Photoreactions of Phenylborylene with Dinitrogen and Carbon Monoxide

Article abstract of DOI:10.1021/jacs.7b08497

Formal removal of two bonding partners from boranes, BR₃, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = C₆H₅; 1) is accessible from phenyldiazidoborane by photochemical extrusion of dinitrogen under matrix isolation conditions. Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of phenylborylene at 375 nm (S₀ → S₂) and tentatively assigned the S₀ → S₁ transition to a very weak band at 518 nm. We also show for the first time that an electrophilic borylene such as 1 can react with N₂ reversibly and with CO irreversibly under photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their small E values are in agreement with the linear arrangements Ph-B-N-N and Ph-B-C-O obtained by density functional theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds are the first examples of boron analogues of diazoalkanes (R₂CNN) and ketenes (R₂CCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.

Full text of DOI:10.1021/jacs.7b08497

Article
pubs.acs.org/JACS
Photoreactions of Phenylborylene with Dinitrogen and Carbon
Monoxide
Klara Edel,† Matthias Krieg,† Dirk Grote,‡ and Holger F. Bettinger*,†
†Institut fur Organische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
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‡Lehrstuhl fur Organische Chemie II, Ruhr-Universitat Bochum, Universitatsstr. 150, 44780 Bochum, Germany
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̈
S
* Supporting Information
ABSTRACT: Formal removal of two bonding partners from boranes, BR3, yields
borylenes, RB, which have been inferred as reactive intermediates in a number of
reactions. Phenylborylene (R = C6H5; 1) is accessible from phenyldiazidoborane
by photochemical extrusion of dinitrogen under matrix isolation conditions.
Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we
used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix
isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of
phenylborylene at 375 nm (S0 → S2) and tentatively assigned the S0 → S1 transition to a very weak band at 518 nm. We also
show for the first time that an electrophilic borylene such as 1 can react with N2 reversibly and with CO irreversibly under
photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their
small E values are in agreement with the linear arrangements Ph−B−N−N and Ph−B−C−O obtained by density functional
theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for
phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds
are the first examples of boron analogues of diazoalkanes (R2CNN) and ketenes (R2CCO), and their formation may serve as a
demonstration of the high reactivity of phenylborylene.
reaction of the transient arylborylene with the 1,2-diketone
(Scheme 1d).12
INTRODUCTION
■
The boron analogues of carbenes, borylenes, which are also
known as borenes or boranediyls, are an underrepresented class
of reactive intermediates in the chemical literature. This is not
due to a lack of interest in these fundamental species of boron
chemistry but rather to the difficulties in obtaining them. For
example, early claims1 of methylborylene (CH3B) generation
by reductive elimination of dibromomethylborane (CH3BBr2)
were later on refuted.2,3 Likewise, the reported photochemical
generation of (1-naphthyl)borylene from tris(1-naphthyl)-
borane4 was shown to be incorrect.5
A number of trapping experiments were interpreted as
evidence for the involvement of borylenes. Timms, for example,
obtained 1,4-diboracyclohexadiene derivatives from trapping of
BF and BCl with ethyne (Scheme 1a).6−9 The borylenes were
obtained by comproportionation of the corresponding
trihalides over solid boron at very high temperatures. Pachaly
and West10 ascribed the products obtained from photolysis of
trisilylboranes in organic glasses in the presence of bis-
(trimethylsilyl)acetylene at 77 K to result from trapping of
silylborylenes (Scheme 1b). Curiously, the authors could not
obtain spectral evidence for the formation of silylborylenes in
the organic glass by optical spectroscopy.10 Grigsby and
Power11 reduced sterically encumbered arylboron dihalides
and interpreted the isolated products as resulting from insertion
of a transient borylene into a C−C σ bond (Scheme 1c).
Tokitoh and co-workers12 photolyzed sterically encumbered
bis(methylseleno)arylboranes in the presence of 1,2-diketones.
