Coupled hydrogenation of alkenes and arenes in the presence of catalytic systems based on cobalt Bis(acetylacetonate) and tributylphosphine

Article abstract of DOI:10.1134/S0023158413040216

The effect of the composition of the catalytic systems based on Co(acac)₂ and tertiary phosphines on the activity and efficiency of cobalt catalysts in the coupled hydrogenation of alkenes and arenes is reported. The process occurs in the presence of cobalt catalysts formed under the action of both organoaluminum compounds and tert-butoxy derivatives of complex aluminum hydrides. NMR and IR spectroscopic methods show that the interaction of the components of the catalytic systems yields mono- and/or trihydrido cobalt phosphine complexes, whose composition depends on the nature of the reducing agent and gas atmosphere. The homogeneous character of the process is hypothesized. The most probable schemes are proposed for the reaction mechanism, according to which the kinetic coupling of alkene (alkadiene) and arene hydrogenations is due to the fact that the reaction proceeds through a σ-alkyl or σ-alkenyl cobalt complex with two phosphorus-containing ligands.

Full text of DOI:10.1134/S0023158413040216

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2013, Vol. 54, No. 4, pp. 431–442. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © Yu.Yu. Titova, L.B. Belykh, G.V. Ratovskii, A.V. Rokhin, O.G. Soroka, F.K. Schmidt, 2013, published in Kinetika i Kataliz, 2013, Vol. 54, No. 4, pp. 453–465.
Coupled Hydrogenation of Alkenes and Arenes in the Presence
of Catalytic Systems Based on Cobalt Bis(acetylacetonate)
and Tributylphosphine
Yu. Yu. Titova, L. B. Belykh, G. V. Ratovskii, A. V. Rokhin, O. G. Soroka, and F. K. Schmidt*
Irkutsk State University, Irkutsk, 664033 Russia
*eꢀmail: fkschmidt@chem.isu.ru
Received October 3, 2012
Abstract—The effect of the composition of the catalytic systems based on Co(acac)2 and tertiary phosphines
on the activity and efficiency of cobalt catalysts in the coupled hydrogenation of alkenes and arenes is
reported. The process occurs in the presence of cobalt catalysts formed under the action of both organoaluꢀ
minum compounds and tertꢀbutoxy derivatives of complex aluminum hydrides. NMR and IR spectroscopic
methods show that the interaction of the components of the catalytic systems yields monoꢀ and/or trihydrido
cobalt phosphine complexes, whose composition depends on the nature of the reducing agent and gas atmoꢀ
sphere. The homogeneous character of the process is hypothesized. The most probable schemes are proposed
for the reaction mechanism, according to which the kinetic coupling of alkene (alkadiene) and arene hydroꢀ
genations is due to the fact that the reaction proceeds through a
phosphorusꢀcontaining ligands.
σ
ꢀalkyl or ꢀalkenyl cobalt complex with two
σ
DOI: 10.1134/S0023158413040216
The catalytic hydrogenation of monoꢀ and polycyꢀ lacetonate), the hydrogenation of alkenes, for examꢀ
clic aromatic compounds is practically and theoretiꢀ ple, styrene, is accompanied by benzene reduction to
cally significant for the development of catalysis theꢀ cyclohexene and cyclohexane. Benzene hydrogenaꢀ
ory. The hydrogenation of benzene to cyclohexane and tion stopped after the conversion of the entire the alkꢀ
that of phenol to cyclohexanone are among the most ene to the alkane. The simultaneous hydrogenation of
important largeꢀscale industrial processes [1]. The styrene and benzene resumed after the introduction of
partial hydrogenation of benzene to cyclohexene and a new portion of the alkene into the catalytic solution.
the reduction of aromatic polymers, such as polystyꢀ That is why the process was accepted to be coupled.
rene, to poly(cyclohexaethylene) are equally signifiꢀ
cant, because the products of polymer hydrogenation
acquire new thermal, oxidation, and optical properties
[2]. The group VIII metals are usually used as catalysts
in the hydrogenation of aromatic compounds. These
metals can conventionally be arranged in the following
order according to their activity in the hydrogenation
of monocyclic arenes: Rh > Ru > Pt > Ni > Pd > Co
[2]. The Raney catalyst and Rh/Al2O3 are the most
popular.
The nature of the phosphorusꢀcontaining ligands
plays the decisive role in the coupled hydrogenation of
alkenes and arenes. The most active cobalt catalysts
are based on trialkylphosphines. However, the
replacement of only one alkyl radical by phenyl in the
tertiary phosphine sharply decreases the rate of benꢀ
zene hydrogenation. No benzene hydrogenation was
observed on the cobalt catalysts formed in the presꢀ
ence of PPh2Bu or PPh3, whereas styrene was reduced
to ethylbenzene at a high rate [10].
The study of arene hydrogenation in the presence
Researchers are still interested in arene hydrogenaꢀ
of homogeneous and nanosized catalysts based on tion [11–13]. Along with the necessity of enhancing
transition metal complexes was started in the 1960s the efficiency and thermal stability of arene hydrogeꢀ
[3–5]. There has been a series of studies on the hydroꢀ nation catalysts, the problem of discriminating
genation of benzene in the presence of cobalt
complexes,
3ꢀС3Н5СоL3, where L is phosphine or cies remains the key issue. The study of the conversion
phosphite [6–8]. In spite of the comparatively low activꢀ products of the metalꢀcontaining precursors by transꢀ
ity of this catalyst, which was ~2 mission electron microscopy demonstrated that most
10–3 (mol С6Н6
(gꢀatom Со)–1 min–1 at 25
and a hydrogen pressure of the arene hydrogenation catalysts based on the
of 1 atm, the results obtained were very significant for rhodium (RhCl3 3H2O, [RhCl(diene)]2) or rutheꢀ
elucidating the mechanism of benzene hydrogenation. nium (RuCl3 3H2O) compounds in combination with
πꢀallyl hypotheses about the nature of catalytically active speꢀ
η
×
)
×
°С
⋅
⋅
It was discovered [9, 10] that, in the presence of the various quaternary ammonium salts, which were iniꢀ
Co(acac)2–PВu3–AlEt3 system (where асас = acetyꢀ tially claimed to be homogeneous, turned out to be
431

432
TITOVA et al.
metal nanoclusters 2–6 nm in diameter [2]. At the solvent) was characterized by the following parameꢀ
same time, the composition of products of the reacꢀ ters:
(СН3) = 1.22 ppm (t, 3H, 1J = 8.24 Hz),
tion between the components of Ziegler catalytic sysꢀ
(СН2) = 0.45 ppm (q, 2H, 1J = 8.24 Hz). Triethylaꢀ
δ
δ
tems was experimentally found to depend substantially luminum was stored in a sealed ampule under argon.
on the proportions of these components, on the reacꢀ Solutions of AlEt3 in hexane or octane were prepared
tion conditions, on the nature of the solvent, on the in a Schlenk flask under argon. The concentration of
presence of moisture and other impurities in the reacꢀ AlEt3 in the solution was determined volumetrically by
tion mixture, and on the introduction of various modꢀ decomposing an aliquot of the solution with water.
