Kinetics and mechanism of cleavage of the oxygen-oxygen bond in hydrogen peroxide and dibenzoyl peroxide by arylplatinum(II) complexes

Article abstract of DOI:10.1039/b103690b

Reaction of [PtAr₂(NN)] (Ar = Ph, p-MeC₆H₄,m-MeC₆H₄ or p-MeOC₆H₄; NN = 2,2′-bipyridyl or 1,10- phenanthroline) with R₂O₂, in which R = H or C(=O)Ph, gives the platinum(IV) complexes cis,trans- [PtAr₂(OH)₂(NN)] or [PtAr₂(OCOPh)₂(NN)] (as a 5:1 mixture of cis,trans and cis,cis isomers), respectively. The kinetics of these oxidative addition reactions in acetone was followed by visible spectrophotometry between 10 and 40°C. The reactions followed simple second-order kinetics, rate = k₂[PtAr₂(NN)][peroxide], and the rates were insensitive to change of solvent. Activation parameters have been determined and large negative values for ΔS≠ were obtained. A mechanism is suggested which involves a concerted three-center transition state, followed by homolytic cleavage of the O-O bond and formation of thefinal products.

Full text of DOI:10.1039/b103690b

View Article Online / Journal Homepage / Table of Contents for this issue
Kinetics and mechanism of cleavage of the oxygen–oxygen bond in
hydrogen peroxide and dibenzoyl peroxide by arylplatinum(II)
complexes
Mehdi Rashidi,* Masoud Nabavizadeh, Rakhshan Hakimelahi and Sirous Jamali
Chemistry Department, College of Sciences, Shiraz University, Shiraz, Iran 71454.
E-mail: rashidi@chem.susc.ac.ir
Received 24th April 2001, Accepted 2nd October 2001
First published as an Advance Article on the web 7th November 2001
Reaction of [PtAr2(NN)] (Ar = Ph, p-MeC6H4, m-MeC6H4 or p-MeOC6H4; NN = 2,2Ј-bipyridyl or 1,10-
phenanthroline) with R O , in which R = H or C(᎐O)Ph, gives the platinum() complexes cis,trans-
᎐
2
2
[PtAr2(OH)2(NN)] or [PtAr2(OCOPh)2(NN)] (as a 5 : 1 mixture of cis,trans and cis,cis isomers), respectively. The
kinetics of these oxidative addition reactions in acetone was followed by visible spectrophotometry between 10
and 40 ЊC. The reactions followed simple second-order kinetics, rate = k2[PtAr2(NN)][peroxide], and the rates
were insensitive to change of solvent. Activation parameters have been determined and large negative values
for ∆S≠ were obtained. A mechanism is suggested which involves a concerted three-center transition state,
followed by homolytic cleavage of the O–O bond and formation of the final products.
mechanistic study of the oxidative addition of this basic
Introduction
reagent, or any other O–O bond containing reagents, to plat-
inum complexes. We found that H2O2 and dibenzoyl peroxide
reacted with the arylplatinum() complexes with rates that
could easily be followed spectrophotometrically.
There has been a great interest in antitumor activity of plat-
inum() complexes, e.g. iproplatin ([Pt(ipa)(OH)2Cl2], ipa =
isopropylamine).1–11 In this connection, carboxylation of
kinetically inert dihydroxoplatinum() complexes with differ-
ent reagents such as acid anhydrides and acyl chlorides to form
bis(carboxylato)platinum() compounds which have useful
antitumor activity has recently been of increasing interest.8–11
One important route to dihydroxoplatinum() complexes is by
the oxidative addition of hydrogen peroxide to platinum()
complexes. Thus, for example trans oxidative addition of
an HO–OH bond to [PtMe2(phen)] or [Pt(C6F5)2(en)] gave
[PtMe2(OH)2(phen)]12 or [Pt(C6F5)2(OH)2(en)],10 respectively.
The oxygen–oxygen bond, along with other Group 16–
Group 16 bonds (e.g. RSe–SeR), should be considered as class
A (non-polar or of low polarity) substrate in the oxidative
addition reactions.13,14 Although there are three distinct mech-
anisms that are proposed for the activation of class A reagents,
a cis-concerted mechanism is the dominant one. The mech-
anistic studies of class A oxidative addition substrates are
mostly concerned with H–H, C–H or C–C bond activation.
