UV-photoelectron spectroscopic studies on 2-arylethynyl-1,3,2- benzodiazaboroles

Article abstract of DOI:10.1021/om100352c

Gas-phase He I photoelectron spectra of a series of 2-arylethynyl-1,3,2- benzodiazaboroles BEP-X [1 (X = H), 2 (X = Me), 3 (X = OMe), 4 (X = SMe), 5 (X = NMe₂), 6 (X = Br)] have been recorded and assessed by density functional theory calculations. The first ionization energies of these benzodiazaboroles are in the order 5 (7.0 eV) < 3 (7.15 eV) < 2 (7.2 eV) < 1 = 4 = 6 (7.3 eV). As an important result of this study it was disclosed that in contrast to the remaining diazaboroles the frontier orbitals of the amino derivative 5 lack contributions from the diazaborole part of the molecule. In keeping with this, the dipole moment of the ground state (5.51 D) is only slightly changed, to 6.60 D, upon electronic excitation. This also rationalizes the low Stokes shift of 2600 cm^-1 in comparison to the other diazaboroles in this study (5900-7300 cm^-1).

Full text of DOI:10.1021/om100352c

5192 Organometallics 2010, 29, 5192–5198
DOI: 10.1021/om100352c
UV-Photoelectron Spectroscopic Studies on 2-Arylethynyl-1,3,2-
benzodiazaboroles†
,‡
‡
‡
‡
ꢀ
Anna Chrostowska,* Mazgorzata Maciejczyk, Alain Dargelos, Patrick Baylere,
Lothar Weber,*,§ Vanessa Werner,§ Daniel Eickhoff,§ Hans-Georg Stammler,§ and
Beate Neumann§
‡
0
2 Avenue du President Angot, BP 1155, 64013 Pau Cedex, France, and Fakultat fur Chemie der Universitat
ꢁ
Equipe Chimie Physique, IPREM, UMR 5254, Universite de Pau et des Pays de l Adour,
ꢁ
§
ꢁ
Bielefeld, D-33615 Bielefeld, Germany
€
€
€
Received April 27, 2010
Gas-phase He I photoelectron spectra of a series of 2-arylethynyl-1,3,2-benzodiazaboroles BEP-X [1
(X=H), 2(X=Me), 3(X=OMe), 4(X=SMe), 5(X=NMe2), 6(X=Br)] have been recorded and assessed
by density functional theory calculations. The first ionization energies of these benzodiazaboroles are in the
order 5 (7.0 eV)<3 (7.15 eV)<2 (7.2 eV)<1=4=6 (7.3 eV). As an important result of this study it was
disclosed that in contrast to the remaining diazaboroles the frontier orbitals of the amino derivative 5 lack
contributions from the diazaborole part of the molecule. In keeping with this, the dipole moment of the
ground state (5.51 D) is only slightly changed, to 6.60 D, upon electronic excitation. This also rationalizes the
low Stokes shift of 2600 cm-1 in comparison to the other diazaboroles in this study (5900-7300 cm-1).
Introduction
quantum yields up to 0.99.3,4 In a more recent paper new
2-arylalkynyl-1,3,2-benzodiazaboroles 2-(40-X C6H4CtC)-1,3-
Et2-1,3,2-N2BC6H4 (X = H, Me, MeO, MeS, Me2N) have been
prepared and characterized by X-ray diffraction. UV-vis and
luminescence spectroscopic studies on these diazaboroles reveal
intense blue/violet fluorescence with very large quantum yields
of 0.89-0.99. The experimental findings have been complemen-
ted by DFT and TD-DFT calculations.5
For the 4-dimethylamino compound an unexpected small
Stokes shift of only 2600 cm-1 has been observed. This is in
stark contrast with Stokes shifts of 5900-7300 cm-1 for the
remaining derivatives and rationalized by different electronic
structures. In the aminophenyl derivative the HOMO and
LUMO are located on the aryl part, whereas in the four other
compounds the HOMOs are mainly represented by the diaza-
borolyl group.5 To shed further light onto this situation, we have
decided to also study the above-mentioned 2-arylalkynyl-1,3,2-
benzodiazaboroles by UV-photoelectron spectroscopy in the
gas phase. This technique previously has proved valuable in the
elucidation of the electronic structures of monocyclic 1,3,2-dia-
zaboroles 2-(R)-1,3-t-Bu2-1,3,2-BN2C2H2 with R = H, Me,
CN, NH2, NMe2, OMe, SMe, and Br.6
The chemistry of 1,3,2-diazabenzoboroles has experienced a
vivid development within recent years.1,2 Investigations on this
class of heterocycles are not only of academic interest but also of
relevance to possible applications in electro-optical devices. In
this context we have carried out studies on the synthesis and
optical properties of extended π-conjugated systems such as
benzene, biphenyl, fluorene, thiophene, and dithiophene with
1,3,2-diazaborolyl and 1,3,2-benzodiazaborolyl substituents.