ifiers [14, 15]. Depending on the process conditions,
Diethylaluminum acetylacetonate was obtained by
either metal nanoclusters, or monoꢀ or polynuclear
the dropwise addition of an acetylacetonate (1 mmol)
complexes of transition metals in different oxidation
solution in benzene to a stirred solution of triethylaluꢀ
states can be formed. Most researchers agree that the
minum (1.1 mmol). 1H NMR (C6H6):
δ
= 4.95 (s, 1
Н
=
,
reactions of the acetylacetonate complexes of Group
VIII metals (Fe, Co, Ni, Pd, Pt) with trialkylalumiꢀ
num yield nanoparticles stabilized by AlR2(аcac) and
AlR3 [16–19]. The redox process between MXn and
CH), 1.61 (s, 6H, CH3), 0.53 (q, 4H, CH2Al, 1J
8.24 Hz), 1.54 (t, 6H, 1J = 8.24 Hz) ppm.
С6Н5СН2MgCl was synthesized using a standard
AlR3 (or AlR2(OR)) in the presence of a tertiary phosꢀ procedure by reacting an alkyl halide with magnesium
phine is one of the methods for the synthesis of the in diethyl ether [28]. Its concentration was determined
hydrido [20, 21], alkyl [22, 23], and nitrogenꢀcontainꢀ by titration of an aliquot with a 0.1 M Н2SO4 solution.
ing phosphine complexes of transition metals in differꢀ
LiAlH(tert
ꢀ
BuO)3 was synthesized by the alcoholyꢀ
for 48 h
ent oxidation states [24] and, at PR3 : M < 2, nanoparꢀ
ticles [18]. The coupled hydrogenation of arenes and
alkenes in the presence of the cobalt catalysts has been
insufficiently studied to date, and the issue of its
nature (homogeneous or nanosized) remains unclear.
The purpose of this work is to study the effect of the
composition of the Co(acac)2–PR3–Red catalytic
systems (PR3 = PBu3, PPh3; Red = reducing agent)
and the nature of the conversion products of Co(II) on
the coupled hydrogenation of alkenes and arenes in
order to discriminate hypotheses about the nature of
catalysis in the systems and to establish the main
causes the influence of tertiary phosphines on the
properties of the cobalt catalysts.
sis of LiAlH4 with tertꢀbutanol in THF at 30 С
°
[29] at a reactant molar ratio of tertꢀBuOH : LiAlH4 = 3.
The course of the reaction was monitored by 27Al
NMR. The 27Al NMR spectrum (THF as the solvent)
exhibited a singlet at = 78 ppm.
δ
Tertiary phosphines (PPh3, PBu3) (SigmaꢀAldrich)
were used in experiments. Triphenylphosphine was addiꢀ
tionally recrystallized from ethanol. Its 31P NMR specꢀ
trum (in C6H6) contains a singlet at
tylphosphine was distilled in vacuo (100
31P NMR spectrum (in C6H6) exhibited a singlet at
δ
= –6 ppm. Tribuꢀ
°
С/8 Torr). Its
δ
= –32 ppm.
The substrates were hydrogenated in a glass temꢀ
peratureꢀcontrolled shaker at 30 С and an initial
°
hydrogen pressure of 2 atm (0.20265 MPa) in the presꢀ
ence of a catalytic system formed in situ. A solution of
PBu3 in benzene (РBu3 : Co = 3) and a solution of the
EXPERIMENTAL
The solvents (benzene, toluene, and tetrahydrofuꢀ
ran (THF)), substrate (styrene, hexꢀ1ꢀene, isoprene,
and butadiene), and reactant (tertꢀbutanol) were puriꢀ
fied according to standard procedures used for orgaꢀ
nometallic compounds [25]. For deeper drying, benꢀ
zene and toluene were additionally distilled from
LiAlH4 using a fractionation column, and they were
stored under argon in sealed ampules over molecular
sieve 4A. After the removal of peroxides, THF was sucꢀ
cessively distilled from sodium, LiAlH4, and benzopheꢀ
none ketyl and was also kept under argon in sealed
ampules. The water concentration in benzene was meaꢀ
reducing agent (AlEt3, AlEt2(асас), С6Н5СН2MgCl
or LiAlH(tert BuO)3) were successively added to a
solution of Со(аcac)2 (from
10–4 to 10–4 mol in
,
ꢀ
2
×
5
×
10 mL of benzene or THF). The Red : Co ratio was
varied between 2 and 8 (Red = AlEt3) or between 2 and
20 (Red = LiAlH(tertꢀBuO)3). The reaction mixture
was stirred for 2–3 min, and styrene (1 mL, 8.7 mmol)
or another alkene (alkadiene) was then introduced.
The total volume of the solution of was 12 mL. Thereꢀ
after, the reactor was sealed with a Teflon stopper with
a rubber septum (for sampling), an excess hydrogen
pressure (1 atm) was produced, and hydrogenation
was carried out. The course of the reaction was moniꢀ
tored by measuring the pressure in the system with a
manometer and analyzing the products by gas liquid
chromatography on a Chromꢀ5 chromatograph
(Czech Republic) with a flameꢀionization detector
using nitrogen as the carrier gas. The packed column,
3.6 m in length and 3 mm in diameter, contained the
sured by Fischer’s method [26] to be 1.1
×
10–3 mol/L,
and that in THF was 1.6
×
10–3 mol/L.
The synthesis of Сo(acac)2 was carried out via a
standard procedure [27]. The resulting pale pink preꢀ
cipitate was dried in vacuo (45°C/10 Torr), recrystalꢀ
lized from toluene, and sublimed at 140–150
pressure of 1–2 Torr. According to thermoanalytical
data, the water content of the complex was 1.12%.
°С
and a
Triethylaluminum was distilled in vacuo (48– Carbowax 20M phase. The catalyzate composition
49°
С/1 Torr). The 1H NMR spectrum of AlEt3
(
С6D6 was determined using the internal standard method.
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COUPLED HYDROGENATION OF ALKENES AND ARENES
433
Table 1. Coupled hydrogenation of benzene and alkenes in the presence of the cobalt catalysts
Product ratio
C6H5C2H5
Amount of
C6H6 convertꢀ
ed, mmol
Amountof
alkene,
mmol
C6H10 formation
selectivity, %
Alkened
Catalytic system
Red : Co
C6H10 + C6H12
17.40.
