Oxidative addition of phthaloyl peroxide to dimethylplatinum(II) complexes

Article abstract of DOI:10.1021/om3009074

Complexes [PtMe₂(NN)], with NN = 2,2′-bipyridine (bipy), 4,4′-di-tert-butyl-2,2′-bipyridine (bu₂bipy), di-2-pyridylamine (dpa), or di-2-pyridyl ketone (dpk), react easily with phthaloyl peroxide to give a mixture of the chelate complex [PtMe ₂{κ²-O,O′-1,2-(O₂C) ₂C₆H₄}(NN)], which was structurally characterized when NN = bu₂bipy, and an oligomer or polymer [PtMe₂{μ-κ²-O,O′-1,2-(O₂C) ₂C₆H₄}(NN)]_n. In the case with NN = dpa, no phthalate chelate complex is formed. These complexes are easily hydrolyzed, and the complexes cis-[PtMe₂(OH){κ¹-O- O₂CC₆H₄-2-CO₂H}(bipy)] and trans-[PtMe₂{κ¹-O-O₂CC₆H ₄-2-CO₂H}(dpkOH)] have been structurally characterized. It is argued that the oxidative addition of phthaloyl peroxide occurs by a polar mechanism and that the hydrolysis is easy because there is no special stability associated with the seven-membered platinum-phthalate chelate ring.

Full text of DOI:10.1021/om3009074

Article
pubs.acs.org/Organometallics
Oxidative Addition of Phthaloyl Peroxide to Dimethylplatinum(II)
Complexes
Kyle R. Pellarin, Matthew S. McCready, Thomas I. Sutherland, and Richard J. Puddephatt*
Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7
S
* Supporting Information
ABSTRACT: Complexes [PtMe2(NN)], with NN = 2,2′-bipyridine (bipy), 4,4′-di-
tert-butyl-2,2′-bipyridine (bu2bipy), di-2-pyridylamine (dpa), or di-2-pyridyl ketone
(dpk), react easily with phthaloyl peroxide to give a mixture of the chelate complex
[PtMe2{κ2-O,O′-1,2-(O2C)2C6H4}(NN)], which was structurally characterized when
NN = bu2bipy, and an oligomer or polymer [PtMe2{μ-κ2-O,O′-1,2-(O2C)2C6H4}-
(NN)]n. In the case with NN = dpa, no phthalate chelate complex is formed. These
complexes are easily hydrolyzed, and the complexes cis-[PtMe2(OH){κ1-O-
O2CC6H4-2-CO2H}(bipy)] and trans-[PtMe2{κ1-O-O2CC6H4-2-CO2H}(dpkOH)]
have been structurally characterized. It is argued that the oxidative addition of
phthaloyl peroxide occurs by a polar mechanism and that the hydrolysis is easy
because there is no special stability associated with the seven-membered platinum-
phthalate chelate ring.
Scheme 2. Possible Polar and Concerted Mechanisms of
Oxidative Addition of Hydrogen Peroxide
INTRODUCTION
■
a
The oxidation of methylplatinum(II) complexes with dioxygen
or hydrogen peroxide can give methyl(hydroxo)platinum(IV)
complexes,1 which may then reductively eliminate methanol.1,2
If the methylplatinum(II) complex can be regenerated by C−H
bond activation of methane, a cycle for oxidation of methane to
methanol can be envisaged (Scheme 1).1−3 There are many
problems to be overcome if this potential is to be realized, and a
deeper understanding of the individual steps is needed.
Scheme 1. Potential Catalysis of Oxidation of Methane to
Methanol
a
NN = diimine ligand.
intermediate D. However, it is possible that the product of cis
oxidative addition, E, could isomerize rapidly to C via
intermediate F1e or that the five-coordinate ionic intermediate
B formed in the polar mechanism could isomerize to F before
hydroxide coordination.4 The product of trans oxidative
addition is usually observed,1 although there are examples of
cis addition.1g,2d Free radical mechanisms might also be
considered, although there is no evidence for them.
