Charge distribution within hypercarbon-halogenated 1-Ph-2-X-1,2-dicarba- closo-dodecaboranes, (X = F, Cl, Br, I): A dipole moment and computational study

Article abstract of DOI:10.1016/j.molstruc.2010.02.027

Vector analyses of experimental dipole moments of a series of halogenated 1-Ph-2-X-1,2-dicarba-closododecaboranes, (X = F, Cl, Br, I) combined with theoretical calculations show that there are intramolecular contacts between heavy halogens (Br and I) and the benzene ring, whereas this attraction is absent for F and Cl.

Full text of DOI:10.1016/j.molstruc.2010.02.027

Journal of Molecular Structure 978 (2010) 246–249
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Charge distribution within hypercarbon-halogenated
1-Ph-2-X-1,2-dicarba-closo-dodecaboranes, (X = F, Cl, Br, I):
A dipole moment and computational study
b
Drahomír Hnyk a, , Václav Všetecka , Ladislav Droz
*
ˇ
ˇ b
ˇ
a
ˇ
Institute of Inorganic Chemistry of the ASCR, v.v.i., No 1001, CZ – 250 68 Husinec-Rez, Czech Republic
b Department of Organic Chemistry, Charles University, Hlavova 2030, CZ – 128 40 Praha 2, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Vector analyses of experimental dipole moments of a series of halogenated 1-Ph-2-X-1,2-dicarba-closo-
dodecaboranes, (X = F, Cl, Br, I) combined with theoretical calculations show that there are intramolecular
contacts between heavy halogens (Br and I) and the benzene ring, whereas this attraction is absent for F
and Cl.
Received 15 December 2009
Received in revised form 9 February 2010
Accepted 9 February 2010
Available online 18 February 2010
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to Professor Heinz Oberhammer
on the occasion of his 70th birthday.
Keywords:
Halogens
Carboranes
Dipole moments
1. Introduction
geometrical solution of triangles, on a series of 1-(4-substituted-
phenyl)-o-carboranes with the orientation of the positive end of
The o-, m- and p-carboranes (1,2-, 1,7- and 1,12-dicarba-closo-
dodecaboranes, 1,2-, 1,7-, 1,12-closo-C2B10H10) represent relatively
large, very symmetrical and remarkably stable molecules. Such
high stabilities of these icosahedral systems have led to their use
in such diverse fields as medicine (neutron capture therapy [1],
molecular scaffolding (rigid building units [2]) and material sci-
ence [3]), m-carboranes (1,7-) attracting much less attention in
contrast to o- and p-carboranes. This is also valid in terms of
researching these distorted icosahedra with an aim at their poten-
tial applications in nanotechnology [4].
the dipole towards the carbon atoms. As a consequence, the dipole
moment vector of 1-Ph-o-carborane, 2a, lies at an angle of 28° with
respect to the exohedral C–Car bond, i.e. practically in the symme-
try axis of the carborane structure (an electron diffraction study of
2a yielded this angle to be 31° [9]).
The knowledge of the dipole moment value of o-carborane cage
and its direction helps us to understand electronic interactions be-
tween substituents on carbon atoms and the o-carborane cage.
Halogens are relatively easily bonded to the o-carborane carbons
experimentally and fluorine was found to act as a weaker
p-elec-
Interest in capacity of these carboranes, regarded as three-
dimensional aromatic systems [5], to communicate electronically
with substituents resulted in our interest in acquiring data that de-
scribe both charge distribution within them and their ability to
serve as models for simple electronic devices on the molecular le-
vel: the dipole moments and computational study of the p-carbo-
rane system with various substituents bonded to both carbon
ends of the icosahedron [6] has revealed that the p-carborane cage
behaves as a moderate conduit for electronic effects. The direction
of the dipole moment in o-carborane [7], 1, was unambiguously
confirmed by a well-established graphical method [8], based on
tron donor in 1-Ph-2-X-1,2-closo-C2B10H10 (2b, Fig. 1a) as revealed
by NMR and computational data [10]. Interaction of halogens with
benzene has been computed elsewhere [11] and it would therefore
be interesting to know how such an interaction proceeds if halogen
and benzene ring are attached to the two neighbouring cage car-
bons of o-carborane. Halogens are also prone to the so-called ‘‘hal-
ogen bonding” in terms of
r-hole (bonding towards nucleophiles
with a region of positive electrostatic potential on the outermost
portion of the surfaces of halogens [12]).
In order to understand this phenomenon further, we prepared a
series of 1-Ph-2-X-1,2-closo-C2B10H10 (X = F, Cl, Br, I, 2b, 2c, 2d, 2e,
respectively, Fig. 1a–d) and measured their dipole moments for
quantifying the corresponding electron distribution. We also car-
ried out theoretical calculations of these systems.
* Corresponding author.
E-mail address: hnyk@iic.cas.cz (D. Hnyk).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.02.027

