Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B′,B′′-trichloro-N,N′,N′′-tri((4-fluoro)phenyl)borazine

Article abstract of DOI:10.1039/c8dt03954b

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B′,B′′-trichloro-N,N′,N′′-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1?:?1 adduct 1a (ArNH₂BCl₃, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k₁ = 3.03(7) × 10^?2 min^?1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k₁ = 3.18 × 10^?2 min^?1) from computed activation parameters (ΔH^?_373.15 = 28.1 kcal mol^?1, ΔS^?_373.15 = 1.56 eu, ΔG^?_373.15 = 27.6 kcal mol^?1). The product of the first hydrogen chloride evolution is anilinodichloroborane ArNHBCl₂ (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of formation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.

Full text of DOI:10.1039/c8dt03954b

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PAPER
Thermal dehydrochlorination in the
4-fluoroaniline–trichloroborane system:
identification of reactive intermediates involved
in the formation of B,B’,B’’-trichloro-N,N’,N’’-
tri((4-fluoro)phenyl)borazine†
Cite this: Dalton Trans., 2018, 47,
17304
Jennifer Hahn,a Matthias Krieg,a Constanze Keck,a Cäcilia Maichle-Mössmer,b
a
Reinhold F. Finkc and Holger F. Bettinger
*
Borazines are used in chemical vapor deposition processes to produce hybrid graphene–boron nitride
nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the
mechanism of formation of B,B’,B’’-trichloro-N,N’,N’’-tri(p-fluorophenyl)borazine (3a) from p-fluoro-
aniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping
experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory.
These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNH2BCl3, Ar = 4-fluorophenyl) with a
dative B–N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a
undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k1 = 3.03(7) ×
10−2 min−1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k1 =
3.18 × 10−2 min−1) from computed activation parameters (ΔH‡373.15 = 28.1 kcal mol−1, ΔS‡
= 1.56 eu,
373.15
ΔG‡373.15 = 27.6 kcal mol−1). The product of the first hydrogen chloride evolution is anilinodichloroborane
ArNHBCl2 (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a
transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than
anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of for-
mation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.
Received 1st October 2018,
Accepted 2nd November 2018
DOI: 10.1039/c8dt03954b
rsc.li/dalton
that can be adjusted through the substitution pattern around
the borazine core. The synthesis of borazines is known for a
Introduction
Band gap engineering of graphene and nanographene mole- long time and methods for their assembly have not significantly
cules by selective doping with boron and nitrogen units is an changed since the discovery of borazine in 1926.28 Carbon-rich
important topic in materials science today.1–23 Most of the tech- borazines are mostly synthesized via the reaction of boron tri-
niques available for single layered systems rely on CVD pro- chloride with amines/anilines to form the corresponding B,B′,
cesses for producing BN-graphene from gaseous boron, nitro- B″-trichloro-N,N′,N″-triorganylborazines that are subjected to
gen, and carbon precursors.2,24–26 Promising starting materials nucleophilic reactions with organometallic compounds after-
include borazines27 as they offer options to incorporate all the wards.29 There are two different ways to synthesize these bora-
needed atoms (B, N, C) in one molecule and in a precise ratio zines: a base (in most cases triethylamine) assisted30 and a
thermal route. The thermal reaction, which was first observed in
1889 by Rideal31 and recognized to yield borazine by Jones and
aInstitut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18,
Kinney32 in 1939, can further be divided into two subgroups of
72076 Tübingen, Germany. E-mail: Holger.Bettinger@uni-tuebingen.de;
reactions that either use free amines and boron trichloride29,32
Fax: +49 7071/29-5244
or ammonium tetrachloroborates33–35 as the starting materials.
Although the synthetic strategy via B,B′,B″-trichlorobor-
azines is widely employed for the synthesis of hexaarylbor-
azines,11,36,37 the available knowledge about reactive inter-
mediates is scarce. Most mechanistic suggestions were made
on the basis of the kinetics of hydrogen chloride evolution29 or
bInstitut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18,
72076 Tübingen, Germany
cInstitut für Physikalische und Theoretische Chemie, Universität Tübingen, Auf der
Morgenstelle 18, 72076 Tübingen, Germany
†Electronic supplementary information (ESI) available. CCDC 1535682, 1535683
and 1590012. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8dt03954b
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Scheme 1 Simple reaction sequence from amine and boron trichloride to B,B’,B’’-trichloroborazine.
the formation of isolated intermediates.38 The general reaction
sequence (Scheme 1) that proceeds via subsequent elimination
Results
The 1 : 1 adduct (1)
of hydrogen chloride has been recognized to be “oversimipli-
fied”.30 For example, the chemical composition of the precipi- The addition of aromatic amines (ArNH2) to boron trichloride
tate formed upon reaction of amine and boron trichloride, tetra- usually results in the formation of solids. Their composition
ammonium tetrachloroborate or a 1 : 1 adduct (1) depends on depends on the aryl moiety and can either have the stoichio-
the substituent R.39 The aminodichloroborane 2 could not be metry of a 1 : 1 complex (1, ArNH2BCl3) or sometimes (e.g. Ar =
identified so far as a reactive intermediate en route to borazine meta-tolyl) that of an ammonium tetrachloroborate
3. If it can be isolated, then it is isolated only with o,o′-di- (ArNH3BCl4).39 The constitution of some of the 1 : 1 complexes
substituted aryl groups, but these derivatives cannot be trans- was described as (ArNHBCl2)·HCl based on IR spectroscopy
formed into 3 subsequently.40
and chemical behavior.39 This proposal was later refuted in
The closely related reaction of ammonia and boron trichlo- favor of a classical Lewis acid–base (dative) structure based on
ride has received some attention due to its importance for the 1H NMR and IR spectral data.49
production of boron nitride under CVD conditions.41–45 The
The dropwise addition of a solution of 4-fluoroaniline in
datively bonded 1 : 1 adduct, BCl3NH3, could be first character- toluene to a toluene solution of boron trichloride at 0 °C pro-
ized by matrix isolation techniques46 and later on also by duces a solid compound. The elemental analysis of the solid
single crystal X-ray analysis.47 Kinetic investigations of the gas- isolated by filtration shows it to be a 1 : 1 complex. The isolated
phase reaction have shown that the first elimination of hydro- solid 1 : 1 adduct shows no decomposition when stored for
gen chloride to give aminodichloroborane is facile,42,43 with a months at room temperature. The stability of such adducts
barrier of 8.4 kcal mol−1 with respect to the energy of NH3 and has already been recognized by Blackborow et al.49,50 Its consti-
BCl3.43 Further steps involve the reaction of aminodichlorobor- tution can be elucidated by spectroscopy. Diffuse reflectance
ane with an excess of ammonia (BCl2NH2 + NH3 → BCl(NH2)2 infrared Fourier transform spectroscopy (DRIFTS) of the solid
+ HCl, a barrier of 18.5 kcal mol−1), but also surface reactions shows two signals at 3190 cm−1 and 3174 cm−1 that can be
have been identified to be important.43
assigned to symmetric and asymmetric NH stretching
A better understanding of the species participating in the vibrations, respectively, similar to earlier data reported by
formation of B,B′,B″-trichloroborazines in solution phase reac- Blackborow and Lockhart.49 The 11B{1H} solid state NMR spec-
tions is certainly important for the optimization of the reac- trum shows only one signal at 6.1 ppm, suggesting the pres-
tion conditions and for gaining access to more complex start- ence of a tetracoordinate boron atom. The 1 : 1 complex shows
ing materials for CVD processes. In view of the limited knowl- sufficient solubility in benzene and dichloromethane to allow
edge, we decided to study the aniline–trichloroborane system solution phase NMR spectroscopy. Its 11B NMR chemical shift
and some aspects of its thermal dehydrochlorination. We here in [D]6-benzene is 6.5 ppm. Compared to the isotropic chemi-
report our investigation of the reaction of trichloroborane with cal shift in the solid state the difference is very low (0.4 ppm),
4-fluoroaniline (Scheme 1, R = 4-fluorophenyl). The choice of indicative of no fundamental structural differences between
4-fluoroaniline was motivated by the favorable properties of the solid and solution phases. In addition, an intensity ratio of
the 19F nucleus in NMR spectroscopy. In contrast to 11B, 19F 4 : 2 is observed in the 1H NMR spectrum for the aromatic
NMR spectroscopy has a broad spectral range, usually yields protons and the NH2 group.
