Nickel-catalyzed reductive hydroesterification of styrenes using CO 2 and MeOH

Article abstract of DOI:10.1021/om300819d

Complexes [(dippe)Ni(μ-H)]₂ (A) (dippe = 1,2-bis-di- isopropylphosphino)ethane) and [(dtbpe)Ni(μ-H)]₂ (B) (dtbpe = 1,2-bis-di-tert-butylphospino)ethane) catalyze the reductive hydroesterification of styrenes with the use of CO₂ and MeOH. The latter acts as a hydrogen source and as an esterificating agent, to yield the corresponding branched and linear esters in moderate to good yields. In all of the studied reactions the linear esters were obtained in higher amounts than the branched ones. When the hydroesterification reaction was carried out using a stoichiometric metal/substrate ratio, the complexes [(P-P)Ni(CO)₂] and [(P-P)Ni(CO₃)] (P-P = dippe or dtbpe) were isolated and characterized by standard spectroscopic methods. Compounds [(dtbpe)Ni(CO) ₂] and [(dtbpe)Ni(CO₃)] were also fully characterized by single-crystal X-ray diffraction.

Full text of DOI:10.1021/om300819d

Article
pubs.acs.org/Organometallics
Nickel-Catalyzed Reductive Hydroesterification of Styrenes Using
CO2 and MeOH
Lucero Gonzalez-Sebastian, Marcos Flores-Alamo, and Juventino J. García*
́
́
́ ́
Facultad de Química, Universidad Nacional Autonoma de Mexico, Mexico City 04510, Mexico
S
* Supporting Information
ABSTRACT: Complexes [(dippe)Ni(μ-H)]2 (A) (dippe = 1,2-bis-di-isopropylphosphino)ethane)
and [(dtbpe)Ni(μ-H)]2 (B) (dtbpe = 1,2-bis-di-tert-butylphospino)ethane) catalyze the reductive
hydroesterification of styrenes with the use of CO2 and MeOH. The latter acts as a hydrogen source
and as an esterificating agent, to yield the corresponding branched and linear esters in moderate to
good yields. In all of the studied reactions the linear esters were obtained in higher amounts than the branched ones. When the
hydroesterification reaction was carried out using a stoichiometric metal/substrate ratio, the complexes [(P−P)Ni(CO)2] and
[(P−P)Ni(CO3)] (P−P = dippe or dtbpe) were isolated and characterized by standard spectroscopic methods. Compounds
[(dtbpe)Ni(CO)2] and [(dtbpe)Ni(CO3)] were also fully characterized by single-crystal X-ray diffraction.
source, is attractive. To date, there are only a few successful
attempts to use CO2 instead of CO to synthesize organic
products.9
1. INTRODUCTION
Carbon dioxide is an extremely attractive carbon source that is
readily available, inexpensive, and inherently renewable. The
thermodynamic stability and kinetic inertness of CO2 represent
a challenge for converting CO2 into useful materials. Its
utilization as a C1 feedstock in both large-scale fixation
processes and small-scale synthesis has gained considerable
attention in recent years.1 Hence, the development of transition
metal-catalyzed reactions for CO2 incorporation into organic
molecules to produce value-added products is of great
importance.2
The oxidative cycloaddition of CO2 and unsaturated
hydrocarbons at low-valent transition metal complexes is one
of the most extensively studied reactions for CO2 incorporation
routes, and some of them are nickel-mediated processes.2b,3
These have been largely known for over 20 years due to the
seminal work by Hoberg, who performed research in the field
of stoichiometric activation of CO2.4 In a broad summary, there
are two general routes: (a) reaction of CO2 with olefins or
alkynes and (b) the insertion of CO2 into metal−element
bonds, along with other co-reactants, leading to a variety of
useful products, such as carboxylates, esters, or carbonates.2a,5
Certainly, reagents such as Grignard and organolithium are
strong nucleophiles, which react directly with CO2 to form
valuable carboxylic acids and derivatives; however, they display
a poor functional group compatibility that ultimately limits
their use.2a,6
Taking advantage of the reduction of CO2 to CO with A, we
envisaged its potential application in the hydroesterification of
styrenes, since carboxylic esters have important industrial
applications as large-volume products and chemical intermedi-
ates. The main pathway for the synthesis of esters is the nickel-,
cobalt-, or palladium-catalyzed hydroesterification reaction of
alkenes, along with carbon monoxide and alcohols.10
Recently in 2010, Rieger and co-workers reported the
synthesis of methyl acrylate from CO2, ethylene, and methyl
iodide, mediated by the fragment [(dpppNi)] (dppp =
bis(diphenylphosphino)propane), being the first report on
the release of acrylate from a metallalactone. Nevertheless, the
reaction is not catalytic, and acrylate is obtained in a low yield
(33%).5b
Herein we report the first example of a nickel-catalyzed
reductive hydroesterification of styrenes using CO2 as CO
source and methanol, the last acting as both a hydrogen source
and esterificating agent.
