Switching the Spin State of Pentafluorophenylnitrene: Isolation of a Singlet Arylnitrene Complex

Article abstract of DOI:10.1021/jacs.8b10792

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements, which diminish the use of arylnitrenes as intermediates in organic synthes

Full text of DOI:10.1021/jacs.8b10792

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Article
Switching the Spin State of Pentafluorophenylnitrene:
Isolation of a Singlet Arylnitrene Complex
Joel Mieres-Perez, Paolo Costa, Enrique Mendez-Vega, Rachel Crespo-Otero, and Wolfram Sander
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Nov 2018
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Switching the Spin State of Pentafluorophenylnitrene: Isolation of a
Singlet Arylnitrene Complex
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*
Joel Mieres-Perez†1, Paolo Costa†1, Enrique Mendez-Vega†, Rachel Crespo-Otero‡, Wolfram Sander†
†Lehrstuhl für Organische Chemie II, Ruhr Universität Bochum, 44780 Bochum, Germany
‡
School of Biological and Chemical sciences, Queen Mary University London, Mile End Road, London E1 4NS, UK
Supporting Information Placeholder
ABSTRACT: The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in
radical-type reactions and unwanted rearrangements which diminish the use of arylnitrenes as intermediates in organic synthesis. While the
closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed shell singlet states
of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis
acid BF3 under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two
molecules of BF3 is formed. Although the closed shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25
kcal/mol above its triplet ground state, the reaction with BF3 results in switching the spin state from triplet to singlet. The formation of the
singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental
findings.
INTRODUCTION
Aryl nitrenes1 are key intermediates in a large number of reactions which found applications in photoaffinity labelling,2-4
photolithography,3 and as building blocks for organic magnetic materials.5-7 The photochemical and thermal decomposition
of arylazides as suitable precursors of arylnitrenes has therefore been extensively studied (Scheme 1).1, 8-12 The chemistry of
arylnitrenes is complex, since, although these nitrenes show robust triplet ground states, they are born as highly reactive,
long-lived open-shell singlets from their precursors. The intersystem crossing rates are frequently low, and therefore open-
shell singlet reactivity (e. g. rearrangements) efficiently competes with the triplet ground state chemistry.13 In the related
carbenes, most of the useful chemistry arises from closed-shell (zwitterionic) singlet states, e. g. insertion reactions,
cycloadditions, Lewis base chemistry, or ylide formation. For nitrenes, the closed-shell singlet states are too high in energy
to be chemically accessible. Here, we suggest a strategy to stabilize the closed-shell singlet states of nitrenes and to control
the chemistry of these species via Lewis acid-base interactions.
Phenylnitrene 1a is conveniently produced by photolysis14 or flash vacuum pyrolysis15 of phenylazide 2a. Nitrene 1a is highly
labile and easily rearranges to the cyclic ketenimine 4a with azirine 3a as intermediate.1, 16-17 The ground state of 1a was found
to be triplet by EPR spectroscopy,14 and the singlet-triplet splitting (∆EST) determined to 14.8 kcal/mol by negative ion
photoelectron spectroscopy.18 The large ∆EST in phenylnitrene is not much dependent on substituents in the phenyl ring,18-
19 in good agreement with predictions from high-level ab initio calculations.20
1 Both authors contributed equally to this work.
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Scheme 1. Rearrangements of phenylnitrene 1a.