The isolated 1,3,2-dioxaboroles were ascribed to result from the
The reductive elimination of the adducts of dichloroboranes
and N-heterocyclic carbenes (NHCs) was investigated more
recently.13,14 The reduction of a mes2BB(Cl)2NHC with
potassium graphite yielded the product that was ascribed to
result from intramolecular CH insertion of a R2BB−NHC
species (Scheme 2a).13 The reduction of NHC−BHCl2 with
sodium naphthalide produced a cycloadduct that was inferred
to result from cycloaddition of NHC−BH to naphthaline
(Scheme 2b).14 This interpretation was challenged, however,
and a boryl radical mechanism was suggested instead.15 An
interesting recent observation is the CO extrusion from
ArB(CO)(CAAC) (CAAC = cyclic (alkyl)(amino)carbene)
to give the ArB(CAAC) adduct, which was trapped intra-
molecularly or by an added neutral donor L (Scheme 2c).16
The reactant in Scheme 2c is an example of compounds of
the type RBL2, with L being a neutral formal two-electron
donor, which are currently receiving some attention as
summarized in a recent review.17 When, e.g., a CAAC or CO
18,19
is used as a neutral donor, stable compounds RBL2
or
RBL20,21 result. These were considered as having boron in the
formal oxidation state +1 and hence were perceived as
stabilized borylenes. The boron center has lost its electrophilic
nature and acts as a Brønsted or Lewis base toward a proton or
a Lewis acid.18,22
Received: August 10, 2017
© XXXX American Chemical Society
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Scheme 1. Trapping Experiments of Borylenes
Scheme 2. Trapping Experiments of Borylene−NHC or Borylene−CAAC Adducts
Direct spectroscopic observation of organoborylenes, RB, is
extremely scarce. Andrews et al.23 observed by IR spectroscopy
that cocondensation of laser-ablated boron atoms and acetylene
in an argon matrix gives ethynylborylene next to a number of
additional products. We have reported that phenylborylene (1)
can be obtained by UV photolysis of phenyldiazidoborane (2)
in a nitrogen matrix along with nitrene 3 (Scheme 3).24 The
subvalent organoboron compound 1 was found to be
photolabile, as longer-wavelength irradiation resulted in the
formation of benzoborirene (4).24 A computational analysis of
various PhBNx isomers identified 3 as the thermodynamically
least stable isomer of PhBN2 stoichiometry and the trans-
formation 2 → 1 + 3 N2 as considerably exothermic (−21 kcal
mol−1).25
The available experimental evidence from trapping studies
suggests that borylenes RB are highly reactive transient species.
Computational analysis of the reactions with prototypical
carbon multiple bonds has revealed that borylenes behave as
electrophilic species because of their vacant p orbitals.26,27 The
transition states for cycloaddition resemble those of carbenes,
and changes in barrier heights, distances, and angles can
similarly be associated with varying borylene philicities.26,27
In our preliminary report,24 we focused on IR character-
ization of the products formed from 2 upon irradiation. We
now report and discuss the observations from UV/vis and
B
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Scheme 3. Photochemical Decomposition of Phenyldiazidoborane (2) Isolated in Solid Nitrogen at 10 K Results in
Phenylborylene (1), B-Nitreno-N-phenyliminoborane (3), and Benzoborirene (4); The Reactions and Reaction Products B-
Isocyanato-N-phenyliminoborane (5), N-Nitreno-B-phenyliminoborane or Phenyldiazoborane (6), and Phenylboraketene (7)
(Drawn in Blue) Are the Subject of the Present Work
electron spin resonance (ESR) spectroscopy along with
additional IR measurements in a nitrogen matrix as well as in
a CO-doped nitrogen matrix. The experimental investigations
are further complemented by detailed computational inves-
tigations. We report for the first time spectroscopic evidence for
the photoreaction of phenylborylene with the N2 and CO
molecules (Scheme 3, blue part). The products PhBL (L = N2,
CO) are shown to have triplet ground states.
RESULTS AND DISCUSSION
■
UV/Vis Spectroscopy. Diazide 2 has its absorption of
maximum intensity at 250 nm in solid nitrogen (Figure S1).
Irrespective of the conformation of diazide 2, a strong
absorption (S0 → S2) is computed in the 4.5−4.6 eV (270−
280 nm) range (Table S5). A much weaker (by a factor of 100)
absorption (S0 → S1) is obtained at longer wavelengths (272−
292 nm). It is thus assumed that the strong band is due to the
S0 → S2 transition.
