Borylene-based direct functionalization of organic substrates: Synthesis, characterization, and photophysical properties of novel π-conjugated borirenes

Article abstract of DOI:10.1021/ja902198z

Room temperature photolysis of aminoborylene complexes, [(CO) ₅MdBdN(SiMe₃)₂] (1: M = Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne,

Full text of DOI:10.1021/ja902198z

Published on Web 05/29/2009
Borylene-Based Direct Functionalization of Organic
Substrates: Synthesis, Characterization, and Photophysical
Properties of Novel π-Conjugated Borirenes
Holger Braunschweig,* Thomas Herbst, Daniela Rais, Sundargopal Ghosh,
Thomas Kupfer, and Krzysztof Radacki
Institut fu¨r Anorganische Chemie, Bayerische Julius-Maximilians-UniVersita¨t Wu¨rzburg Am
Hubland, 97 074 Wu¨rzburg, Germany
Andrew G. Crawford, Richard M. Ward, and Todd B. Marder*
Department of Chemistry, Durham UniVersity, South Road, Durham, DH1 3LE, U.K.
Israel Ferna´ndez*
Dpto. Qu´ımica Orga´nica I, UniVersidad Complutense de Madrid, 28040 Madrid, Spain
Gernot Frenking*
Fachbereich Chemie, Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse,
35043 Marburg, Germany
Received March 30, 2009; E-mail: H.Braunschweig@mail.uni-wuerzburg.de;
Frenking@chemie.uni-marburg.de; Israel@quim.ucm.es; Todd.Marder@durham.ac.uk
Abstract: Room temperature photolysis of aminoborylene complexes, [(CO)5MdBdN(SiMe3)2] (1: M )
Cr, 2: Mo) in the presence of a series of alkynes and diynes, 1,2-bis(4-methoxyphenyl)ethyne, 1,2-bis(4-
(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-
bis(trimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethynyl)thiophene led to the
isolation of novel mono and bis-bis-(trimethylsilyl)aminoborirenes in high yields, that is [(RCdCR)(µ-
BN(SiMe3)2], (3: R ) C6H4-4-OMe and 4: R ) C6H4-4-CF3); [{(µ-BN(SiMe3)2(RCdC-)}2], (5: R ) C6H5 and
6: R ) C6H4-4-OMe); [1,4-bis-{(µ-BN(SiMe3)2(SiMe3CdC)}benzene], 7 and [2,5-bis-{(µ-BN(SiMe3)2 ((C6H4NMe2)-
CdC)}-thiophene], 8. All borirenes were isolated as light yellow, air and moisture sensitive solids. The new
1
borirenes have been characterized in solution by H, 11B, 13C NMR spectroscopy and elemental analysis
and the structural types were unequivocally established by crystallographic analysis of compounds 6 and
7. DFT calculations were performed to evaluate the extent of π-conjugation between the electrons of the
carbon backbone and the empty pz orbital of the boron atom, and TD-DFT calculations were carried out to
examine the nature of the electronic transitions.
sensors, solar cells and electronic circuits.1 Three coordinate
boron based materials, for which a variety of structural patterns
Introduction
Boron-containing molecular and polymeric species have
remarkable linear, nonlinear and electro optical properties, and
have had a significant impact in the area of materials
chemistry.1-3 π-Conjugated organic compounds, in particular,
displaying three coordinate boron centers, constitute a highly
topical area of research, as these functional molecular and
polymeric materials have potential for applications in OLEDs,
have been realized, have the presence of trivalent, sp2-hybridized
boryl groups in common. Hence, their generation relies on
conventional synthetic techniques common in organoboron
chemistry,4-6 for example, hydro- and diboration of C-C
multiple bonds, salt elimination and silicon-boron exchange.
Based on recent work in our laboratory, it was demonstrated
(3) (a) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew. Chem., Int. Ed.
2007, 46, 4273–4276. (b) Qin, Y.; Cheng, G.; Achara, O.; Parab, K.;
Ja¨kle, F. Macromolecules 2004, 37, 7123–7131. (c) Parab, K.; Quin,
Y.; Ja¨kle, F. Poly. Mater. Sci. Eng. Prepr. 2005, 93, 422. (d) Haussler,
M.; Tang, B. Z. AdV. Polym. Sci. 2007, 209, 1–58.
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2927–2931. (b) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004,
16, 4574–4585. (c) Ja¨kle, F. Coord. Chem. ReV. 2006, 250, 1107–
1121. (d) Elbing, M.; Bazan, G. C. Angew. Chem., Int. Ed. 2008, 47,
834–838.
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(b) Brown, H. C. Boranes in Organic Chemistry; Cornell University
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Press: New York, 1975. (d) Beletskaya, I.; Pelter, A. Tetrahedron 1997,
53, 4957–5026.
(2) (a) Noda, T.; Ogawa, Y. S.; Shirota, Y. AdV. Mater. 1999, 11, 283–
285. (b) Shirota, Y.; Kinoshita, T.; Noda, K.; Okumoto, T. O. J. Am.
Chem. Soc. 2000, 122, 11021–11022. (c) Matsumi, N.; Naka, K.;
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Han, L.-B.; Tanaka, M. Chem. Commun. 1999, 395–402. (c) Ishiyama,
T.; Miyaura, N. J. Organomet. Chem. 2000, 611, 392–402.
9
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J. AM. CHEM. SOC. 2009, 131, 8989–8999 8989

A R T I C L E S
Braunschweig et al.
that borylene complexes of the type [LxMdBdNR2] (M ) Cr,
Mo, W; R ) SiMe3)7-10 not only stabilize borylenes in the
coordination sphere of various transition metals but, more
importantly, serve as unprecedented sources for the elusive BR
moiety, under mild conditions.11,12
that might exhibit 2π aromatic stabilization, and have attracted
considerable fundamental interest; however, synthetic methods
yielding borirenes are laborious, low yielding, and severely
limited in scope.22 Herein, we report the photochemical transfer
of the aminoborylene moiety, BN(SiMe3)2 from [(CO)n-
MdBdN(SiMe3)2], (M ) Cr and Mo) to a set of selected
alkynes and diynes, which led to the isolation of novel
π-conjugated borirenes, all of which have been characterized
spectroscopically as well as, in selected cases, by X-ray
diffraction studies. This borylene based functionalization of
carbon-carbon triple bonds turned out to be not only applicable
to a wide range of substrates including alkynes and diynes, but
also to proceed with very high yield. Upon availability of these
borirenes in sufficient quantities, theoretical calculations com-
bined with spectroscopic data were employed to elucidate the
electronic structure of these species particularly with respect to
the extent of π-delocalization within the BCC-ring.
Borylenes represent hypovalent, low coordinate species
which, unlike their low valent carbon relatives, carbenes CR2,
are highly reactive compounds that cannot be generated or
handled under standard conditions. Classical studies by Timms
proved that B-F can be generated at high temperature >2000
°C in dilute gas phase,13 and B-SiPh3 was obtained photo-
chemically in hydrocarbon matrices at -196 °C by West.14
Some notable accounts on this novel class of organometallics
followed,15-18 but development has been slow because conve-
nient routes to these compounds were not available and, owing
to the drastic conditions under which borylenes are accessible
as short-lived species, the chemistry of these compounds has
been severely limited.
