Immobilized Platinum Hydride Species as Catalysts for Olefin Isomerizations and Enyne Cycloisomerizations

Article abstract of DOI:10.1021/acs.organomet.1c00216

Platinum hydride species catalyze a number of interesting organic reactions. However, their reactions typically involve the use of high loadings of noble metal and are difficult to recycle, making them somewhat unsustainable. We have synthesized surface-immobilized Pt-H species via oxidative addition of surface OH groups to Pt(PtBu3)2 (1), a rarely used immobilization technique in surface organometallic chemistry. The hydride species thus made were characterized by infrared, magic-angle spinning nuclear magnetic resonance, and X-ray absorption spectroscopies and catalyzed both olefin isomerization and cycloisomerization of a 1,6 enyne (5) with a high selectivity and low Pt loading.

Full text of DOI:10.1021/acs.organomet.1c00216

pubs.acs.org/Organometallics
Article
Immobilized Platinum Hydride Species as Catalysts for Olefin
Isomerizations and Enyne Cycloisomerizations
Sarah Maier, Steve P. Cronin, Manh-Anh Vu Dinh, Zheng Li, Michael Dyballa, Michal Nowakowski,
Matthias Bauer, and Deven P. Estes*
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ABSTRACT: Platinum hydride species catalyze a number of
interesting organic reactions. However, their reactions typically involve
the use of high loadings of noble metal and are difficult to recycle,
making them somewhat unsustainable. We have synthesized surface-
immobilized Pt−H species via oxidative addition of surface OH groups
to Pt(PtBu3)2 (1), a rarely used immobilization technique in surface
organometallic chemistry. The hydride species thus made were
characterized by infrared, magic-angle spinning nuclear magnetic
resonance, and X-ray absorption spectroscopies and catalyzed both
olefin isomerization and cycloisomerization of a 1,6 enyne (5) with a
high selectivity and low Pt loading.
the catalyst can occur by dimerization of hydrides, necessitating
higher catalyst loadings (usually 5 mol %).4b,6
INTRODUCTION
■
Platinum hydrides are useful catalysts for both olefin isomer-
izations1 and enyne cycloisomerizations (Figure 1).2 These
Immobilizing catalysts on metal oxide surfaces using surface
organometallic chemistry (SOMC) can prevent dimerization
and often reduce metal loadings through site isolation of the
active sites on a metal oxide surface.7 Immobilization has the
added bonus of making the recycling of the catalyst material
simpler. In SOMC, OH groups on metal oxide surfaces typically
act as acids to protonolyze basic ligands and create new M−O
bonds. However, Bergman and Kaplan showed that surface OH
groups can also oxidatively add to Ru(0) complexes.8 Analogous
oxidative addition of surface OHs to Pt(0) complexes would
form immobilized Pt hydride species, as shown in Figure 1.
Here, we immobilize Pt−H species on metal oxides by
oxidative addition of surface OHs to Pt(PtBu3)2 (1) and
characterize them by nuclear magnetic resonance (NMR),
infrared (IR), elemental analysis, X-ray absorption spectroscopy
(XAS), transmission electron microscopy (TEM), and compar-
ison with molecular models. Oxidative addition of surface OH
groups to Pt(0) (as opposed to the protonolysis reactions typical
of SOMC) is analogous to the reactions of Pt(PtBu3)2 with
Brønsted acids. The reactivity of Pt(0) model species with OH
groups is related to their pKa, and immobilization is only possible
Figure 1. Enyne cycloisomerization catalyzed by group 10 metal
hydrides.
reactions are widely used in the fine chemical and
pharmaceutical industries due to their excellent atom econo-
my,1c,3 low waste production, and their ability to make and break
multiple C−C bonds in a single synthetic step.2c,4 However,
these reactions require relatively high catalyst loadings and are
difficult to recycle. Pt−H complexes can easily be made by the
addition of Brønsted acids to Pt(0) complexes.5 Deactivation of
Received: April 6, 2021
Published: June 1, 2021
https://doi.org/10.1021/acs.organomet.1c00216
© 2021 American Chemical Society
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(400 MHz, C6D6, δ) −75.38 (s, −CF3). IR (ATR Diamond Crystal)
2987, 2963, 2937, 2892, 1470, 1383, 1353, 1269, 1245, 1214, 1173,
1021, 969, 808, 725. Elemental analysis (C32H56F18O2P2Pt): Calc:
35.86% C, 5.27% H. Found: 36.16% C, 4.89% H.
on supports with stronger Brønsted acids. These immobilized
Pt−H species are selective catalysts for enyne cycloisomeriza-
tions. Oxidative additions of surface OH groups to M(0)
complexes are uncommon in SOMC and represent a novel
immobilization method in the toolkit of surface organometallic
chemists.
