A mechanistic change results in 100 times faster CH functionalization for ethane versus methane by a homogeneous Pt catalyst

Article abstract of DOI:10.1021/ja504368r

The selective, oxidative functionalization of ethane, a significant component of shale gas, to products such as ethylene or ethanol at low temperatures and pressures remains a significant challenge. Herein we report that ethane is efficiently and selectively functionalized to the ethanol ester of H₂SO₄, ethyl bisulfate (EtOSO₃H) as the initial product, with the Pt^II "Periana-Catalytica" catalyst in 98% sulfuric acid. A subsequent organic reaction selectively generates isethionic acid bisulfate ester (HO₃S-CH ₂-CH₂-OSO₃H, ITA). In contrast to the modest 3-5 times faster rate typically observed in electrophilic CH activation of higher alkanes, ethane CH functionalization was found to be ～100 times faster than that of methane. Experiment and quantum-mechanical calculations reveal that this unexpectedly large increase in rate is the result of a fundamentally different catalytic cycle in which ethane CH activation (and not platinum oxidation as for methane) is now turnover limiting. Facile Pt ^II-Et functionalization was determined to occur via a low energy β-hydride elimination pathway (which is not available for methane) to generate ethylene and a Pt^II-hydride, which is then rapidly oxidized by H₂SO₄ to regenerate Pt^II-X₂. A rapid, non-Pt-catalyzed reaction of formed ethylene with the hot, concentrated H₂SO₄ solvent cleanly generate EtOSO₃H as the initial product, which further reacts with the H₂SO₄ solvent to generate ITA.

Full text of DOI:10.1021/ja504368r

Article
pubs.acs.org/JACS
A Mechanistic Change Results in 100 Times Faster CH
Functionalization for Ethane versus Methane by a Homogeneous Pt
Catalyst
Michael M. Konnick,† Steven M. Bischof,† Muhammed Yousufuddin,‡ Brian G. Hashiguchi,*,†
Daniel H. Ess,*,§ and Roy A. Periana*,†
†The Scripps Energy & Materials Center, Department of Chemistry, The Scripps Research Institute, Jupiter, Florida 33458, United
States
‡Center for Nanostructured Materials, The University of Texas at Arlington, Arlington, Texas 76019, United States
§Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States
S
* Supporting Information
ABSTRACT: The selective, oxidative functionalization of ethane, a
significant component of shale gas, to products such as ethylene or ethanol
at low temperatures and pressures remains a significant challenge. Herein we
report that ethane is efficiently and selectively functionalized to the ethanol
ester of H2SO4, ethyl bisulfate (EtOSO3H) as the initial product, with the PtII
“Periana-Catalytica” catalyst in 98% sulfuric acid. A subsequent organic
reaction selectively generates isethionic acid bisulfate ester (HO3S-CH2-CH2-
OSO3H, ITA). In contrast to the modest 3−5 times faster rate typically
observed in electrophilic CH activation of higher alkanes, ethane CH
functionalization was found to be ∼100 times faster than that of methane.
Experiment and quantum-mechanical calculations reveal that this unexpect-
edly large increase in rate is the result of a fundamentally different catalytic
cycle in which ethane CH activation (and not platinum oxidation as for
methane) is now turnover limiting. Facile PtII-Et functionalization was determined to occur via a low energy β-hydride
elimination pathway (which is not available for methane) to generate ethylene and a PtII-hydride, which is then rapidly oxidized
by H2SO4 to regenerate PtII-X2. A rapid, non-Pt-catalyzed reaction of formed ethylene with the hot, concentrated H2SO4 solvent
cleanly generate EtOSO3H as the initial product, which further reacts with the H2SO4 solvent to generate ITA.
INTRODUCTION
■
Over the past decade there has been a significant increase in the
availability of shale gas and other sources of natural gas.1 In
addition to methane, shale gas contains significant amounts of
ethane.2 Currently, the conversion of ethane to oxy-function-
alized products involves multiple steps beginning with initial
generation of ethylene by expensive, high-temperature (∼900
°C), capital- and energy-intensive steam cracking (Figure 1A).3
generation and separation of ethanol from ethane in sulfuric
acid could be practical based upon the recognition that an early
A lower-temperature (<250 °C) selective oxidative functional-
ization process directly to these products could lead to
substantially lower costs and reduced emissions.4,5 One strategy
commercial process for ethanol production (albeit at lower
temperatures) operated by reaction of ethylene with con-
that has shown promise for lower-temperature alkane
centrated sulfuric acid to generate ethyl bisulfate (EtOSO3H),
conversions is based on the design of catalysts that operate
followed by hydrolysis (eq 2);33 (3) studies indicating that
electrophilic CH activation of higher alkanes could be
somewhat more reactive than that of methane (∼3−5
times),34 which suggested that efficient ethane functionalization
could be achieved at milder conditions; and (4) the presence of
a β-C−H in the putative Pt-Et intermediate from C−H bond
via C−H bond activation6−10 and metal−alkyl (M-R)
functionalization (Figure 1B).6,11−32
The “Periana-Catalytica” system,12 based on (bpym)PtCl2
(bpym = 2,2′-bipyrimidine), PtII, which operates by CH
activation in concentrated H2SO4 at ∼200 °C, is an efficient
system for the selective catalytic conversion of CH4 to methyl
bisulfate (MeOSO3H; see eq 1). Several factors led us to
examine the reactions with ethane: (1) the increasing
availability of ethane from shale gas; (2) the possibility that
Received: May 5, 2014
© XXXX American Chemical Society
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Figure 1. (A) Comparison of current multistep, high-temperature, energy- and capital-intensive processes versus a more direct approach based on
lower-temperature oxidative conversion of ethane to ethylene, ethanol, and ethylene glycol. (B) Generalized catalytic cycle for hydrocarbon C−H
bond functionalization.
activation, which could lead to fundamentally different
chemistry for ethane relative to methane.