Irradiation was performed with the output of a low-pressure
mercury lamp (λ = 254 nm). Under these conditions, a
decrease of the diazide band (250 nm) was accompanied by a
growth of signals due to 3 (709, 623, 557, 504, 413, 325, and
317 nm) and 1 (375 nm) (Figure 1). The assignments are
based on the behavior of the bands in subsequent irradiations at
various wavelengths in the absence and presence of small
amounts of CO (up to 5%), complementary IR and ESR
spectroscopy, and computations as detailed below.
The bands assigned to 3 decreased upon irradiation into the
Figure 1. UV/vis spectra of the photodecomposition of diazide 2. The
long-wavelength transitions (λ > 530 nm) in the presence of
UV/vis spectrum of diazide 2 is not shown. Top panel: black trace,
CO (Figure 2). Although no characteristic new bands were
after irradiation of a nitrogen matrix containing diazide 2 for 35 min at
observed in the UV/vis spectra, the formation of the isocyanate
PhNB(NCO) (5) was confirmed by IR spectroscopy (see
below). The only band growing under these irradiation
conditions is at around 250 nm. The isocyanate has its
strongest and longest-wavelength absorption at 265 nm (f =
0.472), and hence, experiment and theory are in agreement
with respect to the changes in the optical spectra under these
irradiation conditions (λ > 530 nm) in the presence of CO. In
the absence of CO, only slight bleaching of the bands
associated with 3 was observed upon λ > 530 nm irradiation
(Figure 1). There is some similarity to the photochemical
λ = 254 nm; red trace, after irradiation of the black trace for 2 h at 530
nm < λ < 630 nm; blue trace, after irradiation of the red trace at 495
nm < λ < 630 nm for 1 h; green trace, after irradiation of the blue trace
at λ = 254 nm for 25 min. Bottom panel: difference spectra (bands
pointing downward decrease and bands pointing upward increase
during irradiation): red trace, after irradiation at 530 nm < λ < 630 nm
for 2 h; blue trace, after irradiation at 495 nm < λ < 630 nm for 1 h;
green trace, after irradiation at λ = 254 nm for 25 min.
behavior of the triplet borylnitrene CatBN (Cat = catecholato),
which upon long-wavelength irradiation was found to react
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a factor of 4 and is expected to appear at much longer
wavelengths (529 nm).29 The long-wavelength absorption of 1
was difficult to detect in our experiments, and it is tentatively
assigned to a very weak band at 518 nm that shows the same
behavior as the band at 375 nm in irradiation experiments. The
presence of the weak band of 1 can be demonstrated by
irradiation with light of wavelength λ > 495 nm after first
irradiating with λ > 530 nm. Under these irradiation conditions,
the 375 nm band decreases quickly while the bands of 3 are
hardly affected (Figure 1). This photoreaction is partially
reversible, and irradiation with 254 nm light resulted in growth
of the signals of 1 (Figure 1). Complementary IR and ESR
experiments (see below) demonstrated that these conditions
resulted in the photoreaction of 1 with N2 to give PhB(NN)
(6). Bands grew in the 250−260 nm region, around 305 nm,
and at 459 nm (2.7 eV). Computations for 6 arrived at a weak
band at 3.1 eV (396 nm, f = 0.047) and a stronger one at 4.0 eV
(311 nm, f = 0.120), in reasonable agreement with the
experimental observations.
Shorter-wavelength irradiation (λ > 495 nm) in the presence
of CO, i.e., into the very weak transition of 1, also resulted in
the disappearance of the bands of 1 (Figure 2). Under these
conditions, a single band at 303 nm grew. IR and ESR
spectroscopy (discussed below) demonstrated that the product
of the photoreaction of 1 with CO is PhB(CO) (7).
Computations were in agreement with assignment of the 303
nm band to 7, as the strongest absorption was computed to be
at 306 nm (f = 0.320). Further discussion of the electronic
structures of 6 and 7 is presented below.