They all exhibit an intense blue luminescence upon UV-irradia-
tion with Stokes shifts in the range 6000-10 000 cm-1 and
† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar
Seyferth in honor of his outstanding contributions to our science and for his
excellent work as founding editor of Organometallics.
*To whom correspondence should be addressed. E-mail: anna.
chrostowska@univ-pau.fr; lothar.weber@uni-bielefeld.de.
(1) For reviews on 1,3,2-diazaboroles see: (a) Weber, L. Coord. Chem.
Rev. 2001, 215, 39. (b) Weber, L. Coord. Chem. Rev. 2008, 252, 1.
(c) Yamashita, M.; Nozaki, K. Pure Appl. Chem. 2008, 80, 1187. (d) Yamashita,
M; Nozaki, K Bull. Chem. Soc. Jpn. 2008, 81, 1377.
(2) For recent chemistry of 1,3,2-benzodiazaboroles see: (a) Weber,
L.; Domke, I.; Kahlert, J.; Stammler, H. G. Eur. J. Inorg. Chem. 2006,
3419. (b) Weber, L.; Schnieder, M.; Maciel, T. C.; Wartig, H.-B.; Schimmel,
€
M.; Boese, R.; Blaser, D. Organometallics 2000, 19, 5791. (c) Marder, T. B.
Results and Discussion
Science 2006, 314, 69. (d) Segawa, Y.; Yamashita, M.; Nozaki, K. Science
2006, 314, 113. (e) Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem.
Int. Engl. 2007, 46, 6710. (f) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki,
K. J. Am. Chem. Soc. 2007, 129, 9570. (g) Segawa, Y.; Suzuki, Y.;
Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2008, 130, 16069.
(h) Segawa, Y.; Yamashita, M.; Nozaki, K. Organometallics 2009, 28,
6234. (i) Yamashita, M.; Suzuki, Y.; Segawa, Y.; Nozaki, K. Chem. Lett.
2008, 37, 802. (j) Terabayashi, T.; Kajiwara, T.; Yamashita, M.; Nozaki, K.
J. Am. Chem. Soc. 2009, 131, 14162. (k) Segawa, Y.; Yamashita, M.;
Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201. (l) Murphy, J. M.; Lawrence,
K; Kawamura, J. D.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2006,
128, 13688. (m) Weber, L.; Wartig, H.-B.; Stammler, H.-G.; Neumann, B.
Organometallics 2001, 20, 5248. (n) Sugimone, M.; Yamamoto, A.; Murakami,
M. Angew. Chem., Int. Ed. 2005, 44, 2380.
The 2-arylethynyl-1,3,2-benzodiazaboroles 1-5 employed
in this study were conveniently obtained from 2-bromo-1,3-di-
ethyl-1,3,2-benzodiazaborole and the in situ generated lithium
(3) Weber, L.; Werner, V.; Domke, I.; Stammler, H.-G.; Neumann,
B. Dalton Trans. 2006, 3777.
(4) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.;
Brockhinke, A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009, 1339.
(5) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.;
Brockhinke, A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009, 2823.
r
pubs.acs.org/Organometallics
Published on Web 08/17/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5193
Figure 1. Molecular structure of 6 in the crystal. Selected bond
ꢁ
˚
lengths [A] and angles [deg]: B(1)-N(1) 1.426(3), B(1)-N(2)
1.424(3), N(1)-C(1) 1.400(3), N(2)-C(2) 1.394(2), C(1)-C(2)
1.406(3), B(1)-C(11) 1.522(3), C(11)-C(12) 1.209(3), C(12)-
C(13) 1.439(3), C(13)-C(14) 1.396(3), C(14)-C(15) 1.380(3),
C(15)-C(16) 1.376(3), C(16)-C(17) 1.384(3), C(17)-C(18)
1.384(3), C(13)-C(18) 1.395(3), C(16)-Br(1) 1.901(2), N(1)-B-
(1)-N(2) 106.9(2), B(1)-N(1)-C(1) 108.0(2), B(1)-N(2)-C(2)
108.3(2), N(1)-C(1)-C(2) 108.4(2), N(2)-C(2)-C(1) 108.5(2),
N(1)-B(1)-C(11) 128.6(2), N(2)-B(1)-C(11) 124.3(2), B(1)-
C(11)-C(12) 171.7(2), C(11)-C(12)-C(13) 179.5(2).
derivatives of the appropriate arylalkynes as previously
described.5,7
The 40-bromophenylethynyl derivative 6 was synthesized
analogously and obtained as colorless crystals in 82% yield.