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1
N3
N
N
N
ν
h
∆
or
- N2
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8
osS-
2a
1a
3a
4a
9
ISC
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3
N
N
N
1a
T-
5a
Singlet phenylnitrene S-1a is a transient species that was detected by laser flash photolysis showing an absorption maximum
at 350 nm and a lifetime of approximately 1 ns.21-22 The lowest-lying singlet state of 1a is the open-shell singlet state osS-
1a(1A2), whereas the closed-shell singlet state csS-1a (1A1) lies 30 kcal/mol above the triplet ground state T-1a (3A2).8, 23
Characteristic of osS-1a is the delocalization of one of the unpaired electrons resulting in a short C-N bond distance. This
resembles the electronic structure of the benzyl radical, and consequently, this state is stabilized by radical stabilizing
substituents. Based on this hypothesis, Wenthold predicted ∆ΕS-T for 2-furanylnitrene 6 to be 4.5 kcal/mol smaller than that
in 1a.24
N
N
N
osS-
csS-
1A1
1a
1a
1a
T-
3A2
1A2
(
)
(
)
(
)
The only known singlet ground state nitrenes where the nitrene center is attached to a carbon atom are α-oxonitrenes and
related species, such as acylnitrenes. In these nitrenes, the closed-shell singlet state is stabilized by an oxazirine-like resonance
structure.25 For the parent HCNO 7 in its singlet ground state the C-O, C-N, and O-N bond lengths were calculated to 1.307,
1.260, and 1.816 Å, respectively (CCSD(T)/cc-pV TZ), indicating a hybrid nitrene-oxazirine structure with a very strong
N...O interaction.26 Acylnitrenes are intermediates in the Curtius rearrangement, and several derivatives were detected by
time-resolved spectroscopy27 or could be isolated in inert gas matrices.26 In a recent publication, Zeng et al. described that
furoylnitrene 8 could be characterized in their singlet states in inert gas matrices, while in solid toluene at 10 K the triplet
state was observed by EPR spectroscopy.28 This indicates a very small singlet-triplet gap in 8, although it is not clear how the
different matrix environments (argon vs. toluene) influences the gap. The singlet states of nitrenes are also stabilized when
the nitrene center is attached to heteroatoms such as transition metals (metal-nitrido complexes), nitrogen (aminonitrenes)
or phosphorus (phosphinonitrenes).29
N
N
C O
N
N
C O
H
C O
H
O
O
7
6
8
Recently, we demonstrated that the closed-shell singlet states of arylcarbenes such as diphenylcarbene 9 can be stabilized by
interactions with Lewis acids such as water,30-31 methanol,32-33 CF3I,34 or BF3.35 Thus, while the triplet ground state of
diphenylcarbene T-9 lies approximately 5 kcal/mol below the closed-shell singlet state S-9, the singlet state becomes ground
state upon interaction with a single molecule of water or methanol. This allowed us to isolate and spectroscopically
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characterize complexes of singlet states of carbenes that without interactions with the Lewis acids would be excited states.
With BF3, zwitterion 10 is formed in a highly exothermic reaction. Zwitterion 10 consists of a positively charged benzhydryl
and a negatively charged BF3 unit.
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7
in Ar
Ph
Ph
Ph
Ph
Ph
Ph
405 nm
254 nm
1% BF3
F
BF3
10
annealing 25 K
BF2
8
9
T-
9
It is tempting to use the same method to stabilize the closed-shell singlet states of arylnitrenes by interactions of the nitrenes
with Lewis acids. However, given the very large energy gaps between the triplet ground states and the closed-shell singlet
states of arylnitrenes in the order of 30 kcal/mol, only very strong Lewis acids such as BF3 could possibly switch the spin
states of arylnitrenes. While reactions of arylazides with BF3 are well known,36 thorough mechanistic studies of reactions of
arylnitrenes with BF3 are lacking. To suppress unwanted photochemical rearrangements of the arylnitrenes during their
synthesis, we selected pentafluorophenlynitrene 1b as a suitable nitrene that can be synthesized in high yields by photolysis
of the corresponding arylazide 2b. Initial experiments with water, methanol, and CF3I confirmed our expectation that the
singlet-triplet splitting of 1 is far too high to make the singlet state ground state by interaction with these molecules. Here,
we describe the interaction of triplet nitrene T-1b with the strong Lewis acid BF3 that results in the formation of a closed-
shell singlet state nitrene complex.
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Results and Discussion
IR experiments. Triplet pentafluorophenylnitrene T-1b was generated by λ = 254 nm irradiation of pentafluorophenylazide
2b, matrix-isolated in argon at 3 K. Under these conditions, high yields of nitrene T-1b are obtained and only small amounts
of rearranged products.37 If the argon is doped with 1% BF3, photolysis of 2b produces again high yields of T-1b as long as
the matrix is kept at temperatures below 10 K to prevent diffusion of trapped molecules and thus bimolecular reactions
(Figure 1). However, annealing of this matrix at 20 K for several min results in a decrease of the intensity of all IR bands
assigned to T-1b and to BF3 monomer. In addition to the IR bands of the oligomers of BF3 at 1421, 1472 and 1515 cm-1, two
weak bands appear at 984 and 1031 cm-1 which are assigned to cis-1,2-bis(perfluorophenyl)diazene 5b. Additionally, small
shifts in the positions of the IR bands of T-1b are observed as result of the relaxation of the matrix. Similar small shifts were
observed if an argon matrix containing T-1b without BF3 was annealed at 20 K for 10 min (Figure S1, SI).
