Syntheses, crystal structures, photophysical and theoretical studies of 1,3,2-benzodiazaborolyl-functionalized diphenylacetylenes

Article abstract of DOI:10.1039/c0dt01410a

A series of diphenylacetylenes with one 1,3,2-benzodiazaborolyl end group (BDB) and a second end group X (X = H, OMe, NMe₂, SMe, CN and BDB) were synthesized using established 1,3,2-benzodiazaborole methodologies. The 1,3,2-benzodiazaborolyldiphenylacetylenes with X = p-H (4), p-OMe (5), p-NMe₂ (6), p-SMe (7) and p-CN (8) end groups are functionalized with cyano groups at the central ring in an ortho-position to the triple bond. Molecular structures of 2, 3, 5, 6 and 7 were determined by X-ray diffraction. These borylated systems show intense blue luminescence in cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran, whereas green luminescence was observed in acetonitrile solutions. Thereby Stokes shifts in the range 1700-8600 cm^-1 and quantum yields of 0.60-1.00 were observed in cyclohexane solutions. The absorption maxima (308-380 nm) are well reproduced by TD-DFT computations (B3LYP/G-311G(d,p)) and arise from strong HOMO-LUMO transitions. The LUMOs in all the molecules under study are mainly located on the diphenylacetylene bridge, while with the exception of the dimethylamino derivative 6, the HOMO is largely benzodiazaborolyl in character. Thus, the S1←S0 absorption bands are assigned to π(diazaborolyl)- π*(diphenylacetylene) transitions. In contrast to this, in compound 6 the HOMO is mainly represented by the terminal dimethylaminophenyl unit. While calculated ground state dipole moments μ_g are small (1.1-7.5 D), experimentally determined changes of the dipole moments upon excitation are large (14.8-19.7 D) and reflect a significant charge transfer upon excitation. NLO activities of the rod-structured compounds 2, 4, 6 and 8 are indicated by calculated static first-order hyperpolarizabilities β up to 76.8 × 10^-30 esu.

Full text of DOI:10.1039/c0dt01410a

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PAPER
Syntheses, crystal structures, photophysical and theoretical studies of
1,3,2-benzodiazaborolyl-functionalized diphenylacetylenes†
Lothar Weber,*a Daniel Eickhoff,a Vanessa Werner,a Lena Bo¨hling,a Stefanie Schwedler,a
Anna Chrostowska,*b Alain Dargelos,b Małgorzata Maciejczyk,b Hans-Georg Stammlera and Beate Neumanna
Received 18th October 2010, Accepted 14th February 2011
DOI: 10.1039/c0dt01410a
A series of diphenylacetylenes with one 1,3,2-benzodiazaborolyl end group (BDB) and a second end
group X (X = H, OMe, NMe2, SMe, CN and BDB) were synthesized using established
1,3,2-benzodiazaborole methodologies. The 1,3,2-benzodiazaborolyldiphenylacetylenes with X = p-H
(4), p-OMe (5), p-NMe2 (6), p-SMe (7) and p-CN (8) end groups are functionalized with cyano groups
at the central ring in an ortho-position to the triple bond. Molecular structures of 2, 3, 5, 6 and 7 were
determined by X-ray diffraction. These borylated systems show intense blue luminescence in
cyclohexane, toluene, chloroform, dichloromethane and tetrahydrofuran, whereas green luminescence
was observed in acetonitrile solutions. Thereby Stokes shifts in the range 1700–8600 cm-1 and quantum
yields of 0.60–1.00 were observed in cyclohexane solutions. The absorption maxima (308–380 nm) are
well reproduced by TD–DFT computations (B3LYP/G-311G(d,p)) and arise from strong
HOMO–LUMO transitions. The LUMOs in all the molecules under study are mainly located on the
diphenylacetylene bridge, while with the exception of the dimethylamino derivative 6, the HOMO is
largely benzodiazaborolyl in character. Thus, the S1←S0 absorption bands are assigned to
p(diazaborolyl)–p*(diphenylacetylene) transitions. In contrast to this, in compound 6 the HOMO is
mainly represented by the terminal dimethylaminophenyl unit. While calculated ground state dipole
moments mg are small (1.1–7.5 D), experimentally determined changes of the dipole moments upon
excitation are large (14.8–19.7 D) and reflect a significant charge transfer upon excitation. NLO
activities of the rod-structured compounds 2, 4, 6 and 8 are indicated by calculated static first-order
hyperpolarizabilities b up to 76.8 ¥ 10-30 esu.
variation of the length of the conjugated p-system as well as
by the introduction of donor and/or acceptor functionalities at
appropriate positions of the molecules. It has been recognized
that larger net dipoles and linear electron conducting pathways
provide the largest bathochromic shifts.4 Yamaguchi et al. have
demonstrated that oligophenylene–ethynylenes I with donor end
groups and cyano substituents flanking the rod shaped scaffold
are beneficial for the wavelength of the emission and the quantum
efficiencies of these species.5,6 Interestingly, the dipole moments in
the ground- and excited states of I did not differ significantly.
Introduction
Molecular and polymeric compounds based on the phenylene–
ethynylene motif have been the subject of considerable interest
in recent years. Linear and cruciform structured phenylene–
ethynylenes with electron donating and accepting substituents
have been intensively investigated for their potential application
as efficient light emitters in electro–optical devices, as semicon-
ductors, as well as materials with second- and third-order non-
linear properties.1–3 Fundamental knowledge of the structure–
efficiency relationship is required to create compounds with
defined properties.4 This could principally be achieved by the
aFakulta¨t fu¨r Chemie der Univerita¨t Bielefeld, D-33615, Bielefeld, GER-
MANY. E-mail: lothar.weber@uni-bielefeld.de
bEquipe Chimie Physique, IPREM, UMR 5254, Universite´ de Pau et des
Pays de l’Adour, 2 Av. du Pre´sident Angot, BP 1155, 64013, Pau Ce´dex,
FRANCE. E-mail: anna.chrostowska@univ-pau.fr
† Electronic supplementary information (ESI) available: Tables of atomic
coordinates for [B3LYP/6-311G(d,p)] optimized geometries, values of
total energies and ionization energies (IEs) of 2–9. CCDC reference
numbers 780599–780603. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01410a
Boron-containing functionalities in phenylene–ethynylenes type
compounds are scarce.7,8 Only recently Marder et al. reported on
the syntheses and photophysics of dimesitylboryl substituted 4-
(arylethynylene)anilines of type II.9
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Results and discussion
A prerequisite for the estimation of the influence of the cyano
group on our systems is the availability of phenylacetylenes with
one or two benzodiazaboryl substituents as a reference.
The protocol for the synthesis of 2-[4¢-phenylethynyl]phenyl-
1,3-diethyl-1,3,2-benzodiazaborole (2) involved the lithiation
of 4-bromodiphenylacetylene15 by n-butyllithium and coupling
of the obtained lithium aryl with 2-bromo-1,3-diethyl-1,3,2-
benzodiazaborole (1).16 Crystallization of the solid reaction
residue from an n-pentane–dichloromethane mixture afforded
colorless needles of product 2 in 37% yield (Scheme 1).
4,4¢-Dibromo-diphenylacetylene was synthesized by the Sono-
gashira coupling of trimethylsilylacetylene with 2 equiv. of para-
bromoiodobenzene.15 Lithiation of this intermediate by two molar
equiv. of n-butyllithium and the subsequent addition of two equiv.
of 1 to the reaction mixture led to product 3 as colorless needles
after crystallization from methylcyclohexane–CH2Cl2 (yield 48%)
(Scheme 2).
The Sonogashira coupling of 2-bromo-5-iodobenzonitrile17
with p-substituted arylacetylenes furnished 5-bromo-2-
(arylethynyl)benzonitriles,6 which were metallated in THF
solution at -110 ◦C by n-butyllithium and then combined
with 2-bromo-1,3,2,-benzodiazaborole (1). Colorless crystals of
compounds 4–8 were obtained in 38–77% yield by extraction of
the reaction residue with n-hexane and recrystallization of the
crude products (Scheme 3).
