Synthesis and luminescent properties of lewis base-appended borafluorenes

Article abstract of DOI:10.1021/ic402408t

A series of Lewis base adducts of 9-bromo-9-borafluorene (BrBFl-LB, LB = IPr, IPrCH₂, PPh₃, and PCy₃), parent borafluorenes (HBFl-IPr and HBFl-IPrCH₂), and the bisadduct [(DMAP)₂BFl]Br were prepared and structurally characterized (IPr = [(HCNDipp)₂C:], IPrCH₂ = [(HCNDipp)₂Ci - CH₂], Dipp = 2,6-i-Pr₂C₆H₃, and DMAP = N,N-dimethylaminopyridine). The adducts BrBFl-IPr, BrBFl-PPh₃, BrBFl-PCy₃, [(DMAP)₂BFl]Br, BrBFl-IPrCH₂, and HBFl-IPrCH₂ were found to exhibit bright blue luminescence with low to moderately high quantum efficiencies (19 to 63%). Selective irradiation at different excitation wavelengths revealed the presence of two distinct emission processes in the adducts BrBFl-LB, leading to a ligand-independent, presumably borafluorene-based, blue light emission at 435 nm and another less intense emission band in the ultraviolet region (315-324 nm); [(DMAP)₂BFl]Br exhibits an emission profile that tails into the visible region. Time-dependent density functional theory studies are also included for representative borafluorene adducts. With a judicious choice of functional groups at boron, one can envisage the future generation of a whole library of 4-coordinate borafluorene-based luminogens that complement the efficient light-emitting behavior known for the widely studied boron-dipyrromethene analogues.

Full text of DOI:10.1021/ic402408t

Article
pubs.acs.org/IC
Synthesis and Luminescent Properties of Lewis Base-Appended
Borafluorenes
Christopher J. Berger, Gang He, Christian Merten, Robert McDonald, Michael J. Ferguson,
and Eric Rivard*
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: A series of Lewis base adducts of 9-bromo-9-
borafluorene (BrBFl−LB, LB = IPr, IPrCH2, PPh3, and PCy3),
parent borafluorenes (HBFl−IPr and HBFl−IPrCH2), and the
bisadduct [(DMAP)2BFl]Br were prepared and structurally
characterized (IPr = [(HCNDipp)2C:], IPrCH2
=
[(HCNDipp)2CCH2], Dipp = 2,6-i-Pr2C6H3, and DMAP =
N,N-dimethylaminopyridine). The adducts BrBFl−IPr, BrBFl−
PPh3, BrBFl−PCy3, [(DMAP)2BFl]Br, BrBFl−IPrCH2, and
HBFl−IPrCH2 were found to exhibit bright blue luminescence
with low to moderately high quantum efficiencies (19 to 63%). Selective irradiation at different excitation wavelengths revealed
the presence of two distinct emission processes in the adducts BrBFl−LB, leading to a ligand-independent, presumably
borafluorene-based, blue light emission at 435 nm and another less intense emission band in the ultraviolet region (315−324
nm); [(DMAP)2BFl]Br exhibits an emission profile that tails into the visible region. Time-dependent density functional theory
studies are also included for representative borafluorene adducts. With a judicious choice of functional groups at boron, one can
envisage the future generation of a whole library of 4-coordinate borafluorene-based luminogens that complement the efficient
light-emitting behavior known for the widely studied boron-dipyrromethene analogues.
workers.4 In addition the Jakle group has successfully
INTRODUCTION
̈
■
incorporated 3-coordinate boron environments within oligo-
meric and polymeric arrays for fluorescence-based sensing
applications (Figure 1).5 In general, sterically encumbered
substituents (e.g., Trip; Trip = 2,4,6-i-Pr3C6H2) are required at
boron to render the materials stable to oxygen and water, while
concurrently enabling the p orbitals at boron to remain
accessible for electronic interactions leading to luminescence.
In this Paper we report new, well-defined borafluorene-based
emitters containing 4-coordinate environments.1c These species
display photoluminescence in the visible spectral region and
can be considered to be molecular analogues to the well-
documented boron-dipyrromethene (BODIPY) class of
emitters (Figure 1).1a,b
The generation of light-emitting materials based upon boron
represents a widely explored concept in modern main-group
chemistry.1 One common approach in this field is to fuse low-
coordinate boron environments (BR3) with organic π frame-
works to encourage electronic communication/charge-transfer
involving accessible empty p orbitals at boron, leading to
fluorescent behavior.
Of particular relevance to the current study, Wagner and co-
workers have developed light-emitting diboraanthracenes
(Figure 1),2,3 while luminescent materials based on borafluor-
ene scaffolds have been reported by Yamaguchi and co-
RESULTS AND DISCUSSION
■
Preparation of Lewis Base Adducts of 9-Bromo-9-
borafluorene (BrBFl). The central structural motif of the
compounds reported in this paper is the planar borafluorene
array (abbreviated as BFl hereafter). We initially prepared the
haloborafluorene 9-bromo-9-borafluorene BrBFl (1) as a
potential synthon to access boron-containing polymers but
soon uncovered that coordination of this Lewis acidic species
with electron pair donors afforded adducts with bright blue
luminescence when irradiated with UV light (vide infra).
Figure 1. Selected examples of light-emitting materials containing
boron.
Received: September 23, 2013
© XXXX American Chemical Society
A
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The first molecular BrBFl adduct prepared in our study
BrBFl−IPr (2) was obtained by combining the Lewis base IPr
(IPr = [(HCNDipp)2C:]; Dipp = 2,6-i-Pr2C6H3) with 1 in
toluene (eq 1). Isolation of BrBFl−IPr (2) in high purity and
borafluorene 1. The exocyclic B−CIPr interaction in 2 is
1.639(3) Å and is of similar value to the dative B−C
interactions within known carbene adducts of heterocyclic
boron species in the literature, including the tetraphenylborole
adduct BrBC4Ph4−SIMes (1.655(3) Å; SIMes =
[(H2CNMes)2C:]).8
Intrigued by the isolation of a carbene-supported anionic
boryl heterocycle K[SIMes−BC4Ph4] by Braunschweig and co-
workers, we attempted to prepare an analogous carbene-bound
borafluorene.8b The ultimate goal was to use the resulting
nucleophilic boron center to access new borafluorene analogues
with novel optoelectronic properties. In pursuit of this goal, we
combined 2 with 2 equiv of potassium graphite (KC8) in
diethyl ether. A new, upfield-shifted, 11B{1H} NMR resonance
was noted at −19.1 ppm when the reaction mixture was
analyzed; however, this signal split into a doublet in the proton-
coupled 11B NMR spectrum (1JBH = 84 Hz), indicating the
presence of a B−H group in the final product. Multiple
attempts to reduce 2 with KC8 under different conditions in
tetrahydrofuran (THF) and toluene (−78 °C to room
temperature) gave the same boron-containing product. Treat-
ment of 2 with the milder reducing agents Na(s), sodium
naphthalenide, and 5% Na(Hg) amalgam in THF gave no
reaction. Positing that the product formed was the hydridobor-
ane adduct HBFl−IPr (3), we reacted 2 with the hydride
source Li[AlH4] (Scheme 1). As expected, the resulting
yield is straightforward as compound 2 is only sparingly soluble
in toluene and thus precipitates as it is formed in the reaction
mixture. This Lewis base coordinated borafluorene is a colorless
solid, and comprehensive characterization of this species was
achieved by a combination of NMR spectroscopy, single-crystal
X-ray crystallography (Figure 2), and elemental analysis. The
Scheme 1. Generation of the Donor-Stabilized Parent
Borafluorene HBFl−IPr via Treatment of BrBFl−IPr with
Either KC8 or Li[AlH4]
Figure 2. Thermal ellipsoid plot (30% probability) of BrBFl−IPr (2)
with hydrogen atoms and CH2Cl2 solvate molecules omitted for
clarity. Selected bond lengths (Å) and angles (deg): B−Br 2.114(2),
B−C(1) 1.639(3), B−C(51) 1.619(3), B−C(61) 1.619(3); Br−B−
C(1) 101.47(13), Br−B−C(51) 108.32(14), Br−B−C(61)
111.65(14), C(51)−B−C(61) 110.14(17), C(1)−B−C(51)
119.11(18), C(1)−B−C(61) 116.27(17).
1
presence of a coordinated IPr unit in 2 is evident from the H
and 13C{1H} NMR spectra, while a 11B NMR resonance
belonging to 2 appears at −6.4 ppm. This latter resonance is in
line with the presence of a 4-coordinate boron environment,
and this signal is positioned significantly upfield in relation to
the resonance found in the 3-coordinate borafluorene 1 (δ 65.8
ppm).6 BrBFl−IPr (2) is remarkably stable, with no visible
decomposition or melting noted up to 340 °C in the solid state
under a nitrogen atmosphere. Moreover, unlike 1, compound 2
is moisture-stable as the addition of H2O to a solution of 2 in
CDCl3 did not result in any discernible hydrolysis. In addition,
compound 2 yields bright blue luminescence when irradiated in
CH2Cl2 at 252 nm, and this effect (and the photoluminescence
from related BrBFl−LB adducts; LB = Lewis base) will be
discussed in detail later in this paper.