The oxidative addition of dibenzoyl peroxide can be
considered in a similar way. A simple polar mechanism would
give the product of trans oxidative addition, H, via intermediate
G. An ionic intermediate might be stabilized because the
PhCO2 group could act as a three-electron ligand by
coordination of the carbonyl group in K, and this could give
a route to the product of cis oxidative addition, I.1p,5 The
The oxidative addition of hydrogen peroxide to
dimethylplatinum(II) complexes, A, might occur by a polar
stepwise mechanism or a concerted nonpolar mechanism
(Scheme 2). In principle, the mechanisms could be
distinguished by the stereochemistry of the product, with the
polar mechanism leading to the product of trans addition, C, by
way of the ionic intermediate B and the concerted mechanism
leading to the product of cis addition, E, by way of the
Received: September 24, 2012
Published: November 15, 2012
© 2012 American Chemical Society
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concerted mechanism would give I via intermediate J, but this
could be followed by isomerization to H (Scheme 3). The
Scheme 4. Possible Mechanisms and Products of Oxidative
Addition of Phthaloyl Peroxide
Scheme 3. Possible Mechanisms and Products of Oxidative
Addition of Dibenzoyl Peroxide
Chart 1. Dimethylplatinum(II) Reagents
reaction often gives a mixture of the products of cis and trans
oxidative addition, I and H (Scheme 3).1e,m,p,5 Only one kinetic
study has been reported.1e The reaction with [PtPh2(bipy)],
bipy = 2,2′-bipyridine, followed second-order kinetics. It gave a
large negative value of the entropy of activation, normally
indicative of a polar SN2 mechanism, but the rate showed only a
small dependency on the solvent polarity, indicative of a
nonpolar mechanism.6 The authors proposed the concerted
mechanism of oxidative addition.1e A free radical mechanism
involving L and intermediate PhCO2· radicals might be
expected to give the phenylplatinum complex M by
decarboxylation of this PhCO2· radical before coordination,7
but no such products have been observed.1,5
This paper reports the first study of the reactivity of a cyclic
acyl peroxide with dimethylplatinum(II) complexes. The high
reactivity of phthaloyl peroxide has been recognized and is
attributed in part to the nearly eclipsed conformation about the
peroxide, with dihedral angle C−O−O−C = 11°.8−10 In many
stoichiometric organic oxidation reactions, phthaloyl peroxide is
thought to react by free radical or single-electron-transfer
mechanisms,8−10 and it has recently been studied as an
organocatalyst for dihydroxylation of alkenes.11 It was
anticipated that the polar mechanism would give intermediate
N, which could give either oligomers P with trans-PtO2 linkages
(a trimer is the smallest possible oligomer of this type) or the
product of cis oxidative addition, O (Scheme 4). The concerted
mechanism would also give O via intermediate Q. A free radical
mechanism via intermediate R might also give O, or the
intermediate might undergo decarboxylation to give the
arylplatinum complex S, whose five-membered chelate ring is
likely to be stabilized compared to the seven-membered chelate
ring of O. In fact, the chemistry proved to be complicated by
easy hydrolysis of the phthalate complexes.
1b give five-membered chelate rings, while di-2-pyridylamine
(dpa) or di-2-pyridyl ketone (dpk) in 1c and 1d give six-
membered chelate rings on coordination to platinum.
Phthaloyl peroxide reacted rapidly with all of the complexes
[PtMe2(NN)], 1a−1d, according to Scheme 5. For example, an
orange solution of [PtMe2(bipy)], 1a,12 in acetone-d6 was
immediately decolorized on addition of phthaloyl peroxide. The
stereochemistry at platinum in the products 2a and 3a was
readily deduced from the 1H NMR spectrum. Thus, the
product 3a of trans oxidative addition has Cs or C2v symmetry,
with a plane of symmetry bisecting the Me2Pt group, so the 1H
NMR spectrum contains a single methylplatinum resonance
and four resonances for the bipy protons, whereas the product
1
2a of cis oxidative addition has only C1 symmetry, so the H
NMR spectrum contains two methylplatinum resonances and
eight resonances for the bipy protons. In this case, a mixture of
2
2
2a [δ(MePt) 1.09, J(PtH) 69 Hz; δ(MePt) 1.15, J(PtH) 71
2
Hz] and 3a [δ(MePt) 1.88, J(PtH) 70 Hz] was formed. The
ESI-MS in acetone solution contained a major envelope of
peaks centered at m/z = 546.1, corresponding to [2a + H]+,
and a lower intensity peak at m/z = 1091.2, which corresponds
to [{PtMe2(phthalate)(bipy)}2 + H]+ and might be assigned to
3a, with n = 2 (but note that one of the platinum(IV) centers
would be five-coordinate). The highest mass peak observed was
at m/z = 1450, which corresponds to [3a (n = 3) − bipy −
2Me + H]+, indicating that oligomers with at least n = 3 are
present.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The reactions of
phthaloyl peroxide with the dimethylplatinum(II) complexes
shown in Chart 1 were studied. The ligands 2,2′-bipyridine
(bipy) and 4,4′-di-tert-butyl-2,2′-bipyridine (bu2bipy) in 1a and
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Scheme 5. Formation of Phthalate Complexes by Oxidative
a
Addition
Figure 1. Structure of complex 2b. Selected bond parameters (Å and
deg): Pt−O(1) 2.020(5); Pt−O(2) 2.160(4); Pt−N(1) 2.027(5); Pt−
N(2) 2.120(5); Pt−C(19) 2.067(6); Pt−C(20) 2.063(6); O(1)−
C(21) 1.296(8); O(3)−C(21) 1.221(9); O(2)−C(28) 1.291(8);
O(4)−C(28) 1.223(8); N(1)−Pt−N(2) 79.3(2); O(1)−Pt−O(2)
92.3(2).
membered ring, O(1)−Pt−O(2) 92.3(2)o] is naturally greater
than for the bu2bipy ligand [five-membered ring, N(1)−Pt−
N(2) 79.3(2)o]. The folded conformation of the PtO2C4 ring of
the chelating phthalate is achieved by twisting the carboxylate
groups out of the plane of the phenylene ring (twist angles for
the C(28)O(2)O(4) and C(21)O(1)O(3) groups are 38° and
80°, respectively). The carbonyl distances C(21)−O(3) and
C(28)−O(4) are shorter than the C−OPt distances C(21)−
O(1) and C(28)−O(2), indicating significant covalent
character in the Pt−O bonds. The ESI-MS of 2b in acetone
solution gave the major peak at m/z = 658.2, corresponding to
a
Complexes 3 were identified by NMR spectroscopy, and 2c was not
detected (see text).
