Reactive N-protonated isocyanate species stabilized by bis(μ-hydroxo) divanadium(IV)-substituted polyoxometalate

Article abstract of DOI:10.1002/anie.201108205

O- or N-protonated? The bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type polyoxometalate (see picture, left) (TBA) ₄[γ-SiV^IV₂W₁₀O ₃₆(μ-OH)₄] (TBA=tetra(n-butyl)ammonium) was synthesiz

Full text of DOI:10.1002/anie.201108205

Angewandte
Chemie
DOI: 10.1002/anie.201108205
Carbocations
Reactive N-Protonated Isocyanate Carbocation Stabilized by
Bis(m-hydroxo)divanadium(IV)-Substituted Polyoxometalate**
Kazuhiro Uehara, Keisuke Fukaya, and Noritaka Mizuno*
[
8b]
Polyoxometalates (POMs) are anionic nanosized metal oxide
clusters that consist of early transition metals, and stimulate
many current research activities in broad fields of science,
such as catalysis, materials, and medicine, because their
chemical properties, such as acidity and redox potential, can
acylation.
With regard to the isocyanate cations, their
isolation and structural determination have never been
realized. Herein, we report the formation and isolation of
the putative “N-protonated phenyl isocyanate carbocation”
by the reaction of a bis(m-hydroxo)divanadium(IV)-substi-
[1]
IV
2
be finely tuned by constituent elements and countercations.
tuted g-Keggin-type POM, (TBA) [g-SiV W O (m-OH) ],
4
10 36
4
Especially, acidic forms of POMs are active catalysts in
various reactions, and some of them have been industrially
with phenyl isocyanate.
The divanadium(IV)-substituted g-Keggin-type POM was
synthesized by the reaction of VOSO with K [g-SiW O ] in
[
1h,i]
used.
Spectroscopic data indicate that POMs induce
4
8
10 36
unique activities by stabilizing reaction intermediates, such
water, in which the pH value was adjusted to 4.8 with 0.1m
acetate buffer. Addition of TBABr resulted in formation of
reddish brown powders. Recrystallization from a mixed
solvent of 1,2-dichloroethane and toluene gave dark brown
crystals in 28% yield. The IR spectrum of 1 showed n(OÀH)
[
1c,h,i,2]
as onium and carbenium ions.
However, to the best of
our knowledge, isolated unstable organic cationic intermedi-
ates stabilized by POMs that exhibit catalytic activity have
never been reported, and only one structurally determined O-
[3]
À1
alkylated POM has been reported.
vibration bands at 3628 and 3566 cm and n(V=O) vibration
À1
Carbocations play an important role as highly reactive
bands at 1007 and 962 cm (see Figure S9 in the Supporting
intermediates for electrophilic CÀC bond-forming reactions,
Information). Cold-spray ionization mass spectroscopy (CSI-
such as Friedel–Crafts alkylation and acylation, Mukaiyama
MS) showed the parent signal centered at m/z = 3824.22, in
[4,5]
+
aldol reaction, and Tsuji–Trost reaction.
Isolation of such
agreement with {(TBA) H [SiV W O ]} (centered at m/z =
5
4
2
10 40
2
9
51
reactive species is quite difficult because of the instability, and
their existence is usually confirmed by NMR and IR
3824.62; Figure S11). The Si and V NMR spectra in
CD CN did not exhibit any signals, suggesting the para-
3
[6]
1
IV
1
IV
spectroscopy. p Conjugation and hyperconjugation contrib-
ute to stabilization of the reactive species. Bulky conjugated
bases of superacids, such as trifluoromethane sulfonate
magnetic d (V )-d (V ) high spin state in 1 (Figure S10).
The magnetic susceptibility of 1 was 3.011(4) m at 300 K, thus
B
supporting the idea (Figure S24).
À
À
(
CF SO ), tetrafluoroborate (BF ), and carborane, can
The molecular structure of 1 was successfully determined
by single-crystal X-ray crystallography (see Figure 1a, and
Tables S1 and S2 in the Supporting Information). Compound
3
3
4
also stabilize carbocations with electrostatic and hydrogen-
bonding interaction between the anionic and cationic moi-
eties, thus enabling the determination of their structures by X-
IV
1 was identified as (TBA) [g-SiV W O (m-OH) ], which
4
2
10 36
4
[4c,f,7]
IV
ray crystallography.
