Ethylene oxidation by a platinum(II) hydroxo complex. insights into the wacker process

Article abstract of DOI:10.1021/om300626a

The ethylene reaction of hydroxo complex [Pt(COD)(μ-OH)] ₂(OTf)₂ (4) yields acetaldehyde and an equilibrium mixture of Pt(COD)(C₂H₅)(OTf) (6) and [Pt(COD)(C ₂H₅)(η²-C₂H₄)]OTf (7). Dinuclear [Pt₂(COD)₂(μ-OH){μ- κ²C,O-CH₂C(O)H}](OTf)₂ (10) and mononuclear Pt(COD)(CH₂C(O)H)(OTf) (11) are detected as intermediates during the reaction. The reaction displays three phases: (1) an induction period, (2) an accelerating reaction yielding intermediate 11, water, and 6 plus 7, and (3) a linear decrease in the concentration of 11 with simultaneous formation of acetaldehyde and more 6 plus 7. The reaction is proton catalyzed, and phases 1 and 2 are attributed to autocatalytic behavior resulting from the acidic complex of 6 with water, [Pt(COD)(C₂H₅)(OH ₂)]OTf. Phase 3 is initiated by the complete consumption of basic 4 and the resulting release of protons to catalyze the reaction of 11 with water and ethylene to give acetaldehyde and 6 plus 7.

Full text of DOI:10.1021/om300626a

Article
pubs.acs.org/Organometallics
Ethylene Oxidation by a Platinum(II) Hydroxo Complex. Insights into
the Wacker Process
Nandita M. Weliange and Paul R. Sharp*
125 Chemistry, University of MissouriColumbia, Columbia, Missouri 65211, United States
S
* Supporting Information
ABSTRACT: The ethylene reaction of hydroxo complex [Pt(COD)(μ-
OH)]2(OTf)2 (4) yields acetaldehyde and an equilibrium mixture of
Pt(COD)(C2H5)(OTf) (6) and [Pt(COD)(C2H5)(η2-C2H4)]OTf (7).
Dinuclear [Pt2(COD)2(μ-OH){μ-κ2C,O-CH2C(O)H}](OTf)2 (10) and
mononuclear Pt(COD)(CH2C(O)H)(OTf) (11) are detected as
intermediates during the reaction. The reaction displays three phases:
(1) an induction period, (2) an accelerating reaction yielding intermediate
11, water, and 6 plus 7, and (3) a linear decrease in the concentration of
11 with simultaneous formation of acetaldehyde and more 6 plus 7. The
reaction is proton catalyzed, and phases 1 and 2 are attributed to
autocatalytic behavior resulting from the acidic complex of 6 with water,
[Pt(COD)(C2H5)(OH2)]OTf. Phase 3 is initiated by the complete
consumption of basic 4 and the resulting release of protons to catalyze the
reaction of 11 with water and ethylene to give acetaldehyde and 6 plus 7.
insertion reactions, the product of the addition is often drawn
with the OH group dissociated from the palladium center.
However, it seems likely that the OH group would remain
coordinated to give a protonated metallaoxetane,10,11 as shown
in Scheme 1. Mechanistic evidence for both possibilities 1 and 2
has been gathered experimentally12−23 and computation-
ally,7,10,24−31 and it appears that either one may occur
depending on reaction conditions and substrate.10,21,22
INTRODUCTION
■
Wacker catalytic alkene oxidation has been an important
reaction for more than half a century. Originally developed as
an aqueous Pd/Cu system with molecular oxygen as the
terminal oxidant (Scheme 1),1−3 various modified forms of the
Scheme 1
Various reactions and complexes that may serve as models
for the steps and intermediates in the Wacker reaction have
appeared in the literature.32−57 Some time ago we reported
stoichiometric ethylene oxidation to acetaldehyde by Pt(II) oxo
complex 1 and formation of platinaoxetane 3 from the reaction
of the strained alkene norbornene (NB) with 1 (Scheme 2) and
suggested this system as a model for 1,2-addition in Wacker
alkene oxidation.58,59 Since then we have traced the reactivity of
oxo complex 1 to hydroxo complex [Pt(COD)(μ-
OH)]2(OTf)2 (4)60,64 and reported the proton-catalyzed
reaction of 4 with NB (Scheme 2).60,65 The 1,2-addition of
NB to the Pt−OH bond of 4 is rapid and reversible. (For other
alkene 1,2-additions to Pt−OH bonds see refs 57 and 66.)
Here, we report the reaction of 4 with the simple unstrained
alkene ethylene. The reaction displays unusual characteristics,
including an induction period and intermediates on the way to
the final acetaldehyde product. Modifications to the initial
reaction conditions and a study of the intermediates have given
insight into the reaction pathway.
reaction have evolved using different solvents, single metals
(Rh, Pt, Pd), and terminal oxidants other than molecular
oxygen.4−6 An overall mechanism for the classic version has
generally been accepted, but uncertainty remains over the C−O
bond-forming step (hydroxypalladation)7 and subsequent
rearrangement processes.8,9
Two commonly considered possibilities for the C−O bond
forming step are (Scheme 1) (1) anti attack of an external
nucleophilic oxygen species (water or OH−) on the
coordinated alkene (anti addition) and (2) 1,2-addition of
the coordinated alkene to the M−O bond of a coordinated OH
group (syn addition). In analogy to hydride and alkyl migratory
Received: July 5, 2012
Published: September 20, 2012
© 2012 American Chemical Society
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to the presence of water and other polar species in the mixture,
which favor 7. In fact, in the presence of 1 equiv of HOTf only
7 is observed.67 (Water displacement of OTf− from 6 is also a
possible factor in the equilibrium: see below and Scheme 6.)
