O-Carborane-assisted Lewis acidity enhancement of triarylboranes

Article abstract of DOI:10.1039/b918263b

The introduction of an o-carborane cage into the triarylborane significantly enhances the Lewis acidity of the boron atom leading to large increase in fluoride ion affinity.

Full text of DOI:10.1039/b918263b

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o-Carborane-assisted Lewis acidity enhancement of triarylboranesw
Jung Oh Huh,a Hyungjun Kim,a Kang Mun Lee,a Yoon Sup Lee,a
Youngkyu Do*a and Min Hyung Lee*b
Received (in Cambridge, UK) 7th September 2009, Accepted 20th November 2009
First published as an Advance Article on the web 15th December 2009
DOI: 10.1039/b918263b
The introduction of an o-carborane cage into the triarylborane
significantly enhances the Lewis acidity of the boron atom
leading to large increase in fluoride ion affinity.
Triarylboranes have recently attracted great attention in the
fields of dihydrogen activation,1–4 catalytic polymerization,5–7
and anion sensors8–11 due to their high Lewis acidity. Efforts
to enhance their Lewis acidity have thus been the focus of
research interest in such applications. For this purpose, the
introduction of electron-withdrawing groups into the triaryl-
boranes, which involves the use of perfluorinated arenes,12,13
cationic substituents13–19 or metal chelation,20,21 is actively
being investigated. While the first approach is the most
straightforward and effective, this method may adversely
affect the stability in ambient air conditions, the anion
selectivity, and the compatibility with a Lewis basic medium
in proportion to the increasing Lewis acidity, rendering the use
of triarylboranes limited in many applications. In contrast, the
latter two methods utilize the attachment of electron-
withdrawing substituents at the peripheral positions of aryl
moieties to afford the enhanced Lewis acidity while retaining
the steric protection of the boron center by ortho-methyl
groups.
Scheme 1 Reagents and conditions: (i) n-BuLi, ether, ꢀ78 1C.
(ii) Mes2BF, ether, 25 1C. (iii) KF/18-crown-6, toluene, 25 1C.
highly Lewis acidic triarylboranes bearing
a peripheral
o-carborane cage, and describe their fluoride binding
properties to account for the enhanced Lewis acidity in
conjunction with computational studies.
The reaction between the lithium salts derived from
1-bromo-4-(2-R-o-carboran-1-yl)benzene (R = Ph (1a), Me
(1b))24,25 and the dimesitylboron fluoride (Mes2BF) in ether
afforded the o-carborane substituted triarylboranes
[1-(Mes2B)-4-(2-R-o-carboran-1-yl)benzene] (R = Ph (1), Me (2))
as a white solid (Scheme 1).w
Bearing in mind that the Lewis acidity is governed by the
stability of the vacant p-orbital on the boron atom which
contributes largely to the LUMO of triarylboranes, the
effective stabilization of the boron orbital is necessary to attain
high Lewis acidity in the boron center. In this regard, we have
become interested in closo-C2B10H12 carborane as a novel
substituent on the triarylboranes because the carborane not
only constitutes an electron deficient cage, but also possesses
high stability.22 Although the carborane attached on an aryl
group is generally regarded as an electron-withdrawing group
like halogens,23 its electronic effects on the Lewis acidity have
not been clearly disclosed. Thus, it will be intriguing to
investigate whether the electronic properties of the carborane
cage would directly lead to the stabilization of the vacant
p-orbital of the boron atom and whether the extent of such
stabilization could be comparable to that of usual electron-
withdrawing groups. In this report, we provide a novel type of
The identity of 1 and 2 has been fully characterized by
multinuclear NMR spectroscopy, elemental analysis, and
X-ray diffraction methods. While the 1H and 13C NMR
spectra show the expected resonances corresponding to the
2-R-o-carborane and the Mes2B moieties, the 11B NMR
signals detected in the two regions of d 74 ppm and
d ꢀ2 – ꢀ10 ppm confirm the presence of a trigonal planar
boron center and of o-carboranyl boron atoms, respectively.
The crystal structures of 1 (Fig. 1, left)z and 2 (Fig. S3 in
the ESI)w further reveal the trigonal planar geometry
P
(
(C–B–C) = 360.01 for both 1 and 2) around the boron atom
of the triarylborane moiety and the o-carborane cage
appended to the triarylborane. As similarly noted in
the triarylboranes possessing the ortho-methyl groups,
compounds 1 and 2 are also air- and moisture-stable.
Prior to investigation of the fluoride binding properties of 1
and 2, compound 1 was converted into a fluoride adduct.
Treatment of 1 with KF in the presence of 18-crown-6 in
toluene cleanly led to the potassium salt of [1F]ꢀ (Scheme 1).
