Carbon-Oxygen Bond Forming Reductive Elimination from Cycloplatinated(IV) Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00745

The synthesis of the two cycloplatinated(IV) complexes [PtMe(OAc)₂(C^{^}N)(H₂O)] (C^{^}N = 2-phenylpyridinate (2a), benzo[h]quinolate, 2b) by reaction of [PtMe(C^{^}N)(SMe₂)] with PhI(OAc)₂ is described. Complexes 2 undergo carbon-oxygen bond forming reductive elimination instead of C-C reductive elimination to produce MeOAc as protected methanol. The kinetics and mechanism of both Pt-O bond formation and C-O reductive elimination have been experimentally and theoretically investigated. The results suggest that formation of methyl acetate proceeds via nucleophilic attack of the dissociated acetate ligand at the methyl group carbon in a cationic five-coordinate intermediate cycloplatinated(IV) complex.

Full text of DOI:10.1021/acs.organomet.7b00745

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Carbon−Oxygen Bond Forming Reductive Elimination from
Cycloplatinated(IV) Complexes
Marzieh Dadkhah Aseman,†,‡ S. Masoud Nabavizadeh,*,†,§ Fatemeh Niroomand Hosseini,§,∥
Guang Wu,§ and Mahdi M. Abu-Omar*,§
†Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565, Iran
‡Faculty of Chemistry, Kharazmi University, Tehran, Iran
§Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States
∥Department of Chemistry, Shiraz Branch, Islamic Azad University, Shiraz 71993-37635, Iran
S
* Supporting Information
ABSTRACT: The synthesis of the two cycloplatinated(IV) complexes [PtMe(OAc)2(C∧N)(H2O)] (C∧N = 2-phenylpyridinate
(2a), benzo[h]quinolate, 2b) by reaction of [PtMe(C∧N)(SMe2)] with PhI(OAc)2 is described. Complexes 2 undergo carbon−
oxygen bond forming reductive elimination instead of C−C reductive elimination to produce MeOAc as protected methanol.
The kinetics and mechanism of both Pt−O bond formation and C−O reductive elimination have been experimentally and
theoretically investigated. The results suggest that formation of methyl acetate proceeds via nucleophilic attack of the dissociated
acetate ligand at the methyl group carbon in a cationic five-coordinate intermediate cycloplatinated(IV) complex.
In this study we describe the synthesis and characterization
of isolable Pt(IV)−diacetate complexes using PhI(OAc)2 as the
acetate source. These cycloplatinated(IV) diacetate complexes
undergo smooth C(sp3)−O reductive elimination. Competing
C(sp2)−C(sp3) coupling at Pt(IV) center is not observed
during thermolysis. A kinetic study of oxidative addition to the
Pt(II) center by PhI(OAc)2 and reductive elimination of Me−
OAc from the resulting Pt(IV) center is discussed.
INTRODUCTION
■
Oxidative addition reactions of transition-metal complexes are
some of the most fundamental transformations in organo-
metallic chemistry and are often key steps in catalytic reactions.
In contrast to oxidative addition, its microscopic reverse,
reductive elimination, has received considerably less attention.
Among the C−X reductive elimination processes, examples of
reductive eliminations that form carbon−heteroatom (C−O,
C−N, or C−S) bonds have been less investigated in
comparison to reductive elimination reactions, which form
C−H and C−C bonds.1 Reductive elimination reactions to
form C−O bonds can result in the production of valuable
chemicals such as alcohols, ethers, and esters.2−4 Most
examples of C−O bond forming reactions are encountered
for low-valent d8 metal centers which involve aryl or acyl
carbon groups.2,5,6 C−O reductive eliminations of O−Calkyl
bonds have also been reported.7−13 Recently reductive
elimination of C−O bonds from Pd(IV) intermediates has
been studied by several groups (Scheme 1 for some examples),
forming valuable organic products.14−18 In particular, C−OAc
bond formation via reductive elimination is the most studied
case, providing various acetates for further synthetic trans-
formation.11,19−24
EXPERIMENTAL SECTION
■
1
General Remarks. H and 13C NMR spectra were recorded on a
Bruker Avance DPX 400 or Varian 500 or 600 MHz spectrometer and
referenced to external TMS (0.00 ppm). All chemical shifts and
coupling constants are given in ppm and Hz, respectively. UV−vis
spectra and kinetics were recorded on a Cary-60 spectrophotometer
equipped with a temperature controller capable of holding four cells or
a PerkinElmer Lambda 25 spectrophotometer with temperature
control using an EYELA NCB-3100 constant-temperature bath.
[PtMe(ppy)(SMe2)] (1a) and [PtMe(bhq)(SMe2)] (1b) were
prepared as reported previously.26
Synthesis of Cycloplatinated(IV) Complexes. The NMR
labeling for the ligands is shown in Scheme 3 to clarify chemical
shift assignments.
[PtMe(ppy)(OAc)2(H2O)] (2a). [PtMe(ppy)(SMe2)] (40 mg, 0.09
mmol), was dissolved in CH2Cl2 (15 mL), and PhI(OAc)2 (30 mg,
0.09 mmol) was added to the solution. The mixture was stirred at
room temperature over 8 h. The solvent was evaporated from the
C−O bond reductive elimination from Pt(IV) complexes,
including kinetics and mechanisms of the C−O coupling
reaction, has also been investigated, some examples of which
including the C−OAc group from Pt(IV) complexes are shown
in Scheme 2.7−9,12,25
Received: October 6, 2017
© XXXX American Chemical Society
A
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resulting pale yellow solution, and the residue was washed with ether
and hexane. Yield: 88%. Mp: 244 °C dec. Anal. Calcd for
C16H19O5NPt: C, 38.4; H, 3.8; N, 2.8. Found: C, 38.2; H, 3.7; N,
Scheme 1. Examples of C−O Bond Formation via Reductive
a
Elimination from Palladium(IV) Complexes
1
3.0. H NMR in CDCl3: δ 1.75 (s, 6H, CH3 of OAc ligand), 2.20 (s,
2J(PtH) = 68.6 Hz, 3H, MePt), 7.20−7.30 (m, 3H), 7.49 (t, 3J(HH) =
3
3
7.2 Hz, 1H, H5′), 7.70 (dd, J(H5H6) = 5.2 Hz, J(H5H4) = 8.2, 1H,
H5), 7.97 (t, 3J(HH) = 8.2 Hz, 1H, H4), 8.05 (d, 1H, 3J(H3H4) = 8.3
Hz, H3), 9.08 (d, J(H6H5) = 5.6 Hz, 1H, H6). 13C NMR: δ 1.77 (s,
3
1J(PtC) = 609.8 Hz, MePt), 23.38 (s, 2C, J(PtC) = 46.6, acetate
3
ligand), 119.78, 123.44, 124.85, 126.30, 127.82, 130.45, 130.58, 139.62,
2
141.12, 147.52, 159.72 (s, 11C, aromatic carbons), 182.30 (s, J(PtC)
= 43.1, 2C, OAc ligand).