sharp signals, and allows faster measurements, which is
Further evidence for the constitution of the precipitate is
important for kinetic studies. It should be recognized that the gained from single crystal X-ray structural analysis. Adduct 1a
fluorine substituent will influence the nucleophilicity of the crystallizes in the orthorhombic system, space group Pca21
aniline nitrogen and therefore the conclusions may not be and Z = 4. Its molecular structure is shown in Fig. 1. All B–Cl
general, but limited to this particular system. The basicity of bonds have equal lengths of 1.84 Å. They are longer than in
4-fluoroaniline (pKs = 4.65) is, however, similar to that of the free planar BCl3 (1.72–1.74 Å).51 The coordination of chlorine
parent aniline (pKs = 4.87).48 Our study shows that the two around the boron atom is no longer planar, but tetrahedrally
reactants form a Lewis acid–base pair that undergoes a first distorted with Cl–B–Cl angles of around 111°. The B–N bond
order dehydrochlorination reaction to give the monomeric has a length of 1.61 Å. The structure indicates a classic dative
4-(fluorophenyl)aminodichloroborane.
bonding in the 1 : 1 complex 1a.47,52,53 To our knowledge, this is
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ution.38 Addition of five equivalents of boron trichloride to the
isolated complex 1a does not result in any reaction over a
period of one day at room temperature. Addition of five equiva-
lents of 4-fluoroaniline to a solution of isolated 1a at RT
quickly produces a thick suspension. Two new products were
observed in the reaction mixture. One is assigned to tri-
(4-fluoroanilino)borane, B(NHC6H4F)3 (4a), by comparison
with NMR data that were obtained from a sample that we syn-
thesized independently via aminolysis of B(NMe2)3 with
4-fluoroaniline as described by Nöth et al. for trianilino-
borane.54 Compound 4a is poorly soluble in benzene, but could
be completely characterized by 1H, 13C, 11B, and 19F NMR spec-
troscopy. From the benzene solution, crystals could be grown
that were of sufficient quality for single crystal X-ray analysis
(Fig. 2). Compound 4a cystalizes in the triclinic system, space
Fig. 1 The molecular structure of the 1 : 1 complex 1a in the solid state;
selected bond lengths (in Å) and bond angles (in °): N1–B1 1.607(4), B1–
Cl1 1.835(3), B1–Cl2 1.836(3), B1–Cl3 1.836(3); C2–N1–B1 117.7(2), Cl1–
B1–Cl2 111.1(2), Cl1–B1–Cl3 110.8(2), Cl1–B1–N1 109.3(2), Cl2–B1–Cl3
111.4(2), Cl2–B1–N1 105.1(2), Cl3–B1–N1 108.9(2). Hydrogen atoms
omitted for clarity.
ˉ
group P1 with Z = 2. The BN bonds in 4a have similar lengths
(1.428, 1.435 and 1.438 Å), while for trianilinoborane two
shorter and one significantly longer BN bond were reported
(1.428, 1.431 and 1.442 Å).54 The sum of the bond angles
around the boron atom is 360°, confirming the expected trigo-
nal–planar coordination. We also determined a solid state 11B
NMR spectrum of 4a and obtained an isotropic shift of
the first structural characterization of a 1 : 1 complex formed 23.5 ppm that is virtually identical to that measured in
between an aromatic amine and boron trichloride.
benzene solution (23.5 ppm), indicating that the molecular
structures in the solid state and those in solution are
comparable.
Composition of the reaction mixture before heating and the
properties of the 1 : 1 complex (1a)
We investigated by 11B NMR spectroscopy also the mother from the reaction of 1a with five equivalents of 4-fluoroaniline
liquors after separation of the initially formed solid 1 : 1 has NMR spectral properties similar to but distinct from 4a. In
adduct. The signal at 6.5 ppm shows that the 1 : 1 complex is
The other product 5a observed in the mixture obtained
not fully precipitating initially and, upon standing, a further
crop of 1 : 1 adduct crystallizes from the mother liquors. In
addition, roughly 5% of borazine 3a can be detected by
11B NMR. The initial formation of borazine during addition of
aniline and boron trichloride in cold solutions has already
been reported by Gerrard and Mooney.39 Monitoring
a
benzene solution of the isolated 1 : 1 adduct 1a kept at room
temperature by 19F{1H} NMR spectroscopy revealed only minor
decomposition due to the formation of anilinodichloroborane
2a (see below for the assignment) after a period of three days.
Importantly, no borazine formation was observed during this
time. Likewise, stirring the mixture obtained after the addition
of 4-fluoroaniline to boron trichloride at 0 °C does not change
the amount of borazine. These experiments show that the
initial borazine formation does not result from fast decompo-
sition of 1a and ceases once the addition of the components is
completed.
To better understand the initial formation of borazine and
the possible involvement of the 1 : 1 complex 1a, additional
experiments were performed. Heating the suspension of the
1 : 1 adduct in toluene until complete dissolution followed by
quickly cooling to room temperature allows almost quantitat-
ive recovery of the precipitated 1 : 1 adduct 1a as no significant
changes in NMR spectra are observed. This is in agreement
with the observation of Bartlett et al. that dissolution of
Fig. 2 The molecular structure of tri-(4-fluoroanilino)borane 4a;
selected bond lengths (in Å) and bond angles (in °): N1–B1 1.435(2), N2–
B1 1.438(2), N3–B1 1.428(2); N1–B1–N2 119.5(1), N1–B1–N3 120.4(1),
adducts in toluene is faster than hydrogen chloride evol- N2–B1–N3 120.0(1). Hydrogen atoms omitted for clarity.
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the 19F and 11B NMR the corresponding signals are shifted
downfield (19F: −120.1 ppm vs. −122.7 ppm; 11B: 26.3 ppm vs.