Oxidative addition of some S–S or Te–Te bonds to Vaska’s
compound, [IrCl(CO)(PPh3)2], has been kinetically studied and
in each case a mechanism has been suggested.15,16
Results and discussion
Synthesis and characterization of the arylplatinum(IV) complexes
The yellow complexes [PtAr2(NN)] reacted cleanly with an
excess of peroxides R O (R = H or C(᎐O)Ph) in acetone at
᎐
2
2
room temperature to form very pale yellow solutions, from
which air-stable solid products having the general formula
[PtAr2(OR)2(NN)] were obtained. When R = H, the products
of each reaction contained almost exclusively the cis,trans
isomer, and only very small traces of the cis,cis isomer were in
some cases observed. However, when R = C(᎐O)Ph, a mixture of
᎐
both isomers was formed and the ratio of the isomers (cis,trans :
cis,cis), which could not be separated, was approximately 5 : 1.
The complexes were characterized by 1H NMR spectroscopy,
microanalysis and IR spectroscopy (see Experimental section
for necessary details).
1
The H NMR spectrum of [Pt(p-MeC6H4)2(OH)2(phen)] is
3
representative of all the products. The value of J(PtH2) (H2 is
This article describes a kinetic study of the oxidative addition
of hydrogen peroxide to platinum() complexes [PtAr2(NN)]-
(Ar = Ph, p-MeC6H4, m-MeC6H4 or p-MeOC6H4; NN = 2,2Ј-
bipyridyl (bpy) or 1,10-phenanthroline (phen)). A comparative
kinetic study is also performed with dibenzoyl peroxide and a
mechanism is suggested.
The oxidative addition of the electron-rich complexes [PtMe2-
(NN)] with a wide variety of reagents has been studied.14 Also,
the oxidative addition of some alkyl halides to the metalla-
cycle analog [Pt{(CH2)4}(NN)] has recently been reported.17,18
Due to the electronic and steric problems, the related study on
the aryl analogs [PtAr2(NN)] has been limited to reaction with
MeI19 for which the reactions are some 50–1000 times slower
than the corresponding reactions with dialkylplatinum() com-
plexes. However, despite the importance of hydrogen peroxide
in chemistry, the literature contains no report of any kinetic and
an ortho proton of phen) in this platinum() complex is 11.4
Hz, which is considerably lower than the corresponding value
of 18.6 Hz for the starting platinum() complex [Pt(p-Me-
C6H4)2(phen)]. The ortho protons of the aryl ligands (i.e. Ho)
appeared as only one doublet (due to coupling to Hm with
3J(HoHm) = 7.50 Hz) signal. The presence of only one signal
indicates that the complex possesses a plane of symmetry as
3
expected for a product of trans addition. Again, the J(PtHo)
value of 35.20 Hz in this platinum() complex is much lower
than the corresponding value of 68.75 Hz for the starting plat-
inum() complex. The Hm protons also appeared as only one
3
doublet (with J(HoHm) = 7.50 Hz) which further confirms the
existence of a plane of symmetry. Also only one singlet was
observed at δ 2.39 for the Me substituents on the Ar ligands. A
broad resonance at δ 1.23 was assigned to the OH groups, since
addition of D2O led to loss of this peak and appearance of a
3430
J. Chem. Soc., Dalton Trans., 2001, 3430–3434
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103690b

View Article Online
Fig. 1 Changes in the UV–Vis spectrum during the reaction of [Pt-
(p-MeOC6H4)2(bpy)] (4.5 × 10Ϫ4M) and H2O2 (0.24M) in acetone at
T = 20 ЊC; (a) initial spectrum (before adding H2O2) and (b) spectrum
at t = 0; successive spectra were recorded at intervals of 30 s.
Fig. 2 First-order plots for the reaction of [Pt(p-MeC6H4)2(NN)]
with peroxides at different temperatures in acetone: (a) H2O2 (0.21 M),
T = 10 ЊC, NN = bpy; (b) H2O2 (0.21 M), T = 30 ЊC, NN = phen;
(c) dibenzoyl peroxide (0.021 M), T = 20 ЊC, NN = bpy; (d) dibenzoyl
peroxide (0.024 M), T = 40 ЊC, NN = phen.
peak due to HOD at δ 4.78. Two very small peaks at δ 9.56 and
9.87 were tentatively assigned to the two non-equivalent H2 pro-
tons of the very small trace of the cis,cis isomer. The complex
also has a strong ν(OH) band in the IR spectrum at 3570 cmϪ1
.
In the 1H NMR spectrum of [Pt(p-MeOC6H4)2(OCOPh)2-
(phen)] similar assignments were made to confirm the existence
of a plane of symmetry in the cis,trans isomer. In addition,
three multiplets at δ 8.12 (Hp) and 7.33 and 7.80 (Hm and Ho)
were observed for the Ph groups of OC(᎐O)Ph ligands. Two
᎐
weaker signals were observed at δ 9.52 and δ 10.20 due to the
two non-equivalent H2 protons of the cis,cis minor isomer. For
this isomer, two multiplets at δ 7.83 and 7.22 were also clearly
observed for non-equivalent meta protons of Ar ligands. A
singlet at δ 3.85 was assigned to Me substituents of Ar ligands
in the cis,trans isomer, while for the cis,cis isomer the two non-
equivalent Me substituents were observed at δ 3.61 and 3.52.