Unlike 6, which decomposes in hours, the 6, 7, and ethylene
mixture is stable for days. EXSY spectroscopy (1200 ms mixing
time) on the mixture reveals an additional equilibrium. The free
ethylene peak at δ 4.85 is correlated with the ethyl CH3 peak of
6 at δ 0.89, and the CH2 and the CH3 peaks of the ethyl group
are correlated to each other, although a strong COSY
correlation between the CH2 and the CH3 peaks make it
difficult to be certain of the EXSY correlation. These
correlations indicate exchange through reversible β-hydride
elimination to ethylene hydride complex 8 and ethylene
exchange, possibly through 9 (Scheme 4). Careful examination
Scheme 2
RESULTS
■
Reaction Products. Ethylene addition to [Pt(COD)-
(OH)]2[OTf]2 (4) in CD2Cl2 causes the colorless solution to
become pale yellow after several hours (∼2 h), ultimately (∼8
h) yielding acetaldehyde and an equilibrium mixture of
Pt(COD)(C2H5)(OTf) (6) and [Pt(COD)(C2H5)(η2-
C2H4)]OTf (7)58,67 (Scheme 3). While 6 or 7 could not be
Scheme 4
of the high-field region of the 1H NMR spectrum of the mixture
Scheme 3
revealed a small singlet at δ −1.96 with Pt satellites (JPt−H
=
775.5 Hz) that can be assigned to the hydride ligand of 8 or 9.
Similar systems relating metal hydride, alkyl, and olefin
complexes have been reported or proposed for Pt,68,69
Pd,70,71 and Ni72,73 complexes involved in olefin oligomeriza-
tions.
Intermediates. Monitoring the reaction of 4 and ethylene
revealed two intermediates, dinuclear [Pt2(COD)2(μ-OH){μ-
κ2C,O-CH2C(O)H}](OTf)2 (10) and mononuclear Pt(COD)-
(CH2C(O)H)(OTf) (11). These complexes are unstable to
isolation but were identified by their NMR spectral signatures
and by independent synthesis. Thus, both 10 and 11 are also
produced in the reaction of hydroxo complex 4 with
(vinyloxy)trimethylsilane according to Scheme 5. (Related
isolated, they were readily identified by their NMR spectra and
by independent synthesis. The yield of acetaldehyde (by NMR)
varies depending on reaction conditions (see below) but can
reach nearly 100%. The yield of 6 plus 7 is essentially
quantitative (by NMR), with no other Pt-containing products
at the end of the reaction. (Helfer and Atwood have observed
similar products in the aqueous phase ethylene reaction of
water-soluble cis-PtCl2(TPPTS)2 (TPPTS = P(m-
C6H4SO3Na)3.54)
Scheme 5
Acetaldehyde is easily identified by its characteristic quartet
(δ 9.75) and doublet (δ 2.15) in the 1H NMR spectrum of the
mixture. Ethyl complex 6 is identified from the close
1
resemblance of its H NMR signals to those of the known
chloro analogue, Pt(COD)(C2H5)(Cl).58 Upfield triplet and
quartet patterns with Pt satellites at δ 1.54 and 0.89 indicate the
presence of the Pt-bonded ethyl group, while downfield
multiplets with differing Pt coupling constants of 23 (trans to
Et group) and 97 Hz (trans to OTf) at δ 5.81 and 4.31 account
for the olefinic portion of the COD ligand. A 195Pt NMR signal
is observed for 6 at δ −3471. Identical NMR spectra are
observed for 6 prepared from Pt(COD)(C2H5)2 and HOTf.
The known ethyl ethylene complex 7 was also easily identified
complexes have been similarly prepared.32,34,50) Hydroxo
complex 4 is poorly soluble, making it difficult to prevent the
first formed and freely soluble 10 from reacting further with
(vinyloxy)trimethylsilane to give 11. As a result, we were unable
to obtain pure samples of 10; some 11 is always present. Pure
solutions of 11 can be obtained with 6 equiv or more of
(vinyloxy)trimethylsilane. This is more than the 4 equiv
required by the stoichiometry, and the excess is presumably
needed because of the reaction of (vinyloxy)trimethylsilane
with adventitious water in the reaction mixture.
58,67
1
from its H NMR spectrum.
The ethyl group quartet and
triplet patterns of 7 appear at unusually high field shifts of δ
0.31 and −0.41.
Equilibrium between 6 and 7 is evident from the formation
of a mixture of 6 and 7 when ethylene is added to a solution of
6 (prepared from Pt(COD)(C2H5)(Cl) and AgOTf or from
Pt(COD)(C2H5)2 and HOTf). Consistent with the ionic
formulation of 7, the equilibrium constant appears sensitive
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Proton NMR assignments and NOE correlations for 10 are
carbon atom and the signal at δ 5.25 (JPt−H = 68.0 Hz) to that
trans to the μ-OH group.81 This assignment is confirmed from
the NOESY spectrum, which shows NOE correlation of the
COD signal at δ 6.11 with that of the μ-OH group and
correlations of the COD signal at δ 5.25 with those of the
formylmethyl ligand protons (Figure 1).
given in Figure 1. A prominent feature in the 1H NMR
The second set of COD signals at δ 6.26 and 5.67 in 10
appear as broad singlets without visible Pt satellites at 27 °C.