1
23.5 ppm), and the H(N) signal in the H NMR is also shifted
downfield (4.60 ppm vs. 4.14 ppm). Based on these slight
shifts the product is identified as di-(4-fluoroanilino)chloro-
borane ClB(NHC6H4F)2 (5a). The downfield shifts of 2.7 ppm
in the 19F and 2.9 ppm in the 11B NMR are in agreement with
computations (Δδ(19F) = 4.0 ppm and Δδ(11B) = 3.9 at the KT3/
pcS-3//SCS-MP2/cc-pVTZ level of theory). Upon addition of two
equivalents of 4-fluoroaniline to 1a, compound 5a is the only
1
product and can be isolated. In the H NMR the H(N) proton
and aromatic protons integrate in the required 1 : 4 ratio. As
compound 5a does not sublime without decomposition, a
high resolution EI-MS spectrum could not be obtained, but
the detection of a signal at m/z = 36.1 (HCl) is supportive of
the assignment to ClB(NHC6H4F)2.
We then varied the ratio of 4-fluoroaniline and boron tri-
chloride in the initial addition at 0 °C. As expected, a ten-fold
excess of 4-fluoroaniline results in the formation of tri-(4-fluoro-
anilino)borane 4a. On the other hand, a ten-fold excess of
boron trichloride produces a significantly larger amount
(roughly 13%) of borazine 3a. Subsequent stirring of this reac-
tion mixture does not increase the amount of borazine. This
observation is supportive of the above statement that borazine
formation ceases after the addition of components is com-
pleted. Hence, there must exist pathways to borazine that are
only accessible as long as complex formation is not completed.
As complex 1a does neither react with an excess of 4-fluoroani-
line nor react with boron trichloride to borazine, we conclude
that once the formation of 1a is completed, borazine for-
mation will come to a halt. The actual mechanism of
Fig. 3 The molecular structure of 3a in the crystal prepared in refluxing
toluene; selected bond lengths (in Å) and bond angles (in °): N1–B1
1.436(3), N1–B3 1.436(3), N2–B1 1.431(3), N2–B2 1.435(3), N3–B2 1.432(3),
N3–B3 1.421(3), B1–Cl1 1.762(2), B2–Cl3 1.764(3), B3–Cl2 1.770(2),
N1–C1 1.450(3), N2–C7 1.449(3), N3–C13 1.453(3); C1–N1–B1 120.3(2),
C1–N1–B3 120.0(2), B1–N1–B3 119.7(2), C7–N2–B1 120.2(2), C7–N2–
B2 119.9(2), B1–N2–B2 119.8(2), C13–N3–B2 119.6(2), C13–N3–B3
120.0(2), B2–N3–B3 120.3(2), N1–B1–N2 120.0(2), N1–B1Cl1 120.1(2),
N2–B1–Cl1 119.9(2), N2–B2–N3 119.8(2), N2–B2–Cl3 119.9(2), N3–B2–
Cl3 120.2(2), N1–B3–N3 120.1(2), N1–B3–Cl2 119.6(2), N3–B3–Cl2
120.4(2). Hydrogen atoms omitted for clarity.
initial borazine formation is unclear and involvement of trace was added a solution of 4-fluoroaniline in toluene (1.00 eq.)
amounts of impurities cannot be ruled out.
at 0 °C, which resulted in the immediate precipitation of 1a.
Then the mixture was heated under a constant argon flow and
NMR samples were collected from the reaction vessel after
B,B′,B″-Trichloro-N,N′,N″-tri((p-fluoro)phenyl)borazine (3a)
As reported in the literature, borazines 3 can usually be complete dissolution. The samples were immediately diluted
obtained by heating the 1 : 1-adduct 1 in refluxing benzene or with deuterated solvent and measured at room temperature.
toluene.40,50 The same is true for the 1 : 1 adduct 1a that forms Under these conditions, homogeneous solutions were
between p-fluoroaniline and boron trichloride. After heating to obtained. The 19F NMR spectrum only displayed three major
100 °C for 19 hours in toluene, the 11B NMR signal of the tetra- peaks at −112.0 ppm, −117.2 ppm, and −115.5 ppm. The car-
coordinated boron atom has disappeared and a new signal at riers of the signals at −112.0 ppm and −115.6 ppm can be
∼32 ppm is observed. This is within the range typical of tri- assigned to the 1 : 1 adduct 1a (−112.0 ppm) and borazine 3a
chloroborazines. The compound can be isolated by evapor- (−115.6 ppm), respectively, based on the 19F NMR shifts of sep-
ation of the solvent. The EI-MS spectrum gives the M+ peak arately obtained samples (see above). During the course of the
(ArNBCl)3 along with peaks due to fragmentation of the bora- reaction the intensity of the signal at −112.0 ppm (i.e., the 1 : 1
zine core to the diazadiboretidine (ArNBCl)2 and iminoborane adduct 1a) decays and that of −117.2 ppm (anilinodichlorobor-
ArNBCl.
ane 2a, see below for assignment) goes through a maximum
Crystals of the borazine 3a, prepared in refluxing toluene as (Fig. 4). After a standard reaction time29,37,40 of 19 h the data
described above, were obtained by slow evaporation of toluene of the −115.6 ppm signal (i.e., borazine 3a) pointed to a limit.
from a concentrated solution. Compound 3a crystallizes in the As already mentioned in the literature,39 B,B′,B″-trichlorobor-
monoclinic system, space group P21/c and Z = 4 (Fig. 3).
azines are unstable upon prolonged heating and 3a likewise
decomposed after 3 d at 100 °C as seen by the disappearance
of the peak at −115.6 ppm in the fluorine NMR and the for-
NMR reaction monitoring
The transformation of 1a into 3a was followed by 19F and 11B mation of an insoluble material.
NMR spectroscopy. In order to collect the reaction data, to a solu-
Data acquisition at the beginning of the reaction is proble-
tion of boron trichloride (1 M in hexanes, 1.24 eq.) in toluene matic: the zero-point of the kinetic measurements is difficult
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Fig. 4 Data points obtained by 19F{1H} NMR in relative intensities using the signals at −112.0 ppm (1a, red), −117.2 ppm (2a, green) and −115.6 (3a,
blue). (a) Fit of the experimental data to the reaction mechanism shown in Scheme 3; (b) fit of the experimental data to the reaction mechanism of
Scheme 4.
to determine as stable signals can only be obtained after the Assignment and properties of the intermediate of borazine
complete dissolution of the initially formed precipitate. Also, formation
the short sample collection interval did not allow direct trans-
fer to the NMR spectrometer, so the first 15 samples were The NMR investigations confirmed the involvement of an
stored at 0 °C and were measured approximately one hour intermediate that can be assigned to anilinoboron dichloride
later. The measured samples were homogeneous solutions. 2a, as explained in this section. Isolation of the intermediate
The effect of prolonged standing in an ice bath should not 2a was not successful. Stopping the reaction at the highest
have a dramatic effect on the shape of the NMR spectrum as concentration of 2a and evaporating the solvent at room temp-
1a is stable under these conditions and the temperature differ- erature under vacuum resulted in compounds 1a and 3a.
ence of ΔT = 100 K is expected to lead to a drastic slowdown of Attempts to crystallize 2a at this stage of the reaction led again
the reaction of 2a. Nevertheless, the trace of 3a shows an offset to the same disproportionation. Similar observations were
of about 0.15 units that we attribute to a parallel reaction that made by Bartlett et al.,38 who identified the 1 : 1 adduct and
quickly forms borazine 3a at 0 °C (see above). Note that borazine instead of the expected anilinoboron dichloride
Mooney and Gerrard39 could also detect B,B′,B″-trichlorobora- (ArNHBCl2) when they stopped the reaction after the elimin-
zines at −80 °C together with dichloroaminoborane in the di- ation of ca. one mole of hydrogen chloride per mole of 1 : 1
chloromethane phase in the reaction mixture of aniline and adduct. To further characterize 2a, the 15N-1H-HSQC-DEPT
boron trichloride.