Equilibrium Measurements. Stock solutions of 1 and 2d in C6D6
were mixed in J-young NMR tubes to achieve the desired
concentrations of 1 (fixed at 0.0274 M initial concentration) and 2d
(between 0.060 and 0.6 M initial concentrations). These solutions were
then allowed to equilibrate for 10 min (after this amount of time, no
EXPERIMENTAL SECTION
■
1
further change to the NMR spectrum was seen), and H NMR was
All experiments were carried out under an inert atmosphere using
standard Schlenk and glovebox techniques. Hexane, THF, toluene,
deuterated benzene, and chloroform were purified by standard
methods. PtBu3, K2PtCl4, KOH, Zn, alcohols 2a−d, 1-hexene, dimethyl
propargylmalonate, sodium hydride, and 4-bromo-2-methyl-2-butene
were purchased and used as received. SiO2 (Degussa Aerosil-200, 209
m2 g−1), γ-Al2O3 (Gobain, 260 m2 g−1), and SiO2/Al2O3 (Gobain, Si:Al
ratio, 5.87:1, 351 m2 g−1) were compacted by drying of a water
suspension and then sieved to a particle size between 100 and 300 μm
before being dehydroxylated under a flow of synthetic air at 500 °C and
stored in a glovebox. Pt(PtBu3)2 (1) and substrate 6 were synthesized
by known literature procedures.9 Pt/SiO2 was synthesized by a known
procedure10 with a Pt loading of 1 wt % and a Pt dispersion of 80%, as
measured by H2 chemisorption.
measured with a recycle delay of 10 s (enough for all the protons to fully
relax). The Pt species in the solution were quantified using their
t
signature Bu resonances at 1.52 ppm (1) and 1.13 ppm (3). The
fluorinated alcohol concentration was corrected from the initial
concentration based on the amount of 3 that had formed in the
sample. The results are shown in Figure 2 in the text. For the
temperature-dependent samples, two separate samples with [1]0 and
[2d]0 of 0.02 and 0.73 M and 0.015 and 0.31 M, respectively, were
heated in an NMR spectrometer until equilibrium was achieved and no
solid was present. The equilibrium constants were determined as above.
Synthesis and Characterization of Surface Species 4a−c. The
desired solid support (SiO2, Al2O3, or SiO2-Al2O3, 1 g) was suspended
in pentane (5 mL) under an inert atmosphere. A solution of 1 (0.1
mmol, 60 mg) dissolved in 5 mL of pentane was filtered into this
suspension and stirred overnight under Ar. The supernatant was then
filtered away, and the support was washed with toluene (1 × 5 mL) and
3 × 5 mL of pentane and dried under vacuum. Loadings of Pt on the
surface were determined by Pt ICP-OES to be 0.04, 0.02, and 0.09
mmol of Pt g−1 for SiO2, γ-Al2O3, and SiO2-Al2O3 (4a−c), respectively.
TEM, IR, magic-angle spinning (MAS) NMR, X-ray absorption near-
edge structure (XANES), and extended X-ray absorption fine structure
(EXAFS) spectra are shown in Figure 3 and Figures S7−S21 for each
surface complex. XAS spectra were only measured for 4c due to the low
Pt loadings of 4a and 4b.
Liquid-state 1H and 31P{1H} NMR spectra were measured on a 300
MHz Bruker Avance III spectrometer. 13C NMR spectra were measured
on a 400 MHz Bruker AVIII spectrometer. Solid-state NMR spectra
were recorded on a 400 MHz Bruker Avance III spectrometer in 4 mm
zirconia rotors. 1H and 31P CPMAS NMR spectra were recorded at 8−
11 kHz at resonance frequencies of 400.1 or 161.9 MHz, respectively.