101% H2SO4 media, only ITA (and no EtOSO3H) was
observed. Isethionic acid (HO-CH2CH2-SO3H, which is readily
produced upon hydrolysis of ITA) is utilized industrially in the
production of biodegradable ionic surfactants and taurine and is
currently produced via (1) the reaction of aqueous NaHSO3
with ethylene oxide or (2) the sulfonation and subsequent
hydrolysis of ethylene.35,36 To avoid complications from
possible formation of chlorination products, subsequent studies
were carried out with (bpym)Pt(TFA)2, 1. Significantly,
carrying out the reaction with 1 in 98% H2SO4 under otherwise
identical conditions showed formation of both EtOSO3H and
ITA (in a 1:1.7 ratio, respectively) as the only products. This
suggested that EtOSO3H could be the initial product, which is
then subsequently converted into ITA. However, given the
long-known rapid conversion of ethylene to EtOSO3H in
concentrated H2SO4 solvent,33 it is also possible that the initial
product of the reaction is ethylene, which would not be
observed. This result is significant, as if the system could be
modified to selectively generate EtOSO3H as the major
product, it could be possible to isolate EtOH by the procedures
developed in the early commercial synthesis of EtOH based on
reaction of ethylene with 98% H2SO4.33
Catalysis with 1 was efficient and highly selective; in 101%
H2SO4 with 15 mM 1 at 160 °C for 2.5 h, ITA concentrations
of >1 M were observed. After the reactions were completed, the
solution phase remained homogeneous. No other C2 products
(such as CH3CO2H, CH3CHO, or CH3CH2-SO3H), were
observed. Consistent with catalysis by a (bpym)Pt complex,
control experiments show that 1 was required for reaction, and
no reaction was observed with bpym ligand, trifluoroacetate
anions, Pt black, insoluble PtCl2, or soluble K2PtCl4 under
otherwise identical conditions. Importantly, no C1 side-
products (such as CH3OSO3H, CH3SO3H, CH3OH,
CH3TFA, or HCO2H), were observed. To more rigorously
examine reaction selectivity, the reaction was carried out with
100% 13C2H6 in 98% H2SO4 solvent. Analysis of the liquid
phase by 1H and 13C NMR spectroscopy, and the gas phase by
GC-MS, showed only 13CH313CH2-OSO3H (13C-EtOSO3H)
and HO3SO-13CH213CH2-SO3H (13C-ITA) as the products.
Alternative potential products, such as 13C2H4,
13CH313CH2SO3H, 13CH313CO2H, or 13CO2, were not
observed. Additionally, control studies show that added ITA
is essentially inert to over-oxidation to generate CO2 or other
products under catalytic conditions (see Supporting Informa-
tion (SI), Table S1). Consistent with the reaction products
shown above (eq 3), gas-phase analysis (via quantitative
analytical titration, see Experimental Section below) indicated
that a substantial amount of SO2 is generated during the
reaction.
Herein we report that ethane undergoes selective, efficient
functionalization with the Periana-Catalytica system. Function-
alization proceeds by C−H bond activation to selectively
generate ethyl bisulfate as the initial product with no detectable
amount of C−C cleavage, over-oxidation, or decomposition (to
products such as CO2, AcOH, or intractable materials).
Interestingly, rather than observing the expected further
functionalization of EtOSO3H to the bis-bisulfate ester of
ethylene glycol (EG(OSO3H)2), the bisulfate ester of isethionic
acid (HO3SO-CH2CH2-SO3H, referred to herein as ITA),
which possesses a C−S and C−O bond, is cleanly generated as
the final product from EtOSO3H. Combined EtOSO3H and
ITA concentrations of >1 M can be generated with >95%
selectivity. Significantly, the observed 100-fold higher rate of
functionalization of the CH bonds of ethane relative to
methane12 was much faster than anticipated on the basis of
reported studies of electrophilic CH activation.34 Experimental
and quantum-mechanical calculations reveal that this is the
result of a fundamentally different catalytic cycle as compared
to that of methane functionalization. This cycle, involving
ethane CH activation to generate a PtII-Et intermediate, is now
turnover-limiting, and PtII-Et functionalization occurs by fast β-
hydride elimination to generate ethylene and a PtII-H
intermediate, which is rapidly oxidized by hot H2SO4 to
regenerate the active Pt catalyst. Rapid addition of H2SO4 to
ethylene generates EtOSO3H as the initial product of the
reaction, which is then converted (by a classical organic, non-
Pt-mediated sulfonation reaction with the concentrated sulfuric
acid solvent) into the final product, ITA. These results could
lead to new general considerations for the design of catalysts for
oxidative functionalization of higher alkanes that operate by C−
H activation.
RESULTS AND DISCUSSION
■
Catalytic Ethane Functionalization. We began by
examining ethane functionalization under conditions similar
to those reported previously for methane functionalization, in
101% H2SO4 solvent (1 M SO3 in 100% H2SO4) with 15 mM
(bpym)PtX2 (X = Cl, TFA) catalyst at 500 psig of ethane at
160 °C for 1 h in a stirred high-pressure reactor (eq 3). In
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Figure 2. Reaction time course (A), initial rate [PtII] dependence (B), EtH pressure dependence (C), and % H2SO4 dependence (D) for the
oxidation of EtH catalyzed by 1. Reaction conditions: (A) 5 mM 1, 150 psi EtH, 3 mL of 98% H2SO4, 1−4 h, 160 °C; (B) 2−20 mM 1, 150 psi EtH,
3 mL of 98% H2SO4, 1 h, 160 °C; (C) 5 mM 1, 50−500 psi EtH, 3 mL of 98% H2SO4, 1 h, 160 °C; (D) 5 mM 1, 150 psi EtH, 3 mL of 84−98%
H2SO4, 3 h, 160 °C.
The observed functionalization of ethane is significant for
several reasons. First, the reaction with ethane is highly selective
in generating EtOSO3H and ITA as the only observed products.
Second, in contrast to MeH (where no C−S products, such as
CH3SO3H, were observed), the EtH functionalization product
ITA contains both C-SO3H and C-OH functional groups.
