Control of olefin hydroarylation catalysis via a sterically and electronically flexible platinum(II) catalyst scaffold

Article abstract of DOI:10.1021/om400390e

Pt^II complexes supported by dipyridyl ligands have been demonstrated to catalyze olefin hydroarylation. Herein, studies on the influence of dipyridyl motif variation are reported. Increasing the chelate ring size of dipyridyl-ligated Pt^II complexes from five- to six-membered rings by replacing 4,4′-di-tert-butyl-2,2′-bipyridine with 2,2′-dipyridylmethane has been shown to increase catalytic activity and longevity for catalytic ethylene hydrophenylation. For 2,2′-dipyridyl ligands, the presence of methyl groups in the 6/6′-positions of the pyridyl rings reduces the extent of dialkylation to produce diethylbenzenes but also increases the rate of catalyst decomposition. Substituting the methylene spacer between the pyridyl rings of 2,2′-dipyridylmethane with more electron-withdrawing groups also reduces catalytic efficiency. The steric profile of Pt^II complexes with increased chelate ring size or substituents in the 6/6′-positions of the pyridyl rings provides a marked change in regioselectivity for ethylene hydroarylation using ethylbenzene as well as the linear to branched selectivity for the hydrophenylation of propylene.

Full text of DOI:10.1021/om400390e

Article
pubs.acs.org/Organometallics
Control of Olefin Hydroarylation Catalysis via a Sterically and
Electronically Flexible Platinum(II) Catalyst Scaffold
Bradley A. McKeown,† Hector Emanuel Gonzalez,‡ Thoe Michaelos,§ T. Brent Gunnoe,*,†
Thomas R. Cundari,*,‡ Robert H. Crabtree,§ and Michal Sabat∥
†Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
‡Center for Advanced Scientific Computing and Modeling (CASCaM), Department of Chemistry and Energy Sciences Institute,
University of North Texas, Denton, Texas 76203, United States
§Department of Chemistry and Energy Sciences Institute, Yale University, New Haven, Connecticut 06520, United States
∥Nanoscale Materials Characterization Facility, Materials Science and Engineering Department, University of Virginia, Charlottesville,
Virginia 22904, United States
S
* Supporting Information
ABSTRACT: PtII complexes supported by dipyridyl ligands have been
demonstrated to catalyze olefin hydroarylation. Herein, studies on the
influence of dipyridyl motif variation are reported. Increasing the chelate ring
size of dipyridyl-ligated PtII complexes from five- to six-membered rings by
replacing 4,4′-di-tert-butyl-2,2′-bipyridine with 2,2′-dipyridylmethane has
been shown to increase catalytic activity and longevity for catalytic ethylene
hydrophenylation. For 2,2′-dipyridyl ligands, the presence of methyl groups
in the 6/6′-positions of the pyridyl rings reduces the extent of dialkylation to
produce diethylbenzenes but also increases the rate of catalyst decom-
position. Substituting the methylene spacer between the pyridyl rings of 2,2′-
dipyridylmethane with more electron-withdrawing groups also reduces
catalytic efficiency. The steric profile of PtII complexes with increased
chelate ring size or substituents in the 6/6′-positions of the pyridyl rings
provides a marked change in regioselectivity for ethylene hydroarylation using ethylbenzene as well as the linear to branched
selectivity for the hydrophenylation of propylene.
provides a route to alkyl arenes that is complementary to acid-
catalyzed processes and offers the potential to overcome the
limitations of traditional methodologies.1b,4
INTRODUCTION
■
The direct and selective functionalization of hydrocarbons by
C−H bond activation and subsequent C−C bond formation
remains challenging but has the potential to become a valuable
tool for organic synthesis.1 For example, the conversion of
olefins and arenes to alkyl arenes is an important industrial
reaction.2 Current methods for alkyl arene production utilize
acids (Lewis and/or Brønsted) to catalyze the net addition of
an aromatic C−H bond across an olefin CC bond (olefin
hydroarylation) by a Friedel−Crafts pathway.3 Despite being
widely employed, these processes have limitations that result
from the intermediacy of carbocations.1b,4 For example, acid
catalysts are highly selective for the formation of Markovnikov
products when substituted olefins are used. In addition,
directing groups on the arene dictate regioselectivity, and in
cases of “non-functional” groups (e.g., alkyl groups), little
regioselectivity can be achieved. Alkylated arene products are
typically more reactive than starting arenes, and polyalkylation
at high substrate conversion is problematic. While zeolites have
provided improvement over traditional Friedel−Crafts cata-
lysts,2b,5 they operate by the same fundamental mechanism. A
transition-metal-mediated pathway that combines olefin in-
sertion into metal−aryl bonds and aromatic C−H activation
Examples of olefin hydroarylation by a non-acid-catalyzed
mechanism to yield alkyl arenes are limited and often require
chelate assistance6 or substrates activated by heterofunctional
groups.7 The use of unactivated substrates (e.g., benzene and
ethylene) for olefin hydroarylation has been realized with Ru,
Ir, and Pt catalyst precursors.4,8 In order to develop improved
catalyst systems, it is necessary to understand how alterations of
the catalyst’s electronic and steric properties influence
selectivity. However, few detailed studies delineating struc-
ture/activity relationships are available to guide new catalyst
design.4a,9
Platinum has emerged as a promising candidate for catalytic
olefin hydroarylation, with established precedent for metal-
mediated olefin insertion and aromatic C−H activation10 but
inefficient oligomerization/polymerization.11 Examples of olefin
hydroarylation using PtII catalyst precursors have been
reported,8b−d,9b,12 but mechanistic evidence supporting a non-
Received: May 3, 2013
Published: June 28, 2013
© 2013 American Chemical Society
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a
Scheme 1. Proposed Catalytic Cycle for Ethylene Hydrophenylation by Cationic Bipyridyl-Supported PtII Complexes
a
t
[Pt] = [(N∼N)Pt]+ and N∼N = bpy, dpm.
acid-catalyzed pathway has been disclosed for only two
complexes.8c,9b−d Goldberg and co-workers reported a PtII
catalyst precursor ligated by a (pyridyl)pyrrolide ligand for
which mechanistic studies are consistent with a pathway that
incorporates olefin insertion into a Pt−aryl bond and
subsequent activation of an aromatic C−H bond.8c Addition-
ally, we have reported studies of ethylene hydrophenylation
catalyzed by [(N∼N)Pt(Ph)(THF)][BAr′4] (N∼N = 4,4′-di-
tert-butyl-2,2′-bipyridine (tbpy), 2,2′-dipyridylmethane (dpm);
Ar′ = 3,5-(CF3)2-C6H3) that support a similar catalytic cycle
(Scheme 1).9b−d
bridging pyridyl functionality on catalyst activity and selectivity
is disclosed (Chart 1). The catalysis data for [(dpm)Pt(Ph)-
(THF)][BAr′4] have been previously reported, but we have
included them in various tables and discussions for comparative
purposes.9c
Chart 1. Generic Structure for Dipyridyl-Ligated PtII
Complexes, Highlighting the Substituents Studied in This
a
Report
The bipyridyl framework is easily modified and presents an
opportunity to establish structure/activity relationships. In a
previous study, the 4,4′-substituents of the 2,2′-bipyridyl ligand
were modified to probe the influence of electron-donor ability
on catalyst selectivity without alteration of the catalyst steric
profile.9d It was demonstrated that, by varying the donor ability
of the 2,2′-bipyridyl ligand, the metal center can be attenuated
to bias the formation of ethylbenzene versus styrene. In
addition to electronic perturbations, we found that expansion of
the dipyridyl ligand from a five- to six-membered chelate (i.e.,
a
E = CH2, CH2CH2, CO, NH, O; R1, R2 = H, Me.