NMR spectrum was measured immediately after stopping the
Besides 19F{1H} NMR investigations, 11B{1H} NMR spectra reaction at the highest concentration of 2a. This NMR experi-
were acquired. To suppress background signals, measurements ment gives cross-peaks for hydrogen atoms bound to nitrogen,
were run in quartz NMR tubes. The boron spectra showed two and indeed two such signals can be found. One is due to 1a
peaks at 6.5 and 32.0 ppm, but deconvolution of the latter (15N 77.8 ppm; 1H 4.8 ppm) by comparison with the known 1H
peak revealed two signal subsets with different halfwidths (70 NMR shift of the isolated material, and another one is
1
Hz and >200 Hz). Due to the overlap of the 11B NMR signals, assigned to 2a (15N 117.0 ppm; H 5.4 ppm). In addition, the
only a few important data points were measured (ESI, DEPT experiment shows that the phases of the two peaks are
Fig. S2†). The intensities of these three peaks line up perfectly opposite. As 1a has two protons bound to nitrogen; the oppo-
with the data obtained from 19F NMR. Due to the uncertainties site phase for 2a suggests that only one or three protons are
of the deconvolution, the 11B data were not employed for bound to its nitrogen atom. As three protons and the aryl
kinetic analysis.
group would result in an anilinium ion, the only reasonable
Note that the 19F NMR spectra did not display the peak of conclusion is that 2a has one proton bound to nitrogen.
free 4-fluoroaniline. The small excess of boron trichloride Taking into account the 11B NMR shift of 32 ppm as an
(0.24 eq.) was detectable by 11B NMR only during the first additional constraint, only leaves the monomeric aminodi-
60 min before it was presumably taken away by the argon chloroborane (ArNHBCl2) 2a as a reasonable proposal for the
stream. Running the experiment in sealed NMR tubes proved intermediate. Its cyclodimer or cyclotrimer is expected to show
to be disadvantageous because the formation of 3a was very 11B NMR chemical shifts in the range of tetracoordinated
strongly slowed down and a multitude of novel and unassigned boron atoms and hence can be excluded as possible carriers of
19F NMR signals appeared. This demonstrates the importance the signals associated with the intermediate. The assignment
of removal of volatile products (like HCl) from the reaction to 2a is further supported by computations of the chemical
mixture, which was achieved by employing a controlled stream shifts at the KT3/pcS-3//SCS-MP2/cc-pVTZ level of theory (see
of argon (see Experimental details).
Computational methods for details). These arrive at values of
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−114.9 ppm for 19F (exp. −117 ppm), 30.4 ppm for 11B formation of boron nitride from ammonia and boron trichlo-
(exp. 32 ppm), and 119.7 ppm for 15N (exp. 117 ppm).
ride under CVD conditions. Indeed, iminoborane could not be
We also probed the stability of 2a with respect to reaction detected by mass spectrometry in a flow reactor under CVD
with free boron trichloride and 4-fluoroaniline by stopping the conditions.43
reaction close to the maximum concentration of 2a (obtained
We also computed barriers for the reaction of anilinodi-
from an equimolar amount of reagents) and adding either of chloroborane 2a with free 4-fluoroaniline or boron trichloride
the reagents at room temperature. While 2a does not react and subsequent loss of HCl, as these reactions were probed
with boron trichloride to an appreciable amount, it reacts experimentally, but only that with 4-fluoroaniline was
instantaneously with 4-fluoroaniline to give (ArNH)2BCl 5a as observed. Due to system size, the geometry of the compounds
the major product according to NMR. This experiment was optimized with the more economic M06-2X/6-311+G**
suggests that the lifetime of 2a could be limited by the method (for details see Computational methods) while ener-
absence or presence of free 4-fluoroaniline.
gies were refined using CCSD(T)/cc-pVTZ single energy point
computations. Anilinoborane 2a forms an adduct with 4-fluoro-
aniline in a mildly endergonic reaction (Δ373.15G = +5.7
kcal mol−1) to give 2aA (Scheme 2, see ESI, Fig. S29† for com-
puted structures), while the formation of the adduct 2aB with
Computational investigations
To shed light on the barriers for HCl elimination from 1a and
2a, computational investigations were performed at the CCSD
(T)/cc-pVTZ//SCS-MP2/cc-pVTZ level of theory (for details see
Computational methods). As the reaction was run in the non-
polar toluene, the influence of solvent was neglected in the
computations. The boron–nitrogen bond in the 1 : 1 adduct 1a
has a bond dissociation energy D0 of 22.3 kcal mol−1 for the
formation of p-fluoroaniline and boron trichloride. Taking
into account entropy and thermal corrections using the rigid
rotor harmonic oscillator model shows that the dissociation of
the 1 : 1 adduct 1a is endoergic at the temperature of the
experiment (Δ373.15G = +7.7 kcal mol−1). We could identify a
transition state for the unimolecular elimination of hydrogen
chloride from the 1 : 1 adduct 1a. Its structure (ESI, Fig. S28†)
is very similar to that identified for hydrogen chloride elimin-
ation from BCl3NH3 earlier.44,45 It is most remarkable that one
boron–chlorine bond is essentially already broken (distance:
2.68 Å), while the chlorine–hydrogen distance is still rather
long (1.67 Å), indicative of a late transition state. The hydrogen
boron trichloride is strongly endergonic (ΔG0
= +15.7
373.15
kcal mol−1). Elimination of HCl from 2aA to 5a can proceed
with a barrier ΔG‡ = 17.9 kcal mol−1 while that of 2aB to
373.15
6a has a higher barrier ΔG‡373.15 = 24.7 kcal mol−1
.
Based on reactants 2a and aniline or boron trichloride, the
formation of (ArNH)2BCl is much more favorable than that of
ArN(BCl2)2, in agreement with the experimental observations.
These calculations show that the reactive intermediate 2a can
readily react with free aniline under experimental conditions.
Kinetic data fitting
The traces of the compounds 1a, 2a, and 3a were fitted with
the kinetic schemes displayed in Scheme 3 and Scheme 4. The
rate constants were determined in a least-squares fit between
the experimental traces and calculated values obtained by
numerically solving the set of first order initial value differen-
tial equations with the fourth-order Runge-Kutta method.55
The fit was performed using the Marquardt method and incor-
porated variational optimization of the initial concentrations.
