FLP reduction and hydroboration of phenanthrene o-iminoquinones and α-diimines

Article abstract of DOI:10.1039/c7dt01024a

Redox active, or non-innocent, ligands containing O or N heteroatoms are frequently used in transition metal complexes, imparting unique catalytic properties, but have seen comparatively limited use in the chemistry of group 13 elements. In this article we report the frustrated Lewis pair (FLP) hydrogenation and hydroboration of an N-aryl-phenanthrene-o-iminoquinone and two N,N′-diaryl-phenanthrene α-diimines. These reactions exploit B(C₆F₅)₃/H₂, HB(C₆F₅)₂ and H₂BC₆F₅·SMe₂ to give a series of derivatives including 1,3,2-oxaza- and diazaboroles and borocyclic radicals. The reaction pathways leading to these products are outlined and supported by DFT-calculations and experimental insight. The modular and unusual synthetic strategies described herein give access to new boroles as well as air-stable boron-containing radicals, thus extending the chemistry of redox active ligands in main group systems.

Full text of DOI:10.1039/c7dt01024a

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FLP Reduction and Hydroborations of Phenanthrene o-
Iminoquinones and α-Diimines†
Karlee L. Bamford,a Lauren E. Longobardi,a Lei Liu,b Stefan Grimme,b and Douglas W. Stephana*
Received 00th January 20xx,
Accepted 00th January 20xx
Redox active, or non-innocent, ligands containing O or N heteroatoms are frequently used in transition metal complexes,
imparting unique catalytic properties, but have seen comparatively limited use in the chemistry of group 13 elements. In
this article we report the frustrated Lewis pair (FLP) hydrogenations and hydroboration of an N-aryl-phenanthrene-o-
iminoquinone and two N,N’-diaryl-phenanthrene α-diimines. These reactions exploit B(C6F5)3/H2, HB(C6F5)2 and
H2BC6F5·SMe2 to give a series of derivatives including 1,3,2-oxaza- and diazaboroles and borocyclic radicals. The reaction
pathways leading to these products are outlined and supported by DFT-calculations and experimental insight. The modular
and unusual synthetic strategies described herein give access to new boroles as well as air-stable boron-containing
radicals, thus extending the chemistry of redox active ligands in main group systems.
DOI: 10.1039/x0xx00000x
www.rsc.org/
semidiimine ligands,17-21 group 13 derivatives are few in
number and are largely unknown for boron in particular.22-24,25
Recent reactions of boranes and the redox-active ligand N,N’-
1,4-diazabutadienes26, 27 have targeted families of borocations
although the corresponding neutral open-shell analogs have
received limited attention.15,28
Introduction
The synthesis and isolation of stable radicals is an on-going
challenge in main-group chemistry. This is motivated both by
the fundamental interest in such open-shell species1 and by
the potential for embedding them within conjugated materials
to access new optoelectronically active materials.2-5 Examples
of such materials featuring group 13 elements, particularly
boron, have become more abundant in the literature in recent
years. Stable boron-containing radicals have been made
accessible through a variety of synthetic means, including the:
i) reduction of boranes,6 borane complexes,7 diboranes,8 and
boroles9 to give anionic radical species; ii) reduction of donor-
stabilized borenium cations, to neutral boryl radicals;10 and iii)
oxidation of spiroborates to zwitterionic radicals.11
Chart 1. Examples of polyaromatic borocyclic radicals.
Examples of amine-ligated boron radicals are limited in
number and variety, but have been prepared by the chemical
Our group has added a new frustrated Lewis pair (FLP)
hydrogenation approach involving reactions of B(C6F5)3, H2 and
polyaromatic diones to give remarkably air-stable, neutral
1,3,2-dioxoborocyclic radicals (Chart 1, B).29 Most recently we
have examined the reactivity of these radicals with a variety of
nucleophiles affording a unique approach to a series of novel
zwitterions with concurrent electron transfer reactions.30
Herein we expand our approach, exploiting hydrogenation and
hydroboration reactions of phenanthrene o-iminoquinones
and α-diimines to generate a new air-stable 1,3,2-oxaza- and
diazaboroles and borocyclic radicals.
reduction of base-stabilized boranes12, 13 and from reactions of
redox-active ligands with boranes.14,
Early work in the
15
former area revealed that without base-stabilization,
reduction of organohaloboranes affords diamagnetic
borolanylborolanes.16 The reactions of the diaza-homologue of
a semiquinone (i.e. a semidiimine) in anionic form with boron
halides yields 1,3,2-diazaborocyclic radical (Chart 1, A) via salt
metathesis.14 While analogous salt metathesis approaches
have been widely used in the preparation of transition metal
complexes
of
polyaromatic
semiiminoquinone
and
Results and Discussion
An equimolar mixture of N-(2,6-dimethylphenyl)-phenanthren-
o-iminoquinone (1) and B(C6F5)3 in toluene was treated with H2
(4 atm) in a J-young NMR tube and heated to 110 °C for 0.5 h
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(Scheme 1), affording a dark brown solution. 19F and 11B NMR C=C bond lengths, while the dihedral angle between the 2,6-
spectra of the reaction mixture confirmed consumption of dimethylphenyl substituent on N and the plane of the
B(C6F5)3 The 19F NMR signals showed three signals assigned to oxazaborole is 77.7(3)°.
HC6F5, and five other resonances that were broad (wb = 130
Hz) and one very broad signal at -159.9 ppm (wb =1300 Hz)
while a single 11B NMR signal was observed at 10.4 ppm. These
data are consistent with the formation of a new species
exhibits some degree of inhibited rotation about the B–C
bonds and a second product, , that is likely paramagnetic.
3 that
4
Figure 2. Experimental X-band EPR spectrum of
4 in toluene at 298 K (black:
mod. amp. = 1) and simulated EPR spectrum (red line: g = 2.0039; a(11B) = 4.25 G,
a(14N) = 5.17 G, a(1H) = 1.42-3.98 G (8H)).
EPR spectroscopic analysis of
4 (Figure 2) revealed coupling
of the unpaired spin density to 14N, 11B, 19F (10F) and H (8H)
nuclei. The complex fine-structure resulting from these
couplings was not resolved and thus a range of fitted coupling
constants gave good correlation between simulated and
experimental spectra. Similar complications have been
encountered for related group 13 element radicals. 13,32,33 The
simulated spectrum presented herein was optimized from
computed hyperfine coupling constants (PBE0/def2-
QZVP//PBEh-3c, see ESI). Coupling to 1H nuclei is consistent
with delocalization of the unpaired spin density over the ligand
framework. Computational models of the SOMO and spin-
1
Scheme 1. Reaction of compound
dimethylphenyl).
1 and B(C6F5)3 under 4 atm H2 (C6H3Me2 = 2,6-
Fortuitously, a dilute pentane solution of the 3/4 mixture
left to slowly evaporate furnished several bronze crystals that
were shown by single crystal X-ray diffraction to be the amine-
ligated borinic ester C14H8O(NHC6H3Me2)B(C6F5)2
3 (
density distribution of
4 at multiple levels of theory (see Figure
Figure 1a). Compound 3 displays a pseudo-tetrahedral
3 and ESI) are also consistent with such delocalization.
Qualitatively, these spectra are similar to those seen for the
dioxoborocyclic radicals,29 however the indicated LUMO and
geometry about the boron atom, with O-B-C6F5 angles of
106.8(2)° and a B–N bond length of 1.706(6) Å.
SOMO energies (-3.66 eV and -5.87 eV) for
4 are higher than
those determined for the phenanthrene-based dioxoborocyclic
radical (the energies of LUMO and SOMO are -4.323 eV and -
6.550 eV, respectively)29 by the PW6B95/def2-TZVP//PBEh-3c
level. These differences are related to the electronegativity
differences between oxygen and nitrogen, resulting in an
overall lowering of all the molecular orbitals in the dioxo-
radical species. Experimentally, the aromatic stabilization in
manifests as its remarkable stability in air.34
4
Figure 1. POV-ray depictions of (a)
clarity. Atom colour scheme: H: grey; B: yellow-green; C: black; N: blue; O: red; F:
pink.