The contact time of 31P CPMAS was 2 ms and a 70_100 ramp was
applied during the Hartmann−Hahn contact period and spinal64
decoupling during acquisition. Inductively coupled plasma−optical
emission spectrometry (ICP-OES) analysis of Pt was measured in the
Institute of Technical Chemistry using a Perkin-Elmer Avio 200 optical
emission spectrometer calibrated to standard Pt solutions. IR spectra
were measured under an inert atmosphere with a Nicolet 6700 IR
spectrometer in transmission mode with the sample diluted in KBr (for
surfaces) or with a Diamond ATR attachment (for 3). TEM and
energy-dispersive X-ray (EDX) spectra were measured using a Philips
CM200-FEG electron microscope operated at 200 kV.
Catalytic Olefin Isomerization. 1-Hexene (0.1 mL, 0.81 mmol)
was added to the Pt catalyst (4 μmol, 0.5 mol %) suspended in 0.9 mL of
C6D6 under inert gas. This mixture was heated for 18 h at 80 °C, and the
conversion was measured by 1H NMR. The peaks of each product were
confirmed vs authentic samples. Product peaks are labeled in Figure
S22.
Catalytic Cyclization of Substrate 5. Substrate 5 (50 mg, 0.21
mmol) was mixed with the desired loading of the Pt catalyst and
internal standard and dissolved in C6D6 (0.5 mL). The reaction was
then heated at 80 °C for the reported time and monitored periodically
Pt L3-edge XAS spectra were collected as pure solid samples in
transmission mode on the P65 beamline at the PETRA III extension at
DESY.11 The experiments were performed using ionization chambers
with partial pressures of 800 mbar or Ar plus 200 mbar Kr (15.5% of
absorption) and 1000 mbar Kr (49.5% absorption) for detection of the
I0 and I1 signals, respectively. Energy selection was performed with a
Si(311) double-crystal monochromator (DCM), and Pt-coated mirrors
were used for higher harmonic rejection and beam focusing. The DCM
resolving power was between 0.6 × 104 and 1.4 × 104, resulting in an
experimental resolution of 1.2 eV at the Pt L3-edge. The spectra were
measured to 1000 eV over the edge energy. The reported edge energies
here correspond to the first inflection points of the absorption spectra.
No indications of radiation damage were observed. Samples 4a and 4b
were not measured due to their low loading. Modeling of the k and R-
space data was performed using the Artemis program of the Demeter
software package.12 All spectra were modeled with satisfactory χ2 and R
values. The fitted values of R + ΔR, N, and σ02 are given in Table 1.
Synthesis and Characterization of HPt(PtBu3)2[H(OC(CF3)3)2]
(3). In a Schlenk flask, Pt[PtBu3]2 (0.083 mmol, 50 mg) and alcohol 2d
(1.79 mmol, 0.25 mL) were suspended in n-hexane (1.5 mL) under an
inert atmosphere with vigorous stirring. A yellow precipitate formed
after 1 min. The supernatant was filtered away, and the solvent was
removed from the product by flushing it with an argon stream to yield a
yellow solid. Treatment of this solid under vacuum resulted in loss of
alcohol and reproduced the starting material Pt[P(tBu)3]2. NMR was
measured in the presence of an excess of 2d. 1H NMR (300 MHz, C6D6,
δ) 1.13 (t (JPH = 5.7 Hz), −CH3, 54H), −36.34 (s, JPt−H = 2577 Hz, H−
1
by H NMR. The reaction conversion and yield of the product were
determined by 1H NMR versus internal standard. The peaks
corresponding to 6a matched what was known in the literature.9b
Higher catalyst loadings and longer reaction times led to the formation
of another product 6b. Although we were not able to isolate 6b, we can
tentatively assign a structure based on its 1H NMR spectra, as shown in
1
Figures S27 and S28. Cyclized product 6b: H NMR (CDCl3, 300
MHz, d) 5.64 ppm (s, 1H, CHCCH3), 3.68 ppm (s, 6H,
−CO2CH3), 3.04 (s, 2H, CH2CC, overlapping with the peak from
6a), 1.98 (s, 3H, Me), 1.83 (s, 3H, Me), 1.66 (s, 3H, Me).
Hot Filtration Tests. Hot filtration experiments were performed for
both the isomerization of 1-hexene and the cyclization of substrate 5 to
test whether the catalytically active species was present in the solution.