These are assigned to the olefinic protons of Pt2, the Pt center
bonded to the formylmethyl ligand oxygen atom, and this is
1
Figure 1. H NMR assignments and noe correlations for 10.
confirmed by H−195Pt HMQC spectroscopy. Cooling the H
NMR sample and probe to 0 °C caused the Pt2 COD olefinic
signals to sharpen and Pt satellites to appear. The Pt coupling
constants of 73.2 Hz (δ 6.26) and 68.3 Hz (δ 5.67) are
relatively large, as expected for the weak trans influence of the
μ-OH group and the carbonyl group. The signal with the larger
coupling constant at δ 6.26 (JPt−H = 73.2 Hz) is tentatively
assigned to the olefinic protons trans to the carbonyl group.
Warming the sample to 55 °C causes the two Pt2 COD signals
to coalesce, giving a broad singlet. The sample decomposes at
higher temperatures, preventing observation of a fast exchange
spectrum. Although 10 and 11 are in slow equilibrium (see
below), this does not appear to be the process that exchanges
the Pt2 COD signals. If it were, exchange of the Pt1 COD
signals and the formylmethyl ligand signals of 10 with those of
11 would also be observed. An intramolecular process to
exchange only the Pt2 COD protons must be occurring. One
possibility is rotation of the Pt2(COD) fragment. Such an
exchange process has been proposed for an isoelectronic
Rh(COD) fragment bonded to a weakly donating bidentate
ligand (sparteine).87 Supporting DFT calculations indicate a
low-spin, tetrahedral-like transition state.
1
1
spectrum of 10 is the aldehydic proton of the bridging
formylmethyl (μ-κ2C,O-CH2C(O)H) ligand, observed in
CDCl3 as a triplet at δ 9.25, in the same range reported for
nonbridging formylmethyl complexes.32,34,38,45,50,52,74 Samples
of 10 display broad and variable 195Pt satellites for the aldehydic
peak. This behavior is associated with an equilibrium process
involving 11 (see below). The 13C NMR peak for the carbonyl
1
carbon atom (correlated to the aldehydic peak in the H−13C
HMQC spectrum) is found at δ 208.9, downfield from that of
acetaldehyde at δ 200 and in the same range as the carbonyl
carbon atom of bridging ketonyl ligands75−77 and also in the
range for nonbridging formylmethyl complexes.32,34,38,45,50,52,74
Aldehydic proton coupling to 195Pt is clearly resolved in the
1H−13C HMQC spectrum, giving a surprisingly large coupling
constant of 286 Hz. The methylene protons of the μ-κ2C,O-
CH2C(O)H ligand were assigned from a COSY experiment
where a doublet at δ 3.99 (JPt−H = 73.0 Hz) correlates to the
aldehydic peak at δ 9.25. Again, this shift is in the region
observed for formylmethyl32,34,38,45,50,52,74 and related ketonyl
(−CH2C(O)R) ligands.75,76,78−84 1H−195Pt HMQC spectros-
copy (Table 1) shows correlation of the methylene protons to a
The structure of 11 is somewhat less certain, and possible
1
structures are given in Figure 2. The H NMR spectrum of 11
Table 1. 1H−195Pt HMQC Spectroscopy Correlations for 10
and 11
1
195Pt NMR signal (ppm)
correlated H NMR signal (ppm)
−3065 (10)
−3410 (10)
−3342 (11)
9.25, 6.26, 5.67, ∼2.3
6.11, 5.25, 3.99, ∼2.3
4.85, 2.84, ∼2.3
Pt signal at δ −3410 (assigned to Pt1 in Figure 1), while the
aldehydic proton is correlated to an equal-intensity Pt signal at
δ −3065 (assigned to Pt2 in Figure 1). Thus, 1H−195Pt coupling
of the aldehydic proton is through the H−CO−Pt2 linkage,
establishing the bridging character of the formylmethyl ligand.
This appears to be the first example of a bridging formylmethyl
ligand. Several dinuclear Pd(II) complexes with bridging
ketonyl ligands are known, and three have been structurally
characterized.75−77,85
Figure 2. Possible structures for intermediate 11.
The bridging hydroxo group of 10 appears as a singlet at δ
7.46, near that for the parent μ-hydroxo complex 4. Its identity
as an OH group is supported by the 1H−13C HMQC spectrum,
in which a correlating 13C signal is absent, and by the signal’s
disappearance with the addition of 1 drop of D2O. The four
olefinic protons of 10 are found at δ 6.26, 6.11, 5.67, and 5.25
with only the two at δ 6.11 and 5.25 displaying coupling to the
195Pt nucleus of 34.5 and 68.0 Hz, respectively. 1H−195Pt
HMQC spectroscopy reveals them to belong to the COD
ligand of Pt1, the Pt center that is bonded to the formylmethyl
ligand carbon atom. On the basis of the greater trans
influence78,86 afforded by alkyl groups, the signal at δ 6.11
(JPt−H = 34.5 Hz) is assigned to the olefinic group trans to the
shows an aldehydic proton as a triplet at δ 9.67, near that of
dinuclear 10. However, in contrast to the case for 10, there is
no coupling to Pt in the 1H NMR spectrum or any correlation
1
with the 195Pt signal in the H−195Pt HMQC spectrum, which
is evidence against dinuclear structure A. COSY spectroscopy
shows that the aldehydic triplet is correlated to a doublet with
satellites at δ 2.84 (JPt−H = 81.5 Hz), identifying this as the
CH2C(O)H ligand methylene protons. The equivalency of the
methylene protons is inconsistent with a static oxaallyl structure
(C),76,88,89 but a fluxional process involving structure D or B
would interchange the protons. The involvement of structure
type D has been proposed for fluxional complexes with oxaallyl
structures.76,89 Unfortunately, at lower temperatures the
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methylene signal overlaps with COD peaks and we were unable
to determine if its appearance is temperature dependent.
