PtII-catalyzed ethylene hydrophenylation: Influence of dipyridyl chelate ring size on catalyst activity and longevity

Article abstract of DOI:10.1021/cs400231f

Expansion of the dipyridyl ligand from a five- to six-membered chelate for Pt^II-catalyzed ethylene hydrophenylation provides an enhancement of catalyst activity and longevity. Mechanistic studies of [(dpm)Pt(Ph)(THF)] [BAr′₄] [dpm = 2,2′-dipyridylmethane, and Ar′ = 3,5-(CF₃)₂C₆H₃] attribute the improved catalytic performance at elevated temperatures to a favorable change in entropy of activation with an increase in chelate ring size. The Pt ^II catalyst precursor [(dpm)Pt(Ph)(THF)][BAr′₄] is among the most active catalysts for ethylene hydrophenylation by a non-acid-catalyzed mechanism.

Full text of DOI:10.1021/cs400231f

Research Article
pubs.acs.org/acscatalysis
PtII-Catalyzed Ethylene Hydrophenylation: Influence of Dipyridyl
Chelate Ring Size on Catalyst Activity and Longevity
Bradley A. McKeown,† Hector Emanuel Gonzalez,‡ T. Brent Gunnoe,*,† Thomas R. Cundari,*,‡
and Michal Sabat†
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
‡Center for Advanced Scientific Computing and Modeling, Department of Chemistry, University of North Texas, Denton, Texas
76203, United States
S
* Supporting Information
ABSTRACT: Expansion of the dipyridyl ligand from a five- to six-membered
chelate for PtII-catalyzed ethylene hydrophenylation provides an enhancement of
catalyst activity and longevity. Mechanistic studies of [(dpm)Pt(Ph)(THF)]-
[BAr′4] [dpm = 2,2′-dipyridylmethane, and Ar′ = 3,5-(CF3)2C6H3] attribute the
improved catalytic performance at elevated temperatures to a favorable change in
entropy of activation with an increase in chelate ring size. The PtII catalyst
precursor [(dpm)Pt(Ph)(THF)][BAr′4] is among the most active catalysts for
ethylene hydrophenylation by a non-acid-catalyzed mechanism.
KEYWORDS: C−H activation, platinum, aromatic alkylation, olefin hydroarylation, mechanism
5
t
(N∼N = bpy or dpm). Thus, the effect of chelate ring size
on bis(pyridyl) PtII-catalyzed ethylene hydrophenylation can be
directly evaluated without a substantial change in metal electron
INTRODUCTION
■
Synthesis of alkyl arenes from arenes and olefins has historically
been achieved using Lewis acid catalysts.1 Transition metal
catalysts that operate via the insertion of olefin into metal−aryl
bonds and subsequent metal-mediated aromatic C−H
activation offer opportunities for improvement over acid-
catalyzed processes.2 For example, transition metal-mediated
pathways could reduce the level of polyalkylation, bias reactions
with α-olefins toward anti-Markovnikov products, regioselec-
tively produce dialkyl arenes (e.g., 1,2-, 1,3-, or 1,4-dialkyl
benzenes) from monoalkyl arenes, and catalyze oxidative olefin
hydroarylation to directly produce vinyl arenes.2b,c Until
recently, transition metal-catalyzed olefin hydroarylation has
been limited to the use of chelate-assisted or activated
substrates.2a,3 Catalysts based on Ir, Ru, and Pt have been
reported with unactivated substrates (e.g., benzene and
ethylene).2c,d,4 However, these systems have yet to achieve
levels of selectivity and activity sufficient for application to fine
or commodity chemical synthesis, and detailed studies of the
impact of the catalyst’s steric and electronic profile on activity
and/or selectivity are needed for the rational design of
improved catalysts.
t
density by substituting bpy with dpm. Herein, we report that
substituting the bpy ligand of [(tbpy)Pt(Ph)(THF)]+ with
t
dpm provides a more active (at ≥90 °C) and longer-lived
catalyst. In fact, [(dpm)Pt(Ph)(THF)][BAr′4] (2) is among
the most active and longest-lived catalysts for ethylene
hydrophenylation by a non-acid-catalyzed process. Mechanistic
studies suggest that the different activities of the tbpy- and dpm-
supported catalysts are largely a result of entropic factors.
RESULTS AND DISCUSSION
■
Complex 2 was prepared by protonation of (dpm)Pt(Ph)2 (1)
with [H(Et2O)2][BAr′4] at −70 °C in THF (eq 1). Catalytic
ethylene hydrophenylation using 2 was evaluated, and the
results are summarized in Table 1.
Previously, we reported a study of ethylene hydrophenylation
catalyzed by [(tbpy)Pt(Ph)(THF)][BAr′4] [tbpy = 4,4′-di-tert-
butyl-2,2′-bipyridine, and Ar′ = 3,5-(CF3)2C6H3] and proposed
a catalytic cycle that incorporates both ethylene insertion and
benzene C−H bond activation.4b Puddephatt et al. have shown
that tbpy and 2,2′-dipyridylmethane (dpm) have nearly
identical donor abilities when coordinated to PtII, as
determined from a comparison of carbonyl stretching
frequencies of [(N∼N)Pt(CO)(Me)]+ model complexes
At 100 °C under 0.1 MPa of ethylene, a solution of 0.01 mol
% 2 (relative to benzene) results in 55.3 turnovers (TO) of
ethylbenzene, 10.6 TO of diethylbenzenes, and trace quantities
Received: March 26, 2013
Revised: April 15, 2013
Published: May 3, 2013
© 2013 American Chemical Society
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a
Table 1. Catalytic Ethylene Hydrophenylation Using 2
Figure 2. Comparison of TO vs time (≤4 h) for ethylene
hydrophenylation (100 °C) catalyzed by [(N∼N)Pt(Ph)(THF)]+
t
[for N∼N = bpy (squares), R2 = 0.98; for N∼N = dpm (circles),
R2 = 0.99]. At 0.01 mol % Pt in C6H6 with 0.1 MPa of C2H4 and HMB
as an internal standard.
