Synthesis, characterization and Baeyer-Villiger oxidation of ketones by the bis-cationic platinum(II) complex [Pt(μ-OH)(Pom-Pom)]2[BF4]2 [Pom-Pom = (OMe)2PCH2CH2P(OMe)2]

Article abstract of DOI:10.1016/j.molcata.2006.08.011

The synthesis and spectroscopic characterization of the new hydroxo-bridged platinum(II) complex [P(μ-OH)(Pom-Pom)]₂[BF₄]₂ (3) containing the electron-poor P-P ligand 1,2-bis(dimethoxyphosphino)ethane, (OMe)₂PCH₂CH₂P(OMe)₂ (Pom-Pom), is reported. This compound has been used in the Baeyer-Villiger oxidation of a series of cyclic ketones using 35% hydrogen peroxide as oxidant. The reactions were performed at 25 °C and at 70 °C in chlorinated solvent/H₂O biphasic system. The catalytic activity and the Lewis acidity of the complex (3) has been compared to other Pt(II) complexes previously investigated.

Full text of DOI:10.1016/j.molcata.2006.08.011

Journal of Molecular Catalysis A: Chemical 261 (2007) 202–206
Synthesis, characterization and Baeyer–Villiger oxidation of ketones by the
bis-cationic platinum(II) complex [Pt(␮-OH)(Pom-Pom)]2[BF4]2
[Pom-Pom = (OMe)2PCH2CH2P(OMe)2]
Paolo Sgarbossaa, Alessandro Scarsob, Erika Pizzoa, Silvia Mazzega Sbovataa,
Augusto Tassana, Rino A. Michelina,∗, Giorgio Strukulb,∗
a Dipartimento di Processi Chimici dell’Ingegneria, Universita` di Padova, Via F. Marzolo 9, 35131 Padua, Italy
b Dipartimento di Chimica, Universita` Ca’ Foscari di Venezia, Dorsoduro 2137, 30123 Venice, Italy
Received 26 May 2006; accepted 2 August 2006
Available online 8 September 2006
Abstract
The synthesis and spectroscopic characterization of the new hydroxo-bridged platinum(II) complex [P(␮-OH)(Pom-Pom)]2[BF4]2 (3) containing
the electron-poor P–P ligand 1,2-bis(dimethoxyphosphino)ethane, (OMe)2PCH2CH2P(OMe)2 (Pom-Pom), is reported. This compound has been
used in the Baeyer–Villiger oxidation of a series of cyclic ketones using 35% hydrogen peroxide as oxidant. The reactions were performed at 25 ◦C
and at 70 ◦C in chlorinated solvent/H2O biphasic system. The catalytic activity and the Lewis acidity of the complex (3) has been compared to
other Pt(II) complexes previously investigated.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Catalytic oxidation; Hydrogen peroxide; Electron-poor diphosphine; Platinum complex; Baeyer–Villiger
1. Introduction
the ligands coordinated to the metal [4]. The hydroxo-bridged
dimeric cationic compounds of the type {[Pt(␮-OH)(P–P)]2}
2+
Catalytic oxidation reactions are among the most useful
organic transformation [1], and analogously to other reactions,
key goals are productivity, selectivity and broad substrate scope
of the catalyst employed. Moreover, in the recent years oxi-
dation reactions are experiencing the shift from hazardous to
more environmentally friendly terminal oxidants. Among all,
hydrogen peroxide is emerging as a promising choice because
of its easy handling, relatively low cost and production of only
water as by-product [2]. The homogeneous oxidation of organic
substrates by hydrogen peroxide can be performed with sev-
eral transition metal complexes [3], in particular over the past
two decades some of us focused their attention to the employ-
ment of cationic Pd(II) and Pt(II) complexes due to the peculiar
Lewis acid character of such complexes that allows both nucle-
ophilic and electrophilic oxidation as a direct consequence of
(P–P = chelating diphosphine) have been found [4,5] to promote
the Baeyer–Villiger (BV) oxidation of cyclic and acyclic ketones
in the presence of H2O2 to give esters and lactones, respec-
tively. In a recent work [6] we have systematically changed
the electrophilicity of a homologous series of dimeric cationic
Pt(II) centres (and hence their Lewis acidity) by acting on
the degree of fluorine substitution of a series of tetraaryl
diphosphine ligands that were employed for the synthesis of
␮-hydroxo complexes of the type [Pt(␮-OH)(P–P)]2[BF4]2,
where P–P = (C6H5−nFn)2PCH2CH2P(C6H5−nFn)2 (n = 2–5)
(Scheme 1). As a result, we have observed a systematic increase
in activity in the catalytic Baeyer–Villiger oxidation of ketones
with hydrogen peroxide when the Lewis acidity of the metal
center increases.