3 and (b) 4, with C–H atoms omitted for
Solutions of pure
4 are intensely brown in colour and UV-
vis spectroscopic analysis revealed an absorption band at 423
nm having a molar absorptivity of 1.3 x 103 L·cm-1·mol-1 (see
ESI). Compound
4
gives rise to a single broad 19F NMR
resonance (-159.9 ppm) but is otherwise NMR silent and
exhibits an EPR signal (vide infra). CH2Cl2 solutions of
compound
brown crystals. The molecular structure (
Figure b) verified the identity of
oxazaborocyclic radical, [C14H8O(NC6H3Me2)B(C6F5)2]•. The B–O
bond distance is notably increased in (1.514(4) Å) compared
with (1.478(5) Å), while the B–N bond distance is decreased
to 1.601(5) Å versus 1.706(6) Å in (Table 1). The (O)C–C(N)
bond length for (1.405(5) Å) lies between typical C–C and
4 cooled to -35 °C led to the isolation of black-
1
4 as the target 1,3,2-
Figure 3. Calculated SOMO (a) and spin density (b) of compound 4. Contour
surface value: 0.03 a.u. and 0.002, respectively. Computational level:
PW6B95/def2-TZVP//PBEh-3c. Atom colour scheme: H: white; B: pink; C: black;
O: red; F: green.
4
3
3
Cyclic voltammetric studies of 4 (see ESI) using the
ferrocene/ferrocenium couple (Fc/Fc+) as the reference
4
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revealed a pseudo-reversible wave centred at -0.63 V. This
indicates that is less easily reduced than its dioxo-
homologues (E = -0.27 V). This is consistent with the higher
conversion of
3
to
5, accompanied by loss of HC6F5, was
4
apparent (Scheme 2).
½
energy calculated for the SOMO, which suggests a lower
electrophilicity than the the corresponding orbital in the dioxo-
homologue.
Table 1. Selected geometric parameters of compounds
(PBEh-3c level) are given in parentheses for comparison.
3-
5
. Computed values
compound
d[B−O] (Å)
3
4
5
Heating reaction mixtures of
1
and B(C6F5)3 under H2 in
1.478(5)
(1.460)
1.706(6)
(1.723)
1.345(6)
(1.325)
100.9(3)
(100.2)
106.8(2)
(108.5)
111.1(3)
(110.6)
1.514(4)
(1.4964)
1.601(5)
(1.602)
1.405(5)
(1.409)
99.0(2)
(98.8)
1.383(3)
(1.376)
1.415(3)
(1.416)
1.368(3)
(1.361)
109.5(2)
(108.5)
124.0
toluene for an additional 2.5 h at 110 °C did not change the
dark appearance of the solution but led to complete
d[B−N] (Å)
consumption of
C6F5 resonances attributable to a species
signals attributable to HC6F5 (see ESI). The corresponding 11B
NMR spectrum for shows only a single broad resonance at
28.6 ppm. The spectroscopic data attributable to are typical
3
and the growth of ortho-, para-, and meta-
5
alongside those
d[(O)C-C(N)] (Å)
5
O
−B−N (°)
5
110.1(2)
(112.2)
108.4(2)
(108.1)
O−B−C6F5 (°)
N−B−C6F5 (°)
of 1,3,2-oxazaboroles, suggesting the formulation as
C14H8O(NC6H3Me2)BC6F5.35,36 Nonetheless, the isolated solid
proved to be a mechanical mixture of dark brown and white
solids, both of which exhibited moderate-to-excellent
solubility in common organic solvents, precluding facile
separation. Flash chromatography and sublimation proved to
be similarly ineffective. MS analysis of the crude mixture
confirmed the presence of two species having mass-to-charge
(123.5)
126.5
(127.9)
ratios consistent with the 1,3,2-oxazaborocyclic radical (
a 1,3,2-oxazaborole ( ). Successive recrystalizations lead to the
isolation of in 34 % yield.
The challenge of isolating compound
initial hydrogenation reaction prompted efforts to uncover an
alternative route. Treatment of with one equivalent
H2B(C6F5)·SMe2 (“Lancaster’s reagent”)37 in toluene afforded a
clear and colourless solution within 1 h of mixing at ambient
temperature (Scheme 2). 11B and 19F NMR spectra for this
reaction indicated complete consumption of Lancaster's
4) and
5
Scheme
dimethylphenyl).
2. Reaction of compound 1 and H2B(C6F5)·SMe2 (C6H3Me2 = 2,6-
4
5
cleanly from the
The mechanism of formation of 3-5 in the reaction of
1
with B(C6F5)3 and H2 was considered further. By analogy to the
proposed mechanism for the formation of dioxoborocyclic
radicals,29 an amino-alcohol
1
(
6
), resulting from B(C6F5)3-
, is proposed to react with
. It is interesting to note
, the NH···C6F5(ipso) distance is 2.6594
mediated hydrogenation of
1
B(C6F5)3 to give
3 and, subsequently, 5
that in the structure of
3
reagent after 2 h and clean formation of
isolated in 89% yield following purification by sublimation. X-
ray diffraction quality single crystals of were grown from
CH2Cl2 at -35 C. The structure of
related 1,3,2-oxazaboroles with
5, which could be
Å), which is within the sum of the van der Waal radii (2.9 Å),
foreshadows the protonation of the perfluorophenyl ring
5
leading to elimination of HC6F5 en route to
A possible pathway to was considered in which reduction
to C14H8OH(NHC6H3Me2) and subsequent
conproportionation with generated the neutral
5.
°
5
(Figure 4) is similar to
five-membered O-B-N
4
a
of
1
(6)
heterocycle that is not strictly planar as a result of the trigonal
pyramidal geometry about N.38 The cyclic voltammogram for
1
5
semiiminoquinone radical [C14H8O(NHC6H3Me2)]• (Scheme 3).
To the best of our knowledge, the formation of such a neutral
phenanthrene-based semiiminoquinone has not been
reported, although the phenomenon has been documented
for other o-iminoquinone derivatives.39,40,41 To probe this
(see ESI) displays irreversible oxidations at Epa values of 0.73 V
and 1.41 V.
possibility, 6 was independently prepared and structurally
characterized (see ESI) by a Pd-catalyzed hydrogenation of 1 42
.
Treating 6 with one equivalent of 1 resulted in a mixture that
was dark green in colour. The 1H NMR spectrum of this mixture
showed broadened methyl resonances from the N-bound
substituent of
its DFT-supported simulation (see ESI) were consistent with
the formation of free semiiminoquinone radical ( , Scheme 3).
1 and 6 (see ESI), while the EPR spectrum and
Figure 4. POV-ray depiction of
scheme: B: yellow-green; C: black; N: blue; O: red; F: pink.
5, with H atoms omitted for clarity. Atom colour
E
An alternative synthetic route to
4 was derived from the
Although attempts to isolate the semiiminoquinone radical
were unsuccessful, solutions of the in-situ generated
semiiminoquinone treated with a stoichiometric amount of
B(C6F5)3 immediately turned from dark green to dark brown in
reaction of 1 with one equivalent of HB(C6F5)2 (“Piers’
borane”) in toluene at ambient temperature. The immediately
formed dark brown solution shows complete consumption of
HB(C6F5)2 and the formation of compounds
3 and 4
by 11B and
colour, suggesting formation of
4. Multinuclear NMR spectra
19F NMR spectroscopic analysis. After 3 h heating at 110 °C the
were consistent with the formation of
3
and 4 and revealed
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complete consumption of B(C6F5)3 in 0.5 h at ambient kcal/mol). This may be explained by the errors arising from the
temperature. Crucially, reaction of with one equivalent of treatment of solvation or by an energy offset (i.e. a systematic
B(C6F5)3 at room temperature yields only the intermediate error) in the reference point for the zero of free energy.The
as detected by DART-MS and 19F NMR spectroscopy. reaction of Piers’ borane and
was also considered
Subsequent heating of the reaction mixture produced computationally. Pathways involving 1,2- and 1,4-
6
3,
1
compound
mixtures before and after heating were found to be EPR silent. computed. While the products
Thus the proposed mechanism for the formation of and
(Scheme 3) resembles that proposed for the dioxoborocyclic barrier to the imine-enamine tautomerism was found to be
radicals and is supported by the observation of key 29.3 kcal·mol-1. Oxidation of
by to give and is computed
5
and HC6F5. Most convincingly, the reaction hydroborations affording the α-boryloxy imine (
K) and
3
were
K
and are formed from
3
1 with
3
5
small barriers of 5.3 and 6.4 kcal·mol-1, respectively, the
K
1
E
4
intermediates that evaded isolation for the dioxo- to proceed with a remarkably-low activation barrier of 6.0
homologues.29
kcal·mol-1 resulting in an increased yield of
this reaction protocol.