Third, ethane functionalization is substantially more facile than
methane functionalization, as comparable conversions were
observed at much lower temperatures than previously reported
for methane (160 vs 220 °C, respectively). These observations
suggested the possibility for a reaction mechanism different
than that previously determined for methane and prompted our
more detailed analysis, presented below.
of EtOSO3H to ITA has been proposed to involve reversible
formation of ethylene (by elimination of H2SO4 from
EtOSO3H) followed by addition of the S−O bond of disulfuric
acid (e.g., H2S2O7, which is known to be present in
concentrated H2SO4 solutions)45−47 across ethylene, resulting
in both C−S and C−O bond formation to generate ITA
(Scheme 1).38−43 To examine the possibility of EtOSO3H as a
Scheme 1. Proposed Pathways and Relative Rates for the
Conversion of Ethane to EtOSO3H and ITA
Reaction Pathway for ITA Production. To more
rigorously examine the mechanism of ethane oxidation
catalyzed by 1, we studied the reaction kinetics under milder
conditions using 98% H2SO4 solvent at 160 °C. As can be seen
in Figure 2A (see SI for data), a reaction time course indicated
an unchanging, steady-state [EtOSO3H] throughout the
reaction with a linear growth of [ITA]. This data is consistent
with the generation of EtOSO3H (or ethylene, vide supra) as
the initial observed product, which is converted to ITA as the
final, stable product. All reactions were shown to be highly
reproducible, and initial rates studies indicate a first-order
dependence on both the concentration of 1 and ethane
pressure (Figure 2B, 2C). Similar to our previous studies on the
system with methane, the reaction rate is highly dependent
upon the concentration of H2SO4 and asymptotically decreases
at lower concentrations of H2SO4 (Figure 2D). Interestingly,
while lower ethane conversions are observed, the product ratios
increasingly favor EtOSO3H over ITA as the concentration of
H2SO4 is decreased (see SI, Figure S4). This would suggest that
the rate of conversion of EtOSO3H into ITA increases with
increasing concentration of H2SO4.
precursor to ITA under catalytic conditions, the reactions of
EtOSO3H with concentrated sulfuric acid were examined in the
presence and absence of 1. Consistent with a prior
report,37,38,44 in both 101% and 98% H2SO4 media at 160
°C, EtOSO3H was converted into ITA as the sole product (see
SI, Table S1). Significantly, identical results were obtained in
the presence or absence of 1, indicating that catalysis by Pt is
not necessary for EtOSO3H-to-ITA transformation. The
reaction is faster in 101% H2SO4, but ITA is the final product
in both cases with >90% mass balance.
Consistent with earlier reports,37,38,44 replacing EtOSO3H
with ethylene showed identical results and conversion into
primarily ITA with some EtOSO3H in <1 h at 160 °C in the
presence or absence of Pt (Figure 3A). Lowering the
temperature slightly (ca. 140 °C, Figure 3B) allowed us to
observe a mixture of both ITA and EtOSO3H. Consistent with
the very early literature,32,33 and the possibility of reversible
generation of ethylene from EtOSO3H, we confirmed that
ethylene is quantitatively converted solely into EtOSO3H in 1 h
at 50 °C (Figure 3C). These results, as well as the kinetic
Both EtOSO3H and ethylene were shown in very early as
well as later studies by others to readily react by a classical
organic reaction in concentrated H2SO4 or oleum to cleanly
generate ITA.37−44 The reaction mechanism for the conversion
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Figure 4. 1H NMR of the post-reaction mixture with a 1:1 mixture of
methane to ethane. Reaction conditions: 15 mM 1, 250 psi MeH/250
psi EtH (500 psi total pressure), 3 mL of 98% H2SO4, 1 h, 160 °C.
1
Figure 3. H NMR of 98% H2SO4 after exposure to C2H4 for 1 h at
red cycle, R = Et), comparable rates of reaction of ethane and
methane could be anticipated. The ∼100-fold difference in
reaction rates observed between these alkanes suggested that
ethane functionalization may occur by a fundamentally different
mechanism. One plausible mechanism for the functionalization
of ethane is shown below (Scheme 2, blue cycle, R = Et).
Analogous to the methane functionalization cycle, the ethane
cycle begins with the C−H activation step of ethane by PtII to
generate a PtII-Et intermediate, k2. At this point the ethane and
methane mechanisms diverge. Unlike the analogous PtII-Me
intermediate, the PtII-Et intermediate could be expected to
undergo facile β-hydride elimination to give a PtII(H)(C2H4)
intermediate (k5). Subsequent loss of ethylene (k6) and known
reversible addition of H2SO4 or irreversible addition of H2S2O7
to ethylene give EtOSO3H or ITA, respectively.37,38,44 It is also
possible that these functionalization reactions could occur on
the ethylene coordinated to Pt (see SI, Figure S8). However,
since control reactions show that 1 is not required for the
conversion of ethylene to EtOSO3H and ITA, vide supra,
catalysis by 1 for ethylene conversion is not required to explain
the observed results. The PtII-H is then oxidized by H2SO4 to
generate SO2 and regenerate the PtII catalyst (k7). Although β-
hydride elimination is widely known to occur in M-R
complexes,49,50 in catalytic systems for alkane functionalization
it has been utilized almost exclusively for generating alkenes as
the final products.51 If this proposed mechanism is correct
(Scheme 2, blue cycle), it would describe a high-yielding
catalytic system for alkane oxy-functionalization involving a β-
hydride elimination step which leads to products with C−O
bonds. In order to examine this mechanistic possibility, further
studies were undertaken as described below.
C−H Activation Step. Catalytic H/D exchange with alkane
substrates in D-acids is commonly utilized to provide evidence
for C−H activation by reversible generation of a M-R
intermediate. To examine this, we carried out the catalytic
reaction of ethane with 1 in 98% D2SO4 160 °C. Consistent
with a C−H activation mechanism involving generation of Pt-
Et, deuterated ethane (D-ethane) was generated along with
partially deuterated EtOSO3H and ITA. While highly accurate
quantitative analysis of the products when utilizing 98% D2SO4
was complicated by the formation of several ethane isotopologs
(C2HnD6−n) and D-incorporation into the oxidized products
(EtOSO3H and ITA), we found approximately a 1:1 molar
ratio of C2HnD6−n to oxidized products. On the basis of our
proposed mechanism above (Scheme 2, blue cycle), this ratio
would result from the branching rates from reaction of PtII-Et
with D2SO4 to generate D-ethane versus the functionalization
reaction of PtII-Et to generate EtOSO3H and ITA (k−1/k6, R =
Et, see SI). Importantly, the 1:1 ratio of D-ethane to
EtOSO3H/ITA for ethane starkly contrasts with the reported
(A) 160, (B) 140, and (C) 50 °C.
profiles shown above (Figure 2), are consistent with the
reactions of ethane catalyzed by 1, generating either ethylene or
EtOSO3H as the initial product, followed by conversion to ITA
(Scheme 1).