Besides the 11B resonances attributed to the boron atoms of
the o-carborane cage, which appears in a region similar to that
of 1, the 11B NMR signal detected at d 5.0 ppm and the 19F
NMR signal at d ꢀ177 ppm collectively confirm the formation
of four-coordinate triarylfluoroborate, thus indicating the
binding of fluoride to the boron atom of the triarylborane
a Department of Chemistry, KAIST, Daejeon 305–701,
Republic of Korea. E-mail: ykdo@kaist.ac.kr;
Fax: +8242 869 2810; Tel: +8242 869 2829
b Department of Chemistry, University of Ulsan, Ulsan 680–749,
Republic of Korea. E-mail: lmh74@ulsan.ac.kr;
Fax: +8252 259 2348; Tel: +8252 259 2335
w Electronic supplementary information (ESI) available: Experimental
and computational details. CCDC 746881–746883. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b918263b
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Fig. 1 Crystal structures of 1 (left) and [1F]ꢀ (right) (30% thermal ellipsoid). The H-atoms and [K(18-crown-6)]+ were omitted for clarity.
moiety. The crystal structure of [1F]ꢀ determined from the
X-ray diffraction study (Fig. 1, right) is also in good agreement
with the spectroscopic data.
of 1 and 2 by 3 orders of magnitude. Furthermore such
binding constants are even larger than that found in
the neutral bidentate B/Hg Lewis acid [1-{Mes2B}-8-
To determine the fluoride ion affinity of 1 and 2, UV-vis
titrations were carried out. Compounds 1 and 2 feature a
low-energy absorption band at 322 nm (log e = 4.09 for 1 and
4.05 for 2) in THF which is quenched upon addition of
incremental amounts of fluoride (Fig. S6 and S7w). Remark-
ably, the binding constants estimated from the 1 : 1 binding
{(2,6-Me2-4-Me2NC6H2)Hg}C10H6] (K=1.3(ꢂ0.1) ꢃ 102 Mꢀ1 17
)
under the same conditions.
To get more insight into the effect of the o-carborane cage,
we compared the fluoride binding constants of a series of
para-substituted triarylboranes, i.e. Mes2B(4-R-C6H4) (R = F (4);
H (5); Me (6)) (Fig. S8–S10w). The titration results indicate
that the K values are in order of 1 E 2 (K 4 107 Mꢀ1) 4
4 (5.0 ꢃ 107 Mꢀ1) 4 5 (5.0 ꢃ 106 Mꢀ1) 4 6 (2.5 ꢃ 106 Mꢀ1) in
THF, which is in good agreement with the inductive electronic
effect of the substituents being in parallel with their Hammett
constants.23,27 For better discrimination between the binding
constant of 4 and those of 1 and 2, the fluoride binding
constant of 4 was further measured in THF–H2O (9/1 vol.).
The titration result confirms that the fluoride binding constants
of 1 and 2 are higher than that of 4 (K = 3.3(ꢂ0.1) ꢃ 101 Mꢀ1
in THF–H2O 9/1 vol.) by 2 orders of magnitude (Fig. 2b).
These results thus suggest that the high fluoride ion affinity of
1 and 2, which in turn indicates the increased Lewis acidity of
the boron atom, is ascribable to the introduction of the
o-carborane cage at the aryl moiety of the triarylboranes.
isotherms exceed the measurable range (K 4 107
both 1 and 2.
M
ꢀ1) for
Since we note that the fluoride binding constant of Mes3B
(3) is equal to 3.3(ꢂ0.5) ꢃ 105 Mꢀ1 in THF (Fig. S7w),10 this
result indicates that the o-carborane cage significantly
enhances the fluorophilicity of the boron atom of 1 and 2.
To provide a more quantitative measure of the fluoride ion
affinity, the titrations were carried out in a more competitive
medium of THF–H2O (9/1 vol.). As shown in Fig. 2a, the
fluoride complexation by 1 does occur in THF–H2O (9/1 vol.)
with the binding constant of 5.0(ꢂ0.3) ꢃ 103 Mꢀ1. The similar
K value of 7.7(ꢂ0.3) ꢃ 103 Mꢀ1 is also obtained for 2
(Fig. S12w). Comparison of these K values with that of 3
(K = 1(ꢂ0.3))26 reveals the drastic increase of fluorophilicity
Fig. 2 Spectral changes in the UV-vis absorption of a solution of (a) 1 (4.04 ꢃ 10ꢀ5 M) and (b) 4 (4.38 ꢃ 10ꢀ5 M) in THF–H2O (9/1 vol.) upon
addition of a Bu4NF (0–9.20 ꢃ 10ꢀ4 M for 1 and 0–3.27 ꢃ 10ꢀ2 M for 4). The insets show the absorbance of the solution of 1 and 4 at 322 and
314 nm, respectively, as a function of [Fꢀ] (’). The thin lines correspond to the binding isotherm calculated with K = 5.0 ꢃ 103 Mꢀ1 for 1 and
3.3 ꢃ 101 Mꢀ1 for 4.
