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New Impetus for Inorganic Host-Guest Chemistry.

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when the preferred orientation of the reaction partners with
their C=C, C-C, and C-H bonds in the inter- or intramolecular reactions are taken into consideration.
German version: Anpew. Chem. 1992, 104%I208
[ f ] F. A. Cotton, R. A. Walton, Muliiple Bonds Between Meiaf Atoms, Wiley,
New York, 1982.
[2] G . A. Rempel, P. Legzdins, H. Smith, G. Wilkinson, fnorg. Synrh. 1972,
13, 90.
[3] M. P. Doyle, Chenr. Rev. 1986, 86, 919.
[4] M. P. Doyle, Ace. Chem. Res. 1986, f9, 348.
[S] G. Maas, Top. Cum. Chem. 1987, 137, 75.
161 M . P. Doyle, R e d . Trov. Chim. Pqs-Bas 1991, f10,305
(71 P. A. Agaskar, F. A. Cotton, L. R . Falvello, S. Hahn, J. Am. Chem. SOC.
1986, 108, 1214.
181 H . Brunner, H. Kluschanzoff, K. Wutz, BUN.SOC.Chim. Belg. 1989,98,63.
[9] D Ark, M. Jautelat, R. Lantzsch, Angew. Chem. 1981. 93, 719, A n g w
Chem. Inr. Ed. Engl. 1981, 20, 703.
[lo] H. Nozaki, S. Moriuti, H. Takaya, R. Noyori, Tefrahedron Leti 1966,
Ill] T. Aratani, Pure Appl. Chem. 1985, 57, 1839.
[12) H. Fntschi, U . Leutenegger, A. Pfaltz, A n g a t . Chem. 1986, 98. 10ZX:
Angew. Ckenz. h i . Ed. Engl. 1986.8, 1'405.
1131 a) R. E. Lowenthal, A. Abiko, S. Masamune. Tetrahedron Leu. 1990,31,
6005; b) D. A. Evans, K. A. Woerpel, M. M. Hinman, J. Am. C h m Soc.
1991, 113, 726.
[14] Short review: C. Bolm, Angew Ckern, 1991, 103, 556; A n g m . Chrm. liar.
Ed. Engl. 1991, 30, 542.
[lS] M . P. Doyle, B. D. Brandes, A. P. Kazala, R. J. Pieters. M . B. Jarstfer.
L. M. Watkins, C. T. Eagle, Terrahedron L e t / . 1990. 31, 6613.
(161 M. P. Doyle. R. J. Pieters, S. F. Martin, R . E. Austin, C J. Oalmann, P.
Miiller, J. Am. C h ~ mSor.
1991, 113, 1423.
1171 M. N.Protopopova, M . P. Doyle, P. Muller. D. Ene, J. An?. Chem So?.
1992, 114, 2755.
[la] M. P. Doyle, A. v. Oeveren, L. 1. Westrum, M. N. Protopopova, T W.
Clayton. Jr., J. Am. Chem. SOC.1991, f13, 8982.
New Impetus for Inorganic Host-Guest Chemistry
By Hans ReuteP
Exactly 25 years ago the probably coincidental dislodging
of a small stone-the synthesis of the first crown ether and
the discovery of its coordinating properties"I-started
avalanche-supramolecular organic chemistry. The rapid
development of this field, which reached an all-time high in
1987 with the award of the Nobel Prize in Chemistry to C. J.
Pedersen, J.-M. Lehn, and J. Cram, rests on the fact that
through it abiotic counterparts to biological processes, for
example enzyme catalysis, can be studied. It is therefore not
surprising that here a very close interaction between organic
chemistry and biochemistry exists and that receptor-substrate interactions are fundamental in supramolecular chemistry.'']
Under the term supramolecular compounds, as portrayed
graphically in Scheme 1, is meant aggregates of two or more
different building blocks, stable in solution as well as in the
solid state, in which the parts themselves are already compounds with defined physical and chemical properties. Characteristic of this type of host-guest relationship is, first, that
the bonding interactions (hydrogen bonds, van der Waals
forces) between the partners are so weak that the supermolecule can also dissociate reversibly into its starting compounds again under specific conditions, and second, that the
host has a structure that enables It to enclose the guest selectively and securely.
