalization. Within the alkaline
earth series of compounds, the only member other than
LiMgN (r(Li@)/r(Mg'@>= 1.0) whose crystal structure has
been reported is LiSrN.['] LiSrN (r(Li@)/r(Sr'@) < 1) crystallizes in the YCoC structure,"'] which is energetically more
favorable than a hypothetical fluorite structure.["] The homologous compound LiCaN['*] offers the possibility to investigate a crystal-chemical link between the LiMgN and
LiSrN structures. Moreover, a Li-rich phase of the ternary
Li-Sr-N system (Li,SrN,) has been prepared and its crystal
structure determined. The structure bears a close relationship to the Li,N structure.[131Intermediate phase^^'^*'^^
were observed from the systems Li-Be-N and Li-Ba-N, but
no suitable single crystals have so far been obtained.
LiCaN was prepared from the elements. In order to obtain
single crystals suitable for structural analysis a tantalum
crucible was charged with a mixture of lithium and calcium
(molar ratio 6: 1) and fused under argon at 250 "C. The temperature was subsequently increased, under 1 atm nitrogen,
to 850 "C. LiCaN is obtained (along with Li3N) as orangecolored single crystals with a plate-like habit. Single phase,
polycrystalline samples of Li,SrN, (dark metallic luster)
were prepared by reaction of lithium nitride with strontium
(molar ratio Li:Sr = 4:l) under nitrogen (1 atm). The melt
was cooled from 700 "C to room temperature over a 12 hour
The crystal structure of LiCaN[161(Fig. 1) is directly related to the fluorite structure. The calcium atoms occupy half
the tetrahedral holes whereas the lithium atoms are displaced
from the tetrahedral center positions of the approximately
cubic close-packed nitrogen matrix, being only 0.1 35 A from
a tetrahedral face but 3.570(8) A from the fourth nitrogen
atom. Lithium atoms are thus coordinated to three nitrogen
atoms (Li-N: 2.071(4)-2.108(8) A), a situation similar to
that found in the crystal structure of Li,N['31 where Li
atoms are coordinated in a planar arrangement within the
Li,N layers (Li-N: 2.130(1) A). As a result of the special
distribution of the cations (Fig. 1) infinite a[LiN,,,] bands
occur, which run in the [OIO] direction. These are one dimensional sections from the Li,N layers in Li,N['31 which,
through nitrogen atoms shared with CaN, tetrahedra
(Ca-N: 2.436(4)- 2.472(2) A; N-Ca-N: 98.6(1)- 120.1(I)'),
become interconnected into a three-dimensional structural
[*] Prof. Dr. A. Rabendu, Dr. G. Cordier ['I, DipLChem. A. Gudat,
Prof. Dr. R. Kniep ['I
Max-Planck-Institut fur Festkorperforschung
Heisenbergstrasse 1, D-7000 Stuttgart 80 (FRG)
['I Permanent Address:
Eduard-Zintl-Institut der Technischen Hochschule
Hochschulstrasse 10, D-6100 Darmstadt (FRG)
[**I This work was supported by the Fonds der Chemischen Industrie and the
Freunde der TH Darmstadt.
Q VCH Verlagsgesellschajl mbH. 0-6940 Weinheim, 1989
Fig. 1. Section from the crystal structure of LiCaN. Schematic representation:
cubic face centered arrangement of nitrogen atoms; calc~umin tetrahedral
coordination; lithium displaced from the center of the tetrahedral hole to the
tetrahedral face (LiN bands parallel to [OlO],,,,,); pentagonal bipyramiddk coordination of the nitrogen atoms.
Fig. 2. Schematic representation of the coordination polyhedra (hexagondl
and pentagonal bipyramids) around the nitrogen atoms in the structure of Li,N
(top, (0011projection) 1131 and Li,SrN, (bottom, [loo] projection). In the second case, groups of layers are present in which the axes of pentagonal bipyramids lie either in the plane of the diagram (open polyhedra) or perpendicular to
it (shaded polyhedra).
