Platinum(II)-catalyzed ethylene hydrophenylation: Switching selectivity between alkyl- and vinylbenzene production

Article abstract of DOI:10.1021/om400306w

The series of Pt^II complexes [(^xbpy)Pt(Ph)(THF)] [BAr′₄] (^xbpy =4,4′-X-2,2′-bipyridyl, X = OMe, ^tBu, H, Br, CO₂Et, NO₂; Ar′ = 3,5-bis(trifluoromethyl)phenyl) are catalyst precursors for ethylene hydrophenylation. The bipyridyl substituent provides a tunable switch for catalyst selectivity that also has significant influence on catalyst activity and longevity. Less electron donating 4,4′-substituents increase the propensity toward styrene formation over ethylbenzene.

Full text of DOI:10.1021/om400306w

Article
pubs.acs.org/Organometallics
Platinum(II)-Catalyzed Ethylene Hydrophenylation: Switching
Selectivity between Alkyl- and Vinylbenzene Production
Bradley A. McKeown,† H. Emanuel Gonzalez,‡ Max R. Friedfeld,† Anna M. Brosnahan,†
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 (CASCaM), Department of Chemistry, University of North Texas,
Denton, Texas 76203, United States
§Nanoscale Materials Characterization Facility, Materials Science and Engineering Department, University of Virginia, Charlottesville,
Virginia 22904, United States
S
* Supporting Information
ABSTRACT: The series of PtII complexes [(xbpy)Pt(Ph)(THF)][BAr′4] (xbpy
=4,4′-X-2,2′-bipyridyl, X = OMe, tBu, H, Br, CO2Et, NO2; Ar′ = 3,5-
bis(trifluoromethyl)phenyl) are catalyst precursors for ethylene hydrophenylation.
The bipyridyl substituent provides a tunable switch for catalyst selectivity that also
has significant influence on catalyst activity and longevity. Less electron donating
4,4′-substituents increase the propensity toward styrene formation over ethyl-
benzene.
The development of selective catalysts for olefin hydro-
arylation presents several challenges, such as regioselective C−
H activation of substituted aromatic substrates, selectivity for
olefin insertion (e.g., 1,2- versus 2,1-insertion), selectivity for
mono- versus polyalkylation (starting from unsaturated
aromatic substrates), and selectivity for alkyl- versus vinyl
arene production. Despite these obstacles, few detailed
structure/activity studies that could guide new catalyst design
exist.6b,7c,9 In order to design improved catalysts, it is important
to understand how modifications to the transition-metal
complex influence the various facets of selectivity. In some
cases, saturated alkyl arenes are desired while vinyl arenes are
preferred for other applications. For transition-metal-catalyzed
olefin hydroarylation, the selectivity for vinyl arene (pathway
A) versus alkyl arene (pathway B) formation is presumably
controlled by the relative kinetics of the steps shown in Scheme
1, and understanding how to use ligand modification to switch
catalyst selectivity is a potentially important feature.
INTRODUCTION
■
The formation of C−C bonds with aromatic substrates has
received considerable attention due to its importance in both
fine and commodity chemical production.1 While methods for
the functionalization of aromatic C−X bonds (X = halide,
triflate) have been successfully developed,1b,2 atom-economical
catalytic olefin hydroarylation (i.e., the addition of aromatic C−
H bonds across olefin CC bonds) offers potential
advantages.1d,3 For example, halogenation of aromatic sub-
strates can generate substantial waste and reduce the overall
yield of desired products. In addition, the conversion of
aromatic C−X bonds to C−C bonds generally requires
stoichiometric organometallic reagents (e.g., Grignard, tin,
boron, etc.). Thus, the efficient direct functionalization of
aromatic C−H bonds would reduce the generation of waste,
especially that of halogenated and metal-containing byproducts.
Given the substantial efforts to control the stereochemistry of
olefin insertions (e.g., asymmetric olefin hydrogenation4 or
olefin polymerization5), extension of catalytic olefin hydro-
arylation to enantioselective variants is a reasonable proposal.
Despite these potential advantages, examples of catalysts for the
hydroarylation of olefins by a non-acid-catalyzed (i.e., non-
Friedel−Crafts) pathway are relatively rare,3,6 and catalysts for
unactivated substrates, such as benzene with unfunctionalized
olefins, are especially limited.3,6b,7 In addition, the oxidative
coupling of aromatic C−H bonds with alkenes to form vinyl
arenes has typically been restricted to activated olefins.1d,6c,8
Recently, we reported a mechanistic study of ethylene
hydrophenylation catalyzed by cationic PtII supported by 4,4′-
di-tert-butyl-2,2′-bipyridine.7c The bipyridyl ligand is easily
modified to determine the impact of ligand donor ability on
catalysis without altering the catalyst’s steric profile. Herein, we
report the influence of 4,4′-substituents on catalytic hydro-
phenylation of ethylene for the series of complexes [(xbpy)Pt-
Received: April 10, 2013
© XXXX American Chemical Society
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The N1−Pt−N2 bond angle is compressed to 79.4(2)° relative
to the ideal 90° bond angles for a square-planar complex, which
is characteristic of PtII bipyridyl and diimine complexes.10 The
Pt−N1 bond is 0.08 Å shorter than the Pt−N2 bond, indicative
of a greater trans influence of the phenyl ligand relative to THF.
Significant disorder exists for the THF ligand in the refined
structure.
Scheme 1. Likely Control of the Selectivity of Alkyl Arenes
versus Vinyl Arenes (Ethylbenzene versus Styrene in this
Scheme) during Catalytic Ethylene Hydrophenylation by the
Relative Kinetics of Divergent Pathways that Follow Olefin
Insertion
The proposed mechanism for PtII-catalyzed ethylene hydro-
phenylation on the basis of previous experimental and
computational studies7c is shown in Scheme 2. Catalytic
Scheme 2. Proposed Mechanism for Ethylene
Hydrophenylation Catalyzed by Cationic PtII Complexes
Supported by Bipyridyl Ligands
(Ph)(THF)][BAr′4] (xbpy = 4,4′-X-2,2′-bipyridyl, X = OMe,
tBu, H, Br, CO2Et, NO2; Ar′ = 3,5-bis(trifluoromethyl)phenyl).
Of particular note is the ability to control alkyl- to vinyl arene
ratios by adjusting the donor ability of the bipyridyl ligand.
Controlling alkyl- versus vinyl arene production is important
for achieving desired product selectivity. Moreover, the
formation of vinyl arenes likely involves β-hydride elimination,
which is a plausible decomposition route for some catalysts that
mediate alkyl arene synthesis.