a
At 0.01 mol % catalyst in C6H6 with 0.1 MPa of C2H4 and
To compare the rate of catalytic ethylene hydrophenylation
between 2 and [(tbpy)Pt(Ph)(THF)][BAr′4] (3), we used
TOFs calculated after reaction for 4 h, which show that 2
catalyzes ethylene hydrophenylation ∼3.5 times faster than 3 at
100 °C.4b For example, catalysis at 100 °C under 0.1 MPa of
ethylene pressure with [(tbpy)Pt(Ph)(THF)]+ exhibits a TOF
of 1.3(2) × 10−3 s−1 after 4 h, while a TOF of 4.6(8) × 10−3 s−1
was observed for 2. We selected the 4 h time point because TO
versus time plots reveal little evidence of catalyst deactivation
(Figures 1 and 2). The linear regression in Figure 2 does not
extrapolate through the origin because of the heating period
required to bring the reaction solutions to 100 °C.
b
hexamethylbenzene (HMB) as an internal standard. Ratio of 1,2-,
1,3-, and 1,4-diethylbenzene after 4 h. Turnover frequency calculated
on the basis of the total number of turnovers after 4 h. Turnovers
after 4 h as determined by GC/MS. Numbers in parentheses are
c
d
e
f
turnovers after 16 h. Numbers in brackets are turnovers after 36 h.
of styrene after 4 h, corresponding to a turnover frequency
(TOF) of 4.6 × 10−3 s−1 (the formation of diethylbenzene is
counted as a single catalytic TO). At ≤100 °C, plots of TO
versus time reveal minimal catalyst deactivation after 4 h
(Figures 1 and 2). Prolonged reaction times gave 235.2 and
364.4 total TO of alkyl benzenes after 16 and 36 h, respectively.
A final turnover number (TON) after 110 h at 100 °C is 469.
At 100 °C with 0.1 MPa of ethylene, almost complete
catalyst deactivation is observed after 24 h for 3, but 2 remains
active over a period of more than 4 days (Figure 1). Monitoring
catalysis using 2 until deactivation results in a TON of 469,
t
which is an ∼5.6-fold increase compared to that with the bpy
catalyst (TON of 84) under identical conditions. The identity
of the catalyst decomposition product is unknown as a black
material (presumably Pt black) and several intractable products
1
are observed by H NMR spectroscopy at the end of the
reaction. Using plots of TOF versus time, we modeled catalyst
deactivation kinetically. Both complex 2 and 3 display kinetics
that are best modeled by a process that is second-order in
platinum complex concentration.
Our proposed mechanism for cationic PtII-catalyzed ethylene
hydrophenylation, based on detailed studies of 3, is summarized
in Scheme 1.4b Substitution of the THF ligand with ethylene
initiates the cycle, followed by the insertion of ethylene into the
Pt−Ph bond. After insertion, an additional 1 equiv of ethylene
coordinates to form the catalyst resting state [(N∼N)Pt-
Figure 1. Comparison of TO vs time for ethylene hydrophenylation
(100 °C) catalyzed by [(N∼N)Pt(Ph)(THF)]+ [N∼N = tbpy
(squares) or dpm (circles)]. At 0.01 mol % Pt in C6H6 with 0.1
MPa of C2H4 and HMB as an internal standard.
t
(CH2CH2Ph)(η2-C2H4)]+ [N∼N = dpm (4) or bpy (5)],
which has been observed during catalysis by 1H NMR
spectroscopy for both 2 and 3. Mechanistic studies could not
differentiate between reaction pathways in which the resting
state is incorporated into or removed from the catalytic cycle.4b
Displacement of ethylene by benzene and rate-limiting C−H
activation result in the formation of [(N∼N)Pt(η2-C,C-
The influence of temperature on catalyst activity and stability
was evaluated. At 90 °C under 0.1 MPa of ethylene pressure, a
solution of 2 in benzene results in a TOF of 1.4 × 10−3 s−1
(calculated after 4 h). Thus, increasing the temperature to 100
and 110 °C increases the rate of ethylene hydrophenylation
compared to the rate of the reaction at 90 °C by factors of ∼3
and ∼17, respectively; however, catalyst deactivation becomes
more competitive at elevated temperatures. The TON values
after 36 h at 110 °C (∼331) and 120 °C (∼301) are lower than
that observed at 100 °C (∼364).
t
C6H5Et)(Ph)]+. Calculations for the bpy−Pt complex suggest
that benzene C−H bond activation occurs by oxidative addition
to form a PtIV intermediate.4b Ethylbenzene is liberated by
substitution with ethylene.