17.40
17.40
8.70
Co(acac)2–Bu3–AlEt3
Co(acac)2–Bu3– AlH(
4
4
4
4
styrene
styrene
styrene
styrene
2.25
2.70
6.78
8.40
7.7
6.5
2.6
1.1
26.0
22.1
27.0
66.5
i
ꢀC4H9)2
Co(acac)2–PBu3– Et2Al(acac)
Co(acac)2–PBu3–P(ОВu3)2–AlEt3
Co(acac)2–Bu3–C6H5CH2MgClb
a
6
6
styrene
styrene
6.87
8.70
2.60
3.80
2.7
2.3
18.0
13.2
Co(acac)2–Bu3– C6H5CH2MgClc
Co(acac)2–PBu3–AlEt3
4
4
4
hexꢀ1ꢀene
butadiene
isoprene
5.40
0.80
5.10
0.70
0.60
7.7
1.3
7.8
5.3
7.5
Co(acac)2–PBu3–AlEt3
Co(acac)2–PBu3–AlEt3
0.65
12.3
–2
–2
Note:
C
= 1 × 10 mol/L, PR : Co = 3; solution volume, 20 mL; amount of C H , 5.03 × 10 mol; heptane solvent; PH = 2 atm, 30°C.
Co 3 6 6
2
a
PBu : P(OBu ) = 0.5.
3
3
b
T
T
= 20°C.
c
= 7°C.
d
Conversion of alkene and alkadiene is 100%.
ThereactionofCo(acac)2 with AlEt3 or another reducꢀ formed under the hydrogenation conditions from the
ing agent (LiAlH(tert BuO)3, AlH( ꢀBu)2) at different paramagnetic cobalt(0) complexes [19]. If Co(acac)2 is
ꢀ
i
ratios of the starting components in the presence of a
tertiary phosphine was studied in an anhydrous deoxꢀ
ygenated hydrogen atmosphere. For example, a soluꢀ
tion (1 mL) of tributylphosphine (1.8 mmol) in benꢀ
zene was poured dropwise to a stirred solution of
Co(acac)2 (0.1542 g, 0.6 mmol) in benzene (4 mL).
reduced with triethylaluminum in the presence of
tributylphosphine (РBu3 : Со = 3), benzene hydrogeꢀ
nation occurs only over 4–5 min. This might seem to
be reasonable attributable to the rapid deactivation of
the catalyst. However, the hydrogenation of both styꢀ
rene and benzene resumes after styrene is introduced
into the Со(асас)2–РBu3–AlEt3 catalytic system. The
ratio of the hydrogenation products of unsaturated
hydrocarbon and benzene, catalytic activity, and proꢀ
ductivity depend on the nature of the reductant of the
cobalt precursors. When an organoaluminum comꢀ
pound is used as the Со(асас)2 reductant, the benzene
conversion increases in the following order: AlEt3 <
The resulting solution was cooled to
tion (1 mL) of a reducing agent in benzene (Red = AlEt3,
AlH( ꢀBu)3) or in THF (Red = LiAlH(tert BuO)3) was
0
°
С, and a soluꢀ
i
ꢀ
added. The Red : Co molar ratio was varied between 2
and 4. The solution thus prepared, placed in an argonꢀ
purged and then sealed KBr cell with an optical path
length of 0.112 mm, was analyzed on an FTꢀ801 IR
spectrometer (Simex, Novosibirsk, Russia). NMR
studies were carried out in a similar way, using benꢀ
zeneꢀd6 or THFꢀd8 as the solvent. 1Н, 31Р, and 27Al
NMR spectra were recorded on a VXRꢀ500S pulse
spectrometer (Varian, United States). Chemical shifts
of 27Al signals were measured relative to an external
standard (0.1 mol/L Al(NO3)3 solution). The 31P
chemical shifts are presented relative to 85% phosphoꢀ
ric acid, with their positive values corresponding to
downfield shifts.
AlH(iꢀC4H9)2 < AlEt2(acac) (Table 1). In the presence
of the Со(асас)2–PВu3–AlEt2(acac) system, the turnꢀ
over frequency (TOF) in benzene hydrogenation calꢀ
culated from the rate of benzene conversion within the
initial 20 min (when the rate remains unchanged) is
three times higher than in the case of the catalyst conꢀ
taining AlEt3 as the reducing agent (Fig. 1). In addiꢀ
tion, the replacement of AlEt3 by AlEt2(acac) considꢀ
erably extends the stable service life of the catalyst. In
the presence of the Со(асас)2–PВu3–AlEt3 catalytic
system, the rate of benzene hydrogenation decreases
sharply upon the introduction of additional portions
of styrene (after the completion of the hydrogenation
of the previous portion), whereas the decrease is insigꢀ
nificant when AlEt2(acac) is used.
RESULTS AND DISCUSSION
It was earlier shown [30] that benzene is hydrogeꢀ
nated to cyclohexane in the presence of the
Co(acac)2–AlEt3 catalytic system under mild condiꢀ
tions (30°С,
= 2 atm). The reaction occurs on
PН
2
Thus, use of Et2Al(acac) as the reducing agent
cobalt nanoclusters 2–5 nm in diameter, which are affords a threefold increase in the benzene hydrogenaꢀ
KINETICS AND CATALYSIS Vol. 54
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434
TITOVA et al.
Amount of arene converted, mmol
TOF, min–1
0.5
3.0
2.5
2.0
1.5
1.0
0.5
1
0.4
0.3
0.2
0.1
1
2
2
0
1
2
3
4
0
2
4
6
8
Portion no.
РВu3 : Co
Fig. 1. Change in the turnover frequency of benzene
hydrogenation coupled with styrene hydrogenation in the
presence of the Со(асас) –3PBu –4Red catalytic sysꢀ
2
3
Fig. 2. Effect of the РBu : Co molar ratio on the amount
3
tems, where Red is (
1) AlEt (асас) and (2) AlEt , with
of (
their coupled hydrogenation with styrene in the presence
of the Со(асас) –PBu –AlEt catalytic system;
1) benzene and (2) toluene converted in the course of
2 3
successive introduction of 1ꢀmL portions of styrene.
C
=
Co
C =
Co
2
3
3
0.01 mol/L, Al : Co = 4, PH = 1.5 atm, 30°C.
2
0.01 mol/L, Al : Co = 4, 30°C.
tion rate in the process coupled with styrene hydrogeꢀ 35% for РBu3 : Со = 6. The optimum Red : Co ratio
nation, an enhancement of the stability of the that maximizes the rate of hydrogenation of both subꢀ
Со(асас)2–PВu3–AlEt2(acac) catalytic system comꢀ strates depends on the nature of the reducing agent. In
pared to the Со(асас)2–PВu3–AlEt3 system, and, as a the case of triethylaluminum, this ratio is 4 (Figs. 3a, 3b).