In a typical hot filtration experiment, a catalytic reaction was performed
at 80 °C for a typical amount of time (0.5 mol % 4b for 1-hexene and 5
mol % 4c for cyclization of substrate 5). While keeping the solution at
80 °C, half of the supernatant was extracted with a preheated glass
syringe and needle (80 °C) and filtered using a syringe filter into a new
vessel with half as much fresh substrate and solvent as for the initial
reaction preheated to 80 °C and allowed to react for the same amount of
time as for the initial reaction. The Pt leaching was determined by
removing the solvent in vacuo, dissolving the residue in aqua regia, and
measuring the Pt content by ICP-OES. The remaining 50% of the initial
1
Pt). 31P{1H} NMR (121.5 MHz, C6D6, δ) 88.4 (s, JPt−P = 2614 Hz). 13
C
supernatant was removed and analyzed by H NMR. During the hot
APT NMR (400 MHz, C6D6, δ) 120.03 (quartet, −CF3), 76.60
filtration experiment of 5, twice as much solvent was added to the initial
reaction to ease the extraction of the supernatant.
(multiplet, −C−CF3), 39.69 (t, P−C−), 31.20 (t, −CH3). 19F NMR
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and Al2O3 (estimated pKa as low as 4.9 in H2O)18 are good
candidates.
Recycling Experiments. Recycling experiments of the isomer-
ization of 1-hexene (with 4b) and cyclization of substrate 5 (with 4c)
were performed at 80 °C with 0.5 and 5 mol % catalyst for 18 and 36 h,
respectively. For 1-hexene, reactions were filtered by filter canula,
washed successively with toluene and pentane, and dried in vacuo after
which fresh substrate and solvent were added to the dry catalyst. This
process was repeated to perform three total cycles and the results of
each cycle analyzed by 1H NMR. For substrate 5, the reaction
(performed in toluene solvent) was filtered and washed with toluene (3
× 2 mL) into a pre-weighed vial and these washings were dried in vacuo
to weigh the isolated product. Ferrocene as the internal standard was
weighed into this mixture, and the quantities of different products were
[3]
[1]
= Keq[2d]2
(1)
1
measured by H NMR. The data for the recycling experiments are
shown in Figures S32 and S33.
RESULTS
■
Pt(PtBu3)2 (1) reacts with alcohols to produce complexes of the
type HPt(OR)(PtBu3)2.5d We started by reacting 1 with the
molecular silanol HOSi(OtBu)3 (a good model for isolated
silanols) as well as heptaisobutyryl-POSS-trisilanol (a good
model for vicinal silanol groups). Neither of these reacted with
complex 1. Next, we reacted 1 with fluorinated alcohols
HOC(CH3)3−x(CF3)x (x = 0, 1, 2, and 3 denoted as 2a−d,
respectively) as models of surface OH groups (SiOH groups on
SiO2 are thought to have similar electronic properties to 2b and
2c).13 Complex 1 did not react with 2a−c. However, mixing 1
with an excess of alcohol 2d in pentane gave a yellow precipitate
whose elemental analysis showed two equivalents of 2d reacting
with every Pt. We were not able to grow crystals of suitable
quality for X-ray diffraction. However, NMR spectroscopy
showed new peaks in the 1H (1.13 ppm (54H) and −36.3 ppm
(1H) JH−Pt = 2577 Hz; Figure S1) and 31P{1H} spectra (88.4
ppm JP−Pt = 2614 Hz; Figure S2). These data are similar to
platinum hydrides such as [HPt(PtBu3)2][BF4].5a,c In particular,
chemical shifts of ca. −32 to −36 ppm and JH−Pt of 2500−2600
Hz are diagnostic of such three coordinate Pt−H species,
whereas the addition of a fourth ligand results in higher chemical
shifts of −10 to −27 ppm and a much lower JH−Pt of 665−1450
Hz.5b,c This suggests the structure [HPt(PtBu3)2][H(OC-
(CF3)3)2] for 3 in which the charge of the cationic Pt−H is
balanced by the perfluorinated alkoxide hydrogen bonding to a
second alcohol. This assignment is supported by the Pt L3-edge
EXAFS data of 3, which does not show a Pt−O scattering path in
the first coordination sphere (vide infra). Full characterization
data can be found in the Supporting Information (Figures S1−
S5).