In an effort to explain the difference in catalytic activity
between 2 and 3, the insertion of ethylene into the Pt−Ph bond
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Scheme 1. Proposed Catalytic Cycle for Cationic PtII-
Catalyzed Ethylene Hydrophenylation
The reaction of 2 and C6D6 (0.5 M) occurs with a kobs of 9.9(4)
× 10−5 s−1 at 29 °C in CD2Cl2 (eq 3). As observed with olefin
insertion, benzene C−D bond activation by 2 is almost twice as
slow as activation by [(tbpy)Pt(Ph)(THF)]+ [kobs = 1.71(5) ×
10−4 s−1].4b
a
Complexes [(N∼N)Pt(CH2CH2Ph)(η2-C2H4)]+ [N∼N =
dpm (4) or tbpy (5)] have been identified as the resting states
for catalytic ethylene hydrophenylation, and benzene C−H
bond activation is likely the catalytic rate-limiting step for
complex 3.4b Relative rates of stoichiometric benzene activation
by 4 or 5 to produce ethylbenzene and 2 or 3 are similar to that
observed for C6D6 activation by 2 and 3. The dpm complex 4
reacts with benzene (1.5 M) with an observed rate constant of
7.5(3) × 10−5 s−1 at 54 °C. The reaction using 5 proceeds ∼5
times faster with an observed rate constant of ∼4 × 10−4 s−1.
a
[Pt] = (tbpy)Pt or (dpm)Pt.
and benzene C−H bond activation were probed individually for
complex 2. Ethylene readily inserts into the Pt−Ph bond of 2 to
form [(dpm)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (4). Under
pseudo-first-order conditions, the conversion of 2 to 4 in
CD2Cl2 at 23 °C in the presence of 0.4 M C2H4 proceeds with
an observed rate constant of 8.4(9) × 10−4 s−1 (eq 2). Thus,
the observed rate of ethylene insertion for 2 is slower than that
of [(tbpy)Pt(Ph)(THF)]+ (3) (kobs = 1.46 × 10−3 s−1) under
similar conditions by a factor of ∼1.7.4b
1
Highly accurate integration of H NMR spectra was prevented
by coincidental overlap of resonances between 5 and
ethylbenzene, and thus, the ratio of rates is approximate.
The rates of the stoichiometric reactions (<90 °C) are
significantly reduced for the dpm complex, whether utilizing
t
complex 2 or 4, versus those of their bpy analogues 3 and 5,
respectively. It was surmised that entropic factors might be
important to the overall difference in activation barriers
(ΔΔG⧧) for catalytic ethylene hydrophenylation. The rates of
catalytic and stoichiometric reactions were determined over a
range of temperatures. Activation parameters for ethylene
hydrophenylation by each complex were obtained from Eyring
plots (50−100 °C) (Figure 4). Ideally, the rates of catalysis
The structure of 4 is shown in Figure 3. As observed for the
structure of [(tbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ (5),4b the
Figure 3. ORTEP diagram of [(dpm)Pt(CH2CH2Ph)(η2-C2H4)]-
[BAr′4] (4) (50% probability; H atoms and BAr′4 anion omitted for
the sake of clarity). Selected bond lengths (in angstroms): Pt−N1,
2.135(7); Pt−N2, 2.068(6); Pt−C3, 2.049(8); Pt−C1, 2.113(9); Pt−
C2, 2.113(9); C1−C2, 1.37(2). Selected bond angles (in degrees):
N1−Pt−N2, 85.3(3); Pt−C3−C4, 118.9(8); C3−C4−C5, 125.0(1).
Figure 4. Eyring plots for ethylene hydrophenylation catalyzed by
t
[(N∼N)Pt(Ph)(THF)]+ [for N∼N = bpy (squares), R2 = 0.98; for
N∼N = dpm (circles), R2 = 0.98].
phenyl ring is juxtaposed over the cis-pyridyl ring, but the
methylene group of the dpm ligand removes planarity, which
increases the π−π distance for dpm complex 4 compared to
that of tbpy complex 5. In the solid state, the π−π distance in 4
is 4.32 Å, whereas in 5, this distance is only 3.68 Å.6
Comparative rates of benzene C−D bond activation by 2 and
3 were studied using reactions with excess C6D6 to form
[(N∼N)Pt(Ph-d5)(THF)]+ (2-d5 or 3-d5) and free C6H5D.
1
could be determined using in situ H NMR spectroscopy.
However, significant overlap between the resonances of the
catalyst resting state and the alkylbenzene products prevented
accurate integration. We chose instead to compare TOFs
calculated after reaction times in which catalyst decomposition
is negligible (see Figure 2 and Table 2). Using the second-order
rate constants for catalyst decomposition, the percent decrease
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bond activation are almost statistically indistinguishable
between the complexes with values of 23(2) and 19(2) kcal/
mol for 2 and 3, respectively (Table 3). Similar to the ΔS⧧
Table 2. Turnover Frequencies for Ethylene
Hydrophenylation (50−100 °C) and Activation Parameters
a
[(N∼N)Pt(Ph)(THF)]+
dpm (2)
tbpy (3)
Table 3. Activation Parameters and kobs Values for C6D6 C−
a
D Bond Activation at 29−59 °C
TOF
ΔG⧧
TOF
ΔG⧧
calcd
calcd
b
b
temp (°C)
(×10−5 s−1
)
(kcal/mol)
(×10−5 s−1
)
(kcal/mol)
[(N∼N)Pt(Ph)(THF)]+
c
50
1.1(3)
27(3)
27(3)
27(3)
27(3)
1.1(1)
9(1)
27(2)
27(2)
27(2)
27(2)
tbpy (3)
c
dpm (2)
70
6.6(5)
d
kobs
ΔG⧧
kobs
ΔG⧧
90
140(60)
510(20)
44(5)
210(30)
calcd
(kcal/mol)
calcd
(kcal/mol)
b
b
(×10−4 s−1
)
(×10−4 s−1
)
e
temp (°C)
100
ΔH⧧ (kcal/mol)
29(3)
6(9)
23(2)
29
0.99(4)
2.00(5)
6.8(2)
23(2)
23(2)
23(2)
23(2)
23(2)
1.71(4)
5.7(3)
22(2)
22(2)
22(2)
22(2)
22(2)
ΔS⧧ (eu)
−11(6)
37
a
45
9.17(8)
18.5(5)
44(7)
At 0.01 mol % catalyst in C6H6 with HMB as an internal standard.