Having developed a practical synthetic route to borylene
complexes of group 6 metals [(CO)nMdBdN(SiMe3)2], (1: M
) Cr, 2: Mo) in good yields, a variety of reactivity patterns
were investigated. Reactions with cyclopentadienylmetalcarbo-
nyls of groups 8-9 and with alkynes have been found to be
the most profitable.10-12,19-21 The results prior to this work are
shown in Scheme 1 and illustrate the thermal and photochemical
transfer of borylene ligands from borylene complexes to
inorganic (Scheme 1a-b) and organic species (Scheme 1c).
Boracyclopropenes or borirenes are isoelectronic with cy-
clopropenium cations, that is, the smallest aromatic carbocycles
Results and Discussions
Synthesis and Characterization of Mono- and Bis-bis(trimeth-
ylsilyl)aminoborirenes, 3-8 by Photochemical Borylene Transfer
from [(CO)5MdBdN(SiMe3)2] (M ) Cr, Mo). As shown in
Scheme 2, UV irradiation of a pale-yellow solution of
[(CO)5MdBdN(SiMe3)2], (1: M ) Cr and 2: Mo) in THF with
appropriate quantities of the alkynes 1,2-bis(4-methoxyphenyl)-
ethyne, 1,2-bis(4-(trifluoromethyl)phenyl)ethyne, 1,4-diphenylbuta-
1,3-diyne, 1,4-bis(4-methoxyphenyl)buta-1,3-diyne, 1,4-bis(t-
rimethylsilylethynyl)benzene and 2,5-bis(4-N,N-dimethylamino-
phenylethynyl)thiophene, for 4-8 h at room temperature
resulted in the formation of mono and bis-borirenes, 3-8. All
reactions were monitored by multinuclear NMR spectroscopy,
which revealed gradual consumption of the starting materials
and quantitative formation of the new boron containing com-
pounds, as indicated by the presence of a singlet resonance in
the 11B{1H} NMR spectrum at δ 23.7-29.1 ppm.
(6) (a) Cragg, G. M. L. Organoboranes in Organic Synthesis; Marcel
Dekker: New York, 1972. (b) Suzuki, A.; Miyaura, N. Chem. ReV.
1995, 95, 2457–2483. (c) Kaufmann, D. E.; Matteson, D. S. Science
of Synthesis; Boron Compounds; Thieme: Stuttgart, 2005; Vol. 6.
(7) (a) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786–1801.
(b) Braunschweig, H.; Kollann, C.; Englert, U. Angew. Chem., Int.
Ed. 1998, 37, 3179–3180.
[(RCdCR)(µ-BN(SiMe3)2]. (3: R ) C6H4-4-OMe and 4: R
) C6H4-4-CF3): Room temperature photolysis of a pale yellow
solution of [(CO)5CrdBdN(SiMe3)2], 1 in the presence of 1,2-
bis(4-methoxyphenyl)ethyne or 1,2-bis(4-trifluoromethylphe-
nyl)ethyne for 4 h led to the isolation of 3 and 4, respectively.
In both cases, aminoborirenes have been isolated almost in
quantitative yields in the form of pale yellow crystals by
filtration of the reaction mixture and subsequent crystallization
from hexane at -60 °C. Compounds 3 and 4 have been
characterized by comparison of their spectroscopic data with
those of related compounds.23 The 11B{1H} NMR spectra of 3
and 4 show the presence of a single resonance at δ ) 25 and
27 ppm, respectively. These values are in good agreement with
those reported by Habben and Meller for a series of 1-bis(tri-
methylsilyl)aminoborirenes24 which appeared at δ ) 26.5-27.7
ppm. The mass spectrometric data for 3 and 4 suggest molecular
formulas [(MeOC6H4CCC6H4OMe)BN(SiMe3)2] and [(CF3C6H4-
(8) Braunschweig, H.; Colling, M. Eur. J. Inorg. Chem. 2003, 393–403.
(9) Braunschweig, H. AdV. Organomet. Chem. 2004, 51, 163–192.
(10) Blank, B.; Braunschweig, H.; Colling-Hendelkens, M.; Kollann, C.;
Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. R. Chem.-Eur. J.
2007, 13, 4770–4781.
(11) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006,
45, 5254–5274.
(12) Braunschweig, H.; Ferna´ndez, I.; Frenking, G.; Radacki, K.; Seeler,
F. Angew. Chem., Int. Ed. 2007, 46, 5212–5214.
(13) Timms, P. L. Acc. Chem. Res. 1973, 6, 118–123.
(14) Pachaly, B.; West, R. Angew. Chem. 1984, 96, 444–445; Angew. Chem.
Int. Ed. Engl. 1984, 23, 454-455.
(15) Shimoi, M.; Ikubo, S.; Kawano, Y. J. Am. Chem. Soc. 1998, 120,
4222–4223.
(16) Irvine, G. J.; Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright,
L. J. Angew. Chem., Int. Ed. 2000, 39, 948–950.
(17) Cowley, A. H.; Lomeli, V.; Voigt, A. J. Am. Chem. Soc. 1998, 120,
6401–6402.
(18) (a) Aldridge, S.; Coombs, D. L.; Jones, C. Chem. Commun. 2002,
856–857. (b) Coombs, D. L.; Aldridge, S.; Jones, C.; Willock, D. J.
J. Am. Chem. Soc. 2003, 125, 6356–6357. (c) Kays (nee Coombs),
D. L.; Day, K. J.; Ooi, L.-L.; Aldridge, S. Angew. Chem., Int. Ed.
2005, 44, 7457–7460. (d) Aldridge, S.; Jones, C.; Gans-Eichler, T.;
Stasch, A.; Kays (nee Coombs, D. L.; Coombs, N. D.; Willock, D. J.
Angew. Chem., Int. Ed. 2006, 45, 6118–6122. (e) Vidovic, D.; Pierce,
G. A.; Aldridge, S. Chem. Commun. 2009, 1157–1171.
(19) Braunschweig, H.; Colling, M.; Kollann, C.; Englert, U. J. Chem. Soc.,
Dalton Trans. 2002, 2289–2296.
1
CCC6H4CF3)BN(SiMe3)2], respectively. The H NMR spectra
show a single resonance for the N-(SiMe3)2 protons at δ ) 0.31
ppm, indicating a symmetric structure.
(22) (a) Krogh-Jespersen, K.; Cremer, D.; Dill, J. D.; Pople, J. A.; Schleyer,
P.; von, R. J. Am. Chem. Soc. 1981, 103, 2589–2594. (b) Eisch, J. J.;
Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc. 1990,
112, 1847–1853.
(20) (a) Braunschweig, H.; Forster, M.; Radacki, K.; Seeler, F.; Whittell,
G. R. Angew. Chem., Int. Ed. 2007, 46, 5212–5214. (b) Braunschweig,
H.; Forster, M.; Kupfer, T.; Seeler, F. Angew. Chem., Int. Ed. 2008,
47, 5981–5983.
(23) Braunschweig, H.; Herbst, T.; Rais, D.; Seeler, F. Angew. Chem. Int.
Ed. 2005, 44, 7461–7463.
(21) (a) Braunschweig, H.; Dewhurst, R. D.; Herbst, T.; Radacki, K. Angew.
