Oxidative Hydrophenylation of Ethylene Using a Cationic Ru(II) Catalyst: Styrene Production with Ethylene as the Oxidant

Article abstract of DOI:10.1002/ijch.201700099

The complex [(MeOTTM)Ru(P(OCH₂)₃CEt)(NCMe)Ph][BAr′₄] (MeOTMM=4,4’,4’’-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole), BAr′₄=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is used to catalyze the hydrophenylation of ethylene to produce styrene and ethylbenzene. The selectivity of styrene versus ethylbenzene varies as a function of ethylene pressure, and replacing the MeOTTM ligand with tris(1-phenyl-1H-1,2,3-triazol-4-yl)methanol reduces the selectivity toward styrene. For styrene production ethylene serves as the oxidant to produce ethane, as determined by both ¹H NMR spectroscopy and GC-MS. The Ru(III/II) potentials of [(MeOTTM)Ru[P(OCH₂)₃CEt](NCMe)Ph][BAr′₄] (0.86 V) and [(HC(pz⁵)₃)Ru[P(OCH₂)₃CEt](NCMe)Ph][BAr′₄] (0.82 V) (HC(pz⁵)₃=tris(5-methyl-pyrazolyl)methane) are nearly identical. Since catalytic conversion of ethylene and benzene by [(HC(pz⁵)₃)Ru[P(OCH₂)₃CEt](NCMe)Ph][BAr′₄] is known to selectively produce ethylbenzene, the formation of styrene using [(MeOTTM)Ru[P(OCH₂)₃CEt](NCMe)Ph][BAr′₄] is attributed to the substituents on the triazole rings of the MeOTTM ligand.

Full text of DOI:10.1002/ijch.201700099

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DOI: 10.1002/ijch.201700099
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Oxidative Hydrophenylation of Ethylene Using a Cationic
Ru(II) Catalyst: Styrene Production with Ethylene as the
Oxidant
Xiaofan Jia,[a] J. Brannon Gary,[c] Shaojin Gu,[a, d] Thomas R. Cundari,*[b] and T. Brent Gunnoe*[a]
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Professor Bergman’s contributions to chemistry are wide ranging and impactful, and the authors wish to congratulate him for the
well-deserved recognition of the 2017 Wolf Prize in Chemistry.
Abstract: The complex [(MeOTTM)Ru(P(OCH2)3CEt)(NCMe) both 1H NMR spectroscopy and GC-MS. The Ru(III/II)
Ph][BAr’4] (MeOTMM=4,4’,4’’-(methoxymethanetriyl)-tris(1- potentials of [(MeOTTM)Ru[P(OCH2)3CEt](NCMe)Ph][BAr’4]
benzyl-1H-1,2,3-triazole),
BAr’4 =tetrakis[3,5-bis(trifluoro- (0.86 V) and [(HC(pz5)3)Ru[P(OCH2)3CEt](NCMe)Ph][BAr’4]
methyl)phenyl]borate) is used to catalyze the hydrophenyla- (0.82 V) (HC(pz5)3 =tris(5-methyl-pyrazolyl)methane) are
tion of ethylene to produce styrene and ethylbenzene. The nearly identical. Since catalytic conversion of ethylene and
selectivity of styrene versus ethylbenzene varies as a function benzene by [(HC(pz5)3)Ru[P(OCH2)3CEt](NCMe)Ph][BAr’4] is
of ethylene pressure, and replacing the MeOTTM ligand with known to selectively produce ethylbenzene, the formation of
tris(1-phenyl-1H-1,2,3-triazol-4-yl)methanol reduces the se- styrene using [(MeOTTM)Ru[P(OCH2)3CEt](NCMe)Ph][BAr’4]
lectivity toward styrene. For styrene production ethylene is attributed to the substituents on the triazole rings of the
serves as the oxidant to produce ethane, as determined by MeOTTM ligand.
Keywords: Ruthenium · C-H Activation · Olefin Hydroarylation · Ethylbenzene · Styrene
29 1. Introduction
30
barrier for ethylene insertion into Ru–Ph bonds,[27,29,32] and, as
a result, ethylene C–H activation, ultimately to form thermally
stable and catalytically inactive h3-methylallyl com-
plexes,[25,27,29,32] competes with ethylene insertion. Hence, the
most electron-poor complex among the series TpRu
(L)(NCMe)Ph, with L=CO, proved to be the longest-lived
catalyst. Accordingly, more electron-poor Ru(II) complexes
31 Styrene is one of the major building blocks of the plastics
32 industry.[1] Currently, styrene is produced from benzene ethyl-
33 ation, trans-alkylation, and dehydrogenation of ethylbenzene.[2]
34 Ethylbenzene is produced either by traditional Friedel-Crafts
35 alkylation catalyzed by a Lewis acid (e.g., AlCl3) in the presence
36 of a Brønsted acid (such as HCl or HF) or by using a zeolite
37 catalyst.[3–6] Acid-catalyzed alkylation is generally faster for
38 alkyl-substituted arenes, which often gives rise to the undesired
39 polyalkylation of benzene. Thus, in order to optimize yield of
40 ethylbenzene, energy consuming distillation and then trans-
41 alkylation of benzene and polyethylbenzenes are performed.
42 Herein, we disclose new Ru(II) catalysts that convert benzene
43 and two equivalents of ethylene to styrene and ethane in a
44 process that offers possible benefits over existing routes to
45 styrene or other vinyl or alkenyl arenes.