As anticipated from the spectral data, compound 2 contains a
4-coordinate environment about boron (Figure 2) with a
concomitantly longer B−Br bond length [2.114(2) Å] in
comparison to that found in BrBFl (1) [1.909(10) Å].7 In
addition the intraring B−C(borafluorene) distances in 2
[1.619(3) Å average] are expanded by ca. 0.1 Å relative to
the B−C endocyclic bond lengths found within the donor-free
product obtained after crystallization (27% yield) was the
colorless solid HBFl−IPr (3). This compound gave identical
spectroscopic features as the product obtained between the
reaction of 2 with KC8, including a doublet resonance at −19.1
ppm in the 11B NMR spectrum; thus, it appears that the target
borafluorene anion [IPr−BFl]− is unstable under the reaction
conditions explored and undergoes hydrogen abstraction
chemistry either with the solvent or with residual water.
Crystals of 3 suitable for X-ray analysis were grown from a
mixture of CH2Cl2 and hexanes at −35 °C, and the refined
structure is presented in Figure 3.
The metrical parameters within the borafluorene (BFl) unit
in HBFl−IPr (3) are similar to those found in the bromo
adduct 2, while the dative B−CIPr bond length in 3, 1.6232(17)
B
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NMR spectroscopy. It is salient to mention that a cationic
borafluorene was successfully prepared by Narula and Noth in
̈
the form of the acridine adduct [(acridine)BFl]AlCl4.13 With
the intention of expanding the scope of coordination chemistry
involving BrBFl (1), this borafluorene was combined with
triphenylphosphine, tricyclohexylphosphine, and N,N-dimethy-
laminopyridine (DMAP) donors, respectively. The synthesis of
the triphenylphosphine adduct of 1, BrBFl−PPh3 (5),
proceeded in a similar fashion as the N-heterocyclic carbene
analogue 2 by combining a 1:1 mixture of 1 and PPh3 in
toluene (Scheme 2). The sparingly soluble phosphine complex
Scheme 2. Preparation of the Triphenylphosphine- and
Tricyclohexylphosphine-Substituted Borafluorene Adducts
BrBFl−PPh3 (5) and BrBFl−PCy3 (6)
Figure 3. Thermal ellipsoid plot (30% probability) of HBFl−IPr (3)
with the carbon-bound hydrogen atoms omitted for clarity. Selected
bond lengths (Å) and angles (deg): B−H(1B) 1.135(14), B−C(1)
1.6232(16), B−C(51) 1.6232(17), B−C(62) 1.6258(17); H(1B)−B−
C(1) 105.8(7), H(1B)−B−C(51) 109.4(7), Br−B−C(62) 110.6(7),
C(51)−B−C(62) 99.31(9), C(1)−B−C(51) 117.85(9), C(1)−B−
C(62) 113.80(9).
Å, is only slightly shorter than the corresponding distance in 2.
The major difference in the structures of both adducts is that
the IPr unit of 3 is twisted by approximately 90° about the
boron-carbene carbon bond relative to that in 2; as a result, one
of the flanking Dipp groups within the IPr donor in 3 is
positioned above the B−H bond vector. The hydrogen atom at
boron was located in the electron difference map and
isotropically refined to yield a B−H bond length of
1.135(14) Å. The related base-stabilized borafluorenes9
Me2S−HBFl9a and t-Bu2PH−HBFl9c were recently character-
ized by X-ray crystallography, and these species have similar
overall geometric arrangements as in 3.
1
BrBFl−PPh3 (5) was isolated in a 92% yield. The H and
13C{1H} NMR spectra of 5 in CDCl3 gave expected resonances
for the borafluorene and PPh3 units, while a 4-coordinate boron
environment was identified by 11B NMR spectroscopy (δ −7.4
(br)); no discernible P−B coupling was observed, likely due to
low symmetry present at boron, leading to increased
quadrupolar broadening of the resonance. The 31P NMR
resonance of 5 (δ 1.8) is shifted slightly downfield from the
resonance due to free PPh3 (δ −5.3). Repeated attempts to
grow crystals suitable for X-ray analysis were unsuccessful;
however, additional confirmation for the formation of 5 was
provided through mass spectrometry and elemental analysis.
The synthesis of the related tricyclohexylphosphine adduct
BrBFl−PCy3 (6) proceeded in a similar manner as 5, leading to
the formation of 6 as an analytically pure colorless solid in a
87% yield. As will be detailed later, compounds 5 and 6 display
similar bright blue luminescence as 2 (vide infra).
We also investigated the formation of a base-stabilized
borafluorenium cation.10 When an equimolar ratio of Ag[OTf]
−
(OTf = triflate, F3CSO3 ) and BrBFl−IPr (2) were allowed to
react in CH2Cl2, the triflate-substituted borafluorene (TfO)-
BFl−IPr (4) was isolated as a colorless solid in a 45% yield (eq
2). 11B NMR spectroscopy (CDCl3) yields a singlet resonance
Divergent chemistry transpired between DMAP and BrBFl
(1) with respect to what was noted for Ph3P, IPr, and PCy3.
Treatment of 1 with 1 equiv of DMAP in toluene gave a crude
reaction mixture with a new DMAP-containing product (by 1H
and 13C{1H} NMR spectroscopy); however, a significant
quantity of unreacted 1 was also present. By increasing the
amount of DMAP relative to boron in the reaction mixture to a
molar ratio of 2:1, the previously observed product formed in a
much higher yield and could be readily isolated in pure form as
a colorless solid. NMR spectroscopy identified the presence of
2 equiv of DMAP relative to the BrBFl unit, while a 4-
coordinate boron environment with an upfield-shifted reso-
nance (relative to free 1) was detected at 4.6 ppm by 11B NMR
spectroscopy. X-ray crystallography performed on crystals
grown from a mixture of THF and CH2Cl2 at −35 °C
corroborated the incorporation of two DMAP donors at boron
at 2.3 ppm that is slightly downfield-positioned relative to the
11B NMR resonance observed for 2. The OTf group in 4
resonates at −77.8 ppm in the 19F NMR spectrum and is
consistent with the presence of a coordinated OTf group at
boron; for comparison the free OTf− anion in [n-Bu4N]OTf
gives a 19F NMR resonance at −78.7 ppm.11 In addition
rigorously 3-coordinate borenium cations typically have 11B
NMR chemical shifts >20 ppm,12 adding further support for a
boron-bound OTf group in 4. Unfortunately our attempts to
grow X-ray quality crystals of 4 where not successful; but,
satisfactory elemental analyses (C, H, and N) were obtained,
and a molecular ion for [(TfO)BFl−IPr]+ was detected by mass
spectrometry. Attempts to reduce the borafluorene center in 4
with KC8 invariably led to the formation of the hydridobora-
1
fluorene adduct HBFl−IPr (3) as evidenced by H and 11B
C
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to give the borafluorenium salt [(DMAP)2BFl]Br (7) with a
distorted tetrahedral geometry at boron (Figure 4). To be
Å] despite the presence of a 3-coordinate boron center in
[(acridine)BFl]+;13 this effect highlights the strong donating
ability of DMAP.
Hoping to isolate an additional borafluorene adduct with
luminescent properties, attention turned to the use of the N-
heterocyclic olefin (NHO), IPrCH2 (IPrCH2
=
(HCNDipp)2CCH2) as a Lewis base. Note that IPrCH2
has been used as a donor ligand in p-block chemistry,15,16 and
the nucleophilic/ylidic character of the terminal CH2 group is
described by the canonical forms illustrated in Scheme 4.17
Scheme 4. Representative Canonical Forms for IPrCH2
Figure 4. Thermal ellipsoid plot (30% probability) of [(DMAP)2BFl]
Br (7) with hydrogen atoms, bromide anion, and CH2Cl2 solvate
molecules omitted for clarity. Three molecules of 7 are present in the
asymmetric unit; thus, only one of these molecules is presented above.
Selected bond lengths (Å) and angles (deg) with values for the two
other molecules in the asymmetric unit in square brackets: B(1B)−
N(1B) 1.582(5) [1.584(5), 1.576(5)], B(1B)−(N3B) 1.582(5)
[1.597(5), 1.604(5)], B(1B)−C(11B) 1.609(6) [1.606(6),
1.620(6)], B(1B)−C(21B) 1.632(6) [1.606(6), 1.629(6)]; N(1B)−
B(1B)−N(3B) 108.2(3) [105.8(3), 102.2(4)], C(11B)−B(1B)−
C(21B) 100.6(3) [100.8(3), 100.2(3)], N(1B)−B(1B)−C(11B)
112.6(3) [114.0(3), 110.2(3)], N(1B)−B(1B)−C(21B) 108.6(3)
[110.8(3), 113.4(3)], N(3B)−B(1B)−C(11B) 111.8(3) [112.5(4),
114.3(4)], N(3B)−B(1B)−C(21B) 115.0(3) [113.0(3), 109.0(5)].