The olefinic signals for 11 appear at δ 5.95 (JPt−H = 30.5 Hz)
and δ 4.85 (JPt−H = 86.5 Hz) as clear multiplets with Pt
satellites. The signal at δ 5.95 is assigned to the olefinic protons
trans to the CH2C(O)H ligand carbon atom, and that at δ 4.85
can be assigned to the olefinic protons trans to the CH2C(O)H
ligand oxygen atom of structure D or to the triflate anion of
structure B. These signals are temperature independent from
25 to 55 °C but at lower temperatures show temperature-
dependent shifts, with the δ 5.95 signal shifting upfield to δ 5.86
and the δ 4.85 signal shifting more strongly downfield to δ 5.04
at −50 °C. As the shift is greatest for the olefinic signal (δ 4.85)
trans to the weak donor, equilibria between all four structures
may be involved and/or coordination of other donor molecules
present in the solution (e.g., water and acetaldehyde).
other. (This would be expected from the fast fluxional process
observed in the VT 1H MNR spectra of 10 described above.) In
addition, the μ-OH group of 10 is in exchange with a very
broad (as judged by the correlation peak width) peak at δ 2.5,
which is buried under the COD signals. This signal is assigned
to water (adventitious or from the formation of 10), shifted
from its usual position of δ 1.5690 by coordination to 11 and/or
6 (Scheme 6). D2O exchange of the OH proton was already
noted above.
Scheme 6
A
13C NMR peak for the carbonyl carbon atom of 11 could
NMR data at various stages of the reaction were also
collected at low temperatures (see Table S1 in the Supporting
Information). In addition to the 1H NMR signals for 6−11, two
signals were observed at room temperature: a weak unresolved
peak at δ 11.4 and a broad moving signal originating at δ 1.52.
The signal at δ 11.4 is most intense when 10 is at its highest
concentrations (phase 2 to phase 3 transition). The signal is
temperature independent and absent at the end of the reaction.
The low concentration of the compound and the lack of other
visible NMR signals prevented further characterization.
not be located, even in mixtures of 10 and 11 where the
carbonyl carbon atom of 10 is clearly visible. Presumably, the
peak is broadened out from the same equilibria, causing the
temperature-dependent olefinic COD 1H NMR shifts described
above. The methylene carbon of the CH2C(O)H ligand was
readily located at δ 37.8. The 195Pt NMR signal (CDCl3) of 11
is found at δ −3342 and is weaker than would be expected
relative to the signals for 10. The weak signal may again be
attributable to the equilibria discussed above. 1H−195Pt
correlations (Table 1) are observed. The CH2C(O)H ligand
methylene protons at δ 2.84 and the COD olefinic signal at δ
4.85 correlate to the 195Pt NMR signal. A correlation with the
COD peak at δ 5.95 (trans to carbon) was not observed,
presumably due to the weak coupling to Pt. Also observed in
the 1H−195Pt HMQC spectrum of both 10 and 11 is a
correlation of the Pt signals to a broad COD methylene group
signal at ∼δ 2.3.
Thus, the structure of dinuclear 10 is clearly defined but that
of 11 is not and may involve multiple species. However, all are
based on the [Pt(COD)(CH2C(O)H)]+ fragment, which is
well represented by structure B with its labile coordinated
triflate anion, and this structure will be used for all further
references to 11.
Mixtures of 10 and 11 (prepared from 4 and ethylene or
from 4 and (vinyloxy)trimethylsilane) were studied by EXSY
spectroscopy (800 ms mixing time, Table 2). Correlation peaks
The broad signal at δ 1.52 grows and shifts downfield in the
ambient-temperature 1H NMR spectra during phase 2. Its
maximum integrated area occurs at a shift of δ 2.0, coincident in
time with the maximum concentration of 10 (phase 2 to phase
3 transition). Subsequently, it is difficult to determine if the
area changes, since the signal continues to shift downfield and
begins to overlap with the COD methylene signals and
eventually entirely overlaps. The signal can be detected up to δ
2.11. Lowering the temperature causes the signal to shift
downfield and become visible but also very broad. A CD3NO2
reaction displays similar behavior but with the broad signal
originating at δ 2.0. Considering the initial chemical shift, the
broadness, and the dynamic behavior of this signal, it is
assigned to free H2O (δ 1.52 in CD2Cl2)90 exchanging with
H2O coordinated to ethyl complex 6 (Scheme 6). Triflate anion
displacement by water is a known process,91 and an equilibrium
like that in Scheme 6 has been reported.92
The equilibrium in Scheme 6 also affects the 195Pt NMR
spectra. Throughout the reaction, signals for 6 can be seen in
Table 2. Exchange in EXSY Spectrum (800 ms Mixing Time)
of a CDCl3 Solution of 10 and 11
1
the H NMR spectra where coordination of water would have
little influence on the COD and ethyl group shifts. However,
rapid water coordination and loss should more dramatically
affect the 195Pt NMR shifts and room-temperature signals for 6
are absent (exchange broadened) until the reaction is complete
and the water peak has disappeared. During the reaction, 195Pt
NMR signals observed at δ −3501 and −3539 at −20 and −50
°C are believed to be associated with the aqua ethyl complex
13. The rise in H2O concentration along with intermediate 10
and ethyl complex 6 (in equilibrium with 7 and 13) indicates
the stoichiometry of Scheme 7 for phase 2 of the reaction.