[a] X. Jia, S. Gu, T. B. Gunnoe
Department of Chemistry, University of Virginia, Charlottesville,
Virginia 22904, United States
Fax: 434-924-3710
E-mail: tbg7h@virginia.edu
[b] T. R. Cundari
Department of Chemistry, Center for Advanced Scientific Com-
puting and Modeling (CASCaM), University of North Texas,
Denton, Texas 76203, United States
Fax: 940-565-4318
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Transition metal catalyzed olefin hydroarylation or oxida-
E-mail: t@unt.edu
47 tive olefin hydroarylation that does not generate carbocation
48 intermediates offers a strategy for the alkylation or alkenyla-
49 tion of arenes, respectively, that is complementary to current
50 processes.[7–26] Along these lines, our groups have been study-
51 ing a series of neutral ruthenium(II) catalysts with the general
52 formula TpRu(L)(NCMe)Ph (Tp=hydridotris(pyrazolyl)bo-
[c] J. B. Gary
Department of Chemistry & Biochemistry
Stephen F. Austin State University, Nacogdoches, Texas 75962,
United States
Fax: 936-468-7634
E-mail: garyjb@sfasu.edu
[d] S. Gu
53 rate; L=CO, PMe3, P(pyr)3,
, P(OCH2)3CEt;
School of Materials Science and Engineering, Wuhan Textile Uni-
versity, Wuhan, Hubei 430200, People’s Republic of China
54 pyr=N-pyrrolyl) for catalytic ethylene hydrophenylation.[21,27–
33]
55
In those studies, we demonstrated that more strongly
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/ijch.201700099
56 donating ligands “L” result in an increase in the activation
Isr. J. Chem. 2017, 57, 1–11
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have been pursued in efforts to further improve catalytic
activity.
2. Results and Discussion
One strategy to access more electron-deficient Ru(II)
catalyst precursors has been to prepare cationic variants of
TpRu(L)(NCMe)Ph complexes (Scheme 1). Replacement of
Tp with a tetra(pyrazolyl)alkane resulted in intramolecular C–
H activation of a pyrazolyl ring.[34] Thus, the cationic Ru(II)
complex was synthesized using HC(pz5)3 (HC(pz5)3 =tris(5-
methyl-pyrazolyl)methane) in which the pyrazolyl 5-positions
The ruthenium precursor (h6-p-cymene)Ru(P(OCH2)3CEt)(Ph)
Br (1) and the proligand MeOTMM {4,4’,4’’-(methoxymetha-
netriyl)tris(1-benzyl-1H-1,2,3-triazole)} were prepared accord-
ing to literature procedures.[27,35–36] An acetonitrile solution of
complex 1 was heated for 3.5 hours at 708C to yield the
putative complex (NCMe)3Ru[P(OCH2)3CEt](Ph)Br (2).[35]
Complex 2 was then reacted with MeOTTM to produce
[(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br (3) in 76% iso-
lated yield. A metathesis reaction of 3 with NaBAr’4 in THF
gives the Ru(II) complex [(MeOTTM)Ru(P(OCH2)3
CEt)(NCMe)Ph][BAr’4] (4) in 95% isolated yield (Scheme 2).
10 are protected by incorporation of methyl substituents. Under
11 optimized conditions, the cationic Ru(II) complex [(HC(pz5)3)
12 Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] successfully increased
13 the TON (turnover numbers) of the ethylene hydrophenylation
14 reaction by over 30-fold compared to TpRu(P(OCH2)3
15 CEt)(NCMe)Ph.[27,35]
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Scheme 1. Changes in performance and selectivity based upon the
tridentate ligand coordinated to the Ru(P(OCH2)3CEt)(NCMe)Ph
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fragment (BAr’4 =tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, Bn=
benzyl).
With the success of [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)
35 Ph][BAr’4] for catalytic hydrophenylation of ethylene, we have
36 been seeking to explore related motifs that provide reduced
37 electron density at Ru. Tris(triazolyl)methanol ligands have a
38 similar coordination mode as Tp and HC(pz5)3.[36–38] It was
39 anticipated that tris(triazolyl)methanol ligands would have
40 reduced donor ability compared to Tp and HC(pz5)3. Thus, we
41 sought to prepare a Ru(II) catalyst precursor supported by a
42 tris(triazolyl)methanol ligand to test for ethylene hydropheny-
43 lation. Since we have obtained data to directly compare
44 catalytic ethylene hydrophenylation using TpRu(P(OCH2)3
45 CEt)(NCMe)Ph and [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph]
46 [BAr’4],[35] we sought to prepare variations of this motif with
47 tris(triazolyl)methanol in place of Tp and HC(pz5)3. Herein,
48 we report on the synthesis and studies of catalytic ethylene
49 hydrophenylation, including the surprising result of selective
50 styrene production in which ethylene serves as oxidant (see
51 Ru(II) catalyst on right in Scheme 1).
Scheme 2. Synthesis of [(MeOTTM)Ru[P(OCH2)3CEt](NCMe)Ph]
[BAr’4] (4) (MeOTMM=4,4’,4’’-(methoxymethanetriyl)-tris(1-benzyl-
1H-1,2,3-triazole), Bn=benzyl).