[PtMe(bhq)(OAc)2(H2O)] (2b). This compound was made similarly
using [PtMe(bhq)(SMe2)] (45 mg, 1 mmol) and PhI(OAc)2 (32 mg,
1 mmol). Yield: 80%. Mp: 244 °C dec. Anal. Calcd for C18H19O5NPt:
1
C, 41.2; H, 3.6; N, 2.6. Found: C, 41.1; H, 3.8; N, 2.5. H NMR data
2
in CDCl3: δ 1.68 (s, 6H, CH33of acetate ligand), 2.41 (s, J(PtH) =
4
68.1 Hz, 3H, MePt), 7.54 (td, J(HH) = 7.56 Hz, J(HH) = 2.00 Hz,
1H, H10), 7.57 (m, 3J(HH) = 7.53 Hz, 1H, H9), 7.71 (dd, 3J(H11H10)
4
3
= 7.60 Hz, J(H11H9) = 1.11 Hz, 1H, H11), 7.75 (d, J(H6H7) = 8.7
Hz, 1H, H6), 7.79 (dd, J(H3H4) = 7.9 Hz, J(H3H2) = 5.38 Hz, 1H,
3
3
H3), 7.86 (d, 1H, 3J(H7H6) = 8.8 Hz, H7), 8.34 (dd, 1H, J(H4H3) =
3
8.28 Hz, 4J(H4H2) = 1.30 Hz, H4), 9.29 (dd, 1H, 3J(H2H3) = 5.14 Hz,
J(H2H4) = 1.23 Hz, H2). 13C NMR: δ 1.87 (s, 1C, J(PtC) = 578.09
1
3
Hz, MePt), 23.37 (s, 2C, J(PtC) = 43.80, Me of acetate ligand),
122.43, 124.01, 124.98, 127.49, 127.95, 129.13, 129.24, 130.27, 134.20,
137.05, 138.08, 146.94, 149.71 (s, 13C, aromatic carbons), 182.24 (s,
2J(PtC) = 42.77, 2C, acetate ligands).
a
Reprinted (adapted or reprinted in part) with permission from the
corresponding references. Copyright 2009 and 2015 American
Chemical Society and 2014 Royal Chemical Society.
General Procedure for Investigation of the C−O Reductive
Elimination Process. Reductive elimination reactions were carried
out in different solvents such as CDCl3, CD3CN, and C6D6 at 60 °C.
The formation of methyl acetate (MeOAc) was monitored by 1H
NMR spectroscopy. Crystals suitable for X-ray diffraction could not be
obtained for Pt(II) complexes 3 because they decompose and convert
to platinum black under vacuum. Complexes 3 were trapped by SMe2
to form the complexes [Pt(C∧N)(OAc)(SMe2)], which were easily
characterized by NMR spectroscopy.
Scheme 2. Examples of C−O Bond Formation via Reductive
Elimination from Platinum(IV) Complexes
a
[Pt(ppy)(OAc)(H2O)] (3a). 1H NMR in C6D6: δ 2.29 (s, 3H, CH3 of
2
2
acetate ligand), 5.81 (dd, 1H, J(HH) = 7.89 Hz, J(HH) = 12.0 Hz,
3J(PtH) = 24.0 Hz), 5.90−8.20 (br, aromatic protons), 8.25 (d,
2J(HH) = 5.80 Hz, J(PtH) = 41.2 Hz, 1H).
3
[Pt(bhq)(OAc)(H2O)] (3b). 1H NMR in C6D6: δ 2.39 (s, 3H, CH3 of
2
2
acetate ligand), 5.83 (dd, 1H, J(HH) = 5.3 Hz, J(HH) = 7.78 Hz,
3J(PtH) = 18.8 Hz), 5.90−8.20 (br, aromatic protons), 8.25 (d,
2J(HH) = 5.6 Hz, J(PtH) = 45.20 Hz, 1H).
3
[Pt(ppy)(OAc)(SMe2)] (4a). 1H NMR in CDCl3: δ 1.98 (s, methyl of
3
OAc group, 3H), 2.75 (s, J(PtH) = 52.7 Hz, SMe2 trans to N, 6H),
9.51 (d, J(HH) = 5.9 Hz, 3J(PtH) = 36.1 Hz, the C−H proton
adjacent to N of ppy, 1H), other aromatic protons of ppy ligand 7.51−
8.42.
[Pt(bhq)(OAc)(SMe2)] (4b). 1H NMR in CDCl3: δ 2.10 (s, methyl of
3
OAc group, 3H), 2.86 (s, J(PtH) = 53.8 Hz, SMe2 trans to N, 6H),
9.86 (d, J(HH) = 7.0 Hz, 3J(PtH) = 38.7 Hz, the C−H proton
adjacent to N of bhq, 1H), other aromatic protons of bhq ligand 7.51−
8.42.
a
Reprinted (adapted or reprinted in part) with permission from the
corresponding references. Copyright 1999, 2004, and 2007 American
Chemical Society.
Kinetic Study. For oxidative addition reactions, a solution of
complex 1 in CH2Cl2 (3 mL, 3.0 × 10−4 M) in a cuvette was
thermostated at 25 °C and PhI(OAc)2 was added using a microsyringe
under second-order 1:1 stoichiometric conditions ([PhI(OAc)2]0 =
[Pt]0). After rapid stirring, the absorbance at the corresponding
wavelength (360 nm for complex 1a and 400 nm for complex 1b) was
monitored over time. The absorbance−time profiles were analyzed
using the second-order equation (eq 1). For each temperature, at least
three kinetic experiments were run and the mean value was taken as
the second-order rate constant. The data at other temperatures were
obtained similarly, and activation parameters were obtained from the
Eyring equation (eq 2).
Scheme 3. NMR Labeling of Complexes 2
B
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Scheme 4. Reactions Studied in This Work
1
Figure 1. H NMR of complex 2b in CDCl3. The inset shows HH-COSY NMR in the aromatic region.
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Centre, CCDC 1568574.
Computational Details. Gaussian 09 was used29 to fully optimize
all the structures at the B3LYP level of density functional theory. The
effective core potential of Hay and Wadt with a double-ξ valence basis
set (LANL2DZ) was chosen to describe Pt and I.30 The 6-31G(d)
basis set was used for all other atoms. A polarization function (ξf =
0.993 for Pt and ξd = 0.289 for I) was also added. Single-point energy
calculations were performed at the B3LYP level using the def2-QZVP
basis set on Pt and I along with the corresponding ECP and the 6-
311+G(2d,p) basis set on other atoms. Frequency calculations were
carried out at the same level of theory to identify whether the
calculated stationary point is a minimum (zero imaginary frequency)
or a transition-state structure (one imaginary frequency). All data were
calculated at standard temperature and pressure (298.15 K and 1.0
atm). The solvation energies were calculated by the CPCM model in
CHCl3 and CH2Cl2 solvent for reductive elimination and oxidative
addition, respectively, which reflect the operating experimental
conditions. The ionic species were considered as an ion pair.