RESULTS AND DISCUSSION
■
The complexes (N∼N)PtPh2 (N∼N = 6-methyl-2,2′-dipyridyl-
methane (Me-dpm, 1b), 6,6′-dimethyl-2,2′-dipyridylmethane
(Me2-dpm, 1c), 6,6′-dimethyl-2,2′-bipyridine (Me2-bpy, 1d),
1,2-bis(2-pyridyl)ethane (dpe, 1e)) were prepared by reaction
of the appropriate dipyridyl ligand with the binuclear platinum
dimer [Pt(Ph)2(μ-Et2S)]2 (eq 1). Crystals suitable for an X-ray
t
using dpm in place of bpy) provides an enhancement of
catalyst activity and longevity.9c Recent work by Puddephatt et
al. has also examined the influence of dipyridyl chelate ring size
on the reactivity and selectivity of PtII alkyl complexes for arene
C−H activation, H/D exchange, and oxidative addition
reactions.13 Diethylbenzenes constitute a significant portion
(∼20%) of the total alkyl arene products using bipyridyl- and
dipyridylmethane-ligated PtII catalyst precursors and likely
result from aromatic C−H activation competing with ethyl-
benzene dissociation.9b We hypothesized that increased steric
congestion around the Pt center might facilitate ethylbenzene
displacement and provide increased selectivity for monoalky-
lated products. Moreover, we considered that steric perturba-
tions about the Pt center could have an effect on the
regioselectivity of α-olefin hydroarylation (i.e., selectivity for
Markovnikov versus anti-Markovnikov products). In this
report, the impact of changes to the catalyst, including 6,6′-
pyridyl substitution, chelate ring size, and the identity of the
diffraction study were obtained for complexes (dpm)PtPh2 (1a)
and 1c,e (Figure 1 and Table 1). For the dpm complex 1a, the
N−Pt−N angle is increased by ∼9° to 86.0(3)° relative to the
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Figure 1. ORTEP drawings of complexes (dpm)Pt(Ph)2 (1a), (Me2-dpm)Pt(Ph)2 (1c), and (dpe)Pt(Ph)2 (1e) (50% probability; H atoms omitted
for clarity).
Table 1. Comparison of Selected Bond Lengths and Angles among the Complexes (dpm)Pt(Ph)2 (1a), (Me2-dpm)Pt(Ph)2 (1c),
(dpe)Pt(Ph)2 (1e), and (tbpy)Pt(Ph)2
a
1a
1c
1e
(tbpy)Pt(Ph)2
Bond Lengths (Å)
2.138(2)
Pt−N1
Pt−N2
Pt−C1
Pt−C7
2.133(7)
2.147(7)
2.018(8)
2.02(1)
2.124(7)
2.137(6)
2.016(7)
2.002(7)
2.097(3)
2.097(3)
2.023(2)
2.023(2)
2.131(2)
1.997(2)
1.998(2)
Bond Angles (deg)
83.51(8)
N1−Pt−N2
N2−Pt−C1
N1−Pt−C1
C1−Pt−C7
C17−C18−C19
C19−C20−C18−C17
86.0(3)
176.9(4)
91.1(3)
88.2(4)
110.8(8)
87.8(2)
178.6(3)
92.4(3)
89.3(3)
77.1(2)
174.5(1)
97.4(1)
88.2(2)
176.44(8)
94.06(9)
89.0(1)
110.1(2)
53.6(1)
a
Crystallographic data from ref 14.
reported structure of (tbpy)Pt(Ph)2 (77.1(2)°).14 The addition
of methyl groups to the 6/6′-positions in complex 1c results in
an approximate 3° compression of the N−Pt−N bite angle
(83.51(8)°) relative to 1a. The additional methylene spacer to
form a seven-membered chelate ring in complex 1e has a small
effect on the N−Pt−N angle (87.8(2)°) in comparison to the
dipyridylmethane variants. For 2,2′-bipyridyl-ligated Pt com-
plexes, the pyridyl rings reside approximately in the Pt square
plane.14,15 In contrast, the dpm complex 1a adopts a pseudo-
boat conformation that causes the pyridyl rings to be contorted
out of the Pt square plane by ∼42° (average measurement of
the N−Pt−N−C2‑pyridyl torsion angles). The methyl groups in
the 6/6′-positions of complex 1c enhance this distortion with
the pyridyl rings ∼52° out of planarity. Coordination of dpe to
the PtPh2 fragment results in the pyridyl rings being twisted out
of the square plane by ∼58°. The 2,2′-bipyridyl complex
(tbpy)Pt(Ph)2 possesses shorter Pt−N and longer Pt−C bond
lengths in comparison to complexes 1a,c,e, which is consistent
with the tbpy ligand exerting a greater trans influence than dpm,
6,6′-dpm, or dpe.
The complexes [(N∼N)Pt(Ph)(THF)][BAr′4] (2b−e) were
prepared by protonation of 1b−e with [H(Et2O)2][BAr′4] at
−70 °C in THF (eq 2). A suitable crystal of [(dpe)Pt(Ph)-
(THF)][BAr′4] (2e) was obtained for an X-ray diffraction
study (Figure 2). A search of the Cambridge Structural
Database revealed only three examples of solid-state structures
of PtII−THF complexes.9d,16 Comparison of the previously
reported structures of PtII−THF complexes suggests that dpe
exerts the weakest trans influence with the Pt−O bond length
increasing along the series dpe (2.048(4) Å) < 4,4′-Br2-2,2′-
Figure 2. ORTEP drawing of [(dpe)Pt(Ph)(THF)][BAr′4] (2e) (50%
probability; BAr′4 anion and H atoms omitted for clarity). Selected
bond lengths (Å): Pt−N1 = 2.141(4), Pt−N2 = 1.980(4), Pt−O1 =
2.048(4), Pt−C13 = 2.001(5). Selected bond angles (deg): N1−Pt−
N2 = 87.9(2), O1−Pt−C13 = 93.2(2).
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a
Table 2. Comparison of Catalytic Ethylene Hydrophenylation Using Complexes 2a−e
a
b
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa of C2H4. Ratio of
c
d
1,2-, 1,3-, and 1,4-diethylbenzene after 4 h. Turnover frequency calculated after 1 h. Percent of diethylbenzenes in the total arene product after
catalyst deactivation. Turnovers after 4 h as determined by GC/MS. Numbers in parentheses are turnovers after 16 h. Numbers in brackets are
e
f
g
TON.
bipyridine (2.060(7) Å) < cyclooctadiene (2.077(2) Å) <
[Ph2B(CH2PPh2)2]− (2.170(4) Å).
The asymmetric complex 2b, with a methyl group in the 6-
position of one pyridyl ring, provides 35.2 TO of ethylbenzenes
and styrene after 4 h. The orientation of the phenyl ligand and
methyl substituent on the bpy ligand of complex 2b has not
been determined. The inclusion of methyl groups into both 6-
positions of the dipyridyl ligand (complex 2c) results in a total
of 21.2 TO of ethylbenzene and styrene after 4 h. Thus, among
complexes 2a−c using data after 4 h of reaction, the addition of
each methyl group reduces the catalytic turnover by ∼40%.
Furthermore, the presence of 6/6′-substituents results in a
change in ethylbenzene to styrene ratios. For example, after 4 h,
complex 2a produces ethylbenzene and styrene in an
approximate 138:1 ratio, in comparison to ∼11:1 for both 2b
and 2c. For catalysis using [(tbpy)Pt(Ph)(THF)][BAr′4],
computational studies are consistent with β-hydride elimination
following ethylene insertion into the Pt−Ph bond to form
[(tbpy)Pt(H)(η2-CH2CHPh)]+ being rapid and reversible.9b
The increased sterics around the Pt center provided by the
methyl substituents of 2b,c might facilitate styrene displace-
ment and have the effect of decreasing the ethylbenzene/
styrene ratio. Also, if the formation of unstable PtII−hydride
complexes provides the primary pathway for catalyst decom-
position,9b facilitating styrene dissociation would be expected to
decrease the TON, which is consistent with the data shown in
entries 1−3 in Table 2.