Cyclic voltammetry of complex 4 shows a reversible
Ru(III/II) oxidation at 0.86 V (vs. NHE), which is a +0.04 V
shift compared to the previously reported complex, [(HC(pz5)3
Ru(P(OCH2)3)CEt)(NCMe)Ph][BAr’4].[35] Chart 1 shows a
series of Ru(II) complexes with P(OCH2)3CEt, NCMe and Ph
ligands coordinated by different tridentate ligands and Ru(III/
II) potentials.[27,29,35] Surprisingly, the exchange of HC(pz5)3
with MeOTTM has negligible impact on the Ru(III/II) redox
potential. Therefore, we anticipated that complex 4 might
display catalytic activity and selectivity for ethylene hydro-
phenylation similar to [(HC(pz5)3Ru(P(OCH2)3)CEt)(NCMe)
Ph][BAr’4].[35]
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styrene TON of 53. At all temperatures, styrene is favored
over ethylbenzene with styrene/ethylbenzene ratios ranging
from 6.9 to 70. At 1508C, we explored ethylene hydro-
phenylation under different ethylene pressures (Table 2). The
results indicate that the selectivity for styrene increases with
increasing ethylene pressure, and 100% selectivity toward
styrene was observed under 75 psig of ethylene.
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Table 2. TON of styrene and ethylbenzene and % styrene under
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different ethylene pressures.a
The use of complex 4 as a catalyst for ethylene hydro-
15 phenylation was investigated at different temperatures (Ta-
16 ble 1). The catalytic reactions produce substantial quantities of
17 both ethylbenzene and styrene. Heating 10 mL benzene
18 solutions of complex 4 (0.001 mol% relative to benzene) at
19 different temperatures (908C, 1208C, 1508C and 1808C)
20 affords up to 57 TON with a mixture of styrene and
21 ethylbenzene after 4 hours (three experiments were performed
22 at each temperature). The data show that higher temperatures
23 facilitate the catalytic reaction. However, catalyst decomposi-
24 tion is also accelerated at elevated temperatures. The optimal
25 temperature was determined to be 1508C, which provided a
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Table 1. TON for catalytic hydrophenylation of ethylene using
[(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] (4).
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a Conditions: 0.001 mol% of complex 4 dissolved in 10 mL benzene
with decane as an internal standard at 150 8C.b TON were
determined by GC-FID after 4 hours and are the average of three
separate experiments. Standard deviations are given in parentheses.
Since the cationic Ru(II) complex [(HC(pz5)3)Ru(-
P(OCH2)3CEt)(NCMe)Ph][BAr’4] is selective for ethylbenzene
production and is electronically similar to complex 4 (i.e.,
similar Ru(III/II) potentials),[35] we considered that the for-
mation of styrene using 4 might be a result of the steric
influence of the benzyl groups at the triazolyl 4-position.[36]
Thus, we sought to change the identity of the 4-position
substituent to determine the influence of ancillary ligand
sterics on selectivity. We prepared the complex [(PhTTM)
Ru(P(OCH2)3CEt)(NCMe)Ph]Br (5) (PhTTM=tris(1-phenyl-
1H-1,2,3-triazol-4-yl)methanol) as shown in Scheme 3. Cyclic
voltammetry data show that the electron densities change of
complex 4 (0.86 V vs NHE) and complex 5 (0.85 V vs NHE)
are nearly identical, which suggests negligible differences in
the electron donor ability of the MeOTTM and PhTTM
ligands coordinated to Ru(P(OCH2)3CEt)(NCMe)Ph.
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a
Conditions: 0.001 mol% of complex 4 dissolved in 10 mL of
benzene with decane as an internal standard and 40 psig of
ethylene. TON were determined by GC-FID after 4 hours and are
b
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We probed ethylene hydrophenylation with 5 as a catalyst
precursor at variable ethylene pressures (Figure 1). The results
the average of three separate experiments. Standard deviations are
given in parentheses.
Isr. J. Chem. 2017, 57, 1–11
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Table 3. The effect of external oxidants on the oxidative ethylene
hydrophenylation catalyzed by complex 4.
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Scheme 3. Synthesis of [(PhTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br (5).
a Reactions were performed with 0.001 mol% complex 4 dissolved in
10 mL benzene with decane as an internal standard at 1508C with
40 psig of ethylene. 10 equivalents of copper(II) salt relative to
complex 4 were used for entries 2 and 3, 15 psig of air was used in
entry 4. c TON were determined by GC-FID after 4 hours and are the
average of three experiments. Standard deviations are given in
parenthesis.
b
Figure 1. Percent styrene formation as
a function of ethylene
formation of one equivalent of ethane per equivalent of
styrene. The production of ethane was confirmed by heating
the solution of complex 4 in C6D6 with 25 psig ethylene
pressure at 708C for 40 hours. Monitoring by 1H NMR
spectroscopy reveled styrene production as well as a singlet at
0.88 ppm, which is consistent with ethane-d1. Additionally,
ethane-d1 was detected in the head space of the reactor by GC-
MS. Thus, the overall reaction for styrene formation is likely
conversion of benzene and two equivalents of ethylene to
styrene and ethane.
pressure using catalyst precursors [(MeOTTM)Ru(P(OCH2)3
CEt)(NCMe)Ph]Br (3) and [(PhTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br
(5) (Conditions: 0.001 mol% of complex 3 or 5 dissolved in 10 mL
benzene with decane as an internal standard at 1508C).
36 show that the cationic ruthenium catalyst 5 is less selective for
37 styrene production than complex 4. Although the steric A
38 values, which are indicative of the steric bulk of the
39 substituent based on impact on substituted cyclohexane
40 confirmations,[39] show that the steric bulk of a phenyl group
41 (A=3) is larger than that of a benzyl group (A=1.81),[40] the
42 differences in the two-dimensional phenyl versus three-dimen-
43 sional benzyl render any evaluation of relative steric influence
44 challenging. At a minimum, the selectivity difference for
45 ethylbenzene versus styrene production using complexes 3 and
46 5 indicate that the steric profile of the triazole substituent
47 plays a role.