4
relative to
3
for
Figure 5. Reaction coordinate diagram for the conversion of
and HC6F5 using B(C6F5)3/H2, computed at the PW6B95/def2-TZVP (COSMO-RS,
1
to products 4,5
Scheme 3. Proposed mechanism for the formation of products
(C6H3Me2 = 2,6-dimethylphenyl).
3, 4, and 5
toluene)//PBEh-3c level.
DFT methods were used to compute the energy profiles for
the investigated reactions. All structures were optimized with
a composite DFT method PBEh-3c,43 followed by single point
energy calculations at the PW6B95-D3 level of theory44-46 with
a large Gaussian AO def2-TZVP basis set.47, 48 The COSMO-RS
(Conductor-like Screening Model for Real Solvents) solvation
model49, 50 (with toluene as the solvent) was used to compute
solvation free energies (for computational details, see ESI).
According to the DFT analysis, the reaction proceeds via an
initial FLP cleavage of H2 affording an ion-pair that ultimately
generates
6
, which reacts competitively with B(C6F5)3 or
1
. The
the
kinetically-preferred
latter
pathway
generates
semiiminoquinone radical
E which reacts with B(C6F5)3 to yield
4
(Scheme 3). The rate determining step in the formation of
4
Figure 6. Reaction coordinate diagram for the conversion of 1 to products 4, 5,
and HC6F5 using HB(C6F5)2, computed at the PW6B95/def2-TZVP (COSMO-RS,
toluene)//PBEh-3c level (C6H3Me2 = 2,6-dimethylphenyl).
is the elimination of HC6F5 from the B(C6F5)3 adduct of the
semiiminoquinone radical (
kcal·mol-1.
F
), with an energy barrier of 34.2
The thermodynamically favoured pathway
, which subsequently eliminates HC6F5 to give
similarly involves the
Efforts to extend the above reactivity to homologous α-
diimines were undertaken. Reaction of (N,N’-(2,6-
produces
3
5.
The energy barrier to the formation of
5
dimethyl)phenyl)phenanthrene-9,10-diimine
eference source not found.) with B(C6F5)3 affords the weak
adduct C14H8(NC6H3Me2)(N(C6H3Me2)B(C6F5)3) , while addition
of H2 prompts reduction of the diimine to the give the α-
diamine, C14H8(NHC6H3Me2)2 quantitatively (see ESI for
structural characterization). While combination of compounds
and showed no evidence of conproportionation (see ESI),
did react with HB(C6F5)2 to give a new species 10 (Scheme ). 19
7
(Error!
protonation of a perfluorophenyl ring and was computed to be
41.4 kcal·mol-1. It is important to point out that the computed
high energy barriers are qualitatively consistent with the
8
experimental conditions. For example, the formation of
occurs at high temperature (110 C) and the conversion of
to requires an even higher temperature of 150 C for a fixed
3 and
9
4
°
3
5
°
7
9
7
reaction time of 0.5 h. However, on an absolute scale, the
computed barriers seem inordinately high (about 5-10
F
4 | J. Name., 2012, 00, 1-3
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NMR spectra of 10 obtained at elevated temperatures (25-100 that prolonged heating at 150 °C in bromobenzene did not
°C) showed averaging of signals, consistent with hindered promote conversion of the monohydroborated species 11 to
rotation about the B–N bond. While purification of the crude compound 12. This suggests that the geometry of the initial
product was achieved by sublimation (79% isolated yield), hydroboration product determines the elimination of H2,
single crystals of 10 were obtained from CH2Cl2/pentane consistent with the steric demands associated with rotation
layered solution stored at -35°C. The resulting crystallographic about the boron-substituted N-C bond.
study
confirmed
the
identity
of
10
as
Computations of the Gibbs energies of reactions of 7 (see
C14H8(NHC6H3Me2)(N(C6H3Me2)B(C6F5)2)
(
Error! Reference ESI) reveal that access to the 2,6-dimethylphenyl-substituted
ource not found., Figure 7a). In this α-amino borylamide, the 1,3,2-diazaborocyclic radical is thermodynamically feasible.
average distance between the H(N) atom and the nearest However, the experimental isolation of compounds 10 and 11
perfluorophenyl ipso-carbon for the two molecules of the suggests that steric demands result in an elevated energy for
asymmetric unit was found to be 3.98(3) Å, considerably the transition state towards loss of HC6F5 and, thus, the
longer than that seen in
3.
corresponding diazaborocyclic radical. To probe this
hypothesis, the less encumbered phenanthrene α-diimine,
C14H8(NC6H4Me)2 (13), bearing a p-tolyl substituent at nitrogen
was prepared. Equimolar reactions of B(C6F5)3 and compound
13 formed the adduct C14H8(NC6H4Me)(NC6H4Me(B(C6F5)3))
(
14) at ambient temperature (Scheme ).
Scheme 4. Reactions of compound 7 involving B(C6F5)3/H2, HB(C6F5)2, and
H2B(C6F5)·SMe2 (C6H3Me2 = 2,6-dimethylphenyl).
Scheme 5. Reactions of compound 13 with B(C6F5)3/H2, HB(C6F5)2, and H2B(C6F5)·SMe2
(C6H4Me = p-tolyl).
Reaction of 13 with B(C6F5)3/H2 heated to 110 °C for 1-2 h
gave a mixture of products, including trace amount of the α –
diamine, C14H8(NHC6H4Me)2 15. Flash column chromatography
of the crude hydrogenation reaction (95:5 pentane/CH2Cl2
eluent) gave partial separation, affording severals fractions
Figure 7. POV-ray depictions of (a) 11 and (b) 12. Disordered solvent molecules
(a, toluene; b, CH2Cl2) and C–H atoms have been omitted for clarity. Atom colour
scheme: H: grey; B: yellow-green; C: black; N: blue; F: pink.
containing
a mixture of two species formulated as
C14H8(NHC6H4Me)(NC6H4Me(B(C6F5)2) 16 and the 1,3,2-
diazaborole, C14H8(NC6H4Me)2B(C6F5) 17. While single crystals
of 16 could not be isolated, 19F and 11B NMR resonances and
DART-MS data support its formulation as an analog of the α-
amino boryl amide 10. The identity of 17 was confirmed by its
direct synthesis from the reaction of 13 and H2B(C6F5)·SMe2,
affording 17 in 62 % isolated yield (Scheme 5) and its
subsequent characterization by X-ray crystallography (
While heating of
reaction (>4 days, 150 °C),
(Scheme to generate the mono-hydroborated species
C14H8(NHC6H3Me2)(NC6H3Me2)BH(C6F5) 11 and the 1,3,2-
diazaborole C14H8(NC6H3Me2)2BC6F5 12 Error! Reference
9
with B(C6F5)3 or pure 10 resulted in no
7
reacts with H2B(C6F5)·SMe2
)
(
ource not found.). The two products could not be readily
separated, though single crystals suitable for X-ray diffraction
studies of 12 were successfully grown from a concentrated
CH2Cl2 solution cooled to -35 °C. The molecular structure of 12
(Figure 7b) is typical of a 1,3,2-diazaborole, with B–N distance
of 1.419(5) Å and N-B-N angle of 107.2(3)°. It is noteworthy
Figure 8a). The remaining fractions from the hydrogenation
reaction cumulatively yielded a trace amount of the radical
[C14H8(NC6H4Me)2B(C6F5)2]• 18 as evidenced by DART-MS and
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Dalton Transactions
EPR spectroscopy. Nonetheless, there was insufficient material compound 18 (see ESI) shows a reversible 1-electron reduction
for bulk isolation.
process with E1/2 = -0.94 V (vs. Fc/Fc+). The decreasing redox
potentials for the dioxo-, oxaza-, and diazaborocyclic radicals
indicates that incorporation of N-atoms in the borocyclic
radical framework is consistent with the diminishing reduction
ability of the boron radicals. The radical 18 and its solutions
exhibit remarkable stability, remaining unaffected by wet and
oxygenated benchtop solvents for up to 5 days and an
indefinite amount of time in the solid state under an inert
atmosphere of N2.
