Oxidative reactivity of (N2S2)PdRX complexes (R = Me, Cl; X = Me, Cl, Br): Involvement of palladium(III) and palladium(IV) intermediates

Article abstract of DOI:10.1021/om400286j

A series of (N2S2)PdRX complexes (N2S2 = 2,11-dithia[3.3](2,6) pyridinophane; R = X = Me, 1; R = Me, X = Cl, 2; R = Me, X = Br, 3; R = X = Cl, 4) were synthesized, and their structural and electronic properties were investigated. X-ray crystal structures

Full text of DOI:10.1021/om400286j

Article
pubs.acs.org/Organometallics
Oxidative Reactivity of (N2S2)PdRX Complexes (R = Me, Cl; X = Me,
Cl, Br): Involvement of Palladium(III) and Palladium(IV) Intermediates
Jia Luo,† Nigam P. Rath,‡ and Liviu M. Mirica*,†
†Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, United States
‡Department of Chemistry and Biochemistry, One University Boulevard, University of Missouri−St. Louis, St. Louis, Missouri
63121-4400, United States
S
* Supporting Information
ABSTRACT: A series of (N2S2)PdRX complexes (N2S2 =
2,11-dithia[3.3](2,6)pyridinophane; R = X = Me, 1; R = Me, X
= Cl, 2; R = Me, X = Br, 3; R = X = Cl, 4) were synthesized,
and their structural and electronic properties were investigated.
X-ray crystal structures show that for the corresponding Pd(II)
complexes the N2S2 ligand adopts a κ2 conformation, with the
pyridine N donors binding in the equatorial plane. Cyclic
voltammetry (CV) studies suggest that the Pd(III) oxidation
state is accessible at moderate redox potentials. In situ EPR,
ESI-MS, UV−vis, and low-temperature electrochemical studies
were employed to detect the formation of Pd(III) species during the oxidation of Pd(II) precursors. In addition, the
[(N2S2)PdIVMe2](PF6)2 ([12+](PF6)2) complex was isolated by oxidation of 1 with 2 equiv of FcPF6, and its structural
characterization reveals an octahedral Pd(IV) center. The reversible PdIV/III redox couple for the Pd(IV) species supports the
observed formation of the Pd(III)−dimethyl species upon chemical reduction of 12+. In addition, reactivity studies reveal ethane,
MeCl, and MeBr elimination upon one-electron oxidation of 1 (as well as the one-electron reduction of 12+), 2, and 3,
respectively. Mechanistic studies suggest the initial formation of a Pd(III) species, followed by methyl group transfer/
disproportionation and subsequent reductive elimination from a Pd(IV) intermediate, although a halogen radical pathway cannot
be completely excluded during C−halide bond formation. Interestingly, computational results suggest that the N2S2 ligand
stabilizes to a greater extent the Pd(IV) vs the Pd(III) oxidation state, likely due to steric rather than electronic effects.
group transfer reactivity of the dinuclear species [(N2S2)-
PdIIMe]2(OTf)2 (5).27 Reported herein are the synthesis,
characterization, and reactivity studies of a series of organo-
metallic (N2S2)PdRX complexes (Scheme 1) that can promote
C−C or C−Cl/Br bond formation reactions. These reactions
INTRODUCTION
■
Pd-catalyzed transformations are being extensively utilized in C−
C and C−heteroatom bond formation reactions,1−3 as key steps
in target-oriented syntheses to afford complex natural products,
functional advanced materials, pharmaceuticals, and other high-
value chemicals.4 While the common Pd oxidation states
involved in these catalytic cycles are Pd(0) and Pd(II),5−8
Pd(III) and Pd(IV) species are also proposed as reactive
intermediates,2−4,9−12 yet they have been less commonly isolated
and characterized.13−21 We have recently reported a series of
mononuclear organometallic Pd(III) complexes stabilized by
Scheme 1. (N2S2)PdIIRX Complexes
R
tetradentate N4 ligands (RN4 = N,N′-dialkyl-2,11-diaza[3.3]-
(2,6)pyridinophane, R = tBu, iPr, Me)22,23 and studied their
aerobic oxidation and C−C/C−heteroatom bond formation
reactivity.24,25 In addition, we have shown that the tridentate
ligand N′,N″,N′′′-trimethyltriazacyclononane (Me3tacn) can
stabilize halogen-bridged dinuclear PdIII complexes that are
active catalysts for the Kharasch radical addition.26 These results
suggest that Pd(III) complexes are more common than
previously anticipated, and thus it prompted us to synthesize
and characterize Pd(III) species supported by various ligand
systems. In this regard, we have recently employed the N- and S-
donor tetradentate ligand N2S2 (N2S2 = 2,11-dithia[3.3](2,6)-
pyridinophane) and investigated the oxidatively induced methyl
Received: April 5, 2013
Published: May 29, 2013
© 2013 American Chemical Society
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Synthesis of (N2S2)PdII(CD3)2 (1-d6). (COD)PdII(CD3)2 was
freshly made24 and used immediately after preparation. Solid samples of
(COD)PdII(CD3)2 (39.5 mg, 0.157 mmol) and N2S2 (43.0 mg, 0.157
mmol) were combined in 20 mL of anhydrous diethyl ether, and the
reaction mixture was stirred at 0 °C for 3 h and then at 20 °C for 0.5 h.