Chem., Int. Ed. 2008, 47, 5978–5980. (b) Apostolico, L.; Braunsch-
weig, H.; Crawford, A. G.; Herbst, T.; Rais, D. Chem. Commun. 2008,
497–498.
(24) (a) Habben, C.; Meller, A. Chem. Ber. 1984, 117, 2531–2537. (b)
Brown, C.; Cragg, R. H.; Miller, T. J.; Smith, D. O. J. Organomet.
Chem. 1983, 244, 209–215.
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Borylene-Based Direct Functionalization
A R T I C L E S
Scheme 1. (a) Thermolytic Borylene Transfer to Group 9 Cyclopentadienylmetalcarbonyls, (b) Formation of Half-Sandwich Vanadium
Complex by Metal to Metal Borylene Transfer, and (c) Photolytic Borylene Transfer to Alkynes to Give Borirenes
[{(µ-BN(SiMe3)2(RCdC-)}2]. (5: R ) C6H5 and 6: R ) C6H4-
4-OMe). By employing diynes with adjacent triple bonds, that
is 1,4-diphenylbuta-1,3-diyne and 1,4-bis(4-methoxyphenyl)buta-
1,3-diyne, two bisborirenes, 5 and 6, have been isolated in good
yields upon photolysis with 1. The presence of a 11B{1H}
resonance at δ ) 24.0 ppm in each case is consistent with the
)1.367(2) Å) are comparable to those of structurally character-
ized borirenes with boron-bound mesityl (B-C ) 1.450(1) and
1.464(1), C-C ) 1.380(9) Å),22b [(η5-C5Me5)(OC)2Fe]- (B-C
) 1.490(4), C-C ) 1.371(3) Å),12 and amino substituents
(B-C ) 1.485(3), C-C ) 1.376(4) Å).23
These structural findings have been interpreted as indicative
of extensive delocalization of the two π-electrons over a three-
center bonding molecular orbital comprised of the pz-atomic
orbitals of boron and carbon.25 In the case of 6, this kind of
delocalization is likely to be attenuated by the presence of the
π-donating amino substituent. Indeed, a slightly elongated B-N
separation of 1.417(2) Å, which matches that of the aforemen-
tioned aminoborirene (1.421(4) Å),23 possibly indicates a weaker
than normal π interaction between the boron and nitrogen
centers as a result of the aromatic character of the boracyclo-
propene ring. Thus, there remains a question of the degree of
B-N π-bonding vs aromaticity of the borirene ring, which will
be addressed further by dynamic NMR spectroscopic studies
and DFT calculations, Vide infra. Finally, it should be mentioned
that the central C2-C3 distance of 1.421(2) Å is significantly
shorter than the central C-C single bond length of 1.483(1) Å
in butadiene, linking two sp2-hybridized C-atoms,25 but is
comparable to the central C-C bond in [{Rh(PMe3)3(Cl)}2(µ-
(1,2-η2):(3,4-η2)-4-F3C-C6H4-CtC-CtC-C6H4-4-CF3)]
1
proposed structure, while single H NMR resonances for the
SiMe3 groups at δ ) 0.29 (5) and 0.27 ppm (6) indicate highly
symmetrical molecules in solution, and thus rapid rotation
around the B-N bond at room temperature. On the basis of
the NMR data combined with mass spectrometric analysis, 5
and 6 are formulated as [{BN(SiMe3)2(RCC)}2] (5: R ) C6H5
and 6: R ) C6H4-4-OMe).
Single crystals suitable for X-ray diffraction analysis of 6
were obtained from a hexane solution at -60 °C, thus allowing
for the first structural characterization of a bisborirene (Figure
j
1). The molecule crystallizes in the space group P1 with Z ) 2
and is composed of two almost identical halves defined by the
borirene rings and their exo-substituents, which are linked via
the central C2-C3 bond; thus, an approximate C2 symmetry is
adopted. As indicated by a dihedral angle of 90.4°, the two
boracyclopropene moieties are oriented almost perpendicular
to each other, thus minimizing steric congestion imposed by
the bulky N(SiMe3)2 substituents. As the geometry of both
borirene subunits is identical within the experimental error, only
one set of data will be discussed. The distances within the BC2
ring (C1-B1 ) 1.482(2), B1-C2 ) 1.500(2) and C1-C2
(25) Marais, D. J.; Sheppard, N.; Stoicheff, B. P. Tetrahedron 1962, 163–
169.
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A R T I C L E S
Braunschweig et al.
Scheme 2. Synthesis of Mono- and Bis-bis-(trimethylsilyl)aminoborirenes, from Borylene Complexes, [(CO)5MdBdN(SiMe3)2] (M ) Cr and
Mo)
(1.405(4) Å), which connects the two conjugated CtC-triple
bonds each coordinated to one rhodium center.26 We interpret
this short C2-C3 distance as indicative of a strong electronic
interaction between the boracyclopropene rings. DFT calcula-
tions agree with the latter interpretation. The Wiberg bond
order for the C2-C3 bond order is >1 for both bis-borirenes
(1.10 for 5 and 1.09 for 6). Moreover, the calculated
hybridization of the carbon atoms C2/C3 is sp1.57 in borirene
5 and sp1.56 in compound 6 which indicates a high degree of
s-character in the C2-C3 bonding orbitals at the carbon
atoms. The results thus confirm the strong electronic interac-
tion which is responsible for the observed rather short C-C
distances. The influence of lateral boryl groups on the
electronic structure of π-conjugated oligomers and polymers
has been reported by Ja¨kle et al. in bis(dimesitylboryl)
substituted oligothiophenes and by Yamaguchi et al. in
(26) Ward, R. M.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B. Inorg.
Chim. Acta 2006, 359, 3671–3676.
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Borylene-Based Direct Functionalization
A R T I C L E S
7.4 kcal mol-1 corresponding to 31.0 kJ mol-1, in good
agreement with the measured barrier for 7. These findings,
together with the aforementioned structural data, lend further
support to the description of the BN linkage in such aminobor-
irenes as a rather weak double bond, whose π-contribution is
reduced by the incorporation of the pz-orbital at boron into the
2π-electron aromatic ring. Indeed, as shown in Figure 2 (top
right), one observes only a small drop in the boron pz-orbital
occupancy from ca. 0.43 to ca. 0.37 electrons upon rotation to
a dihedral angle between the BNSi2 and BC2 planes of 90°,
indicative of the relatively small B-N π-component even at 0°
dihedral.
In the case of 8, spectroscopic data in addition to the
aforementioned 11B NMR resonance clearly indicate the forma-
tion of a bisborirene. The NMR behavior of 8 is very similar
indeed to that of 7, differing most noticeably in small changes
1
of proton shielding. H NMR spectrum of 8 shows singlets at
δ ) 0.46 for the four SiMe3 groups and at 7.64 ppm for the
two protons in the thiophene ring. These data together with
characteristic 13C NMR resonances at δ ) 3.6 for the SiCH3
groups, and 130.4 ppm for the two CH groups in the thiophene
ring, suggest rapid rotation around the B-N bond.