(Abs0 − Abs∞)
1 + [Pt]0 × k2 × t
Abst = Abs∞
+
(1)
ΔS⧧
R
ΔH⧧
RT
⎛
⎜
⎝
⎞
⎟
⎠
⎛
⎜
⎝
⎞
k2
T
kB
h
⎟
⎠
ln
= ln
+
−
(2)
The C−O reductive elimination process was monitored by 1H
NMR spectroscopy as follows: a small sample (10 mg) of complex 2
was dissolved in 0.75 mL of deuterated solvent in a sealed NMR tube
and heated to the desired temperature. Triphenylmethane (Ph3CH)
was used as an integral reference. NMR spectra were recorded several
times over about 24 h until the reaction was gradually completed. A
kinetic study of the reductive elimination reaction was also monitored
by UV−vis spectroscopy. In this case, a solution of complex 2b in
solvent (3 mL, 3.0 × 10−4 M) in a cuvette was thermostated at 60 °C
and the absorbance changes at the corresponding wavelength were
monitored over time. These UV−vis experiments for complex 2b were
repeated in the presence of excess water and N(n-Bu)4OAc to study
the effect of these species. The concentrations used for these
experiments were 0.018−0.18, (3.3−8.3) × 10−3 and 3.0 × 10−4
for water, N(n-Bu)4OAc, and 2b, respectively.
M
RESULTS AND DISCUSSION
■
Crystallographic Data. Single-crystal X-ray diffraction data of
complex 2b were collected on a Bruker KAPPA APEX II
diffractometer equipped with an APEX II CCD detector using a
TRIUMPH monochromator with a Mo Kα X-ray source (λ = 0.71073
Å). The crystal was mounted on a cryoloop under Paratone-N oil and
kept under nitrogen. Absorption correction of the data was carried out
using the multiscan method SADABS.27 Subsequent calculations were
carried out using SHELXTL.28 Structure determination was done
using intrinsic methods. Structure solution, refinement, and creation of
publication data were performed using SHELXTL. Crystallographic
information is presented in Table S1 in the Supporting Information.
The reaction studied in this work is divided into two processes
as illustrated in Scheme 4; (a) oxidative addition involving Pt−
O bond formation and (b) C−O bond formation via reductive
elimination from platinum(IV) to give MeOAc.
Oxidative Addition Process. Synthesis and Character-
ization of Cycloplatinated(IV) Acetate. As depicted in Scheme
4, the reaction of complexes [PtMe(SMe2)(C∧N)] (1) with
PhI(OAc)2 in CH2Cl2 cleanly yields the air-stable complexes
[PtMe(OAc)2(C∧N)(H2O)] (2) by oxidative addition of two
acetate groups onto the Pt(II) center in a trans manner. The
C
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resulting Pt(IV) center probably absorbs H2O (from moisture)
to replace the SMe2 ligand. The reaction in dry solvent under
an inert atmosphere forms [PtMe(OAc)2(C∧N)(SMe2)]
complexes, which are not stable over prolonged times. When
the NMR tube samples of these complexes are opened in air or
a solid sample is dissolved in an NMR solvent that is not dried,
the SMe2 complexes are converted to aqua complexes 2, as
confirmed by NMR spectroscopy. The Pt(IV) complexes 2 are
pale yellow solids and are stable in common organic solvents
such as chloroform, benzene, and CH2Cl2 for several days at
room temperature. This stability enabled their full character-
ization by multinuclear (1H and 13C) NMR spectroscopy, mass
spectrometry (ESI-MS), and X-ray crystal structure determi-
nation. As an example, the 1H NMR spectrum of 2b (Figure 1)
indicated methylplatinum groups at δ 2.45, which was coupled
carboxylate π bond is rather delocalized. The ESI mass
spectrum of 2b (positive ion mode in Figure 3) shows a base
peak at m/z 509, assignable to the species [Pt(OAc)2(bhq)-
(H2O)]+, which confirms the formation of cyclometalated
complex 2b containing two acetate groups.
2
with Pt to give satellites with J(PtH) = 68.1 Hz, confirming
that the methyl is directly bonded to platinum(IV). The two
methyls of acetate appeared as a singlet at δ 1.75. The
equivalency of the two acetate CH3 groups suggests that the
acetate ligands must be trans to each other. The spectroscopic
data are consistent with two acetate ligands bonded to 1b, as
the integration ratio of Me and OAc groups is 1:2. The inset of
Figure 1 shows the HH-COSY NMR of 2b, which was used to
assign the aromatic hydrogens.
Figure 3. Expanded (left) and simulated (right) isotopic pattern ESI
mass spectrum of the cation [Pt(bhq)(OAc)2(H2O)]+ fragment of
complex 2b at m/z 509, showing the expected intensity due to isotopic
distribution.
The solid-state structure of complex 2b was further
confirmed crystallographically and is shown in Figure 2. The
NMR Monitoring of Formation of Cycloplatinated(IV)
Complexes. The reaction of [PtMe(bhq)(SMe2)] (1b) with
1
PhI(OAc)2 was also monitored by H NMR spectroscopy.
Attempts were made to detect reaction intermediates, but no
intermediates could be detected under our reaction conditions.
1
The H NMR spectra (aromatic region) of a 1:1 reaction
mixture of 1b with PhI(OAc)2 in CD2Cl2 at 23 °C is shown in
Figure 4. Comparison of the spectra in Figure 4 shows that,
after addition of PhI(OAc)2 to Pt(II) complex 1b, the signals
due to starting complex 1b gradually disappeared and those for
the complex 2b and PhI emerged. A study of the reaction at low
temperature (from −40 °C) followed by warming to room
temperature gave similar NMR spectra, showing no detectable
intermediates.
Kinetic Study of Oxidative Addition Process. Complexes 1
contain an MLCT band in the visible region that could be used
to monitor their reactions with PhI(OAc)2 by using UV−vis
spectroscopy. The kinetics of oxidative addition reactions were
studied in dichloromethane solution. The reactions were
conveniently followed under second-order conditions by
using equimolar concentrations of 1a or 1b and PhI(OAc)2.
A typical set of UV−visible spectra taken during a kinetic run
is shown in Figure 5. Under these conditions, the reactions
followed second-order kinetics, first order in each reactant, the
corresponding Pt complex, and PhI(OAc)2. The resulting rate
constants are given in Table 1.
Eyring plots for the addition of PhI(OAc)2 to Pt(II)
complexes are shown in Figure 6, from which the activation
parameters were calculated (Table 1). The negative entropies
of activation are typical values for an associative mechanism.31
On the basis of kinetic data, a possible mechanism is shown in
Scheme 5. The reaction is initiated by nucleophilic attack of the
Pt center onto the iodide atom of PhI(OAc)2, which can be
considered as a ligand exchange process at I. The next step
involves elimination of PhI to give a five-coordinate
intermediate (see the next paragraph for DFT calculations).
