Synthesis and catalytic properties of niobium indenyl peroxo complexes

Article abstract of DOI:10.1134/S107036321206014X

Synthesis of four niobium sandwich peroxo complexes containing fragments of substituted indenes was performed and their structure was proved by NMR spectroscopy and ESI-MS mass spectrometry. Catalytic activity of the complexes in the oxidation of sulfides

Full text of DOI:10.1134/S107036321206014X

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 6, pp. 1118–1121. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.V. Rakhmanov, Zhong Sinyan, A.V. Tarakanova, A.V. Anisimov, A.V. Akopyan, N.S. Baleeva, 2012, published in Zhurnal
Obshchei Khimii, 2012, Vol. 82, No. 6, pp. 960–963.
Synthesis and Catalytic Properties
of Niobium Indenyl Peroxo Complexes
E. V. Rakhmanov, Zhong Sinyan, A. V. Tarakanova,
A. V. Anisimov, A. V. Akopyan, and N. S. Baleeva
Lomonosov Moscow State University, Leninskie Gory 1, building 3, Moscow, 119992 Russia
e-mail: erakhmanov@petrol.chem.msu.ru; sulfur45@mail.ru
Received March 22, 2011
Abstract—Synthesis of four niobium sandwich peroxo complexes containing fragments of substituted indenes
was performed and their structure was proved by NMR spectroscopy and ESI-MS mass spectrometry. Catalytic
activity of the complexes in the oxidation of sulfides with hydrogen peroxide was studied, and a high
selectivity in the oxidation of sulfides to sulfoxides and sulfones of the sandwich peroxo complexes was found.
DOI: 10.1134/S107036321206014X
A large class of transition metal complexes are
sandwich complexes of titanium, zirconium, niobium,
and vanadium with ligands containing fragments of
cyclopentadiene and its derivatives [1, 2]. Many of
these complexes are able to catalyze various organic
reactions with high efficiency and selectivity [3].
Among these reactions a significant synthetic value
belongs to the oxidation reaction opening a way to the
direct asymmetric synthesis of optically active oxygen-
and sulfur-containing compounds [1, 2].
The aim of this work was to obtain niobium indenyl
peroxo complexes with different structures and to use
them as catalysts of oxidation of some organosulfur
compounds with peroxides.
The niobium peroxo complexes with different
indenyl ligands were obtained according to the
following scheme:
R1
R1
R3
R3
n-BuLi
_
R2
R2
THF, _78oC
R3
R3
R1
R1
R3
R3
R2
R2
R3
R3
O
H2O2
THF
Nb
Nb
Cl
Cl
R2
Cl
NbCl4(THF)2
CH2Cl2
R3
R3
R3
R3
O
R2
R1
R1
I, R1 = H, R2 = H, R3 = H (31%); II, R1 = Ph, R2 = H, R3 = H (37%); III, R1 = H, 2R2 = (CH2)2, R3 = H (38%); IV, R1 = H,
R2 = H, R3 = OMe (50%).
The starting substances were commercially available
indenes with substituents in the five-membered ring, as
well as 5,6-dimethoxyindene synthesized specially from
the available 5.6-dimethoxyindanone [4, 5].
1118

