Stepwise intramolecular photoisomerization of NHC-chelate dimesitylboron compounds with C-C bond formation and C-H bond insertion

Article abstract of DOI:10.1021/ja304211v

C,C-chelate dimesitylboron (BMes₂) compounds containing an N-heterocyclic carbene (NHC) donor have been obtained. Single-crystal X-ray diffraction analyses established that the boron atom in these compounds is bound by four carbon atoms in a distorted tetrahedral geometry. Compared to previously reported N,C-chelate dimesitylboron compounds, the new C,C-chelate boron compounds have a much larger HOMO-LUMO energy gap (>3.60 eV). They do, however, respond to UV irradiation (300 nm) in the same manner as N,C-chelate BMes₂ compounds do, undergoing photoisomerization and converting to an intensely colored (yellow or orange) isomer A quantitatively, with a high quantum efficiency (0.60-0.75). NMR and single-crystal X-ray diffraction analyses established that the structure of A is similar to the dark isomers obtained from N,C-chelate BMes₂ compounds. However, unlike the N,C-chelate dark isomers that have the tendency to thermally reverse back to the light colored isomers, the isomers A of the C,C-chelate BMes₂ are thermally stable and no reverse isomerization was observed even when heated to 80 °C (or 110 °C) for hours. The most unusual finding is that isomers A undergo further photoisomerization when irradiated at 350 nm, forming a new colorless species B nearly quantitatively. NMR and single-crystal X-ray diffraction analyses established the structure of isomer B, which may be considered as an intramolecular C-H insertion product via a borylene intermediate. Mechanistic aspects of this unusual two-step photoisomerization process have been examined by DFT computational studies.

Full text of DOI:10.1021/ja304211v

Article
pubs.acs.org/JACS
Stepwise Intramolecular Photoisomerization of NHC-Chelate
Dimesitylboron Compounds with C−C Bond Formation and C−H
Bond Insertion
Ying-Li Rao, Leanne D. Chen, Nicholas J. Mosey, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: C,C-chelate dimesitylboron (BMes2) com-
pounds containing an N-heterocyclic carbene (NHC) donor
have been obtained. Single-crystal X-ray diffraction analyses
established that the boron atom in these compounds is bound
by four carbon atoms in a distorted tetrahedral geometry.
Compared to previously reported N,C-chelate dimesitylboron
compounds, the new C,C-chelate boron compounds have a
much larger HOMO−LUMO energy gap (>3.60 eV). They
do, however, respond to UV irradiation (300 nm) in the same manner as N,C-chelate BMes2 compounds do, undergoing
photoisomerization and converting to an intensely colored (yellow or orange) isomer A quantitatively, with a high quantum
efficiency (0.60−0.75). NMR and single-crystal X-ray diffraction analyses established that the structure of A is similar to the dark
isomers obtained from N,C-chelate BMes2 compounds. However, unlike the N,C-chelate dark isomers that have the tendency to
thermally reverse back to the light colored isomers, the isomers A of the C,C-chelate BMes2 are thermally stable and no reverse
isomerization was observed even when heated to 80 °C (or 110 °C) for hours. The most unusual finding is that isomers A
undergo further photoisomerization when irradiated at 350 nm, forming a new colorless species B nearly quantitatively. NMR
and single-crystal X-ray diffraction analyses established the structure of isomer B, which may be considered as an intramolecular
C−H insertion product via a borylene intermediate. Mechanistic aspects of this unusual two-step photoisomerization process
have been examined by DFT computational studies.
investigating organoboron compounds that contain an NHC
chelating ligand is to develop new photoresponsive organo-
boron compounds. We recently reported a new class of boron-
based photochromic compounds based on B(ppy)Mes2 (ppy =
2-phenylpyridine),11 shown in Scheme 1.
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) are well-known for their
ability to stabilize rare main group species1 such as borylenes,
boroles,2 silylenes, disilylenes, disilynes,3 germylenes,4 dibor-
6
enes,5 P2, P4, As2 , and Ga27 and so forth. Unusual and exciting
chemical properties and reactivity have been frequently
observed for NHC-stabilized main group compounds,1−7
which further drives the intense research interest and activities
in NHC-main group chemistry. Nonetheless, much earlier
research on main group NHC compounds has focused on
monodentate NHC ligands. Main group compounds that
contain chelate ligands involving NHC donors have hardly
been explored, although NHC-containing chelate ligands
involving a donor atom such as oxygen or nitrogen are well-
known and have been extensively explored for transition metal
chemistry.8,9
Scheme 1. Photoisomerization of B(ppy-R)Mes2
We have been particularly interested in aryl-cyclometallating
or C,C-chelate NHC ligands based on the consideration that
these ligands contain two strongly σ-donating carbon atoms
and should be capable of acting as a new type of chelate
scaffolds for building new main-group π-conjugated materials.
Many examples of transition metal complexes with aryl-
cyclometallating NHC ligands have been reported previously
and used successfully in catalysis and OLEDs.9 In contrast,
main-group examples especially those with a boron as the
central atom remain unexplored.10 Our other motivation in
This class of compounds display a unique photoisomeriza-
tion phenomenon, transforming from a colorless or light
colored isomer to a dark colored isomer A quantitatively, upon
irradiation by light (Scheme 1), with moderate to high
quantum yields.11 The N,C-chelate ligand plays a critical role
in this unusual photoisomerization phenomenon. The steric
Received: May 7, 2012
Published: June 11, 2012
© 2012 American Chemical Society
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congestion imposed by the mesityl groups and the charge
transfer transition from the mesityl to the N,C-chelate π*
orbital were found to be the key driving forces for the
photoisomerization of the N,C-chelate BMes2 compounds. The
pyridyl moiety in B(ppy)Mes2 and derivatives, albeit not
affected by isomerization, plays a vital anchoring role, keeping
the boron chromophore intact through the photoisomerization
process. NHC ligands are much stronger σ donors than pyridyl.