consequence, an increase in the TOF of the reaction. The benzene hydrogenation rate decreases sharply
However, Et2Al(acac) exerts almost no effect on the with a further increase in the Al : Co ratio, and only
cyclohexene formation selectivity. As follows from styrene is reduced at Al : Co = 8. An extremum in
Table 1, the Со(асас)2–РBu3–Red (Red = AlEt3, dependence of TOF on the reactant ratio (Red : Co)
AlH(iꢀC4H9)2, and AlEt2(acac)) catalytic systems are was earlier observed for Ziegler catalysts formed under
characterized by similar values of cyclohexene formaꢀ the action of trialkylaluminum in the absence of terꢀ
tion selectivity. If tributylphosphine is partially tiary phosphines [14, 15, 18]. The poisoning effect of
replaced by tributyl phosphate (in a ratio of 1 : 2), the excess AlEt3 can be due to several factors. On the one
selectivity increases to 66.5%, while the reaction rate hand, if metal nanoclusters form in the system considꢀ
decreases by a factor of 3. The increase in the selectivꢀ ered, as in the MXn–AlR3 system, the inhibiting effect
ity of the process with the sharp decrease in its rate is is most likely explained by the blocking of the active
not due to catalyst deactivation, because the converꢀ centers because of the formation of surface comꢀ
sion of benzene in this system is higher than in the pounds. On the other hand, since the coupled hydroꢀ
Со(асас)2–РBu3–AlEt3 system. Both the benzene genation of styrene and benzene occurs only in the
conversion and cyclohexene formation selectivity presence of tributylphosphine at РBu3 : Со
≥
2 and the
decrease when styrene is replaced by hexꢀ1ꢀene, isoꢀ Со(асас)2–PВu3–AlEt3 system contains both a Lewis
prene, or butadiene (Table 1).
acid and a Lewis base, the inhibiting effect of excess
triethylaluminum can be caused by the displacement
of tributylphosphine molecules from the coordination
sphere of the catalytically active cobalt complexes via
the interaction between PBu3 and AlEt3, which yields
an adduct and thau changes the nature of catalytic
activity carriers.
The РBu3 : Со and Red : Co molar ratios exert a
significant effect on the process considered. The couꢀ
pled hydrogenation of styrene and benzene in the
presence of the Со(асас)2–PВu3–Red catalytic sysꢀ
tems was studied at РBu3 : Со
Со < 2, only the hydrogenation of styrene to ethylbenꢀ
≥
2 (Fig. 2). At РBu3 :
zene occurs. This suggests that the complexes active in
This hypothesis is consistent with the experimental
coupled hydrogenation most likely contain two tribuꢀ results for the Со(асас)2–PВu3–LiAlH(tertꢀBuO)3
tylphosphine molecules in the coordination sphere of system (Table 2). The fundamental distinction
cobalt. Note that, as the РBu3 : Со molar ratio is between the cobalt catalysts, whose formation involves
increased, the selectivity of the Со(асас)2–PВu3– LiAlH(tertꢀBuO)3, and the organoaluminum comꢀ
AlEt3 catalytic system in benzene conversion to cycloꢀ pounds is that the reducing agent does not exert an
hexene increases from 25–26% for РBu3 : Со = 3 to inhibition effect up to Red : Co = 20. The total rate of
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COUPLED HYDROGENATION OF ALKENES AND ARENES
435
coupled styrene and benzene hydrogenation in the
presence of the Со(асас)2–PВu3–LiAlH(tert BuO)3
system increases sharply as the LiAlH(tert BuO)3 : Co
ratio increases from 1 to 5 and slowly increases as this
ratio further increases to 20 (Fig. 4). The average rate
(а)
0.24
0.16
0.08
0
ꢀ
3.0
ꢀ
2
1
of benzene hydrogenation is maximum at LiAlH(tert
ꢀ
1.5
0.6
BuO)3 : Co = 10.
The systems considered were studied by spectroꢀ
scopic methods in order to discriminate the hypotheꢀ
ses about the nature of catalytic activity carriers.
Cobalt bis(acetylacetonate) exists in benzene soluꢀ
tion as the tetranuclear association species
[Co(acac)2]4, which interacts with tributylphosphine
to form the Co(acac)2 (PBu3)2 complex [10]:
2
4
Al : Co
6
8
PBu
H3C
HC
CH3
CH
3
C O O C
Co
0.625
0.500
(b)
3.0
2.0
1/4[Co(acac2)]4 + 2PBu3
C O O C
PBu
H3C
CH3
3
.
2
0.375
0.250
0.125
The introduction of triethylaluminum into a
Co(acac)2–3PBu3 solution under argon changes the
color of the solution from blue to dark brown. The
brown color of the solutions for the reduction of tranꢀ
sition metal compounds in the Ziegler systems is an
indirect indication of the formation of metal nanoꢀ
1
1.0
1
clusters [2]. However, the H NMR spectrum of the
Co(acac)2–3PBu3–4AlEt3 reaction system contains a
2
4
Al : Co
6
8
resonance signal in the strong field at = –20.9 ppm
δ
2
(q, JP–H = 49 Hz). The appearance of the resonance
signal in this region is characteristic of transition metal
hydrido complexes, and the splitting of the signal indiꢀ
cates that the coordination sphere of cobalt includes
Fig. 3. Effect of the Red : Co molar ratio on the turnover
frequency of the coupled hydrogenation of ( ) benzene
and ( ) styrene in the presence of the Со(асас) –PBu –
Red catalytic system, where Red is (a) AlEt and (b)
1
2
2
3
1
3
three tributylphosphine ligands. The H NMR specꢀ
AlEt (асас)
;
C
= 0.01 mol/L, [styrene] = 0.45 mol/L,
2
Co
trum of the Co(acac)2–3PPh3–4AlEt3 reaction system,
with PBu3 replaced by PPh3, in argon also contains a
PВu
: Со = 3, PH = 1.5 atm, 30°C.
3
2
resonance signal in the strong field at = –19.8 ppm
δ
(q, 2JP–H = 51 Hz), indicating the formation of a cobalt
hydrido complex with three PPh3 ligands. The presꢀ
ence of a doublet with a chemical shift of 47 ppm
system exhibits a quartet at
δ
= –13.4 ppm, which is
transformed into a singlet upon broadband phosphoꢀ
rus decoupling, indicating that the proton interacts
with the three equivalent phosphorus nuclei. The resꢀ
onance signals in the strong field in the spectra of the
31
(d, 2JP–H = 50 Hz) in the P NMR spectrum of this
reaction system indicates the formation of a cobalt
monohydrido complex with three triphenylphosphine
Co(acac)2–3PR3–4LiAlH(tert
tems (R = Bu, Ph) appear at
= –13.6 ppm (q, 2JP–H
= –12.8 ppm
ꢀBuO)3 reaction sysꢀ
ligands. It should be mentioned that a similar chemiꢀ
2
δ
=
cal shift,
δ
= –19 ppm (q, JP–H = 50 Hz), was also
observed in the 1H NMR spectrum of the cobalt monoꢀ
hydrido complex Co(H)(N2)(PPh3)3 with a trigonal
bipyramid structure [31, 32]. At the same time, the
cobalt carbonyl hydrido complexes CoH(CO)(PR3)3
(R = Ph, PMePh2) with a similar structure, but with a
37 Hz) for tributylphosphine and at
δ
(q, 2JP–H = 45 Hz) for triphenylphosphine. It should
be mentioned that, in this case, the cobalt hydrido
complexes formed under the action of the aluminum
hydride derivatives have similar spectral characterisꢀ
strong electronꢀwithdrawing ligand (CO), are characterꢀ tics regardless of the nature of the phosphine ligand.