Free energy of activation calculated using experimental enthalpies and
b
54
17.3(4)
30(2)
c
d
entropies of activation. TOF calculated after 16 h. TOF calculated
59
e
ΔH⧧ (kcal/mol)
ΔS⧧ (eu)
23(2)
0(4)
19(2)
after 4 h. TOF calculated after 1 h.
−10(6)
a
1
Determined by H NMR spectroscopy with 0.03 M Pt and 0.5 M
in catalyst concentration was determined after 1 h at 100 °C.
For catalysis using complex 2, a decrease in active catalyst
concentration is calculated to be <1%. Catalysis with complex 3
is calculated to have ∼3.5% catalyst decomposition after 1 h at
100 °C. The negligible degree of catalyst decomposition at 100
°C and the fact that catalyst decomposition at lower
temperatures will proceed at slower rates support the use of
TO to calculate the TOF for the Eyring plots. The TOF and
activation parameters are summarized in Table 2. The values of
ΔH⧧ are 29(3) and 23(2) kcal/mol for 2 and 3, respectively,
with a ΔΔH⧧ of 6(4) kcal/mol in favor of the bipyridyl-
supported complex 3. Interestingly, the ΔS⧧ value for 2 [6(9)
eu] is positive, while the ΔS⧧ value for 3 [−11(6) eu] is
negative and larger in magnitude. Although the deviations for
the ΔS⧧ values are relatively large, as is often observed, it is
clear that 2 has an entropic advantage over 3, and the Eyring
plot (Figure 4) shows that 2 is a more active catalyst than 3 at
≥90 °C.
b
C6D6 in CD2Cl2 with hexamethyldisilane as an internal standard. Free
energy of activation calculated using experimental enthalpies and
entropies of activation.
values for catalytic ethylene hydrophenylation, 3 suffers a larger
entropic penalty, compared to that of 2, with a ΔS⧧ value of
−10(6) eu for the activation of a C−D bond of C6D6, while the
reaction is almost entropically neutral for 2 [ΔS⧧ = 0(4) eu].
Unfortunately, deviations in ΔS⧧ for C6D6 activation are too
large to provide a meaningful comparison.
DFT studies were used to compare catalysis with the dpm
and tbpy complexes. Because [(N∼N)Pt(CH2CH2Ph)(η2-
C2H4)]+ [N∼N = dpm (4) or bpy (5)] is observed as the
t
catalyst resting state,4b activation parameters for benzene C−H
bond activation were calculated relative to these intermediates
(full details are provided in the Supporting Information). Given
π-arene stacking between a pyridine ring of N∼N and the
phenyl group of the phenethyl ligand in the crystal structures of
[(N∼N)Pt(CH2CH2Ph)(η2-C2H4)]+, calculations with and
without this interaction in the resting states and transition
states were modeled. We focused on C−H oxidative addition of
benzene because previous research indicated this to be the
preferred pathway for benzene C−H bond activation.4b
Because of the lack of highly accurate integration in 1H NMR
spectra for stoichiometric reactions of 4 and 5 with benzene,
activation parameters for stoichiometric C−D bond activation
of C6D6 by 2 and 3 were obtained from Eyring plots [29−59
°C (Figure 5)]. Enthalpies of activation for C6D6 (0.5 M) C−D
t
For both the dpm and bpy resting states, 4 and 5,
respectively, the nonstacked conformations are calculated to
be more favorable at 100 °C (Scheme 2) and were used to
calculate activation parameters for benzene C−H bond
activation. Table 4 shows calculated activation parameters for
the lowest-energy conversions of 4 and 5 to the transition states
for benzene C−H bond activation. Consistent with exper-
imental observations, calculated activation parameters for 4 and
5 are similar. At 100 °C, the model predicts complex 4 to have
an ∼3 kcal/mol advantage over complex 5. Importantly, DFT
calculations indicate a more favorable ΔS⧧ for 4 compared with
that for 5. Given the anticipated limits of such calculations and
experimental uncertainties, the DFT calculations are in
agreement with experiment.
CONCLUSIONS
■
In summary, an increase in the chelate size for bis-pyridyl
ligands on PtII catalysts for ethylene hydrophenylation provides
an enhancement of the activity and longevity. Using the dpm
catalyst precursor 2 provides a TON of ∼470, which, to the
best of our knowledge, is comparable to the best catalyst for
Figure 5. Eyring plots for C6D6 C−D bond activation by
t
[(N∼N)Pt(Ph)(THF)]+ [for N∼N = bpy (squares), R2 = 0.98; for
N∼N = dpm (circles), R2 = 0.99]. Because of their small values, the
deviations are obscured by the size of the data point markers.
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dichloromethane-d2 were used as received and stored under a
N2 atmosphere over 4 Å molecular sieves. H NMR spectra
Scheme 2. Calculated Equilibria (100 °C) between
Conformations of Catalyst Resting States with or without
π−π Arene Interaction
1
were recorded using a Varian Mercury 300, Unity Innova 500
MHz, or Bruker 800 MHz spectrometer. 13C NMR spectra
were recorded using a Varian Mercury 300 (operating
frequency of 75 MHz), Unity Innova 500 MHz (operating
frequency of 125 MHz), or Bruker 800 MHz spectrometer
1
(operating frequency of 201 MHz). All H and 13C NMR
spectra are referenced against residual proton signals (1H
NMR) or the 13C resonances (13C NMR) of the deuterated
solvents. 19F NMR (operating frequency of 282 MHz) spectra
were obtained on a Varian 300 MHz spectrometer and
referenced against an external standard of hexafluorobenzene
(δ −164.9). GC/MS was performed using a Shimadzu GCMS-
QP2010 Plus system with a 30 m × 0.25 mm SHRXI-5MS
column with a 0.25 mm film thickness using negative chemical
ionization (NCI), which also allows for simulated electron
impact (SEI) ionization, or electron impact (EI) ionization.