Figure 1. Molecular structure of 6. Relevant bond lengths [Å] and angles
[deg]: N1-B1 1.417(2), C1-B1 1.482(2), B1-C2 1.500(2), B2-N2
1.420(2), B2-C4 1.483(2), B2-C3 1.497(2), C2-C3 1.421(2), N1-Si1
1.758(1), Si1-C31 1.861(2), C1-C2 1.367(2), C3-C4 1.370(2), C1-B1-N1,
153.62(1), C1-B1-C2 54.59(8), N2-B2-C3 145.63(1), B1-N1-Si1
113.13(9), B2-N2-Si3 121.38(9), N1-Si1-C31 108.48(6). Ellipsoids
drawn at the 50% probability level.
Crystals of 7, were obtained by cooling a concentrated hexane
solution to -60 °C. The single crystal X-ray diffraction structure
of 7, shown in Figure 3, confirms the structural inferences made
on the basis of the spectroscopic data. The molecule crystallizes
oligo(phenyleneethynylenes) (OPE′s) containing bis(dimesi-
tylboryl) functionalities.2d,e
j
in the triclinic space group P1 with Z ) 1, and thus lies on a
[1,4-Bis-{(µ-BN(SiMe3)2(SiMe3CdC)}C6H4], 7 and [2,5-Bis-{(µ-
BN(SiMe3)2((C6H4-4-NMe2)CdC)}thiophene], 8. In addition to the
aforementioned alkynes and 1,3-diynes, a different type of
substrate was employed, in which the two alkyne functionalities
are separated by a conjugated spacer. Analogous to the
preparation of 3-6, photolysis of 1 with 1,4-bis(trimethylsilyl-
ethynyl)benzene and 2,5-bis(4-N,N-dimethylaminophenylethy-
nyl)thiophene furnished aminoborirenes 7 and 8, respectively,
in good yield. Singlet 11B{1H} NMR resonances at δ ) 29.1
(7) and 24.2 ppm (8) are in the expected range for borirene
products. The mass spectrum of 7 is consistent with a formula
containing two B, two N and six Si atoms and the parent ion
mass of 612 corresponds to addition of two BN(SiMe3)2 units
to the dialkyne scaffold. Consistent with these observations is
the presence of two singlets in the room temperature 1H NMR
spectrum at δ ) 0.27 and 0.34 ppm in a 1:2 ratio, which are
associated with the carbon and nitrogen bound SiMe3-groups,
respectively. The observation of only one N(SiMe3)2 resonance
implies rapid rotation around the B-N bond at room temper-
ature. However, the 1H NMR spectrum at -90 °C clearly shows
two separate signals for the nitrogen-bound trimethylsilyl groups
separated by 68 Hz. From 700 MHz VT 1H NMR spectroscopy,
a value of ∆Gqj ) 38.7 kJ/mol at the coalescence temperature
of -79 °C was computed for the rotational barrier, which is
significantly smaller (by ca. 30 kJ/mol) than those commonly
observed for B-N π-bonds.24 Given the fact that there is an
additional signal of equal intensity for the carbon-bound SiMe3
group close by, NOESY spectroscopy at -90 °C was used to
confirm which pair of SiMe3 groups was exchanging. We also
measured, as above, a similar barrier of 43.1 kJ/mol at the
coalescence temperature of -68 °C for the related, previously
reported23 bis(borirene) [{µ-BN(SiMe3)2(Me3SiCdC)}2] (9)
formed via the transfer of two borirene groups to 1,4-
bis(trimethylsilyl)buta-1,3-diyne. Via DFT calculations, we also
computed the barrier to rotation about the B-N bond in the
previously reported23 symmetric borirene compound [(µ-
BN(SiMe3)2(Me3SiCdCSiMe3)] (10) (Figure 2, top left) to be
crystallographic inversion center. The dihedral angles of 56.21(3)°
between the spacer (phenyl) and the boracyclopropene units is
considerably smaller than in 6 in which the rings are almost
perpendicular to each other. This may be due to the presence
of a phenyl ring which minimizes the steric overcrowding
imposed by the bulky N(SiMe3)2 groups. The overall geometry
of the borirene rings in 7 resembles that of 6, displaying B-C
separations of 1.473(3) and 1.498(3) Å, and a C1-C2 distance
of 1.362(3) Å. Interestingly, the carbon-carbon bond length
of 1.470(3) Å, connecting the BCC and phenyl rings, falls
between the corresponding C-C single bond distance observed
in butadiene (1.483(1) Å)25 and bisborirene 6 (1.421(2) Å). The
bond length is similar to that of the C-C bond linking the
phenyl group with the olefin in styrene derivatives, for example
(disodium)1,4-bis(boratastyryl)benzene (1.464(4) Å).27
To analyze the bonding situations in the synthesized borirenes
3-8 in more detail we first carried out DFT calculations on the
model compound (H3Si)2NBC2(SiH3)2 (10M). The nature of the
bonding interactions was investigated with the AIM (atoms in
molecules)28 method and with the EDA (energy decomposition
analysis) partitioning procedure,29 which has previously been
used by us to investigate chemical bonds in other boron
compounds.30
Figure 4 shows the contour line diagrams of the Laplacian
distribution 32F(r) in the plane of the three-membered ring. The
B-N bond in 10M has an area of charge concentration (32F(r)
< 0, solid lines) at the nitrogen end, while the boron end carries
(27) Lee, B. Y.; Bazan, G., Z. J. Am. Chem. Soc. 2000, 122, 8577–8578.
(28) Bader, R. F. W. Atoms in Molecules. A Quantum Theory; Oxford
University Press: Oxford, 1990.
(29) See Computational Details.
(30) (a) Uddin, J.; Frenking, G. J. Am. Chem. Soc. 2001, 123, 1683–1693.
(b) Chen, Y.; Frenking, G. J. Chem. Soc., Dalton Trans. 2001, 434–
440. (c) Do¨rr, M.; Frenking, G. Z. Anorg. Allg. Chem. 2002, 628,
843–850. (d) Bessac, F.; Frenking, G. Inorg. Chem. 2003, 42, 7990–
7994. (e) Erhardt, S.; Frenking, G. Chem.-Eur. J. 2006, 12, 4620–
4629.
9
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A R T I C L E S
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Figure 2. (Top) Relative energy of [(µ-BN(SiMe3)2(Me3SiCdCSiMe3)] (left) and boron pz orbital occupation (right) as a function of the dihedral angle
between the BNSi2 and BC2 planes. (Bottom) Ball and stick representations of the calculated structures at the energy minimum (left) and transition state for
B-N bond rotation (right). Hydrogen atoms were omitted for clarity.
Figure 3. Molecular structure of 7. Relevant bond lengths [Å] and angles
[deg]: B1-N1 1.423(3), B1-C2 1.473(3), B1-C1 1.498(3), N1-Si2
1.751(2), C1-Si1 1.858(2), C2-C1 1.362(3), C2-C3 1.470(3), N1-B1-C2
154.95(2), N1-B1-C2 150.33(2), C2-B1-C1 54.59(1), B1-N1Si2
120.35(1), B1-N1-Si3 113.90(1), C1-C2-C3 136.72(2), C1-C2-B1
63.63(2), C3-C2-B1 159.48(2). Ellipsoids drawn at the 50% probability
level.
Figure 4. Contour line diagrams 32F(r) of 10M in the BC2 ring plane.