The direct reaction of IPrCH2 with BrBFl (1) progressed in a
somewhat complex fashion, necessitating considerable opti-
mization of the reaction conditions to generate the target
adduct BrBFl−IPrCH2 (8) in high yield. When IPrCH2 and 1
were allowed to react in a 1:1 molar ratio in toluene, a white
precipitate formed that contained BrBFl−IPrCH2 (8) (vide
infra) and the methylimidazolium salt [IPrMe]Br (9);18 the
latter species was prepared independently by bubbling HBr gas
through a solution of IPrCH2 in toluene. The soluble fraction
from the reaction of IPrCH2 and 1 contained unreacted starting
materials along with a new boron-containing product (11B
NMR resonance at 2.6 ppm). This product has been tentatively
assigned as the vinyl-substituted borafluorene (IPr=CH)BFl
(10)19 and would form via the IPrCH2-induced deprotonation
of 8, leading to the generation of the [IPrMe]Br coproduct
observed; related chemistry has been reported recently in our
group.15b Thus far, attempts to generate 10 in a definitive
fashion by reacting 1 with excess IPrCH2 have been
unsuccessful. Fortunately, if the ratio between 1 and IPrCH2
is kept at 2:1 (i.e., excess 1 according to Scheme 5), then
sparingly soluble BrBFl−IPrCH2 (8) can be recovered from
toluene in a nearly quantitative fashion. Compound 8 gives a
11B NMR resonance at 1.4 ppm, while the methylene CH2
protons within the IPrCH2 donor resonate at 2.66 ppm
discussed in more detail later, compound 7 is also photo-
luminescent in solution. Furthermore, the PPh3 donor in
BrBFl−PPh3 (5) can be readily displaced by 2 equiv of DMAP
to generate [(DMAP)2BFl]Br (7) and free PPh3 in a
quantitative fashion (Scheme 3).
Scheme 3. Preparation of the Doubly Substituted
Borafluorene-DMAP Adduct, [(DMAP)2BFl]Br (7)
Scheme 5. Synthesis of BrBFl−IPrCH2 (8)
The boron−carbon bond distances within the borafluorene
unit in 7 are the same within experimental error as those seen
in BrBFl−IPr (2) and HBFl−IPr (3) (Figures 2 and 3). The
boron−nitrogen bond distances in 7 range from 1.576(5) to
1.604(5) Å and are similar to the reported pyridine−
borafluorene N−B bond lengths found in the literature;
specifically, the t-Bu-susbtituted hydridoborafluorene adduct
[2,7-t-Bu2C6H6BH−pyridine]9b has a dative B−N bond length
of 1.564(5) Å, while the azidoborafluorene complex N3BFl−
pyridine14 has a B−N distance of 1.619(2) Å. Interestingly the
B−N distance within Noth’s acridine borafluorenium adduct
̈
[(acridine)BFl]+ is slightly elongated relative to 7 [1.650(11)
D
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(CDCl3) in the 1H NMR spectrum. For comparison, the
IPrCH2 aminoborane adduct IPrCH2−BH2NMe2BH3 has a
CH2 resonance at 1.96 ppm (CDCl3). Structural authentication
of 8 was provided by single-crystal X-ray diffraction, and the
refined structure is presented in Figure 5.
Scheme 6. Formation of the Amidoborafluorene
FlB[N(SiMe3)2] (11) by Two Different Routes
Figure 5. Thermal ellipsoid plot (30% probability) of BrBFl−IPrCH2
(8) with all hydrogen atoms, except those at C(2), and CH2Cl2 solvate
molecules omitted for clarity. Selected bond lengths (Å) and angles
(deg): B−Br 2.124(2), B−C(2) 1.660(3), B−C(51) 1.609(3), B−
C(62) 1.610(3), C(1)−C(2) 1.473(2); C(51)−B−C(62) 100.29(15),
C(51)−B-Br 106.29(12), C(62)−B−Br 106.94(13), C(51)−B−C(2)
115.38(16), C(62)−B−C(2) 116.19(15), Br−B−C(2) 110.75(12),
B−C(1)−C(2) 121.83(15).
potentially π-donating −N(SiMe3)2 group at boron in 11
reduces the electrophilicity (and Lewis acidity) of the
borafluorene unit, preventing coordination of IPrCH2 from
transpiring.
Drawing inspiration from the successful synthesis of HBFl−
IPr via Br/H exchange (Scheme 1), we prepared the
nucleophilic olefin-capped borafluorene HBFl−IPrCH2 (12)
by combining BrBFl−IPrCH2 (8) with an excess of Li[AlH4] in
CH2Cl2 (Scheme 7). Proton-coupled 11B NMR spectroscopy
As noted previously, the metrical parameters within the
borafluorene (BFl) units are rather insensitive to the nature of
the Lewis base(s) coordinated to boron. The B−CH2 bond
length in BrBFl−IPrCH2 (8) (1.660(3) Å) is slightly elongated
compared to the analogous B−CIPr bond in 2 (1.639(3) Å),
and is similar in value to the corresponding bond lengths in the
IPrCH2−borane adducts prepared in our group (e.g., B−C
bond length of 1.659(3) Å in IPrCH2−H2BNMe2−BH3).15c
The C(1)−C(2) bond length within the IPrCH2 donor in 8
[1.473(2) Å] approaches the length expected for a carbon−
carbon single bond (approximately 1.54 Å), indicating that π
electron density in IPr=CH2 is being drawn away to form a new
B−C coordinative bond in 8; for comparison, the related
carbon−carbon distance in free IPrCH2 is 1.331(8) Å
(average).20
Scheme 7. Synthesis of the IPrCH2-Capped Parent
Borafluorene Adduct HBFl−IPrCH2 (12) and Instability of
This Species in Solution
Attempts were made to eliminate HBr from 8 to form the
alkenyl-substituted borafluorene (IPr=CH)BFl 10. When the
potential Brønsted base, IPr, was combined with 8, it was
discovered that IPr had exchanged with IPrCH2 at boron,
resulting in the formation of BrBFl−IPr (2) and free IPrCH2
(Scheme 6). This observation is in line with previous chemistry
from our group illustrating that IPrCH2 is a weaker donor than
IPr.15a,c Addition of the weakly nucleophilic base K[N(SiMe3)2]
to 8 gave a new boron-containing product and free IPrCH2 in
the crude reaction mixture (by 1H and 11B NMR spectroscopy).
The new product had a 11B NMR resonance at 54.2 ppm (in
CDCl3), which strongly suggested the presence of a 3-
identified the presence of a B−H unit (doublet at −15.1; 1JBH
=
82 Hz), while spectral features for the remaining borafluorene
unit were identifiable by 1H, 11B, and 13C{1H} NMR
spectroscopy. Attempts to grow crystals suitable for X-ray
analysis from a mixture of CH2Cl2 and hexanes exclusively gave
the known salt [IPrMe]Cl15b as a crystalline product. To verify
if HBFl−IPrCH2 (12) was decomposing during the above-
mentioned crystallization attempts, a solution of 12 in CH2Cl2
was stirred for 3 d in the absence of Li[AlH4]. Analysis of this
product showed that the [IPrMe]Cl salt was present (ca. 70%
by 1H NMR), along with several unidentified boron-containing
compounds. Elemental analyses on 12 routinely gave low values
for carbon; thus, copies of the NMR spectra for 12 have been
deposited as part of the Supporting Information.18
1
coordinate boron center. Given that well-defined H, 13C{1H},
and 29Si{1H} resonances were present for both the borafluorene
and N(SiMe3)2 moieties, the product (red semisolid) was
formulated as the amidoborafluorene FlB[N(SiMe3)2] (11).18
Furthermore, this product can be prepared from the direct
reaction of K[N(SiMe3)2] with BrBFl 1 (Scheme 6). These
results show that the presence of a sterically encumbered and
Photophysical Properties of BrBFl−IPr (2), BrBFl−PPh3
(5), BrBFl−PCy3 (6), [(DMAP)2BFl]Br (7), BrBFl−IPrCH2 (8),
and HBFl−IPrCH2 (12). As stated earlier, the adducts in this
study, BrBFl−IPr (2), BrBFl−PPh3 (5), BrBFl−PCy3 (6),
[(DMAP)2BFl]Br (7), and XBFl−IPrCH2 (X = Br and H; 8
E
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and 12), exhibit bright blue luminescence in halogenated
solvents when irradiated with ultraviolet (UV) light (λexcit.
=
252 nm for compounds 2, 5, 6, 8, and 12; λexcit. = 320 nm for
compound 7). Figures 6 and 7 show the ultraviolet−visible
Figure 6. (top panel) UV−vis absorbance spectrum (solid line) of
BrBFl−PPh3 (5) in CH2Cl2, with the calculated spectrum (dotted
line) as derived from TD-DFT. The calculated line spectrum reflecting
the oscillator strengths for the various transitions present is also
shown. (bottom panel) Fluorescence excitation (dashed line) and
emission spectrum (solid line) for compound 5. λexcit. = 252 nm.