Ethylene (99.5%) was purchased in a gas cylinder from GTS-
Welco and used as received. All other reagents were used as
purchased from commercial sources. The preparation, isolation,
and characterization of [Ir(μ-acac-O,O,C3)-(acac-O,O)-(acac-
C3)]2,7 [H(Et2O)2][BAr′4] [Ar′ = 3,5-(CF3)2C6H3],8 [Pt-
(Ph)2(Et2S)]2,9 2,2′-dipyridylmethane,10 [(tbpy)Pt(Ph)-
(THF)][BAr′4],4b and [(tbpy)Pt(CH2CH2Ph)(η2-C2H4)]-
[BAr′4]4b have been previously reported.
Table 4. Calculated Activation Parameters (100 °C) for
Benzene Activation by [(N∼N)Pt(CH2CH2Ph)(η2-C2H4)]+
dpm (4)
tbpy (5)
ΔG⧧ (kcal/mol)
ΔH⧧ (kcal/mol)
ΔS⧧ (eu)
36.8
37.0
0.5
39.9
39.4
−1.6
Computational Methods. All calculations were conducted
utilizing Gaussian03.11 The B3LYP12 functional was employed
in conjunction with the Stevens effective core potentials and
valence basis sets13 augmented by a d polarization function for
carbon (ξC = 0.8) and nitrogen (ξN = 0.8), viz. CEP-31G(d).
Closed-shell (diamagnetic) species were modeled within the
restricted Kohn−Sham formalism. All systems were fully
optimized without symmetry constraints, and analytic calcu-
lations of the energy Hessian were performed to confirm
stationary points as minima or transition states and to obtain
vibrational frequencies that were used in the calculation of gas-
phase free energies at 1 bar and 298.15 K.
ethylene hydrophenylation by a non-acidic pathway. For
example, Periana, Goddard, and co-workers have reported
that [Ir(μ-acac-O,O,C3)(acac-O,O)(acac-C3)]2 (6) catalyzes
ethylene hydrophenylation with a TON of 455 after 3 h at
180 °C, which is, to the best of our knowledge, among the most
effective transition metal catalysts for ethylene hydrophenyla-
tion.4a Because the conditions for catalysis using 2 and 6 are
different, we synthesized and tested 6 for catalytic activity under
the conditions used in this study. At 120 °C under 0.1 MPa of
ethylene, the iridium-catalyzed reaction (0.01 mol % Ir) yielded
4.0 and 12.9 TO of ethylbenzene after 4 and 16 h, respectively,
which corresponds to a TOF of 2.8 × 10−4 s−1. Complex 2
gives a TOF (120 °C) of 1.8 × 10−2 s−1, which is 65 times more
active than the Ir catalyst. In addition, we probed the TON
using the Ir catalyst 6 at 180 °C using the same catalyst loading
that was used in the Pt-catalyzed reactions (i.e., 0.01 mol %).
Under these conditions, complex 6 provided an ethylbenzene
TON of 161. Thus, complex 2 is among the most active and
longest-lived catalysts for the non-acid-catalyzed hydropheny-
lation of ethylene. Analyses of the elementary steps of the
catalytic cycle (e.g., ethylene insertion and benzene C−H bond
activation) demonstrate that the larger chelate ring of the dpm
ligand provides an entropic advantage compared to that of tbpy,
which results in increased catalytic activity at elevated
temperatures.
Synthesis of (dpm)PtPh2 (1). To a suspension of
[Pt(Ph)2(Et2S)]2 (0.35 g, 0.40 mmol) in Et2O (30 mL) was
added 2 equiv of 2,2′-dipyridylmethane (0.14 g, 0.80 mmol).
The suspension was stirred at room temperature for ∼12 h.
The mixture, with a substantial precipitate observed, was
reduced in vacuo, and hexanes were added (∼20 mL). The
solution was filtered, and the white precipitate was washed with
diethyl ether (2 × 5 mL) and hexanes (2 × 5 mL) and dried
1
under vacuum: isolated 0.35 g (83%); H NMR (800 MHz,
3
3
CD2Cl2) δ 8.45 (d, JHH = 6 Hz, 2H, dpm), 7.76 (td, JHH = 8
Hz, JHH = 2 Hz, 2H, dpm), 7.47 (m, 6H, dpm and Ho-Ph),
4
3
3
4
7.07 (ddd, JHH = 8 Hz, JHH = 6 Hz, JHH = 2 Hz, 2H, dpm),
6.90 (t, 3JHH = 8 Hz, 4H, Hm-Ph), 6.77 (t, 3JHH = 7 Hz, 2H, Hp-
Ph), 4.74 (br s, 2H, dpm-CH2); 13C NMR (201 MHz, CD2Cl2)
δ 155.9, 151.9, 145.1, 138.8, 138.1, 127.0, 124.7, 124.2, 121.9
(dpm and Ph aromatic), 47.7 (dpm-CH2). Anal. Calcd for
PtN2C23H20 (%): C, 53.17; H, 3.89; N, 5.39. Found: C, 52.65;
H, 3.98; N, 5.31.