The white precipitate of (N2S2)PdII(CD3)2 was vacuum-filtered and
washed with diethyl ether and pentane to give 24.0 mg for fraction 1.
The filtrate was rotary-evaporated at 0 °C to give a pale yellow solid,
which was suspended in 2 mL of anhydrous diethyl ether, vacuum-
filtered, and washed with ether and pentane to give 23.1 mg for fraction
involve both Pd(III) and Pd(IV) intermediates, as suggested by
spectroscopic, electrochemical, and mechanistic studies.
EXPERIMENTAL SECTION
■
Reagents and Materials. All manipulations were carried out under
a nitrogen atmosphere using standard Schlenk and glovebox techniques,
if not indicated otherwise. All chemicals were commercially available
from Aldrich, Fisher, or Strem Chemicals and were used as received
without further purification. Acetylferrocenium tetrafluoroborate
([AcFc+]BF4),28 (COD)PdIIMe2,29 (COD)PdII(CD3)2,24 (COD)-
PdIIMeCl,30 (N2S2)PdIICl2 (4),31 and [(N2S2)PdIIMe]2(OTf)2 (5)27
were prepared according to the literature procedures. Solvents were
purified prior to use by passing through a column of activated alumina
using an MBraun solvent purification system. On the basis of their redox
potential,28 ferrocenium hexafluorophosphate ([Fc+]PF6), acetylferro-
cenium tetrafluoroborate ([AcFc+]BF4), and nitrosonium tetrafluor-
oborate ([NO+]BF4) were used as chemical oxidants for the oxidation of
1, 2/3, and 4, respectively, while cobaltocene (CoIICp2) was used as a
reducing agent for the reduction of 12+.
1
2. Yield: 47.1 mg, 72%. The H NMR spectrum of the product is
identical with that of 1 except for the missing singlet of the PdMe group
at −0.027 ppm. ESI-MS (m/z): 398.0081, calcd for [(N2S2)Pd(CD3)]+
398.0056.
Synthesis of (N2S2)PdII(13CH3)2 (1-13C). (COD)PdII(13CH3)2 was
prepared by following the same procedure as for the preparation of
(COD)PdIIMe2,29 except with 13CH3I instead of MeI. Freshly made
(COD)PdII(13CH3)2 (104.1 mg, 0.423 mmol) and N2S2 (116.1 mg,
0.423 mmol) were combined in 100 mL of anhydrous diethyl ether, and
the reaction mixture was stirred at 0 °C for 3 h and then at 20 °C for 0.5
h. The white precipitate of (N2S2)PdII(13CH3)2 was vacuum-filtered
and washed with diethyl ether and pentane to give 87.0 mg for fraction 1.
The filtrate was rotary-evaporated at 0 °C to give a pale yellow solid,
which was suspended in 5 mL of anhydrous diethyl ether, vacuum-
filtered, and washed with ether and pentane to give 35.0 mg for fraction
1
Physical Measurements. H (300.121 MHz) NMR spectra were
recorded on a Varian Mercury-300 spectrometer. Low-temperature
1
(−20 °C) H (600 MHz) NMR spectra were recorded on a Varian
Unity Inova-600 spectrometer. Chemical shifts are reported in ppm and
referenced to residual solvent resonance peaks. Abbreviations for the
multiplicity of NMR signals are s (singlet), d (doublet), t (triplet), q
(quartet), sep (septet), m (multiplet), and br (broad). UV−vis spectra
were recorded on a Varian Cary 50 Bio spectrophotometer. EPR spectra
were recorded on a JEOL JES-FA X-band (9.2 GHz) EPR spectrometer
in 1/3 MeCN/PrCN (v/v) at 77 K. Simulations of EPR spectra were
performed using WinEPR SimFonia v. 1.25. Elemental analyses were
carried out by the Columbia Analytical Services Tucson Laboratory.