Figure 7. (top panel) UV−vis absorbance spectrum (solid line) of
BrBFl−IPrCH2 (8) in CH2Cl2, with the calculated spectrum (dotted
line) as derived from TD-DFT. The calculated line spectrum reflecting
the oscillator strengths for the various transitions present is also
shown. (bottom panel) Fluorescence excitation (dashed line) and
emission spectrum (solid line) for compound 8. λexcit. = 252 nm.
(UV−vis) absorbance and fluorescence excitation/emission
spectra for two representative adducts BrBFl−PPh3 (5) and
BrBFl−IPrCH2 (8), while the related UV−vis and emission
data for BrBFl−IPr (2), BrBFl−PCy3 (6), [(DMAP)2BFl]Br
(7), and HBFl−IPrCH2 (12) have been included as part of the
Supporting Information.18 Initially we expected the lumines-
cence in our borafluorene adducts to be quenched due to the
lack of a vacant p orbital on boron to participate in
conjugation/emission, as noted previously.3 However after
surveying the literature it was noted that Yamaguchi’s
borafluorenes TripBFl′ (Figure 1) show blue luminescence
when placed in dimethylformamide (DMF) solvent;4 thus, it
now appears that their blue luminescent species could be 4-
coordinate borafluorene TripBFl′−DMF adducts of similar
structure to the BrBFl−LB complexes presented in this Study.21
The UV−vis absorbance maxima for compounds 2, 5−8, and
12 each occur between 240 and 290 nm, while tailing of the
absorption profile to ca. 300 nm was noted in the BrBFl−LB
adducts. The corresponding molar absorptivity coefficients for
each major absorbance band were large (0.53−2.84 × 103 M−1
cm−1) and increased with respect to the Lewis base in the order
IPr < DMAP < PCy3 < PPh3 < IPrCH2. Most striking was the
presence of nearly identical emission bands at 435 nm for each
of the monoadducts (BrBFl−LB) 2, 5, 6, 8, and 12 when
irradiated at 252 nm in CH2Cl2. A second emission band was
seen in each complex from 315 to 325 nm that was much less
intense than the peak at 435 nm than the peak at 435 nm. The
invariant nature of the positions of these emission bands, as the
Lewis base (LB) bound at boron is varied, strongly suggests
that the fluorescence occurs from the borafluorene unit without
significant participation from the accompanying Lewis basic
donors (vide infra).22 The structurally distinct DMAP complex
[(DMAP)2BFl]Br (7) also yielded a discernible emission
spectra with a maxima of ca. 355 nm (λexcit = 320 nm) with
significant tailing into the violet/blue region (Supporting
Information, Figure S11). The quantum yields, measured
relative to quinine sulfate in 1 N H2SO4, were found to be 19,
50, 31, 63, and 19% for BrBFl−IPr (2), BrBFl−PPh3 (5),
BrBFl−PCy3 (6), BrBFl−IPrCH2 (8), and HBFl−IPrCH2
(12), respectively; the fluorescence data for 12 should be
interpreted with great caution, as this species consistently
contained an unidentifiable impurity, despite repeated
recrystallization attempts.18 The quantum yield of
[(DMAP)2BFl]Br (7) in DCM is 51%, relative to naphthalene
in cyclohexane. These quantum yields were found to be in
agreement with those obtained from BODIPY derivatives with
4-coordinate boron centers.1b,23 To our knowledge, this is the
first report of well-defined fluorescent Lewis base−borafluorene
adducts, and given that the luminescent compounds contain
reactive boron−bromine bonds, functionalization at boron
should be possible to generate substituted borafluorenes with
potentially tunable light-emitting properties.
The luminescent borafluorene systems reported by Yama-
guchi and co-workers have 3-coordinate boron centers leading
to significant pπ−π* conjugation in the LUMO states involving
the vacant p orbital of boron and the π* manifold of the
ancillary biaryl moiety.4,24 The corresponding HOMO levels
are localized on the aromatic C atoms of the borafluorene unit.
Upon coordination of an incoming donor species (i.e., F− or
CN−) the delocalization of the LUMO throughout the pπ−π*
F
dx.doi.org/10.1021/ic402408t | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
system is disrupted, leading to the emergence of a new
fluorescence profile accompanied by a dramatic fluorescence
color change.4 Noting the possible parallel between Yamagu-
chi’s donor-bound borafluorenes and our stable 4-coordinate
adducts, we performed TD-DFT studies (B3LYP/6-31G(d,p))
to gain added insight into the electronic transitions involved in
the UV−vis spectra of the borafluorene adducts 5, 7, and 8
(Figures 7 and 8).
that compounds 5 and 8 show nearly identical emission profiles
(Figures 7 and 8), despite the presence of distinct PPh3 and
IPrCH2 donors, respectively, the search for identifiable π to π*
absorptions on the borafluorene units was conducted. As
illustrated in Figures 7 and 8, this search was complicated due
to the vast numbers of electronic transitions present in BrBFl−
PPh3 (5) and BrBFl−IPrCH2 (8), including those that are
entirely based on the Lewis basic PPh3 and IPrCH2 units and
those involving orbital participation from both a BFl unit and
the adjacent donor. At this stage we have not been able to
directly identify which specific absorption events (from 250 to
325 nm) are leading to luminescence.18 To highlight the
complicated nature of the calculated absorption profile in
BrBFl−IPrCH2 (8), selected transitions, corresponding to S1,
S7, S8, and S9 in Figure 7a, are presented (see Figure 9). We did
note that the strong emission feature at 435 nm in each adduct
(leading to blue luminescence) was absent when the BrBFl−LB
compounds were irradiated at longer wavelengths in the range
of 285−305 nm. Thus, the high-energy absorptions centered at
250 nm are leading to blue light emission; in addition, the same
long-wavelength excitation led to the retention of the minor
intensity emission band at 315−325 nm for compounds 2, 5, 8,
and 12. In each case, irradiation at a longer wavelength (450
nm) did not yield any discernible luminescence. Lastly, the
Cy3P adduct BrBFl−PCy3 (6) was prepared to see if donor-
based π* orbitals were directly involved in luminescence.
Compound 6 gave similar emission data as the corresponding
IPr, IPrCH2, and PPh3 adducts, lending added support for the
exclusive participation of the borafluorene unit in emission;
however, excitation could involve the occupation of ligand-
based Sn (n > 1) excited states that decay nonradiatively to
lower emissive states.
The absorption spectrum for the DMAP adduct
[(DMAP)2BFl]Br (7) was also evaluated computationally by
TD-DFT, with good agreement between the experimental and
computational data (see Supporting Information, Figure
S11).18 Because of a relatively lower number of electronic
transitions in 7, a more detailed analysis could be conducted.
Two major absorptions near 280 nm were identified by TD-
DFT, and in each band, contributions from DMAP (π to π*)
localized transitions are mixed with borafluorene (π) to DMAP
(π*) electronic transitions and states involving orbital
population from both fragments. In addition, some contribu-
tion associated with borafluorene-localized π−π* transitions are
present; however, as with compounds 5 and 8, a clear path
toward emission cannot be discerned at this time.
Figure 8. Computationally derived HOMO and LUMO plots for (a)
BrBFl−PPh3 (5) and (b) [(DMAP)2BFl]Br (7).
Figure 9. Selected electronic transitions computed for BrBFl−IPrCH2
(8); see the Supporting Information for a detailed listing.18
CONCLUSIONS
■
A series of complexes featuring electron-deficient borafluorene
units was prepared and comprehensively characterized.
Specifically bright blue luminescence was noted in BrBFl−IPr
(2), BrBFl−PPh3 (5), BrBFl−PCy3 (6), [(DMAP)2BFl]Br (7),
and XBFl−IPrCH2 (X = Br and H; 8 and 12), with quantum
efficiencies as high as 63% noted. Furthermore, computational
studies (TD-DFT) were used to analyze the electronic
transitions available to these species, and the invariant nature
of the emission profiles in the series BrBFl−LB suggests that
the borafluorene unit is largely responsible for the observed
luminescence. This study is significant as it involves the
syntheses of a new class of well-defined tetravalent organo-
borane emitter, and future work will be geared toward
modifying the anionic substituent at boron (by replacing Br
The calculated HOMO and LUMO plots for compounds 5,
7, and 8 are shown in Figures 8 and 9. In each compound, the
HOMO levels were of C−C π-bonding character and located
entirely on the borafluorene biaryl units; the LUMO states were
exclusively positioned on the Lewis bases and had C−C or C−
N π* character. However, these charge-transfer HOMO−
LUMO transitions do not appear to be important in dictating
the luminescence of compounds 5, 7, and 8, as the oscillator
strengths were found to be zero in all cases (see Supporting
Information for a summary of selected calculated electronic
transitions computed);18 thus, the light absorption leading to
luminescence involved alternate electronic transitions. Given
G
dx.doi.org/10.1021/ic402408t | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Crystallographic Data for Compounds 2, 3, and 7−9
2·CH2Cl2
3
7·1/3 CH2Cl2·1/2 THF
8·2 CH2Cl2
9·2 CH2Cl2
formula
formula wt
cryst. dimens. (mm)
cryst. syst.
