Sulfur and selenium activation by frustrated NHC/B(C6F 5)3 Lewis pairs; conformational flexibility of products

Article abstract of DOI:10.1515/znb-2011-0406

Frustrated Lewis pairs consisting of N-heterocyclic carbenes (NHC) and the borane B(C₆F₅)₃ react with elemental sulfur or selenium to give products of the type NHC-E-B(C₆F₅) ₃, where E is S

Full text of DOI:10.1515/znb-2011-0406

Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs;
Conformational Flexibility of Products
Dirk Holschumacher, Constantin G. Daniliuc, Peter G. Jones, and Matthias Tamm
Institut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t Braunschweig,
Postfach 3329, 38023 Braunschweig, Germany
Reprint requests to Prof. Dr. M. Tamm. Fax: +49-531-391-5387. E-mail: m.tamm@tu-bs.de
Z. Naturforsch. 2011, 66b, 371 – 377; received February 18, 2011
Frustrated Lewis pairs consisting of N-heterocyclic carbenes (NHC) and the borane B(C6F5)3 react
with elemental sulfur or selenium to give products of the type NHC-E-B(C6F5)3, where E is S or Se.
Three such products, two with sulfur and one with selenium, were characterized by X-ray diffraction
and shown to exhibit considerable conformational flexibility, as revealed by differing torsion angles
in the atom sequence N–C–E–B–Cipso–Cortho. In the sulfur derivatives, the S–B bonds are all long
˚
˚
(ca. 2.05 A), and the C–S bonds (ca. 1.73 A) are clearly lengthened compared to imidazole-2-thiones.
˚
The Se–B distance of 2.2111 A is the first selenone-borane bond length to be determined by X-ray
analysis.
Key words: N-Heterocyclic Carbenes, Frustrated Lewis Pairs, Sulfur, Selenium
Introduction
The reaction behavior of free NHCs with sulfur
or selenium has been studied [16– 19]. Mixing the
starting materials grey selenium or S8 with the car-
benes at r. t. in THF or toluene resulted in the forma-
tion of imidazole-2-thiones 3 [16, 17] or imidazole-2-
selenones 5 [20, 21] within a few hours. After filtration
and precipitation of the product from the filtrate with
hexane the thiones or selenones could be obtained in
almost quantitative yield. The selenium compounds are
more strongly polarized than the corresponding sulfur
compounds, because selenium forms only weak Se–C
π bonds [22].
The system (tBu)2PC6F4B(C6F5)2 is unable to form
an FLP adduct with S8; only an oxidation of the phos-
phane unit without further coordination of the bo-
rane to sulfur was achieved [12]. However, S–S bond
cleavage in organo-disulfides with (tBu)3P/B(C6F5)3
or carbogenic FLP combinations is known, because of
the weaker non-polar covalent S–S bond in disulfides
[12, 13].
Since 2006 a novel type of metal-free reactivity be-
tween Lewis acids and Lewis bases in the presence of
dihydrogen, based on the so called frustrated Lewis-
pairs (FLP), has been observed by Stephan and oth-
ers [1 – 5]. The classical Lewis theory [6 – 8] postu-
lates that the Lewis acid and the Lewis base form stable
donor-acceptor adducts, in which the unshared pair of
electrons of the base forms a covalent bond through the
vacant orbital of the acid. In FLP systems, however, no
such normal adduct formation is possible for steric rea-
sons [3 – 5], but there is a great latent tendency to react
when suitable substrates are available.
ThesystemN-heterocycliccarbene(NHC)/B(C6F5)3
represents the most reactive FLP combination [9].
We were able to show that 1,3-di-tert-butylimidazo-
lin-2-ylidene 1a and 1,3-di-tert-butylimidazolidin-2-
ylidene 1b together with B(C6F5)3 are FLP systems
for dihydrogen activation (with H+ adding to the
carbene and H− to the borane) and for THF ring
opening [10, 11]. Furthermore, sterically encumbered
Lewis acids and Lewis bases react with enthalpically
strong bonds such as S–S and P–P and thus with other
small molecules such as organic disulfides [12, 13],
polyphosphides [14] or P4 [15]. Herein we demon-
strate carbene/B(C6H6)3 FLP reactivity towards sulfur
and selenium.