Such behaviour, in agreement with a general observation
common to oxidation reactions and recently reviewed [7],
prompted us to explore the use of new Lewis acidic Pt(II)
complexes with the aim to develop more active and selective cat-
alysts. In this work we report the synthesis and characterization
of the complex [Pt(␮-OH)(Pom-Pom)]2[BF4]2 containing the
∗
Corresponding authors. Tel. +39 049 8275522; fax: +39 049 8275525
(R.A. Michelin); Tel.: +39 041 2348931; fax: +39 041 2348517 (G. Strukul).
E-mail addresses: rino.michelin@unipd.it (R.A. Michelin),
strukul@unive.it (G. Strukul).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.08.011

P. Sgarbossa et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 202–206
203
sidearm fitted with a screw-capped silicone septum to allow
sampling. Stirring was performed by a Teflon-coated bar driven
externally by a magnetic stirrer. Constant temperature (20 0.1
or 70 0.1 ◦C) was maintained by water circulation through an
external jacket connected with a thermostat. The concentration
of the commercial 35% H2O2 solution was checked iodometri-
cally prior to use. The following general procedure was used: the
solid catalyst (0.017 mmol) was placed in the reactor, which was
evacuated and filled with N2 and then 1,2-dichloroethane (3 mL)
was added under N2 flow, followed by purified N2-saturated
substrate (1.7 mmol). After termostating at the required temper-
ature for a few minutes, the H2O2 solution in the appropriate
amount (1.7 mmol) was injected through the septum and time
was started. All reactions were monitored with GLC by direct
injection of samples taken periodically from the reaction mix-
tures with a microsyringe. Prior quenching of the samples by
adding an excess of LiCl was found to be unnecessary. Initial
rate data were determined from conversion vs time plots. Sepa-
ration of the products was performed on a 25 m HP-5 capillary
column using a flame ionization detector.
Scheme 1. Baeyer–Villiger oxidation of cyclic ketones with H2O2 catalyzed by
electron-poor fluorinated dimeric Pt(II) complexes.
Scheme 2. Dimeric structure of the electron-poor complex [Pt(␮-OH)(Pom-
Pom)]2[BF4]2 (3).
electron-poor diphosphine 1,2-bis(dimethoxyphosphino)eth-
ane, (OMe)2PCH2CH2P(OMe)2 (Pom-Pom) (Scheme 2). The
catalytic activity of the above reported derivative was tested in
theBaeyer–Villigeroxidationofcyclicandacyclicketonesusing
hydrogen peroxide as oxidant and its activity and Lewis acidity
discussed.
2.3. Synthesis of [PtCl2(Pom-Pom)] (1)
[PtCl2(COD)] (2.03 g, 5.42 mmol) was suspended in chloro-
form (100 mL) at room temperature and subsequently treated
with Pom-Pom (1.16 mL, 5.42 mmol) under nitrogen. The reac-
tion mixture was stirred until complete dissolution of the solid.
Thesolutionwasthenconcentratedto10 mLunderreducedpres-
sure and diethyl ether was added. The white solid precipitate was
filtered off, washed with diethyl ether and dried under vacuum.