A proposed catalytic cycle is shown in Scheme 4. Initial
exchange of coordinated NCMe with ethylene forms
[(MeOTTM)Ru(P(OCH2)3CEt)(h2-C2H4)Ph][BAr’4].
Then
ethylene inserts into the Ru–Ph bond and results in the
formation of
phenethyl intermediate.[8] The complex
[(MeOTTM)Ru(P(OCH2)3CEt)(h2-styrene)(H)]+
can be
a
formed via b-hydride elimination from [(MeOTTM)Ru(-
P(OCH2)3CEt)(CH2CH2Ph)]+. Net ligand exchange of styrene
and ethylene completes the process for styrene formation.
Ethylene then inserts into the RuÀH bond of [(MeOTTM)
Ru(P(OCH2)3CEt)(h2-C2H4)(H)]+ to form a Ru-ethyl complex.
Finally, the coordination and C–H activation of benzene
liberates ethane and regenerates the RuÀPh moiety.
48
We recently reported that Rh catalyst precursors efficiently
49 catalyze oxidative olefin hydroarylation with Cu(II) oxidants
50 to form alkenyl arene.[41–43] Thus, we attempted to improve
51 catalysis using 4 by adding external oxidants. However, when
52 O2 or Cu(II) salts were added to catalytic reaction using 4, no
53 improvement in catalysis was observed (Table 3).
One likely key step in olefin hydroarylation catalyzed by 4
and 5 is the dissociation of the coordinated acetonitrile to
create a coordination site for ethylene. We expected that the
cationic nature of complexes 4 and 5 might slow the rate of
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Under anaerobic conditions and in the absence of an
55 external oxidant {e.g., Cu(II)}, we assumed that ethylene is
56 serving as the oxidant for styrene production with the
Isr. J. Chem. 2017, 57, 1–11
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Scheme 6. Kinetic isotope effect experiment using [(MeOTTM)
Ru(P(OCH2)3CEt)(Ph)(NCMe)][BAr’4] (4).
C6D6, the previously reported complexes TpRu(CO)(NCMe)
Ph and [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] ex-
hibit KIEs of 2.1(1) and 2.11(5), respectively.[29,35] We probed
catalysis (0.001 mol% of 4 in benzene at 1508C and 40 psig
ethylene) in a 1:1 molar ratio of C6H6 to C6D6. Presumably,
this leads to the formation of both [(MeOTTM)Ru(P(OCH2)3
CEt)(CH2CH2C6H5)][BAr’4] and [(MeOTTM)Ru(P(OCH2)3
CEt)(CH2CH2C6D5)][BAr’4] as intermediates, which would
result in the production of per-protio styrene and styrene-d5.[41]
The ratio of styrene and styrene-d5 provides a determination of
the KIE for catalytic styrene formation. After 1 hour, a kH/kD
of 3.2(6) was determined by examining the ratio of perprotio-
styrene (m/z=104) to styrene-d5 (m/z=109) (Scheme 6). In
addition, benzene H/D exchange is observed when heating up
4 in benzene-d6 solution, indicating that the stoichiometric
C6D6 activation occurs. This observation supports the possi-
bility that the cleavage of the C–H bond of benzene precedes
or is the rate-determining step in the catalytic cycle.
In an effort to gain more insight into the proposed
mechanism of this transformation, density functional theory
(DFT) calculations were employed. Complex 5 was employed
as the model system as it eliminates the multiple possible
confirmations associated with the benzyl substituents of the
MeOTMM analog in complex 3. Examining the proposed
mechanism given in Scheme 4, the species were optimized
using the Gaussian09 software package[44] in the gas phase
using the B3LYP functional[45–46] and LANL2dz basis set for
Ru and 6-31G(d,p) basis set on all other atoms. Single point
energies of the optimized geometries were performed using
LANL2DZ/6-311G(d,p) basis sets in benzene solvent using
the SMD solvation model[47] with GD3BJ empirical disper-
sion,[48] and thermal frequency corrections at 423.15 K were
employed to calculate Gibbs energy values.
Scheme 4. Proposed catalytic cycle for oxidative ethylene hydro-
phenylation using [(MeOTTM)Ru(P(OCH2)3CEt)(Ph)(NCMe)][BAr’4]
(4) ([Ru]=[(MeOTTM)Ru(P(OCH2)3CEt)]+).
29 NCMe dissociation compared to the charge neutral complex
30 TpRu(P(OCH2)3CEt)(NCMe)Ph. The rate of exchange be-
31 tween coordinate NCMe and free NCMe-d3 was determined
32 using 1H NMR spectroscopy by heating acetonitrile-d3
33 solutions of complex 4 at 70, 75, 90, and 1058C (Scheme 5).
34 Values for kobs of 1.53(2)3 10À5 sÀ1 (708C), 2.8(3)3 10À5 sÀ1
35 (758C), 2.1(2)3 10À4 sÀ1 (908C) and 1.0(1)3 10À3 sÀ1 (1058C),
1
36 were determined by monitoring the reactions using H NMR
37 spectroscopy. As anticipated, at 708C the exchange rate for 4
38 is slower by ~2 fold compared to the charge neutral complex
39 TpRu(P(OCH2)3CEt)(NCMe)Ph (kobs =3.2(2)3 10À5 sÀ1 at
40 708C.[29]
41
We probed for a possible kinetic isotope effect (KIE)
42 comparing the rates of catalysis using C6H6 and C6D6
43 (Scheme 6). For hydrophenylation of ethylene using C6H6 and
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Calculations were referenced to the starting precatalyst 5
as the zero point in Gibbs energy (Scheme 7). Loss of
acetonitrile is calculated to be moderately endergonic by
12.1 kcal/mol, and binding of ethylene is slightly stabilizing
compared to the 5-coordinate intermediate at 10.6 kcal/mol.
