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nido-Five-Vertex Clusters In and Out of Boron Chemistry.

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Five-Vertex Clusters
nido-Five-Vertex Clusters: In and Out of Boron
Yves Canac and Guy Bertrand*
boron · carboranes · cations · phosphorus ·
valence isomerization
In the early days of borane chemistry
the nido-pentaborane B5H9 (1 a) was
called the “stable pentaborane” whereas
arachno-B5H11 was called
the “unstable pentaborane”.[1, 2] Since a BH vertex is isolobal with a CH
unit as well as a naked P
fragment, one could have
expected that a variety of carba-, phospha-, phosphacarba-nido-pentaboranes
or even all-carbon or all-phosphorus
analogues would be readily obtained.[3]
Indeed, the first carborane cage, the
1,2-C2B3H7 (1 b), isoelectronic and isostructural with B5H9 was discovered by
Grimes and co-workers as early as
1970.[4] This compound was prepared in
low yield (10 %) from the reaction of
B4H10 with acetylene (Scheme 1). Compound 1 b was stable in the gas phase up
to 50 8C, but irreversibly polymerizes
within minutes in the liquid phase at
room temperature. The 1,2-dicarba-ni-
Scheme 1. Synthesis of the first five-vertex
nido-carborane in the gas phase. * = BH.
[*] Prof. G. Bertrand, Dr. Y. Canac
UCR-CNRS Joint Research Chemistry
Laboratory, UMR 2282
Department of Chemistry
University of California, Riverside, CA
92521-0403 (USA)
Fax: (+ 1) 909-787-2725
[**] Financial support by the NSF
(CHE0213510) and ACS PRF (38192-AC4)
is gratefully acknowledged.
do-pentaborane structure of 1 b was
originally assigned from IR, 11B and
H NMR spectroscopy, and mass spectrometry data. The results of a microwave study of 1 b, although announced
as a private communication in 1972,[5]
were finally published in 1988[6a] and in
1998 a combined analysis of gas-phase
electron-diffraction data and rotation
constants restrained by ab initio calculations was reported.[6b]
It was only in 2002 that the second
and only other known heteroborane
with a nido-five-vertex geometry was
reported, namely the phosphacarba-nido-pentaborane 1 c (Scheme 2).[7] Of
Scheme 2. Synthesis of the first five-vertex
nido-heterocarborane in the gas phase.
* = BH.
note is that the synthesis of 1 c, in 15 %
yield, was by a route very similar to that
which affords 1 b (Scheme 1). Instead of
an alkyne, Greatrex et al. used a phosphaalkyne, and they also performed the
reaction in the gas phase at 70 8C. Like
dicarbaborane 1 b, the phosphacarbaborane 1 c is only stable in the gas phase
and decomposes in the liquid state at
room temperature. The nido-structure
has been assigned from multinuclear
NMR spectroscopy and mass spectrometry. The optimized geometry has been
calculated at the MP2/6-31G* level.
Before leaving boron chemistry, it is
worth mentioning that the first neutral
closo-borane featuring a square-pyramidal structure, compound 2, has recently
been prepared by hydroboration of a
distorted diamond-shaped tetraborane
(Scheme 3).[8] This compound appears
to be thermally quite stable (m.p. 113 8C,
decomposition) and has been fully characterized including a single-crystal Xray diffraction study.
As mentioned above, there is, in
principle, no reason to restrict the nidofive-vertex structure to boron-containing compounds. Indeed in 1972 Stohrer
and Hoffmann[9] suggested that the antiaromatic [C5H5]+ ion 3 (R =
H) does not maintain
the planar cyclopentadienyl structure, but rearranges to a square-pyramidal geometry as in 1 d (Scheme 4), which would be
the unique stable structure. Although
recent calculations have shown that this
statement was not correct,[10] one should
admit that to date, with the exception of
derivatives with several strongly electron-donating substituents,[11] [C5R5]+
Scheme 3. Synthesis of the first neutral closo-borane with a square-pyramidal structure. * = BH.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200301648
Angew. Chem. Int. Ed. 2003, 42, 3578 – 3580
Scheme 4. Formation of nido-cyclopentadienyl cations in superacid solutions.
cyclopentadienyl cations of structure
3[12–14] have only been detected in matrices at 195 8C by ESR spectroscopy.[15] In contrast, nido structures of type
1 d have been reported to be stable for
several hours at 73 8C even for simple
(R = Me;
Scheme 4).[16] These cations have been
prepared from 3-hydroxyhomotetrahedrane derivatives in superacid media.