Discussion
We have investigated the reactivity of the FLP com-
binations 1/B(C6F5)3 towards sulfur (S8) and grey se-
lenium. In the hope of splitting one S–S bond of the S8
molecule heterolytically, the combinations 1/B(C6F5)3
◦
were mixed in a toluene solution of S8 at 0 C. After
c
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D. Holschumacher et al. · Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs
Scheme 1. Reactions of 1/B(C6F5)3 with sulfur and sele-
nium.
about 1 h the colour of the mixture changed from
yellowish to golden yellow at ambient temperature.
Fig. 1a. Displacement ellipsoid plot (30 %) of the first in-
dependent molecule of compound 3a. Hydrogen atoms have
been omitted for clarity.
◦
Adding pentane and cooling to −35 C led to preci-
pitation of yellow products 3a,b. 3a was obtained after
filtration in 91 % yield, whereas 3b was isolated af-
ter crystallisation in only 41 % yield (Scheme 1). The
products are pale yellow, very unpleasantly smelling
solids, which decompose at room temperature and in
chlorinated solvents.
The 11B NMR spectrum revealed a broad singlet
resonance at 10.3 ppm for 3a, which is about 53 ppm to
high field of free B(C6F5)3. However, the signal of the
saturated complex 3b at 37.2 ppm is only moderately
high-field shifted, indicating a weaker coordination of
the B(C6F5)3 moiety at sulfur. The resonances in the
11B NMR spectra show in comparison to related thio-
borates (≈ –10 ppm) [12, 23] a distinct downfield shift
consistent with the known ability of soft donors to form
only weak adducts with Lewis acids [24]. The reso-
nances of the former carbene carbon atom in the 13C
NMR spectra appear for 3a at 152.0 ppm and for 3b at
180.9 ppm, and thus are minimally shifted in contrast
to the thiones (2a: 160.6 ppm; 2b: 183.6 ppm; [25]).
The 19F NMR spectra of 3a, b exhibit three broad res-
onances for the ortho-, para- and meta-positions of the
C6F5 group. The 1H NMR spectra show resonances of
the tert-butyl groups and of the imidazol skeleton (see
Experimental Section).
Fig. 1b. Least-squares fit of both independent molecules
of compound 3a. Full bonds: molecule 2; dashed bonds:
molecule 1 (inverted). Fitted atoms: NHC ring, B, S.
Recrystallisation from toluene/pentane solution af-
forded single crystals suitable for X-ray diffraction
analysis. The molecular structures reveal the forma-
tion of the adducts 3a, b, which contain just one sul-
fur atom between the former carbene and the B(C6F5)3
unit (Figs. 1a and 2). The unsaturated compound 3a
crystallised with two molecules in the asymmetric unit, Fig. 2. Displacement ellipsoid plot (30 %) of compound 3b.
Hydrogen atoms have been omitted for clarity.
while the saturated compound 3b crystallised with one
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D. Holschumacher et al. · Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs
373
1
Table 1. Crystallographic data
for compounds 3a, 3b, and 5b.
3a
3b·C5H12
5b·C5H12· /2C7H8
C35.5H33BF15N2Se
862.41
Formula
Mr
C29H20BF15N2S
724.34
C34H34BF15N2S
798.50
Crystal size, mm3
T, K
0.15×0.14×0.11
0.17×0.12×0.07
0.14×0.11×0.09
100(2)
100(2)
100(2)
Crystal system
Space group
Cell constants
triclinic
triclinic
monoclinic
P21/n
¯
¯
P1
P1
˚
a, A
10.3616(6)
13.8457(4)
20.2999(11)
90.557(3)
91.302(4)
90.907(3)
2911.0
11.9017(8)
12.3662(8)
13.0292(8)
99.796(6)
105.035(6)
102.704(6)
1753.3
11.5674(4)
12.4107(4)
25.5403(10)
90
90.750(8)
90
˚
b, A
˚
c, A
α, deg
β, deg
γ, deg
3
˚
V, A
3666.2
4
Z
4
2
F(000), e
1456
2.1
1.54184
152
0.688, 1.000
816
1.8
1.54184
152
0.884, 1.000
1736
2.4
1.54184
152
0.880, 1.000
µ, (CuKα ), mm−1
˚
λ, A
2θmax, deg
Transmissions
Reflections meas. / indep. / Rint 31666 / 11660 / 0.021 25399 / 7252 / 0.032 52696 / 7624 / 0.029
Ref. parameters / restraints
wR (F2, all refl.)