Phase 3 of the reaction is marked by the decomposition of
intermediate 10, the formation of acetaldehyde, and the
continued production of 6 plus 7. The ideal stoichiometry for
this phase is given in Scheme 8 and requires the consumption
of the water produced in phase 2. Phase 3 commences at
concentrations of 10 and 6 (plus 7) that correspond to the
complete consumption of hydroxo complex 4 according to
a
peak for 10
exchange peak
9.25
7.46
6.26
6.11
5.25
3.99
9.67 (11)
2.54 (water)
5.67 (10)
5.95 (11)
4.85 (11)
2.84 (11)
a
Peak assignment given in parentheses.
between the formylmethyl ligand aldehydic and methylene
protons and between the COD olefinic protons of 10 and 11
are observed, indicating slow exchange. Of the four olefinic
signals for dinuclear 10, the two olefinic signals for Pt1 at δ 6.11
and 5.25 exchange with their respective counterparts in
mononuclear 11. The only exchange detected for the two
olefinic signals (δ 6.26 and 5.67) for Pt2 of 10 is with each
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In contrast to water, HOTf and Pt(COD)(OTf)2 additions
strongly affect the reaction. Four mole percent HOTf nearly
eliminates the induction period, and the phase 2 and 3
reactions are accelerated by a factor of 3−4 (Figure 4).
Scheme 7
Scheme 8
Scheme 7 (i.e., at about two-thirds of the starting concentration
of 4). Thus, the presence of 4 appears to inhibit the
decomposition of 10. This is confirmed by studies of the
ethylene reaction of dinuclear intermediate 10 (with some 11)
independently prepared from 4 and (vinyloxy)trimethylsilane
(see below).
Figure 4. Ethylene reaction profile for [Pt(COD)(OH)]2[OTf]2 (4)
in CD2Cl2 at 27 °C with 4% HOTf, [4] ≈ 9 mM, and [C2H4] = 200
mM. Slopes are from least-squares fits to linear portions of the plots.
Reaction Variables. To help understand the ethylene
reaction of 4, various changes were made to the initial reaction
conditions. In the first four experiments (water, HOTf, or
Pt(COD)(OTf)2 addition and reduced ethylene concentration)
described below, conditions were held as close as possible to
those for the control reaction in Figure 3 (same batch of 4,
CH2Cl2 solvent, and general procedure).
However, the acetaldehyde yield appears to be reduced from
∼80% to ∼65%. In a separate set of experiments, ethylene
reactions of 4 were run under identical conditions except for
added HOTf amounts of 10%, 20%, and 100% (with respect to
4). The respective acetaldehyde yields decreased from 70% to
43% to 17%, respectively. Similar to the case for HOTf, 10 mol
% Pt(COD)(OTf)2 eliminates the induction period (Figure S2,
Supporting Information). The phase 2 period is shortened by a
factor of 10 from the untreated reaction, and the phase 3
reaction is accelerated by a factor of 4−5. The acetaldehyde
yield is reduced to ∼70%.
Reducing the ethylene concentration from 200 mM to 56
mM also has a large influence on the reaction (Figure 5). The
induction period increases from 50 min to about 200 min, and
the phase 2 period extends from a total of 100 min to about
400 min. The phase 3 rate remains similar for the
decomposition of 10 and the formation of 6, but the
Figure 3. Ethylene reaction profile for [Pt(COD)(OH)]2[OTf]2 (4)
in CD2Cl2 at 27 °C with [4] = 9 mM and [C2H4] = 200 mM. Slopes
are from least-squares fits to linear portions of the plots.
Addition of 10 mol % water (with respect to 4) at the
beginning of the reaction has a relatively small effect, causing a
slight reduction in the induction period (from ca. 60 to 50 min)
and a slight acceleration (10−20%) of the phase 2 and 3
reactions (Figure S1, Supporting Information). In addition,
there is a small shift in the equilibrium between ethyl
complexes 6 and 7, with ionic 7 being more abundant,
probably due to water-induced solvent changes favoring ionic
species. A greater concentration of ionic 7 is observed in
nitromethane (see below), and only 7 is observed in CD2Cl2 in
the presence of 1 equiv of HOTf.67
Figure 5. Ethylene reaction profile for [Pt(COD)(OH)]2[OTf]2 (4)
in CD2Cl2 at 27 °C with [4] = 9 mM and [C2H4] = 56 mM. Slopes are
from least-squares fits to linear portions of the plots.
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acetaldehyde formation rate is reduced by a factor of about 2,
giving a final acetaldehyde yield of only ∼50%.
Switching the solvent from CD2Cl2 to CD3NO2 virtually
eliminates the phase 1 induction period and accelerates the
phase 2 reaction (Figure S2). The phase 3 decomposition rate
of 10 appears similar to that in CD2Cl2, but the acetaldehyde
formation rate is reduced, as is the final acetaldehyde yield
(∼50%). The concentration of 6 could not be monitored
1
during the reaction because the H NMR ethyl peaks of ethyl
triflate complex 6 are very broad in CD3NO2 and the two
olefinic COD peaks of 6, while sharp, overlap with a COD peak
of 10 and the protio residue signal of CD3NO2. Ionic ethyl
ethylene complex 7 is observed at somewhat higher
concentrations than in CH2Cl2, while the mononuclear
intermediate 11 is not seen at all. After 48 h, the known93
allyl complex [(COD)Pt(η3-CH2CHCHCH3)]+ is the only Pt-
containing product.
Figure 6. Ethylene reaction profile for a 10:1 mixture of 10 and 11
(prepared in situ from 4 (∼9 mM) and (vinyloxy)trimethylsilane) in
CD2Cl2 at 27 °C with [C2H4] = 200 mM. Acetaldehyde from the
preparation of 10 has been subtracted. Slopes are from least-squares
fits to linear portions of the plots.