The transition state for ethylene insertion into the Ph ligand is
calculated to be 31.6 kcal/mol, about 21 kcal/mol above the
precursor [(PhTTM)Ru(P(OCH2)3)CEt](h2-C2H4)Ph][BAr’4].
Scheme 5. Degenerate NCMe/NCMe-d3 exchange for [(MeOTTM)
Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] (4).
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Scheme 7. Calculated free energies (in kcal/mol) for oxidative ethylene hydrophenylation to form styrene using 5 {[Ru]=(PhTTM)Ru(P(OCH2)3
CEt)}.
27 This insertion results in an phenethyl ligand that possesses an
benzene with the phenethyl unit (the transition state that would
likely be responsible for ethylbenzene formation) is calculated
to be 3.6 kcal/mol higher in energy (40.5 kcal/mol) than the
same benzene CÀH activation by the Ru-ethyl fragment
(36.9 kcal/mol). If one envisions a Curtin-Hammett scenario,
then all of the olefin insertion steps are reversible and the
product selectivity would be controlled by the relative energy
barriers for benzene CÀH activation (Scheme 8). That is, the
overall calculated free energy of activation (using 5 as the
benchmark) for ethylbenzene formation is 40.5 kcal/mol while
that for styrene formation is 36.9 kcal/mol. Another possible
product controlling feature could be relative binding of styrene
versus ethylene to the 5-coordinate hydride intermediate
[(PhTTM)Ru(P(OCH2)3CEt)(H)]+. Calculations predict the
ethylene binding is favored over styrene coordination by
3.4 kcal/mol, again indicating a preference for styrene for-
mation. In this explanation, it is the rapid equilibrium between
[(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-styrene)]+ and [(PhTTM)
Ru(P(OCH2)3CEt)(H)(h2-C2H4)]+, which the calculations pre-
dict favors the ethylene complex, and the relative rates of
ethylbenzene formation from the styrene complex and styrene
and ethane formation from ethylene complex that dictates the
ethylbenzene/styrene ratio. The calculated free energy of
activation from [(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-styrene)]+
to the benzene CÀH activation transition state that produces
ethylbenzene is 31.5 kcal/mol, while the calculated free energy
of activation from [(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-C2H4)]+
to the benzene CÀH activation transition state that produces
styrene/ethane is 30.3 kcal/mol. It is this more complete
assessment that provides a rationalization for increased styrene
28 agostic CÀH bond interaction that is calculated to be
29 17.0 kcal/mol relative to the precatalyst 5. A b-hydride
30 elimination step is calculated to have a very low barrier at
31 only 0.8 kcal/mol above the phenethyl intermediate with an
32 overall free energy of 17.8 kcal/mol above 5. The resulting
33 Ru-styrene/hydride intermediate is calculated to be endergonic
34 from the precatalyst 5 by 9.0 kcal/mol. Loss of styrene to yield
35 the coordinatively unsaturated hydride species is 9.7 kcal/mol
36 or approximately energetically equivalent to the styrene
37 hydride. Ethylene binding to the hydride is calculated to be
38 slightly more favorable, having an energy of 6.6 kcal/mol
39 relative to the precatalyst. The transition state for ethylene
40 insertion into the hydride is also quite low with a free energy
41 of 15.0 kcal/mol, which results in an agostic stabilized ethyl
42 intermediate with an energy of 12.3 kcal/mol. The largest
43 calculated barrier in the system is the transition state for a s-
44 bond metathesis CÀH activation in which benzene is
45 exchanged for the ethyl ligand and has an energy of 37.0 kcal/
46 mol relative to the precatalyst. An off-cycle intermediate is the
47 binding of NCMe to the 5-coordinate hydride, which is
48 calculated to be the likely resting state of the system with an
49 energy of À4.5 kcal/mol relative the starting precatalyst.
50
The calculations indicate that the proposed mechanism
51 shown in Scheme 4 is plausible with the highest barrier
52 resulting from CÀH activation of benzene to release ethane,
53 which is consistent with the observed kinetic isotope effect
54 (kH/kD) of 3.2(6) (see above). Two possible explanations for
55 the styrene selectivity (versus ethylbenzene) arise from the
56 calculations. First, the s-bond metathesis CÀH activation of
Isr. J. Chem. 2017, 57, 1–11
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Ph]+. Thus, the change in selectivity for styrene versus
ethylbenzene is likely a result of steric profile of the ligands.
That is, the benzyl and phenyl substituents on the MeOTTM
and PhTTM ligands biases the catalysis toward styrene
production. DFT calculations provide an explanation: the
equilibrium between [(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-
C2H4)]+ to [(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-styrene)]+ fa-
vors the ethylene complex, which ultimately leads to styrene
formation. Furthermore, assessment under Curtin-Hammett
conditions is consistent with favorable styrene production (see
above). Since ethylene is produced directly from ethane, this
new process for styrene production without an added external
oxidant is promising as outlined in Scheme 9.
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Scheme 9. Vision for a new process for styrene production.