R1 [F ≥ 4 σ(F)]
S
893 / 24
0.085
0.032
1.02
0.36 / −0.33
530 / 92
0.117
0.039
1.09
0.39 / −0.27
439 / 0
0.069
0.026
1.06
0.31 / −0.37
−3
˚
∆ρmax/min, e A
molecule of pentane. The tetrahedral thioborate unit molar ratio of sulfur to the FLP combinations 1a,
shows S–B bond lengths of 2.0568 (15) and 2.0490 b/B(C6F5)3 at various temperatures; only compounds
˚
˚
(15) A for 3a and 2.0572 (17) A for 3b, which are with one sulfur atom between the NHC and B(C6F5)3
longer than those determined for [RSB(C6F5)3]− an- moiety could be obtained.
˚
˚
ions (1.953 (13) A and 1.991 (3) A; [3]) reflecting
the weak coordination of the borane at the sulfur
atom. Even longer B–S bonds have been recorded for
(tetrahydrothiophene)B(C6F5)3 (2.0843 (16) A; [26])
and Me2SB(C6F5)3 (2.091 (5) A; [27]). The C–S dis-
Because of the chemical similarity of sulfur and se-
lenium, an obvious extension of this investigation was
to the possible reactivity of the FLP combinations 1a,
b/B(C6F5)3 towards grey selenium. After about 1 h the
reaction mixture changed from yellowish to orange and
an oily product was obtained. No adduct formation be-
tween the FLP and the selenium was observed; instead
rearrangement to an abnormal carbene or selfdehydro-
genation of 1a, b/B(C6F5)3 was detected [10, 11]. The
failure of Se adduct formation could be a result of the
high insolubility of grey selenium in toluene, which
leads to fast deactivation rather than selenium activa-
tion.
However the formation of imidazole-2-selenones
from free carbenes is known [20, 21], and the se-
lenones 4a, b can be prepared by the reaction of the
carbenes 1a, b with grey selenium. Interestingly, the
77Se resonances of 4a (188.8 ppm) and 4b (309.0 ppm)
are markedly different, and this trend is in agreement
˚
˚
˚
tances of 1.7320 (13), 1.7335 (14) A for 3a and 1.7327
˚
(15) A for 3b are clearly lengthened compared to re-
˚
˚
lated imidazole-2-thiones (1.686 (3) A; 1.674 (4) A),
because of the loss of the partial double bond character
[25]; a search of the Cambridge Database [28] revealed
ca. 150 hits for the system C–S with three-coordinate
C and one-coordinate S, with bond lengths in the range
˚
1.60– 1.74 A (excluding obvious outliers such as be-
taines and disordered structures), av. 1.66 (3) A. In-
deed, the C–S bond lengths of 3a, b correspond well
with those of the zwitterionic species (NHC)-S-(4,6-
˚
˚
dioxo-1,3-dioxanide) 1.763 (2) A [29]. Accordingly,
the angles at sulfur were found to be 108.69 (6)◦ and
110.88 (6)◦ for 3a and 111.07 (7)◦ for 3b.
It is noteworthy that no single S–S bond cleavage with the presence of a more strongly polarised C–Se
of the S8 molecule to form opened S8 bridge adducts bond in the 2-selenoimidazoline 4a in comparison with
could be observed, despite deliberate variation of the the 2-selenoimidazolidine 4b [30].
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D. Holschumacher et al. · Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs
Compounda
E–C1
E–B
3ab
3b
5b
Table 2. Selected molecular di-
˚
mensions (A, deg) for com-
1.7320(13), 1.7335(14)
2.0568(15), 2.0490(15)
108.69(6), 110.88(6)
−89.31(12), 86.83(12)
12.79(11), −32.67(11)
−111.77(9), 93.12(10)
135.64(9), −154.59(9)
−67.49(14), 66.87(14)
−49.84(13), 51.60(14)
−7.74(18), 8.43(8)
C8–N2–C1–S
1.7327(15)
2.0572(17)
111.07(7)
68.03(15)
−19.99(12)
102.92(10)
−145.78(10)
63.49(15)
51.80(15)
7.03(19)
1.9187(14)
2.2111(15)
107.086)
pounds 3a, 3b, and 5ba.
