Heterogeneous and homogeneous hydrogenation of styrene and stilbene chromium tricarbonyl complexes

Article abstract of DOI:10.1007/s11172-007-0008-1

Heterogeneous hydrogenation of the styrene and stilbene chromium tricarbonyl complexes by molecular hydrogen on skeletal nickel and palladium on carbon as catalysts was studied. As compared to styrene and stilbene, their arene chromium tricarbonyl analogs

Full text of DOI:10.1007/s11172-007-0008-1

Russian Chemical Bulletin, International Edition, Vol. 56, No. 1, pp. 45—48, January, 2007
45
Heterogeneous and homogeneous hydrogenation of styrene
and stilbene chromium tricarbonyl complexes
A. N. Artemov,ꢀ E. V. Sazonova, and D. S. Lomakin
Research Institute of Chemistry, N. I. Lobachevsky Nizhnii Novgorod State University,
Building 5, 23 prosp. Gagarina, 603950 Nizhnii Novgorod, Russian Federation.
Fax: +7 (831) 265 7343. Eꢀmail: ilis@uic.nnov.ru
Heterogeneous hydrogenation of the styrene and stilbene chromium tricarbonyl complexes
by molecular hydrogen on skeletal nickel and palladium on carbon as catalysts was studied. As
compared to styrene and stilbene, their arene chromium tricarbonyl analogs are hydrogenated
considerably more slowly, which is related, most likely, to strong adsorption of the πꢀcomꢀ
plexes on the catalyst surface. For the homogeneous hydrogenation of these complexes using
6
a H2PtCl6—SnCl2—LiBr system, styrene and η ꢀstyrene chromium tricarbonyl are reduced
with a high rate, whereas stilbene and its chromium tricarbonyl complex are hydrogenated very
slowly. A possibility of reduction of the unsaturated arene chromium tricarbonyl complexes by
sodium borohydride in the presence of cobalt(II) chloride as a catalyst was shown.
Key words: arene chromium tricarbonyl complexes, homogeneous and heterogeneous hydroꢀ
genation, catalysis, Raney nickel, palladium on carbon, hexachloroplatinic acid, cobalt(II)
chloride, sodium borohydride.
Reactivity of the double bond in the arene chromium
tricarbonyl complexes of alkenes is insufficiently studꢀ
ied.1 In particular, the effect of the chromiumtricarbonyl
group on the properties of the double bond in the
αꢀposition toward the πꢀbonded arene cycle remains unꢀ
clear. The approach to this question can be found in the
study of the reaction behavior of the double bond in charꢀ
acteristic reactions of alkenes, for instance, in hydrogeꢀ
nation. The reduction with hydrogen in the presence of
catalysts occurs rapidly, the hydrogenation procedure is
simple, and few byꢀproducts are formed.2
double bond activation due to conjugation with the pheꢀ
nyl rings. However, compared to the nonꢀcoordinated
compounds (Fig. 1), the arene chromium tricarbonyl comꢀ
plexes are hydrogenated with a low initial rate, which is
probably associated with the strong adsorption of the
πꢀcomplexes on the catalyst surface. To confirm this asꢀ
sumption, we carried out experiments with different addiꢀ
tives introduced into the reaction mixture. The additives
were benzene, chromium hexacarbonyl, bis(η6ꢀbenꢀ
zene)chromium, and η6ꢀtoluene chromium tricarbonyl,
i.e., compounds containing the same structural fragments
as styrene and stilbene chromium tricarbonyls.
As can be seen from the presented curves (Fig. 2),
benzene and bis(η6ꢀbenzene)chromium exert a very weak
effect on the hydrogenation of octꢀ1ꢀene and stilbene,
respectively, whereas the chromium carbonyl complexes
substantially retard the hydrogenation of octꢀ1ꢀene. Esꢀ
pecially strong retardation is observed for additives of the
arene chromium tricarbonyl complexes, probably, due to
their stronger chemisorption on the catalyst surface.
This is also favored by the results of competitive hydroꢀ
genation of stilbene and η6ꢀstilbene chromium tricarbonyl.
As can be seen from the data in Fig. 3, when the solution
simultaneously contains equimolar amounts of these comꢀ
pounds, η6ꢀstilbene chromium tricarbonyl is predomiꢀ
nantly activated, whereas stilbene is reduced with hydroꢀ
gen much more slowly. It can be assumed that the arene
chromium tricarbonyl complexes occupy the most part of
the active catalyst surface, while the free stilbene ligands
Styrene and stilbene chromium tricarbonyl πꢀcomꢀ
plexes of the general formula R—CH=CH—PhCr(CO)3
(R = H, Ph) were chosen for the study. These compounds
were reduced in a setup for alkene hydrogenation with
hydrogen3 at room temperature. The catalysts were skelꢀ
etal nickel,4 palladium on carbon,5 the platinum—tin
catalyst for homogeneous hydrogenation,6 and
a
CoCl2—NaBH4 system7 capable of reducing alkene withꢀ
out use of molecular hydrogen.
The results obtained for the hydrogenation of nonꢀ
coordinated compounds on the heterogeneous catalysts
(Ni, Pd/C) agree with literature data.8,9 The hydrogenaꢀ
tion reactions of free ligands are characterized by a linear
dependence of the conversion on the duration of the proꢀ
cess in the initial regions, although their slope ratios,
halfꢀlives, and reaction rates are different. The halfꢀlives
in the series octꢀ1ꢀene < styrene < stilbene are related as
1 : 3 : 7. This order of changing the reactivity is caused by
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 44—47, January, 2007.
1066ꢀ5285/07/5601ꢀ0045 © 2007 Springer Science+Business Media, Inc.