Solid lines indicate areas of charge concentration (32F(r) < 0) while dashed
lines show areas of charge depletion (32F(r) > 0). The solid lines connecting
the atomic nuclei are the bond paths. The solid lines separating the atomic
basins indicate the zero-flux surfaces crossing the molecular plane.
an area of charge depletion (32F(r) > 0, dashed lines) which is
typical for a polar electron-sharing bond.28,31 As expected, 10M
is characterized by the AIM method as three-membered cyclic
species possessing two B-C and one C-C bond paths and one
BC2 ring critical point.
More detailed information about the boron bonds is given
by the EDA results which are summarized in Table 1.
The B-N electron-sharing bond in 10M was analyzed using
the fragments (H3Si)2N and BC2(SiH3)2 in the electronic doublet
state. The EDA indicates that the attractive B-N interactions
in 10M mainly result from orbital (covalent) interactions
(59.8%), rather than from electrostatic attraction (40.2%). The
most interesting EDA data come from the breakdown of the
(31) Frenking, G.; Koch, W.; Gauss, J.; Cremer, D.; Sawaryn, A.; Schleyer,
P. v. R. J. Am. Chem. Soc. 1986, 108, 5732–5740.
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8994 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Borylene-Based Direct Functionalization
A R T I C L E S
Table 1. Results of the EDA at BP86/TZ2P (Energy Values in kcal/mol)
a Percentage values in parentheses give the contribution to the total attractive interactions ∆Eelstat + ∆Eorb
.
b Percentage values in parentheses give the
contribution to the total orbital interactions ∆Eorb
.
Table 2. Computed Second Order Perturbation Energies (in
kcal/mol)a Associated with the Two-Electron Donations to the
pz-Atomic Orbital at the Boron Atom (see Figure 5) in Compounds
3-8 and 10
orbital term ∆Eorb into σ and π contributions which clearly
indicates that the B-N linkage is indeed a rather weak double
bond. The ∆Eπ term contributes only 8.1% (-21.8 kcal/mol)
to the total orbital interactions. Interestingly, this π-contribution
is reduced by the incorporation of the pz-orbital at boron into
the 2π-electron aromatic ring in 10M compared to
(H3Si)2NB(CH3)2 (11). The EDA data clearly shows an increase
in the ∆Eπ term in 11 which lacks the contribution of the three-
membered ring. Table 1 also gives the EDA results for the
B-C2(SiH3)2 bonds in 10M using the interacting fragments in
the triplet state. The breakdown of ∆Eint into the attractive and
repulsive interactions reveals that the main contribution to the
total interaction energy comes from the orbital term ∆Eorb
(60.5%). The most interesting information is derived from the
calculated values for the B-C2(SiH3)2 π-interactions in the
three-membered ring. The ∆Eorb(π) value in the parent borirene
10M is -30.6 kcal/mol, indicating substantial cyclic delocal-
ization which suggests some aromatic character of the ring. The
aromaticity is supported by the computed NICS(0) and NICS(1)
values, -15.3 and -12.9 ppm respectively.32 This means that
the boron atom in 10M has a stronger π-interaction with the
CdC π electrons than with the (SiH3)2N lone pair. The lack of
a CdC double bond in 11 leads to a rather small ∆Eorb(π)
contribution (-12.8 kcal/mol) which is even smaller than the
value for the π-interaction in the B-N bond.
compound
LP(N) f p(B)
π(C-C) f p(B)
10
3
4
5
6
7
8
-46.9
-45.7
-43.5
-65.7
-64.3
-61.2
-52.3
-93.8
-94.2
-100.6
-99.1
-100.7
-94.2
-101.2
a All values have been computed at the B3LYP/def2-SVP level.
Figure 5. Schematic representation of the two-electron stabilizing donation
to the p-atomic orbital at boron atom in borirenes 3-10.
or nitrogen lone pair to pz-(boron) donation in the borirenes
3-8 (Figure 5).
Table 2 shows that the replacement of the trimethylsilyl
groups in 10 by a p-methoxyphenyl substituent in 4 yields a
small increase in the π(CdC) f pz(B) donation due to the
presence of the π-donor methoxy groups and a slight decrease
of the LP(N) f pz(B) donation. However, the presence of the
weak π-acceptor groups CF3 in 3 does not lead to an expected
decrease of the π(CdC) f pz(B) donation which remains
practically the same. This means that the electronic effect exerted
by the trifluoromethylphenyl substituent in borirene 3 is
comparable to the trimethylsilyl group in 10. Strikingly, the
presence of a second borirene moiety in compounds 5-8
compared to 10 yields in all cases a clear enhancement of the
LP(N) f pz(B) donation which is a consequence of the extended
π-conjugation. This effect is nicely reflected in the shortening
of the B-N bond length (see Figure 6). In the latter bis-borirenes
the π(CdC) f pz(B) donation remains higher than the LP(N)
f pz(B) donation which is comparable to the value in 4 due to
the presence of the phenyl groups attached to the CdC linkage.
Similar conclusions can be drawn using the second order
perturbation energies from the NBO method. As readily seen
in Table 2 the two-electron stabilizing donation from the
π(CdC) molecular orbital to the pz-atomic orbital at boron in
borirene 10 is substantially higher (associated second order
energy, ∆E(2) ) -93.8 kcal/mol) than the corresponding
electronic donation from the nitrogen atom lone pair (∆E(2)
)
-46.9 kcal/mol, Table 2). These data are in nice agreement
with the EDA results and therefore we shall use the computed
NBO-data to explain the changes in the strength of the π(CdC)
(32) NICS(0) value was computed at the (3,+1) ring critical point of the
electron density due to its high sensitivity to diamagnetic effects and
its unambiguous character. See: (a) Ferna´ndez, I.; Sierra, M. A.; Coss´ıo,
F. P. J. Org. Chem. 2006, 71, 6178. (b) Ferna´ndez, I.; Coss´ıo, F. P.;
Sierra, M. A. J. Org. Chem. 2007, 72, 1488. (c) Ferna´ndez, I.; Sierra,
M. A.; Coss´ıo, F. P. J. Org. Chem. 2008, 73, 2083, and references
therein.
9
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A R T I C L E S
Braunschweig et al.
Figure 6. Optimized geometries of borirenes 3-10. Hydrogen atoms were omitted for clarity.
Table 3. UV-Vis Absorption Spectra and TD-DFT Computed
Transitions for the Borirene Compounds
Finally, the presence of a π-spacer connecting both borirene
moieties in compounds 7 and 8 also enhances the donation from
the nitrogen atom lone pair to the boron atom thus confirming
the extended π-conjugation in these species. As expected, the
π(CdC) f pz(B) donation is higher in 8 due to the attachment
of the π-donor 4-N-dimethylphenyl substituent to the borirene
ring compared to the TMS group in 7.
f, oscillator
strength,a (ꢀ)b
compound
transitiona
λ /nmb
weighta
c
c
3
4
HOMOfLUMO
HOMO-1fLUMO
HOMOfLUMO
HOMO-1fLUMO
0.616
0.183
0.634
0.494
0.510
0.479
0.448
0.133
0.484
0.393
0.229
0.110
0.549
0.507
0.356
0.668
0.124
0.135
0.461
0.104
0.680
0.346 (12,200)
0.567
(37,400)
302 (269)
309
(279)
c
c
d
d
5
6
0.448 (24,200)
280 (257)
272
HOMO-1fLUMO+1 0.227
UV-vis spectra of the borirene compounds were recorded
in either CH2Cl2 or hexane solutions. In contrast to the spectra
of the corresponding alkyne or diyne precursors, which show
sharp absorptions featuring well-defined vibrational fine struc-
ture, most likely arising from the CtC triple bonds, those of
the borirenes display broad, featureless absorptions with maxima
occurring at higher energies (Table 3). TD-DFT calculations in
the gas phase (Table 3) give computed oscillator strengths which
are in general accord with (i.e., generally follow the same trend)
the experimental values of the extinction coefficients, with the
notable exception of compound 8, for which the observed low
energy absorption is especially broad and thus the extinction
coefficient is likely not a good measure of oscillator strength.