0570-0833/89/1212-1702 $02.50/0
Angew. Chem. In!. Ed. Engl. 28 (1989) No. 12
arrangement. The nitrogen atoms are coordinated in the
form of a distorted pentagonal bipyramid, axially by two
calcium atoms and equatorially by two calcium and three
lithium atoms. The Ca,,-N-Ca,,
angle is nearly linear at
177.5(3)"; the Ca,,-N-(Ca, Li)eqangles lie between 81.5(2)"
and 96.9(2)". In the equatorial plane the angle subtended at
nitrogen between two calcium atoms is 94.7(1)"; that between two lithium atoms is 62.0(2)".
In the crystal structure of Li4SrN,1'61(bottom of Fig. 2)
the N atoms are also coordinated within distorted pentagonal bipyramids, in this case involving 2 x Li,, and 2 x
Sreq+ 3 x Li,,. The angular relationships are comparable to
those in the calcium compound (vide supra): LiaX-N-Li,,
174.4(3)"; Liax-N-(Sr,Li)eq 87.2(2)"-92.0(2)"; Sreq-N-Sreq
92.4(2)"; Lieq-N-Lieq 62.8(2)". As implied in Figure 2 (bottom), the Li,SrN, structure can be derived directly from the
structure of Li,N (Fig. 2, top)['31. The three-dimensional
framework of hexagonal bipyramids interconnected through
equatorial edges and axial vertices in Li,N (NLi,,zLi,,,) is
cut into layers which in Li,SrN, (NLiZ/,Li3,,Sr,/,) are in turn
made up of pentagonal bipyramids. The layers run along
[OOl]; each is turned 90" relative to the next, and they are
connected to one another through shared strontium atoms.
Strontium is therefore coordinated to four nitrogen atoms in
a distorted tetrahedral geometry (Sr-N: 2.648(3) b;; N-SrN: 92.4(2)"- 118.6(3)"). The Li-N bond lengths within the
layers (Liax-N: 1.913(2) A, Lie,-N: 2.112(2)-2.149(2) A) are
just as graded as those in the structure of Li,N113](Liax-N:
1.938 A, Lie,-N: 2.130(1). Impedance measurements show
Li,SrN, to be an ion conductor (activation energy 0.9(5) eV)
but with a considerably lower ionic conductivity than
Li,N'"'. At 430 "C the ionic conductivity of Li,SrN, is still
an order of magnitude lower than that of pure Li3N measured at room temperature.
Received: May 26,1989 [ Z 3359 IE]
German version: Angew. Chem. 101 (19S9) 1689
[l] R. Juza, K. Langer, K. von Benda, Angew. Chem. 80 (1968) 373; Angew.
Chem. Int. Ed. Engl. 7 (1968) 360.
121 R. D. Shannon, Aria Crystullogr. A 3 2 (1976) 751.
[3] R. Juza, F. Hund, Z. Anorg. Allg. Chem. 257 (1948) 1.
[4] R. Juza, F. Hund, Z. Anorg. Allg. Chem. 257 (1948) 13.
[ S ] R. Juza, H. H. Weber, E. Meyer-Simon, Z. Anorg. Allg. Chem. 273 (1953)
[6] R. Juza, W. Gieren, J. Haug, Z. Anorg. Allg. Chem. 300 (1959) 61.
171 R. Juza, E. Anschiitz. H. Puff, Angew. Chem. 71 (1959) 161.
[S] A. Gudat, R. Kniep, A. Rabenau, German Chem. Soc., 5th Annu. Conf.
Solid State Chem., Erlangen, September 28-30, 1988,
[9] G. Cordier, A. Gudat, R. Kniep, A. Rabenau, Angew. Chem. fOt (1989)
204; Angew. Chem. Int. Ed. Engl. 28 (1989) 201.
[lo] M. H. Gerss, W. Jeitschko, 2. Naturforsch. 8 4 1 (1986) 946.
[ l l ] R. Nesper, Z . Kristnllogr. manuscript in preparation. The observed distance d(Sr-N) = 2.6 A leads to a decidedly larger volume for the fluorite
structure type than for the YCoC structure type. For d(Sr-N) > 2.44 A,
the fluorite structure type will be energetically less favorable than the
[l2] J. Aubry, M. Fromont, R. Streiff, C.R. Hebd. Seances Acad. Sci. Ser. C262
(1966) 1785.
1131 A. Rabenau, H. Schulz, J. Less Common M e / . 50 (1976) 155.
[14] J:F. Brice, J.-P. Motte, R. Streiff, C.R. Hebd. SiancesAcad. Sci. Ser. C269
(1969) 910.