1
[2b + H]+, with no dimer or trimer peaks. The H NMR and
Table 1 lists the ratio of the products 3a/2a as a function of
ESI-MS data both indicate that the equilibration of 2b with its
isomer 3b is slow at room temperature.
solvent and reaction time for reactions carried out in a sealed
The mixture of 2a and 3a was difficult to crystallize, but
single crystals were finally obtained by the slow diffusion of
pentane into a solution in dichloromethane, stored at 5 °C for
several weeks. The complex that crystallized was formed by
hydrolysis and had the stoichiometry either
[PtMe2(O2CC6H4CO2H)(OH)(bipy)]·4.5H2O, 4, Scheme 5,
or [PtMe2(O2CC6H4CO2)(OH2)(bipy)]·4.5H2O, 4′, with
some uncertainty in the positions of hydrogen atoms. Note
that 4 is expected to be the kinetic product of hydrolysis of 2a
because its formation would involve cleavage of the weaker Pt−
O bond of 2a trans to the methyl group. There are two
independent but similar molecules in the crystal of 4, and the
structure of a pair of Pt(1) molecules, connected by hydrogen-
bonded water molecules, is shown in Figure 2. As in complex
2b, the platinum(IV) centers have octahedral stereochemistry
with the cis,cis,cis-PtO2N2C2 arrangement of donor groups, but
in complex 4 the phthalate groups act as monodentate ligands,
and there is a coordinated hydroxide (4) or water (4′) group as
the second oxygen-donor ligand. The two molecules in Figure 2
are related by an inversion center, so they represent a racemic
pair and are connected through hydrogen bonds to the water
molecules O(9S) and O(9SA), as well as being involved in
hydrogen bonding to several other water molecules. The
hydrogen atoms could not be located directly, so the nature of
the hydrogen bonding is not obvious. Based on OO distances,
hydrogen bonds are present between the atoms O(2)···O(5) =
2.60(2), O(5)···O(9S) = 2.64(2), and O(4)···O(9SA) =
2.82(2) Å, as well as O(3)···O(7S) = 2.70(2) and
O(4)···O(5S) = 2.70(2) Å. Another clue is that the carbon−
oxygen distances of the C(11)O(1)O(2) carboxyl group are
consistent with the presence of a single and double bond
[C(11)−O(1) = 1.32(1) Å, C(11)−O(2) = 1.24(2) Å] but that
Table 1. Ratio of Products 3a/2a As a Function of Solvent
and Reaction Time
solvent
5 min
1 day
7 days
CD3OD
(CD3)2CO
CD2Cl2
1.0
1.6
1.5
1.0
3.5
5.7
1.0
4.1
25
NMR tube. It can be seen that in the polar solvent CD3OD
essentially equal amounts of 3a and 2a were formed and that
the product ratio did not change over the course of a week at
room temperature. In the least polar solvent CD2Cl2, the initial
ratio of 3a/2a was 1.5, but this ratio increased slowly but
dramatically to 25 in a week. The mixture in acetone-d6 gave an
intermediate degree of isomerization over time (Table 1).
The product ratio was strongly dependent on the diimine
ligand. Thus, for reaction in dry acetone-d6 solution, the initial
product ratio for NN = bipy was 3a/2a = 1.6, but, for the
similar ligand NN = bu2bipy, it was 3b/2b = 0.12. For NN =
dpa, only the trans adduct 3c was detected. For NN = dpk, the
1
isomer ratio could not be determined by integration of the H
NMR spectrum because the trans product 3d was sparingly
soluble and partly precipitated from solution.
From these initially formed complexes 2 and 3, only complex
2b could be recrystallized successfully, and its structure is
shown in Figure 1. The platinum(IV) center has octahedral
stereochemistry with the cis,cis,cis-PtO2N2C2 arrangement of
donor groups. The complex is therefore chiral, but it crystallizes
as a racemic mixture of C and A enantiomers. The high trans
influence of the methyl groups causes the distances Pt−O(2)
and Pt−N(2) to be longer than Pt−O(1) and Pt−N(1),
respectively. The chelate bite angle of the phthalate [seven-
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Figure 2. Structure of a racemic pair of molecules of complex 4, with
connecting water molecules. Selected bond parameters (Å): Pt(1)−
O(1) 2.013(9); Pt(1)−O(5) 2.206(9); Pt(1)−N(1) 2.10(1); Pt(1)−
N(2) 2.01(1); Pt(1)−C(19) 2.04(1); Pt(1)−C(20) 2.06(1); O(1)−
C(11) 1.32(1); O(2)−C(11) 1.24(2); O(3)−C(18) 1.24(2); O(4)−
C(18) 1.25(2). Symmetry equivalent A: 2−x, 1−y, −z.