RESULTS AND DISCUSSION
■
The complexes [(xbpy)Pt(Ph)(THF)][BAr′4] (2a−f) were
prepared according to the procedure previously reported for
[(tbpy)Pt(Ph)(THF)][BAr′4] (2b; bpy = 4,4′-di-tert-butyl-
t
2,2′-bipyridyl) (eq 1).7c All complexes 2 have been isolated in
ethylene hydrophenylation using complexes 2a−f was probed
by heating benzene solutions of 2 (0.01 mol %) at 100 °C with
0.1 MPa of ethylene. The results are summarized in Table 1.
Plots of turnovers (TO) versus time for 2a−c reveal no
evidence of catalyst deactivation after 4 h (Figure 2). Thus, the
Table 1. Catalytic Ethylene Hydrophenylation using
a
Complexes 2a−f with 0.1 MPa of Ethylene
1
≥80% yield and characterized by H and 13C NMR spectros-
copy as well as elemental analysis. A crystal of complex 2d
suitable for an X-ray diffraction study was grown (Figure 1).
a
Conditions: 0.01 mol % catalyst dissolved in C6H6 with
hexamethylbenzene as an internal standard at 100 °C with 0.1 MPa
b
Figure 1. ORTEP drawing of [(Brbpy)Pt(Ph)(THF)][BAr′4] (2d)
(30% probability; H atoms and BAr′4 anion omitted for clarity).
Selected bond lengths (Å): Pt−N1 = 1.998(6), Pt−N2 = 2.075(6),
Pt−O1 = 2.060(7), Pt−C1 = 2.014(8). Selected bond angles (deg):
N1−Pt−N2 = 79.4(2), C1−Pt−O1 = 89.4(3).
of ethylene. Ratio of 1,2-, 1,3-, and 1,4-diethylbenzene after 4 h.
c
Turnover frequency calculated on the basis of total turnovers after 4
d
e
h. Turnovers after 4 h as determined by GC/MS. Numbers in
parentheses are turnovers after 16 h. Numbers in brackets are TON
f
g
values after catalyst deactivation. Reference 7c
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ratio of 27.8 (after 4 h), in comparison to 0.1 for complex 2f
(NO2, σp = 0.78). A Hammett plot was constructed using
product ratios and the Hammett parameter σp (Figure 4).11
Figure 2. Plot of ethylbenzene and diethylbenzene TO values as a
function of time for ethylene hydrophenylation catalyzed by complexes
2a−c at 100 °C with 0.1 MPa of ethylene pressure.
Figure 4. Hammett plot for the ratios of ethylbenzene to styrene from
[(xbpy)Pt(Ph)(THF)]+-catalyzed ethylene hydrophenylation after 4 h
at 100 °C with 0.1 MPa of ethylene (slope −2.3, R2 = 0.77).
TO after 4 h for these catalysts should reasonably reflect
relative catalyst activities. For complexes 2a−c, the relative rates
of catalysis (based on total product formation after 4 h) are
OMe (turnover frequency (TOF) 5.9 × 10−4 s−1) < tBu (TOF:
13.8 × 10−4 s−1) < H (TOF 15.3 × 10−4 s−1), which is
consistent with less donating 4,4′-substituents providing a slight
rate enhancement. Results with catalyst precursors 2d−f, which
possess less donating 4,4′-substituents than catalyst precursors
2a−c, indicate less effective catalysis. Complex 2d provides only
2.3 total TO, with more styrene than ethylbenzene, after 4 h,
but a plot of TO versus time for 2d reveals no signs of catalyst
deactivation after 4 h (Figure 3). Although catalysis with 2e is
The effects of substituted pyridyl ligands are rarely amenable to
Hammett correlations, since Hammett σp parameters do not
accurately reflect substituent effects upon the basicity of
pyridine, as the inductive and resonance interactions from the
substituents differ from those found in benzoic acids.12 In
addition, π interactions with the metal center influence the
correlation.12 Thus, it is not surprising that the fit of the
Hammett plot is not good (R2 = 0.77). However, the plot
demonstrates that less donating 4,4′-substituents result in a
decrease in the ratio of ethylbenzene to styrene. Using
Hammett σp parameters as a relative gauge of substituted
bipyridyl donation to PtII, plots of ethylbenzene to styrene ratio
versus substituent Hammett parameters further demonstrate
this trend (Figure 5). Complex 2d (Br, σp = 0.23) exhibits an
ethylbenzene/styrene ratio similar to that of 2f and falls outside
of the observed linear trend shown in Figure 5. The deviation
of 2d from the remaining five catalysts is not currently
understood.
Figure 3. Plot of ethylbenzene and diethylbenzene TO values as a
function of time for ethylene hydrophenylation catalyzed by complexes
2d−f at 100 °C with 0.1 MPa of ethylene pressure.
more efficient than that with 2d, it also performs less effectively
than complexes 2a−c with no evidence of substantial
deactivation after 24 h. The nitro complex 2f provides slightly
more than 1 TO and undergoes relatively rapid deactivation to
multiple intractable complexes within approximately 1 h.
The ratio of ethylbenzene to styrene is influenced by the
donor ability of the 4,4′-bipyridyl functional groups. For
example, catalysis using complex 2a (OMe, σp = −0.27) and 0.1
MPa of ethylene (100 °C) results in an ethylbenzene/styrene
Figure 5. Ethylbenzene/styrene ratios from [(xbpy)Pt(Ph)(THF)]+-
catalyzed ethylene hydrophenylation after 4 h at 100 °C with 0.1 and
0.3 MPa of ethylene versus Hammett parameters (σp) for the 4,4′-
substituent. Complex 2d (X = Br) is not included in either linear fit
(0.1 MPa, R2 = 0.98; 0.3 MPa, R2 = 0.96).
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We sought to determine if ethylene concentration would
influence ethylbenzene/styrene ratios. Catalysis performed
under the conditions outlined above but with 0.3 MPa of
ethylene results in decreased catalytic activity (Table 2), as
Complexes 2d,e produce >1.0 TO of styrene. For example, at
0.1 MPa of ethylene, complex 2d produces a TON of 7.5 for
styrene after 4 days at 100 °C. The production of ≥1 equiv
(relative to Pt) of styrene requires a hydrogen acceptor.
Heating a CD3NO2 solution of complex 2d and benzene under
ethylene results in the formation of styrene and ethane, as
Table 2. Catalytic Ethylene Hydrophenylation using
a
1
Complexes 2a−f with 0.3 MPa of Ethylene
observed by H NMR spectroscopy. Confirmation of ethane
formation was achieved using isotopically labeled 13C2H4. In
1
the H and 13C NMR spectra, ethane is clearly observed and
identified using a comparison to an analytically pure standard
(Figure 6). Therefore, the observed catalytic oxidative hydro-
phenylation of ethylene by 2d uses ethylene as the oxidant (eq
3).
a
Conditions: 0.01 mol % catalyst dissolved in C6H6 with
hexamethylbenzene as an internal standard at 100 °C with 0.3 MPa
b
of ethylene. Ratio of 1,2-, 1,3-, and 1,4-diethylbenzene after 4 h.
c
Turnover frequency calculated on the basis of total turnovers after 4
d
e
h. Turnovers after 4 h as determined by GC/MS. Numbers in
parentheses are turnovers after 16 h. Reference 7c.
f
previously reported for 2b.7c Two observations relevant to
styrene/ethylbenzene production are made. First, for all
complexes, the ethylbenzene/styrene ratio decreases at higher
ethylene pressure (Table 3). Second, similar to reactions at 0.1
Figure 6. 13C{1H} NMR spectrum (top) and 1H NMR spectrum (top
1
inset, JCH = 120 Hz) of 13C2H6 in CD3NO2 resulting from the
formation of styrene by complex 2d and benzene under 13C2H4
pressure and the 13C{1H} NMR spectrum (bottom) of an analytically
pure sample of C2H6 in a CD3NO2/benzene solution.