2. RESULTS AND DISCUSSION
Reductive Hydroesterification Catalyzed by [(dippe)-
Ni(μ-H)]2. The reactivity of [(dippe)Ni(μ-H)]2 (A) toward the
hydroesterification reaction using CO2, methanol, and styrene
as a substrate was first assessed at a stoichiometric level. The
reaction of A with 2 equiv of styrene in methanol yielded the
complex [(dippe)Ni(η2-C,C-styrene)] (5), identified by the
characteristic signals on the 31P{1H} NMR spectrum, showing
two doublets at 73.5 and 60.0 ppm, with JP−P = 64.1 Hz. A
methanol solution of 5 was stirred under a CO2 stream for 20
min at room temperature and further heated at 120 °C for 36 h;
after this time the reaction was completed and the crude was
Ni(0) has a particular affinity for CO2, as evidenced by a
number of characterized monomeric and oligomeric Ni-CO2
complexes.7 We have recently reported the CO2 reduction to
CO using the nickel complex [(dippe)Ni(μ-H)]2 (A) under
mild conditions to yield mono- and bimetallic nickel complexes
(Scheme 1).8
Although carbon monoxide is widely used as a raw material
in synthesis, it is highly toxic and raises several issues related to
personal safety and environmental protection. For that reason,
replacement of CO by CO2, as a nontoxic and abundant C1
Received: August 24, 2012
© XXXX American Chemical Society
A
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Scheme 1. Reactivity of CO2 with [(dippe)Ni(μ-H)]2
Scheme 2. Reaction of 5 with CO2 and MeOH
a
Table 1. Reductive Hydroesterification with Different Amounts of Catalyst
b
entry
ratio 5a:A
conv (%)
5b (%)
5c (%)
5d (%)
5e (%)
5f (%)
1
2
3
4
1:1
100
100
100
100
5
7
6
8
4
16
29
60
73
4
3
3
68
62
21
10
1:0.2
1:0.1
1:0.01
11
13
a
All reactions were carried out in a Schlenk flask with a Rotaflo valve using 5 mL of MeOH/THF, 8:2. Yields of the organic products were quantified
b
by GC-MS. Yield of 5d is the sum of the isomers E and Z.
analyzed by CG-MS and NMR, yielding the following
hydroesterification products: methyl 2-phenylpropanoate (5e,
4%), methyl 3-phenylpropanoate (5f, 68%), and the byproducts
5b−5d (Scheme 2).
those obtained in stoichiometric conditions, and therefore,
these conditions were used in the following hydroesterification
reactions. The low yields of hydroesterification products
observed on using lower amounts of catalyst (10 and 1%)
can be related to a deactivation of the catalyst by the formation
of the rather stable compounds 6, 7, and dippeO2 from the
active fragment [(dippe)Ni]. Noteworthy, the independent
preparation of 6 revealed to be very stable and not to be active
in the hydroesterification process (see Experimental Section).
A mechanistic proposal for this process is depicted in Scheme
3. In the above proposal, the initial step is the styrene
coordination to yield 5, followed by the oxidative addition of
methanol to produce I11 and a subsequent insertion of metal
hydride to afford II. The CO is generated in the process by the
reduction of CO2 as already demonstrated (vide supra), and the
formed CO is involved in the catalytic process to produce III
and, then, carrying out a migratory insertion into the metal−
carbon bond to form an acyl group, generating IV. Finally, IV
undergoes a reductive elimination to yield the corresponding
ester, thus closing the catalytic cycle.
The 31P{1H} NMR spectrum of the reaction mixture
displayed signals assigned to complexes [(dippe)Ni(CO)2]
(6), a singlet at 73.9 ppm (CD3OD); [(dippe)Ni(CO3)2] (7), a
singlet at 86.1 ppm (CD2Cl2); and dippeO2, a singlet at 48.8
ppm (C7D8). In addition, in the 13C{1H} NMR, complex 6
displayed a key signal at 204.3 (t, 4.1 Hz) assigned to the CO
group, and 7 showed a key signal at 167.1 ppm assigned to the
CO3 moiety. All products were purified by chromatographic
column and fully characterized (see Experimental Section and
the Supporting Information).
Considering the above, experiments were performed using
different amounts of [(dippe)Ni(μ-H)]2 (A), in order to find
the optimized conditions for the hydroesterification. The main
results for these experiments are summarized in Table 1.
According to these results, the yield of the hydroesterification
products decreases with the decreasing amount of A. On using
a catalyst load of 20% mol of A, good yields are obtained
toward hydroesterification products, only slightly lower than
To the best our knowledge, this is the first example of a
nickel-catalyzed reductive hydroesterification of styrenes using
B
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the use of CO2, and no hydroesterificaton product was
obtained.
Scheme 3. Mechanistic Proposal for the Hydroesterification
Reaction
Encouraged by these results, we turned our attention to
extend the scope of this reaction to a wide variety of substituted
styrenes (Table 2).
As shown in Table 2, the best yields were obtained with
styrenes bearing σ-electron-withdrawing groups (entries 2a−
5a), while electron-donating groups gave low yields (6a−9). In
all of these reactions there is selectivity toward linear esters over
the branched ones; this is in accordance with previous
reports,15 in which the dependence of the regioselectivity on
the nature of the used ligand has been established. A chelating
diphosphine ligand leads to the stabilization of the cis
intermediate, which yields the linear ester as a major product.
On the other hand, the use of a monophosphine ligand favors
the trans intermediate species, leading to the formation of
branched esters. For entries 10 to 12 lower yields were
observed, perhaps due to a competing coordination of the
oxygen or nitrogen donor atoms at the p-styrene.