There is also reasonable agreement of the TD-DFT computed
lowest energy transition wavelengths with those measured in
solution (i.e., within ca. 13-33 nm), with the exception of
compounds 7 and 8, which feature more extended conjugation
paths between the two borirene moieties. In these latter cases,
the computed values are significantly red-shifted from the
measured ones, most likely a result of the fact that the computed
values are based on optimized geometries which are relatively
planar, maximizing conjugation. However, the rotational barriers
around the C-C bonds linking the borirene moieties with the
HOMO-1fLUMO
HOMOfLUMO+1
HOMO-2fLUMO
HOMO-1fLUMO+1
HOMOfLUMO
0.763
(34,600)
0.424
283
(266)
c
c
272
7
HOMO-1fLUMO
HOMO-1fLUMO+2
HOMOfLUMO
0.256
(16,800)
330
(262)
d
d
HOMO-2fLUMO
0.426
298
HOMOfLUMO
c
c
8
HOMOfLUMO
HOMO-1fLUMO+2 0.461
HOMO-1fLUMO+3
HOMO-1fLUMO+4
HOMO-1fLUMO
HOMOfLUMO
0.935 (8,900)
434 (387)
267
(276)
c
c
(13,100)
10
0.0031
326
(313)
d
d
(487)
a TD-DFT data calculated at the B3LYP/def2-SVP level.
b Experimental values in parentheses. c Recorded in CH2Cl2. d Recorded
in n-hexane.
central phenylene or thienyl groups are likely to be relatively
small, giving rise to free rotation in solution and thus, all
rotamers will likely be populated at room temperature. As a
result, the observed solution spectra contain Franck-Condon
transitions arising from this emsemble of rotamers including
9
8996 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Borylene-Based Direct Functionalization
A R T I C L E S
Figure 7. Molecular orbitals of compounds 4, 5, 8, and 10 calculated at B3LYP/def2-SVP level (isosurface value of 0.035). Hydrogen atoms were omitted
for clarity. See Figure 2 caption for additional details.
not only the highly conjugated planar conformations, but also
twisted rotamers for which the conjugation length is greatly
decreased. Compound 8 displays a strong, broad emission (λmax
) 463 nm) in CH2Cl2 solution, red-shifted compared with that
seen for its precursor, 2,5-bis(4-N,N-dimethylamino-phenyl-
ethynyl)-thiophene, which occurs at 434 nm.33 It is not
unreasonable to assume that for 8, the emission maximum is
closely related to the 0,0 transition energy,33 and is thus a more
reasonable value to compare with the TD-DFT computed value,
with which it is in much closer agreement, again, bearing in
mind that the latter is a gas-phase value whereas the former
was recorded in a moderately polar solvent. The large Stokes
shift observed for this compound also supports the above
reasoning. Key frontier orbitals involved in the optical transitions
for several of the compounds are illustrated in Figure 7.
characterized bisborirenes and have an extended π-conjugation
incorporating three-coordinate boron centers. The joint com-
putational-experimental study presented here supports the view
of the B-N linkage in these borirenes as a rather weak double
bond whose π-contribution is reduced by the incorporation of
the empty pz-orbital at boron into the 2π-electron aromatic ring.
Experimental Section
General. All manipulations were performed either under dry
argon or in Vacuo using standard Schlenk line and glovebox
techniques. Solvents (THF, ether, toluene and hexane) were
purified by distillation under dry argon from sodium and stored
under the same inert gas over molecular sieves. NMR spectra
were acquired on Varian VNMRS-700 (1H: 699.74; 29Si: 139.02
MHz), Varian Unity 500 (1H: 499.834; 11B: 160.364; 13C:
125.697 MHz) or Bruker Avance 500 (1H: 500.133; 11B: 160.472;
1
13C: 125.777 MHz) NMR spectrometers. H and 13C{1H} NMR
Conclusion
spectra were referenced to external TMS via the residual protio
solvent (1H) or the solvent itself (13C). 11B{1H} and
29Si{1H}NMR spectra were referenced to external BF3 · OEt2 and
TMS, respectively. NMR probe temperatures were calibrated
using a MeOH standard for VT NMR spectroscopic studies.
UV-vis and fluorescence spectra were recorded in toluene if
not otherwise indicated. UV-vis absorption spectra and extinc-
tion coefficients were obtained on a Hewlett-Packard 8453 diode
array and a Shimadzu 1240 mini spectrophotometer using
standard 1 cm width quartz cells. Fluorescence spectra were
recorded at right angles to the excitation source on dilute
solutions using a Horiba Jobin Yvon Fluoromax 3 spectropho-
tometer. The emission spectra were fully corrected using the
manufacturer’s correction curves for the spectral response of
the emission optical components.
The present article describes a facile synthetic method for a
borylene based functionalization of organic substrates for which
a variety of reagents of the type R-CtC-R and R-CtC-
(spacer)-CtC-R were employed in stoichiometric amounts.
This high yield synthetic procedure for the generation of
borirenes not only confirmed the function of complexes 1-2
as valuable borylene sources, but also proved to be general with
respect to the variety of alkynes that undergo the transformation.
The bisborirenes, 6 and 7, reported here are the first structurally
(33) Siddle, J. S.; Ward, R. M.; Collings, J. C.; Rutter, S. R.; Porre`s, L.;
Applegarth, L.; Beeby, A.; Batsanov, A. S.; Thompson, A. L.; Howard,
J. A. K.; Boucekkine, A.; Costuas, K.; Halet, J.-F.; Marder, T. B. New
J. Chem. 2007, 31, 841–851.
9
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A R T I C L E S
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Microanalyses were performed on either a Carlo Erba model
1106 or a Leco CHNS-932 Elemental Analyzer. Na2[M(CO)5]34
(M ) Cr, and Mo), and Cl2B(SiMe3)2 were prepared according
Compound 6. In a 5 mm quartz NMR tube, a pale-yellow
solution of 1 (100 mg, 0.25 mmol) and 1,4-bis(4-methoxyphe-
nyl)buta-1,3-diyne (35.9 mg, 0.14 mmol) in 1 mL of THF was
irradiated for 8 h at room temperature. The volatile components
were removed in Vacuo, and the brown residue was extracted with
hexane (2 × 3 mL) followed by filtration through a pad of silica
gel. The filtrate was stored at -30 °C for 2 weeks to afford yellow
35
to the literature. Borylene complexes, (OC)5MdBN(SiMe3)2 (1: Cr
and 2: Mo),7 2,5-bis(4-N,N-dimethylamino-phenylethynyl)th-
iophene,33 alkynes and diynes36 were synthesized as reported in
the literature. NMR spectroscopic experiments were performed in
quartz NMR tubes. The light source was a Hg/Xe arc lamp
(400-550 W) equipped with IR filters, irradiating at 210-600 nm.
