Oxo/imido heterometathesis of N-sulfinylamines and carbonyl compounds catalyzed by silica-supported vanadium oxochloride

Article abstract of DOI:10.1016/j.jcat.2011.07.011

A series of silica-supported vanadium oxo complexes has been prepared via water-free grafting VOCl₃ onto the silica surface. The obtained materials have been characterized with Raman, diffuse reflectance FTIR (DRIFT) and UV-vis, ⁵¹V

Full text of DOI:10.1016/j.jcat.2011.07.011

Journal of Catalysis 283 (2011) 108–118
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Oxo/imido heterometathesis of N-sulfinylamines and carbonyl compounds
catalyzed by silica-supported vanadium oxochloride
a
Pavel A. Zhizhko a,b, Anton A. Zhizhin a, Dmitry N. Zarubin a, , Nikolai A. Ustynyuk ,
⇑
Dmitry A. Lemenovskii b, Boris N. Shelimov c, Leonid M. Kustov c, Olga P. Tkachenko c,
Gayane A. Kirakosyan d
a A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia
b Chemistry Department, Moscow State University, Vorob’evy Gory 1, 119992 Moscow, Russia
c N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, 119991 Moscow, Russia
d N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prospect 31, 119991 Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of silica-supported vanadium oxo complexes has been prepared via water-free grafting VOCl3
onto the silica surface. The obtained materials have been characterized with Raman, diffuse reflectance
FTIR (DRIFT) and UV–vis, 51V solid-state NMR and X-ray photoelectron spectroscopies, elemental analysis
and N2 physisorption. The samples with a low vanadium loading (1–5%) have been found to be comprised
primarily of the isolated tetrahedral d0 units („SiO)V(@O)Cl2, whereas the materials with a higher vana-
dium content are contaminated with a significant amount of d1 vanadium species. The silica-supported
vanadium complexes act as heterogeneous catalysts for oxo/imido heterometathesis between N-sulfinyl-
amines and carbonyl compounds affording imines and SO2. Grafting VOCl3 onto silica leads to a dramatic
enhancement of its catalytic activity. A novel water-free express method for preparation of imines of
wide range of aldehydes and some ketones has been developed. Noteworthy, this is the first example
of transition metal mediated heterometathetical imidation of ketones.
Received 2 May 2011
Revised 28 July 2011
Accepted 29 July 2011
Available online 31 August 2011
Keywords:
Heterometathesis
Heterogeneous catalysis
N-sulfinylamine
Imine
Imido
Vanadium
Silica-supported
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
temperature [9], alkane metathesis [10], and other related trans-
formations [11].
Olefin metathesis has already become a powerful practical tool
of modern organic synthesis [1,2]. Since the middle sixties, the ole-
fin metathesis catalysts have passed the long way from the first
examples of ill-defined heterogeneous systems to a varied stock
of well-defined homogeneous catalysts. In recent years, having
successfully combined both directions, Basset, Copéret, and others
have developed a new generation of highly efficient well-defined
heterogeneous olefin metathesis catalysts based on Schrock car-
bene complexes via grafting a molecular precursor of a determi-
nate structure onto the surface of an inorganic support [3–7].
The immobilized olefin metathesis catalysts displayed the activity
and stability superior to their homogeneous analogs, especially in
metathesis of functionalized olefins bearing polar substituents
[5–7]. Application of the grafting methodology to some other tran-
sition metal complexes has recently led to the discovery of well-
defined heterogeneous catalysts for such nontrivial processes as
polyolefin depolymerization [8], nitrogen activation at ambient
Catalytic heterometathesis (i.e., carbon–heteroatom and hetero-
atom–heteroatom multiple bonds metathesis) is quite a novel class
of transition metal mediated reactions. Although numerous stoi-
chiometric heterometathetical transformations as well as [2 + 2]
cycloadditions across M@O, M@N and other metal–ligand multiple
bonds were described in literature [12–15], only a few examples of
transition metal catalyzed heterometathesis, mainly imido-trans-
fer reactions, were reported to date. These are metathesis of imines
[16–20], carbodiimides [21–23], diphosphenes [24], and nitriles
[25,26]; condensation of isocyanates and N-sulfinylamines into
carbodiimides [27,28] and sulfurdiimines [29], respectively; imida-
tion of dimethylformamide with N-sulfinylamines affording form-
amidines [29] and metathetical imido-deoxygenation of aldehydes
with isocyanates [20], N-sulfinylamines [30] and iminophosphor-
anes [31,32]. Among them, only two examples, namely imine
metathesis [20] and condensation of isocyanates into carbodii-
mides [33] catalyzed by CH3ReO3/Nb2O5 and VOCl3/SiO2 respec-
tively, belong to heterogeneous catalysis.
Since heterometathesis implies transformations of the polar
substrates, heterometathesis catalysts immobilized on polar
inorganic supports could be expected to exhibit a higher activity
⇑
Corresponding author. Fax: +7 499 135 50 85.
E-mail address: zaroubine@ineos.ac.ru (D.N. Zarubin).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.07.011

P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
109
The catalysts were placed in an ampule (supplied with a CaF2 win-
dow) in air and immediately evacuated at 350 °C for 2 h. The probe
molecule CD3CN was adsorbed at ambient temperature and the
saturation vapor pressure of 96 Torr.
R'
H
R'
H
cat. (3-4 mol%)
heptane or toluene, Δ
RN S O
+
O
RN
+
SO2
cat. = VOCl3, MoOCl3, MoO2Cl2, (2,4,6-Br3C6H2N=)2Mo(OMes)2
X-ray photoelectron spectra (XPS) were measured using a
Scheme 1. Oxo/imido heterometathesis between aldehydes and N-sulfinylamines.