Mechanism of Ethane Functionalization. Given the
expected similarities of methane and ethane,32,34 we anticipated
that functionalization of ethane catalyzed by 1 produces
EtOSO3H and ITA via a C−H activation mechanism. However,
it is also possible that free radical reactions are involved. Sen
and co-workers had previously reported that the non-transition
metal-mediated oxidation of ethane in either 98% H2SO4 or
oleum with radical initiators (Ce(SO4)2 or K2S2O8) generated
ITA at ∼5% selectivity with predominantly C1 products (via
C−C bond cleavage) and over-oxidation products such as
CH3CO2H.26,44,48 The poor selectivity for ITA, generation of
C−C bond cleavage products, and over-oxidized products led
Sen and co-workers to propose a mechanism involving free-
radical chain reactions initiated by K2S2O8. Since C1 and over-
oxidized products are not observed in our work on the selective
functionalization of ethane described above, we considered a
radical chain mechanism unlikely. Additionally, the well-defined
and reproducible kinetic data, along with no induction period
and lack of side products, strongly support our proposal for a
nonradical mechanism based on ethane CH activation.
To further examine the similarities between the reactions of
methane and ethane with 1, we examined the relative rates of
functionalization through competition reactions. Competition
studies were carried out in 98% H2SO4 solvent containing 1 at
identical pressures of methane and ethane, under conditions
where the reactions were not mass transfer limited and at <10%
conversion of both alkanes to ensure pseudo-first-order
conditions. Unexpectedly, the amount of the C2 products
(EtOSO3H + ITA) produced from ethane functionalization was
found to be ∼144 times higher (suggesting a ∼100-fold higher
rate when correcting for the number of CH bonds) than the
amound of MeOSO3H from methane functionalization (Figure
4). This was much higher than anticipated on the basis of
reported rates of electrophilic CH activation of various
hydrocarbons.34
We recently showed that one of two plausible pathways for
methane functionalization in the Periana-Catalytica system
(Scheme 2, red cycle, R = Me) involves fast C−H activation to
reversibly generate a (bpym)PtII-Me intermediate (k1),
followed by reaction with of non-methylated PtIV that is
generated by the turnover-limiting step, oxidation of non-
methylated PtII to PtIV (k4) that does not involve methane.32
Thus, if ethane reacted by this mechanistic pathway (Scheme 2,
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Scheme 2. Proposed Catalytic Cycles for the Functionalization of Methane (Red) and Ethane (Blue) by (bpym)Pt(TFA)2, 1, in
H2SO4
∼20:1 ratio of D-methane to functionalized product from
reactions of methane catalyzed by PtII (k−1/k2, R = Me).32 This
substantial difference in the ratio of products from catalysis with
1 in D2SO4 provides further evidence that, while methane and
ethane substrates could both undergo C−H activation with PtII,
the functionalization steps for the PtII-R intermediates (R = Me
and Et) are different.52
ethane with 1 in D2SO4 solvent at 160 °C, a branching ratio of
∼1:1 for ethane to functionalized product was expected from
the stoichiometric reaction of 2 with D2SO4 solvent at the same
temperature. The stoichiometric reactivity of 2 was examined
under the catalytic conditions (but in the absence of ethane) by
adding 100 μL of a 1 M solution of 2 in DMSO all at once to
5.0 mL of a stirred solution of 98% H2SO4 at 160 °C in a sealed
glass vial under Ar (Scheme 3). The vial was removed and
Pt-Ethyl Functionalization. To examine the PtII-Et
functionalization step (k5) in more detail, the complex
(bpym)PtII(Et)(TFA), 2, was synthesized and fully charac-
terized (Figure 5, see SI) and examined as a model for the PtII-
Et intermediate. Based on the reaction mechanism shown
above (Scheme 2), kinetic analysis, and the ∼1:1 ratio of D-
ethane to EtOSO3H and ITA from the catalytic reaction of
Scheme 3. Functionalization and Protonolysis of
(bpym)PtII(Et)(TFA), 2, upon Addition to 98% H2SO4 at
160 °C
allowed to cool to room temperature immediately after addition
of 2. The gas phase and liquid phase after reaction were
analyzed by GC-MS and NMR, respectively (see SI for specific
details and controls for this procedure). This reaction showed
the formation of EtOSO3H and ethane in 65% and 35% yields,
respectively, based on added 2 (Table 1, entry 1).
Significantly, consistent with the proposed oxidation of a
putative PtII-H intermediate, k7, functionalization is accom-
panied by oxidation with H2SO4, as 1 equiv of SO2 relative to
EtOSO3H was also generated (see Experimental Section for
details). Consistent with the high selectivity of the reaction and
Figure 5. ORTEP structure of (bpym)Pt(Et)(TFA), 2, with 30%
probability ellipsoids.
E
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Table 1. Reaction of (bpym)PtII(R)(TFA), 2 and 3, in HX at
160 °C
In addition to the β-hydride elimination step (Scheme 2, k6),
PtII-Et functionalization requires two additional steps: (i)
functionalization of ethylene to give EtOSO3H and ITA, which
can occur by ethylene dissociation (Scheme 2, k6), followed by
non-Pt-catalyzed, classical reversible addition of H2SO4 and
sulfonation with H2S2O7, and (ii) PtII-H oxidation to generate
SO2 (Scheme 2, k7). To account for the ∼1:1 ratio of branching
rates for protonolysis and functionalization from PtII-Et, the
rate of step k5 must be comparable to k−1, and step k6 must be
comparable to or faster than re-formation of PtII-Et by olefin
insertion on PtII(H)(C2H4), k−6 (not shown). Additionally, to
account for the formation of 1 equiv of SO2 in the
stoichiometric reaction of 2 with D2SO4, step k7 step must
also be comparable to or faster than k5.