One of our current research interests is focused on the chemistry
of 1,3,2-diazaboroles.10,11 Thereby, we carried out studies on
the syntheses and optical properties of extended p-conjugated
systems with 1,3,2-diazaborolyl- and 1,3,2-benzodiazaborolyl-
substituents, which gave intense blue luminescence when irradiated
with UV-light.12–14 Thereby it became evident that the 10 p-electron
diazaborole (BDB) fragment functions as a p-donor despite the
presence of a three-coordinate boron contact atom. The donor
strength of the BDB substituent has not been quantified as yet.
Here we attempt to estimate the donor capacity of BDB experi-
mentally by comparison with more common donors in line with
Marder’s method by evaluating first-order hyperpolarizabilities.8
Thus, molecules of type III, where the diazaborolyl unit is
separated from a substituent X by a p-conducting spacer, should
be designed to study their photophysical properties, particularly
dipole moments and first-order hyperpolarizabilities b.
For the lithiation of the precursor with n-butyllithium and
subsequent treatment of the transient aryllithium species with 2-
bromo-1,3,2-benzodiazaborole (1) it was crucial to◦maintain the
temperature within the reaction vessel below -100 C. Thus, the
n-butyllithium solution as well as the solution of 1 had to be added
sufficiently slow to avoid local warming, which invariably led to
side reactions at the cyano function.
With donors of similar strength as the BDB unit the dipole
moments and b-values are expected to decrease markedly, and
in case of an ideal match they should vanish completely. As
the p-spacer we selected diphenylacetylene as it leads to rod-like
molecules with a longer p-skeleton and red shifts in absorption and
emission in comparison to the previously studied 2-phenylethynyl-
1,3,2-benzodiazaboroles.14 Moreover, the decreased sensitivity to
moisture of the B–C-linkage in phenylboranes with respect to
ethynylboranes should be advantageous for the use of the new
materials in OLEDs.
With respect to Yamaguchi’s work5,6 the incorporation of cyano
side groups into the molecules is envisaged. In doing so, LUMO
energies are lowered and electron transport by these species in
OLEDs should be facilitated as well.
All the compounds synthesized here can be stored under an
argon atmosphere for several weeks without decomposition. The
1
11B{ H}-NMR spectra for the diazaboroles 2–8 displayed singlets
at 27.4–28.6 ppm in line with other 2-aryl-1,3,2-benzodiazaboroles
which revealed singlets at d = 28.6 to 29.3 ppm.13
X-Ray crystallography
Molecular structures were determined for five benzodiazaborolyl-
functionalized diarylacetylenes 2, 3, 5–7 (Fig. 1, Table 7). Bond
lengths and angles of interest are listed in Table 1.
Molecule 3 lies on a crystallographic inversion center at the
middle of bond C(17)–C(17A). For compound 2 the unit cell
Scheme 1 Synthesis of compound 2.
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Scheme 2 Synthesis of compound 3.
Scheme 3 Synthesis of compounds 4–8.
contains two independent conformers, the bonding parameters
of which are identical within 3 esd’s.
The BN, CN and CC bond lengths within the 1,3-diethyl-1,3,2-
arene ring range from 39.2 to 56.7◦ (av. 46.6◦), where the extreme
values are measured for the two independent conformers of 2. In
1,4-bis- or 1,3,5-tris-p-benzodiazaborolylphenylbenzene values of
33.9–49.8◦ (average 46.2◦) for the respective torsion angles were
found previously.13
In the centrosymmetric molecule 3 the central diphenylacetylene
part is planar, whereas in the two conformers of 2 both arene
rings are tilted by 11.1◦ and 78.7◦, respectively. In the remaining
compounds the deviation from planarity increases on going from
5 (25.3◦) via 7 (50.5◦) to 6 (78.3◦), which is in agreement with a
very low rotational barrier about the carbon–carbon triple bond
in all five compounds. A similar result was recently obtained for
2-[arylethynyl]-1,3-diethyl-1,3,2-benzodiazaboroles.14b
˚
benzodiazaborolyl groups [av. 1.43, 1.40, 1.41 A] (see ESI†) in
all the structures are virtually identical and coincide with those
12–14
in numerous benzodiazaborole structures previously studied
The benzodiazaborole units are linked to the central arene moiety
˚
by single bonds B(1)–C(11) of 1.566 A (av.). It is remarkable that
in compounds 2, 3 and 5 the p-phenylene rings contain two C–C
bonds (C(12)–C(13) and C(15)–C(16)) which are slightly shorter
than the remaining C–C contacts, indicating a small degree of
quinoid character. In contrast to this in 6 and 7 no such behavior
is evident. Both aryl rings are linked by a linear C–C triple bond
˚
of 1.20 A (av.). The mutual orientations of the aromatic p-systems
Thus, it is obvious that at least in the solid state no satisfactory
and their orientation with respect to the diazaborolyl groups
are important, as better p-conjugation between the p-orbitals
on boron and on the diphenylacetylene fragment would occur
if the aromatic rings and the diazaborolyl groups are coplanar.
The interplanar angles between the heterocycle and the adjacent
p delocalization between the three ring units is present. It is
worth mentioning that the distance N(4)–C(23) of 1.383(1) A is
consistent with multiple bonding, despite the fact that the amino
group is slightly pyramidal (sum of angles 353.5◦) and twisted by
24.4◦ out of the plane of the adjacent ring.
˚
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Fig. 2 Emission maxima of 2–8 in CH2Cl2.
Fig. 3 Photograph of the emission of 7 in different solvents.
of the UV-Vis absorption maximum labs (303 nm in 2 vs.
306 nm for BDB–C C–Ph in c-C6H12). A similar effect has
been noted previously for donor/acceptor substituted phenylene–
ethynylene oligomers and was attributed to statistically poorer
donor/acceptor interactions in the ground state of the longer
system due to an increased number of rotameric conformers of the
more than two rings.1j,9 The emission maximum of 2 (380 nm) is red
shifted in comparison to the 2-phenylethynyl benzodiazaborole
(361 nm), which is in agreement with the elongation of the p-
system and an increased planarization of the molecule in the
excited state. The extension of the rod-like structure of 2 by a
second benzodiazaborolyl group to afford derivative 3 gives rise to
a bathochromically shifted absorption maximum (labs = 314 nm).
The emission maximum, however, (386 nm) is only slightly shifted.
Upon incorporation of a cyano group at the central ring adjacent
to the triple bond in 2 to give 4, both, the absorption and emission
maxima are red shifted by 21 and 27 nm respectively (in c-C6H12).
In derivates 5 (X = OMe), 7 (X = SMe) and 6 (X = NMe2) the
donor end groups cause red shifts in the UV absorption of about
15, 25 and 56 nm in comparison to compound 4 (X = H) in c-
C6H12. Remarkably, the emission maxima of 4 (407 nm), 5 (403
nm), 6 (403 nm) and 7 (408 nm) are not essentially influenced by
the nature of the substituent in the para-position of the terminal
aryl ring.
Fig. 1 Molecular structures of 2, 3, 5–7.
UV-visible and luminescence spectroscopy
The UV-Vis absorption and emission spectra of compounds 2–
8 were determined in a variety of solvents (Fig. 2 and 3). Their
absorption and emission maxima and their extinction coefficients
are given in Table 2. In general all chromophores show intense
absorption bands, which are not markedly influenced by the
solvents (max. difference 10 nm). These small solvatochromic
absorption shifts are consistent with small ground-state dipole
moments. Interestingly, the insertion of a phenylene unit into
the B–C (alkyne) bond of BDB–C C–Ph14a to afford 2 with
an elongated p-system was accompanied by a slight blue shift
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Table 1 Selected bond lengths and angles for 2, 3, 5–7
2
3
5
6
7
˚
Bond lengths (A)
B–C
B–N (av.)