space group
unit cell
a (Å)
C40H46BBr Cl2N2
716.41
C39H45BN2
552.58
C28.33H32.67BBr Cl0.67N4O0.5
551.60
C42H50BBr Cl4N2
815.36
C30H43Br Cl4N2
653.37
0.41 × 0.24 × 0.23
monoclinic
P21/c
0.45 × 0.42 × 0.35
monoclinic
P21/n
0.30 × 0.26 × 0.18
monoclinic
0.36 × 0.14 × 0.11
monoclinic
P21/n
0.31 × 0.13 × 0.07
triclinic
C2/c
P1
̅
11.4012 (4)
21.2295 (8)
15.8472 (6)
11.3096 (4)
14.8357 (6)
20.1829 (8)
29.9685 (7)
12.7445 (3)
43.8064 (10)
12.5593 (9)
22.5211 (15)
15.1550 (10)
11.6745 (3)
12.1104 (3)
12.2440 (3)
79.3360 (10)
85.4340 (10)
89.7850 (10)
1695.69 (7)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
104.2470 (10)
91.3286 (4)
101.145 (3)
103.6023 (5)
4
3282.3 (2)
4
16726.6 (7)
24
4205.7 (5)
4
Z
ρcalcd (g cm−3
)
1.276
1.118
1.314
1.288
1.280
μ (mm−1
)
1.276
0.064
1.564
3.895
4.698
T (K)
173(1)
52.82
173(1)
52.80
173(1)
52.95
173(1)
140.43
27 177
7844 (0.0217)
7335
173(1)
2θmax (°)
total data
136.70
29 726
7649 (0.0406)
6154
26 087
6735 (0.0214)
5571
66 493
11 375
unique data (Rint)
observed data [I > 2σ(I)]
params.
17236 (0.0370)
11962
5928 (0.0125)
5299
415
383
897
452
362
a
R1 [I > 2σ(I)]
0.0374
0.0940
0.782/−0.886
0.0400
0.1090
0.0659
0.0350
0.0914
0.877/−0.625
0.0419
a
wR2 [all data]
difference map, Δρ (e Å−3
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo2 − Fc2)2/∑ w(Fo )]1/2
0.1996
0.1166
)
0.217/−0.183
1.510/−1.198
0.674/−0.533
a
4
adducts were measured relative to quinine sulfate in 1 N H2SO4 (Φ =
0.55).28b
with a variety of nucleophiles) to develop new air-stable
materials for OLED technologies.1
X-ray Crystallography. Crystals of suitable quality for X-ray
crystallography were removed from a vial in a glovebox and coated
immediately with a thin layer of hydrocarbon oil (Paratone-N). A
suitable crystal was picked and mounted on a glass fiber and then
quickly placed in a low-temperature stream of nitrogen on the X-ray
diffractometer.29 All data were collected using a Bruker APEX II CCD
detector/D8 diffractometer with Mo Kα radiation or Cu Kα radiation,
with crystals cooled to −100 °C. The data were corrected for
absorption30 through Gaussian integration [BrBFl−IPr (2), [IPrMe]Br
(9), BrBFl−IPrCH2 (8)] or multiscan SADABS31 (HBFl−IPr (3))
from the indexing of the crystal faces. Structures were solved using
direct methods SHELXD ([IPrMe]Br (9)), SHELXS-97 (HBFl−IPr
(3), BrBFl−IPrCH2 (8)), intrinsic phasing SHELXT32
([(DMAP)2BFl]Br (7)), or Patterson/structure expansion facilities
within the DIRDIF-200833 program suite (BrBFl−IPr (2)) and refined
using SHELXS-97.32 Hydrogen atoms were assigned positions based
on the sp2 or sp3 hybridization geometries of their attached carbon
atoms and were given thermal parameters 20% greater than those of
their parent atoms. Table 1 contains selected X-ray crystallographic
data for each of the reported compounds.
Special Refinement Conditions. Compound 7. Attempts to
refine peaks of residual electron density as disordered or partial-
occupancy solvent tetrahydrofuran oxygen or carbon atoms were
unsuccessful. The data were corrected for disordered electron density
through use of the SQUEEZE procedure34 as implemented in
PLATON.35 A total solvent-accessible void volume of 2100.4 Å3
with a total electron count of 448 (consistent with 12 molecules of
solvent THF, or one-half THF molecule per formula unit of 7, found
in the unit cell. Restraints were applied to distances involving the
disordered 4-dimethylaminopyridine group: d(N3C−C41C) = d-
(N3C−C45C) = d(N3D−C41D) = d(N3D−C45D) = 1.36(1) Å;
d(N3C−B1C) = d(N3D−B1C) = 1.59(1) Å; d(N4C−C46C) =
d(N4C−C47C) = d(N4D−C46D) = d(N4D−C47D) = 1.45(1) Å;
d(C41C−C42C) = d(C44C−C45C) = d(C41D−C42D) = d(C44D−
C45D) = 1.36(1) Å; d(C42C−C43C) = d(C43C−C44C) =
EXPERIMENTAL SECTION
■
General. Author: All reactions were performed using standard
Schlenk line techniques under an atmosphere of nitrogen or in an inert
atmosphere glovebox (MBraun Inc.). Solvents were dried using a
Grubbs-type solvent purification system25 manufactured by Innovative
Technology Inc., degassed (freeze−pump−thaw method), and stored
over molecular sieves under a nitrogen atmosphere prior to use. Boron
tribromide (1.0 M solution in hexanes), allylmagnesium bromide (1.0
M solution in diethyl ether), n-butyllithium (n-BuLi, 2.5 M solution in
hexanes), N,N-dimethylaminopyridine (DMAP), Li[AlH4], K[N-
(SiMe3)2], Ag[OTf], PPh3, and Cy3P were obtained from Aldrich
and used as received. 1,2-Dibromobenzene was purchased from Matrix
Scientific and used as received. Hydrogen bromide gas was purchased
from Matheson Gas Products Canada and used as received.
[Rh(COD)Cl]2 was purchased from Strem Chemicals, Inc. and used
as received. 1,3-Bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr),26
1,3-bis-(2,6-diisopropylphenyl)-2-methyleneimidazoline (IPrCH2),15a
and 2,2′-dibromobiphenyl27 were prepared following literature
procedures. 11B{1H}, 11B, 19F{1H}, and 31P NMR spectra were
recorded on a Varian iNova-500 spectrometer referenced externally to
F3B−OEt2 (11B), CFCl3 (19F{1H}), and 85% H3PO4 (31P),
1
1
respectively. H, H{11B}, and 13C{1H} NMR spectra were recorded
on a Varian VNMRS-500 spectrometer and referenced externally to
SiMe4. Elemental analyses were performed by the Analytical and
Instrumentation Laboratory at the University of Alberta. Mass spectra
were obtained on an Agilent 6220 spectrometer. Melting points were
measured in a sealed glass capillary under nitrogen using a Meltemp
melting point apparatus and are uncorrected. UV/vis spectra were
obtained from a Cary 400 UV/vis spectrometer. Photoluminescence
spectra were obtained from a Photon Technology International (PTI)
MP1 Fluorescence System. The quantum yield of [(DMAP)2BFl]Br
(7) was measured relative to naphthalene in cyclohexane, assuming a
quantum yield (Φ) of 0.23;28a the quantum yields for the remaining
H
dx.doi.org/10.1021/ic402408t | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
13C{1H} NMR (C6D6, 125 MHz): δ 149.7 (ArC), 146.2 (ArC),
136.1 (ArC), 134.3 (ArC), 131.1 (ArC), 129.1 (ArC), 126.5
(ArC), 125.1 (ArC), 124.8 (ArC), 118.7 (−N−CH−), 30.2
(CH(CH3)2), 29.3 (CH(CH3)2), 26.0 (CH(CH3)2), 22.2
(CH(CH3)2). 11B{1H} NMR (C6D6, 160 Hz): δ −6.4 (s).
UV/vis (in CH2Cl2): λmax = 272 nm (ε = 5.3 × 103 L mol−1
cm−1). Luminescence emission (in CH2Cl2): λem = 324, 435
nm, luminescence quantum yield: Φ = 0.19, relative to quinine
sulfate in 1 N H2SO4. HR-MS EI (positive mode, m/z): Calcd.
for [M−Br]+: 551.35974. Found: 551.35892 (Δppm = 1.5).
Anal. Calcd. for C39H44BBrN2: C, 74.18; H, 7.02; N, 4.44.
Found: C, 73.53; H, 7.17; N, 4.28%. Mp (°C): no melting or
decomposition up to 340.
d(C42D−C43D) = d(C43D−C44D) = 1.41(1) Å. Distances within
the disordered solvent CH2Cl2 molecule were restrained during
refinement: d(Cl1S−C1S) = d(Cl2S−C1S) = d(Cl3S−C2S) =
d(Cl4S−C2S) = 1.75(1) Å; d(Cl1S−−−Cl2S) = d(Cl3S−−−Cl4S)
= 2.85(1) Å.