Scheme 8. The lower free energy of the transition state for CÀH
activation by the Ru-ethyl complexes (right) than that of the Ru-
phenethyl complex (left) favors styrene production over ethylbenzene
{[Ru]=(PhTTM)Ru(P(OCH2)3CEt)}.
Experimental Section
General Considerations: Unless otherwise noted, all synthetic
procedures were performed under anaerobic conditions in a nitrogen-
filled glovebox or by using standard Schlenk techniques. Glovebox
purity was maintained by periodic nitrogen purges and was monitored
by an oxygen analyzer (O2 <15 ppm for all reactions). Tetrahydrofur-
an and diethyl ether were dried by distillation from sodium/
benzophenone, respectively. Benzene, methylene chloride, and hex-
anes were purified by passage through a column of activated alumina.
29 production at higher ethylene pressures since these conditions
30 should increase the ratio of [(PhTTM)Ru(P(OCH2)3
31 CEt)(H)(h2-C2H4)]+ to [(PhTTM)Ru(P(OCH2)3CEt)(H)(h2-
32 styrene)]+.
Benzene-d6, acetone-d6, CD3CN, and THF-d8 were used as received
and stored under a N2 atmosphere over 4 A molecular sieves. H NMR
spectra were recorded on a Varian 600, Varian 500 MHz or a Bruker
600 MHz or 800 MHz spectrometer, and 13C{1H} NMR spectra were
recorded on a Varian 600 MHz (operating frequency 125 MHz),
33
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35 3. Summary and Conclusions
36
37 The new cationic Ru(II) complexes supported by tris(triazolyl)
38 methanol ligand [(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]+
39 and [(PhTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]+ catalyze oxida-
40 tive ethylene hydrophenylation to produce styrene. In contrast,
41 similar Ru(II) catalysts with trispyrazolylborate (Tp) or
42 trispyrazolylmethane ligands are highly selective for ethyl-
43 benzene production.[29,35] The complex [(MeOTTM)Ru(-
44 P(OCH2)3CEt)(NCMe)Ph][BAr’4] produces 53 TON of styrene
45 under 40 psig of ethylene at 1508C, and the selectivity toward
46 styrene production increases when higher ethylene pressures
47 are used. The work reported herein is unique in its direct use
48 of ethylene, a reactant, as the oxidant for this process whereas
Bruker 600 MHz (operating frequency=150 MHz) or
a Bruker
800 MHz (operating frequency=201 MHz). All 1H and 13C spectra are
referenced against residual proton signals (1H NMR) or 13C resonances
(13C NMR) of the deuterated solvents. 31P{1H} NMR spectra were
obtained on a Varian 500 MHz (operating frequency=201 MHz) or
Varian 600 MHz (operating frequency=243 MHz) spectrometer and
referenced against an external standard of H3PO4 (d=0). GC/MS was
performed using a Shimadzu GCMS-QP2010 Plus system with a
30 m3 0.25 mm RTx-Qbond column with 8 mm thickness using
electron impact ionization. GC/FID was performed using a Shimadzu
GC-2014 system with a 30 m3 90.25 mm HP5 column with 0.25 mm
film thickness. Styrene production was quantified using linear
regression analysis of gas chromatograms of standard samples of
authentic product. A plot of peak area ratios versus molar ratios gave a
regression line. For the GC/FID system, the slope and correlation
coefficient of the regression line were 1.34 and 0.99, respectively. FID
response factors for other products were determined in a similar
fashion, using authentic standards of products. All other reagents were
used as received from commercial sources. The preparation, isolation
and characterization of [(h6-p-cymene)Ru(Br)(m-Br)]2, NaBAr’4 (so-
dium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), Ph2Mg[THF]2,
49 previously disclosed work by our group and that of Milstein
[22]
50 and co-workers utilized copper(II) salts[41–43] or O2
as
51 oxidant. The reduced ethylene, to form ethane, can in principle
52 be easily recycled to ethylene. Determination of reversible
53 Ru(III/II) potentials using cyclic voltammetry indicate minor
54 differences in electron-density at Ru for [(HC(pz5)3Ru(-
55 P(OCH2)3)CEt)(NCMe)Ph]+,
[(MeOTTM)Ru(P(OCH2)3
MeOTTM
{4,4’,4’’-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-
56 CEt)(NCMe)Ph]+ and [(PhTTM)Ru(P(OCH2)3CEt)(NCMe)
Isr. J. Chem. 2017, 57, 1–11
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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triazole)}, PhTTM {4,4,4’’-(hydroxymethanetriyl)-tris(1-phenyl-1H-
1,2,3-triazole)}, (h6-p-cymene)Ru(P(OCH2)3CEt)(Br)Ph were per-
formed according to literature procedures.[27,35–36,49–50] Elemental analy-
ses were performed by Atlantic Microlab, Inc.
C75H58BO4N10PF24Ru: C, 51.12; H, 3.22; N, 7.95. Found: C, 50.88; H,
3.46; N, 8.17.
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[(PhTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br (5): Following the above
procedure for 3 and using PhTTM, 5 was obtained as a white powder
[(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br (3): The complex (h6-
p-cymene)Ru(P(OCH2)3CEt)(Br)Ph (1) (0.55 g, 1.0 mmol) was dis-
solved in NCMe (20 mL), added to a pressure tube, and heated for
3.5 h at 708C. The reaction was allowed to cool to room temperature.