Two bands in the IR spectrum of the complex at 1650 cmϪ1
(νasym(CO2)) and 1280 cmϪ1 (νsym(CO2)) are assigned to the
carboxylate coordination (see ref. 10).
Fig. 3 Plots of first-order rate constants (kobs/sϪ1) for the reaction
of [Pt(p-MeC6H4)2(phen)] with H2O2 (᭿, T = 10 ЊC; ᭝, T = 20 ЊC; ꢀ,
T = 30 ЊC; ×, T = 40 ЊC) and dibenzoyl peroxide (᭺, T = 10 ЊC; ᮀ,
T = 40 ЊC) in acetone at different temperatures versus concentration of
peroxide.
The kinetic study
The kinetics of oxidative addition of hydrogen peroxide in
acetone (or a 2 : 1 mixture of acetone–benzene) or dibenzoyl
peroxide in acetone (or benzene) to [PtAr2(NN)], was studied
by using UV–vis spectroscopy. In each case, an excess of per-
oxide was used and disappearance of the MLCT band at the
corresponding λmax (see the Experimental section) was used
to monitor the reaction. The change in the spectrum during a
typical run is shown in Fig. 1.
The reactions followed good first-order kinetics (Fig. 2). Plots
of these first-order rate constants against concentration of
the peroxides gave good straight line plots passing through the
origin, showing a first-order dependence of the rate on the con-
centration of peroxides (Fig. 3). The results indicate that the
reactions obey a simple second-order rate law [eqn. (1)].
in which the transition state involves the formation of a cationic
intermediate is not considered. Also, given that these reactions
follow good second-order kinetics with remarkable reproduc-
ibility and the fact that radical scavengers did not affect the rate
would rule out the possibility of any radical mechanism.
We therefore suggest that a concerted three-centered tran-
sition state is involved in each of the reactions (Scheme 1). This
association is not only consistent with a large negative ∆S≠
value, but also with the simple second-order rate law given by
eqn. (1).
Goddard and Low have presented an elegant ab initio
model20–22 for oxidative addition processes in which plat-
inum() or platinum() is considered as a neutral atom with a
d9s1 or d8s2 electronic configuration, respectively. According to
this model, the ligands in platinum() should significantly distort
from square-planar geometry at the transition state to allow the
electronic promotion required for the availability of cis
coordination sites and formation of the six-coordinate transi-
tion state (Scheme 1) and eventually the formation of two, new
covalent bonds (two Pt–OR) in the six-coordinate platinum()
Ϫd[PtAr2(NN)]/dt = k2[PtAr2(NN)][peroxide]
(1)
The activation parameters were also determined from
measurement at different temperatures, and the data are given
in Table 1.
The large negative value of ∆S≠ in each reaction indicates
that an association has taken place in the transition state. How-
ever, examination of the second-order rate constants upon
changing the solvent (from acetone to benzene for the dibenz-
oyl peroxide reaction and from acetone to a 2 : 1 mixture of
acetone–benzene for the hydrogen peroxide reaction) shows
that the rate of reactions of Pt() complexes with peroxides are
not significantly sensitive to a fairly large change in the polarity
of the solvent. Thus the possibility of an SN2-type mechanism
product. Goddard and Low have estimated the Pt(d9s1
d8s2)
transition for the free atom to be endothermic by 17.2 kcal
molϪ1. If this is taken as an approximate value, then the ∆H≠
values of approximately 40–50 kJ molϪ1 (≈10–13 kcal molϪ1
)
for the reactions in Scheme 1 are probably consistent with the
