Reactions of five-coordinate platinum(IV) complexes with molecular oxygen

Article abstract of DOI:10.1021/om4003363

The reactions of various five-coordinate Pt(IV) complexes with molecular oxygen have been studied. In arene solution, the complexes ( ^tBuMe2nacnac⁰)PtMe₃ (1; ^tBuMe2nacnac^- = [((4-tBu-2,6-Me₂C ₆H₂)NC(CH₃))₂CH]^-), (^Me3Me-nacnac)PtMe₃ (2; ^Me3Me-nacnac ^- = [((2,4,6-Me₃C₆H₂)NC(CH ₃))₂CCH₃]^-), and ( ^tBu2PyPyr)PtMe₃ (3; ^tBu2PyPyr^- = 3,5-di-tert-butyl-2-(2-pyridyl)pyrrolide) reacted immediately with oxygen to form peroxo species in which two oxygen atoms bridge between the metal center and a carbon atom in the ligand backbone. In contrast, no reaction between (^iPr2AnIm)PtMe₃ (4a; ^iPr2AnIm^- = [o-C₆H₄-{N(C₆H₃ⁱPr ₂)}(CHi - NC₆H₃ⁱPr ₂)]^-) or (^Me3AnIm)PtMe₃ (4b; ^Me3AnIm^- = [o-C₆H₄-{N(C ₆H₂Me₃)}(CHi - NC₆H ₂Me₃)]⁰) and oxygen was observed. As activation of oxygen by five-coordinate Pt(IV) species was found to involve cooperation between the metal center and the ligand, the ability of the ligand to participate in the oxygen binding appears to be a vital component. Oxygen atom transfer reactions of the novel peroxo species are also presented.

Full text of DOI:10.1021/om4003363

Article
pubs.acs.org/Organometallics
Reactions of Five-Coordinate Platinum(IV) Complexes with Molecular
Oxygen
Margaret L. Scheuermann, Avery T. Luedtke,† Susan K. Hanson,‡ Ulrich Fekl,§ Werner Kaminsky,
and Karen I. Goldberg*
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
S
* Supporting Information
ABSTRACT: The reactions of various five-coordinate Pt(IV) complexes
with molecular oxygen have been studied. In arene solution, the complexes
(
tBuMe2nacnac)PtMe3 (1; tBuMe2nacnac− = [((4-tBu-2,6-Me2C6H2)NC-
(CH3))2CH]−), (Me3Me-nacnac)PtMe3 (2; Me3Me-nacnac− = [((2,4,6-
Me3C6H2)NC(CH3))2CCH3]−), and (tBu2PyPyr)PtMe3 (3; tBu2PyPyr− =
3,5-di-tert-butyl-2-(2-pyridyl)pyrrolide) reacted immediately with oxygen
to form peroxo species in which two oxygen atoms bridge between the
metal center and a carbon atom in the ligand backbone. In contrast, no reaction between (iPr2AnIm)PtMe3 (4a; iPr2AnIm− = [o-
i
i
C6H4-{N(C6H3 Pr2)}(CHNC6H3 Pr2)]−) or (Me3AnIm)PtMe3 (4b; Me3AnIm− = [o-C6H4-{N(C6H2Me3)}(CH
NC6H2Me3)]−) and oxygen was observed. As activation of oxygen by five-coordinate Pt(IV) species was found to involve
cooperation between the metal center and the ligand, the ability of the ligand to participate in the oxygen binding appears to be a
vital component. Oxygen atom transfer reactions of the novel peroxo species are also presented.
respectively.3 As oxidative addition and reductive elimination
INTRODUCTION
■
are key reactions in homogeneous catalysis, understanding how
these unsaturated intermediate species react with molecular
oxygen will be essential to the rational development of novel
oxidation catalysts for use with O2. While five-coordinate Pt
species are usually highly reactive, often eluding even
spectroscopic observation, a limited number of isolable species
have recently been reported and characterized.3−7 In this
contribution we describe the reactivity of several isolable five-
coordinate Pt(IV) trimethyl complexes bearing chelating
anionic nitrogen ligands with molecular oxygen. We have
found that activation of dioxygen by these complexes requires
the involvement of both the electrophilic metal center and a
Lewis basic site on the ligand.
The use of homogeneous transition-metal complexes as
catalysts for organic transformations has grown tremendously
in the past several decades. Mild reaction conditions and the
realization of high selectivities with these catalysts have fueled
remarkable progress in synthetic organic chemistry. For
oxidation reactions, a challenge facing the field is how to
enable the widespread use of molecular oxygen as an oxidant in
combination with organometallic catalysts.1 Dioxygen (partic-
ularly if it can be used directly from air) is a tremendously
appealing stoichiometric oxidant for catalytic oxidation
reactions; it is readily available, is inexpensive, and is
environmentally benign, making it an ideal “green” oxidant.
However, controlling reactions with molecular oxygen has often
proven difficult. Overoxidation of the substrates and a lack of
selectivity in the products can limit synthetic utility.