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Figure 1. IR spectra showing the reaction of T-1b with BF3. a) IR spectrum of T-1b in an argon matrix doped with 1% of BF3 at 3 K generated by λ =
254 nm photolysis of 2b. b) Difference IR spectrum of the same matrix showing the changes after annealing at 20 K for 10 min. Bands pointing
downwards assigned to T-1b and BF3 are disappearing, and bands pointing upwards assigned to 5b and oligomers of BF3 are appearing. c) Difference
IR spectrum of the same matrix showing changes after annealing at 35 K for 5 min. Bands pointing downwards assigned to T-1b are disappearing, and
bands pointing upwards assigned to S-1b…2BF3 are appearing. d) IR spectrum of an argon matrix containing 1% of BF3 annealed from 3 K to 20 K for
10 minutes. e) Calculated spectra of csS-1b…2BF3 at the M06-2X/6-311++G(d) level of theory considering the natural abundancies of the boron
isotopes. The black trace is the corresponding sum of the spectra of the different isotopes. f) Difference IR spectrum showing changes after irradiation
of the matrix with λ = 530 nm for 20 min. Bands pointing downwards assigned to complex csS-1b…2BF3, T-1b and dimer cis-5b are disappearing. Bands
pointing upwards correspond to 3b, trans-5b and some unidentified products.
Interestingly, additional annealing of the matrix for several min at 35 K results in a characteristic change of the color of the
matrix from colorless to red, (Figure S6, SI). During this annealing process, all IR bands corresponding to T-1b and BF3
decrease in intensity and a new set of bands appears with the strongest absorptions at 1021.5, 1059 and 1605.6 cm-1 (Figure
1). If the matrix contained only 0.2% of BF3, annealing resulted in less coloration and smaller IR peaks of the new product
(Figure S2, SI). This indicates that nitrene T-1b reacts with BF3, and that this reaction requires high concentrations and a
high mobility (at temperatures above 20 K in argon) of BF3. The species formed by the interaction between T-1b and BF3 is
photolabile, and irradiation with λ = 530 nm produces benzazirine 3b plus further IR bands of unidentified products. In
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addition, the trans form of 5b was found in the spectrum by comparison with an authentic matrix isolated sample. This
selective photochemistry helped to identify IR bands of the new species.
1
2
The IR spectrum of the reaction product between T-1b and BF3 was compared to calculated spectra of a variety of possible
reaction products. The best agreement was found for the closed-shell singlet nitrene csS-1b interacting with two BF3
molecules (Figure 1, Figure S3). In this singlet complex csS-1b...2BF3, the two BF3 molecules interact via an unusual
fluoronium bridge (Figure 1). The IR spectrum of csS-1b…2BF3 shows a weak IR band at 555 cm-1, which is assigned to the
asymmetric B-F-B stretching vibration of the fluoronium bridge, (Table S1, see SI). A similar B-F-B bridge with a vibration
at 537 cm-1 was recently reported for the complex between tetrachlorocyclopentadienylidene 11 and two molecules of BF3.38
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EPR experiments. The EPR spectrum of T-1b was recorded in argon doped with 1% of BF3 at 5 K, under similar conditions
as in the IR experiments. The intense transition at 713 mT observed in the EPR spectrum (Figure S4) is typical for triplet
arylnitrenes. Annealing at 20 K for 5 min and subsequent cooling to 3 K resulted in a loss of 10% of the EPR signal intensity
of T-1b. A loss of about 50% of T-1b is found when the matrix is annealed at 30 K (Figure S4). During these annealing
experiments the concentration of radical species increases only slightly, indicating that T-1b reacts with BF3 to form a
diamagnetic species.
UV-vis Experiments. Pentafluorophenylazide 2b was co-deposited with argon doped with 1% of BF3 onto a sapphire
window cooled at 8 K. Irradiation of the matrix with UV light (254 nm) produced the UV-vis spectrum of T-1b,
characterized by maxima at λ = 228 nm, 281 nm, 312 nm and 380 nm (Figure 2 – black trace).39a Upon annealing at 25 K
and 35 K for 10 min each, new maxima appear at λ = 324 nm and 341 nm (Figure 2 – blue trace), which are assigned to
dimer 5b by comparison with an authentic matrix isolated sample.