The reaction of 1 with 2d is fully reversible. Treating solid 3
under vacuum for several hours reproduces 1. We measured the
equilibrium between 1 and 3 by varying [2d] (Figure 1). The
equilibrium varies with [2d]2 (as prescribed by the equilibrium
in eq 1) and has Keq = 107(4) at 23 °C. Since some catalytic
reactions of 3 require elevated temperatures, we tested the
temperature dependence of this equilibrium up to 75 °C (Figure
S6). While [2d] = 0.73 M gave nearly quantitative conversion of
1 to 3 at room temperature, between 65 and 75 °C, the 3:1 ratio
was between 0.3 and 0.15. We estimate ΔH and ΔS of this
reaction to be −23(1) kcal mol−1 and − 68(4) cal mol−1 K−1,
respectively. More acidic OH groups may be necessary to
prevent catalyst leaching and Pt nanoparticle formation at higher
temperatures, both of which are common for late transition
metal surface organometallic species.14 Therefore, only surfaces
containing OH groups with pKa values lower than those of
alcohol 2d (20.55 in MeCN15 or 5.33 in H2O16) will immobilize
1 as 3. SiO2 (with estimated pKa values of 4.5 and 8.9 in H2O)17
Figure 2. Measurement of the equilibrium between 1 and 3 at 23 °C by
varying [2d].
Reaction of 1 with SiO2, Al2O3, and SiO2-Al2O3 (all
dehydroxylated at 500 °C) made materials 4a−c, respectively.
These were characterized by ICP-OES, IR, 1H MAS, 31P
CPMAS NMR, TEM, and Pt L3-edge X-ray absorption
spectroscopies (of 4c). ICP-OES shows relatively low Pt
loadings of 0.04, 0.02, and 0.09 mmol of Pt g−1 (0.8, 0.4, 1.8
wt % Pt) for 4a−c, respectively. These are lower than typical for
protonolysis reactions in SOMC. We saw above that more acidic
OH groups are more reactive toward Pt−H formation. The low
loadings may be the result of only a small percentage of all OH
groups being reactive enough to add to the Pt(0) complex.
Silica-alumina immobilized two to four times as much Pt as the
other supports. This is due to both its higher surface area and its
higher acidity than either silica or alumina (likely due to the
formation of bridging and pseudo-bridging Si−OH−Al
groups).19 TEM of all three samples (Figures S7−S9) confirmed
that no Pt nanoparticles were formed during the synthesis. IR
(Figures S10−12) shows C−H stretching and bending modes of
tBu groups, with a higher intensity for a higher Pt loading. No
Pt−H stretching band (expected around 2850 cm−1) is seen in
the IR spectra of 3 or 4a−c. This is likely due to its low intensity,
low loading, and overlap with C−H stretches, similar to
HPt(PtBu3)2BF4.5c
Immobilized species 4a has two 31P CPMAS NMR signals at
78 and 75 ppm having visible JPt−P = ca. 3.0(3) kHz (Figure S14)
that are similar to other HPt(PR3)2X.5d This doubling is also
seen in 1H MAS NMR of the tBu protons, indicating that there
are two different Pt species present on the surface of SiO2
(Figure S13). The presence of multiple chemically similar Pt−P
species suggests that these are Pt−H species that are interacting
with two different Si−O sites (e.g., isolated and vicinal) on the
surface. No hydride signals were observed in 1H MAS NMR of
4a. However, this is not unusual given the extremely large
chemical shift anisotropy of transition metal hydrides20 and the
low loading of the complex on SiO2. There is also a signal around
62 ppm in 31P CPMAS NMR most likely due to adsorbed
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OPtBu3, indicating that some phosphines were oxidized on the
metal oxide surface.21
Table 1. Edge Energies and Fitted Parameters of the EXAFS
Spectra of 1, 3, and 4c
The 31P CPMAS NMR (Figure S16) spectrum of 4b showed
two signals, one at 77 ppm (JPt−P = 2.0(3) kHz) consistent with a
HPt species and a separate signal at −19 ppm (JPt−P = 3.9(3)
kHz) with a very large anisotropy. These Pt−P coupling
constants suggest that the signal at 77 ppm is a Pt(II)−H species,
while the signal at −19 ppm is due to an unknown Pt(0) species.
