Oxo/Imido heterometathesis reactions catalyzed by a silica-supported tantalum imido complex

Article abstract of DOI:10.1021/om4001499

Grafting Ta(=N^tBu)(CH₂CMe₂Ph)₃ onto the surface of silica partially dehydroxylated at 300°C leads to the formation of the surface imido complex (≡SiO)₂Ta(=N ^tBu)(CH₂CMe₂Ph) as a major species, which was characterized with EXAFS, ¹³C CP/MAS NMR, diffuse reflectance FTIR, elemental analyses, and chemical reactivity. The obtained material acts as an efficient heterogeneous catalyst for various oxo/imido heterometathesis transformations: imidation of ketones and DMF with N-sulfinylamines and condensation of N-sulfinylamines into sulfurdiimines and phenyl isocyanate into diphenylcarbodiimide.

Full text of DOI:10.1021/om4001499

Article
pubs.acs.org/Organometallics
Oxo/Imido Heterometathesis Reactions Catalyzed by a Silica-
Supported Tantalum Imido Complex
Pavel A. Zhizhko,† Anton A. Zhizhin,† Olga A. Belyakova,† Yan V. Zubavichus,†,‡ Yuriy G. Kolyagin,§
Dmitry N. Zarubin,*,† and Nikolai A. Ustynyuk†
†A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia
‡National Research Center “Kurchatov Institute”, Acad. Kurchatov sq. 1, 123182 Moscow, Russia
§Chemistry Department, Lomonosov Moscow State University, Leninskie Gory, 1/3, 119991 Moscow, Russia
S
* Supporting Information
ABSTRACT: Grafting Ta(NtBu)(CH2CMe2Ph)3 onto the
surface of silica partially dehydroxylated at 300 °C leads to the
formation of the surface imido complex (SiO)2Ta(
NtBu)(CH2CMe2Ph) as a major species, which was charac-
terized with EXAFS, 13C CP/MAS NMR, diffuse reflectance
FTIR, elemental analyses, and chemical reactivity. The obtained
material acts as an efficient heterogeneous catalyst for various
oxo/imido heterometathesis transformations: imidation of
ketones and DMF with N-sulfinylamines and condensation of
N-sulfinylamines into sulfurdiimines and phenyl isocyanate into diphenylcarbodiimide.
Scheme 1. Oxo/Imido Heterometathesis between N-
Sulfinylamines and Oxo Compounds
INTRODUCTION
■
Catalytic heterometathesis (i.e., carbon−heteroatom and heter-
oatom−heteroatom multiple-bond metathesis) is quite a novel
class of transition-metal-mediated reactions. Despite a plethora
of stoichiometric metathetical reactions of various multiply
bonded transition-metal−ligand units with heteroolefins and
heterocumulenes being described,1−3 the examples of catalytic
heterometathesis of the latter are still scarce. The only reports
to date are metathesis of imines,4,5 carbodiimides,6 diphos-
phenes,7 and nitriles,8 condensation of isocyanates and N-
sulfinylamines into carbodiimides9 and sulfurdiimines,10
respectively, and metathetical imido-deoxygenation of alde-
hydes with isocyanates5 and iminophosphoranes.11
Recently we have successfully combined two well-docu-
mented stoichiometric heterometathesis reactionsimidation
of transition-metal oxo complexes with N-sulfinylamines12 and
imido group transfer from imido complexes to oxo com-
pounds1−3,5,13−16into a catalytic cycle (Scheme 1). The
overall transformation, catalyzed by vanadium and molybde-
num oxo and imido complexes (MOCl3, M = V, Mo; Mo(
NAr)2Z2, Z = OR, Alk, etc.), results in oxo/imido heterometa-
thesis between N-sulfinylamines and aldehydes (X = CHR) to
afford aldimines.17
The low activity of the molybdenum and vanadium catalysts
and quite limited scope of reactive oxo substrates stimulated us
to seek more efficient catalysts based on other transition metals.