Figure 8. POV-ray depictions of (a) 17 and (b) 18. C–H atoms have been omitted
for clarity. Atom colour scheme: H: grey; B: yellow-green; C: black; N: blue; F:
pink.
Efforts to generate the radical 18 by reaction of the
semidiimine radical [C14H8(NHC6H4Me)(NC6H4Me)]• derived
from conproportionation of 13 and 15 afforded undentified
products. However, reaction of 13 with Piers’ borane (Scheme
5
Scheme ) at ambient temperature, following flash column
chromatography (95:5 pentane/CH2Cl2 eluent), yields fractions
containing 16 17 18 and a new compound 19. Heating the
crude reaction mixture to 110°C for 2-4 prior to
,
,
h
Figure 10. Calculated SOMO (a) and spin density (b) of compound 18. Contour
surface value: 0.03 a.u. and 0.002 a.u., respectively. Computational level:
PW6B95/def2-TZVP//PBEh-3c. Atom colour scheme: H: white; B: pink; C: black;
O: red; F: green.
chromatography appears to convert 16 to 17 and HC6F5, while
compounds 18 and 19 presist. Through mass spectrometric
analysis (DART, EI) compound 19 was identified as
C14H10(N(C6H3Me2)B(C6F5)2)2, a minor product (<5 % isolated
yield) from double hydroboration of compond 13. While single
crystals could not be obtained, compound 19 was
comprehensively characterized by NMR spectroscopy (see ESI).
Conclusions
A selection of non-innocent o-iminoquinone and α-diimine
ligands have been shown to produce stable 1,3,2-oxaza- and
diazaborocyclic radicals in FLP-type hydrogenations with
B(C6F5)3, or in hydroboration reactions with HB(C6F5)2. The
mechanism for the formation of these radicals has been
comprehensively modelled through DFT-calculations and
corroborated with control reactions involving independently
synthesized intermediates and by-products. Whereas the o-
iminoquinone was found to produce the targeted
oxazaborocyclic radical (4) in either FLP-hydrogenation or
hydroboration reactions, only the hydroboration route proved
effective for the less sterically hindered of the two α-diimine
substrates studied and yielded isolable diazaborocyclic radical
18. The radicals afforded from these reactions represent rare
examples of structurally-authenticated and comprehensively
characterized polyaromatic borocyclic radicals. The key
intermediate in the FLP hydrogenation pathway is postulated
to be the free semiiminoquinone or semidiimine radical while
the hydroboration route to borocyclic radicals is believed to
involve oxidation of 1,2-hydroborated intermediates by
unreacted o-iminoquinone or α-diimine substrates. The
inability of the bulky 2,6-dimethylphenyl N-substituted α-
diimine to produce a diazaborocyclic radical is attributed to a
steric limitation. Efforts to extend these synthetic approaches
to radicals containing other group 13 elements as well as to
explore the properties and reactivity of such borocyclic
radicals are on-going and will be reported in due course.
Figure 9. Experimental X-band EPR spectrum of 18 in toluene at 298 K (black:
mod. amp. = 1) and simulated EPR spectrum (red line: g = 2.0044; a(11B) = 5.06 G,
a(14N) = 5.25, 4.63 G (2N), a(1H) = 0.83-2.12 G (8H)).
The 1,3,2-diazaborocyclic radical 18 was isolated in 14 %
yield as a violet microcrystalline material. In contrast to the
1,3,2-oxazaborocyclic radical 4
, radical 18 exhibits 11B and 19F
NMR characteristics consistent with a pseudo-tetracoordinate
boron environment. Compound 18 affords magenta coloured
solutions in CH2Cl2 that exhibit multiple absorption bands (λ
=523, 562, 666nm; 1.7-6.4 x 103 L·cm-1·mol-1, see ESI) in the
visible region. EPR studies (Figure 9) reveal that while
hyperfine coupling constants to B and N are greater than those
seen for
4
and the related dioxoborocyclic radicals,29 the
hyperfine coupling to H nuclei in 18 are smaller. This infers
that the unpaired spin density is delocalized over the
phenanthrene backbone to
a lesser extent for 18 in
comparison to 3 and the dioxo-analog. This view is supported
by calculated models of the SOMO and spin density
distribution (Figure 10). The measured cyclic voltammogram of
6 | J. Name., 2012, 00, 1-3
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Dalton Transactions
toluene (2 mL) and transferred to a 50 mL Schlenk bomb
containing a stir bar. The vial containing remnant B(C6F5)3 was
rinsed with fresh toluene (3 x 1 mL) and these aliquots were
Experimental Section
Materials and general methods All preparative procedures
were performed in an inert atmosphere of dry, deoxygenated
(O2 < 0.5 ppm) nitrogen, using glovebox techniques or
standard Schlenk techniques unless otherwise specified.
Solvents were stored over activated 4 Å molecular sieves
following drying procedures. Pentane, dichloromethane, and
toluene (Sigma Aldrich) were dried using a Grubbs-type
Innovative Technologies solvent purification system.
Bromobenzene was dried over and distilled from CaH2.
Deuterated solvents (CDCl3, toluene-d8) were purchased from
Cambridge Isotope Laboratories, Inc. and stored over activated
transferred to a vial of
solution subsequently transferred to the Schlenk bomb. The
solution, containing the adduct of and B(C6F5)3, appeared
1 (93 mg, 0.3 mmol) and the combined
1
dark green-brown. The bomb was degassed using three freeze-
pump-thaw cycles prior to the addition of H2 (4 atm) at -196
°C. The solution was subsequently heated to 110 °C for 1 h,
resulting in a dark brown homogenous solution, which was
cooled to room temperature and dried in vacuo. The solid
obtained was dark brown. Method 2: A solution of compound
1
(93 mg, 0.3 mmol) in DCM (2 x 0.5 mL) was added to a
4
Å molecular sieves. 9,10-phenanthrenequinone (>95%
concentrated solution of HB(C6F5)2 (104 mg, 0.3 mmol) in DCM
(1 mL) while stirring. Instantaneously the mixture exhibited a
dark brown colour. The homogenous solution was stirred for 1
h prior to removal of volatiles under reduced pressure,
furnishing a dark brown solid. Method 3: A solution of
purity), p-toluidine, titatnium (IV) chloride (puriss) and
activated Pd/C were obtained from Sigma Aldrich and used
without further purification. 9,10-phenanthrenequinone
(>90% purity) was obtained from Alfa Aesar. B(C6F5)3 was
purchased from Boulder Scientific and sublimed under vacuum
at 95 °C prior to use. 2,6-dimethylaniline was obtained from
compound 6 (94 mg, 0.3 mmol) in DCM (2 x 0.5 mL) was added
to a solution of B(C6F5)3 (153 mg, 0.3 mmol) in DCM (1 mL),
with stirring. The mixture immediately produced a light brown
coloured solution. The solution was left to stir for 0.5 h prior to
removal of volatiles under reduced pressure, furnishing a
Acros
Organics.
N-(2,6-dimethylphenyl)-phenanthren-o-
), N,N’-(2,6-dimethylphenyl)-phenanthrene-
), N,N’-(p-tolyl)-phenanthrene-9,10-diimine
13), HB(C6F5)2, and H2B(C6F5)·SMe2 were prepared according
iminoquinone
9,10-diimine
(1
(
7
(
brown solid. Compound
solution, to give compound
compound from large scale syntheses by any of the three
above methods consistently resulted in mixtures of
compounds and . Crystals of suitable for X-Ray diffraction
3
eliminates HC6F5 over time, in
to previously reported synthetic procedures.51,52,53,37 o-
Iminoquinone and α-diimine substrates were purified by
column chromatography using Silicycle Silia-P Flash Silica Gel.
A Nanochem Weldassure purifier column was used to dry H2
(grade 5.0, supplied by Linde) used in hydrogenation reactions.
Thin-layer chromatography (TLC) was performed on EMD Silica
Gel 60 F254 aluminum plates and visualization of the developed
plates was achieved using a UV lamp (254 nm). Silica gel used
in glovebox manipulations was dried under vacuum at 150 °C
and plastic syringes were used in place of glass columns.
5.