Figure 3. Part of the hydrogen-bonding network in complex 4. Pt(1)
and Pt(2) molecules, and the connecting water molecules O(9S) and
O(8S), are shown in red and blue, respectively, and only the NCCN
atoms of the bipy groups are shown, for clarity. Only the numeric
labels of the O-atoms are given, with S indicating a water molecule
[e.g., 3 = O(3), 1S = O(1S)].
the carbon−oxygen distances of the C(18)O(3)O(4) carboxyl
group are equal within experimental error [C(18)−O(3) =
1.24(2) Å, C(18)−O(4) = 1.25(2) Å], suggesting the presence
of a carboxylate anion. Chart 2 indicates that only a small
and only four pyridyl resonances and two phthaloyl resonances,
clearly indicating that it has at least Cs symmetry, as expected
for a product of trans oxidative addition. The complex with dpa
is the only one studied that gives no detectable amount of a cis
product. We tentatively suggest that this could be because, in
the polar intermediate N (Scheme 6), the free carboxylate
Chart 2. Hydrogen Bonding and Structures of 4 and 4′
Scheme 6. Possible Structures of Intermediate N and
a
Product 5 or 5′
motion of hydrogen atoms is needed to interconvert the
structures 4 and 4′, and so there is no easy way to determine
which of them is the dominant form in the crystalline state. In
complexes [Pt(OH)(OH2)Me2(NN)]+, it has been shown that
the acidity of the aqua group is similar to that of a carboxylic
acid,1c so 4 and 4′ are likely to have similar stability provided
that they can form the same number and strength of
intermolecular and intramolecular hydrogen bonds.
a
Only the HN(CN)2 backbone atoms of the dpa ligand (inset) are
shown, for clarity.
Figure 3 shows a section of the complex hydrogen-bonded
network structure formed by self-assembly of complex 4 and
the nine independent water molecules. Racemic pairs of Pt(1)
and Pt(2) molecules are connected through hydrogen bonding
to O(9S) and O(8S), respectively, and Pt(1) and Pt(2)
molecules are connected by hydrogen bonding of the
uncoordinated carboxylic acid/carboxylate groups to O(7S).
There are also longer range connections of the complexes
through chains of bridging water molecules (Figure 3).
group can hydrogen bond to the NH proton of dpa. Molecular
modeling indicates that this hydrogen bond cannot be formed
by a cis isomer, and the effect may be strong enough to prevent
rearrangement of N to form the chelating phthalate group. The
isolated complex analyzed as [PtMe2(phthalate)(OH2)(dpa)],
consistent with either structure 5 or 5′ (Scheme 6). The ESI-
MS in acetone did not give a parent ion but contained
prominent peaks centered at m/z = 413.1 and 561.1,
corresponding to [5-phthalate-H]+ and [5-OH]+, respectively.
The peak at m/z = 413.1 supports the presence of a
coordinated hydroxide or water molecule, but the data do
Complex 5 was sparingly soluble and did not give single
crystals, so its proposed structure is based on spectroscopic
1
studies only. The H NMR spectrum contains a single methyl
platinum resonance at δ = 1.61, with coupling 2J(PtH) = 67 Hz,
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not clearly distinguish between the two possible structures 5
and 5′. As in complex 4, it is likely that there is extensive
intermolecular hydrogen bonding in the solid state, and this
may rationalize the low solubility of the complex.
The red complex [PtMe2(dpk)], 1d, reacted rapidly with
phthaloyl peroxide. The initial products are thought to be 2d
and 3d (Scheme 5), but, following purification, the two
colorless complexes 6 and 7, which are formed by hydration,
were isolated (Scheme 7). These complexes could be separated
Scheme 7. Formation and Structures of Complexes 6 and 7
Figure 4. Structure of complex 7, showing the dimer formed by
intermolecular hydrogen bonding. Selected parameters: Pt(1)−N(1)
2.146(8); Pt(1)−N(2) 2.153(9); Pt(1)−O(1) 2.009(6); Pt(1)−O(2)
2.046(7); Pt(1)−C(12) 2.06(1); Pt(1)−C(13) 2.03(1); O(1)−C(6)
1.438(7); O(2)−C(14) 1.30(1); O(3)−C(14) 1.23(1); O(4)−C(21)
1.31(1); O(5)−C(21) 1.20(1) Å. Symmetry equivalent A: 1−x, y, −1/
2−z.
in complex 1d gives a strong peak at 1682 cm−1 in the IR
spectrum, but complex 6 gives only carbonyl stretches in the
range 1650−1605 cm−1, characteristic of the phthalate group.
This observation indicates that complex 6 contains the
dpkOH− ligand rather than the original dpk ligand. The ESI-
MS, in the presence of NaI to aid ionization, gave a peak at m/z
= 614.1, which is assigned to the ion [Pt(C8H5O4)-
Me2(dpkOH)Na]+. Together, these data strongly support the
proposed structure of complex 6. The complex is expected to
self-associate through intermolecular hydrogen bonding in the
solid state, but we have no data to determine how this occurs.
Computational Studies. The synthetic and structural
study has given insight into the mechanism of reaction of the
dimethylplatinum(II) complexes with phthaloyl peroxide, PP,
but several puzzles remain. These were addressed by computa-
tional studies using DFT. Scheme 4 outlines the potential polar,
concerted, or radical mechanisms, which involve intermediates
(or transition states) N, Q, or R, respectively. The polar
because complex 7 was sparingly soluble and precipitated from
solution in moist acetone. Complexes 6 and 7 can be
considered as derivatives of the products of cis and trans
oxidative addition, 2d and 3d, respectively (Scheme 7). Once
formed, the complexes did not easily interconvert. The overall
stereochemistry at platinum(IV) was readily determined from
the 1H NMR spectra. Thus, complex 6 gave two equal intensity
methylplatinum resonances at δ = 1.94 and 2.04, each with
2J(PtH) = 70 Hz, while 7 gave a single methylplatinum
2
resonance at δ = 1.64, with J(PtH) = 70 Hz.