J. Chem. Soc., Dalton Trans., 2001, 3430–3434 3431

View Article Online
Table 1 Second-order rate constants and activation parametersa for reaction of [PtAr2(NN)] with the peroxides (H2O2 or (O2CPh)2) in acetoneb
Reaction with H2O2
103k2 (10 ЊC)/
103k2 (20 ЊC)/
103k2 (30 ЊC)/
103k2 (40 ЊC)/
∆S≠ c/J KϪ1
Complex
L molϪ1 sϪ1
L molϪ1 sϪ1
L molϪ1 sϪ1
L molϪ1 sϪ1
∆H≠/kJ molϪ1
molϪ1
[PtPh2(bpy)]
7.8 0.1
12.2 0.1
15.7 0.8
11.2 0.1
12.8 0.1
17.0 0.1
[20.4 0.5]
19.4 0.1
27.5 0.4
34.5 0.3
24.4 0.3
29.1 0.3
35.5 0.5
[40.3 0.3]
35.5 0.2
53.5 0.3
59.2 0.6
47.2 0.3
55.6 0.4
60.6 0.6
[79.9 0.5]
73.7 0.9
90.9 1.1
99.8 0.7
89.0 0.6
94.0 0.8
102.3 0.9
[130.5 0.4]
51.7 2.4
46.9 2.2
42.5 2.5
48.3 7.8
46.5 2.4
41.2 1.8
[43.7 1.6]
Ϫ102
8
7
8
3
8
6
[Pt(p-MeC6H4)2(bpy)]
[Pt(p-MeC6H4)2(phen)]
[Pt(m-MeC6H4)2(bpy)]
[Pt(p-MeOC6H4)2(bpy)]
[Pt(p-MeOC6H4)2(phen)]
Ϫ115
Ϫ128
Ϫ111
Ϫ116
Ϫ132
[Ϫ122 5]
Reaction with (O2CPh)2
Complex
104k2 (10 ЊC)/
104k2 (20 ЊC)/
104k2 (30 ЊC)/
104k2 (40 ЊC)/
∆S≠ c/J KϪ1
L molϪ1 sϪ1
L molϪ1 sϪ1
L molϪ1 sϪ1
L molϪ1 sϪ1
∆H≠/kJ molϪ1
molϪ1
[PtPh2(bpy)]
[Pt(p-MeC6H4)2(bpy)]
7.5 0.1
11.4 0.1
(15.8 0.1)
19.7 0.1
10.8 0.3
13.7 0.1
(18.2 0.2)
21.4 0.4
(27.3 0.1)
14.6 0.1
23.3 0.1
(28.6 0.1)
36.3 0.7
21.9 0.6
25.6 0.2
(32.4 0.2)
39.5 0.9
(48.9 0.5)
28.6 0.1
40.5 0.2
(46.2 0.3)
59.1 0.8
38.9 0.2
43.2 0.2
(54.6 0.4)
61.4 2.9
(79.6 0.5)
47.5 0.3
64.6 0.5
(71.6 0.3)
99.8 0.2
63.7 0.4
69.1 0.6
(90.2 0.5)
101.4 3.1
(118.9 0.5)
43.3 1.3
40.2 2.1
(34.5 1.1)
37.0 0.8
41.2 1.6
37.2 1.0
(36.9 0.2)
35.2 4.4
(33.7 1.4)
Ϫ113
Ϫ120
(Ϫ138 4)
Ϫ128
Ϫ117
Ϫ129
(Ϫ128 1)
Ϫ133
(Ϫ136 5)
4
7
[Pt(p-MeC6H4)2(phen)]
[Pt(m-MeC6H4)2(bpy)]
[Pt(p-MeOC6H4)2(bpy)]
3
5
3
[Pt(p-MeOC6H4)2(phen)]
8
a Values given based on 95% confidence limits from least-square regression analysis. b Values in square brackets are from a 2 : 1 mixture of acetone–
benzene and the values in parentheses are in benzene. c Obtained from the Arrhenius equation.
to the ROOR reagents used in the present study, one might
predict that the bonding in dibenzoyl peroxide should have a
significant contribution from resonance form III, due to the
electron withdrawing property of C(᎐O)Ph groups. It is thus
᎐
not surprising to observed that dibenzoyl peroxide reacts nearly
10 times faster than hydrogen peroxide with the platinum()
complexes, if we consider that association of platinum()
complexes with peroxides to form the three-centered transition
state via overlapping of the corresponding orbitals proceed by
symmetrical platinum attack into the oxygen–oxygen bond.
An increase in rate of the reactions of dibenzoyl peroxide on
going from acetone solvent to the non-polar solvent benzene,
although not great (in fact by a factor of about 1.3), could also
be interpreted by this model.
The rates increase slightly with electron-donating substi-
tuents on the phenyl ligands (by a factor of about 1.5 from Ph
to p-MeOC6H4), probably due to an increased energy of the
filled d orbitals which makes the transition Pt(d9s1
d8s2)
easier. In the reaction of MeI with [PtAr2(bpy)] complexes19
which proceeds by an SN2-type mechanism by a nucleophilic
attack of platinum() on the carbon atom of the substrate to
afford the transient cationic platinum() intermediate, such a
rate increase is more pronounced (by a factor of nearly 4 by
changing the organic ligand from Ph to p-MeOC6H4). Along
the same lines, a slight increase in the rate is observed on going
from bpy to phen in the arylplatinum() complexes.