The lines in Fig. 4a represent the fittted result of this pro-
cedure for Scheme 3 which provide first order rate constants
chloride elimination is mildly exergonic (ΔG0
= −3.8
373.15
kcal mol−1) with ΔG‡
= 27.6 kcal mol−1. Using the Eyring
373.15
equation, this free energy of activation corresponds to k = 3.18
× 10−2 min−1. This is in very good agreement with the rate con-
stant k1 = 3.03(7) × 10−2 min−1 derived from fitting the kinetic
data (see below).
k1 = 3.03(7) × 10−2 min−1, k2 = 2.5(3) × 10−3 min−1, and k3
=
1.97(3) × 10−3 min−1. It was attempted to perform the fit with
various alternative fitting schemes such as avoiding the back
reaction from 2a to 1a or including a second order reaction in
2a from 2a to 3a and second order reactions of 2a with the
boron trichloride or aniline concentrations. However, these
The computations show that the elimination of another
molecule of hydrogen chloride from the anilinodichloroborane
2a to give the iminoborane ArNBCl has a much higher free
energy of activation, ΔG‡
= 67.9 kcal mol−1. For this
373.15
reason, the formation of the iminoborane (followed by its
rapid cyclotrimerzation to 3a) is not feasible under experi-
mental conditions. Due to a similarly high barrier for unimole-
cular hydrogen chloride elimination from the parent aminodi-
chloroborane (BCl2NH2 → BClNH + HCl) this reaction was
judged to be unimportant by Allendorf and Melius44 for the
Scheme 3 Kinetic scheme used to fit the observed traces in Fig. 4a.
Scheme 2 Reaction of 2a with 4-fluoroaniline and boron trichloride.
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Scheme 4 Alternative kinetic scheme involving dissociation of 1a used to fit the traces in Fig. 4b.
fits provided a significantly poorer representation of the with boron trichloride that undergoes successive hydrogen
measured values. chloride evolution and proceeds via the aminodichloroborane
As Scheme 3 is not consistent with the very high barrier 2 (Scheme 1).39
computed for the unimolecular HCl elimination from 2a (to
Bartlett et al. observed that the thermal evolution of hydro-
give iminoborane ArNBCl, see the section Computational gen chloride from the aniline–trichloroborane adduct is
investigations), we also considered the alternative kinetic initially first order and turns to second order in the course of
scheme that involves the reaction of free 4-fluoroaniline the reaction.38 Hence, these authors specifically ruled out
with 2a (Scheme 4).
Gerrard and Mooney’s mechanistic proposal. Their suggestion
Here, 1a is in equilibrium with 2a and HCl via first order (Scheme 5) is bimolecular formation of the four- and six-mem-
reactions in these compounds. Hereby we assume a constant bered chains 7 and 8, respectively, by connecting three amino-
(steady state) concentration of HCl, which is formed in the borane molecules 2. The last step transforms the open chain
reaction and simultaneously evaporates from the reaction array into the borazine ring system 3 by a unimolecular ring
mixture. Furthermore, 1a is in equilibrium with its dis- closure.38
sociation products, the trichloroborane in the liquid phase,
Atkinson et al.40 likewise assumed the intermediacy of
BCl3(l), and the aniline, RNH2. The latter reacts in a second short chains like 7 and 9 formed in bimolecular reactions with
order reaction with 2a to give an adduct (2aA, Scheme 2) hydrogen chloride and boron trichloride elimination
that leads, in several steps, to borazine 3a. In that reaction, (Scheme 6). The produced boron trichloride closes the chain
two further molecules of 2a are consumed and three molecules to the borazine ring 3.
of HCl and one molecule of RNH2 are produced. In an additional
This proposal rests on the finding that monomeric amino-
step, BCl3(l) evaporates from the solution to the gas phase dichloroboranes are known to form from the reaction of boron
molecule, BCl3(g), with a rate constant k6. The latter is likely to trichloride with aromatic amines with o,o′-disubstitution, and
happen under the reaction conditions due to the low boiling in this case these can be isolated.40,49,50,56 However, such o,o′-
point of BCl3 (12.5 °C) and the argon flow conditions. Rate disubstituted anilinodichloroboranes cannot be transformed
constants and initial concentrations were optimized to repro- thermally into borazines. Further thermal hydrogen chloride
duce the measured concentrations. However, the NMR spectra
were measured at room temperature where boron trichloride
and fluoroaniline react with 1a, which was assumed in the
simulation of the observed concentrations. A fit of the rate
constants provided the concentration profiles shown in
Fig. 4b.
As shown in Fig. 4b, Scheme 4 allows for an appropriate
reproduction of the evolution of the observed concentrations.
Thus, a kinetic scheme involving the reaction of 2a with
aniline discussed above as a key step is a realistic alternative to
Scheme 5 Mechanism proposed by Bartlett et al.38 (b: R = Ph).
the pleasantly simple Scheme 3.
Discussion
Previous mechanistic suggestions
In the literature, three mechanistic suggestions were made
that will be briefly sketched below. A first-order hydrogen
chloride evolution obtained for the last phase (roughly the last
10% due to poor solubility of the 1 : 1 adduct in benzene) of
the reaction led Gerrard and Mooney39 to the assumption that
Scheme 6 Mechanism proposed by Atkinson et al.40 (a: R = pFPh, e:
aniline, p-toluidine, and p-bromoaniline first form an adduct 1 R = mClPh, f: R = oBrPh, g: R = mBrPh).
17310 | Dalton Trans., 2018, 47, 17304–17316
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elimination produces chains,38,40 but such chains do not in solution. These authors thus concluded that catalysis is
cyclize to borazines with the added boron trichloride.40 required for the transformation of 2 into 3, a conjecture that
However, o,o′-disubstituted anilinodichloroboranes form bora- also seems likely for the system studied here.
zines upon reaction with triethylamine.38
An important observation in our study is the finding that 2a
It should also be noted that there may exist mechanistic reacts rapidly with free 4-fluoroaniline to afford the dianilino-
alternatives depending on the acidity of the aniline and steric chloroborane [(C6H4FNH)2BCl] 5a, and the computational
demand of substituents. For example, ortho-toluidine confirmation of a low-energy pathway. 4-Fluoroaniline could
(o-CH3C6H4NH2) was found to undergo hydrogen chloride arise along with boron trichloride from the dissociation of the
elimination by two consecutive first-order reactions with quite adduct 1a in hot toluene. According to our CCSD(T)/cc-pVTZ//
different rates.30 After loss of one mole of hydrogen chloride, SCS-MP2/cc-pVTZ computations this process is endergonic by
the aminoborane intermediate can be isolated.30 Furthermore, +7.7 kcal mol−1, which would result in a low concentration of
adducts of aliphatic primary amines and boron trichloride the free 4-fluoroaniline and BCl3 (log K373 = −4.5). The volatile
evolve hydrogen chloride in fractional-order reactions and BCl3 would be removed along with the HCl reaction product in
initial equilibrium and a chain reaction were suggested for the argon stream. We would not have been able to detect free
explanation.30
4-fluoroaniline by NMR spectroscopy, as this was done at room
temperature where the equilibrium is very far on the side of
the adduct 1a.