ized by a chemical shift of –13 ppm (q, 2JP–H = 50 Hz) in
The considerable difference between the chemical
shifts in the spectra of the cobalt hydrido complexes
the 1H NMR spectrum [33].
obtained under the action of triethylaluminum and a
hydride derivative (LiAlH(tert BuO)3 or AlH( ꢀBu)2
The spectrum changes if a hydride derivative
AlH( ꢀBu)2, LiAlH(tertꢀBuO)3) is used as the reducing
ꢀ
i
)
(
i
is possibly due to the different symmetries of the
resulting cobalt complexes. Cobalt phosphine monoꢀ
hydrido complexes (Co(H)(N2)(PR3)3 [22, 31],
agent for the cobalt(II) phosphine complexes: the resoꢀ
nance signals from the cobalt hydrido complexes appear
predominantly between –13 and –14 ppm. In particuꢀ
lar, the spectrum of the Co(acac)2–3PBu3–AlH(
i
ꢀBu)2 Co(H)(СН2=СН2)(dppe)2 [34], and Co(H)(PR3)4
KINETICS AND CATALYSIS Vol. 54
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436
TOF, min–1
TITOVA et al.
47 ppm) and in free triphenylphosphine (δ = –6 ppm).
Similar P NMR characteristics were also observed
for the Co(acac)2–4PPh3–Red reaction solution
31
10
1
(Red = LiAlH(tertꢀBuO)3 or AlH(iꢀBu)2). The interꢀ
8
action of the components was also studied under a
hydrogen atmosphere in order to bring the analysis
conditions closer to the real conditions of the catalytic
reaction. The reduction of Co(acac)2 by triethylalumiꢀ
num or diethylethoxyaluminum under hydrogen in
the presence of a tertiary phosphine (PPh3, PBu3) also
results in the formation of cobalt hydrido complexes.
Along with the resonance signal in the range from –20
to –21 ppm, the 1H NMR spectrum contains an unreꢀ
solved broad signal between –12.1 and –12.2 ppm.
The broadening of the 1H and 31P NMR signals can be
due to several factors, namely, the presence of paraꢀ
magnetic species, interꢀ or intramolecular exchange,
and the formation of a microheterogeneous system.
Since all of the signal undergo broadening, the main
causes are likely the interꢀ or intramolecular exchange
and the presence of paramagnetic cobalt complexes in
the solution.
6
4
2
4
3
2
0
5
10
15
20
Red : Co
Fig. 4. Initial rates of styrene and benzene hydrogenation
in the presence of the Co(acac) –PBu –LiAl(tert BuO)
3
catalytic system vs. LiAl(tert
ꢀ
2
3
ꢀBuO) : Co molar ratio upon
3
1
the successive introduction of the (
) first, (
2) second, (3)
third, and (
–2
4
) fourth 1ꢀmL portions of styrene;
С
=
Со
1.67 × 10 mol/L, PH = 2 atm, 30°С.
2
Thus, the NMR study of the Co(acac)2–PR3–Red
catalytic systems showed that they involve the formaꢀ
tion of cobalt hydrido complexes, whose nature
depends considerably on whether the reducing agent
contains the hydride ion. The interaction of the comꢀ
ponents of the catalytic systems was studied by IR
spectroscopy to gain additional information. Since the
gas atmosphere in the process (Ar, H2, N2) affects the
composition of the resulting cobalt complexes, the IR
spectra were recorded under different conditions.
Absorption bands at 1585, 1533, 1380, and 1296 cm–1,
which are characteristic of vibrations of the acac
ligand in AlEt2(асас), were detected in the spectrum of
the Co(acac)2–3PPh3–4AlEt3 reaction system under
argon 5 min after the mixing of the components [18, 19].
The absence of the characteristic absorption bands of
[23]) and a trihydrido complex (H3Co(PR3)3 [21]) are
known. Most of the hydrido complexes, whose strucꢀ
ture was proved by Xꢀray diffraction, were not characꢀ
terized previously by NMR because of their low soluꢀ
bility and the insufficient sensitivity of the spectromeꢀ
ters [34, 35]. The signals from the hydrides of the
H3Co(PPh3)3 complex synthesized using a standard
procedure [21] were detected in the 1H NMR spectrum
of a benzene solution at
NMR spectrum they occur at 68 ppm. The hydride ions
of the iron trihydrido complex PhBP( ꢀPr)3Fe(H)3
show themselves in the same spectral region (
12.4 ppm, q, 3JP–H = 7.3 Hz) [36].
δ
= –11.8 ppm, and in the 31P
i
δ
=
⎯
Note that the 31P NMR spectrum of the these ligands in the spectrum of Co(acac)2 (1592,
H3Co(PPh3)3 complex under argon contains the resoꢀ 1520, and 1259 cm–1) indicates their almost quantitaꢀ
nance signal at 68 ppm and also broad lowꢀintensity tive transfer from cobalt to aluminum. In addition to
signals corresponding to the phosphorus nuclei in the the absorption bands listed above, the IR spectrum
cobalt monohydrido complex with PPh3 ligands (
δ
=
also includes bands at 1484, 1434, 774, and 744 cm–1
Table 2. Coupled hydrogenation of benzene and styrene in the presence of the Co(acac)2–PBu3–LiAlH(tertꢀBuO)3 cataꢀ
lytic system
Amount of substrate converted, mmol
C6H5C2H5
Cyclohexene formation
selectivity, %
Red : Co
Product ratio
C6H6
C6H10 + C6H12
styrene
2.5
5
17.4
26.1.
34.8
68.0
52.2
1.3
2.2
3.3
3.2
2.5
13.9
18.5
8.8
7.5
5.3
5.7
11.7
10.5
21.6
21.0
10
15
20
–2
–2
Note:
C
= 2
×
10 mol/L; PBu : Co = 3; solution volume, 25 mL; amount of C H , 5.03
×
10 mol, heptane solvent; PH = 2 atm; 30
°C.
Co
3
6 6
2
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COUPLED HYDROGENATION OF ALKENES AND ARENES
437
corresponding to vibrations of the phenyl moiety in
Wave number
PPh3 and also weak bands similar in intensity at 2130
2150 2100 2050 2000 1950 1900 1850 1800 1750
and 2090 cm–1 (Fig. 5).
100
90
80
70
60
The presence of an absorption band at 2090 cm–1 in
the spectrum of the Co(acac)2–3PPh3–4AlEt3 reacꢀ
tion system formed in argon was unexpected, because
1
4
2
2130
3
3
1
the
N≡N stretching vibrations in the nitrogenꢀconꢀ
taining triphenylphosphine cobalt complexes show
themselves in this region [21, 23, 24, 31, 33]. A similar
spectrum was observed for the process under helium.