ESI-MS experiments were performed on a Bruker Maxis QTOF mass
spectrometer with an electron spray ionization source. ESI mass
spectrometry was provided by the Washington University Mass
Spectrometry Resource, an NIH Research Resource (Grant No.
P41RR0954).
1
2. Yield: 122 mg, 70%. H NMR (acetone-d6, 300 MHz; δ (ppm)):
−0.028 (d, 6H, 13CH3, J(13C−1H) = 127.2 Hz), 4.20 (d, J = 14.1 Hz, 4H,
CH2), 6.05 (d, J = 13.8 Hz, 4H, CH2), 7.25 (d, J = 7.5 Hz, 4H, Py Hmeta),
7.65 (t, J = 7.5 Hz, 2H, Py Hpara). ESI-MS (m/z): 395.9913, calcd for
[(N2S2)Pd(13CH3)]+ 395.9902.
Synthesis of [(N2S2)PdIVMe2](PF6)2 ([12+](PF6)2). A suspension of
1 (20.0 mg, 48.9 μmol) in anhydrous acetone (3 mL) was added
dropwise to a stirred solution of 2 equiv of FcPF6 (32.4 mg, 97.8 mmol)
in anhydrous acetone (3 mL). After 1 h, the solution was set up for
crystallization by diethyl ether diffusion. Light brown crystals formed
1
after 1 day. Yield: 23.1 mg, 67%. H NMR (acetone-d6, 300 MHz; δ
(ppm)): 2.61 (s, 6H, Me), 5.67 (d, J = 18 Hz, 4H, CH2), 5.97 (d, J = 18
Hz, 4H, CH2), 7.80 (d, J = 7.5 Hz, 4H, Py Hmeta), 8.11 (t, J = 7.5 Hz, 2H,
Py Hpara). UV−vis (MeCN; λ, nm (ε, cm−1 M−1)): 318 (4000). Anal.
F o u n d : C , 3 0 . 2 1 ; H , 3 . 3 7 ; N , 3 . 7 4 . C a l c d f o r
C16H20N2PdS2P2F12·CH3COCH3, C, 30.07; H, 3.45; N, 3.69. ESI-MS
(m/z): 205.0063, calcd for [(N2S2)PdIVMe2]2+ 205.0051.
Electrochemical Measurements. Electrochemical grade
Bu4NClO4 or Bu4NBF4 from Aldrich was used as the supporting
electrolyte. Cyclic voltammetry was performed with a BASi EC Epsilon
electrochemical workstation or a CHI 660D Electrochemical Analyzer.
Electrochemical measurements were carried out under a flow of
nitrogen, and the analyzed solutions were deaerated by purging with
nitrogen. A glassy-carbon electrode (GCE, d = 1 mm) was used as the
working electrode, while a Pt wire was used as the auxiliary electrode.
The nonaqueous reference electrode containing Ag/0.01 M AgNO3 in
0.1 M Bu4NClO4/MeCN was calibrated against Fc; the potential of the
Fc+/Fc couple vs the Ag/0.01 M AgNO3/0.1 M Bu4NClO4/MeCN
reference electrode is +0.105 V.
Synthesis of (N2S2)PdIIMeCl (2). N2S2 (85.0 mg, 0.310 mmol)
and (COD)PdIIMeCl30 (82.4 mg, 0.310 mmol) were stirred in 50 mL of
anhydrous diethyl ether for 1 day. The yellow precipitate was filtered off,
washed with 10−20 mL of anhydrous diethyl ether and 2−3 mL of
pentane, and dried under vacuum. Yield: 105 mg, 79%. UV−vis
(CH2Cl2; λ, nm (ε, cm−1 M−1)): 390 (sh, 700), 330 (sh, 1900). 1H NMR
(CDCl3, 300 MHz; δ (ppm)): 0.79 (s, 3H, PdMe), 4.13 (d, J = 14 Hz,
2H, CH2), 4.23 (d, J = 14 Hz, 2H, CH2), 5.99 (d, J = 14 Hz, 2H, CH2),
6.21 (d, J = 14 Hz, 2H, CH2), 7.05 (d, J = 7 Hz, 2H, Py Hmeta), 7.16 (d, J =
7 Hz, 2H, Py Hmeta), 7.51 (t, J = 7 Hz,1 H, Py Hpara), 7.60 (t, J = 7 Hz, 1H,
Py Hpara). Anal. Found: C, 40.61; H, 3.85; N, 6.11. Calcd for
C15H17ClN2PdS2·0.5H2O, C, 40.92; H, 4.12; N, 6.36. ESI-MS (m/z):
394.9897, calcd for [(N2S2)PdIIMe]+ 394.9868; 414.9341, calcd for
[(N2S2)PdIICl]+ 414.9322.