Consequently, there has been intense effort toward developing
new reaction methodologies employing transition-metal-cata-
lyzed oxidation reactions with dioxygen.1
To fully exploit dioxygen’s potential as an oxidant in
homogenously catalyzed reactions, a better understanding of
how dioxygen interacts with various organometallic species that
may be present as intermediates in a catalytic system is needed.
To this end, we have been studying how molecular oxygen
interacts with various types of saturated and unsaturated
organometallic complexes.1c,2 In this contribution, we present
our investigations of the reactivity of a series of 16e five-
coordinate Pt(IV) centers with molecular oxygen. Five-
coordinate Pt(IV) species have been identified as ubiquitous
intermediates on the reaction paths of C−H, C−C, and C−X
bond cleavage and bond formation reactions via oxidative
addition to Pt(II) and reductive elimination from Pt(IV),
The five-coordinate complexes (tBuMe2nacnac)PtMe3, (1;
tBuMe2nacnac− = [((4-tBu-2,6-Me2C6H2)NC(CH3))2CH]−),5,8
(
Me3Me-nacnac)PtMe3 (2; Me3Me-nacnac− = [((2,4,6-
Me3C6H2)NC(CH3))2CCH3]−), (tBu2PyPyr)PtMe3 (3; tBu2Py-
Pyr− = 3,5-di-tert-butyl-2-(2-pyridyl)pyrrolide),6
(
iPr2AnIm)-
PtMe3 (4a; iPr2AnIm− = [o-C6H4-{N(C6H3 Pr2)}(CH
i
7
NC6H3 Pr2)]−), and (Me3AnIm)PtMe3 (4b; Me3AnIm− = [o-
C6H4-{N(C6H2Me3)}(CHNC6H2Me3)]−), shown in Chart
1, were exposed to oxygen in benzene-d6 or toluene-d8
solutions. Complexes 1−3 reacted rapidly, forming peroxo
intermediates in which one oxygen atom is bound to the
platinum center and the other to a carbon atom of the ligand
backbone. No reaction was observed with complexes 4a,b. This
difference in reactivity is attributed to the inability of the AnIm
i
Received: April 17, 2013
Published: August 26, 2013
© 2013 American Chemical Society
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which the Me groups can exchange positions. Five-coordinate
d6 complexes are typically fluxional, and the methyl groups of
the starting five-coordinate Pt(IV) trimethyl complexes have
been observed to exchange position rapidly on the NMR time
scale even at temperatures of −80 °C.3
Chart 1. Five-Coordinate Pt(IV) Complexes Studied in Our
Laboratory
Spectroscopic data for 5 also support the binding of oxygen
to the ligand backbone. The one-bond carbon−hydrogen
coupling constant (1JC−H) of the central carbon of the ligand
backbone decreased from 157 Hz in 1 to 144 Hz in 5. This
decrease in the coupling constant is consistent with a
hybridization change from sp2 to sp3.10 Likewise, addition of
alkene or alkyne to 1 also results in formal cycloaddition with
the same change in hybridization of the central carbon (from
sp2 to sp3), which is reflected in the measured JC−H values.8
1
The structure of 5 as depicted in Scheme 1 is well supported
by NMR spectroscopic evidence. Efforts to obtain crystals of 5
suitable for X-ray crystallography were unsuccessful as 5
underwent further reaction under ambient conditions to form
the Pt−alkoxide complex 6 (Scheme 1). All attempts to grow
crystals of 5 yielded complex 6 instead.8 The alkoxide complex
6 was characterized by NMR spectroscopy and X-ray
crystallography, confirming both the coordination of oxygen
to the metal and the formal insertion of an oxygen atom into
the C−H bond of the central carbon of the ligand backbone.
Complex 6 is structurally similar to Pt hemiketal ring systems
prepared by Puddephatt and co-workers.11 In the room-
ligand to interact with oxygen, thus preventing concurrent
binding of oxygen to the metal and the ligand.
RESULTS AND DISCUSSION
Reactions of Five-Coordinate Pt Complexes with
■
1
temperature H NMR spectrum of 6 in toluene-d8, resonances
for the axial Pt−Me group (0.72 ppm, 2JPt−H = 72 Hz) and the
equatorial Pt−Me groups (1.21 ppm, 2JPt−H = 70 Hz) could be
identified but were somewhat broadened, while the resonances
of the ligand were sharp. The Pt−Me resonances sharpened at
lower temperatures and broadened into the baseline at 50 °C,
suggesting fluxionality similar to that observed for 5.
Molecular Oxygen. When a bright orange solution of
(
tBuMe2nacnac)PtMe3 (1) in benzene-d6 or toluene-d8 was
mixed with dioxygen, the solution rapidly became pale yellow
and a new species, 5 (Scheme 1), formed.8 At room
Scheme 1
The conversion of peroxo complex 5 to the alkoxo species 6
involves cleavage of the oxygen−oxygen bond and the insertion
of an oxygen atom into a carbon−hydrogen bond. We reasoned
that replacement of the carbon−hydrogen bond on the central
carbon of the nacnac ligand with a more oxidation resistant
carbon−carbon bond could inhibit this transformation of the
peroxo intermediate. Thus, the analogous five-coordinate Pt
complex (Me3Me-nacnac)PtMe3 (2) was prepared, incorporat-
ing a recently reported nacnac-based ligand bearing a methyl
group rather than a hydrogen atom at the central carbon of the
ligand backbone.12 In analogy to the method used for preparing
1 and 3,6,8 Me3Me-nacnac-H was deprotonated with KH in
THF, and a toluene solution of the deprotonated ligand was
then added to a toluene solution of [PtMe3OTf]4, resulting in
immediate formation of bright red 2.