Large-scale experiments were performed in a 150-mL Schlenk flask
equipped with a quartz cooling jacket into which a Hg lamp (125
W) was inserted vertically.
1
crystals of 6 (54.7 mg, 68%). H NMR: δ ) 7.81 (m, 4H, CH-m
of C6H4-OCH3), 6.63 (m, 4H, CH-o of C6H4-OCH3), 3.17 (s, 6H,
O-CH3), 0.40 (s, 36H, Si(CH3)3); 13C{1H}-NMR: δ ) 160.7 (s,
C-OCH3), 149.4 (bs, C bonded to boron), 142.3 (bs, C bonded to
boron), 132.2 (s, CH-m of C6H4), 125.6 (s, C-i of C6H4), 114.5 (s,
CH-o of C6H4), 54.7 (s, OCH3), 3.26 (s, Si(CH3)3); 11B{1H}-NMR:
δ ) 24.0 (s). EI-MS m/z ) 604 (M+), 73 (M - SiMe3)+. Elemental
analysis calcd. [%] for C30H50Si4B2N2O2: C 59.59, H 8.33, N 4.63;
found: C 59.73, H 8.41, N 4.38.
Compound 7. In a 150 mL quartz Schlenk flask, a pale-yellow
solution of 1 (350 mg, 0.96 mmol) or 2 (391 mg, 0.96 mmol) and
1,4-bis(trimethylsilylethynyl)benzene (131 mg, 0.48 mmol) in 15
mL of THF was irradiated for 4 h at room temperature. The volatile
components were removed in Vacuo, and the black residue was
extracted with 10 mL of hexane followed by filtration using glass-
fiber filter paper. The filtrate was stored at -30 °C overnight to
remove a brown unknown solid as well as Cr(CO)6. After removing
the residue the mother liquor was kept at -60 °C to afford colorless
crystalline 7 (232 mg, 79%). 1H NMR: δ ) 7.53 (s, 4H, of C6H4),
0.34 (s, 36H, N(Si(CH3)3)2), 0.27 (s, 18H, Si(CH3)3); 13C{1H}-NMR:
δ ) 184.0 (s, dC-SiMe3), 172.8 (s, dC-C6H4), 137.4 (s, C-i of
C6H4), 127.5 (s, CH of C6H4), 3.3 (s, N-Si(CH3)3), 0.2 (s,
dC-SiCH3); 11B{1H}-NMR: δ ) 29.1 (s). 29Si{1H}-NMR: δ )
5.95 (s, N-(SiMe3)2), -14.92 (s, )C-SiMe3). EI-MS m/z ) 612
(M+), 98 (BdN-SiMe3+). Elemental analysis calcd. [%] for
C28H58Si6B2N2: C 54.87, H 9.54, N 4.57; found: C 54.89, H 9.62,
N 4.07.
Synthesis of Mono- and 1-Bis(trimethylsilyl)aminoborirenes,
3-8. Compound 3: In a 5 mm quartz NMR tube, a pale yellow
solution of 1 (125 mg, 0.34 mmol) and 1,2-bis(4-(trifluorometh-
yl)phenyl)ethyne (104 mg, 0.33 mmol) in 1.5 mL of THF was
irradiated for 5 h at room temperature. The volatile components
were removed in Vacuo, and the brown residue was extracted with
5 mL of hexane followed by centrifugation. The brown hexane
solution was decanted from the brown residue and stored at -60
°C overnight to separate Cr(CO)6. After filtration, the solvent was
removed in Vacuo and 3 was isolated as an analytically pure yellow
1
solid (0.11 g, 72% yield). H NMR: δ ) 7.30 (m, 4H, CH-m or
CH-o of C6H4), 7.24 (m, 4H, CH-m or CH-o of C6H4), 0.31 (s,
18H, Si(CH3)3); 13C{1H}-NMR: δ ) 161.4 (bs, C bonded to boron),
2
137.6 (s, CH-m of C6H4-CF3), 130.3 (q, JC-F ) 33 Hz, C-CF3),
1
125.9 (q,3JC-F ) 4 Hz, CH-o of CF3), 124.8 (q, JC-F ) 271 Hz,
CF3), 3.1 (s, Si(CH3)3); 11B{1H}-NMR: δ ) 23.7 (s). EI-MS m/z
) 485 (M+), 98 (BdN-SiMe3+). Elemental analysis calcd. [%]
for C22H26Si2BNF6: C 54.43, H 5.40, N 2.89; found: C 54.32, H
5.45, N 2.83.
Compound 4. In a 5 mm quartz NMR tube, a pale-yellow
solution of 1 (110 mg, 0.30 mmol) and 1,2-bis(4-methoxyphenyl)-
ethyne (73 mg, 0.30 mmol) in 1.5 mL of THF was irradiated for
5 h at room temperature. The volatile components were removed
in Vacuo, and the brown residue was extracted with 8 mL of hexane
followed by centrifugation. The brown hexane solution was
decanted from the black residue and stored at -60 °C overnight to
separate Cr(CO)6. After filtration, the solvent was removed in Vacuo
and 4 was isolated as an analytically pure yellow solid (90.4 mg,
Compound 8. In a 5 mm quartz NMR tube, a pale-yellow
solution of 1 (100 mg, 0.28 mmol) or 2 (114 mg, 0.28 mmol) and
2,5-bis(4-N,N-dimethylaminophenylethynyl)-thiophene (51.0 mg,
0.14 mmol) in 1.5 mL of THF was irradiated for 6 h at room
temperature. The volatile components were removed in Vacuo, and
the brown residue was extracted with hexane (2 × 3 mL) followed
by centrifugation. The brown hexane solution was decanted from
the black residue and stored at -60 °C overnight to separate
Cr(CO)6 and an unidentified residue. The mother liquor was
evaporated to dryness and 8 was isolated as an analytically pure
1
74% yield). H NMR: δ ) 7.72 (m, 4H, CH-m of C6H4-OCH3),
6.77 (m, 4H, CH-o of C6H4-OCH3), 3.28 (s, 6H, OCH3), 0.42 (s,
18H, Si(CH3)3); 13C{1H}-NMR: δ ) 160.2 (s, C-OCH3), 157.7 (bs,
C bonded to boron), 130.9 (s, CH-m of C6H4), 126.9 (s, C-i of
C6H4), 114.6 (s, CH-o of C6H4), 54.8 (s, OCH3), 3.4 (s, Si(CH3)3);
11B{1H}-NMR: δ ) 24.8 (s). EI-MS m/z ) 409 (M+). Elemental
analysis calcd. [%] for C22H32Si2BNO2: C 64.53, H 7.88, N 3.42;
found: C 64.50, H 7.62, N 3.42.
1
yellow oily solid (60.2 mg, 61% yield). H NMR: δ ) 7.99 (m,
4H, H-m of C6H4), 7.64 (s, 2H, C4H2S), 6.54 (m, 4H, H-o of C6H4),
2.47 (s, 12H, N(CH3)2), 0.46 (s, 36H, (SiCH3)3); 13C{1H}-NMR: δ
) 150.9 (s, C of C6H4 bonded to NMe2), 137.8 (s, C of C4H2S),
131.9 (s, CH-m of C6H4), 130.4 (s, CH, of C4H2S), 121.6 (s, C-i of
C6H4), 112.2 (s, CH-o of C6H4), 39.8 (s, N(CH3)2), 3.6 (s, Si(CH3)3);
11B{1H}-NMR: δ ) 24.2 (s). EI-MS m/z ) 713 (M+), 73 (M -
SiMe3)+. Elemental analysis calcd. [%] for C36H58Si4B2N4S: C
60.65, H 8.20, N 7.86; found: C 60.17, H 8.03, N 7.43.