Compound 12. The C−Cl distances (C2S−Cl3S, C2S−Cl4S,
C3S−Cl5S, C3S−Cl6S) within the disordered dichloromethane
solvent molecule were restrained to be the same.
Theoretical Studies. All calculations were carried out using the
Gaussian 09 software package at the B3LYP/6-31G(d,p) level of
theory.36 Implicit solvent effects were taken into account by using the
integral equation formalism version of the polarizable continuum
model (IEF-PCM) for dichloromethane.37 If available, crystal
structures were used as input geometries. Default convergence criteria
were chosen for the geometry optimizations. The obtained optimized
geometries were confirmed to be local energy minimum structures by
performing vibrational frequency analysis. For the calculation of the
vertical excitation spectra using TD-DFT, nonequilibrium state-
specific solvation of the excited states were accounted for. The
reported transition wavelengths were shifted by ca. 25 nm to higher
wavelengths than the calculated values. The electronic absorbance
spectra were then obtained by assigning a uniform Gaussian band
shape of 0.3 eV half-width at 1/e− height. The molecular orbitals were
extracted from the output using GaussView 5 and assuming an isovalue
of 0.02.
Synthesis of HBFl−IPr (3). To a mixture of compound 2
(102.2 mg, 0.162 mmol) and Li[AlH4] (24.5 mg, 0.646 mmol)
was added 15 mL of toluene. The resulting mixture was allowed
to stir for 5 days. The mixture was then filtered through a small
(ca. 1 cm) plug of silica gel. The volatile components were
removed from the filtrate in vacuo, and the remaining crude
solid was washed with 5 mL of diethyl ether to yield product 3
as a white solid (24.2 mg, 27%). Crystals suitable for X-ray
analysis were grown from a 50/50 mixture of dichloromethane
and hexanes at −35 °C.
1H NMR (CDCl3, 500 MHz): δ 7.34 (d, 3JHH = 6.5 Hz, 2H,
ArH), 7.28 (d, 3JHH = 7.5 Hz, 2H, ArH), 7.23 (d, 3JHH = 6.0 Hz,
4H, ArH), 7.10 (s, 2H, −N−CH−), 7.08 (d, 3JHH = 8.0 Hz, 4H,
SYNTHETIC PROCEDURES
■
Synthesis of 9-Bromo-9-borafluorene (BrBFl, 1). A
solution of 2,2′-dibromobiphenyl (3.140 g, 10.06 mmol) in
toluene (50 mL) was prepared and sparged with N2. The
solution was then cooled to −78 °C, and n-BuLi (14.0 mL, 22
mmol, 1.6 M solution in hexanes) was added dropwise. The
resulting mixture was allowed to slowly warm to room
temperature and stirred for 48 h. The mixture was then cooled
to −78 °C before a solution of BBr3 (11.0 mL, 11 mmol, 1.0 M
solution in hexanes) was added dropwise. The resulting mixture
was allowed to slowly warm to room temperature and stirred
for 12 h. The supernatant was decanted, and the remaining
precipitate was extracted with 3 × 50 mL of toluene; the
toluene extracts were combined with the initial supernatant
before the volatile components were then removed in vacuo.
The crude product was recrystallized from hexanes at −35 °C
to afford the product 1 as yellow needles (2.208 g, 90%).
1H NMR (CDCl3, 500 MHz): δ 7.56 (d, 3JHH = 7.2 Hz, 2H,
3
4
ArH), 6.94 (td, JHH = 7.5 Hz, JHH = 1.5 Hz, 2H, ArH), 6.90
(td, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, 2H, ArH), 2.81 (septet, 3JHH
= 7.0 Hz, 4H, CH(CH3)2), 1.17 (d, JHH = 7.0 Hz, 12H,
CH(CH3)2), 1.11 (d, JHH = 6.5 Hz, 12H, CH(CH3)2).
3
3
13C{1H} NMR (CDCl3, 125 MHz): δ 149.2 (ArC), 145.2
(ArC), 134.2 (ArC), 131.4 (ArC), 130.1 (ArC), 124.9 (ArC),
124.1 (ArC), 123.5 (ArC), 118.4 (−N−CH−), 29.8 (CH-
(CH3)2), 29.0 (CH(CH3)2), 26.4 (CH(CH3)2), 22.3 (CH-
(CH3)2). 11B{1H} NMR (CDCl3, 160 MHz): δ −19.1 (s). 11B
NMR (CDCl3, 160 MHz): δ −19.1 (d, 1JBH = 84 Hz). HR-MS
EI (positive mode, m/z): Calcd. for [M]+: 552.36755. Found:
552.36558 (Δppm = 3.6). Anal. Calcd. for C39H45BBrN2: C,
84.77; H, 8.21; N, 5.07. Found: C, 84.77; H, 8.30; N, 5.13%.
Mp (°C): 308−310.
Synthesis of [BFl−IPr]OTf (4). To a mixture of compound
2 (500.1 mg, 0.792 mmol) and Ag[OTf] (203.1 mg, 0.791
mmol) was added 15 mL of CH2Cl2. The reaction mixture was
protected from light and allowed to stir for 12 h. The resulting
mixture was filtered and all volatiles were removed from the
filtrate in vacuo to afford the crude product as a pale yellow
solid. The crude product was recrystallized from 10 mL of a
50/50 mixture of dichloromethane/hexanes at −35 °C to afford
product 4 as a colorless solid (250.9 mg, 45%).
3
4
ArH), 7.37 (td, JHH = 7.6 Hz, JHH = 1.2 Hz, 2H, ArH), 7.33
(d, 3JHH = 7.0 Hz, 2H, ArH), 7.15 (td, 3JHH = 7.0 Hz, 4JHH = 1.2
Hz, 2H, ArH). 13C{1H} NMR (CDCl3, 125 MHz): δ 152.9
(ArC), 135.5 (ArC), 133.4 (ArC), 128.7 (ArC), 119.7 (ArC).
11B{1H} NMR (CDCl3, 160 MHz): δ 65.8 (s).6
Synthesis of BrBFl−IPr (2). To a mixture of compound 1
(503.9 mg, 2.074 mmol) and IPr (804.4 mg, 2.070 mmol) was
added 20 mL of toluene. The mixture was allowed to stir for 12
h, during which time a colorless precipitate formed. The
supernatant was decanted, the remaining solid was washed with
3 × 20 mL of toluene, and the insoluble solid was dried under
vacuum to afford product 2 as a white powder (1.218 g, 93%).
Crystals suitable for X-ray analysis were grown from a 50/50
mixture of dichloromethane and hexanes at −35 °C.
3
1H NMR (CDCl3, 500 MHz): δ 7.35 (t, JHH = 7.6 Hz, 2H,
ArH), 7.27 (d, 3JHH = 6.8 Hz, 2H, ArH), 7.15 (d, 3JHH = 7.6 Hz,
2H, ArH), 7.11 (s, 2H, −N−CH−), 7.10 (d, 3JHH = 7.6 Hz, 4H,
3
4
ArH), 7.02 (td, JHH = 7.2 Hz, JHH = 1.2 Hz, 2H, ArH), 6.96
(td, 3JHH = 7.2 Hz, 4JHH = 1.2 Hz, 2H, ArH), 2.68 (septet, 3JHH
3
= 6.8 Hz, 4H, CH(CH3)2), 1.22 (d, JHH = 6.8 Hz, 12H,
3
3
1H NMR (C6D6, 500 MHz): δ 7.43 (d, JHH = 7.6 Hz, 2H,
CH(CH3)2), 1.09 (d, JHH = 6.8 Hz, 12H, CH(CH3)2).
13C{1H} NMR (CDCl3, 125 MHz): δ 150.0 (ArC), 144.6
(ArC), 134.0 (ArC), 131.0 (ArC), 130.7 (ArC), 127.9 (ArC),
126.5 (ArC), 125.4 (ArC), 123.8 (ArC), 119.0 (−N−CH−),
29.8 (CH(CH3)2), 29.2 (CH(CH3)2), 26.6 (CH(CH3)2), 22.0
(CH(CH3)2). 11B{1H} NMR (CDCl3, 160 MHz): δ 2.3 (s).
19F{1H} NMR (CDCl3, 376 MHz): δ −77.8 (s). HR-MS EI
ArH), 7.25 (t, 3JHH = 7.6 Hz, 2H, ArH), 7.01 (d, 3JHH = 7.6 Hz,
3
4
4H, ArH), 6.97 (td, JHH = 7.6 Hz, JHH = 1.2 Hz, 2H, ArH),
6.57 (td, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 2H, ArH), 6.40 (d, 3JHH
= 7.2 Hz, 2H, ArH), 6.28 (s, 2H, −N−CH−), 2.79 (septet, 3JHH
3
= 6.8 Hz, 4H, CH(CH3)2), 1.08 (d, JHH = 6.4 Hz, 12H,
3
CH(CH3)2), 0.90 (d, JHH = 7.2 Hz, 12H, CH(CH3)2).
I
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(positive mode, m/z): Calcd. for [M]+: 700.31177. Found:
700.31221 (Δppm = 0.6). Anal. Calcd. for C40H44BF3N2O3S:
C, 68.57; H, 6.33; N, 4.00. Found: C, 68.35; H, 6.43; N, 4.03%.
