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Derivatives of the Simplest Polyhedral Carborane Anion Structures at the Borderline between Two- and Three-Dimensional Aromatic Compounds.

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Derivatives of the Simplest Polyhedral Carborane
Anion: Structures at the Borderline between Twoand Three-Dimensional Aromatic Compounds**
Yksel Sahin, Carsten Prsang, Matthias Hofmann,
Gertraud Geiseler, Werner Massa, and Armin Berndt*
Dedicated to Professor Paul von Ragu Schleyer
on the occasion of his 75th birthday
Salts of tertiary alkyl cations were recently described to be
stable with carborane anions.[1] The latter represent impressive examples of the unusual properties of three-dimensional
aromatics.[2] Characteristic for these is an increase of their
aromatic stabilization energies per center with the number of
framework atoms, that is also with the number of delocalized
s electrons, as shown by computations by Schleyer and
Najafian.[2] As a result, the smallest representatives of a
homologous series of three-dimensional aromatics are the
least stabilized, that is, “the weakest”. In contrast, for the twodimensional aromatics, the smallest members of the series
with (4n+2) p electrons, that is those with the lowest number
of delocalized p electrons, are “the strongest”: Two-electron
(2e) aromatics have significantly higher aromatic stabilization
energies—particulary per center—than comparable six-electron aromatics.[3] Along the homologous series of anions
CBnHn+1 ,[2, 4] which includes the three-dimensional aromatics
1 and 2 (Scheme 1) as well as the puckered and planar twodimensional 2e aromatics 3 and 4,[5, 6] the most weakly
stabilized aromatics of one group are next to the most
strongly stabilized aromatics of the other group.
Here we report on 2 a (Schema 2), the first derivative of
the smallest anionic three-dimensional aromatic. Furthermore we describe two protonation products of 2 a, namely, 6 a
and 8 a. Computations[7] on model compounds show that the
three-dimensional aromatic character—retained upon protonation of 1 and higher homologues (to give 5 and its higher
homologues)[8, 9]—is lost upon protonation of anions 2, the
[*] Dr. Y. Sahin,[+] Dr. C. Prsang, G. Geiseler, Prof. Dr. W. Massa,
Prof. Dr. A. Berndt
Fachbereich Chemie
Universitt Marburg
35032 Marburg (Germany)
Fax: (+ 49) 6421-282-8917
Dr. M. Hofmann
Anorganisch-Chemisches Institut
Universitt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[+] Current address:
Adnan Menderes University
Faculty of Science and Arts
Department of Chemistry
09010 Aydin (Turkey)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2005, 44, 1643 –1646
DOI: 10.1002/anie.200462397
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. The carborane anion 1 and its protonated form 5, as well as the
frameworks of the smaller anions CBnHn+1 and molecules CBnHn+2. In the case
of 7, R may not be an H atom. The circle and the ellipsoids symbolize two cyclic
delocalized electrons, dashed lines indicate multicenter bonds and, in 7 and 8,
3c-2e s bonds.
Figure 1. Structures of anion 2 a (a) as well as of 6 a (b) and 8 a (c) in
the crystal. For clarity only the boron-bound H atoms are shown
Scheme 2. Synthesis of the three-dimensional aromatic 2 a from the
puckered 2e aromatic 7 a, and protonation of 2 a to 6 a and 8 a.
Dur = 2,3,5,6-tetramethylphenyl, R = SiMe3.
Table 1: Selected distances [pm] and angles [8] for 2 a and 6 a–8 a
(determined experimentally) as well as for 8 b and 8 u (computed at the
B3LYP/6-31G* level). Remarkable values are printed in italics.
weakest three-dimensional aromatics of this series. Molecules
of type 8 are two-dimensional 2e aromatics,[10] and those of
type 6 only exist in the crystal, that is, with additional
stabilization by a polar medium.
The lithium salt of anion 2 a forms upon treatment of 7 a[11]
with lithium in diethyl ether at 10 8C. The reaction of 2 a
with HBF4·OMe2 provides the isomeric protonation products
6 a and 8 a, which surprisingly coexist in the crystal.
Figure 1 shows the structures of anion 2 a, as determined
with a crystal of the potassium salt [K(dme)4]-2 a[12] as well as
those of 6 a and 8 a.[13] Selected structural data of these
molecules as well as of 7 a are given in Table 1 along with the
corresponding values computed for the model compounds 8 b
and 8 u (Scheme 3).
As predicted for the prototype 2 u,[2] compound 2 a is a
trigonal bipyramid with an apical C atom. The bond lengths in
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
the framework of 2 a are in good agreement with the values
computed for the model compounds (2 u, 2 b: CB 155.6,
157.4, BB(basal, basal) 183.6, 184.4, BB(basal, apical) each
Angew. Chem. Int. Ed. 2005, 44, 1643 –1646
Scheme 3. Molecules 2 b,u, 6 b,u, and 8 b,u computed for comparison.