As can be seen in the mechanistic proposal, Scheme 3, the
alcohol plays an important role in the reactivity since it is
involved in the oxidative addition and reductive elimination
steps; therefore, the nature of the alcohol was changed to study
this effect. The use of alcohols such as i-PrOH and EtOH
yielded styrene oligomers, probably due to the higher steric
hindrance of these alcohols and, therefore, to a decrease in the
rate of the nucleophilic attack step, as it has been previously
observed.16
Reductive Hydroesterification Catalyzed by [(dtbpe)-
Ni(μ-H)]2. Hillhouse and co-workers recently reported17 the
structure of [dtbpeNi(η2-CO2)] derived from the reaction of
[(dtbpe)Ni]2(η2,μ-C6H6) with CO2. Such a compound is a
close example of a η2-C,O coordination mode stabilized by the
fragment (dtbpeNi). Despite the similarity between the
complexes [(dippe)Ni(μ-H)]2 (A) and [(dtbpe)Ni(μ-H)]2
(B), the latter is less reactive and does not generate CO2
reduction products at room temperature. However, when the
complex [(dtbpe)Ni(η2-CO2)] is heated at 80 °C, it activates
the η2-CO2 to yield [(dtbpe)Ni(CO)2] (10) and the
corresponding phosphine monoxide dtbpeO.
Therefore, we envisaged complex B as a good candidate to
perform either CO2 insertion or CO transfer in a hydro-
esterification reaction under the optimized reaction conditions
used with A. The main results of these reactivity tests are
summarized in Table 3.
only CO2 and methanol. A closely related methodology using
nickel was recently reported by Rieger and co-workers5b for the
formation of methyl acrylate from CO2 and ethylene via
methylation of nickelalactones.
Other examples for the formation of esters using an alkene,
CO2, and methanol have been described for Rh and Ru as
catalysts. For instance, in a 1981 patent by Rohm GmnbH12 it
is reported that propene, CO2, and methanol in the presence of
a Ru catalyst yield the butanoic esters (30% yield). Noteworthy,
the author stated that the carbonyl group could be originated
from either CO2 or methanol, and similar observations were
́
reported by Marko using a rhodium-based decarbonylation
process for primary alcohols during the alkene hydrogenation.13
Therefore, CO is originated from methanol activation initiated
by the formation of an acyl species, which is subsequently
attacked by methanol, ultimately yielding the esters.14
Considering this, in order to assess the source of the carbonyl
group in the hydroesterification reaction catalyzed by [(dippe)-
Ni(μ-H)], experiments using 13C-labeled CO2 were performed,
using the same reaction conditions for entry 2. Figure 1 shows
the corresponding mass spectra for methyl 3-phenylpropanoate
(5f) from (a) regular CO2 and (b) 13CO2.
The reaction proved to be active in generating esters from
styrenes, MeOH, and CO2. Nevertheless, the results in Table 3
show in general lower yields using B compared with those
obtained with A; again, the best yields were obtained with
styrenes bearing σ-electron-withdrawing groups.
The 31P{1H} NMR spectrum of the reaction mixture using 1
equiv of [(dtbpe)Ni(μ-H)]2 with 2 equiv of styrene in
methanol under a CO2 atmosphere at room temperature
revealed the formation of the complex [(dtbpe)Ni(η2-C,C-
styrene)] (8), identified by the characteristic signal in the
31P{1H} NMR spectrum showing two doublets at 93.1 and 82.4
ppm, with JP−P = 68 Hz.
Comparing both products (5e and 5f) the molecular ion
peak was shifted from m/z = 164 (CO2) to m/z = 165 (13CO2),
also (M+ − OCH3) changed from m/z = 133 to m/z = 134.
Such results confirmed the CO insertion originated from CO2.
Further evidence, confirming the last statement, was obtained
in the 13C{1H} NMR spectrum; here the signal assigned to the
carbonyl group increased its relative intensity (see the
Supporting Information). This reaction was also tested without
When the reaction mixture was heated to 120 °C for 36 h,
the complete consumption of styrene was observed along with
the production of the hydroesterification products 5e, 5f, and
the byproducts 5b−5d. The 31P{1H} NMR spectrum displays
signals located at 97.8 ppm assigned to [(dtbpe)Ni(CO)3] (9),
signals at 94.3 ppm assigned to [(dtbpe)Ni(CO)2] (10), a
C
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Figure 1. Mass spectrum for methyl 3-phenylpropanoate from styrene, methanol, and (a) CO2 and (b) 13CO2.
singlet at 63.7 ppm assigned to [dtbpeO2], and two doublets at
62.7 and 36.4 with JP‑P = 44.8 Hz for [dtbpeO].