1151 J:F. Brice, J. Aubry, C.R. Hebd. Seances Acad. Sci. Ser. C271 (1970) 825.
[16] Crystallographic data: LiCaN: orthorhombic, Pnma. a = 8.471(3), b =
3.676(2), c = 5.537(3) A. Z = 4 ; 670 measured, 268 independent reflections; R = 0.034. Li,SrN,: tetragonal, I4,/amd, a = 3.822(2), c =
27.042(9) A, Z = 4; 1192 measured, 188 independent reflections; R =
0.045. Phillips PW-1100 single crystal diffractometer, Mo,,. 300 K. Further details of the crystal structure investigation are available on request
from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG) hy quoting the depository number CSD 54206, the names
of the authors, and the journal citation.
(171 A. Rabenau, Solid Stale Ionics 6 (1982) 277.
Angen'. Chem. In!. Ed. Engi. 28 (1989) No. 12
Chirality and Isomerism in
Binuclear Iron Complexes with Sulfur Ligands:
[Fe(CO)(p-"S,")]z., a Model Complex
for Oxidoreductases **
By Dieter Sellrnann,* Robert Weiss, and Falk Knoch
Numerous oxidoreductases, including nitrogenases and
hydrogenases, have an active center consisting of iron atoms
in a coordination sphere of sulfur atoms. However, much
uncertainty remains about the precise structures of such
multinuclear centers and the molecular processes associated
with the corresponding catalytic reactions." Previously described model compounds of these centers are all only of the
structural type; i.e., they do incorporate iron atoms and
sulfido or thiolato ligands,"bl are usually highly symmetric,
but fail to react with such relevant substrates as CO, N,, or
H,. What has generally not been considered with respect to
model complexes is the fact that the metal centers in oxidoreductases are always chirotopic as a consequence of bonding to the protein shell.1z1This generalization applies even
when the catalyzed reaction involves only achiral substrates
or, as in the case of the iron centers in ferredoxins, when the
iron centers are constituents of highly symmetrical subunits
such as [Fe,S,(SR),] clusters.[1a1Metallic centers in oxidoreductases must also provide free or potentially free coordination sites for the attachment of substrates.
In our search for model complexes of the active centers in
nitrogenases and hydrogenases we have attempted to elaborate the iron complex 1 in such a way as to create multicentered chiral units with free or potentially free coordination
sites at the Fe centers. Since 1 itself is chiral, it was of particular interest to ascertain what diastereomers would form in
the course of a dimerization. We now report the isolation and
structural characterization of the binuclear complex
which represents a new type of chiral complex. 2 contains
two homochiral fragments with low-valent Fe centers that
bind 0'-n ligands, and it is formed upon dimerization of
racemic precursor complexes according to equation (a).'"]
Theoretically, the dimerization of [Fe(CO)("S,")] fragments might lead to ten diastereomers (Scheme 1, I-X). This
assertion is based on the assumption of configurational stability at the Fe centers, however, since otherwise the number
of diastereomers would be even larger (e.g., isomers with
planar orientation of the sulfur ligands). Molecular models
reveal that the conformation of the C,H, bridge in 1 is fixed
due to the rigid C,H,S, units (i.e., "inversion" of the bridge
would lead to serious distortion of the angles at the Fe atom).
Thus, dimerization can only lead to RR, RS and SS isom e r ~ . [In
~ ]contrast to previously known binuclear complexes with chiral fragments,r61each of these combinations in 2 is
in principle the source of four isomers. Members of two of
the resulting ''pairs'' are in fact identical, so the overall result
Prof. Dr. D. Sellmann, Dip1.-Chem. R. Weiss, Dr. F. Knoch
Institut fur Anorganische Chemie der Universitat Erlangen-Niirnberg
Egerlandstrasse 1, D-8520 Erlangen (FRG)
(''S,")20 = 2,2'-(ethylenedithio)dibenzenethiolate. Transition Metal
Complexes with Sulfur Ligands, Part 51. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Part 50: D. Sellmann, 0. Kappler, F. Knoch, M. Moll, 2. Narurforsch., in press.
0 VCH VerlagsgesellschuftmhH. 0-6940
Wernherm. 1989
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structure, fluorite, nitride, licam, derivatives, lithium, li4srn2
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