It is well-known that the reactivity of transition-metal imido
complexes toward the CO unit of carbonyl compounds (X =
CR1R2), isocyanates (X = CNR), carbon dioxide (X = CO), etc.
increases to the left and to the down across the d block of the
periodic table, which could be explained in terms of
nucleophilicity of the imido group18 or oxophilicity of the
metal atom. Thus, among the most reactive are the group 4 and
heavy group 5 metal imides (Ti,3,15 Zr,16 Ta1,14). However, the
pronounced tendency of their terminal oxo derivatives to form
catalytically inactive oligomers with bridging oxo ligands
prevents their application as catalysts. The only example has
been reported recently9c and utilized a TiO species stabilized
with a bulky phthalocyanine ligand.
Meanwhile, the stabilization of terminal MO fragments
could be simply achieved by the fixation of the catalytically
active species on the surface of the support. Indeed, the
possibility of in situ generation of isolated TaO species on
silica and even its stoichiometric heterometathetical imidation
Received: February 22, 2013
© XXXX American Chemical Society
A
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with tert-butyl isocyanate has been already demonstrated by
Basset.19 Thus, we suppose that application of modern
techniques of the rational design of well-defined heterogeneous
catalysts, such as Surface Organometallic Chemistry,20 to the
reactive imido complexes of Ti, Zr, Hf, and Ta provides an
opportunity to develop a new generation of highly efficient
heterogeneous oxo/imido heterometathesis catalysts. Herein
we report the preparation, characterization, and catalytic
performance of a silica-supported tantalum imido complex
the first tantalum-based oxo/imido heterometathesis catalyst.
should be also noted that no olefin was observed in this
reaction, which excludes the formation of a surface alkylidene
complex.
In order to retrieve information on the number of atoms and
interatomic distances within the first and second coordination
spheres of the Ta atoms in Ta/SiO2-300, extended X-ray
absorption fine structure (EXAFS) spectroscopy was applied.
The Fourier transform (FT) of the experimental EXAFS
spectrum together with the best-fit theoretical curve are shown
in Figure 1, whereas the best-fit parameters corresponding to
the structural model are summarized in Table 2. The EXAFS
spectrum of the sample is fully compatible with the bis-grafted
structure A (Scheme 2) described above. The short TaN
bond (at ca. 1.81 Å, which is close to values observed in other
similar surface complexes22a,b) is clearly attributable in the
experimental spectrum, which strongly supports the notion that
the imido ligand survived grafting. Three neighbors at ca. 1.95
Å are assigned to oxygen and carbon atoms of the siloxy and
alkyl ligands, respectively (not resolved), the observed distance
lying between the literature values of Ta−O (∼1.9 Å)22 and
Ta−C (∼2.1 Å)22c bond lengths in similar surface complexes.
Finally, long nonbonded Ta···Si contacts are observed at ca.
3.26 Å.
The structure of the surface complex was also investigated
with solid-state 13C CP/MAS NMR spectroscopy (Figure 2).
The assignments of the peaks observed in the 13C NMR
spectrum of Ta(NtBu)(CH2CMe2Ph)3 (C6D6) and the solid-
state spectrum of Ta/SiO2-300 are juxtaposed in Table 3. All of
the resonances in the solution spectrum of the initial complex
are clearly attributable. The solid-state spectrum of Ta/SiO2-
300 shows two series of peaks in the region of CH3 groups
(28−36 ppm) and aromatic CH atoms (124−132 ppm), three
distinct resonances of quaternary carbons with chemical shifts
practically identical with those of the molecular complex at 41.8
(CMe2Ph), 68.1 (NCMe3), and 153.5 ppm (Phipso), and an
additional peak at 53.4 ppm.
