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Topomerization of a Distorted Diamond-Shaped Tetraborane(4) and Its Hydroboration to a closo-Pentaborane(7) with a nido Structure.

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Angewandte
Chemie
Isomerization of Boron Aromatics
A
Topomerization of a Distorted Diamond-Shaped
Tetraborane(4) and Its Hydroboration to a closoPentaborane(7) with a nido Structure**
A
B
A
B
A
+2e
B
B
A
Carsten Prsang, Matthias Hofmann, Gertraud Geiseler,
Werner Massa, and Armin Berndt*
B
- 2 Cl
A
B
A
2b
A
A
B
H
B(NMe2)2
B
B
B
B
H
B
R
B
B
B
R
R
B
B
NMe2
(Me2N)2B
1a,b
2a
B
H
B
H
H
B
H
H
3a
Scheme 1. Known tetrahedral tetraorganyltetraboranes(4) 1 a
(R = tBu)[1] and 1 b (R = 2,4,6-trimethylphenyl),[2] the planar tetraborane(4) 2 a,[3] as well as the unknown pentaborane(7) 3 a. The ellipse
denotes two cyclically delocalized p electrons.
compound 2 b, a tetraalkyltetraborane(4) with a distorted
diamond-shaped structure (Scheme 2). Density functional
computations[4] for the tetramethyltetraborane(4) 2 c
(Scheme 3) show that this distortion is a characteristic
property of planar tetraalkyltetraboranes(4). The exchange
of the short and long diagonal of a diamond, which, as the
diamond–square–diamond (dsd) rearrangement,[5] plays a
central role in isomerizations of boranes, carboranes, and
metallaboranes (Scheme 4), is investigated for the first time,
by way of the topomerization of 2 b, in a diamond that is not
part of a polyhedron. Furthermore, 2 b provides access to the
first derivative 3 b of the hitherto unknown pentaborane(7)
(3 a).
The tetraalkyltetraborane(4) 2 b is obtained by reaction of
a solution of 4[6] in diethyl ether at 100 8C with two
equivalents of lithium naphthalenide in tetrahydrofuran
(Scheme 2). Since 2 b is thermally stable (m.p.: 140 8C without
decomposition), the naphthalene formed can be removed
[*] Prof. Dr. A. Berndt, Dr. C. Pr<sang, G. Geiseler, Prof. Dr. W. Massa
Fachbereich Chemie
Universit<t Marburg
35032 Marburg (Germany)
Fax: (+ 49) 6421-282-8917
E-mail: berndt@chemie.uni-marburg.de
Dr. M. Hofmann
Anorganisch-Chemisches Institut
Universit<t Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2003, 42, No. 9
4
A
B
+ ClBH2
B
H
B
A
A
B
A
H
B
A
_
A
A
B
B
A
A
3b, c
Me2N
A
Cl
H
B
A
A
B
+ NaBHEt3
+ BH3
H
R
A
B
_
A
Known tetraorganyltetraboranes(4) 1 a and 1 b have a tetrahedral structure,[1, 2] the recently described planar 1,3-diamino-2,4-diboryltetraborane(4) 2 a[3] is diamond-shaped with
edges of about equal length (Scheme 1). We present here
A
Cl
_
5
Scheme 2. Synthesis of the tetraalkyltetraborane(4) 2 b from the tetraborane(6) 4 with retention of the two-electron aromaticity, as well as
conversion of 2 b into the pentaborane(7) 3 b or into the aromatic triboracyclopropanate 5. The ellipses and the circle denote two cyclically
delocalized p electrons, three dashed lines two s electrons, which are
delocalized over more than two centers. (A = SiMe3, 3 c: A = H).
easily at 55 8C under vacuum. Compound 2 b reacts with
BH3·SMe2 to form 3 b (ca. 11 % yield), which is obtained in
85 % yield if 2 b is allowed to react initially with NaBHEt3 and
then with H2BCl·SMe2. The triboracyclopropanate 5 is the
intermediate in this reaction.[7] The constitutions of the new
compounds are established by NMR data (Table 1) and X-ray
structure analyses.[8] Figure 1 shows the structures of 2 b and
3 b in the crystal. Selected structural data of 2 b and 3 b are
compared with those computed for the model molecules 2 c, d
(Scheme 3) and 3 a, c (Scheme 1 and 2) in Table 2.
