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Self-Assembly of Molecular-Sized Boxes.

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Self-Assembly of Molecular-Sized Boxes
C. A. Hunter"
Molecular recognition of guests by synthetic hosts is a rapidly
developing field, and many of the basic principles that govern
the efficiency and selectivity of intermolecular complexation have
The following requirements have emerged
for the design of an effective receptor for a particular target
1. The host should be polymacrocyclic and contain a large
cavity in which the number of interactions with the bound guest
can be maximized.
2. The van der Wads surfaces and electrostatic potential surfaces of the host and guest should be complementary.
3. The host should be relatively rigid so that the loss of conformational entropy on binding a guest is minimized.
While these principles are clear. the realization of rigid polymacrocyclic structures is a difficult synthetic challenge. The use
of high-dilution conditions is one way to encourage macrocyclization and reduce polymer formation, but yields can be low,
and chromatographic separation of mixtures of linear and cyclic
oligomers with similar properties can be difficult. Templatedirected synthesis has proved to be a very successful method for
generating macrocycles.[2J Since it works well when the interactions between the template and the acyclic precursor are strong,
its use has been restricted to metal ion/ligand interactions and
the construction of hosts for ionic or coordinating guests. Templating is not a viable approach for the construction of macrocyclic hosts for neutral organic guests where the individual interactions between the template and the acyclic precursor are
orders of magnitude weaker.
Thus new techniques are needed for the construction of polymacrocyclic structures. Over the last few years, self-assembly
has emerged as a very promising approach to the generation of
compounds with large molecular-sized cavities. Self-assembled
complexes are discrete, structurally well-defined species composed of at least two molecular units connected by noncovalent
interactions.[31The use of noncovalent interactions to generate
macrocyclic structures is particularly attractive since the cyclization is under thermodynamic control. Provided that the design
of the components is sufficiently accurate, entropically unfavorable polymerization is avoided. Moreover, the self-assembly
process should become increasingly favorable as the complexity
of the structure increases from simple macrocyclic to polymacrocyclic architectures. There are obvious parallels in nature where
[*] Dr. C. A. Hunter
Department of Chemistry, University of Sheffeld
GB-Sheffield S37HF (UK)
Telefax: Int. code + (1 14)273-8673
large three-dimensional cavities are almost always generated by
the self-assembly of many proteins, for example in the coats of
The first self-assembled macrocyclic host (1) was reported in
1986 by Maverick et aLL4]This work showed how coordination
chemistry could be used to
generate cyclic structures in
good yield by simply mixing
the appropriate components in
solution. 1 also functioned as a
host: it selectively cornplexed
in chloroform ( K = 220 M - ').
An appealing design for a
host molecule is a regular box
shape--construction in three dimensions is conceptually simpler
if we use three orthogonal axes, in other words, if we use right
angles in our building blocks. An attractive feature of inorganic
complexes is that they afford a range of 90" linkages for the
construction of macrocyclic boxes. The square-planar geometry
of platinum(r1) and palladium(rr) complexes makes them ideal
building blocks, and in 1990. Fujita et al. first demonstrated the
utility of palladium(rr) in the self-assembly of a large macrocyclic cavity (Fig. 1) .['] The thermodynamic cyclization resulted
in a quantitative yield of the macrocycle, which also formed a
stable 1 : 1 complex ( K = 750 M - ') with 1,3,5-trimethoxybenzene in water.
Fig. 1. Fujita and Ogura's box forms in quantitative yield on mixing the components [S].M = Pd", L, = ethylenediamine.