In the 11B{1H} NMR spectrum of 6 a singlet is observed at
δ = 21.0 ppm, in agreement with respective 11B{1H} NMR
shifts of 1-5 (δ = 20.5-21.2 ppm).
X-ray Structural Analysis of 6. Single crystals of com-
pound 6 suitable for an X-ray structural study (Table 4)
were grown from an n-pentane solution at -35 °C.
The molecule (Figure 1) is constructed from a benzodia-
zaborole ring that is linked to the arylalkynyl unit by a
Figure 2. UV-photoelectron spectra of BEP-X with X = H (1),
Me (2), and Br (6).
ꢁ
˚
B(1)-C(11) single bond of 1.522(3) A. The length of the triple
ꢁ
˚
bond C(11)-C(12) is 1.209(3) A. Valence angles at the nearly
linear bridge, B(1)-C(11)-C(12) and C(11)-C(12)-C(13),
between the two rings are 171.7(2)° and 179.5(2)°. In the crystal,
the planes of the heterocycle and the arene ring are tilted by
56.1°. Thus, at least in the solid state, no significant π-commu-
nication between both ring systems is present. All other bonding
parameters within the heterocycles are similar to those observed
inalargeseriesofbenzodiazaborolesandarenotdiscussedhere.
UV-Photoelectron Spectra and Theoretical Studies. The
UV-photoelectron spectra of the 2-arylethynyl-1,3-diethyl-
1,3,2-benzodiazaboroles 1, 2, 6 and 3, 4, 5 have been re-
corded, and their spectra are presented in Figures 2 and 3.
The PE spectrum of 1 (X = H) displays three first bands at
7.3, 7.9, and 8.7 eV. The next intense band with a left-side
shoulder at 9.35 eV is located at 9.7 eV. For the methyl
derivative 2 first ionizations at 7.2, 7.8, 8.35, 9.2, and 9.55 eV
are clearly distinguished. The spectrum of 6 (X = Br) is well re-
solved, and the first three PE bands are located at 7.3, 7.9, and
8.55 eV. They are followed by three sharp bands at 9.7, 10.1, and
10.6 eV. For 3, 4, and 5 the lowest energy PE bands are located
at 7.15 eV for 3 (X = OMe), at 7.3 eV for 4 (X = SMe), and at
7.0 eV for amino derivative 5. In the spectrum of 3 a second,
more intense band with a shoulder at 8.05 eV is observed at
7.9 eV. The spectrum of 4 exhibits a second ionization at 7.9 eV
with shoulders at 8.2 and 8.3 eV. The amino compound 5 has a
second, very intense ionization at 7.5 eV, which also contains a
right-side shoulder at 7.65 eV. More energetic and intense bands
are clearly identified at 9.3, 9.5, and 9.85 eV for 3, at 9.3 and
9.6 eV for 4, and at 9.0, 9.3, and 9.4 eV for 5.
For a correct assignment of these different PE bands, density
functional theory calculations have been carried out on com-
pounds 1-6. Table 1 lists selected geometrical parameters for
these benzodiazaboroles and shows that the different substitu-
ents X in the para-position of the aryl ring have little influence
on the bond lengths. Comparisons between the experimental
X-ray geometries of 1, 2, 3, 5, and 6 and the calculated
(6) Weber, L.; Domke, I.; Greschner, W.; Miqueu, K.; Chrostowska,
ꢀ
A.; Baylere, P. Organometallics 2005, 24, 5455.
(7) Weber, L.; Penner, A.; Domke, I.; Stammler, H.-G.; Neumann, B.
Z. Anorg. Allg. Chem. 2007, 633, 563.

5194 Organometallics, Vol. 29, No. 21, 2010
Chrostowska et al.
geometrical parameters are in good agreement, which is parti-
cularly evident by the CN(amino) multiple bonding in 5 [1.380
ꢁ
˚
vs 1.381(1) A]. As an exception to this, the experimental values
for the bonds B-N and C(11)-C(12) are shorter than the
calculated ones. For bonds B(1)-C(11) and C(12)-C(13),
however, the opposite is true. These discrepancies between
the experimental and theoretical data indicate that the DFT
calculations might have overestimated the extent of electron
delocalization.