close to the expected ideal tetrahedron. The P−Ni−CO angles
are also in agreement with a tetrahedral geometry, with
109.0(9)° (av), and are shorter than those reported for the
complex [Ni(CO)(np3)] (112.7.0(9)° av)20 (np3 = tris(2-
diphenylphosphinoethyl)amine). On the other hand, the Ni−
CO bond distance in 10 (1.787 Å av) is relatively close to the
Ni−CO bond distance in [Ni(CO)(np3)] (1.74 Å),20 but
shorter than those reported for Ni(CO)4 (1.84 Å) and
[(CO)3Ni(PPh2)2Ni(CO)3)] (1.8003(8) Å).21 The Ni−P
bond distances of 2.1652(9) Å are shorter than those reported
for other tetrahedral d10 nickel complexes such as [Ni(P-
(OCH2)3CCH3)3NO]BF4 (2.186 Å av) and Ni(CO)(np3)]
(2.215 Å av).20,22
Suitable crystals for single-crystal X-ray studies for
compounds 9 and 10 were obtained after separation by column
chromatography (see Experimental Section). Complex 9
crystallizes as [(dtbpe)Ni(CO3)]·2CH3OH. The corresponding
ORTEP representation for 9 is shown in Figure 2, and
crystallographic data are summarized in Table 4. The nickel
atom is in a slightly distorted square-planar geometry and
coordinated by the chelating dtbpe and two oxygen atoms from
the carbonate ion. Ni−P (2.1652 Å average) and Ni−O (1.900
Å average) distances are comparable with the values reported
for closely related square planar nickel(II) complexes.18 The
distances and angles for the carbonate ion in 9 are comparable
with the interatomic distances and angles of similar structures
containing such a counterion.18b,19 In addition, 13C{1H} NMR
spectra confirmed the presence of the carbonate ligand on the
nickel center at 169.7 ppm. The IR spectrum for 9 shows
strong peaks at 1663 and 1627 cm−1, also corroborating the
presence of the carbonate ligand coordinated to nickel.
In the case of complex 10, colorless crystals suitable for X-ray
studies were obtained spontaneously crystallizing from the
reaction mixture, and the corresponding ORTEP representa-
tion is depicted in Figure 3. Complex 10 exhibits a nickel center
in a tetrahedral geometry, coordinated by two P atoms from
dtbpe and two terminal CO ligands. The bite angle for the
chelating dtbpe ligand of 92.07(7)° is similar to other dtbpeNi0
complexes,17 while the OC−Ni−CO angle of 108.1(3)° (av) is
The 13C{1H} NMR spectrum for pure 10 shows a key
resonance at 204.6 ppm as a triplet (JC−P= 3.27 Hz) assigned to
the carbonyl group coupled to two equivalent phosphorus.
Despite the fact that complex B does not produce the
analogue complexes to 1 and 2 containing the ligand dtbpe,
only the stable complex [(dtbpe)Ni(CO)2], since complex B
resulted in being active in the hydroesterification reaction (vide
supra), we speculate that probably under these reaction
conditions a reductive disproportionation reaction,23 namely,
2CO2 + 2e− = CO + CO32−, may be operating. A pathway for
this type of CO2 reduction could involve a “head-to tail”
dimerization of CO2 to make a −C(O)−O−C(O)−O type of
intermediate followed by fragmentation to CO and CO32−. A
report supporting this pathway has been documented for the
reaction of [IrCl(C8H14)(PMe)3] with CO2.24 Therefore, the
D
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Table 2. Reductive Hydroesterification of Substituted
Styrenes
Table 3. Reductive Hydroesterification of Styrenes with CO2
Catalyzed by [(dtbpe)Ni(μ-H)]2
a
a
a
All reactions were carried out in a Schlenk flask equipped with a
a
All reactions were carried out in a Schlenk flask with a Rotaflo valve
Rotaflo valve using 5 mL of MeOH/THF, 8:2. Yields of the organic
products were quantified by GC-MS. Standard conditions: 1:5:Patm of
[(dippe)Ni(μ-H)]2, styrene, CO2, respectively, were used. Entry 11*
afforded the same esters as 10.
using 5 mL of MeOH/THF, 8:2. Yields of the organic products were
quantified by GC-MS. Standard conditions: 1:5:Patm of [(dippe)Ni(m-
H)]2, styrene, CO2, respectively, were used. Entry 11* afforded the
same esters as 10.
and distilled from solutions of magnesium/iodine. Deuterated solvents
for NMR experiments were purchased from Cambridge Isotope
Laboratories and stored over 3 Å molecular sieves in the glovebox.
Celite and silica gel were dried by heating at 200 °C under vacuum for
CO generation from CO2 and the formation of the complex
[(dtbpe)Ni(CO3)] are consistent with that proposal.
CONCLUSION
25
■
20 h and stored in the glovebox. Compounds [(dippe)Ni(μ-H)]2
(A) and [(dtbpe)Ni(μ-H)]26 were prepared according to the reported
procedure. The bisphosphine ligands dippe (1,2-bis-
diisopropylphosphino)ethane and dtbpe (1,2-bis-di-tert-
butylphospino)ethane were synthesized according to the reported
methods.27 All other chemicals and filter aids were reagent grade and
were used as received. The purification of the new compounds was
made by crystallization or by column chromatography. Carbon dioxide
was supplied by Air Products and Chemicals Inc. (purity >99.99%)
and was used without further purification. All reagents for the catalytic
reactions were loaded in the glovebox using Schlenk flasks equipped
with Rotaflo high vacuum stopcocks and further loaded with CO2. The
crude reaction mixtures for each catalytic run were immediately
analyzed by GC-MS. GC-MS determinations were performed using an
Agilent 5975C system equipped with a 30 m DB-5MS capillary (0−32
mm i.d.) column. FT-IR spectra were determined on a Perkin-Elmer
1600 series. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded at
room temperature on a 300 MHz Varian Unity spectrometer in
toluene-d8, CD3OD, or CD2Cl2, unless otherwise stated. 1H and
13C{1H} chemical shifts (δ, ppm) are reported relative to the residual
proton resonance in the corresponding deuterated solvent. 31P{1H}
NMR spectra were referred to an external 85% H3PO4 solution. All air-
sensitive NMR samples in this work were handled under an inert
atmosphere using thin wall (0.38 mm) WILMAD NMR tubes
equipped with J. Young valves. Mass spectrometry (MS-EI+) of pure
compounds was performed on a Thermo-Electron DFS spectrometer
at USAI-UNAM. Single-crystal X-ray diffraction determinations were
performed on an Oxford Diffraction Gemini-Atlas diffractometer.