Complex 6 has not given good single crystals, but complex 7
could be crystallized and its structure is shown in Figure 4. The
platinum(IV) center has octahedral stereochemistry with the
trans,cis,cis-PtO2N2C2 arrangement of donor atoms, corre-
sponding to trans oxidative addition. The phthalate is present
in the monoprotonated form, and there is a strong intra-
molecular hydrogen bond to an oxygen atom of the
coordinated carboxylate group, with O(4)···O(3) = 2.46(1)
Å. The dpk ligand has been converted to the ketal derivative
dpkOH−, which is bound as a fac-tridentate ligand. This
derivatization of the dpk ligand has been observed previously,
for example in the oxidative addition of hydrogen peroxide to
complex 1d, and may involve hydroxide attack on the carbonyl
group of dpk.1n,2d The dimer shown in Figure 4 is formed by
self-assembly through complementary hydrogen bonding
between the C(OH)(OPt) units of neighboring molecules,
with O(6)···O(1A) = O(1)···O(6A) = 2.70(1) Å.
2
mechanism with 1a involves end-on attack with the 5dz orbital
of platinum (HOMO) as nucleophile and the σ*(OO) orbital
of PP (LUMO) as acceptor, as shown in Figure 5a, leading to
cleavage of the O−O bond and formation of the zwitterionic
intermediate N. This is calculated to be energetically favorable
(Figure 6) with a low activation energy. This polar mechanism
has previously been supported for oxidative addition of
halogens, many peroxides, and alkyl halides.1,6,13 The radical
mechanism is expected give an intermediate R with similar
structure to N, but the calculation does not support this
mechanism since spin-paired structures are found to be more
stable. The concerted mechanism is expected to involve side-on
approach with transfer of electrons from a 5dπ orbital
(HOMO−1) of 1a to the σ*(OO) orbital of PP, as shown in
Complex 6 is structurally characterized only by elemental
analysis, which indicates the presence of a molecule of water,
and by spectroscopic methods. The 1H NMR spectrum
indicates the cis,cis,cis stereochemistry of the PtO2N2C2 donors
(Figure S1). The carbonyl group of the coordinated dpk ligand
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of two new bonds in 2a and because the side-on approach leads
to strong repulsion between the filled 2pπ orbitals of PP and the
filled 5d orbitals of 1a. Concerted oxidative addition to
platinum(II), or reductive elimination from platinum(IV),
usually occurs after dissociation of a ligand to make a
coordinatively unsaturated reagent, but this is difficult for 1a.
The present theory supports this pattern of behavior and
predicts easy reaction through the polar mechanism.1−6,14
Figure 6 shows that the rearrangement of the intermediate N
to give 2a is calculated to be favorable. This could occur
directly or via its isomer N′ (Figure 6). In N′, the coordinative
unsaturation at platinum is relieved by semichelation by a single
carboxylate group (calculated Pt−O = 2.05, Pt···O 2.39 Å), and
it lies about midway in energy between N and 2a. The
hydrolysis of 2a to give 4 is also favored. The gas phase
calculation indicates that the hydroxo form 4 is more stable
than the aqua complex isomer, but the calculated and
experimental structures differ (Figure 2, Chart 2), because
there is intermolecular hydrogen bonding to water in the
structure determined experimentally. Figure 6 also indicates
that the trans isomer, labeled trans-4, is calculated to be more
stable than 4. Results from similar calculations for the reactions
of the dpa complex 1c are shown in Figure 7. The relative
energies are similar to those for compounds formed from 1a
(Figure 6), but the reaction of 1c did not give detectable
amounts of either 2c or cis-5 as products. The intermediate N is
calculated to have a hydrogen bond between the carboxylate
and NH groups (Figure 7), and so the calculation lends support
Figure 5. Frontier orbitals for (a) polar mechanism and (b) concerted
mechanism of reaction of phthaloyl peroxide with 1a.
Figure 6. (Above) Calculated relative energies (kJ mol−1) and (below)
calculated structures for intermediates and products of the reaction of
phthaloyl peroxide (PP) with complex 1a.
Figure 7. (Above) Calculated relative energies (kJ mol−1) and (below)
calculated structures for intermediates and products of the reaction of
phthaloyl peroxide (PP) with complex 1c.
Figure 5b. This mechanism would lead directly to complex 2a,
but no easy route could be found. There are difficulties because
the rigid planar structure of 1a must distort to allow formation
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to the hypothesis (Scheme 6) that rearrangement to 2c is
slower than either oligomerization to give 3c or addition of
water to give 5 (Figure 7). There seems to be no
thermodynamic reason that 2c could not be formed.
The structure of the proposed oligomeric compounds 3
(Scheme 5) remains undetermined. Figure 8 shows calculated
concerted mechanism of reaction through intermediate O
(Scheme 4) would be expected to give the chelating phthalate
complex more selectively, while the radical mechanism through
intermediate R might be expected to give products of
rearrangement or decarboxylation, such as S (Scheme 4).