XSAM-800 spectrometer (Kratos) with Mg K
a nonmonochromatic
radiation; the Mg K source was operated at 90 W. The spectra
a
were measured under the conditions of a constant relative energy
resolution in steps of 0.1 eV at ambient temperature and
ꢀ4 Â 10À10 Torr pressure. The spectral lines recorded were fitted
with a Gauss profile or a sum of Gauss profiles after the subtraction
of the linear background. The energy scale of the spectrometer was
calibrated using the standard procedure; the following binding
energies were used: Cu 2p3/2, 932.7 eV; Ag 3d5/2, 368.3 eV; and
Au 4f7/2, 84.0 eV. The XPS spectra were referenced to the CA/CAH
component of the C 1s spectrum of adventitious carbon, assuming
its binding energy to be 285.0 eV.
as compared to their homogeneous analogs, especially in nonpolar
media. Herein, we apply this hypothesis to the heterometathesis
reaction between aldehydes and N-sulfinylamines that has been
recently shown in our group to be catalyzed by the vanadium
and molybdenum oxo- and imido complexes in hydrocarbon sol-
vents (Scheme 1) [30]. VOCl3 grafted onto silica (VOCl3/SiO2) was
chosen as a model catalytic system to compare its activity with
that of unsupported VOCl3. This system was selected since it is
generally accepted that, at least at low loadings, vanadium oxide
species on the silica are composed of isolated tetrahedral
(„SiO)nV(@O)X3Àn units (where X = Hal, OR, etc. and n = 1, 2, 3)
[33–47], whose structure closely resembles the structure of the
tetrahedral vanadium oxo complexes in solution. This structural
similarity suggests that silica-supported vanadium oxochloride
could be involved in the same heterometathetical transformations
as molecular VOCl3. In addition, silica-supported vanadium oxo
complexes were extensively studied in recent decades and diverse
physical techniques were found suitable for their characterization.
This work demonstrates convincingly that the immobilization
of VOCl3 on the silica support dramatically enhances its catalytic
activity; VOCl3/SiO2 is shown to be a highly efficient catalyst for
express water-free preparation of imines of wide range of alde-
hydes and some ketones.
The 51V NMR spectra of solid samples were recorded at 297 K
using a Bruker Avance II 300 spectrometer operating at a frequency
of 78.94 MHz. The spin system was excited by a one-pulse
sequence using short pulses (pulse width, 2.0
ls; dead time,
6.0 s; number of scans, 10,000). Exponential multiplication with
l
a 100-Hz line broadening was used. The 51V NMR chemical shifts
were referenced to an external VOCl3 solution in CDCl3. The chem-
ical shifts of solid-state signals in static 51V NMR spectra were
measured with an accuracy of 10 ppm. The solid samples were
transferred into NMR tubes equipped with Teflon stopcocks under
argon.
2.2. Starting materials
VOCl3 (Strem Chemicals) was distilled under argon, degassed
through three freeze–pump–thaw cycles and stored in a Schlenk
tube equipped with a Teflon stopcock. Tri(tert-butoxy)silanol
(tBuO)3SiOH and polyhedral oligomeric silsesquioxane trisilanol
iBu7Si7O9(OH)3 (iBu-POSSH3) were purchased from Aldrich. The
corresponding vanadyl complexes [(tBuO)3SiO]3VO [49] and [iBu-
POSS]VO [50] were prepared according to the previously reported
procedures. Reagent grade solvents were dried by refluxing over
sodium/benzophenone ketyl (tetrahydrofuran and diethyl ether),
sodium (toluene and n-heptane), phosphorus pentoxide (chloro-
form), and calcium hydride (dichloromethane) and distilled prior
to use. CDCl3 was dried over phosphorus pentoxide and stored over
activated 4 Å molecular sieves in a Schlenk tube equipped with a
Teflon stopcock. Details for organic compounds used are given in
Supplementary material.
2. Experimental
2.1. Instrumentation
If not otherwise stated, all manipulations were carried out
under an argon atmosphere using standard Schlenk techniques.
NMR spectra were recorded using Bruker AMX 400 and Avance
300 spectrometers. Infrared spectra were registered with Specord
M80 and M82 spectrophotometers. Elemental analyses were per-
formed in the Laboratory of Microanalysis of INEOS RAS. The
amount of chemisorbed vanadium was determined by X-ray fluo-
rescence analysis.
Nitrogen adsorption–desorption isotherms were measured at
77 K using an ASAP 2000 instrument (Micromeritics). The samples
were preheated for 3 h in a vacuum at 200 °C before the measure-
ments. The specific surface area was calculated by the BET method.
The pore size and the pore size distribution were calculated by the
BJH method using the desorption isotherm branch.
Silica gel 60 for column chromatography (MerckÒ; surface area,
480–540 m2/g; particle size, 63–200
lm; mean pore size, 60 Å)
was used as a support in all experiments. The designations for
the silicas used are as follows. Silica-25 was just outgassed in a
vacuum (10À2 Torr) at ambient temperature for 10 min. Silica-
300 was preheated at 300 °C in air for 2 h and cooled down to
ambient temperature in a vacuum (10À2 Torr). This procedure is
supposed to remove only physically adsorbed water without sig-
nificant dehydroxylation [51]. Silica-500 was preheated at 500 °C
in a vacuum (10À2 Torr) for 12 h.
Raman spectra in the 100–1200 cmÀ1 region were recorded
using a Laser Raman spectrometer LabRAM (Horiba Jobin Yvon)
equipped with a cooled CCD detector and a microscope. The exci-
tation was accomplished by the red 632.8 nm line of a He-Ne laser
with an output power less than 5 mW. The samples for the analysis
were transferred into capillary tubes and sealed under argon.
UV–vis diffuse reflectance spectra (UV–vis DRS) in the 260–
800 nm region were recorded using a Hitachi M-340 UV–vis–NIR
spectrophotometer equipped with a R-10A integrating sphere
accessory using a homemade Schlenk-type quartz cell under argon.
A disk of pressed MgO powder was used as a reference sample.
FTIR diffuse reflectance (DRIFT) spectra were recorded in the
6000–400 cmÀ1 range at a data point resolution of 4 cmÀ1 with a
Nicolet ‘‘Protege’’ 460 spectrometer equipped with a diffuse reflec-
tance attachment developed at IOC RAS [48]. To obtain a satisfac-
tory signal-to-noise ratio, 500 scans were collected per spectrum.
2.3. Catalyst preparation
Silica-supported vanadium catalysts were prepared via impreg-
nation of silica with VOCl3 in dry n-heptane. A typical procedure
for 5V/SiO2-300 preparation: VOCl3 (0.30 ml, 3.2 mmol) was added
to a vigorously stirred suspension of silica-300 (3.92 g, 65.3 mmol)
in 30 ml of n-heptane. The mixture was stirred at room tempera-
ture (RT) for 5 h, and then, all volatiles were removed in a vacuum.
The resulting material was twice washed with boiling n-heptane

110
P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
IR spectra [52]. 1H NMR (300 MHz, CDCl3): d 7.05 (d, 3J = 7.9 Hz,
1H), 7.27 (t, 3J = 7.7 Hz, 1H), 7.45–7.56 (m, 4H), 7.68 (d,
3J = 7.7 Hz, 1H), 7.91–7.95 (m, 2H), 8.37 (s, 1H, CH@N); 19F NMR
Table 1
Designations and elemental analysis data of silica-supported vanadium catalysts.