Table 1: Selected physical and spectroscopic properties of 2 b, 3 b, and
5·Na(THF)2.
2 b: yellow solid, m.p. 140 8C, yield 95 %; 1H NMR (300 MHz, C6D6,
27 8C): d = 2.60 (m, 4 H, C2CHSi and B3BCHSi), 2.30 (m, 2 H, B2BCHSi),
0.28, 0.26, 0.12 ppm (each s, each 18 H, SiMe3); 13C NMR (75 MHz,
C6D6, 27 8C): d = 39.9 (d, 1J(C,H) = 122 Hz, C2CHSi), 31.8 (br. d,
1
J(C,H) = 110 Hz, B2BCHSi), 23.1 (br. d, 1J(C,H) = 109 Hz, B3BCHSi), 0.5,
0.1, 3.7 ppm (each q, SiMe3); 11B NMR (96 MHz, C6D6, 27 8C): d = 125,
33 ppm.
3 b: colorless solid, m.p. 113 8C (decomp), yield: 90 % (from 5) or 11 %
(NMR spectrocopy, from 2 b); 1H NMR (300 MHz, CDCl3, 27 8C):
d = 2.51 (br. q, 1 H, 1J(H,B) = 158 Hz, BH), 2.34 (br. s, 2 H, BHB), 1.97 (t,
2 H, C2CHSi), 0.91 (s, 4 H, BCHSi), 0.06, 0.09 ppm (each s, in total
54 H, SiMe3); 13C NMR (75 MHz, CDCl3, 27 8C): d = 38.3 (d,
1
J(C,H) = 125 Hz, C2CHSi), 18.2 (br. d, 1J(C,H) = 115 Hz, BCHSi), 0.3,
3.2 ppm (each q, SiMe3); 11B NMR (96 MHz, CDCl3, 27 8C): d = 8 (s,
4 B), 14 ppm (d, 1 B, 1J(B,H) = 158 Hz).
5·Na(THF)2 : orange solid, m.p. 103 8C (decomp), yield: 83 %; 1H NMR
(300 MHz, C6D6, 27 8C): d = 5.82 (br. s, 1 H, BH), 3.44, 1.38 (THF), 2.96
(pseudo t, 1 H, C2CHSi), 2.33 (d, 1 H, BCHSi), 2.22 (pseudo t, 1 H,
BCHSi), 2.06 (pseudo t, 1 H, C2CHSi), 1.58, 1.45 (each d, each 1 H,
BCHSi), 0.49, 0.46, 0.41, 0.39, 0.18, 0.16 ppm (each s, each 9 H, SiMe3);
13
C NMR (75 MHz, C6D6, 27 8C): d = 68.7, 25.4 (THF), 44.3 (C2CHSi),
38.0, 36.2, 30.2 (each br., BCHSi), 30.1 (C2CHSi), 26.1 (br., BCHSi), 1.8,
1.6, 1.1, 0.9, 2.0, 2.6 ppm (SiMe3); 11B NMR (96 MHz, C6D6, 27 8C):
d = 74, 54 (2 B), 26 ppm.
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Table 2: Selected structural data [pm] of 2 a, 2 b, and 3 b (exp.), 2 c, 2 d, and 3 a, 3 c (computed at the B3LYP/6-31G* level).