SYNTHESIS AND CATALYTIC PROPERTIES
MeO
1119
MeO
MeO
MeO
MeO
NaBH4
EtOH
CCl4, Py
PPh3, Δ
MeO
O
OH
87%
73%
In the first stage of the reaction, the action of n-
BuLi on the substituted indene leads to the formation
of a yellow solution of the salt, which at the action of
NbCl4(THF)2 followed by addition of 37% hydrogen
peroxide affords sandwich niobium peroxo complexes
I–IV as pale yellow powders via the intermediate
halogenated complexes.
The oxidation of sulfides by hydrogen peroxide was
carried out in acetonitrile at 70°C for 4 h. The com-
position of the formed mixture was analyzed by gas-
liquid chromatography. Benzo- and dibenzothio-
phenes are practically not oxidized in these conditions,
in contrast to methyl phenyl sulfide, which is oxidized
to 96–100%.
The synthesized niobium peroxo complexes are
unstable in air and suffer gradual decomposition. Study
of the complexes by NMR spectroscopy shows a
downfield shift of proton signals and change in their
multiplicity, which confirms the formation of metallo-
cene complexes of sandwich type. The disappearance
of the signals of protons of the indene CH2 fragments
and comparison of chemical shifts with literature data
allowed a reliable identification of the compounds [3].
Yield, %
Peroxo complex
PhSOMe
97
PhSO2Me
3
0
I
100
II
3
97
4
III
IV
96
The ESI–MS mass spectrometry data of indenyl peroxo
complexes
Comp.
no.
I
An additional information about the structure of the
synthesized peroxo complexes I–IV was obtained
using ESI-MS (electrospray ionization mass spectro-
metry). A characteristic feature of the mass spectrum
of each peroxo complex is a peak of the cation formed
at the cleavage of the chloride anion from the parent
peroxo complex molecule (see the table).
Ligand
Indene
Ion
m/z
355
O
O
Nb+
As an example, the figure shows the mass spectrum
of the peroxo complex II. The presence of low-inten-
sity ion peaks is due to the instability of the indenyl
peroxo complexes under the analysis conditions. A
similar pattern is typical of the mass spectra of the
other peroxo complexes.
2-Pheylindene
507
II
Ph
O
Nb+
O
Ph
As the model substances for the study of catalytic
activity of indenyl niobium peroxo complexes methyl
phenyl sulfide, benzothiophene, and dibenzothiophene
were taken. Such a choice of the objects originates
from the difference in the reactivity of these sulfides in
the oxidation reaction ranging from the readily
oxidized methyl phenyl sulfide to difficultly oxidized
benzo- and dibenzothiophenes.
1,2-Bis(1-
indenyl)ethane
381
475
III
IV
Nb+
O
O
5,6-dimeth-
oxyindene
MeO
MeO
R1
O
O
O
Nb+
Nb
Cl
O
MeO
MeO
R1
RSX
RSOX + RSO2X
H2O2, CH3CN
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 6 2012

1120
RAKHMANOV et al.
Irel, %
m/z
Mass spectrum of bis[η5-(2-phenylindenyl)]chloroniobium(V)peroxide II.
Attention is attracted by the selectivity of oxidation
of methyl phenyl sulfide in the presence of peroxo
complexes of different structures. The complexes with
the ligands that are separate indenyl fragments lead to
MeSPh oxidation to sulfoxide, while the peroxo
complex III with bis-indenyl ligand catalyzes the
oxidation to sulfone. The presence of substituents not
binding two indenyl moieties does not affect the
selectivity or the rate of the methyl phenyl sulfide
oxidation. Apparently, the presence of short –(CH2)2–
fragment makes the catalytic center of the peroxo
complex III more sterically accessible, which increases
its catalytic activity.
ionization (ESI) mode of the sample ionization and
with the registration of positive ions. The samples
were prepared as solutions in chloroform (LS MS
grade, Aldrich), the concentration 1 mg ml–1. Condi-
tions of analysis: flow rate 0.5 ml min–1, pressure
50 psi, temperature 150°C, drying gas flow 12 l min–1,
voltage 4500 V.
The composition of reaction products and the purity
control of the starting materials were monitored by the
gas-liquid chromatography (GLC) on a Kristall-2000M
chromatograph with a flame ionization detector,
column Zebron, l = 30 m, d = 0.32 mm, the liquid
phase ZB-1, ramp from 100 to 250°C.
The data on the composition of oxidation products
indicate that the nature of the indenyl peroxo complex
plays an important role in the oxidation process. The
high selectivity of the peroxo complexes makes it
possible to expect that other complexes with rigidly
connected ligands may lead to new highly selective
catalytic systems.
Synthesis of the niobium indenyl peroxo com-
plexes I–IV. A substituted indene (0.86 mmol) was
dissolved in 2.6 ml of THF, the mixture was cooled to
–78°C, and 0.6 mmol of 2.5 M n-BuLi in heptane was
added. The resulting yellow solution was stirred for
30 min. Then 0.43 mmol of complex NbCl4(THF)2
(0.86 mmol in the case of compound III) dissolved in
THF was added dropwise, the mixture was stirred at
room temperature for 3 h. The resulting solution was
evaporated to dryness on a rotary evaporator. The
resulting product was dissolved in 6 ml of CH2Cl2 in a
dry box, and 12 ml of hexane was added, the separated
precipitate was filtered off, the organic phase was
evaporated to dryness at 30°C. The resulting yellow
material was dissolved in 6 ml of CH2Cl2, and 7.21
EXPEIMENTAL
1
The H NMR spectra were recorded on a Bruker
DRX-400 spectrometer with the operating frequency
400 MHz, solvent CDCl3, internal reference HMDS.
Chemical shifts were measured with an accuracy of
0.01 ppm. The ESI-MS spectra were recorded on an
Agilent LC/MS 1100 SL instrument in the electrospray
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 6 2012