Heterogeneous
catalyst
Supporta
V (mol%)b
V (wt.%)c
Cl/Vc atomic
ratio
(282 MHz, CDCl3): d 17.51 (s); IR (n-heptane):
m
1636 cmÀ1 (C@N).
10V/SiO2-25
1V/SiO2-300
5V/SiO2-300
10V/SiO2-300
2V/SiO2-500
5V/SiO2-500
Silica-25
10
1
5
10
2
5
7.1
0.9
3.9
6.4
1.4
3.3
0.4
1.5
1.3
1.0
2.0
1.7
Silica-300
Silica-300
Silica-300
Silica-500
Silica-500
3. Results and discussion
3.1. Catalyst preparation
a
b
c
Most grafting techniques are based on the interaction of the
surface hydroxyl groups of the support with metal chlorides, alk-
oxides, amides, alkyls, etc., leading to a substitution of one or sev-
eral ligands. Interaction of VOCl3 with the silica surface is depicted
in Scheme 2.
The designations of silicas are explained in Section 2.2.
Nominal value.
Determined from elemental analysis.
(1 h reflux followed by filtering or decanting) and dried at RT in a
vacuum yielding 4.25 g of white powder. For the designations of
the catalysts, see Table 1. Calcined samples were prepared by heat-
ing the sample for several hours at 500 °C in a quartz tube in a flow
of air flowing through a column filled with dry Al2O3. Preheated sil-
icas, and all the heterogeneous catalysts were stored in Schlenk
flasks under argon.
In this work, immobilization was accomplished via impregna-
tion of the support with a certain amount of VOCl3 in excess of
n-heptane followed by evaporation of volatiles in a vacuum. This
water-free grafting procedure is usually referred to as atomic layer
deposition (ALD) [45]. It is believed that aggregation and formation
of oligomeric vanadia species and V2O5 crystallites can thus be
avoided providing a better dispersion of the precursor on the sur-
face. A series of heterogeneous catalysts with different VOCl3 load-
ings (1–10 mol%) were prepared using silicas dehydroxylated
under different conditions. Their designations and elemental anal-
ysis data are summarized in Table 1.
All the manipulations were carried out in the inert atmosphere
since the catalysts, substrates (N-sulfinylamines), and intermedi-
ates (imido complexes) are sensitive to moisture. The exposure
of the catalysts to air leads to an immediate color change from ini-
tial white or beige (or gray for 10V/SiO2-25) to yellow or orange
caused by hydrolysis of VACl bonds (the chlorine loss was con-
firmed by elemental analysis). After air exposure for several days,
the samples finally become green, which is typical of fully hydrated
supported vanadia [41,42,45–47].
2.4. Catalytic tests
A Schlenk flask was charged with the catalyst, and a certain vol-
ume of a freshly prepared 0.15 M n-heptane solution of benzalde-
hyde and N-sulfinyl-2,4,6-trichloroaniline (1:1) was added under
argon. The reactor was equipped with a backflow condenser,
purged with argon, and heated under reflux (98 °C) for 20 min.
Then, the reaction mixture was cooled down for ꢀ1 min, the inter-
nal standard (C6Me6) was added (if the heterogeneous catalyst was
used, it was filtered off before), and the conversion was determined
from 1H NMR spectra.
When the test reaction was carried out for a longer time
(>20 min), it was followed with IR spectroscopy by taking aliquots
from the reaction mixture and monitoring the benzaldehyde car-
bonyl (1712 cmÀ1) and imine C@N (1648 cmÀ1) bands. The sub-
strate to catalyst molar ratio was calculated with respect to the
total vanadium amount in the sample determined from elemental
analysis.
On the other hand, we focused primarily on the synthetic labo-
ratory application of our catalysts and hence tried to simplify the
techniques used as much as possible and to avoid the procedures
that are not commonly used in organic laboratories (e.g., calcina-
tion). The same reason accounts for the choice of Merck silica gel
60 as a widespread and easily available sorbent for column
chromatography.
2.5. Preparation of imines
3.2. Catalyst characterization
The detailed procedures for the synthesis of imines 1a–15a, 1b–
5b (Tables 4 and 5) as well as their spectral characteristics and ele-
mental analysis data are given in Supplementary material.
A number of physicochemical techniques were applied to
characterize the obtained materials. The influence of the grafting
2.6. Stoichiometric reactions of surface complexes
n
SiOH
+
VOCl3
( SiO)nV(=O)Cl3-n + nHCl
2V/SiO2-500 (558 mg,ꢀ0.15 mmol V) was placed into a Schlenk
flask, and then, n-heptane (10 ml) and N-sulfinyl-2-trifluorometh-
ylaniline (0.23 ml, 1.5 mmol) were added. The mixture was stirred
for 20 min under reflux and then filtered via a cannula filter, and
the residue was dried at 50 °C in a vacuum (7.5 Â 10À4 Torr). The
purple powder was obtained (580 mg). Elemental analysis: V,
1.45%; N, 0.45%; C, 2.85%; F, 2.01%; S 6 0.1% (V:N:C:F =
0.9:1:7.4:3.3).
Scheme 2. Interaction of VOCl3 with surface hydroxyl groups of silica.
Table 2
Textural characteristics of the supports and immobilized materials.
Heterogeneous
catalyst
BET surface
area (m2/g)
Surface
density
(V/nm2)
Pore
size (Å)
Pore volume
(cm3/g)
A portion of the material obtained (102 mg, ꢀ0.03 mmol V) was
placed into a Schlenk flask. n-Heptane (4 ml) and benzaldehyde
(0.06 ml, 0.6 mmol) were added. The mixture was heated under re-
flux for 20 min and then cooled down, filtered, and washed with
20 ml of a pyridine–heptane (1:10) mixture. The filtrate was evap-
orated in a vacuum, and an internal standard (C6Me6) was added.
N-benzylidene-2-trifluoromethylaniline (ꢀ50% to the initial vana-
dium content in the sample) was identified in 1H, 19F NMR, and
Silica-300
467
451
425
423
405
500
480
477
–
49
48
47
44
48
49
48
45
0.72
0.69
0.62
0.58
0.59
0.71
0.68
0.60
1V/SiO2-300
5V/SiO2-300
10V/SiO2-300
10V/SiO2-25
Silica-500
0.24
1.08
1.79
2.07
–
2V/SiO2-500
5V/SiO2-500
0.34
0.82

P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
111
Fig. 1. Pore size distribution profiles of the supports and immobilized materials.