B(1)–C(11) 1.566(2)
B(1)–N(1) 1.431(1)
B(1)–N(2) 1.432(2)
B(1)–C(11) 1.564(2)
B(1)–N(1) 1.432(2)
B(1)–N(2) 1.438(2)
B(1)–C(11) 1.563(1)
B(1)–N(1) 1.438(1)
B(1)–N(2) 1.437(1)
1.568(1)
1.434(1)
1.433(1)
1.568(1)
1.434(2)
1.428(2)
Aryl links
C–C
C(11)–C(12) 1.403(2)
C(12)–C(13) 1.384(2)
C(13)–C(14) 1.400(2)
C(14)–C(15) 1.401(2)
C(15)–C(16) 1.384(2)
C(11)–C(16) 1.404(2)
C(11)–C(12) 1.407(2)
C(12)–C(13) 1.386(2)
C(13)–C(14) 1.397(2)
C(14)–C(15) 1.402(2)
C(15)–C(16) 1.390(2)
C(11)–C(16) 1.405(2)
C(11)–C(12) 1.403(1)
C(12)–C(13) 1.396(1)
C(13)–C(14) 1.410(1)
C(14)–C(15) 1.405(1)
C(15)–C(16) 1.385(1)
C(11)–C(16) 1.406(1)
C(20)–C(21) 1.405(1)
C(21)–C(22) 1.381(1)
C(22)–C(23) 1.394(1)
C(23)–C(24) 1.394(1)
C(24)–C(25) 1.390(1)
C(20)–C(25) 1.399(1)
C(23)–O(1) 1.366(1)
1.402(1)
1.397(1)
1.414(1)
1.399(1)
1.394(1)
1.406(1)
1.401(2)
1.387(1)
1.410(1)
1.411(1)
1.388(1)
1.400(2)
1.402(2)
1.394(2)
1.422(2)
1.398(2)
1.387(2)
1.405(2)
1.403(2)
1.381(2)
1.402(2)
1.395(2)
1.386(2)
1.396(2)
C–X
C(23)–N(4) C(23)–S(1)
1.383(1)
1.758(1)
Inter-ring
C–C
C(14)–C(17) 1.435(2)
C(17)–C(18) 1.202(2)
C(18)–C(19) 1.436(2)
C(14)–C(17) 1.437(2)
C(17)–C(17A) 1.202(3)
C(14)–C(18) 1.432(1)
C(18)–C(19) 1.203(1)
C(19)–C(20) 1.435(1)
1.433(1)
1.205(1)
1.435(1)
1.434(2)
1.204(2)
1.436(2)
Bond Angles (◦)
C–C–C
C(14)–C(17)–C(18) 177.3(1)
C(17)–C(18)–C(19) 178.1(1)
C(14)–C(17)–C(17A) 178.2(2) C(14)–C(18)–C(19) 176.1(1)
C(18)–C(19)–C(20) 177.9(1)
175.3(1)
176.5(1)
178.5(1)
177.8(1)
Torsion Angles (◦)
N(1)–B(1)–C(11)–C(16) 56.7
C(13)–C(14) ◊ ◊ ◊ C(17)–
C(18) ◊ ◊ ◊ C(19)–C(24)
11.1
N(1)–B(1)–C(11)–C(17) 45.9 N(1)–B(1)–C(11)–C(16) 41.0
47.9
78.3
49.3
50.5
C(13)–C(14)..C(17)–
C(17A) ◊ ◊ ◊ C(14A)–C(13A)
0.0
C(13)–C(14) ◊ ◊ ◊ C(18)–
C(19) ◊ ◊ ◊ C(20)–C(25)
25.3
The similar energies for the luminescence of 2 and 3 are
surprising and the reason for this is not completely clear at the
moment. Similar luminescence energies of ca. 400 nm for 4, 5
and 7 agree with transitions from excited states of similar energy
and geometry regardless of the terminal substituent, which might
perhaps be due to an unfavorable orientation of the aryl–X unit.
After radiative relaxation a state of identical geometry is acquired,
which relaxes to the ground state by rotational processes. In 6 the
situation is different. Here the HOMO is located on the aniline part
of the molecule and by electronic excitation a charge transfer to
the adjacent cyanophenylene ring takes place. The comparatively
small Stokes shift agrees with the lack of significant geometrical
changes during this process. The situation in 8, where in contrast to
derivatives 4–7 an electron acceptor is placed in the para-position
of the terminal ring, is different again. The absorption maximum
of compound 8 (labs. = 300 nm in c-C6H12) is blue shifted relative to
4 by 24 nm and like compound 4 no significant solvatochromism of
the absorption is observed. In cyclohexane the emission maximum
of 8 is bathochromically shifted by 126 nm with respect to the
absorption, whereas in 2 and in 4 the corresponding red shifts
amount to only 77 nm or 83 nm, respectively. Compounds 2, 4–8
show pronounced solvatochromism in line with large ICTs upon
excitation and large dipole moments in the excited states. The
largest bathochromic shift (123 nm) is observed for 4 upon going
from cyclohexane to acetonitrile as the solvent. Dipole moment
changes between the ground and excited state. Dm were estimated
following the Onsager–Lippert–Mataga model. According to a
in the narrow range of 18.6–19.7 D were found (Table 6). This
could be rationalized by emissive states of similar polarity.
It has been pointed out previously that the size of the dipole
moment change Dm is largely independent from the donor and
acceptor strength of the substituents present in the molecule.3n
The quantum yields of the new benzodiazaboroles are also
solvent sensitive and decrease with increasing solvent polarity.
This is impressively documented for derivative 5 (R = OMe)
with a quantum yield of 1.00 in c-C6H12 and only 0.04 in
acetonitrile. Obviously the stabilization of the excited state favors
the probability of a non-radiative decay.
As anticipated the insertion of a cyanophenylene unit into the
B–C-bond of the phenylenethynyl-1,3,2-benzodiazaboroles leads
to significant bathochromic shifts for the absorption and emission
bands (for X = OMe: Dlabs = 31 nm, Dlem = 91 nm; X = SMe:
Dlabs = 31 nm, Dlem = 59 nm; X = NMe2: Dlabs = 53 nm, Dlem
=
123 nm; solvent THF).