When helium is replaced by nitrogen, the absorption
band intensity at 2090 cm–1 in the IR spectrum of the
Co(acac)2–3PPh3–4AlEt3 reaction system formed
under helium (or argon)increases sharply (by a factor
of ~30) and the intensity of the band at 2130 cm–1 in
this spectrum decreases considerably. The reverse
replacement of nitrogen by helium or argon induces
opposite changes: the intensity of the band at
2090 cm–1 decreases by a factor of ~10 and that of the
band at 2130 cm–1 increases by a factor of ~6. The
intensity ratio of the bands at 2090 and 2130 cm–1
takes the initial value. The band at 1894 cm–1 also
appears in the spectrum (Fig. 5). Absorption bands at
2090 and 2130 cm–1 with an intensity ratio close to
2 were detected at the initial moment in the IR specꢀ
trum in experiments on the interaction of the compoꢀ
nents of the Co(acac)2–3PPh3–4AlEt3 catalytic sysꢀ
tem directly under argon. Within 1 h, the intensity of
the band at 2090 cm–1 increased by a factor of ~10 and
that of the band at 2130 cm–1 decreased by a factor of
~6 times. Within the next 2 h, the intensity of the
former band increased by a factor of 5 and that of the
latter decreased to zero. The replacement of nitrogen by
helium or argon was again accompanied by a decrease
in the intensity of the band at 2090 cm–1 and an increase
in the intensity of the band at 2130 cm–1 and the
appearance of an absorption band at 1890 cm–1.
2
2090
4
1894
Fig. 5. IR spectra of the of the Co(acac) –3PPh –4AlEt
3
2
3
reaction system in benzene recorded (
after the beginning of the reaction under an argon atmoꢀ
sphere, ( ) 30 min after argon was replaced by nitrogen,
and ( ) after another purging of the solution with argon;
= 0.10 mol/L, 25
1) 30 and (2) 15 min
3
4
C
°C.
Co
of the Co(Н)(N2)(PPh3)3 complex determined by Xꢀray
diffraction [31], one can suggest that the vibrations of
the
N N and Co–H bonds occur along one axis and are
≡
coupled to each other through the cobalt atom. The freꢀ
quencies of these vibrations are close. This suggests that
a joint vibration of the
N N and Co–H bonds appears,
≡
and the intensity of the corresponding band is mainly
determined by the change in the dipole moment of the
N
≡N
bond, since the integral intensity of the same band
in the spectrum of the Fe(N2)(H2)(PPh3)3 complex is
4.9
104 L mol–1 cm–2 [24].
Based on the aforesaid, we assigned the absorption
band at 2090 cm–1 to the joint vibration of the
and Co–H bonds, although its intensity and shape are
mainly determined by the bond vibration. In our
opinion, the splitting of this band into absorption
×
N N
≡
N≡N
The observed correlation between the spectroꢀ bands at 2100 and 2083 cm–1 [23, 31] is due to the sepꢀ
scopic data and the composition of the gas atmoꢀ aration of the joint vibration into two joint vibrations
sphere, with the relevant literature taken into account, under the action of the crystal field. These two joint
data, suggests the IR band assignment presented in vibrations may differ in the ratio of contributions
Table 3. The absorption band at 2090 cm–1 is usually introduced into them by a change in the
N≡
N
and
attributed to the stretching vibrations of the
N≡N
Co–H bond lengths. In addition, these changes may
bond of coordinated nitrogen in the cobalt complexes be cophasal or noncophasal. Note that, for the
with triphenylphosphine ligands Co(Н)(N2)(PPh3)3 Co(Н)(N2)(PPh3)3 complex synthesized according to a
[23, 35]. However, there is another opinion [31]. standard procedure under nitrogen [31], a weakly proꢀ
Although the band of the M–H bond was not detected nounced shoulder at 2130 cm–1 is detected along with
in the spectra of some hydrido complexes of transition the fairly intense band at 2090 cm–1 corresponding to
metals, the IR spectrum of a suspension of single crysꢀ the vibrations described above. Since the stretching
tals of the Co(Н)(N2)(PPh3)3 complex in Nujol exhibꢀ bands of the Co–H bond in the spectra of the
ited two intense bands at 2100 and 2085 cm–1, which Co(H)(PR3)4 cobalt monohydrido complexes (in the
were assigned to the stretching vibrations of the
N≡N and solid phase) shift to higher frequencies with a decreasꢀ
Co–H bonds [31]. The highꢀfrequency 100–200 cm–1 ing basicity of the phosphine ligand (νСо–Н in the specꢀ
shift of the Co–H band can be due to the transꢀ tra of Co(H)(Me2PPh)4 and Co(H)(MePPh2)4 are
arrangement of hydrogen with respect to nitrogen [31]. 1880 and 1958 cm–1, respectively) [37], we assigned
The bands at 2100 and 2083 cm–1 were assigned [23] to the band at 2130 cm–1 to the stretching vibrations of
the crystal effect, since only one band at 2088 cm–1 was the Со–Н bond in the monohydrido complex
detected in the spectrum of a solution of the Co(Н)(PPh3)3(Solv), where Solv is solvent. The
Co(Н)(N2)(PPh3)3 complex. Based from the structure numerical value of the frequency of the absorption
KINETICS AND CATALYSIS Vol. 54
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438
TITOVA et al.
Table 3. Assignment of absorption bands in the IR spectra of the Co(acac)2–3PR3–4AlEt3 systems
ν
N + Co–H
2090
, cm–1
Catalytic system
Gas atmosphere
Ar, He or N2
Assumed complex
N≡
Co–H
Co(acac)2–PPh3–4AlEt3
Co(H)(N2)(PPh3)3
Co(H)(PPh3)3(Solv)
[Co(H)(PPh3]n
2130
1894
Co(acac)2–3PBu3–4AlEt3
Co(acac)2–3PPh3–4AlEt3
Ar, He or N2
2045
2090
Co(H)(N2)(PBu3)3
Co(H)(PBu3)3(Solv)
[Co(H)(PBu3)3]n
2065
1884
H2
Co(H)(N2)(PPh3)3
Co(H)(PPh3)3(Solv)
[Co(H)(PPh3]n
2130
1894
1754
H3Co(PPh3)3
Co(acac)2–3PBu3–4AlEt3
H2
2045
Co(H)(N2)(PBu3)3
H3Co(PBu3)3
1717
1884
[Co(H)(PBu3)3]n
band at 1894 cm–1 and an increase in its intensity upon the 1800–2100 cm–1 range. This band is characteristic
the replacement of nitrogen by helium or argon allow of the cobalt monohydrido complex
one to assign this band to the stretching vibrations of Со(Н)(N2)(PBu3)3 [23]. During the purging of the
the Co–H bridging bond in the association species reaction solution with argon, the intensity of the
resulting
from
the
dissociation
of
the absorption band at 2045 cm–1 decreased, which was
accompanied by the appearance of a band at 2065 cm–1
in the highꢀfrequency region and a band at 1884 cm–1.