Ethylene hydrophenylation was evaluated using 2b−e as
catalyst precursors, and the results are summarized in Table 2.
The catalyst data for the previously reported dipyridylmethane-
ligated PtII complex [(dpm)Pt(Ph)(THF)][BAr′4] (2a) are
included for comparative purposes.9c Using data after 4 h at 100
°C under 0.1 MPa of ethylene, complex 2a is the most effective
catalyst with 65.9 turnovers (TO) of ethylbenzenes (the
formation of diethylbenzene is counted as one TO) and trace
styrene. Using 2a, an ultimate turnover number (TON) of 469
is achieved after approximately 4 days. The incorporation of
methyl substituents in the 6/6′-positions of dpm or 2,2′-
bipyridine ligands (2b−d) or an increase in the chelate ring size
from a six-membered to a seven-membered ring using 1,2-
bis(2-pyridyl)ethane (2e) results in a decrease in overall
catalyst efficiency relative to 2a (see Table 2 and Figure 3).
Figure 3 shows TO versus time for the hydrophenylation of
ethylene using catalysts 2a−c. Unlike 2a, complexes 2b,c
exhibit signs of decomposition in less than 4 h at 100 °C with
0.01 mol % of Pt catalyst. In order to mitigate the influence of
catalyst decomposition and offer a direct comparison between
the complexes, turnover frequencies (TOF) were evaluated
early in the reaction. After 1 h at 100 °C, complex 2a catalyzes
the formation of 18.6 TO of ethylbenzenes and styrene for a
TOF of 5.1 × 10−3 s−1. Complexes 2b,c perform similarly with
18.2 (TOF = 5.1 × 10−3 s−1) and 16.5 TO (TOF = 4.6 × 10−3
s−1), respectively. Thus, the decreased catalytic TON observed
with increasing 6/6′-substitution of the dpm ligand is a likely
result of increased rates of catalyst decomposition as opposed
to decreased catalytic activity. We have modeled the rate of
catalyst decomposition using first-order and second-order plots
Figure 3. Plot of TO versus time for ethylene hydrophenylation (100
°C) catalyzed by complexes 2a−c using 0.01 mol % of catalyst
dissolved in C6H6 with 0.1 MPa of C2H4 and hexamethylbenzene as an
internal standard. The inset highlights turnovers as function of time for
the first 4 h of catalysis.
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of TOF as a function of time (see the Supporting Information
for details). Excellent fits (R2 ≥ 0.98) were obtained for
complexes 2a−c using second-order plots (Figure 4). The
membered counterparts)18 or facile displacement of 1,2-
bis(pyridyl)ethane from transient PtIV intermediates.15
One issue with ethylene hydrophenylation catalyzed by
formally cationic bipyridyl PtII complexes is the formation of
diethylbenzenes.8b,9b−d For example, diethylbenzenes consti-
tute approximately 20% of the total alkyl arene product from
ethylene hydrophenylation with 2a and [(tbpy)Pt(Ph)(THF)]-
[BAr′4] (tbpyPt). This result has been attributed to
competition between ethylbenzene C−H activation and net
dissociation of ethylbenzene from the metal center.9b While the
percent composition of diethylbenzenes from catalysis using 2a
is not improved relative to catalysis using tbpyPt, the
regioselectivity of diethylbenzene formation is influenced. For
complex 2a, the ratio of meta/para to ortho diethylbenzenes is
∼17:1 at 100 °C, which is 4 times greater than that reported for
tbpyPt (eq 4). Introducing steric perturbations in the 6/6′-
Figure 4. Plot of 1/TOF versus time demonstrating the second-order
decomposition of complexes 2a−c (R2 ≥ 0.98 for all three fits).
kinetic analysis of catalyst decomposition reveals that the
second-order rate constant for decomposition of complex 2a is
1.61(8) × 10−3 s−1 M−1. In comparison, the rate of
decomposition of the monomethyl complex 2b is close to 1
order of magnitude larger (1.38(3) × 10−2 s−1 M−1), and the
rate of decomposition for the dimethyl complex 2c is
accelerated by a factor of ∼2.5 (3.4(1) × 10−2 s−1 M−1)
relative to 2b.
positions of the dipyridylmethane ligand influences the
selectivity for monoalkylation versus dialkylation of benzene.
Analysis of the reaction mixture from a reaction of benzene and
ethylene after 16 h using complex 2b yields a total of 60.6 TO,
of which only ∼4% are diethylbenzenes, similar to the
observation with a reported formally neutral PtII catalyst
precursor.8c With complex 2c, diethylbenzenes represent <2%
of the total arene product after complete catalyst deactivation,
and catalysis with complexes 2d,e results in no observable
diethylbenzene formation. Therefore, while methyl groups in
the 6-positions of the pyridyl rings decrease catalytic TO, the
formation of diethylbenzenes is suppressed.
Ethylene hydroarylation using ethylbenzene as the aromatic
substrate was probed (Table 3). Using data after 4 h of
reaction, the observed catalytic activity was significantly
reduced in comparison to hydroarylation of ethylene using
benzene for complexes 2a−e. Note that this comparison
Using [(Me2-bpy)Pt(Ph)(THF)][BAr′4] (2d) as a catalyst
precursor for ethylene hydrophenylation results in styrene
production with 0.7 TO (relative to 2c) along with small
quantities of (vinyl)ethylbenzene isomers, which were not
quantified. No evidence for the formation of ethylbenzenes was
obtained. The selectivity for vinylarene production is in
contrast to that for a previously reported unsubstituted
bipyridine ligated PtII precursor for ethylene hydrophenyla-
tion.17 The reaction of 2d with ethylene at room temperature in
CD2Cl2 readily produces styrene quantitatively with the
formation of multiple intractable Pt complexes, as observed
1
by H NMR spectroscopy (eq 3). The putative intermediate
Table 3. Comparison of Catalytic Ethylene Hydroarylation
a
with Ethylbenzene Using Complexes 2a−e
[(Me2-bpy)Pt(CH2CH2Ph)]+ likely undergoes rapid β-hydride
elimination and net dissociation of styrene, with the resulting
Pt−H complex [(Me2-bpy)Pt(H)]+ decomposing rapidly.
Expansion to a seven-membered chelate using [(dpe)Pt(Ph)-
(THF)][BAr′4] (2e) results in 1.2 TO of ethylbenzene and
styrene in an approximate 2:1 ratio upon heating in benzene
with 0.1 MPa of ethylene. Decreasing the temperature to 80 °C
for complex 2e did not extend catalyst longevity, as only 1.5
total TO of ethylbenzene and styrene were observed. Rapid
decomposition of the catalyst is likely due to the poor stability
of seven-membered chelates (in comparison to their five- or six-
a
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
of C2H4. Turnovers after 4 h as determined by GC/MS. Numbers in
parentheses are turnovers after 16 h.
b
c
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assumes negligible catalyst deactivation after 4 h (see above).
Thus, the observed relative rates of ethylene hydroarylation
using benzene and ethylbenzene are opposite to those of
catalysis using a Friedel−Crafts catalyst.19 For the PtII catalysts,
the decreased hydroarylation activity using ethylbenzene as the
aromatic substrate and the low concentration of ethylbenzene
during ethylene hydrophenylation (due to a low catalyst
loading) strongly suggest that the formation of diethylbenzenes
from the reaction of benzene with ethylene does not originate
from the catalyst reacting with free ethylbenzene generated in
situ.