The experimentally determined angles N(1)-B(1)-C(11)
and N(2)-B(1)-C(12) in 1 [127.3(1)°; 125.7(1)°] and in 3
[128.4(1)°; 124.8(1)°] also deviate from the calculated ones.
According to our calculations, both angles should be equal. Cal-
culations suggest linearity about the atoms B(1), C(11), C(12),
and C(13). Experimentally, this is only realized in 5 [B(1)-
C(11)-C(12) 179.4(1)°; C(11)-C(12)-C(13) 178.3(1)°]. In
compounds 1-3 and 6 angles B(1)-C(11)-C(12) range from
171.7(2)° to 175.6(1)°, whereas deviations from linearity are
less pronounced for angles C(11)-C(12)-C(13) [176.1(2)-
179.5(2)°]. Due to the calculations, both rings are orientated
in the same plane. However, the rotational barrier of 1 does not
exceed 0.33 kcal/mol in the case of orthogonality. In the crystal,
the planes of the heterocycle and arene ring enclose dihedral
angles of 65.8° (1), 70.4° (2), 79° (3), 76.4° (4), and 56.1° (6).
The nature of each of the first six MOs (Table 2) does not
change considerably for X = Me and Br in comparison with
X = H. Thus, the HOMO corresponds to the antibonding in-
teraction between the benzodiazaborole ring and the ethynyl
system π3-πNBN-πCtCπ. The first IEs calculated with cam-
B3LYP/6-311G(d,p) (italics) (7.17 (1); 7.10 (2); 7.25 (6)) are in
good agreement with the experimental values [7.3 (1), 7.3(6), 7.2
(2)]. The second MO is associated with the antibonding inter-
action between the π CdC bonds and the nitrogen π lone pairs
(πCdC--nNπ) and is calculated to be 7.83 eV for 1, at 7.78 eV
for 2, and at 7.90 eV for 6. For compounds 1 and 6 the band
at 7.9 eV and for 2 that at 7.8 eV are attributed to the ejection
of an electron from this MO. The third IEs (HOMO-2) des-
cribe the antibonding interaction between the CtC unit (in
the π plane) and the directly linked X-substituted phenyl ring
(πCꢀCπ-π2-nXπ(X)) and are calculated to be 8.65 eV for 1,
8.42 eV for 2, and 8.61 eV for 6, in excellent agreement with the
experimental IEs [8.7 eV for 1, 8.35 eV for 2, and 8.55 eV for 6].
The two next, higher energy PE bands are attributed for three
Figure 3. UV-photoelectron spectra of BEP-X with X = OMe
(3), SMe (4), and NMe2 (5).
˚
Table 1. Selected Bond Lengths [A] and Angles [deg] Calculated [B3LYP/6-311G(d,p)] for Compounds BEP-X with X = H (1), Me (2),
OMe (3), SMe (4), NMe2 (5), and Br (6) (experimental values in brackets)
X (compound)
H (1)
Me (2)
OMe (3)
SMe (4)
NMe2 (5)
Br (6)
B(1)-N(2)
1.439 [1.429(1)]
1.439 [1.428(1)]
1.394 [1.394(1)]
1.394 [1.394(1)]
1.417 [1.411(1)]
1.515 [1.524(2)]
1.215 [1.208(1)]
1.424 [1.439(1)]
1.084 -
108.2 [108.2(1)]
108.2 [108.1(1)]
106.6 [106.8(1)]
126.7 [127.3(1)]
126.7 [125.7(1)]
179.9 [175.6(1)]
179.9 [178.8(1)]
-0.2
1.439 [1.430(1)]
1.439 [1.431(1)]
1.394 [1.392(1)]
1.394 [1.398(1)]
1.417 [1.413(1)]
1.515 [1.530(1)]
1.215 [1.207(1)]
1.423 [1.438(1)]
1.509 [1.509(1)]
108.1 [108.0(1)]
108.1 [107.7(1)]
106.6 [107.1(1)]
126.7 [128.7(1)]
126.7 [124.2(1)]
180.0 [174.0(1)]
179.9 [178.0(1)]
0.8