Mp (°C): 312 (decomposition).
1H NMR (CDCl3, 500 MHz): δ 8.05 (m, 4H, ArH in
DMAP), 7.64 (dt, 3JHH = 7.5 Hz, 4JHH = 0.5 Hz, 2H, ArH), 7.49
3
4
3
(dt, JHH = 7.5 Hz, JHH = 0.5 Hz, 2H, ArH), 7.31 (td, JHH
=
7.5 Hz, 4JHH = 1.0 Hz, 2H, ArH), 7.20 (td, 3JHH = 7.5 Hz, 4JHH
= 1.0 Hz, 2H, ArH), 6.80 (m, 4H, ArH in DMAP), 3.16 (s,
12H, N(CH3)2 in DMAP). 13C{1H} NMR (CDCl3, 125 MHz):
δ 156.3 (ArC), 149.2 (ArC), 143.4 (ArC), 130.2 (ArC), 128.9
(ArC), 128.2 (ArC), 127.5 (ArC), 125.3 (ArC), 120.0 (ArC),
40.1 (N(CH3)2). 11B{1H} NMR (CDCl3, 160 MHz): δ 4.6 (s).
UV/vis (in CH2Cl2): λmax = 320 nm (ε = 1.85 × 104 L mol−1
cm−1). Luminescence emission (in CH2Cl2): λem = 355 nm,
luminescence quantum yield: Φ = 0.42, relative to naphthalene
in cyclohexane. HR-MS ES (positive mode, m/z): [M−Br]+;
407.2399. Anal. Calcd. for C26H28BBrN4: C, 64.09; H, 5.79; N,
11.50. Found: C, 65.30; H, 5.81; N, 10.31%. Mp (°C): 144−
147.
Synthesis of BrBFl−PPh3 (5). To a mixture of compound
1 (93.1 mg, 0.383 mmol) and PPh3 (100.0 mg, 0.381 mmol)
was added 15 mL of toluene. The resulting mixture was allowed
to stir for 12 h. All volatile components were removed in vacuo
and the remaining solid was washed with 10 mL of hexanes and
dried before the product 5 was isolated as a white solid (176.7
mg, 92%).
1H NMR (CDCl3, 500 MHz): δ 7.57 (dd, 3JHP = 9.2 Hz, 3JHH
= 7.5 Hz, 6H, ArH in PPh3), 7.49 (td, 3JHH = 7.0 Hz, 4JHH = 1.5
Hz, 2H, ArH), 7.42 (d, 3JHH = 7.5 Hz, 2H, ArH), 7.37−7.32 (m,
3
4
9H, ArH in PPh3), 7.14 (tt, JHH = 7.5 Hz, JHH = 1.0 Hz, 2H,
3
4
ArH), 6.99 (td, JHH = 7.0 Hz, JHH = 1.0 Hz, 2H, ArH).
13C{1H} NMR (CDCl3, 125 MHz): δ 148.8 (d, JCP = 7.0 Hz,
3
Synthesis of BrBFl−IPrCH2 (8). To a mixture of
compound 1 (504.5 mg, 2.08 mmol) and IPrCH2 (412.7 mg,
1.03 mmol) was added 20 mL of toluene. The mixture was
allowed to stir for 72 h, during which time a colorless
precipitate formed. The supernatant was decanted, and the
remaining solid was washed with 3 × 15 mL of toluene. All
volatile components were removed from the solid in vacuo to
afford compound 8 as a white powder (638.5 mg, 96%).
Crystals suitable for X-ray analysis were grown from a 50/50
mixture of dichloromethane and hexanes at −35 °C.
3
5
ArC), 134.2 (d, JCP = 8.2 Hz, ArC), 132.2 (d, JCP = 1.8 Hz,
ArC), 131.8 (d, JCP = 2.5 Hz, ArC), 128.6 (d, JCP = 10.3 Hz,
4
2
4
4
ArC), 127.8 (d, JCP = 2.0 Hz, ArC), 126.5 (d, JCP = 2.3 Hz,
ArC), 125.4 (d, JCP = 59.3 Hz, ArC), 119.4 (s, ArC). 11B{1H}
1
NMR (CDCl3, 160 MHz): δ −7.4 (s). 31P{1H} NMR (CDCl3,
202 MHz): δ 1.8 (s). UV/vis (in CH2Cl2): λmax = 248 nm (ε =
2.30 × 104 L mol−1 cm−1), 254 nm (ε = 2.64 × 104 L mol−1
cm−1). Luminescence emission (in CH2Cl2): λem = 315, 435
nm, luminescence quantum yield: Φ = 0.50, relative to quinine
sulfate in 1 N H2SO4. HR-MS EI (positive mode, m/z, %):
262.0909 (M+ − BrBFl, 100), 184.0411 (Ph2P+, 9), 183.0363
(Ph2P+ − H, 51), 152.0627 (Ph22+, 9), 108.0128 (PhP+, 25),
91.0548 (BrB+, 3), 77.0389 (Ph+, 4). Anal. Calcd. for
C30H23BBrP: C, 71.32; H, 4.59. Found: C, 72.18; H, 4.81%.
Mp (°C): 209−212.
3
1H NMR (CDCl3, 500 MHz): δ 7.68 (t, JHH = 8.0 Hz, 2H,
ArH), 7.50 (d, 3JHH = 8.0 Hz, 4H, ArH), 7.36 (d, 3JHH = 7.5 Hz,
3
2H, ArH), 7.22 (s, 2H, −N−CH−), 6.95 (td, JHH = 7.0 Hz,
3
4
4JHH = 1.5 Hz, 2H, ArH), 6.69 (td, JHH = 7.0 Hz, JHH = 1.5
3
Hz, 2H, ArH), 5.94 (d, JHH = 9.0 Hz, 2H, ArH), 3.05 (septet,
3JHH = 7.0 Hz, 4H, CH(CH3)2), 2.66 (s, 2H, −CH2−B), 1.34
3
3
Synthesis of BrBFl−PCy3 (6). To a mixture of compound
1 (0.161 g, 0.663 mmol) and PCy3 (0.186 g, 0.663 mmol) was
added 10 mL of toluene. The resulting mixture was allowed to
stir for 2 h. The solution was decanted, and the remaining solid
was washed with toluene (3 × 10 mL) and dried before
(d, JHH = 6.5 Hz, 12H, CH(CH3)2), 1.37 (d, JHH = 7.0 Hz,
12H, CH(CH3)2). 13C{1H} NMR (CDCl3, 125 MHz): δ 160.5
(ArC), 146.7 (ArC), 146.1 (ArC), 131.6 (ArC), 131.5 (ArC),
129.3 (ArC), 126.1 (ArC), 125.8 (ArC), 125.7 (ArC), 122.1
(ArC), 118.5 (−N−CH−), 29.2 (−CH2−B), 26.4 (CH-
(CH3)2), 22.9 (CH(CH3)2). 11B{1H} NMR (CDCl3, 160
MHz): δ −1.4 (s). UV/vis (in CH2Cl2): λmax = 248 nm (ε =
2.84 × 104 L mol−1 cm−1), 254 nm (ε = 2.91 × 104 L mol−1
cm−1). Luminescence emission (in CH2Cl2): λem = 320, 435
nm, luminescence quantum yield: Φ = 0.63, relative to quinine
sulfate in 1 N H2SO4. HR-MS EI (positive mode, m/z): Calcd.
for [BrBFl]+: 243.98819. Found: 243.98620 (Δppm = 8.2).
Calcd. for [IPrCH2]+: 402.30350. Found: 402.30281 (Δppm =
1.7). Anal. Calcd. for C40H46BBrN2: C, 74.42; H, 7.18; N, 4.34.
Found: C, 73.15; H, 7.05; N, 4.20%. Mp (°C): 228−231.
Synthesis of [IPrMe]Br (9). Gaseous HBr was bubbled
through a solution of IPrCH2 (100.2 mg, 0.249 mmol) in 10
mL of toluene until a colorless precipitate formed (approx-
imately 30 s). The resulting precipitate was isolated by filtration
and dried in vacuo to give compound 9 as a white solid (115.3
mg, 96%).
1
compound 6 was isolated as a white solid (0.301 g, 87%). H
NMR (CDCl3, 500 MHz): 7.49 (dd, 3JHH = 7.0 Hz, ArH), 7.42
(d, 3JHH = 7.5 Hz, 2H, ArH), 7.14 (tt, 3JHH = 7.5 Hz, 4JHH = 1.0
3
4
Hz, 2H, ArH), 6.99 (td, JHH = 7.0 Hz, JHH = 1.0 Hz, 2H,
ArH), 2.30 (m, 3H, PCy3), 2.07 (s, 6H, PCy3), 1.76 (s, 3H,
PCy3), 1.68 (d, J = 10.0 Hz, 6H, PCy3), 1.17 (m, 15H, PCy3).