Thus, replacing the pyridyl with a NHC donor may allow us to
achieve a new class of photoresponsive organoboron
compounds and discover new photochemical reactivity of
organoboron compounds.
With this in mind, we designed and synthesized two families
of C,C-chelate BMes2 compounds, namely, compounds 1 and
2, and their methylated derivatives 1-Me and 2-Me (Scheme
2). We have found that 1-Me and 2-Me undergo unusual
Scheme 2. Synthetic Procedures and the Two Resonance
Structures of 1, 2, 1-Me, and 2-Me
Figure 1. Crystal structures of 1-Me (top) and 2-Me (bottom) with
50% thermal ellipsoids and labeling schemes for key atoms.
Table 1. Important Bond Lengths (Å) and Angles (deg) of 1,
2, 1-Me, and 2-Me
compd.
B−CPh
B−CNHC
B−CMes
1
1.650(7)
1.653(7)
1.638(2)
1.663(2)
1.672(6)
1.644(7)
1.653(2)
1.639(2)
1.657(6), 1.643(7)
1.638(7), 1.646(7)
1.639(2), 1.666(2)
1.651(2), 1.653(2)
CMes−B−CMes
1-Me
2
stepwise photoisomerization, generating two distinct isomers in
the process. The details of our investigation on the new
photoresponsive organoboron system are reported herein.
2-Me
compd.
CPh−B−CNHC
1
92.9(4)
94.3(3)
94.3(2)
94.2(1)
117.5(4)
115.1(4)
116.5(1)
113.6(1)
1-Me
2
RESULTS AND DISCUSSION
■
Synthesis and Structures of 1, 2, 1-Me, and 2-Me. The
syntheses of compounds 1 and 2 were accomplished by a
straightforward one-pot procedure shown in Scheme 2. Double
lithiation of N-phenyl imidazole or N-phenyl benzimidazole at
−78 °C in diethylether, followed by the addition of 1 equiv of
BMes2F yielded the lithium salt Li[B(C,C-chelate)Mes2]. The
possibility to double lithiate N-phenylimidazole was previously
demonstrated by Canac and co-workers.12 Addition of CH3OH
to the corresponding solution of the lithium salt generated
compounds 1 and 2 in moderate yields (50% for 1 and 30% for
2). 1 and 2 can be then converted quantitatively to the
methylated derivatives 1-Me and 2-Me by the addition of a
base (KOtBu) and CH3I. Compounds 1, 2, 1-Me, and 2-Me are
colorless, air-stable, and can be purified by column chromatog-
raphy. They are fully characterized by NMR, single-crystal X-
ray diffraction, and elemental analyses. The two key resonance
structures of these compounds are also shown in Scheme 2.
The crystal structures of 1-Me and 2-Me are shown in Figure
1. The structural diagrams of 1 and 2 can be found in the
Supporting Information. As shown by Figure 1 and the key
bond lengths and angles around the boron atom in Table 1,
compounds 1, 2, 1-Me, and 2-Me have structural features
similar to those of B(ppy)Mes2 and derivatives.11 For example,
2-Me
the B−CPh and B−CMes bond lengths are all considerably
longer than the typical B−C bonds observed in noncongested
four-coordinate boron compounds (e.g., B(ppy)Ph2)11c but
similar to those in sterically congested four-coordinate boron
that contain a BMes2 group.11,13 The B−CNHC bond lengths are
also all longer than previously reported typical B−CNHC bond
lengths in tetrahedral boron compounds.2 One notable
difference is that the B−CNHC bond lengths in the non-
methylated compounds 1 and 2 (1.672(6) and 1.653(2) Å) are
longer than those in the methylated compounds 1-Me and 2-
Me (1.644(7) and 1.639(2) Å), which may be attributed to the
greater electron donating ability of methyl, compared to a H
atom.
Electronic Properties of 1, 2, 1-Me, and 2-Me.
Compared to N,C-chelate B(ppy)Mes2, the new C,C-chelate
compounds have a much larger HOMO−LUMO or optical
energy gap and do not have any absorption in the visible region
at all. Instead, they all have intense absorption bands in the UV
region of 250−350 nm, as shown in Figure 2. The absorption
maxima of 1 and 1-Me are about 5−10 nm higher in energy
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Table 2. Absorption and Electrochemical Data
absorp. λ,
nm,
Ered
Eox
optical gap LUMO
HOMO
(eV)
a
a
compd. (log ε)
(V)
(V)
(eV)
(eV)
b
1
264
4.06
(3.89)
b
d
f
f
1-Me
1-MeA
1-MeB
2
265
−2.32
−2.30
3.87
2.45
4.18
3.78
3.65
2.30
3.75
−1.94
−1.80
−5.81
(3.92)
c
e
426
−0.01
−4.24
(3.50)
b
260
(3.86)
b
286
(4.03)
b
d
f
f
2-Me
2-MeA
2-MeB
290
−2.06
−2.00
−2.14
−2.09
−5.79
(4.06)
Figure 2. UV−vis absorption spectra of 1, 1-Me, 1-MeB, 2, 2-Me, and
2-MeB in CH2Cl2 (∼1.0 × 10−5 M).
c
e
456
0.13
−4.38
(4.06)
b
280
than those of 2 and 2-Me. The absorption edges (∼320 nm) of
1 and 1-Me are about 20−30 nm higher in energy than those
(∼345 nm) of 2 and 2-Me, which can be attributed to the
greater conjugation of the C,C-chelate ligand in 2 and 2-Me.
Electrochemical data indicate that the narrowing of the
HOMO−LUMO gap from 1-Me to 2-Me is mainly caused by
the stabilization of the LUMO level (−1.94 to −2.14 eV),
which is in agreement with the trend suggested by DFT
calculation results (Figure 3, see Supporting Information for 2-
(4.05)
a
+/0
Relative to Ag/AgCl reference electrode, recorded in DMF, FeCp2
b
c
d
= 0.55 V. λmax in CH2Cl2. λmax in toluene. From the reduction
potential. Determined from the oxidation potential. From the optical
energy gap and the reduction potential or the oxidation potential.
e
f
HOMO−LUMO diagrams of 1-Me and 2-Me bear striking
resemblance to those of B(ppy)Mes2. Thus, based on the
similarity of molecular and electronic structures of 1-Me and 2-
Me with those of B(ppy)Mes2, we anticipated that the new
C,C-chelate BMes2 compounds are very likely to undergo
photoisomerization in the same manner as the N,C-chelate
compounds.