Complex 2a catalyzes the formation of 9.7 and 23.0 TO of
diethylbenzenes after 4 and 16 h, respectively. The ratio of
meta to para substitution was ∼1.4:1, and the formation of 1,2-
diethylbenzene was observed only in trace amounts. Using
complex 2b, the meta to para ratio is nearly identical with that
observed for 2a and the formation of 1,2-diethylbenzene was
not observed in quantifiable amounts. Catalysis with 2c exhibits
little selectivity, as the formation of 1,2-diethylbenzene is equal
to the formation of 1,4-diethylbenzene. No quantifiable
amounts of diethylbenzenes were detected with complexes
2d,e over a period of 16 h from the reaction with ethylene and
ethylbenzene.
to the 6-positions of the pyridyl rings of dipyridylmethane
heightens the propensity for cumene formation over n-
propylbenzene. Using complex 2b, a ratio of branched to
linear isomers of 5.3 is observed after 4 h of catalysis. The
symmetrically substituted Me2-dpm complex 2c is completely
selective for the formation of cumene in substoichiometric
amounts. The formation of several isomers of (propenyl)-
propylbenzene was detected by GC/MS but not quantified. As
observed during catalysis with ethylene, complex 2d readily
undergoes β-hydride elimination following propylene insertion
into the Pt−Ph bond, and only 2-phenylpropylene is observed
after 16 h but was not quantified. Complex 2e is selective for
the formation of cumene in ∼40% yield, relative to 2e. Whether
the cumene to n-propylbenzene ratios are controlled by the
regioselectivity of propylene insertion or by the relative rates of
subsequent reactions (i.e., Curtin−Hammett conditions) is not
known; thus, rationalizing any trend is difficult.
The substitution of tbpy with dpm provides enhanced activity
and longevity for PtII-catalyzed ethylene hydrophenylation.9c
We sought to determine the influence of the dipyridyl linkage
identity on catalytic activity and selectivity. A series of catalyst
precursors were synthesized in which the methylene bridge
between pyridyl rings of dpm was substituted with CO (4a),
NH (4b), and O (4c) following the procedure for complexes 2
(eq 5). Under conditions of 100 °C and 0.1 MPa of ethylene,
Catalytic olefin hydroarylation using substituted olefins and a
nonacidic pathway offers an opportunity to control the
selectivity for Markovnikov/anti-Markovnikov products. For
example, catalysts that can bias 2,1- over 1,2-insertion with α-
olefins could selectively produce linear alkyl arenes, which are
not accessible with current acid-based methodologies. Propy-
t
lene hydrophenylation with bpyPt results in the formation of
both cumene and n-propylbenzene in an approximate 3:1
ratio.8b Catalysis using propylene with complexes 2a−e was
performed to determine the influence of steric modifications on
regioselectivity (Table 4). At 100 °C under 0.1 MPa of
propylene, cumene (9.6 TO) and n-propylbenzene (2.2) are
produced in a 4.4:1 ratio after 4 h with complex 2a. The ratio of
branched to linear products is almost invariant with time. Thus,
the six-membered chelate of 2a increases the branched to linear
ratio by ∼50% in comparison to that observed for the bipyridyl-
supported catalyst.8b The sequential addition of methyl groups
the catalytic activities of complexes 4a−c were evaluated (Table
5). Comparison of the TO observed after 4 h demonstrates that
catalytic efficiency decreases in the following order: CH2 (2a) >
NH (4b) > CO (4a) > O (4c). Note that catalyst
Table 4. Comparison of Catalytic Propylene
Hydrophenylation Using Complexes 2a−e and bpyPt
a
t
Table 5. Comparison of Catalytic Ethylene
a
Hydrophenylation Using Complexes 4a−c
a
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with
a
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with
b
c
of C3H6. Ratio of cumene to n-propylbenzene after 4 h. Turnovers
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
d
b
after 4 h as determined by GC/MS. Numbers in parentheses are
of C2H4. Ratio of 1,2-, 1,3-, and 1,4-diethylbenzene after 4 h.
e
f
c
d
turnovers after 16 h. Catalysis data from ref 8b. Minor production of
Turnovers after 4 h as determined by GC/MS. Numbers in
2-phenylpropylene observed.
parentheses are turnovers after 16 h.
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decomposition complicates any comparison of activity using
these data. The dipyridylamine complex [(dpa)Pt(Ph)(THF)]-
[BAr′4] (4b; dpa = bis(2-pyridyl)amine) provides a total of
25.1 TO after 4 h, which is more than a 60% decrease in TO
compared to the case for 2a. Catalysis with [(dpk)Pt(Ph)-
(THF)][BAr′4] (4a; dpk = bis(2-pyridyl)ketone) results in
only 3.3 and 1.9 TO of ethylbenzene and styrene, respectively,
after 4 h. Using dipyridyl ether in [(dpo)Pt(Ph)(THF)][BAr′4]
(4c; dpo = bis(2-pyridyl) ether) provides a total of 4.5 TO after
4 h, favoring styrene production. In all cases, substitution of the
methylene linkage reduces catalyst TO and longevity; the latter
is evident by the marginal changes in TO after 16 h in
comparison to the TO after 4 h for catalysis using 4a−c.
Diethylbenzenes were only observed during catalysis with 4b
and constitute ∼16% of the total arene products. As previously
observed with bipyridyl donor variations,9d changing the
identity of the pyridyl linkage has an influence on styrene
production. As the linkage becomes more electron withdrawing,
the ratio of ethylbenzene to styrene decreases. For example, the
amine linkage in complex 4b produces ethylbenzene and
styrene in a 7.4:1 ratio, which is a 95% decrease in comparison
to that for the parent dpm ligand. The ratio further decreases
for carbonyl and ether linkages with observed ratios of 1.7:1
and 0.6:1 for complexes 4a,c, respectively.
Table 7. Comparison of Catalytic Propylene
Hydrophenylation Using Complexes 4a−c
a
a
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
b
c
of C3H6. Ratio of cumene and n-propylbenzene after 4 h. Turnovers
d
after 4 h as determined by GC/MS. Numbers in parentheses are
turnovers after 16 h.
catalytic activity. Propylene hydrophenylation with the
dipyridyl ether complex 4c after 4 h results in a product
distribution nearly identical with that of 4a with 4.9 TO of
cumene and n-propylbenzene in a 7.2:1 ratio. As the TO values
of cumene are the same for 4a and 4c and the TO value of n-
propylbenzene differs by only 0.1, the regioselectivity of
insertion into the Pt−Ph bond is approximately the same for
these complexes.
The results of catalysis employing ethylbenzene as the
aromatic substrate for complexes 4a−c are similar to those
observed for complexes 2 (Table 6). For all complexes, 1,2-
CONCLUSION
■
We have studied the systematic variation of ligand steric and
electronic properties for dipyridyl PtII catalyzed olefin hydro-
arylation. Modifications include substitution at the 6,6′-
position(s) of the pyridyl rings with methyl groups, expansion
to a seven-membered chelate ring with 1,2-(dipyridyl)ethane,
and changes in the identity of the functionality that bridges
pyridyl rings. In comparison to the parent dipyridylmethane-
ligated PtII catalyst precursor, introducing steric modification
proximal to the metal center with methyl groups in the 6/6′-
positions of the pyridyl rings increases selectivity for
monosubstituted benzenes with catalytic activity similar to
that of the dpm complex 2a for ethylene hydrophenylation;
however, the rate of catalyst decomposition is accelerated. If
conditions to stabilize similar catalysts can be developed, these
sterically bulky ligands are promising targets for both active and
selective ethylbenzene synthesis. The ratios of ethylbenzene to
styrene decrease with substitution at the 6,6′-positions likely
due to facilitation of styrene displacement. 2,2′-Bipyridyl-
ligated PtII precursors featuring 6/6′-substitution are catalyti-
cally inactive, as β-hydride elimination is preferred over
aromatic C−H activation. Replacing the methylene bridge
with carbonyl, amine, or ether groups also results in decreased
catalytic activity for ethylene hydrophenylation. All the
complexes discussed herein have significantly reduced catalytic
efficiency for ethylene hydroarylation using ethylbenzene,
which is in contrast with acid-catalyzed pathways. The
formation of Markovnikov addition products using propylene
is favored with increasing steric congestion around the metal
center and an electron-withdrawing pyridyl bridge functionality.