Stang and others have recently produced several variations on
this basic idea (2 and 3).[61These structures are obtained in high
yields by simply mixing the components. Interestingly, covalent
analogues of these macrocycles are known (4 and 5).['] These
require multistep syntheses concluding with a low-yielding
macrocyclization reaction which produces large quantities of
acyclic oligomers. The similarity between these self-assembled
2, W = X = N; Y = Z = M ( N H ~ C H ~ C H Z N H ~ ) ~ ,
3,W = N; X = C; Y = M(Ph2PCH2CHZCHzPPh2)2;Z = I’
4,W = X = C; Y = 2 = I t
5,W = X = N’; Y = Z = CH2
M - M
M = Pt, Pd
M - M
macrocycles and Stoddart’s “big blue box” (5) suggests that
they will have a bright and colorful future in supramolecular
Fujita and Ogura have now extended this work to the Construction of a three-dimensional cavity (Fig. 2).[81This self-as-
R = H,Me, OMe
R’ = CH2C02Na, S03Na
Fig. 2. This macrobicyclic structure does not assemble efficiently unless a complementary guest is added to the mixture [S]. M = Pd”, L, = ethylenediamine.
sembly process is significantly less efficient than that in Figure 1
because the ligands are more flexible; less than 60% of the
molecules exist as the macrobicyclic complex which is in equilibrium with other oligomers. However, addition of a complementary guest such as sodium 4-methoxyphenylacetate shifts
the equilibrium dramatically such that only the cage complex
can be detected. Guests that are too large to fit inside the cage
can still bind but shift the equilibrium in the opposite direction,
in favor of open oligomers.
Three-dimensional macrobicyclic assemblies have also been
described by Baxter et al. and by Saalfrank et al. The Lehn
group used the coordination of copper(1) by 2,2’-bipyridyl
derivatives to generate a large cylindrical box (Fig. 3 a) .I9] In
total, five ligands are complexed by six metal centers in this
singularly impressive example of the power of self-assembly to
generate polymacrocyclic architectures. Saalfrank et al. have
#-? VCH ~ r l a g s g e s e l l ~ c l i umhH.
0-69451 Weinheini. 1995
Fig. 3. Schematic representations of cavities assembled on metal centers. a) Lehn’s
cylindrical box based on copper(1) and coordination of 2.2’-bipyridine 191.
b) Saalfrank‘s tetrahedral iron-malonate assembly [lo]. c) Fujita and Ogura’s
square box, which could be extended into a cube (d) by using an octahedral metal
developed the coordination chemistry of iron with malonic acid
derivatives to produce tetrahedral tetranuclear complexes which
enclose cavities of variable dimensions (Fig. 3 b).[”] The smallest
structure encapsulates cations such as ammonium. Both of these
systems use bifunctional chelating ligands rather than pyridyl
ligands: the cylinder in Figure 3 a is generated by a distorted
tetrahedral metal center and the tetrahedron in Figure 3 b is
generated by an octahedral metal center.
In principle, such three-dimensional assemblies should be
more stable than simple macrocycles since the number of interactions per component is larger. It remains to be seen whether
the two-dimensional self-assembly of square boxes based on
pyridyl ligands and square-planar metal centers can be extended
into three-dimensional cubic structures by the use of octahedral
metal centers (Fig. 3 c and d).
Self-assembly of macrocyclic hosts is not restricted to metal
complexes, and macrocyclic hosts have been reported in which
hydrogen bonding alone controls the assembly process.[’ ‘I The
most striking example is Rebek’s “tennis ball”.“ ‘‘1 Two selfcomplementary molecules form a three-dimensional spherical
cavity, which complexes small nonpolar guests such as methane
in organic solvents.
Self-assembly appears to be an extremely promising method for
the construction of polymacrocyclic structures and will facilitate
the construction of large cavities for use in molecular recognition. An interesting feature of these systems is that assembly of
the host structure may itself be coupled with guest recognition
such that the host only exists in the presence of the guest. The
structures generated to date all have a high degree of symmetry,
which may limit the range of substrates that can be recognized,
but strategies for avoiding symmetry have yet to be explored.
German version: Angew. Chewa. 1995, 107, 1181
Keywords: molecular recognition . noncovalent interactions
0570-0833195/1010-1080$ 10.00 f ,2510
Angen. Chem. hi.Ed. Engl. 1995, 34* No. 10
[l] J. M . Lehn. Angun. Chem. 1988. 100. 91-116; A n g w . Chent. I n / . Ed. Engl.
1988,.?7, 89 - 1 12: D. J. Cram. ihid. 1988, 100. 1041-1052and 1988.27,10091020.