IR Spectroscopy. The previous IR spectroscopic inves-
tigation found that irradiation at λ = 350−450 nm turned 1 into
4.24 In the 1417−1387 cm−1 range a number of weak bands
that could not be assigned at that time also increased under
these irradiation conditions.24 As we deduced from the UV/vis
experiments and the computations29 that there are two low-
lying excited states of 1 (see above), we irradiated the mixture
of 1 and 3 with long-wavelength light. Upon >530 nm
irradiation, 1 was photostable while IR bands associated with 3
were hardly changed (Figure 3). Upon >495 nm irradiation, the
bands of 1 quickly decreased in intensity and new signals grew.
These bands do not belong to 4 but are assigned to 6 on the
basis of computations and isotopic labeling studies using 15N2.
The IR signals for 15N-labeled 6 are observed at 1373−1367
cm−1. The measured isotopic shift of approximately 25−28
cm−1 is in agreement with the computed one (24.9 cm−1). The
band of the ν(BNN) stretching vibration is tentatively assigned
to the feature observed at 1691 cm−1 (1752.8 cm−1 calculated
at the B3LYP/6-311+G** level). However, the band for the
isotopically labeled compound PhB(15N15N) could not be
assigned because of overlap with other signals. As observed by
UV/vis spectroscopy, the photoreaction PhB + N2 is reversible
upon 254 nm irradiation, as the signals due to 6 decreased
while those of 1 increased in intensity (Figure 3).
Figure 2. UV/vis spectra of the photodecomposition of diazide 2 in a
CO (5%) doped N2 matrix. UV/vis spectrum of diazide 2 is not
shown. Top panel: black trace, after irradiation of diazide 2 for 33 min
at λ = 254 nm; red trace, after irradiation of the black trace for 2 h at
530 nm < λ < 630 nm; blue trace, after irradiation of the red trace at
495 nm < λ < 630 nm for 1 h; green trace, difference spectrum
obtained from the red and black traces showing decreasing signals of 3.
Bottom panel: difference spectrum obtained from the blue and red
traces in the top panel, showing the decreasing signal of 1 and
increasing signal of 7. In the difference spectra, bands pointing
downward decrease and bands pointing upward increase during
irradiation.
readily with N2 to form its azide CatBN3 and with CO to form
its isocycanate CatB(NCO).28
The absorption spectrum of 3 computed at the TD-B3LYP
level places the S0 → S2 transition at 2.0 eV (634 nm) with
considerable oscillator strength. The experimental transition
energy (1.7 eV, 709 nm) is in reasonable agreement with the
computations. The S0 → S1 transition of 3 (1.6 eV) is not
dipole-allowed because S1 is 13A1 according to these
computations. The spacings between the signals in the visible
are 1947, 1902, and 1888 cm−1, indicating that the three bands
of lower intensity (623, 557, and 504 nm) are due to a vibronic
progression that is associated with the BN stretching vibration
of 3. Low-energy excited states (up to 600 nm) and a
pronounced vibrational progression were also observed in the
absorption spectrum of the triplet borylnitrene CatBN in
argon.28
The band at 375 nm was the only signal that did not change
intensity upon λ > 530 nm irradiation in the presence of CO
(Figure 2). However, the signal was quickly bleached in the
absence of CO upon shorter-wavelength irradiation (λ = 350−
450 nm), while the other bands associated with 3 were hardly
affected. We have previously shown by IR spectroscopy that
350−450 nm irradiation results in the photoreaction of 1 to
form 4.24 Hence, the feature at 375 nm is assigned to 1. A
computational investigation of the low-lying excited states of
various borylenes was reported recently.29 For borylenes such
as 1, two low-energy excited states, 11B1 (S1) and 11B2 (S2),
exist as a result of excitation from the HOMO (13a1) to the
LUMO (3b1) or LUMO+1 (8b2). The stronger absorption is
due to the S0 → S2 transition that is computed for 1 at 392 nm
(TD-B3LYP, f = 0.041).29 The S0 → S1 transition is weaker by
In a CO-doped N2 matrix, nitrene 3 reacts with CO upon
>530 nm irradiation to form PhNB(NCO), as shown by the
growth of the typical isocyanate bands at 2315−2332 cm−1 and
the stretching vibration of the iminoborane unit (Figure 4).