DFT calculations
The geometries of compounds 2, 4, 6, 8 and 9 were optimized by
DFT calculations at the B3LYP/6-311G(d,p) level of theory. With
the exception of 5 global minima occur when both benzene rings
are co-planar, whereby the plane of the heterocycle deviates by
49.7◦ for 2 to 57.9◦ for 4. The optimized structure of 4 exhibiting
this particular interplanar angle is 48.0 kcal mol-1 more stable than
the completely planar structure. In contrast to the experimental
observation (sum of angles 353.5◦) the nitrogen atom of the amino
˚
radius of 5.74 A for the p-substituted molecules 4–6, 8 Dm values
4438 | Dalton Trans., 2011, 40, 4434–4446
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Table 2 Photophysical data of compounds 2–8
lmax,abs (nm)
lmax,ex (cm-1)
e (L mol-1 cm-1)
lmax,em (nm)
lmax,em (cm-1)
Stokes shift (cm-1)
Ufl
2a
2b
2c
2d
2e
2f
303
306
305
305
305
303
314
317
314
314
317
314
324
326
327
323
327
323
339
343
340
338
340
336
380
382
382
378
380
376
349
351
349
346
348
341
300
301
301
300
302
298
32 700
32 300
32 500
32 500
32 500
32 800
31 900
31 600
31 900
31 900
31 600
31 900
30 500
29 600
30 200
31 000
30 200
30 600
29 300
29 000
29 000
29 300
29 200
29 600
26 400
25 800
25 700
26 100
25 900
26 200
28 500
28 000
28 100
28 400
28 400
28 800
31 900
31 200
31 500
30 700
31 400
—
24 300
31 000
35 300
36 400
40 900
15 100
8 500g
21 200
33 600
33 800
35 200
4 500h
27 500
25 700
25 800
32 800
28 200
31 900
13 700
30 500
23 800
31 500
29 300
30 500
32 400
35 800
27 200
40 200
33 900
35 100
35 900
28 600
33 300
29 700
39 600
19 100
31 200
24 000
33 900
33 900
34 000
15 100
380
403
421
437
438
462
386
406
426
431
444
471
407
441
464
480
487
530
403
433
455
465
477
510
403
441
459
478
476
522
408
439
462
469
486
528
426
470
497
525
530
—
26 100
24 300
23 300
23 200
23 100
21 300
25 900
24 600
23 500
23 200
22 500
21 200
24 500
22 200
20 900
20 200
20 100
18 300
24 700
22 800
21 600
21 200
20 600
19 100
24 700
22 400
21 600
20 700
20 800
18 800
24 400
22 400
21 300
20 700
20 200
18 400
23 300
21 000
19 800
18 500
18 500
—
6 600
8 000
9 200
9 300
9 400
11 500
6 000
7 000
8 400
8 700
9 100
10 700
6 000
7 400
9 300
10 800
10 100
12 300
4 600
6 200
7 400
8 100
8 600
10 500
1 700
3 400
4 100
5 400
5 100
7 400
4 100
5 600
6 800
7 700
8 200
10 400
8 600
10 200
11 700
12 200
12 900
—
0.83
0.71
0.46
0.42
0.72
0.40
1.00
0.90
0.40
0.37
0.69
0.34
0.63
—
3a
3b
3c
3d
3e
3f
4a
4b
4c
4d
4e
4f
—
—
0.40
—
5a
5b
5c
5d
5e
5f
1.00
0.74
0.69
0.05
0.60
0.04
1.00
0.94
0.55
0.55
0.63
0.06
0.77
0.77
0.55
0.02
0.55
0.02
0.60
0.49
0.34
0.16
0.13
—
6a
6b
6c
6d
6e
6f
7a
7b
7c
7d
7e
7f
8a
8b
8c
8d
8e
8f
Solventsa cyclohexane, b toluene, c CHCl3, d THF, e CH2Cl2, f MeCN. g Small value is due to poor solubility in c-C6H12. h Small value is due to partial
decomposition in CH3CN into non-fluorescing fragments.
group in 6 is planar (sum of angles 359.2◦). This plane is slightly
twisted out of the adjacent benzene plane as is evident from the
torsion angles of 5.5◦ and 4.9◦. In the experimentally determined
structure this angle was measured as 24.4◦. Thus, according to the
calculation, a maximum p-delocalization of the amino group and
arene ring is observed. Table 3 lists selected geometrical parameters
for 2, 4–6 and 8. Comparison of the experimental by determined
bond lengths and angles and the optimized geometries reveal very
good agreement. The calculations and the X-ray studies differ,
however, in the mutual orientation of the ring planes.
The calculated absorption maxima from TD–DFT computa-
tions on the compounds clearly depend on the mutual orientation
of all three rings (Table 4). These absorptions arise from low energy
HOMO–LUMO transitions. In comparison to the experiment,
the measured absorption values for 2, 4–6, 8 (in c-C6H12) are red
shifted by 3–32 nm, which may be rationalized by the presence
of all conformers in the solvent. In the gas phase, planarity
between the arene units and the absence of solvent favors long wave
absorptions. In contrast, the calculated absorption maximum of 3
is blue shifted by 4 nm.
Table 5 lists the HOMO–LUMO gaps and the molecular orbitals
for compounds 2–6, 8 and 9. In derivatives 2–5 and 8 the HOMOs
correspond to the antibonding interaction p3–pNBN inside the ben-
zodiazaborole ring. The LUMO for all species under investigation
has no orbital contributions from the diazaborole unit, leading
to HOMO–LUMO gaps varying from 5.72 eV in 8 (X = CN,
Y = CN) to 6.39 eV in 2 (X = H, Y = H). Substitution of the
3-position in the central ring of 3 by the electron-withdrawing
cyano function affords derivative 9 whereby the symmetry of the
molecule is broken. As a consequence the orbital [p3–pNBN]+ is no
longer the HOMO of the resulting species, but the HOMO-1. The
HOMO of 9 is the antibonding combination p3–pNBN inside the
benzodiazaborole ring at the position X of the terminal benzene
ring.
Similarly, the introduction of the cyano group into molecule
3 leads to a narrowing of the HOMO–LUMO gap from 6.24 to
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Dalton Trans., 2011, 40, 4434–4446 | 4439
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◦
˚
Table 3 Selected bond lengths [A] and angles [ ] calculated [B3LYP/6-311G(d,p)] for optimized geometries of 2, 4–6 and 8
2
4
5
6
8
X = Y = H
X = H, Y = CN
X = OMe, Y = CN
X = NMe2, Y = CN
X = Y = CN
˚
Bond lengths (A)
B–C
B–N
C–N
1.565
1.442
1.397
1.211
1.423
1.084
1.568
1.439
1.398
1.210
1.420
1.084
1.568
1.443
1.398
1.210
1.418
1.360
1.565
1.441
1.392
1.212
1.415
1.373
1.568
1.439
1.398
1.210
1.418
1.430
C
C
C–C( C)
C–X
Bond angles (◦)
N–B–N
106.2
108.6
126.9
180.0
106.2
108.2
126.8
178.5
106.2
108.3
126.9
178.0
106.2
108.2
126.8
178.4
106.6
108.1
126.7
178.2
B–N–C
N–B–C
C–C
C
Torsion angles (◦)
N–B–C–C
49.7
0.0
57.1
1.1
51.9
19.4
50.0
0.9
49.8
0.3
C
C–( )–C C
Table 4 Comparison of calculated [B3LYP/6-311G(d,p)] data for opti-
mized geometries of 2–6, 8 and 9 and observed UV absorption maxima
(in c-C6H12)
the scaffold. The dipole moment has also changed its direction
from the ground to the excited state.
The ground state dipole moments mg, molecular polarizabilities
a and the static first hyperpolarizabilities b of compounds 2–6,
8 and the hypothetical molecule 9 have been calculated using the
two state model18 and are listed in Table 7. The magnitudes of mg
are all found to be rather small and depend on the substituents
on the terminal aryl ring. They range from 1.1 D for the cyano-
free derivative 2, via 3.5 D for the CN-side group containing 4 to
7.5 D for 8, where the donating benzodiazaborole group and the
terminal CN-acceptor are ideally separated. These low values are
consistent with only a little charge transfer in the ground state of
the compounds. The electronic transitions upon excitation were
accompanied by significant charge transfer processes as evident
from the pronounced solvatochromism of the emissions and the
experimentally derived change of the dipole moment of 14.8–19.7
D.
The molecular polarizabilities a of compounds 4–8 featuring
CN-side groups vary little in the range 5.3–6.2 ¥ 10-23 esu. The a-
values for derivatives 3 and 9 are 7.6 ¥ 10-23 esu and 7.9 ¥ 10-23 esu,
which suggests that a second benzodiazaborole unit at the terminus
of the rod-like molecule confers more to its polarizability than the
remaining substituents in this study.