Fig. 2. DRIFT spectra of the C„N stretching vibration region of: (a) silica-300, (b) 1V/SiO2-300, (c) 5V/SiO2-300 and (d) 10V/SiO2-300 after adsorption and desorption of
CD3CN.
process on the porous structure of the support was estimated using
N2 physisorption. The nature of the surface acidic sites was probed
by the adsorption of CD3CN monitored by DRIFT spectroscopy. The
structure of vanadium centers was investigated with Raman, UV–
vis DRS, XPS, and 51V solid-state NMR spectroscopies.
The textural characteristics of the parent supports and immobi-
lized materials are given in Table 2, and the pore size distribution
profiles are presented in Fig. 1. It is clear that the porous structure
of the support is not strongly affected by grafting. As should be ex-
pected, a gradual decrease in the surface area, pore size, and pore
volume is observed with increasing vanadium loading.
spectrum of silica-300 indicates that the parent support does not
contain isolated silanols. Yet, Fig. 3 clearly demonstrates partial
consumption of silanol groups caused by grafting and appearance
of the band at ꢀ3740 cmÀ1 characteristic of isolated SiAOH. The
interaction of hydrogen-bonded hydroxyl groups with CD3CN
was also confirmed by the red shift of SiAOH bands. It should be
noted that the acidity of hydrogen-bonded OH groups increases
in parallel with the V content. The red shifts of OH groups in the
spectra of 1V/SiO2-300, 5V/SiO2-300, and 10V/SiO2-300 are equal
Fig. 2 demonstrates the DRIFT spectra of silica-300, 1V/SiO2-
300, 5V/SiO2-300, and 10V/SiO2-300 in the region of C„N stretch-
ing vibrations after adsorption of CD3CN (upper curves) and after
removal of adsorbed CD3CN upon evacuation for 100 min (lower
curves). Only one band at 2273 cmÀ1 was observed in the spectrum
of the parent support, while two bands at 2313–2315 cmÀ1 and
2267–2273 cmÀ1 were found in the spectra of the vanadium-con-
taining samples. Note that the spectrum of 1V/SiO2-300 exhibits
the first of these two bands as a low-intensity shoulder. The band
at 2115 cmÀ1 in all spectra is due to the bending vibrations of CAD
bonds of CD3CN.
The C„N vibration at 2267–2273 cmÀ1 belongs to CD3CN mol-
ecules adsorbed on moderate Brönsted acidic sites (BAS). This band
is blue shifted by 14–20 cmÀ1 compared to the C„N vibration fre-
quency in the free CD3CN molecule in the gas phase at 2253 cmÀ1
[53–55]. These sites are the surface hydrogen-bonded silanol
groups. This was confirmed by DRIFT spectra in the region of sila-
nol stretching vibrations (Fig. 3). The broad SiAOH band in the
Fig. 3. DRIFT spectra of the silanol stretching vibration region of: (a) silica-300, (b)
1V/SiO2-300, (c) 5V/SiO2-300, (d) 10V/SiO2-300.

112
P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
Fig. 4. Raman spectra of: (a) silica-300, (b) 5V/SiO2-300, (c) 10V/SiO2-300, (d) 2V/SiO2-500, (e) 5V/SiO2-500 and (f) 10V/SiO2-25.
to 273, 283 and 315 cmÀ1, respectively, whereas for silica-300 it
equals to 300 cmÀ1
are at 994, 697, 518, 404, 303, 284, and 144 cmÀ1 [42,46]). The
sharp band at 448 cmÀ1 in the spectrum of 5V/SiO2-500 (Fig. 4e)
corresponds to the VACl vibration (cf. 411 cmÀ1 for VOCl3). It
was not observed in the spectra of other samples, since they con-
tain less chlorine, and the corresponding band is apparently
masked by the intense bands of the support.
.
The 2313–2315 cmÀ1 band (Fig. 2b–d) corresponds to the C„N
vibration of CD3CN molecules adsorbed on strong Lewis acidic sites
(LAS) (vanadium cations); the blue shift is 60–62 cmÀ1 [53–55].
The increase in the concentration of vanadium is accompanied by
increasing relative intensity of the 2313–2315 cmÀ1 band. The
desorption curves show that the adsorption on LAS is more stable
than on BAS. In particular, evacuation for 100 min at ambient tem-
perature appears insufficient to fully remove CD3CN adsorbed on
vanadium cations, whereas CD3CN adsorbed on hydroxyl groups
is almost completely removed.
An intense broad band centered at ꢀ940 cmÀ1 was observed in
the Raman spectra of the samples with a high vanadium loading
(10V/SiO2-25 and 10V/SiO2-300). The origin of this band will be
discussed below.
UV–vis diffuse reflectance spectroscopy further proves the pres-
ence of isolated tetrahedral vanadyl units on the surface of 1V/
SiO2-300, 5V/SiO2-300, 2V/SiO2-500, and 5V/SiO2-500. It appears
that the materials with a higher vanadium content (10V/SiO2-25
and 10V/SiO2-300) have a more complex structure. All UV–vis
DRS spectra of our samples (Fig. 5) are dominated by the intense
charge-transfer O2À ? V5+ band in the region below 400 nm. For
10V/SiO2-300 and 10V/SiO2-25 (Fig. 5c and d), an additional band
(or several bands) is apparently present in the 400–800 nm region
as a shoulder on the low energy side of the main CT band. The po-
sition of the CT band maximum for surface vanadia species is
known to be strongly dependent on the vanadium coordination
number and degree of polymerization of vanadium centers
[40,42–44]. Polymerization and change of the coordination num-
ber of vanadium atoms from 4 to 5 or 6 that usually occur at high
loadings or after hydration give rise to a significant red shift of the
Application of DRIFT spectroscopy to vanadium oxides grafted
to silica entails significant difficulties, since the silica is virtually
nontransparent in the spectral region below 1200 cmÀ1 and the
V@O bands appear to be totally disguised by the intense SiAOASi
fundamental modes. Conversely, Raman spectroscopy is widely
used for characterization of surface vanadium oxo species on var-
ious oxide supports since the latter are very weak Raman scatters
[46,47].
The Raman spectra of silica-300 and the samples 5V/SiO2-300,
10V/SiO2-300, 2V/SiO2-500, 5V/SiO2-500 and 10V/SiO2-25 are
shown in Fig. 4 (only silica bands were observed for 1V/SiO2-300
possibly because of the low vanadium concentration). The silica
support exhibits Raman features at ꢀ400, ꢀ490, ꢀ570, ꢀ800, and
980 cmÀ1. The 980 cmÀ1 band is associated with SiAOH stretching
mode of the surface hydroxyls [42,44]. This band is absent in the
spectra of the vanadium-containing samples, thus providing an
additional proof of silanol groups being consumed during grafting
in agreement with the DRIFT data.