SYNTHESIS AND CATALYTIC PROPERTIES
1121
mmol of 37% H2O2 was added dropwise to the
solution. After 30 min, to the resulting yellow-orange
solution was added 5 ml of water, the organic layer
was separated and dried over Na2SO4. After the
evaporation of solvent the target peroxo complexes
were isolated as pale yellow powders. Yields of the
peroxo complexes I–IV were 31, 37, 38, and 50%
respectively.
Compound I. 1H NMR spectrum (CDCl3, 400 MHz):
δ 7.44 m (8H, HAr), 5.39 m (2H, HCp), 5.22 m (2H,
HCp), 4.40 (dd, 2H, HCp). ESI mass spectrum: m/z 355
[M+ – Cl–].
Oxidation of sulfides by hydrogen peroxide in
the presence of the Nb5+ indenyl peroxo complexes.
Under argon, to a solution of 0.8 mmol of sulfide in
5 ml of CH3CN was added 0.066 ml of 37% H2O2 and
0.008 mmol of a Nb5+ peroxo complex. The reaction
mixture was kept for 4 h at 70°C, and then cooled.
Excess hydrogen peroxide was neutralized by adding
sodium thiosulfate, the solution was dried over
Na2SO4, filtered and evaporated. The reaction products
were analyzed by GLC.
ACKNOWLEDGMENTS
Compound II. 1H NMR spectrum (CDCl3, 400 MHz):
δ 7.84 m (4H, HAr), 7.74 d (2H, HAr), 7.54 d (2H, HAr),
7.58 m (4H, HAr), 7.00 m (2H, HAr), 6.81 m (4H, HCp),
5.56 s (2H, HCp), 5.01 s ( 2H, HCp). ESI mass
spectrum: m/z 507 [M+ – Cl–].
Compound III. 1H NMR spectrum (CDCl3, 400 MHz):
δ = 7.69 m (4H, HAr), 7.52 m (4H, HAr), 7.52 m (4H,
HAr), 4.20 m (4H, HCp) 3.33 s (4H, CH2). ESI mass
spectrum: m/z 381 [M+ – Cl–].
Compound IV. 1H NMR spectrum (CDCl3, 400 MHz):
δ 7.22 s (2H, HAr), 7.16 s (2H, HAr), 5.25 d (2H, HSr),
5.02 d (2H, HCp) 4.35 m (2H, HCp), 3.90 s (3H, CH3),
3.89 s (3H, CH3).
This work was conducted as a part of the Federal
target program “Scientific and scientific-pedagogical
staff of innovative Russia”, supported by the Russian
Foundation for Basic Research (grant no. 09-03-00108).
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