The latter band shows the typical 1:4 intensity ratio of the 10B
and 11B isotopologues at 2131 and 2081 cm−1. The assignment
is supported by the finding that the isocyanate band shifts upon
isotopic substitution (60−61 cm−1 for 13CO and 16−17 cm−1
for C18O) while the iminoborane stretching vibration is
essentially insensitive. The observed isotopic shifts are in
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Figure 3. IR difference spectra showing the photoreversible reaction
between 1 and 6. Signals of 1 are framed in a red box. Black traces
show spectra of a dinitrogen matrix and blue traces those of a 15N2
matrix. Before irradiation at 495 nm < λ < 630 nm, matrixes containing
diazide 2 were irradiated at λ = 254 and 530−630 nm. (a, b)
Difference spectra after irradiation at 495 nm < λ < 630 nm for 80
min; (c, d) difference spectra after irradiation at λ = 254 nm for 30
min. In the difference spectra, bands pointing downward decrease and
bands pointing upward increase during irradiation.
Figure 5. IR difference spectra showing the photoreaction between 1
and CO to form 7 through irradiation at 495 nm < λ < 630 nm for 1 h
in a dinitrogen matrix doped with 5% isotopically labeled carbon
monoxide. Before irradiation at 495 nm < λ < 630 nm, matrixes
containing diazide 2 were irradiated at λ = 254 nm for 30 min and at
530−630 nm for 2 h. Decreasing signals of 1 are framed in a red box.
In the difference spectra, bands pointing downward decrease and
bands pointing upward increase during irradiation.
The same set of signals was already observed in the previous
irradiation of the same CO-doped matrix at λ = 254 nm. The
signals at 2012/2006 cm−1 belong to the ν(BCO) stretching
vibration and show isotopic shifts of 51 and 26 cm−1 upon the
use of 13CO and C18O, respectively, in good agreement with
the computed values of 52.8 and 26.1 cm−1 for PhB(13CO) and
PhB(C18O), respectively, obtained at the B3LYP/6-311+G**
level of theory. Signals around 1400 cm−1 overlap with signals
of 6 which is formed under the same irradiation conditions
because of the presence of an excess of dinitrogen in the matrix.
Besides the signals of 7, a new band at 1980 cm−1 was observed
upon irradiation at >495 nm. This signal behaves differently
than the signals of 7 upon irradiation at λ = 254 nm. Because of
the shift of the band in the presence of 13CO, its assignment to
PhBO (ν
̃
= 1977−1971 cm−1 in Ar)32 is excluded. While the
Figure 4. IR difference spectra showing the photoreaction between 3
and CO to form 5 through irradiation at 530 nm < λ < 630 nm for 2 h
in a dinitrogen matrix doped with 5% isotopically labeled carbon
monoxide. Before irradiation at 530 nm < λ < 630 nm, matrixes
containing diazide 2 were irradiated at λ = 254 nm for 30 min. The top
trace shows a spectrum of 5 computed at the B3LYP/6-311+G** level
of theory. In the difference spectra, bands pointing downward decrease
and bands pointing upward increase during irradiation.
newly formed species cannot be assigned, we note that a
possible candidate could be the bis-CO derivative PhB(CO)2
(Table S4).19
In summary, the behaviors of the bands assigned to 1 and 3
under various irradiation conditions are identical under both IR
and UV/vis spectroscopic detection in support of the
assignments to 1 and 3.
ESR Spectroscopy. The earlier assignment of triplet 3 as
the major product of photodecomposition of diazide 2 was
based on IR spectroscopy and supported by an ESR
spectroscopy investigation.24 A triplet signal observed in
argon with zero-field splitting (zfs) parameters |D/hc| = 1.240
cm−1 and |E/hc| = 0.0021 cm−1 was observed after extended
irradiation (λ = 254 nm) and assigned to 3.24 In the present
work, additional measurements were performed in solid
nitrogen at 4 K. This results in slightly changed zfs parameters
for 3 (|D/hc| = 1.247 cm−1 and |E/hc| = 0.002 cm−1).