D. Hnyk et al. / Journal of Molecular Structure 978 (2010) 246–249
247
Fig. 1. 1-Ph-2-X-1,2-dicarba-closo-dodecaboranes, (X = F 2b, Cl 2c, Br 2d, I 2e).
Table 1
2. Experimental and computational details
11B chemical shifts (in ppm) for 1-Ph-2-X-1,2-dicarba-closo-dodecaboranes, (X = H 2a,
F 2b, Cl 2c, Br 2d, I 2e) with respect to BF3ÁOEt2.
2.1. Syntheses
X
B(9)
B(12)
B(4,5)
B(7,11)
B(3,6)
B(8,10)
Ha
À2.8
0.0
À4.8
À3.7
À6.6
À11.3
À6.4
À6.8
À5.3
À6.0
À3.8
À4.5
À10.9
À10.5
À11.3
14.0
À12.8
À13.3
À12.6
À15.6
À10.8
À11.6
À9.2
À10.9
À15.2
À13.5
À16.1
À10.8
À12.3
À10.9
À12.8
À9.4
À9.1
À9.6
Compounds 2b, 2c, 2d, and 2e were prepared by allowing to
halogenate phenyl-ortho-carborane or lithiated 2a with N-fluoro-
benzenesulfonimide/benzene, Cl2/P4O10, Br2/toluene, and I2/tetra-
hydrofuran, respectively, according to known or slightly modified
literature procedures [13]. The purity of all compounds was checked
by analytical TLC and the structures of the molecules were con-
firmed on the basis of 1H{11B} and 11B{1H} NMR spectra recorded
on a Varian Unity-500 instrumentin CDCl3 solution. Table 1 provides
11B chemical shifts and their comparison with computed values at
GIAO-B3LYP, for basis sets and geometries used (see below).
Calc.a
F
À3.8
À7.1
À4.5
À5.1
À4.2
À4.8
À3.2
À4.4
À14.5
À16.4
À11.8
À13.5
À10.9
À13.1
À10.3
À12.8
Calc.b
Cl
À9.8
Calc.b
Br
À11.6
À9.1
Calc.b
I
À11.2
À7.3
À12.0
À9.4
Calc.b
À11.2
À11.6
À12.6
a
Ref. [10] in which 2a calculated at GIAO-B3LYP/6-311G*//MP2/6-31G*. There
are a few minima of 2a but only that of the same molecular shape as 2b–e (i.e.
phenyl group perpendicular to the C1–C2 plane) was considered in magnetic
properties calculations.
2.2. Dipole moments
b
Calculated at the GIAO-B3LYP/II//MP2/basis set level, for the basis set used see
Section 2.
Dipole moments were measured at 25 °C in benzene (five solu-
tions, weight fraction 1.8 Â 10À4–1.1 Â 10À3) by the method of
Guggenheim and Smith [14,15]. Relative permittivities were mea-
sured at 6 MHz on a home-made DK-meter with direct frequency
reading. Refractive indices were measured on an Aerograph refrac-
tive index detector (Varian). The following dipole moments were
obtained: 2b 3.67 D, 2c 3.61 D, 2d 4.13 D, 2e 4.72, the estimated
uncertainty was 0.05 D.