This unknown Pt(0) species does not match the spectrum of 1
(isotropic chemical shift of 100.8 ppm) but could potentially be
a surface-stabilized Pt(PR3) fragment.
edge (eV)
11566.3
atom
N
R + ΔR (Å)
σ02 (Å2)
1
P
2.0(1)
4.2(4)
2.7(7)
5.4(1.6)
2.1(2)
4.2(4)
1.9(6)
4.3(7)
4.8(8)
1.8(1)
1.0(1)
2.1(3)
4.1(4)
2.6(2)
0.9(2)
6.3(6)
2.26(3)
3.39(3)
3.93(3)
5.05(4)
2.27(3)
3.38(3)
3.81(3)
3.94(3)
4.13(3)
2.29(3)
2.01(3)
3.11(5)
3.17(4)
3.47(5)
3.87(6)
4.80(6)
0.0032(4)
0.0046(6)
0.0054(7)
0.0066(9)
0.0042(8)
0.006(1)
0.007(1)
0.006(1)
0.007(1)
0.0070(2)
0.0073(5)
0.012(1)
0.0103(3)
0.0105(3)
0.0142(9)
0.0145(4)
C
C
C
P
C
C
C
C
P
O
Si
C
C
Si
C
3
11565.9
11565.2
Immobilization of 1 on SiO2-Al2O3 was the cleanest. The 31
P
CPMAS NMR spectrum of 4c (Figure S18) shows only one
peak at 84 ppm having a full width at half max larger than our
expected JPt−P. However, due to the higher loading of Pt on 4c,
we were able to observe a broad Pt−H resonance at −35 ppm in
1H MAS NMR (Figure S17) with JPt−H = 2.6(3) kHz, very
similar to the molecular model 3. This suggests that the
immobilized complex on 4c has a similar structure to that of 3. A
small peak at 54 ppm in the 31P CPMAS spectrum corresponds
to a small amount of HPtBu3+ on the surface.22
Pt L3-edge X-ray absorption spectroscopy was measured for
species 1, 3, and 4c. Their XANES spectra (Figure 3) all have
similar edge energies (within the spectral resolution of 1.2 eV).
Based purely on the oxidation state, the expected edge energy of
1 should be lower than that of 3 or 4c. The L3-edge is dominated
by the allowed 2p → 5d electronic transition. Since 1 has a filled
5d shell, this transition will be of low intensity.23 However,
protonation of 1 to give 3 or 4c creates empty states in the 5d
shell, resulting in a larger contribution of the 2p → 5d transition
to the overall edge intensity, counteracting the oxidation state
effect and leaving the edge energy unchanged.
4c
within the range of other Pt(II)−O distances in the literature
(between 1.95 and 2.06 Å) but 0.06 Å longer than other
Pt(OSi) distances.14b,26 Given that the NMR data of 4c
(especially the Pt−H chemical shift and JPt−H) were consistent
with a cationic three-coordinate platinum hydride, this would
suggest a structure intermediate between a Pt−O bond and a
true ion pair as in 3.
Complexes 3 and 4a−c catalyze the isomerization of 1-hexene
at 80 °C to form mixtures of 2-hexene and 3-hexene (Table 2
Table 2. Activity and Selectivity of 1-Hexene Isomerization
Using 3 and 4a−c as Catalysts
The parameters obtained from modeling the EXAFS spectra
are shown in Table 1 (full data shown in Figures S19−S21).
a
a
catalyst catalyst
% conv % 2-hexene (cis:trans) TON TOF (h−1
)
3
4.7%
0%
5.1%
78.6%
7.0%
100% (0.4)
9
0.5
Pt/SiO2
4a
4b
100% (0.6)
96.2% (0.7)
100% (0.5)
10
142
14
0.6
b
4.4
0.8
4c
a
Standard conditions: 0.5 mol % (4 μmol of Pt), with 0.8 mmol of 1-
hexene (0.1 mL), in a total volume of 1 mL, heated at 80 °C for 18 h
b
with stirring. Since the conversion of 1-hexene in this case was so
high, the TOF listed in this table is a lower limit. The real value could
be as high as 17.4 h−1.
and Figure S22). Species 3 and 4a−c all had lower activity for
olefin isomerization than the commonly used H2PtCl6 but
higher activity than the similar complex HPtCl(PEt3)2 (only
active at 180 °C).1a This matches a previous report in which the
olefin insertion into similar Pt−H complexes is slow.5c
Figure 3. XANES spectra for Pt complexes 1, 3, and 4c.