One last point remains regarding the stereochemistry of the
products. Goddard and Low22 have pointed out that in Pt()
(d8s2), electronegative ligands favor bonding to the sp hybride
orbital and thus expect that in Pt(Cl)2(CH3)2(PH3)2 the Pt–Cl
bonding favors Pt sp and Pt–CH3 should favor Pt d, and there-
fore bonding both Cl to sp orbitals should favor trans Cl
ligands. Consistent with this, it may not be surprising to observe
that the dihydroxy platinum() complexes in the present study
have been formed with an almost exclusively trans arrangement
of the OH groups. (Electronegativities of OH and Cl are close,
being 3.16 and 3.42, on the Pauling scale, respectively.25) The
observation of almost exclusively trans oxidative addition is not
consistent with the concerted mechanism proposed above
(Scheme 1), but it is quite possible that rapid isomerization of
Scheme 1
transition state in each case being “early”, i.e. the interaction of
peroxides with platinum with concomitant weakening of the
oxygen–oxygen bond in the transition state is not extensive
(considering that the homolytic O–O bond dissociation energy
in H2O2 is 51.1 kcal molϪ1 23). This “early” transition state
may be responsible for the fact that the ∆H≠ values in the
present study are not significantly sensitive to the nature of the
incoming peroxide.
Bridgeman and Rothery24 have presented an ab initio model
for bonding in halogen and hydrogen peroxides. Thus, for
XOOX they have described that when X is H, Br or F, the
resonance forms I, II or III (Scheme 2), respectively, are of con-
siderable importance. If this electronegativity effect is extended
Scheme 2
3432
J. Chem. Soc., Dalton Trans., 2001, 3430–3434

View Article Online
the kinetic cis complex to the thermodynamic trans complex
has occurred.
The related organo-dihydroxoplatinum() complex [Pt-
Me2(OH)2(phen)]12 has also an exclusively trans dihydroxy
arrangement. The dominant platinum() isomer formed upon
oxidative addition of dibenzoyl peroxide to the arylplatinum()
3J(H2H3) 5.06 Hz, 3J(PtH2) 11.4 Hz, H2 of phen], 7.90 [2 H, dd,
3J(H3H2) 5.06 Hz, H3 of phen], 8.57 [2 H, d, 3J(H4H3) 8.17 Hz,
H4 of phen], 8.07 (2 H, s, H5 of phen); νmax/cmϪ1: 3570 (OH).
The following complexes were made similarly by using the
appropriate [PtAr2(NN)] complex. The NN data in the 1H
NMR are very similar to those obtained for the above represent-
ative complexes and thus are not quoted. [Pt(Ph)2(OH)2(bpy)].
Yield 70%; mp 195 ЊC (decomp.) (Found: C, 48.7; H, 3.4; N, 4.9.
C22H20N2O2Pt requires C, 48.9; H, 3.7; N, 5.1%); δH 1.06 (1 H,
br s, OH), 7.50 [4 H, d, 3J(HoHm) 7.84 Hz, 3J(PtHo) 35.00 Hz, Ho
of Ph], 7.15 [4 H, m, 3J(HmHo) 7.84, Hm of Ph], 7.20 (2H, Hp of
Ph); νmax/cmϪ1: 3612 (OH). [Pt(m-MeC6H4)2(OH)2(bpy)]. Yield
84%; mp 210 ЊC (decomp.) (Found: C, 49.7; H, 4.1; N, 4.9.
C24H24N2O2Pt requires C, 50.7; H, 4.2; N, 4.9%); δH 1.04 (1 H,
complexes is also that with trans orientation of the OC(᎐O)Ph
᎐
ligands (see also ref. 12).
Experimental
1
The H NMR spectra were recorded as CDCl3 solutions on a
Bruker Avance DPX 250 MHz spectrometer and TMS was
used as external reference. All the chemical shift and coupling
constants are in ppm and Hz, respectively. UV–vis spectra
were recorded using a Philips PU 7850 spectrometer. Kinetic
studies were carried out by using a Philips PU 8675 vis spec-
trometer, fitted with a Pentium (III) computer and with tem-
perature control using a Polyscience 900 constant temperature
bath. IR spectra were recorded on a Perkin-Elmer IR 1310
spectrometer as KBr pellets. Melting points were recorded on a
Buchi 530 apparatus and are uncorrected. The known complex
[PtPh2(bpy)]26 was prepared by a literature method.27 The
following starting materials were prepared similarly with good
yields (86–95%) using the corresponding cis-[PtAr2(SMe2)2]
3
br s, OH), 2.19 (6 H, s, ArCH3), 7.36 [4 H, J(PtHo) 35.02 Hz,
Ho of Ar], 7.02 (4 H, other H of Ar); νmax/cmϪ1: 3580 (OH).
