Facile Styrene Formation from Ethylene and a Phenylplatinum(II) Complex Leading to an Observable Platinum(II) Hydride

Article abstract of DOI:10.1021/acs.organomet.6b00892

A new 2-(di-tert-butylphosphanyl)benzenesulfonate-supported phenylplatinum(II) complex instantaneously but reversibly binds ethylene at room temperature. Direct and rapid styrene formation at room temperature via insertion of the Pt^II-bound ethylene into the Pt^II-Ph fragment followed by β-hydride elimination results in the formation of a solution-stable Pt^II-H complex. The Pt^II-H fragment is resistant toward protonolysis by acetic acid. Oxidation of the Pt^II-H fragment by excess Cu^II(OTf)₂ leads to an inorganic Pt^II complex incapable of C-H activation.

Full text of DOI:10.1021/acs.organomet.6b00892

Communication
pubs.acs.org/Organometallics
Facile Styrene Formation from Ethylene and a Phenylplatinum(II)
Complex Leading to an Observable Platinum(II) Hydride
Shrinwantu Pal, Shuhei Kusumoto, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
S
* Supporting Information
ABSTRACT: A new 2-(di-tert-butylphosphanyl)-
benzenesulfonate-supported phenylplatinum(II) complex instan-
taneously but reversibly binds ethylene at room temperature.
Direct and rapid styrene formation at room temperature via
insertion of the PtII-bound ethylene into the PtII−Ph fragment
followed by β-hydride elimination results in the formation of a
solution-stable PtII−H complex. The PtII−H fragment is resistant toward protonolysis by acetic acid. Oxidation of the PtII−H
fragment by excess CuII(OTf)2 leads to an inorganic PtII complex incapable of C−H activation.
onventional energy-inefficient technologies for the
production of styrene, the monomer for the synthesis of
the ubiquitous polystyrene, involve arene alkylation followed by
subsequent dehydrogenation.1 The Pd-catalyzed Fujiwara−
Moritani reaction2 (Scheme 1), considerable improvements3−5
role of the oxidant (e.g., CuII) is to regenerate the PdII catalyst
(step v) by oxidation of 5.
C
Given the demonstrated ability of PtII complexes to activate
C−H bonds,8 attempts to catalytically produce styrene from
ethylene and benzene have also been made; however, the
decomposition of PtII−H species, presumably via disproportio-
nation, has been suggested to limit turnover.9−12 Thus, by using
RhI, for which the thermodynamics of disproportionation are
unfavorable, an efficient catalytic cycle for styrene production
has been realized.13 However, to the best of our knowledge, the
purported hydride complexes have never been isolated in
related catalytic transformations.14
Scheme 1. Fujiwara−Moritani Reaction for Oxidative
Coupling of Benzene and Ethylene
We rationalized that by using a mixed-donor ligand such as 2-
(di-tert-butylphosphanyl)benzenesulfonate (L), the supported
PtII−H complexes might allow isolation. The soft P donor
would disfavor formation of PtIV, while the hard O donor would
disfavor formation of Pt0. Indeed, the intermediacy of LPdII−H
complexes in olefin polymerization15 has been confirmed by
isolation.16 Thus, we chose to explore the reactivity of the
derived phenylplatinum(II) complexes toward olefins.17
As shown in Scheme 2, reaction of KL with 1 equiv of
Ph2PtII(SMe2)2 resulted in clean formation of the K[LPtIIPh2]
complex 6 (see section 1.3 in the Supporting Information).
Careful protonolysis of one Pt−Ph fragment of 6 in THF with
HBF4·OEt2 leads to the formation of 7-THF. The air-stable
complex 7-SMe2, featuring SMe2 as an ancillary ligand, was
synthesized by reacting the Ph2PtII(SMe2)2 precursor with HL.
Single crystals of complexes 6, 7-THF, and 7-SMe2 were
analyzed by single-crystal X-ray diffraction (see the Supporting
Information for details). The molecular structures of complexes
7-THF and 7-SMe2, as shown in Figure 1, feature THF and
SMe2 (respectively) bound to the PtII center trans to the
phosphine fragment. Consistent with a highly trans influencing
of which have been made in recent years, is a lucrative
alternative strategy for styrene production via oxidative
coupling of benzene and ethylene. The proposed mechanism6,7
involves C−H activation of benzene by 1 (step i) to form a
phenylpalladium complex (2), which undergoes migratory
insertion (step ii) of ethylene to form the phenethylpalladium
complex (3). β-Hydride elimination (step iii) from 3 leads to
the formation of styrene and a hydridopalladium complex (4),
which directly undergoes formation of a Pd0 species (5). The
Received: December 1, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00892
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Organometallics
Communication
Scheme 2. Synthesis of New
Phosphinesulfonatophenylplatinum(II) Complexes
Scheme 3. Reversible Coordination of Ethylene to Form an
1
Observable Pt−(η2-C2H4)Intermediate (Top) and H (Inset
1
a) and H{31P} (Inset b) NMR Resonances of the Pt−(η2-
C2H4) Fragment in THF-d8
in stabilizing η2-ethylene bound five-coordinated PtII com-
plexes,18−20 the intermediate observed upon coordination of
ethylene (7-C2H4) was identified as a four-coordinated PtII
complex.21 Interestingly, no coordination of styrene was
observed when 10 equiv of styrene was added to a THF-d8
solution of 7-THF (see section 1.7 and Figure S2.8 in the
Supporting Information).