(b: R1 = SiH3, R2 = CH3 ; u: R1 = R2 = H).
2e p bond and two additional 3c-2e s bonds between the
boron atoms B1,B2,B4 and B3,B2,B4.
Compound 7 d, a stereoisomer of 7 c, does not exist. Its B
H bond at the methyl-substituted boron atom forms a BHB
bridge. This leads to 8 c where the coordination number of a
carbon-bound boron center is increased to five. In analogy to
the tetracoordinate boron atom in 10, this boron atom can no
longer participate in the cyclic delocalization of the p
electrons. As a result, this delocalization only extends over
three centers. While 10 is a homoaromatic with a classical
borata bridge and a classical s skeleton, compounds 8 c and 8 a
166.7 pm). Substituents have only a small
influence on the distances within the framework of the three-dimensional aromatics.[14, 15]
Compound 6 a derives from 2 a—just as 5[8]
derives from 1—by protonation of a B3 face.
The B1B3 distance in 6 a is elongated as
compared to that in 2 a by 10 pm more than in
5 with respect to 1. However, according to
calculations, the Cs-symmetrical model compounds 6 u and 6 b are transition states with
energies that are very close to those of the
edge-protonated minima 8 u and 8 b (0.5 and
0.0 kcal mol1, respectively). Compound 6 a
Scheme 4. Bicyclobutanes such as 9* and 7 c* with two boron atoms as electron-deficient centers
exists in the crystal due to its dipole moment,
are puckered four-membered 2e aromatics. Upon increasing the coordination number of one of
which provides additional stabilization in the
the two electron-deficient centers by addition of a hydride ion (9!10) or formation of a BHB
bridge (7 d!8 c) the cyclic delocalization of the p electrons is reduced to three centers.
polar environment of the crystal.[16] We conclude this from the free enthalpies of solvation
DGsolv calculated with water as solvent.[7] The
are 2e homoaromatics with a partial nonclassical BH2 bridge
DGsolv values are larger for 6 u and 6 b than for 8 u and 8 b
(3.1 and 9.7 compared to 0.6 and 4.5 kcal mol1).
and a partial nonclassical s framework. Substituents have a
comparatively small influence on the geometrical parameters
Compound 8 a differs from 7 a mainly in the periphery:
of the aromatic portion of molecules of type 8 as on the entire
replacement of a chloride ion by a hydride ion, an additional
geometry of compounds of type 2, in which all the framework
BHB bridge, and a different substitution pattern for the boron
atoms participate in the aromatic system.
atoms of the CB3 ring.[17] Similarities predominate in the
Compound 2 a represents the first derivative of the
framework: Remarkably short and long distances (given in
simplest three-dimensional aromatic in the series of carboritalics in Table 1) are found in the framework between
ane anions CBnHn+1 to be synthesized and characterized.
corresponding centers: C1B1 in 8 a is 148.6 pm and thus
relatively short, as are C1B1,3 in 7 a (149.4, 150.2 pm). Bond
Upon protonation of 2 a the three-dimensional aromatic
lengths B1B3 and B1B4 in 8 a are fairly long in comparison
character is lost, whereas it is retained upon protonation of
to those in 2 a and thus are more similar to those in 7 a. The
the more strongly stabilized higher homologues of 2.
strong dependence of the elongation of the framework
Received: October 22, 2004
distances on the substituents is also striking, a feature that
was not observed for the three-dimensional aromatics 2.
Keywords: aromaticity · boron · carboranes ·
To analyze the similar bond lengths in the frameworks of
density functional calculations · multicenter bonds
7 a and 8 a, we carried out DFT calculations[7] and NBO
analyses.[18] These showed that the initial formulation[11] of a
boryltriborabicyclobutane with an open three-center, twoelectron (3c-2e) bond between the framework boron atoms
[1] T. Kato, C. A. Reed, Angew. Chem. 2004, 116, 2968; Angew.
Chem. Int. Ed. 2004, 43, 2908 and references therein.
does not sufficiently describe the electronic structure of 7 a.
[2] P. von R. Schleyer, K. Najafian, Inorg. Chem. 1998, 37, 3454 and
As early as 1986, through the use of isodesmic equations,
references therein.