In compounds of the type Mes2B–C C–C6H4–4–R the polar-
izabilities a vary between 4.4 ¥ 10-23 esu (R = H) and 5.0 ¥ 10-23
esu, except for Mes2B–C C–C6H4–4–NO2, where a = 8.8 ¥ 10-23
esu were found.8
lmax(calcd) Oscillator (calcd) lmax
Dlmax
(calcd - exp)
Compound (nm)
strength (f)
(exp, nm)
2
3
4
5
6
8
9
307
310
327
360
390
332
330
0.92
1.09
0.55
0.54
1.10
1.14
0.59
303
314
324
339
380
300
—
+4
-4
+3
+21
+10
+32
—
5.93 eV. The stabilization of the HOMO of 3 from -6.82 to -6.93
eV is much less pronounced than that of the LUMO from -0.59 to
-1.00 eV. In going from 2 to 4 the introduction of the CN-function
leads to a narrowing of the HOMO–LUMO gap from 6.40 to 6.13
eV. The situation is different for the dimethylamino derivative 6.
Due to the rather strong p-donor effect of the amino group
the orbital pC p–p2(Ph¢¢)–nNp(X) is considerably stabilized and
C
corresponds to the HOMO of this system, while the MO p3–pNBN
now becomes the HOMO-1. Here it has to be noted that CAM-
B3LYP/6-311G(d,p) tends to overestimate HOMO–LUMO gaps.
With regard to the nature of the frontier orbitals in compound
8 the UV-absorption induces an intramolecular charge-transfer
transition from the benzodiazaborolyl group into the LUMO
of the molecule which is virtually located at the terminal p-
cyanophenylene–ethynyl unit.
In the UV-spectrum of 2 the low energy absorption was
attributed to the ICT transition from the benzodiazaborole ring
to the central phenylene (ethynylene) unit. Similar ICT transitions
occur in 4 and 5 by UV excitation. Due to the different situation in
the frontier orbitals of the dimethylamino derivative 6 the HOMO–
LUMO transition is described as an ICT from the electron-
releasing aminophenyl group into the CN-functionalized part of
Donor/acceptor substituted p-conjugated organic molecules
having low-lying charge transfer excited states exhibit large first-
order non-linear properties. The molecular hyperpolarizabilities
b of 2–6, 8 and 9 were computed as analytical third derivatives
of the energy and correspond to the static hyperpolarizabilities
(frequency = 0). An increase in the range b = 5.0 ¥ 10-30 esu (2) £
5.6 ¥ 10-30 esu (4) < 21.1 ¥ 10-30 esu (8) < 33.8 ¥ 10-30 esu (5) ꢀ
76.8 ¥ 10-30 esu (6) was calculated. An interpretation of these data
4440 | Dalton Trans., 2011, 40, 4434–4446
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Table 5 Calculated [CAM-B3LYP/6-311G(d,p)] HOMO–LUMO gaps and MO nature of 2–6, 8 and 9 (BDB = 1,3-diethyl-1,3,2-benzodiazaborolyl).
Contour values are plotted at 0.04 e bohr-3)1/2
X
H
Y
H
HLG
6.40
HOMO
LUMO
2
3
4
5
BDB
H
6.24
6.13
6.16
H
CN
CN
OMe
6
8
NMe2
CN
CN
5.82
5.72
CN
9
BDB
CN
5.93
on the basis of the two state model, whereby b is proportional to
the dipole moment change between the ground and the excited
state Dm and the square of the transition moment integral for
the optical transition between two states (oscillator strength).
Moreover, the hyperpolarizability is inversely proportional to
the square of the energy of the respective transition (HOMO–
LUMO gap). According to the Lippert–Mataga model Dm does
not change significantly (ca. 19 D), and with the exception of 4 and
5 (f = 0.55) the oscillator strength for the remaining derivatives
is ca. 1. As observed with donor/acceptor functionalized tolanes
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Dalton Trans., 2011, 40, 4434–4446 | 4441
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Table 6 CAM-B3LYP/6-311G+(d,p) calculated ground state (mg) dipole
moments, molecular polarizabilities (a), static first-order molecular hy-
perpolarizabilities (b) and experimental change of dipole moments (Dm)
of compounds 2–6, 8 and 9
even though the HOMO–LUMO gap (6.17 eV) and the oscillator
strength of 0.54 are smaller than in 8 and Dm is similar (18.6 D).
Anyway, our results are virtually consistent with observations by
Marder and others8 that a large charge transfer upon excitation
leads to higher dipole moments, pronounced solvatochromism
and significant NLO activities. Thereby, hyperpolarizabilities b of
molecules Mes2B–C C–C6H4-4-R range from 2.5 ¥ 10-30 to 25.7
¥ 10-30 esu.
Compound
mg [D]
a [10-23 esu]
b [10-30 esu]
Dm [D]
2
3
4
5
6
8
9
1.1
0.0
3.5
4.7
4.7
7.5
3.9
5.1
7.6
5.3
5.7
6.2
5.7
7.9
5.0
0.0
5.6
33.8
76.8
21.1
22.6
14.8
—
19.7
18.9
19.4
18.6
—
Conclusion
It has been demonstrated that 1,3,2-benzodiazaboroles, func-
tionalized by variously substituted diphenylacetylenes have been
synthesized using known methodologies. Unlike their absorp-
tion maxima, their emission maxima are characterized by large
solvatochromic shifts in solvents of increasing polarity, which
is indicative of significant dipole moments in the excited state.
Using the Onsager–Lippert–Mataga approximation changes of
the dipole moment between the ground state and the excited
state of ca. 19 D for the cyano substituted derivatives were
estimated. Static first-order molecular hyperpolarisibilities b for
the novel compounds range from 5.0 ¥ 10-30 to 76.8 ¥ 10-30 esu. A
straightforward explanation for these data, however, could not be
obtained on the basis of the familiar two-state model.
[3m]
4-D-C6H4–C C–C6H4-4-A
or arylethynyl-dimesityl-boranes
8
4-D-C6H4–C C–BMes2 the magnitude of b in 2, 4–8 may
also be dominated by the size of the HOMO–LUMO gaps
(reflecting the donor/acceptor strengths). Ordering schemes for b
by the donor/acceptor strengths, however, are at best approximate
as various donor–acceptor pairs couple differently across the
conjugated framework. In keeping with this, the large b-value
of 6 (76.8 ¥ 10-30 esu) would agree with a small HOMO–LUMO
gap (5.82 eV), an oscillator strength of 1.10 and a Dm of 19.4
D. Here, however, as pointed out before, the ICT transition reflects
the donation of charge from the electron rich dimethylaminoaryl
ring onto the central cyanophenyl ring, which differs from all
remaining derivatives in this study. In 8 the ICT transition from
the benzodiazaborole to the terminal cyanophenyl acceptor has a
similar oscillator strength (f = 1.14), a similar Dm (= 18.6 D) and
an even smaller HOMO–LUMO gap than 6. Against intuition a
b-value of only 21.1 ¥ 10-30 esu was calculated for 8. For compound
5 the hyperpolarizability b was computed to be 33.8 ¥ 10-30 esu,
Experimental
All experiments were performed under dry, oxygen-free argon
