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Carboranes Anti-Crowns and Big Wheels.

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HIGHLIGHTS
Carboranes, Anti-Crowns, and Big Wheels
By Riisscll N . Grimrs*
Carboranes polyhedral boranes containing carbon in
the framework-have been around for over 30 years, and
their intrinsic stability, versatility, structural variety, and
electronic properties have been put to use in a number of
diverse areas.[’] for example in the synthesis of extraordinarily heat-stable polymers, in boron neutron capture therapy
(BNCT). its ligands in metallacarborane catalysts, as complexing agents for extraction of metal ions, as precursors to
ceramics. conducting polymers. and nonlinear optical materials. as anticancer agents, and as carriers for radioactive
metals in radioimmunodetection and radioimmunotherapy.
Although polyhedral carboranes having as few as five and
as many as twelve vertices (fourteen if metal atoms are included) are known,”] most research and applications have
centered on the exceptionally stable seven- and twelve-vertex
cage systems. Largely for reasons of accessibility, the twelvevertex C,B,,H,, icosahedral carboranes have been most
widely studied. The three known isomers. in which the carbon atoms occupy ortl7o (1,2), mefa (1.7), or pnrii (1,12)
vertices. are white solids that rank among the most stable
molecular compounds known. These clusters can be viewed
as “superaromatic” systems in which 26 electrons fill the
13 bonding molecular orbitals on the polyhedral framework: moreover, the cage volume approximates that displaced by a benzene molecule rotating on one of its twofold
axes. The very high intrinsic stability of the C,B,,H,, isomers. together with the acidic character of the hydrogen
atoms bound to the cage carbon atoms (which allows facile
introduction of functional groups at carbon), is the basis of
extensive development of icosahedral carborane chemistry
over three decades and its application to practical problems.“, 2l
The remarkable versatility of carboranes presents an almost limitless range of possible roles in designed synthesis, a
Fact that is drawing increasing attention in organic and inorganic chemistry, materials science, engineering, and biologically related areas. A particularly elegant recent example is
the design and construction of novel carborane-based
rnacrocycles, in which both the electron-withdrawing character of the cdrbordne cage and the close geometric relationship of icosahedral C,B,,H I > to planar C,H, are exploited.
In a series of papers,[31Hawthorne et al. report the synthesis
of “mercuracarborands” which incorporate three or more
C,B,,,H,, cages linked by an equal number of mercury
atoms. The reaction of 1,2-dilithio-ortho-carborane
Li,C2B,,Hi, with HgCI, afforded the tetramer shown in
Figure 1 a, which binds to a CI- ion, resulting in a nearly
square-planar geometry for this ion (Fig. 1 c), an unprecedented coordination mode for halide
Since the uncomplexed T H F adduct of the host macrocycle has been shown by X-ray crystallography to have s,
[*I Proi’. Dr R . N. Grimes
Department of Chemistry
University of Virginia
Charlottesville. VA 22901 (USA)
Teldar: Int. code + (804)924-3710
symmetry with the centers of the icosahedra well outside the
plane of the four mercury atoms,[3b1it has been proposed
that the chloride ion functions as a template, creating the
square-planar conformation. The carborane macrocycle is a
Lewis acid and coordinates nucleophiles; it is therefore an
“ a n t i - c r o ~ n ” , [that
~ ~ is, a charge-reversed analogue of the
well-known family of nucleophilic hosts such as [I 21crown-4
(Fig. 1 b) that have figured prominently in molecular recognition studies.[51
H
‘g’
Fig. 1. a) Structure of the host tetramer (1.2-C,B,,H,,Hg), in its planar conformation [3a]. b) Structure of [12]crown-4. c) Structure of the tetramer with a
bound chloride ion [3a]. d) Structure of the host trimer (1.2-C,B,,,H,,,Hg),
(bound CH,CN molecules not shown) [3f].
Treatment of the 1,2-dilithiocarborane with HgI, generated the dianionic species [(1,2-C,BIoH,,),Hg,I,]*~ in which
two iodide ions bind to the four mercury atoms;13“ the corresponding reaction of the 1,2-dilithio-3-phenylcarborane
gave a cyclic tetramer with only one 1- ion bound in the
sterically encumbered cavity.r3d1Surprisingly, of the four
possible stereoisomers, the only one formed was that having
two phenyl rings directed “up” and two “down” with respect
to the cavity. Addition of Ag’ ions to the mercuracarborand
complexes was found to remove the halide ion(s) and afford
the free host m ~ l e c u l e . ~ ~ “ ~ ’ ~
A related example of complexation by the mercuracarborand tetramer involves two polyhedral B,,Hf; guest dianions that are bound to the four H g atoms through three-center, two-electron B- H- Hg bonds.[3e1 The reaction of
dilithiocarborane with mercuric acetate’”’] yielded yet another macrocycle, this time a trimer (Fig. 1 d) which coordi-
nates acetonitrile in a most unusual manner: the solid-state
structure features two cocrystallized adducts having three
and five CH,CN-bound molecules, respectively. This anticrown is analogous to trimeric ortho-phenylene mercury, but
its mean Hg-Hg distance of more than 3.7 8, implies a larger
central cavity than the latter
On treatment of
Hg3(C2BloHlo)3
with LiCI, an anionic chloride complex was
formed in which the C1- ion is proposed to reside at the
center of the Hg, triangle. The binding ability of these mercuracarborands toward Lewis bases implies that nitrogencontaining bases of biological relevance-adenine, guanine,
and the like-may be similarly
In a different synthetic tour de force that combined chemistry with art, Wade et aLr6]prepared the macrocycle shown
in Figure 2. In this case the starting reagent was the dicopper
@0
OH
Fig. 2. Structure of (1,7-C,B,,H,,-1’,3‘-C~H~)~
[6]
rneta-carborane, 1,7-Cu,C,Bl0H,, , which reacted with
meta-diiodobenzene to give the desired trimeric product in
low yield. As revealed by X-ray crystallography, the carborane cages are tilted away by 17” from the plane defined by
their carbon atoms, while the benzene rings are tilted in the
opposite direction by the same amount; consequently, the
molecule has a dish-shaped structure whose central cavity is
defined by three inward-directed carborane hydrogen atoms
(mean separation 3.16 A) and three phenylene hydrogen
atoms that lie almost in the same plane; three other carborane hydrogen atoms are much further apart (ca. 4.47 A).