Attempts to crystallize
3
3
5
3
were obtained by slow evaporation of a dilute pentane
solution over a span of 3 days at ambient temperature inside
an inert atmosphere glovebox. The solution was decanted,
affording bronze crystals (~15 mg). 11B NMR (128 MHz, CDCl3,
298 K): δ 10.0 (br s); 19F NMR (377 MHz, CDCl3, 298 K): δ -131.9
(br s, 1F, o-C6F5), -137.0 (br s, 1F, o-C6F5), -155.8 (br pseudo d,
2F, p-C6F5), -162.6 (br s, 2F, m-C6F5), -164.0 (br s, 2F, m-C6F5);
HRMS (DART) calcd for [C34H19BF10NO] ([M+H]+) 658.1400,
Found 658.1406.
All NMR spectra were collected at 298 K on Bruker Avance
III 400, Agilent DD2 500, or Agilent DD2 600 spectrometers in 3
or 5 mm diameter NMR tubes. H and 13C NMR chemical shifts
1
are reported relative to protio-solvent signals, while 11B and
19F NMR chemical shifts are reported relative to (Et2O)BF3 and
CFCl3 external standards, respectively. For compounds
generated in situ only 11B and 19F NMR data are reported,
while isolated compounds are accompanied by additional 1H
and 13C{1H} NMR data. All chemical shifts (δ) are reported in
ppm and coupling constants are given in Hz (see ESI for atom
numbering). Departmental facilities were used for all
elemental analyses (Perkin Elmer 2400 Series II CHNS Analyser)
and high resolution mass spectrometry (HRMS-DART; JEOL
AccuTOF).
Synthesis of [C14H8O(NC6H3Me2)B(C6F5)2]• (4)
.
Method 1:
Following the same procedure as for compound
3 (method 1),
and using the same quantities of reagent, the reaction mixture
was heated to 110 °C for 3 h, resulting in a dark brown
homogenous solution. The solution was cooled to room
temperature and dried in vacuo. The solid obtained was dark
brown. Crystalline material, grown from concentrated DCM
solutions cooled to -35 °C, was decanted, washed with cold
pentane and successively recrystallized in this fashion until
spectroscopically pure and X-ray diffraction quality dark brown
crystals of compound
13 % (crystals). Method 2: Following a similar procedure as
used for compound (method 2), and using the same
quantities of reagent, compound and HB(C6F5)2 were
4 were obtained. Isolated yield: 26 mg,
Generation of C14H8O(NC6H3Me2)B(C6F5)3 (2). Attempts to
isolate this species were unsuccessful. A green-brown solution
in toluene, prepared by combining equimolar amounts of
3
1
compound
1 and B(C6F5)3 at ambient temperature in an NMR
combined in 3 mL toluene. The mixing immediately gave the
solution a dark brown colouration. The homogenous reaction
mixture was heated to 110 °C and left to stir for 3 h prior to
removal of volatiles under reduced pressure at room
tube. 11B NMR (128 MHz, toluene-d8, 298 K): δ 39.5 (br s); 19F
NMR (377 MHz, toluene-d8, 298 K): δ -130.5 (br s, 6F, o-C6F5), -
147.6 (br s, 3F, p-C6F5), -161.5 (br s, 6F, m-C6F5).
Generation of C14H8O(NHC6H3Me2)B(C6F5)2 (3). Method 1:
B(C6F5)3 (154 mg, 0.3 mmol) was dissolved in a minimum of
temperature, furnishing
a dark brown solid. Following
fractional crystallization, dark brown microcrystals were
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Dalton Transactions
1
obtained. Isolated yield: 37 mg, 18 % (crystals). H NMR (400 H10 & H11), 7.02 (d, 3JH-H = 7.6 Hz, 2H, H18), 6.86 (t, 3JH-H = 7.3 Hz,
MHz, CDCl3, 298 K): NMR silent; 19F NMR (377 MHz, CDCl3, 298 1H, H19), 5.16 (br s, OH), 2.12 (s, 3H, H17); 13C{1H} NMR (126
K): δ
-159.9 (br s, C6F5); 11B NMR (128 MHz, CDCl3, 298 K): NMR MHz, CDCl3, 298 K): δ 146.7 (s, C1), 142.8 (s, C15), 131.5 (s, C13),
silent; 13C{1H} NMR (101 MHz, CDCl3, 298 K): NMR silent; HRMS 130.0 (s, C7) 129.9 (s, C18), 127.2 (s, C11), 126.9 (s, C4), 126.6 (s,
(DART) calcd for [C34H17BF10NO] ([M]+) 656.1243, found 656. C5), 126.4 (s, C8), 125.9 (s, C16), 124.8 (s, C14), 124.0 (s, C10)
1257;
Elemental
Analysis
calcd
(%) 123.0 (br s, C9 & C6), 122.5 (s, C3), 121.8 (s, C12), 121.1 (s, C19),
C34H17BF10NO⋅
1/2(CH3CH2CH2CH2CH3) : C 63.32; H 3.35; N 2.02; 117.7 (s, C2), 18.8 (s, C17); HRMS (DART) calcd for [C22H19NO]+
Found: C 63.58; H 3.43; N 2.45.
([M+H]+) 314.1544, found 314.1541; Elemental Analysis calcd
Synthesis of C14H8O(NC6H3Me2)BC6F5 (5). H2B(C6F5)·SMe2 (73 (%) for C22H19NO: C 84.31; H 6.11; N 4.47; Found*: C 83.00; H
mg, 0.3 mmol) was dissolved in a minimum of DCM (1 mL) and 6.15; N4.39. Elemental analysis was consistently low for
added to a vial of compound
1 (93 mg, 0.3 mmol) in 0.5 mL carbon.
DCM, equipped with a stir bar. A further 0.5 mL of DCM was Generation of C14H8(NC6H3Me2)(N(C6H3Me2)B(C6F5)3) (8). An
used to quantitatively transfer the remaining H2B(C6F5)·SMe2 orange-red solution in toluene was prepared from equimolar
to the reaction mixture. Within several minutes of initial amounts of compound
7 and B(C6F5)3 at room temperature.
mixing the reaction exhibited a teal-green colour. Over the 11B NMR (128 MHz, , toluene-d8, 298 K): δ 58.5 (br s); 19F NMR
course of 1 h stirring at ambient temperature the reaction (377 MHz, , toluene-d8, 298 K): δ -128.7 (br s, 6F, o-C6F5), -
turned colourless, and then became a faintly yellow, clear 142.0 (br s, 3F, p-C6F5), -160.1 (br s, 6F, m-C6F5).