46
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Artemov et al.
С (%)
С (%)
100
80
60
40
20
100
80
60
40
20
a
2
1
2
3
1
4
5
t/min
3
0
100
200
300
t/min
Fig. 3. Dependence of the conversion (C) of the components of
an equimolar mixture of stilbene (1) and stilbene chromium
tricarbonyl (2) on the duration of the reaction on the catalyst
palladium on carbon; Т = 18.5 °C.
0
10
20
30
С (%)
100
80
60
40
20
b
2
1
To avoid chemisorption of the arene chromium triꢀ
carbonyl complexes on the solid catalyst surface, we carꢀ
ried out the homogeneous hydrogenation of free and coꢀ
ordinated olefins using the H2PtCl6—SnCl2 system6 by a
procedure proposed10 for the homogeneous hydrogenaꢀ
tion of Dewar hexamethylbenzene.
4
5
For homogeneous catalytic alkene hydrogenation on
the platinum—tin mixed catalyst, hydrogen addition difꢀ
fers strongly from that observed in the case of heteroꢀ
geneous hydrogenation. As can be seen from the data in
Fig. 4, a, styrene is characterized by the highest hydrogeꢀ
nation rate, whereas stilbene is hydrogenated very slowly.
This can be explained by the fact that the hydrogenation
on the heterogeneous catalysts involves adsorption of
hydrogen and olefin on a number of catalytic sites and
steric factors are insignificant. For homogeneous catalysis
hydrogen and olefin are accommodated on one transition
metal atom. The stilbene ligand containing two bulky
phenyl groups represents considerable steric hindrance
for stilbene coordination on a metal atom, which cerꢀ
tainly reflects the process rate.
η6ꢀStyrene chromium tricarbonyl is rapidly hydroꢀ
genated on this catalyst. The reaction affords ethylꢀ
benzene chromium tricarbonyl, which was isolated from
the reaction mixture in 87% yield. Its physicochemiꢀ
cal characteristics (melting point, IR spectrum, etc.)
correspond completely to the substance prepared by
us using the direct synthesis from ethylbenzene and
chromium hexacarbonyl. All attempts to hydrogenate
η6ꢀstilbene chromium tricarbonyl on this catalytic system
failed.
0
10
20
30
t/min
Fig. 1. Dependence of the conversion (C) of olefins and the
arene chromium tricarbonyl complexes on the duration of the
reaction on the catalysts Raney nickel (a) and palladium on
carbon (b): stilbene (1), styrene (2), octꢀ1ꢀene (3), stilbene chroꢀ
mium tricarbonyl (4), and styrene chromium tricarbonyl (5);
Т = 18.5 °C.
С (%)
80
1
2
60
40
3
20
4
0
10
20
30
t/min
Fig. 2. Effect of additives on the conversion of olefins on the
catalyst palladium on carbon: stilbene + bis(benzene)chroꢀ
mium (1), octꢀ1ꢀene + benzene (2), octꢀ1ꢀene + chromium
hexacarbonyl (3), and octꢀ1ꢀene + toluene chromium triꢀ
carbonyl (4); Т = 18.5 °C.
We hydrogenated the unsaturated arene chromium
tricarbonyl complexes using hydride inorganic reducing
agents. It is known,7 for example, that the complex obꢀ
tained from cobalt(II) salts and sodium borohydride easily
reduces alkenes in a high yield and with a very high selecꢀ
tivity.
are hydrogenated on a few number of remaining active
sites.