Table 3. Ratio of Ethylbenzene to Styrene as a Function of
Ethylene Pressure
The complex [(tbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ (3b) has
been shown to be the catalyst resting state using 2b as the
catalyst precursor.7c Catalysis using 2a−f was monitored by 1H
NMR at 90 °C over 4 h to confirm that [(xbpy)Pt-
(CH2CH2Ph)(η2-C2H4)]+ is the resting state for each bpy
x
a
b
Ethylbenzene/styrene ratio after 4 h at 100 °C. Ratio after 4 h with
ligand. This species is observed as the catalyst resting state
using complexes 2a−e. Note that for complexes 2d,e the
insertion product [(xbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ is ob-
served but is slowly consumed as the complexes [(xbpy)Pt-
(Et)(η2-C2H4)]+ are formed, as a result of β-hydride
elimination and styrene displacement. The PtII ethyl complexes
[(xbpy)Pt(Et)(η2-C2H4)]+ [X = Br (2d), CO2Et (2e)]
eventually decompose. Consistent with the observation of ∼1
TO under catalytic conditions (see above), complex 3f is
unstable and is consumed within minutes to yield stoichio-
metric equivalents of ethylbenzene and styrene as well as
multiple Pt decomposition products.
c
d
0.3 MPa of ethylene at 100 °C. Reference 7c. Only styrene observed.
MPa of ethylene, decreasing the donor ability of the 4,4′-
substituents results in a decrease in the ethylbenzene/styrene
ratio (Figure 3). Again, complex 2d deviates from the observed
linear correlation of ethylbenzene/styrene ratio versus
Hammett σp value. At 0.3 MPa, complex 2f gives exclusive
formation of styrene after 4 h (eq 2). The dependence of
Previously, we reported that heating [(tbpy)Pt(CH2CH2Ph)-
(η2-C2H4)]+ under ethylene pressure in CD3NO2 results in
stoichiometric styrene production as well as the formation of
[(tbpy)Pt(C2H5)(η2-C2H4)]+.7c Styrene formation is not
observed in the absence of excess ethylene. For example, the
thermolysis (100 °C) of 3f in benzene results in the formation
ethylbenzene/styrene ratios for all catalyst precursors on
ethylene concentration is consistent with the possibility that
the rate of styrene displacement by ethylene is a key factor in
the ethylbenzene/styrene ratios (see below).
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of ethylbenzene in quantitative yield (eq 4). Thus, for complex
3f the formation of styrene is dependent on the presence of
ethylene, which indicates that ethylene plays a role in the
formation of free styrene and is consistent with the trends in
ethylbenzene/styrene ratios as a function of ethylene pressure
(see above). Therefore, it is likely that release of styrene occurs
via an associative ligand exchange with ethylene.
The rates of stoichiometric styrene production from the
thermolysis (45 °C) of [(xbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ (X
Figure 7. Hammett plot for styrene formation from [(xbpy)Pt-
(CH2CH2Ph)(η2-C2H4)]+ at 45 °C with 0.3 MPa of ethylene (R2 =
0.83, slope 1.7).
t
= OMe (3a), Bu (3b), H (3c), CO2Et (3e), NO2 (3f); eq 5)
Table 4. Observed Rate Constants for Stoichiometric
a
Styrene Production from Complexes 3a−3f
Figure 8. Plot of pseudo-first-order rate constants (kobs) for styrene
formation from [(xbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ at 45 °C with 0.3
MPa of ethylene versus Hammett σp parameter of the 4,4′-bipyridyl
functionality (R2 = 0.99).
M ethylene at 45 °C. In contrast, complex 3f produces styrene
∼60 times faster with an observed rate constant of [1.6(2)] ×
10−4 s−1. The relative rates of styrene formation cannot be
directly compared to the results from catalysis, since the
conditions used for catalysis and stoichiometric styrene
production are different. Also, in addition to the relative rates
of styrene formation, the relative rates of ethylbenzene
formation play a role in ethylbenzene/styrene ratios. However,
it can be stated definitively that the trend in the rates of
stoichiometric styrene production from the five complexes
[(xbpy)Pt(CH2CH2Ph)(η2-C2H4)]+ (3a−c,e,f) is identical with
the trend in ethylbenzene/styrene ratios observed during
catalysis. Interestingly, the rate of styrene formation from 3d,
which is the complex that deviates from the linear plots in
Figure 3, is much faster than that of the other complexes. For
example, at room temperature the reaction of 2d with ethylene
is complete within approximately 10 min.
a
Determined by 1H NMR spectroscopy at 45 °C using hexamethyldi-
silane as an internal standard. [Pt] = 0.03 M.
were measured by 1H NMR spectroscopy (Table 4). Similar to
the Hammett plot for ethylbenzene and styrene ratios, a
Hammett plot using the rate constants for styrene formation
from 3a−f (without 3d) reveals a poor linear correlation (R2 =
0.83; Figure 7). The curvature in the plot might indicate a
change in mechanism or rate-determining step; however, given
the precedent for poor Hammett correlations for substituted
pyridyl groups,12 it is difficult to interpret the plot definitively.
Despite the poor linear correlation, the identity of the 4,4′-
substituent has a clear effect on the rate of styrene evolution
(Figure 8). Decreasing the electron donor ability of the 4,4′-
substituent results in more rapid styrene production. For
example, the formation of styrene from 3a occurs with a
pseudo-first-order rate constant of [2.6(2)] × 10−6 s−1 with 0.3
The production of styrene by these PtII complexes is clearly
facilitated by less donating bipyridyl ligands. The formation of
styrene from complexes 3 is likely a multistep reaction
involving ethylene dissocation, β-hydride elimination, and net
dissociation of styrene. Possible explanations for the trends in
styrene production include (i) the barrier to the reinsertion of
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x
styrene after β-hydride elimination increases with less donating
ligands, (ii) styrene is more readily displaced by ethylene for
the Pt complexes with less donating ligands, or (iii) a
combination of both effects.
completes the process for styrene formation. For most bpyPt
complexes (excluding 2d,e), we presume that the PtII−H
complexes are unstable and result in catalyst decomposition,
since only ∼1 TO of styrene is observed. For X = Br (2d),
CO2Et (2e), ethylene insertion into the Pt−H bond and
subsequent benzene C−H activation liberates ethane and
regenerates the [(xbpy)Pt(Ph)]+ fragment; however, catalytic
production of styrene is not sustained over a long period, as
evidenced by the low TON for styrene production (Table 1).