Schleyer et al. demonstrated that the puckered 2e aromatic 9
[3] P. H. M. Budzelaar, P. von R. Schleyer, J. Am. Chem. Soc. 1986,
is highly favored for the related bicyclobutane with two boron
108, 3967, and references therein; C. S. Wannere, P. von R.
atoms (9*, Scheme 4; 49.5 kcal mol1 at the level of theory
Schleyer, Org. Lett. 2003, 5, 865.
used here).[19] Calculations on boryltriborabicyclobutane 7 c*
[4] Representative of the series CBnHn+1 with n = 6 were recently
provide the analogous electronic structure 7 c with a CB3 4cdescribed: B. Stibr, O. L. Tok, W. Milius, M. Bakardjiev, J. Holub,
Angew. Chem. Int. Ed. 2005, 44, 1643 –1646
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
D. Hnyk, B. Wrackmeyer, Angew. Chem. 2002, 114, 2230;
Angew. Chem. Int. Ed. 2002, 41, 2126; A. Franken, D. L. Ormsby,
C. A. Kilner, W. Clegg, M. Thornton-Pett, J. D. Kennedy, J.
Chem. Soc. Dalton Trans. 2002, 2807.
Y. Sahin, C. Prsang, P. Amseis, M. Hofmann, G. Geiseler, W.
Massa, A. Berndt, Angew. Chem. 2003, 115, 693; Angew. Chem.
Int. Ed. 2003, 42, 669.
A. A. Korkin, P. von R. Schleyer, U. von Arx, R. Keese, Struct.
Chem. 1995, 6, 225, and references therein.
Geometry optimizations were carried out with the B3LYP
hybrid functional using the 6-31G(d) basis set. The stationary
points were characterized by analytical frequency calculations.
Relatives energies are based on energy calculations with 6-311 +
G(d,p) and are corrected for zero-point vibrational energies. The
energies of solvation were calculated with the conductor-like
polarizable continuum model (CPCM). a) Gaussian 98 (Revision A.7), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich,
J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas,
J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson,
P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B.
Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres,
M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc.,
Pittsburgh, PA, 1998; b) A. D. Becke, J. Chem. Phys. 1993, 98,
1372; A. D. Becke, J. Chem. Phys. 1993, 98, 5648; c) C. Lee, W.
Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
J. Jaballas, T. Onak, J. Organomet. Chem. 1998, 550, 101, and
references therein; M. L. McKee, M. Bhl, O. P. Charkin,
P. von R. Schleyer, Inorg. Chem. 1993, 32, 4549.
M. McKee, Inorg. Chem. 2001, 40, 5612; I. A. Koppel, P. Burk, I.
Koppel, I. Leito, T. Sonoda, M. Mishima, J. Am. Chem. Soc.
2000, 122, 5114.
Molecules of type 7 are also two-dimensional aromatics; they
formally result upon reaction of the three-dimensional aromatics
2 with electrophiles (e.g. 2 a + Cl+!7 a).
Y. Sahin, C. Prsang, M. Hofmann, G. Subramanian, G. Geiseler,
W. Massa, A. Berndt, Angew. Chem. 2003, 115, 695; Angew.
Chem. Int. Ed. 2003, 42, 671. In that work 7 a (referred to there as
4 a) is insufficiently described as a triborabicyclobutane with an
open 3c-2e BBB bond.
The potassium salt of 2 a crystallized from a solution of the
lithium salt in dimethoxyethane (DME) containing small
amounts of potassium/sodium alloy. Anion 2 a formed a solvent-separated ion pair with a potassium ion coordinated with
four DME molecules. [K(dme)4]-2 a: colorless solid, m.p.
> 190 8C 8C (no decomp), yield 87 %; 1H NMR (500 MHz,
[D8]THF, 10 8C): d = 6.42 (s, 2 H, p-H), 2.12, 2.05 (each s,
each 12 H; o-, m-CH3), 0.54, 0.35 (each s, each 2 H, BCH2),
0.01, 0.15, 0.31 ppm (each s, each 9 H, Me3Si); 13C NMR
(125 MHz, [D8]THF, 10 8C): d = 149.7 (br s, 2 C, i-C), 136.2,
130.9 (each s, each 4C; o-, m-C), 127.3 (d, 2 C, p-C), 68.8, (br s,
1 C, CB3, confirmed by measurements in [D10]DME: 69.8), 21.2
(q, 8 C; o-, m-CH3), 5.3, 3.3 (each br t, each 1 C, BCH2), 2.5, 2.1,
1.7 ppm (each q, each 3 C, Me3Si); 11B NMR (96 MHz, [D8]THF,
10 8C): d = 36 (1 B), 6 ppm (3 B).