It is worth noting that no CH2 resonance was observed in the
spectrum of the solid sample. This observation seems to
suggest that the Ta atom in the surface complex is no longer
bearing the alkyl ligands, all three of them being eliminated as
PhtBu and not washed out completely. However, a series of
observations and additional experiments strongly supports the
presence of the organometallic fragment on the surface.23 First,
almost constant Ta/N/C ratios and reproducible elemental
analyses, consistent with elimination of nearly two alkyl groups,
for the independently prepared samples could hardly result
from a coincidence. Second, additional washing of the samples
with a large excess of heptane, Et2O, and CH2Cl2 did not affect
the elemental analyses, thus confirming that the remaining
“PhtBu” is somehow bound in the solid. Meanwhile, it is
quantitatively washed out after treatment of the sample with
RESULTS AND DISCUSSION
■
Catalyst Preparation and Characterization. Grafting
Ta(NtBu)(CH2CMe2Ph)3 (5 mol % to SiO2) onto the
surface of silica partially dehydroxylated at 300 °C (SiO2-300)
was accomplished via addition of a heptane solution of the
complex to a suspension of silica and stirring at ambient
temperature overnight. Subsequent filtering, washing with
heptane, and drying in vacuo gave a pale yellow solid (Ta/
SiO2-300). The elemental analysis of several independently
prepared samples of Ta/SiO2-300 (Table 1) showed that the
Table 1. Elemental Analysis Data for the Independently
Prepared Samples of Ta/SiO2-300
a
ac
,
sample
Ta (wt %)
mmol of Ta/g
Ta/N/C
1/1/16
a
b
1
2
3
10.5
9.2
0.58 (0.61 )
0.51 (0.64)
0.56 (0.60)
1/0.8/17
1/0.9/17
10.2
a
b
According to elemental analysis. Maximum value expected at total
c
chemisorption of the complex. Atomic ratio.
composition of the obtained material was reproducible, the
tantalum content (∼10 wt %) was only slightly lower than that
expected at total chemisorption of the complex, and the Ta/N/
C ratio was close to 1/1/14, corresponding to the bis-grafted
surface complex with an imido group and one alkyl ligand
remaining. Elimination of tert-butylbenzene during grafting was
confirmed using NMR spectroscopy. When grafting was carried
out in the presence of the internal standard (C6Me6), the
amount of tert-butylbenzene in the filtrate was estimated from
1H NMR as ∼1.6 equiv per Ta atom. The mass balance during
grafting was also consistent with the elimination of 1.5−1.7
PhtBu/Ta. Thus, these data suggest the preliminary formulation
of the surface complex as (SiO)2Ta(NtBu)(CH2CMe2Ph)
(Scheme 2).21
The presence of the [NtBu] group was confirmed by the
stoichiometric heterometathesis reaction of Ta/SiO2-300 with 5
equiv of PhCHO that gave 57% (to Ta) of spectroscopically
identified PhCHNtBu after 3 h of reflux in heptane and
subsequent washing of the solid with Et2O (Scheme 3a). It
Scheme 2. Proposed Scheme of Grafting Ta(NtBu)(CH2CMe2Ph)3 onto SiO2-300
B
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Scheme 3. Stoichiometric Reactions of (SiO)2Ta(NtBu)(CH2CMe2Ph)
Table 3. 13C Chemical Shifts of Ta(NtBu)(CH2CMe2Ph)3
and Ta/SiO2-300
δ (ppm)
Ta(NtBu)(CH2CMe2Ph)3
Ta/SiO2-300
assignment
33.8
34.4
41.4
NCMe3
CMe2Ph
CMe2Ph
NHCMe3
NCMe3
CH2
30.6−33.6
41.8
53.4
68.4
106.9
125.7
125.9
128.7
153.3
68.1
not observed
Phmeta
126.4, 129.3
153.5
Phpara
Phortho
Phipso
ppm, JCD = 19.3 Hz) NMR. Finally, the 13C resonances that
appear at δ 41.4 and 41.8 ppm in the spectra of the complex
and Ta/SiO2-300, respectively, are characteristic of the
quaternary carbon in the TaCH2CMe2Ph moiety, whereas the
corresponding atom of PhtBu has a chemical shift of 34.6 ppm
(CDCl3).
Thus, consistent with EXAFS, elemental analysis, and the
literature, 13C NMR confirms the presence of the surface
complex (SiO)2Ta(NtBu)(CH2CMe2Ph). Meanwhile, the
observation of the 13C resonance at 53.4 ppm and a series of
multiple poorly resolved peaks in the methyl group region
indicates the possible coexistence of other surface complexes.