The PtII catalysts eventually decompose to multiple intract-
able complexes, and understanding the exact pathway for
catalyst deactivation is challenging. However, inspection of the
TON values for catalysts 2a−f (Table 1) shows that complexes
2a−c, which possess more donating bipyridyl ligands, give
higher TON values than 2d−f. Since complexes 2d−f, which
possess less donating bipyridyl ligands, exhibit a greater
predilection for styrene production, one possible explanation
for reduced TON for 2d−f in comparison to 2a−c is that the
PtII−H complexes that result from β-hydride elimination
(Scheme 3) are unstable and more prone to decomposition.
We sought to measure the rate of styrene displacement by
ethylene as a function of the 4,4′-substituent using [(xbpy)Pt-
t
(H)(η2-styrene)]+ (X = Bu, NO2). Attempts to synthesize the
PtII hydride complexes were unsuccessful. Instead, the PtII
methyl complexes [(xbpy)Pt(Me)(η2-styrene)]+ (X = tBu
(5b), NO2 (5f)) were used as models for the Pt−H variants.
Unfortunately, the displacement of styrene by ethylene from
both 5b and 5f was too rapid for measurement even at −120
°C. The Pt complexes were dissolved in a solvent mixture of
CD2Cl2, CDCl3, and CCl4 (60/27/13, v/v/v) and then frozen.
The tube was pressurized with 0.3 MPa of ethylene and allowed
to thaw in the spectrometer. The first NMR spectrum showed
complete conversion to [(xbpy)Pt(η2-C2H4)(Me)]+ and free
styrene. The structure of [(NO2bpy)Pt(η2-C2H4)(Me)]+ (6f) is
shown in Figure 9.
SUMMARY AND CONCLUSIONS
■
Direct oxidative olefin hydroarylation to produce vinyl arenes is
a desirable target, and the availability of a tunable “switch” that
dictates alkyl- versus vinyl arene selectivity is potentially useful.
For [(xbpy)Pt(Ph)(THF)]+ complexes, we have shown that
catalyst selectivity for the production of vinyl arenes versus
alkyl arenes can be controlled by the 4,4′-substituents on the
bipyridyl ligand. Less donating 4,4′-substituents result in an
increased propensity toward styrene production. Of course, for
the PtII catalysts reported herein, application toward vinyl arene
production will require conditions that permit catalytic
turnover with oxidants other than ethylene. In addition, such
structure/activity relationships are important, since the
formation of vinyl arenes is a possible deactivation pathway
for this series of PtII catalysts and possibly for other transition-
metal catalysts.
Figure 9. ORTEP drawing of [(NO2bpy)Pt(η2-C2H4)(Me)][BAr′4]
(6f) (50% probability; H atoms and BAr′4 anion omitted for clarity).
Selected bond lengths (Å): Pt−N1 = 2.113(7), Pt−N2 = 2.063(6),
Pt−C1 = 2.021(9), Pt−C2 = 2.073(13), Pt−C3 = 2.113(12), C2−C3
= 1.349(16). Selected bond angles (deg): N1−Pt−N2 = 77.5(2), N1−
Pt−C1 = 173.1(3).
A plausible mechanism for styrene formation is shown in
Scheme 3. Ethylene insertion into the Pt−Ph bond results in a
β-agostic phenethyl intermediate, which coordinates ethylene
to form the catalyst resting state, complex 3. Complex 3 may
either exchange ethylene with benzene and continue along the
ethylene hydrophenylation catalytic cycle7c or dissociate
ethylene and undergo β-hydride elimination to form [(xbpy)-
Pt(H)(η2-styrene)]+. Displacement of styrene with ethylene
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).
Tetrahydrofuran and diethyl ether were dried by distillation over
sodium/benzophenone and CaH2, respectively. n-Pentane was distilled
over P2O5. Methylene chloride and benzene were purified by passage
through a column of activated alumina. Benzene-d6, acetone-d6,
nitromethane-d3 and dichloromethane-d2 were used as received and
Scheme 3. Proposed Mechanism for Styrene Formation
during PtII-Catalyzed Ethylene Hydrophenylation
1
stored under a N2 atmosphere over 4 Å molecular sieves. H NMR
spectra were recorded using a Varian Mercury 300 or 500 MHz
spectrometer or using a Bruker 800 MHz spectrometer. 13C NMR
spectra were recorded using a Varian Mercury 300 or 500 MHz
spectrometer (operating frequency 75 or 125 MHz, respectively) or
using a Bruker 800 MHz spectrometer (operating frequency 201
1
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 (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 electron impact 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
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4
3
preparation, isolation, and characterization of [H(Et2O)2][BAr′4] (Ar′
= 3,5-(CF3)2C6H3),13 [Pt(Ph)2(Et2S)]2,14 (bpy)Pt(Ph)2 (1c; bpy =
6 Hz, JHH = 2 Hz), 7.14 (t, 2H, Hm-Ph, JHH = 7 Hz), 7.04 (m, 1H,
Hp-Ph), 6.74 (dd, 1H, H5-mbpy, JHH = 7 Hz, JHH = 3 Hz), 4.12 (m,
4H, α-THF), 4.02 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 1.84 (m, 4H,
β-THF). 13C NMR (126 MHz, CD2Cl2): δ 169.0, 168.5, 159.1, 155.5,
155.2, 148.1, 139.2, 136.5, 128.5, 125.3, 112.7, 112.1, 111.2, 110.5
(mbpy and Ph), 77.8 (α-THF), 57.3 (OCH3), 57.1 (OCH3), 25.1 (β-
THF). Anal. Calcd for PtN2O3BF24C54H37: C, 45.55; H, 2.62; N, 1.97.
Found: C, 45.38; H, 2.81; N, 2.10.
3
4
2,2′-bipyridine),15 (tbpy)Pt(Ph)2 (1b; bpy = 4,4′- di-tert-butyl-2,2′-
t
bipyridine),10a [(tbpy)Pt(Ph)(THF)][BAr′4] (2b),7c [(tbpy)Pt-
(CH2CH2Ph)(η2-C2H4)][BAr′4] (3b),7c [Pt(Me)2(Et2S)]2,16 and
16
tbpyPtMe2 have been previously reported.
General Procedure for the Synthesis of (xbpy)PtPh2
Complexes 1a−f. To a suspension of [Pt(Ph)2(Et2S)]2 in diethyl
ether (30 mL) was added 2 equiv of the appropriate bipyridyl ligand.