Finally, the ability of small amounts of HOTf to nearly
eliminate the induction period and speed the reaction suggests
proton catalysis. To determine if protons are essential for the
reaction, the effect of added polymer-bound diethylamine
(∼NEt2) base was tested. Addition of the base at the same time
as ethylene to a CD2Cl2 solution of 4 causes broadening of the
1H NMR signals for 4 and a number of new unidentified peaks
to appear. Complexes 6−11 and acetaldehyde do not form.
Base addition in phase 2 of the ethylene reaction of 4 also alters
the reaction. When ∼NEt2 is added to a reaction mixture that
has progressed to form 6−10 and a trace amount of
Scheme 9
1
acetaldehyde (beginning of phase 3), then within 30 min H
NMR signals of 10 disappear and signals for unknown
compound(s) appear. No further acetaldehyde is formed.
The decay of 10 is also much faster and fits an exponential
instead of being approximately linear. This is again consistent
with proton catalysis of phase 3.
Also observed is a newly resolved signal at δ −1.96 (JPt−H
=
775.5 Hz), assigned to hydride complex 8 or 9 (Scheme 4).
1
When 4 is only partially converted to 10 by (vinyloxy)-
trimethylsilane and the mixture is exposed to ethylene, instead
of decreasing, the concentration of 10 increases as remaining 4
is converted to 10 and 6 plus 7 (Figure 7). Only after 4 is
completely consumed does 10 begin to decompose, producing
acetaldehyde and 6 plus 7 along with small amounts of 11. In
this case, the final yield of 6 plus 7 is quantitative relative to the
initial amount of 4 and that of acetaldehyde is 80%.
The H NMR spectrum also displays a broad signal at δ 1.99
and a sharp singlet with Pt satellites at δ 3.18 (JPt−H = 56.7 Hz).
1H−13C HMQC reveals the latter to be a Pt-bonded ethylene
moiety. The compound to which the ethylene belongs is
unknown but may be 8. After 12 h the signal at δ 1.99 shifts
upfield to δ 1.87 and is assigned to the bound water molecule of
Pt ethyl aqua complex 13 (see above).
Reaction Chemistry of 10 and 11. To gain further insight
into phase 3, the reaction chemistry of independently prepared
10 and 11 was investigated. A solution of 10 (with some 11)
was prepared by completely reacting 4 with (vinyloxy)-
trimethylsilane (Scheme 5). Ethylene addition to this solution
resulted in an immediate reaction with formation of
acetaldehyde and 6 plus 7. The reaction profile is given in
Figure 6 and resembles that of phase 3 of the ethylene reaction
of 4. However, the amount of acetaldehyde and 6 plus 7 that
can form in this reaction is limited by the absence of the water
that is generated in phase 2 of the reaction of 4 (see Scheme 7).
Only an amount equal to the initial concentration of 10 is
observed, suggesting the stoichiometry of Scheme 9 for this
reaction. By this stoichiometry, mononuclear intermediate 11
should be a product. While the concentration of 11 does
increase at the beginning of the reaction, it does not continue
to grow to reach the concentrations of 6 plus 7 and
acetaldehyde. No other Pt species is detected, and we are
unable to account for the missing Pt-containing product(s).
When the same reaction is run with 10% added HOTf, the
final products and amounts are essentially the same as without
the added HOTf, but the initial formation rate of acetaldehyde
and 6 is about doubled (Figure S4, Supporting Information).
Figure 7. Ethylene reaction profile for a mixture of 4 and 10 (from 4
(∼9 mM) and (vinyloxy)trimethylsilane) in CD2Cl2 at 27 °C with
[C2H4] = 200 mM. Acetaldehyde from the preparation of 10 has been
subtracted. Slopes are from least-squares fits to linear portions of the
plots.
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The effect of H2O on a mixture of 10 and 11, prepared from
(vinyloxy)trimethylsilane, was investigated (Scheme 10). To a
are considered and can be categorized as autocatalysis and slow
catalyst generation.
Autocatalysis typically occurs when a product is a catalyst for
the reaction or when a product undergoes further reaction to
produce a catalyst.94−96 Autocatalytic reactions give induction
periods and accelerating rates, as observed in phases 1 and 2
(Figure 3). The induction period of an autocatalytic reaction
usually involves either a small amount of the catalytic product
already present or a slow uncatalyzed reaction which produces a
small amount of product to initiate the catalytic reaction.97,98
The products over phase 2 of the ethylene reaction of 4 are
H2O, ethyl triflate complex 6, and dinuclear intermediate 10
(Scheme 7). Water on its own can be eliminated as a catalyst, as
it has a relatively small effect on the induction period and the
reaction rate when added at the beginning of the reaction
(Figure S1). Complex 10 can also be eliminated as a catalyst by
the ethylene reaction profile of 4 when 10 is initially present
(Figure 7). If 10 were a catalyst, a rapid reaction would be
expected instead of the slow initial rate that is observed. We
have no data to eliminate ethyl triflate complex 6 as a catalyst,
although it is difficult to see how it could be a catalyst on its
own. However, 6 is believed to bond water (see above) and the
acidic Pt-bonded water molecule would be a proton source.
Thus, the combination of 6 and water would be an effective
catalyst.