1.439 [1.433(1)]
1.439 [1.428(1)]
1.394 [1.396(1)]
1.394 [1.396(1)]
1.417 [1.413(1)]
1.514 [1.524(1)]
1.216 [1.207(1)]
1.422 [1.440(2)]
1.360 [1.361(3)]
108.1 [108.0(1)]
108.1 [108.3(1)]
106.5 [106.8(1)]
126.7 [128.4(1)]
126.8 [124.8(1)]
179.8 [173.8(2)]
179.9 [176.1(1)]
0.5
1.438
1.438
1.394
1.394
1.417
1.515
1.215
1.423
1.797
108.0
108.0
106.7
126.7
126.7
179.9
179.8
0.0
1.441 [1.431(1)]
1.441 [1.429(1)]
1.394 [1.395(1)]
1.394 [1.395(1)]
1.418 [1.411(1)]
1.512 [1.525(2)]
1.217 [1.209(1)]
1.420 [1.435(2)]
1.381 [1.380(1)]
108.2 [108.1(1)]
108.2 [108.3(1)]
106.4 [106.7(1)]
126.8 [127.0(1)]
126.8 [126.3(1)]
179.6 [179.4(1)]
179.8 [178.3(1)]
0.0
1.438 [1.424(3)]
1.438 [1.426(3)]
1.394 [1.394(2)]
1.394 [1.400(3)]
1.418 [1.406(3)]
1.516 [1.522(3)]
1.216 [1.209(3)]
1.423 [1.439(3)]
1.915 [1.901(2)]
108.0 [108.3(2)]
108.0 [108.0(2)]
106.7 [106.9(2)]
126.7 [128.6(2)]
126.7 [124.3(2)]
180.0 [171.7(2)]
179.9 [179.5(1)]
0.3
B(1)-N(1)
C(2)-N(2)
C(1)-N(1)
C(1)-C(2)
B(1)-C(11)
C(11)-C(12)
C(12)-C(13)
C(16)-X
C(2)-N(2)-B(1)
C(1)-N(1)-B(1)
N(1)-B(1)-N(2)
N(1)-B(1)-C(11)
N(2)-B(1)-C(11)
B(1)-C(11)-C(14)
C(11)-C(12)-C(13)
N(2)-B(1)-C(11)-C(14)

Article
Organometallics, Vol. 29, No. 21, 2010 5195
Table 2. Experimental (bold) and Calculated [cam-B3LYP/6-311G(d,p) (italics)] TD-DFT Ionization Energies (eV) and MOLEKEL MO
Visualization for BEP-X with X = H (1), Me (2), and Br (6)
compounds to the ionization of an X-substituted phenyl ring
[π3, HOMO-3: exptl 9.35 eV (calcd 9.35 eV) for 1; exptl 9.2 eV
(calcd 9.26 eV) for 2; exptl 9.7 (calcd 9.67 eV) for 6] and to the
triple-bond ionization [πCtCσ, HOMO-4, exptl 9.7 eV (calcd
9.64 eV) for 1; exptl 9.55 eV (calcd 9.55 eV) for 2, and exptl 9.7
eV (calcd 10.16 eV) for 6]. As for the first series, the attribution
of the main PE bands of 3, 4, and 5 is corroborated by the TD-
DFT calculated IE values, which are summarized in Table 3.
The HOMO is the same for the three compounds 3, 4, and 5 and
corresponds, as for the first group (1, 2, 6), to the antibonding
interaction between the benzodiazaborole ring and the ethynyl
system (π3-πNBN-πCꢀCπ). However, it should be remarked
the significant influence of the NMe2 substituent in 5 on the
decrease of the boron contribution in this MO.
corresponding to the antibonding interaction between the CtC
unit (in the π plane) and directly linked X-substituted phenyl
ring is attributed to the experimental value of 8.05 eV for 3
(HOMO-2, π3-πNBNþπCtCπ-π2-nOπ, calcd 8.11 eV), 8.3
eV for 5 (HOMO-3, πCtCπ-π2-nNπ(X), calcd 9.06 eV). The
ionization of the phenyl ring moiety (π3) corresponds to the
band at 9.3 eV in the spectrum of 3 (HOMO-3, 9.36 eV), of 4
(HOMO-4, 9.45 eV), and of 5 (HOMO-4, calcd 9.19 eV). The
triple-bond ionization (πCtCσ) is calculated as HOMO-4 for 3
(exptl 9.5 eV; calcd 9.50 eV) but as HOMO-5 for 4 (exptl 9.6
eV; calcd 9.71 eV) and 5 (exptl 9.4 eV; calcd 9.64 eV). The
ejection of an electron from the sulfur lone pair in 4is observed at
8.2 eV (HOMO-2, calcd 8.57 eV).