The common feature of all spectra is a sharp band at 1038 cmÀ1
which is always present in Raman spectra of dehydrated silica-sup-
ported vanadium oxides. There is an unanimous agreement among
all researchers that it should be attributed to the V@O stretching
vibration of the tetrahedrally coordinated d0 vanadium centers
bearing an oxo ligand (cf. 1034 cmÀ1 for liquid VOCl3) [40–47].
The majority of the researchers consider these centers as isolated,
though there is an opinion that Raman spectroscopy may not be
reliable in discriminating the monomeric and polymeric vanadyl
species [40]. Anyway, the VAOAV vibrations that usually appear
as intense bands in the 200–300 and 500–800 cmÀ1 region
[41,42,46,47] were not observed in the spectra of our samples, thus
supporting the presence of isolated structures. We also did not ob-
serve Raman bands attributed to V2O5 crystallites (these typically
Fig. 5. UV–vis DRS spectra of: (a) 1V/SiO2-300, (b) 5V/SiO2-300, (c) 10V/SiO2-300,
(d) 10V/SiO2-25, (e) 2V/SiO2-500 and (f) 5V/SiO2-500.

P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
113
CT band to 400–500 nm. The CT bands in the region below 400 nm
are attributed to isolated tetrahedral („SiO)3V@O units. The sur-
face complex („SiO)V(@O)Cl2 described by Rice and Scott also
exhibited similar UV–vis DRS spectrum [33].
2V/SiO2-500, and 5V/SiO2-500 that are not contaminated with re-
duced vanadium species are composed of isolated tetrahedral d0
units of a general structure („SiO)nV(@O)Cl3Àn. The values of n
can be determined from the elemental analysis. The values of the
Cl/V atomic ratio for all the catalysts are presented in Table 1.
Three independent processes can lead to the chlorine loss: (a) sub-
stitution of Cl with the surface hydroxyl groups according to
Scheme 2, (b) partial hydrolysis with adsorbed water that should
be taken into account for the catalysts with silica-300 and espe-
cially with silica-25 as supports, and (c) reduction of vanadium ob-
served for the catalysts with a high vanadium loading (10 mol%).
Thus, 10V/SiO2-25 has the least chlorine content because of the ac-
tions of all three factors. 10V/SiO2-300 contains on the average 1
chlorine per 1 vanadium atom. The Cl/V ratio of 1V/SiO2-300, 5V/
SiO2-300, and 5V/SiO2-500 lies between 1 and 2, and 2V/SiO2-
500 has exactly 2 chlorine atoms per 1 vanadium atom. Therefore,
we believe that 2V/SiO2-500 is comprised of single mono-grafted
(„SiO)V(@O)Cl2 units, 1V/SiO2-300, 5V/SiO2-300, and 5V/SiO2-
500 likely contain the admixtures of the other mononuclear tetra-
hedral vanadyl surface species, e.g., („SiO)2V(@O)Cl and
(„SiO)3V@O, whereas the structures of 10V/SiO2-25 and 10V/
SiO2-300 may be very complex and will not be discussed further.
These assignments correlate with the 51V NMR spectra of 2V/
SiO2-500, 5V/SiO2-500, and 5V/SiO2-300 solid samples (Fig. 7).
The complications in the structure of the samples with a
higher vanadium content (10V/SiO2-25 and 10V/SiO2-300), which
manifested themselves as the appearance of the additional Ra-
man band at 940 cmÀ1 and the absorbance in the visible region,
were hypothesized as a result of a partial reduction of the vana-
dium centers to yield the d1 configuration. This assumption was
confirmed by X-ray photoelectron spectroscopy. The XPS spectra
of 5V/SiO2-300 and 10V/SiO2-25 are shown in Fig. 6. The V 2p3/2
peak of 5V/SiO2-300 was fitted with a single Gauss profile at a
binding energy (Ebe) of 519.0 eV (Fig. 6a) and assigned to the
V5+ state. The V 2p3/2 peak of 10V/SiO2-25 was fitted with a
sum of two Gauss profiles at Ebe = 519.0 and Ebe = 517.6 eV
(Fig. 6b) related to V5+ and V4+ states with relative intensities
ca. 2:1. The full width at half maximum of all the peaks is
1.9 eV. Therefore, the appearance of the features discussed above
in the Raman and UV–vis spectra can be correlated with the
presence of d1 vanadium centers, whereas the samples that do
not contain the reduced vanadium species in significant amounts
do not display these features. The absorbance in the visible re-
gion thus should be attributed to the d–d transitions of the re-
duced vanadium atoms and the Raman band at 940 cmÀ1
results from the corresponding V@O vibration. It was found that
all the catalysts underwent similar slow transformations when
they were kept under argon for several months. The reduction
of V5+ can be slowed down or even completely prevented by
keeping the samples in the dark. The samples with a higher
vanadium loading were affected by this process already at the
preparation stage. The indications of partial reduction of VOCl3
during immobilization [36] or further evolution of surface vana-
dium and other metal chlorides [35] were mentioned earlier in a
number of studies where the immobilized complexes were not
hydrolyzed or annealed to remove the ligands. It was also re-
ported that silica-supported vanadia could be reduced with HCl
evolved during grafting [37].
The spectra show a broad resonance (
D
m
½
ꢁ 10 kHz) at À378,
À370, and À395 ppm, respectively. The line shape is somewhat
distorted, most likely due to the chemical shift anisotropy deter-
mined in [36]. The signal position and shape are close to those in
the static 51V NMR spectrum of silica-supported VOCl3 [36]. The
isotropic chemical shift measured in the 51V MAS NMR spectrum
of the same sample (À295 ppm [36]) is consistent with that re-
ported for („SiO)V(@O)Cl2 species by other authors [34,56]. Bis-
and tris-siloxy units („SiO)2V(@O)Cl [56] and („SiO)3V(@O) [41]
were reported to show the isotropic chemical shifts of À540 and
À710 ppm, respectively. No such signals were detected in the
MAS NMR spectra of silica-supported VOCl3 in [36]. Thus, on
the basis of the literature data, the signals in the spectra of
Summarizing all the above characterization data, we can
conclude that the catalytic centers of 1V/SiO2-300, 5V/SiO2-300,
Fig. 7. 51V solid-state NMR spectra of: (a) 2V/SiO2-500, (b) 5V/SiO2-500
and (c) 5V/SiO2-300.
Fig. 6. XPS spectra of: (a) 5V/SiO2-300 and (b) 10V/SiO2-25.