Synthesis of 2,11-Dithia[3.3](2,6)pyridinophane (N2S2). N2S2
was synthesized by following the reported procedure.31,32 The final
product was first extracted in MeOH before being recrystallized out of
toluene.
Synthesis of (N2S2)PdIIMe2 (1). (COD)PdIIMe2 was freshly
made29 and used immediately after preparation. Solid samples of
(COD)PdIIMe2 (100 mg, 0.41 mmol) and N2S2 (112 mg, 0.41 mmol)
were combined in 150 mL of anhydrous diethyl ether, and the reaction
mixture was stirred at 0 °C for 3 h and then at 20 °C for 0.5 h. The white
precipitate of (N2S2)PdIIMe2 was vacuum-filtered and washed with
diethyl ether and pentane to give 111.9 mg for fraction 1. The filtrate was
rotary-evaporated at 0 °C to give a pale yellow solid, which was
suspended in 5 mL of anhydrous diethyl ether, vacuum-filtered, and
washed with ether and pentane to give 38.8 mg for fraction 2. Total
Synthesis of (N2S2)PdIIMeBr (3). Acetyl bromide (5.9 μL, 78.0
μmol, 1.0 equiv) was added to a solution of 1 (31.9 mg, 78.0 μmol) in 3
mL of anhydrous CH2Cl2. The reaction mixture was stirred at room
temperature for 6 h. The resulting pale yellow precipitate was filtered off,
washed with 2 mL of ether and 2 mL of pentane, and dried under
vacuum. Recrystallization was performed in CH2Cl2 solution by diethyl
ether diffusion. Yellow-orange crystals formed after 1−2 days at room
1
1
temperature. Yield: 21.4 mg, 58%. H NMR (acetone-d6, 300 MHz; δ
yield: 150.7 mg, 90%. H NMR (acetone-d6, 300 MHz; δ (ppm)):
−0.027 (s, 6H, Me), 4.20 (d, J = 14.1 Hz, 4H, CH2), 6.05 (d, J = 13.8 Hz,
4H, CH2), 7.24 (d, J = 7.8 Hz, 4H, Py Hmeta), 7.65 (t, J = 7.8 Hz, 2H, Py
Hpara). UV−vis (acetone; λ, nm (ε, cm−1 M−1)): 361 (sh, 800). Anal.
Found: C, 46.57; H, 5.62; N, 6.76. Calcd for C16H20N2PdS2: C, 46.77; H,
4.91; N, 6.82. ESI-MS (m/z): 394.9891, calcd for [(N2S2)PdIIMe]+
394.9868.
(ppm)): 0.64 (s, 3H, PdMe), 4.28 (d, J = 15 Hz, 2H, CH2), 4.49 (d, J =
14 Hz, 2H, CH2), 5.87 (d, J = 14 Hz, 2H, CH2), 6.14 (d, J = 14 Hz, 2H,
CH2), 7.28 (d, J = 7 Hz, 2H, Py Hmeta), 7.44 (d, J = 7 Hz, 2H, Py Hmeta),
7.69 (t, J = 7 Hz, 1H, Py Hpara), 7.81 (t, J = 7 Hz, 1H, Py Hpara). UV−vis
(MeCN; λ, nm (ε, cm−1 M−1)): 368 (sh, 980). ESI-MS (m/z):
394.9891, calcd for [(N2S2)PdIIMe]+: 394.9868; 460.8806, calcd for
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Figure 1. X-ray crystal structures of 1 and 12+ with 50% thermal ellipsoids. Selected bond distances (Å): 1, Pd1−C1 = 2.043(5), Pd1−C2 = 2.035(4),
Pd1−N1 = 2.184(3), Pd1−N2 = 2.194(3); 12+, Pd1−C1/C1′ = 2.082(11), Pd1−N1/N1′ = 2.151(8), Pd1−S1/S1′ = 2.323(2).