[Pt(p-MeOC6H4)2(OH)2(bpy)]. Yield 94%; mp 173 ЊC (decomp.)
(Found: C, 47.7; H, 4.0; N, 4.4. C24H24N2O4Pt requires C, 48.0;
H, 4.0; N, 4.6%); δH 1.90 (1 H, br s, OH), 3.82 (6 H, s, ArOCH3),
6.78 [4 H, d, 3J(HoHm) 8.04 Hz, Hm of Ar], 7.42 [4 H, d,
3
3J(HmHo) 8.04, J(PtHo) 39.23 Hz, Ho of Ar]; νmax/cmϪ1: 3612
(OH). [Pt(p-MeOC6H4)2(OH)2(phen)]. Yield 85%; mp 243 ЊC
(decomp.) (Found: C, 49.7; H, 4.1; N, 4.2. C26H24N2O4Pt
requires C, 50.0; H, 3.9; N, 4.5%); δH 1.23 (1 H, br s, OH), 3.85
3
(6 H, s, Ar-OCH3), 6.83 [4 H, d, J(HoHm) 8.67 Hz, Hm of
Ar], 7.53 [4 H, d, 3J(HmHo) 8.67, 3J(PtHo) 34.00 Hz, Ho of Ar].
νmax/cmϪ1: 3612 (OH).
1
precurssor28 and NN. The H NMR data are given for two
representative complexes and the others gave very similar
spectra. The NMR labeling for NN ligands are those that are
conventionally used (see for example ref. 17). [Pt(p-MeC6H4)2-
(bpy)]: mp 253 ЊC (decomp.); λmax/nm: (acetone) 447, (benzene)
484; δH 2.25 (6 H, s, ArCH3), 6.85 [4H, d, 3J(HoHm) 7.63 Hz, Hm
of Ar], 7.40 [4 H, d, 3J(HmHo) 7.63 Hz, 3J(PtHo) 69.75 Hz, Ho of
[Pt(Ph)2(OCOPh)2(bpy)]. To a solution of [Pt(Ph)2(bpy)] (40
mg) in acetone (25 ml) was added dibenzoyl peroxide (20 mg)
with stirring for 1 h. The yellow solution was decolorized. The
solvent was removed and the white solid residue was recrystal-
lized from acetone–pentane. Yield 88%; mp 132 ЊC (decomp.)
(Found: C, 57.4; H, 3.7; N, 3.0. C36H28N2O4Pt requires C, 57.8;
H, 3.7; N, 3.7%); νmax/cmϪ1: 1760 (asym, CO2) and 1453 (sym,
3
3
Ar], 8.70 [1 H, d, J(H6H5) 5.40 Hz, J(PtH6) 18.8 Hz, H6 of
bpy], 7.35 [2 H, m, 3J(H5H6) 5.40 Hz, H5 of bpy], 7.99 [2 H, m,
3J(H4H3) 8.25 Hz, H4 of bpy], 8.05 [2 H, d, J(H3H4) 8.25 Hz,
3
1
H3 of bpy]. [Pt(p-MeOC6H4)2(phen)]: mp 220 ЊC (decomp);
λmax/nm: (acetone) 438, (2 : 1 mixture of acetone–benzene) 444,
(benzene) 484; δH 3.80 (6 H, s, ArOCH3), 6.79 [4 H, d, 3J(HoHm)
CO2). The H NMR data for this and following complexes are
not quoted; the data are very similar to the corresponding
dihydroxoplatinum() complexes mentioned above expect that
in this case three signals close to δ 8.1 (2H), 7.7 (4H) and 7.1
7.65 Hz, Hm of Ar], 7.47 [4 H, d, J(HmHo) 7.65 Hz, J(PtHo)
3
3
69.26 Hz, Ho of Ar], 9.00 [1 H, d, J(H2H3) 5.08 Hz, J(PtH2)
3
3
(4H) were in addition observed for Ph groups on C(᎐O)Ph
᎐
18.6 Hz, H2 of phen], 8.58 [2 H, d, 3J(H3H4) 8.16 Hz, H4
ligands. Also for the minor product, typical data are given in the
main text.
of phen], 7.98 (2 H, s, H5 of phen), 7.77 [2 H, dd, J(H4H3)
3
8.16 Hz, 3J(H2H3) 5.08 Hz, H3 of phen]. [Pt(m-MeC6H4)2(bpy)]:
mp 273 ЊC (decomp.); λmax/nm (acetone): 444. [Pt(p-MeO-
C6H4)2(bpy)] mp 218–222 ЊC (decomp.); λmax/nm: (acetone)
438, (benzene) 488. The complexes gave satisfactory analytical
results.