114
P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
2V/SiO2-500, 5V/SiO2-500, and 5V/SiO2-300 can be assigned to the
vanadium monosiloxy dichloro species („SiO)V(@O)Cl2 bound to
the support through one SiAOAV bond with the vanadium atom
in a distorted tetrahedral local environment, which is consistent
with the conclusions made on the basis of other techniques.
According to some early works [37,38], the interaction of excess
gaseous VOCl3 with silica leads to a mixture of mono-, bis-, and
tris-grafted vanadyl units depending on the temperature of silica
dehydroxylation, mono-grafted units being formed on silica pre-
heated at 500 °C and higher. Other authors reported on the exclu-
sive formation of a monosiloxy complex, irrespective of the
temperature of the support pretreatment [33–36]. We conclude
that under our experimental conditions, single mono-grafted spe-
cies („SiO)V(@O)Cl2 are unambiguously formed on silica-500 at
low vanadium loadings. These surface complexes still dominate
on silica-300, but the presence of other forms in minor amounts
cannot be excluded. The increase in the vanadium loading leads
to a partial reduction of the vanadium atoms already at the stage
of preparation.
1.5 h (5 mol% of V). Thus, the immobilization of VOCl3 onto the sil-
ica surface apparently leads to a dramatic enhancement of its cat-
alytic activity in the studied oxo/imido heterometathesis reaction.
Interestingly, [(tBuO)3SiO]3VO and [iBu-POSS]VO that are often
used as molecular models to mimic silica-supported vanadyl
species do not display any catalytic activity at all (no imine
was observed in the reaction mixture after 5 h of reflux). This
finding excludes the idea that the high activity of silica-sup-
ported VOCl3 arises from the unique electronic properties of
the siloxy ligands.
A series of catalytic tests with 1 mol% of catalysts were carried
out to reveal the effects of the vanadium loading and the support
pretreatment on the catalytic activity and to rank the activities of
the heterogeneous catalysts. It was found that the catalysts pre-
pared with the silica pretreated at higher temperatures displayed
a higher catalytic activity compared to the samples with the same
vanadium content on less-dehydroxylated silica. The catalysts with
the lower vanadium loading display the higher specific activity in
the series prepared with the same silica. Thereby, 2V/SiO2-500
exhibited the highest activity whereas 10V/SiO2-25 was the least
active. The low activity of the samples with higher vanadium con-
tents is probably related to the presence of V(d1) and their lower
stability. Thus, with 1 mol% of 2V/SiO2-500, the test reaction
reaches a quantitative conversion in 2 h; while it practically stops
in 4–5 h at a ca. 75–80% conversion when 1 mol% of 10V/SiO2-25 is
used. Moreover, the activity of the catalysts that were stored for
months without light protection is considerably lower than their
initial activity. This observation confirms the idea that the pres-
ence of reduced vanadium disfavors the high catalytic
performance.
3.3. Catalytic activity tests
The reaction between benzaldehyde and N-sulfinyl-2,4,6-tri-
chloroaniline in boiling n-heptane (batch reactor) was used to
probe the catalytic activity of the silica-supported catalysts and
their homogeneous analogs (Table 3). The turn over frequencies
presented in Table 3 were measured after 20 min of the reaction.
It should be noted that the substrates selected do not interact in
the absence of the catalyst and less than 5% of imine was de-
tected in the reaction mixture after 5 h of reflux with parent
SiO2.
The reactions with 5 mol% of the catalysts were duplicated no
less than three times. The test reaction reaches a quantitative con-
version within 20 min with any of the heterogeneous catalysts
used. However, it takes about 7 h to achieve a quantitative conver-
sion when unsupported VOCl3 is used. In contrast to homogeneous
VOCl3, the supported catalysts display a reasonably high activity
even at room temperature: a 70–80% conversion was achieved in
3.4. Imidation of aldehydes
In order to estimate the scope of applicability of the heteroge-
neously catalyzed oxo/imido heterometathesis reactions, a wide
range of both aldehydes and N-sulfinylamines was tested (Table 4).
10V/SiO2-25 (5 mol%) was used for all aldimine syntheses despite
its poorest catalytic activity in the series of vanadium surface com-
plexes described above. The reason for this selection is that its
preparation procedure is the most simple, and this makes this cat-
alyst more attractive from the standpoint of express laboratory
application. Some syntheses were also duplicated with other heter-
ogeneous catalysts, with the reaction time being reduced.
At room temperature the oxo/imido heterometathesis proceeds
fast only for couples of highly reactive sterically unhindered N-sul-
finylamines and aldehydes (Table 4, entry 1a). In the other cases,
the reaction is to be preferentially performed under reflux to rap-
idly produce the aldimines in a high yield. The N-sulfinylamines
having electron-withdrawing substituents (Table 4, entries 1a–
3a, 7a, 8a) turned out to be more reactive than aniline derivatives
bearing electron-donating groups (Table 4, entries 4a, 9a, 11a).
Imidation of well-coordinating aldehydes, e.g., containing a pyri-
dine core (Table 4, entries 9a–11a), is more difficult in comparison
with imidation of other aldehydes. Sterically hindered aldehydes
like mesitaldehyde (Table 4, entries 4a, 12a, 13a) react more
slowly; however, even in combination with sterically hindered N-
sulfinylanilines bearing such bulky o-substituents as i-propyl
groups or bromine atoms (Table 4, entries 14a, 15a), azomethines
can be obtained in a high yield at prolonged reaction times. It
should be noted that neither pyridinecarboxaldehydes nor couples
of sterically hindered reagents enter into the same reaction in the
presence of molecular VOCl3.
Table 3
Catalytic activities of the homogeneous and heterogeneous vanadium catalysts in the
test reactiona:
Cl
Cl
Ph
H
N
O
N
Ph
cat.
n-heptane,
S
+
+
SO2
O
Cl
Cl
Cl
Cl
Catalyst
Catalyst/substrate
(mol% of V)
Conversion (%)
TOFb (hÀ1
)
SiO2
VOCl3
–
5
5
5
Traces
20
0
–
12
–
[(tBuO)3SiO]3VO
[iBu-POSS]VO
0
–
10V/SiO2-25
1V/SiO2-300
5V/SiO2-300
5
5
5
P95
P95
P95
60
60
60
10V/SiO2-300
2V/SiO2-500
5V/SiO2-500
5
5
5
P95
P95
P95
60
60
60
10V/SiO2-25
1V/SiO2-300
5V/SiO2-300
1
1
1
17
30
21
51
90
63
10V/SiO2-300
2V/SiO2-500
5V/SiO2-500
1
1
1
20
40
25
60
120
75
Thus, the water-free method based on VOCl3/SiO2 catalyzed
imidation of aldehydes with N-sulfinylamines is suitable for the
fast preparation of a wide range of imines of aromatic and hetero-
cyclic aldehydes. This method is especially useful for the synthesis
a
Reaction conditions: refluxing n-heptane (98 °C), 20 min.
