Multiply borylated arenes: X-ray crystal structure analyses and quantum chemical calculations

Article abstract of DOI:10.1002/zaac.200400021

Optimised synthesis procedures and results of X-ray single crystal structure analyses for 4-(dibromoboryl)toluene, 1,3-bis(dibromoboryl)benzene, 1,4-bis(dibromoboryl)benzene, and 1,3,5-tris(dibromoboryl)benzene are reported. These compounds have also been studied by Hartree-Fock (HF), density functional theory (DFT), and Moller-Plesset second-order perturbation (MP2) methods in combination with the polarized double-ζ valence (SVP) and polarized triple-ζ valence (TZVP) basis sets of Ahlrichs and coworkers. A comparison of the quantum chemical results for optimised geometries and computed NMR chemical shifts with experiment is presented to test the quality of the various methods for this class of compounds. All DFT methods tested yield optimised geometries within the experimental error bars of 3σ for bond lengths, whereas larger deviations among the methods are observed for computed NMR chemical shifts. This calibration recommends the B3LYP/SVP combination as a reliable and computationally efficient level of theory to assess the structures and absolute and relative ¹H-, ¹³C- and ¹¹B NMR shift values of borylated aromatic compounds in future investigations.

Full text of DOI:10.1002/zaac.200400021

Multiply Borylated Arenes: X-ray Crystal Structure Analyses and Quantum
Chemical Calculations
Monika C. Haberechta, Julia B. Heilmanna, Alireza Haghiria, Michael Boltea, Jan W. Batsb,
Hans-Wolfram Lernera, Max C. Holthausenc,* and Matthias Wagnera,*
Frankfurt am Main, a Institut für Anorganische Chemie and b Institut für Organische Chemie der J. W. Goethe-Universität
Marburg, c Fachbereich Chemie der Philipps-Universität
Received January 30th, 2004.
Abstract. Optimised synthesis procedures and results of X-ray
single crystal structure analyses for 4-(dibromoboryl)toluene, 1,3-
bis(dibromoboryl)benzene, 1,4-bis(dibromoboryl)benzene, and
1,3,5-tris(dibromoboryl)benzene are reported. These compounds
have also been studied by Hartree-Fock (HF), density functional
theory (DFT), and Møller-Plesset second-order perturbation
(MP2) methods in combination with the polarized double-ζ valence
(SVP) and polarized triple-ζ valence (TZVP) basis sets of Ahlrichs
and coworkers. A comparison of the quantum chemical results for
optimised geometries and computed NMR chemical shifts with
experiment is presented to test the quality of the various methods
for this class of compounds. All DFT methods tested yield op-
timised geometries within the experimental error bars of 3σ for
bond lengths, whereas larger deviations among the methods are
observed for computed NMR chemical shifts. This calibration rec-
ommends the B3LYP/SVP combination as a reliable and compu-
tationally efficient level of theory to assess the structures and abso-
1
lute and relative H-, 13C- and 11B NMR shift values of borylated
aromatic compounds in future investigations.
Keywords: Lewis acids; Arylboranes; Crystal structures; Quantum
chemical calculations
Mehrfach borylierte Arene: Kristallstrukturanalysen and quantenchemische
Rechnungen
Inhaltsübersicht. Wir berichten über optimierte Synthesevorschrif-
ten und die Ergebnisse der Kristallstrukturanalysen von 4-(Dibro-
moboryl)toluol, 1,3-Bis(dibromoboryl)benzol, 1,4-Bis(dibromobo-
ryl)benzol und 1,3,5-Tris(dibromoboryl)benzol. Diese Verbindun-
gen wurden weiterhin mittels Hartree-Fock (HF), Dichtefunktio-
naltheorie (DFT) und Møller-Plesset-Störungstheorie (MP2) in
Kombination mit dem polarisierten Double-ζ Valenz- (SVP) und
dem polarisierten Triple-ζ Valenzbasissatz (TZVP) von Ahlrichs et
al. untersucht. Ein Vergleich der optimierten Geometrien und be-
rechneten NMR-Verschiebungen mit den entsprechenden experi-
mentellen Daten dient als Test für die Qualität der unterschiedli-
chen theoretischen Methoden im Hinblick auf eine zuverlässige Be-
handlung der hier untersuchten Verbindungsklasse. Alle eingesetz-
ten DFT-Methoden liefern optimierte Molekülstrukturen, deren
Bindungslängen im Rahmen des 3σ-Fehlerintervalls mit dem Expe-
riment übereinstimmen. Bei der Berechnung von NMR-Verschie-
bungen treten signifikante Abweichungen zwischen den einzelnen
Methoden auf. Für zukünftige Untersuchungen borylierter aroma-
tischer Verbindungen empfiehlt sich die Verwendung der B3LYP/
SVP Kombination als zuverlässiges und effizientes theoretisches
Niveau zur Berechnung von Molekülstrukturen sowie absoluter
1
und relativer H-, 13C- und 11B NMR-Verschiebungen.
1 Introduction
substituted 1-4 times upon reaction with the appropriate
amount of BBr3 (Ia-d, Figure 1) [2Ϫ6]. The resulting ferro-
cenylboranes have been successfully employed for the gener-
ation of charge transfer polymers, redox-active macrocycles
and ferrocenophanes with switchable ansa bridges (see refs
[7, 8] for reviews). Recently, even polymeric boron-contain-
ing Lewis acids (e.g. dibromoborylated polystyrene II, Fig.
1) have been reported by Jäkle et al. [9].
Molecules containing several Lewis acidic functional
groups (e.g. BR2) are currently gaining growing attention
due to potential applications as (co)catalysts in organic
transformations [1]. Among the smaller molecules that may
serve as backbones of multiply borylated compounds, ferro-
cene is of particular interest since it can conveniently be
Our group is interested in borylarene derivatives, which
may serve as useful building blocks for the synthesis of oli-
gonuclear Lewis acids and oligotopic tris(1-pyrazolyl)bo-
rate ligands [10]. In contrast to ferrocene, borylation of ben-
zene is not achieved by the direct reaction of the aromatic
compound with boron trihalides. However, treatment of
(trimethylsilyl)arenes with BX3 (X ϭ Cl, Br, I) results in the
clean substitution of the silyl group for a BX2 substituent
(Scheme 1) [11]. Using this method, various monoboryl-
ated, 1,2-, 1,3- and 1,4-diborylated as well as 1,3,5-triboryl-
* Prof. Dr. M. Wagner
Institut für Anorganische Chemie
der J. W. Goethe-Universität
Marie-Curie-Str. 11
D-60439 Frankfurt (Main)
Fax: ϩ49(0)69-79829260
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
* Dr. M. C. Holthausen
Fachbereich Chemie der Philipps-Universität
D-35032 Marburg
904
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
DOI: 10.1002/zaac.200400021
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Multiply Borylated Arenes
equiv.; 2, 3, 5: 3-4 equiv.; 4: 6 equiv.) The general reaction
is outlined in Scheme 1. The first BBr2 group is easily intro-
duced into the aryl ring already at ambient temperature,
whereas further substitution generally requires higher tem-
peratures.