π
eV for 4 (HOMO-3, πCtCπ-π2-nS , calcd 8.66 eV), and 9.0
The lowest IEs are calculated for 5 (X = NMe2, 6.71 eV). For
derivatives 3 (X = OMe, 7.02 eV) and 4 (X = SMe, 7.22 eV)
slightly higher values are obtained. Thus, the removal of an
electron from this MO corresponds to the experimental value of
7.15 eV for 3, 7.3eVfor4, and 7.0 eV for 5. For the following IEs
the natures of the MO do not follow the same sequence. Thus,
the MO description of the antibonding interaction between the π
CdC bonds and the nitrogen π lone pairs (πCdC--nNπ) corres-
ponds to the HOMO-1 of 3 (calcd 7.75; exptl 7.9 eV) and of 4
(calcd 7.88; exptl 7.9 eV), but in the case of 5 to the HOMO-2
(calcd 7.65; exptl 7.65 eV). In the latter case, the HOMO-1 is
mainly localized on π3-πNBN-π2-nNπ(X) and appears at 7.5
eV (calcd 7.42 eV). The ejection of an electron from an MO
Figure 4 gives a summary of the UV-PES data of the six
differently substituted 2-arylethynyl-1,3-diethyl-1,3,2-benzodia-
zaboroles 1-6. Thus, the required energy to remove an electron
from the orbital localized on the diazabenzoborole ring and the
ethynyl system (π3-πNBN-πCtCπ) increases in the range 5
(7.0 eV)<3 (7.15 eV)<2 (7.2 eV)<1=4=6 (7.3 eV). The
ionization of the (πCdC--nN-π) MO is very slightly affected by
the presence of the X-substituent and corresponds to 7.9 eV IE
for 1, 6, 3, and 4 and to the 7.8 eV for 2. In the case of 5 the
energy of this MO ((πCdC--nN-π, now HOMO-2) is lowered
(7.65 eV). Also, due to the antibonding (destabilizing) interac-
tion of the nitrogen (X) lone pair with the phenyl ring (π3-
πNBN-π2-nNπ(X)) corresponding to the orbital HOMO-1,

5196 Organometallics, Vol. 29, No. 21, 2010
Chrostowska et al.
Table 3. Experimental (bold) and Calculated [cam-B3LYP/6-311G(d,p) (italics)] TD-DFT Ionization Energies (eV), and MOLEKEL MO
Visualization for BEP-X with X = OMe (3), SMe (4), and NMe2 (5)
Table 4. Crystallographic Data and Structure Refinement for 6
ethynyl bond in the π plane, and the antibonding interaction
between the phenyl ring and the X-atom lone pair. Thus, in
comparison with the unsubstituted representative 1 (8.7 eV), the
mesomeric electron-donating methoxy group strongly destabi-
lizes this MO (8.05 eV); a less important destabilization is
effected by the thiomethyl group in 4 (8.3 eV) and the bromine
substitutent in 6 (8.55 eV), while the methyl group acts as an
inductive electron donor (8.35 eV). The removal of an electron
from the phenyl ring (π3) is only very little influenced by the
presence of X and corresponds to the IE at 9.3 eV (experimental
value) for molecules 3, 4, and 5. For 1 this ionization is observed
at 9.35 eV and, thus, is only slightly destabilized by the methyl
group (9.2 eV). In the case of Br in 6 a stabilization of 0.25 eV
(9.7 eV) is observed. Finally, the ionization of the triple bond
in the sigma plane (πCtCσ) appears at 9.4-9.7 eV as one of
the most intense bands in the PE spectra. As presumed above,
the slightly lower calculated ionization energies might be due
to the overestimation of electron delocalization by the DFT
method.
The PE spectroscopic results compliment our previous
photophysical findings on compounds 1-6, where for the
amino derivative 5 a comparatively low Stokes shift points to
a small charge transfer upon irradiation.5 This observation
was rationalized by the fact that the HOMO and LUMO of
this species are mainly localized on the X-substituted aryl
part of the molecule. In contrast to this, in the remain-
ing diazaboroles 1-4 and 6, the HOMO is characterized
by the diazaborole moiety, and LUMO by the aryl group
empirical formula
M [g mol-1
C18H18BBrN2
353.06
]
cryst dimens (mm)
cryst syst
space group
0.21 ꢁ 0.14 ꢁ 0.08
orthorhombic
Pccn
15.8927(4)
26.1619(5)
8.0594(2)
3550.96(13)
8
˚
a [A]
˚
b [A]
˚
c [A]
3
V [A ]
˚
Z
ς
calc [g cm-1
]
1.400
2.450
1440
μ [mm-1
F(000)
]
θ [deg]
3-27.5
36 484
3834
0.058
2854
no. reflns collected
no. reflns unique
R(int)
no. reflns [I > 2σ(I)]
refined params
GOF
201
1.049
Rf [I > 2σ(I)]
wRF2 (all data)
Δςmax/min [e A
0.0320
0.0863
0.469/-0.499
-3
˚
]
the experimentally determined IE value is very low (7.5 eV).