by using standard Schlenk technique. Solvents were dried with
appropriate drying agents and freshly distilled under argon
before use. The following compounds were prepared according
Table 7 Crystal data for 2, 3, 5, 6 and 7
2
3
5
6
7
Empirical formula
M [g mol-1]
Crystal dimensions (mm)
Crystal system
C24H23BN2
350.25
C34H36B2N4
522.29
0.22 ¥ 0.14 ¥ 0.06
Monoclinic
P21/n
9.8599(3)
11.9000(4)
12.5138(3)
90
103.875(2)
90
1425.44(7)
2
1.217
0.071
556
3–27.5
19844
3195
0.042
2625
183
C26H24B3N3O
405.29
C27H27BN4
418.34
C26H24BN3S
421.35
0.30 ¥ 0.25 ¥ 0.20
0.30 ¥ 0.28 ¥ 0.24
0.30 ¥ 0.21 ¥ 0.10
0.30 ¥ 0.27 ¥ 0.20
Triclinic
Triclinic
Triclinic
Triclinic
¯
¯
¯
¯
Space group
P1
P1
P1
P1
˚
a [A]
8.2289(1)
935640(1)
26.2207(3)
90.276(1)
94.182(1)
102.184(1)
1967.66(4)
4
1.182
0.068
744
3–27.5
39445
7.6596(2)
8.2312(1)
18.3644(3)
100.449(1)
90.037(1)
108.983(1)
1074.49(4)
2
1.253
0.077
428
3–27.5
24875
8.1333(3)
8.8483(2)
15.7389(5)
92.105(2)
94.804(2)
91.021(2)
1127.68(6)
2
1.232
0.073
444
3–30.1
22633
10.1351(2)
10.7913(2)
10.8348(2)
82.583(1)
73.105(1)
80.018(1)
1112.80(4)
2
1.257
0.164
444
3.4–25.0
22441
˚
b [A]
˚
c [A]
a [◦]
b [◦]
g [◦]
3
˚
V [A ]
Z
ycalc [g cm-1]
m [mm-1]
F (000)
H [◦]
No refl. collected
No refl. unique
R (int)
No refl. [I > 2s(I)]
Refined parameters
GOF
9002
0.033
7048
491
4894
0.028
4247
283
6329
0.029
5322
293
3882
0.026
3612
376
1.046
1.046
1.058
1.022
1.075
Rf [I > 2s(I)]
0.0394
0.1016
0.243/-0.215
780599
0.0428
0.1116
0.249/-0.249
780600
0.0379
0.1018
0.271/-0.267
780601
0.0430
0.1156
0.312/-0.232
780602
0.0301
0.0785
0.258/-0.255
780603
wRF2 (all data)
-3
˚
Dymax/min [e A ]
CCDC number
4442 | Dalton Trans., 2011, 40, 4434–4446
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©

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to literature procedures: 2-bromo-1,3-diethyl-1,3,2-benzodia-
1H-NMR (CDCl3): d = 1.32 (t, JHH = 7.2 Hz, 12H, CH2CH3),
3.79 (q, JHH = 7.2 Hz, 8H, CHCH3), 7.06 (m, 4H, CH CH–
zaborole (1),16 4-bromo-diphenylacetylene,15 5-bromo-2-{[4-
(methoxy)phenyl]ethynyl}benzonitrile,6
5-bromo-2-{[4-(dime-
CH CH), 7.13 (m, 4H, CH CH–CH CH), 7.57 (d, JHH =
thylamino)phenyl]ethynyl}benzonitrile,6 5-bromo-2-{[4-(methyl-
thio)phenyl]ethynyl}benzonitrile,6 5-bromo-2-[(4-cyanophenyl)-
ethynyl]benzonitrile,6 bis(4-bromophenyl)acetylene,17 2-bromo-
7.5 Hz, H–Ph), 7.64 (d, JHH = 7.5 Hz, H–Ph) ppm; 13C-{ H}-NMR
(CDCl3): d = 16.3 (s, CH2CH3), 37.7 (s, CH2CH3), 90.2 (s, C C),
109 (s, CH–CH CH–CH), 118.8 (s, CH CH–CH CH), 123.4
(s), 131.1 (s), 133.4 (s), 133.7 (s, C–Ph), 137.1 (s, C2N2) ppm; 11B-
1
5-iodobenzonitrile.15
4-Bromophenylacetylene,
4-methoxy-
1
phenylacetylene, 1,4-bromoiodobenzene, trimethylsilylacetylene,
[(Ph3P)2PdCl2] and CuI were purchased commercially.
{ H}-NMR (CDCl3): d = 28.5 (s) ppm; MS–EI (m/z): 522.3 [M+].
2[3¢-Cyano-4¢[phenylethynyl]phenyl]-1,3-diethyl-1,3,2-
benzodiazaborole (4)
1,3-Diethyl-2[4¢-phenylethynyl]phenyl-1,3,2-benzodiazaborole (2)
A sample of an 1.6 M solution of n-butyllitium (2.4 mL, 3◦.9 mmol)
in n-hexane was slowly added to chilled solution (-110 C) of 5-
bromo-2-(phenylethynyl)benzonitrile (1.1 g, 3.9 mmol) in 40 mL
of THF. After stirring for 20 min a solution of 1 (0.98 g, 3.9
mmol) in THF (10 mL) was added dropwise. Stirring the resulting
mixture at -110 ◦C was continued for 4 h. Then it was allowed
to slowly warm up to room temperature. After 16 h of stirring
the solution was evaporated to dryness. The solid residue was
suspended in 30 mL of n-hexane, heated to ca. 60 ◦C and filtered
while still hot. The filter-cake was extracted with 20 mL of hot
n-hexane. The combined filtrates were freed from solvent and
volatile components in vacuo, and the solid residue was purified by
crystallization from n-hexane at -35 ◦C. The crude product 4 was
obtained as a dark green powder (1.14 g, 78%). The green impurity
was removed by dissolving the powder in diethylether and slowly
cooling to -35 ◦C (0.81 g, 55% yield).
A stirred THF solution (50 mL) of 4-bromodiphenylacetylene
(0.55 g, 2.14 mmol) was combined dropwise at -78 ◦C with
2.26 mmol n-butyllithium dissolved in 1.41 mL of n-hexane.
Stirring at this temperature was continued for 30 min. Then the
dry ice bath was removed and the reaction mixture was stirred for
30 min at 20 ◦C. After re-cooling to -78 ◦C a sample of 2-bromo-
1,3-diethyl-1,3,2-benzodiazaborole (1) (0.54 g, 2.14 mmol) was
added. Stirring at -78 ◦C was pursued for 1 h, and then overnight
at room temperature. The mixture was evaporated to dryness, the
residue triturated with dichloromethane (20 mL) and the obtained
slurry was filtered. Solvent and volatile components were removed
from the filtrate in vacuo, the off-white crude product was then
washed with n-pentane (2 ¥ 5 mL, 0 ◦C) before it was crystallized
from a dichloromethane–n-hexane mixture. Compound 2 was
obtained as colorless crystals in 39% yield.
Found C 80.27, H 6.85, N 8.46%; C24H23BN3 requires C 82.20, H
6.62, N 8.00%; repeated experiments did not provide improved C-
values. Obviously, boron carbide formation has led to incomplete
Found C 79.20, H 5.92, N 11.20%; C22H22BN2 requires C 80.01,
1
H 5.91, N 11.20%; H-NMR (C6D6): d = 0.96 (t, JHH = 7.2 Hz,
6H, CH2CH3), 3.33 (q, JHH = 7.2 Hz, 4H, CH2CH3), 6.98 (m, 3H,
H–Ph and 2H, CH CH–CH CH), 7.15 (m, 2H, CH CH–
CH CH), 7.22 (dd, JHH = 7.6, 2.2 Hz, 1H, H-benzonitrile),
1
combustion of the compound. H-NMR (CDCl3): d = 1.31 (t,
JHH = 7.2 Hz, 6H, CH2CH3), 3.78 (q, JHH = 7.2 Hz, 4H, CH2CH3),
7.08 (m, 5H, H–Ph, H-borole), 7.35 (m, 4H, H–Ph, H-borole),
7.32 (d, JHH = 7.6 Hz, 1 Hz, 1H, H-benzonitrile), 7.46 (d, JHH
=
3
7.55 (d, JHH = 7.6 Hz, 2H, H–Ph), 7.61 (d, JHH = 7.6 Hz, H–
2.2 Hz, 1H, H-benzonitrile), 7.64 (m,2H, H–Ph) ppm; 13C-{ H}-
NMR (C6D6): d = 16.0 (s, CH2CH3), 37.4 (s, CH2CH3), 86.4,
96.6 (2 s, C C), 109.4 (s, CH CH–CH CH), 115.8 (s, C–
CN), 117.8 (s, C–CN), 119.5 (s, CH CH–CH CH), 122.3 (s,
C–Ph), 127.1 (s, C-benzonitrile), 128.6 (s, m-CH–Ph), 129.2 (s,
p-CH–Ph), 131.3 (s, CH-benzonitrile), 132.2 (s, o-CH–Ph), 136.7
(s, CH-benzonitrile), 137.0 (s, C2N2), 137.3 (s, CH-benzonitrile);
1
1
Ph) ppm; 13C-{ H}-NMR (CDCl3): d = 16.3 (s, CH2CH3), 37.6 (s,
CH2CH3), 89.5, 90.0 (2, s, C C), 108.9 (s, CH–CH CH–CH,
borole), 118.7 (s, CH CH–CH CH, borole), 123.3 (s), 133.4 (s,
1
C–Ph), 137 (s, C2N2) ppm; 11B-{ H}-NMR (CDCl3): d = 28.6 (s)
ppm; MS–EI (m/z): 350.2 [M+].