The solution was stirred at room temperature overnight. The solution
was reduced in vacuo, and hexanes was added (∼20 mL). The solution
was filtered, and the precipitate was dried under vacuum.
[(bpy)Pt(Ph)(THF)][BAr′4] (2c): 89% isolated yield, 0.201 g. 1H
NMR (800 MHz, CD2Cl2): δ 8.49 (d, 1H, H6-bpy, 3JHH = 5 Hz), 8.31
(d, 1H, H6-bpy, 3JHH = 6 Hz), 8.24 (td, 1H, H4-bpy, 3JHH = 8 Hz, 4JHH
= 2 Hz), 8.15 (d, 1H, H3-bpy, 3JHH = 8 Hz), 8.11 (td, 1H, H4-bpy, 3JHH
mbpyPtPh2 (1a). The bipyridyl ligand was 4,4′-dimethoxy-2,2′-
= 8 Hz, JHH = 1 Hz), 8.03 (d, 1H, H3-bpy, JHH = 8 Hz), 7.78 (ddd,
4
3
1
3
3
4
bipyridine (mbpy; 96% isolated yield, 0.288 g). H NMR (300 MHz,
H5-bpy, JHH = 8 Hz, JHH = 5 Hz, JHH = 1 Hz), 7.47 (d, 2H, Ho-Ph,
CD2Cl2): δ 8.24 (d, 2H, H6-mbpy, 3JHH3 = 6 Hz), 7.52 (d, 2H, H3-mbpy,
3JHH = 8 Hz), 7.30 (ddd, 1H, H5-bpy, 3JHH = 8 Hz, 3JHH = 6 Hz, 3JHH
=
3
4JHH = 3 Hz), 7.42 (d, 4H, Ho-Ph, JHH = 8 Hz, JPtH = 69 Hz, Pt
2 Hz), 7.17 (t, 2H, Hm-Ph, JHH = 8 Hz), 7.09 (m, 1H, Hp-Ph), 4.16
(m, 4H, α-THF), 1.88 (m, 4H, β-THF). 13C NMR (126 MHz,
CD2Cl2): δ 158.0, 154.4, 154.0, 146.9, 141.0, 140.5, 138.8, 136.1,
128.7, 128.4, 128.2, 125.6, 123.7, 123.4 (bpy and Ph), 77.9 (α-THF),
25.1 (β-THF). Anal. Calcd for PtN2OBF24C52H33: C, 45.80; H, 2.44;
N, 2.05. Found: C, 45.76; H, 2.34; N, 2.01.
3
3
satellites), 6.95 (t, 4H, Hm-Ph, JHH = 7 Hz), 6.87 (dd, 2H, H5-mbpy,
4
3JHH = 6 Hz, JHH = 2 Hz), 6.82 (m, 2H, Hp-Ph), 3.94 (s, 6H,
OMe-mbpy). 13C NMR (201 MHz, CD2Cl2): δ 167.0, 158.0, 151.5,
146.7, 138.7, 127.3, 121.8, 111.9, 109.8 (mbpy and Ph), 56.6 (OCH3).
Anal. Calcd for PtN2O2C24H22: C, 50.97; H, 3.93; N, 4.95. Found: C,
51.02; H, 3.99; N, 5.01.
[(Brbpy)Pt(Ph)(THF)][BAr′4] (2d): 94% isolated yield, 0.105 g. 1H
NMR (300 MHz, CD2Cl2): δ 8.32 (m, 2H, H3-bpy and H6-bpy), 8.20
(d, 1H, H3-bpy, 3JHH = 2 Hz), 8.12 (d, 1H, H6-bpy, 3JHH = 6 Hz), 8.00
BrbpyPtPh2 (1d). The bipyridyl ligand was 4,4′-dibromo-2,2′-
1
bipyridine (Brbpy; 78% isolated yield, 0.176 g). H NMR (300 MHz,
(dd, 1H, H5-bpy, JHH = 6 Hz, JHH = 2 Hz), 7.50 (dd, 1H, H5-bpy,
3JHH = 6 Hz, 4JHH = 2 Hz), 7.42 (d, 2H, Ho-Ph, 3JHH = 7 Hz), 7.17 (t,
2H, Hm-Ph, 3JHH = 7 Hz), 7.08 (m, 1H, Hp-Ph), 4.14 (m, 4H, α-THF),
1.84 (m, 4H, β-THF). 13C NMR (75 MHz, CD2Cl2): δ 157.6, 154.5,
147.3, 138.1, 138.0, 135.8, 132.3, 132.1, 128.1, 127.9, 127.5, 126.8
(Brbpy and Ph), 78.3 (α-THF), 25.0 (β-THF), remaining two aromatic
resonances obscured due to coincidental overlap. Anal. Calcd for
PtN2Br2BF24C52H31: C, 41.05; H, 2.06; N, 1.84. Found: C, 41.27; H,
2.05; N, 1.83.
3
4
4
acetone-d63): δ 8.90 (d, 2H, H3-Brbpy, JHH = 2 Hz), 8.29 (d, 2H,
3
H6-Brbpy, JHH = 6 Hz, JPtH = 21 Hz, Pt satellites), 7.90 (dd, 2H,
H5-Brbpy, 3JHH = 6 Hz, 4JHH = 2 Hz), 7.40 (d, 4H, Ho-Ph, 3HH = 8 Hz,
3JPtH = 71 Hz, Pt satellites), 6.89 (t, 4H, Hm-Ph, 3JHH = 8 Hz), 6.75 (m,
2H, Hp-Ph). 13C NMR (75 MHz, acetone-d6): δ 150.8, 146.6, 139.0,
134.7, 131.8, 128.2, 127.5, 127.0, 122.3 (Brbpy and Ph). Anal. Calcd for
PtN2Br2C22H16: C, 39.84; H, 2.44; N, 4.22. Found: C, 39.82; H, 2.30;
N, 4.22.