On this point inorganic chemistry need not be modest,
since after all, if has dealt with host-guest compounds for
more than LSO years. Already in 1826 J. Berzeliusr3I described the transformation of ammonium molybdate (1)
with phosphoric acid (2), formulated in Equation (a), in
which he obtained a yellow precipitate. Today we know that
it must have been the compound 3, the ammonium salt of
12-molybdatophosphoric acid. The anion of 3 consists of a
[*] Dr. Hans Reuter
Institut fur Anorganische Chernie der Universitat
Gerhard-Domagk-Strasse 1, D-W-5300 Bonn (FRG)
Angew. Chem. In(. Ed. Engl. 1992, 31, N o 9
in solid state
Scheme 1
shell of twelve {MOO,) octahedrons linked together according to the principle of the a-Keggin structural type, leaving
a hole in the middle that accommodates a single phosphorus
+ H,PO,
( N ~ , ) , ~ ( P 0 4 ) M o , , ~ ,i, ] 21 NH,
+ 12H,O
A significant difference between such a compound and the
typical supramolecular compounds of organic chemistry,
however, lies in the special position of the oxygen atoms of
the phosphate group of this heteropolyanion, which are
simultaneously components of the host compound. Thus
according to the narrow definition of organic chemistry, 3 is
strictly speaking not a supramolecular compound. In this
case one prefers to call it a molecular
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This rigid distinction is, however, only of secondary importance for the compound class of the heteropolyacids described here,[51especially since depending on the pH of the
solution, the compounds also dissociate again into the starting compounds. The dissociation shows very clearly the influence of the solvent on the formation of such compounds,
since-in the sense of the above-mentioned definition-the
solvent itself enters more or less strongly into a supramolecular interaction with the two starting materials and thereby
influences their reactivity and availability decisively.
Furthermore, this example depicts very clearly another
aspect of inorganic host-guest chemistry. The structure of
the heteropolyanion can only be explained if the host is built
up around the guest in the condensation reaction in solution,
a phenomenon known as the template effect.
If in a limiting case the interactions between the units in a
host-guest complex are extremely weak, the supercomplex
may be stable in the crystalline state but dissociate to its
individual components in solution, because these enter into
significantly stronger interactions with the solvent. Such
host-guest complexes existing only in the solid state are
termed inclusion compounds or clathrates. A known example from inorganic chemistry is the chlorohydrate
C1;7.27 H,O, described in many
Although the field of inclusion compounds today is mainly the domain of organic chemistry, there have recently been
interesting developments in inorganic chemistry. By far the
most remarkable discovery was certainly the compound
in which the Br, dumbbells are only marginally
extended and embedded in a tetragonally distorted CsF arrangement of the NaCl structural type.
Another example of recent developments in inorganic
clathrates is the compound Cd(CN), .neopentane (4).[81In
contrast to the layered structure of Ni(CN),NH,, benzeneJgl
the very first structurally elucidated inclusion compound,
the host lattice of [Cd(CN),] in 4 is based on a spatial network structure (Fig. 1 ) in which structural elements such as
those in SiO, and H,O modifications are present. Cadmium
no longer constitutes the host; instead the inorganic compound does.
The structural analogy of the two clathrate types introduced here to other AB and AB, compounds allows the
imagination free rein and now, since this conceptual hurdle
has been crossed, the discovery of many original examples of
such inorganic host-guest compounds can be predicted.
In contrast to these clathrates, "true" supramolecular inorganic compounds remained unknown for a long time.
However, many years ago evidence was found that the
large isopolyvanadates can possibly enclose water molecules
in the interior of the vanadium-oxygen cage. Nevertheless,
it came as a great surprise when Day et al. first showed
that these compounds also incorporate organic solvent
molecules. In the compound (Ph,P),[(CH,CN) c V,,O,,] .
3 CH3CN.4H,0 (5) prepared and studied by X-ray diffraction, the water molecules and three of the four acetonitrile
molecules are built into the spaces in the crystal lattice, while
the other, as shown in Figure 2, is almost completely immersed (its methyl group projects) in the interior of the
[v,,O,,]"- ion, which is shaped like a basket. Although the
distances between the atoms of the guest and the host are
very large, and the interactions between them therefore only
relatively weak, this "open" aggregate is retained in solution,
as proven by NMR spectroscopy.'"]
Fig. 2. Crystal structure of the [V,,0,,]4- ion, showing the incorporated acetonitrile molecule (from [ll]).