Discussion of the mechanism
Taking all information and arguments together, we suggest a
The reaction mechanism of Scheme 4 is based on these
reaction sequence for the formation of borazine 3a from arguments and indeed is able to provide an acceptable fit of
4-fluoroaniline and boron trichloride that starts with the for- the experimental data. We therefore suggest that the avail-
mation of a poorly soluble 1 : 1 Lewis acid–base adduct 1a at ability of 4-fluoroaniline to react with the transient reactive
0 °C, which fully dissolves at 100 °C and undergoes unimole- intermediate 2a to afford 5a and HCl limits the rate of the reac-
cular hydrogen chloride elimination to give the anilinodichloro- tion to borazine. The importance of free 4-fluoroaniline for the
borane 2a. This first step is reversible.
course of the reaction can also explain why a small fraction of
1
3
For the reaction anilinodichloroborane (2a) → borazine borazine 3a is already formed during the addition of reagents
(3a) + HCl no additional intermediates could be identified in before heating. In this scenario, formation of the Lewis acid–
the NMR study. The fact that the conversion of intermediate base adduct 1a is a dead end, and heating is required for the
2a is first order in 2a allows discarding mechanisms that production of reactive compounds with HCl elimination being
require the bimolecular reaction of 2a to give chains or the the first step.
cyclodimer and cyclotrimer of 2a as rate limiting steps. The
As is always the case with kinetic analyses, statements on
most obvious mechanistic alternative would be the transform- reactions after the rate-determining step are speculative.
ation of 2a into chloroiminoborane RNBCl and hydrogen Hence, it is not clear how 5a transforms into 3a, but a com-
chloride. However, the computations show that this pathway is parison with amine-borane adducts, RNH2BH3, may be
associated with a prohibitively high energy barrier. We there- instructive. There, the involvement of cyclotriborazanes,
fore discard this mechanism even though it is simple and (RNHBH2)3, has been established in catalyzed dehydrogena-
compatible with the measured kinetics (see Fig. 3a). Note that tion reactions leading to borazines (RNBH)3.61,62 In the case of
the involvement of iminoboranes was implied in early work on aromatic amines, Helten et al.63 concluded that borazines “are
borazine formation and borazine decomposition by dry hydro- not simply formed by dehydrogenation of cyclotriborazanes”.
gen chloride reported by Wiberg and co-workers, but the Rather, dianilinoborane (PhNH)2BH was found to transform
corresponding reactions were run at much higher tempera- the cyclotriborazane into the borazine with concomitant regen-
tures than those studied here.57–60 Avent et al. suggested the eration of the aniline–borane adduct.63 Thermal dehydrochlor-
involvement of the iminoborane BClNH in the closely related inations are generally more facile than dehydrogenations, and
reaction of ammonia and boron trichloride that produces tri- accordingly, fewer intermediates were observed in our study.
chloroborazine (BClNH)3, but conceded that the evidence for While the intermediacy of chains or cycles, such as the chloro-
intermediates other than the 1 : 1 adduct, which they could derivatives of cyclotriborazane, cannot be excluded, the kine-
characterize by X-ray structural analysis and NMR spectroscopy tics are not in agreement with their formation as rate limiting
in solution, is circumstantial.47 Indeed, the gas phase study of steps.
the reaction of boron trichloride and ammonia concluded that
the corresponding iminoborane is not involved (see above).43
The first order disappearance of anilinodichloroborane 2a
is difficult to reconcile with a plausible mechanism, unless it
Conclusions
is pseudo first order due to a stationary concentration of some The present investigation used NMR spectroscopy (19F and
intermediate. This suggestion is in agreement with 11B) to investigate the mechanism of borazine formation from
Blackborow et al.,50 who observed previously that 2-bromoanili- 4-fluoroaniline (ArNH2) and boron trichloride in toluene at
nodichloroborane did not undergo borazine formation once 100 °C. The study shows that the initially formed 1 : 1 adduct
isolated from the reaction mixture and heated in a pure form (1a) transforms into borazine 3a involving ArNHBCl2 (2a) as
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the only detectable reactive intermediate. The assignment to aniline in 10 mL of toluene (abs.) within 5 min, immediately
2a is based on NMR spectral data and is supported by com- leading to a colorless precipitate. Then the dropping funnel
puted chemical shifts using density functional theory. The was washed with 5 mL of toluene (abs.) and the cold suspen-
hydrogen chloride elimination from 1a is a first order reaction sion was filtered through a frit. The remaining solid was dried
in 1a. The computed activation free energy (CCSD(T)/cc-pVTZ// under vacuum (1.7 g; 35.5%).
SCS-MP2/cc-pVTZ) is in good agreement with the measured
EA for C6H6BCl3FN: calc. C 31.57%, N 6.14%, H 2.65%,
rate constant for the hydrogen chloride elimination (1a → 2a + found C 31.51%, N 6.30%, H 2.57%; DRIFT-IR (KBr, cm−1):
HCl). As no intermediates were observed for the transform- 3190 s, 3174 s, 3090 m, 2599 w, 2530 w, 2525 w, 2495 w, 2398
ation 2a → 1/3 3a + HCl, definite conclusions are difficult to w, 1888 w, 1604 w, 1562 m, 1511 s, 1444 w, 1308 s, 1240 m,
draw. The formation of the iminoborane (RNBCl) from 2a by 1230 m, 1199 m, 1161 m, 1100 m, 1014 w, 939 w, 908 s, 893 s,
hydrogen chloride elimination is excluded based on the very 851 s, 831 m, 800 m, 792 m, 774 s, 717 m, 700 m, 686 m,
1
high barriers associated with this step. More likely are pro- 667 m, 630 w, 547 s, 492 m, 429 w; H (400 MHz, C6D6): 4.84
cesses involving free aniline and boron trichloride, which may (s, 2H, NH2), 6.47 (s, 2 H, Ar), 6.46 (s, 2 H, Ar); 19F{1H}
be produced in low concentrations from the 1 : 1 adduct 1a. (376 MHz, C6D6): −112.0; 11B{1H} (80 MHz, C6D6): 6.5; 11B
The observed facile reaction of 2a with 4-fluoroaniline to (64 MHz, MAS, Rf 10 kHz): δiso 6.1.
afford di-(4-fluorophenyl)aminochloroborane 2aA suggests
Compound 3a. A solution of 1 g (4.4 mmol) 1a in 20 mL
that this is a key reaction in the mechanism, which is compati- toluene (abs.) was heated to 100 °C with a gentle argon flow
ble with the measured concentration profiles.
passing over the mixture. After 19 h the solvent was evaporated
under vaccum at room temperature. Slightly yellow micro-
crystals were obtained with impurities due to partial decompo-
sition of 3a (475 mg; 23%).