The signal at 53.4 ppm could arise from the quaternary carbon
of a tert-butylamido ligand.24 Indeed, application of diffuse
reflectance FTIR (DRIFT) spectroscopy allowed us to confirm
the presence of the surface amido species. In addition to the
intense CH vibrations in the aliphatic (3000−2800 cm−1) and
aromatic regions (3088, 3062, and 3028 cm−1) and broad Si−
OH bands (3800−3300 cm−1), the DRIFT spectrum of Ta/
SiO2-300 (Figure 3) clearly demonstrates distinct bands in the
NH vibration region (at 3306 and 3250 cm−1).
1
Figure 1. FT of the experimental Ta LIII-edge EXAFS spectrum of Ta/
SiO2-300 (solid line) compared to the best-fit model curve (open
circles).
Table 2. Parameters of the Local Environment of Ta Atoms
a
in Ta/SiO2-300 Refined from EXAFS Data
b
sphere
CN distance (Å) Debye−Waller factor (Å2)
NtBu
1
1.81
1.95
3.26
0.0021
0.0021
0.0074
c
−OSi, −CH2CMe2Ph
−OSi
3
2
a
Fitting ranges: k = 2.5−14.0 Å−1, R = 0.8−3.2 Å; ΔE = 9 eV common
to all spheres, Rf = 0.025. Coordination numbers were fixed at
anticipated values and not refined. Not resolved.
b
c
Thereby we can conclude that at least two different passways
take place during grafting: the initially proposed substitution of
two alkyl ligands by silanol groups leading to the surface imido
complex A (Scheme 2) and the substitution of one alkyl ligand
along with the direct addition of the SiO−H bond across the
MN linkage that results in the formation of the amido
species B (Scheme 4). Such transformations of multiply bonded
ligands during grafting (and the imido ligand particularly) have
been reported previously.25
Figure 2. Solid-state 13C CP/MAS NMR spectrum of Ta/SiO2-300.
The asterisks mark the spinning sidebands.
The amount of form B could be tentatively estimated as 10−
40 mol % on the basis of (a) the amount of PhtBu eliminated
during grafting (∼1.6 equiv/Ta) and after alcoholysis of Ta/
SiO2-300 (∼1.1 equiv/Ta), (b) the amount of imine formed in
the reaction with PhCHO (∼0.6 equiv/Ta), and (c) the Ta/C
ratio found by elemental analysis (the experimental value 1/17
1
methanol (∼1.1 equiv of PhtBu per Ta according to H NMR
with internal standard). Furthermore, tert-butylbenzene turns
out to be monodeuterated after treating the sample with
1
methanol-d4 (Scheme 3b), which is clearly evidenced by H
(triplet at 1.30 ppm, JHD ≈ 2 Hz) and 13C (triplet at ∼31.0
2
C
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Table 4. Catalytic Performance of Various Homogeneous
and Heterogeneous Oxo/Imido Heterometathesis Catalysts
conversion
a
catalyst
time
(%)
1
2
3
4
5
6
Ta(NtBu)(CH2CMe2Ph)3/SiO2-300
20 min ≥95
b
Mo(NMes)2(CH2CMe2Ph)2/SiO2-300
1 h
5 h
5 h
5 h
≥95
90
c
2% VOCl3/SiO2-500
Ta(NtBu)(CH2CMe2Ph)3
82
Mo(NMes)2(CH2CMe2Ph)2
37
MOCl3 (M = V, Mo)
no reaction
a
b
Spectroscopically estimated. Obtained via grafting Mo(
NMes)2(CH2CMe2Ph)2 onto SiO2-300.27 Obtained via grafting 2
mol % of VOCl3 onto SiO2-500.26
c
Figure 3. DRIFT spectra of (a) SiO2-300 and (b) Ta/SiO2-300.
Table 5. Imidation of Ketones with N-Sulfinylamines
lies between 1/14 and 1/24 expected for forms A and B,
respectively, and corresponds to 70% A and 30% B).
Unfortunately, EXAFS spectroscopy also used to identify the
nature of the Ta active site cannot provide an accurate estimate
of the fraction of the B form. The abundance of form B would
decrease the effective coordination number for the shortest
imido TaN bonds and slightly increase the coordination
number for the Ta−O/Ta−C bonds to an extent that is beyond
the nominal accuracy given by EXAFS.