1
Note: For the assignment of H- and 13C NMR reson-
ances, a numbering scheme following the standard IUPAC
nomenclature is employed, which is different from the atom
labelling in the crystal structure analyses. The 11B NMR
signals of 1-5 lie in the range between 56.6 ppm (1) and
58.3 ppm (3) characteristic of trivalent aryl-BBr2 species
[15]. Since the boron atoms possess rather uniform coordi-
nation spheres in 1-5, the strongest influence on the mag-
netic shielding can be expected to come from the π charge
density on the boron nuclei which is obviously very similar
in all five compounds. As an experimental probe for the
charge density distribution within the aromatic rings, 13C
NMR spectroscopy provides a more reliable diagnostic tool
than 1H NMR spectroscopy, because proton shift values
may be greatly influenced by magnetic anisotropy effects
[16]. In the monoborylated derivative 1, the carbon atoms
C-3 and C-5 appear at δ ϭ 138.3 and are thus deshielded
by 9.8 ppm compared to the corresponding carbon nuclei
in parent toluene (δ ϭ 128.5 [16]). In contrast, only a negli-
gible shift difference is observed between the carbon atoms
C-2 and C-6 of 1 (δ ϭ 129.4) and the complementary car-
bon atoms of toluene (δ ϭ 129.3 [16]). Introduction of a
second BBr2 substituent into the 3-position of the benzene
ring (2) leads to three 13C NMR resonances with chemical
shift values of δ ϭ 147.0 (C-2), 143.5 (C-4,6) and 128.1 (C-
5; C-1,3 are not observed due to an extreme broadening of
their 13C NMR signals, which has to be attributed to the
quadrupolar relaxation of the boron nucleus [15]). Again,
the chemical shift of the carbon atom in the meta position
to both boryl substituents (C-5) is virtually unchanged
compared to the benzene molecule [δ(C6H6) ϭ 128.5 [16]].
The other three carbon nuclei, which experience a π elec-
tron-withdrawing effect of two boryl groups, show an ad-
ditional deshielding of 8.7 ppm (C-2) and 5.2 ppm (C-4,6)
with respect to the monoborylated species 1 (18.5 ppm and
15.0 ppm with respect to benzene). In the 1,4-diborylated
derivative 3, the atoms C-2,3,5,6 give rise to one 13C NMR
signal at δ ϭ 136.5 in agreement with the resonances of C-
3,5 in 1 (δ ϭ 138.3). In the triborylated molecule 4, each of
the carbon atoms C-2, C-4 and C-6 is located in a position
ortho to two boryl substituents and para to the third one.
Consequently, the respective NMR resonance appears at
extremely low field [δ(C-2,4,6) ϭ 151.4; ∆δ(4-benzene) ϭ
22.9]. As to be expected, the 13C NMR spectrum of the
Figure 1 Mono- to tetraborylated ferrocene derivatives (Ia Ϫ Id)
and the polymeric Lewis acid II.
Scheme 1 Borylated arenes 1Ϫ5 and the general synthesis strategy
via silyl/boryl exchange.
ated benzene derivatives have been synthesized by Kauf-
mann [11] and Siebert [12]. Siebert et al. also obtained hexa-
borylbenzenes by the CpCo(CO)2-catalysed cyclotrimeris-
ation of diborylacetylenes [13].
The purpose of this paper is to summarize optimised syn-
thetic procedures for the (dibromoboryl)arenes 1-5 (Scheme
1), to provide a full NMR spectroscopical characterization
of these compounds, and to discuss their molecular struc-
tures in the solid state. The experimental data is augmented
with density functional (DFT) calculations [14] on 1-4 to
1
unambiguously assign H- and 13C NMR chemical shifts
and to get insight into the degree of aryl-boron π bonding
within these compounds. In order to test the applicability
of contemporary DFT for this class of compounds, several
density functionals widely in use have been tested in combi-
nation with standard basis sets.
biphenyl derivative
5
[δ(C-3,3Ј,5,5Ј)
ϭ
138.6, δ(C-
2,2Ј,6,6Ј) ϭ 127.1], which bears one BBr2 substituent per
C6H4 ring, closely resembles the spectrum of 4-(dibromobo-
ryl)toluene 1 [δ(C-3,5) ϭ 138.3, δ(C-2,6) ϭ 129.4]. Qualitat-
ively similar trends as discussed for the 13C NMR shifts are
observed for the proton resonances of 1 Ϫ 5 (see Exper-
imental Section).
2 Results and Discussion
2.1 Synthesis and Spectroscopy
All borylarenes 1Ϫ5 were synthesized by treating the corre-
sponding aryl(trimethyl)silanes with excess BBr3 (1: 1.5
Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913
zaac.wiley-vch.de
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905

M. C. Haberecht, J. B. Heilmann, A. Haghiri, M. Bolte, J. W. Bats, H.-W. Lerner, M. C. Holthausen, M. Wagner
Table 1 Crystallographic data of the compounds 1 Ϫ 5
1
2
3
4
5
formula
fw
C7H7BBr2
261.76
C6H4B2Br4
417.35
C6H4B2Br4
417.35
C6H3B3Br6
586.97
C12H8B2Br4
493.44
colour, shape
crystal size /mm
temp /K
radiation (MoKα) /A
crystal syst.
colourless, plate
0.45ϫ0.28ϫ0.06
143(2)
0.71073
orthorhombic
Pca21
20.161(4)
4.0467(7)
10.542(2)
colourless, needle
0.49ϫ0.11ϫ0.09
100(2)
0.71073
orthorhombic
Pna21
20.1283(15)
4.0393(3)
13.0422(13)
colourless, needle
0.26ϫ0.09ϫ0.07
173(2)
0.71073
monoclinic
P21/c
4.0998(10)
10.805(2)
12.275(3)
91.54(2)
543.6(2)
2
colourless, needle
0.28ϫ0.05ϫ0.03
173(2)
0.71073
hexagonal
P6122
11.0560(9),
11.0560(9),
20.376(2),
colourless, plate
0.33ϫ0.25ϫ0.05
173(2)
0.71073
monoclinic
P21/n
4.0864(12)
20.970(4)
8.573(3)
100.66(3)
722.0(4)
2
˚
space group
˚
a /A
˚
b /A
˚
c /A
β /deg
3
˚
V /A
860.1(3)
4
2.021
9.344
10551
1888
1060.39(15)
4
2.614
15.121
11711
1878
2157.0(3)
6
2.711
16.715
15122
1042
Z
Dcalcd. /g cmϪ3
2.550
14.749
4399
787
2.270
11.124
3139
745
µ /mmϪ1
no. of rflns. coll.
no. of rflns. obs.
(I > 2σ(I))
no. of indep. rflns.
R(int)
data/restr./ param.
GOF
2643
0.0848
2643/1/ 92
1.044
2064
0.0610
2064/1/109
0.995
1098
0.0728
1098/0/55
1.005
1297
0.1511
1297/0/70
1.045
1296
0.1909
1296/0/82
0.983
F(000)
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
496
760
380
1584
460
0.0498, 0.0947
0.0838, 0.1047
0.896, Ϫ0.891
0.0261,0.0589
0.0294, 0.0598
0.660, Ϫ0.330
0.0420, 0.0832
0.0685, 0.0921
0.701, Ϫ0.714
0.0507, 0.0719
0.0730, 0.0771
0.382, Ϫ0.573
0.1058, 0.2578
0.1473, 0.2879
1.287, Ϫ1.377
Ϫ3
˚
largest diff. peak and hole /e A
2.2 X-ray crystallography
X-ray quality crystals of 1 (orthorhombic; Pca21) were ob-
tained by slow evaporation of its n-pentane solution. 2 (or-
thorhombic; Pna21), 3 (monoclinic; P21/c), 4 (hexagonal;
P6122) and 5 (monoclinic; P21/n) precipitated from the re-
action mixtures [solvents: pentane (2), toluene (3, 4, 5)]
upon cooling from reflux temperature to ambient tempera-
ture. Details of the crystal structure analyses of 1-5 are sum-
marized in Table 1.
