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Supercarboranes.

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Highlights
Beyond the Icosahedral Carborane
Supercarboranes
Russell N. Grimes*
Keywords:
boron · carboranes · cluster compounds ·
supercarboranes
The icosahedron is to boron what the
hexagon is to carbon. The familiar
layered structure of graphite, the thermodynamically preferred form of elemental carbon, is paralleled in boron
chemistry by the B12 icosahedral unit,
which is a feature of all known allotropes
of the pure element.[1] The icosahedral
[B12H12]2 dianion is the most stable
covalently bonded molecule in all of
chemistry (it is unaffected by 3 m aqueous HCl or concentrated NaOH, and its
dicesium salt is unchanged at 810 8C);[2]
it is, in fact, a three-dimensional “superaromatic” counterpart of benzene that
has 26 delocalized valence electrons in
its s-bonded skeletal framework.[3] An
icosahedral architecture is also found in
the isoelectronic carborane analogues of
[B12H12]2 , which include the [CB11H12]
monoanion and the neutral dicarbon
carboranes 1,2-, 1,7-, and 1,12-C2B10H12,
all of which exhibit extremely high
oxidative and thermal stability and have
given rise to thousands of characterized
derivatives.[1, 4] Additionally, there are
many types of heterocarboranes—clusters having main-group atoms such as P,
As, S, Se, Ge, Sn, or Pb as well as carbon
in the cage framework—and metallacarboranes, in which transition-metal atoms
are incorporated into the cluster skeleton.[1, 4]
The carboranes and their cousins
vary not only in composition, but in
cluster size as well. For example, the nvertex closed polyhedral (closo) carboranes of the general formula C2Bn 2Hn
are known for all values of n from 5 to 12
[*] Prof. Dr. R. N. Grimes
Department of Chemistry
University of Virginia
Charlottesville, VA 22901 (USA)
Fax: (+ 1) 434-924-3710
E-mail: rng@virginia.edu
1198
(in some cases as more than one isomer), and a similar size range is found in
the hetero- and metallacarboranes. Boron clusters with more than 12 vertices
are confined to the 13- or 14-vertex
MxC2B10 or MxC4B8 metallacarboranes
where M is a transition metal and x is 1
or 2;[4a, 5] the first clear example of a pblock 13-vertex cluster, SnC2B10H12, was
reported very recently.[6] Until now, the
“icosahedral barrier” has remained intact for binary boron hydrides and for
carboranes lacking cage heteroatoms,
despite many theoretical predictions of
the viability of supra-icosahedral boron
hydride clusters.[7] Because of the extraordinary stability of the B12 icosahedron,
cage-expansion methods that work with
smaller clusters have, to date, been
unsuccessful when applied to C2B10,
CB11, or B12 polyhedra; although attempted insertions of heteroatoms may
in some cases form 13-vertex species,
they are not isolable and revert to
icosahedral products. So-called macropolyhedral boranes, which are covalently linked or fused assemblies of borane,
carborane, or metallaborane cage units,
have long been known,[1] but while their
architectures are often interesting[8]
these molecules contain no individual
cluster modules larger than 12 vertices.
In light of this history, the achievement of Welch and his co-workers in
breaching this formidable barrier, as
described recently,[9] is impressive. More
importantly, their approach is an eminently rational one that may well open
the way to the controlled synthesis of
even larger carborane polyhedra—a
truly fascinating prospect that could
lead to unprecedented new carboranebased chemistry. Their synthetic strategy employs a long-known technique in
polyhedral borane chemistry, but with a
novel twist.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/03/4211-1198 $ 20.00+.50/0
Polyhedral expansion, a reaction
type originated by Hawthorne and his
students in the 1960s[10] and subsequently exploited greatly in metallacarborane
chemistry,[1, 4] is normally a two-stage
process: a closo n-vertex cage system
undergoes a cage-opening reduction
that is followed by insertion of a new
vertex into the “hole” to create an
(n + 1)-vertex closo polyhedron. The
reverse process, oxidative cage-closure
or polyhedral contraction,[10] entails the
extraction of a boron vertex from an nvertex cluster and the oxidation of the
resulting anionic species to afford a
neutral (n 1)-vertex product.[11] As applied to an icosahedral cage (structure
A, Scheme 1), the addition of two electrons generates an open 12-vertex dianion, for example B, into which one
can, in principle, insert [BR]2+ (where R
is H or a substituent group) to complete
a neutral 13-vertex cluster such as C.