The second possibility we consider is slow proton generation
by impurity levels of Pt(COD)(OTf)2 (9). This complex is
used to prepare 4 and is known to give HOTf on reaction with
ethylene.67 Thus, a slow reaction of impurity 9 in the induction
period would lead to increasing concentrations of HOTf. The
reaction of 9 with ethylene would also explain the lack of an
induction period and the acceleration in phase 2 when 9 is
added at the beginning of the reaction: higher concentrations of
9 would give faster rates of proton generation. However, 9 also
readily adds H2O, yielding 4 and HOTf (see the Supporting
Information). Thus, added 9 and traces of water, as well as
water generated in the reaction, would also give H+ to catalyze
the reaction.
Scheme 10
90/10 mixture of 10 and 11 in CD2Cl2 was added H2O
(equimolar to the amount of 4 used to prepare the mixture).
1H NMR spectroscopy indicated acetaldehyde formation
(∼16% yield based on the sum of 10 and 11). Signals for
mononuclear 11 disappeared, while those for 10 were
broadened and slightly decreased. After 1.5 h, 1H NMR signals
for dinuclear 10 further diminished and an olefinic COD signal
for hydroxo complex 4 appeared. The 195Pt NMR spectrum
confirmed the presence of 4 and remaining 10. After 36 h, only
signals for acetaldehyde and 4 remained. At this point, the
acetaldehyde yield was ∼50% (based on the sum of 10 and 11).
DISCUSSION
■
The induction period (phase 1) in the ethylene reaction of 4
and the linearity of the reaction profile (zero-order kinetics)
over major portions of phases 2 and 3 (Figure 3) suggest
reaction catalysis for both the phase 2 and 3 reactions. The
dramatic acceleration with catalytic amounts of HOTf (Figure
4) confirms this. Catalysis is also consistent with our previous
work on the norbornene (NB) reaction of 4, which is limited
by proton-catalyzed breakup of the hydroxo-bridged structure
of 4 and NB trapping of monomer 16 (Scheme 11).65
Both of the above scenarios were computer-modeled and fit
to the kinetic data of Figure 3 over phases 1 and 2 (see the
Supporting Information). Both models give a good fit to the
data. However, with reasonable impurity levels of 9 (1−10%)
the sensitivity to 4% initial proton concentration of the slow
proton generation model is too great and there is no response
to initial water. In contrast, the 6 plus water model responds
correctly to a 4% initial proton concentration and to a 10%
initial water concentration (decreased phase 2 reaction time
while leaving the induction period nearly unchanged). Of the
two possibilities considered, we therefore favor autocatalysis
with H+ generated by water coordinated to 6.
Scheme 11
A proposed scheme for the phase 2 reaction pathway is given
in Scheme 12. Quite a few steps must occur in this reaction,
including the C−O bond forming step, but no intermediates
are detected. As with the NB reaction (Scheme 11), a proton-
catalyzed breakup of 4 and ethylene coordination yields
ethylene hydroxo complex 20 (step 1). The analogous NB
hydroxo complex 17 is not observed in the reaction of 4 with
NB, but its intermediacy is indicated by the formation of
protonated platinaoxetane 5 via syn addition of the coordinated
OH group to NB. In analogy, 1,2-addition is proposed in step 2
for the ethylene reaction. However, Vedernikov99 reports that
It would seem reasonable that a similar process would occur
for the ethylene reaction and that proton catalysis would also
be required. However, ethylene’s weaker bonding makes it a
much less effective trap for monomer 16 than NB and places
greater demands on the catalyst. We believe this is the reason
for the induction period in the reaction without added HOTf.
Proton impurity levels, while adequate for the NB reaction, are
too low to give a significant reaction rate and the induction
period involves a buildup of H+ concentration. Two scenarios
for the induction period in phase 1 and the reaction in phase 2
Pt(dpms)(OH)(C2 H4 ) (dpms
= bis(2-pyridyl)-
methanesulfonate) in water does not undergo 1,2-addition,
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Scheme 12
Scheme 13
phase is also proton catalyzed, as evidenced by the linear
portions of the reaction profile (Figure 3) and the HOTf
acceleration of the decomposition both in the reaction of 4
(Figure 4) and in the decomposition of independently prepared
10 (Figure 6). As such, we believe the stability of 10 in the
presence of 4 is due to a preeminent reaction of 4 with H+ (i.e.,
4 is more basic than 10). Only after the concentration of 4 has
been depleted does H+ become available for catalyzing the
breakup of 10 to 11 and hydroxo triflate complex 16 (step 1).
Our independent study of solutions of 10 and 11 shows that 11
reacts with water to produce acetaldehyde while 10 is relatively
stable to water (step 2). The Pt-containing product from this
reaction is expected to be hydroxo triflate complex 16, which is
also produced in the breakup of 10. Complex 16 reacts with
ethylene to give the hydroxoethylene hydride complex 23
(steps 3 and 4). In phase 2 of the reaction, 23 is rapidly trapped
by hydroxo dimer 4 to give 10, but in phase 3 complex 4 has
been consumed, allowing an alternative ethylene displacement
of the coordinated hydroxoethylene with subsequent rearrange-
ment to acetaldehyde (step 5). Insertion of ethylene into the
Pt−H bond then yields 6.
Finally, the variable yield of acetaldehyde in the ethylene
reaction of 4 with always a quantitative yield of 6 plus 7 is
explainable. We have previously shown that the ethylene
reaction of Pt(COD)(OTf)2 or [Pt(COD)(OTf)(THF)]-
(OTf) yields 7 and HOTf.67 This suggests the proton-catalyzed
reaction shown in Scheme 14, where the aqua complex
[Pt(COD)(OTf)(H2O)](OTf) would show reactivity similar
to that of the THF complex. Under certain conditions this
reaction may operate in parallel with acetaldehyde formation,
even though the analogous NB and cis-cyclooctene complexes
do.57 Pt(dpms)(OH)(C2H4) does undergo reversible anti
attack by free OH− to give the hydroxo hydroxoethyl complex
[Pt(dpms)(OH)(C2H4OH)]−. Free OH− attack on the
coordinated ethylene of 20 is not reasonable under our
reaction conditions, given the nonaqueous solvent and the
absence of free OH−. Water is present and could attack the
coordinated ethylene of 20, but added water has little effect on
the phase 2 reaction. Thus, we believe the most reasonable C−
O bond forming step is 1,2-addition for ethylene hydroxo
complex 20 to give protonated platinaoxetane 21.