1 (Figure 2) adopts an almost planar conformation in the
solid state (dihedral angle between the plane of the phenyl
ring and the plane spanned by the BBr2 substituent: 6.5°).
The Br(1)-B(1)-Br(2) angle of 114.4(4)° is significantly
smaller than the adjacent angles C(1)-B(1)-Br(1) [122.4(5)°]
and C(1)-B(1)-Br(2) [123.3(5)°; sum of angles around the
boron atom 360°]. The angle with the boryl substituent at-
tached to its vertex [C(2)-C(1)-C(6) ϭ 116.6(5)°], deviates by
3.4° from the value of 120° expected for an ideal sp2-hy-
bridized carbon atom. The crystal lattice features intermolec-
Figure 2 Structure plot of 1 in the solid state. Displacement ellips-
oids are drawn at the 50 % probability level.
˚
Bond lengths/A, angles/° and dihedral angles/°: B(1)-C(1) 1.499(9), B(1)-
Br(1) 1.943(7), B(1)-Br(2) 1.931(7), C(1)-C(2) 1.421(8), C(2)-C(3) 1.373(9),
C(3)-C(4) 1.383(9), C(4)-C(5) 1.384(9), C(5)-C(6) 1.369(9), C(6)-C(1)
1.395(8); Br(1)-B(1)-Br(2) 114.4(4), C(1)-B(1)-Br(1) 122.4(5), C(1)-B(1)-Br(2)
123.3(5), C(1)-C(2)-C(3) 120.8(6), C(2)-C(3)-C(4) 121.3(6), C(3)-C(4)-C(5)
118.2(6), C(4)-C(5)-C(6) 121.3(6), C(5)-C(6)-C(1) 121.6(5), C(6)-C(1)-C(2)
116.6(5); C(1)C(2)C(3)C(4)C(5)C(6)//B(1)Br2 6.5.
˚
ular C-H···Br contacts with distances ranging from 3.18 A to
substituent: 4.3(5)°]. The carbon-carbon bonds C(1)-C(2) ϭ
˚
˚
˚
˚
3.23 A, and C-H···π(phenyl) contacts of about 3.0 A.
1.405(9) A and C(1)-C(3A) ϭ 1.412(9) A are somewhat
The planar 1,3-diborylated compound 2 [dihedral angles:
longer than those bonds of the aromatic ring which connect
non-borylated carbon atoms [C(2)-C(3) ϭ 1.385(9) A]. 3
˚
C(1)C(2)C(3)C(4)C(5)C(6)//B(1)Br2
C(1)C(2)C(3)C(4)C(5)C(6)//B(2)Br2
structural features similar to 1 (Figure 3). 2 crystallizes in
ϭ
5.6(5)°;
ϭ
6.0(5)°] exhibits
forms a layer structure in the solid state. Within these layers,
the B···B axis of each molecule is oriented orthogonal to
the B···B axes of its four next neighbors. This arrangement
parallel stacks and shows intermolecular C-H···Br contacts
˚
˚
˚
in a range from 3.08 A to 3.12 A.
results in intermolecular C-H···Br contacts of 3.98(1) A,
˚
˚
Compound 3 (Figure 4) contains an inversion center in
the solid state and features an almost planar molecular
structure similar to 1 and 2 [dihedral angle between the
plane of the phenyl ring and the plane spanned by the BBr2
3.89(1) A and 3.76(3) A. Consecutive layers are shifted with
respect to each other in such a way that the bromine atoms
of one layer are placed on top of the boron atoms of the
other.
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Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913

Multiply Borylated Arenes
Figure 3 Structure plot of 2 in the solid state. Displacement ellips-
Figure 5 Structure plot of 4 in the solid state. Displacement ellips-
oids are drawn at the 50 % probability level.
Bond lengths/A, angles/° and dihedral angles/°: B(1)-C(1) 1.543(9), B(2)-C(3)
oids are drawn at the 50 % probability level.
Bond lengths/A, angles/° and dihedral angles/°: B(1)-C(1) 1.520(20), B(2)-
˚
˚
1.526(9), B(1)-Br(1) 1.919(7), B(1)-Br(2) 1.910(7), B(2)-Br(3) 1.921(6), B(2)-
Br(4) 1.919(6), C(1)-C(2) 1.401(9), C(2)-C(3) 1.422(8), C(3)-C(4) 1.404(8),
C(4)-C(5) 1.385(10), C(5)-C(6) 1.382(10), C(6)-C(1) 1.410(9); Br(1)-B(1)-
Br(2) 116.7(4), C(1)-B(1)-Br(1) 121.0(5), C(1)-B(1)-Br(2) 122.3(5), C(1)-C(2)-
C(3) 122.3(5), C(2)-C(3)-C(4) 117.3(5), C(3)-C(4)-C(5) 121.3(6), C(4)-C(5)-
C(6) 120.1(6), C(5)-C(6)-C(1) 121.6(6), C(6)-C(1)-C(2) 117.3(5);
C(1)C(2)C(3)C(4)C(5)C(6)//B(1)Br2 5.6(5)°, C(1)C(2)C(3)C(4)C(5)C(6)//
B(2)Br2 6.0(5)°.
C(3) 1.571(12), B(1)-Br(1) 1.908(11), B(2)-Br(2) 1.888(13), B(2)-Br(3)
1.903(14), C(1)-C(2) 1.418(13), C(2)-C(3) 1.423(16), C(3)-C(4) 1.388(13);
Br(1)-B(1)-Br(1A) 117.3(11), Br(2)-B(2)-Br(3) 118.8(6), C(1)-B(1)-Br(1)
121.4(5), C(3)-B(2)-Br(2) 122.2(9), C(3)-B(2)-Br(3) 119.0(9), C(1)-C(2)-C(3)
122.7(8), C(2)-C(3)-C(4) 117.2(8), C(3)-C(4)-C(3A) 124.0(13), C(2A)-C(1)-
C(2) 116.3(13); C(1)C(2)C(3)C(4)C(3A)C(2A)//B(1)Br2 8.8(1), C(1)C(2)C(3)-
C(4)C(3A)C(2A)//B(2)Br2 5.8(1). Symmetry transformations used to gene-
rate equivalent atoms: Ϫx, Ϫxϩy, Ϫzϩ2/3.
Figure 6 Structure plot of 5 in the solid state. Displacement ellips-
oids are drawn at the 50 % probability level.
˚
Bond lengths/A, angles/° and dihedral angles/°: B(1)-C(1) 1.53(2), B(1)-Br(1)
Figure 4 Structure plot of 3 in the solid state. Displacement ellips-
1.90(2), B(1)-Br(2) 1.90(2), C(1)-C(2) 1.44(2), C(2)-C(3) 1.38(3), C(3)-C(4)
1.41(3), C(4)-C(5) 1.41(2), C(5)-C(6) 1.31(3), C(6)-C(1) 1.46(2), C(4)-C(4A)
1.50(3); Br(1)-B(1)-Br(2) 117.1(8), C(1)-B(1)-Br(1) 120.6(12), C(1)-B(1)-Br(2)
122.2(12), C(1)-C(2)-C(3) 122.9(17), C(2)-C(3)-C(4) 119.3(15), C(3)-C(4)-
C(5) 118.5(15), C(4)-C(5)-C(6) 122.7(16), C(5)-C(6)-C(1) 122.5(15), C(6)-
C(1)-C(2) 113.9(14); C(1)C(2)C(3)C(4)C(5)C(6)//B(1)Br2 1(1), C(1)C(2)-
C(3)C(4)C(5)C(6)//C(1A)C(2A)C(3A)C(4A)C(5A)C(6A) 0.0(9). Symmetry
transformations used to generate equivalent atoms: 1Ϫx, 2Ϫy, 2Ϫz.
oids are drawn at the 50 % probability level.