Experimental details
General
1H (400 MHz, C6D6): 6.7–6.8 (m, 4H, Ar); 19F{1H} (376 MHz,
C6D6): −115.5; 11B{1H} (80 MHz, C6D6): 32.0 (>200 Hz); EI-MS
(m/z): 465 [M•+, (FArNBCl)3], 310 [M•+, (FArNBCl)2], 155 [M•+,
(FArNBCl)]; HR-EI-MS for C18H12B3Cl3F3N3: calc. 465.032808,
found 465.03587.
All reactions were carried out under an inert atmosphere using
argon as a protective gas. Dry toluene was obtained from a
solvent purification system (MBraun SPS-800). Boron trichlo-
ride (1 M in hexanes) was purchased from Acros, 4-fluoro-
aniline was obtained from ABCR and distilled (25 mbar, 74 °C)
prior to use. All NMR spectra were recorded on Bruker DRX
250, Avance II 400, Avance II+, Avance III HD 400, and Avance
III HDX 600 spectrometers. The NMR spectra were measured
at room temperature in C6D6 (dried over molecular sieves 4 Å)
that was purchased from Deutero GmbH. The spectra were
referenced either to residual solvent signals (1H, 13C: SiMe4)64
or externally (19F: CCl3F, 11B: BF3·OEt2). 11B solid-state NMR
spectra were measured with a Bruker DSX-200 instrument at
Compound 4a. To a solution of trisdimethylaminoborane
(0.5 mL, 2.86 mmol) in toluene (22 mL) was added 4-fluoro-
aniline (0.81 mL, 8.58 mmol). The mixture was refluxed for
36 hours and turned from yellow to red-brown during this
time. During slow cooling to room temperature, crystals
formed overnight, which were isolated (0.41 g, 61%).
1H (400 MHz, C6D6): 6.72 (m, 6 H), 6.47 (m, 6 H), 4.14 (br s,
3 H); 13C (100 MHz, C6D6): 116.1 (d, J = 22.32 Hz), 121.8 (d, J =
7.44 Hz), 139.9 (d, J = 2.67 Hz), 158.3 (d, J = 239.50 Hz); 19F{1H}
(376 MHz, C6D6): −122.7; 11B{1H} (80 MHz, C6D6): 23.5; 11B
(64 MHz, MAS, Rf 10 kHz): δiso 23.5 ppm, χ = 2.40 Hz, η = 0.17;
64 MHz in an 4 mm ZrO2 rotor referenced to NaBH4 and δiso
,
χ, and η were determined using the program WSolids1.65 The
deconvolution of the 11B spectra was done with TopSpin.66
DRIFTS measurements were performed with a NICOLET 6700
FTIR spectrometer (Thermo Scientific) and dried KBr powder
was used as a dilution matrix (10 wt% analyte related to
manalyte + mKBr = 100%). The sample–KBr mixture was always
intimately mixed by grinding in an agate mortar prior to the
measurements and put into a DRIFTS cell with KBr windows.
The collected data were converted using the Kubelka–Munk
refinement. For EI-MS measurements a MSD 5977 Agilent
MSD and for HR-EI-MS measurements a MAT95 Finnigan
spectrometer was used. Elemental analysis was done with a
varioMICROcube (Elementar Analysensysteme GmbH (Hanau))
in the CHNS modus.
EI-MS (m/z): 341 [M•+], 230 [FArNBNHArF+], 111 [FArNH2 ];
+
HR-EI-MS: calc. 341.13056 Da, found 341.13210 Da.
Compound 5a. To a solution of 12 mg (0.05 mmol) 1a in
benzene (2.5 mL) were added 12 µL (0.11 mmol) 4-fluoro-
aniline. The immediate formation of a very fine colorless sus-
pension was observed. The NMR spectral data of this suspen-
sion given below are in agreement with essentially quantitative
formation of 5a. Attempts to isolate 5a by removal of the
solvent under vacuum always resulted in mixtures of 5a and 4a
that could not be separated.
1H (400 MHz, C6D6): 4.60 (br s, 2 H), 6.49–6.52 (m, 4 H),
6.64–6.70 (m, 4 H); 13C (100 MHz, C6D6): 115.9 (d, J = 22.52
Hz), 124.1 (d, J = 7.73 Hz), 137.7 (d, J = 2.85 Hz), 159.5 (d, J =
242.41 Hz); 19F (376 MHz, C6D6): −120.1; 11B (80 MHz, C6D6):
26.3.
Synthesis
Kinetic measurements
Compound 1a. At 0 °C, a solution of 21 mL (21 mmol)
boron trichloride (1 M in hexanes) in 40 mL of toluene (abs.) To guarantee optimal argon flow, the reaction exhaust is
was charged with a solution of 1631 µL (17 mmol) 4-fluoro- passed only through CaCl2 and a commonly used water or pot-
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assium hydroxide washing flask is omitted. During the Paratone-N (Hampton) and fixed on a nylon loop. Data for all
reaction monitoring, each collected sample (0.3 ml crystals were collected using a Bruker APEX DUO instrument
reaction mixture) was transferred via a syringe to a NMR equipped with an IμS microfocus sealed tube and QUAZAR
tube filled in the glovebox with 0.2 ml C6D6 and sealed optics for MoKα radiation (λ = 0.71073 Å). The data collection
with
a
septum before 19F{1H} or 11B{1H} spectra were strategy for all was determined using COSMO67 employing ω-
measured. Although no information relevant for the reaction and ϕ scans. Raw data were processed using APEX67 and
can be gathered from measuring hydrogen nuclei (due to the SAINT,67 and corrections for absorption effects were applied
manyfold excess of the nondeuterated toluene solvent) using SADABS.68 The structures were solved by direct methods
1H NMR spectra were acquired for every sample to check the and refined against all data by full-matrix least-squares
field homogeneity.
methods on F2 using SHELXT69 and Shelxle.70 Further details
Kinetic experiment 1. At 0 °C, to a solution of 21 mL of the refinement and crystallographic data are given in
(21 mmol) boron trichloride (1 M in hexanes) in 40 mL of Table 1. CCDC numbers 1535682, 1535683, and 1590012†
toluene (abs.) was added a solution of 1631 µl (17 mmol) contain all the supplementary crystallographic data for this
4-fluoroaniline in 10 mL of toluene (abs.) within 5 min, paper.
immediately leading to a colorless precipitate. Then the
dropping funnel was washed with 5 ml of toluene (abs.) and
Computational methods
the argon inlet was switched so that argon is passing gently
Geometry optimizations were performed using spin-com-
over the mixture. The suspension was stirred for another
ponent scaled71,72 Møller-Plesset second-order perturbation
20 min at 0 °C and then heated to 100 °C within 20 min
theory (SCS-MP2) in conjunction with the resolution-of-the-
while the precipitate dissolved after 10 min. The first sample
was collected when the reaction reached 100 °C and the
following samples at intervals of 30 min (up to 3.5 h, samples
1–8). Sample 9 was taken after 4.5 h, sample 10 after 19 h and
identity (RI)73 and frozen-core approximations. Dunning’s74
correlation consistent triple-ζ basis set (cc-pVTZ) and the rec-
ommended75 RI fitting basis set were employed. Due to the
samples 11, 12 and 13 after 43.5, 68.5 and 116.5 h, res-
pectively. 11B{1H} NMR measurements were performed on
samples 1–6 and 10, 11, and 13. The boron and fluorine
Table 1 Crystal data and structure refinements
NMR spectra of samples 11–13 showed increasing amounts
of decomposition products while the product peak
1a
3a
4a
(−115.5 ppm, 32.0 ppm) steadily vanished. After 116.5 h, the
decomposition products were almost insoluble and precipi-
tated from solution. The graph received from the fluorine
NMR spectra of experiment 1 can be found in the ESI
(Fig. S2†).