C1–E–B
B–E–C1–N1
C1–E–B–C20
C1–E–B–C30
C1–E–B–C40
E–B–C20–C21
E–B–C30–C31
E–B–C40–C41
largest
−88.79(12)
44.71(10)
−81.79(10)
166.97(9)
−72.97(14)
−52.41(15)
13.64(18)
C4–N1–C1–Se
−28.63(18)
C4–N1–C1–S
(34.22)
a
b
E = S or Se; two independent
molecules.
Cbutyl–N–C1–E
19.9(2), 22.0(2)
The complexes 5a, b could be obtained by adding
B(C6F5)3 to a solution of the selenones 4a, b in
toluene. After 30 min stirring at r. t., the addition of
pentane led to the precipitation of white solids at
−35 ◦C, which were identified as the compounds 5a, b.
The solids are highly reactive and decompose over a
few hours at r. t. under argon to a reddish oil. The com-
pounds 5a, b were characterised by elemental analysis,
NMR and in the case of 5b by X-ray diffraction analy-
sis.
The 11B NMR spectra revealed a broad singlet res-
onance for 5a at 9.6 ppm and for the saturated imida-
zole complex 5b at 24.2 ppm, similar to the complexes
3a,b and indicating a weak coordination of B(C6F5)3
at selenium (see Experimental Section). The 13C NMR
resonance of the former carbene carbon atom could
only be detected for the molecule 5b at 174.9 ppm,
which is high-field shifted in comparison to the se-
lenone 4b at 180.4 ppm. The remaining spectroscopic
data of the 1H NMR and 19F NMR spectra are in good
agreement with a cationic imidazole and an anionic bo-
rate moiety (Table 1).
The expected structure of the selenium compound
5b was also confirmed by X-ray diffraction anal-
ysis, as shown in Fig. 3. The selenium adduct of
the saturated carbene crystallised with one disordered
molecule of pentane and half a disordered molecule
Fig. 3. Displacement ellipsoid plot (30 %) of compound 5b.
Hydrogen atoms have been omitted for clarity.
is about 4◦ smaller. A possible reason for the smaller
angle is afforded by the VSEPR model, which states
that the bond angles in compounds containing diva-
lent chalcogen atoms decrease from sulfur to tellurium
[29]. Analogous to the reaction pathway to 5a, b, the
sulfur adducts 3a, b can be also formed through com-
plexation of B(C6F5)3 at the corresponding thiones 2a,
b (Scheme 1).
All three structures are characterised by a poten-
of toluene per asymmetric unit. The structural data tially high degree of conformational flexibility via the
for the tetrahedral selenoborate unit shows a C–Se torsion angles N–C–E–B, C–E–B–Cipso and E–B–
˚
distance of 1.9187 (14) A, which is slightly longer
Cipso–Cortho (E = S or Se). Selected values are given
˚
than 1.869 (6) A in dimethylimidazolidin-2-selenone in Table 2. For 3a and 5b, the N–C–E–B angles are
[31], but in the range of cationic dimethylimidazolidin- close to 90◦, whereas for 3b, N1–C1–S–B is 68◦.
˚
2-selenonehalogen species 1.894– 1.910 A [32]. The The three torsion angles C–E–B–Cipso per structure
Se–B distance of 2.2111 (15) A represents the first differ, as expected, by steps of ca. 120◦; a low abso-
˚
selenone-borane bond length to be determined by lute value means that the ring is bent back towards the
X-ray diffraction analysis. The C–Se–B bond angle of NHC, often causing steric pressure against the butyl
107.08 (6)◦ is close to a tetrahedral angle, but in com- groups. These display some E–C–N–Cbutyl torsion an-
parison to the corresponding sulfur compound 3b it gles that differ markedly from 0◦, with the central butyl
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D. Holschumacher et al. · Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs
375
◦
carbon being pushed back significantly out of the NHC added. At −35 C yellowish precipitates of the sulfur com-
plane (e. g. S–C1–N1–C4 34◦ for 3b). The ring with
plexes were formed. These were isolated by filtration, im-
mediately washed three times with 5 mL of pentane and
dried in vacuo. The products 3 were collected as yellowish
solids.
the largest absolute value of C–E–B–Cipso generally
has the smallest value of E–B–Cipso–Cortho, e. g. C40
in 3b. The variation in molecular conformation can be
recognised in Figs. 1a, 2, 3, in which the B(C6F5)3
group has been drawn with approximately the same
orientation (the ring at C40 is presented horizontally
and perpendicular to the plane of the paper, whereas
the other two rings point out of the paper) [33]. Fig.