contains a V (m-OH) diamond core, in which the g-Keggin
2 2
Isocyanates are reactive heterocumulenes that show
a variety of reactivities for dehydration, deoxygenation,
framework was maintained by introduction of vanadium(IV)
moieties. Four TBA cations per anion were observed. The
bond-valence sum (BVS) values for O(113), O(114), O(126),
and O(128) were 1.22, 1.21, 1.25, and 1.25, respectively,
suggesting that these oxygen atoms are monoprotonated and
two protons are generated on WÀOÀW bridging oxygen
[
8]
insertion, oligomerization, and polymerization. Especially
the N-protonated isocyanate cation is energetically favorable
[
8j]
by theoretical calculation, whereas the O-protonated one
has been proposed as an intermediate for Friedel–Crafts
IV
atoms (O(126) and O(128)) on introduction of two V atoms
[
9]
(
Table S3). The BVS values for the other bridging and
[
*] Dr. K. Uehara, Dr. K. Fukaya, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
terminal oxygen atoms were 1.70–2.06 and 1.59–2.00, respec-
tively, suggesting that the valences are À2. The BVS values for
Si (3.93), V (4.18 and 3.96), and W (5.64–6.42) suggest that the
respective valences are + 4, + 4, and + 6. All the data show
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
IV
[
**] We thank Prof. J. Shimoyama (The University of Tokyo) for magnetic
susceptibility measurements and his valuable discussion. This work
was supported by the global COE program (GCOE) “Chemistry
Innovation through Cooperation of Science and Engineering”, the
Ministry of Education, Culture, Science, Sports, and Technology of
Japan (MEXT) (Japan), and the Funding Program for World-Leading
Innovative R&D on Science and Technology (FIRST Program)“ of
the Japan Society for the Promotion of Science (JSPS).
formation of (TBA) [g-SiV W O (m-OH) ].
4
2
10 36
4
The V···V (3.101(5) ꢀ), V=O (1.605(17) and 1.64(2) ꢀ),
and VÀO(ÀV) (1.95(2)–2.01(2) ꢀ) distances in 1 were similar
to those in the vanadium(V) derivative, (TBA) [g-
4
V
SiV W O (m-OH) ] 1’ (3.096(3), 1.631(14), and 1.577(14),
2
10 38
2
and 1.957(13)–1.980(12) ꢀ). On the other hand, the V···O(À
Si) distances in 1 were 2.90(1) and 2.91(1) ꢀ and much longer
[10]
than those in 1’ (2.511(12) and 2.531(11) ꢀ; Table S2).
Therefore, the interaction between the diamond core and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208205.
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.
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Communications
at m/z = 4062.27, in agreement with {(TBA) H [SiV W O -
5
4
2
10 40
+
(
C H NCO) ]} (centered at m/z = 4062.70), thus supporting
6 5 2
the idea (Figure S16). All these NMR and CSI-MS results
support the existence of the phenylisocyanate moiety.
2
9
On the other hand, no signals were observed in Si and
5
1
V NMR spectra because of the paramagnetic nature of 2
[
16]
(
Figure S15).
The magnetic susceptibility of
2 was
2
.527(4) m at 300 K. These results indicate the retention of
B
valence of vanadium centers as + 4 (Figure S25).
The molecular structure of 2 was successfully determined
by single-crystal X-ray crystallography (see Figure 1b, and
Figure S2 and Tables S1 and S2 in the Supporting Informa-
tion). Four TBA cations per anion were observed. The BVS
values for O(113) (1.30) and O(114) (1.25) indicate that these
oxygen atoms are monoprotonated. No proton was observed
on O(126) and O(128), respectively, thus suggesting the
reaction of two protons on O(126) and O(128) in 1 with
phenyl isocyanate (Table S4). The BVS values of the other
bridging and terminal oxygen atoms were 1.73–2.09 and 1.46–
1
.86, respectively, suggesting that the valences of these oxygen
atoms are À2. Therefore, the valences of O(126) and O(128)
in 2 were decreased to À2 by the deprotonation of 1.
The
W(104)ÀO(126)
(2.010(13) ꢀ),
W(110)ÀO(126)
(
1.873(13) ꢀ), W(106)ÀO(128) (1.996(12) ꢀ), and W(112)À
IV
O(128) (1.882(12) ꢀ) distances were shortened, thus support-
Figure 1. Thermal ellipsoid plots of a) (TBA) [g-SiV W O (m-OH) ] 1,
4
2
10 36
4
IV
ing the idea. The N(11)ÀO(128) and N(21)ÀO(126) distances
and b) (TBA) [g-SiV W O (m-OH) (PhNHCO) ] 2, drawn at 50%
4
2
10 38
2
2
probability level (TBA cations were omitted for clarity).
in 2 were 2.885 and 2.839 ꢀ, respectively, suggesting the
hydrogen-bonding interaction between the carbocation and
POM, which would stabilize the carbocations.
8
À
the [g-SiW O ] fragment in 1 is weaker than that in 1’,
We next focused on the organic moieties depicted in
1
0
36
probably because of the Jahn–Teller effect, which is charac-
Figure 2, and Figure S3 in the Supporting Information. Phenyl
1
[11]
teristic of d transition-metal complexes.