A survey of known phthalate coordination complexes,
published in 2004, identified no less than 26 different bonding
modes of the phthalate group.15 The chelate structure
established for complex 2b is relatively uncommon and is
established mostly with small metal ions such as Be2+ and
Ti4+.16 Complex 2b appears to be the first crystallographically
characterized example of a platinum complex with chelating
phthalate (Figure 1). The chelate structure has previously been
suggested, based on spectroscopic characterization, for several
platinum(II) complexes (T, Chart 3) but not for platinum(IV)
Chart 3. Some Known Platinum Phthalate Complexes
Figure 8. Possible structures for complex 3a: (above) a dimer
containing a coordinatively unsaturated platinum(IV) center and an
uncoordinated carboxylate group; (below) the cyclic trimer.
complexes.17 The monodentate structure, established for
complexes 4 and 7 (Figures 2 and 4), is also unusual,15 but it
has previously been established in the platinum(IV) complex U
(Chart 3).18 The bridging structure, suggested here for
complexes 3a−3d (Scheme 5), is much more common15 and
has been established for one platinum(II) complex, V (Chart
3).19 An unspecified bridging mode has also been suggested for
phthalate “platinum blues”.20 Clearly there is no special stability
associated with the seven-membered ring formed by the
chelating phthalate group.
The overall proposed mechanisms of formation of some of
the characterized complexes is shown in Scheme 8 and will be
discussed for the case with NN = bipy. The primary
intermediate is thought to be N, which can react rapidly to
give the oligomer 3a, with bridging phthalate groups, or the
monomeric complex 2a, with chelating phthalate groups. The
easy isomerization of N to its isomer N′ may precede formation
of 2a. The ratio of the initially formed products 2a and 3a is
dependent on these fast reactions (Table 1). Once formed,
complexes 2 and 3 can equilibrate more slowly by dissociation
of one of the Pt−O bonds to re-form N or N′ (Scheme 8). The
hydrolysis reactions to give 4 or trans-4 must also involve the
intermediates N and/or N′. Once formed, these complexes
models for a dimer or trimer for 3a. It can be seen that the
dimer (and higher linear oligomers) must contain a five-
coordinate platinum(IV) center and a free carboxylate group.
The coordinative unsaturation at platinum(IV) might be
relieved by solvent coordination, but these structures would
contain nonequivalent platinum(IV) centers and so would be
1
expected to have more complex H NMR spectra than those
observed. A cyclic trimer is the smallest oligomer that might
contain equivalent platinum(IV) centers (Figure 8).
CONCLUSIONS
■
The combination of experimental and computational studies of
the oxidative addition reactions of complexes 1a−1d with
phthaloyl peroxide supports the polar mechanism of reaction,
in which the zwitterionic complex N (Scheme 4) is the key
intermediate. This intermediate can rearrange to give the
chelate complexes 2a−2d or oligomerize to give the bridging
phthalate complexes 3a−3d. Further slow isomerization of 2a
to 3a has been observed in solution (Table 1). The equilibria
and rates of equilibration are strongly dependent on both the
nature of the supporting diimine ligand and the solvent. The
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Organometallics
Article
66 Hz, PtMe); from complex 1d: 3d, δ(1H) 1.71 (s, 6H, 2J(PtH) = 69
Hz, PtMe).
Scheme 8. Proposed Mechanisms for Formation of the
Isolated Complexes 2, 4, and 5
[PtMe2(bu2bpy)(C8H4O4)], 2b. Phthaloyl peroxide (6.0 mg,
0.0405 mmol) was added to a solution of [PtMe2(bu2bpy)] (20.0
mg, 0.0405 mmol) in acetone (5 mL). The color changed immediately
from orange to colorless. The product was precipitated by addition of
pentane, then separated and washed with pentane (3 × 2 mL) and
ether (3 × 2 mL) and dried in vacuo. It was recrystallized from
CH2Cl2/pentane by slow diffusion. NMR in acetone-d6: δ(1H) 1.01 (s,
3H, 2J(PtH) = 68 Hz, PtMe trans to N), 1.10 (s, 3H, 2J(PtH) = 70 Hz,
Pt-Me trans to O), 1.40 (s, 9H, t-Bu), 1.44 (s, 9H, t-Bu), 7.30−7.70
3
4
(m, 4H, phth), 7.73 (dd, 1H, J(H5′H6′) = 6 Hz, J(H5′H3′) = 1 Hz,
H5′), 8.01 (dd, 1H, 3J(H5H6) = 6 Hz, 4J(H5H3) = 1 Hz, H5), 8.50 (d,
1H, J(H5H6 = 6 Hz, J(PtH6) = 22 Hz, H6), 9.06 (d, 1H, 3J(H5′H6′)