Photoisomerization. 1-Me/2-Me to 1-MeA/2-MeA. To
examine the response of the new C,C-chelate compounds
toward light, we irradiated them using UV light at 300 nm,
approximately the absorption maxima. The UV−vis spectral
1
change was recorded in toluene. H NMR spectral change was
recorded in C6D6. As shown in Figure 4, upon irradiation by
Figure 3. DFT calculated total energy, HOMO/LUMO energy, and
diagrams of 1-Me, 1-MeA, and 1-MeB (isocontour value = 0.02).
Me data). Furthermore, electrochemical data support that the
large HOMO−LUMO gaps of 1-Me and 2-Me, compared to
those of B(ppy)Mes2 (LUMO level = −2.50 eV, determined
from CV data) are caused mainly by the high LUMO level of
the C,C-chelate compounds. 2 and 2-Me display weak purple-
blue fluorescence at λmax = 410 nm in toluene, which is also
much higher in energy than that of B(ppy)Mes2 (λmax = 480
nm). The fluorescence quantum efficiency was determined to
be 0.11 for 2-Me, and 0.10 for 2. No appreciable fluorescence
was observed for 1 and 1-Me. Absorption spectral data and
electrochemical data are summarized in Table 2.
TD-DFT computational results confirmed that the transition
from the ground state to the first excited state of the C,C-
chelate compounds is dominated by the HOMO to LUMO
transition, which is a charge transfer from the mesityl to the π*
orbital of the C,C-chelate backbone as shown in Figure 3. The
Figure 4. UV−vis spectra showing the conversion of 1-Me (left) and
2-Me (right) to 1-MeA and 2-MeA, respectively, in toluene irradiated
at 300 nm, recorded with time intervals of a few seconds between each
spectrum. Inset: photographs showing the color change of the samples
before and after irradiation.
UV light, the C,C-chelate compounds undergo a distinct
spectral change, with a new low energy absorption band
appearing and growing with irradiation time, which can be
attributed to the new isomer (λmax = 426 nm for 1-MeA, λmax
=
456 nm for 2-MeA). This spectral change accounts for the
change of the compounds from colorless to bright yellow (1-
Me) or orange (2-Me).
1H and 11B NMR experiments showed that the methylated
compounds 1-Me and 2-Me undergo clean and quantitative
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isomerization upon irradiation at 300 nm, forming the new
isomers 1-MeA and 2-MeA, respectively (see Supporting
Information ), while the response of 1 and 2 is complex with
evident decomposition, which may be attributed to the acidic
proton of the imidazolium. Thus, we focused our study on
photoisomerization of 1-Me and 2-Me.
that can be attributed to the Ha and Hb protons of the
cyclohexadienyl group, similar to those observed for the
B(ppy)Mes2-A isomer shown in Scheme 1. Furthermore, the
11B NMR spectral change (see Supporting Information ) from
−9.26 to −25.86 ppm for 1-Me, and −8.98 to −19.70 ppm for
2-Me follows the same trend as that observed for B(ppy)Mes2.
Thus, the NMR data support that the yellow and orange
isomers from 1-Me and 2-Me likely have similar structures as
those of B(ppy)Mes2-A. To firmly establish the structures of 1-
MeA and 2-MeA, 2D NMR experiments (COSY and NOESY,
see Supporting Information ) were performed for both
compounds, which confirmed that they indeed have the
structures shown in Figures 5.
Unlike N,C-chelate compounds, where the C−C bond
formation is restricted between a mesityl and the carbon donor
atom of the chelate ligand, in the C,C-chelate compounds 1-Me
and 2-Me because both donor atoms from the chelate ligand
are carbon atoms, the C−C bond formation could occur at
either the phenyl carbon site or the imidazolyl carbon site. As
1
illustrated by the H NMR spectra in Figure 5, the isomer 1-
Further evidence supporting the structures of 1-MeA and 2-
MeA were obtained from the single-crystal X-ray diffraction
analysis data of 2-MeA. The crystal structure of 2-MeA is
shown in Figure 6. Its geometrical parameters are similar to
MeA or 2-MeA shows two characteristic olefinic proton peaks
Figure 6. The crystal structure of 2-MeA with 50% thermal ellipsoids
and labeling schemes for key atoms (left) and the side view of the
structure with H atoms omitted (right). Important bond lengths (Å)
and angles (°): B(1)−C(7) 1.539(5), B(1)−C(15) 1.684(5), B(1)−
C(20) 1.621(5), B(1)−C(24) 1.601(5), C(1)−C(15) 1.488(5),
C(15)−C(20) 1.548(5), C(15)−C(16) 1.484(4), C(16)−C(17)
1.346(5), C(17)−C(18) 1.443(5), C(18)−C(19) 1.355(4), C(19)−
C(20) 1.466(5); C(7)−B(1)−C(24) 117.3(3), C(15)−B(1)−C(20)
55.8(2), B(1)−C(15)−C(20) 60.2(2), B(1)−C(20−C(15) 64.2(2).