However, the above result does not rule out the possibility
that β-hydride elimination and oxidation by H2SO4 to generate
SO2 occur in a single step with no discrete PtII-H intermediate
and that is competitive with k−1. Another alternative, though we
consider this unlikely based on reactions of the PtII-Me model
complex (vide infra), is the direct oxidation and functionaliza-
tion of PtII-Et by H2SO4 to generate EtOSO3H without the
involvement of a β-hydride elimination step (k2, Scheme 2, red
cycle, R = Et). Since these alternative mechanisms both require
oxidation by H2SO4 to generate EtOSO3H from PtII-Et, we
examined the reaction of 2 in neat CF3SO3H, a non-oxidizing
acid, under conditions similar to those of the reaction with
H2SO4. Significantly, both the ethyl ester, CF3SO3Et, and
ethane were formed in this reaction in ∼15% and ∼80% yields,
respectively, based on added 2 (Table 1, entry 3). Importantly,
no Pt-black (Pt0) or SO2 was observed. While the molar ratio of
these products (∼1.5:8) is different from that found in H2SO4
(∼2:1), CF3SO3H is a stronger acid than H2SO4, and more
protonolysis of 2 to generate ethane is expected. These
observations show that oxidation of PtII-Et by H2SO4 to give
SO2 is not required for PtII-Et functionalization to generate
EtOSO3H, in stark contrast to PtII-Me functionalization.
a
entry
Pt(bpym)(TFA)-R
acid
RX (%)
RH (%)
1
2
3
4
R = Et (2)
R = Me (3)
R = Et (2)
R = Me (3)
H2SO4
H2SO4
HOTf
HOTf
65
5
35
95
15
0
80
100
a
Reactions were carried out at 160 °C in 98% H2SO4 or neat HOTf
with 10 mM 2 or 3.
the absence of any stable intermediates or side products, mass
balance of >90% based on 2 was observed. Importantly, no ITA
was observed at this short reaction time, and EtOSO3H was
observed as the sole product. This is consistent with studies,
vide supra, that show that while EtOSO3H is converted to ITA
under the reaction conditions, conversion is not instantaneous.
Within the experimental error, this molar ratio of ethane to
EtOSO3H is comparable to the ∼1:1 molar ratio of D-ethane
(in D2SO4) to the total number of moles of EtOSO3H and ITA
(in H2SO4) obtained from the functionalization of ethane by
H2SO4 catalyzed by 1. Consistent with a different mechanism
for reactions with methane, previous studies showed that the
stoichiometric reaction of (bpym)PtII(Me)(TFA), 3 (a model
of PtII-Me), in H2SO4 showed primarily CH4 generated by
protonolysis and almost no MeOSO3H formation that would
be anticipated for functionalization of PtII-Me with H2SO4
(Table 1, Entry 2).32 These observations provide further
evidence for the alternate mechanism shown above (Scheme 2,
blue cycle, R = Et) involving reversible C−H bond activation of
ethane to generate PtII-Et, followed by functionalization.
Significantly, these results would suggest that, unlike the
methane system where oxidation of PtII to PtIV is the slow step,
the turnover-limiting step in the case of ethane is CH
activation, k1, R = Et.
Figure 6. B3LYP free energy surface for ethane C−H activation, β-hydride elimination, and Pt-hydride oxidation. M06 free energies are given in
parentheses. Energies are reported in kcal/mol.
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Consistent with this difference between methane and ethane,
the reaction of the PtII-Me model, 3, in non-oxidizing
CF3SO3H (Table 1, entry 4) generated only methane; no
MeOTf was observed within the limits of detection. Analysis of
the mixture after reaction of PtII-Et in CF3SO3H showed no
detectable PtII-H intermediate. However, this does not rule out
the possibility of PtII-H intermediacy, since previous studies of
PtII-Me species in H2SO4 solvent showed that rapid proton
exchange with the ligands and anions of the Pt complex led to
ill-defined and very broad resonances. It is also possible that
protonolysis of PtII-H generates H2 that we could not identify.
Attempts at synthesizing and characterizing [PtII(H)-
(C2H4)]+[TFA]− as a model of PtII(H)(C2H4) were
unsuccessful, likely due to the instability of the complex.
These results provide strong support for our proposed
mechanism involving β-hydride elimination, loss of ethylene
(that reacts with CF3SO3H to generate CF3SO3Et), and
generation of a PtII-H intermediate.
Computational Analysis. We have also performed density
functional theory (DFT) calculations to examine the proposed
catalytic mechanism (Figure 6). These calculations are also
consistent with CH activation as the turnover-limiting step in
the functionalization of ethane catalyzed by 1. Similar to the
computed methane functionalization mechanism,53−56 we
identified a pathway involving ethane coordination and C−H
bond cleavage by [(H-bpym)PtII(OSO3H)2]+, A (PtII), the
presumed active catalyst in H2SO4. The B3LYP ΔG⧧ for C−H
activation via transition state B is 27.6 kcal/mol, leading to the
generation of the Pt-ethyl intermediate, C (PtII-Et). This
endergonic intermediate (ΔG = 16.3 kcal/mol) is consistent
with the observed H/D exchange with ethane and D2SO4 that
would be expected from reversible formation of C. From C, loss
of a bisulfate ligand to give the tri-coordinate species D
thermodynamically requires ΔG = 27.3 kcal/mol. An intra-
molecular agostic interaction between the ethyl group and the
Pt metal center (structure E) occurs prior to facile β-hydride
elimination via transition state F with ΔG⧧ = 16.3 kcal/mol to
give the Pt-hydride ethylene complex G (PtII(H)(C2H4)) with
ΔG = 6.4 kcal/mol. From complex G, dissociation of ethylene
followed by bisulfate coordination gives complex H (PtII-H),
which thermodynamically requires ΔG = 17.3 kcal/mol. As
shown experimentally above (Figure 3), ethylene would then
be rapidly converted into EtOSO3H (and subsequently ITA)
by the hot H2SO4 solvent. Additionally, calculations also
suggest that at Pt meditated pathway is also energetically viable,
via direct functionalization of the Pt-bound ethylene species G
(see SI for details).