˚
Bond lengths/A, angles/° and dihedral angles/°: B(1)-C(1) 1.559(11), B(1)-
Br(1) 1.925(8), B(1)-Br(2) 1.920(9), C(1)-C(2) 1.405(9), C(2)-C(3) 1.385(9),
C(1)-C(3A) 1.412(9); Br(1)-B(1)-Br(2) 116.6(4), C(1)-B(1)-Br(1) 121.4(6),
C(1)-B(1)-Br(2) 122.0(5), C(1)-C(2)-C(3) 120.5(6), C(2)-C(3)-C(1A) 121.2(6),
C(2)-C(1)-C(3A) 118.3(6); C(1)C(2)C(3)C(1A)C(2A)C(3A)//B(1)Br2 4.3(5).
Symmetry transformations used to generate equivalent atoms: 2Ϫx, 1Ϫy,
1Ϫz.
Similar to 1, 2, and 3, the 1,3,5-triborylated species 4 is
effectively planar with dihedral angles between the BBr2
substituents and the phenyl ring of 5.8(1)° and 8.8(1)° (Fig-
breaking in the crystal structure spurred us to perform
quantum chemical calculations to gain further insights
(see below).
˚
ure 5). A value of 1.520(20) A is found for the B(1)-C(1)
The crystal structure analysis of 5 suffers from large error
margins because the crystals were just weakly diffracting
thin plates. Bond lengths and angles (Figure 6) will there-
fore not be discussed in any detail. 5 shows a dihedral angle
between the BBr2 substituent and the phenyl ring of only
1(1)°. The biphenyl backbone also adopts a planar confor-
˚
bond while the B(2)-C(3) bond length is 1.571(12) A. The
C-C bond lengths of the central six-membered ring fall in
˚
˚
a range between 1.388(13) A [C(3)-C(4)] and 1.423(16) A
[C(2)-C(3)]. The molecules are arranged into helical stacks,
all of them having the same sense of rotation (chiral space
group P6122). The only symmetry element present in the
solid state structure is a C2 axis along the atoms B(1), C(1)
and C(4). Given an essentially planar structure for 4, how-
ever, we would expect an effective D3h symmetry for this
species. The lack of any obvious reason for the symmetry
mation
[C(1)C(2)C(3)C(4)C(5)C(6)//C(1A)C(2A)C(3A)-
C(4A)C(5A)C(6A) ϭ 0.0(9)°]. 5 crystallizes in parallel
stacks featuring a herringbone pattern. There are several C-
H···Br contacts with distances in the interval between
˚
˚
3.07 A and 3.28 A but no C-H···π(phenyl) interactions.
Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913
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907

M. C. Haberecht, J. B. Heilmann, A. Haghiri, M. Bolte, J. W. Bats, H.-W. Lerner, M. C. Holthausen, M. Wagner
2.3 Quantum Chemical Calculations
within an interval of 3σ: e.g., C-C bonds involving the bory-
lated carbon atom are significantly longer than bonds be-
Quantum chemical calculations have been performed by
means of the programs Turbomole [17] and Gaussian 03
[18]. As ’production quality’ method, for example, the
B3LYP/cc-pVTZ functional/basis set combination could be
considered the most reliable quality of approximation
within the framework of DFT established so far [14]. Using
contemporary workstation hardware, however, calculations
at this level are computationally quite expensive already for
the present systems. As we intend to study the chemistry of
much larger borylated aromatic systems in the future we
included other, potentially more efficient functional/basis
set combinations in our study. At the GGA level we used
the older BP86 and BLYP functionals, both of which are
quite commonly in use, as well as the HCTH407 functional
of Boese et al. [19], which has been newly implemented in
the Gaussian 03 program (HCTH for short). All GGA
functionals can make use of the resolution of identity (RI)
approximation [20], which can improve the computational
efficiency and scalability with system size substantially.
However, use of the very small DGA1 Coulomb density
fitting basis available in the program Gaussian 03 resulted
in a substantial pyramidalisation of BBr2 groups in several
test calculations. This has been clearly identified as an arte-
fact due to the neglect of higher angular momentum func-
tions in the atomic fitting basis for Br. As we are interested
here only in the general performance of functionals and
basis sets, we avoided the use of the RI approach in the
current study Ϫ more careful testing of available density
fitting basis sets for use with Gaussian 03 is clearly indi-
cated. As a newly developed hybrid method we employed
the B98 functional of Schmider and Becke [21]. DFT results
are compared to Hartree-Fock (HF) and Møller-Plesset se-
cond order perturbation theory (MP2) calculations Ϫ com-
putationally the latter level is nearly prohibitively de-
manding for the present system size if used with basis sets
of polarized triple-ζ quality, and not an efficient means to
treat larger molecular systems. All calculations have been
done in combination with the SVP and TZVP basis sets of
Ahlrichs and coworkers [22, 23]. Additional B3LYP calcu-
lations have been performed using the large cc-pVTZ basis
of Dunning [24]. All structures have been optimised within
the highest applicable symmetry point group (Cs, C2ν, C2ν,
and D3h for 1, 2, 3 and 4, respectively). The minimum
character of the resulting stationary points has been con-
firmed by the absence of negative eigenvalues in the analyti-
cally computed Hessian matrices. Calculations of nuclear
magnetic shielding tensors were performed based on the
gauge-independent atomic orbital (GIAO) approach as im-
plemented in Gaussian 03 [25].
˚
tween non-borylated carbon atoms (e. g. 3: 1.409 A and
˚
1.385 A, respectively). Some trends become obvious from
inspection of the computed data presented in Tables 2-5:
quite independent of the method a minor but systematic
˚
shortening of C-C and C-B bonds (by 0.009Ϫ0.004 A) oc-
curs when going from the SVP to the TZVP basis, whereas
˚
for B-Br bonds an elongation (by 0.007Ϫ0.002 A) is ob-
served. For B3LYP, the choice of the larger cc-pVTZ basis
does not lead to any significant changes of bond lengths
in comparison to the computationally much more efficient
TZVP basis. When going from HF to MP2, the inclusion
of electron correlation leads to a constant elongation of C-
˚
C bonds by about 0.010 A, whereas B-Br bonds are signifi-
˚
cantly shortened by 0.021 A. C-B bonds hardly change at
all. Only minor differences occur between MP2, HCTH,
B98, and B3LYP calculations. In contrast, the older GGA
functionals BP86 and BLYP give systematically longer C-C
˚
˚
(by about 0.010 A) and B-Br bonds (by about 0.024 A).
This observation is in line with the tendency quite com-
monly observed for these functionals to overestimate bond
lengths [14]. With all other optimised parameters being vir-
tually identical, the only difference in structures optimised
with these two GGA functionals is the B-Br bond, which
˚
is systematically longer (by 0.012 A) in BLYP calculations.
Interestingly, all methods investigated give almost identical
C-B bond lengths. All in all, the computational methods
applied yield optimised geometries within the experimental
˚
error bars of 3σ (between 0.009 and 0.048 A) Ϫ hence,
based only on results from the present applications no
method seems preferable over another in terms of quality.