solution. The reaction mixture was subsequently concentrated Synthesis of C14H8(NHC6H3Me2)2 (9)
in vacuo and stored at -35 °C, affording colourless crystals preparation of compound (125 mg, 0.3 mmol scale of
suitable for X-Ray diffraction; yield: 131 mg, 89 % (crystal). compound ; 3 mg, 10 mol % Pd/C), only, using twice the
Crude material (faintly yellow in colour) from larger scale volume of toluene (6 mL) given the reduced solubility of
syntheses may alternatively be purified by sublimation under compound in comparison with compound . The crude
. Identical to the
6
7
7
1
1
reduced pressure with heating (~163 °C). H NMR (500 MHz, material obtained following removal of volatiles was washed
CDCl3, 298 K): δ 8.79 (d, 3JHH = 8.6 Hz, 1H, H9), 8.75 (d, 3JHH = 8.2 with cold pentane (3 x 0.2 mL) to give a white powder; Yield:
3
3
Hz, 1H, H3), 8.44 (d, JHH = 8.1 Hz, 1H, H6), 7.75 (tm, JHH = 7.6 119 mg, 95 % (powder). Colourless single crystals of the
Hz, 1H, H5), 7.65 (tm, 3JHH = 7.6 Hz, 1H, H4), 7.54 (tm, 3JHH = 7.6 product, used in X-Ray diffraction studies, were isolated from a
Hz, 1H, H10), 7.33 – 7.20 (m, 4H, H11 & H18 & H19), 7.06 (d, 3JHH
=
DCM solution layered with pentane and stored at -35 °
C. 1H
3
8.3 Hz, 1H, H12), 2.12 (s, 6H, H17); 11B NMR (128 MHz, CDCl3, NMR (400 MHz, CDCl3, 298 K): δ 8.65 (d, JH-H = 7.9 Hz, 2H,
298 K): δ 28.9 (br s); 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ H3/H12), 7.84 (d, 2H, 3JH-H = 7.8 Hz, H6/H9), 7.50 (t, 3JH-H = 7.6 Hz,
1
1
3
3
148.0 (dm, JCF ~ 247 MHz, o-C6F5), 142.6 (dm, JCF ~ 249 MHz, 2H, H4/H11), 7.41 (t, JH-H = 7.6 Hz, 2H, H5/H10), 6.93 (d, JH-H
=
p-C6F5), 142.5 (s, C1), 137.3 (dm, 1JCF ~ 249 MHz, m-C6F5), 137.0 7.0 Hz, 4H, H18), 6.82 (t, 3JH-H = 7.4 Hz, 2H, H19), 5.27 (br s, N-H),
(s, C15), 136.3 (s, C16), 128.7 (s, C18), 128.2 (s, C19), 128.0 (s, C8), 1.85 (s, 12H, H17); 13C{1H} NMR (126 MHz, CDCl3,298 K): δ 142.3
127.3 (two overlapping s, C2 & C5), 126.9 (s, C11), 126.3 (s, C14), (s, C15), 130.1 (s, C7/C8), 129.3 (s, C18), 129.0 (s, C2/C13), 128.5
125.2 (s, C4), 124.8 (s, C10), 124.1 (s, C13), 123.7 (s, C9), 123.4 (s, (s, C1/C14), 128.0 (s, C16), 126.7 (s, C5), 125.1 (s, C4), 123.0 (two
C7), 123.1 (s, C3), 120.3 (s, C6), 119.8 (s, C12), 103.4 (br s, i-C6F5), overlapping s, C3/C12 & C6/C9), 121.3 (s, C19), 18.9 (s, C17);
18.2 (s, C17); 19F NMR (377 MHz, CDCl3, 298 K): δ -129.3(m, 2F, HRMS (DART) calcd for [C30H29N2]+ ([M+H]+) 417.2331, found
3
4
o-C6F5), -150.2 (tt, JFF = 20.2 Hz, JFF = 3.0 Hz, 1F, p-C6F5), - 417.2339; Elemental Analysis calcd (%) for C30H28N2: C 86.50; H
160.9 − -161.1 (m, 2F, m-C6F5); HRMS (DART) calcd for 6.78; N 6.72; Found: C 85.65; H 6.81; N 6.66. Elemental
[C28H18BF5NO] ([M+H]+) 490.1402, Found 490.1391; Elemental analysis was consistently low for carbon.
Analysis calcd (%)C28H17BF5NO : C 68.74, H 3.50, N 2.86; Found: Synthesis of C14H8(NHC6H3Me2)(N(C6H3Me2)B(C6F5)2) (10). To
C 68.60,H 3.66, N 2.81.
Synthesis of C14H8OH(NHC6H3Me2) (6). Following the methods compound
used to prepare 9,10-phenanthrene diol,42 a 25 mL bomb were added. Dropwise addition of the clear, colourless
containing a 3 mL toluene solution of compound (93 mg, 0.3 solution of HB(C6F5)2 to the stirring red-orange suspension of
separate vials containing HB(C6F5)2 (104 mg, 0.3 mmol) and
7
(124 mg, 0.3 mmol) 1 mL volumes of toluene
1
7
mmol) and 10 mol% (3 mg) Pd/C was sealed and degassed resulted in the immediate formation of a tan-orange coloured
using three freeze-pump-thaw cycles. The bomb was then solution. The reaction mixture was left to stir for 4 h before
charged with 4 atm H2 at -196 °C and warmed to room removal of volatiles in vacuo. The crude product was purified
temperature using
a
water bath. The solution was by flash column chromatography (eluent = 7:3 pentane:DCM).
subsequently heated for 2 h at 110 °C and filtered through Yield: 180.2 mg, 79 % (yellow powder). A concentrated DCM
celite in the glovebox after cooling. Removal of volatiles in solution of the product layered with pentane and stored at -35
vacuo and repeated washes with cold pentane (3 x 0.5 mL) °C readily yielded pale yellow crystals suitable for X-ray
afforded an analytically pure white powder. Isolated yield: 90 diffraction. 1H NMR (500 MHz, CDCl3, 298 K): δ 8.60 (dm, 3JHH
=
=
3
3
mg, 96 %. X-ray diffraction quality colourless single crystals 8.4 Hz, 1H, H9), 8.56 (dm, JHH = 8.5 Hz, 1H, H6), 8.36 (d, JHH
3
were grown from a DCM solution layered with pentane and 8.5 Hz, 1H, H3), 7.59 (t, JHH = 7.3 Hz, 1H, H4), 7.55−7.49
1
stored at -35 °C. H NMR (500 MHz, CDCl3, 298 K): δ 8.71-8.62 (overlapping tm & d, 2H, H10 & H12), 7.45 (tm, 3JHH = 7.5 Hz, 1H,
3
(m, 2H, H3 & H9), 8.38 (d, JH-H = 7.3 Hz, 1H, H6), 7.73-7.65 (m, H5), 7.19 (tm, , 3JHH = 7.6 Hz,1H, H11), 7.10 (d, 3JHH = 7.6 Hz, 1H,
2H, H4 & H5), 7.61 (d, 3JH-H = 7.6 Hz, 1H, H12), 7.51-7.40 (m, 2H, H18b/a), 6.89−6.72 (m, 4H, H18c & H18d & H19a & H19c), 6.64 (d,
8 | J. Name., 2012, 00, 1-3
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3JHH = 7.2 Hz, 1H, H18a/b), 5.83 (br s, N-H), 2.46 (s, 3H, H17a/b), diffraction studies, were isolated from a DCM solution layered
2.42 (s, 3H, H17d/c), 1.89 (s, 3H, H17c/d), 0.90 (s, 3H, H17b/a); 11B with pentane and stored at -35 °C. The product is stable under
NMR (128 MHz, CDCl3, 298 K): δ 40.0 (br s); 13C{1H} NMR (126 the inert atmosphere of the glovebox for an indefinite amount
MHz, CDCl3, 298 K, CDCl3): δ 147.8 (dm, 1JCF ~ 258 MHz, o-C6F5), of time, however, trace amounts of O2 causes clear, yellow
1
1
147.4 (dm, JCF ~ 246 MHz, o-C6F5), 146.3 (dm, JCF ~ 248 MHz, solutions of 15 to oxidize and turn dark purple over the course
1
of 5-6 h (see ESI for further description of stability). H NMR
1
o-C6F5), 145.8 (dm, JCF ~ 255 MHz, o-C6F5), 142.2 (dm, JCF
1
~
1
256 MHz, p-C6F5), 141.6 (dm, JCF ~ 256 MHz, p-C6F5), 141.6 (s, (500 MHz, CDCl3, 298 K): δ 8.74 (d, JHH = 8.4 Hz, 2H, H3/H12),
3
1
3
3
C15c), 140.5 (s, C15a), 136.9 (br dm, JCF ~ 245 MHz, m-C6F5), 8.03 (d, JHH = 8.1 Hz, 2H, H6/H9), 7.63 (t, JHH = 7.9 Hz, 2H,
135.6 (s, C16a/b), 135.42 (s, C16c/d), 134.1 (s, C16b/a), 130.6 (s, H4/H11), 7.53 (t, 3JHH = 7.6 Hz, 2H, H5/H10), 6.96 (d, 3JHH = 8.2 Hz,
3
C18c/d), 130.5 (s, C2), 129.9 (s, C8), 129.7 (s, C13 or C14), 129.4 (s, 4H, H17), 6.57 (d, JHH = 8.0 Hz, 4H, H16); 2.26 (s, 6H, H19),
C18a/b), 129.1 (s, s, C14 or C11), 128.8 (s, C19c), 128.6 (s, 16d/c), 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ 144.4 (s, C15), 130.2 (s,
128.6 (s, C18b/a), 127.0 (s, C10), 126.9 (s, C7), 126.6 (s, 18d/c), C1/C14), 130.0 (s, C7/C8), 129.9 (br m, C17 & C2/C13), 128.9 (s,
126.3 (s, C11), 126.0 (s, C4), 125.8 (s, C1), 124.3 (s, C5), 124.3 (br C18), 126.9 (s, C5/10), 126.3 (s, C4/C11), 125.1 (br s, C6/C9), 123.0
t, C3), 123.8 (s, C12), 122.9 (s, C6), 122.7 (s, C9), 122.5 (s, C19a), (s, C3/C12), 115.6 (s, C16), 20.6 (s, C19); (DART) calcd for
112.9 (br s, i-C6F5), 21.8 (d, JCF ~ 6 Hz, C17c/d), 20.0 (br t, C17d/c), [C28H25N2] ([M+H]+) 389.2018 , Found 389.2013; Elemental
19.1 (d, JCF ~ 3 Hz, C17b/a), 17.4 (s, C17a/b); 19F NMR (377 MHz, Analysis calcd (%) C28H24N2·C7H8 : C 87.46, H 6.71, N 5.83;
3
CDCl3, 298 K): δ -128.0 (dm, JFF = 24 Hz, 1F, o-C6F5(a/b)), - Found: C 85.14, H 6.45, N 6.34. Elemental analysis was
130.9 (dm, 3JFF = 24Hz, 1F, o-C6F5(c/d)), -131.7 (dm, 3JFF = 24 Hz, consistently low for carbon and solid samples handled (~ 5
1F, o-C6F5(b/a)), -132.8 (dm, 3JFF = 24Hz, 1F, o-C6F5(d/c)), -150.3 minutes) under an inert atmosphere of N2 for elemental
3
3
(t, JFF = 20 Hz, 1F, p-C6F5(a)), -151.1 (t, JFF = 20 Hz, 1F, p- analysis repeatedly reverted to compound 12 due to trace
C6F5(c)), -160.8 − -161.4 (m, 4F, m-C6F5(a,b,c,d)); HRMS (DART) oxygen.