1b, in contrast, shows a least-squares fit of the NHC
ring plus the Se and B atoms for the two independent
molecules of compound 3a; the difference in orienta-
tion of the borane moiety is obvious.
3a: Yield 645 mg (91 %). Crystals of 3a were obtained
from a mixture of toluene and pentane at −35 ◦C. Note:
slow decomposition takes place at r. t. – 1H NMR (400 MHz,
C6D6): δ = 1.15 (s, 18H, CH3), 6.11 (s, 2H, CH). – 11B
NMR (96 MHz, C6D6): δ = 10.3 (br. s). – 13C NMR
(150 MHz, C6D6): δ = 28.9 (CH3), 62.8 (C(CH3)3), 117.2
1
(N2(CH)2), 137.2 (dm, JCF = 249 Hz, m-C6F5), 148.4
(dm, JCF = 244 Hz, o-C6F5), 152.0 (CSB). – 19F NMR
1
(376 MHz, C6D6): δ = −129.3 (br s, o-C6F5), −154.7 (br
s, p-C6F5), −163.9 (br s, 6F, m-C6F5). – C29H20BF15N2S:
calcd. N 3.87, C 48.09, H 2.78; found N 3.75, C 48.97,
In conclusion, we have shown that carbene/
B(C6F5)3 FLP combinations are effective in the acti- H 3.30.
vation of sulfur and in the fixation of selenium. The
weak interactions between the chalkogen atoms and
FLP combinations in these systems might find appli-
cation in S/Se transfer reactions.
3b: Yield 291 mg (41 %). Crystals of 3b were obtained
from a mixture of toluene and pentane at −35 ◦C. Note: de-
composition takes place at r. t. and in chlorinated solvents.
The yield could be increased if the thione 2b was stirred for
30 min with B(C6F5)3 at r. t. – 1H NMR (400 MHz, C6D6):
δ = 1.33 (s, 18H, CH3), 2.66 (s, 4H, CH2). – 11B NMR
(96 MHz, C6D6): δ = 37.22 (br s). – 13C NMR (150 MHz,
C6D6): δ = 27.18 (CH3), 44.16 (CH2), 57.64 (C(CH3)3),
180.9 (CSB). – 19F NMR (376 MHz, C6D6): δ = −129.17
Experimental Section
All operations with air and moisture-sensitive compounds
were performed in a glovebox under a dry argon atmosphere
(MBraun 200B) or on a high-vacuum line using Schlenk
techniques. All solvents were purified by a solvent purifica-
tion system from MBraun and stored over molecular sieve
3
(br d, JFF = 19.7 Hz, 6F, o-C6F5), −149.18 (br s, 3F,
p-C6F5), −162.34 (br s, 6F, m-C6F5). – C29H22BF15N2S:
calcd. N 3.86, C 47.95, H 3.05; found N 3.77, C 48.54,
H 3.53.
(4 A) prior to use. The 1H, 13C, 11B, and 19F NMR spec-
˚
tra were recorded on Bruker DPX 200 (200 MHz), Bruker
DRX 400 (400 MHz), and Bruker DPX 600 (600 MHz) in-
struments. The chemical shifts are expressed in parts per mil-
lion (ppm) using tetramethylsilane (TMS) as internal stan-
dard (1H, 13C) or CFCl3 (19F), BF3·OEt2 (11B) and Me2Se
NMR spectra of the selenones 4a and 4b
4a: 1H NMR (300 MHz, C6D6): δ = 1.92 (s, 18H, CH3),
7.01 (s, 2H, CH). – 13C NMR (50 MHz C6D6): δ = 28.8
(CH3), 60.67 (C(CH3)3), 115.8 (CH), 150.0 (CSe). – 77Se
NMR (57 MHz, C6D6): δ = 188.8 (s).
(
77Se) as external standards, respectively. Coupling constants
(J) are reported in Hertz (Hz), and splitting patterns are in-
dicated as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), sept (septet) and br (broad). Elemental analy-
sis (C, H, N) was performed by combustion. Unless oth-
erwise indicated, all starting materials were obtained from
Aldrich and were used without further purification. 1,3-
Di-tert-butylimidazolin-2-ylidene (1a) [34, 35], 1,3-di-tert-
butylimidazolidin-2-ylidene (1b) [36, 37], the imidazole-2-
thiones 2a and 2b [17, 16], the imidazole-2-selenones 4a and
4b [20, 21], and B(C6F5)3 [38], were prepared according to
the literature procedures.