Compound 5. In a 5 mm quartz NMR tube, a pale-yellow
solution of 1 (100 mg, 0.25 mmol) and 1,4-diphenylbuta-1,3-diyne
(25 mg, 0.13 mmol) in 0.5 mL of d6-benzene was irradiated for
6 h at room temperature. The volatile components were removed
in Vacuo, and the brown residue was extracted with hexane (2 ×
3 mL) followed by filtration using glass fiber. The filtrate was
evaporated to dryness to afford an analytically pure yellow oil of
X-Ray Structure Determinations. The crystallographic data of
6 and 7 were collected on a Bruker Apex diffractometer with a
CCD area detector and graphite monochromated Mo KR radiation.
The structures were solved using direct methods, refined with the
Shelx software package (G. Sheldrick, University of Go¨ttingen
1997) and expanded using Fourier techniques. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were assigned
idealized positions and were included in structure factor calculations.
Crystal Data for 6. C30H50B2N2O2Si4, Mr ) 604.70, yellow plate,
1
5 (63.7 mg, 90%). H NMR: δ ) 7.83 (m, 4H, CH-o of C6H5),
7.09 (m, 6H, CH-m and CH-p of C6H5), 0.35 (s, 36H, Si(CH3)3);
13C{1H}-NMR: δ ) (C bonded to boron not detected), 132.8 (s,
C-i of C6H5), 130.4 (s, CH-m or CH-o), 129.1 (s, CH-m or CH-o),
128.6 (s, CH-p), 3.2 (s, Si(CH3)3); 11B{1H}-NMR: δ ) 24.0 (s).
EI-MS m/z ) 544 (M+). Elemental analysis calcd. [%] for
C28H46Si2BN2: C 61.75, H 8.51, N 5.14; found: C 61.43, H 8.70,
N 4.50.
3
j
0.30 × 0.18 × 0.07 mm , triclinic space group P1, a ) 9.5049(11)
(34) Maher, J. M.; Beatty, R. P.; Cooper, N. J. Organometallics 1985, 4,
1354–1361.
(35) Haubold, W.; Kraatz, U. Z. Anorg. Allg. Chem. 1976, 421, 105–110.
(36) Batsanov, A. S.; Collings, J. C.; Fairlamb, I. J. S.; Holland, J. P.;
Howard, J. A. K.; Lin, Z.; Marder, T. B.; Parsons, A. C.; Ward, R. M.;
Zhu, J. J. Org. Chem. 2005, 70, 703–706.
Å, b ) 13.4023(14) Å, c ) 15.0311(18) Å, R ) 81.229(5)°, ꢀ )
72.047(5)°, γ ) 79.572(5)°, V ) 1781.9(4) Å3, Z ) 2, Fcalcd
)
1.127 g·cm-3, µ ) 0.195 mm-1, F(000) ) 652, T ) 98(2) K, R1
) 0.0529, wR2 ) 0.1146, 10597 independent reflections [2θ e
61.12°] and 361 parameters.
9
8998 J. AM. CHEM. SOC. VOL. 131, NO. 25, 2009

Borylene-Based Direct Functionalization
A R T I C L E S
2
Crystal Data for 7. C28H58B2N2Si6, Mr ) 612.92, clear block,
ˆ
〈φ*|F|φ〉
(2)
φφ*
3
j
∆E ) -nφ
0.14 × 0.07 × 0.04 mm , triclinic space group P1, a ) 6.6063(9)
Å, b ) 10.7222(14) Å, c ) 14.846(2) Å, R ) 68.987(7)°, ꢀ )
82.634(7)°, γ ) 77.599(7)°, V ) 957.3(2) Å3, Z ) 1, Fcalcd ) 1.063
g·cm-3, µ ) 0.237 mm-1, F(000) ) 334, T ) 101(2) K, R1 )
0.0587, wR2 ) 0.1053, 3736 independent reflections [2θ e 52.18°]
and 172 parameters.
εφ* - εΦ
*
ˆ
where F is the DFT equivalent of the Fock operator and φ and φ
are two filled and unfilled Natural Bond Orbitals having εφ and εφ*
energies, respectively; nφ stands for the occupation number of the
filled orbital.
The nature of the interatomic interactions was investigated by
means of the energy decomposition analysis (EDA) which was
carried out with the program package ADF 2007.01.46 For the EDA
calculations, the geometries of the molecules were reoptimized at
the gradient corrected DFT level using the exchange functional of
Becke in conjunction with the correlation functional of Perdew
(BP86).47 Uncontracted Slater-type orbitals (STOs) were employed
as basis functions for the SCF calculations.48 The basis sets have
triple-ꢁ quality augmented by two sets of polarization functions,
that is, p and d functions for the hydrogen atoms and d and f
functions for the other atoms. This level of theory is denoted as
BP86/TZ2P. An auxiliary set of s, p, d, f, and g STOs was used to
fit the molecular densities and to represent the Coulomb and
exchange potentials accurately in each SCF cycle.49 The EDA has
proven to give comprehensive information about the nature of the
bonding in main-group compounds and transition-metal com-
plexes.50 Details of the energy partitioning analysis can be found
in the literature.50,51
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-722578 and 722579. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Computational Details. The geometries of the molecules have
been optimized with GAUSSIAN 0337 using the gradient corrected
hybrid functional B3LYP38 in conjunction with the def2-SVP basis
sets.39 Energy minima and transition states have been verified by
calculation of the Hessian matrices.40 All stationary points which
are reported here have only real frequencies and the transition states
have one imaginary mode. Nucleus independent chemical shifts
(NICS)41 were evaluated using the gauge invariant atomic orbital
(GIAO) approach at the GIAO-B3LYP/def2-SVP//B3LYP/def2-
SVP level. Calculations of absorption spectra were accomplished
using time-dependent density functional theory (TD-DFT)42 method
at the same level. The assignment of the excitation energies to the
experimental bands was performed on the basis of the energy values
and oscillator strengths. The B3LYP Hamiltonian was chosen
because it was proven to provide accurate structures and reasonable
UV-vis spectra for a variety of chromophores43 including orga-
nometallic complexes.44
Acknowledgment. We thank the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie for financial support.
R.M.W. and A.G.C. thank the EPSRC (U.K.) for PhD studentships.
Part of the work has been carried out within the GRK 1221. We
thank Catherine Heffernan and Dr. Alan M. Kenwright for obtaining
some of the VT NMR spectra reported herein.
The atomic orbital occupations and the strength of the orbital
interactions have been computed using the natural bond orbital
(NBO) method.45 The energies associated with the two-electron
interactions have been computed according to the following
equation:
Supporting Information Available: CIF file for 6, and 7,
Cartesian coordinates and total energies of all calculated
compounds. Full list of authors for refs 37 and 46. This material
is available free of charge via the Internet at http://pubs.acs.org.
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