The current report shows the first catalytic reductive hydro-
esterification reaction of styrenes, using CO2 as a C1 source
and methanol as a reductant, catalyzed by nickel complexes.
Depending on the catalytic precursor used, esters were
obtained from moderate to good yields. Best results were
always produced with the use of complex [(dippe)Ni(μ-H)]
and styrenes bearing σ-electron-withdrawing groups. The use of
a chelating phosphine allowed a higher linear/branched ratio.
Complexes of the type [(P−P)Ni(CO)2] and [(P−P)Ni-
(CO3)], where (P−P) = dippe and dtbpe, are the resting state
compounds and are ultimately responsible for the catalyst
deactivation. An important aspect of this transformation is the
dependence on the used alcohol. Methanol turned out to be a
suitable reagent for the hydroesterification. Current studies are
under way to extend the scope of this reaction to other
transformations to incorporate CO from CO2 in the
preparation of value-added organic molecules and fine
chemicals.
EXPERIMENTAL SECTION
■
Unless otherwise noted, all the operations were carried out in an
MBraun glovebox (<1 ppm H2O and O2) or by using high-vacuum
and standard Schlenk techniques under an argon atmosphere. THF
was dried and distilled from dark purple solutions of sodium/
benzophenone ketyl. Ethanol, methanol, and 2-propanol were dried
E
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Table 4. Crystallographic Data for 9 and 10
9
10
empirical formula
fw
C19H4oNiO3P2·2CH3OH
501.24
C20H40NiO2P2
433.17
temperature (K)
cryst syst
space group
a (Å)
126 (2)
monoclinic
C2
100(2)
orthorhombic
Pna21
13.2110(13)
10.2690(6)
10.9440(10)
90
16.4750(12)
11.2499(8)
12.5950(15)
90
b (Å)
c (Å)
α (deg)
β (deg)
118.711(12)
90
90
γ (deg)
V (Å3)
90
1302.16(19)
2
2334.4(4)
4
Z value
D(calcd) (Mg/m3)
1.278
1.233
F(000)
cryst size (mm3)
544
936
0.4765 × 0.3469 × 0.3354 0.3657 × 0.084 ×
0.0702
θ range (deg)
reflns collected
4.61 to 68.20
4233
4.76 to 66.59
5129
Figure 2. ORTEP drawing (50% probability) for complex 9. Selected
distances (Å) and angles (deg): C(10)−O(1) = 1.323(4), C(10)−
O(1)#1 = 1.323(4), C(10)−O(2) = 1.252(6), C(10)−Ni(1) =
2.296(5), O(1)−Ni(1) = 1.900(2), Ni(1)−O(1)#1 = 1.900(2), P(1)−
Ni(1) = 2.1652(9), Ni(1)−P(1)#1 = 2.1652(9); O(2)−C(10)−O(1)
#1 = 124.2(2), O(2)−C(10)−O(1) = 124.2(2), O(1)#1−C(10)−
O(1) = 111.7(4), O(2)−C(10)−Ni(1) = 180.0, O(1)#1−C(10)−
Ni(1) = 55.8(2), O(1)−C(10)−Ni(1) = 55.8(2), C(10)−O(1)−
Ni(1) = 89.0(2), O(1)−Ni(1)−O(1)#1 = 70.38(17), O(1)−Ni(1)−
P(1)#1 = 169.61(9), O(1)#1−Ni(1)−P(1)#1 = 99.23(9), O(1)−
Ni(1)−P(1) = 99.23(9), O(1)#1−Ni(1)−P(1) = 169.61(9), P(1)#1−
Ni(1)−P(1) = 91.16(5), O(1)−Ni(1)−C(10) = 35.19(8), O(1)#1−
Ni(1)−C(10) = 35.19(8), P(1)#1−Ni(1)−C(10) = 134.42(2), P(1)−
Ni(1)−C(10) = 134.42(2).
independent reflns
(Rint/%)
2240 [2.67]
2922 [5.39]
completeness to theta
(%)
99.9
99.8
refinement method
full-matrix least-squares on full-matrix least-
F2
squares on F2
no. data/restraints/
params
goodness-of-fit on F2
2240/2/144
2922/1/238
1.048
1.027
final R indices [I > 2σ
(I)] (%)
R = 4.52, Rw = 11.68
R = 6.89, Rw = 15.94
R indices (all data) (%) R = 4.69, Rw = 11.87
largest diff peak and 1.349 and −0.444
hole (e Å−3
R = 8.12, Rw = 16.83
1.577 and −0.625
)
General Procedure for the Reductive Hydroesterification of
Styrenes with [(dippe)Ni(μ-H)]2 in Stoichiometric Conditions. A
representative methodology is described as follows: A 25 mL Schlenk
flask equipped with a Rotaflo valve and with an inner magnetic stirring
bar was loaded in a glovebox with a THF solution (1 mL) of
[(dippe)Ni(μ-H)]2 (0.05 g, 0.077 mmol) and a methanol solution (4
mL) of styrene (17.8 μL, 0.154 mmol). A color change from wine red
to brown was observed. The solution was stirred for 10 min, and then
a CO2 stream was bubbled at room temperature for 20 min. Then the
Schlenk flask was closed and heated in an oil bath at 120 °C for 36 h.