No other bands in the UV-vis region have been detected except those of 5b. In an additional experiment,
pentafluorophenylazide 2b was co-deposited with argon doped with 1% of BF3 in larger amount compared to the previous
measurement. Irradiation with UV light at λ = 254 nm of such matrix generated T-1b, whose UV transitions cannot be
detected anymore due to the signal saturation caused by the high concentration used. Upon annealing to 25 K, a broad visible
band appeared at 522 nm, which increased remarkably in intensity after warming up the matrix at 35 K. This visible band
was assigned to the csS-1...2BF3, in agreement with TD-DFT calculations (M06-2X/6-311++G(d)) that predict a weak band
in the visible region at 429 nm (Figure 2). Several arylnitrenium ions were produced by protonation of singlet arylnitrenes
and studied by time resolved spectroscopy and in HCl-doped argon matrices.39b These arylnitrenium ions also exhibit
absorptions in the visible region, similar to that of csS-1...2BF3.
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Figure 2. Panel A – UV-vis experiment with low concentration of pentafluorophenylazide 2b. Dotted line: UV-vis spectrum of pentafluorophenylazide
2b in an argon matrix doped with 1% of BF3. (Black line) UV-vis spectrum of T-1b generated by photolysis of 2b with light at λ = 254 nm. Blue line:
UV-vis spectrum of the same matrix after annealing from 8 to 25 K for 10 minutes. Red line: UV-vis spectrum of the same matrix after annealing to 35
K for 10 minutes. Panel B – UV-vis experiment with high concentration of pentafluorophenylazide 2b. Dotted line: UV-vis spectrum of
pentafluorophenylazide 2b in an argon matrix doped with 1% of BF3. Black line: UV-vis spectrum of T-1b generated by photolysis of 2b with light at λ
= 254 nm. Blue line: UV-vis spectrum of the same matrix after annealing from 8 to 25 K for 10 minutes. Red line: UV-vis spectrum of the same matrix
after annealing to 35 K for 10 minutes. The bottom region of the figure shows the spectral transitions of csS-1b…2BF3 calculated at the M06-2X/6-
311++G(d) level of theory.
Calculations. The three electronic states of pentafluorophenyl nitrene T-1b were considered: the triplet ground state T-1b
(3A2), the open-shell singlet state osS-1b (1A2), and the closed-shell singlet state csS-1b (1A1), as well as their complexes with
BF3. The energies of T-1b and csS-1b were calculated at the CCSD(T)/ 6-311++G(d) //M06-2X/6-311++G(d) level of
theory, whereas the open shell singlet state osS-1b requires to use multireference methods, and therefore
CASPT2(10,10)/6-311GG(d)//CASSCF(10,10)/6-311G(d) was used. These computations suggest that osS-1b lies 17.6
kcal/mol and csS-1b even 25.7 kcal/mol above the triplet ground state T-1b (Figure 3). These values are similar to what was
reported for the parent phenyl nitrene 1a.40
CCSD(T) calculations of the BF3 complexes of nitrene 1b predict that the closed shell singlet complex csS-1b...BF3 is
stabilized by 24.6 kcal/mol, while the triplet complex T-1b…BF3 is stabilized by only 5.2 kcal/mol for (Figure 3). As a result,
the energy gap between T-1b and csS-1b is reduced from 25.7 kcal/mol in the free nitrene to only 6.3 kcal/mol in the
complex, whereas the gap between T-1b and osS-1b remains still large at 17.6 and 19.5 kcal/mol with respect to the
corresponding triplet complexes (Figure 3). The B-N distance in csS-1b...BF3 is 1.63 Å (M06-2X/6-311++G(d)) similar to
that in H3N...BF3,41 whereas in T-1b…BF3 it is considerably longer (2.43 Å). Other structural parameters are also markedly
different: the triplet complex T-1b…BF3 is linear with a C-N-B bond angle of 180°, whereas in csS-1b...BF3 the C-N-B bond
angle is 125.8° and the boron atom lies in the ring plane (Figure 3).
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Figure 3. Relative energies of the triplet and singlet complexes of 1b with one and two BF3 molecules at the CCSD(T)/6-311++G(d)//M06-2X/6-
311++G(d) level of theory. The energy gap obtained at the CASPT2(10,10)/6-311G(d)//CASSCF(10,10)/6-311G(d) level of theory between
triplet and open-shell singlet states of the nitrene and its complex with one BF3 molecule is represented in dashed blue. The structure of the crossing
point between the singlet and triplet surfaces of complex 1b...2BF3 is shown in Figure S10 (SI).