The supposed intermediate can absorb an acetate ion to give
Figure 2. Structure of [PtMe(bhq)(OAc)2(H2O)] (2b). Selected
geometrical parameters (Å, deg): Pt1−O1 2.022(2); Pt1−O3
2.020(2); Pt1−O5 2.174(8); Pt1−C11 2.015(3); Pt1−C18
2.052(17); Pt1(6)−N1 2.161(15); O1−Pt1−O3 173.4(4); O3−
Pt1−O5 90.4(2); O1−Pt1−O5 95.3(8); N1−Pt1−C11 82.0(1);
C11−Pt1−C18 84.2(1); O1−Pt1−N1 88.31(9).
crystal was grown by slow diffusion of hexane into a CH2Cl2
solution of 2b in ambient air and contains a coordinated water
molecule. It confirms the characterization of 2b as an
octahedral platinum(IV) complex as a result of trans oxidative
addition of acetate groups of PhI(OAc)2. The Pt(IV)−C and
Pt−O bond lengths are comparable to those in other reported
structures with similar trans ligands.8 The two Pt−O distances
of acetate ligands are approximately equal (2.022(2) and
2.020(2) Å). The formal carbon−oxygen single and double
bonds in the acetate fragment have small differences in bond
lengths (1.298(4) and 1.237(4) Å), suggesting that the
D
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Figure 4. Monitoring the platinum(II)/platinum(IV) transformation process by 1H NMR spectroscopy (aromatic region) in CD2Cl2 at 23 °C. The
first spectrum is obtained from the starting complex [PtMe(bhq)(SMe2)] (1b), and the next spectra are obtained after addition of PhI(OAc)2 to the
starting complex 1b. The peaks of oxidant (PhI(OAc)2, #) and consumed oxidant (PhI, *) are assigned.
Figure 6. Eyring plot for the reaction of [Pt(C∧N)(Me)(SMe2)] (1)
with PhI(OAc)2 in CH2Cl2.
To shed some light on the suggested mechanism (depicted
in Scheme 5), DFT calculations were carried out on the
precursor complex 1b, final product 2b, the potential transition
state, and intermediates. The conductor-like polarizable
continuum model (CPCM) was used to model general
solvation effects by CH2Cl2. The reaction is initiated by
Figure 5. Changes in the UV−visible spectrum during the reaction of
[PtMe(ppy)(SMe2)] (1a; 3 mL of 3 × 10−4 M solution) with
PhI(OAc)2, under second-order 1:1 stoichiometric conditions, in
CH2Cl2 at 25 °C. The inset shows the variation of absorbance at 360
nm (the MLCT band) over time.
the final cyclometalated Pt(IV) complex.32 A similar mecha-
nism was suggested for the reaction of PhI(OAc)2 with
nucleophiles.33 As reported in Table 1, the rate constant for
oxidative addition process is almost 2 times faster for complex
1b (having bhq ligand) than for complex 1a (with ppy as
chelating ligand), which could be attributed to the more
extended π conjugation system in the bhq complex in
comparison to that for ppy. It should be noted that the
cyclometalated Pt(II) complexes are among the most active of
the noble-metal complexes in oxidative addition reactions.31,34
nucleophilic attack by the 5dz HOMO of the Pt(II) center of
2
1b onto the iodide atom of PhI(OAc)2. Substitution of an
acetate group by the Pt center, followed by simultaneous partial
removal of an acetate ion, results in formation of the transition
state TS (Figure 7). The free energy barrier for this step was
calculated to be 16.1 kcal mol−1 in CH2Cl2 at 298.15 K, which
is in excellent agreement with the experimental value of 16.4
kcal mol−1. In TS, the bond angle Pt−I−Ph (96.7°) is close to
90°, showing a vertical arrangement with the iodide atom.
During the formation of TS, the most significant changes in
a
Table 1. Rate Constants and Activation Parameters for the Reaction of Complexes [PtMe(C∧N)(SMe2)] (1) with PhI(OAc)2
in Dichloromethane
k2 (L mol−1 s−1
)
complex
15 °C
20 °C
25 °C
30 °C
35 °C
ΔH‡ (kcal mol−1
)
ΔS⧧ (cal mol−1 K−1
)
ΔG⧧ (kcal mol−1
)
1a
1b
1.23
2.97
1.84
4.03
2.51
5.12
3.88
6.83
5.14
8.68
12.3 0.3
8.8 0.1
−15
−26
1
5
16.8 0.3
16.4 0.3
a
Estimated errors are 6%.
E
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Scheme 5. Proposed Mechanism for Reaction of Pt(II) Complexes with PhI(OAc)2
an acetate ion to the platinum center gives intermediate IM2.
The latter intermediate can absorb another acetate ion released
in an earlier step to form IM3, which then proceeds to
cyclometalated Pt(IV) complex 2b by replacement of SMe2 by
H2O. Product 2b has an octahedral geometry around the
Pt(IV) center (see Scheme 5 and Figure 7) with Pt−OAc bond
lengths of 2.062 Å, in excellent agreement with the
experimental X-ray value of 2.022 Å.
Reductive Elimination Process. Carbon−Oxygen Bond
Formation. Complexes 2 are prone to lose methyl acetate by
heating in different organic solvents through a carbon−oxygen
reductive elimination process (see Scheme 6). The Pt(II)
products obtained from reductive elimination were charac-
terized by NMR and trapped by addition of SMe2.
Monitoring of MeOAc Formation. The thermolyses of
complexes 2 at 60 °C were studied in different organic solvents
such as C6D6, acetone-d6, CD3CN, and CDCl3. During heating
C(sp3)−O bond forming reductive elimination was observed,
forming methyl acetate along with [Pt(C∧N)(OAc)(H2O)]
(3). No competitive reductive elimination of the carbon−
carbon bond was observed. To investigate the detail of the
mechanism of C−O bond reductive elimination, thermolyses of
complexes 2 were monitored in CD3CN in a sealed NMR tube
at 60 °C (see Figure 8). As the time was passing at this
temperature, the signals due to starting complex 2b at 1.57 and
2.36 ppm gradually disappeared and those for methyl acetate at
2.01 and 3.63 ppm appeared. The product 3b is not stable at
high temperature for a long time and decomposes.