After this time, the heating was stopped and the flask vented. An
aliquot of the reaction mixture was immediately analyzed by GC-MS.
Complexes [(dippe)Ni(CO3)], [(dippe)Ni(CO)2], and dippeO2 were
isolated from the stoichiometric reaction. The reaction mixture was
concentrated to dryness, giving a brown residue, and then redissolved
with THF (2 mL) to yield a green-yellow solid, which was filtered and
dried under vacuum for 6 h. This solid was identified as
[(dippe)Ni(CO3)]. Yield: 4.3%. Anal. Calcd for C15H32NiO3P2 (7):
C, 47.28; H, 8.46. Found: C, 47.26; H, 8.45. 31P{1H} NMR (22 °C,
1
300 MHz, CD2Cl2): δ 86.1 (s, P(i-Pr)2CH2CH2P(i-Pr)2). H NMR
(22 °C, 300 MHz, CD2Cl2): δ 2.2−2.07 (m, CH, 4H), 1.82−1.73 (m,
CH2, 4H), 1.53−1.3 (m, CH3, 24H). 13C{1H} NMR (22 °C, 300
MHz, CD2Cl2): δ 167.1 (s, CO32−), 24.36 (m, CH2CH2), 19.9 (m,
(CH(CH3)2), 18.5 (m, (CH(CH3)2). The FT-IR (KBr pellet) display
bands at 1666 and 1632 cm−1 assigned to the carbonate ligand.
The resulting THF mother liquors were evaporated to dryness, and
the remaining mixture was separated by column chromatography using
silica gel, eluting with hexane/ethyl acetate, 80:20. The solvent was
removed from all fractions, and the corresponding residues were
vacuum-dried for 6 h. Compound [(dippe)Ni(CO)2] (6) was
characterized by standard spectroscopic methods. Yield for complex
6: 93%. Anal. Calcd for C16H32NiO2P2 (6): C, 50.97; H, 8.55. Found:
Figure 3. ORTEP drawing (50% probability) for complex 10. Selected
distances (Å) and angles (deg): C(20)−O(1) = 1.131(10), C(21)−
O(2) = 1.132(8), C(20)−Ni(1) = 1.780(8), C(21)−Ni(1) =
1.794(6), Ni(1)−P(1) = 2.2396(19), Ni(1)−P(2) = 2.2443(19),
O(1)−C(20)−Ni(1) = 175.9(6), O(2)−C(21)−Ni(1) = 176.9(7),
C(20)−Ni(1)−C(21) = 108.1(3), C(20)−Ni(1)−P(1) = 113.9(2),
C(21)−Ni(1)−P(1) = 114.6(2), C(20)−Ni(1)−P(2) = 114.9(2),
C(21)−Ni(1)−P(2) = 112.7(2), P(1)−Ni(1)−P(2) = 92.07(7).
C, 50.77; H, 8.52. 31P{1H} NMR (22 °C, 300 MHz, CD3OD): δ 73.9
(s, (i-Pr2)2PCH2CH2P(i-Pr2)2). 13C{1H} NMR (22 °C, 300 MHz,
F
dx.doi.org/10.1021/om300819d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
CD3OD): δ 204.3 (t, CO, JC−P = 4.1 Hz), 25.8 (t, (CH(CH3)2), JC−P
=
obtained from the mixture reaction as yellow, air-stable crystals. These
were filtered and dried under vacuum for 6 h. Yield for [(dtbpe)Ni-
(CO3)]: 24.68%. Anal. Calcd for C19H40NiO3P2 (9): C, 52.20; H, 9.22.
Found: C, 52.18; H, 9.20. 31P{1H} NMR (22 °C, 300 MHz, CD2Cl2):
9.8 Hz), 23.8 (dd, (CH(CH3)2), JC−P = 8.6, 6.9 Hz), 19.6 (m,
1
(CH2CH2)). H NMR (22 °C, 300 Mz, CD3OD): δ 2.0−1.82 (m,
CH, 4H), 1.56−1.48 (m, CH2, 4H), 1.1−0.9 (m, CH2, 24H).
1
Isolated yield for dippeO2 (4): 2.8%. 31P{1H} NMR (22 °C, 300
δ 97.83 (s, P(i-Pr)2CH2CH2P(i-Pr)2). H NMR (22 °C, 300 MHz,
MHz, C7D8): δ 48.8 (s, OP(i-Pr2)CH2CH2(i-Pr)2PO). H NMR
CD2Cl2): δ 1.84−1.82 (m, CH2, 4H), 1.57−1.45 (m, 36H). 13C{1H}
NMR (22 °C, 300 MHz, CD2Cl2): δ 169.9 (s, CO32−), 36.9 (m, CH2),
30.24 (s, (CH(CH3)2), 22.9 (m, (CH(CH3)2). FT-IR (KBr) showed
CO3 bands at 1663, 1626, and 1471 cm−1. Suitable crystals of this
complex for X-ray diffraction studies were obtained from the reaction
mixture spontaneously (vide infra).