The mixture was filtered through Celite, and the filtrate was
concentrated to dryness yielding the putative complex (NCMe)3Ru(-
P(OCH2)3CEt)(Br)Ph.[35] The resulting solid was taken up in CH2Cl2
(10 mL) and added to a 50 mL thick-wall glass pressure tube with
MeOTTM (0.57 g, 1.1 mmol) in CH2Cl2 (10 mL). The solution was
heated to 708C for 15 h after which it was cooled to room temperature
and filtered through Celite. The volatiles were removed from the
filtrate under reduced pressure. Benzene was added, and the mixture
was stirred for 10 min. The solution was filtered through Celite, and
the filtrate was discarded. The remaining white solid was dissolved in
CH2Cl2 and filtered through Celite. The filtrate was concentrated to
2 mL, and hexanes were added to induce precipitation. The colorless
precipitate was collected on a fine porosity frit. The solid was washed
(44% yield). H NMR (600 MHz, CD2Cl2) d 8.86 (s, 1H, triazole-H),
1
8.75 (s, 1H, triazole-H), 8.64 (s, 1H, triazole-H), 7.83–7.39 (m, 15H,
phenyl from benzyl), 7.26 (d, 3JHH = 7.4 Hz, 2H, phenyl ortho-H), 6.82
(t, 3JHH =7.3 Hz, 2H, phenyl meta-H), 6.75 (t, 3JHH =7.1 Hz, 1H,
2
phenyl para-H), 4.27 (dt, JHH =6.3, 2.8 Hz, 6H, P(OCH2)3CCH2CH3),
3
2.32 (s, 3H, NCMe), 1.21 (d, JHH =8 Hz, 2H, P(OCH2)3CCH2CH3),
0.82 (t, 3JHH =8 Hz, 3H, P(OCH2)3CCH2CH3). 31P{1H} NMR
(121 MHz, CD2Cl2) d 134.6. Anal. Calcd for C39H38O4N10PBrRu: C,
50.76; H, 4.15; N, 15.18. Found: C, 51.82; H, 4.15; N, 16.26.
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Catalytic Oxidative Hydrophenylation of Ethylene: A representa-
tive catalytic reaction is described. A stock solution containing 4
(0.040 g, 0.023 mmol), decane (88 mL, 0.46 mmol), and benzene
(200 mL) was prepared in a volumetric flask. Thick-walled Fisher-
Porter reactors were charged with stock solution (10 mL). The vessels
were sealed, pressurized with ethylene (40 psig), and subsequently
stirred and heated to 1508C. The reaction was sampled every 1 h for
the first 2 h, then every 2 h. At each time point, the reactors were
cooled to room temperature, sampled, recharged with ethylene
(40 psig), and heated. Aliquots of the reaction mixture were analyzed
by GC/FID using relative peak areas versus the internal standard.
1
with pentane and dried in vacuo to yield a tan solid (73%). H NMR
(600 MHz, CD2Cl2) d 8.38 (s, 1H, triazole-H), 8.36 (s, 1H, triazole-
H), 8.21 (s, 1H, triazole-H), 7.38–7.29 (m, 15H, phenyl form benzyl),
3
6.91 (d, JHH =7 Hz, 2H, phenyl ortho-H), 6.83-6.76 (m, 3H, phenyl
meta and para-H), 5.65 (m, 1H, ÀCH2À), 5.59-5.54 (m, 2H, ÀCH2À),
5.53 (s, 1H, ÀCH2À), 5.50 (s, 1H, ÀCH2À), 5.46 (s, 1H, ÀCH2À), 4.19-
4.11 (m, 6H, P(OCH2)3CCH2CH3), 4.07 (s, 3H, ÀOCH3), 2.30 (s, 3H,
Temperature Variation Experiments: A stock solution containing 4
(0.001 mol% relative to benzene), decane (20 equiv. relative to 4), and
benzene (200 mL) was prepared in a volumetric flask. Thick-walled
Fisher-Porter reactors were charged with stock solution (10 mL). The
vessels were sealed, charged with ethylene (40 psig), and subsequently
stirred and heated to 90, 120, 150, or 1808C (3 reactors per
temperature). The reaction was sampled every 1 h for the first 2 h,
then every subsequent 2 h. At each time point, the reactors were
cooled to room temperature, sampled under N2, recharged with
ethylene pressure, and reheated. Aliquots of the reaction mixture were
analyzed by GC/FID using relative peak areas versus an internal
standard.
3
3
NCMe), 1.20 (q, JHH =8 Hz, 2H, P(OCH2)3CCH2CH3), 0.83 (t, JHH
=
8 Hz, 3H, P(OCH2)3CCH2CH3).31P{1H} NMR (121 MHz, CD2Cl2) d
134.7. Anal. Calcd for C43H46O4N10PBrRu: C, 52.18; H, 4.80; N,
14.49. Found: C, 52.43; H, 4.92; N, 14.51.
[(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] (4): The Ru(II)
complex
(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph]Br
(0.978 g,
0.300 mmol) was suspended in THF (10 mL) in a round bottom flask
to form a heterogeneous mixture. NaBAr’4 (0.270, 0.303 mmol) in
THF (5 mL) was slowly added, resulting in a colorless homogenous
solution. The reaction was stirred at room temperature for 2.5 h during
which time it turned from colorless to grey. The solution was filtered
through Celite, and the filtrate was concentrated to dryness. The
resulting solid was reconstituted in Et2O and filtered through Celite.
The filtrate was concentrated to dryness to yield a golden solid. The
golden solid was reconstituted in benzene and filtered through Celite.