The W(104)À isocyanate is a planar molecule as a result of p conjugation of
O(126) and W(106)ÀO(128) distances were 2.163(13) and
the C(Ph)ÀN=C moiety. When protonation takes place at the
O atom of the phenyl isocyanate, the p conjugation is
retained and those atoms can keep linear conformation. On
the other hand, when protonation takes place at the N atom,
2
.185(13) ꢀ, respectively, and thus longer than the others,
because of monoprotonation at O(126) and O(128).
Next, the reactivity of 1 with phenyl isocyanate was
investigated. Compound 1 was dissolved in a mixed solvent of
acetone and 1,2-dichloroethane containing an excess amount
of phenyl isocyanate. Keeping the reaction solution for one
day gave dark brown crystals 2 in 71% yield. The IR spectrum
+
the p conjugation collapses and the C(Ph)ÀNHÀC =O
3
moiety becomes bent because of sp hybridization. Those
À1
of 2 showed strong bands at 3335, 1736, 1597, and 1537 cm ,
[
12]
which are assignable to n(NÀH),
n(C=O), n(C=N), and
n(C=C) vibration modes, respectively, suggesting the exis-
tence of a phenyl isocyanate moiety (Figure S12). The
1
H NMR spectrum of 2 was measured (Figure S13). Just
after dissolution of 2 in CD CN, four broad signals of the
3
phenyl isocyanate carbocation were observed at 8.77, 7.70,
7
1
.40, and 7.03 ppm with the respective integration ratio of
[14]
:2:2:1.
Generally, the protons of alcohols have been
observed in the range of 4–5 ppm, whereas those of imines
and amides have been observed in the range of 8–9.5 ppm. For
example, the NH protons of N,N’-diphenylurea were
[
10]
observed at 8.66 ppm in [D] DMSO.
Therefore, the
6
1
H NMR signal at 8.77 ppm is assignable to the NH moiety
and those at 7.70, 7.40, and 7.03 ppm are assignable to the o-,
Figure 2. Representative molecular orbitals illustrating interaction
between the N-protonated phenyl isocyanate and POM in 2 (isosurface
value 0.01), a) a(212) orbital, b) a(212) orbital with emphasis on
cationic moiety, c) a(210) orbital, and d) a(210) orbital with emphasis
1
3
m-, and p-C H moieties, respectively. In addition, the C{H}
6
5
[
15]
NMR signals of CO (148.86 ppm) and Ph (130.27, 130.15,
26.76, and 119.38 ppm) were observed (Figure S14). The
CSI-MS spectrum of 2 also showed the parent signal centered
1
[
17]
on cationic moiety.
2
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Angewandte
Chemie
phenomena were confirmed by DFT calculation (Table S7
[
18]
and Figure S8). The C(11)ÀN(11) distance (1.41(2) ꢀ) and
C(11)-N(11)-C(17) angle (125.9(18)8) of the carbocation in 2
show the single bond and bent characters, respectively. The IR
1
band of NÀH stretching vibration and H NMR signal of NH
of the N-protonated phenylisocyanate carbocation were
Scheme 1. Cyclotrimerization of phenyl isocyanate catalyzed by 2
(yields in parentheses are those of isolated products). N,N’-diphenyl-
À1
observed at 3335 cm and 8.77 ppm, respectively. The DFT
urea would be formed by hydration and decomposition of phenyl
calculation showed that the N-protonated cation is by
[
8c]
À1
isocyanate.
3
1.13 kJmol more stable than the O-protonated one (Fig-
ure S8). All these results show that protonation takes place at
the N atom of the phenyl isocyanate. Two protons at O(126)
and O(128) in 1 probably transfer to the N atoms of phenyl
isocyanates in close proximity to O(126) and O(128) in 2.
The C(17)ÀO(11) and C(17)ÀN(11) distances were 1.20(3)
synthesized and fully characterized by IR and CSI-MS
spectroscopy and X-ray crystallography. Two protons at WÀ
OÀW bridging oxygen reacted with two phenyl isocyanate
IV
and 1.33(3) ꢀ, respectively, exhibiting double bond charac-
ters, whereas the C(17)ÀO(124) distance was 1.44(3) ꢀ and
thus much longer than the C(17)ÀO(11) distance. DFT
molecules
to
form
(TBA) [g-SiV W O (m-OH) -
4 2 10 38 2
(PhNHCO) ] 2 with the theoretically proposed N-protonated
2
phenyl isocyanate carbocations. The cations weakly inter-
acted with the bridging oxygen atoms of the POM through the
polar p interaction mainly contributing from the POM with
small contribution of the s bond as well as the hydrogen-
bonding interaction. Compound 2 catalyzed the cyclotrime-
rization of phenyl isocyanate to produce 1,3,5-triphenyliso-
cyanurate 3 and 2-(phenylimino)-1,3,5-triphenylhexahydro-s-
triazine-4,6-dione 4.
calculation of the anionic part of 2 also supports the
hypothesis of the interaction between the N-protonated
phenyl isocyanate cation and POM. The representative
orbitals a(212) and a(210) prove that the C(17)ÀO(124)
bond mainly consists of the polar p interaction (electrostatic
attraction; coefficients of p orbitals (0.035 for C(17) and
y
0
.113 for O(124)) with slight contribution of the s bond
(
coefficients of p orbitals (0.047 for C(17) and À0.046 for
x
[
19]
O(124)).