[(N2S2)PdIIBr]+ 460.8821. The (N2S2)PdIIBrCl side product
ESI-MS Studies of the Formation of Pd(III) Intermediates. All
solutions were precooled to −40 °C. To a solution of Pd complex in 1
mL of MeCN was added a solution of 1 equiv of chemical oxidant at −40
°C. The mixture was thoroughly mixed and diluted to the required
concentration for ESI-MS, followed by immediate injection into the
instrument.
cocrystallized with 3 and was estimated to be present in 10−20%
1
amount on the basis of H NMR and 25% on the basis of the X-ray
crystal structure. However, it is expected that (N2S2)PdIIBrCl, by
analogy to (N2S2)PdIICl2, has a PdII/III oxidation potential higher than
that of 3, and thus (N2S2)PdIIBrCl should not be oxidized by AcFc+ and
should not affect the oxidative reactivity of 3.
UV−Vis Study of the Formation of the [(N2S2)PdIIICl2]+ (4+)
Complex. A 3 mL solution of 4 (0.74 mM) in MeCN was mixed with an
MeCN solution of 1 equiv of NOBF4 in a quartz cuvette (10 mm path
length) equipped with a septum-sealed cap. UV−vis spectra were
recorded immediately after thorough mixing. However, the attempted
isolation of 4+ in the formation of a product mixture included the
sulfoxide complex (N2S2-O)PdIICl2 (7), characterized by 1H NMR. 1H
NMR (CD3CN, 300 MHz; δ (ppm)): 4.49 (d, J = 15,0 Hz, 2H, CH2),
5.05 (d, J = 14.1 Hz, 2H, CH2), 6.27 (d, J = 15.0 Hz, 2H, CH2), 6.48 (d, J
= 14.1 Hz, 2H, CH2), 7.15 (d, J = 7.5 Hz, 2H, Py Hmeta), 7.41 (d, J = 7.5
Hz, 2H, Py Hmeta), 7.74 (t, J = 7.8 Hz, 2H, Py Hpara).
Synthesis of (N2S2)PdII(13CH3)Br (3-13C). Acetyl bromide (5.2 μL,
70.0 μmol, 1.0 equiv) was added to a solution of 1-13C (28.8 mg, 70.0
μmol) in 3 mL of anhydrous CH2Cl2. The resulting pale yellow
precipitate was filtered off, washed with 2 mL of ether and 2 mL of
pentane, and dried under vacuum. Recrystallization was performed in
CH2Cl2 solution by diethyl ether diffusion. Yellow-orange crystals
formed after 1−2 days at room temperature. Yield: 20.9 mg, 63%.1H
NMR (acetone-d6, 300 MHz; δ (ppm)): 0.64 (d, 3H, 13CH3, J(13C−1H)
= 134.4 Hz), 4.28 (d, J = 15 Hz, 2H, CH2), 4.49 (d, J = 14 Hz, 2H, CH2),
5.87 (d, J = 14 Hz, 2H, CH2), 6.14 (d, J = 14 Hz, 2H, CH2), 7.28 (d, J = 7
Hz, 2H, Py Hmeta), 7.44 (d, J = 7 Hz, 2H, Py Hmeta), 7.69 (t, J = 7 Hz, 1H,
Py Hpara), 7.81 (t, J = 7 Hz, 1H, Py Hpara). ESI-MS (m/z): 395.9914,
calcd for [(N2S2)Pd(13CH3)]+ 395.9902; 460.8815, calcd for [(N2S2)-
PdIIBr]+ 460.8821.
Low-Temperature Controlled-Potential Electrolysis (CPE) of
4 and in Situ Cyclic Voltammetry (CV). Low-temperature (−40 °C)
electrochemical oxidation was performed in a two-compartment H-
shaped bulk electrolysis cell with a medium-frit glass junction and
equipped with two septa and a magnetic stirring bar. Reticulated vitreous
carbon was used as the working electrode for electrolysis, while a
platinum wire was used as the working electrode for CV. Platinum gauze
(25 mm × 10 mm) and a Ag/AgCl wire were used as the auxiliary
electrode and the pseudo reference electrode, respectively. Electro-
chemical oxidation was performed at a constant potential at −40 °C
under a blanket of nitrogen, and CVs were recorded before and after
electrolysis.