cbpyPtPh2 (1e). The bipyridyl ligand was 4,4′-diethoxycarbonyl-
2,2′-bipyridine (cbpy; 85% isolated yield, 0.171 g). 1H NMR (300
MHz, acetone-d6): δ 8.99 (d, 2H, H3-cbpy, 4JHH = 1 Hz), 8.67 (d, 2H,
H6-cbpy, 3JHH = 6 Hz), 8.10 (dd, 2H, H5-cbpy, 3JHH = 6, 4JHH = 2 Hz),
[(cbpy)Pt(Ph)(THF)][BAr′4] (2e): 86% isolated yield, 0.099 g. 1H
NMR (300 MHz, CD2Cl2): δ 8.87 (s, 1H, H3-cbpy), 8.73 (d, 1H,
H3-cbpy, JHH = 2 Hz), 8.68 (d, 1H, H6-cbpy, JHH = 6 Hz), 8.51 (d,
4
3
1H, H6-cbpy, 3JHH = 6 Hz), 8.39 (dd, 1H, H5-cbpy, 3JHH = 6 Hz, 4JHH
=
3
7.40 (d, 4H, Ho-Ph, 3JHH = 8 Hz, JPtH = 70 Hz, Pt satellites), 6.91 (t,
2 Hz), 7.85 (dd, 1H, H5-cbpy, 3JHH = 6 Hz, 4JHH = 2 Hz), 7.45 (d, 2H,
Ho-Ph, 3JHH = 8 Hz), 7.20 (t, 2H, Hm-Ph, 3JHH = 8 Hz), 7.11 (m, 1H,
3
4H, Hm-Ph, JHH = 8 Hz), 6.77 (m, 2H, Hp-Ph), 4.48 (q, 4H,
3
3
OCH2CH3, JHH = 7 Hz), 1.43 (t, 6H, OCH2CH3, JHH = 7 Hz). 13C
NMR (75 MHz, acetone-d6): δ 164.7, 157.3, 151.1, 146.6, 140.1,
139.1, 127.8, 127.6, 123.8, 122.5 (cbpy and Ph), 63.2 (OCH2CH3),
14.4 (OCH2CH3). Anal. Calcd for PtN2O4C28H26: C, 51.76; H, 4.04;
N, 4.31. Found: C, 52.01; H, 3.94; N, 4.16.
Hp-Ph), 4.55 (q, 2H, OCH2CH3, JHH = 7 Hz), 4.48 (q, 2H,
3
3
OCH2CH3, JHH = 7 Hz), 4.18 (m, 4H, α-THF), 1.88 (m, 4H, β-
THF), 1.47 (t, 3H,, OCH2CH3, 3JHH = 7 Hz), 1.42 (t, 3H, OCH2CH3,
3JHH = 7 Hz). 13C NMR (126 MHz, CD2Cl2): δ 163.0, 162.8, 158.3,
155.3, 154.4, 147.8, 142.6, 141.7, 135.8, 128.3, 128.0, 123.7, 123.5
(cbpy, Ph and CO2Et), 78.3(α-THF), 64.0 (OCH2CH3), 25.1 (β-
THF), 14.3 (OCH2CH3), remaining five resonances obscured due to
coincidental overlap. Anal. Calcd for PtN2O5BF24C58H41: C, 46.20; H,
2.75; N, 1.86. Found: C, 46.22; H, 2.79; N, 1.91.
NO2bpyPtPh2 (1f). The bipyridyl ligand was 4,4′-dinitro-2,2′-
bipyridine (NO2bpy; 89% isolated yield, 0.779 g). 1H NMR (300
MHz, CD2Cl2): δ 8.97 (d, 2H, H6-NO2bpy, 3JHH = 6 Hz), 8.92 (d, 2H,
H3-NO2bpy, 4JHH = 2 Hz), 8.16 (dd, 2H, H5-NO2bpy, 3JHH = 6 Hz, 4JHH
= 2 Hz), 7.39 (d, 4H, Ho-Ph, 3JHH = 8 Hz), 7.06 (t, 4H, Hm-Ph, 3JHH
=
8 Hz), 6.91 (t, 2H, Hp-Ph, JHH = 8 Hz). 13C NMR (201 MHz,
CD2Cl2): δ 157.5, 153.6, 153.1, 144.0, 138.1, 127.8, 123.2, 122.2,
117.1(NO2bpy and Ph). Anal. Calcd for PtN4O4C22H16: C, 44.37; H,
2.71; N, 9.41. Found: C, 44.63; H, 2.82; N, 9.37.
[(NO2bpy)Pt(Ph)(THF)][BAr′4] (2f): 92% isolated yield, 0.334 g. H
3
1
NMR (300 MHz, CD2Cl2): δ 9.13 (d, 1H, H3-NO2bpy, JHH = 2 Hz),
4
8.97 (d, 1H, H3-NO2bpy, 4JHH = 2 Hz), 8.95 (d, 1H, H6-NO2bpy, 3JHH
=
6 Hz), 8.81 (d, 1H, H6-NO2bpy, JHH = 6 Hz), 8.68 (dd, 1H,
H5-NO2bpy, 3JHH = 6 Hz, 4JHH = 2 Hz), 8.16 (dd, 1H, H5-NO2bpy, 3JHH
= 6 Hz, 4JHH = 2 Hz), 7.43 (m, 2H, Ho-Ph), 7.25 (m, 2H, Hm-Ph), 7.17
(m, 1H, Hp-Ph), 4.21 (m, 4H, α-THF), 1.91 (m, 4H, β-THF). Note:
complex 2f decomposes over the course of hours at room temperature
in CD2Cl2, which prevented the acquisition of 13C NMR data. Anal.
Calcd for PtBN4O5F24C52H31: C, 44.08; H, 2.58; N, 3.67. Found: C,
43.71; H, 2.38; N, 3.93.
3
General Procedure for the Synthesis of [(xbpy)Pt(Ph)(THF)]-
[BAr′4] Complexes 2a−f. A solution/suspension of (xbpy)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 volatiles were removed in vacuo. The residue was treated with n-
pentane (∼2 mL), which was then removed under vacuum to afford a
low-density solid. The solid was dried in vacuo.
General Procedure for the Synthesis of [(xbpy)Pt-
(CH2CH2Ph)(η2-C2H4)][BAr′4] Complexes 3a−f. Complex 2 was
dissolved in dichloromethane (∼5 mL). The solution was transferred
to a stainless steel pressure reactor and pressurized with ethylene (0.3
MPa). After 12 h, the volatiles were removed in vacuo, and n-pentane
(∼2 mL) was added to the crude solid. The pentane was removed
under vacuum to afford a low-density solid. The solid was collected
and dried in vacuo.
Spectroscopic Data for BAr′4 Anion. The chemical shifts for the
BAr′4 anion of various Pt complexes are virtually identical. The NMR
1
spectroscopy data for the anion are as follows. H NMR (300 MHz,
CD2Cl2): δ 7.72 (s, 8H, Ho-BAr′4), 7.56 (s, 4H, Hp-BAr′4). 13C NMR
1
(75 MHz, CD2Cl2): δ 162.3 (q, Ar′, JB‑Cipso = 49 Hz), 135.4 (Ar′),
129.5 (q, m-Ar′, 2JC−F = 32 Hz), 125.2 (q, Ar′, 2JC−F = 272 Hz), 118.1
(Ar′). 19F NMR (282 MHz, CD2Cl2): δ −63.1 (s, CF3-Ar′).