248
D. Hnyk et al. / Journal of Molecular Structure 978 (2010) 246–249
2.3. Computational details
Structure optimizations (no symmetry constraints), harmonic
vibrational frequency analyses, calculations of NMR chemical shifts
(without spin–orbit corrections to them), and dipole moments em-
ployed the Gaussian 03 suite of programs [16]. Structures were
optimized both at B3LYP [17,18] and at the frozen-core MP2
*
[MP2(fc)] levels, using the 6-31G [19] basis for F and Cl, and quasi-
relativistic energy-consistent pseudopotentials [20] with DZP va-
lence basis sets for Br and I. NMR chemical shift calculations
were done with gauge-including atomic orbitals (GIAO) [21] at
*
B3LYP level, at the MP2/6-31 (F, Cl) and DZP (Br, I) level. These cal-
culations used the same ECPs for Br and I as the optimizations and
IGLO-II all-electron basis sets [22] for all atoms, the latter being
well-designed for magnetic properties calculations.
3. Results and discussion
The electron deficiency of the o-carborane cage brings about its
behavior both towards alkyl and aryl groups: the dipole moments
are: o-carborane, 4.50 D [7], 1-methyl-o-carborane, 4.75 D [23],
and 1-phenyl-o-carborane, 4.93 D [7]. From these values the carbo-
rane acts as an electron acceptor. The direction of the dipole in the
latter was experimentally determined and, in essence of this ap-
proach, coincides within experimental error with that in o-carbo-
rane [7] and the same direction may be anticipated in the case of
1-methyl-o-carborane.
In organic chemistry, there are two sets of the so-called bond
moments [8]that mainly incorporate electronic interaction be-
tween a substituent and a substrate: i.e. one for a bond between
a substituent and methyl carbon, the other for a bond between a
substituent and phenyl carbon, [8] as exemplified for halogens:
X–Cmethyl, F = 1.79, Cl = 1.87, Br = 1.82, I = 1.65 D, X–Cphenyl the cor-
responding values are 1.47 D, 1.59 D, 1.57 D, 1.40 D, respectively.
These vectors turned out to be very helpful in structural analyses
of organic substances.
Fig. 2. Possible explanation of the dipole moment of 2e.
reduces the value of l(2a). However, for X = Br and predominantly
for X = I this tendency is compensated by an ‘‘internal” attraction
directing from I (i.e. from its positively charged outer shell, see
[12], note also that NPA [25] charge of I in 2e is 0.3e at MP2/
DZP) towards the negative ‘‘region” of the benzene quadrupole mo-
ment [26]. This ‘‘vector” (Fig. 2, dashed and full arrows within the
cage correspond to the direction of the dipole moments in 2a and
2e, respectively) roughly lies at an angle of ca. 120°–140° with
respect to the direction of
l(2a) (the angle X–C(2)–I is ca. 80° at
MP2/DZP, where X is the centre of the phenyl ring) This is also
partly true for Br in 2d (NPA = 0.19) and far from being true for
Cl and mainly for F (NPA = 0.1 and À0.31, respectively). That bro-
mine and iodine are involved in this intramolecular halogen bond-
ing can be demonstrated in terms of HOMO’s as compared with
HOMO’s for 2b and 2c: there is no contribution to this orbital for
F and Cl (Fig. 3). Note that the both sets of calculations (Table 2)
follow the experimental trend but DFT values perform slightly bet-
ter in terms of comparison with experimental data.
Dipole moments of 2b–e indicate that neither of these bond
moments can be associated with
l of a X–C bond in 1-phenyl-2-
X-o-carboranes (see Table 2). In other words, we are unable to ver-
ify experimental dipole moments of 2b–e by a simple vector addi-
tion of that for 2a [l(2a), a substrate] and a bond moment applied
for C(2)–X (a substituent), which is an usual approach in organic
chemistry used e.g. in conformational analysis and feasible e.g.
for a series of 12-X-1-SB11H11 (X = Cl, Br, I) where X–B bond dipole
Due to overlaps of many peaks in experimental 11B NMR spectra
of 2b–e, the assignments provided in Table 1 are mainly based on
the ‘‘best-fit” with the calculated values. The correct assignment of
moments were found to be very close to
l(X–Cphenyl) [24]. In the
present study, the corresponding ’s differ from either set of
D
l
group moments in their absolute values, for X = F and Cl to a lesser
extend (see Table 2).
It is very likely that the opposite direction of ‘‘
l(C(2)–X)” in
relation to
l(2a), mainly due to their electron-withdrawing effect,
Table 2
Dipole moments (in D) of 1 and 2a–e.
l
D
la
l
(MP2)d
l
(B3LYP)d
1b
4.50
4.93
3.67
3.61
4.13
4.72
–
4.53
5.62
4.68
4.72
4.88
5.23
4.28
5.49
4.58
4.70
4.84
5.09
2ac
2b
2c
2d
2e
0.00
1.26
1.32
0.80
0.21
a
[
l
(2a) À
(2a) À
l
(2x)], x = b–e.
b
[l
l(1)] is considered to be so-called mesomeric dipole moment and is
interpreted as an electron shift from the benzene ring into the carborane skeleton
[7].
c
See Ref. [10].
For the basis set used, see Section 2.
d
Fig. 3. HOMO for 2b–e at HF level (for geometry used see Section 2).