The following complexes were made similarly by using the
appropriate [PtAr2(NN)] complex. [Pt(p-MeC6H4)2(OCOPh)2-
(bpy)]. Yield 85%; mp 166 ЊC (decomp.) (Found: C, 58.1; H,
4.2; N, 3.5. C38H32N2O4Pt requires C, 58.8; H, 4.2; N, 3.6%);
νmax/cmϪ1: 1660 (asym, CO2) and 1295 (sym, CO2). [Pt(m-Me-
C6H4)2(OCOPh)2(bpy)]. Yield 84%; mp 115 ЊC (decomp.)
(Found: C, 58.1; H, 4.3; N, 3.5. C38H32N2O4Pt requires C, 58.8;
H, 4.2; N, 3.6%); νmax/cmϪ1: 1670 (asym, CO2) and 1300 (sym,
CO2). [Pt(p-MeOC6H4)2(OCOPh)2(bpy)]. Yield 83%; mp 104 ЊC
(decomp.) (Found: C, 56.3; H, 3.5; N, 3.7. C38H32N2O6Pt
requires C, 56.5; H, 4.0; N, 3.5%); νmax/cmϪ1: 1645 (asym, CO2)
and 1275 (sym, CO2). [Pt(p-MeOC6H4)2(OCOPh)2(phen)].
Yield 80%; mp 101–103 ЊC (decomp.) (Found: C, 57.5; H,
4.2; N, 3.4. C40H32N2O6Pt requires C, 57.8; H, 3.8; N, 3.4%);
νmax/cmϪ1: 1650 (asym, CO2) and 1280 (sym, CO2).
Syntheses
[Pt( p-MeC6H4)2(OH)2(bpy)]. To a solution of [Pt(p-MeC6H4)2-
(bpy)] (50 mg) in acetone (50 ml) was added an excess (1 ml) of
30% w/w hydrogen peroxide in water with stirring. The yellow-
ish solution turned pale yellow. The solvent was removed and
the solid residue was recrystallized from acetone–pentane. Yield
80%; mp 220.5 ЊC (decomp.) (Found: C, 50.3; H, 4.0; N, 5.3.
C24H24N2O2Pt requires C, 50.7; H, 4.2; N, 4.9%); δH 1.47 (1 H,
br s, OH), 2.36 (6 H, s, ArCH3), 6.97 [4 H, d, 3J(HoHm) 7.52 Hz,
Hm of Ar], 7.38 [4 H, d, 3J(HmHo) 7.52, 3J(PtHo) 35.04 Hz, Ho of
Kinetic studies of the reaction of [PtAr2(bpy)] with dibenzoyl
peroxide
3
3
Ar], 8.82 [2 H, d, J(H6H5) 5.01 Hz, J(PtH6) 10.9 Hz, H6 of
bpy], 7.58 [2 H, m, 3J(H5H6) 5.01 Hz, H5 of bpy], 8.07 [2 H, m,
3J(H4H3) 8.50 Hz, H4 of bpy], 8.29 [2 H, d, 3J(H3H4) 8.5 Hz, H3
of bpy]; νmax/cmϪ1: 3570 (OH).
In a typical experiment, a solution of [Pt(p-MeC6H4)2(bpy)] in
acetone or benzene (3 ml, 3 × 10Ϫ4M) in a cuvette was thermo-
stated at 20 ЊC and a known excess of dibenzoyl peroxide was
added using a microsyringe. After rapid stirring, the absorb-
ance at λ = 484 nm (in benzene) or λ = 447 nm (in acetone) was
monitored with time and a plot of Ϫln((At Ϫ A∞)/(A0 Ϫ A∞))
versus time gave a good straight line (Fig. 2) from which the
observed first-order rate constants and standard deviations
were obtained. A plot of kobs versus [dibenzoyl peroxide] was
[Pt( p-MeC6H4)2(OH)2(phen)]. This was prepared similarly
using [Pt(p-MeC6H4)2(phen)]. Yield 80%; mp 261 ЊC (decomp.)
(Found: C, 52.0; H, 4.7, N, 4.0. C26H24N2O2Pt requires C, 52.8;
H, 4.7; N, 4.0%); δH 1.23 (1 H, br s, OH), 2.39 (6 H, s, ArCH3),
7.02 [4 H, d, 3J(HoHm) 7.50 Hz, Hm of Ar], 7.45 [4 H, d,
3J(HmHo) 7.50, 3J(PtHo) 35.20 Hz, Ho of Ar], 9.15 [2 H, d,
J. Chem. Soc., Dalton Trans., 2001, 3430–3434
3433

View Article Online
9 C. M. Giandomenico, M. J. Abrams, B. A. Murrer, J. F. Vollano,
M. I. Rheinheimer, S. B. Wyer, G. E. Bossard and J. D. Higgins,
Inorg. Chem., 1995, 34, 1015.