Empirical
formula
Formula weight 228.28
Temperature
Crystal system
Space group
Unit cell
C6H6BCl3FN
C18H12B3Cl3F3N3 C18H15BF3N3
466.09
100(2) K
341.14
100(2) K
100(2) K
Triclinic
ˉ
P1
Orthorhombic Monoclinic
Pca21 P21/c
a = 11.2213(3) Å a = 5.7426(2) Å
b = 12.4120(3) Å b = 27.2822(10) Å b = 10.109(9) Å
c = 6.7073(2) Å c = 13.2120(5) Å
α = 90°
β = 90°
Kinetic experiment 2. At 0 °C, to a solution of 21 mL
(21 mmol) boron trichloride (1 M in hexanes) in 40 ml of
toluene (abs.) was added a solution of 1631 µl (17 mmol)
4-fluoroaniline in 10 ml of toluene (abs.) within 7.5 min,
immediately leading to a colorless precipitate. Then the
dropping funnel was washed with 10 ml of toluene (abs.) and
the argon flow was set at 21 sccm min−1. The first sample was
collected and the suspension was stirred for 15 min at 0 °C.
After that, the reaction vessel was heated to 100 °C within
20 minutes while the precipitate dissolved at around 70 °C.
The first 20 samples were collected with a 5 min (sample 20 at
107 min), samples 21–32 with a 15 min (sample 32 at
287 min), samples 33–38 with a 30 min (sample 38 at
467 min), and samples 39–47 with a 60 min interval. The last
sample (48) was taken after 1107 min. The first three data
points were obtained with samples that still contained some
solid materials in the reaction mixture and were thus omitted
from the kinetic analysis.
a = 8.377(7) Å
dimensions
c = 10.309(9) Å
α = 99.27(2)°
β = 112.227
(16)°
α = 90°
β = 101.990(2)°
γ = 90°
γ = 90°
γ = 90.341(17)°
795.5(12) Å3
2
Volume
Z
Density
(calculated)
Crystal size
934.18(4) Å3
4
2024.78(13) Å3
4
1.623 Mg m−3
1.529 Mg m−3
1.424 Mg m−3
0.193 × 0.075 × 0.313 × 0.080 ×
0.045 mm3 0.064 mm3
0.268 × 0.133 ×
0.080 mm3
2.047 to
Theta range for 1.641 to 28.306° 1.576 to 29.596°
data collection
Index ranges
27.102°
−14 ≤ h ≤ 14
−16 ≤ k ≤ 16
−8 ≤ l ≤ 8
10 743
−7 ≤ h ≤ 7
−37 ≤ k ≤ 37
−18 ≤ l ≤ 18
38 758
−10 ≤ h ≤ 10
−12 ≤ k ≤ 12
−13 ≤ l ≤ 13
24 544
Reflections
collected
Independent
reflections
Completeness to 99.9%
theta = 25.242°
2231 [R(int) =
0.0305]
5668 [R(int) =
0.0407]
100.4%
3483 [R(int) =
0.0471]
99.7%
Goodness-of-fit
1.047
1.095
1.041
X-ray crystallography
on F2
Final R indices
[I > 2sigma(I)]
R indices (all
data)
R1 = 0.0258,
wR2 = 0.0629
R1 = 0.0275,
wR2 = 0.0641
R1 = 0.0439,
wR2 = 0.1163
R1 = 0.0521,
wR2 = 0.1258
R1 = 0.0402,
wR2 = 0.0977
R1 = 0.0505,
wR2 = 0.1037
Crystals were grown by standard techniques from saturated
solutions using toluene. Suitable single crystals for X-ray struc-
ture analyses were selected in a glovebox and coated with
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computational demand of the SCS-MP2 method, the geome-
tries of larger systems (reactions ArNHBCl2 + BCl3 and
ArNHBCl2 + ArNH2) were optimized using the M06-2X76 hybrid
density functional along with the 6-311+G**77 basis set
employing the Gaussian 09 program.78 The nature of station-
ary points as minima or transition structures was confirmed
by computing second derivatives by finite differences of ana-
lytic gradients (SCS-MP2/cc-pVTZ)79 or analytically (M06-2X/
6-311+G**). This produced harmonic vibrational frequencies
that were employed without a scaling factor for computing
enthalpy and entropy corrections using the approximation of
an ideal gas and no coupling between degrees of freedom.
The optimized geometries were employed in subsequent
single point energy computations using coupled-cluster theory
with singles, doubles, and a perturbative estimate of triple
excitations, CCSD(T),80 using the cc-pVTZ basis set.74 The
resolution of the identity (RI) approximation was used along
with the corresponding fitting basis set.75 All SCS-MP2 and
CCSD(T) computations employed the frozen core approxi-
mation and were performed with the Turbomole program
package.81
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The Dalton program82,83 was employed for the computation
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motivated by the recommendations of Krivdin86 based on the 11 D. Bonifazi, F. Fasano, M. M. Lorenzo-Garcia, D. Marinelli,
performance when calculating 15N chemical shifts. The struc-
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were computed with reference to the experimental values
of boron trichloride (11B 46.5 ppm) and 4-fluoroaniline
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
16 M. Krieg, F. Reicherter, P. Haiss, M. Ströbele, K. Eichele,
M.-J. Treanor, R. Schaub and H. F. Bettinger, Angew. Chem.,
Int. Ed., 2015, 54, 8284–8286.
M. K. and C. K. thank the Fonds der chemischen Industrie
for fellowships. We thank Dr Klaus Eichele for the measure-
ment and interpretation of the solid state 11B NMR spectra. 17 M. Fingerle, C. Maichle-Mössmer, S. Schundelmeier,
The computations were performed on the BwForCluster
JUSTUS. The authors acknowledge support from the state of
B. Speiser and H. F. Bettinger, Org. Lett., 2017, 19, 4428–
4431.
Baden-Württemberg through bwHPC and the German 18 J. Dosso, J. Tasseroul, F. Fasano, D. Marinelli, N. Biot,
Research Foundation (DFG) through grant no. INST 40/467-1
FUGG.
A. Fermi and D. Bonifazi, Angew. Chem., Int. Ed., 2017, 56,
4483–4487.
19 B. Neue, J. F. Araneda, W. E. Piers and M. Parvez, Angew.
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20 H. Helten, Chem. – Eur. J., 2016, 22, 12972–12982.
21 H. Wei, Y. Liu, T. Y. Gopalakrishna, H. Phan, X. Huang,
L. Bao, J. Guo, J. Zhou, S. Luo, J. Wu and Z. Zeng, J. Am.
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