It should be pointed out that according to our initial
hypothesis (the catalytic cycle presented in Scheme 1) only
form A is capable of participating in heterometathetical
transformations, and the reaction with PhCHO seems to
confirm this supposition. Thus, we believe that all the catalytic
results which are presented below reflect the chemical behavior
of the surface imido complex A, whereas form B remains
inactive.
Imidation of Ketones with N-Sulfinylamines. The
obtained material was tested in the oxo/imido heterometathesis
reaction between N-sulfinyl-p-toluidine and acetophenone and
compared to its molecular precursor and other previously
developed homogeneous10,17 and heterogeneous26,27 catalysts
(Table 4). As was earlier observed for vanadium26 and
molybdenum27 systems, the grafted tantalum complex dis-
played activity significantly higher than that of its molecular
analogue. Moreover, Ta/SiO2-300 appears to be the most
active to date among all tested homogeneous and heteroge-
neous catalysts, in full agreement with the higher imidating
ability of tantalum imido complexes over vanadium and
molybdenum complexes.
a
Isolated yield.
To further investigate the catalytic performance of Ta/SiO2-
300 in the imidation of ketones, we studied the interaction of a
range of N-sulfinylamines and ketones in the presence of the
heterogeneous catalyst (Table 5). All reactions were carried out
in refluxing heptane (98 °C) and followed with IR spectros-
copy. The method can be regarded as general for the derivatives
of acetophenone, benzophenone, fluorenone, and acetylferro-
cene. It is particularly attractive in the case of electron-deficient
anilines (2, 5, and 9) that are poorly reactive in the classical
Scheme 4. Formation of the Surface Amido Complex
D
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Table 6. Imidation of DMF with N-Sulfinylamines
a
conversion of RNSO (%)
entry no.
R
time (h)
into formamidine
into sulfurdiimine
1
2
3
2-FC6H4
3
5
5
100
45
4-MeC6H4
2-MeOC6H4
10
36
32
a
Spectroscopically estimated.
acid-catalyzed condensation of anilines and ketones. It is worth
noting that even such sterically hindered substrates as N-
sulfinyl-2,4,6-trimethylaniline and N-sulfinyl-2,4,6-tribromoani-
line react smoothly (6 and 9). Only in the case of N-sulfinyl-
2,6-diisopropylaniline does the reaction not give quantitative
conversion (7). Prolonged heating was also required in the case
of substrates with pronounced coordinating ability: e.g.,
containing a pyridine core (12).
Scheme 5. Condensation of PhNCO into PhNCNPh
24 h of reflux in heptane), it is worth noting that the conditions
are significantly milder than those reported.
Imidation of DMF and Condensation of N-Sulfinyl-
amines into Sulfurdiimines. Dimethylformamide can be
used as a carbonyl source instead of a ketone, the
corresponding formamidines being formed. Three N-sulfinyl-
amines were tested as imidating agents (Table 6). The observed
reactivity of N-sulfinylamines correlates with their electron
deficiency, N-sulfinyl-2-fluoroaniline being remarkably more
active than methyl- and methoxy-substituted species.
It was noticed that DMF imidation was accompanied by the
formation of the corresponding sulfurdiimines as byproducts
(Table 6). Analogously, this was reported earlier for the same
reaction catalyzed by Mo(NMes)2(CH2CMe2Ph)2.10 This
observation reveals the occurrence of an alternative oxo/imido
heterometathesis process, where N-sulfinylamine acts as both
imidating and oxo component (Scheme 1, X = SNR). In the
absence of a ketone, DMF, or any other oxo compound the
reaction can be used as a preparative method for sulfurdiimine
synthesis. Table 7 demonstrates the performance of Ta/SiO2-
300 in condensation of the same series of N-sulfinylamines. o-
Fluoro-substituted sulfinylaniline was again found to be the
most reactive.