Both 1 and 3 have first coordination shells with only two P
atoms. The Pt−P distances modeled for 1 and 3 (2.26(3) and
2.27(3) Å, respectively) are within the error of known
crystallographic data for complex 1 (2.25 Å) and similar
Pt(II)−H species (2.28 Å).24 The EXAFS of 4c also showed two
P scatterers as well as an additional O scattering path in the first
coordination sphere, without which the data could not be
modeled. The Debye−Waller factors of 4c are consistent with a
previous report of immobilized catalysts on amorphous
supports.25 The Pt−P distance of 2.29(3) Å is similar to other
four coordinate Pt(II) hydride complexes (e.g., 2.33 Å for
HPt(PtBu3)2Cl).24 The modeled Pt−O distance of 2.01(3) Å is
Catalyst 4b is 10 times more active than the other catalysts
and also gives the highest selectivity for the cis-product and even
gives a small percentage of 3-hexene. Control reactions showed
that the supports were inactive for catalysis, and therefore, the
higher activity of 4b is not due to the Al2O3 support. Attempts at
isomerization of 1-hexene using platinum nanoparticles
supported on silica (denoted Pt/SiO2) under the same
conditions resulted in no detectable isomerization. Complex 1
was also inactive for olefin isomerization. Hot filtration showed
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that the supernatant from 1-hexene isomerization with 4b was
not catalytically active. Recycling experiments for 4b-catalyzed
1-hexene isomerization gave 78, 9, and 13 turnovers of
isomerized products (Figure S31), showing deactivation after
the first cycle (with some losses of the product due to the
recycling procedure). TEM of 4a and 4c after isomerization
showed no signs of Pt particles, while 4b showed some signs of
Pt particle formation after catalysis (Figures S23−S25). As
shown by NMR, catalyst 4b contains both Pt−H species and
unidentified Pt(0)−PtBu3 complexes. These experiments
suggest that the Pt(0) centers are likely more active for olefin
isomerization but agglomerate to Pt particles during the first
cycle of catalysis, which are inactive under these conditions.
After the first cycle, the activity of 4b is comparable to those of
4a and 4c, suggesting that the nanoparticles do not play a role in
catalysis.
Finally, we tested 4a−c as catalysts for enyne cyclo-
isomerization activity using the test substrate 5. The results
are shown in Table 3. Complex 1 did not react with 5, while 3
and 4a−c were all active catalysts. The cyclization reactions are
relatively slow, consistent with other Pt-catalyzed enyne
reactions.27 Reaction of 5 with 4a−c (0.5 mol %) at 80 °C
gave 78% (4b), 35% (4a), and 82% (4c) conversions and 76, 10,
and 81% yields of 6a, respectively, after 336 h (97 and 99%
selectivity for 4a and 4c, respectively). The time profiles of the
reactions at 0.5 mol % loading are shown in Figure 4. Catalysts
is that it produces less of this side product 6b. A tentative
assignment of the structure of 6b is given below, which is likely
the result of isomerization of the terminal olefins of 6a to internal
positions.
Table 3. Cycloisomerizations Using Catalysts 3 and 4a−c
a
catalyst
loading
temp, time
% 6a (% 6b)
1
3
5 mol %
0.5 mol %
5 mol %
5 mol %
5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
80 °C, 22 h
80 °C, 2 h
n.o.
82% (6%)
62% (27%)
56% (32%)
6%()
b
4a
4c
Pt/SiO2
4a
4b
4c
80 °C, 35 h
80 °C, 35 h
80 °C, 35 h
80 °C, 336 h
80 °C, 336 h
76% ()
10% ()
81% ()
80 °C, 336 h
a
b
Measured by NMR vs internal standard. Solvent (0.8 mL) was
necessary to wet all the catalysts.
The activity of 4a−c was much higher than Pt/SiO2, which
gave only 5.8% yield under the same conditions. In addition,
TEM of the catalysts after cyclization of substrate 5 (Figures
S28−S30) showed no signs of Pt particle formation. Both of
these results suggest that the Pt particle formation does not play
a role in enyne cycloisomerization. The bare supports were also
not active for cyclization of 5. Hot filtration of the 5 mol %
reactions at 80 °C showed roughly 11% Pt leaching into the
supernatant, suggesting that Pt mostly remains on the surface.
The hot filtration test also showed that the solution was not
catalytically active. Recycling tests of 5 mol % 4c for cyclization
of substrate 5 (Figure S32) gave 74% (15 TON), 76% (15
TON), and 62% (12 TON) yields over three cycles, showing
that the catalyst can be reused without significant loss of activity
(the minimal loss of activity here is due to loss of catalyst during
filtration).