(
The Mulliken charge of the carbonyl moiety
Received: November 22, 2011
Revised: April 17, 2012
Published online: && &&, &&&&
0.809) in the N-protonated phenyl isocyanate cation was
almost the same as that without the POM (0.827), thus
supporting the idea that the interaction between O(124) and
C(17) is weak (Table S6). N(11)-C(17)-O(124), O(11)-C(17)-
O(124) and O(11)-C(17)-N(11) angles were 111.5(18), 117(2),
and 132(2)8, respectively, and the sum of these angles was
approximately 3608. The W(103)ÀO(124) and W(106)ÀO-
Keywords: carbocations · cyclotrimerization · isocyanates ·
.
polyoxometalates · protonation
(
124) distances were 2.101(13) and 2.118(13) ꢀ, respectively,
and longer than those of the other WÀO(ÀW) bonds
1.855(15)–2.010(13) ꢀ) These facts show that N(11), C(11),
[
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(
O(11), and C(17) atoms are placed in the same plane by the
sp hybridization. In addition, N(11)···O(128) and N(21)···O-
(
ing the presence of the hydrogen-bonding interaction, which
would stabilize the carbocations. All the results lead to the
conclusion that compound 2 is assignable to (TBA) [g-
SiV W O (m-OH) (PhNHCO) ], which is the first example
of isolation and stabilization of the N-protonated phenyl
isocyanate carbocation by using the bulky POM.
The reaction of 1 with phenyl isocyanate at room temper-
ature for about three months produced 1,3,5-triphenylisocya-
nurate 3 and 2. In addition, compound 1 catalyzed cyclo-
trimerization of phenyl isocyanate to form 3 in 39% yield at
2
126) distances were 2.885 and 2.839 ꢀ, respectively, suggest-
4
IV
2
10 38
2
2
[20]
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33 K. Compound 2 catalyzed the same reaction at the lower
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[
21]
triphenylhexahydro-s-triazine-4,6-dione 4 in 49% and 25%
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IV
2
substituted POM, (TBA) [g-SiV W O (m-OH) ] 1 was
4
10 36
4
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3
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the strong asymmetric and very weak symmetric NÀH vibration
À1
bands appear at 3410 and 3412 cm , respectively (without any
À1
corrections). Therefore, the band observed at 3335 cm in 2 is
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2
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IV
broadening as a result of the paramagnetic effect by V in the
1
13
POM framework. The H and C{H} NMR spectra of TBA and
phenyl isocyanate carbocations in 2 also showed peak broad-
ening. Since TBA in 1 and TBA and phenyl isocyanate
carbocation in 2 are not included in the paramagnetic POM
framework, the effect is weak and their signals are observed. A
similar peak broadening was observed for TBA in TBA H [g-
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2
II
SiW O Cu (m-1,1-N ) ] (3.15, 1.64, 1.43, 0.99 ppm in CD CN),
1
0
36
2
3
2
3
thus supporting the idea.
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À1
[
12] A new IR band appeared at 3335 cm upon transformation
4
12 40
v
from 1 to 2. Generally, the NÀH stretching vibration bands have
polyoxometalate ((TBA) [g-SiV W O (m-OH) ]) 1’, thus dem-
4
2
10 38
2
À1 [13]
been observed in the range of 3200–3500 cm
.
In addition, the
onstrating the uniqueness of 1.
frequency calculation of the anionic moiety of 2 also shows that
4
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Carbocations
O- or N-protonated? The bis(m-hydroxo)-
divanadium(IV)-substituted g-Keggin-
K. Uehara, K. Fukaya,
N. Mizuno*
type polyoxometalate (see picture, left)
IV
&&&& — &&&&
(TBA) [g-SiV W O (m-OH) ] (TBA=
4
2
10 36
4
tetra(n-butyl)ammonium) was synthe-
sized and characterized by X-ray crystal-
lography. Its reaction with phenyl isocya-
Reactive N-Protonated Isocyanate
Carbocation Stabilized by Bis(m-
hydroxo)divanadium(IV)-Substituted
Polyoxometalate
IV
nate gave (TBA) [g-SiV W O (m-OH) -
4
2
10 38
2
(PhNHCO) ], which contains two N-pro-
2
tonated phenyl isocyanate carbocations
and catalyzes the cyclotrimerization of
phenyl isocyanate.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!