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unique reflns = 5207, Rint = 0.0395, R1 = 0.0519, wR2 = 0.1335
(I
4
2s(I)) and GOF
=
1.005; For [Kꢄ(18-crown-6)][1F]:
C45H65B11FKO6, Mw = 866.97, l = 0.71073 A, Monoclinic, Pn,
a = 14.5518(5), b = 11.4399(4), c = 15.0625(6) A, a = 90.00, b =
94.463(2), g = 90.001, V = 2499.87(16) A3, Z = 2, rc = 1.152 g cmꢀ3
,
m = 0.153 mmꢀ1, T = 296(2) K, measd reflns = 25774, unique reflns =
6986, Rint = 0.0339, R1 = 0.0769, wR2 = 0.2162 (I 4 2s(I)) and GOF =
1.024; For 2: C27H39B11, Mw = 482.49, l = 0.71073 A, Monoclinic,
P21/n, a = 7.9560(4), b = 27.7402(15), c = 13.6954(8) A, a = 90.00,
b = 93.533(3), g = 90.001, V = 3295.4(3) A3, Z = 4, rc
=
1.062 g cmꢀ3, m = 0.054 mmꢀ1, T = 296(2) K, measd reflns = 56 299,
unique reflns = 6956, Rint = 0.0437, R1 = 0.0597, wR2 = 0.1658
(I 4 2s(I)) and GOF = 1.038. CCDC reference numbers 1, 746 882;
[Kꢄ(18-crown-6)][1F], 746 881; 2, 746 883.
Fig. 3 HOMO and LUMO of 1 (isovalue = 0.04 a.u.).
To elucidate the increase in the Lewis acidity, the DFT
calculations were carried out at the B3LYP/6-31G(d) level of
theory. The frontier orbitals of 1 show that while the HOMO
is mainly located on the mesityl rings with an orbital
contribution of 87%, the LUMO bears a substantial contri-
bution from the o-carborane cage (16%), as well as from the
empty p-orbital of the boron atom (22%) and the phenylene
ring (32%) (Fig. 3). Further inspection of the LUMO indicates
that the delocalization occurs through exo-p-interaction28
between the tangential p-orbital on the C1 atom of the
o-carborane cage and the p*-system of the triarylborane
moiety.
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is found to be substantially lower than that calculated for the
LUMO of 4 (ꢀ1.63 eV) at the same level of theory, indicating
that 1 is much more Lewis acidic than 4 (Fig. S14w). This result
is in fact in excellent agreement with the enhanced fluoride ion
affinity of 1 and 2. Despite the almost identical compositions
of HOMO for 1 and 4, the HOMO level of 1 (ꢀ6.19 eV) is also
lower than that of 4 (ꢀ6.06 eV), implying the strong inductive
electron-withdrawing effect of the o-carborane cage. However,
since the relative stabilization of the LUMO of 1 with respect
to 4 is greater than that of the HOMO, it can be suggested that
in addition to the inductive effect, the direct contribution of
the o-carborane cage to the LUMO of 1 is strongly associated
with the large stabilization of the LUMO.
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In conclusion, the introduction of an o-carborane cage into
the triarylborane significantly enhances the Lewis acidity of
the boron atom. It is suggested that the contribution of the
o-carborane cage to LUMO coupled with a strong inductive
effect leads to the large stabilization of the LUMO of
triarylboranes.
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Financial support from the Korea Science and Engineering
Foundation (No. R01-2007-000-20299-0 and No. R11-2007-
012-03001-0) and the Priority Research Centers Program of
the National Research Foundation of Korea (No. 2009-
0093818) are gratefully acknowledged.
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z Crystal Data for 1: C32H41B11, Mw = 544.56, l = 0.71073 A,
Monoclinic, P21/c, a = 11.3388(7), b = 21.7577(12), c = 13.6469(8) A,
a = 90.00, b = 101.822(4), g = 90.001, V = 3295.4(3) A3, Z = 4, rc =
1.098 g cmꢀ3, m = 0.057 mmꢀ1, T = 296(2) K, measd reflns = 24356,
ꢁc
This journal is The Royal Society of Chemistry 2010
1140 | Chem. Commun., 2010, 46, 1138–1140