temperature, the three Pt−Me groups of 5 are represented by
a single broad resonance in the 1H NMR spectrum centered at
0.83 ppm in toluene-d8. When a sample of 5 in toluene-d8 was
cooled to −40 °C, the 1H NMR spectrum revealed two distinct
signals for the Pt−Me groups. A singlet integrating to one
methyl group was evident at 0.55 ppm with 2JPt−H = 68 Hz, and
a singlet integrating to two methyl groups appeared at 1.10
Upon exposure of a benzene solution of 2 to 1 atm of air or
O2, the bright red color of 2 rapidly disappeared and a pale
yellow solution with a pale yellow precipitate of a new species,
7 (Scheme 2), was obtained. In addition to being only sparingly
soluble in benzene, 7 is also insoluble in THF and acetonitrile.
2
ppm with JPt−H = 71 Hz. These values are consistent with
those expected for an axial and two equivalent equatorial Pt−
Me groups, respectively, in an octahedral geometry. The
coupling constant between the protons of the axial methyl
group and the Pt center (2JPt−H), 68 Hz, is of note because it
strongly indicates that a ligand is coordinated trans to the axial
methyl group. If there were no ligand in this position or only a
weakly coordinating one, a significantly larger coupling (>80
Hz) would be expected.4e,9 The broad resonance for the three
Pt−Me groups that is observed at room temperature is readily
explained by a fluxional process in which the Pt−Me groups
exchange positions on the NMR time scale. Most likely this
exchange proceeds through reversible opening of one of the
ligand arms to generate a transient five-coordinate species in
Scheme 2
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Dissolution of compound 7 in dichloromethane-d2 allowed for
Co macrocycle was reported to cyclize with dioxygen to form a
peroxo complex, but no X-ray structural evidence was
presented.18 We recently reported in situ spectroscopic
characterization of complex 5;8 however, the crystallographic
characterization of 7 provides further confirmation that oxygen
can indeed react by cycloaddition involving the β-diketiminate
ligand backbone. This new evidence is important in light of the
widespread use of β-diketiminate ligands in inorganic chemistry
and catalysis.19 In addition, it should be noted that such peroxo
structures wherein the oxygen molecule is bound to both the
metal center and the ligand backbone are reminiscent of those
proposed in the mechanisms of heme oxygenase20 and extradiol
ring-cleaving dioxygenases.21
To better understand the reactivity of a range of five-
coordinate Pt(IV) species with molecular oxygen, we
investigated the reaction of (tBu2PyPyr)PtMe3 (3; Scheme 3)
with oxygen.6 A benzene-d6 solution of 3 was exposed to
oxygen, and within minutes nearly quantitative formation of a
new species, 8, was observed by 1H and 13C NMR spectroscopy
at room temperature. In addition to a single set of well-resolved
resonances corresponding to the chelating ligand, the 1H NMR
spectrum of 8 contains three well-resolved Pt−Me resonances
indicative of a static six-coordinate Pt complex. This static
structure is in contrast to 5 and 7, which both exhibit exchange
of the axial and equatorial Pt−Me groups on the NMR time
scale at room temperature. When 8 was warmed in toluene-d8,
the Pt−Me resonances broadened relative to other ligand
resonances but were not fully coalesced at 80 °C.
1
characterization by H and 13C NMR spectroscopy. At 25 °C
the resonances attributable to the chelating ligand of 7 are
sharp; however, the Pt−Me groups appear as a single broad
resonance centered at 0.27 ppm. At −40 °C, two distinct
resonances for the axial and equatorial Pt−Me groups of 7 can
be observed (axial, −0.11 ppm, 2JPt−H = 69 Hz; equatorial, 0.30
2
ppm, JPt−H = 71 Hz). These coupling constants are nearly
identical with those of 5 (vide supra): 68 and 71 Hz for the
axial and equatorial Pt−Me groups, respectively. As in
compound 5, these data are most consistent with a six-
coordinate octahedral Pt(IV) complex.
Complex 7, with a methyl group on the central carbon of the
ligand backbone, is more stable than compound 5, with a
hydrogen atom at that position. The peroxo complex 7 persists
in dichloromethane solution for several days before decaying to
a mixture containing multiple species. X-ray-quality crystals of 7
were grown from a dichloromethane and pentane solution at
−10 °C. The crystal structure confirms that the product 7 is
indeed a peroxo complex with one oxygen atom bound to the
Pt center and the other bound to the central carbon of the
ligand backbone (Figure 1). The crystal structure reveals a
In air, a benzene/pentane solution of 8 was allowed to
evaporate slowly, resulting in small crystals suitable for X-ray
diffraction. Analysis of these crystals revealed an asymmetric
unit containing two independent molecules of 8 (Figure 3).