Yield 1.79 g, 69%. Anal. calc. for C6H16Cl2O4P2Pt: C, 15.01;
H, 3.36. Found: C, 15.03; H, 3.11%. IR (ν¯, PE): 311, 290 (s, Pt-
Cl). 1H NMR (δ, (CD3)2CO): 4.04 (d, OCH3, 3JP–H = 13.0 Hz);
2. Experimental
2.1. General procedures and materials
All work was carried out with the exclusion of atmospheric
oxygen under a dinitrogen atmosphere using standard Schlenck
techniques. Solvents were dried and purified according to stan-
dard methods. Substrates were purified by passing through neu-
tral alumina and stored in the dark at low temperature. Hydrogen
peroxide (35% Fluka), HBF4 54% in diethyl ether (Aldrich) and
all the ketone substrates were commercial products and used
without purification. IR spectra were taken on a FT-IR AVATAR
320 of the Nicolet Instrument Corporation spectrophotometer
in CH2Cl2 solution using CaF2 windows; the wavenumbers are
1
1.84 (m, PCH2). 31P{ H} NMR (δ, (CD3)2CO): 136.4 (s,
1JPt–P = 4645 Hz).
2.4. Synthesis of [PdCl2(Pom-Pom)] (2)
A suspension of [PdCl2(MeCN)2] (0.31 g, 1.19 mmol) in
chloroform (50 mL) at room temperature was treated with Pom-
Pom (0.27 mL, 1.25 mmol) under nitrogen. The reaction mixture
was stirred overnight until complete dissolution of the solid
to afford a yellow solution, which was concentrated to ca.
5 mL under reduced pressure and diethyl ether was added. The
white solid precipitate was filtered off, washed with diethyl
ether and dried under vacuum. Yield 0.46 g, 98%. Anal. calc.
for C6H16Cl2O4P2Pd: C, 18.41; H, 4.42. Found: C, 18.70; H,
3.81%. IR (ν¯, cm−1, PE): 317, 289 (s, Pd-Cl); 1060 (s, BF4). 1H
1
given in cm−1. 1H NMR and 31P{ H} spectra were run at 298 K,
unless otherwise stated, on a Bruker AC200 spectrometer oper-
ating at 200.13 and 81.015 MHz, respectively; δ values in ppm
are relative to SiMe4 and 85% H3PO4. GLC measurement were
taken on a Hewlett-Packard 5890A gas chromatograph equipped
with a FID detector (gas carrier He). Identification of products
was made with GLC by comparison with authentic samples.
The elemental analyses were performed by the Department of
Analytical, Inorganic and Organometallic Chemistry of the Uni-
3
NMR (δ, CD2Cl2): 4.10 (d, OCH3, JP–H = 13.0 Hz); 1.92 (m,
1
PCH2). 31P{ H} NMR (δ, CD2Cl2): 165.9 (s).
´
versita di Padova. The compounds Pom-Pom [8], [PtCl2(COD)]
[9] (COD 1,5-cyclooctadiene) and [PdCl2(CH3CN)2] [10] were
prepared according to published procedures.
2.5. Synthesis of [Pt(μ-OH)(Pom-Pom)]2[BF4]2 (3)
To a suspension of 1 (0.30 g, 0.62 mmol) in methanol
(15 mL) at room temperature was added 1.46 mL (1.25 mmol)
of a 0.9 M acetone solution of AgBF4. The reaction mixture
was stirred under nitrogen for 3 h and then the solid AgCl
formed was filtered off. After concentration, the solution was
2.2. Kinetic studies
These were carried out in a 10 mL round-bottomed flask
equipped with a stopcock for vacuum/N2 operations and a

204
P. Sgarbossa et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 202–206
treated with diethyl ether to give a white solid, which was fil-
tered off and dried under vacuum. Yield 0.30 g, 94.0%. Anal.
calc. for C12H34B2F4O10P4Pt2: C, 14.05; H, 3.34. Found: C,
14.00; H, 3.11%. IR (ν¯, cm−1, Nujol): 3500 (s, OH); 1060
1
(s, BF4). H NMR (δ, CD2Cl2): 4.14 (s, OH); 3.90 (d, OCH3,
3JP–H = 12.5 Hz); 1.92 (m, CH2). 31P{ H} NMR (δ, CD2Cl2):
1
Scheme 4. Preparation of the isocyanide complex (4).
123.5 (s, 1JPt–P = 4762 Hz).