= 6 Hz, H6′), 9.08 (d, 1H, 4J(H3H5) = 1 Hz, H3), 9.12 (d, 1H,
4J(H3′H5′) = 1 Hz, H3′). ESI-MS: m/z 658.2; calcd for [2b+H]+,
658.22. Anal. Calcd for C28H34N2O4Pt·CH2Cl2: C, 48.89; H, 5.04; N,
4.00. Found: C, 48.82; H, 4.93; N, 3.61.
3
3
[PtMe2(O2CC6H4CO2H)(OH)(bipy)], 4. Phthaloyl peroxide (7.70
mg, 0.0525 mmol) was added to a solution of [PtMe2(bpy)] (20.0 mg,
0.0525 mmol) in acetone (5 mL). The color changed from orange to
colorless. Pentane was added to precipitate the product as a white
solid, which was separated, washed with pentane (3 × 2 mL) and ether
(3 × 2 mL), and then dried in vacuo. NMR analysis indicated the
presence of both 4 and trans-4, and recystallization from CH2Cl2/
pentane gave 4·4.5H2O. NMR in acetone-d6: trans-4, δ(1H) 2.20 (s,
2
6H, J(PtH) = 66 Hz, Me), 7.2−7.7 (m, 4H, phth), 7.84 (dd, 2H,
3
3
3J(H5H6) = 6 Hz, J(H4H5) = 8 Hz, H5), 8.23 (dd, 2H, J(H4H5) = 8
Hz, 3J(H4H3) = 7 Hz, H4) 8.60 (d, 2H, 3J(H3H4) = 7 Hz, H3), 9.38 (d,
2H, J(H6H5) = 6 Hz, H6). 4, δ(1H) = 1.10 (s, 3H, J(PtH) = 70 Hz,
3
2
PtMe), 1.15 (s, 3H, 2J(PtH) = 69 Hz, PtMe), 7.2−7.7 (m, 4H, phth),
7.81 (dd, 1H, J(H5H6) = 7 Hz, J(H5H4) = 8 Hz, H5) 8.05 (dd, 1H,
3
3
3J(H5′H6′) = 6 Hz, 3J(H4′H5′) = 8 Hz, H5′), 8.36 (dd, 1H, 3J(H4′H3′)
= 8 Hz, J(H4′H5′) = 8 Hz, H4′), 8.37 (dd, 1H, J(H3H4) = 8 Hz,
3
3
3J(H4H5) = 8 Hz, H4), 8.73 (d, 1H, J(H3H4) = 8 Hz, H3), 8.79 (d,
3
1H, J(H3′H4′) = 8 Hz, H3′), 9.20 (d, 1H, J(H6′H5′) = 6 Hz, H6′),
3
3
9.39 (d, 1H, J(H6H5) = 7 Hz, H6). ESI-MS: m/z 546.1; calcd for
3
equilibrate only slowly. The surprisingly complex chemistry can
therefore be rationalized in terms of Scheme 8.
[4+H]+, 546.10. Anal. Calcd for C20H18N2O4Pt·H2O: C, 42.63; H,
3.58; N, 4.97. Found: C, 42.36; H, 3.41; N, 4.51.
[PtMe2(O2CC6H4CO2H)(OH)(dpa)], 5. This was prepared in a
similar way to 4, but using [PtMe2(dpa)], 1c. Yield: 73%. NMR in
CD3OD: δ(1H) 1.61 (s, 6H, 2J(PtH) = 67 Hz, PtMe), 7.30 (ddd, 2H,
EXPERIMENTAL SECTION
■
Reactions were carried out using standard Schlenk techniques, unless
otherwise stated. NMR spectra were recorded using a Varian Mercury
400 or Varian INOVA 400 or 600 MHz spectrometer. 1H NMR
chemical shifts are reported relative to TMS. Mass spectrometric
analysis was carried out using an electrospray PE-Sciex mass
spectrometer (ESI-MS) coupled with a TOF detector.
The complexes [Pt2Me4(μ-SMe2)2] and 1a−1d were prepared
according to the literature.1n,12,21 Phthaloyl peroxide was prepared by
the method of Russell and recrystallized twice from benzene/pentane
until pure by NMR.9 NMR in CDCl3: δ(1H) 8.04 (m, 2H, H3); 8.30
(m, 2H, H2). ESI-MS: m/z 165.03, calcd for [C8H4O4+H]+ m/z
165.01.
3
4
3J(H5H6) = 6 Hz, J(H5H4) = 8 Hz, J(H5H3) = 1 Hz, H5), 7.40 (d,
2H, 3J(H3H4) = 8 Hz, H3), 7.58 (m, 2H, phth), 7.99 (ddd, 2H,
3
4
3J(H4H5) = 8 Hz, J(H4H3) = 8 Hz, J(H4H6) = 2 Hz, H4), 8.19 (m,
2H, phth), 8.45 (dd, 2H, 3J(H6H5) = 6 Hz, 4J(H6H4) = 2 Hz, H6). ESI-
MS: m/z = 413.1, 561.1; calcd for [PtMe2(OH)(dpa)]+, 413.1;
[PtMe2(dpa)(C8H5O4)]+, 561.1 g/mol. Anal. Calcd for
C20H21N3O5Pt·1.5H2O: C, 39.67; H, 4.00; N, 6.94. Found: C,
39.71; H, 3.79; N, 6.62.
[PtMe2(O2CC6H4CO2H)(dpkOH)], 6 and 7. To a stirring solution
of [PtMe2(dpk)] (126 mg, 0.307 mmol) in acetone was added a
solution of phthaloyl peroxide (50 mg, 0.307 mmol) in acetone (5
mL). The initial red color of the solution changed to colorless, and a
white precipitate formed after 10 min. The solution was filtered to give
7 as a white solid, which was washed with pentane (3 × 2 mL) and
ether (3 × 2 mL) and dried in vacuo. Pentane was added to the filtrate
to precipitate 6 as a white solid, which was separated and washed as
above. cis-[PtMe2(O2CC6H4CO2H)(dpkOH)], 6: NMR in acetone-d6:
δ(1H) 1.94 (s, 3H, 2J(PtH) = 70 Hz, PtMe), 2.04 (s, 3H, 2J(PtH) = 70
DFT calculations were carried out by using the Amsterdam Density
Functional program based on the BLYP functional, with double-ζ basis
set and first-order scalar relativistic corrections.22 The reported results
are from gas phase calculations. The energy minima were confirmed by
vibrational frequency analysis in each case.