those of the dark isomer of a N,C-chelate compound,
BMes2(Py-N-Ph-IN)-A (Py-N-Ph-IN = 1-Phenyl-2-(2-Pyrid-
yl)-indolyl), the only other dark isomer crystal structure we
reported recently.11e The B(1)−C(7) bond (1.539(5) Å) in 2-
MeA is much shorter than that of 2-Me. The C(15)−C(20)
bond (1.548(5) Å) is typical of a C−C single bond while the
bond lengths (1.346(5), 1.355(4) Å) of C(16)−C(17) and
C(18)−C(19) agree well with typical CC bonds, thus,
confirming that the transformed mesityl group can indeed be
described as a cyclohexadienyl. The structural features of the
BC2-cyclohexadienyl moiety in 2-MeA are also similar to a BC2-
naphthyl moiety in boron-carbene compounds reported by
Braunschweig and co-workers.2b The C,C-chelate benzene ring
and the benzimidazolyl ring are twisted out of coplanarity with
a dihedral angle of 19.4°, attributable to the interactions of the
two ortho-H atoms. The NMR data and the crystal structure of
2-MeA confirmed that the C−C bond coupling occurs
exclusively at the phenyl site of the C−C chelate. This finding
indicates that the NHC-carbon donor atom in these molecules
Figure 5. 1H NMR spectral change (olefin and aromatic region) of 1-
Me (top)/2-Me (bottom) upon irradiation at 300 and 350 nm,
respectively, showing their full conversion to 1-MeA/2-MeA, and the
subsequent conversion to 1-MeB/2-MeB. The spectra were recorded
in C6D6 at 25 °C. The aliphatic region of the same spectra can be
found in the Supporting Information . The spin side bands are marked
by asterisks (*). Additional time-lapsed NMR spectra showing the
two-step isomerization can be found in Supporting Information .
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play an anchoring role, in the same manner as the pyridyl does
in B(ppy)Mes2.
1-MeA/2-MeA to 1-MeB/2-MeB. In sharp contrast to the
behavior of B(ppy)Mes2-A and its derivative compounds that
can thermally reverse back to the more stable colorless or light
colored isomers,11 1-MeA and 2-MeA have been found to have
The photoisomerization of 1-Me/2-Me to 1-MeA/2-MeA is
very efficient and the quantum efficiency has been determined
in toluene using ferrioxalate actinometry at 297 nm excitation.14
For 1-Me to 1-MeA, the quantum efficiency was found to be
∼0.75, similar to that of B(ppy)Mes2 (∼0.80), while for 2-Me
to 2-MeA, the efficiency is lower, ∼0.60. These data support
that the C,C-chelate BMes2 compounds undergo very efficient
photoisomerization.
Despite the structural similarity of 1-MeA and 2-MeA with
the dark isomers of N,C-chelate BMes2 compounds, 1-MeA
and 2-MeA have a much greater stability toward oxygen in
solution and the solid state, compared to the N,C-chelate
analogues. In fact, the orange crystals of 2-MeA are stable
under air for days.
1
a very high thermal stability. The H NMR spectra of 1-MeA
and 2-MeA show no change at all after being heated at either
110 °C in d8-toluene or 80 °C in C6D6 for hours (see
Supporting Information). However, remarkably, we observed
that both 1-MeA and 2-MeA can be sensitized by light. When
irradiated at 350 nm, 1-MeA and 2-MeA gradually lose their
color, converting to a new colorless isomer 1-MeB and 2-MeB,
respectively (Scheme 3). This process can be monitored by
Scheme 3. Stepwise Photoisomerization of 1-Me and 2-Me
Electrochemical analysis data (CV) show that 1-MeA and 2-
MeA have a distinct low oxidation potential (−0.55 and −0.41
V, vs FeCp2+/0) similar to that11c of the dark blue isomer
B(ppy)Mes2-A (Figure 7 and Table 2). The greater stability of
1
UV−vis spectra (see Supporting Information), 11B and H
NMR spectra. NMR spectra show that 1-MeA and 2-MeA
transform cleanly to 1-MeB and 2-MeB (Figure 5 and
Supporting Information), respectively, although this photo-
isomerization step is much less efficient, compared to the
isomerization of 1-Me/2-Me to 1-MeA/2-MeA. Full con-
version can only be achieved after the samples were irradiated
at 350 nm for a few days at the typical NMR concentration
scale (∼1 mg sample in 0.5 mL of solvent), while under the
same conditions, using 300 nm excitation, the full conversion of
1-Me/2-Me to 1-MeA/2-MeA can be achieved in less than 30
min. Interestingly, the conversion of 1-MeA and 2-MeA to 1-
MeB and 2-MeB is considerably less efficient and slower, when
irradiated at their absorption maxima (420 and 450 nm,
respectively), compared to excitation at 350 nm, under the
same conditions.
Figure 7. CV diagrams showing the appearance and the growth of the
oxidation peak of 2-MeA with time (t) when the DMF solution of 2-
Me is irradiated at 300 nm with a hand-held UV lamp, with NBu4PF6
as the electrolyte, and a scan rate of 150 mV s−1. E1/2 (FeCp2+/0) =
0.55 V.
1-MeB and 2-MeB are air-stable and colorless. The
1
structures of 1-MeB and 2-MeB were first established by H,
1-MeA and 2-MeA toward oxygen, compared to the N,C-
chelate analogues is therefore quite surprising. On the basis of
UV−vis and CV data, the HOMO level of 1-MeA and 2-MeA
is more than 1 eV above that of 1-Me and 2-Me while the
LUMO level experiences little change (see Table 2). The color
difference between the yellow 1-MeA/orange 2-MeA and the
dark blue B(ppy)Mes2-A is therefore caused mainly by the
relatively high LUMO level (∼−2.0 eV) of 1-MeA and 2-MeA,
compared to that of B(ppy)Mes2-A (∼-2.50 eV), leading to a
higher S0 → S1 transition energy for the C,C-chelate
compounds. This trend is in good agreement with the DFT
computational results shown in Figure 3. The electron density
distribution in 1-MeA and 2-MeA was found to be similar to
that of B(ppy)Mes2-A with the HOMO level being dominated
by contributions from the BC2 triangular ring (Figure 3). The
computational results also show that 1-MeA and 2-MeA are
about 70−80 kJ less stable than 1-Me and 2-Me. This energy
difference is much less than that between B(ppy)Mes2 and
B(ppy)Mes2-A (∼115 kJ). Thus, the carbene donors appear to
exert a greater stabilization effect to the isomers A, compared to
the pyridyl.