1
11B-{ H}-NMR (C6D6): d = 27.45 (s) ppm; MS–EI (m/z): 375.1
4,4¢-Bis[2¢¢-1¢¢,3¢¢-diethyl-1¢¢,3¢¢,2¢¢-benzodiazaborolyl]-
diphenylacetylene (3)
[M+].
A solution of bis(4-bromophenyl)acetylene (0.51 g, 1.53 mmol)
in 40 mL of THF was treated at -78 ◦C with 2.15 mL (3.44
mmol) of a 1.6 M solution of n-butyllithium in n-hexane. After
stirring for 30 min at -78 ◦C the reaction mixture was warmed
up to 20 ◦C and stirred for another 30 min. Re-cooling to -78 ◦C
was followed by the addition of 2-bromo-1,3,2-benzodiazaborole
(1) (0.80 g, 3.17 mmol). After stirring the chilled solution for
1 h and warming to room temperature it was stirred over night.
Solvent and volatile components were removed in vacuo, the
residue stirred with 30 mL of dichloromethane and filtered. The
filtrate was liberated from volatiles and the solid ◦residue was
purified by short-path distillation at 10-6 bar and 300 C. The light
yellow solid distillate was crystallized from methylcyclohexane–
dichloromethane to afford product 3 as colorless yellow rods
(yield: 0.38 g, 48%).
2[3¢-Cyano-4¢[4¢¢-methoxyphenylethynyl]phenyl]-1,3-diethyl-1,3,2-
benzodiazaborole (5)
The solution of 5-bromo-2-{[4-(methoxy)phenyl]ethynyl}-
benzonitrile (2.0 g, 6.4 mmol) in THF (50 mL) was cooled
to -110 ◦C, and was then slowly combined with 6.7 mmol
n-butyllitium (4.2 mL, 1.6 M in n-hexane). Stirring at -110 ◦C was
continued for 1 h before a THF solution (5 mL) of 1 (1.87 g, 6.4
mmol) was added dropwise. The resulting mixture was stirred for
5 h at -110 ◦C and then warmed up to room temperature. During
warm-up the solvent was removed in vacuo. The green–brown
residue was suspended in 40 mL of dichloromethane and filtered.
The filtrate was stored for 12 h at -20 ◦C, whereby light brown
crystals separated. The supernatant solution was decanted and
the crystals were washed with 2 mL of cold dichloromethane.
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 4434–4446 | 4443
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Recrystallization from n-hexane–dichloromethane yielded 0.89 g
(38%) of 5 as colorless crystals.
in n-hexane (1.75 mL, 2.8 mmol) and after 20 min at the same
temperature a THF solution (5 mL) of 0.70 g (2.77 mmol) 1 was
added. After an analogous workup (16 h, extraction of the residue
with n-C6H14) a microcrystalline light red solid was separated.
Recrystallization from CHCl3 furnished 0.81 g (69%) of colorless
crystalline 7.
Found C 76.78, H 5.92, N 10.45%; C26H24BN3O requires C
1
77.05, H 5.97, N 10.37%; H-NMR (C6D6): d = 0.96 (t, JHH
=
7.2 Hz, 6H, CH2CH3), 3.17 (s, 3H, OCH3), 3.34 (q, JHH = 7.2 Hz,
4H, CH2CH3), 6.58 (d, JHH = 8.8 Hz, 2H, CH3O–C CH), 6.97 (m,
2H, CH CH–CH CH), 7.15 (m, 2H, CH CH–CH CH),
Found C 73.58, H 5.70, N 9.80, S 7.32%;C26H24BN3S requires
1
7.25 (dd, JHH = 7.7, 1.3 Hz, 1H, H-benzonitrile), 7.38 (d, JHH
=
C 74.11, H 5.74, N 9.97, S 7.61%; H-NMR (C6D6): d = 0.97 (t,
7.7 Hz, 1H, H-benzonitrile), 7.49 (d, JHH = 1.3 Hz, H-benzonitrile),
JHH = 7.2 Hz, 6H, CH2CH3), 1.85 (s, 3H, SCH3), 3.35 (q, JHH =
1
7.61 (d, JHH = 8.8 Hz, 2H, CH3O–C CH–CH); 13C{ H}-NMR
7.2 Hz, 4H, CH2CH3), 6.86 (d, JHH = 8.5 Hz, 2H, CH3S–C CH–
CH), 6.98 (m, 2H, CH CH–CH CH), 7.15 (m, 2H, CH CH–
CH CH), 7.25 (dd, JHH = 7.8, 1.3 Hz, 1H, H-benzonitrile), 7.37
(d, JHH = 7.8 Hz, 1H, H-benzonitrile), 7.48 (d, JHH = 1.3 Hz, H-
benzonitrile), 7.49 (d, JHH = 8.5 Hz, 2H, CH3S–C CH–CH);
(C6D6): d = 16.0 (s, CH2CH3), 37.4 (s, CH2CH3), 54.6 (s, OCH3),
85.9, 97.6 (2 s, C C), 109.4 (s, CH CH–CH CH), 114.3 (s, C–
Ph), 114.4 (s, H3CO–C CH), 115.6 (s, C–CN), 117.9 (s, C–CN),
119.5 (s, CH CH–CH CH), 127.6 (s, C-benzonitrile), 131.0 (s,
CH-benzonitrile), 133.8 (s, H3CO–C CH–CH), 136.7 (s, CH-
benzonitrile), 137.0 (s, C2N2), 137.3 (s, CH-benzonitrile), 160.7
1
13C{ H}-NMR (C6D6) d = 14.3 (s, SCH3), 16.0 (s, CH2CH3),
37.5 (s, CH2CH3), 86.5, 97.6 (2 s, C C), 109.4 (s, CH CH–
CH CH), 115.7 (s, C–CN), 117.9 (s, C–CN), 118.1 (s, C–Ph),
119.5 (s, CH CH–CH CH), 125.7 (s, H3CS–C CH), 127.3
(s, C-benzonitrile), 131.0 (s, CH-benzonitrile), 132.4 (s, H3CS–
1
(s, C–OCH3); 11B{ H}-NMR (C6D6): d = 27.4 (s); MS–EI (m/z):
405.2 [M+].
C
CH–CH), 136.7 (s, CH-benzonitrile), 137.0 (s, C2N2), 137.3
2[3¢-Cyano-4¢[4¢¢-(dimethylamino)phenylethynyl]phenyl]-1,3-
diethyl-1,3,2-benzodiazaborole (6)
1
(s, CH-benzonitrile), 141.5 (s, C–SCH3); 11B{ H}-NMR (C6D6):
d = 27.5 (s); MS–EI (m/z): 421.1 [M+].