4b: 1H NMR (300 MHz,C6D6): δ = 1.63 (s, 18H, CH3),
2.69 (s, 4H, CH2). – 13C NMR (50 MHz C6D6): δ = 28.5
(CH3), 44.9 (CH2), 57.3 (C(CH3)3), 180.4 (CSe). – 77Se
NMR (57 MHz, C6D6): δ = 309.0 (s).
Synthesis of the selenium complexes 5a and 5b
To 10 mL of a toluene solution of the appropriate selenone
5 (0.59 mmol; 136.6 mg), was added B(C6F5)3 (0.59 mmol;
300 mg), and the mixture was stirred at r. t. for 30 min. To the
reaction mixture was added pentane at −35 ◦C, and a yellow-
ish precipitate of the selenium complexes was formed. The
precipitate was isolated by filtration, immediately washed
Synthesis of the sulfur complexes 3a and 3b
◦
To 20 mL of toluene were added at 0 C S8 (0.14 mmol; three times with 5 mL of pentane and dried in vacuo. The
37 mg), B(C6F5)3(0.98 mmol; 500 mg) and the appropri- products 5 were collected as colourless solids. Note: pro-
ate NHC (0.98 mmol). The mixture was stirred for 2 h longed stirring decreases yields; the compounds decompose
at r. t., the solution concentrated to 10 mL and pentane over a few hours at r. t. to red oils.
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D. Holschumacher et al. · Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs
5a: Yield 362 mg (89 %); an accurate elemental anal- an Oxford Diffraction Nova A diffractometer using mirror-
ysis could not be obtained due to the instability at r. t. – focussed CuKα radiation. Absorption corrections were ap-
1H NMR (300 MHz, C6D6): δ = 1.15 (s,18H, CH3), 6.11 plied using the multi-scan method. Structures were solved
(s, 2H, CH). – 13C NMR (75 MHz, C6D6): δ = 29.2 (CH3), with routine Direct Methods and refined on F2 using the pro-
63.2 (C(CH3)3), 118.4 (CH). – 19F NMR (376 MHz, C6D6): grams SHELXS/L-97 [39]. Hydrogen atoms were included
δ = −128.9 (br s, 6F, o-C6F5), −151.3 (br s, 3F, p-C6F5), using rigid methyl groups or a riding model.
−163.0 (br s, 6F, m-C6F5). – 11B NMR (96 MHz, C6D6):
δ = 9.6 (br).
Exceptions and special features: In compound 3a one tert-
butyl group (atoms C9ꢀ, C10ꢀ, C11ꢀ) is disordered over two
positions, while in the asymmetric unit of 3b one pentane
molecule disordered over two positions was found. Several
restraints (ISOR, DELU, SAME, SADI and SIMU) were
used in order to improve refinement stability of the disor-
dered moieties. One disordered pentane molecule and one
disordered half molecule of toluene were found in the asym-
metric unit of 5b, but could not be refined satisfactorily. For
this reason, the program SQUEEZE (part of the PLATON pro-
gram suite [40]) was used to remove mathematically the ef-
fects of the solvents.
5b: Yield 357 mg (87 %); an accurate elemental analysis
could not be obtained due to the instability at r. t. Crystals
of 5b w◦ere obtained from a mixture of toluene and pentane
at −35 C. – 1H NMR (300 MHz, C6D6): δ = 1.4 (s,18H,
CH3), 2.6 (s, 4H, N2(CH2)2). – 13C NMR (75 MHz, C6D6):
δ = 28.6 (CH3), 45.3 (CH2), 59.3 (C(CH3)3), 135.9 (dm,
m-C6F5), 149.9 (dm, o-C6F5), 174.9 (CSeB). – 19F NMR
(376 MHz, C6D6): δ = −130.6 (br. s, 6F, o-C6F5), −156.8
(br s, 3F, p-C6F5), −163.8 (br s, 6F, m-C6F5). – 11B NMR
(96 MHz, C6D6): δ = 24.2 (br).
CCDC 813555 (3a), 813556 (3b), 813557 (5b) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
X-Ray structure determinations
Crystal data and details of data collection and refine- Crystallographic Data Centre via www.ccdc.cam.ac.uk
ment are summarised in Table 1. Data were registered on /data request/cif.
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