Kinetic Study of C−O Bond Formation. The rate constants
for the C−O reductive elimination product were obtained using
1H NMR spectroscopy by following the signal for MeOAc
formation in different solvents. Triphenylmethane (Ph3CH)
was used as an integral reference. The data are collected in
Table 2. A first-order kinetic plot of thermolysis of 2b, as an
Figure 7. Free energy diagram and calculated structures of starting
material, transition state, intermediates, and product of oxidative
addition (2b) in CH2Cl2 solvent. The H atoms are omitted for clarity.
bond distances are observed for I−Ac and Pt−I distances. The
I−OAc bond increases from 2.182 Å in PhI(OAc)2 to 2.444 Å
in TS, whereas the Pt−I distance decreases from being far apart
in the reactant to 2.975 Å in TS. This confirms that oxidative
addition of PhI(OAc)2 to 1b involves simultaneous cleavage of
the I−OAc bond and formation of a Pt−I bond, which is
followed by formation of the cationic intermediate IM1 by
completely breaking the I−OAc bond and forming Pt−I. The
intermediate IM1 has a square-pyramidal geometry around Pt,
with the incoming PhI(OAc) group occupying the apical
position and the acetate ion being in the outer sphere of IM1.
The elimination of PhI from IM1 followed by coordination of
Scheme 6. Carbon−Oxygen Bond Formation by Reductive Elimination from Pt(IV) Complexes
F
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Figure 8. 1H NMR spectra (aliphatic region) of thermolysis of complex 2b at 60 °C in CD3CN. The time interval is 30 min. The residual peak of
CD3CN appeared at 1.94 ppm. Small traces of impurities around 2.10−2.15 ppm are due to the water of the deuterated solvent and Pt complex after
decomposition. Asterisks show Pt satellites.
a
b
was used to determine the rate of the reaction as a function of
Pt(IV) concentration over a concentration range of (1.5−9.0)
× 10−4 M (see Figure 9C). Under our reaction conditions, the
C−O reductive elimination from cycloplatinated(IV) complex
2b showed a first-order dependence on Pt complex (1.06
0.06). The rate constants for carbon−oxygen bond reductive
elimination were obtained and are reported in Table 2. The
obtained rate constants from UV−vis studies are in good
agreement with those obtained from NMR experiments.
The C−O reductive elimination reaction of complex 2b was
also investigated at different excess amounts of N(n-Bu)4OAc
(over a concentration range of (3.3−8.3) × 10−3 M) in
comparison to Pt(IV) complex 2b under pseudo-first-order
conditions. Under our reaction conditions, the C−O reductive
elimination from cycloplatinated(IV) complex 2b showed a
zero-order dependence on N(n-Bu)4OAc (see Figure 10A). As
is clear from Figures 9D and 10A, the results indicate that the
reaction obeys the simple first-order rate law rate = k[Pt(IV)].
The same experiment was also done to investigate the effect of
added water on the rate of C−O reductive elimination. It was
found that water has no significant effect on the rate and the
order of the reaction in water is zero (see Figure 10B).
Theoretical Mechanistic Study of C−O Coupling Reac-
tions. Because of the carbon−heteroatom bond forming
reductive elimination from Pt(IV), commonly observed to
proceed through a dissociative mechanism,9,35 and the key five-
coordinate intermediates accessed through ligand loss from the
ground-state octahedral structure, three different mechanisms
for C−O reductive elimination can be proposed (Scheme 7).
The first possible mechanism (mechanism A) is thermally
induced dissociation of an acetate ligand to form a cationic five-
coordinate cycloplatinated(IV) complex, followed by nucleo-
philic attack of dissociated acetate to the Me group carbon to
give methyl acetate and Pt(II) product. Alternatively, the
remaining coordinated acetate can concertedly form a bond to
the Me group and produce methyl acetate. The second
mechanism (mechanism B), or the concerted bond formation
accompanied by chelate dissociation, involves direct C−O
reductive elimination bond formation from a five-coordinate
platinum(IV) complex. The third possible mechanism (mech-
Table 2. Rate Constants and Yields for Thermolysis of
Complexes 2a,b in Different Organic Solvents at 60 °C
2a
2b
dielectric
constant
105k
yield
(%)
yield
(%)
solvent
C6D6
(s−1
)
105k (s−1
)
2.27
4.81
20.7
37.5
1.8
3.1
2.1
3.2
68
77
16
87
0.9 (1.3)
1.6 (1.1)
1.2 (2.0)
1.7 (3.2)
79
74
20
CDCl3
acetone-d6
CD3CN
88
a
b
Estimated error is 10%, using NMR (or UV−vis) spectroscopy. By
NMR on the basis of MeOAc formation.
example, is shown in Figure 9A. The rate constants of MeOAc
formation from 2a are generally higher than those for 2b,
showing that complex 2a is more reactive toward C−O
reductive elimination than 2b. Dielectric constants of solvents
used for reductive elimination process are also included in
Table 2. The solvent polarity has no effect on the rate of methyl
acetate formation. For example, the rate of reaction in the more
polar solvent acetone is almost identical with that obtained in
the less polar solvent benzene. The yield of MeOAc, reported
in Table 2, shows a range of about 16% in acetone to 88% in
acetonitrile. Although the only organic product obtained during
the reductive elimination process is MeOAc (see the NMR
experiment shown in Figure 8), Pt complexes did decompose in
solution when they were heated for a long time (black Pt metal
was usually observed). This may be a reason for the low yield in
acetone and the less than quantitative yield of MeOAc in the
other solvents.
The carbon−oxygen bond reductive elimination from
complex 2b at 60 °C has also been monitored by UV−vis
spectroscopy. During this process, the cycloplatinated(IV)
complex 2b is converted to complex 3b by losing methyl
acetate. Using UV−vis spectroscopy a known concentration of
2b is used and the appearance of the MLCT band at the
corresponding λmax (400 nm for CH3CN, CHCl3, and benzene
and 380 nm for acetone) is used to monitor the C−O reductive
elimination reaction. The change in the spectrum during a
typical run is shown in Figure 9B. The method of initial rates
G
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1
Figure 9. (A) First-order kinetic plot of thermolysis of 2b, in CD3CN at 60 °C using H NMR spectroscopy. (B) Changes in the UV−visible
spectrum during thermolysis of [Pt(bhq)(Me)(OAc)2(H2O)] (2b; 3 mL, 3 × 10−4 M solution) in CH3CN at 60 °C. (C) First-order kinetic plot of
the thermolysis of 2b showing first-order dependence of rate on [2b]. (D) Plot of initial rate versus concentration of 2b fit to y = m1*m0∧m2 where
y = initial rate, m0 = [2b], m1 = rate constant (k), and m2 = order in Pt(IV).
Figure 10. Plot of initial rate versus concentration of N(n-Bu)4OAc (A) and H2O (B) showing zero-order dependence of rate on free ion acetate and
H2O.
anism C) initiates with predissociation of H2O ligand followed
by internal unimolecular C−O coupling bond formation.
To find the most suitable mechanism for C−O bond forming
reductive elimination from the cycloplatinated(IV) acetate
complex, the proposed mechanisms were theoretically inves-
tigated using density functional theory calculations to gain
more insight into the mechanism, as well as to determine the
geometries of the transition states, intermediates, and energy
barriers of each proposed mechanism (see Figure 11). The
optimized geometry for complex 2b is in excellent agreement
with X-ray crystallographic data, and our calculation success-
fully reproduces the bond angles and bond distances within
precisions of 1.2° and 0.05 Å, respectively. This indicates the
reliability of the selected theoretical method for this study, and
therefore we employ it for our mechanistic investigation.