In terms of future olefin hydroarylation catalyst design, the
following conclusions are pertinent: the key to long-lived
catalytic production of alkyl arenes appears to be avoiding
vinylarene formation. If vinylarene production is desired, a
sterically crowded 2,2′-bipyridyl ligand is preferred, such as 2d.
For efficient styrene production, the issue of catalyst
decomposition will need to be overcome, which is possibly
Table 6. Comparison of Catalytic Ethylene Hydroarylation
a
with Ethylbenzene Using Complexes 4a−c
a
Conditions: 0.01 mol % of catalyst dissolved in C6H6 with
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
b
c
of C2H4. Turnovers after 4 h as determined by GC/MS. Numbers in
parentheses are turnovers after 16 h.
diethylbenzene was not detected after 16 h. Complex 4a
produces approximately equimolar amounts of 1,3- and 1,4-
diethylbenzene with complete cessation of catalytic activity
after 4 h. Meta- and para-substituted diethylbenzenes are also
observed in an approximate 1:1 ratio using complex 4b after 4 h
with a total of 4.3 TO after 16 h, slightly favoring meta
substitution. Complex 4c yields substoichiometric amounts of
diethylbenzenes for an approximate 50% yield after 16 h.
Catalysis using propylene and benzene with complexes 4a−c
was then studied to probe the influence of pyridyl linker on
selectivity (Table 7). For 4a−c, the regioselectivity is biased
toward cumene, and the ratio of Markovnikov to anti-
Markovnikov addition products increases slightly for 4a−c in
comparison to 2a. At 100 °C under 0.1 MPa of propylene,
complex 4b was found to be the most active catalyst precursor
for propylene hydrophenylation with 25.4 TO of cumene and
n-propylbenzene in a 5.2:1 ratio after 4 h. Using complex 4a,
only 5.0 TO are observed after 4 h with a branched to linear
ratio of 6.1:1, and extended reaction times result in no further
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Article
sealed and subjected to microwave heating (Initiator EXP, Biotage) at
200 °C with a maximum pressure of 1.8 MPa for 1 h. The solid was
removed by filtration and washed with acetonitrile. The deep orange
filtrate was then dried by rotary evaporation to give a thick oily residue.
Purification by column chromatography on silica gel with 3:2 ethyl
acetate:hexanes afforded the product as an oil which solidifies at room
temperature. Isolated yield: 0.56 g (51%). 1H NMR (400 MHz,
due to the formation of unstable Pt−H intermediates that result
from β-hydride elimination and styrene dissociation. Thus, it is
clear that square-planar PtII complexes provide a sterically and
electronically flexible catalyst platform by which both activity
3
and selectivity for the synthesis of Caryl−Csp linkages,
important in both bulk and fine chemical synthesis, may be
produced directly from hydrocarbon starting materials without
the need for intermediate functionalization steps.
3
4
5
CDCl3): δ 8.29 (ddd, JHH = 5 Hz, JHH = 2 Hz, JHH = 1 Hz, 2H, 6-
3
3
4
dpo), 7.76 (ddd, JHH = 8 Hz, JHH = 7 Hz, JHH = 2 Hz, 2H, 4-dpo),
7.09 (m, 4H, 3- and 5-dpo). 13C NMR (125 MHz, CDCl3): δ 162.3,
148.5, 139.9, 120.3, 114.4. ESI-FTICR MS: calcd for [C10H9N2O]+
173.0709, found 173.0707. Anal. Calcd for C10H8N2O: C, 69.76; H,
4.68; N, 16.27. Found: C, 69.55; H, 4.77; N, 16.32.
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 (O2 <15 ppm for all reactions).
Acetonitrile and diethyl ether were dried by distillation over CaH2.
Tetrahydrofuran and n-pentane were distilled over sodium/benzophe-
none and P2O5, respectively. Methylene chloride and benzene were
purified by passage through a column of activated alumina. Benzene-
d6, acetone-d6, and dichloromethane-d2 were used as received and
General Procedure for the Synthesis of (N∼N)PtPh2
Complexes (1b−e and 3a−c). To a suspension of [Pt(Ph)2(Et2S)]2
in Et2O (30 mL), was added 2 equiv of the appropriate ligand. The
solution was stirred at room temperature for approximately 12 h. The
solution was reduced in vacuo, and hexanes were added (∼20 mL).
The solution was filtered, and the precipitate was washed with Et2O (1
× 5 mL) and hexanes (2 × 5 mL) and dried under vacuum.
(Me-dpm)PtPh2 (1b). The ligand was 6-methyl-2,2′-dipyridyl-
1
methane (Me-dpm). Isolated yield: 0.19 g (92%). 1H NMR (800
stored under a N2 atmosphere over 4 Å molecular sieves. H NMR
3
MHz, CD2Cl2): δ 8.54 (ddd, JHH = 6 Hz, 4JHH = 2, 5JHH = 1 Hz, 1H,
spectra were recorded on a Varian Mercury 300 MHz, Unity Avance
500 MHz, Bruker 400 MHz, or Bruker 800 MHz spectrometer. 13C
NMR spectra were recorded using a Varian Mercury 300 MHz
(operating frequency 75 MHz), Unity Avance 500 MHz (operating
frequency 125 MHz), or Bruker 800 MHz (operating frequency 201
MHz) spectrometer. All 1H and 13C NMR spectra are referenced
against residual proton signals (1H NMR) or the 13C resonances (13C
NMR) of the deuterated solvents. 19F NMR (282 MHz operating
frequency) spectra were obtained on a Varian 300 MHz spectrometer
and referenced against an external standard of hexafluorobenzene (δ
−164.9 ppm). GC/MS was performed using a Shimadzu GCMS-
QP2010 Plus system with a 30 m × 0.25 mm SHRXI-5MS column
with 0.25 mm film thickness using negative chemical ionization (NCI),
which also allows for simulated electron impact (SEI) ionization.
Microwave synthesis was performed using a Biotage Initiator EXP, in
addition to caps and vials purchased from Biotage. The mass spectral
data were obtained from a Bruker 9.4 T Apex-Qe Hybrid Qe-Fourier
Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometer
equipped with an Apollo II electrospray ionization source. The sample
was reconstituted with 200 μL of acetonitrile, and a 3.5 μL aliquot was
diluted with 50 μL of 60% acetonitrile/0.1% formic acid prior to
directly infusing into the FT-ICR MS via a TriVersa NanoMate
(Advion BioScience). Exact masses were obtained for the entire broad-
band spectrum. Instrument parameters were adjusted to maximize the
signals around the peak of interest. Bruker Daltonics DataAnalysis
software (v.3.4) was utilized for the analysis of the data, and
assignments were made on the basis of exact mass measurements and
fit of isotopic peaks to theoretical isotopic patterns (IsotopePattern
algorithm, Bruker). Errors between observed and theoretical peaks are
reported in ppm. Elemental analysis was performed by Atlantic
Microlabs, Inc. Ethylene (99.5%) and propylene (99.5%) were
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 [H(Et2O)2][BAr′4]
(Ar′ = 3,5-(CF3)2b6H3),20 [Pt(Ph)2(Et2S)]2,21 6-methyl-2,2′-dipyridyl-
methane,22 6,6′-dimethyl-2,2′-dipyridylmethane,22 1,2-bis(2-pyridyl)-
ethane,23 (dpm)Pt(Ph)2 (1a),9c [(dpm)Pt(Ph)(THF)][BAr′4] (2a),9c
and [(tbpy)Pt(Ph)(THF)][BAr′4] (tbpyPt)9b have been previously
reported.