−12.7 kcal/mol). Subsequent SO2 extrusion via transition state
L (ΔG⧧ = 17.2 kcal/mol) and Pt−OH protonation gives the
aqua complex M prior to regenerating (bpym)PtII(OSO3H)2, A
(PtII).
A critical feature of the energy landscape shown is that the
ΔG⧧ for Pt-hydride oxidation (G → K) is lower than that
computed for the C−H activation step (A → C). As noted
previously, stoichiometric reaction of 2 with H2SO4 gave a
mixture of ethane and EtOSO3H. In contrast, stoichiometric
reaction of TFA-PtII-Me showed predominantly CH4 and
almost no MeOSO3H. Consistent with these experiments, the
computed transition state for the oxidation of PtII-methyl by
SO3 (Figure 6, inset), analogous to that for the oxidation of the
PtII-hydride species H, was found to be ∼6 kcal/mol higher
(ΔG⧧ = 30.1 kcal/mol), since proton transfer is more facile
than methyl group transfer.
CONCLUSION
■
In summary, we have shown that ethane is functionalized
rapidly and selectively to EtOSO3H as the initial product at
rates that are ∼144 times higher (∼100 per CH bond) than for
methane with the Periana-Catalytica system. EtOSO3H is
converted to ITA, the final product, by a well-established, non-
metal-mediated electrophilic sulfonation of ethylene (reversibly
generated from EtOSO3H) in concentrated sulfuric acid media.
Our studies show that the faster rate of ethane functionalization
to EtOSO3H (compared to methane to MeOSO3H) is the
result of a fundamentally different mechanism that results in
much faster functionalization of PtII-Et compared to PtII-Me.
Experiments and calculations have provided evidence that
ethane functionalization involves turnover-limiting C−H bond
activation (albeit the activation barrier is only slightly lower),
followed by fast β-hydride elimination, ethylene functionaliza-
tion, and Pt-hydride oxidation. This fundamentally different
mechanistic pathway could help to guide new catalyst designs
for the functionalization of higher-order alkanes which operate
by C−H bond activation, as (1) it could be possible to generate
ethanol from reactions of ethane with H2SO4 as the oxidizing
agent if the reaction could be optimized for the formation of
EtOSO3H (by designing catalysts that can operate at lower
temperatures in concentrated H2SO4); (2) it may be possible to
trap and functionalize alkene intermediates to a wide range of
products; and (3) oxidation of the M-H complex should occur
more readily than that of a corresponding M-R complex,
potentially with a wider scope of oxidants.
EXPERIMENTAL SECTION
■
Importantly, transition state B and intermediate D are close
in relative free energies, which is in accord with experiments
showing competitive protonolysis versus Pt-ethyl group
functionalization when the model of PtII-Et, complex 2, was
treated with H2SO4. Another key feature of our proposed
catalytic cycle is the fast regeneration of PtII, (bpym)-
PtII(OSO3H)2, from the Pt-hydride intermediate, H. Indeed,
our calculations have identified several low-energy Pt-hydride
oxidation mechanisms. One pathway involves SO3 coordination
to intermediate H to give intermediate I with ΔG = 10.8 kcal/
mol (generation of SO3 from concentrated H2SO4 is calculated
to be endergonic by ∼10 kcal/mol). From I there is a low-
energy pathway that first involves turnstile ligand rearrange-
ment followed by transition state J with ΔG⧧ = 24.3 kcal/mol
for proton transfer to an oxygen atom of the SO3 group.
Transition state J leads to the exergonic intermediate K (ΔG =
General Procedures. All manipulations were carried out using an
argon-filled MBraun glovebox and standard Schlenk techniques using
over-dried glassware (>1 h at 110 °C under vacuum, −30 mmHg)
unless otherwise stated. H2O was degassed and stored under argon
before use. Reagent-grade chemicals and solvents were purchased from
Sigma-Aldrich, Alfa Aesar, or EMD and used as received unless
otherwise specified. 2,2′-Bipyrimidine (bpym) was purchased from
Sigma-Aldrich. Deuterated solvents were purchased from Cambridge
Isotope Inc. and degassed and stored under argon before use unless
otherwise noted. Electrospray ionization (ESI) mass spectroscopy was
performed at the University of Illinois at Urbana−Champaign Mass
Spec Facility (Urbana, IL) on a Q-Tof Ultima mass spectrometer.
Liquid-phase organic products were analyzed with a Shimadzu
QP2010S gas chromatograph−mass spectrometer equipped with a
cross-linked RTX-5 methyl silicone gum capillary column. Gas
measurements were performed using an Agilent GasPro or HP Q-
plot column. The retention times of the products were confirmed by
G
dx.doi.org/10.1021/ja504368r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
comparison with known standards. NMR spectra were obtained on a
Bruker Digital Avance III 400 spectrometer (400.132 MHz for H,
temperature. The solution was concentrated (ca. ∼5 mL) using a
rotary evaporator, and 30 mL of pentane was added to precipitate an
orange solid. The solid was centrifuged, and the supernatant was
1
100.623 MHz for 13C, and 376.461 MHz for 19F). Chemical shifts are
given in ppm relative to residual solvent resonances (1H and 13C) or to
an external capillary containing neat HTFA (δ = −76.55, 19F).
Pt(bpym)Cl232,57−59 and diethyl(1,5-cyclooctadiene)platinum(II) (Pt-
(cod)Et2)60 were synthesized via previously reported procedures.