Mechanism A. According to this mechanism, the
cycloplatinated(IV) complex 2b with coordination number 6
first loses an acetate ion to give a cationic five-coordinate (IMa)
as the reaction intermediate. The free energy barrier for this
step is calculated to be 23.6 kcal mol−1 in CHCl3 solution. It
should be noted that no significant structural geometry change
was found in intermediate IMa. The corresponding inter-
mediate in which coordinated acetate ion acts as a bidentate
ligand is found to be less stable in comparison to the five-
coordinate intermediate IMa due to ring strain present in the
four-membered ring of this structure with an O−Pt−O angle of
H
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a
Scheme 7. Proposed Mechanisms for Carbon−Oxygen Reductive Elimination from Cycloplatinated(IV) Complex 2b
a
The main mechanism is pathway 2b → IMa → IMa1 → TSa2 → 3b.
Figure 11. Calculated geometries of the species involved in the C−O reductive elimination process from complex 2b and the energy barriers of each
proposed mechanisms (A in black, B in red, and C in blue). Anionic components of ionic species are omitted, and only DFT-optimized structures of
Pt complexes are shown for more clarity.
58.6°. Although the intermediate IMa could undergo a
carbon−oxygen bond formation to give transition state TSa
(not shown in Figure 11; see Scheme 7), this transition state
cannot be located and instead TSa1 is found. The intermediate
IMa then isomerizes to IMa1, in which the methyl ligand
occupies an axial position. Formation of such intermediates
with the axial methyl group is well-established in a number of
C−O coupling reactions at Pt(IV).10,25 There are two
I
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possibilities from this intermediate. The first is an intra-
molecular C−O reductive elimination of the remaining
coordinated acetate ion and Me group (transition state
TSa1) to give methyl acetate. The energy barrier for this step
is calculated as 69.4 kcal mol−1, which is higher than that found
for nucleophilic attack of dissociated acetate onto the Me
group. Therefore, this path (IMa1 → TSa1 → 3b + MeOAc) is
ruled out. The second possible pathway for IMa1 is
nucleophilic attack of dissociated acetate to the Me group
carbon to give methyl acetate and Pt(II) product through
TSa2, which includes a Pt−CMe−O bond angle of 157.0°.
During the formation of TSa2, the most significant changes in
bond distances are observed for Pt−CMe and C−O bonds. The
Pt−CMe length increases from 2.054 Å in IMa1 to 2.272 Å in
transition structure TSa2, whereas the O···CMe distance
decreases from being far apart to 2.185 Å. The accuracy of
transition state TSa2 is confirmed by the observation of an
imaginary frequency (−347 cm−1), which corresponds to a Pt−
C−O stretching vibrational mode in which the Pt−O bond
ruptures and the C−O bond forms. TSa2 has a free energy of
+24.1 kcal mol−1 in CHCl3 in comparison to starting complex
2b. The formation of the transition state TSa2 is followed by
complete breaking of the Pt−CMe bond and formation of Me−
OAc. The overall free energy change of this C−O bond
forming reductive elimination process is −21.9 kcal mol−1.
Mechanism B. In this mechanism, the starting cyclo-
metalated Pt(IV) complex 2b, as the starting material,
undergoes a reductive elimination process. Here, the reactant
concurrently releases a pyridyl arm of the bhq ligand to form
the five-coordinate Pt(IV) transition sate TSb. In TSb, the O−
Pt−CMe bond angle decreases from 90.4° (in 2b) to 49.2°,
while the Pt−N bond increases from 2.237 Å (in 2b) to 2.765
Å. The Pt−C and Pt−O bond distances also increase from
2.067 to 2.549 Å and from 2.062 to 2.086 Å, respectively. The
free energy barrier for this step is +35.7 kcal mol−1 in CHCl3
solution. The direct product of this mechanism is the square-
planar cycloplatinated(II) complex 3b with an overall free
energy of −21.9 kcal mol−1 in comparison to reactant 2b. This
mechanism is ruled out due to the high energy barrier
calculated for this pathway in comparison to those calculated
for Path A.
Mechanism C. The third possible C−O coupling mecha-
nism, C, involves preliminary water dissociation to produce the
five-coordinate neutral intermediate IMc. Direct C−O coupling
reductive elimination from this intermediate is ruled out due to
the high energy barrier of 63.9 kcal mol−1 calculated for TSc
(see Scheme 7 and Figure 11). Isomerization of IMc to IMc3 is
also not possible because the latter is located at higher energy
in comparison to IMc. Therefore, the formation of the
intermediate IMc is followed by isomerization to intermediate
IMc1 and then intermediate IMc2, in which the Me group is
located in an axial position of Pt(IV) complex. Although
transition state TSc1 (shown in Scheme 7) from IMc1 could
not be located, the reaction may proceed through IMc1 and
then transition state TSc2, where Me ligand in an axial position
can make a bond with the OAc group trans to C of the bhq
ligand. Complete Pt−O and Pt−C bond breaking and C−O
bond forming results in the neutral four-coordinate [Pt(bhq)-
(OAc)(H2O)] product 3b, having a square-planar geometry,
with the water group located in a position trans to the carbon
atom of the cyclometalated bhq ligand. The energy barrier for
this mechanism is calculated as +36.9 kcal mol−1 in CHCl3.
In all of these proposed mechanisms studied above using
DFT calculations, the rate-determining step is C−O bond
forming reductive elimination. The free energy barriers for
mechanisms A−C (through transition states TSa2, TSb, and
TSc2) are +24.1, +35.7, and +36.9 kcal mol−1, respectively.
These values suggest that the favored mechanism should be
mechanism A with the lowest energy barrier. On the other
hand, using transition state theory with the transmission
coefficient equal to 1, the first-order rate constants at a
temperature of 60 °C for mechanisms A−C are calculated as
1.2 × 10−5, 3.7 × 10−14, and 5.1 × 10−15 s−1, respectively. If we
compare these calculated rate constants with our measured
experimental value of 1.6 × 10−5 or 1.1 × 10−5 s−1 (from NMR
and UV−vis measurements, respectively) in CHCl3, the best
theoretical value in agreement with experimental finding is 1.2
× 10−5 s−1, which provides further support for the pathway 2b
→ IMa → IMa1 → TSa2 → 3b.