The replacement of argon by nitrogen or performing
the process under a nitrogen atmosphere increased the
intensity of the absorption band at 2045 cm–1 by a facꢀ
tor of 30. The opposite situation is observed for the
back replacement of nitrogen by argon (Fig. 6). Based
Co(Н)(N2)(PPh3)3 complex in the solution
Co(H)(N2)(PPh3)3
←⎯⎯→Co(H)(PPh3)3 ←⎯→1 n[Co(H)(PPh3)3]n.
−N2
Therefore, the reduction of Co(acac)2 by triethylaꢀ
luminum in the presence of triphenylphosphine in an
inert gas (helium, argon) or nitrogen atmosphere
affords the cobalt monohydrido complexes
Co(Н)(N2)(PPh3)3 and Co(Н)(PPh3)3(Solv) and the
association species [Co(Н)(PPh3)3]n. The ratio of
their concentrations is determined by the process conꢀ
ditions. It follows from the experimental data that,
under the conditions of the dominant formation of the
Co(Н)(PPh3)3(Solv) complex, the band at 2130 cm–1
is many times weaker than the band at 2090 cm–1 in
under the conditions of the dominant formation of the
Co(Н)(N2)(PPh3)3 complex. This confirms the deterꢀ
on the above considerations, we ascribed the absorpꢀ
tion band at 2045 cm–1 to the joint vibration of the
N≡N and Co–H bonds in the Со(Н)(N2)(PBu3)3 comꢀ
plex, and the band at 2065 cm–1 was assigned to the
Co–H stretching vibrations in the Со(Н)(PBu3)3(Solv)
monohydrido complex. The slight change in the
absorption band intensity at 1884 cm–1, depending on
the reaction conditions, indicates weak association
between the cobalt monohydrido tributylphosphine
complexes, as distinct from the triphenylphosphine
hydrido complexes (Fig. 6).
A band at 1754 cm–1 is present in the IR spectrum
mining role of the contribution from the
N N bond of the Co(acac)2–3PPh3–4AlEt3 reaction system in
≡
vibrations to the intensity of the band at 2090 cm–1. hydrogen along with the absorption bands at 2090,
The formation of the cobalt monohydrido complex 2130, and 1894 cm–1 corresponding to stretching
Co(Н)(N2)(PPh3)3 in argon or helium is probably due vibrations of the cobalt monohydrido complexes
to the presence of traces of nitrogen in these gases and Co(Н)(N2)(PPh3)3 and Co(Н)(PPh3)3(Solv) and their
on the walls of the reactor. The reduction of Co(acac)2 association species [Co(Н)(PPh3)3]n. The intensities of
by triethylaluminum in helium (or argon) in the presꢀ the bands at 2090, 2130, and 1894 cm–1 decrease and
ence of PBu3 results in the appearance of a weak that of the band at 1754 cm–1 increases during the proꢀ
absorption band at 2045 cm–1 (Fig. 6) in the IR specꢀ cess. In the system with tributylphosphine, the
trum of the solution of Co(acac)2–3PBu3–4AlEt3 in replacement of argon or helium by hydrogen exerts
KINETICS AND CATALYSIS Vol. 54
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COUPLED HYDROGENATION OF ALKENES AND ARENES
439
only an insignificant effect on the spectral characterisꢀ
Wave number
2100 2050 2000 1950 1900 1850 1800 1750
tics of the Co(acac)2–3PBu3–4AlEt3 system: an
absorption band of the Co–H stretching vibrations
appears at 2045 cm–1 and the intensity of the band at
1717 cm–1 increases slightly. It is known that the trihyꢀ
drido complex Н3Co(PPh3)3 can result from the substiꢀ
tution of hydrogen for nitrogen in the
Co(Н)(N2)(PPh3)3 monohydrido complex [23, 35]:
100
3
4
1
1
2
2045
2056
2065
90
3
2
4
80
1884
Co(H)(N2)(PPh3)3 + H2 ↔ H3Co(PPh3)3 + N2.
70
2045
The stretching vibrations of the Co–H bond in the
H3Co(PPh3)3 complex synthesized via the procedure
reported by Sacco and Rossi [21] show themselves in
the IR spectrum in Nujol at 1745 (s) and 1934 (m) cm–1,
which is in agreement with the literature [35]. In a
benzene solution of the H3Co(PPh3)3 complex, the
Co–H vibration is detected at 1754 cm–1. The second
band of the Co–H stretching vibrations is overlapped
with the band at 1945 cm–1 from overtones of benzene.
Fig. 6. IR spectra of the of the Co(acac) –3PBu –4AlEt
3
2
3
reaction system in benzene recorded (
1
) 20 min after the
beginning of the reaction under an argon atmosphere,
(
(
2
4
) after argon was replaced by nitrogen, and ( ) 30 and
3
) 120 min after the beginning of the second purging of
= 0.10 mol/L, 25
the solution with argon;
C
°C.
Co
Thus, the study of the Co(acac)2–3PR3–4Red catꢀ
alytic systems by IR and NMR spectroscopy showed
that the reactions of the components both in the presꢀ
ence of tributylphosphine and in the presence of triphꢀ
enylphosphine yield the cobalt monohydrido and/or
trihydrido complexes, whose composition depends on
the choice of tertiary phosphine, but is mainly deterꢀ
mined by the nature of the reducing agent and the gas
atmosphere in the reaction medium. The formation of
the cobalt hydrido phosphine complexes in the
Co(acac)2–3PR3–4Red systems radically distinꢀ
guishes them from the classical Ziegler systems, in
which metal nanoclusters form due to reduction. Of
course, the cobalt nanoclusters forming in the
Co(acac)2–AlEt3 system are active in the hydrogenaꢀ
tion of various substrates (both benzene and styrene)
under mild conditions [14, 19]. It is seemingly reasonꢀ
able to assume that these nanoclusters, even if they
form in insignificant amounts along with the cobalt
hydrido complex in the Co(acac)2–3PR3–4Red sysꢀ
tem, are responsible for the catalytic styrene and benꢀ
zene hydrogenation reactions, which should compete
in this case. If this were so, benzene hydrogenation
would occur in the absence of the alkene as well, but
this contradicts the experimental data. This fact and
the finding that benzene hydrogenation in the
Co(acac)2–3PBu3–6C6H5СН2MgCl system also
occurs in the absence of styrene and the nature of terꢀ
tiary phosphine substantially affects the process sugꢀ
attributed to the change in the character of catalysis.
Since the redox process involving Co(acac)2 and triꢀ
ethylaluminum in the presence of PBu3 proceeds
through alkyl complexes of Co(I) with the tribuꢀ
tylphosphine ligands [22], they are probably responsiꢀ
ble for the hydrogenation of arenes in the Co(acac)2–
РBu3–AlEt3 system in the absence of styrene.
In spite of the spectral and catalytic studies carried
out, the cause of the determining effect of the phosꢀ
phine nature on the coupled hydrogenation of alkenes
with arenes in the Co(acac)2–3PBu3–4Red catalytic
system remains unclear. The available experimental data
array does not allow one to unambiguously elucidate the
causes of the influence of the tertiary phosphine nature
on the activity of the Co(acac)2–3PR3–4Red system in
coupled alkene and arene hydrogenation. In our opinꢀ
ion, at least three factors play the most important role.