In terms of computational efficiency we note the excellent
performance of the HCTH functional, which can make use
of the RI approximation rendering the calculations faster
by a factor of 2-3 for the present system sizes, and poten-
tially more for larger species. Yet, this functional compares
well with MP2 or the two hybrid functionals tested.
Trends in the computed data become obvious also for
NMR shifts (Tables 6Ϫ9) [26]: When going from the SVP
to the TZVP basis set a systematic low-field shift results for
the 1H NMR data (0.5-0.7 for MP2 and 0.2-0.5 for all other
methods) and for 13C NMR shifts. For the latter, however,
low field shifts are much more pronounced for DFT meth-
ods compared to HF and MP2 (mean low field shifts
SVPǞTZVP in ppm: HF: 4.8, MP2: 3.5, BP86: 7.5, BLYP:
8.9, B98: 6.8, B3LYP: 7.8). This large dependence on the
one-particle basis renders a comparison of computed data
with experiment rather arbitrary: e.g., while for B3LYP the
13C NMR results obtained with the SVP basis nicely agree
with experiment (mean error Ϫ1.8 ppm) a significant deteri-
oration results with the TZVP basis (shifts are systemati-
cally overestimated by a mean error of 5.9 ppm). A similar
situation becomes visible for the B98 functional and for HF.
The BLYP data, in turn, show a similar basis set depen-
dence but both data sets are somewhat shifted with respect
to experimental data (SVP: Ϫ4.2 ppm, TZVP: ϩ2.6 ppm).
For a comparison of calculated and experimental data a
common numbering scheme following the standard IUPAC
nomenclature is employed. Here, only the mean experimen-
tal values of symmetry-related bond lengths are discussed.
First of all we note that for all computational models ap-
plied all trends observable in the experimental bond lengths
(Tables 2Ϫ5), angles, and dihedral angles are reproduced
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Multiply Borylated Arenes
Table 2. Comparison between experimental and theoretical data for selected bond lengths in 1
HF
MP2
BP86
BLYP
HCTH407
B98
B3LYP
Exp.
Mean 3σ
SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP cc-pVTZ
C4-C5 1.421(8)
1.408 0.024 1.403 1.398 1.412 1.406 1.422 1.417 1.426 1.420 1.414 1.408 1.417 1.411 1.414 1.407
1.371 0.027 1.383 1.377 1.394 1.387 1.399 1.392 1.401 1.394 1.391 1.385 1.394 1.387 1.392 1.385
1.384 0.027 1.395 1.388 1.404 1.397 1.414 1.407 1.416 1.409 1.405 1.398 1.408 1.401 1.405 1.398
1.407
1.384
C3-C4 1.395(8)
C5-C6 1.373(9)
C2-C3 1.369(9)
C1-C6 1.383(9)
1.397
1.537
1.934
C1-C2 1.384(9)
C4-B1 1.499(9) 1.499 0.027 1.553 1.544 1.549 1.541 1.551 1.544 1.552 1.546 1.554 1.546 1.547 1.540 1.546 1.538
B1-Br1 1.943(7)
1.937 0.021 1.934 1.939 1.913 1.920 1.937 1.940 1.949 1.953 1.927 1.932 1.930 1.933 1.934 1.937
B1-Br2 1.931(7)
Table 3 Comparison between experimental and theoretical data for selected bond lengths in 2
HF
MP2
BP86
BLYP
HCTH407
B98
B3LYP
Exp.
Mean 3σ
SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP cc-pVTZ
C1-C2 1.401(9)
1.412 0.026 1.400 1.395 1.408 1.402 1.417 1.410 1.421 1.415 1.410 1.404 1.412 1.406 1.410 1.404
1.407 0.026 1.404 1.398 1.411 1.405 1.421 1.418 1.425 1.419 1.413 1.407 1.416 1.410 1.414 1.408
1.384 0.030 1.385 1.379 1.396 1.389 1.402 1.394 1.404 1.397 1.393 1.387 1.397 1.390 1.395 1.387
1.535 0.027 1.560 1.552 1.557 1.549 1.558 1.552 1.560 1.554 1.562 1.556 1.556 1.550 1.554 1.550
1.920 0.020 1.929 1.932 1.908 1.911 1.929 1.931 1.942 1.946 1.922 1.925 1.924 1.926 1.927 1.929
1.915 0.020 1.931 1.936 1.910 1.913 1.932 1.934 1.946 1.949 1.924 1.928 1.927 1.930 1.931 1.933
1.402
1.407
1.386
1.546
1.927
1.930
C2-C3 1.422(8)
C3-C4 1.404(8)
C1-C6 1.410(9)
C4-C5 1.385(10)
C5-C6 1.382(10)
C1-B1 1.543(9)
C3-B2 1.526(9)
B1-Br1 1.919(7)
B2-Br3 1.921(6)
B1-Br2 1.910(7)
B2-Br4 1.919(6)
Table 4 Comparison between experimental and theoretical data for selected bond lengths in 3
HF
MP2
BP86
BLYP
HCTH407
B98
B3LYP
Exp.
Mean 3σ
SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP cc-pVTZ
C1-C2 1.405(9)
C1-C6 1.412(9)
1.409 0.009 1.401 1.396 1.413 1.407 1.421 1.414 1.425 1.418 1.413 1.407 1.416 1.409 1.413 1.407
1.406
C2-C3 1.385(9) 1.385 0.027 1.384 1.380 1.393 1.386 1.398 1.391 1.400 1.393 1.390 1.384 1.394 1.387 1.392 1.385
C1-B1 1.559(11) 1.559 0.033 1.569 1.563 1.557 1.549 1.560 1.555 1.563 1.557 1.564 1.558 1.559 1.553 1.557 1.551
1.384
1.550
B1-Br1 1.925(8)
1.923 0.026 1.927 1.932 1.909 1.912 1.930 1.932 1.943 1.946 1.921 1.926 1.924 1.926 1.927 1.930
B1-Br2 1.920(9)
1.927
For MP2 best results are obtained with the larger TZVP
basis. Interestingly, for the 13C(CH3) NMR shift in 1 sub-
stantially lower deviations from experiment are observed
compared to the data for carbon nuclei of the aromatic
rings (Table 6). These trends, together with the large depen-
dency on the basis set, point to strong error compensation
effects. In order to identify the source of these findings, we
also computed the 13C NMR shifts for benzene (Table 10).
Indeed, a compilation of signed errors for the carbon nuclei
constituting the aromatic subsystems (Table 11) reveals that
error trends are already present in the parent benzene sys-
tem to a large extent. In other words: most of the deviations
Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913
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 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
909

M. C. Haberecht, J. B. Heilmann, A. Haghiri, M. Bolte, J. W. Bats, H.-W. Lerner, M. C. Holthausen, M. Wagner
Table 5 Comparison between experimental and theoretical data for selected bond lengths in 4
HF
MP2
BP86
BLYP
HCTH407
B98
B3LYP
Exp.
Mean 3σ
SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP SVP TZVP cc-pVTZ
C1-C2 1.418(13)
C2-C3 1.423(16) 1.410 0.042 1.400 1.394 1.407 1.401 1.417 1.410 1.421 1.414 1.409 1.403 1.412 1.406 1.410 1.403
C3-C4 1.388(13)
C1-B1 1.520(20)
1.402
1.550
1.926
1.546 0.048 1.562 1.554 1.561 1.554 1.561 1.556 1.564 1.559 1.565 1.559 1.559 1.553 1.557 1.552
C3-B2 1.571(12)
B1-Br1 1.908(11)
B2-Br2 1.888(13) 1.900 0.038 1.928 1.932 1.906 1.909 1.928 1.930 1.942 1.945 1.920 1.924 1.923 1.925 1.926 1.928
B2-Br3 1.903(14)
1
Table 6 Experimental and computed H-, 13C-, and 11B NMR shifts for 1
HF
MP2
TZVP
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
Exp.