The astonishment was even greater when a short time later
A. Miiller et al. proved for the first time with complexes of
the type (NMe,),[(X) c VI5O3,] (6) that simple (X = C1-,
Br-) and complex anions (X = COZ-) can also be incorporated into such polyvanadates.[' But not only the inclusion
of an anion in an anion is surprising; the widely divergent sizes
of the included anions is exceptional. As can be seen from
Figure 3, all vanadium atoms of the [v,,0,,]5-ion of 6 are
pentacoordinate; the oxygen atoms are arranged in a square
Fig. 1. Section of the crystal structure of Cd(CN),.neopentane (from [S])
is tetrahedrally surrounded by four cyanide groups, which in
turn bridge two metal atoms, building cavities in which stoichiometric amounts of neopentane molecules are enclosed in
an ordered fashion. In this compound the relationship between host and guest is thus inverted: the organic molecule
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Fig. 3. Crystal structure of the [v,,0,,]5-ion, showing the included carbonate
ion (from [12]).
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Angew. Chem. In[. Ed. Engl. 1992, 31, N o . 9
pyramid around them with the apexes directed outwards.
For the first time this basal linkage of the 15 individual
polyhedrons leads to a closed structure with a cavity of such
dimensions that different sized components can be accommodated.
That in all the previous cases the structure and charge of
the guest determined the geometry of the cavity and thereby
the constitution of the isopolyvanadate was demonstrated
by the same research group only a little later with
the complexes (NEt4)6[(C104)c V,,O,,(OH)] (7) and
(NEt,),[(N,) c V,,O,,(OH),]
(8). In 7 a tetrahedral
perchlorate ion guest is present in a [V,,0,3(OH)]5- host
and in 8 a linear azide ion guest in a [V,804,(OH),]4- host,
which indicates that the formation of the isopolyvanadate
shells is controlled by the molecular recognition of the template. Also remarkable is that the two hosts are structurally
directly related and that as in the previous cases mixed-valent VIV-Vv complexes are involved.[’31
That is not yet the end. In this issue A. Miiller et al.
describe the compound (Me2NH2)5(NH4)[
(NH,CI), c
V,,0,,(OH)4(H,0),(PhP0,)8]~5 H 2 0 . 4 D M F (9) in which
for the first time two anions are simultaneously incorporated
in a large cavity divided into two by two coordinatively
bound water molecules. The shell of this cage, as depicted in
Figure 4, is formed from two half shells in which isopolyvanadate building blocks are linked internally and to one
another by phenylphosphonate groups. Furthermore, the
crystals will be built into supramolecular inorganic compounds, but also three-dimensional fragments. The inclusion
and thus the directed synthesis of nanocrystals would be a
good deal closer.
Naturally the hetero- and isopolyacids are in the forefront
of the research activities in the search for such supramolecular inorganic compounds, because decades of practical experience is available. But one must not forget that supramolecular effects can also occur in other sectors of inorganic
chemistry. Recently we found the first example for the
inclusion of ions in organometallic compounds by use
of the template effect.[I5] In the compound [(Na) c
(iPrSn),,04(OH),,][Ag,I,,]C1~lODMSO.H,O (lo), each
atom of the tin-organic cation is surrounded by a distorted
octahedron of five oxygen atoms and one isopropyl group.
The individual octahedrons are linked to each other as in the
y-Keggin structural type. This produces a cavity in the center
that is the right size to take up a sodium ion, which is then
coordinated tetrahedrally to four p 3 - 0 atoms.
The construction and type of octahedron linkage in the
cation of 10 might still strongly resemble a heteropolycation
surrounded by organic residues, but the supramolecular unit
of the compound (NH,),[(Na) c Au,,Se,] (ll),described
only recently by S.-P. Huang and M. G. Kanatzidis, bears no
trace of similarity to such a structure.[’61In the crystal lattice, besides the ammonium ions, the centrosymmetric
[Au,,Se,14- ions exist in the form of an almost regular cube.
The selenium atoms occupy the eight corners, and the gold
atoms the centers of the twelve edges. Exactly in the middle
of this gold selenide, a sodium atom is situated, forming
twelve short distances to gold atoms and eight long distances
to selenium atoms (Fig. 5).
Fig. 5. Crystal structure of the cubic [ A u , , S e J - ion, showing the included
sodium ion (from [16]).