TOF = (moles of imine produced)/[(moles of V) Â time].
b

P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
115
Table 4
Imidation of aldehydes with N-sulfinylamines.
R'
H
R'
H
10V/SiO2-25 (5 mol%V)
n-heptane, Δ
RN S O
+
O
RN
+ SO2
1a-15a
Reaction
Time
Isolated Yield
(%)
№
N-sulfinylamine Aldehyde
Aldimine
CF3
CF3
CHO
Cl
Cl
1a
30 mina
20 min
20 min
97
85
96
N
NSO
Cl
Cl
Cl
Cl
Cl
CHO
Cl
NSO
Cl
Cl
N
2a
3a
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
CHO
NSO
Cl
N
Cl
SMe
N
CHO
4a
5a
6a
1 h
94
95
95
NSO
SMe
CHO
NSO
N
1 h
O2N
O2N
NO2
COOMe
N
COOMe
NSO
CHO
1 h 45 min
NO2
Cl
Cl
NSO
Cl
Cl
N
7a
O
30 min
84
CHO
O
S
Cl
Cl
CF3
N
S
8a
9a
40 min
1.5 h
98
91
CHO
CHO
NSO
CF3
OMe
OMe
NSO
N
N
N
N
Cl
Cl
NSO
Cl
Cl
N
N
10a
11a
2 h
5 h
90
81
CHO
O
Cl
Cl
SMe
NSO
N
N
N
N
O
SMe MeS
NSO
CHO
12a
13a
1 h 20 min
15 h
94
80
N N
Mes
Mes
OSN
COOMe
N
COOMe
NSO
CHO
CHO
CHO
14a
15a
26 h
36 h
73
62
NSO
Br
N
Br
NSO
Br
N
Br
Br
Br
a At room temperature.

116
P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
of azomethines from poorly nucleophilic anilines that are difficult
to access by classic acid-catalyzed condensation of primary amines
with aldehydes.
considerably expand the scope of applicability of the catalytic
reaction.
3.6. Leaching and recycling
3.5. Imidation of ketones
All the catalysts were always twice washed with boiling n-
heptane after grafting to model the conditions of the heterome-
tathesis reaction studied. However, this procedure seems to be
not necessary, since in most cases, the amount of vanadium
grafted found by elemental analysis was consistent with that ini-
tially introduced into the grafting procedure. It is not surprising,
since the polar vanadium species, even in a physisorbed state,
should exhibit more affinity to silica rather than to an aliphatic
solvent. Chemically bound vanadium requires at least a stoichi-
ometric amount of a protic reagent to cleave VAOASi bonds,
such a cleavage being often observed in oxidation reactions
where hydroperoxides, alcohols, or other protic substrates are
involved. Indeed, treatment of the catalysts with MeOH followed
by filtration led to a significant loss of vanadium in the sample.
We also performed a simple leaching test under reaction condi-
tions [57]. The test reaction was carried out in the presence of
1 mol% 5V/SiO2-300, and the catalyst was filtered off after
20 min. The procedure was accomplished under argon, and the
reaction mixture was maintained under reflux during the filtra-
tion. The filtrate was monitored for further reaction up to 10 h
after filtration, and no reaction was observed. However, this does
not exclude leaching, as we know that vanadium complexes are
of low catalytic activity in homogeneous media. Therefore, the
crude product of the reaction was subjected to elemental analy-
sis and was found to contain less than 0.005% of vanadium.
The activity of all heterogeneous catalysts studied lowers signif-
icantly already after the first catalytic run. Washing the catalysts
Only few reports on the stoichiometric imido-transfer reactions
from transition metal imides to ketones have been reported. Pri-
marily, these examples are restricted to the imido complexes of
titanium and zirconium that are characterized by pronounced oxo-
philicity. As far as transition metal catalyzed heterometathetical
ketone imidation is concerned, to the best of our knowledge, there
is no report on the processes of such type to date. Much higher
activity of the vanadium heterogeneous catalysts compared to
their homogeneous analog VOCl3 encouraged us to probe ketones
as a carbonyl component in the oxo/imido heterometathesis reac-
tion under consideration. Here, we report the first examples of ke-
tone imidation (Table 5), where 5V/SiO2-300 was used as a catalyst
in loading of 10 mol% to reach a high conversion of the reagents in
a reasonable period of time.
We found that highly reactive o-monofluorinated ketones, e.g.
2-fluoroacetophenone, react readily (Table 5, entry 1b), whereas
in other cases, prolonged heating was necessary to obtain keti-
mines in a high yield. Aside from N-sulfinyl-2-trifluoromethylani-
line, the variety of reactive N-sulfinylamines turned out to be
limited to N-sulfinylanilines having no o-substituents. This indi-
cated that the reaction is sensitive to the steric factor. It should
be noted that homogeneous catalysis by using molecular VOCl3 is
not efficient in imido-deoxygenation of ketones with N-sulfinylam-
ines. Thus, grafting VOCl3 onto the silica surface not only leads to
the enhancement of its catalytic activity, but also allows one to
Table 5
Imidation of ketones with N-sulfinylamines.
R'
R'
5V/SiO2-300 (10 mol%V)
n-heptane, Δ
RN S O
+
O
RN
+ SO2
R''
R''
1b-5b
Isolated Yield
(%)
№
N-sulfinylamine
Ketone
Ketimine
Reaction Time
1.5 h
CF3
O
CF3
F
1b
95
N
NSO
F
CF3
CF3
O
N
Ph
2b
3b
18 h
12 h
52
NSO
Ph Ph
Ph
CF3
O
CF3
N
90
NSO
O
O
N
NSO
4b
5b
12 h
12 h
96
94
MeO
N
NSO
MeO

P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
117
after the reaction with boiling heptane, CH2Cl2, etc., or heating in
a vacuum did not restore their initial activity. However, calcina-
tion at 500 °C in a stream of air followed by heating in a vacuum
at the same temperature led to the materials with the activity
close to that of the initial catalyst. Obviously, such a treatment
has to be accompanied by some structural changes of the initial
catalyst (e.g., the resulting material does not contain chlorine
according to elemental analysis). These changes and their influ-
ence on the catalytic behavior are currently being under
investigation.
complex. It should be mentioned that heterometathetical imida-
tion of grafted oxo complexes with isocyanates (with liberation
of CO2) had already been introduced for tantalum [69], rhenium
[70] and vanadium [33] surface oxo species.