[(mbpy)Pt(Ph)(THF)][BAr′4] (2a): 80% isolated yield, 0.148 g. 1H
NMR (300 MHz, CD2Cl2): δ 8.24 (d, 1H, H6-mbpy, JHH = 6 Hz),
[(mbpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (3a): 92% isolated yield,
3
3
4
1
3
8.01 (d, 1H, H6-mbpy, JHH = 7 Hz), 7.55 (d, 1H, H3-mbpy, JHH = 2
0.087 g. H NMR (300 MHz, CD2Cl2): δ 8.61 (br d, 1H, bpy, JHH
= 7 Hz), 7.81 (br d, 1H, bpy, 3JHH = 7 Hz), 7.64 (br s, 1H, bpy), 7.61
Hz), 7.45 (m, 3H, H3-mbpy and Ho-Ph), 7.21 (dd, 1H, H5-mbpy, 3JHH
=
G
dx.doi.org/10.1021/om400306w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(br s, 1H, bpy), 7.30−7.10 (m, 6H, bpy and Ph), 4.17−3.88
(overlapping resonances, 10H, OMe and C2H4), 2.68 (t, 2H, Pt-
CH2CH2Ph, 3JHH = 8 Hz), 1.39 (t, 2H, Pt−CH2CH2Ph, 3JHH = 8 Hz).
13C NMR (201 MHz, CD2Cl2): δ 170.68, 169.4, 169.0, 159.5, 156.2,
150.3, 147.3, 144.0, 129.3, 128.9, 128.8, 126.8, 126.1 (mbpy and Ph),
69.0 (C2H4), 58.0 (OMe), 57.6 (OMe), 37.7 (CH2CH2Ph), 16.4
(CH2CH2Ph), remaining resonance obscured due to coincidental
overlap. Anal. Calcd for PtBN2O2F24C54H37: C, 46.07; H, 2.65; N,
1.99. Found: C, 46.24; H, 2.61; N, 2.11.
Ph), 6.29 (dd, 1H, PhCH=CH2, 3JHHTrans = 14 Hz, 3JHHCis = 8 Hz) 4.49
3
(d, 1H, PhCHCH2, JHHTrans = 14 Hz), 4.19 (d, 1H, PhCHCH2,
t
t
3JHHCis = 8 Hz), 1.44 (s, 9H, bpy), 1.37 (s, 9H, bpy), 0.67 (br s, 3H,
Me, JPtH = 71 Hz, Pt satellites). 13C NMR (126 MHz, CD2Cl2): δ
2
175.2, 168.6, 165.8, 157.3, 154.0, 147.6, 145.7, 136.0, 130.1, 129.1,
128.8, 125.2, 124.8, 120.3, 120.2, 90.9 (tbpy and styrene), 36.2
(CCH3), 35.8 (CCH3), 29.7 (CCH3), 29.6 (CCH3), methyl resonance
not observed due to broadening. Anal. Calcd for PtBN2F24C59H47: C,
49.01; H, 3.28; N, 1.94. Found: C, 49.31; H, 3.42; N, 2.10.
[(bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (3c): 81% isolated yield, 0.098
[(NO2bpy)Pt(Me)(η2-styrene)][BAr′4] (5f): 96% isolated yield, 0.582 g.
1H NMR (300 MHz, CD2Cl2): δ 9.25 (br d, 1H, H6-NO2bpy, 3JHH = 5
Hz), 9.14 (br s, 1H, H3-NO2bpy), 9.04 (br s, 1H, H3-NO2bpy), 8.65 (br
s, 1H, H6-NO2bpy), 8.30 (br s, 1H, H5-NO2bpy), 8.23 (br d, 1H,
H5-NO2bpy, 3JHH = 6 Hz), 7.60 (m, 2H, Ho-styrene), 7.42 (m, 1H, Hp-
styrene), 7.33 (m, 2H, Hm-styrene), 6.64 (dd, 1H, PhCH=CH2,
3JHHtrans = 15 Hz, 3JHHcis = 8 Hz), 4.78 (dd, 1H, PhCHCH2, 3JHHtrans
1
g. H NMR (800 MHz, CD2Cl2): δ 8.80 (br s, 1H, bpy), 8.34−8.22
(br m, 4H, bpy), 8.05 (br s, 1H, bpy), 7.84 (br s, 1H, bpy), 7.26 (m,
4H, Ho/m-Ph), 7.17 (m, 1H, Hp-Ph), 4.19 (br s, 4H, C2H4), 2.72 (t,
2H, Pt-CH2CH2Ph, 3JHH = 8 Hz), 1.54 (t, 2H, Pt-CH2CH2Ph, 3JHH = 8
Hz). 13C NMR (126 MHz, CD2Cl2): δ 157.5, 154.1, 148.5, 145.8,
143.5, 141.4, 130.5, 129.4, 128.6, 126.6, 124.1 (bpy and Ph), 70.6
(C2H4), 37.4 (CH2CH2Ph), 17.0 (CH2CH2Ph), remaining three
resonances obscured due to coincidental overlap. Anal. Calcd for
PtBN2F24C52H31: C, 46.41; H, 2.33; N, 2.08. Found: C, 46.61; H, 2.41;
N, 2.19.
= 15 Hz, 2JHHgem = 1 Hz), 4.50 (dd, 1H, PhCHCH2, 3JHHcis = 8 Hz,
2
2JHHgem = 1 Hz), 1.04 (s, 3H, Pt-CH3, JPtH = 74 Hz, Pt satellites). 13
C
NMR (126 MHz, CD2Cl2): δ 158.9, 156.5, 155.5, 155.2, 152.5, 150.7,
135.7, 131.5, 129.6, 129.4, 122.7, 118.7, 118.3, 100.5, 68.1 (NO2bpy and
[(cbpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (3e): 88% isolated yield,
1
styrene), −0.8 (s, Pt-CH3, JPt−C = 684 Hz, Pt satellites), remaining
1
c
0.127 g. H NMR (300 MHz, CD2Cl2): δ 8.90 (m, 3H, bpy), 8.28
(m, 3H, cbpy), 7.26 (m, 5H, Ph), 4.54 (overlapping m’s, 4H,
OCH2CH3), 4.30 (br s, 4H, C2H4, 3JPtH = 34 Hz, Pt satellites), 2.71 (t,
2H, Pt-CH2CH2Ph, 3JHH = 8 Hz), 1.60 (t, 2H, Pt-CH2CH2Ph, 3JHH = 8
Hz), 1.47 (overlapping m’s, 6H, OCH2CH3). 13C NMR (201 MHz,
CD2Cl2): δ 157.8, 154.7, 149.8, 146.9, 144.3, 143.2, 142.9, 129.2,
128.6, 128.3, 126.8, 124.2 (cbpy and Ph), 71.9 (C2H4), 64.4
(OCH2CH3), 64.2 (OCH2CH3), 37.5 (CH2CH2Ph), 17.5
(OCH2CH3), 14.3 (CH2CH2Ph), remaining five resonances obscured
due to coincidental overlap. Anal. Calcd for PtBN2O4F24C58H41: C,
46.69; H, 2.78; N, 1.88. Found: C, 46.90; H, 2.78; N, 2.00.
resonance obscured due to coincidental overlap. Anal. Calcd for
PtBN4O4F24C51H29: C, 43.03; H, 2.05; N, 3.94. Found: C, 43.33; H,
1.92; N, 3.90.