The complexes (1)–(3) are new compounds and they have
been characterized by IR, multinuclear NMR and elemental
analysis. TheIRspectraof(1)and(2)showtypicalM–Clstretch-
2.6. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)
(Pom-Pom)][BF4] (4)
ings around 300 cm−1. The 31P{ H} NMR spectrum of (1)
1
To a suspension of 1 (0.48 g, 1.00 mmol) in dichloromethane
(70 mL) was added, under stirring, 1.25 mL (1.07 mmol) of an
acetone solution of AgBF4. The reaction mixture was stirred
under nitrogen for 30 min and then the solid AgCl formed was
filtered off. To the solution was added 2,6-dimethylphenyl iso-
cyanide (0.11 g, 0.83 mmol) and stirred for 5 min. It was then
concentrated under vacuum and diethyl ether was added. The
yellowish solid precipitated was filtered off, washed with diethyl
ether and dried under vacuum. Yield 0.49 g, 74%. Anal. calc. for
C15H25BClF4NO4P2Pt: C, 27.19; H, 3.80; N, 2.11%. Found:
C, 27.81; H, 3.60; N, 2.48%. IR (ν¯, cm−1, CH2Cl2): 2219 (s,
shows a singlet at 136.4 ppm flanked by 195Pt satellites (1JP–Pt
4645 Hz), while (2) shows the 31P resonance at 165.9 ppm. The
IR spectrum of (3) shows a medium-intensity O–H stretching
band at 3500 cm−1 and a strong broad band at ca. 1060 cm−1
1
typical of the BF4 group. The 31P{ H} NMR spectrum gives
rise to a singlet at δ 123.5 ppm (1JP–Pt 4762 Hz). In the 1H NMR
spectrum in anhydrous CD2Cl2 the OH resonance was observed
at 4.14 ppm, a value which is a clear indication of the bridg-
ing nature of the anionic ligand [11a]. The high values of the
Pt–P coupling constants of (1) and (3) with respect to those
observed for alkyl and aryl-substituted diphosphines, which are
reported to be ca. 1000 Hz lower [12], may be explained with
the presence of the strong electron-withdrawing methoxy groups
directly bonded to the P atoms giving rise to low trans-influence.
Thus, for instance, the complex cis-[PtCl2{P(OPh)3}] contain-
ing the even more electron-withdrawing phenoxy groups, shows
the 1JPt–P of 5793 Hz [13].
1
C N). IR (ν¯, cm−1, PE): 319 (s, Pt-Cl). H NMR (δ, CDCl3):
3
7.4–7.0 (m, Ph); 3.98, 4.08 (d, OCH3, JP–H = 12.7 Hz); 2.45
(m, PCH2); 2.50 (s, CH3). 31P{ H} NMR (δ, CDCl3): 138.1 (d,
1
1
2
PCl-trans, JPt–P = 4253 Hz, JP–P = 13.4 Hz); 144.4 (d, PC-trans
1JPt–P = 3830 Hz, 2JP–P = 13.4 Hz).
,
The isocyanide complex (4) was specifically prepared in
order to correlate the catalytic activity of (3) with the Lewis
acidity of the metal center arising from the withdrawing ability
of the coordinated electron-poor diphosphine. The (4) was pre-
pared as previously described for similar isocyanide complexes
[6] of the type [PtCl(C N–R)(P–P)][BF4] (R = 2,6–(Me)2C6H3;
P–P = variousfluorinateddiphosphines)byinitialhalideabstrac-
tion from (1) with an equivalent amount of AgBF4 in CH2Cl2
and subsequent treatment with a stoichiometric amount of the
2,6-dimethylphenyl isocyanide ligand (Scheme 4).
It is known [14] that the value of the C N stretching (or
the wavenumber shift ꢀν¯ = (ν¯(C N))coord − ν¯(C N)free of a
transition metal coordinated isocyanide gives information about
the electrophilicity of the isocyanide carbon atom, which in
turn is related to the electron density and hence to the Lewis
acidity of the metal center. Thus, it is expected that a larger
electron-withdrawing effect by the metal will give rise to a larger
wavenumber shift. The general assumption is confirmed by the
ꢀν¯ observed for (4) (92 cm−1) which is a rather high value,
comparable to that observed for partially fluorinated aryl diphos-
phine complexes with the same general dimeric structure [6],
and generally higher then analogous complexes bearing normal
alkyl diphosphine ligands [15]. This electronic effect positively
influences the catalytic activity, as reported later on.