The structure of 8 has a peroxo group that bridges between the
Pt center and a carbon atom on the ligand backbone,
reminiscent of the peroxo complexes 5 and 7. The Pt−O
Figure 1. POV-Ray14 rendition of the ORTEP15 of compound 7 at the
50% probability level with hydrogen atoms and aryl rings omitted for
clarity. Selected bond lengths (Å) and angles (deg): Pt1−C1 =
2.062(3), Pt1−C2 = 2.041(2), Pt1−N1 = 2.1787(17), Pt1−O1 =
2.103(2), O1−O2 = 1.465(3), O2−C5 = 1.438(4); O2−O1−Pt1 =
117.05(16), C5−O2−O1 = 117.0(2), C2−Pt1−C1 = 84.27(13), C2−
Pt1−C1 = 86.46(11), C2−Pt1−O1 = 89.99(8), C2−Pt1−N1 =
94.99(8), C1−Pt1−N1 = 98.78(9), O1−Pt1−N1 = 84.71(7).
bond lengths in 7 (2.103(2) Å) and in 8 (2.11
0.02 Å,
average of two molecules) are similar, perhaps not surprisingly,
as the oxygen atoms of both 7 and 8 are trans to strongly
donating methyl groups. The O−O bond length in 8 (1.49
0.02 Å average) is slightly longer than that in 7 (1.465(3) Å),
and the O−O−Pt bond angles for the two complexes differ
substantially with 7 having an angle of 117.05(16)° and 8 an
slightly distorted octahedral geometry about the Pt center with
angles between cis ligands ranging from 84.3 to 98.8°. The O−
O bond length is 1.465(3) Å. A related iridacyclohexa-2,5-
diene−peroxo complex was reported to have an O−O bond
length of 1.466(7) Å (Figure 2).13
angle of 109
1° (average). The smaller angle in 8 is
consistent with the presence of smaller five-membered
platinacycles, in contrast to 7, which has only six-membered
platinacycles. Efforts to isolate 8 on a large scale were not
successful.
Unlike the peroxo complexes 5 and 7, complex 8 is not stable
to vacuum. A benzene solution of 8 was allowed to evaporate
slowly under vacuum over 1 week. Again X-ray-quality crystals
were obtained, but analysis of these revealed a new bimetallic
peroxo species, 9 (Figure 4). Such a structure could form from
the loss of O2 from 8 to form 3, which then coordinates to a
second equivalent of 8. The structure of 9 with a second Pt
atom bound to the peroxo oxygen atom proximal to the Pt
center suggests that the proximal oxygen atom is a relatively
electron rich and nucleophilic site. Efforts to crystallize 9 on a
larger scale were not successful.
Surprisingly, no reaction was observed upon mixing a
benzene-d6 solution of the five-coordinate complex (iPr3AnIm)-
PtMe3 (4a; Chart 1) with dioxygen at room temperature. This
is in contrast with the case for the five-coordinate Pt species 1−
3, which react with oxygen immediately upon mixing. When a
Figure 2. Previously reported Ir−peroxo complex with an O−O bond
length of 1.466(7) Å.13
While complexes with β-diketiminate-based ligands have
been shown to cyclize a wide range of substrates with multiple
bonds,4b,8,16 evidence for the formal 4 + 2 cyclization of
dioxygen has been more limited. Oxidation of the central
carbon of a β-diketiminate ligand by dioxygen has been
observed in Cu and Zn complexes, but in these reactions no
peroxo intermediates were detected.17 A β-diketiminate-based
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Scheme 3
Figure 4. POV-Ray14 rendition of the ORTEP15 of compound 9 at the
50% probability level with hydrogen atoms, tBu groups, and disorder
in the pyridine rings omitted for clarity. Selected bond lengths (Å) and
angles (deg): Pt1−C3 = 2.013(12), Pt−C2 = 2.037(12), Pt1−C1 =
2.082(12), Pt1−N1 = 2.12(3), Pt1−O1 = 2.188(7), Pt1−N2 =
2.198(9), Pt2−C23 = 2.021(12), Pt2−C22 = 2.031(11), Pt2−C21 =
2.076(12), Pt2−N3 = 2.02(3), Pt2−O1 = 2.202(7), Pt2−N4 =
2.209(9), O1−O2 = 1.490(9), O2−C9 = 1.388(12); C3−Pt1−C2 =
85.1(5), C3−Pt1−C1 = 92.7(6), C2−Pt1−C1 = 87.3(6), C3−Pt1−
N1 = 91.6(12), C2−Pt1−N1 = 98.6(14), N1−Pt1−O1 = 80.0(12),
C3−Pt1−N2 = 100.8(4), C1−Pt1−N2 = 98.2(5), N1−Pt1−N2 =
75.6(13), O1−Pt1−N2 = 77.9(3), C23−Pt2−C22 = 86.2(5), C22−
Pt2−C21 = 86.1(5), C22−Pt2−O1 = 89.9(4), N3−Pt2−O1 =
94.3(17), O2−O1−Pt1 = 108.6(5), O2−O1−Pt2 = 106.9(5), Pt1−
O1−Pt2 = 144.5(3), C9−O2−O1 = 110.7(7).