Consequently, the bonding interaction between the nitrogen (X)
lone pair and the phenyl ring (πCtCπ-π2þnNπ(X)) results in a
significant stabilization and is found at 9.0 eV. This fact clearly is
due to the electron-donating mesomeric effect of the amino
group. For derivatives 6, 3,and4this MO (πCtCπ-π2-nXπ(X))
corresponds mainly to the antibonding interaction between the

Article
Organometallics, Vol. 29, No. 21, 2010 5197
Figure 4. First five ionization energies for the 2-arylethynyl-1,3-diethyl-1,3,2-diazabenzodiazaborole series of compounds, according
to UV-photoelectron spectra of BEP-X with X = H (1), Me (2), Br (6), OMe (3), SMe (4), and NMe2 (5).
Figure 5. Calculated [B3LYP/6-311G(d,p)] dipole moments (Debye) in the ground and excited states for a 2-arylethynyl-1,3-diethyl-
1,3,2-diazabenzodiazaborole series of compounds BEP-X, where X = H (1), Me (2), Br (6), OMe (3), SMe (4), and NMe2 (5).
(X-substituted). This situation is also reflected in the dipole
moments of 1-6 in the ground and excited states (Figure 5).
It is obvious that the change of dipole moment (as a vector)
for the dimethylamino derivative 5 is only 1.09 D, whereas
for compounds 1-4 and 6 changes of 10.36 (X = OMe) to
22.16 (X = Br) are calculated.
prior to use. The benzodiazaboroles 2-(40-X C6H4 CtC)-1,3-
Et2-1,3,2-N2BC6H4 (X=H (1), Me (2), OMe (3), SMe (4), NMe2
(5)) and 2-bromo-1,3-diethyl-1,3,2-benzodiazaborole8 were
prepared according to literature methods. 4-Bromophenyla-
cetylene and n-butyllithium were purchased from commercial
sources (Aldrich). NMR spectra (CDCl3) were recorded on a
Bruker AM Avance DRX 500 spectrometer (1H, 11B, 13C) using
SiMe4 and BF3OEt2 as external standards. Absorption is mea-
sured with a UV/vis double-beam spectrometer (Shimadzu UV-
2550) using the solvent as a reference. The setup used to acquire
excitation-emission spectra was similar to that employed in
commercial static fluorimeters and described in detail in ref 5.
2-(40-Bromophenylethynyl)-1,3-diethyl-1,3,2-benzodiazabo
role (6). A 1.6 M n-butyllithium solution in hexane (2.0 mL, 3.2
mmol) was added at 20 °C to the solution of 4-bromophenylace-
tylene (0.572 g, 3.17 mmol) in 30 mL of n-pentane. After stirring for
30 min a sample of neat 2-bromo-1,3-diethyl-1,3,2-benzodiazabor-
ole (0.802 g, 3.17 mmol) was added and stirring of the mixture was
continued for 16 h. The resulting slurry of the mixture was filtered,
and the filter-cake was extracted with 20 mL of n-pentane. The
filtrate was freed from solvent and volatile components in vacuo.
The solid residue was dissolved in a minimum amount of n-pentane
and stored overnight at -35 °C. The product was thus obtained as
colorless crystals (0.92 g, 82% yield).
Conclusion
With regard to the bands in the PE spectrum and their asso-
ciation with the molecular orbitals and ionization energies, it is
obvious that the most significant perturbation of the electronic
structure of the BEP-X compounds under investigation is caused
by the NMe2 group. This fact is experimentally confirmed by
the significant lowering of the ionization energy correspond-
-
ing to HOMO (π3-πNBN-πCtCπ) and HOMO-2 (πCdC
-
nN-π). In addition, the antibonding (HOMO-1) and bonding
(HOMO-3) interaction of the nitrogen atom of the amino group
with the phenyl π system is only relevant for this compound; as a
result, the electron excitation is not accompanied by a significant
charge transfer, as evident from the calculated dipole moments of
5 in the ground and excited states (5.51 vs 6.60 D) and experi-
mentally from a Stokes shift of only 2600 cm-1 versus 5900-
7300 cm-1 for the remaining derivatives.5
Anal. Found: C 61.17; H 5.17; N 7.99. C18H18BBrN2 requires:
C 61.23; H 5.14; N 7.93. 1H NMR (C6D6): δ 1.20 (t, JHH = 7.2
Hz, 6H, CH2CH3), 3.75 (q, JHH = 7.2 Hz, 4H, CH2CH3), 6.96
(m, 2H, CHdCH-CHdCH), 7.09 (m, 4H, H-Ph), 7.13 (m, 2H,
Experimental Section
CHdCH-CHdCH). 13C{1H }NMR (C6D6):
δ 16.0 (s,
General Procedures. All manipulations were performed under
an atmosphere of dry argon using standard Schlenk techniques.