An additional but much weaker signal with smaller zfs
parameters (in argon,24 |D/hc| = 0.870 cm−1 and |E/hc| =
0.0007 cm−1; in nitrogen, |D/hc| = 0.882 cm−1 and |E/hc| <
0.001 cm−1) was also observed after irradiation of 2 at λ = 254
nm. The carrier of this signal could not be firmly assigned in
our previous investigation.24 Monitoring the subsequent longer-
wavelength irradiation (λ = 350−450 nm) by ESR spectroscopy
agreement with those computed for 5 (63.4 cm−1 for 13CO and
13.6 cm−1 for C18O, see Table S1). Interestingly, computations
arrived at a linear structure for this isocyanate (see Figure S2).
This is most likely due to the adjacent electron-deficient
dicoordinate boron center, similar to the situation in electron-
deficient linear ketenimines.30 While isocyanates of iminobor-
anes were not considered previously, dimethylisocyanatobor-
ane, (CH3)2BNCO, was found to feature a bent isocyanato
group with a very low barrier for isomerization,31 in agreement
with our computations (see Figure S2).
After much of nitrene 3 was converted to 5, switching the
wavelength of irradiation to >495 nm resulted in a quick
decrease of the bands of 1 and the growth of a set of new
signals (Figure 5). The photoproduct is assigned to boraketene
7 on the basis of isotopic shifts (13CO and C18O) and
comparison with computed harmonic vibrational frequencies.
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Figure 6. ESR spectra (4 K) obtained after 254 nm irradiation of 2 in (a−c) N2 and (d−f) CO-doped N2 (5%). Black, spectra before irradiation
given in the panel; red, spectra after irradiation given in the panel. Arrows pointing up or down indicate an increase or decrease in the corresponding
absorption under the irradiation conditions given.
Figure 7. Natural orbitals and occupation numbers of the B−C−O unit of PhBCO (3A2) computed at the UB3LYP/6-311+G** level of theory.
in the present work showed that the weak signal intensifies
while the strong one associated with 3 stays almost constant
(Figure S3). Under these irradiation conditions, 1 undergoes
the photoreaction to form benzoborirene,24 an ESR-silent
singlet species, and reacts with N2 to afford 6, as shown above
by IR spectroscopy. Switching the wavelength of irradiation to
λ = 308 nm (XeCl excimer laser) reduces the intensity of the
signal with the smaller D value, while the signal due to 3 again
stays constant.
As the IR and UV/vis experiments showed wavelength-
selective photochemistry of 3 (>530 nm) and 1 (>495 nm) in
the absence or presence of CO, additional experiments were
performed. In the absence of CO, irradiation at 630 nm using a
light-emitting diode (LED) results in weakening of the ESR
signal of 3 (Figure 6a), probably as a result of the reaction with
N2 to give PhNBN3. Switching the irradiation wavelength to
505 nm (LED) causes an increase in the weaker signal (Figure
6b), while this signal decreases again upon short-wavelength
irradiation (254 nm) (Figure 6c). As under long-wavelength
irradiation conditions 1 reacts with molecular nitrogen to afford
6 and as this can be cleaved to 1 + N2 upon short-wavelength
irradiation according to IR and UV/vis measurements, the weak
ESR signal is assigned to the xy2 transition of the triplet ground
state of 6, in agreement with the previous suggestion.24
In the presence of CO, the strongest ESR signal is still due to
3 after photodecomposition of the diazide by 254 nm
irradiation (Figure 6d). This strongest signal decreases in
intensity upon irradiation with 630 nm light (Figure 6d). This
observation is in agreement with the IR and UV/vis
measurements and is due to the formation of isocyanate 5.
Besides the weak signal of 6, another triplet signal (|D/hc| =
0.525 cm−1 and |E/hc| < 0.0005 cm−1) of higher intensity can
be observed already after 254 nm irradiation (Figure 6d). The
signal of medium intensity and that of 6 increase upon 505 nm
irradiation (Figure 6e). Upon short-wavelength irradiation, only
the signal of 6 decreases (Figure 6f). As the IR and UV/vis
measurements confirmed the formation of 6 and 7 under these
irradiation conditions, the new signal is assigned to the latter
compound. The small E value (<0.0005 cm−1) is indicative of a
structure that is almost cylindrical, in agreement with the
computed linear arrangement of the Ph−B−C−O unit.