Hydrogenation of arene chromium tricarbonyls
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
47
С (%)
Experimental
100
1
2
Synthesis of organometallic compounds and all procedures
with them were carried out under purified argon. Styrene, octꢀ
1ꢀene, and stilbene were commercially purchased (analytical
3
80
60
40
20
a
6
purity grade). η ꢀStyrene chromium tricarbonyl was prepared
6
by the Rausch method,11 and η ꢀstilbene chromium tricarbonyl
was synthesized by the direct reaction of stilbene with chroꢀ
mium hexacarbonyl.12 All solvents used were dried and purified
directly before use. The catalysts, viz., skeletal nickel,13 pallaꢀ
dium on carbon,14 and platinum—tin for homogeneous hydroꢀ
genation,10 were prepared according to earlier published proceꢀ
dures.
Compounds were hydrogenated in a setup described for alkꢀ
ene reduction with hydrogen.4 Gas chromatographic analysis
was carried out on a Tsvetꢀ500 M instrument with a dumped
packing column (15% Apiezon L on Chromaton NꢀAWꢀDMCS,
4
0
20
40
60
80
100 t/min
column length 2 m, rate of the carrier gas (helium) 26 mL min–1
flameꢀionization detector).
,
С (%)
Heterogeneous hydrogenation. The catalyst (0.1 g, Raney Ni
or Pd/C) and ethyl acetate (25 mL) were introduced under
argon into a reaction flask equipped with a magnetic stirrer and
injection of gaseous hydrogen. The reaction mixture was mainꢀ
tained at a constant temperature for 15 min purging hydrogen
from the Kipp apparatus, then the magnetic stirrer was switched
on, and an unsaturated compound (1.0 mmol) dissolved in ethyl
acetate (10 mL) was syringed into the flask through the side
neck. The reaction course was monitored by the volume of abꢀ
sorbed hydrogen measured with a gas burette. These data reꢀ
ferred to normal conditions were used to calculate the converꢀ
sion and yields of the reaction products. The reaction mixture
obtained after hydrogenation was filtered, and its composition
was determined by gas chromatography. Before analysis the arene
chromium tricarbonyl complexes were decomplexated by crysꢀ
talline iodine. The hydrogenation products were identified by
retention times in the chromatographic column.
80
60
40
20
1
2
3
b
4
0
100
200
300
t/min
Fig. 4. Dependence of the conversion (C) of olefins and the arene
chromium tricarbonyl complexes on the H2PtCl6—SnCl2—LiBr
mixed catalyst (a) and CoCl2—NaBH4 system (b) on the duraꢀ
tion of the reaction: styrene (1), octꢀ1ꢀene (2), styrene chroꢀ
mium tricarbonyl (3), and stilbene (4); Т = 18.5 °C.
Homogeneous hydrogenation on the Pt/Sn catalyst. To preꢀ
pare a solution of the catalyst, H2PtCl6•6H2O (0.13 g,
0.25 mmol), LiBr (0.39 g, 0.45 mmol), and ethyl acetate (10 mL)
were introduced under argon into a roundꢀbottom threeꢀnecked
flask equipped with a magnetic stirrer, a dropping funnel, and
a tube for gas injection. Then a solution of tin chloride (0.29 g,
1.5 mmol) in ethyl acetate (15 mL) was added dropwise
with stirring from the dropping funnel. The solution turned
stramineousꢀcolored and then redꢀcolored after the flask was
filled with molecular hydrogen. Stirring was continued for 1 h,