Fig. 4. Crystal structure of the [V,,0,,(OH),(H,0),(PhP03)8]6-ion, showing
the incorporated chloride and ammonium ions (from [14]). Color code: V light
brown; 0 red; P yellow; C black; H white; CI green; NH, blue.
cage is formed and polarized by the combination of apolar
organic residues (phenyl groups) and polar inorganic ligands
(terminal oxygen atoms) in such a way that two niches are
created, in which two ammonium ions can be incorporated
as in a coronand. The anions (CI-) and cations (NH,+) are
not only approximately as far apart as in the ionic crystal,
but also in a square planar environment as there.[’41
It seems to be only a question of time and the dexterity of
the preparative chemists until not only single ions, ion pairs,
or as in the case of 9, two-dimensional fragments of ionic
Angew. Chem. Int. Ed. Engl. 1992, 31, No. 9
Similarly unusual host-guest relationships are also found
in the compound Ph,P[(($-C5Me,TiF,),F2}Na] (12),which
was recently described by Roesky et aI.[”] In 12 two [($C,Me,TiF,),F,]- fragments form the nucleophilic host, and
its fluoride surface binds the Na’ ion in a similar way as the
oxygen-containing analogues, the crown ethers. The sodium
ion is thereby surrounded by eight fluorine atoms.
All these examples show that the inorganic host-guest
chemistry is experiencing an upswing. Already the range of
isolated observations demonstrates the scope of this field.
German version: Angew. Chem. 1992, 104, 1210
[l] C. J. Pedersen, J. Am. Chem. Soc. 1967, 89, 2495, 7017; C. J. Pedersen.
H. K. Frensdorff, Angew. Chem. 1972,84,16;Angew. Chem. Int. Ed. Engl.
1972, 11, 16.
Verlagsgesellschaft mbH, W-6940 Weinheim, 1992
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121 J.-M. Lehn, Pure Appl. Chem. 1980, 52, 2441; Science 1985, 227, 849;
Angew. Chem. 1988, 100, 91; Angew. Chem. I n t . Ed. Engl. 1988, 27, 89;
h i d . 1990, 102, 1347 bzw. 1990, 29, 1304.
[3] J. Berzelius, Ann. Phys. (Leipiig) 1826, 6, 369, 380.
[4] Cf. F. Vogtle, Supramolekulare Chemie, Teubner, Stuttgart, 1989, p. 15 ff.
[S] On account of the relatively long bonds berween the transition metal atoms
and the oxygen atoms on the phosphorus atom, an alternative description
has also been suggested for this heteropolyanion (C. J. Clark, D. Hall, Acta
Cry.$lal/ogr.Sect. B 1976, 32. 1545). i n which the shell comprises twelve
square-pyramidal {MOO,) units linked to each other through their basal
oxygen atoms. In this case a further largely isolated phosphate ion is
situated in the center of the anion, and a true supramolecular compound
is indeed present; for a comprehensive discussion of the bonding, see G. M.
Brown, M.-R. Noe-Spirlet, W R. Busing, H. A. Levy, Acta Crystallogr.
Sect. B 1977, 33, 1038.
[6] K. W. Allen, J. Chem. Soc. 1959,4131.
[7] D. D. DesMarteau, T. Grelbig, S.-H. Hwang, K. Seppelt, Angew. Chem.
1990,102, 1519; Angew. Chem. Itit. Ed. Engl. 1990,29, 1448.
[8] T. Kitazawa, S. Nishikiori, A. Yamagishi, R. Kuroda, T. Iwamoto, .
Chem. Soc. Chem. Commun. 1992,413.
[9] H. J. Powell, J. H. Rayner, Nature 1949, 163, 566.
[lo] G. J. Johnson, E. 0. Schlemper, J. A m . Chem. Soc. 1978, fO0,3645.
[ I l l V. W. Day, W. G. Klemperer, 0. M. Yaghi, J. Am. Chem. Soc. 1989, l f l ,
[12] A. Muller, M. Penk, R. Rohlfing, E. Krickmeyer, J. Doring, Angew. Chem.
1990, 102, 927; Angew. Chem. I n t . Ed. Engl. 1990, 29,926.
[I31 A. Muller, E. Krickmeyer, M. Penk, R. Rohlfink, A. Armatage, H. Bogge,
Angew. Chem. 1991,103,1720;Angew. Chem. In[. Ed. Engt 1991,30,1674.
[I41 A. Muller, K. Hovemeier, R. Rohlfing, Angew. Chem. 1992, 104. 1214;
Angew. Chem. Inl. Ed. Engl. 1992, 31. 1192.
[15] H. Reuter, Angew. Chem. 1991, 103, 1487; Angew. Chem. Int. Ed. Engl.
1991,30, 1482.
[I61 S.-P. Huang, M. G. Kanatudis, Angew. Chem. 1992, 104, 799; Angew.
Chem. I n f . Ed. Engl. 1992, 3f, 787.
[I71 H. W. Roesky, M. Sotoodeh, M. Noltemeyer, Angew. Chem. 1992, 104,
869; Angew. Chem. I n t . Ed. Engl. 1922, 31, 864.
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