Crystal structure determination: [K(dme)4]-2 a: Measurements
were carried out on a colorless crystal (0.35 0.20 0.20 mm) at
193 K with an IPDS-II area detector system (Stoe) with MoKa
radiation. C48H97B4KO8Si3, Mr = 968.87, orthorhombic, space
group P212121, Z = 4, a = 1233.1(1), b = 1394.8(1), c = 3557.1(2) pm, V = 6118.0(7) 1030 m3, 1calcd = 1.052 Mg m3. Of 37 258
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
reflections to q = 26.38, 12 166 were independent (Rint = 0.0321)
and 9488 were with I > 2s(I). The structure was solved with
direct methods and refined against all F 2 data with full matrix.
The hydrogen atoms of the methylene groups were refined with
isotropic displacement parameters. The remaining hydrogen
atoms were placed as “riding” atoms in calculated positions with
displacement parameters set to 1.5 times Ueq of the bonding
partner. wR2 = 0.0993 for all reflections, R = 0.0416 for observed
reflections. The correctness of the absolute structure is proved by
refinement of the Flack parameter to x = 0.02(4). Compounds 6 a
and 8 a: colorless crystal (0.54 0.21 0.07 mm), C32H58B4Si3,
Mr = 570.29, monoclinic, space group P21/n, Z = 8, a = 1445.5(3),
b = 1381.4(2),
c = 3732.4(6) pm,
b = 95.07(2)8,
7424(2)·1030 m3, 1calcd = 1.020 Mg m3. Measurements were carried out on an IPDS-II area detector system (Stoe) with MoKa
radiation. Of 55 536 reflections to q = 26.08, 13 828 were independent (Rint = 0.0541) and 8952 were with I > 2s(I). The
structure solution was analogous to that of [K(dme)4]-2 a;
wR2 = 0.1018 for all reflections and R = 0.0413 for observed
reflections. The asymmetrical unit contains two isomers (6 a and
8 a) for which the hydrogen atoms on boron and on the
methylene groups were freely refined. CCDC-196908 ([K(dme)4]-2 a) and CCDC-196907 (6 a and 8 a) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
The aromatic stabilization energy (ASE) of 2 u can be estimated
as 27 kcal mol1 by interpolation of the ASEs calculated for
C2B3H5 and B5H52 (19.8 and 34.8 kcal mol1, respectively[15]).
P. von R. Schleyer, G. Subramanian, A. Dransfeld, J. Am. Chem.
Soc. 1996, 118, 9988.
For the influence of the environment on the stabilization of polar
molecules, see: V. Jonas, G. Frenking, M. T. Reetz, J. Am. Chem.
Soc. 1994, 116, 8741, and references therein.
A 1:1 mixture of 7 a and 8 a: colorless solid, m.p. 132–133 8C
(decomp), yield 85 %. Only 8 a is present in solution: 1H NMR
(500 MHz, C6D6, 27 8C): d = 6.94 (s, 2 H, p-H), 3.09 (br s, 1 H,
BHB), 2.40, 2.17 (each s, each 12 H; o-, m-CH3), 0.95, 0.75 (each
s, each 2 H, BCH2), 0.28, 0.23, 0.09 ppm (each s, each 9 H,
Me3Si); 13C NMR (125 MHz, C6D6, 27 8C): d = 137.5, 133.7 (each
s, each 4 C; o-, m-C), 132.9 (d, 2 C, p-C), 80.3 (br s, 1 C, CB3), 22.9,
20.3 (each q, each 4 C; o-, m-CH3), 5.0, 4.9 (each br t, each 1 C,
BCH2), 1.6, 0.5, 0.4 ppm (each q, each 3 C, Me3Si), the signal for
the i-C atom was observed at 134.0 ppm in [D8]dioxane at 70 8C;
B NMR (160 MHz, C6D6, 27 8C): d = 27.0, 21.6 (2 B), 15.7 ppm.
In solution a rapid exchange of the two duryl-substituted boron
atoms takes place for 8 a: Only one signal is observed for the p-H
or p-C atoms of the duryl groups in the NMR spectra of 8 a down
to 80 8C. At even lower temperature the signals broaden. This
can be explained by the enantiomerization of 8 a via a Cssymmetric transition state whose mirror plane contains the BHB
bridge and bisects the B1B2 bond. Cleavage of the B2B3 bond
and further movement of the bridging H atom towards B2 results
in the enantiomer of 8 a in which the duryl-substituted boron
atoms have formally switched places, in fact, however,
exchanged their coordination environments. For 8 b a value of
8.2 kcal mol1 can be calculated for the energy required for the
exchange to take place.
A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899.
P. H. M. Budzelaar, E. Kraka, D. Cremer, P. von R. Schleyer, J.
Am. Chem. Soc. 1986, 108, 561, and references therein.
Angew. Chem. Int. Ed. 2005, 44, 1643 –1646
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polyhedra, two, structure, dimensions, compounds, simplest, carborane, borderlines, anion, three, aromatic, derivatives
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