The very different interaction of BF3 with T-1b and csS-1b can be also demonstrated by following the interaction energies
between 1b and BF3 and the changes in hybridization of the lone pair at the nitrene center (Figure 4). For both T-1b and
csS-1b the sigma orbital has a high s character of 70% and 65%, respectively. Once the BF3 molecule approaches, the s
character in csS-1b is drastically reduced to 33% by efficient rehybridization with the doubly filled px orbital, resulting in two
sp2 orbitals. This rehybridization is also reflected in the C-N-B bond angle in the complex of 125.8°. The hybridization of
the approaching BF3 molecule changes in a similar way from sp2 in the initial stage to sp3 in the final complex, resulting in a
strong pyramidalization of BF3 in csS-1b...BF3 and a very large energetic stabilization. In T-1b, the rehybridization of the
nitrene lone pair is not possible, and the s character remains at 70%, irrespective of the B-N distance. The BF3 molecule
approaches the nitrene along the B-N axis, and the resulting stabilization energy is small.
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Figure 4. Relax scan of the energy of csS-1b…BF3 and T-1b…BF3 along the C–B distance. Plot and s-character of the doubled-occupied NBMOs are
shown at selected points to visualize the rehybridization of the orbitals at the N center of S-1b, upon complexation with BF3. Energies and electronic
properties were calculated at the (U)M06-2X/6-311++G(d) level of theory.
The interaction energy ∆E between the nitrene 1b and a BF3 molecule can be partitioned into the deformation (EN + EBF
and instantaneous energies (Eint).42-43
)
3
∆E = Eint + (EN + EBF3
)
EN and EBF are the energies required to deform the nitrene and the BF3 from their relaxed nuclear configurations to the
3
geometries in the complex. Eint is the instantaneous interaction energy of the deformed fragments in the complex (Table 1).
Since in the triplet complex T-1b…BF3 the fragments are only very slightly deformed compared to the non-interacting
fragments, Eint is with -6.0 kcal/mol only by 0.8 kcal/mol more negative than ∆E. For S-1b…BF3, the energy required for
the deformation of the nitrene (EN = 2.4 kcal/mol) and the BF3 (EBF3 = 26.9 kcal/mol) is much larger, however, more than
compensated by the huge Eint of -53.9 kcal/mol. The interaction energy ∆E of csS-1b…BF3 is very similar to that of ammonia
borane N3H…BF3, while the intrinsic interaction energy Eint is even higher (Table 1). This indicates that the closed-shell
singlet state csS-1b acts as a strong Lewis base, comparable to ammonia, whereas T-1b is only weakly basic.
Table 1. Deformation of the BF3 fragment and energetics of Lewis acid-base complexes.
EN
0.1
2.4
0.2
EBF3
23.0
26.9
0.6
Eint
∆E
∠ FFFB
26.6°
28.5°
5.0°
NH3...BF3
csS-1b…BF3
T-1b…BF3
–47.3
–53.9
–6.0
–24.2
–24.6
–5.2
Complexation energy between the fragments (∆E) partitioned into interaction energy of the frozen fragments (Eint) and deformation energy of the base
fragment (EN) and the BF3 fragment (EBF3). Energies were calculated at the CCSD(T)/6-311++G(d) //M06-2X/6-311++G(d) level of theory and
are given in kcal/mol. The FFFB dihedral angle correspond to the angle between the FFF and FFB intersecting planes and has a zero value for the non-
distorted BF3 molecule.
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These calculations indicate that forming the BF3 complex drastically reduces the S-T gap of nitrene 1b, however, not enough
to switch the ground state of the nitrene complex to singlet. Thus, to achieve a singlet ground state Lewis acid-base complex
of 1b, a stronger Lewis acid than BF3 is required.
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Scheme 2. Reaction of pentafluoronitrene 1b with BF3 and structure of S-11...2BF3.
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Cl
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We recently observed that the Lewis acidity of BF3 in a carbene complex increases by adding a second BF3 molecule to the
complex resulting in ternary complexes.44 In the complex between tetrachlorocyclopentadienylidene S-11 and two
molecules of BF3, the second BF3 interacts with the first BF3 via a strong B-F-B fluoronium bridge, but does not directly
interact with the carbene as Lewis base (Scheme 2). In a very similar way, the singlet state csS-1b forms a ternary complex
csS-1b…2BF3 with two molecules of BF3 in which one of the BF3 molecules is directly interacting with the nitrene center of
csS-1b, while the second BF3 is attached to the first via a B-F-B fluoronium bridge (Figure 5). In csS-1b…2BF3, the BF3
molecule directly interacting with 1b adopts a quasi-tetrahedral geometry with B-F bonds considerably longer than in the
isolated BF3 molecule. The bridging F atom is almost equally shared between the two boron atoms, resulting in a less negative
partial charge at this fluorine atom compared to the other fluorine atoms (see electrostatic potential in Figure 5 and natural
charges in Figure S9, SI). Furthermore, one of the fluorine atoms of the second BF3 molecule donates electron density to the
empty π orbital in the nitrene, resulting in additional stabilization of the ternary complex. The stabilization of csS-1b…2BF3
is with 39.3 kcal/mol large enough to become more stable (by 2.2 kcal/mol) than the triplet complex T-1b…2BF3, thus
switching the ground state of the complex from triplet to singlet (Figure 3), in accordance with our experimental results.