CONCLUSION
■
In summary, we have shown that surface organometallic
chemistry opens new possibilities for the design of oxo/imido
heterometathesis catalysts due to the stabilization of in situ
generated highly reactive terminal oxo complexes on the surface
of the support. The simply prepared tantalum surface imido
complex (SiO)2Ta(NtBu)(CH2CMe2Ph) acts as an
efficient heterogeneous catalyst for the imidation of ketones
and dimethylformamide with N-sulfinylamines and catalyzes
condensations of N-sulfinylamines into sulfurdiimines and
phenyl isocyanate into diphenylcarbodiimide.
EXPERIMENTAL SECTION
■
General Considerations. If not otherwise stated, all manipu-
lations were carried out under an argon atmosphere using standard
Schlenk techniques. NMR spectra were recorded using Bruker AMX
400 and Avance 300 spectrometers. Chemical shifts are measured
1
relative to residual H and 13C resonances of the solvents: 7.24 (1H,
CDCl3) and 7.16 (1H, C6D6); 77.0 (13C, CDCl3) and 128.0 (13C,
C6D6). 19F spectra were referenced externally to TFA. Chemical shifts
are given in ppm and coupling constants in Hz. Infrared spectra were
registered with a Specord M82 spectrophotometer. Elemental analyses
were performed in the Laboratory of Microanalysis of the INEOS
RAS.
Table 7. Condensation of N-Sulfinylamines into
Sulfurdiimines
Reagent grade solvents were dried by refluxing over sodium/
benzophenone ketyl (tetrahydrofuran and diethyl ether), sodium
(toluene and n-heptane), magnesium (methanol), phosphorus
pentoxide (chloroform, DMF), or calcium hydride (dichloromethane)
and distilled prior to use. Deuterated solvents were dried over sodium/
potassium alloy (C6D6) or phosphorus pentoxide (CDCl3) and stored
over activated 4 Å molecular sieves. Methanol-d4 was used as received.
N-Sulfinylamines were prepared according to the standard proce-
dures17,28 using Merck SOCl2 (“for synthesis” grade) and freshly
distilled or recrystallized anilines.
entry no.
R
time (h)
conversion (%)
a
b
1 (13)
2 (14)
3 (15)
2-FC6H4
5
94 (66 )
72 (50)
4-MeC6H4
2-MeOC6H4
24
24
87 (70)
a
b
Spectroscopically estimated conversion. Isolated yield.
Silica (Merck silica gel 60 for column chromatography; surface area,
480−540 m2/g; particle size, 63−200 μm; mean pore size, 60 Å) was
preheated in vacuo (10−2 Torr) to 300 °C for 5 h and stored under
argon.
Condensation of Phenyl Isocyanate into Diphenyl-
carbodiimide. Condensation of isocyanates into carbodii-
mides with liberation of CO2 catalyzed by transition-metal
imido complexes is a mechanistic equivalent of the con-
densation of N-sulfinylamines. It was reported for several
homogeneous catalysts, the reaction conditions being usually
quite severe (140−190 °C).9a−c We have investigated the
catalytic capability of Ta/SiO2-300 in the condensation of
PhNCO into PhNCNPh (Scheme 5). Although the observed
activity is rather low (only 25% conversion was achieved after
Preparation of Ta(NtBu)(CH2CMe2Ph)3. A 1.76 M solution of
CMe2(Ph)CH2MgCl (13.9 mL, 24.5 mmol) in diethyl ether was added
dropwise to a stirred suspension of Ta(NtBu)Cl3(py)229 (4.22 g, 8.2
mmol) in diethyl ether (100 mL) cooled to −78 °C. The reaction
mixture was stirred at room temperature overnight, the resulting white
suspension was filtered, and the residue was washed with diethyl ether
(2 × 25 mL). The filtrate was evaporated and dried in vacuo to give
5.23 g (98%) of a yellow oil. 1H NMR (400 MHz, C6D6): δ 7.31 (d, 3J
E
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3
3
= 7.3, 6H, Phortho), 7.22 (t, J = 7.3, 6H, Phmeta), 7.08 (t, J = 7.3, 3H,
Phpara), 1.47 (s, 9H, NCMe3), 1.39 (s, 18H, CMe2Ph), 0.76 (s, 6H,
CH2). 13C NMR (100 MHz, C6D6): δ 153.3 (Phipso), 128.7 (Phortho),
125.9 (Phpara), 125.7 (Phmeta), 106.9 (CH2), 68.4 (NCMe3), 41.4
(CMe2Ph), 34.4 (CMe2Ph), 33.8 (NCMe3).
the formation of carbodiimide. The conversion after 24 h of reflux was
∼25% (GC).