The preference of a triplet electronic ground state (3A2) for 3
and 6 was discussed previously on the basis of results obtained
with multireference second-order perturbation theory
(MRMP2).25 Briefly, simple molecular orbital (MO) analysis
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revealed that each of the two compounds features two
orthogonal π systems with three π orbitals at the NBN or
BNN termini that are almost degenerate. As each of the termini
has six π electrons, a high-spin state results, and this was
confirmed by the MRMP2 computations.25 For isoelectronic 7
a similar electronic structure was expected and confirmed by
computations (see the MOs in Figure 7).
The ground state of linear 7 is the A2 state, while the A2
and 1A1 states are higher in energy by 10.1 and 12.0 kcal mol−1
at the CASPT2-CAS(12,12)/cc-pVTZ//B3LYP/6-311+G**
level of theory (Table 1). The triplet ground state of 7 is in
line with the ESR spectroscopy experiments discussed above.
that phenylborylene (1) forms along with nitrene PhNBN (3),
which is the major persistent photoproduct. PhNBN could be
characterized by ESR, IR, and UV/vis spectroscopy. This
nitrene absorbs in the visible region beyond 700 nm with rather
sharp absorption bands and a pronounced vibrational
progression. PhNBN reacts photochemically with CO upon
>530 nm irradiation to give the linear isocyanate PhNB(NCO)
(5).
Compound 1 is a minor product of the photodecomposition
of 2. We could determine the energies of the two lowest-energy
excited states, which are in the visible (tentatively 518 nm) and
UV (375 nm) regions. This marks the first observation of the
absorption energies of a borylene larger than a diatomic one
and is expected to be very important for future transient
absorption spectroscopy experiments with phenylborylene, in
particular as the experimental observations confirm earlier
computations.29
Our study provides the first example of the direct observation
of a reaction of a borylene (RB). Phenylborylene reacts under
photochemical conditions with visible light (>495 nm) with N2
and CO to give PhBNN (6) and PhBCO (7), respectively. The
photoproducts are ground-state triplet species with a linear
arrangement at the boron center according to ESR and
computational investigations. The |D/hc| value of 0.5 cm−1 for
7 is similar to that of phenylcarbene (PhCH), showing that the
isoelectronic relationship of the dicoordinate boron and carbon
centers is also reflected in the electronic properties.
3
1
Table 1. Vertical Excitation Energies (in kcal mol−1)
Computed at the CASPT2-CAS(12,12)/cc-pVTZ//B3LYP/
6-311+G** Level of Theory
compound
3A2
1A2
1A1
PhNBN
PhBNN
PhBCO
0
0
0
16.1
15.5
10.1
18.5
18.2
12.0
The spin density at the terminal nitrogen center is larger in 3
than in 6 according to computations (Figure 8). This is in
agreement with the larger |D/hc| value of 3 (1.247 cm−1), which
is in the range that is typical for nitrenes with weak
delocalization. For example, the closely related boryl nitrenes
CatBN and PinBN (Pin = pinacolato) have |D/hc| values of
1.49 and 1.57 cm−1, respectively,28,33,34 while the |D/hc| value
for phenylnitrene is 0.99 cm−1.35 The spin density at the boron
center is quite large in 6, and in agreement with the stronger
delocalization, the |D/hc| value is smaller (0.882 cm−1). In
comparison with 6, the spin density at the boron center is still
larger in 7. The boron atom in 7 is isoelectronic to the carbon
center in a carbene (RCR), and indeed, the |D/hc| value of
0.525 cm−1 is similar to that of phenylcarbene (|D/hc| = 0.518
cm−1 in Fluorolube at 77 K).36
EXPERIMENTAL DETAILS
■
Synthesis and Handling of PhB(N3)2. CAUTION! Phenyl-
diazidoborane is explosive. While no explosions were experienced in
our laboratory, utmost care must be taken when handling the
compound. We used a face shield, Kevlar gauntlets and gloves, and a
leather apron. The compound was only prepared in small amounts
(max. 10 mg) from commercially available dichlorophenylborane and
chlorotrimethylsilane in dichloromethane following the procedure
described by Mennekes and Paetzold.37
Matrix Isolation Experiments. Matrix experiments were
performed by standard techniques38 using a SHI CKW-21A displex
closed-cycle helium cryostat (IR and ESR) and a CTI Cryogenics 8200
compressor (Brooks Automation) (UV). Phenyldiazidoborane was
sublimed from a glass flask at 0 °C and condensed onto a cold CsI
(IR) or sapphire (UV) window or on a copper rod (ESR) with a large
CONCLUSIONS
The present investigation of the photodecomposition of
phenyldiazidoborane (2) upon 254 nm irradiation showed
■
Figure 8. Computed (UB3LYP/6-311+G**) spin densities.