Figure 3. POV-Ray14 rendition of the ORTEP15 for the two
crystallographically independent molecules of compound 8 present
in the asymmetric unit at the 50% probability level with hydrogen
atoms omitted for clarity. Full details and measurements for each
independent molecule can be found in the Supporting Information.
Selected bond lengths (Å) and angles (deg) averaged over the two
independent molecules (estimated errors 0.02 Å and 1°):22 (C1−
Pt1)* = 2.04, (C2−Pt1)* = 2.01, (C3−Pt1)* = 2.05, (N1−Pt1)* =
2.14, (N2−Pt1)* = 2.21, (O1−O2)* = 1.49, (O1−Pt1)* = 2.11;
(O2−C9−N2)* = 110, (O2−C9−C8)* = 115, (N2−C9−C8)* =
103, (C2−Pt1−C1)* = 88, (C1−Pt1−C3)* = 91, (C2−Pt1−O1)* =
89, (C2−Pt1−N1)* = 95, (O1−Pt1−N1)*= 82, (C3−Pt1−N2)* =
103, (N1−Pt1−N2)* = 75, (O2−O1−Pt1)* = 109, (C9−O2−O1)*
= 111.
reaction times and heating were avoided, as 4b was found to
decompose in the absence of oxygen under these conditions
and the details of these reactions are not well understood.
Each of the five-coordinate Pt complexes studied has an open
site at the metal center, yet only 1−3 react readily with oxygen.
In each reaction, the oxygen molecule forms a bond to the open
site at the Pt center and also forms a bond to a carbon atom on
the ligand backbone. Of note, each of the carbon atoms in the
ligand that participates in the reaction with oxygen is a
relatively electron-rich site where a partial anionic charge
localization is suggested by a resonance structure. On the basis
of the observation that 1−3 react with oxygen but 4a,b do not,
it seems that subtle structural variations in the ligand are
important in controlling the reactivity of five-coordinate Pt
species with oxygen. In all five of these Pt(IV) complexes, the
Pt bears a formally anionic bidentate ligand with a delocalized
negative charge and all of the atoms in the platinacycle
containing the chelating ligand are sp2 hybridized. Upon
cooperative binding of oxygen, one of those sp2-hybridized
atoms must take on sp3 hybridization and move out of the
plane. In complexes 1 and 2 the strain of this conformational
change is easily accommodated, as the nacnac ligand localizes
its electrons to form two imine bonds. The peroxo complexes 5
benzene-d6 solution of 4a was pressurized with 1 atm of O2, the
solution retained its dark red color and no signals other than
those attributable to 4a itself were detected by 1H NMR
spectroscopy. Heating 4a in the presence of dioxygen resulted
in loss of ethane followed by a previously reported activation of
an aryl isopropyl group.7 The thermal reactivity of 4a with
oxygen was identical with that observed on heating 4a in the
absence of dioxygen.7
To test whether the sterically bulky isopropyl substituents on
the aryl rings could be preventing oxygen from accessing the
metal center of 4a, (Me3AnIm)PtMe3 (4b; Chart 1) was
prepared. The steric properties of this complex should be
similar to those of complexes 1 and 2, which also have methyl
groups at the 2- and 6-positions of the aryl rings. A solution of
4b was pressurized with 3 atm of O2. After 1 day, no new
1
species were observed in solution by H NMR spectroscopy
and the deep red color of the solution persisted. Longer
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Scheme 4
and 7 are formed rapidly and irreversibly. The monometallic
and bimetallic peroxo complexes derived from 3, complexes 8
and 9, indicate that the pyridyl−pyrrolide ligand structure can
also demonstrate this change of hybridization. The lack of
reactions involving 4a,b may be rationalized by considering that
a reaction with dioxygen analogous to that seen in 1−3 would
involve breaking the aromaticity of the six-membered arene ring
in the ligand backbone, a more thermodynamically uphill
transformation.23
Reactivity of the Pt(IV)−Peroxo Complexes. The
reactivity of the new peroxo complexes was also explored. As
was noted, the peroxo complex 5 undergoes further reaction in
arene solvents in the absence of any added substrate to form
the Pt alkoxide complex 6 (Scheme 1).8 This reaction to form 6
is interesting, as the O−O bond is cleaved and the C−H
functionality at the central carbon of the ligand backbone is
oxidized to a C−OH, suggesting that Pt(IV) peroxo complexes
might be effective oxidants for incorporating oxygen atoms into
organic substrates.
Figure 5. POV-Ray14 rendition of the ORTEP15 of compound 10 at
the 50% probability level with hydrogen atoms, aryl groups, and
cocrystallized pentane omitted for clarity. Selected bond lengths (Å)
and angles (deg): C1−Pt1= 2.028(8), C2−Pt1= 2.028(7), C3−Pt1 =
2.029(7), N1−Pt1 = 2.145(7), N2−Pt1 = 2.248(11), O1−Pt1 =
2.054(9); C2−Pt1−C1 = 83.6(6), C2−Pt1−C3 = 87.2(6), C2−Pt1−
O1 = 102.2(5), C3−Pt1−O1 = 97.4(5), C1−Pt1−O1 = 172.1(4),
C1−Pt1−N1 = 95.1(5), O1−Pt1−N1 = 79.5(3), O1−Pt1−N2 =
75.6(3).
carbon atom on the chelating ligand. This reaction with oxygen
requires the participation of both the metal center and a site on
the ligand backbone. In complexes 4a,b, there is no adequate
Lewis basic site on the ligand to participate in the binding of
oxygen and consequently no reaction with oxygen is observed.