SVP
TZVP
SVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
H-3,5
H-2,6
H-CH3
C-1
C-3,5
C-2,6
C-4
8.2
7.0
2.0
146.8
138.3
129.4
n.o.
21.8
56.6
8.4
7.2
2.2
152.8
145.2
126.7
129.6
20.6
55.6
8.6
7.4
2.4
159.8
150.4
131.3
134.8
21.0
54.8
8.2
7.4
2.2
135.6
128.8
125.7
129.4
22.7
54.8
8.8
8.0
2.7
141.1
131.9
129.3
133.6
19.0
55.7
8.0
7.0
2.1
141.5
133.2
123.9
128.1
23.0
49.5
8.3
7.3
2.3
151.9
140.3
131.1
137.4
23.6
50.3
7.9
6.9
2.1
142.2
133.3
123.5
128.9
22.8
54.2
8.2
7.3
2.3
154.3
141.9
132.2
139.4
23.9
55.3
8.1
7.1
2.1
145.1
136.9
125.8
129.7
23.0
52.6
8.4
7.4
2.3
154.8
143.7
132.5
138.0
23.4
52.8
8.1
7.0
2.1
144.9
136.7
125.6
130.2
22.6
55.1
8.5
7.5
2.4
155.8
144.5
133.3
139.4
23.2
55.8
8.6
7.4
2.3
154.9
144.3
133.3
139.1
23.8
54.9
C-CH3
B
from experiment observed above relate to shortcomings in
the description of aromatic NMR shielding properties in
relation to the TMS 13C NMR standard. Much higher com-
putational accuracy can be obtained by the choice of a ben-
zene NMR-standard instead of TMS. While this is an ob-
servation quite typical for energies, vibrational frequencies,
and other properties computed with contemporary density
functional methods [14], also HF and MP2 follow these
general trends here. However, particularly large deviations
from experiment occur for 4 at the HF and MP2 levels of
theory, whereas both methods show constantly smaller er-
rors for computed 13C NMR shifts in 1, 2, 3. Apparently,
the correlation problem is especially pronounced in 4. The
DFT methods applied, in turn, show rather constant errors,
independent of the species studied. For computed 11B
NMR shifts the choice of a particular basis set has hardly
any influence on the results (below 1 ppm in most cases,
Table 12), and HF, MP2, BLYP, and B3LYP results nicely
compare with experiment. Larger errors become visible for
the B98 (Ϫ3.3 to Ϫ4.0 ppm) and BP86 (Ϫ5.4 to Ϫ7.1 ppm)
functionals. Trends between the molecules, however, are
equally well reproduced by all methods. This finding indi-
cates a problem related to the choice of the B2H6 11B NMR
standard for the latter two functionals.
practicability we find this level of theory computationally
efficient and suitable to describe the NMR properties of the
present set of systems.
Overall, theoretically predicted relative shift values within
each molecule as well as between molecules are in excellent
agreement with the experiment. For example, a pronounced
deshielding of the carbon nuclei C-2,4,6 in compound 4 has
not only been found experimentally [δ(13C)exp ϭ 151.4] but
is fully reproduced at all levels of theory [e.g. δ(13C)B3LYP/
ϭ 152.2]. The finding of nearly identical 11B NMR
SVP
shifts for 1-4 is intuitively in agreement with essentially
identical NBO charges on boron for all four species (1, 2:
ϩ0.41; 3, 4: ϩ0.40). Thus, substitution of the para-methyl
group in 1 for a BBr2 substituent has apparently no appreci-
able effect on the charge density around the boron centre.
Ipso carbon atoms bearing a boryl substituent possess large
negative NBO charges (1, 2, 4: Ϫ0.46; 3: Ϫ0.41). Carbon
atoms C-2,6 in 1 and C-5 in 2 bearing NBO charges of
Ϫ0.22 and Ϫ0.21, respectively, are not affected by the pres-
ence of BBr2 substituents in the meta position(s) of the aro-
matic ring (carbon NBO charges in benzene: Ϫ0.20). In
contrast, charges of carbon atoms in a position ortho to one
boryl group show a considerably reduced negative charge
(e.g. C-3,5 in 1: Ϫ0.12), which becomes even smaller upon
introduction of a second ortho BBr2 group (C-2 in 2:
Ϫ0.06). A Wiberg bond order analysis assigns a single bond
between C and B (1: 0.98; 2: 0.96; 3: 0.95; 4: 0.94) and a
partial double bond character to the B-Br bonds (1: 1.19;
2, 3: 1.21; 4: 1.22; for comparison C-C in benzene: 1.44).
In conclusion, for the present set of systems the best over-
all agreement with experiment is found for the B3LYP/SVP
1
level for H-, 13C-, and 11B NMR shifts alike. As alluded
to above, we relate this good performance largely to fortu-
itous error compensation. Notwithstanding the limited
physical basis underlying such a statement, in terms of
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 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913

Multiply Borylated Arenes
Table 7 Experimental and computed H-, 13C-, and 11B NMR shifts for 2
1
HF
MP2
TZVP
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
Exp.
SVP
TZVP
SVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
H-2
H-4,6
H-5
C-2
C-4,6
C-5
8.9
8.1
6.9
147.0
143.5
128.1
n.o.
9.6
8.8
7.4
158.0
153.2
123.8
130.4
56.4
9.8
9.1
7.6
162.9
158.5
128.5
135.3
55.8
9.2
8.4
7.7
137.5
132.9
125.4
132.0
56.1
9.9
9.1
8.3
139.5
135.8
128.7
135.7
57.1
9.0
8.2
7.2
143.2
138.4
122.5
129.9
51.1
9.3
8.6
7.6
150.0
145.9
130.2
138.8
52.0
8.9
8.1
7.1
143.6
138.4
122.1
130.6
55.8
9.3
8.5
7.6
151.9
147.3
131.1
140.9
57.0
9.2
8.4
7.4
147.7
142.7
124.1
131.4
54.1
9.5
8.8
7.7
154.1
149.7
131.2
139.4
54.3
9.1
8.4
7.3
147.4
142.3
124.0
131.9
56.5
9.6
8.8
7.7
154.8
150.3
131.9
140.9
57.4
9.8
9.0
7.8
154.6
150.3
131.9
140.6
56.5
C-1,3
B
57.6
1
Table 8 Experimental and computed H-, 13C-, and 11B NMR shifts for 3
HF
MP2
TZVP
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
Exp.
SVP
TZVP
SVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
H-2,3,5,6
C-2,3,5,6
C-1,4
7.9
8.5
8.7
8.4
9.1
8.1
8.5
8.0
8.4
8.3
8.6
8.2
8.6
8.7
136.5
n.o.
58.3
139.3
140.8
57.6
144.2
146.5
56.3
130.8
133.6
56.2
133.5
137.6
57.2
132.2
134.1
51.7
139.1
143.6
52.4
132.1
134.9
56.3
140.5
145.7
57.4
135.0
136.8
54.8
141.6
145.3
55.0
134.8
137.2
57.2
142.3
146.7
57.9
142.2
146.2
57.2
B
1
Table 9 Experimental and computed H-, 13C-, and 11B NMR shifts for 4
HF
MP2
TZVP
BP86
TZVP
BLYP
B98
B3LYP
TZVP
Exp.