The filtrate was concentrated to dryness to yield a golden foam solid
(53%). 1H NMR (600 MHz, CD2Cl2) d 7.74 (br s, 8H, BAr’4, ortho-H),
7.64 (s, 1H, triazole-H), 7.63 (s, 1H, triazole-H), 7.59 (s, 1H, triazole-
H), 7.57 (br s, 4H, BAr’4, para-H), 7.38-7.31 (m, 6H, phenyl), 7.24–
7.23 (m, 4H, phenyl), 6.95-6.94 (m, 2H, phenyl ortho-H), 6.87-6.83
Ethylene Pressure Experiments: A stock solution containing 4
(0.001 mol% relative to benzene), decane (20 equiv. relative to 4), and
benzene (200 mL) was prepared in a volumetric flask. Thick-walled
Fisher-Porter reactors were charged with stock solution (10 mL). The
vessels were sealed, charged with ethylene (15, 25, 40, 50, or 75 psig,
3 reactors at each pressure), and subsequently stirred and heated to
1508C. The reaction was sampled every 1 h for 6 h. At each time
point, the reactors were cooled to room temperature, sampled,
recharged with ethylene pressure, and reheated. Aliquots of the
reaction mixture were analyzed by GC/FID using relative peak areas
versus the internal standard (decane).
2
(m, 3H, phenyl meta and para-H), 5.63 (d, JHH =15 Hz, 2H, ÀCH2À),
5.56 (d, 2JHH =15 Hz, 2H, ÀCH2À), 5.51 (d, 2JHH =14 Hz, 2H, ÀCH2À),
5.43 (m, 5H), 4.21 (m, 6H, P(OCH2)3CCH2CH3), 3.86 (s, 3H, ÀOCH3),
Degenerate NCCH3/NCCD3 Exchange for [(MeOTTM)Ru(-
P(OCH2)3CEt)(NCMe)Ph][BAr’4]: In
a 1 mL volumetric flask
3
[(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr’4] (0.054 g) was dis-
solved in CD3CN. A small crystal of hexamethylbenzene was added as
an internal standard. The solution was divided between three J. Young
2.33 (s, 3H, NCMe), 1.21 (q, 2H, JHH =8 Hz, P(OCH2)3CCH2CH3),
0.84 (t, 3H, 3JHH =8 Hz, P(OCH2)3CCH2CH3). 13C NMR (151 MHz,
CD2Cl2) d 166.8 (d, 2JCP =17 Hz, ipso of phenyl), 162.8, 162.5, 162.2,
161.8 (four line pattern, 1JCB =50 Hz, BAr’4 ), 145.8, 145.8, 145.6,
142.4, 134.0, 133.8, 129.9-129.6 (m) (each a s, phenyl groups of
benzyl substituents), 135.4 (s, BAr’4), 128.9 (d, 2JCP =7 Hz), 126.1,
125.8, 124.3 (each a s,triazole-H), 121.1–120.9 (m, ÀCH2À), 118.1 (s,
NCCH3)., 74.6 (d, 2JCP =7 Hz, P(OCH2)3CCH2CH3), 56.3-56.1(m),
55.6(ÀOCH3), 35.8(d, 3JCP =31 Hz, P(OCH2)3CCH2CH3), 24.1 (s,
P(OCH2)3CCH2CH3), 7.5 (s, P(OCH2)3CCH2CH3), 4.9 (s, NCCH3).
31P{1H} NMR (121 MHz, CD2Cl2) d 134.6. 19F NMR (282 MHz,
1
NMR tubes (300 mL per tube). H NMR spectra were taken every 15
minutes. Each spectrum required 2 minutes to complete. Eight scans
were acquired for each spectrum. The delay time was set to 12.8 s, and
the acquisition time was set to 2.2 s. The exchange reaction was
repeated at 708C, 908C and 1108C only the tubes were reheated in a
1
temperature-controlled oil bath. H NMR spectra using a 12.8 s pulse
delay time were acquired periodically. All reactions were monitored
through at least three half-lives.
CD2Cl2)
d
À62.9. CV (NCMe):
E1/2 =0.86 V. Anal. Calcd for
Isr. J. Chem. 2017, 57, 1–11
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Kinetic Isotope Effect Experiments: A stock solution containing 4
(0.001 mol% relative to benzene) and a 1:1 molar mixture of C6H6
and C6D6 (30 mL) was prepared in a volumetric flask. Thick-walled
Fisher-Porter reactors were charged with stock solution (10 mL). The
vessels were sealed, charged with ethylene (40 psig), and subsequently
stirred and heated to 1508C. The reaction was sampled at 1, 2, and
4 h. At each time point, the reactors were cooled to room temperature,
sampled, recharged with ethylene, and reheated. Aliquots of the
reaction mixture were analyzed by GC/MS. KIE was determined by
examining the ratio of styrene (m/z=104) to styrene-d5 (m/z=109) in
the mass spectrum, accounting for the initial isotopic distribution and
natural abundance. No change in the isotopic distribution for benzene
was observed over the course of the reaction, and the observed isotopic
distribution of product was consistent with the initial distribution. No
d6-8 products were observed, except those predicted by the natural
abundance of deuterium in ethylene.
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17 Acknowledgements
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19 This work was supported by the U.S. Department of Energy,
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Received: September 6, 2017
Accepted: October 4, 2017
Published online on && &&, 0000
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© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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X. Jia, J. B. Gary, S. Gu, T. R.
Cundari*, T. B. Gunnoe*
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Oxidative Hydrophenylation of
Ethylene Using a Cationic Ru(II)
Catalyst: Styrene Production with
Ethylene as the Oxidant
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