EXPERIMENTAL SECTION
■
General Methods. 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
(<15 ppm O2 for all reactions). Tetrahydrofuran and diethyl
ether were dried by distillation from sodium/benzophenone
and CaH2, respectively. n-Pentane was distilled over P2O5.
Methylene chloride and benzene were purified by being passed
through a column of activated alumina. Benzene-d6 and
Synthesis of [(dpm)Pt(Ph)(THF)][BAr′4] (2). A suspen-
sion of (dpm)Pt(Ph)2 (1) (0.07 g, 0.1 mmol) in THF (30 mL)
was cooled to approximately −70 °C. One equivalent of
[H(Et2O)2][BAr′4] (0.15 g, 0.10 mmol), dissolved in THF
(∼10 mL, 303 K), was added. The solution was immediately
placed under vacuum, and the volatiles were removed under
reduced pressure. The residue was treated with n-pentane (∼2
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Research Article
mL), which was then removed under vacuum to afford a fluffy
blue-green solid. The solid was dried in vacuo: isolated 0.17 g
production of styrene, 1,3-diethylbenzene, 1,4-diethylbenzene,
and 1,2-diethylbenzene. The slope and correlation coefficients
(R2) for these species are 0.51 and 0.99, 0.52 and 0.99, 0.53 and
0.99, and 0.55 and 0.99, respectively.
1
3
(90%); H NMR (500 MHz, CD2Cl2) δ 8.47 (t, JHH = 6 Hz,
2H, dpm), 7.92 (td, 3JHH = 8 Hz, 4JHH = 2 Hz, 1H, dpm), 7.80
(td, 3JHH = 8 Hz, 4JHH = 2 Hz, 1H, dpm), 7.73 (s, 8H, Ho-Ar′),
Determination of the Catalyst Resting State for
Ethylene Hydrophenylation Using Complex 2. Complex
2 (0.02 g, 0.01 mmol) was weighed into a J-Young NMR tube
and dissolved in C6D6 (0.4 mL). The tube was then pressurized
with 0.1 MPa of ethylene and placed in a temperature-
equilibrated NMR probe (90 °C setting). The actual
temperature of the probe (89 °C) was determined using a
solution of 80% ethylene glycol in DMSO-d6.14 Spectra were
collected every 15 min for 4 h with eight scans and a 5.0 s pulse
delay. Beginning with the initial spectrum, the only observable
Pt species in solution was [(dpm)Pt(CH2CH2Ph)(η2-C2H4)]-
[BAr′4].
3
7.62 (d, JHH = 8 Hz, 1H, dpm), 7.56 (s, 4H, Hp-Ar′), 7.48
(overlapping m, 2H, dpm), 7.37 (dd, 3JHH = 8 Hz, 4JHH = 1 Hz,
2H, Ho-Ph), 7.09 (t, 3JHH = 8 Hz, 2H, Hm-Ph), 7.04 (ddd, 3JHH
3
4
3
= 8 Hz, JHH = 6 Hz, JHH = 2 Hz, 1H, dpm) 6.98 (t, JHH = 7
Hz, 1H, Hp-Ph), 4.64 (v br s, 2H, dpm-CH2), 4.05 (s, 4H, α-
THF), 1.78 (s, 4H, β-THF); 13C NMR (75 MHz, CD2Cl2) δ
1
162.3 (q, Ar′, JBCipso = 49 Hz), 156.7, 154.6, 153.7, 149.8,
140.8, 140.3, 136.5, 135.2 (Ar′), 129.4 (q, m-Ar′, 2JCF = 32 Hz),
128.7, 128.2, 126.4, 125.9, 125.5, 125.4, 125.2 (dpm and Ph
aromatic), 125.0 (q, CF3-Ar′, 1JCF = 272 Hz), 117.9 (Ar′), 77.4
(α-THF), 47.4 (dpm-CH2), 24.9 (β-THF); 19F NMR (282
MHz, CD2Cl2) δ −63.1 (s, CF3−Ar′). Anal. Calcd for
PtN2OBF24C53H35 (%): C, 46.20; H, 2.57; N, 2.03. Found:
C, 45.96; H, 2.44; N, 2.13.
Kinetics of Ethylene Insertion. A representative kinetic
experiment is described. Complex 2 (0.057 g, 0.041 mmol) and
hexamethyldisilane (HMDS, 2.0 μL as an internal standard)
were dissolved in 1.3 mL of CD2Cl2. The solution was then
divided (0.4 mL for each sample) and added to three high-
pressure NMR tubes. Each tube was pressurized with 0.4 MPa
of ethylene and placed into a temperature-equilibrated (25 °C
setting) NMR probe. The actual temperature of the probe (23
°C) was determined using a sample of methanol-d4.15 Kinetic
runs were performed in triplicate. The concentration of
ethylene in solution was determined by integration against
Synthesis of [(dpm)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (4).
A solution of 2 (0.042 g, 0.031 mmol) in CD2Cl2 (20 mL) was
transferred to a stainless steel reactor. The reactor was then
pressurized with 3.5 bar of C2H4. The reaction mixture was
stirred for 6 h. The volatiles were removed under vacuum. The
residue was treated with n-pentane (∼2 mL), which was then
removed under vacuum to afford a fluffy solid. The solid was
1
then dried in vacuo: isolated 0.037 g (89%); H NMR (500
MHz, CD2Cl2) δ 8.62 (d, 3JHH = 6 Hz, 1H, dpm), 8.19 (d, 3JHH
1
the internal standard, HMDS. H NMR spectra were collected
3
4
= 6 Hz, 1H, dpm), 8.01 (td, JHH = 8 Hz, JHH = 2 Hz, 1H,
every 1 min with four scans and a 5.0 s pulse delay. The
product peaks were integrated against that of HMDS, and from
a plot of ln(1 − [4]t/[2]o) versus time (seconds), the rate
constants were extracted. The rate of formation of 4 from
complex 2, in the presence of 0.5 M C2H4, was 8.4(9) × 10−4
s−1 with a correlation coefficient (R2) of 0.99 for each plot.