Extended X-ray Absorption Fine Structure Spectroscopy
(EXAFS). The Ta LIII-edge EXAFS spectrum for Ta/SiO2-300 was
measured at the Structural Materials Science beamline of the
Kurchatov Synchrotron Radiation Center (Moscow, Russia).31 The
measurements were performed in the transmission mode using a
Si(111) channel-cut monochromator and two ionization chambers
filled with appropriate N2−Ar mixtures. Since the sample is susceptible
to humidity, freshly prepared substance was used and all sample
manipulation steps were performed under argon. Standard processing
and quantitative analysis of the experimental spectrum were performed
using the IFEFFIT software suite32 with the amplitude and phase
functions calculated by FEFF as implemented therein.
Preparation of Ta/SiO2-300. Ta(NtBu)(CH2CMe2Ph)3 (5.23
g, 8.0 mmol) was dissolved in heptane (30 mL) and added slowly to a
stirred suspension of silica (9.78 g, 162.8 mmol) in heptane (100 mL).
The mixture was stirred overnight at room temperature and then
filtered, and the residue was washed twice with heptane and dried in
vacuo to give 13.05 g of a pale yellow solid. Anal. Found: Ta, 10.5; N,
0.85; C, 11.72.
Alcoholysis of Ta/SiO2-300. A vial was charged with Ta/SiO2-
300 (93 mg, 0.052 mmol Ta) and C6Me6 (0.043 mmol, internal
standard), and methanol (∼0.2 mL) was added. The vial was left open
for several days to evaporate the excess methanol under atmospheric
pressure. The solid was then washed with CDCl3 (3 × 0.2 mL). The
Solid-State 13C CP/MAS NMR Spectroscopy. Solid-state 13C
CP/MAS NMR spectra were recorded on a Bruker Avance-II 400
NMR spectrometer operating at a frequency of 100.4 MHz using a
solid-state 7 mm H/X MAS probe. The sample was packed into a 7
mm zirconia rotor (in a glovebox) and spun at the magic angle at a
rate of 5.0 kHz. A RAMP cross-polarization pulse sequence was used33
with ramping of 1H-channel power from 100 to 70% and 2 ms contact
time. For high-power proton decoupling during the acquisition the
SW-TPPM sequence (τ = 8 μs, φ = 15°) was used.34 The recycle delay
was 2 s, and the length of the proton 90° pulse was 5.5 μs. The
number of transients collected for spectrum was 2048. The sweep
width was 55 kHz, and the chemical shift scale was referenced relative
to the CH2 group of solid adamantane.35
1
amount of tert-butylbenzene was estimated from H NMR as 0.056
1
3
mmol (1.1 equiv/Ta). H NMR (300 MHz, CDCl3): δ 7.39 (d, J =
7.3, 2H, Phortho), 7.30 (t, 2H, Phmeta), 7.16 (t, 1H, Phpara), 1.32 (s, 9H,
CMe3). 13C NMR (100 MHz, CDCl3): δ 151.0 (Phipso), 128.1 (Ph),
125.4 (Ph), 125.2 (Ph), 34.61 (quat CMe3), 31.36 (CMe3). When
methanol-d4 was used, signals assigned to monodeuterated tert-
butylbenzene were observed. 1H NMR (400 MHz, CDCl3): δ 1.32 (s,
6H, CMe2), 1.30 (t, JH−D ≈ 2, 2H, CH2D). 13C NMR (100 MHz,
2
CDCl3): δ 34.54 (quat CMe2CH2D), 31.34 (CMe2), 31.04 (t, 1JC−D
19.3, CH2D).