6-dpm), 7.72 (td, 3JHH = 8 Hz, 4JHH = 2 Hz, 1H, 4-dpm), 7.64 (t, 3JHH
= 8 Hz, 1H, 4-dpm), 7.51 (dd, 3JHH = 8 Hz, 4JHH = 1 Hz, 2H, Ho-Ph),
3
3
7.46 (d, JHH = 7 Hz, 1H, 3-dpm), 7.37 (d, JHH =8 Hz, 1H, 3-dpm),
7.19 (dd, JHH = 8 Hz, JHH = 1 Hz, 1H, Ho-Ph), 7.09 (ddd, JHH = 7
3
4
3
Hz, 3JHH = 6 Hz, 4JHH = 1 Hz, 1H, 5-dpm), 7.05 (d, 3JHH = 7 Hz, 1H,
5-dpm), 6.88 (t, JHH = 8 Hz, 2H, Hm-Ph), 6.77−6.72 (m, 3H, Hm-
3
and Hp-Ph), 6.68 (tt, JHH = 8 Hz, JHH = 1 Hz, 1H, Hp-Ph), 5.67 (d,
2JHH = 14 Hz, 1H, dpm-CH2), 4.26 (d, 2JHH = 14 Hz, 1H, dpm-CH2),
2.38 (s, 3H, dpm-Me). 13C NMR (201 MHz, CD2Cl2): δ 162.7, 156.3,
156.1, 151.1, 146.2, 141.0, 138.5, 138.4, 137.9, 137.8, 126.9, 126.2,
124.9, 124.7, 123.8, 121.9, 121.5, 121.5 (Me-dpm and Ph aromatic),
47.7 (dpm-CH2), 26.7 (dpm-Me). Anal. Calcd for PtN2C24H22: C,
54.02; H, 4.16; N, 5.25. Found: C, 54.06; H, 4.28; N, 5.21.
3
4
(Me2-dpm)PtPh2 (1c). The ligand was 6,6′-dimethyl-2,2′-dipyridyl-
1
methane (Me2-dpm). Isolated yield: 0.32 g (94%). H NMR (800
MHz, acetone-d6): δ 7.75 (t, 3JHH = 8 Hz, 2H, 4-dpm), 7.60 (d, 3JHH
=
3
4
8 Hz, 2H, 3-dpm), 7.37 (dd, JHH = 8 Hz, JHH =1 Hz, 4H, Ho-Ph),
3
3
7.15 (d, JHH = 8 Hz, 1H, 5-dpm), 6.68 (t, JHH = 8 Hz, 4H, Hm-Ph),
3
2
6.57 (t, JHH = 8 Hz, 2H, Hp-Ph), 6.11 (d, JHH = 14 Hz, 1H, dpm-
2
CH2), 4.69 (d, JHH = 14 Hz, 1H, dpm-CH2), 2.53 (s, 6H, dpm-Me).
13C NMR (201 MHz, acetone-d6): δ 162.4, 157.3, 141.6, 139.1, 138.8,
126.4, 124.9, 122.7, 121.4 (dpm and Ph aromatic), 47.5 (dpm-CH2),
26.1 (dpm-Me). Anal. Calcd for PtN2C25H24: C, 54.83; H, 4.43; N,
5.12. Found: C, 54.33; H, 4.30; N, 5.11.
(Me2-bpy)PtPh2 (1d). The ligand was 6,6′-dimethyl-2,2′-bipyridine
(Me2-bpy). Isolated yield: 0.230 g (69%). 1H NMR (800 MHz,
3
3
acetone-d6): δ 8.38 (d, JHH = 8 Hz, 2H, bpy), 8.16 (t, JHH = 8 Hz,
3
3
2H, bpy), 7.48 (d, JHH = 7 Hz, 2H, bpy), 7.40 (d, JHH = 7 Hz, 4H,
Ho-Ph), 6.80 (t, JHH = 7 Hz, 4H, Hm-Ph), 6.69 (t, JHH = 7 Hz, 2H,
Hp-Ph). 13C NMR (201 MHz, acetone-d6): δ 163.3, 157.8, 142.8,
139.4, 139.0, 127.5, 126.8, 121.6, 120.3 (bpy and Ph aromatic), 26.7
(CH3-bpy). Anal. Calcd for PtN2C24H22: C, 54.02; H, 4.16; N, 5.25.
Found: C, 54.26; H, 4.32; N, 5.17.
3
3
(dpe)PtPh2 (1e). The ligand was 1,2-bis(2-pyridyl)ethane (dpe).
1
Isolated yield: 0.19 g (76%). H NMR (500 MHz, CD2Cl2): δ 8.71
(dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 2H, 6-dpe), 7.57 (td, 3JHH = 8 Hz, 4JHH
= 2 Hz, 2H, 4-dpe), 7.45 (dd, 3JHH = 8 Hz, 3JHH = 1 Hz, 3JPtH = 72 Hz
Pt satellites, 4H, Ho-Ph), 7.23 (d, 3JHH = 8 Hz, 2H, 3-dpe), 6.99 (ddd,
3
4
3
3JHH = 8 Hz, JHH = 6 Hz, JHH = 1 Hz, 2H, 5-dpe), 6.82 (t, JHH = 7
Hz, 4H, Hm-Ph), 6.71 (t, 3JHH = 7 Hz, 2H, Hp-Ph), 4.21 (br s, 4H, dpe-
CH2). 13C NMR (125 MHz, CD2Cl2): δ 160.40, 151.52, 143.01,
138.81, 137.30, 126.57, 126.35, 122.99, 121.60 (dpe and Ph aromatic),
34.83 (dpe-CH2). Anal. Calcd for PtN2C24H22: C, 54.02; H, 4.16; N,
5.25. Found: C, 53.98; H, 4.13; N, 5.17.
Synthesis of Bis(2-pyridyl) Ether (dpo). A method for
synthesizing bis(2-pyridyl) ether was developed from previous
protocols24 using convenient starting materials. Microwave-assisted
synthesis in sealed vials reduces reaction times and increases the yield
of desired product over that of prior work without resorting to high-
boiling solvents such as DMF. In an oven-dried microwave vial with a
magnetic stir bar, 2-bromopyridine (1.0 g, 6.3 mmol), 2-hydroxypyr-
idine (0.90 g, 9.5 mmol), cesium carbonate (6.2 g, 19 mmol), and dry
acetonitrile (15 mL) were added under a flow of nitrogen. The vial was
(dpk)PtPh2 (3a). The ligand was bis(2-pyridyl)ketone (dpk).
Isolated yield: 0.21 g (91%). 1H NMR (300 MHz, acetone-d6): δ
3
4
5
8.49 (ddd, JHH = 6 Hz, JHH = 2 Hz, JHH = 1 Hz, 2H, 6-dpk), 8.25
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3
4
3
(td, JHH = 8 Hz, JHH = 2 Hz, 2H, 4-dpk), 8.14 (ddd, JHH = 8 Hz,
3JHH = 2 Hz, 5JHH = 1 Hz, 2H, 3-dpk), 7.59 (ddd, 3JHH = 8 Hz, 3JHH = 6
Hz, 4JHH = 2 Hz, 2H, 5-dpk), 7.28 (dd, 3JHH = 8 Hz, 4JHH = 2 Hz, 3JPtH
dpm and Ph aromatic), 76.6 (α-THF), 48.4 (dpm-CH2), 28.7 (dpm-
Me), 24.8 (β-THF), 24.5 (dpm-Me). Anal. Calcd for
PtN2OBF24C54H37: C, 46.99; H, 2.80; N, 1.99. Found: C, 46.79; H,
2.76; N, 2.05.
= 71 Hz Pt satellites, 4H, Ho-Ph), 6.77 (t, JHH = 7 Hz, 4H, Hm-Ph),
3
6.65 (tt, 3JHH = 7 Hz, 4JHH = 2 Hz, 2H, Hp-Ph). 13C NMR (125 MHz,
CD2Cl2): δ 189.4 (CO-dpk), 153.3, 152.1, 143.9, 138.8, 138.4, 128.4,
127.1, 125.9, 122.4 (dpk and Ph aromatic). Anal. Calcd for
PtN2OC23H18: C, 51.80; H, 3.41; N, 5.25. Found: C, 51.70; H,
3.54; N, 5.21.
[(Me2-bpy)Pt(Ph)(THF)][BAr′4] (2d). Isolated yield: 0.13 g (91%).