SVP
TZVP
SVP
SVP
SVP
TZVP
SVP
TZVP
SVP
cc-pVTZ
H-2,4,6
C-2,4,6
C-1,3,5
B
9.0
9.9
10.1
9.4
10.0
9.2
9.5
9.0
9.4
9.4
9.7
9.3
9.7
10.0
151.4
n.o.
57.9
165.7
128.6
56.6
170.7
132.9
55.9
140.6
133.0
56.6
142.5
136.3
57.5
147.3
129.5
51.6
154.3
137.9
52.5
147.7
130.2
56.3
156.0
140.0
57.5
152.5
130.8
54.6
159.0
138.3
54.6
152.2
131.3
56.9
159.7
139.7
57.9
159.5
139.8
57.0
1
Table 10 Experimental and computed H-, and 13C NMR shifts for benzene.
HF
MP2
TZVP
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
Exp.
SVP
TZVP
SVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
H
C
7.3
128.5
7.3
130.1
7.6
135.7
7.3
121.3
7.9
125.2
7.1
122.7
7.4
130.8
7.0
122.5
7.4
132.1
7.2
125.3
7.5
132.9
7.1
125.2
7.6
133.8
7.5
133.4
Table 11 Deviations from experiment for computed 13C NMR shifts (signed errors in ppm). For 1 and 2 a mean value only for the aromatic
carbon nuclei (three data points) is given.
HF
MP2
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
SVP
TZVP
SVP
TZVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
benzene
1.6
3.4
5.5
2.8
14.3
7.2
9.0
10.4
7.8
19.3
Ϫ7.2
Ϫ8.1
Ϫ7.6
Ϫ5.7
Ϫ10.8
Ϫ3.3
Ϫ4.1
Ϫ4.9
Ϫ3.0
Ϫ8.9
Ϫ5.8
Ϫ5.3
Ϫ4.8
Ϫ4.3
Ϫ4.1
2.3
2.9
2.5
2.6
2.9
Ϫ6.0
Ϫ5.2
Ϫ4.8
Ϫ4.4
Ϫ3.7
3.6
4.6
3.9
4.0
4.6
Ϫ3.2
Ϫ2.2
Ϫ1.4
Ϫ1.5
1.1
4.4
5.5
5.5
5.1
7.6
Ϫ3.3
Ϫ2.4
Ϫ1.6
Ϫ1.7
0.8
5.2
6.4
6.1
5.8
8.3
4.9
6.0
6.1
5.7
8.1
1
2
3
4
As to an assessment of the quality of thermochemical
properties of the present class of species, Table 13 presents
a comparison of energy differences obtained at all levels of
theory applied in this study. We note a pleasing agreement
of all correlated methods in the description of the relative
stabilisation of 2 compared to 3: with the exception of HF
Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913
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911

M. C. Haberecht, J. B. Heilmann, A. Haghiri, M. Bolte, J. W. Bats, H.-W. Lerner, M. C. Holthausen, M. Wagner
Table 12 Deviations from experiment for computed 11B NMR shifts (signed errors in ppm).
HF
MP2
BP86
TZVP
BLYP
TZVP
B98
B3LYP
TZVP
SVP
TZVP
SVP
TZVP
SVP
SVP
SVP
TZVP
SVP
cc-pVTZ
1
2
3
4
Ϫ1.0
Ϫ1.2
Ϫ0.7
Ϫ1.3
Ϫ1.8
Ϫ1.8
Ϫ2.0
Ϫ2.0
Ϫ1.8
Ϫ1.5
Ϫ2.1
Ϫ1.3
Ϫ0.9
Ϫ0.5
Ϫ1.1
Ϫ0.4
Ϫ7.1
Ϫ6.5
Ϫ6.6
Ϫ6.3
Ϫ6.3
Ϫ5.6
Ϫ5.9
Ϫ5.4
Ϫ2.4
Ϫ1.8
Ϫ2.0
Ϫ1.6
Ϫ1.3
Ϫ0.6
Ϫ0.9
Ϫ0.4
Ϫ4.0
Ϫ3.5
Ϫ3.5
Ϫ3.3
Ϫ3.8
Ϫ3.3
Ϫ3.3
Ϫ3.3
Ϫ1.5
Ϫ1.1
Ϫ1.1
Ϫ1.0
Ϫ0.8
Ϫ0.2
Ϫ0.4
0.0
Ϫ1.7
Ϫ1.1
Ϫ1.1
Ϫ0.9
Table 13 Energy difference between 2 and 3 and reaction energy for the isodesmic reaction: toluene ϩ 4 Ǟ 1 ϩ 2 (total energy differences
in kcal/mol).
HF
TZVP
MP2
TZVP
BP86
TZVP
BLYP
TZVP
HCTH
SVP TZVP
B98
TZVP
B3LYP
TZVP
SVP
SVP
SVP
SVP
SVP
SVP
cc-pVTZ
∆E(3Ϫ2)
∆E(reaction)
2.0
2.2
6.0
2.4
0.2
1.8
0.3
1.7
0.6
2.7
0.6
3.0
0.5
2.6
0.5
3.1
0.5
2.5
0.5
2.8
0.8
2.7
0.8
2.9
0.8
2.6
0.8
3.0
0.8
3.1
[27], the results do not even show any significant basis set
dependence. Excellent agreement is also found for the iso-
desmic reaction: toluene ϩ 4 Ǟ 1 ϩ 2 (Table 13). Although
error compensation should be highly efficient for the cases
studied, we take these results as a confirmation of the gen-
eral reliability of the quantum chemical methods tested for
the present class of systems.
the charge density on this boron atom as compared to the
corresponding monoborylated species but rather leads to a
further depletion of the aromatic π electron system. DFT
calculations on 1-4 (geometry optimisations; population
analyses; prediction of 1H-, 13C- and 11B NMR parameters)
lead to the conclusion, that the best overall agreement with
experimental data is found for the B3LYP/SVP level of the-
ory.
Acknowledgement. This work was supported by the Deutsche For-
schungsgemeinschaft (DFG). The generous allotment of computer
time by the CSC Frankfurt is gratefully acknowledged.
3 Conclusions
The strong Lewis acids 4-(dibromoboryl)toluene (1), 1,3-
bis(dibromoboryl)benzene (2), 1,4-bis(dibromoboryl)ben-
zene (3), 1,3,5-tris(dibromoboryl)benzene (4), and 4,4Ј-
bis(dibromoboryl)biphenyl (5) have been synthesized from
the corresponding trimethylsilyl derivatives and boron trib-
romide. Each of the compounds adopts an almost perfectly
planar conformation in the solid state. According to DFT
calculations, any deviations of the molecular structures
from the highest applicable symmetry point groups (Cs, C2ν,
C2ν, and D3h for 1, 2, 3, and 4, respectively) are due to
crystal packing effects (e.g. compound 4 possesses only C2
symmetry in the solid state). The 11B NMR signals of 1-5
lie in the very narrow range between 56.6 ppm (1) and
58.3 ppm (3). Since the strongest influence on the magnetic
shielding of three-coordinate boron nuclei usually comes
from the π charge density, the boron p orbitals are obvi-
ously equally populated in all five compounds. This in-
terpretation is supported by the finding of essentially ident-
ical NBO charges on boron for 1 (ϩ0.41), 2 (ϩ0.41), 3
(ϩ0.40), and 4 (ϩ0.40). 13C NMR spectroscopy shows the
resonances of carbon atoms in ortho and para position to
the BBr2 group(s) to experience a continuous downfield
shift as the number of boryl substituents increases [cf.