1
Monitoring by H NMR Spectroscopy. In a typical experiment,
phthaloyl peroxide (1.6 mg, 0.01 mmol) was added to a solution of 1a
(3.8 mg, 0.01 mmol) in acetone-d6 (1 mL) in an NMR tube. The tube
was sealed, and spectra were recorded at intervals over a period of 1
week. The reaction gave a mixture of 2a and 3a. NMR in acetone-d6,
3
Hz, PtMe), 7.5 (m, 2H, H5, H5′), 7.80 (dd, 1H, J(H3H4) = 8 Hz,
4J(H3H5) = 1 Hz, H3), 7.92 (dd, 1H, 3J(H3′H4′) = 8 Hz, 4J(H3′H5′) =
2 Hz, H3′), 8.04 (ddd, 1H, 3J(H4H3) = 8 Hz, 3J(H4H5) = 8 Hz,
2a: δ(1H) 1.09 (s, 3H, 2J(PtH) = 69 Hz, MePt), 1.15 (s, 3H, J(PtH)
2
3
4J(H4H6) = 1 Hz, H4), 8.11 (ddd, 1H, J(H4′H3′) = 8 Hz, 3J(H4′H5′)
= 71 Hz, MePt). 3a: δ(1H) 1.88 (s, 6H, J(PtH) = 70 Hz, MePt),
2
4
3
= 9 Hz, J(H4′H6′) = 1 Hz, H4′), 8.50 (dd, 1H, J(H6′H5′) = 6 Hz,
4J(H6′H4′) = 1 Hz, H6′), 9.21 (dd, 1H, 3J(H6H5) = 6 Hz, 4J(H6H4) = 1
Hz, H6). ESI-MS: m/z 614.1; calcd for [Pt(C8H5O4)Me2(dpkOH)-
Na]+ 614.1. IR (Nujol mull, cm−1): 1652 ν(CO), 1605 ν(CO). Anal.
Calcd for C21H20N2O6Pt: C, 42.63; H, 3.41; N, 4.74. Found: C, 42.94;
7.30−9.25 [m, bipy, phthalate]. Integration of the methylplatinum
resonances gave the ratio of 2a:3a (Table 1). Similarly were identified,
from complex 1b: 2b, δ(1H) 1.01 (s, 3H, J(PtH) = 68 Hz, PtMe),
2
1.10 (s, 3H, 2J(PtH) = 70 Hz, Pt-Me); 3b, δ(1H) 1.87 (s, 6H, 2J(PtH)
= 71 Hz, PtMe); from complex 1c: 3c, δ(1H) 1.82 (s, 6H, J(PtH) =
2
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H, 3.66; N, 4.62. trans-[PtMe2(O2CC6H4CO2H)(dpkOH)], 7: NMR
in methanol-d4: δ(1H) 1.64 (s, 6H, 2J(PtH) = 70 Hz, PtMe), 7.50 (dd,
(5) (a) Canty, A. J. Dalton Trans. 2009, 10409. (b) Canty, A. J.;
Denney, M. C.; van Koten, G.; Skelton, B. W.; White, A. H.
Organometallics 2004, 23, 5432. (c) Canty, A. J.; Jin, H.; Skelton, B.
W.; White, A. H. Inorg. Chem. 1998, 37, 3975. (d) Canty, A. J.; Jin, H.
J. Organomet. Chem. 1998, 565, 135.
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(7) Abel, B.; Assmann, J.; Botschwina, P.; Buback, M.; Kling, M.;
Oswald, R.; Schmatz, S.; Schroeder, J.; Witte, T. J. Phys. Chem. A 1980,
102, 5279.
2
2
2H, J(H5H4) = 8 Hz, J(H5H4) = 5 Hz, H5), 7.52−7.92 (m, 4H,
phth), 7.80 (d, 2H, 3J(H3H4) = 8 Hz, 4J(H3H5) = 1 Hz, H3), 8.07 (dd,
2H, 3J(H4H3) = 8 Hz, 3J(H4H5) = 8 Hz, H4), 9.05 (d, 2H, 3J(H6H5) =
4
5 Hz, J(H6H4) = 1 Hz, H6).
X-ray Structure Determinations. The crystals were mounted on
a glass fiber, and data were collected at 150(2) K by using a Bruker
Smart Apex II CCD diffractometer. The unit cell parameters were
calculated and refined from the full data set, and the structure was
solved and refined by using the SHELX software.23 Details are given in
Table S1 and in the CIF files. The hydrogen atoms of the water
molecules in complex 4·4.5H2O could not be located, so the water
molecules were treated as O-atoms only.
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ASSOCIATED CONTENT
* Supporting Information
Figure S1 and Table S1. Crystallographic data in electronic CIF
form only is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the NSERC (Canada) for financial support and
Seowoo Kim for able experimental assistance.
■
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