Based on these results, we can draw some conclusions about
catalysis. Complex 1 does not react with substrate 5, while 3, 4a,
and 4c all catalyze the cycloisomerization to product 6a with
high selectivities. Based on the hot filtration and recycling
experiments, the catalytic reactions likely occur at the supported
Pt−H species on the catalyst surface and not in solution. The
high selectivity for the 1,4-diene product suggests the
mechanism shown in Figure 5, involving the insertion of alkyne
into the Pt−H bond, insertion of pendant alkene into the Pt−
vinyl bond, and finally β-hydride elimination to reform the Pt−
H bond.2c The minor product 6b may form by isomerization of
the terminal olefins in 6a to their more stable internal positions.
Catalyst 4b is the most active for olefin isomerization but the
least active for cycloisomerization of 5. This can be attributed to
the presence of both Pt−H and Pt(0) species on its surface,
which are known to be active for olefin isomerization29 but do
not catalyze cycloisomerization of enynes, as shown for complex
1 above. Therefore, the Pt(0) species has a much higher activity
for isomerization of 1-hexene but does not contribute to the
overall activity for cycloisomerization. Indeed, if we consider the
fact that only ca. 40% of the Pt sites are in the form of Pt−H on
4b, then a conversion of 35% is comparable to the results with 4a
Figure 4. Time profile of cycloisomerizations using 4a−c.
4a and 4c both give time profiles that showed no induction
period and fit to a simple first-order expression with essentially
the same initial TOF of 2 h−1. The activity and selectivity of 4b
are both much lower than either 4a or 4c; therefore, it was not
used further. Both 4a and 4c were about 40 times slower than the
homogeneous complex 3 (with a TOF of >84 h−1). Such slowing
of catalysts upon immobilization is common for heterogenized
catalyst systems13,28 and may be the result of the additional Pt−
O bond to the surface, inhibiting the catalytic activity. Using a
higher loading (5 mol %) of catalysts 4a and 4c required only 35
h to come to full conversion. At a higher catalyst loading, an
unidentified isomer 6b was also made along with the desired
product, which was inseparable from 6a. Monitoring the
reaction over time showed that 6b is a secondary product due
to further reaction of initial product 6a (Figure S27). One
advantage of the heterogeneous over the homogeneous catalyst
1755
https://doi.org/10.1021/acs.organomet.1c00216
Organometallics 2021, 40, 1751−1757

Organometallics
pubs.acs.org/Organometallics
Article
https://pubs.acs.org/10.1021/acs.organomet.1c00216
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors sincerely thank the University of Stuttgart for
funding this research. We also thank Heike Fingerle for ICP
analysis, Dr. Qian Song for TEM measurements, and Sascha
Wegner for assistance with NMR spectroscopy.
Figure 5. Proposed mechanism of the Pt-catalyzed cyclization of 5 to
6a.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00216.
■
sı
Experimental details, synthesis and characterization data
of 3 and 4a−c, equilibrium reactions, details of catalytic
reactions, XAS data, and modeling (PDF)
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AUTHOR INFORMATION
Corresponding Author
Deven P. Estes − Institute of Technical Chemistry, University of
Stuttgart, Stuttgart D-70569, Germany; orcid.org/0000-
0002-3215-3461; Email: deven.estes@itc.uni-stuttgart.de
■
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Authors
Sarah Maier − Institute of Technical Chemistry, University of
Stuttgart, Stuttgart D-70569, Germany
Steve P. Cronin − Institute of Technical Chemistry, University
of Stuttgart, Stuttgart D-70569, Germany; Present
Address: Department of Chemistry, Princeton University,
Princeton, New Jersey 08544, United States
Manh-Anh Vu Dinh − Institute of Technical Chemistry,
University of Stuttgart, Stuttgart D-70569, Germany
Zheng Li − Institute of Technical Chemistry, University of
Stuttgart, Stuttgart D-70569, Germany
Michael Dyballa − Institute of Technical Chemistry, University
of Stuttgart, Stuttgart D-70569, Germany; orcid.org/
0000-0002-8883-1145
Michal Nowakowski − Department of Chemistry, University of
Paderborn, Paderborn D-33098, Germany
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through Surface Organometallic Chemistry. Angew. Chem., Int. Ed.
2003, 42, 156. (b) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes,
Matthias Bauer − Department of Chemistry, University of
Paderborn, Paderborn D-33098, Germany; orcid.org/
0000-0002-9294-6076
́
D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Nunez-Zarur, F.; Zhizhko, P.
̃
Complete contact information is available at:
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