CONCLUSION
■
In the presented work, first the synthesis of two stable
cycloplatinated(IV) complexes, [PtMe(C∧N)(OAc)2(H2O)], is
described. PhI(OAc)2 is used as the acetate source to make Pt−
O bonds through an oxidative addition process. Kinetic studies
show that the bhq complex reacts more rapidly with PhI(OAc)2
than its ppy analogue. These cyclometalated Pt(IV) complexes
are used to study the kinetics and mechanism of C−O bond
forming reductive elimination. On the basis of kinetics and
DFT investigations, we propose that C−O reductive
elimination proceeds via acetate dissociation to generate a
cationic five-coordinate intermediate, followed by nucleophilic
attack of acetate to give methyl acetate and Pt(II) product. On
the basis of this mechanism, simplified in eqs 3−5, the rate law
can be derived as eq 6.
k1
2b HooI IMa + OAc−
k−1
(3)
(4)
(5)
fast
IMa ⎯⎯→ IMa1
k2
IMa1 + OAc− → 3b + MeOAc
k1k2[OAc−][2b]
k−1[OAc−] + k2[OAc−]
k1k2[2b]
k−1 + k2
rate =
=
(6)
The rate equation shown in eq 6 is consistent with the
experimental data, as it predicts first order in [2b] (see Figure
9) and zero order in [OAc−] (see Figure 10A): i.e., rate = k[2b]
where k = k1k2/(k−1 + k2). The same mechanism has been
previously suggested for C−O bond reductive elimination from
Pt(IV) complexes. For example, Williams et al.12 suggested the
same mechanism in thermolysis of platinum(IV) hydroxide
complexes which occurs from dissociation of hydroxide ion
followed by nucleophilic attack on the Pt(IV) methyl group.
Bercaw and co-workers have also shown that a C−O reductive
elimination from their Pt(IV) system proceeds via an SN2
reaction.3 Similarly, the same mechanism for other organo-
metallic systems including Pd(IV) (see Scheme 1) and Rh(III)
had been proposed.36 The calculated rate constant using DFT
for the proposed mechanism (mechanism A through transition
state TSa2) is in agreement with the experimentally determined
rate constant. It is noteworthy that the acetate leaving group
attacks the Me group of IMa1 in which the Me ligand is located
at an axial position. As a result, a cationic five-coordinate Pt
J
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intermediate that is susceptible to C−O reductive elimination
emerges. One more piece of evidence for this mechanism is the
lack of solvent effect. As shown in Table 2, the rate of C−O
reductive elimination from complex 2a in the more polar
solvent acetonitrile (ε = 37.5) is almost identical with that
obtained in the less polar solvent chloroform (ε = 3.1);
therefore, the rate of C−O coupling is almost independent of
solvent polarity, which is consistent with the suggested
mechanism through acetate ion dissociation.8
REFERENCES
■
(1) Desnoyer, A. N.; Love, J. A. Recent advances in well-defined, late
transition metal complexes that make and/or break C−N, C−O and
C−S bonds. Chem. Soc. Rev. 2017, 46, 197−238.
(2) Hartwig, J. F. Carbon−heteroatom bond formation catalysed by
organometallic complexes. Nature 2008, 455, 314.
(3) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Homogeneous oxidation
of alkanes by electrophilic late transition metals. Angew. Chem., Int. Ed.
1998, 37, 2180−2192.
(4) Canty, A. J.; Denney, M. C.; van Koten, G.; Skelton, B. W.;
White, A. H. Carbon− Oxygen Bond Formation at Metal (IV)
Centers: Reactivity of Palladium (II) and Platinum (II) Complexes of
the [2, 6-(Dimethylaminomethyl) phenyl-N, C, N]-(Pincer) Ligand
toward Iodomethane and Dibenzoyl Peroxide; Structural Studies of M
(II) and M (IV) Complexes. Organometallics 2004, 23, 5432−5439.
(5) Hartwig, J. F. Carbon− heteroatom bond-forming reductive
eliminations of amines, ethers, and sulfides. Acc. Chem. Res. 1998, 31,
852−860.
(6) Racowski, J. M.; Dick, A. R.; Sanford, M. S. Detailed Study of C−
O and C−C Bond-Forming Reductive Elimination from Stable
C2N2O2−Ligated Palladium(IV) Complexes. J. Am. Chem. Soc. 2009,
131, 10974−10983.
(7) Khusnutdinova, J. R.; Newman, L. L.; Zavalij, P. Y.; Lam, Y.-F.;
Vedernikov, A. N. Direct C(sp3)−O Reductive Elimination of Olefin
Oxides from PtIV-Oxetanes Prepared by Aerobic Oxidation of PtII
Olefin Derivatives (Olefin= cis-Cyclooctene, Norbornene). J. Am.
Chem. Soc. 2008, 130, 2174−2175.
(8) Williams, B. S.; Goldberg, K. I. Studies of Reductive Elimination
Reactions To Form Carbon− Oxygen Bonds from Pt (IV) Complexes.
J. Am. Chem. Soc. 2001, 123, 2576−2587.
To confirm that the same product is formed in the presence
of excess N(n-Bu)4OAc, complex 2b was heated at 60 °C in
CD3CN using [2b] = 0.02 M and [N(n-Bu)4OAc] = 0.2 M and
1
the formation of MeOAc was confirmed by H NMR with
lower yield in comparison to that observed in the absence of
acetate ion. On the basis of the first step of the suggested
mechanism, which is an equilibrium (eq 3), it can be expected
that the formation of IMa could be suppressed by the addition
of OAc ion to the thermolyses of 2b. Although, on the basis of
eq 5, the addition of acetate ion would be expected to increase
the amount of product, an overall lower amount of product
formation in the C−O reductive elimination could be explained
on the basis of a larger influence of the free acetate ion on the
equilibrium step.8
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00745.
(9) Williams, B. S.; Holland, A. W.; Goldberg, K. I. Direct
Observation of C− O Reductive Elimination from Pt (IV). J. Am.
Chem. Soc. 1999, 121, 252−253.
Crystallographic data, UV−vis spectra, and DFT data
(PDF)
Cartesian coordinates for the calculated structures
(TXT)
(10) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova,
J. R. Stoichiometric Aerobic PtII− Me Bond Cleavage in Aqueous
Solutions to Produce Methanol and a PtII(OH) Complex. J. Am. Chem.
Soc. 2006, 128, 82−83.
Accession Codes
́
(11) Camasso, N. M.; Perez-Tempran, M. n. H.; Sanford, M. S. C
(sp3)−O Bond-Forming Reductive Elimination from PdIV with
Diverse Oxygen Nucleophiles. J. Am. Chem. Soc. 2014, 136, 12771−
12775.
CCDC 1568574 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(12) Smythe, N. A.; Grice, K. A.; Williams, B. S.; Goldberg, K. I.
Reductive Elimination and Dissociative β-Hydride Abstraction from Pt
(IV) Hydroxide and Methoxide Complexes. Organometallics 2009, 28,
277−288.
́
(13) Iglesias, A.; Muniz, K. Studies on Alkyl-Nitrogen Bond
̃
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.M.N.: nabavizadeh@shirazu.ac.ir.
*E-mail for M.M.A.-O.: abuomar@chem.ucsb.edu.