Note that species 1 d have also been
postulated as intermediates to explain
the carbon skeletal rearrangements and
loss of alkyne in the gas-phase pyrolysis
of substituted cyclopentadienyl cations
As far as Group 14 elements are
concerned, the recent synthesis of the
first “non-transition-metal 1,3-diphosphacyclobutadienyl compound” 1 e is
noteworthy (Scheme 5).[18] This fascinating species has been obtained in 86 %
yield from the reaction between SnCl2
and a 1,3-diphosphabicyclo[1.1.0]butane
zirconium complex. Compound 1 e is
unusually stable, it melts without decomposition at 154–156 8C and, accord-
least up to 30 8C, and in the solid state,
which allowed a single-crystal X-ray
diffraction study to be carried out.
Interestingly, regardless of the elements constituting the backbone, all the
compounds 1 a–f described here have in
common a very distinctive spectroscopic
property: in the NMR spectrum the
resonance signal corresponding to the
Scheme 6. Chloride abstraction with formation of the nido cation 1 f. The structure in parenthesis is the energetically less favored isomer 1 g.
apical nucleus appears at extremely high
field (1 a: d(11B) = 52 ppm; 1 b:
d(13C) = 21 ppm;
1 c:
d(31P) =
501 ppm; 1 d: d( C) = 23 ppm (R =
Me); 1 e: d(119Sn) = 2129 ppm; 1 f:
d(31P) = 532 ppm (calcd)). To emphasize this statement, white phosphorus
(P4) is usually taken to be
the high-field limit in
and has a resonance signal
at only d 488 ppm. In
agreement with Masamune's
Scheme 5. A five-vertex nido complex with tin in the apical pothese very unusual chemsition.
ical shift values can be
attributed to the especially
ing to NMR spectroscopy, does not constrained geometry of the apical posidecompose in solution even when ex- tion. Not surprisingly, for all of these
posed to the air for 24 h.
compounds, the apical–basal bond
The first example of a nido-five- lengths are substantially longer than
vertex structure featuring phosphorus the basal–basal bond lengths (1 d (R =
and carbon was recently reported.[19] H): 156 and 146 ppm;[20] 1 e: 200 and
180 pm, respectively).
The salt [3,5-tBu2--1,2,4-C2P3]+ [AlCl4]
The characteristics of the closo-pen(1 f) was prepared by abstraction of a
chloride anion from the P Cl bond of a taborane 2 with the square-pyramidal
tricyclic derivative (Scheme 6), a syn- structure[8] are very different from those
thetic route reminiscent of that used for of 1 and especially of nido-B5H9 (1 a).
the all-carbon analogues 1 d. This com- The apical boron atom in 2 is strongly
pound seems to be stable in solution, at deshielded (d = 13.6 ppm) and the
Angew. Chem. Int. Ed. 2003, 42, 3578 – 3580
Bapical–Bbasal bonds (169 pm) are even
shorter than one of the Bbasal–Bbasal bond
lengths (168 and 187 pm).
Despite their similarities, compounds 1 a–f behave very differently,
especially 1 f. As demonstrated by labeling experiments, no exchange between
the basal and apical carbon atoms was
observed for dicarbaborane 1 b and the
all-carbon analogues 1 d. In contrast,
and in agreement with the prediction of
Hoffmann[9] for the [C5H5]+, an exchange is observed between the apical
and basal phosphorus atoms of 1 f. This
process, which is calculated to occur via
a C2v transition state is so fast that it
prevents the observation of resonance
signals in the 31P NMR spectrum.