G
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
excess of dinitrogen 6.0 (Westfalen AG), 15N2 2.2 (Westfalen AG), or
dinitrogen doped with 5% carbon monoxide 3.7 (Westfalen AG),
C18O 2.0 (95% 18O) (Sigma-Aldrich), or 13CO 2.3 (99.1% 13C)
(Westfalen AG). The gases or gas mixtures were dosed to 2.0 sccm
(IR), 1.5 sccm (UV), or 3.5 sccm (ESR) by a mass flow controller
(MKS mass flow PR400B). The deposition temperature was 28 K.
FTIR spectra were measured between 4000 and 400 cm−1 on a Bruker
Vertex 70 spectrometer using a resolution of 0.5 cm−1. UV/vis spectra
were measured on a PerkinElmer Lambda 1050 spectrophotometer. X-
band ESR spectra were recorded on a Bruker Elexsys E500 ESR
spectrometer with an ER077R magnet (75 mm pole cap distance), an
ER047 XG-T microwave bridge, and an oxygen-free high-conductivity
copper rod (75 mm length, 3 mm diameter) cooled by a closed-cycle
cryostat. Computer simulations of the ESR spectra were performed
using the XSophe computer simulation software suite (version
1.0.4),39 developed by the Centre for Magnetic Resonance and
Department of Mathematics, University of Queensland (Brisbane,
Australia) and Bruker Analytik GmbH (Rheinstetten, Germany).
Irradiations were achieved with a low-pressure mercury lamp (UVP,
253.7 nm) and an Osram HBO-500-W/2 high-pressure mercury lamp
in an Oriel housing with quartz optics and dichroic mirrors (350−450
and 420−630 nm). Appropriate cutoff filters (Schott) were used. For
the ESR experiments, LEDs with maximum outputs at 505 and 630
nm and a XeCl excimer laser (λ = 308 nm) were used.
Computations. Geometries were optimized using the B3LYP
hybrid density functional40,41 as implemented42 in Gaussian 0943 using
the 6-311+G** basis set. Harmonic vibrational frequencies were
computed at the B3LYP/6-311+G** level analytically. The B3LYP/6-
311+G** geometries were employed for the computation of excitation
energies and oscillator strengths using time-dependent DFT (TD-
B3LYP/6-311+G**).44
Vertical excitation energies were computed for PhNBN, PhB(N2),
and PhB(CO) at the B3LYP/6-311+G** geometry of the X3A2 state
using internally contracted complete-active-space second-order
perturbation theory (CASPT2) as described by Werner45,46 in
conjunction with Dunning’s correlation-consistent triple-ζ basis set
(cc-pVTZ).47 The active space chosen was identical to that used in
earlier adiabatic MRMP2 computations, which were run, however,
with the much smaller 6-31G* basis set.25 In particular, the active
space consisted of 12 orbitals (π1 to π6 of the phenyl ring as well as
three pπ and three pσ orbitals at the X−Y−Z terminus) and was
occupied by 12 electrons. The correlated computations applied the
frozen core approximation and were run using the Molpro
program.48,49
performed on the BwForCluster JUSTUS. The authors
acknowledge support by the state of Baden-Wurttemberg
̈
through bwHPC and the DFG through Grant INST 40/467-1
FUGG.
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AUTHOR INFORMATION
Corresponding Author
*holger.bettinger@uni-tuebingen.de
ORCID
Holger F. Bettinger: 0000-0001-5223-662X
Notes
The authors declare no competing financial interest.
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