D. Hnyk et al. / Journal of Molecular Structure 978 (2010) 246–249
249
Table 3
MP2 geometries (in Å, for the basis set used see Section 2) of 2a–e.
X
C1–C2
C1–X
C2–B3/6
C1–B4/5
C1–B3/6
B3/4–B6/5
B4–B5
Ha
Fa
Cl
Br
I
1.659
1.088
1.352 1.373(5)
1.758
1.911 1.888(6)
2.143 2.107(6)
1.701
1.703
1.707
1.706
1.707
1.705
1.704
1.702
1.700
1.703
1.731
1.734
1.734
1.735
1.734
1.769
1.768
1.770
1.775
1.774
1.773
1.781
1.780
1.780
1.780
1.674 1.606(12)b
1.692
1.695 1.692(8)c
1.695 1.696(8)b
a
For 2b see also Ref. [10]. There are a few minima of 2a, the less stable one with phenyl ring perpendicular to C1–C2 is listed for direct comparison with 2b–e. The more
stable form has phenyl group in the C1–C2 plane.
b
Further nearest-neighbour separations in this X-ray structure also compare quite well with the theoretical geometry, see Ref. [13].
Further nearest-neighbour separations in this X-ray structure also compare quite well with the theoretical geometry, see Ref. [27].
c
d(11B) for 2a have also been helpful in assigning individual signals.
Comparison of the spectra of 2b–e reveals that the corresponding
signals do not differ so much. This observation indicates that the
cage geometries are not heavily influenced by individual X when
relating them to the geometry of parent 2a [10]. This finding is
supported by their calculated geometries (Table 3): there is no sig-
nificant influence of X on the cage geometry. Although no symme-
try constraints have been applied in optimizations of these
systems, the final structures obtained look like as if they had a
symmetry of Cs.
ˇ
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offer further support for accounting for the intramolecular contact
of Br (I) with the benzene ring.
Acknowledgements
Financial support from the Ministry of Education of the Czech
Republic (Project LC523) is greatly appreciated. We thank Dr.
M.A. Fox (University of Durham, UK) for providing MP2 geometries
ˇ
and helpful comments and Dr. Jan Machácek of the Institute of
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