Kinetics of Benzene-d6 C−D Bond Activation. A
representative kinetic experiment is described. Complex 2
(0.064 g, 0.046 mmol) and hexamethyldisilane (HMDS, 2.0 μL
as an internal standard) were dissolved in 1.2 mL of CD2Cl2.
The solution was then divided (0.375 mL for each sample) and
added to three NMR tubes. To each tube was added C6D6
(0.019 mL, 0.21 mmol). The tube was placed into a
temperature-equilibrated (30 °C setting) NMR probe. The
actual temperature of the probe (29 °C) was determined using
a solution of 80% ethylene glycol in DMSO-d6.14 Kinetic runs
3
4
dpm), 7.85 (td, JHH = 8 Hz, JHH = 2 Hz, 1H, dpm), 7.74 (s,
8H, Ho-Ar′), 7.62−7.45 (m, 7H, Hp-Ar′, Ho-Ph and dpm), 7.40
(ddd, 3JHH = 8 Hz, 3JHH = 6 Hz, 4JHH = 2 Hz, 1H, dpm), 7.18−
7.04 (m, 3H, Hm-Ph and Hp-Ph), 6.99−6.93 (m, 2H, dpm),
4.15−3.91 (m, 6H, dpm-CH2 and C2H4), 2.25 (t, 3JHH = 7 Hz,
2H, −CH2CH2Ph), 1.45−1.23 (m, 2H, −CH2CH2Ph); 13C
1
NMR (125 MHz, CD2Cl2) δ 162.2 (q, Ar′, JBCipso = 49 Hz),
154.7, 153.0, 149.6, 147.8, 143.6, 142.3, 141.2, 135.2 (Ar′),
2
129.3 (q, m-Ar′, JCF = 32 Hz), 129.0, 128.3, 127.0, 126.7,
1
126.4, 126.1, 125.0 (q, Ar′, JCF = 272 Hz), 117.9 (Ar′), 100.9
1
(dpm and Ph aromatic), 73.0 (s with Pt satellites, JPtC = 210
Hz, C2H4), 46.2 (dpm-CH2), 36.9 (−CH2CH2Ph), 15.3
(−CH2CH2Ph); 19F NMR (282 MHz, CD2Cl2) δ −63.1 (s,
CF3−Ar′). Anal. Calcd for PtN2BF24C53H35 (%): C, 46.74; H,
2.60; N, 2.06. Found: C, 46.35; H, 2.73; N, 2.10.
Catalytic Olefin Hydrophenylation. A representative
catalytic reaction is described. [(dpm)Pt(Ph)(THF)][BAr′4]
(2) (0.019 g, 0.013 mmol) was dissolved in 12.0 mL of benzene
containing 0.01 mol % hexamethylbenzene (HMB) relative to
benzene as an internal standard. The reaction mixture was
placed in a stainless steel pressure reactor, charged with
ethylene (0.1 MPa), pressurized to a total of 0.8 MPa with N2,
and heated to 100 °C. After a given time period, the reaction
mixture was allowed to cool to room temperature and was
analyzed by GC/MS. Peak areas of the products and the
internal standard were used to calculate product yields.
Ethylbenzene production was quantified using linear regression
analysis of gas chromatograms of standard samples. A set of five
known standards were prepared consisting of 2:1, 4:1, 6:1, 8:1,
and 10:1 molar ratios of ethylbenzene to HMB in benzene. A
plot of the peak area ratios versus molar ratios gave a regression
line. For the GC/MS system, the slope and correlation
coefficient (R2) for ethylbenzene were 0.68 and 0.99,
respectively. Identical procedures were used to quantify the
1
were performed in triplicate. H NMR spectra were collected
every 10 min with eight scans and a 5.0 s pulse delay.
Resonances for complex 2 were integrated against that of the
internal standard, HMDS, and from a plot of ln([2]t) versus
time (seconds), the rate constants were extracted. The rate of
formation of complex 2-d5 and C6H5D in the presence of 0.5 M
C6D6 was 9.9(4) × 10−5 s−1 with a correlation coefficient (R2)
of 0.99 for each plot.
Substrate Concentration Corrections for the Deter-
mination of Catalytic Activation Parameters. Catalysis
with complexes 2 and 3 was performed under conditions that
are inverse first-order in ethylene concentration. Therefore, for
the Eyring analysis, the observed rate constants were multiplied
by the concentration of ethylene. The ethylene concentration
under catalytic conditions was simulated by sparging a sample
of C6D6 in a J-Young NMR tube with ethylene and pressurizing
it according to the preparation of catalytic reaction mixtures.
The concentration of ethylene was then determined in triplicate
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Research Article
6772−6779. (c) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev.
2010, 110, 624−655. (d) Dyker, G.; Muth, E.; Hashmi, A. S. K.; Ding,
L. Adv. Synth. Catal. 2003, 345, 1247−1252. (e) Ritleng, V.; Sirlin, C.;
Pfeffer, M. Chem. Rev. 2002, 102, 1731−1769. (f) Kakiuchi, F.; Murai,
S. Acc. Chem. Res. 2002, 35, 826−834.
(4) (a) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.;
Yoshida, H. J. Am. Chem. Soc. 2000, 122, 7414−7415. (b) McKeown,
B. A.; Gonzalez, H. E.; Friedfeld, M. R.; Gunnoe, T. B.; Cundari, T. R.;
Sabat, M. J. Am. Chem. Soc. 2011, 133, 19131−19152. (c) McKeown,
B. A.; Foley, N. A.; Lee, J. P.; Gunnoe, T. B. Organometallics 2008, 27,
4031−4033.