B1 B2’
B1 B2
B1 B1’
2a
2b
2c
2d
3a
3b
3c
160.5(2)
163.2(2)
163.3(2)
153.6(3)
179.1(3)
169.3(5)
154.0
175.6
165.8
153.0
182.3
168.9
165.9
192.1
168.6
167.9(3)
187.3(3)
246.3(3)[a]
165.5
192.8
168.7
[a] B1 B3 168.8(3); B2 B3 168.2(3).
Figure 1. Structures of 2 b (top) and 3 b (bottom) in the crystal. The
methyl groups at the Si atoms are omitted for clarity. Selected bond
lengths [pm] and angles [8] (see also Table 2). 2 b: B1-C1 161.1(3), B2C3 152.9(3), C1-C2 158.1(3), C2-C3 156.9(3); B1’-B2-C3 174.6(2), B2’B1-C1 145.9(2), C1-B1-B2 94.8(2), B1-B2-C3 114.1(1); B2’-B1-B1’-B2
177.1(2). 3 b: B1-C1 157.3(3), B2-C3 156.6(3) C1-C2 159.3(3), C2-C3
159.3(2); B1-B2-B1’ 87.6(1), B2-B1-B2’ 92.3(1), C1-B1-B3 137.5(2), C3B2-B3 135.8(2); B1’-B2-B1-B2’ 4.5(2).
Scheme 3. Computed[4] structures of planar and tetrahedral tetraalkyltetraboranes(4), relevant angles, and energy differences.
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The pentaborane(7) derivative 3 b is the first structurally
characterized neutral closo-borane.[9, 10] With six skeletal
electron pairs for five centers, 3 b is to be classified as a
closo compound. As a tetragonal pyramid, it has, however, a
nido structure.[11, 12] The distances between the boron atoms in
the base (167.9 (H-bridged) and 187.3 pm) are considerably
different from the value of 181.1 pm in nido-B5H9.[13] Furthermore, remarkable NMR shifts are observed for 3 b: The
apical boron atom (d = 13.6 ppm ) is strongly deshielded
(compared to d = 48.3 ppm for nido-B5H9, d = 53.1 ppm
for 2,3,4-trimethylpentaborane(9)),[14] as are the bridging
hydrogen atoms (d = + 2.34 compared to d = 2.28 ppm in
nido-B5H9). The unusual values are in agreement with the
results of computations for 3 a and 3 c[4] which gave 11B NMR
shifts of d = 1.0 (1 B) and 7.0 (4 B), and 16.9 (1 B) and 3.9
(4 B) ppm, respectively, as well as NMR shifts of the bridging
hydrogen atoms of d = 0.1 and 2.1 ppm, respectively. At the
same level of theory, shifts computed for B5H9 were close to
those determined experimentally (11B NMR: d = 60.8 (1 B),
18.1 ppm (4 B), 1H NMR: d = 2.5 ppm).
The molecule 2 b has a C2 axis perpendicular to the B4
plane. Its boron atoms form an almost planar
(B2,B1,B1’,B2’ = 1778) diamond with two short (153.6 pm)
and two long (179.1 pm) edges and a short diagonal (B1–B1’)
of 169.3 pm (2 a: 160.5 and 163.2 as well as 163.3 pm,
respectively). The orientations of the bonds to the substituents at the tips of the B4 ring deviate strongly (by 37.68) from
the extended long diagonal in 2 b. In contrast the deviation in
2 a is much smaller (2.48). These distortions, however, are not
the result of the five-membered rings fused to the B4 ring of
2 b: Computations[4] for tetramethyltetraborane(4) 2 c
(Scheme 3) give a similar distorted structure (154.0 and
175.6, as well as 165.9 pm).[15] Also in 2 c, the bonds to the
boron-bound carbon atoms at the tips of the B4 diamond do
not lie along an extended long diagonal (D = 21.28).