The activation of oxygen by five-coordinate Pt(IV) species can
be regarded as another class of reaction involving metal−ligand
cooperation,24 opening new possibilities for the discovery of
other transition-metal oxygen activation reactions. Both intra-
and intermolecular oxygen transfer from the new Pt ligand
peroxo complexes were observed.
The related Pt(IV) peroxo complex 7 lacks a C−H bond on
the central C of the β-diketiminate backbone to which the
oxygen molecule attaches. The substitution of a methyl group
in this position appears to stabilize the peroxo intermediate and
has allowed us to probe the reactivity of the peroxo
functionality with added substrates. Complex 7 in dichloro-
methane-d2 solution was observed to react with PPh3, SMe2,
and CO. In each reaction, there was no change in the visual
1
appearance of the samples but H NMR spectra recorded after
4 h revealed complete consumption of 7. Regardless of which
substrate was added, the same new Pt-containing species, 10
(Scheme 4), formed in >90% yield (as determined by
EXPERIMENTAL SECTION
■
1
integration of the H NMR resonances relative to an internal
General Considerations. Unless otherwise specified, all manip-
ulations were carried out in a nitrogen-filled glovebox, using a Schlenk
line or using a high-vacuum manifold. Glassware was oven-dried prior
to use. Benzene, dichloromethane, THF, toluene, and pentane were
dried by passage through activated alumina and molecular sieve
columns under argon. Benzene-d6 and toluene-d8 were vacuum-
transferred from sodium/benzophenone ketyl. Dichloromethane-d2
was vacuum-transferred from CaH2. Complexes 1,8 3,6 4a,b,7 5,8 and
standard). In the cases of PPh3 and SMe2, OPPh3 and DMSO
were also detected, respectively. The three Pt−Me groups in
the Pt product 10 appear as a single broad resonance at 0.53
ppm in the 1H NMR spectrum. When 10 was cooled to −5 °C,
the broad resonance resolved into two singlets with Pt satellites
at 0.58 ppm (2JPt−H = 75 Hz) and 0.12 ppm (2JPt−H = 71 Hz)
for the equatorial and axial Pt−Me groups, respectively.
Crystals of 10 were obtained by slow evaporation of a solution
containing a mixture of dichloromethane-d2 and pentane. X-ray
crystallography confirmed that complex 10 is an octahedral
Pt(IV) species in which an O−O bond has been broken and
one oxygen atom has been lost such that the bridging peroxo
moiety from 7 is now an alkoxo (Figure 5).
25
[PtMe3OTf]4 and the neutral proligand for complex 2, Me3Me-
nacnac-H,12 were prepared using previously reported procedures. KH
was obtained as a suspension in mineral oil and washed with pentane
prior to use. All other reagents and gases were obtained from
commercial vendors and used as received. Gas pressures in excess of 1
atm were attained using a previously described pressurization
apparatus.26 NMR spectra were recorded on Bruker DRX500,
AV500, AV 700, and AV800 spectrometers and were referenced to
the residual protiated solvent signal. Data from HMQC and HMBC
experiments were used in the assignment of 13C resonances.
Abbreviations: s = singlet, d = doublet, dd = doublet of doublets, m
= multiplet, br = broad.
SUMMARY
■
The reactivity of oxygen with a series of five-coordinate Pt(IV)
trimethyl species with monoanionic N,N or N,N′ chelating
ligands was explored. Three of these complexes, 1−3, react with
oxygen immediately to form new Pt(IV) peroxo species that
were characterized spectroscopically and, in two cases,
crystallographically. The peroxo products exhibit bonding of
one of the oxygen atoms to the metal center and the other to a
Synthesis of (Me3Me-nacnac)PtMe3 (2). Me3Me-nacnac-H (69
mg, 0.20 mmol) was dissolved in 5 mL of THF in a glass reaction
vessel with a Teflon stopper. Potassium hydride (40 mg, 1.0 mmol)
was added, and the solution was stirred for 20 h. The bright yellow
suspension was filtered through a 0.2 μm PTFE filter, and the volatiles
were removed in vacuo, giving a yellow residue which was dissolved in
4756
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Organometallics
Article
15 mL of toluene. This solution was slowly added to a solution of
[PtMe3OTf]4 (78 mg, 0.050 mmol) dissolved in 5 mL of toluene.