Early carborane investigators found that
the first step works easily with the
C2B10H12 isomers, and isomeric nido[C2B10H12]2 ions are well known.[4c]
However, attempted insertions of nonmetal atoms into these ions have never
produced the desired 13-vertex target
species. Nonetheless, there have been
some intriguing hints. For example,
when the Welch group treated the
nido-[C2B10H12]2 dianion with BI3, the
isolated product was the boron–iodo
derivative, 1,2-C2B10H11-3-I, which suggests that a B I unit may have been
initially incorporated to generate a 13vertex cage that subsequently loses
BH.[9]
Since it is well established that the
carbon atoms become separated when
the neutral carborane is reduced to
nido-[C2B10H12]2 , these workers reasoned that if the carbon separation
could be prevented, one might then
Angew. Chem. Int. Ed. 2003, 42, No. 11
Angewandte
Chemie
Scheme 1.
successfully achieve insertion, and thus
complete the expansion to a 13-vertex
carborane. A way to accomplish this,
they conjectured, would be to prepare a
derivative of 1,2-C2B10H12 in which the
cage carbon atoms are firmly tethered
together by a suitable linking group.
Accordingly, they prepared 1,2-m{C6H4(CH2)2}-1,2-C2B10H10, a carborane
that has an a,a-o-xylene bridge, and
reduced it to the nido-[7,8-m{C6H4(CH2)2}-7,8-C2B10H11]
monoanion. X-ray crystallographic data on this
ion proved that its carborane carbon
atoms remain adjacent and constitute
one edge of a six-vertex C2B4 open face.
To facilitate insertion into this face, its
bridging proton had to be removed, and
this was accomplished by treatment with
butyllithium to generate the nido-[7,8-m{C6H4(CH2)2}-7,8-C2B10H10]2
dianion
(the same species can also be obtained
directly from the neutral 1,2-m{C6H4(CH2)2}-1,2-C2B10H10
through
two-electron reduction with Na metal).
Reaction of the dianion with phenylboron dichloride gave the desired 13vertex
target
species
1,2-m{C6H4(CH2)2}-1,2-C2B11H10-3-Ph,
the
first example of a supra-icosahedral
boron cluster having no metal atoms.
This compound is unprecedented
not only as the first 13-vertex carborane,
Angew. Chem. Int. Ed. 2003, 42, 1198 – 1200
but also in light of its surprising cage
geometry; instead of the docosahedron
C that has been predicted[7d] for the asyet unknown [B13H13]2 anion (and is
well established in structurally characterized 13-vertex MC2B10 metallacarboranes[1a, 4a]), the C2B11 cluster adopts a
henicosahedral structure D which features a four-sided trapezoidal face. As
the authors note, these two geometries
D and C are, in fact, in principle easily
interconverted by a diamond-square
operation (based on the diamondsquare-diamond or dsd process proposed by Lipscomb)[12] in which two
diagonal corners of the four-sided face
of D are moved closer to form a new
edge, generating the all-triangulated
polyhedron C. To my knowledge, the
henicosahedron has not previously been
observed in a boron cluster.
The synthetic approach that allowed
this breakthrough should enable further
advances, since theory suggests that the
energy barrier of 12-to-13 vertex conversion is higher than those involving
further expansion to larger polyhedra
having 14, 15, or more vertices.[7d]
Hence, as Welch and co-workers propose, it may be feasible to expand the
C2B11 polyhedron through cage-opening
and boron insertion to afford successively larger C2Bn clusters. For those
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
having more than 13 vertices, they calculate that it might be possible to
eliminate the external group tethering
the cage carbons together, thus enlarging the scope of synthetic possibilities
even further.
To those of us who have labored in
the carborane vineyards for years, this
work potentially opens new possibilities
that may well have ramifications extending far beyond boron chemistry;
thus empty (no internal atoms) metalcontaining polyhedral clusters of extraordinary size may now be within reach.