1
(22 °C, 300 MHz, C7D8): δ 1.62−1.5 (m, CH, 4H), 1.4−1.22 (m,
CH2, 4 H), 1.05−0.85 (m, CH3, 24H).
Catalytic Reductive Hydroesterification of Styrenes with
[(dippe)Ni(μ-H)]2. A typical experiment for the catalytic hydro-
esterification reaction was performed as follows: A 25 mL Schlenk flask
equipped with a Rotaflo valve and a magnetic stirrer was charged in a
glovebox with a THF solution (1 mL) of [(dippe)Ni(μ-H)]2 (0.123 g,
0.192 mmol) and a methanol solution (4 mL) of styrene (111.1 μL,
0.961 mmol). The reaction mixture was stirred for 10 min, and then a
CO2 stream was bubbled at room temperature for 20 min, followed by
heating in an oil bath up to 120 °C for 36 h. After this time the
reaction mixture was analyzed by CG-MS.
Compound [(dtbpe)Ni(CO)2] (10) was characterized by standard
spectroscopic methods. Yield for complex 10: 71.4%. Anal. Calcd for
C20H40NiO2P2 (10): C, 55.45; H, 9.31. Found: C, 55.41; H, 9.28.
31P{1H} NMR (22 °C, 300 MHz, CD3OD): δ 94.3 (s, (i-
1
Pr2)2PCH2CH2P(i-Pr2)2). H NMR (22 °C, 300 MHz, CD3OD2): δ
1.79−1.72 (m, CH2, 4H), 1.51−1.40 (m, 36H). 13C{1H} NMR (22
°C, 300 MHz, CD3OD): δ 203.7 (t, CO, JC−P = 4.4 Hz), 35.9 (m,
CH2), 30.94 (s, (CH(CH3)2), 22.4 (m, (CH(CH3)2). Suitable crystals
for X-ray diffraction studies for complex 6 were obtained by slow
evaporation of a solution in deuterated methanol and separated by
decantation.
The corresponding esters were purified by column chromatography
using silica gel, eluting with hexane/ethyl acetate, 80:20. The solvent
was removed from all fractions, and the corresponding residues were
vacuum-dried for 6 h.
For methyl 2-phenylpropanoate (5e). Yield: 3%. Anal. Calcd for
C10H12O2 (5e): C, 73.15; H, 7.37. Found: C, 73.11; H, 7.34. 1H NMR
(22 °C, 300 MHz, CDCl3): δ 7.39−7.37 (m, CH, 2H), 7.29−7.27 (m,
CH, 3H), 3.8 (q, CH, H), 3.68(s, CH3, 3H), 1.6 (d, CH3, 3H).
13C{1H} NMR (22 °C, 300 MHz, CDCl3): δ 173.7 (s, C), 135.5 (s,
Yield for (dtbpeO2) (11): 1.7%. 31P{1H} NMR (22 °C, 300 MHz,
CD3OD): δ 63.7 (s, OP(i-Pr2)CH2CH2(i-Pr)2PO). 1H NMR (22
°C, 300 MHz, CD2Cl2): δ 1.95−1.92 (m, CH2, 4H), 1.78−1.70 (m,
36H).
Yield for (dtbpeO) (12): 2.2%. 31P{1H} NMR (22 °C, 300 MHz,
CD3OD): δ 62.7 (d, 44.8) and δ 36.4 (d, 44.8).
C), 129.7 (s, CH, 2C), 129.1 (s, CH, 2C), 127.5 (s, CH), 52.5 (s,
CH3), 40.1 (s, CH), 13.7 (s, CH3).
Catalytic Reductive Hydroesterification of Styrenes with
[(dtbpe)Ni(μ-H)]2. A typical experiment for the catalytic hydro-
esterification reaction was performed as follows: A 25 mL Schlenk flask
equipped with a Rotaflo valve and a magnetic stirring bar was charged
in a glovebox with a THF solution (1 mL) of [(dippe)Ni(μ-H)]2
(0.147 g, 0.192 mmol) and a toluene solution (4 mL) of styrene (111
μL, 0.96 mmol). The reaction mixture was stirred for 10 min, and then
a CO2 stream was bubbled at room temperature for 20 min. Then the
Schlenk flask was closed and heated in an oil bath up to 120 °C for 36
h. After this reaction time, the reaction mixture was analyzed by CG-
MS.
Attempts of Styrene Hydroesterification with [(dippe)Ni-
(CO)2]. The complex [(dippe)Ni(CO)2] was independently prepared
and tested as a catalyst precursor in the hydroesterification reaction. A
25 mL Schlenk flask equipped with a Rotaflo valve and a magnetic
stirring bar was charged in a glovebox with a THF solution (1 mL) of
[(dippe)Ni(CO)2] (0.100 g, 0.27 mmol) and a toluene solution (4
mL) of styrene (30.7 μL, 0.27 mmol). The reaction mixture was stirred
for 10 min, and then a CO2 stream was bubbled at room temperature
for 20 min. Then the Schlenk was closed and heated in an oil bath up
to 120 °C for 36 h. After this time the reaction mixture was cooled to
room temperature, and an aliquot was analyzed by CG-MS. No
hydroesterification products were observed, and only small amounts of
homocoupling products derived from styrene were obtained. The
31P{1H} NMR spectrum of this sample corroborated the presence of
For methyl 3-phenylpropanoate (5f). Yield: 62%. Anal. Calcd for
C10H12O2 (5f): C, 73.15; H, 7.37. Found: C, 73.13; H, 7.32. 1H NMR
(22 °C, 300 MHz, CDCl3): δ 7.32−7.25 (m, CH, 2H), 7.23−7.17 (m,
CH, 3H), 3.7 (s, CH3, 3H), 2.95 (t, CH2, 2H), 2.6 (t, CH2, 2H).