calcd for [C42H28BF10N2] ([M+H]+) 761.2186, Found 761.2186; Generation of C14H8(NHC6H4Me)(NC6H4Me(B(C6F5)2) (16)
.
Elemental Analysis calcd (%) C42H27BF10N2 : C 66.33, H 3.58, N Similar to the preparation of compound
3.68; Found: C 64.93,H 3.59, N 3.57. Elemental analysis was 0.31 mmol of B(C6F5)3; 126 mg, 0.33 mmol compound 13), but
consistently low for carbon. owing to the reduced solubility of compound 13 in comparison
Generation of C14H8(NC6H3Me2)2BC6F5 (12). Similar to the with compound , a greater volume of toluene (5 mL) was
preparation of compound (128 mg, 0.31 mmol of compound used. The flask charged with hydrogen gas was subsequently
; 76 mg, 0.31 mmol of H2B(C6F5)·SMe2), but owing to the heated to 110 °C for 2 h, resulting in a dark red homogenous
reduced solubility of compound in comparison with solution, which was cooled to room temperature and dried in
compound , a greater volume of toluene (3 mL) was used. vacuo. The solid obtained was dark magenta. Complete
3 (method 1) (156 mg,
1
5
7
7
1
After heating for 1 h at 110 °C, compounds 11 and 12 were separation from compound 18 was achieved by flash column
formed. Solutions containing mixtures of 11 and 12 were chromatography (eluent = 95:5 pentane:DCM). Compounds 16
yellow in colour. Single crystals used in X-ray crystallography and 17 could not be separated by column chromatography,
studies were obtained from a concentrated DCM solution fractional crystallisation, or sublimation (see ESI). 11B NMR
stored at -35 °C. Attempted separation of compounds 11 and (128 MHz, CDCl3, 298 K): δ 36.0 (br s); 19F NMR (377 MHz,
3
12 by sublimation, fractional crystallization, flash column CDCl3, 298 K): δ -129.1 (m, 1F, o-C6F5), -131.4 (br d, JFF = 24.7
3
chromatography, and repeated washing failed, precluding Hz, 1F, o-C6F5), -132.7 (br d, JFF = 21.6 Hz, 1F, o-C6F5), -133.3
3
their complete characterization. 11B NMR (128 MHz, CDCl3, 298 (m, 1F, o-C6F5), -151.4 (t, JFF = 19.8 Hz, 1F, p-C6F5), -152.0 (t,
K): δ 25.3 (br s); 19F NMR (377 MHz, CDCl3, 298 K): δ -128.3 (dd, 3JFF = 20.4 Hz ,1F, p-C6F5), -159.9 (m, 1F, m-C6F5), -160.7 (m, 1F,
3
4
3JFF = 23 Hz, 4JFF = 9 Hz, 2F, o-C6F5), -151.7 (t, 3JFF = 21 Hz, 1F, p- m-C6F5), -161.4 (ddd, JFF = 24.7 Hz & 20.0 Hz, JFF = 10.2 Hz,
C6F5), -161.6 (td, JFF = 23 Hz, JFF = 9 Hz, 2F, m-C6F5) ; HRMS 1F, m-C6F5), -161.8 (ddd, 3JFF = 24.2 Hz & 19.8 Hz, 4JFF = 9.7 Hz,
(DART) calcd for [C36H27BF5N2] ([M+H]+) 593.2187, Found 1F, m-C6F5). HRMS (DART) calcd for [C40H24BF10N2] ([M+H]+)
3
4
593.2199.
733.1873, Found 733.1872.
Generation of C14H8(NC6H4Me)(NC6H4Me(B(C6F5)3)) (14). A Synthesis of C14H8(NC6H4Me)2B(C6F5) (17). Identical to the
dark green solution in toluene, prepared from equimolar preparation of compound 12 (123 mg, 0.32 mmol of
amounts of compound 13 and B(C6F5)3. 11B NMR (128 MHz, compound 13; 79 mg, 0.33 mmol of H2B(C6F5)·SMe2) using 4
toluene-d8, 298 K): δ 2.6 (br s), 11.9 (s); 19F NMR (377 MHz, mL of bromobenzene and heating the reaction mixture to 150
toluene-d8, 298 K): δ -131.8 – -134.1 (br m, 6F, o-C6F5), -158.5 °C for 3 h. immediately forming a dark red-orange coloured
(br s, 3F, p-C6F5), -164.6 (br s, 6F, m-C6F5).
solution. The solution changes from a dark red-orange colour
Identical to the solution (room temperature) to pale yellow with heating
Synthesis of C14H8(NHC6H4Me)2 (15)
.
.
preparation of compounds
6 and 9 (121 mg, 0.3 mmol of Unreacted compound 13 could not be removed by washing or
compound compound 13; 3 mg, 10 mol% Pd/C) using 4 mL of recrystallization, however filtration of the crude reaction
toluene and heating the reaction mixture to 110 °C for 1 h. The mixture through silica gel using a 95:5 pentane:DCM eluent
crude material obtained from filtration over Celite and furnished pure compound 17 in 62 % (yellow powder). Single
removal of volatiles in vacuo was washed with cold pentane (3 crystals suitable for X-ray crystallography were grown at -35 °C
1
x 0.2 mL) to give a bright yellow powder; Yield: 89.4 mg, 92 % from a concentrated DCM solution. H NMR (600 MHz, CDCl3,
(powder). Yellow single crystals of the product, used in X-Ray 298 K): δ 8.75 (br d, 3JHH = 8.8 Hz, 2H, H3/H12), 7.45 (ddd, 3JHH
=
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3
4
8.3 Hz, JHH = 6.9 Hz, JHH = 1.3 Hz, 2H, H4/H11), 7.34 (dd, 3JHH
=
Found 628.1307; MS (EI) calcd for [C52H24B2F20N2] ([M]+):
4
8.5 Hz, JHH = 1.2 Hz, 2H, H6/H9), 7.18-7.29 (m, 10H, H5 & H16
H17), 2.43 (s, 6H, H19); 11B NMR (128 MHz, CDCl3, 298 K): δ 25.9 both DART-MS and EI-MS is the fragmentation product from
(br s); 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ 139.3 (s, C15), loss of [N(C6H3Me)B(C6F5)2].