[2] S. Anderson. H. L. Anderson. J. K. M. Sanders. Acc. Chem. RES.1993. 26,
469 -475; R. Hoss, F. Vogtle, Angew. Chem. 1994, 106, 389-398. Angeiv.
(‘hn77. I n / . Ed. Enyl. 1994. 33. 375-384.
131 J. S. Lindsey. N w J. Chem. 1991. 15. 153-180.
[4] A . W. Maverick, S. C . Buckingham. Q. Yao, J. R. Bradbury, G. G. Stanley, J.
.4n1 Chenz. Sol. 1986. /OK, 7430-7431
[S] M . Fujita. J. Yzzaki. K. Ogura, J. Am. Cheni. Soc. 1990. 112, 5645-5647.
[6] P. J. Stang. D. H. Cao, J. Am. Chern. Suc. 1994, 116. 4981 -4982; P. J. Stang,
K. Chen. ihfd. 1995, 117, 1667-1668; M. Fujita, S. Nagao, M. Iida, K. Ogata,
K. Ogura. ihid 1993, l l S , 1574-1576: H. Rauter. E. C. Hillgeris. A. Erxleben,
B. Lippert. ;hid. 1994. 116.616-624; P. J. Stang. J. A. Whiteford, Organomefulfic.s 1994. 1.3. 3776-3777: C. M. Drain, J. M. Lehn, J. Chem. Soc. Cl7em. Comrnnn. 1994. 2313-2315, see also A. W. Schwabacher, J. Lec, H. Lei. 1 Am.
Cfwm SW. 1992. 114. 7597-7598: R. Riittimann, G. Bernardinelli. A. F. Wilhams. Ang~ii‘.Chcvn. 1993. 105, 432-434; Angew. Chem. h t . Ed. Engl. 1993,
32.392 394: C. A. Hunter. L. D. Sarson. ibid. 1994,106,2424-2426and 1994,
33. 2313-2316: P. Kliifers. J. Schuhmacher. rhid. 1994. 106, 192.5-1927 and
1994. 33, 1863- 186.5
[7] P. J. Stang, V. V. Zhdankin, J. Am. Cl7em. Soc. 1993. 115. 9808- 9809: P. R.
Ashton, C. L. Brown. E. J. T. Chrystal, T. T. Goodnow, A. E. Kaifer. K. P.
Parry, A. M. Z. Slawin. N. Spencer, J. F. Stoddart. D. J. Williams, Angrir..
Chem. 1991, 103. 1055-1061; Angew. Chem. Inr. E d En,?/. 1991. 30. 10391042.
[8] M. Fujita. S. Nagao. K. Ogura. J. Am. Chern. So<. 1995. 117, 1649-1650.
[9] P. Baxter. J. M. Lehn, A. DeCian, J. Fischer, Angew. Chrrn 1993. iOS, 92-95;
Angew. Chem. I n f . Ed. Engl. 1993, 32. 69 - 72.
[lo] R. W. Saalfrank. B. Homer, D. Stalke. J. Salbeck. A n g ~ i i .Chrrn. 1993. f05,
1223-1225; Angeu. Chem. Int. Ed. Engl. 1993. 32. 1179-1182; R. W. Saalfrank, R. Burak, A. Breit, D. Stalke, R. Herbst-Inner, J. Daub, M. Porsch. E.
Bill, M. Miither, A. X. Trautwein, ibid. 1994, 106. 1697 1699 and 1994, 33,
1621-1623; R. W. Saalfrank. R. Burak, S. Reihs, N. Low. F. Hampel. H.-D.
Stachel, J. Lentmaier. K. Peters, E. M. Peters. H. G. von Schnering. ibid. 1995,
107. 1085-1088 and 1995, 34,993-995.
[11] a ) M . Kim. G. W. Gokel, J. Chem. S o ( . Chcm. Commurr. 1987. 1686-1688;
b) C. M. Drain, R. Fischer, E. G. No1en.J. M Lehn,ihir/. 1993,243-245;c) R.
Wyler, J. de Mendoza, J. Rebek, Jr., Angrw. Chenz. 1993. 105, 1820-1821;
Angen. Chern. I n f . Ed. Engl. 1993. 32. 1699- 1701; d ) N. Branda. R. Wyler, J.