Formation via Reductive Elimination from Monomeric Palladium
Complexes in High Oxidation State. Helv. Chim. Acta 2012, 95, 2007−
2025.
(14) Oloo, W.; Zavalij, P. Y.; Zhang, J.; Khaskin, E.; Vedernikov, A.
N. Preparation and C−X Reductive Elimination Reactivity of
Monoaryl PdIV−X Complexes in Water (X= OH, OH2, Cl, Br). J.
Am. Chem. Soc. 2010, 132, 14400−14402.
(15) Oloo, W. N.; Zavalij, P. Y.; Vedernikov, A. N. Palladium(IV)
Monohydrocarbyls: Mechanistic Study of the Ligand-Enabled
Oxidation of Palladium(II) Complexes with H2O2 in Water.
Organometallics 2013, 32, 5601−5614.
(16) Canty, A. J.; Denney, M. C.; Skelton, B. W.; White, A. H.
Carbon− Oxygen Bond Formation at Organopalladium Centers: The
Reactions of PdMeR (L2)(R= Me, 4-tolyl; L2= tmeda, bpy) with
Diaroyl Peroxides and the Involvement of Organopalladium (IV)
Species. Organometallics 2004, 23, 1122−1131.
(17) Hartwig, J. F. Electronic Effects on Reductive Elimination To
Form Carbon−Carbon and Carbon−Heteroatom Bonds from
Palladium(II) Complexes. Inorg. Chem. 2007, 46, 1936−1947.
(18) Liu, G.; Stahl, S. S. Highly regioselective Pd-catalyzed
intermolecular aminoacetoxylation of alkenes and evidence for cis-
aminopalladation and SN2 C−O bond formation. J. Am. Chem. Soc.
2006, 128, 7179−7181.
■
ORCID
S. Masoud Nabavizadeh: 0000-0003-3976-7869
Fatemeh Niroomand Hosseini: 0000-0002-5856-8104
Mahdi M. Abu-Omar: 0000-0002-4412-1985
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.M.N. and F.N.H. wish to acknowledge Shiraz University and
the Islamic Azad University, Shiraz Branch, for their sabbatical
leave at University of California, Santa Barbara (UCSB). We
acknowledge support from the Department of Chemistry and
Biochemistry at UCSB and Shiraz University. The Center for
Scientific Computing from the CNSI, MRL: NSF MRSEC
(DMR-1121053) and NSF CNS-0960316 is also acknowl-
edged.
K
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Organometallics
Article
(19) Li, J.; Yang, W.; Yan, F.; Liu, Q.; Wang, P.; Li, Y.; Zhao, Y.;
Dong, Y.; Liu, H. Pd-Catalyzed oxidative isomerization of propargylic
acetates: highly efficient access to α-acetoxyenones via alkenyl Csp2−
O bond-forming reductive elimination from PdIV. Chem. Commun.
2016, 52, 10644−10647.
(20) Cook, A. K.; Sanford, M. S. Mechanism of the palladium-
catalyzed arene C−H acetoxylation: A comparison of catalysts and
ligand effects. J. Am. Chem. Soc. 2015, 137, 3109−3118.
(21) Campbell, M. G.; Zheng, S.-L.; Ritter, T. One-dimensional
palladium wires: influence of molecular changes on supramolecular
structure. Inorg. Chem. 2013, 52, 13295−13297.
(22) Powers, D. C.; Geibel, M. A.; Klein, J. E.; Ritter, T. Bimetallic
Palladium Catalysis: Direct Observation of Pd (III)− Pd (III)
Intermediates. J. Am. Chem. Soc. 2009, 131, 17050−17051.
(23) Fu, Y.; Li, Z.; Liang, S.; Guo, Q.-X.; Liu, L. Mechanism for
Carbon− Oxygen Bond-Forming Reductive Elimination from
Palladium (IV) Complexes. Organometallics 2008, 27, 3736−3742.
(24) Dick, A. R.; Kampf, J. W.; Sanford, M. S. Unusually stable
palladium (IV) complexes: detailed mechanistic investigation of C− O
bond-forming reductive elimination. J. Am. Chem. Soc. 2005, 127,
12790−12791.
(25) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. C−O
Coupling of LPtIVMe(OH) X Complexes in Water (X= 18OH, OH,
OMe; L= di (2-pyridyl) methane sulfonate). Organometallics 2007, 26,
3466−3483.
(26) Haghighi, M. G.; Nabavizadeh, S. M.; Rashidi, M.; Kubicki, M.
Selectivity in metal−carbon bond protonolysis in p-tolyl-(or methyl)-
cycloplatinated (II) complexes: kinetics and mechanism of the
uncatalyzed isomerization of the resulting Pt (II) products. Dalton
Trans. 2013, 42, 13369−13380.
(27) Sheldrick, G., SADABS, Empirical Absorption Correction Program;
University of Gottingen, Gottingen, Germany, 1997−2005.
̈
̈
(28) SHELXTL PC, Version 6.12; Bruker AXS Inc., Madison, WI,
2005.
(29) Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G.; Robb, M.;
Cheeseman, J.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.,
et al. Gaussian 09, revision D.01; Gaussian, Inc., Wallingford, CT, 2009.
(30) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for
molecular calculations. Potentials for the transition metal atoms Sc to
Hg. J. Chem. Phys. 1985, 82, 270−283.
(31) Crespo, M.; Martínez, M.; Nabavizadeh, S. M.; Rashidi, M.
Kinetico-mechanistic studies on C X (X= H, F, Cl, Br, I) bond
activation reactions on organoplatinum (II) complexes. Coord. Chem.
Rev. 2014, 279, 115−140.
(32) Ochiai, M. Reactivities, properties and structures. In Hypervalent
Iodine Chemistry; Springer: Berlin, 2003; pp 5−68.
(33) Yoshimura, A.; Zhdankin, V. V. Advances in synthetic
applications of hypervalent iodine compounds. Chem. Rev. 2016,
116, 3328−3435.
(34) Nabavizadeh, S. M.; Hoseini, S. J.; Momeni, B. Z.; Shahabadi,
N.; Rashidi, M.; Pakiari, A. H.; Eskandari, K. Oxidative addition of n-
alkyl halides to diimine−dialkylplatinum (II) complexes: a closer look
at the kinetic behaviors. Dalton Trans. 2008, 2414−2421.
́
(35) Geier, M. J.; Dadkhah Aseman, M.; Gagne, M. R. Anion-
Dependent Switch in C−X Reductive Elimination Diastereoselectivity.
Organometallics 2014, 33, 4353−4356.
(36) Sanford, M. S.; Groves, J. T. Anti-Markovnikov Hydro-
functionalization of Olefins Mediated by Rhodium−Porphyrin
Complexes. Angew. Chem., Int. Ed. 2004, 43, 588−590.
L
DOI: 10.1021/acs.organomet.7b00745
Organometallics XXXX, XXX, XXX−XXX