3. Results and discussion
3.1. Synthesis of the complexes
The dichloro complexes [MCl2(Pom-Pom)] (M = Pt (1); Pd
(2)) were readily prepared by ligand exchange from the precur-
sors [MCl2L2] as illustrated in Scheme 3. The Pt(II) bridging
hydroxo dimer [Pt(␮-OH)(Pom-Pom)]2[BF4]2 (3) was prepared
in high yield according to a general method reported in the liter-
ature [11] and previously experienced in the preparation of the
catalysts for the Baeyer–Villiger oxidation of cyclic and acyclic
ketones [5], which involves chloride abstraction from 1 by treat-
ment with AgBF4 in wet methanol/acetone solution.
The analogous reaction starting from the Pd(II) derivative (2)
did not give the corresponding ␮-hydroxo complex, but instead
the formation of black metallic decomposition products was
observed.
3.2. Catalytic studies
Catalytic studies were performed in the oxidation of a series
of cyclic and acyclic ketones with commercial 35% hydrogen
Scheme 3. Synthetic pathway for the synthesis of the catalyst (3).

P. Sgarbossa et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 202–206
205
far the most reactive substrate. The electron-withdrawing effect
of the oxygen atoms on the Pt(II) metal center exerts a posi-
tive effect on the catalytic activity analogously to what observed
with aryl diphosphines [4]. In fact the productivity towards all
the substrates for complex (3) is more than twice as much as
that of the isosteric but less electron-poor complex [depePt(␮-
OH)]2(BF4)2 (depe = bis-diethylphosphino ethane) [15]. Fig. 1
shows that the leveling off in conversion is practically complete
after about 100 min at 25 ◦C and about 60 min at 70 ◦C. Some
randomly performed analyses of the residual hydrogen peroxide
in the final reaction mixture indicate that the consumed oxi-
dant corresponds to the maximum conversion observed. In other
words, there are no side reactions leading to consumption of the
oxidant. These observations imply that reactions stop because
of catalyst deactivation.
Fig. 1. Oxidation of ketones catalyzed by [Pt(␮-OH)(Pom-Pom)]2[BF4]2 (3)
with 35% hydrogen peroxide. Reaction conditions: Pt, 0.017 mmol; ketone,
1.7 mmol; H2O2, 1.7 mmol; 1,2-dichloroethane as solvent, 3 mL; T, 25 ◦C.
1
In order to check this point, a 1H and 31P{ H} NMR study
was performed with the aim of investigating the mechanism
by detecting the active species involved in the catalytic pro-
cess. The complex [Pt(␮-OH)(Pom-Pom)]2[BF4]2 (3) (8.6 mg)
1
was dissolved in CD2Cl2 and the 31P{ H} NMR spectrum was
peroxide using (3) as catalyst. The reactions were carried out
in two-phase solvent systems (1,2-dichloroethane/water) at two
different temperatures (25 and 70 ◦C). Some typical reaction
profiles (% conversion versus time) are reported in Fig. 1, while
initial rates and conversions are collected in Table 1.