Computations[4] also show that tetrahedral 1 c (Scheme 3)
is 18.4 kcal mol 1 lower in energy than planar 2 c, in contrast,
tetrahedral 1 d is 40 kcal mol 1 higher in energy than planar
2 d. This can be explained by the strong distortion (by ca. 368)
of four B-B-C angles in 1 d (108.8 or 109.98) compared to the
angle of 1458 in undistorted 1 c.
Compound 2 b provides the first opportunity to study the
exchange of tri- and tetracoordinate boron atoms, that is the
long and short diagonal of a diamond, which is not part of a
polyhedron.[5] Upon raising the temperature, two of the three
signals for trimethylsilyl groups broaden in the 1H NMR
spectra of 2 b. By line shape analyses we have determined a
barrier of 22 kcal mol 1 for the exchange of these silyl groups,
which are adjacent to the tri- and tetracoordinate boron
atoms. Computations[4] for the models 2 c and 2 d give barriers
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Angew. Chem. Int. Ed. 2003, 42, No. 9
Angewandte
Chemie
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 98
(Revision A.7), 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.
[5] W. N. Lipscomb, Science 1966, 153,
373; S. Wu, M. Jones, Jr., J. Am.
Chem. Soc. 1989, 111, 5373, and
references therein; A. J. Welch, A. S.
Scheme 4. Exchange of the short and long diagonal of diamonds. Top: By way of the diamond–
Weller, J. Chem. Soc. Dalton Trans.
square–diamond rearrangement in polyhedral boranes, carboranes, and metallaboranes.[5] Bottom:
1997, 1205, and references therein;
In the tetraboranes(4) 2 c and 2 d.
R. B. King, Inorg. Chem. 1999, 38,
5151, and references therein.
[6] C. PrGsang, M. Hofmann, G. Geiseler, W. Massa, A. Berndt,
of 26.6 and 20.9 kcal mol 1, respectively, and transition states
Angew. Chem. 2002, 114, 1597; Angew. Chem. Int. Ed. 2002, 41,
TS2 c/2 c’ and TS2 d/2 d’ (Scheme 4), whose B4 skeletons are
1526.
strongly distorted as compared to the square of the dsd
[7] C. PrGsang, A. Mlodzianowska, Y. Sahin, M. Hofmann, G.
rearrangement.
Geiseler, W. Massa, A. Berndt, Angew. Chem. 2002, 114, 3529;
According to natural bond orbital (NBO) analyses the
Angew. Chem. Int. Ed. 2002, 41, 3380.
[8] Crystal structure analyses: 2 b: A pale yellow crystal (0.50 O
transition states TS2 c/2 c’ and TS2 d/2 d’, as well as the ground
0.05 O 0.05 mm) was measured at 193 K on an IPDS area
states 2 c and 2 d, are each characterized by six electrons for
detector system (Stoe) with MoKa radiation. C24H60B4Si6, orthothe s skeleton and two cyclically delocalized p electrons. The
rhombic, space group Pbcn, Z = 4, a = 1914.3(1), b = 1451.0(1),
topomerizations of 2 c and 2 d—and thus also that of 2 b—
c = 1335.7(1) pm, V = 3710.1(4)O10 30 m3, 1calcd = 1.003 Mg m 3,
therefore occur with retention of the aromaticity. A square
20 960 reflections up to q = 25.948, 3613 independent (Rint =
transition state is prohibited due to the crossing of p and s
0.1105), 2206 with I > 2s(I). The structure was solved by direct
molecular orbitals.
methods and refined against F2 with full matrix. The hydrogen
atoms of the SiMe3 groups were refined according to a riding
Received: September 2, 2002 [Z50092]
model, the remainder were refined free with isotropic displacement factors; wR2 = 0.0825 for all reflections, R = 0.0383 for the
observed. 3 b: Under similar conditions a colorless crystal (0.40 O
0.15 O 0.15 mm) was measured. C24H63B5Si6, orthorhombic, space
[1] T. Davan, J. A. Morrison, J. Chem. Soc. Chem. Commun. 1981,
group Pbcn, Z = 4, a = 1993.1(1), b = 1425.2(1), c =
250; T. Mennekes, P. Paetzold, R. Boese, D. BlGser, Angew.