&
1078.18, Found 1078.2. The major peak for compound 19 by
137.2 (s, C18), 130.2 (s, C17), 127.9 (s, C7/C8), 127.9 (s, C2/C13), X-ray crystallography X-ray crystallographic data were
127.4 (s, C16), 126.0 (s, C8/C10), 124.1 (s, C1/C14), 123.9 (s, collected on a Bruker Apex2 X-ray diffractometer at 150±2 K
C4/C11), 123.7 (s, C3/C12), 122.1 (s, C6/C9), 21.4 (s, C19); 19F NMR using a graphite monochromator with MoΚ(α) radiation (λ =
(377 MHz, CDCl3, 298 K): δ -129.1 (dd, , 3JFF = 24.5 Hz, 4JFF = 9.7, 0.71073 Å) and the Bruker APEX-2 software54 package. Suitable
3
2F, o-C6F5), -152.0 (t, JFF = 20.2 Hz, 1F, p-C6F5), -161.8 (ddd, , crystals were selected and mounted in Paratone-N oil on a
4
3JFF = 23.9 & 19.4 Hz, JFF = 9.8 Hz, 2F, m-C6F5); HRMS (DART) MiTeGen cryoloop, then placed in the cold (N2) stream of the
calcd for [C34H23BF5N2] ([M+H]+) 565.1874, Found 565.1867; diffractometer. Unit cell parameters were determined from
Elemental Analysis calcd (%) C34H22BF5N2 : C 72.36, H 3.93, N consecutive scans at different orientations. The data were
4.96; Found: C 72.36, H 3.93, N 4.96.
Synthesis of [C14H8(NC6H4Me)2B(C6F5)2]• (18). Identical to the scan absorption correction was applied using SADABS56. All
preparation of compound (method 2) (120 mg, 0.3 mmol of structures were solved and refined by direct methods in the
integrated using the SAINT software package55 and a multi-
4
compound 13; 107 mg, 0.3 mmol of HB(C6F5)2) using 2.5 mL SHELXTL suite of programs using XS and refinement by full-
toluene. The ruby red-coloured reaction mixture was left to matrix least-squares on F2 using XL.57,58 All non-hydrogen
stir at for 2 h before removal of volatiles in vacuo. The crude atoms were subjected to anisotropic refinement and carbon-
product (18) was a magenta colour solid and was purified by bound hydrogen atoms were placed in calculated positions
flash column chromatography (eluent = 95:5 pentane:DCM). using an appropriate riding model and coupled isotropic
Washing the crude product (or mixed fractions containing 16 temperature factors (Uiso(H)
= 1.2Ueq(C)). Oxygen- and
and 17) with cold pentane was also performed during nitrogen-bound hydrogen atoms were located in the
purification. Isolated yield: 31.4 mg, 14 % (violet powder). difference Fourier map and were refined with restraints of the
Crystals used in X-ray diffraction studies were collected from a O-H (0.84(2) Å) and N-H (0.91(2) Å) bond lengths and with
concentrated solution of 18 in approximately 70:30 Uiso(H) = 1.5Ueq(O).
1
pentane:DCM solvent mixture stored at -35 °C. H NMR (500 EPR spectroscopy A Bruker ECS-EMX X-band EPR spectrometer
MHz, CDCl3, 298 K): δ NMR silent; 11B NMR (128 MHz, CDCl3, equipped with an ER4119HS cavity was used for all
298 K): δ -11.2 (br s); 13C{1H} NMR (126 MHz, 298 K): δ NMR electron paramagnetic resonance (EPR) measurements.
silent ; 19F NMR (377 MHz, CDCl3, 298 K): δ -136. 8 (br s, 4F, o- Samples were measured at 298 K as dilute toluene solutions.
3
3
C6F5), -158.8 (t, JF-F = 18.1 Hz, 2F, p-C6F5), -161.8 (tm, JF-F
=
EPR simulations were performed using PEST WinSIM Software.
22.0 Hz, 4F, m-C6F5); HRMS (DART) calcd for [C40H23BF10N2] Cyclic Voltammetry Electrochemistry was performed with a
([M+H]+) 732.1794, Found 732.1809; Elemental Analysis calcd BASi Epsilon potentiostat and BASi RDE-2 Electrode cell stand.
·
(%) C41H26BF10N2 : C 65.69, H 3.03, N 3.83; Found: C 38.67 ,H A three-electrode cell with a 3.0 mm diameter glassy carbon
2.30, N 3.83. Elemental analysis was inexplicably low for working electrode, a Ag/AgCl reference electrode, and a Pt
carbon and hydrogen, despite HRMS(DART) demonstrating wire counter electrode was used, collected at a 100 mV s-1
purity.
scan rate (unless otherwise stated). 0.5 mM sample solutions
Generation of C14H10(N(C6H3Me2)B(C6F5)2)2 (19)
.
The were prepared in the dry-nitrogen atmosphere of a glovebox,
procedure used for compound 18, using the same quantities of using dry dichloromethane solution, and measured in the
reagents, was followed. Separation of 19 from compounds 17 presence of 0.5 mM ferrocene (Fc/Fc+ reference standard) and
and 18 was achieved by flash column chromatography (eluent 0.1
M
tetrabutylammonium tetrafluoroborate electrolyte.
(98% purity) and tetrabutylammonium
= 95:5 pentane:DCM) and was isolated in very low yield (~ 2 %) Ferrocene
as grains of a magenta coloured solid. 1H NMR (400 MHz, tetrafluoroborate (99% purity) were obtained from Sigma
CDCl3, 298 K): δ 7.63 (d, 3JHH = 7.4 Hz, 2H, H3/H12), 7.38 (t, 3JHH
=
Aldrich and used without any further purification.
3
7.9 Hz, 2H, H4/H11), 7.23 (t, JHH = 7.6 Hz, 2H, H5/H10), 6.98 (d, UV-visible spectroscopy An Agilent 8453 UV-Vis
3JHH = 7.7 Hz, 2H, H6/H9), 6.75-6.60 (br m, 4H, H17a & H16a), 6.47 spectrophotometer and 1 cm pathlength quartz cuvettes with
(br s, 2H, H16b), 6.16 (brs, 2H, H17b), 5.34 (s, 2H, N-H), 2.10 (s, PTFE stoppers were used to collect all UV-Vis absorption
6H, H19); 11B NMR (128 MHz, CDCl3, 298 K): δ 36.4 (br s); spectra. Analyte stock solutions were prepared in volumetric
13C{1H} NMR (126 MHz, CDCl3, 298 K): 140.8 (s, C15), 137.3 (s, flasks in an inert nitrogen atmosphere glovebox, using dry
C18), 135.0 (s, C7/C8), 131.5 (s, C3/C12), 130.4 (s, C2/C13), 129.8 dichloromethane, and were diluted accordingly.
(s, C5/C10), 128.9 (s, C17a), 128.7 (s, C4/C11), 128.1 (s, C16a), 127.6 Computational details All density functional theory (DFT)
(s, C16b), 127.1 (s, C17b), 122.7 (s, C6/C9), 66.6 (s, C1/C14), 29.9 (s, calculations were performed by employing Turbomole 7.0
C19); 19F NMR (377 MHz, CDCl3, 298 K): δ –128.2 (br 1F, o-C6F5), software.59 Harmonic vibrational frequency calculations were
–130.3 (br 1F, o-C6F5), -131.0 (m, 1F, o-C6F5), -132.3 (m, 1F, o- conducted at the PBEh-3c level to characterize the nature of
3
C6F5), -153.4 (t, JFF = 20.3 Hz, 1F, p-C6F5), -161.0 (br s, 2F, p- the stationary points along the reaction coordinates: no
C6F5), -161.8 (ddd, 3JFF = 22.6 Hz & 21.8 Hz, 4JFF = 9.1 Hz, 1F, m- imaginary frequencies were found for the local minima, and
C6F5), -162.0 (ddd, 3JFF = 22.4 Hz & 22.0 Hz, 4JFF = 9.4 Hz, 1F, m- one and only one imaginary frequency for the transition states.
C6F5), HRMS (DART) calcd for [C33H17BF10N] ([M+H]+): 628.1294, The thermostatistical contributions to the free energy in the
10 | J. Name., 2012, 00, 1-3
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373.15 K, respectively, and for 1 atm pressure. The density-
fitting RI-J61,62 approach for the Coulomb integrals was used to
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D.W.S. gratefully acknowledges the financial support from
NSERC Canada and the award of Canada Research Chair. K.L.B.
and L.E.L are thankful for Alexander Graham Bell Canada
Graduate Scholarships and L.E.L. also gratefully acknowledges
the Walter C. Sumner Foundation and Digital Specialty
Chemicals for scholarships. This work was supported by the
DFG in the framework of the Gottfried-Wilhelm-Leibniz prize
(S.G.). The authors also thank Levy Cao for technical assistance
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