11B, COSY and NOESY NMR spectra. In the 1H{11B-
decoupled} NMR spectra of both 1-MeB and 2-MeB, a peak
at 4.23 and 4.35 ppm, respectively, was observed, which can be
assigned to a B−H proton. In the 11B{1H-coupled} NMR
spectra, 1-MeB and 2-MeB display a distinct doublet at −19.4
and −19.3 ppm, respectively, with a coupling constant of 85
Hz, characteristic of JB−H coupling2b,c,e,15,16 (see Supporting
1
Information). The 6 methyl groups on the two mesityls are all
resolved with 6 distinct chemical shifts in the 1H NMR spectra
of 1-MeB and 2-MeB, due to the asymmetric environment
around the B atom. The absorption spectra of 1-MeB and 2-
MeB resemble those of 1-Me and 2-Me (Figure 2), with the S0
→ S1 transition being a charge transfer from a mesityl to the
chelate (Figure 3), shown by DFT computational results.
The crystal structure of 1-MeB was determined by single-
crystal X-ray diffraction analysis, which fully corroborates the
structural features established by NMR experiments. As shown
in Figure 8, the C,C-chelate remains bound to the B atom in 1-
MeB. In addition, the B atom is bound to one mesityl group
and one H atom (located directly from a difference Fourier
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energy difference between B(ppy)Mes2 and B(ppy)Mes2-A, the
activation barrier (determined experimentally to be ∼110
kJmol−1) for the reverse transformation of B(ppy)Mes2-A to
B(ppy)Mes2 is thermally accessible.
Our attempts to map out the transition states and calculate
the activation barrier for the thermal transformation of 1-Me to
1-MeA using DFT methods were unsuccessful. Nonetheless,
we have identified an intermediate (I1) that is similar to that
involved in the transformation of B(ppy)Mes2, but at a much
higher energy (∼204 kJmol−1 higher than 1-Me, and 130
kJmol−1 than 1-MeA), as shown in Figure 9. This suggests that
Figure 8. Crystal structure of 1-MeB with 50% thermal ellipsoids and
labeling schemes for key atoms. Important bond lengths (Å) and
angles (°): B(1)−C(1) 1.621(11), B(1)−C(7) 1.618(11), B(1)−
C(11) 1.609(11), B(1)−H(1) 1.20(5); C(1)−B(1)−C(7) 95.3(6),
C(11)−B(1)−H(1) 113(2), C(1)−B(1)−C(11) 117.5(7), C(7)−
B(1)−C(11) 117.5(7).
map) in a distorted tetrahedral geometry. The B(1)−H(1)
bond length was determined to be 1.20(5) Å, in agreement
with typical terminal B−H bond lengths reported previous-
ly.2b,c,e,15,16 The second mesityl group forms a C−C bond with
a carbon atom of the phenyl ring with a typical bond length
(C(5)−C(20) = 1.488(10) Å). Compared to those of 1-Me
and 2-Me, the B−C bonds in 1-Me-B are considerably shorter,
which can be attributed to the reduced steric congestion around
the B center.
Mechanism. Mechanistically, the photoisomerization of the
C,C-chelate compound 1-Me/2-Me to 1-MeA/2-MeA likely
follows a similar pathway as that of B(ppy)Mes2 via a transition
state M1, as shown in Scheme 4. Therefore, we will first
Figure 9. Energy diagrams showing the relative energies of 1-Me, 1-
MeA, and 1-MeB at the ground state (S0) and at the first excited state
(S1). The arrows indicate the transformation pathways between the
three isomers. The structures and relatively energy levels of two
intermediates at the ground state identified by DFT calculations are
also shown. Peaks in the dashed curves connecting the structures
represent estimated positions of transition states separating those
structures.
Scheme 4. Proposed Mechanism for the Stepwise
Transformation of 1-Me and 2-Me
the thermal activation barriers for both forward and reverse
transformation of 1-Me are much higher than those of
B(ppy)Mes2 and as a result, 1-Me and 1-MeA cannot
interconvert thermally. Preliminary computational results on
the first excited state indicates that [1-Me]* to [1-MeA]*
transformation is a downhill process with an energy difference
∼50 kJmol−1, thus, the transformation of 1-Me to 1-MeA can
occur readily via the excited state.
A computational study of the isomerization 1-MeA to 1-MeB
was also performed. At the ground state, a highly energetic
“borylene”-like intermediate I2 was identified (∼140 kJmol−1
above 1-MeA). The transition state connecting 1-MeA and I2
could not be located; however, the energy of the transition state
must be at least as high as that of I2, suggesting a large
activation energy for the transformation from 1-MeA to 1-
MeB, rendering that process inaccessible via a thermal pathway.
The formation of I2 may be described as a cheletropic boron
elimination18 from 1-MeA.
comment on the B(ppy)Mes2 system. Our computational study
on B(ppy)Mes2 indicated that its transformation to the dark
blue isomer B(ppy)Mes2-A is not accessible on the ground state
due to a very large activation energy. However, at the first
excited state, [B(ppy)Mes2]* is less stable than [B(ppy)Mes2-
A]*, thus, B(ppy)Mes2 can readily transform to the dark isomer
B(ppy)Mes2-A, via the excited state.17 Because of the large
At the first excited state, [1-MeB]* is less stable than [1-
MeA]* by ∼30 kJmol−1. Thus, in order to convert 1-MeA to 1-
MeB via the excited state, 1-MeA needs to be excited above the
first excited state. This provides a plausible explanation as to
why a 350 nm excitation energy is required to achieve
reasonable conversion of 1-MeA/2-MeA to 1-MeB/2-MeB and
the fact that the second photoisomerization step is much less
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Fluorescence quantum yield of compound 2-Me was measured in
dilute degassed toluene solution (Abs. = ∼0.1) at room temperature
using the relative quantum yield method using 9,10-diphenylanthra-
cene as the reference standard (Φ = 0.90).19
Computational Studies. DFT and TD-DFT calculations were
performed at the CAM-B3LYP/SVP level of theory.20 Test
calculations demonstrated that this level of theory accurately
reproduced the experimentally determined UV−vis spectrum of a
reference compound in the same class as those considered here. All
reactant, product and intermediate structures were optimized without
constraints, and characterized as minima on the potential energy
surface through frequency calculations. Excited state energies were
determined through TD-DFT calculations.
