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Helices Supramolecular Chemistry and Metal-directed Self-Assembly.

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Helices, Supramolecular Chemistry, and Metal-directed Self-Assembly
By Edwin C. Constable*
Chemists have been concerned with shape ever since the
realization that molecules possess three-dimensional structure. An aspect of shape which has attracted recent attention
is conveniently categorized under the title molecular topology and is concerned with the interlinking and intertwisting of
molecular threads.“] The spatial orientation of the molecular
threads is critical and has been the subject of some very
elegant organic chemistry. More recently, it has been recognized that the required pre-organisation of molecular threads
may be achieved by utilization of the specific stereochemical
requirements of transition metal ions. All that is required is
the incorporation of suitable donor atoms into the molecular
threads, and this approach has been exploited in the synthesis of many “exotic” molecular systems. The beauty of using
transition metal ions as the templates for the spatial control
of molecular threads lies in the fact that all of the stereochemical information is contained at the metal center. The
result is that the chemist is then a passive observer of the
assembly of the organized molecular system-a
phenomenon termed spontaneous self-assembly.
The helical geometry is particularly appealing, and this
structural motif appears in many important biological
molecules. The metal-directed assembly of double helices has
been established by a number of groups, and in a paper in
this issue, Williams et al. report the extension of this methodology to the synthesis of a binuclear triple-helical complex.[’]
The principle used for the assembly of the triple helix is very
simple. A bis-bidentate ligand is allowed to react with a
metal ion which favors six-coordinate octahedral geometry;
each of two such metal ions coordinates to one bidentate
grouping at each end of three such ligands (Fig. 1).
consisting of two 2-(2’-pyridyl)benzimidazolyl units linked together, each of which can act as a bidentate N, donor, spontaneously forms a triple-helical complex cation [ C O , ( ~ ) , ] ~ +
upon treatment with cobalt(i1) salts. The X-ray crystal structure of the organge-red complex [Co,(l),][CIO,], . 2.5 MeCN
reveals that the complex is indeed a triple helix. The two
cobalt@) centers are in slightly distorted octahedral environments, and the ligand-imposed geometry places the metals
over 8 A apart within the triple-helical array.
This paper is important for a number of reasons beyond
the remarkable beauty of the resultant structure. Williams
et a1.I2]has demonstrated that new molecular topologies may
be designed and synthesized by a logical combination of ligands and metals with specific donor and acceptor properties.
The spontaneous manner in which the highly ordered structure assembles illustrates the efficacy and the remarkable
potential of this approach. This is the first example of a
triple-helical coordination compound, although Shanzer
et al. have described a diiron triple helix resulting from the
metal-controlled intermolecular twisting of a tripodal ligandr3]and Lehn et al. have mentioned in passing a diiron
complex with a bis-bidentate ligand which may well possess
this geometry.I4]
The synthesis of this triple-helical structure is a logical
extension of investigations by a number of groups into the
metal-controlled intertwisting of molecular threads to give
double helical arrays. Although a considerable variety of
ligands have been used for the preparation of double-helical
coordination compounds, they all exhibit structural features
Fig. 1. The formation of a triple-helical complex by interaction of a bis-bidentate ligand with a six-coordinate metal center.
The key to assembling helicate structures lies in the design
of a ligand with the correct linker groups between the metalbinding termini. Williams, Piguet, and Bernardinelli[21have
now shown that the relatively rigid bis-bidentate ligand 1,
Dr. E. C. Constable
University Chemical Laboratory
Lensfield Road, Cambridge, CB2 IEW (UK)
Verlagsgesellschafl mbH, W-6940 Weinheim, 1991
0570-0833j9l~lll1-14SO$3.50+ ,2510
Angew. Chem. Int. Ed. Engl. 30 (1991) N o . 11
Fig. 2. The formation of a double-helical complex by interaction of a bisbidentare ligand with a four-coordinate metal center M.
in common with 1. The basic pattern consists of two or more
polydentate ligands linked by spacer groups. Ligands which
have been shown to give rise to double-helical structures
include tetrapyrroles,[’] cyclohexane-l,2-diirnine~,~~l
bishydrazones)’] quater-,‘’’ q ~ i n q u e - ,and
~ ~ ]sexipyridinesf1’]. But
the essential structural features are most simply seen in the
linked oligobipyridines (2)[4s and oligophenanthrolines[’*]. In the same way that a triple helix is built up by
coordination of bis-bidentate ligands to a six-coordinate
metal center, a double helix is assembled by interaction with
a four-coordinate (usually tetrahedral) one (Fig. 2).“ 21
nal surface of a double-helical structure may interact with
other molecules, of which helicate-nucleic acid interactions
are an obvious area of interest.[l3I
The future for metal-directed topological chemistry is very
bright. The wide variety of coordination numbers and stereochemistries which are available offer a new “playground”
for molecular chemists to prepare new structures in designed
and high-yielding syntheses. A fusion of inorganic and organic chemistry is leading to an exciting and beautiful new
interdisciplinary area.
[l] C. 0. Dietrich-Buchecker, J.-P. Sauvage, Chem. Rev. 87 (1987) 795;
Bioorg. Chem. Front. 2 (1991) 195; J.-P. Sauvage, Ace. Chem. Res. 23
(1990) 319; V. I. Sokolov, Russ. Chem. Rev. 42 (1973) 452.