After 30 min, the bright red suspension was filtered through a 0.2 μM
PTFE filter, and the volatiles were removed in vacuo to give a red solid
which was recrystallized from pentane at −35 °C to give red crystals of
2 (36 mg, 31% yield). 1H NMR of 2 (benzene-d6, 700 MHz, 298 K): δ
Generation of 10 using CO. An NMR tube fitted with a Teflon
valve was charged with 2 (2.2 mg, 3.7 mmol) and benzene (0.4 mL)
with a small amount of 1,3,5-trimethoxybenzene added as an internal
standard. The sample was pressurized with 3 atm of O2. After 20 min
all of the volatiles were removed in vacuo, dichloromethane-d2 was
added by vacuum transfer, and an initial spectrum of 7 was recorded
and integrated relative to the internal standard. The tube was then
pressurized with 1−3 atm of CO. Within 4 h, 10 was observed in
>90% yield (integration of 1H NMR spectrum relative to 1,3,5-
trimethoxybenzene). Crystals of 10 were obtained by layering a
dichloromethane solution of 10 with pentane and allowing it to stand
at approximately −10 °C for several days. 1H NMR of 10
(dichloromethane-d2, 500 MHz, 298 K): δ 6.95 (s, 2H, Ar-H), 6.91
(s, 2H, Ar-H), 2.29 (s, 6H, Arpara-CH3), 2.07 (s, 6H, Arortho-CH3), 2.07
(s, 6H, Arortho-CH3), 1.95 (s, 6 H, NCCH3), 1.84 (s, 3H, OCCH3),
0.53 (br, 9H, PtCH3). 1H NMR of 10 (dichloromethane-d2, 500 MHz,
268 K): δ 6.95 (s, 2H, Ar-H), 6.91 (s, 2H, Ar-H), 2.28 (s, 6H), 2.05 (s,
6H), 2.04 (s, 6H), 1.94 (s, 6 H, NCCH3), 1.85 (s, 3H, OCCH3),
6.90 (s, 4H, Ar-H), 2.24 (s, 6H, Arpara-CH3), 2.07 (s, 12H, Arortho
-
CH3), 1.97 (s, 3H, (NC)2CH3), 1.77 (s, 6 H, NCCH3), 1.12 (s,
2
9H, JPt−H = 74 Hz, PtCH3). 13C NMR of 2 (benzene-d6, 176 MHz,
298 K): δ 157.1 (NCbackbone), 148.4 (NCaryl), 133.6 (CAr‑para), 131.9
(CAr‑ortho), 129 (CAr‑meta), 100 (Cbackbone center), 21.4 (NCCH3), 20.9
(Arpara-CH3), 20.4 (Cbackbone center-CH3), 19.5 (Arortho-CH3), 2.0
(PtCH3). Anal. Calcd for C27H40N2Pt: C, 55.18; H, 6.86; N, 4.77.
Found: C, 55.26; H, 6.97; N, 4.72.
Reaction of 2 with Air To Form 7. Complex 2 (27.6 mg, 0.0470
mmol) was partially dissolved in 2 mL of benzene. The suspension was
exposed to air, upon which the solution color changed from orange to
pale yellow, accompanied by the formation of a pale yellow precipitate.
The suspension was allowed to settle, and after 1 h the benzene
supernatant was pipetted away. In air the remaining solid was rinsed
with 3 × 1 mL of THF and then dried under vacuum, yielding 7 (21.7
mg, 0.036 mmol, 76%). Crystals suitable for X-ray diffraction were
obtained by cooling a solution of 9 in dichloromethane/pentane to
approximately −10 °C. 1H NMR of 7 (dichloromethane-d2, 500 MHz,
2
2
0.58 (s, 6H, JPt−H = 75 Hz, PtCH3), 0.12 (s, 3H, JPt−H = 71 Hz,
PtCH3). 13C NMR of 10 (dichloromethane-d6, 201 MHz, 298 K): δ
190.8 (NC), 140.8 (NCaryl), 135.6 (CAr‑para), 130.4 (CAr‑ortho), 129.5
(CAr‑meta), 129.2 (CAr‑meta), 129.1 (CAr‑ortho), 94.6 (OC), 27.2 (OCCH3),
20.8 (Arpara-CH3), 20.7 (NCCH3), 19.9 (Arortho-CH3), 18.2 (Arortho
-
CH3).27 Anal. Calcd for C27H40N2OPt: C, 53.72; H, 6.68; N, 4.64.
Found: C, 53.79; H, 6.69; N, 4.61.
298 K): δ 6.98 (s, 2H, Ar-H), 6.95 (s, 2H, Ar-H), 2.31 (s, 6H, Arpara
−
Reaction of PPh3 or SMe2 with 7 To Yield 10. A solution of 7 in
dichloromethane-d2 was prepared as above. PPh3 (1.7 mg, 6.5 mmol)
was added. Within 4 h, 10 was observed in >90% yield (integration of
1H NMR spectrum relative to 1,3,5-trimethoxybenzene). Overlapping
signals in the 1H NMR spectrum made quantification of the OPPh3
difficult. A similar procedure was used for the reaction of 7 with SMe2.
The yield of DMSO (1H NMR integration vs the internal standard)
was in excess of 90% relative to the initial amount of 7 present.