Synthesis of [(N2S2)PdIICl](OTf) (6). A solution of AgOTf (17.6
mg, 68.4 μmol) in MeCN (1 mL) was added to a stirred solution of 4
(30.9 mg, 68.4 μmol) in MeCN (10 mL). A white precipitate of AgCl
appeared immediately, and the solution changed from yellow to brown.
After the mixture was stirred at room temperature for 2 h in the dark, the
solvent was removed by rotary evaporation. The solid residue was
redissolved in a minimum amount of MeCN, and the resulting brown
solution was filtered through Celite. A clear dark red filtrate was set to
crystallize by anhydrous diethyl ether vapor diffusion at room
temperature . Dark red crystals formed after several days. Yield: 18.0
mg, 47%. 1H NMR (CD3CN, 300 MHz; δ (ppm)): 4.38 (d, J = 16.2 Hz,
4H, CH2), 4.93 (d, J = 16.5 Hz, 4H, CH2), 7.14 (d, J = 7.8 Hz, 4H, Py
Hmeta), 7.50 (t, J = 7.8 Hz, 2H, Py Hpara). UV−vis (MeCN; λ, nm (ε,
cm−1 M−1)): 422 (2500), 341 (4100). Anal. Found: C, 31.95; H, 2.75;
N, 4.98. Calcd for C15H14ClF3N2O3PdS3: C, 31.87; H, 2.50; N, 4.96.
ESI-MS (m/z): 414.9335, calcd for [(N2S2)PdIICl]+ 414.9322.
EPR Studies of the Formation of Pd(III) Intermediates. An EPR
tube was charged with a solution of Pd complex in 1/3 MeCN/PrCN
(v/v) and immersed into liquid nitrogen. Another solution containing 1
equiv of chemical oxidant or reductant in the same solvent mixture was
quickly added and frozen as a second layer in the EPR tube. An initial
EPR spectrum was taken at 77 K. The sample was then carefully warmed
for 10−30 s to allow the two layers to mix and quickly refrozen, and the
EPR spectrum was recorded. The warming step was repeated if
necessary.
General Procedure for NMR Studies of Reactivity of Pd
Complexes upon Chemical Oxidation or Reduction. An NMR
tube capped with a rubber septum was charged with a solution of Pd
complex in 1.0 mL of acetone-d6. A solution of 1 equiv of chemical
oxidant or reductant in 1.4 mL of acetone-d6 was added in portions with
a microsyringe, and the mixture was thoroughly mixed after each
addition. The yield of products was determined by NMR integration
using dioxane as an internal standard, calculated as [moles of product]/
[moles of starting complex] × 100% and reported as the average of two
runs. Pd products were assigned on the basis of a comparison of the 1H
NMR spectra to those of independently synthesized complexes. For
crossover experiments, a 1/1 mixture of nonlabeled and labeled
complexes was used.
X-ray Structure Determination of 1, [12+](PF6)2, and 6. Crystals
of X-ray diffraction quality were obtained by anhydrous diethyl ether
vapor diffusion into a CH2Cl2 solution for 1, an acetone solution for
[12+](PF6)2, and an MeCN solution for 6. Preliminary examination and
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data collection were performed using a Bruker Kappa Apex-II Charge
Coupled Device (CCD) Detector single-crystal X-ray diffractometer
equipped with an Oxford Cryostream LT device. Data were collected
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) from
a fine focus sealed tube X-ray source. Apex II and SAINT software
packages33 were used for data collection and data integration. Data were
corrected for systematic errors using SADABS on the basis of the Laue
symmetry using equivalent reflections.33 Structure solutions and
refinement were carried out using the SHELXTL-PLUS software
package.34 Complete listings of geometrical parameters, positional and
isotropic displacement coefficients for hydrogen atoms, and anisotropic
displacement coefficients for the non-hydrogen atoms are available as
Supporting Information.35
Electrochemical Studies. The electrochemical properties of
(N2S2)PdII complexes were studied by cyclic voltammetry (CV,
Table 1). Complex 1 exhibits a complicated CV, and no PdII/III
Table 1. Cyclic Voltammetry (CV) Data for (N2S2)Pd
a
Complexes
Complex
EpaII/III (V)
EpcIII/II (V)
EpaIII/IV (V)
EpcIV/III (V)
b
1
−0.4−0.27 (broad)
d
12+
c
−0.93
c
−0.56 (ΔEp = 70 mV)
0.78
2
0.23
0.15
∼0.65
0.55
0.81
0.62
e
3
−0.21
0.48
0.09
0.61
0.80
0.58
d
4
0.75 (ΔEp = 70 mV)
5
0.88
RESULTS AND DISCUSSION
■
6-Cl
c
c
Synthesis and Characterization of (N2S2)Pd Com-
plexes. X-ray Crystal Structures of (N2S2)Pd Complexes.