The
solution
of
5-bromo-2-{[4-(dimethylamino)phenyl]-
2[3¢-Cyano-4¢(4¢¢-cyanophenylethynyl)phenyl]-1,3-diethyl-1,3,2-
benzodiazaborole (8)
ethynyl}benzonitrile (0.88 g, 7.71 mmol) in THF (40 mL)
was chilled to -110 ◦C before 1.7 mL (2.72 mmol) of a 1.6 M
solution of n-butyllithium in n-hexane were added dropwise. The
mixture was stirred at -110 ◦C for 30 min. Then a solution of
2-bromo-1,3,2-benzodiazaborole (1) (1.87 g, 6.4 mmol) in 5 mL
of THF was added dropwise. The solution was allowed to reach
room temperature within 2 h, and then stirred overnight. Solvent
and volatile component were completely removed in vacuo. The
resulting residue was extracted with n-hexane during a period of
4 d. A green microcrystalline solid separated from the extract.
It was filtered and the filter-cake was dried in vacuo and then
recrystallized from THF to give product 6 as light yellow crystals
(yield: 0.80 g, 71%).
Analogously a chilled solution (-110 ◦C, 40 mL) of 5-bromo-2-[(4-
cyanophenyl)ethynyl]benzonitrile (0.88 g, 2.87 mmol) was treated
with a 1.6 M n-butyllithium solution in n-hexane (1.8 mL, 2.88
mmol) before an equimolar amount of 1 (0.70 g, 2.77 mmol) in
5 mL was added. Solvent was removed as described and the residue
extracted with n-hexane for 3 d, to yield a microcrystalline light
green precipitate. This solid was separated and recrystallized from
THF to afford 0.89 g (77% yield) of colorless crystalline 8.
Found C 77.90, H 5.27, N 13.71%; C26H21BN4 requires C 78.01,
H 5.29, N, 14.00%; 1H-NMR (CDCl3): d = 1.32 (t, JHH = 7.2 Hz,
6H, CH2CH3), 3.76 (q, JHH = 7.2 Hz, 4H, CH2CH3), 7.09 (m,
2H, CH CH–CH CH), 7.15 (m, 2H, CH CH–CH CH),
Found C 77.30, H 6.37, N 13.43%; C27H27BN4 requires C 77.52,
H 6.51, N 13.39%; 1H-NMR (CDCl3): d = 1.34 (t, JHH = 7.2 Hz,
6H, CH2CH3), 3.02 (s, 6H, N(CH3)), 3.79 (q, JHH = 7.2 Hz,
4H, CH2CH3), 6.86 (d, JHH = 8.8 Hz, 2H, (CH3)2N–C CH–
CH), 7.09 (m, 2H, CH CH–CH CH), 7.16 (m, 2H, CH CH–
CH CH), 7.54 (d, JHH = 8.8 Hz, 2H, (CH3)2N–C CH–CH),
7.66 (d, JHH = 7.9 Hz, 1H, H-benzonitrile), 7.72 (dd, JHH = 7.9,
1.3 Hz, 1H, H-benzonitrile), 7.84 (d, JHH = 1.3 Hz, H-benzonitrile);
7.66 (d, JHH = 8.2 Hz, 2H, CN–C CH–CH), 7.71 (d, JHH
=
8.2 Hz, 2H, CN–C CH–CH), 7.73 (d, JHH = 7.9 Hz, 1H, H-
benzonitrile), 7.79 (dd, JHH = 7.9, 1.2 Hz, 1H, H-benzonitrile),
1
7.89 (d, JHH = 1.2 Hz, H-benzonitrile); 13C{ H}-NMR (CDCl3)
d = 16.4 (s, CH2CH3), 37.8 (s, CH2CH3), 89.7, 94.3 (2 s, C C),
109.4 (s, CH CH–CH CH), 112.6 (s, CN–C CH–CH), 115.5
(s, C–CN), 117.7 (s, CN–C CH–CH), 118.4 (s, C–CN), 119.4 (s,
CH CH–CH CH), 126.0 (s, C–Ph), 126.9 (s, C-benzonitrile),
131.7 (s, CH-benzonitrile), 132.2 (s, CN–C CH), 132.5 (s, CN–
1
13C{ H}-NMR (CDCl3) d = 15.3 (s, CH2CH3), 36.6 (s, CH2CH3),
39.1 (s, N(CH3)2), 83.6 (s, C-acetylene), 97.8, 107.4 (s, C C),
107.4 (s, C–Ph), 108.1 (s, CH CH–CH CH), 110.6 (s, (H3C)2N–
C
CH–CH), 136.8 (s, C2N2), 137.3 (s, CH-benzonitrile), 137.4
C
CH), 113.3 (s, C–CN), 117.2 (s, C–CN), 118.1 (s, CH CH–
1
(s, CH-benzonitrile); 11B{ H}-NMR (CDCl3): d = 27.4 (s); MS–
CH CH), 127.2 (s, C-benzonitrile), 129.9 (s, CH-benzonitrile),
132.3 (s, (H3C)2N–C CH–CH), 135.8 (s, C2N2), 136.0 (s, CH-
benzonitrile), 136.2 (s, CH-benzonitrile), 149.7 (s, C–N(CH3)2);
EI (m/z): 400.2 [M+].
X-ray Crystallography. Crystallographic data were collected
with a Bruker Nonius Kappa CCD diffractometer with Mo-Ka
1
11B{ H}-NMR (CDCl3): d = 27.6 (s); MS–EI (m/z): 418.3 [M+].
˚
(graphite monochromator, l = 0.71073 A) at 100 K. Crystallo-
graphic programmes used for structure solution and refinement
were from SHELX-97.19 The structures were solved by direct
methods and were refined by using full-matrix least squares on F2
of all unique reflections with anisotropic thermal parameters for all
non-hydrogen atoms. Hydrogen atoms were included at calculated
positions with U(H) = 1.2 Ueq for CH2 groups and U(H) = 1.5 Ueq
2[3¢-Cyano-4¢[4¢¢-(methylthio)phenylethynyl]phenyl]-1,3-diethyl-
1,3,2-benzodiazaborole (7)
Analogously the chilled solution (-110 ◦C) of 5-bromo-2-{[4-
(methylthio)phenyl]ethynyl}benzonitrile (0.91 g, 2.77 mmol) in
THF (25 mL) was combined with a 1.6 M n-butyllithium solution
4444 | Dalton Trans., 2011, 40, 4434–4446
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for CH3 groups, except compound 7, where hydrogens were refined
isotropically. Supplementary crystallographic data for this paper
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.†
Howard, D. P. Lydon, P. J. Low, I. J. S. Fairlamb and T. B. Marder,
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Computational methods
All calculations were performed using the Gaussian 0920 program
package with the 6-311G(d,p) basis set (6-311+G(d,p) for po-
larisability). DFT has been shown to predict various molecular
properties successfully.21 All geometry optimizations were carried
out with the B3LYP22 functional and were followed by frequency
calculations in order to verify that the stationary points obtained
were true energy minima. Ionization energies (IE) were calculated
using the CAM-B3LYP22d functional (which is particularly well
suited for the Ionization Energies evaluation—see for example
ref. 14c) with DSCF/TD–DFT, which means that separate SCF
calculations were performed to optimize the orbitals of the ground
state and the appropriate ionic state (IE = Ecation - Eneutral). The
advantages of the most frequently employed DSCF/TD–DFT
method of calculations of the first ionization energies have been
demonstrated previously.23 The TD–DFT24 approach provides a
first principal method for the calculation of excitation energies
within a density functional context taking into account the low-
lying ion calculated by the DSCF method.
Acknowledgements
This work was financially supported by the French Ministry of
Research and High School Education Grant for M. M. and the
Deutsche Forschungsgemeinschaft, Bonn, Germany.
4 J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley, J. Am.
Chem. Soc., 2005, 127, 2464.
5 Y. Yamaguchi, T. Tanaka, S. Kobayashi, T. Wakayima, Y. Matsubara
and Z.-i. Yoshida, J. Am. Chem. Soc., 2005, 127, 9332.
6 Y. Yamaguchi, T. Ochi, T. Wakayima, Y. Matsubara and Z.-i. Yoshida,
Org. Lett., 2006, 8, 713.
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