Boron atoms on the inside of the macrocycle could be removed and replaced with metals,[61 which opens truly intriguing possibilities for catalysis and other applications
wherein the three metal centers act in concert.
Macroycles incorporating the 1,2-C2B,,H
cage and
linked by trimethylene or 1,3-xylyl groups have been prepared in Hawthorne’s laboratory, and these include both
trimers and t e t r a m e r ~ . ~The
” X-ray crystallographically determined structure of a xylyl-linked tetramer that features a
remarkable 28-membered ring is shown in Figure 3.
While the geometries of the 1,2- and 1,7-C2BIocages are
made to order for the synthesis of macrocycles through substitution at the carbon atoms, the 1,12-carbordne (paru-isomer) has carbon atoms at opposite ends of the polyhedron
and is ideally suited for the assembly of rigid linear rods
1290
0 VCH
Verlagsgrsellsrl~aflmbH, 0-69451 Weinhelm, 1993
(71
through direct C -C connections, as has been demonstrated
by two research groups.I8] As a11 these examples illustrate,
the special stereochemistry of the icosahedral cdrboranes can
be used to advantage in very innovative and creative ways.
Molecular engineering employing nonicosahedral carborane
o r organoborane clusters is also under active investigation,
as in the multidecker sandwich systems that incorporate seven-vertex MC,B,M’ or MC3B,M’ pentagonal bipyramidal
P
OC
Fig. 3. Structure of (1,2-C2B,,H,,-1‘,3’-CH,C,H,CH,),
The beauty of the chemistry highlighted in this article lies
in its essential simplicity, in that preorganized cyclic host
molecules are generated in one or two steps from readily
available reagents-an
advantage not afforded by crown
ethers and cryptands. These synthetic advances vividly exemplify the rapidly evolving art of “designer chemistry” in
which inorganic and organometallic assemblies of specified
architectures are obtained in directed reactions from available building-block molecules. It seems clear that carboranes
and other boron clusters are destined to play a significant
role in this field. Philosophically and historically, designed
synthesis is derived from organic chemistry; but for inorganic chemists, given the entire periodic table of elements to play
with, the scope of possibility seems far larger and the ultimate achievements hardly imaginable at present.
German version: Angeu. Chem. 1993, 105, 1350
[I] J. Plesek, Chem. Rev. 1992, 92, 269, and references therein.
[2] a) V. I. Bregddze, Chem. Rev. 1992, 92, 209; b) B. Stibr, ;bid. 1992. 92, 225;
c) R. N. Grimes, Carboranes. Academic Press. New York, 1970.
[3] a) X . Yang, C. B. Knobler, M. F. Hawthorne, A n g e s . Chem. 1991, 103.
1519; Angew. Chem. i n [ . Ed. Engl. 1991, 30, 1507; b) X. Yang, S . E. Johnson, S. I. Khan. M. F. Hawthorne, ibid. 1992, 104, 886 and 1992. 31, 893;
c ) X. Yang, C. B. Knobler, M. F. Hawthorne, J. Am. Chem. Soc. 1992,114,
380; d) Z. Zheng, X . Yang. C. B. Knobler, M. F. Hawthorne, ;bid. 1993,
115, 5320; e ) X . Yang, C. B. Knobler, M. F. Hawthorne, ibid. 1993. 115,
4904; f ) X . Ymg. Z. Zheng. C. B. Knobler, M. F. Hawthorne, ibid. 1993,
11s. 193.
[4] For a summary of published reports o n anion complexation by multidentate Lewis acid hosts, see ref. [ 3 f l .
IS] a ) D. J. Cram, Science 1983, 219, 1177; b) E Vogtle, E. Weber, Host-Guest
Conip/e.x Chemisrry/Maeroc,vc/e~,
(Eds.: F. Vogtle, E. Weber), Springer,
Berlin, 1985; c) L.F. Lindoy, The Chemistry o f M u u o c j c i i c Ligunds, Cambridge University Press, Cambridge, 1989.
[6] W. Clegg, W. R. Gill, J. A. H. MacBride, K. Wade, Anger.. Chem. 1993,105,
1402: A n g w . Chern. Inr. Ed. Engl. 1993.32, 1328.
[7] I. T. Chizhevsky, S. E. Johnson, C. B. Knobler, F. A. Gomez. M. F
Hawthorne, J An?. Cliem. Sue , 1993, 115. 6981.
[ 8 ] a) X. Ymg, W. Jidng, C B. Knobler, M. F. Hawthorne, J. Am. Chem. SUC.
1992, 114, 9719; b) J. Miiller. K . Base, T. F. Magnerd. J. Michl, rbrd. 1992,
114, 9721.
[9] a) R. N. Grimes, Chem. Rev. 1992,92,251: b) W. Siebert. Pure Appl. Chem.
1987,5Y, 947;c) X . Meng, M. Sabat, R. N. Grimes, J. Am. Chem. Soc. 1993,
115, 6143.
0570-0H33/93/0909-1290 S 10.00i .2S/0
Angeir.. Chem hi.Ed. Engl. 1993. 32, N o . 9
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