(5) Zhang, F. B.; Kirby, C. W.; Hairsine, D. W.; Jennings, M. C.;
Puddephatt, R. J. J. Am. Chem. Soc. 2005, 127, 14196−14197.
(6) Distance measured from the Ph centroid to the cis-pyridyl
centroid using the graphical program ChemCraft and atom
coordinates from single-crystal X-ray diffraction.
by integration of the ethylene resonance versus the internal
standard, HMDS, at 90 °C.
Substrate Concentration Corrections for the Deter-
mination of C6D6 Activation Parameters. The reactions
with C6D6 and complexes 2 and 3 were performed under
conditions that are first-order in the concentration of C6D6.
Therefore, for Eyring analysis, the observed rate constants were
divided by the concentration of C6D6.
Crystal Structure of [(dpm)Pt(CH2CH2Ph)(η2-C2H4)]-
[BAr′4] (3). X-ray intensities were measured on a Bruker
Apex II Kappa Duo CCD diffractometer (Mo Kα radiation, λ =
0.71073 Å) at 120 K; 2358 frames were collected up to 2θ of
48.9°. Intensities were corrected for absorption by applying
Bruker SADABS.16 The structure was determined using direct
methods and standard difference map techniques and was
refined by full-matrix least-squares procedures on F2 with
SHELXTL (version 6.14).17 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included in
calculated positions and were refined using a riding model:
C53H35BF24N2Pt, MW = 1361.73, monoclinic, space group
P2(1)/c, a = 18.556(2) Å, b = 17.006(2) Å, c = 17.621(2) Å, β
= 103.297(2)°, V = 5411.3(9) Å3, Z = 4, ρcalcd = 1.671 g cm−3.
(7) Bennett, M. A.; Mitchell, T. R. B. Inorg. Chem. 1976, 15, 2936−
2938.
(8) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 2002, 21,
3920−3922.
(9) Steele, B. R.; Vrieze, K. Transition Met. Chem. (Dordrecht, Neth.)
1977, 2, 140−144.
(10) Canty, A.; Minchin, N. Aust. J. Chem. 1986, 39, 1063−1069.
(11) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van
Koten, G. Organometallics 1989, 8, 2907−2917.
Of 59891 measured reflections, 8865 were independent (Rint
=
(12) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372−1377. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. 1998, B37, 785−789.
(13) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 610−613.
0.0627); 746 parameters, R1 = 0.0562 [for reflections with I >
2σ(I)], wR2 = 0.1707 (for all reflections). CCDC-916579
contains the supplementary crystallographic data for this paper,
which can be obtained free of charge from the Cambridge
Crystallographic Data Centre (http://www.ccdc.cam.ac.uk/
data_request/cif).
(14) The temperature was verified using the following equation
provided by the Bruker Instruments, Inc., VT-Calibration Manual:
T(K) = (4.218 − X)/0.009132, where X is the parts per million of OH
− the parts per million of CH2.
(15) The temperature was verified using the following equation
provided by experimental data from the University of Nebraska
(2001): T(K) = −23.1902x2 − 31.1062x + 399.081, where x is the
parts per million of OD vs the parts per million of CD3.
(16) Bruker Apex 2 User Manual; Bruker AXS Inc.: Madison, WI,
2010.
ASSOCIATED CONTENT
* Supporting Information
Full computational study with discussion. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: tbg7h@virginia.edu (T.B.G.) or t@unt.edu (T.R.C.).
■
(17) Sheldrick, G. Acta Crystallogr. 2008, A64, 112−122.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.B.G. and T.R.C. acknowledge The Office of Basic Energy
Sciences, U.S. Department of Energy, for support of this work
(DE-SC0000776 and DE-FG02-03ER15387) and the National
Science Foundation for purchase of an X-ray diffractometer
(CHE-1126602).
REFERENCES
■
(1) (a) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley-
Interscience: New York, 1995. (b) Roberts, R. M.; Khalaf, A. A.
Friedel-Crafts Alkylation Chemistry: A Century of Discovery; Marcel
̌
́
Dekker, Inc.: New York, 1984. (c) Cejka, J.; Wichterlova, B. Catal. Rev.
2002, 44, 375−421. (d) Kocal, J. A.; Vora, B. V.; Imai, T. Appl. Catal.,
A 2001, 221, 295−301. (e) Perego, C.; Ingallina, P. Green Chem. 2004,
6, 274−279.
(2) (a) Goj, L. A.; Gunnoe, T. B. Curr. Org. Chem. 2005, 9, 671−685.
(b) Andreatta, J. R.; McKeown, B. A.; Gunnoe, T. B. J. Organomet.
Chem. 2011, 696, 305−315. (c) Foley, N. A.; Lee, J. P.; Ke, Z.;
Gunnoe, T. B.; Cundari, T. R. Acc. Chem. Res. 2009, 42, 585−597.
(d) Luedtke, A. T.; Goldberg, K. I. Angew. Chem., Int. Ed. 2008, 47,
7694−7696.
(3) (a) Yoshikai, N. Synlett 2011, 2011, 1047−1051. (b) Harada, H.;
Thalji, R. K.; Bergman, R. G.; Ellman, J. A. J. Org. Chem. 2008, 73,
1171
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