CH3), 2.17 (s, 6H, Arortho-CH3), 2.07 (s, 6H, Arortho-CH3), 2.02 (s, 6 H,
NCCH3), 1.92 (s, 3H, OOCCH3), 0.27 (br, 9H, PtCH3). 1H NMR
of 7 (dichloromethane-d2, 500 MHz, 233 K): δ 6.95 (s, 2H, Ar-H),
6.93 (s, 2H, Ar-H), 2.28 (s, 6H, Arpara-CH3), 2.11 (s, 6H, Arortho-CH3),
2.02 (s, 6H, Arortho-CH3), 2.01 (s, 6 H, NCCH3), 1.91 (s, 3H,
OOCCH3), 0.30 (s, 6H, 2JPt−H = 71 Hz, PtCH3), −0.11 (s, 3H, 2JPt−H
= 69 Hz, PtCH3). 13C NMR of 7 (dichloromethane-d2, 201 MHz, 298
K): δ 177.5 (NC), 141.7 (NCaryl), 136.1 (CAr‑para), 130.7 (CAr‑ortho),
129.6 (CAr‑meta), 129.4 (CAr‑meta), 128.0 (CAr‑ortho), 88.1 (OOC, 3JPt−H
=
ASSOCIATED CONTENT
* Supporting Information
■
48 Hz), 23.6 (OOCCH3), 21.6 (NCCH3), 21.0 (Arpara-CH3), 19.4
(Arortho-CH3), 18.3 (Arortho-CH3).27 Anal. Calcd for C27H40N2O2Pt: C,
52.33; H, 6.51; N, 4.52. Found: C, 51.96; H, 6.61; N, 4.39.
S
Text, tables, figures, and CIF files giving crystallographic data,
details on X-ray methods, and selected NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Generation of 8 from 3 and O2. An orange benzene-d6 (0.4 mL)
solution of (tBu2PyPyr)PtMe3 (3; 2.3 mg, 0.0046 mmol) containing
toluene as an internal standard was prepared in an NMR tube fitted
with a Teflon stopcock. O2 gas (1 atm) was added to the tube. The
solution became bright yellow, and complex 8 formed in >90% yield,
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.I.G.: goldberg@chem.washington.edu.
1
■
as measured by integration of the H NMR spectrum. Crystals of 8
suitable for X-ray diffraction were obtained by slow evaporation of a
1
benzene/pentane solution of 8 in air. H NMR of 8 (benzene-d6, 500
MHz, 298 K): δ 8.00 (d, 2JH−H = 5 Hz, 3JPt−H = 19 Hz, 1H, Cpyridyl-H),
Present Addresses
2
2
†Sigma-Aldrich, 6000 N. Teutonia, Milwaukee, WI 53209,
United States.
7.21 (d, JH−H = 8.0 Hz, 1H, Cpyridyl-H), 6.73 (t, JH−H = 7.5 Hz, 1H,
2
Cpyridyl-H), 6.33 (t, JH−H = 7.0 Hz, 1H, Cpyridyl-H), 6.22 (s, 1H, N
2
2
‡Chemistry Division, Los Alamos National Laboratory, Los
Alamos, New Mexico 87544, United States.
§University of Toronto Mississauga, 3359 Mississauga Road N,
Mississauga, Ontario, Canada L5L 1C6.
CCH), 1.79 (s, 3H, JPt−H = 73 Hz, PtCH3), 1.60 (s, 3H, JPt−H = 77
2
Hz, PtCH3), 1.29 (s, 9 H, (CH3)3), 1.12 (s, 3H, JPt−H = 68 Hz,
PtCH3), 0.99 (s, 9H, (CH3)3). 13C NMR of 8 (benzene-d6, 126 MHz,
298 K): δ 184.2 (2JPt−C = 15 Hz, CC(CH3)3), 175.0 (CC(CH3)3),
158.9(2JPt−C = 8 Hz, Cpyridyl), 145.5 (2JPt−C = 16 Hz, Cpyridyl-H), 137.7
(Cpyridyl-H), 127 (NCC), 125.3 (JPt−C = 15 Hz, Cpyridyl-H), 122.8
(Cpyridyl-H), 119.1 (2JPt−C = 23 Hz, NCsp3), 36.6(C−(CH3)3), 35.5
(C(CH3)3), 32.6 (CH3)3), 28.1 (CH3)3), −6.5 (1JPt−C = 735 Hz,
PtCH3), −9.8 (1JPt−C = 707 Hz, PtCH3), −15.2 (1JPt−C = 683 Hz,
PtCH3).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
under Grant Nos. CHE-1012045 and DGE-0718124. We thank
Thomas R. Porter and Jason B. Benedict for assistance with the
collection of crystallographic data.
Crystallization of 9. In a glass reaction vessel fitted with a Teflon
stopcock (tBu2PyPyr)PtMe3 (3; 20 mg, 0.040 mmol) was dissolved in
benzene (4 mL). The solution was degassed by three freeze−pump−
thaw cycles, and then oxygen gas (1 atm) was introduced, causing a
color change from red to greenish yellow. The solvent volume was
reduced under vacuum over 1 week, resulting in crystals of the
bimetallic species 9, which were suitable for X-ray diffraction.
Typical Procedure for Testing 4a,b for Reaction with O2. A
benzene-d6 solution of 4b (10 mM) and the internal standard 1,3,5-
trimethoxybenzene in an NMR tube fitted with a Teflon valve was
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