10 G. B. Deacon, E. T. Lawarenz, T. W. Hambley, S. Rainone and L. K.
Webster, J. Organomet. Chem., 1995, 493, 205.
11 M. Galanski and B. K. Keppler, Inorg. Chim. Acta, 1997, 265,
271.
12 K. T. Aye, J. J. Vittal and R. J. Puddephatt, J. Chem. Soc., Dalton
Trans., 1993, 1835.
13 J. P. Collman, L. S. Hegedus, J. R. Norton and R. A. Finke,
Principles and Applications of Organotransition Metal Chemistry,
University Science Books, Mill Valley, CA, 2nd edn., 1987, ch. 5.
14 L. M. Rendina and R. J. Puddephatt, Chem. Rev., 1997, 97, 1735.
15 C. T. Lam and C. V. Senoff, Can. J. Chem., 1973, 51, 3790.
16 R. T. Mehdi and J. D. Miller, J. Chem. Soc., Dalton Trans., 1984,
1065.
linear (Fig. 3), and the slope gave the second-order rate con-
stant. The same method was used at other temperatures, and
activation parameters were obtained from the Arrhenius equa-
tion. A similar procedure was used to obtain the rate constants
and activation parameters for reaction of dibenzoyl peroxide
with other [PtAr2(NN)] complexes at the corresponding λmax
values.
A similar procedure was carried out for the reaction of
[PtAr2(NN)] (at the corresponding λmax) with H2O2 in acetone
(or a 2 : 1 mixture of acetone–benzene). The concentration of
H2O2 was determined by standard method as 9.07 M.
Acknowledgements
17 M. Rashidi and B. Z. Momeni, J. Organomet. Chem., 1999, 574,
We thank the Shiraz University Research Council, Iran (Grant
No, SC-76-999-592) for financial support and Mr Maleki for
technical assistance.
286.
18 M. Rashidi, A. R. Esmaeilbeig, N. Shahabadi, S. Tangestaninejad
and R. J. Puddephatt, J. Organomet. Chem., 1998, 568, 53.
19 J. K. Jawad and R. J. Puddephatt, J. Chem. Soc., Dalton Trans.,
1977, 1466.
20 J. J. Low and W. Goddard III, J. Am. Chem. Soc., 1984, 106, 6928.
21 J. J. Low and W. Goddard III, Organometallics, 1986, 5, 609.
22 J. J. Low and W. Goddard III, J. Am. Chem. Soc., 1986, 108, 6115.
23 P. Politzer, Inorg. Chem., 1977, 16, 3350.
24 A. J. Bridgeman and J. Rothery, J. Chem. Soc., Dalton Trans., 1999,
4077.
25 J. E. Huheey, Inorganic Chemistry, Harper Collins, New York, NY,
2nd edn., 1978, ch. 4.
References
1 G. K. Anderson, in, Chemistry of the Platinum Group Metals, ed.
F. R. Hartley, Elsevier, Amsterdam, 1991, ch. 12.
2 D. J. Evans and M. Green, Inorg. Chim. Acta, 1987, 130, 183.
3 R. Kuroda, S. Neidle, I. M. Ismail and P. Sadler, Inorg. Chem., 1998,
22, 3620.
4 H. E. Bryndza and W. Tam, Chem. Rev., 1988, 88, 1163.
5 C. M. Jensen and W. C. Trogler, J. Am. Chem. Soc., 1986, 108, 723.
6 S. Otsuka, J. Organomet. Chem., 1980, 200, 191.
7 G. Strukul and R. A. Michelin, J. Am. Chem. Soc., 1985, 107, 7563.
8 S. Choi, C. Filotto, M. Bisanzo, S. Delaney, D. Lagasee, J. L.
Whitworth, A. Jusko, C. Li, N. A. Wood, J. Willingham, A.
Schwenker and K. Spaulding, Inorg. Chem., 1998, 37, 2500.
26 N. Chaudhury and R. J. Puddephatt, J. Organomet. Chem., 1975, 84,
105.
27 M. Rashidi, Z. Fakhroeian and R. J. Puddephatt, J. Organomet.
Chem., 1991, 406, 261.
28 M. Rashidi, M. Hashemi, M. Khorasani-Motlagh and R. J.
Puddephatt, Organometallics, 2000, 19, 2751.
3434
J. Chem. Soc., Dalton Trans., 2001, 3430–3434