1H NMR (300 MHz, CD2Cl2): δ 8.09−7.79 (m, 4H, bpy), 7.51 (d,
3JHH = 7 Hz, 1H, bpy), 7.17 (m, 3H, bpy and Ho-Ph), 6.98 (m, 3H, Hm
and Hp-Ph), 3.96 (m, 4H, α-THF), 2.66 (s, 3H, bpy-Me), 2.05 (s, 3H,
bpy-Me), 1.70 (m, 4H, β-THF). 13C NMR (125 MHz, CD2Cl2): δ
167.0, 160.7, 159.3, 155.7, 140.2, 139.8, 137.0, 128.7, 128.4, 128.0,
127.5, 125.5, 120.6, 120.3 (bpy and Ph aromatic), 76.2 (α-THF), 29.1
(bpy-Me), 24.7 (β-THF), 23.7 (bpy-Me). Anal. Calcd for
PtN2OBF24C54H37: C, 46.60; H, 2.69; N, 2.01. Found: C, 46.18; H,
2.52; N, 1.92.
(dpa)PtPh2 (3b). The ligand was bis(2-pyridyl)amine (dpa).
Isolated yield: 0.16 g (84%). 1H NMR (500 MHz, acetone-d6): δ
9.51 (s, 1H, NH-dpa), 8.06 (dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 2H, 6-dpa),
3
3
4
7.82 (ddd, JHH = 8 Hz, JHH = 7 Hz, JHH = 2 Hz, 2H, 4-dpa), 7.39
(dd, 3JHH = 8 Hz, 4JHH = 2 Hz, 3JPtH = 70 Hz Pt satellites, 4H Ho-Ph),
7.27 (d, JHH = 8 Hz, 2H, 3-dpa), 6.76 (t, JHH = 8 Hz, 4H, Hm-Ph),
6.72 (ddd, 3JHH = 7 Hz, 3JHH = 6 Hz, 4JHH,= 2 Hz, 2H, 5-dpa), 6.62 (t,
3JHH = 7 Hz, 2H, Hp-Ph). 13C NMR (201 MHz, acetone-d6): δ 152.4,
151.1, 146.7, 139.5, 139.4, 127.2, 121.6, 119.0, 114.7. Anal. Calcd for
PtN3C22H19: C, 50.76; H, 3.69; N, 8.07. Found: C, 50.97; H, 3.79; N,
7.90.
3
3
[(dpe)Pt(Ph)(THF)][BAr′4] (2e). Isolated yield: 0.12 g (89%). 1H
3
NMR (500 MHz, CD2Cl2): δ 8.69 (d, JHH = 6 Hz, 1H, 6-dpe), 8.52
(d, 3JHH = 6 Hz, 1H, 6-dpe), 7.69 (m, 10H, Ho-Ar′ and 4-dpe), 7.56 (s,
4H, Hp-Ar′), 7.33 (m, 5H, Ho-Ph, 3-dpe and 5-dpe), 7.02 (t, 3JHH = 7
Hz, 3H, Hm-Ph and 5-dpe), 6.91 (t, 3JHH = 7 Hz, 1H, Hp-Ph), 3.88 (br
m, 8H, dpe-CH2 and α-THF), 1.77 (s, 4H, β-THF). 13C NMR (125
MHz, CD2Cl2): δ 162.5, 159.8, 153.4, 150.8, 139.5, 139.4, 136.6,
130.5, 128.7, 127.8, 126.3, 125.2, 124.8, 124.0 (dpe and Ph aromatic),
77.3 (α-THF), 36.16 (dpe-CH2), 34.07 (dpe-CH2), 25.08 (β-THF).
Anal. Calcd for PtN2OBF24C54H37: C, 46.60; H, 2.69; N, 2.01. Found:
C, 46.85; H, 2.81; N, 2.14.
(dpo)PtPh2 (3c). The ligand was bis(2-pyridyl) ether (dpo). Isolated
yield: 0.14 g (93%). 1H NMR (300 MHz, CD2Cl2): δ 8.19 (d, 3JHH = 6
3
3
Hz, JPtH = 25 Hz Pt satellite, 2H, H6-dpo), 7.94 (t, JHH = 8 Hz, 2H,
H4-dpo), 7.42 (m, 6H, Ho-Ph and H3-dpo), 7.06 (t, JHH = 7 Hz, 2H,
3
H5-dpo), 6.91 (t, 3JHH = 7 Hz, 4H, Hm-Ph), 6.79 (t, JHH = 7 Hz, 2H,
3
Hp-Ph). 13C NMR (75 MHz, CD2Cl2): δ 159.1, 150.0, 143.7, 141.4,
138.5, 127.2, 123.1, 122.0, 115.9 (dpo and Ph aromatic). Anal. Calcd
for PtON2C22H18: C, 50.67; H, 3.49; N, 5.37. Found: C, 50.68; H,
3.56; N, 5.21.
[(dpk)Pt(Ph)(THF)][BAr′4] (4a). Isolated yield: 0.22 g (92%). 1H
NMR (300 MHz, CD2Cl2): δ 8.63 (d, 3JHH = 6 Hz, 1H, dpk), 8.33 (d,
3
3JHH = 6 Hz, 1H, dpk), 8.24 (td, JHH = 8 Hz, 4JHH = 2 Hz, 1H, dpk),
8.10 (m, 2H, dpk), 7.85 (m, 10H, Ho-BAr′ and dpk), 7.63 (s, 4H, Hp-
BAr′), 7.37 (m, 3H, dpk and Ho-Ph), 7.11 (m, 3H, Hm- and Hp-Ph),
4.06 (m, 4H, α-THF), 1.78 (m, 4H, β-THF). 13C NMR (126 MHz,
CD2Cl2): δ 185.8 (CO-dpk), 154.6, 153.3, 150.8, 149.3, 141.4, 141.1,
136.1, 130.2, 128.6, 128.5, 127.0 (dpk and Ph aromatic), 77.8 (α-
THF), 24.9 (β-THF), remaining three aromatic resonances obscured
due to broadening or coincidental overlap. Anal. Calcd for
PtN2O2BF24C53H33: C, 45.74; H, 2.39; N, 2.01. Found: C, 45.76; H,
2.57; N, 2.15.
General Procedure for the Synthesis of [(N∼N)Pt(Ph)(THF)]-
[BAr′4] Complexes (2b−e and 4a−c). A solution/suspension of
(N∼N)Pt(Ph)2 in THF (30 mL) was cooled to approximately −70
°C. One equivalent of [H(Et2O)2][BAr′4] dissolved in THF (∼10 mL,
−70 °C) was added. The solution was immediately placed under
vacuum, and the volatiles were removed. The residue was treated with
n-pentane (∼2 mL), which was then removed under vacuum to afford
a solid. The solid was dried in vacuo.
[(dpa)Pt(Ph)(THF)][BAr′4] (4b). Isolated yield: 0.19 g (95%). 1H
NMR (800 MHz, CD2Cl2): δ 8.20 (dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 1H,
dpa), 8.11 (dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 1H, dpa), 8.06 (s, 1H, NH-
dpa), 7.88 (ddd, 3JHH = 8 Hz, 3JHH = 7 Hz, 4JHH = 2 Hz, 1H, dpa), 7.74
Spectroscopic Data for the [BAr′4] Anion. The resonances for the
BAr′4 anion demonstrate negligible differences in chemical shift among
the complexes. Therefore, for simplicity the NMR data for the anion
are given here. H NMR (300 MHz, CD2Cl2): δ 7.72 (s, 8H, Ho-
1
BAr′4), 7.56 (s, 4H, Hp-BAr′4). 13C NMR (75 MHz, CD2Cl2): δ 162.3
(q, Ar′, 1JB‑Cipso = 49 Hz), 135.4 (Ar′), 129.5 (q, m-Ar′, 2JC−F = 32 Hz),
3
(m, 9H, Ho-BAr′ and dpa), 7.56 (s, 4H, Hp-BAr′), 7.36 (dd, JHH = 8
4
3
3
Hz, JHH = 2 Hz, 2H, Ho-Ph), 7.18 (ddd, JHH = 7 Hz, JHH = 6 Hz,
125.2 (q, Ar′, JC−F = 272 Hz), 118.1 (Ar′). 19F NMR (282 MHz,
2
4JHH = 1 Hz, 1H, dpa), 7.13 (d, 3JHH = 8 Hz, 1H, dpa), 7.07 (t, 3JHH
8 Hz, 2H, Hm-Ph), 6.98 (m, 2H, Hp-Ph and dpa), 6.65 (ddd, 3JHH = 7
=
CD2Cl2): δ −63.1 (s, CF3-Ar′).