All solvents were dried by common methods and freshly distilled
CH2CH3), 38.2 (s, CH2CH3),105.7 (s, B-CtC), 109.2 (s,
CHdCH-CHdCH), 119.5 (s, CHdCH-CHdCH), 122.2 (s,
C-Ph), 123.1 (s, C-Ph), 131.7 (s, C-Ph), 133.4 (s, C-Ph), 137.0
(s, C2N2). 11B{1H }NMR (C6D6): δ 21.0 (s) ppm. MS/EI (70 eV)
(m/z): 352.1 [Mþ, 90], 337 [Mþ - CH3, 100]. UV/vis (THF):
(8) Weber, L.; Wartig, H.-G.; Stammler, H.-G.; Neumann, B. Z.
Anorg. Allg. Chem. 2001, 627, 2663.
λ
max= 314 nm. Fluorescence (THF): λmax= 412 nm. Φf = 49%.

5198 Organometallics, Vol. 29, No. 21, 2010
Chrostowska et al.
UV-Photoelectron Spectra. The UV-PES spectra were re-
corded on a home-built, three-part spectrometer equipped with
a main body device (Meca2000), He I radiation source (Omicron
NanoTechnology HIS13),9a and a spherical analyzer (Omicron
NanoTechnology EA125).9b The spectrometer works at con-
stant analyzer energy and is monitored by a microcomputer
supplemented by a digital-analogue converter. The spectra
resulting from a single scan are built from 2048 points and are
accurate within 0.05 eV. Spectra were calibrated with lines of
xenon (12.13 and 13.44 eV) and of argon (15.76 and 15.94 eV).
The accuracy of the ionization potentials is þ0.03 eV for sharp
peaks and (0.05 eV for broad and overlapping signals. Com-
pounds 1-6 were slowly vaporized (in all cases below their
melting points) under low pressure (10-7 mmHg) in the ioniza-
tion chamber, and the gaseous flow was then continuously
analyzed.
functionals and were followed by frequency calculations in order to
verify that the stationary points obtained were true energy minima.
Ionization energies (IE) were calculated with ΔSCF-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 ΔSCF-DFT method of calculations of the first ioniza-
tion energies have been demonstrated previously.14 The TD-
DFT15 approach provides a first-principal method for the calcula-
tion of excitation energies within a density functional context
taking into account the low-lying ion calculated by the ΔSCF
method. Finally, the so-called “corrected” IEs15c were evaluated
applying a uniform shift, x=-IEvexp - εKSHOMO, where εKS
HOMO
is the B3LYP/6-311G(d,p) (or cam- B3LYP/6-311G(d,p)) Kohn-
Sham energy of the highest occupied MO of the molecule in the
ground state, and IEvexp is the lowest experimental IE energy of this
species, as was suggested previously by Stowasser and Hoffmann16
and in our studies on different methods for the calculation of IEs.17
MOLEKEL18 has been used as a visualization tool for MOs.
Computational Methods. All calculations were performed
using the Gaussian 0910 program package with the 6-311G(d,p)
basis set. DFT has been shown to predict various molecular pro-
perties of similar compounds successfully.11 All geometry optimi-
zations were carried out with the B3LYP12 and cam-B3LYP13
Acknowledgment. This work was financially supported
by the French Ministry of Research and High School
Education Grant for M.M. and the Deutsche Forschungs-
gemeinschaft, Bonn, Germany.
(9) (a) http://www.omicron.de/products/excitation_sources/photon_
sources/his_13/media/his_13_1.pdf. (b) http://www.omicron.de/products/
electron_spectrometers//ea_125/media/ea_125_1.pdf.
(10) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.;
Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu,
G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian 09, Revision E.01; Gaussian, Inc.: Wallingford, CT,
2007.
Supporting Information Available: Tables of atomic coordi-
nates for [B3LYP/6-311G(d,p)] optimized geometries, values of
total energies and optimized geometrical parameters of 1-6;
B3LYP/6-311G(d,p) and [cam-B3LYP/6-311G(d,p)] calculated
ionization energies for 1-6. This material is available free of
charge via the Internet at http://pubs.acs.org.
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