When it is kept under 1 atm of ethylene, over the course of
∼4 h at room temperature, the purported ethylene complex 7-
C2H4 underwent direct formation of the hydridoplatinum(II)
complex (8-THF) and styrene (Scheme 4) in 83% yield, as
Scheme 4. Facile Formation of Styrene from 7-THF
Figure 1. ORTEP representation of complexes 7-THF (top) and 7-
SMe2 (bottom) at the 50% probability level. Hydrogen atoms are
omitted for clarity.
indicated by NMR spectroscopy. Additionally, the formation of
1
styrene was confirmed by GCMS. The H and 31P{1H} NMR
resonances corresponding to the Pt−H fragment and Pt−P
fragments of 8-THF are shown in parts a and b of Figure 2,
respectively. The olefinic protons of styrene are shown in
Figure 2c. Since no PtII-bound olefin fragment was seen in the
1H NMR spectrum (see Figure S2.9 in the Supporting
Information), we assigned the doublet (with corresponding
195Pt satellites) at −25.6 ppm (1JH−Pt = 1305 Hz, 2JH−P(cis) = 8.3
Hz) to the hydridoplatinum(II)−THF-d8 adduct 8-THF rather
than styrene or ethylene adducts. Although isolation attempts
resulted in progressive decomposition, complex 8-THF could
be unambiguously characterized in THF-d8 solution by 1H and
31P NMR spectroscopy (see Figure S2.9).
Since 8-THF could not be isolated cleanly, we rationalized
that the SMe2 analogue might be sufficiently stable to allow
isolation, especially since 7-SMe2 was found to be much more
robust than 7-THF. Upon treatment of 7-SMe2 with ethylene,
no visible coordination of ethylene to form 7-C2H4 was
observed at 298 K (see Figure S2.10 in the Supporting
Information), presumably because the substitution of SMe2 by
ethylene was much less favorable than the substitution of THF
by ethylene. However, gently heating a benzene solution of 7-
SMe2 under 1 atm of ethylene at 333 K for 48 h led to the
Pt-bound phenyl fragment in complex 6, and weakly trans
influencing Pt-bound THF and SMe2 fragment in complexes 7-
THF and 7-SMe2, respectively, a substantial increase in
31P−195Pt coupling is observed by 31P NMR spectroscopy in
going from complex 6 (1JP−Pt = 1874 Hz) to 7-THF (1JP−Pt
5439 Hz) or 7-SMe2 (1JP−Pt = 4195 Hz).
=
First we explored the reactivity of complex 7-THF with
ethylene. Immediate coordination of ethylene, indicated by the
observation of a doublet at 4.92 ppm (3JH−P = 3.3 Hz) with
corresponding 195Pt satellites (2JH−Pt = 53 Hz), as shown in
1
1
Scheme 3 (inset a for H NMR and inset b for H{31P} NMR
resonances) was observed upon exposure of a THF-d8 solution
of 7-THF to 1 atm. of ethylene. In THF-d8 solution, both 7-
1
THF and 7-C2H4 (in 1:2.5 ratio) were observed by H NMR
spectroscopy, while in C6D6, only 7-C2H4 was observed (see
Figures S2.5 and S2.6 in the Supporting Information). When
the THF-d8 solution was exposed to dynamic vacuum, complex
7-THF was recovered completely, which indicated that
coordination of ethylene is reversible. Although both steric
congestion and electron-donating ability have been implicated
B
DOI: 10.1021/acs.organomet.6b00892
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Organometallics
Communication
Scheme 6. Formation of Olefin Oligomers with Ethylene
Pressurized at 4 MPa
Supporting Information). While the aliphatic oligomers (n =
2−6) arise from complex 8-THF via ethylene insertion and
subsequent β-hydride elimination (reaction i, Scheme 6),
phenylbutene isomers arise from the 2-phenylethylplatinum(II)
intermediate 9-THF (reaction ii, Scheme 6).