Based on the results of calculations
concerning the various isomers of 1 f
(with H instead of tBu), there is an
interesting observation concerning the
P/C diagonal relationship. In contrast to
the thesis that “phosphorus is a carbon
copy”,[21] the authors demonstrate that
replacement of three CR groups by
three isolobal P atoms induces significant changes in the potential-energy
surface. In marked contrast to the allcarbon analogues 1 d, the planar triphosphacyclopentadienyl, methylenecyclobutenyl, and even vinylcyclopropenyl
isomers are at least 80 kJ mol 1 higher
in energy than the square-pyramidal
structure 1 f. This occurrence is readily
explained by the inefficient p–p overlap
in double bonds between atoms of the
third period, but also to the aptitude of
these elements to form small-ring structures. The latter point is nicely illustrated by the relative energies of 1 f and 1 g
(Scheme 6), which has a carbon fragment in an apical position: 1 f is calculated to be more than 75 kJ mol 1 lower
in energy than 1 g.[22, 23]
These results open interesting perspectives and challenges. The aptitude
of boron, phosphorus, but also carbon
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 7. An example for an induced, reversible rearrangement of a planar structure into a nido
and tin to occupy the apical position of
nido-five-vertex structures leads us to
imagine that in the near future many
other clusters of this type will be prepared. A fascinating target would be the
pentaphosphorus cation nido-[P5]+.[24]
On the other hand, to date, all of the
compounds featuring a nido-five-vertex
structure have been prepared by a route
designed to directly access the threedimensional structure. Thus, could it be
possible to prepare heteroatomic analogues of planar cyclopendanienyl cations that rearrange into the nido structure or vice versa? An example of a
related process is known (see
Scheme 7).[25]
[1] A. Stock, Hydrides of Boron and Silicon,
Cornell University Press, Ithaca, NY,
[2] For a review on polyhedral boranes and
related molecules:R. B. King, Chem.
Rev. 2001, 101, 1119.
[3] From molecular orbital arguments Lipscomb predicted the existence of squarepyramidal carboranes isoelectronic with
B5H9 :W. N. Lipscomb, Boron Hydrides,
W. A. Benjamin, New York, NY, 1963.
[4] a) D. A. Franz, R. N. Grimes, J. Am.
Chem. Soc. 1970, 92, 1438; b) D. A.
Franz, V. R. Miller, R. N. Grimes, J.
Am. Chem. Soc. 1972, 94, 412.
[5] In ref. [4b], it is mentioned that for 1 b a
square-pyramidal structure analogous to
B5H9 has been postulated from spectroscopic evidence and the preliminary
results of a microwave study performed
by R. A. Beaudet.
[6] a) R. A. Beaudet, Advances in Boron
and the Boranes (Eds.: J. F. Liebman, A.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Greenberg, R. E. Williams), VCH, New
York, 1988, Chap 20; b) M. A. Fox, R.
Greatrex, A. Nikrahi, P. T. Brain, M. J.
Picton, D. W. H. Rankin, H. E. Robertson, M. Buhl, L. Li, R. A. Beaudet,
Inorg. Chem. 1998, 37, 2166.
P. N. Condick, M. A. Fox, R. Greatrex,
C. Jones, D. L. Ormsby, Chem. Commun. 2002, 1448.
C. Prasang, M. Hofmann, G. Geiseler,
W. Massa, A. Berndt, Angew. Chem.
2003, 115, 1079; Angew. Chem. Int. Ed.
2003, 42, 1049.
W. D. Stohrer, R. Hoffmann, J. Am.
Chem. Soc. 1972, 94, 1661.
For reviews on cyclopentadienyl cations: a) K. B. Wiberg, Chem. Rev. 2001,
101, 1317; b) A. D. Allen, T. T. Tidwell,
Chem. Rev. 2001, 101, 1333.