run showing a singlet at 123.5 ppm flanked by 195Pt satellites
with a 1JP–Pt coupling constant of 4762 Hz. Addition of 91 ␮L
1
of 2-methylcyclohexanone did not change the 31P{ H} NMR
spectrum, while in the 1H NMR spectrum a series of multiplets
at 2.39–1.38 ppm (CH2) and a doublet at 1.022 ppm (CH3) of
the ketone were observed. Further addition of 73 ␮L 35% hydro-
gen peroxide (Pt:ketone:H2O2 = 5:100:100) produced right after
Fig. 1showsthatreactionsgenerallybeganwithrelativelyfast
rates but tend to level off rather quickly. Reactions performed
at 25 ◦C were generally finished in about 200 min, with yield
from moderate to good. The only acyclic ketone 3,3-dimethyl-2-
butanone (pinacolone) was not oxidized, while for 2,6-dimethyl-
cyclohexanone, the steric hindrance of the substrate hampered
the oxidation and reaction was sluggish under the experimental
conditions employed. Increasing the reaction temperature logi-
cally resulted in a significant increase in both the initial rate and
the maximum conversion. At 70 ◦C reactions may be controlled
by the diffusion of H2O2 from the aqueous to the organic phase,
as was found for example in the case of 2-methylcyclohexanone
and 2-methylcyclopentanone or in the case of cyclobutanone,
even at 25 ◦C. The latter ketone showed very high conversions
and rates, but this is not surprising as ring strain makes it by
a new species, which gives rise in the 31P{ H} NMR spec-
1
trum a downfield resonance at 130.6 ppm with a 1JP–Pt coupling
1
constant of 4572 Hz. On the other hand, the H NMR spec-
trum showed the presence of a growing multiplet at 4.42 ppm
(OCH–Me) for the product lactone and a new doublet at
1.15 ppm (OCH–CH3). After 140 min the singlet present in the
1
31P{ H} NMR disappeared as well as the signals of the catalyst
at 1.92 ppm (m, CH2) and 3.96 ppm (d, OCH3, 3JPt–H = 12.8 Hz)
1
in H NMR spectrum and the water phase assumed a yellow
color. ThisspeaksfortheprogressivetransitionofthePt(II)com-
plex from the organic to the aqueous phase, aided by the presence
of oxygen atoms in the ligand. This was confirmed by running
1
a 31P{ H} spectrum of the surnatant aqueous phase, after addi-
tion of a small volume of D2O, that showed initially the presence
of a singlet at 119 ppm flanked by satellites signals with 1JP–Pt
5120 Hz attributed to [Pt(␮-OH)(Pom-Pom)]2[BF4]2 dissolved
in water, together with a small singlet at 34 ppm with no satel-
lite signals, attributed to the free oxidated ligand Pom-PomO2
by comparison with an authentic sample prepared by direct oxi-
dation of the ligand with H2O2. Over the time, the latter signal
increases and that of the Pt complex decreases as indication of
degradation of the catalyst by ligand oxidation.
Table 1
Summary of the kinetic data for the oxidation of ketones at 25 and 70 ◦C with
35% hydrogen peroxide catalyzed by [Pt(␮-OH)(Pom-Pom)]2[BF4]2 (3)
Ketone
Temperature
(◦C)
Initial rate
(×105 M s−1
Conversion
(%)
)
25
70
0.03
1.3
1
2
2,6-Dimethylcyclohexanone
2-Methylcyclohexanone
25
70
2.2
22
16
29
In summary, the presence of the oxidant in solution trans-
forms the dimeric complex into a new symmetric species with
equivalent P atoms, but still nothing can be said at present
about the monomeric or dimeric nature of such species. Nev-
ertheless, the decrease in the 1JP–Pt coupling constant is indica-
tive of a slightly higher trans-effect compared to the original
␮-OH ligand, which could be at first interpreted by the for-
mation of a symmetric monomeric or dimeric species bearing
25
70
1.7
7.8
4
14
2-Methylcyclopentanone
Cyclobutanone
25
24
95
25
70
–
–
0
0
Pinacolone
Experimental conditions: Pt, 0.017 mmol; ketone, 1.7 mmol; H2O2, 1.7 mmol;
1,2-dichloroethane as solvent, 3 mL.

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P. Sgarbossa et al. / Journal of Molecular Catalysis A: Chemical 261 (2007) 202–206
anionic HOO− ligands. During the reaction progress, such active
species migrates into the aqueous phase because of the intrinsic
hydrophilic nature of the ligand, while concomitant degrada-
tion of the catalyst by oxidation of Pom-Pom to phosphonic
methyl ester causes the progressive inactivation of the cata-
lyst and brings the reaction to stop. The electronic properties
of the complex seems peculiar for the nucleophilic oxidation
of ketones to lactones, in fact, attempts to perform epoxidation,
which is a typical electrophilic oxidation, of cycloctene with
catalyst (3) and 35% hydrogen peroxide at 25 and 70 ◦C were
unsuccessful.
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RAM and GS thank MIUR, the University of Padova and
University of Venice for financial support (PRIN and COFIN
2003). RAM thanks CIRCC (Consorzio Interuniversitario Reat-
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`
tivita Chimica e Catalisi) for a fellowship to EP. GS wishes to
[15] A. Scarso, P. Sgarbossa, R.A. Michelin, G. Strukul, unpublished results.
thank Johnson-Matthey for the loan of platinum.