Finally, in order to realize the catalytic cycle similar to that
presented in Scheme 1, the Pt−H fragment in 8-THF must be
oxidized rapidly. We found that the PtII−H fragment could not
be protonated by acetic acid. Indeed, formation of both styrene
and PtII−H was also observed in acetic acid-d4. However,
treatment of the THF-d8 solution containing complex 8-THF
with 5 equiv of CuII(OTf)2 resulted in immediate obliteration
of the PtII−H signal (see section 1.11 and Figure S2.12 in the
Supporting Information). Analysis of the reaction mixture by
ESI-MS reveals a signal at 496.06 amu corresponding to the
cationic LPtII fragment.22 Attempts at catalytically converting
benzene and ethylene in the presence of excess CuII(OAc)2
produced only a substoichiometric amount of styrene (see
section 1.12 in the Supporting Information). We presume that
C−H activation of benzene at the 2-(di-tert-butylphosphanyl)-
benzenesulfonato−PtII center is a high-energy process and
prevents turnover under the current conditions. In order to
indirectly probe a model C−H activation step, complex 7-THF
was heated in C6D6 (Scheme 7). No scrambling of the Pt−
C6H5 fragment for C6D5 was observed at 100 °C.23
1
Figure 2. (a) H NMR resonance of the Pt−H fragment of 8-THF
and (b) 31P{1H} NMR spectrum of 8-THF and (c) olefinic protons
(*) of styrene.
formation of 8-SMe2, as shown in Scheme 5(see section 1.9 and
Figure S2.11 in the Supporting Information). Single crystals of
Scheme 5. Synthesis and Molecular Structure of a Stable
a
PtII−H Complex (8-SMe2)
Scheme 7. Attempted C−H Activation in C6D6
a
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms other than the Pt−H are omitted for clarity.
In conclusion, we have synthesized new 2-(di-tert-
butylphosphanyl)benzenesulfonatoplatinum representatives of
this popular (nickel, palladium) platform for catalytic olefin
polymerization.15 The supported monophenylplatinum(II)−
THF complex reacts with ethylene at room temperature to
directly produce styrene and a hydridoplatinum(II) complex,
the latter being stable toward disproportionation or proto-
nolysis. The molecular structures of these new complexes were
characterized by X-ray diffraction. To the best of our
knowledge, formation of styrene from a phenylplatinum(II)
complex at room temperature has never been reported. Efforts
are being undertaken to investigate the potential of this
phosphine-sulfonate platinum platform toward catalytic styrene
formation.
8-SMe2 were analyzed by X-ray diffraction (see the Supporting
Information for details), and the molecular structure is shown
in Scheme 5.
We monitored the evolution of styrene and complex 8-THF
1
in THF-d8 at room temperature by H NMR spectroscopy,
1
paying particular attention to the aliphatic region of the H
NMR spectra of the reaction mixture. However, no evidence of
the 2-phenylethylplatinum(II) intermediate (9-THF; Scheme
6) was observed. Indirect but conclusive evidence for the
concurrent formation of 8-THF and 9-THF is obtained from
the following experiment. Upon exposure of complex 7-THF to
higher pressures of ethylene, in addition to styrene, olefin
oligomers with and without phenyl substituents, as shown in
Scheme 6, were observed by GCMS (see section 1.10 in the
C
DOI: 10.1021/acs.organomet.6b00892
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
(18) Benedetti, M.; Antonucci, D.; Girelli, C. R.; Fanizzi, F. P. Eur. J.
Inorg. Chem. 2015, 2015, 2308−2316.
ASSOCIATED CONTENT
* Supporting Information
■
S
(19) Benedetti, M.; Antonucci, D.; De Pascali, S. A.; Girelli, C. R.;
Fanizzi, F. P. J. Organomet. Chem. 2012, 714, 60−66.
(20) Margiotta, N.; Bertolasi, V.; Capitelli, F.; Maresca, L.; Moliterni,
A. G. G.; Vizza, F.; Natile, G. Inorg. Chim. Acta 2004, 357, 149−158.
(21) Nakano, R.; Chung, L. W.; Watanabe, Y.; Okuno, Y.; Okumura,
Y.; Ito, S.; Morokuma, K.; Nozaki, K. ACS Catal. 2016, 6, 6101−6113.
(22) This signal was not observed for any of the other Pt complexes,
implying a new “inorganic” Pt complex as its origin.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00892.
1
Synthetic procedures, H, 13C, and 31P NMR spectra,
details of attempted catalysis, GC traces, and X-ray
characterization of complexes 6, 7-THF, 7-SMe2, and 8-
SMe2 (PDF)
Crystallographic data for complexes 6, 7-THF, 7-SMe2,
and 8-SMe2 (ZIP)
(23) At temperatures higher than 100 °C, formation of Pt black was
observed.
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.N.: nozaki@chembio.t.u-tokyo.ac.jp.
■
ORCID
Shrinwantu Pal: 0000-0001-6260-1803
Kyoko Nozaki: 0000-0002-0321-5299
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the JSPS KAKENHI (Grant Number JP15K21707
and JP15H05796) in “Precisely Designed Catalysts with
Customized Scaffolding” for financial support of this work.
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DOI: 10.1021/acs.organomet.6b00892
Organometallics XXXX, XXX, XXX−XXX