and then olefin (1.0 mmol) was syringed into the catalyst soluꢀ
tion. The reaction course was monitored by the amount of abꢀ
sorbed hydrogen. After the end of reduction, the reaction mixꢀ
ture was hydrolyzed with 30 mL of 2 M HCl, and the organic
layer was separated, washed with water, dried, evaporated, and
analyzed by gas chromatography. In the case of the unsaturated
Our studies showed that the CoCl2—NaBH4 system in
isopropyl alcohol (Fig. 4, b) is most efficient for the hydroꢀ
genation of styrene, octꢀ1ꢀene, and η6ꢀstyrene chromium
tricarbonyl, whereas sterically hindered stilbene is hydroꢀ
genated very slowly (3% dibenzyl are formed for 5 h), and
η6ꢀstilbene chromium tricarbonyl is not virtually reduced
under these conditions.
Despite some restrictions, all the three procedures used
by us can be applied for the hydrogenation of the unsaturꢀ
ated arene chromium tricarbonyl complexes. The hydroꢀ
genation on the heterogeneous metallic catalysts occurs
very slowly, whereas homogeneous hydrogenation, alꢀ
though being rather rapid in the case of terminal olefins,
is hardly suitable for the reduction of olefins with steriꢀ
cally hindered double bonds.
6
arene chromium tricarbonyl complexes, viz., η ꢀstyrene and
6
η ꢀstilbene chromium tricarbonyls, the evaporation of the orꢀ
6
ganic layer and sublimation in vacuo gave η ꢀethylbenzene chroꢀ
mium tricarbonyl (69.5% yield, m.p. 47 °C (cf. Ref. 15: m.p.
Thus, the most convenient procedure for the hydrogeꢀ
nation of η6ꢀstyrene chromium tricarbonyl is its homoꢀ
geneous reduction with molecular hydrogen on the
H2PtCl6—SnCl2—LiBr complex system.
6
48—49 °C)) and η ꢀdibenzyl chromium tricarbonyl (73.1% yield,
m.p. 97.5 °C (cf. Ref. 16: m.p. 98—100 °C)), respectively.
Reduction by the CoCl2—NaBH4 system. A solution of
NaBH4 (0.76 g, 20 mmol) in isopropyl alcohol (10 mL) was

48
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Artemov et al.
introduced into a roundꢀbottom threeꢀnecked flask equipped
with a magnetic stirrer, a dropping funnel, and a tube for gas
injection. A solution of CoCl2 (0.26 g, 10 mmol) in the same
solvent was added from a dropping funnel to this solution for 1 h
at ~20 °C under argon. Upon mixing the reactants, the solution
darkened and molecular hydrogen evolved. Stirring was conꢀ
tinued for 1 h, and the solution turned black and a precipitate
formed. Then alkene under study (2.0 mmol) was introduced into
the vigorously stirred mixture under argon with a microsyringe
through the silicone plug. To monitor the course of the reaction,
samples were taken at different stages and analyzed by chromaꢀ
tography. After the end of reduction (3—7 h), the reaction mixꢀ
ture was poured into 3 M HCl. The aqueous layer was extracted
with ether. The ethereal layer was washed with water and dried
above anhydrous Na2SO4. After ether was evaporated, the reꢀ
sulting product was identified.
[Skeletal Catalysts, Their Properties and Application in Orꢀ
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Received October 7, 2006;
in revised form December 26, 2006