3.7.2. Reaction of the surface imido complex with an aldehyde
The obtained material was introduced into the reaction with an
excess of benzaldehyde in n-heptane in the absence of N-sulfinyl-
amine (reflux, 20 min). The imine formed was not observed in the
solution by IR directly after the reaction, as it was physisorbed on
the surface or coordinated to vanadium. Therefore, the mixture
was filtered and washed with various eluents (pyridine–heptane
(1:10) mixture, triethylamine, etc.) to desorb the product. The
imine band at 1636 cmÀ1 appeared in the IR spectrum of the fil-
trate. The amount of imine desorbed was determined from the
NMR spectrum with an internal standard in the range 35–65% of
the initial vanadium amount in the sample (it is dependent on
the eluent). No free aniline or other compounds apart from N-ben-
zylidene-2-trifluoromethylaniline and benzaldehyde were ob-
served in the mixture according to 1H and 19F NMR.
We also analyzed the filtrate obtained after refluxing the cata-
lyst 2V/SiO2-500 with excess of benzaldehyde, filtering, and wash-
ing it with py–heptane mixture (1:10) and did not detect any
products of benzaldehyde transformation. And finally, when the
catalyst treated with benzaldehyde is subjected to prolonged heat-
ing in a vacuum (100 °C/7.5 Â 10À4 Torr) to remove benzaldehyde
and then N-sulfinylamine is added, no imine formation is observed.
This observation confirms that in the absence of N-sulfinylamine,
benzaldehyde is only adsorbed or coordinated to vanadium as a
ligand.
Although alternative mechanisms cannot be excluded at this
point, these data suggest that the homogeneous and heteroge-
neous oxo/imido heterometathesis reactions follow the same gen-
eral sequence of the stoichiometric steps. However, the activation
effect of the support that is observed in the latter case indicates
that some details of the heterogeneous reaction are apparently dif-
ferent. The activation could arise from various factors. First of all,
the immobilization of the catalytically active species on the surface
should prevent them from decomposition via kinetic stabilization.
On the other hand, the polar substrates in nonpolar media are able
to concentrate in the near-surface layer due to a strong adsorption,
which should increase the reaction rate. This is confirmed by
changing the solvent for more polar toluene, in which the reaction
proceeds slower than in heptane. Furthermore, the adsorption
could also cause an activation of the substrates by enhancing their
polarization, e.g., via hydrogen bonding between the carbonyl oxy-
gen and remaining silanol groups close with vanadium centers, etc.
These hypotheses are currently under investigation.
3.7. Mechanistic considerations and stoichiometric reactions of surface
complexes
The idea of the catalytic activity of surface vanadium oxo com-
plexes in heterometathetical imidation of carbonyl compounds
was initially suggested on the basis of the mechanism taken from
the molecular catalysis (Scheme 3) [30]. The two steps of this cy-
cle—imidation of the transition metal oxo complex with N-sulfinyl-
amine [58–64] and imido group transfer from the imido complex
to an aldehyde [14,15,20,65–68]—are well-documented stoichiom-
etric heterometathesis reactions. To demonstrate that the hetero-
geneous reaction follows the same route, we carried out the
stoichiometric reactions involved in the catalytic cycle step by step
(Scheme 4).
3.7.1. Imidation of the surface oxo complex with N-sulfinylamine
The sample of 2V/SiO2-500 was allowed to react with an excess
of N-sulfinyl-2-trifluoromethylaniline in n-heptane (reflux,
20 min). The purple product was obtained after removal of the sol-
vent and desorption of the excess of sulfinylamine in a vacuum at
50 °C. The elemental analysis revealed that the stoichiometry of
the reaction was approximately 1 0.1 imido ligand per vanadium
atom (average of several experiments). It was also confirmed that
no sulfur remained in the solid product, which excludes N-sulfinyl-
amine being only coordinated to the metal center or physisorbed
on the surface and suggests the formation of the surface imido
R
[M] N
O S
RNSO
SO2
O
[M]=O
[M]=NR
R
[M] N
O
R'CHO
RN=CHR'
R'
H
4. Conclusion
Scheme 3. The catalytic cycle of oxo/imido heterometathesis between aldehydes
and N-sulfinylamines.
A series of silica-supported vanadium catalysts were prepared
via water-free grafting VOCl3 onto the silica surface. The materials
obtained were characterized with Raman, diffuse reflectance FTIR
(DRIFT) and UV–vis, 51V solid-state NMR and X-ray photoelectron
spectroscopies, elemental analysis and N2 physisorption. It was
found that at low vanadium loadings (1–5%), the catalyst surface
is mainly comprised of the isolated tetrahedral d0 units
(„SiO)V(@O)Cl2, whereas the materials with higher vanadium
concentrations contain significant admixtures of reduced vana-
dium species.
F3C
CF3
O
V
Cl
O
V
Cl
N
V
Cl
PhCHO
NSO
O
Si
Cl
O
Si
Cl
O
Si
Cl
CF3
N
SO2
The activities of the heterogeneous catalysts in oxo/imido het-
erometathetical imidation of carbonyl compounds with N-sulfinyl-
amines were found to be significantly higher than the activity of
homogeneous VOCl3. Thus, it can be concluded that the
Ph
Scheme 4. Stoichiometric heterometathesis reactions of vanadium surface
complexes.

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P.A. Zhizhko et al. / Journal of Catalysis 283 (2011) 108–118
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immobilization of the heterometathesis catalysts on the heteroge-
neous support indeed dramatically enhances their catalytic activ-
ity, as it was predicted.
On the basis of the heterogeneous catalysts obtained, a novel
water-free express method for the preparation of organic imines
of a wide range of aldehydes and some ketones was developed.
Noteworthy that we report on the first successful example of tran-
sition metal mediated heterometathetical imidation of ketones.
The simplicity of the catalyst preparation and product isolation
procedures makes this method especially attractive for easy prep-
aration of azomethines from poorly nucleophilic anilines that are
difficult to access by classic acid-catalyzed condensation of pri-
mary amines with aldehydes.
It is important to note that the synthetic potential of heterom-
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ketones. It is possible that the most intriguing catalytic heterome-
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field of homogeneous catalysis by transition metal complexes but
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This work was partially supported by the Russian Foundation
for Basic Research (Grant 07-03-00939). The authors thank the
Laboratory of Microanalysis of INEOS RAS; S. Bukalov and L. Leites
for the registration and discussion of the Raman spectra; A. Naum-
kin and I. Volkov for the registration and discussion of the XPS
spectra; A. Smirnov for the N2 physisorption measurements.
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Supplementary data associated with this article can be found, in
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