R. Gompper, H. Glockner, Angew.
Chem. 1984, 96, 48; Angew. Chem. Int.
Ed. Engl. 1984, 23, 53.
The recently reported pentamethylcyclopentadienyl cation[13] was in fact a
pentamethylcyclopentenyl cation.[14]
a) J. B. Lambert, L. Lin, V. Rassolov,
Angew. Chem. 2002, 114, 1487; Angew.
Chem. Int. Ed. 2002, 41, 1429; b) J. B.
Lambert, Angew. Chem. 2002, 114, 2383;
Angew. Chem. Int. Ed. 2002, 41, 2278.
a) M. Otto, D. Scheschkewitz, T. Kato,
M. M. Midland, J. B. Lambert, G. Bertrand, Angew. Chem. 2002, 114, 2379;
Angew. Chem. Int. Ed. 2002, 41, 2275;
b) T. MNller, Angew. Chem. 2002, 114,
2380; Angew. Chem. Int. Ed. 2002, 41,
2276; c) J. N. Jones, A. H. Cowley,
C. L. B. Macdonald, Chem. Commun.
2002, 1520.
M. Saunders, R. Berger, A. Jaffe, J. M.
McBride, J. O'Neill, R. Breslow, J. M.
Hoffmann, C. Perchonock, E. Wasserman, R. S. Hutton, V. J. Kuck, J. Am.
Chem. Soc. 1973, 95, 3017.
[16] a) S. Masamune, M. Sakai, H. Ona J.
Am. Chem. Soc. 1972, 94, 8956; b) G.
Olah, G. K. S. Prakash, R. E. Williams,
L. D. Field, K. Weid, Hydrocarbon
Chemistry, Wiley, New York, 1987.
[17] H. Schwarz, H. Thies, W. Franke, Ionic
Processes in the Gas Phase (Ed.:
M. A. A. Ferreira), Reidel Co., Dordrecht, The Netherlands, 1984, pp. 267 –
[18] M. D. Francis, P. B. Hitchcock, Chem.
Commun. 2002, 86.
[19] J. M. Lynam, M. C. Copsey, M. Green,
J. C. Jeffery, J. E. McGrady, C. A. Russell, J. M. Slattery, A. C. Swain, Angew.
Chem. 2003, 115, 2884; Angew. Chem.
Int. Ed. 2003, 42, 2778.
[20] For calculations of the geometrical parameters and 13C NMR spectroscopic
chemical shifts of the parent nido[C5H5]+ ion see:G. K. S. Prakash, G.
Rasul, G. A. Olah, J. Phys. Chem. A
1998, 102, 2579.
[21] K. B. Dillon, F. Mathey, J. F. Nixon,
Phosphorus, The Carbon Copy, Wiley,
Chichester, 1998.
[22] In view of these results, it is rather
surprising that the alternative isomer of
1 c (not shown), with the carbon fragment in the apical position, has been
computed to be only approximately
9.6 kJ mol 1 higher in energy.
[23] Another illustration is the relative energy of benzene which is calculated to be
608 kJ mol 1 for favorable than its still
unknown valence isomer anti-tricyclohexylene, whereas the unknown tetraphosphabenzene is calculated to be only
33 kJ mol 1 more stable than its tricyclic
isomer, of which one derivative has been
isolated. Y. Canac, D. Bourissou, A.
Baceiredo, H. Gornitzka, W. W. Schoeller, G. Bertrand, Science 1998, 279, 2080.
[24] a) M. Gonsior, I. Krossing, L. MNller, I.
Raabe, M. Jansen, L. van Wullen, Chem.
Eur. J. 2002, 8, 4475; b) I. Krossing, J.
Chem. Soc. Dalton Trans. 2002, 500.
[25] C. Balzereit, H.-J. Winkler, W. Massa, A.
Berndt, Angew. Chem. 1994, 106, 2394;
Angew. Chem. Int. Ed. Engl. 1994, 33,
Angew. Chem. Int. Ed. 2003, 42, 3578 – 3580
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