Preparation of (bpym)Pt(TFA)2, 1. The complex (bpym)PtCl2
(468.0 mg, 1.103 mmol) was charged to a 30 mL reaction vial with 5
mL of HTFA and a magnetic stir bar. AgTFA (487.5 mg, 2.207 mmol,
2.00 equiv) was dissolved into 20 mL of HTFA, and this solution was
added all at once to the HTFA suspension of (bpym)PtCl2. Almost
immediately a color change of the reaction mixture from burnt orange
to yellow occurred, with dissolution of (bpym)PtCl2 and the formation
of a fine white precipitate. The reaction mixture was sonicated
periodically for the next hour and then allowed to stir in the dark
overnight at room temperature. The reaction mixture was filtered
through a Celite plug to remove the formed AgCl, and the solid was
washed three times with HTFA (5 mL). The combined HTFA filtrates
were poured into 250 mL of diethyl ether, inducing the formation of a
yellow precipitate. The solution was allowed to stand for 30 min to
complete precipitation, and then the solid was collected via filtration,
washed with excess Et2O, and dried under vacuum overnight to yield 1
as a fine, bright yellow powder: 571 mg (89%). 1H NMR spectroscopic
analysis of 1 indicated that the TFA groups remained bound when
examined in d6-acetone solution; however, in CD3CN, moderate
dissociation of TFA to generate (presumably) [(bpym)Pt(TFA)-
1
removed and dried using high vacuum to give a 91% yield. H NMR
3
(400.132 MHz, CD2Cl2): δ 0.86 (t+m, 3H, JH−H = 7.6 Hz,), 1.85 (q
3
2
+d, 2H, JH−H = 7.6 Hz, JPt−H = 86.2 Hz, PtCH2CH3), 7.65 (dd, 1H,
bpym H5/5′), 7.79 (dd, 1H, bpym H5/5′), 8.88 (dd, 1H, bpym H4/4′),
9.17−9.40 (m+m, 3H, H4/4′, bpym H6/6′, Pt-bpym H6/6′). 13C{1H}
1
NMR (100.623 MHz, CD2Cl2): δ 1.66 (s+d, JPt−C = 784.4 Hz,
1
-CH2CH3), 16.3 (s+d, JPt−C = 32.9 Hz, -CH2CH3), 116.0 (q,
1
3
-OCOCF3, JC−F = 289.7 Hz), 124.7 (s+d, JPt−C = 47.3 Hz, bpym
C
4/4′), 125.3 (s, bpym), 154.9 (s, bpym), 156.5 (s+d, 4JPt−C = 37.3 Hz,
bpym), 157.9 (s, bpym), 159.8 (s, bpym), 160.8 (s, bpym C2/2′), 162.9
2
(q, -OCOCF3, JC−F = 36.4 Hz), 164.2 (s, bpym C2/2′). 19F NMR
4
(376.461 MHz, CD2Cl2): δ −74.7 (s+d, -OCOCF3, JPt−F = 19.5 Hz,
Pt-OCOCF3). HRMS(ESI): calcd for [M−TFA]+ 382.0632, found
382.0631. Anal. Calcd for C11H9F3N4O2Pt (Mr = 495.32): C, 29.10; H,
2.24; N, 11.31. Found: C, 29.37; H, 2.45; N, 11.25. X-ray-quality
orange needles of Pt(bpym)(Et)(TFA)·CH2Cl2 were obtained by
crystallization from dichloromethane.
Experimental Procedures. General Procedure for Ethane
Oxidation to Ethanol and Isethioinic Acid. In a typical experiment,
a 15 mL stainless steel reactor with a glass or PTFE insert and a cross
stir bar was loaded with 3 mL of a 15 mM solution of
Pt(bpym)(TFA)2 in either 98% H2SO4 or 101% H2SO4. The reactor
was sealed and flushed with 500 psi of ethane (five times) while the
solution was stirred. The reactor was then pressurized with 500 psi of
ethane and sealed. It was placed on a preheated aluminum block
maintained by a temperature controller at the desired reaction
temperature (160 °C). Next, the reactor was then heated for 1 h while
stirring continued at 1000 rpm. After the reaction, the reactor was
removed from the aluminum block and the reaction quenched by
placing the reactor in a dry ice/acetone bath until cooled to room
temperature. Headspace analyses were performed by venting the
reactor into an evacuated, septa-capped 30 mL vial with an outlet
connected to a one-way gas check valve and analyzing the gas phase by
GC-MS using neon as an internal standard. Liquid-phase analysis was
performed by opening the reactor, adding 50 μL of HOAc or MeOH
as an internal standard, and taking a 0.6 mL aliquot for analysis by 1H
and 13C NMR utilizing a C6D6 external capillary. Yields of isethionic
acid (ITA) were determined by comparison of the integration values
of HOAc to the two triplets of ITA, which were averaged prior to
calculation of yield.
Stoichiometric Reactions of Model Complexes. The stoichio-
metric reactions were carried out by directly injecting 0.1 mL of a 0.52
M solution of the model Pt-Et (or Pt-Me) complex in DMSO all at
once into a magnetically stirred 8 mL glass vial, equipped with a Teflon
seal, containing 5 mL of concentrated H2SO4 or HOTf, and heated to
a 160 °C on an aluminum block. Upon addition of the solution, the
vial was immediately removed and cooled in a dry ice bath, and 2.5 mL
of methane (or ethane for Pt-Me) and 5 μL of AcOH were added via
syringe as gas and liquid standards, respectively. The liquid and gas
phases were then sampled and analyzed as described above. Control
experiments showed that the DMSO solvent is stable for the time scale
of the reaction described above and did not generate any C2 or C1
products in the presence or absence of the Pt complex 1. The reaction
mixtures remained homogeneous and light orange, and no solids were
observed.
1
(CD3CN)][TFA] was observed (∼20%). H NMR (d6-acetone, 400
MHz): δ 9.49 (dd, J = 4.8 Hz, J = 2.0 Hz, 2H), 8.95 (dd, J = 5.6 Hz, J =
2.0 Hz, 2H), 8.12 (dd, J = 5.6 Hz, J = 4.8 Hz, 2H). 1H NMR (CD3CN,
400 MHz): for 1, δ 9.28 (dd, 2H), 8.79 (dd, 2H), 7.83 (2H); for
[(bpym)Pt(TFA)(CD3CN)][TFA], δ 9.36 (dd, 1H), 9.34 (dd, 1H),
9.07 (1H), δ 8.75 (dd, 1H), 7.95 (dd, 1H), 7.88 (dd, 1H). 13C{1H}
2
NMR (d6-acetone, 100 MHz): δ 163.27 (Cbpyz), 162.35 (q, TFA, JCF
= 37 Hz), 161.54 (Cbpyz), 157.12 (Cbpyz), 125.65 (Cbpyz), 115.77 (q,
1
TFA, JCF = 289 Hz). 19F NMR (d6-acetone, 376.5 MHz): δ −72.79
(s). HRMS(ES): calcd for [M−TFA]+ 466.0091, found 466.0107.