13C{1H} NMR (CDCl3, 125 MHz): δ 148.2 (s, ArC), 132.2 (s,
ArC), 127.6 (s, ArC), 126.5 (s, ArC), 119.7 (s, ArC), 32.3 (d,
CyC), 28.2 (s, CyC), 27.4(s, CyC), 25.9(s, CyC). 11B{1H}
NMR (CDCl3, 160 MHz): δ −6.8 (s). 31P{1H} NMR (CDCl3,
202 MHz): δ −1.3 (s). UV/vis (in CH2Cl2): λmax = 254 nm (ε
= 2.30 × 104 L mol−1 cm−1). Luminescence emission (in
CH2Cl2): λem = 316 nm, 435 nm, luminescence quantum yield:
Φ = 0.31, relative to quinine sulfate in 1 N H2SO4. Anal. Calcd.
for C30H41BBrP: C, 68.85; H, 7.90. Found: C, 69.18; H, 7.81%.
Mp (°C): 225−226.
1H NMR (CDCl3, 500 MHz): δ 8.15 (s, 2H, −N−CH−),
Synthesis of [(DMAP)2BFl]Br (7). To a mixture of
compound 1 (199.4 mg, 0.821 mmol) and N,N-dimethylami-
nopyridine (DMAP, 202.8 mg, 1.660 mmol) was added 15 mL
of toluene. The resulting mixture was allowed to stir for 12 h.
The supernatant was decanted, and the remaining solid was
washed with 15 mL of toluene and dried in vacuo to afford
compound 7 as a white solid (354.3 mg, 89%). Crystals suitable
for X-ray analysis were grown from a 50/50 mixture of CH2Cl2
and THF at −35 °C.
3
3
7.62 (t, JHH = 7.5 Hz, 2H, ArH), 7.39 (d, JHH = 7.5 Hz, 4H,
3
ArH), 2.28 (septet, JHH = 7.0 Hz, 4H, CH(CH3)2), 2.08 (s,
3H, −CH3), 1.28 (d, 3JHH = 7.0 Hz, 12H, CH(CH3)2), 1.18 (d,
3JHH = 7.0 Hz, 12H, CH(CH3)2). 13C{1H} NMR (CDCl3, 125
MHz): δ 145.0 (N−C−N), 144.9 (ArC), 132.6 (ArC), 129.1
(ArC), 126.8 (ArC), 125.4 (−N−CH−), 29.2 (CH(CH3)2),
24.8 (CH(CH3)2), 23.5 (CH(CH3)2), 11.0 (−CH3). HR-MS EI
(positive mode, m/z): Calcd. for [IPrMe]+: 403.3108. Found:
J
dx.doi.org/10.1021/ic402408t | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
403.3101 (Δppm = 1.6). Anal. Calcd. for C28H39BrN2: C,
69.55; H, 8.13; N, 5.79. Found: C, 70.46; H, 7.80; N, 4.92%.
Mp (°C): 225 (decomposition).
NMR spectroscopy revealed that ∼70% conversion to
[IPrMe]Cl15b had occurred and 11B NMR spectroscopy
revealed two new boron environments. 11B NMR (CDCl3,
160 MHz): δ 2.3 (br, new environment), −6.7 (s, new
Synthesis of (Me3Si)2NBFl (11). To a solution of
compound 1 (45.5 mg, 0.187 mmol) in 5 mL of toluene was
added a solution of K[N(SiMe3)2] (32.9 mg, 0.165 mmol) in 5
mL of toluene. The resulting mixture was allowed to stir
overnight before being filtered. The volatile components were
removed in vacuo to afford compound 11 as a red semisolid
(40.6 mg, 76%).
1
environment), −15.1 (d, JBH = 82 Hz, 12, ca. 30%).
ASSOCIATED CONTENT
* Supporting Information
■
S
Includes crystallographic data for (IPrMe)Br, NMR spectra for
compounds 11 and 12, absorbance and fluorescence spectra for
compounds 2, 6, 7, and 12, and calculated electronic transitions
for compounds 5, 7, and 8. This material is available free of
charge via the Internet at http://pubs.acs.org.
1H NMR (CDCl3, 500 MHz): δ 7.63 (dt, 3JHH = 7.2 Hz, 4JHH
= 1.2 Hz, 2H, ArH), 7.44 (dt, 3JHH = 7.2 Hz, 4JHH = 1.2 Hz, 2H,
3
4
ArH), 7.30 (td, JHH = 7.6 Hz, JHH = 1.2 Hz, 2H, ArH), 7.14
(td, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 2H, ArH), 0.37 (s, 18H, Si−
CH3). 13C{1H} NMR (CDCl3, 125 MHz): δ 152.6 (ArC),
133.1 (ArC), 132.0 (ArC), 127.4 (ArC), 119.2 (ArC), 4.6 (Si−
CH3). 11B{1H} NMR (CDCl3, 160 MHz): δ 54.2 (s). 29Si{1H}
(CDCl3, 100 MHz): δ 3.4 (s). See the Supporting Information
for copies of the NMR spectra.18
AUTHOR INFORMATION
Corresponding Author
*Corresponding Author E-mail: erivard@ualberta.ca.
■
Notes
The authors declare no competing financial interest.
Synthesis of HBFl−IPrCH2 (12). To a mixture of
compound 8 (202.2 mg, 0.313 mmol) and Li[AlH4] (25.7
mg, 0.677 mmol) was added 10 mL of CH2Cl2. The resulting
mixture was allowed to stir for 5 days. The mixture was then
filtered through a small (ca. 1 cm) plug of silica gel. The volatile
components were removed from the filtrate in vacuo to yield
compound 12 as a white solid (152.4 mg, 86%).
ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada (Discovery
Grant for E.R.), the Canada Foundation for Innovation (CFI),
Alberta Innovates-Technologies Futures (New Faculty Award
to E.R.), and Suncor Energy Inc. (Petro-Canada Young
Innovator Award to E.R.). C.M. acknowledges the Alexander
von Humboldt foundation for a Feodor Lynen Postdoctoral
Fellowship. We also gratefully acknowledge access to the
computing facilities provided by the Western Canada Research
Grid (Westgrid). We also thank Wayne Moffat for his
assistance in collecting luminescence and quantum yield data.
1H{11B} NMR (CDCl3, 500 MHz): δ 7.64 (t, 3JHH = 8.0 Hz,
2H, ArH), 7.49 (d, 3JHH = 7.5 Hz, 2H, ArH), 7.43 (d, 3JHH = 8.0
3
Hz, 4H, ArH), 7.08 (s, 2H, −N−CH−), 6.94 (td, JHH = 7.5
4
3
4
Hz, JHH = 1.0 Hz, 2H, ArH), 6.73 (td, JHH = 7.5 Hz, JHH
=
3
1.0 Hz, 2H, ArH), 6.12 (d, JHH = 7.0 Hz, 2H, ArH) 2.77
(septet, 3JHH = 7.0 Hz, 4H, CH(CH3)2), 2.35 (broad, 1H, −BH,
assignment made by broadband H{11B} decoupling), 2.15 (d,
1
3JHH = 3.5 Hz, 2H, −CH2−B) 1.25 (d, JHH = 7.0 Hz, 12H,
3
3
REFERENCES
CH(CH3)2), 1.21 (d, JHH = 6.5 Hz, 12H, CH(CH3)2).
■
13C{1H} NMR (CDCl3, 125 MHz): δ 163.5 (ArC), 148.1
(ArC), 145.8 (ArC), 131.6 (ArC), 131.3 (ArC), 130.1 (ArC),
125.2 (ArC), 124.3 (ArC), 123.5 (ArC), 121.5 (ArC), 118.2
(−N−CH−), 29.2 (CH(CH3)2), 25.8 (CH(CH3)2), 22.6
(CH(CH3)2). 11B{1H} NMR (CDCl3, 160 MHz): δ −15.1
(s). 11B NMR (CDCl3, 160 MHz): δ −15.1 (d, 1JBH = 82 Hz).
UV/vis (in CH2Cl2): λmax = 315 nm (ε = 8.64 × 103 L mol−1
cm−1). Luminescence emission (in CH2Cl2): λem = 324, 435
nm, luminescence quantum yield: Φ = 0.19, relative to quinine
sulfate in 1 N H2SO4. HR-MS EI (positive mode, m/z): Calcd.
for [M]+: 566.38324. Found: 566.38218 (Δppm = 1.9). Anal.
Calcd. for C40H47BN2: C, 84.79; H, 8.36; N, 4.94. Found: C,
79.65; H, 8.04; N, 4.56%. Mp (°C): 218−220. Despite repeated
attempts, combustion analysis gave consistently low values for
carbon content (lower by ca. 5%). See the Supporting
Information for copies of the NMR spectra.18
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adduct, formulated herein as TripBFl′−DMF, has not been reported;
see reference 4.
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solution in the presence of a large excess of added bromide, in the
form of [n-Bu4N]Br, indicating that the emission is not likely arising
from a cationic borenium species [LB−BFl]+, although we cannot
completely rule out this possibility at this stage.
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L
dx.doi.org/10.1021/ic402408t | Inorg. Chem. XXXX, XXX, XXX−XXX