Figure 5. Some structural parameters of the optimized structure of csS-1b…2BF3 computed at the M06-2X/6-311++G(d) level of theory. Plots of the
electrostatic potential (isosurface value 0.05) are shown below (blue: positive potential, red: negative potential).
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Conclusions
1
The chemistry of arylnitrenes is dominated by open-shell triplet and singlet states frequently resulting in unwanted
polymeric byproducts.13, 45 The strategy presented here to overcome this deficiency is to stabilize the closed-shell singlet
states by interactions with strong Lewis acids. Although in arylnitrenes all three relevant electronic states – the triplet, the
open shell singlet, and the closed shell singlet state – have lone pairs at the nitrene center and thus principally could act as
Lewis base, only the closed shell singlet state shows the required strong Lewis basicity. In the open shell triplet and singlet
states the lone pair occupies an orbital rich in 2s character and thus low in energy.8, 46 Consequently, these states are only
weakly stabilized by interactions with a Lewis acid. In contrast, the second (perpendicular) lone pair in csS-1b is high in
energy with a large p character. Interaction with BF3 results in a re-hybridization to give two in-plane lone pairs of nearly sp2
character. Therefore, csS-1b exhibits strong Lewis basicity, comparable to that of ammonia. However, the strong interaction
with BF3 is not sufficient to stabilize the closed shell singlet state csS-1b below the triplet state T-1b, and thus in the
experiment only a weakly interacting triplet complex is formed.
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To achieve a singlet ground state, it is necessary that the nitrene interacts with two molecules of BF3. This is realized by
doping matrices with higher concentrations of BF3 and by prolonged annealing the matrices at temperatures where BF3 is
able to diffuse. Under these conditions, the red color (corresponding to the 522 nm absorption) of csS-1b…2BF3 appears
and the characteristic EPR signals of T-1b disappear. The formation of the ternary complex csS-1b…2BF3 can also be
monitored by IR spectroscopy. Similar ternary complexes with two molecules of BF3 interacting via a fluoronium bridge
were observed when the matrix-isolated triplet ground state tetrachlorocyclopentadienylidene 11 was allowed to interact
with an excess of BF3. The major difference between this carbene and nitrene 1b is the much larger singlet-triplet splitting
∆EST in the nitrene. Consequently, interaction with one molecule of BF3 with the carbenes is enough to stabilize the singlet
states of the Lewis acid-base complexes of carbenes, whereas two molecules of BF3 are necessary for the nitrene. The
resulting ternary complex csS-1b…2BF3 is highly polar with the nitrene moiety resembling a nitrenium ion.39b
These results demonstrate that the closed shell singlet states of aryl nitrenes become accessible by Lewis acid-base
interactions. However, very strong Lewis acids are necessary to overcome the large singlet – triplet gap in arylnitrenes. It
remains to be proven that this method can be applied to solution phase chemistry.
EXPERMENTAL SECTION
Synthesis. Pentafluorophenylazide 2b was synthesized according to a literature procedure.
Matrix isolation spectroscopy. Matrix isolation experiments were performed using standard techniques as reported
previously.30, 32, 34 Matrices were generated by co-deposition of pentafluorophenylazide 2b and 1% of BF3 with a large excess
of argon (99.99%). The matrices were irradiated at 254 nm using a low pressure mercury arc lamp.
Computational details. Gas-phase geometry optimizations and frequency calculations were performed using the M06-2x
functional47 employing the 6-311++G(d,p) basis set48, 49 as implemented in Gaussian 09 (Revision D.01).50 Multirreference
calculations were performed at the CASPT2(10,10)/6-311G(d)//CASSCF(10,10)/6-311G(d) level of theory, using state
average for the singlets states, as implemented in Molpro.51
Supporting Information
Additional spectroscopic and computational data. “This material is available free of charge via the Internet at
http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
Wolfram.sander@rub.de
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
1
This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche
Forschungsgemeinschaft (DFG).
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