Catalytic Olefin Hydrophenylation. A representative catalytic
reaction is described. [(mbpy)Pt(Ph)(THF)][BAr′4] (2a; 0.019 g,
0.013 mmol) was dissolved in 12.0 mL of benzene containing 0.01 mol
% (relative to benzene) of hexamethylbenzene (HMB) as an internal
standard. The reaction mixture was placed in a stainless steel pressure
reactor, charged with ethylene, pressurized to a total of 0.8 MPa with
N2, and heated to 100 °C. After 4 and 16 h, 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, diethylbenzene, and styrene amounts were
quantified using linear regression analysis of gas chromatograms of
standard samples. For example, 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 0.53 and 0.98,
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 as follows, respectively: 0.55, 0.99; 0.56, 0.99; 0.56, 0.99; 0.52, 0.99.
Kinetics of Styrene Formation. A representative kinetic
experiment is described. Complex 3e (0.044 g, 0.029 mmol) and
hexamethyldisilane (HMDS, 1.5 μL), an internal standard, were
dissolved in 1.0 mL of nitromethane-d3. The solution was then divided
(0.3 mL for each sample) and added to three high-pressure NMR
tubes. The tube was pressurized with 0.3 MPa of ethylene and placed
into a temperature-equilibrated (45 °C) NMR probe. The temperature
of the probe was determined using a solution of 80% ethylene glycol in
DMSO-d6. Kinetic runs were performed in triplicate, and standard
deviations are based on the average kobs values from the three
experiments. The concentration of ethylene in solution was
determined by integration against the internal standard. 1H NMR
spectra were collected every 10 min with eight scans and a 5.0 s pulse
delay. Styrene resonances were integrated against that of HMDS, and
from a plot of ln(1 − [styrene]t/[starting material]0) versus time
(seconds) the rate constants were extracted. The rate of formation of
styrene from complex 3e, in the presence of 0.34 M C2H4, was
[1.1(2)] × 10−4 s−1 with a correlation coefficient (R2) of 0.99 for each
plot.
[(NO2bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] (3f): 87% isolated yield,
0.108 g. 1H NMR (300 MHz, CD2Cl2): δ 9.14 (d, 1H, NO2bpy, 3JHH
=
2 Hz), 8.57 (br m, 2H, NO2bpy), 7.28 (br d, 2H, Ho-Ph, 3JHH = 7 Hz),
7.23 − 7.06 (m, 3H, Hm and Hp-Ph), 4.42 (br s, 4H, η2-C2H4), 2.70 (t,
2H, Pt-CH2CH2Ph, 3JHH = 7 Hz), 1.71 (t, 2H, Pt-CH2CH2Ph, 3JHH = 7
Hz), remaining NO2bpy signals obscured due to broadening or
coincidental overlap. 13C NMR (201 MHz, CD2Cl2): δ 152.3, 149.1,
142.3, 128.9, 128.5, 126.8, 123.3, 118.2 (NO2bpy and Ph), 68.1 (C2H4),
37.0 (CH2CH2Ph), 18.0 (CH2CH2Ph), remaining five resonances
obscured due to coincidental overlap. Anal. Calcd for
PtBN4O4F24C52H31: C, 43.44; H, 2.18; N, 3.90. Found: C, 43.73; H,
2.15; N, 3.86.
Synthesis of (NO2bpy)PtMe2. A heterogeneous mixture of
[(Me)2Pt(μ-SEt2)]2 (0.526 g, 0.834 mmol) and NO2bpy (0.4125 g,
1.68 mmol) in diethyl ether was stirred at room temperature for 16 h.
The solvent volume was partially reduced under vacuum, and the
resulting mixture was filtered. The filtrate was discarded, and the solid
1
was dried in vacuo to afford a purple solid (0.737 g, 94%). H NMR
(300 MHz, acetone-d6): δ 9.78 (d, 2H, H6-NO2bpy, 3JHH = 6 Hz, 3JPtH
=
4
21 Hz, Pt satellites), 9.44 (d, 2H, H3-NO2bpy, JHH = 2 Hz), 8.53 (dd,
2H, H5-NO2bpy, 3JHH = 6 Hz, 4JHH = 2 Hz), 1.38 (s, 6H, Pt-CH3, 2JPtH
= 91 Hz, Pt satellites). The complex was too insoluble in organic
solvents to obtain 13C NMR data. Anal. Calcd for PtN4O4C12H12: C,
30.58; H, 2.57; N, 11.89. Found: C, 30.70; H, 2.56; N, 11.62.
General Procedure for the Synthesis of [(xbpy)Pt(Me)(η2-
styrene)][BAr′4] (x = tBu, NO2). A solution of (xbpy)Pt(Me)2 and 1
equiv of styrene in dichloromethane (30 mL) was cooled to
approximately −70 °C. One equivalent of [H(Et2O)2][BAr′4]
dissolved in dichloromethane (∼10 mL, −70 °C) was added to the
Pt solution. The solution was reduced to approximately half volume in
vacuo and filtered through Celite with dichloromethane as eluent. The
volatiles were removed from the filtrate in vacuo. The residue was
treated with n-pentane (∼2 mL), which was then removed under
vacuum to afford a low-density solid. The solid was dried in vacuo.
[(tbpy)Pt(Me)(η2-styrene)][BAr′4] (5a): 87% isolated yield, 0.084 g.
Thermolysis of [(NO2bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] in
C6H6. In a glass pressure tube, complex 3f (0.024 g, 0.02 mmol)
and benzene (4 mL) containing HMB as an internal standard were
added. The reaction mixture was heated to 100 °C for 4 h, cooled to
room temperature, and analyzed by GC/MS. Only ethylbenzene was
detected (in quantitative yield).
3
1H NMR (300 MHz, CD2Cl2): δ 8.68 (d, 1H, H6-tbpy, JHH = 6 Hz,
3JPtH = 48 Hz, Pt satellites), 8.17 (s, 1H, H3-tbpy), 8.09 (s, 1H, H3-
tbpy), 7.85 (m, 3H, tbpy), 7.59 (m, 2H, Ho-Ph), 7.33 (m, 3H, Hm/Hp-
H
dx.doi.org/10.1021/om400306w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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ASSOCIATED CONTENT
* Supporting Information
CIF files giving crystallographic data for the crystal structure
determinations in this paper. 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.); t@unt.edu (T.R.C.).
Notes
■
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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). T.B.G. acknowl-
edges the National Science Foundation for the purchase of X-
ray diffraction instrumentation at the University of Virginia
(CHE-1126602).
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dx.doi.org/10.1021/om400306w | Organometallics XXXX, XXX, XXX−XXX