1336.8(1) pm, V = 3797.3(4)O10 30 m3, 1calcd = 1.005 Mg m 3,
Chem. 1991, 103, 199; Angew. Chem. Int. Ed. Engl. 1991, 30, 173;
20 957 reflections up to q = 25.968, 3675 independent (Rint =
D. Hnyk, Polyhedron 1997, 16, 603; A. Neu, T. Mennekes, P.
0.0801),
2258 with I > 2s(I). The structure was treated analoPaetzold, U. Englert, M. Hofmann, P. von R. Schleyer, Inorg.
gously to that of 2 b. The hydrogen atoms H1–H6 were refined
Chim. Acta 1999, 289, 58.
freely; wR2 = 0.0680 for all reflections and R = 0.0341 for the
[2] Tetramesityltetraborane(4): H. NHth, personal communication;
observed. CCDC-192460 (2 b) and CCDC-192459 (3 b) contain
W. Ponikwar, Dissertation, UniversitGt MInchen, 2000.
the supplementary crystallographic data for this paper. These
[3] A. Maier, M. Hofmann, H. Pritzkow, W. Siebert, Angew. Chem.
data can be obtained free of charge via www.ccdc.can.ac.uk/
2002, 114, 1600; Angew. Chem. Int. Ed. 2002, 41, 1529.
conts/retrieving.html (or from the Cambridge Crystallographic
[4] All geometries were optimized by employing the B3LYP hybrid
Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+
functional together with the 6-31G(d) basis set. Relative
44) 1223-336033; or deposit@ccdc.cam.ac.uk).
energies are based on energy computations with 6-311 +
[9] The neutral closo-borane B4tBu4H2 has been characterized
G(d,p) and are corrected for zero-point energies. a) M. J.
unambiguously by spectroscopy and by computations on
Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
model compounds, in addition, its deprotonated form has been
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
structurally characterized.[10]
R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D.
Angew. Chem. Int. Ed. 2003, 42, No. 9
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/03/4209-1051 $ 20.00+.50/0
1051
Communications
[10] A. Neu, T. Mennekes, U. Englert, P. Paetzold, M. Hofmann,
P. von R. Schleyer, Angew. Chem. 1997, 109, 2211; Angew. Chem.
Int. Ed. Engl. 1997, 36, 2117.
[11] The closo structure of a trigonal bipyramid was computed for
B5H52 , the doubly deprotonated form of B5H7.[12] Our calculations for B5H6 give a distorted trigonal bipyramid. Evidently,
the presence of two additional protons is the reason for the
opening of the closo form, which is expected according to the
rules, to the nido form. For singly or doubly deprotonated 3 c,
relative energies of + 7.7 and + 19.0 kcal mol 1, respectively,
were computed for the nido structure in comparison with the
closo structure.
[12] M. McKee, Z.-X. Wang, P. von R. Schleyer, J. Am. Chem. Soc.
2000, 122, 4781, and references therein; S. Kalvoda, B. Paulus, M.
Dolg, H. Stoll, H.-J. Werner, Phys. Chem. Chem. Phys. 2001, 3,
514.
[13] R. Greatrex, N. N. Greenwood, D. H. Rankin, H. E. Robertson,
Polyhedron 1987, 6, 1849, and references therein.
[14] P. M. Tucker, T. Onak, J. B. Leach, Inorg. Chem. 1970, 9, 1430.
[15] Computations[4] reveal that tetramethyltetraborane(4) in the
distorted form 2 c is only about 3.7 kcal mol 1 lower in energy
than a regular form with C2v symmetry, which, however,
corresponds to a fifth-order saddle point.
1052
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Angew. Chem. Int. Ed. 2003, 42, No. 9
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