C6H6: δ(13C) ϭ 128.5 [16]; 1: δ(13C-ortho) ϭ 138.3; 2: δ(13C-
ortho) ϭ 147.0, 143.5; 4: δ(13C) ϭ 151.4], in agreement with
decreasing NBO charges on these atoms. Interestingly, even
the introduction of a second BBr2 group into the para posi-
tion of another boryl substituent (cf. 3) does not reduce
4 Experimental Section
General Considerations. All reactions and manipulations of air-sen-
sitive compounds were carried out in dry, oxygen-free argon using
standard Schlenk ware. Solvents were freshly distilled under Ar
˚
from Na-benzophenone (toluene, n-pentane) or stored over 4 A
molecular sieves prior to use (C6D6). NMR: Bruker Avance 400,
Bruker AMX 250, Bruker DPX 250 spectrometers. 11B NMR spec-
tra are reported relative to external BF3 · Et2O. All NMR spectra
were run at ambient temperature. Abbreviations: s ϭ singlet; d ϭ
doublet; t ϭ triplet; n.r. ϭ multiplet expected in the 1H NMR spec-
trum but not resolved; n.o. ϭ signal not observed. 2, 3 [11], and 5
[28] have been prepared previously by slightly different procedures.
Preparation of 1: 4-(Trimethylsilyl)toluene (3.28 g, 19.96 mmol) was
dissolved in 10 ml of n-pentane. Boron tribromide (7.50 g,
29.95 mmol) was added via syringe at ambient temperature. The
reaction mixture was stirred for 2 h. Slow evaporation of all vol-
atiles in vacuo gave colourless crystals of 1. Yield: 5.09 g (97 %).
1H-NMR (250.1 MHz, C6D6): δ 8.15 (d, 2H, 3J(H,H) ϭ 7.9 Hz; H-3,5), 6.95
(d, 2H, 3J(H,H)
ϭ
7.9 Hz; H-2,6), 2.02 (s, 3H; CH3); 13C-NMR
(100.6 MHz,): δ 146.8 (C-1), 138.3 (C-3,5), 129.4 (C-2,6), 21.8 (CH3), n.o.
(C-4); 11B-NMR (128.4 MHz,): δ 56.6 (h1/2 ϭ 150 Hz).
Preparation of 2: 1,3-Bis(trimethylsilyl)benzene (3.00 g; 13.48
mmol) was dissolved in 10 ml of n-pentane. After boron tribromide
(14.56 g; 58.16 mmol) had been added via syringe, the resulting
clear solution was heated at reflux temperature for 7 h. Colourless
needles of 2 precipitated from the reaction mixture upon cooling
912
 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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Z. Anorg. Allg. Chem. 2004, 630, 904Ϫ913

Multiply Borylated Arenes
to ambient temperature. Slow evaporation of the solvent from the
mother liquor in vacuo gave a second crop. Yield: 4.50 g (80 %).
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3
1H-NMR (250.1 MHz, C6D6): δ 8.94 (n.r., 1H; H-2), 8.07 (dd, 2H, JHH
ϭ
7.6 Hz, JHH ϭ 1.5 Hz; H-4,6), 6.87 (t, 1H, JHH ϭ 7.6 Hz; H-5); 13C-NMR
(62.9 MHz, C6D6): δ 147.0 (C-2), 143.5 (C-4,6), 128.1 (C-5), n.o. (C-1,3);
11B-NMR (128.4 MHz, C6D6): δ 57.6 (h1/2 ϭ 210 Hz).
4
3
[9] Y. Quin, G. Cheng, A. Sundararaman, F. Jäkle, J. Am. Chem.
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Preparation of 3: 1,4-Bis(trimethylsilyl)benzene (4.80 g, 21.58
mmol) was dissolved in 10 ml of toluene. After boron tribromide
(16.00 g, 63.87 mmol) had been added via syringe, the resulting
clear solution was heated at reflux temperature for 3 h. Colourless
needles of 3 precipitated from the reaction mixture upon cooling
to ambient temperature. Slow evaporation of the solvent from the
mother liquor in vacuo gave a second crop. Yield: 6.95 g (77 %).
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Functional Theory, 2nd ed., Wiley-VCH, Weinheim, 2001.
[15] H. Nöth, B. Wrackmeyer, Nuclear Magnetic Resonance Spec-
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1H-NMR (400.1 MHz, C6D6):
δ
7.86 (s, 4H, H-2,3,5,6); 13C-NMR
(100.6 MHz, C6D6): δ 136.5 (C-2,3,5,6), n.o. (C-1,4); 11B-NMR (128.4 MHz,
C6D6): δ 58.3 (h1/2 ϭ 240 Hz).
Preparation of 4: The compound was prepared similar to 3 from
1,3,5-tris(trimethylsilyl)benzene (3.11 g, 10.55 mmol) and boron tri-
bromide (16.00 g, 63.87 mmol) in 10 ml of toluene. Colourless
needles of 4 precipitated from the reaction mixture upon cooling
to ambient temperature. Slow evaporation of the solvent from the
mother liquor in vacuo gave a second crop. Yield: 5.90 g (95 %).
1H-NMR (400.1 MHz, C6D6): δ 9.03 (s, 3H, H-2,4,6); 13C-NMR (62.9 MHz,
C6D6): δ 151.4 (C-2,4,6), n.o. (C-1,3,5); 11B-NMR (128.4 MHz, C6D6): δ 57.9
(h1/2 ϭ 270 Hz).
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Preparation of 5: The compound was prepared similar to 3 from
4,4Ј-bis(trimethylsilyl)biphenyl (0.60 g, 2.01 mmol) and boron trib-
romide (1.41 g, 5.63 mmol) in 5 ml of toluene. Colourless needles of
5 precipitated from the reaction mixture upon cooling to ambient
temperature. Slow evaporation of the solvent from the mother
liquor in vacuo gave a second crop. Yield: 0.85 g (86 %).
1H-NMR (250.1 MHz, C6D6): δ 8.11 (d, 4H, JHH ϭ 8.2 Hz, H-3,3Ј,5,5Ј),
3
3
7.23 (d, 4H, JHH ϭ 8.2 Hz, H-2,2Ј,6,6Ј); 13C-NMR (62.9 MHz, C6D6): δ
146.2 (C-1,1Ј), 138.6 (C-3,3Ј,5,5Ј), 127.1 (C-2,2Ј,6,6Ј), n.o. (C-4,4Ј); 11B-
NMR (128.4 MHz, C6D6): δ 57.0 (h1/2 ϭ not determined).
X-ray crystal structure determinations of 1؊5
The data for 1 were collected on a SIEMENS SMART dif-
fractometer, the data for 2, 3, 4 and 5 were collected on a STOE-
IPDS two-circle diffractometer. All structures were solved by direct
methods and refined with full-matrix least-squares techniques.
Non-H atoms were refined anisotropically, whereas H atoms were
refined using a riding model. The molecules of 3 are located on a
centre of inversion with half a molecule in the asymmetric unit.
The molecules of 4 are located on a two-fold rotation axis with half
a molecule in the asymmetric unit.
CCDC reference numbers:212678 (1), 212676 (2), 212675 (3),
212677 (4), 229267 (5).
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NMR chemical shielding tensors is not implemented for the
HCTH functional.
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