3
4
[(Me-dpm)Pt(Ph)(THF)][BAr′4] (2b). Isolated yield: 0.14 g (93%).
Hz, JHH = 6 Hz, JHH = 1 Hz, 1H, dpa), 4.01 (m, 4H, α-THF), 1.73
(m, 4H, β-THF). 13C NMR (201 MHz, CD2Cl2): δ 153.4, 151.7,
149.7, 146.1, 141.2, 141.0, 136.4, 128.3, 125.3, 120.7, 119.8, 115.5,
114.4 (dpa and Ph aromatic), 77.2 (α-THF), 24.9 (β-THF), remaining
aromatic resonance obscured due to broadening or coincidental
overlap. Anal. Calcd for PtN3OBF24C52H34: C, 45.30; H, 2.49; N, 3.05.
Found: C, 45.45; H, 2.63; N, 3.03.
1H NMR (800 MHz, CD2Cl2): δ 8.56 (d, JHH = 6 Hz, 1H, 6-dpm),
3
7.90 (td, 3JHH = 8 Hz, 3JHH = 2 Hz, 1H, 4-dpm), 7.69 (t, 3JHH = 8 Hz,
3
3
1H, 4-dpm), 7.63 (d, JHH = 8 Hz, 1H, 3-dpm), 7.48 (t, JHH = 7 Hz,
1H, 5-dpm), 7.42 (d, 3JHH = 8 Hz, 1H, 3-dpm), 7.08 (d, 3JHH = 8.0 Hz,
1H, 5-dpm), 6.99−6.90 (m, 5H, Ph), 5.55 (d, 2JHH = 15 Hz, 1H, dpm-
3
CH2), 4.44 (d, JHH = 15 Hz, 1H, dpm-CH2), 4.17 (m, 2H, α-THF),
[(dpo)Pt(Ph)(THF)][BAr′4] (4c). Isolated yield: 0.12 g (98%). 1H
NMR (500 MHz, CD2Cl2): δ 8.25 (dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 1H,
dpo), 8.22 (dd, 3JHH = 6 Hz, 4JHH = 2 Hz, 1H, dpo), 8.11 (td, 3JHH = 8
3.98 (m, 2H, α-THF), 2.55 (s, 3H, dpm-Me), 1.88 (m, 4H, β-THF).
13C NMR (201 MHz, CD2Cl2): δ 165.2, 157.3, 154.2, 150.8, 140.6,
134.0, 136.1, 127.4, 126.1, 125.9, 125.3, 125.2, 123.4 (Me-dpm and Ph
aromatic), 77.2 (α-THF), 48.3 (dpm-CH2), 29.1 (β-THF), 25.4 (dpm-
Me), remaining aromatic resonance obscured due to broadening or
coincidental overlap. Anal. Calcd for PtN2OBF24C54H37: C, 46.60; H,
2.69; N, 2.01. Found: C, 46.51; H, 2.70; N, 2.07.
4
3
4
Hz, JHH = 2 Hz, 1H, dpo), 7.99 (td, JHH = 8 Hz, JHH = 2 Hz, 1H,
dpo), 7.72 (s, 8H, Ho-Ar′), 7.54 (m, 5H, Hp-Ar′ and dpo), 7.50 (ddd,
3JHH = 7 Hz, 3JHH = 6 Hz, 4JHH = 1 Hz, 1H, dpo), 7.39 (m, 3H, Ho-Ph
and dpo), 7.11 (t, JHH = 8 Hz, 2H, Hm-Ph), 7.02 (m, 2H, Hp-Ph and
3
dpo), 4.09 (m, 4H, α-THF), 1.77 (m, 4H, β-THF). 13C NMR (125
MHz, CD2Cl2): δ 157.6, 152.9, 146.6, 144.1, 143.8, 136.1, 128.7,
128.5, 125.8, 124.5, 123.9, 117.5, 116.6 (dpo and Ph aromatic), 77.9
(α-THF), 24.9 (β-THF), remaining aromatic resonances obscured due
to broadening or coincidental overlap. Anal. Calcd for
PtN2O2BF24C52H33: C, 45.26; H, 2.42; N, 2.03. Found: C, 45.00; H,
2.38; N, 2.15.
[(Me2-dpm)Pt(Ph)(THF)][BAr′4] (2c). Isolated yield: 0.18 g (95%).
1H NMR (300 MHz, CD2Cl2): δ 7.76 (t, JHH = 8 Hz, 1H, 4-dpm),
3
3
3
7.64 (t, JHH = 8 Hz, 1H, 4-dpm), 7.46 (d, JHH = 8 Hz, 1H, 3-dpm),
3
3
7.38 (d, JHH = 8 Hz, 1H, 5-dpm), 7.33 (d, JHH = 8 Hz, 1H, 3-dpm),
7.12 (d, JHH = 7 Hz, 2H, Ho-Ph), 7.04 (d, JHH = 8 Hz, 1H, 5-dpm),
3
3
7.02−6.86 (m, 3H, Hm and Hp-Ph), 5.82 (d, JHH = 15 Hz, 1H, dpm-
2
2
CH2), 4.48 (d, JHH = 15 Hz, 1H, dpm-CH2), 3.92 (m, 4H, α-THF),
2.94 (s, 3H, dpm-Me), 2.67 (s, 3H, dpm-Me), 1.73 (m, 4H, β-THF).
13C NMR (201 MHz, CD2Cl2): δ 164.7, 160.8, 157.5, 154.0, 140.5,
139.9, 137.2, 128.7, 127.4, 125.8, 125.5, 125.3, 123.38, 123.34 (Me2-
Catalytic Olefin Hydroarylation. A representative catalytic
reaction is described. [(Me-dpm)Pt(Ph)(THF)][BAr′4] (2b; 0.019
g, 0.013 mmol) was dissolved in 12.0 mL of benzene containing 0.01
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Article
mol % of hexamethylbenzene (HMB) relative to benzene as an
internal standard. The reaction mixture was placed in a stainless steel
pressure reactor, and the reactor was charged with ethylene (1.0 bar),
pressurized to a total of 7.5 bar with N2, and heated to 100 °C. After a
given duration, the reaction mixture was cooled to room temperature
and 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 was
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 production
of styrene, 1,3-diethylbenzene, 1,4-diethylbenzene, and 1,2-diethyl-
benzene. The slope and correlation coefficients (R2) for these species
are respectively as follows: 0.51, 0.99; 0.52, 0.99; 0.53, 0.99; 0.55, 0.99.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and CIF files giving details of the kinetic analysis
of catalyst decomposition and X-ray structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tbg7h@virginia.edu (T.B.G.); t@unt.edu (T.R.C.).
■
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
(grant numbers DE-SC0000776 and DE-FG02-03ER15387,
respectively). T.B.G. acknowledges the National Science
Foundation for the purchase of X-ray diffraction instrumenta-
tion at the University of Virginia (CHE-1126602). R.H.C. and
T.M. thank Gary Brudvig for useful discussions and also the
Office of Basic Energy Sciences, U.S. Department of Energy, for
support under DE-PS02-08ER15944 (bis(2-pyridyl) ether
synthesis). B.A.M. thanks Benjamin Vaughan for assistance in
refinement of X-ray diffraction data.
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