For example, the expansion of carboranes by the insertion of metal–ligand
units such as {(h5-C5Me5)M} is well
established and has produced, among
other things, the previously mentioned
13-and 14-vertex metallacarboranes. Extension of this approach by metal insertion into supra-icosahedral carborane
frameworks to generate new, structurally unprecedented macrometallacarboranes appears feasible. It does not
seem excessively speculative to think in
terms of novel metal-containing nanostructured materials constructed from
such molecules. Now that the icosahedral barrier has been surmounted, it will
be fascinating indeed to discover what
lies beyond.
[1] a) F. A. Cotton, G. Wilkinson, C. A.
Murillo, M. Bochmann, Advanced Inorganic Chemistry, 6th ed., Wiley-Interscience, New York, 1999, chap. 5;
b) N. N. Greenwood, The Chemistry of
Boron, Pergamon, Oxford, 1973.
[2] For a recent review, see I. B. Sivaev, V. I.
Bregadze, S. SjIberg, Collect. Czech.
Chem. Commun. 2002, 67, 679.
[3] a) R. B. King, Chem. Rev. 2001, 101,
1119; b) J.-i. Aihara, Inorg. Chem. 2001,
40, 5042; c) M. L. McKee, Inorg. Chem.
2002, 41, 1299.
[4] a) Comprehensive
Organometallic
Chemistry II, Vol. 1 (Eds.: E. W. Abel,
F. G. A. Stone, G. Wilkinson), Pergamon, Oxford, UK, 1995; b) Contemporary Boron Chemistry (Eds.: M. Davidson, A. K. Hughes, T. B. Marder, K.
Wade), Royal Society of Chemistry,
Cambridge, UK, 2000; c) R. N. Grimes,
Carboranes, Academic Press, New York,
1970.
[5] a) W. J. Evans, M. F. Hawthorne, J.
Chem. Soc. Chem. Commun. 1974, 38;
b) W. M. Maxwell, R. F. Bryan, E. Sinn,
R. N. Grimes, J. Am. Chem. Soc. 1977,
1433-7851/03/4211-1199 $ 20.00+.50/0
1199
Highlights
99, 4016; c) J. R. Pipal, R. N. Grimes,
Inorg. Chem. 1978, 17, 6.
[6] N. M. M. Wilson, D. Ellis, A. S. F. Boyd,
B. T. Giles, S. A. Macgregor, G. M. Rosair, A. J. Welch, Chem. Commun. 2002,
464.
[7] a) L. D. Brown, W. N. Lipscomb, Inorg.
Chem. 1977, 16, 2989; b) J. Bicerano,
D. S. Marynick, W. N. Lipscomb, Inorg.
Chem. 1978, 17, 2041; c) J. Bicerano,
D. S. Marynick, W. N. Lipscomb, Inorg.
Chem. 1978, 17, 3443; d) P. von R.
1200
Schleyer, K. Najafian, A. M. Mebel,
Inorg. Chem. 1998, 37, 6765.
[8] J. Bould, M. G. S. Londesborough, D. L.
Ormsby, J. A. H. MacBride, K. Wade,
C. A. Kilner, W. Clegg, S. J. Teat, M.
Thornton-Pett, R. Greatrex, J. D. Kennedy, J. Organomet. Chem. 2002, 657,
256.
[9] A. Burke, D. Ellis, B. T. Giles, B. E.
Hodson, S. A. Macgregor, G. M. Rosair,
A. J. Welch, Angew. Chem. 2003, 115,
235; Angew. Chem. Int. Ed. 2003, 42,
225.
[10] M. F. Hawthorne, G. B. Dunks, Science
1972, 178, 462.
[11] Both processes have recently been exploited in the synthesis and interconversion of small six- and seven-vertex
metallacarboranes: See H.-J. Schanz,
M. Sabat, R. N. Grimes, Angew. Chem.
2001, 113, 2777; Angew. Chem. Int. Ed.
2001, 40, 2705.
[12] W. N. Lipscomb, Science 1966, 153, 373.
Angew. Chem. Int. Ed. 2003, 42, 1198 – 1200
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