Rebek, Science 1994.263. 1267-1268.
Does the Death Knell Toll for the Metallic Bond?**
J. Christian Schon*
“There is an appointed time for everything under the heavens--a time to be born, and a time to die.” Judging from two
recent publications,[” the latter fate awaits the concept of the
metallic bond. Based on two somewhat different trains of
thought. the authors conclude that the metallic bond is a disposable quantity in descriptions of bonding in solids, and should be
subsumed into the supposedly better established concepts of the
covalent and ionic bond.
Analyzing their arguments in more detail, we notice that Allen et al.[’] base their conclusion on an attempt to quantify the
so-called van Arkel -Ketelaar triangle.[3s41They suggest that a
configuration energy (CE) should be defined as an average “valence shell energy”, in other words, the average energy needed to
remove a single valence electron. Here, it is assumed that the
remaining valence electrons remain in their original states during this “ionization” process. Thus, according to Koopman’s
theorem. these valence shell energies correspond to the one-particle energies in the Hartree-Fock a p p r ~ x i r n a t i o n . [ ~ ~ [For
* * *a~
compound composed of A and B, Allen et al. then identify CE,
and CE, with the diagonal elements of the interaction matrix
within the approximation of the extended Hiickel theory
(EHT) .I1,21 Using a common approximation for the off-diagonal
matrix elements,[61 Hij =1/2(Hi + H j ) , they find that
H A , = (CE, + CE,)/2 holds. Since the off-diagonal matrix elements explain (within the context of the EHT, at least) the
Dr. J. C. Schon
Institut fur Anorganische Chemie der Universitat
Gerhard-Domagk-Strasse 1 . D-53121 Bonn (Germany)
Teletdx: Int. code + (228)73-5660
e-mail. tlnc419(
[**I I would like to thank Prof. Dr. M. Jansen. Bonn. for many valuable comments
and discussions.
[***I The interpretation of the one-particle energies is subtle: the total energy ofthe
solid IS not the sum of all one-particle energies, since we are dealing with an
interacting system. In particular. a one-particle energy is in general not identical with the standard ionization energy.
A n g w Chcrn. I n f . Ed. Engl. 1995. 34, No. 10
Peierls distortion,[7.s1 which accompanies the change from
“metallic” to “covalent” band structures, Allen et al. conclude
that (CE, + CE,)/2 is a natural coordinate along the metallic +covalent edge of the van Arkel-Ketelaar triangle.
The second coordinate in the description of the triangle would
have to reflect the degree of ionicity in the bonds. Here, Allen
et al.[91 refer to the classical concept of the difference in electronegativities for the characterization of the polarity of the
bond. Within their model, this concept is most easily implemented by employing the difference in configuration energies,
(CE, - CE,)/2.[*] From these considerations it follows that the
three traditional bonding concepts should be expressed in a
natural way through the two “coordinates” derived from the
configuration energy.[**]
At the same time, Anderson, Burdett, and Czechf2’( = ABC)
have addressed the issue that certain materials should not necessarily be described in terms of typical metallic bonding, although they show macroscopic metallic behavior. Examples are
doped molecular crystals and ionic systems, for instance fullerenes and compounds related to the high-temperature superconductors, respectively. The band structures of such “metals” can
be determined within the tight-binding (TB) approach,[5, and
it is not necessary to invoke the metallic bond with its somewhat
vague concept of delocalized vs. localized electrons to explain
the metallike behavior. Thus, ABC suggest that one should
reevaluate the concept of metallic bonding in light of the current
understanding of theoretical models of the bonding in solids.
They point out that most of the properties associated with
metallic behavior are related to the position of the Fermi surface
in the band approximation[”] for the theoretical analysis of the
Based on these considerations, Allen draws the conclusion that CE should be
used as a general characteristic quantity for the chemical behavior of an atom
on the same level as its position in the periodic table.
For an alternative approach to go beyond the van Arkel-Ketelaar triangle by
applying quantum mechanical concepts, see the work by Urland [lo].
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