Anal. Calcd for C12H6F6N4O4Pt: C, 24.88; H, 1.04; N, 9.67. Found: C,
24.96; H, 1.10; N, 9.77.
Preparation of (bpym)PtEt2. We followed a previously published
procedure61 with minor modifications, as our complex was not stable
when subjected to the published purification procedure. A solution of
2,2′-bipyrimidine (0.44 g, 2.8 mmol) in acetonitrile (50 mL) was
added to a solution of (cod)PtEt2 (1.0 g, 2.8 mmol) in acetonitrile (10
mL) at room temperature. The solution began to turn red immediately
and was stirred at 55 °C for 16 h. After cooling, all solvents were
removed on a rotary evaporator, leaving a dark red solid. The residue
was dissolved in 8 mL of CH2Cl2 and added to 30 mL of pentane. A
dark red solid precipitated from solution, the slurry was centrifuged,
and the supernatant was removed. The recrystallization procedure was
repeated once more, followed by washing with pentane (3 × 30 mL).
Drying under high vacuum gave 870 mg (76% yield) as a dark red
powder. Attempts to obtain elemental analysis were unsuccessful, as
1
residual bpym ligand (∼5 mol %) remained in the H NMR. This
compound was taken on to the next step without further purification.
3
1H NMR (400.132 MHz, CD2Cl2): δ 1.13 (t+d, 6H, JH−H = 7.9 Hz,
2JPt−H = 84.8 Hz, PtCH2CH3), 1.78 (q+d, 4H, 3JH−H = 7.9 Hz, 2JPt−H
=
89.4 Hz, PtCH2CH3), 7.67 (dd, 2H, bpym H5/5′), 9.29 (dd, 2H, bpym
Determination of SO2 Generated from PtII-X. The SO2 reactions
were carried out in duplicate by directly injecting 0.1 mL of a 0.52 M
solution of the model Pt-Et (2), Pt-Me (3), or 1 (as a “background”)
in DMSO all at once into a magnetically stirred 8 mL glass vial,
equipped with a Teflon seal, containing 5 mL of concentrated H2SO4
or HOTf, and heated to a 160 °C with an aluminum block. Upon
addition of the solution, the vial was immediately removed and cooled
in a dry ice bath until cool to the touch. To the vials containing Pt-Et
were added 2.5 mL of methane (or ethane for the vials containing Pt-
Me) and 5 μL of AcOH as gas and liquid standards, respectively. The
second vial was cooled in an ice bath to lower the vapor pressure of the
solvent. The Teflon seal was pierced with a cannula, the other end of
3
H
4/4′), 9.37 (dd+dd, 2H, JPt−H = 18.0 Hz, bpym H6/6′). 13C{1H}
1
NMR (100.623 MHz, CD2Cl2): δ −0.95 (s+d, JPt−C = 890.0 Hz,
-CH2CH3), 17.9 (s+d, JPt−C = 40.1 Hz, -CH2CH3), 124.6 (s+d, JPt−C
= 12.9 Hz, bpym C5/5′), 153.6 (s+d, JPt−C = 25.4 Hz, bpym C6/6′),
2
3
2
156.9 (s, bpym C4/4′), 163.2 (s, bpym C2/2′). HRMS(ESI): calcd for
[M−Et]+ 382.0632, found 382.0633.
Preparation of (bpym)Pt(Et)(TFA), 2. HTFA (37 mL, 0.486
mmol) was dissolved in 3 mL of CH2Cl2 and added dropwise to a
solution containing 200 mg (0.486 mmol) of Pt(bpym)(CH2CH3)2 in
25 mL of CH2Cl2 at −78 °C. The red solution slowly changed to an
orange solution during addition of the acid, accompanied by gas
evolution. The reaction mixture was allowed to warm to room
H
dx.doi.org/10.1021/ja504368r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
which was immersed in a stirred solution of 10 mL of 0.3% H2O2(aq)
with a few drops of bromothymol blue (as a pH indicator). Bubbling
was observed, with an instantaneous color change of the solution (to
yellow, indicating an acidic shift in pH) upon piercing the Teflon
septum. An additional needle was inserted into the reaction vial with
constant flowing N2. After 10 min of sparging the solution, the flow
was stopped. The solution containing H2O2 and indicator was back-
titrated with a 0.01 M solution of KOH(aq) until it returned to basic
(blue in color). The SO2 obtained in the H2SO4 runs correlated with
product observed in the liquid phase. The background reaction (with
1) showed no detectable concentration of SO2. No SO2 was observed
from the HOTf reactions.
Determination of SO2 Generated from Catalytic Reaction. The
test utilized for the stoichiometric reaction was used, except the
catalytic reactor was vented into the H2O2 solution containing of
bromothymol blue (as a pH indicator).
Details of Computational Analysis. All calculations were carried
out in Gaussian 09.62 Geometries were optimized with the B3LYP/6-
31G(d,p)[LANL2DZ] functional.63 B3LYP enthalpy and entropy
corrections were used for all thermodynamic corrections. B3LYP and
M0664 energies were further refined with the 6-311+G(2d,p)/
LANL2TZ(f) and 6-311++G(2df,2dp)/LANL2TZ(f) basis sets.
Solvation was treated using the SMD65 polarizable continuum model
augmented for H2SO4 (see SI).
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ASSOCIATED CONTENT
* Supporting Information
Additional experimental procedures, computational methods,
and X-ray crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
■
Corresponding Authors
bhashigu@scripps.edu
dhe@chem.byu.edu
rperiana@scripps.edu
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
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This work was supported as part of the Center for Catalytic
Hydrocarbon Functionalization, an Energy Frontier Research
Center Funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under award no. DE-
SC0001298 (to R.A.P. and D.H.E.).
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