Firstly, trialkylphosphines are stronger bases than triꢀ
arylphosphines and, therefore, they should induce a
significant trans effect. Therefore, one of the causes of
the effect of the tertiary phosphine nature can be the
electronic factor, namely, the enhancement of the
Co–H bond strength upon the replacement of tribuꢀ
tylphosphine (stronger base) by triphenylphosphine.
This conception is consistent with the IR spectroꢀ
scopic data. Secondly, the steric factor cannot be
neglected, since the conic angle in PBu3 (129
°
) is
smaller than that in PPh3 (145 [38]. In addition,
°
)
triphenylphosphine has a more rigid structure.
Thirdly, the cobalt hydrido complexes with tripheꢀ
nylphosphine ligands are more prone to association.
gests that the catalytic cycle includes the
σ
ꢀalkyl
cobalt complex formation stage rather than occurring
on active surface sites of the cobalt nanoclusters
formed in the classical Ziegler systems. In the frameꢀ
The NMR and IR spectroscopic identification of
work of the proposed hypothesis, the more rapid deacꢀ the cobalt hydrido complexes with phosphine ligands
tivation of the Co(acac)2–РBu3–AlEt3 system during along with the results of catalytic experiments suggests
benzene hydrogenation in the absence of the alkene the homogeneous character of the process and allows
compared to the deactivation of the cobalt nanoclusꢀ one to propose (taking into account published and
ters formed in the Co(acac)2–AlEt3 system can be earlier obtained data) the most probable reaction
KINETICS AND CATALYSIS Vol. 54
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440
TITOVA et al.
mechanism. In a hydrogen atmosphere, the reaction and trihydrido (coordinatively saturated) cobalt comꢀ
system contains an equilibrium mixture of the monoꢀ plexes:
H
2
Co(acac)2 + 3PBu3 + Red
Co(H)(PBu3)3
Co(H)3(PBu3)3
Co(H)(PBu3)2 + PBu3.
The reaction of the cobalt monohydrido complex
with the alkene or alkadiene yields the cobalt ꢀalkyl
or ꢀalkenyl complex, respectively, with two phosphoꢀ
π
ꢀС3Н5Со[P(OC3H7)3]3 complex (kH/kD = 1.4–1.5),
σ
with the first order with respect to hydrogen taken into
account, suggests that hydrogen is involved in the rateꢀ
determining step. However, since, the isotope effect is
weak and the hydrogenation rate decreases with an
increasing number of cyclic structures in the arene, it
can be assumed that the concerted addition of hydroꢀ
gen to the complex containing arene as a ligand occurs
in the rateꢀdetermining step. Based on the aforesaid,
we propose the following mechanism for the coupled
σ
rusꢀcontaining ligands. In our opinion, it is this comꢀ
plex that is the active species in benzene hydrogenaꢀ
tion. The specified stages ensure the kinetic coupling
of the hydrogenation reactions of alkenes (alkadienes)
and arenes. As was established by the kinetic studies,
the reactions are firstꢀorder with respect to hydrogen
and cobalt, and their order with respect to benzene is
close to zero [30]. The kinetic isotope effect observed
[39] for benzene hydrogenation in the presence of the hydrogenation of alkenes with arenes.
B
+
C
C
P
Co
P H
Co
P
P
1/n
P
H
Co
C C
Co
P H
5
n
P
P
A
1
2
CoHP2
+
+
C
C
C
C
+H
2
+H
2
C C
C
C
Co
C
C
C
C
C
P
P
Co
P
P
Co
H
H
P
P
P
+H
3
Co
2
P
H
4
C C
+H
2
C
+H
2
C
C
C
C
C
Co
Co
P
P
P
P
C
C
+
C
C
C C
Co
–
P
P
+H
2
–
Here
Р
is tributylphosphine and several stages (oxidaꢀ
Route A includes twoꢀstep hydrogen transfer to the
unsaturated bond and corresponds to the commonly
tive hydrogen addition) are omitted for simplicity.
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COUPLED HYDROGENATION OF ALKENES AND ARENES
441
accepted scheme of the mechanism of alkene hydroꢀ
genation in the presence of metal complex catalysts.
According to the proposed mechanism of the couꢀ
pled hydrogenation of the alkene and arene, the cycloꢀ
hexene formation selectivity should be determined by
the ratio of the rates of cyclohexene hydrogenation to
cyclohexane and cyclohexene substitution by other
ligands (styrene or another alkene, or a phosphorusꢀ
containing compound) in the coordination sphere of
cobalt. This ratio depends, in turn, on the nature of the
phosphorusꢀcontaining compound and alkene, P to
Co ratio, and temperature. More detailed elaboration
of the mechanism of coupled hydrogenation requires
identification of intermediate complexes by physical
methods and a study of their reactivity.
Route B predicts the transformation of cobalt
hydrido complex 1 with two phosphine ligands into
complex 2 with the cyclohexadienyl ligand in the
coordination sphere of cobalt. The possibility of this
stage taking place was convincingly proved [40] by the
finding that the reaction of the ꢀallyl cobalt complex
π
P
P
Co
(
P
= Су2РС2Н4РСу2, Cy = cyclohexyl) with
P
Thus, arenes, on the one hand, and alkenes, alkaꢀ
dienes, or arylalkenes, on the other hand, can particiꢀ
pate in coupled hydrogenation. The process occurs
under mild conditions and is catalyzed by the cobaltꢀ
containing systems formed only in the presence of triꢀ
alkylphosphines under the action of various reducing
agents, namely, organoaluminum and organomagneꢀ
sium compounds and tertꢀbutoxy derivatives of lithꢀ
ium tetrahydroaluminate. The IR and NMR identifiꢀ
cation of the cobalt hydrido complexes with phosꢀ
phine ligands, whose composition depends, to a
considerable extent, on the reducing agent and gas
atmosphere in the reaction, along with the results of
catalytic experiments, suggested that this process is
homogeneous. The most probable schemes of the
reaction mechanism were proposed.
hydrogen under mild conditions at 20°С results in the
hydrogenation of the allyl ligand to propane and the
resulting hydrido complex [Co(H)P2] interacts with
benzene, turning into the cyclohexadienyl diphosꢀ
phine complex
Co
P
P
.
This complex reacts with hydrogen very slowly to
form cyclohexene. Unfortunately, no quantitative
characteristics of this process were presented [40]. In
the scheme proposed by us, route B includes alkene
coordination with cobalt complex 2 and the subseꢀ
quent addition of hydrogen to the coordinated alkene
and to the cyclohexadienyl ligand, which yields comꢀ
plex 3 containing cyclohexadiene in the coordination
sphere of cobalt.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educaꢀ
tion and Science of the Russian Federation (agreeꢀ
ment no. 14.V37.21.0795).
Route C includes alkene coordination to complex 1
and the formation of the ꢀalkyl complex 4. Its subseꢀ
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