=
DRIFT spectroscopy. Diffuse reflectance FTIR spectra were
recorded in the 6000−400 cm−1 range at a data point resolution of 4
cm−1 with a Nicolet “Protege” 460 spectrometer equipped with a
diffuse reflectance attachment developed at IOC RAS.36 The catalyst
was placed in an ampule supplied with a CaF2 window using a
glovebag filled with argon. The ampule was then evacuated at ambient
temperature for 10 min, and the spectra were recorded (500 scans per
spectrum).
Stoichiometric Reaction of Ta/SiO2-300 with PhCHO. A
Schlenk flask was charged with Ta/SiO2-300 (140 mg, 0.087 mmol
Ta), benzaldehyde (0.045 mL, 0.435 mmol, 5 equiv to Ta), and
heptane (3 mL), and the mixture was stirred under reflux for 3 h. The
solid was then filtered off and washed with diethyl ether (3 × 4 mL).
C6Me6 (0.012 mmol, internal standard) was added to the filtrate. A
57% amount (to Ta) of N-benzylidene-tert-butylamine was identified
in 1H NMR (300 MHz, CDCl3): δ 8.28 (s, 1H, CHN), 1.30 (s, 9H,
NCMe3).
ASSOCIATED CONTENT
* Supporting Information
Text and figures giving spectral characteristics and analytical
data for compounds 1−15. This material is available free of
charge via the Internet at http://pubs.acs.org.
Ketimine Synthesis. A Schlenk flask was charged with Ta/SiO2-
300 (5 mol % of Ta), ketone (1 equiv), N-sulfinylamine (1 equiv), and
heptane (C = 0.15 M), and the mixture was stirred under reflux. The
reaction was followed with IR spectroscopy by taking aliquots of the
reaction mixture until total disappearance of the carbonyl band was
achieved. The resulting solution was rapidly filtered in air through a
glass filter. The filtrate was concentrated in vacuo and cooled to −20
°C. The precipitated solid was separated by filtration. Spectral
characteristics and elemental analysis data for ketimines 1−12 are
given in the Supporting Information.
Sulfurdiimine Synthesis. A Schlenk flask was charged with Ta/
SiO2-300 (5 mol % of Ta), N-sulfinylamine, and heptane (C = 0.30
M), and the mixture was stirred under reflux. The reaction vessel was
purged several times with argon to remove the liberated SO2. The
conversion was determined from the NMR spectra of the aliquots of
the reaction mixture. The catalyst was filtered off under argon. The
filtrate was concentrated in vacuo and cooled to −78 °C. The
precipitated solid was separated by filtration. Spectral characteristics
and elemental analysis data for compounds 13−15 are given in the
Supporting Information.
Imidation of DMF. A Schlenk flask was charged with Ta/SiO2-300
(5 mol % of Ta), DMF (1 equiv), N-sulfinylamine (1 equiv), C6Me6
(internal standard), and heptane (C = 0.15 M), and the mixture was
stirred under reflux for 3−5 h. The reaction was followed with IR
spectroscopy by taking aliquots of the reaction mixture. The products
of the reaction were identified through 1H NMR spectroscopy by
comparison with literature data.10,30 The conversion was determined
relative to the internal standard.
Condensation of PhNCO. A Schlenk flask was charged with Ta/
SiO2-300 (170 mg, 0.10 mmol Ta, 5 mol % of Ta), and a 0.15 M
solution of PhNCO (0.22 mL, 2.1 mmol) in heptane (14 mL) was
added. The mixture was followed with IR spectroscopy by taking
aliquots while being heated under reflux. A band at 2142 cm−1
appeared and increased during the course of the reaction, indicating
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.N.Z.: zaroubine@ineos.ac.ru.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the Russian Foundation
for Basic Research (Grant 07-03-00939). We thank Olga
Tkachenko and Leonid Kustov (IOC RAS) for the registration
and discussion of DRIFT spectra and the Laboratory of
Microanalysis of INEOS RAS. EXAFS measurements were
performed at the User Facility “Kurchatov Center for
Synchrotron Radiation and Nanotechnology” partially sup-
ported by the Russian federal contract 16.552.11.7055.
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