[2] A. F. Williams,C. Piguet,G. Bernardinelli, Angew. Chem. 103(1991) 1530;
Angew. Chem. Inf. Ed. Engl. 30 (1991) 1490.
[3] J. Libman, Y Tor, A. Shanzer, J. Am. Chem. SOC.109 (1987) 5880.
141 M. T. Youinou, R. Ziessel, J.-M. Lehn, Inorg. Chem. 30 (1991) 2144.
[5] W. S. Sheldrick, J. Engel, Arta Crystallogr. S e r f . B 37 (1981) 250; G.
Struckmeier, U. Thewalt, J.-H. Fuhrhop, J. Am. Chem. SOC.98 (1976) 278;
Fig. 3. The assembly of a trefoil knot from a functionalized double helical
precursor. R = H, n = CH,(CH20CH,),CH,, M = Cu.
Why is there such interest in this esoteric activity? There
are a number of answers to this question. Perhaps the simplest answer lies in the aesthetic beauty of the molecules
which are obtained- helices and interlinked networks have
long been associated with artistic works, and the molecular
analogues are equally satisfying to the observer. A more
serious reason to investigate these systems arises from some
of the consequences of intertwisting molecular threads.
Suuvuge and Dietrich-Buchecker have demonstrated that the
key intermediate for the synthesis of the trefoil knot 3 is a
suitably functionalized double helix, and have used the
metal-directed assembly of a double-helical complex as the
key step in their elegant preparation of a molecular knot.[’*’
The important step is the formation of a double-helical dicopper([) complex of a functionalized bis-l,lo-phenanthroline ligand, followed by cyclization in a conventional macrocyclization step (Fig. 3).
The functionalization of helicate complexes offers another
reason for their investigation. These are the first examples Of
a new class of chiral molecules, which may possess novel
Optical Or
properties‘ Lehn et
are investigating
extrahelical interactions in which substituents on the exter-
Angeti. Chem. h i . Ed. Engl. 30 (1991) No. 11
D. Dolphin, R. L. N. Harris, J. L. Huppatz, A. W. Johnson, I. T. Kay, J.
Leng, J. Chem. SOC.C 1966,98.
[6] G. C. van Stein, G. van Koten, F. Blank, L. C. Taylor, K. Vrieze, A. L.
Spek, A. J. M. Duisenberg, A. M. M. Schreurs, B. KojiC-ProdiC, C. Brevard, Inorg. Chim. Acfa 98 (1985) 107; G. C. van Stein, G. van Koten, K.
Vrieze, A. L. Spek, E. A. Klop, C. Brevard, Inorg. Chem. 24 (1985) 1367,
and references therein.
[7] D. Wester, G. J. Palenik, Inorg. Chem. 15 (1976) 755; J. Chem. SOC.Chem.
Commun. 1975,74; E. C. Constable, J. M. Holmes, P. R. Raithhy, Pol.vhedron 10 (1991) 127.
181 J.-P. Gisselhrecht, M. Gross, J.-M. Lehn, J.-P. Sauvage, R. Ziessel, C.
Piccinni-Leopardi, J. M. Arrieta, G. Germain, M. van Meerssche, Nouv. J.
Chim. 8 (1984) 661; J.-M. Lehn, J.-P. Sauvage, J. Simon, R. Ziessel, C.
Piccinni-Leopardi, G. Germain, J.-P. Declercq, M. van Meerssche, ibid. 7
(1983) 413.
[9] E. C.Constable, S . M. Elder, P. R. Raithby, M. D. Ward, Polyhedron 10
(1991) 1395, and references therein.
Dalton Trans.
. , E. C. Constable. M. D. Ward. D. A. Tocher. J. Chem. SOC.
1991, 1675, and references therein.
11 11 T. M. Garrett, U. Koert, J.-M. Lehn, A. Rigault, D. Meyer, J. Fischer, J.
Chem. SOC.Chem. Commun. 1990, 557; J.-M. Lehn, A. Rigault, J. Siegel,
J. Harrowfield, B. Chevrier. D. Moras, Proc. Natl. Acad. Sci. USA 8 4
(1987) 2565; J.-M. Lehn, A. Rigault, Angew. Chem. 100 (1988) 1121;
Angew. Chem. Inf. Ed. Engl. 27 (1988) 1095; C. Piguet, G. Bernardinelli,
A. F. Williams, Inorg. Chem. 28 (1989) 2920.
[12] c. 0.Dietrich-Buchecker, J. Guilhem, c. Pascard, J.-P. Sauvage, Angew.
Chem. 102(1990) 1202; Angew. Chem. I n f . Ed. Engl. 29(1990) 1154; C. 0.
Dietrich-Buchecker, J.-P. Sauvage, ibid. 101 (1989) 192 and 28 (1989) 189.
[13] E. C, Constable, Nature (London) 346 (1990) 319; U, Koert, M. M, Harding, J.-M. Lehn. ibid. 346 (1990) 339.
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chemistry, self, assembly, metali, supramolecular, helices, directed
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