The stationary points were optimized a second time at the B3LYP/
6-311++G** level of theory21 in order to generate the orbital
eigenvalues of the HOMO and LUMO given in Figure 3. This was
done to be consistent with previous computational work on similar
compounds. All calculations were performed with the Gaussian 09
Software.22
efficient than the first step. It is very likely that a “borylene”-like
species I2 is involved in the excited state transformation of
isomer A to B. A simple rotation around the N−C bond in I2
would bring the ortho-H atom of the benzene ring close to the
boron center (M2), activating the C−H bond (Scheme 4). The
possible involvement of a borylene species in intramolecular
C−H insertion reactions was proposed in several previously
reported organoboron systems, where the proposed “borylene”
species was generated in situ via chemical reduction of a
borane, followed by C−H insertion.1e,2c,16 It is conceivable that
in our system, the C,C-chelate ligand stabilizes the “borylene”
intermediate I2 generated in the excited state, leading to C−H
bond activation and the isolation of compounds 1-MeB and 2-
MeB. The selective activation of an aryl C−H bond over an
aliphatic C−H bond (e.g., the methyl group in the mesityl) may
be facilitated by the involvement of the π* orbital of the
benzene ring in stabilizing the transition state and the greater
stability of the resulting 5-membered ring in 1-MeB (versus the
6-membered ring in methyl activation).
Syntheses of 1 and 1-Me. n-Butyllithium (2.5 M, 1.7 mL, 4.2
mmol) was slowly added to a solution of 1-phenylimidazole (0.3 g, 2.1
mmol) in Et2O (100 mL) at −78 °C. The solution was warmed to
room temperature and stirred for 3 h. A diethyl ether solution of
BMes2F (0.62 g, 2.1 mmol) was thenadded dropwise at −78 °C. The
resulting solution was slowly warmed to room temperature and stirred
overnight. After the addition of excess methanol, the solution was then
concentrated under vacuum. Purification by chromatography on silica
gel (hexane/CH2Cl2) yielded compound 1 as a white solid, which was
On the basis of the calculated data, among the three isomers,
1-MeB is the most stable isomer, 75 kJmol−1 below 1-Me and
145 kJmol−1 below 1-MeA. Thus, the transformation of 1-MeA
to 1-MeB is highly favored thermodynamically. Schematically,
the photoisomerization pathways based on experimental
observations and DFT computational results are shown in
Figure 9.
1
recrystallized from hexane/CH2Cl2 (0.40 g, 50%). H NMR (CDCl3,
25 °C, ppm): 9.02 (s), 7.67 (d, 1H, J = 7.0 Hz), 7.38 (d, 1H, J = 1.5
Hz), 7.15−7.31 (m, 3H), 7.87 (d, 1H, J = 1.5 Hz), 6.70 (s, 4H), 2.23
(s, 6H), 1.88 (s, 12H). 13C NMR (CDCl3, 25 °C, ppm): 140.2, 140.2,
134.7, 133.1, 129.0, 126.7, 124.9, 120.0, 111.6, 110.3, 77.3, 77.0, 76.8,
24.9, 20.7. 11B NMR (CDCl3, 25 °C, ppm): −9.66 (s). 11B NMR
(CDCl3, 160 MHz 25 °C, ppm): −9.57 (s). Anal. Calcd for
C27H30BN2: C, 82.44; H, 7.69; N, 7.12. Found: C, 82.58; H, 7.49;
N, 7.09.
CONCLUSIONS
■
We have shown that C,C-chelate BMes2 compounds that
contain one NHC-donor atom have structures and electronic
properties similar to those of N,C-chelate BMes2 compounds,
and as a result, this class of compounds undergo highly efficient
and clean photoisomerization in the same manner as the N,C-
chelate BMes2 compounds do. There are, however, several
distinct differences between the N,C-BMes2 and C,C-BMes2
compounds. First of all, the carbene donor greatly stabilizes the
dark isomers of the C,C-chelate compounds, such that they are
much more stable toward air than the corresponding N,C-
chelate compounds, and cannot thermally reverse back to the
colorless isomers. Second, the dark isomers of the C,C-chelate
compounds can undergo further photoisomerization, producing
a new isomer that involves intramolecular C−H bond
activation. The same phenomenon has not been observed at
all in any of the N,C-chelate BMes2 compounds we investigated
previously. The robustness of the C,C-chelate BMes2
compounds toward photolysis and their ability to undergo
clean and stepwise photoisomerization are truly remarkable,
opening many new research opportunities in accessing unusual
structures/species/reactivity via the excited state.
Compound 1 (100 mg, 0.25 mmol) and KOtBu (40 mg, 0.35
mmol) were dissolved in 5 mL of THF in a vial; excess CH3I (0.5 mL)
was then added to the solution. The resulting solution was stirred
overnight at room temperature, with white precipitate generated
during the progress of the reaction. After filtration of the KI salt and
concentration of the filtrate under vacuum, 1-Me was obtained almost
quantitatively, which was recrystallized from hexane/CH2Cl2. 1H
NMR (CD2Cl2, 25 °C, ppm): 7.60 (d, 1H, J = 7.5 Hz), 7.52 (d, 1H, J
= 1.5 Hz), 7.33 (d, 1H, J = 7.5 Hz), 7.18 (td, 1H, 3J = 7.5 Hz, 4J = 1.5
Hz), 7.09 (td, 1H, 3J = 7.5 Hz, 4J = 1.5 Hz), 7.00 (d, 1H, J = 1.5 Hz),
6.66 (s, 4H), 3.57 (s, 6H), 2.20 (s, 6H), 1.83 (s, 12H). 13C NMR
(CD2Cl2, 25 °C, ppm): 140.4, 140.1, 132.8, 132.6, 129.1, 126.1, 125.3,
124.5, 111.5, 110.8, 35.2, 24.7, 20.4. The carbene carbon could not be
1
observed in 13C NMR spectra. In 2D H−13C HMBC spectra, the
cross peak between the carbene carbon and the protons from N-
methyl showed up at 174.5 ppm. 11B NMR (CD2Cl2,160 MHz, 25 °C,
ppm): −9.75 (s). Anal. Calcd for C28H31BN2·0.17CH2Cl2: C, 80.45;
H, 7.51; N, 6.66. Found: C, 80.39; H, 7.79; N, 6.65. The presence of
CH2Cl2 (∼0.5 per molecule) in the crystal lattice was confirmed by X-
ray crystallographic analysis.