13C{1H} NMR (22 °C, 300 MHz, CDCl3): δ 173.3 (s, C), 140.5 (s,
C), 128.5 (s, CH, 2C), 128.3 (s, CH, 2C), 126.2 (s, CH), 51.6 (s,
CH3), 37.7 (s, CH2), 30.9 (s, CH2).
Synthesis of [(dippe)Ni(η2-C,C-styrene)] (5). A 25 mL Schlenk
flask was loaded with a THF solution (1 mL) of A (0.05 g, 0.077
mmol) and a methanol solution (4 mL) of styrene (17.8 μL, 0.154
mmol). The solution was stirred for 20 min. After this time, the
solvent was removed to yield a brown powder, which was further dried
under vacuum for 6 h. Yield for complex 5: 99%. 31P{1H} NMR (22
°C, 300 MHz, C7D8): δ 73.5 (d, P(i-Pr)2CH2CH2P(i-Pr)2, JP−P = 64.1
Hz), 60.0 (d, P(i-Pr)2CH2CH2P(i-Pr)2, JP−P = 63.4 Hz).
Reductive Hydroesterification of Styrenes with 13CO2
Catalyzed by [(dippe)Ni(μ-H)]2. This procedure was similar to the
above-described procedure, using a THF solution (1 mL) of
[(dippe)Ni(μ-H)]2 (0.124 g, 0.192 mmol) and a methanol solution
(4 mL) of styrene (111.1 μL g, 0.961 mmol). The solution was stirred
for 10 min, and then a 13CO2 stream was added for 5 min. The
Schlenk flask was closed and heated in an oil bath up to 120 °C for 36
h. At the end of that reaction time, the reaction mixture was cooled to
rt, vented in a hood, and analyzed by CG-MS and NMR. The
corresponding esters were purified by column chromatography and
fully characterized as described (vide supra).
[(dippe)Ni(CO)2] by a key signal at 73.9 ppm, confirming the high
stability of this complex.
X-ray Structure Determinations. A yellow block (9) and
colorless prism (10) crystals, suitable for X-ray diffraction studies,
were obtained; crystals of 9 were obtained as described above.
The crystals of compounds 9 and 10 were mounted under LVAC
FOMBLIN Y on glass fibers and immediately placed under a cold
nitrogen stream on an Oxford Diffraction Gemini “A” diffractometer
with a CCD area detector, with a radiation source of λCu Kα = 1.5418 Å
using graphite-monochromatized radiation. CrysAlisPro and CrysAlis
RED software packages28 were used for data collection and data
integration. Data sets consisted of frames of intensity data collected
with a frame width of 1° in ω, a counting time of 1.7 to 6.8 s/frame,
and a crystal-to-detector distance of 55.00 mm. The double pass
method of scanning was used to exclude any noise. The collected
frames were integrated by using an orientation matrix determined from
the narrow frame scans. Final cell constants were determined by a
General Hydroesterification Procedure with [(dtbpe)Ni(μ-
H)]2 in Stoichiometric Conditions. The same procedure above-
described was followed using [(dtbpe)Ni(μ-H)]2 (B), a THF solution
(1 mL) of [(dtbpe)Ni(μ-H)]2 (0.05 g, 0.066 mmol), and a methanol
solution (4 mL) of styrene (15.2 μL, 132 mmol). The reaction mixture
was stirred for 10 min; after this time a CO2 stream was bubbled at
room temperature for 20 min. Later, the reaction mixture was heated
in an oil bath up to 120 °C for 36 h. After this time, the reaction
mixture was analyzed by CG-MS.
Complexes [(dtbpe)Ni(CO3)] and [(ditbpe)Ni(CO)2] were
isolated from this reaction, the first one by adding THF to precipitate
it and the second one by column chromatography using silica gel,
eluting the THF mother liquors with hexane/ethyl acetate, 80:20. The
solvent was removed from all the fractions, and the corresponding
residues were vacuum-dried for 6 h. Complex [(dtbpe)Ni(CO3)] was
G
dx.doi.org/10.1021/om300819d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
global refinement; collected data were corrected for absorbance by
using analytical numeric absorption correction29 using a multifaceted
crystal model based on expressions on the Laue symmetry using
equivalent reflections.
Structure solution and refinement were carried out with the
program(s) SHELXS97,30 SHELXL97; for molecular graphics,
ORTEP-3 for Windows;31 and to prepare the material for publication,
WinGX.32
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the minor twin component refined to 48.7(3)%. Crystal data and
experimental details of the structure determination are listed in the
Supporting Information.
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ASSOCIATED CONTENT
■
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(b) Behr, A., Carbon Dioxide Activation by Metal Complexes;
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* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: juvent@unam.mx.
Notes
(17) Anderson, J. S.; Iluc, V. M.; Hillhouse, G. L. Inorg. Chem. 2010,
49, 10203−10207.
The authors declare no competing financial interest.
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