A key difference between the NB reaction and the ethylene
reaction is that the NB reaction stops at the formation of
protonated platinaoxetane 5. The ethylene reaction continues
on from analogous 21, ultimately to the oxidized alkene
product, acetaldehyde. At least part of the reason for this
difference is the rigidity of the NB fragment. After coupling of
the alkene and the hydroxo group to form the protonated
platinaoxetane, the Pt−OH bond becomes a weak dative bond
and dissociation is facile, giving an open coordination site for β-
hydride elimination. (β-hydride elimination from a protonated
rhodaoxetane has been observed.48) In the case of NB
platinaoxetane 5, the rigidity of the ring system prevents
rotation about the C1−C2 bond and access of the C1 β-
hydrogen atoms to the open coordination site. The C3 β-
hydrogen atom does have access but is in a bridgehead position,
unfavorable for β-hydride elimination. The more flexible
ethylene platinaoxetane 21 can readily rotate about the C1−
C2 bond, positioning the C1 β-hydrogen atoms for facile β-
hydride elimination in 22 (Scheme 12, step 3). The facile β-
hydride elimination, combined with the slower breakup of 4,
results in 21 not being observed. The product of the β-hydride
elimination is the vinyl alcohol hydride complex 23. The vinyl
alcohol group of 23 is expected to be acidic,32,34 and this
complex is also not observed. Instead, it is deprotonated by 4 to
form the phase 2 products water, dinuclear intermediate 10,
and ethyl complex 6, after insertion of ethylene into the Pt−H
bond of 9 (Scheme 12, steps 4 and 5).
Scheme 14
A proposed pathway for phase 3 of the ethylene reaction, the
decomposition of intermediate 10, is given in Scheme 13. This
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reducing the acetaldehyde yield without affecting the yield of 6
plus 7.
resistance of electrophilic Pt alkyl complexes also limits other
potential catalytic systems.100
Potential Catalytic Cycles. In Pd Wacker and Wacker-like
catalytic cycles, product is released through a reductive
elimination or a β-hydride elimination, leaving a Pd(0) complex
or a Pd(II) hydride complex that must be returned to the
electrophilic Pd2+ initiator. It is at this step that analogous
Pt(II) catalytic cycles often fail.54,100 Platinum(II) hydride
complexes are more stable and less reactive than the analogous
Pd hydride complexes. This limitation has been overcome by
using hydride abstractors (e.g., Ph3C+) to regenerate the
electrophilic Pt(II) center, and catalytic systems have
resulted.101−103 Presumably, such an approach would work
here to produce a catalytic system (Scheme 15). Pt hydride 9 is
SUMMARY AND CONCLUSIONS
■
Complex 4, under proton catalysis, is remarkably reactive with
alkenes. As has been proposed for related systems,104
protonation most likely disrupts the OH bridge, allowing
alkene coordination and activation. The [Pt(COD)(OH)]+
fragment is probably particularly effective at alkene activation.
An incoming alkene will be trans to a COD π-acceptor olefinic
unit, which will minimize back-donation from the Pt center.
Although there is no strong evidence for 1,2-addition of the
OH group to coordinated ethylene, the analogous norbornene
reaction does show 1,2-addition60 and most likely it is
happening with ethylene as well.
Irrespective of how the C−O bond forms, the subsequent
reaction chemistry is fairly complex. As in the Wacker reaction,
the hydroxoethyl complex (21) undergoes rapid β-hydride
elimination. However, rather than rearranging and releasing
acetaldehyde, the resulting acidic hydroxoethylene hydride
complex 23 is rapidly trapped by basic 4 and more ethylene,
allowing the observation of the dinuclear formylmethyl
complex intermediate 10. Only after 4 is consumed does 10
undergo a proton-catalyzed reaction with ethylene and water to
release acetaldehyde and form more 6 and 7 and 23. In the
absence of 4, 23 eliminates acetaldehyde and inserts ethylene to
give 6 and 7. But for one missing step (ethyl group protonation
on 6 or hydride oxidation), the reaction would be catalytic.
Scheme 15
ASSOCIATED CONTENT
* Supporting Information
■
S
present in equilibrium with 6 and should be available for
hydride abstraction, and we have shown that 4 forms from
water and Pt(COD)(OTf)2 (see the Supporting Information).
Unfortunately, this possibility was not recognized until after our
work on the alkene reactions of 4 was terminated.
An alternative catalytic cycle that does not involve oxidation
of the Pt hydride complex is possible (Scheme 16) if the ethyl
group of 6 were more susceptible to protonation.67 This
reaction is observed for a water-soluble Pt phosphine complex,
but the system is slow and is limited to 90 turnovers at 95 °C
before catalyst decomposition to Pt black.54 Protonation
Text, tables, and figures giving experimental details and NMR
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sharpp@missouri.edu.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
Scheme 16
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the U.S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-88ER13880) is gratefully acknowl-
edged. We thank Dr. Wei Wycoff for NMR assistance and Dr.
Hanbaek Lee for the corannulene sample.
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