EXPERIMENTAL SECTION
■
General Procedure. Diethyl ether was used directly from Pure
Solv Solvent Purification System (Innovative Technology, Inc.,
Amesbury, MA). All starting materials were purchased from Aldrich
Chemicals Co. All reactions were carried out under nitrogen
atmosphere, using Schlenk and vacuum line techniques. The 1H,
13C, 11B, and 2D HH COSY, NOESY NMR spectra were recorded on
Bruker Avance 500 MHz spectrometer. UV−vis spectra were recorded
on a Cary50 UV−visible spectrometer. Fluorescence spectra were
recorded on a Photon Technologies International Quanta Master
model C-60 sepctrometer. High-resolution mass spectra (HRMS)
were obtained from an Applied Biosystems Qstar XL spectrometer.
Elemental analyses were conducted at Laboratorie d’Analyze
Syntheses of 2 and 2-Me. The ligand 1-phenyl-benzimidazole
was synthesized according to a literature procedure.23 Compound 2
was synthesized and purified in the same manner as described for
compound 1, and recrystallized from hexane/CH2Cl2 (30% yield) as a
colorless solid. 1H NMR (CD2Cl2, 25 °C, ppm): 9.69 (s), 8.15 (d, 1H,
J = 10.8 Hz), 7.84 (d, 1H, J = 8.4 Hz), 7.58−7.66 (m, 3H), 7.49 (t, 1H,
J = 10 Hz), 7.37(t, 1H, J = 10 Hz), 7.23(t, 1H, J = 10 Hz), 6.87 (s,
4H), 2.21 (s, 6H), 1.92 (s, 12H). 13C NMR (CD2Cl2, 25 °C, ppm):
141.7, 140.1, 134.9, 134.1, 133.2, 129.0, 128.2, 125.9, 124.8, 113.2,
112.8, 112.1, 25.2, 20.4. 11B NMR (CD2Cl2, 160 MHz 25 °C, ppm):
−9.65(s). HRMS (TOF MS EI+): Calcd, 442.2580; Observed,
442.2591.
́
́ ́ ́
Elementaire de l’Universite de Montreal.
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Journal of the American Chemical Society
Article
Compound 2-Me was obtained almost quantitatively in the same
way as 1-Me, and recrystallized from hexane/CH2Cl2. 1H NMR
(CD2Cl2, 25 °C, ppm): 8.20 (d, 1H, J = 7.5 Hz), 7.81 (d, 1H, J = 8.0
Hz), 7.68 (d, 1H, J = 7.5 Hz), 7.61 (td, 1H, 3J = 6.0 Hz, 4J = 2.0 Hz),
7.56−7.60 (m, 2H), 7.30 (t, 1H, J = 7.5 Hz), 7.13 (t, 1H, J = 7.0 Hz),
6.69 (s, 4H), 3.57 (s, 6H), 2.21 (s, 6H), 1.88 (s, 12H). 13C NMR
(CD2Cl2, 25 °C, ppm): 140.3, 140.7, 137.2, 133.1, 132.3, 129.2, 128.0,
125.8, 124.9, 124.4, 112.9, 112.4, 111.5, 31.3, 24.9, 20.4. The carbene
J = 7.5 Hz), 7.06 (d, J = 7.5 Hz), 7.02 (s, 1H), 6.97 (s, 1H), 6.93 (s,
1H), 6.85 (m,a 2H), 6.51 (m,a 1H), 5.58 (m,a 1H), 3.30 (s, 3H), 2.96
(s, 3H), 2.48 (s, 3H), 2.39 (s, 3H), 2.21 (s, 3H), 2.06 (s, 3H), 1.78 (s,
1
3H). The B−H peak was observed in a H {11B decoupled} NMR
spectrum: 4.35 (s, 1H). (aThe coupling constant was not obtained due
to the complexity induced by second order coupling.)
1
11B NMR (25 °C, 160 MHz, C6D6): −19.7 ppm, doublet, JB−H
=
85 Hz. HRMS (TOF MS EI+): calcd, 456.2737; observed, 456.2749.
X-ray Crystallographic Analysis. Single crystals of compounds 1,
2, 1-Me, and 2-Me were obtained from the solution of CH2Cl2/
toluene/hexanes while single-crystals of 1-MeB and 2-MeA were
obtained from toluene/hexanes solutions at 298 K. Data were
collected on a Bruker AXS Apex II single-crystal X-ray diffractometer
with graphite-monochromated Mo Kα radiation, operating at 50 kV
and 30 mA at 180 K. Data were processed on a PC with the aid of the
Bruker SHELXTL software package (version 6.14)24 and corrected for
absorption effects. All structures were solved by direct methods. The
crystal lattice of 1-Me contains disordered solvent molecules which are
most likely CH2Cl2 (∼0.5 per molecule of 1-Me). To improve the
quality of the structural refinements, solvent contributions were
removed by the Squeeze routine of Platon program.25 The crystals of
1-MeB are very small and diffract weakly, resulting in a low ratio of
observed reflections versus parameters, despite the 60 s/frame
exposure time employed for data collection. Nonetheless, we were
able to fully refine the structure of 1-MeB. The hydrogen atom bound
to the boron atom in 1-MeB was located directly from a difference
Fourier map and refined successfully. All non-hydrogen atoms were
refined anisotropically. The positions of hydrogen atoms other than
the one bound to boron were calculated, and their contributions in
structural factor calculations were included. The details of crystallo-
graphic data can be found in the Supporting Information. Crystal data
for all structures have been deposited to the Cambridge Crystal Data
Center (CCDC 877955−877960).
1
carbon could not be observed in 13C NMR spectra. In 2D H−13C
HMBC spectra, the cross peak between the carbene carbon and the
protons from N-methyl showed up at 183.1 ppm. 11B NMR (C6D6,
160 MHz 25 °C, ppm): −8.98 (s). Anal. Calcd for C32H33BN2: C,
84.21; H, 7.29; N, 6.14. Found: C, 83.85; H, 7.31; N, 6.18.
Photoisomerization of 1-Me and 2-Me and Syntheses of 1-
MeA, 1-MeB, 2-MeA, and 2-MeB. General Procedures. Photo-
isomerization experiments were carried out in toluene or benzene
under nitrogen using a Rayonet Reactor RPR-100. The progress of the
1
photoisomerization was monitored by H NMR spectra.
The photoisomerization quantum efficiencies of 1-Me to 1-MeA
and 2-Me to 2-MeA at 297 nm were determined using ferrioxalate
actinometry.14 An Ocean Optics fiber optic spectromphotometer
connected to a four-way temperature-controlled cuvette holder from
Quantum Northwest via 400 μm optical fibers was used to measure
the absorbance. The irradiation source was a 200 W Hg/Xe lamp
attached to a monochromator (Photon Technology International).
1-Me to 1-MeA. Compound 1-MeA was obtained quantitatively via
irradiation of 1-Me at 300 nm under nitrogen. The photoisomerization
quantum efficiency was determined to be ∼0.75. On a typical NMR
concentration scale (e.g., 1 mg of compound in ∼0.5 mL of C6D6),
this conversion is completed in less than 0.5 h. Compound 1-MeA can
be crystallized readily from toluene/hexanes as a yellow crystalline
1
solid. H NMR (25 °C, 500 MHz, ppm, C6D6): 7.69 (d, 1H, J = 10
Hz), 7.15 (td, 1H, J = 10 Hz, J = 2.5 Hz), 7.10 (s, 1H), 7.06 (m, 2H),
6.92 (s, 1H), 6.84 (d, 1H, J = 2.5 Hz), 6.03 (s, 1H), 5.78 (d, 1H, J =
2.5 Hz), 5.64 (s, 1H), 3.01 (s, 3H), 2.73 (s, 3H), 2.33 (s, 3H), 2.23 (s,
3H), 2.07 (s, 3H), 1.97 (s, 3H), 0.85 (s, 3H). 11B NMR (25 °C, 160
MHz, C6D6): −25.9 ppm.
ASSOCIATED CONTENT
■
S
* Supporting Information
1D and 2D NMR data, photoisomerization data (UV−vis and
NMR), computational data, electrochemical analysis data and
diagrams, crystal structural data. This material is available free
of charge via the Internet at http://pubs.acs.org.
1-MeA to 1-MeB. Compound 1-MeB was obtained nearly
quantitatively via continuous irradiation of 1-MeA at 350 nm under
nitrogen. The photoisomerization quantum efficiency is ∼0.001. 1-
MeB can be isolated as a colorless crystalline solid from a C6D6/
hexanes solution. Because the conversion of 1-MeA to 1-MeB is very
slow and inefficient (taking a few days to reach completion for a
solution of ∼1 mg of 1-MeA in 0.5 mL of solvent), bulk conversion
was not performed. 1H NMR (25 °C, 500 MHz, ppm, C6D6): 7.87 (d,
1H, J = 7.5 Hz), 7.32 (t, 1H, J = 7.5 Hz), 7.33 (s, 1H), 7.01 (d, 1H, J =
7.5 Hz), 6.98 (s, 2H), 6.97 (s, 1H), 6.10 (d, 1H, J = 2.0 Hz), 5.58 (d,
1H, J = 2.0 Hz), 3.26 (s, 3H), 2.63 (s, 3H), 2.46 (s, 3H), 2.37 (s, 3H),
2.32 (s, 3H), 2.11 (s, 3H), 1.73 (s, 3H). The B−H peak was observed
in a 1H {11B decoupled} NMR spectrum: 4.23 (s, 1H). 11B NMR (25
AUTHOR INFORMATION
■
Corresponding Author
Wangs@chem.queensu.ca.
Notes
The authors declare no competing financial interest.
1
°C, 160 MHz, C6D6): −19.4, ppm, JB−H = 85 Hz. HRMS (TOF MS
ACKNOWLEDGMENTS
■
EI+): calcd, 406.2580; observed, 406.2591.
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support, the High Performance
Computing Virtual Laboratory (HPCVL) and WestGrid High
Performance Computing consortium for computing facilities.
We are in debt to Professor Victor Snieckus for his insights and
suggestions on the isomerization mechanism shown in Scheme
4.
2-Me to 2-MeA. Compound 2-MeA was obtained quantitatively via
irradiation of 2-Me at 300 nm under nitrogen. The photoisomerization
quantum efficiency is ∼0.60. Compound 2-MeA can be crystallized
1
readily from toluene/hexanes as a orange crystalline solid. 2-MeA H
NMR (25 °C, 500 MHz, ppm, C6D6): 7.92 (dd, 1H, J = 10 Hz, J = 1.0
Hz), 7.83 (m,a 1H), 7.70 (dd, 1H, J = 7.5 Hz, J = 1.5 Hz), 7.20 (td,
1H, J = 7.0 Hz, J = 1.5 Hz), 7.15 (td, 1H, J = 8.0 Hz, J = 1.5 Hz), 7.12
(s, 1H), 7.00 (m,a 2H), 6.86 (s, 1H), 6.57 (m,a 1H), 6.08 (s, 1H), 5.61
(s, 1H), 3.04 (s, 3H), 2.97 (s, 3H), 2.33 (s, 3H), 2.26 (s, 3H), 1.95 (s,
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■
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