The (N2S2)PdIIRX complexes (R = X = Me, 1; R = Me, X = Cl,
2; R = Me, X = Br, 3; R = X = Cl, 4; Scheme 1) were synthesized
using common Pd(II) precursors and synthetic proce-
dures.27,31,32 The X-ray crystal structure of (N2S2)PdIIMe2 (1;
Figure 1) reveals a square-planar coordination at the Pd(II)
center, with cis-positioned methyl groups and the two N donors
from N2S2, in line with the typical κ2 conformation of
pyridinophane ligands in Pd(II) complexes.23,31,36 The average
Pd−C and Pd−N bond lengths are 2.039 and 2.189 Å,
respectively, consistent with the reported value of the (tmeda)-
PdIIMe2 complex,36 but with Pd−N distances significantly longer
than those of 4 (Pd−N = 2.049 Å, Pd−Cl = 2.289 Å),31 likely due
to the stronger trans influence of the Me vs Cl ligands.37 Though
no crystal structure was obtained for (N2S2)PdIIMeCl (2), the
preliminary crystal structure of (N2S2)PdIIMeBr (3) reveals a
square-planar geometry and the atom connectivity around the
Pd(II) center, with N2S2 binding in a κ2 conformation as in 1.35
Interestingly, chemical oxidation of 1 with 2 equiv of FcPF6 in
acetone yields a brown product that could be crystallized by ether
vapor diffusion. X-ray structure analysis reveals a Pd(IV)
complex, [(N2S2)PdIVMe2](PF6)2 (12+(PF6)2; Figure 1), with
pseudo-octahedral geometry at the Pd center, which interacts
with two N atoms and two S atoms of N2S2 (that adopts a κ4
conformation) as well as with two cis-positioned methyl groups.
The relatively longer Pd−C (2.082 Å) and shorter Pd−N bond
distances (2.151 Å), in comparison to those of 1, are likely a
result of the oxidation of Pd(II) to Pd(IV), which favors an
octahedral geometry, while the Pd−S bond lengths (2.323 Å) are
similar to those for other reported systems.38 Surprisingly, the
presence of soft S donors can stabilize a high-valent Pd(IV)
center. While there are Pd(II) and even Pd(III) complexes
stabilized by thioether ligands,19 12+ is to the best of our
knowledge only the second Pd(IV) complex with thioether
donors.39 In addition, very few Pd(IV)−thiolate or −sulfonate
complexes have been reported.40,41 On the basis of our recent
studies on a series of (RN4)Pd complexes (RN4 = N,N′-dialkyl-
2,11-diaza[3.3](2,6)pyridinophane, R = tBu, iPr, Me) containing
Pd(II), Pd(III), and Pd(IV) oxidation states,23 we propose that
the ligand steric effects control the stability of Pd(III) vs Pd(IV)
complexes (vide infra).21 The isolation of 12+(PF6)2 provides
further evidence for the paramount role of ligand steric effects vs
electronic effects, as the N2S2 ligand can accommodate the
octahedral geometry of the Pd(IV) center, similar to the case for
MeN4 and in contrast to the case for the bulkier iPrN4 and tBuN4
ligands that disfavor the stabilization of the Pd(IV) oxidation
state.23
a
Reported vs Fc at room temperature. The scan rate was 100 mV/s, if
b
c
d
e
not specified. 500 mV/s scan rate. Not observed. Reversible. 50
mV/s scan rate.
redox wave could be identified, except some oxidation peaks that
have potentials of Epa = 0.47 V (vs Fc) and E1/2 = −0.60 V (ΔEp =
70 mV) (Figure S1, Supporting Information), similar to the
PdII/III oxidation potential of [(N2S2)PdIIMe]2(OTf)2 (5; Figure
S2, Supporting Information) and the PdIII/IV redox potential of
1
2+ (Figure S3, Supporting Information), respectively. It is likely
that the broad oxidation feature from −0.4 to 0.27 V is due to
multiple oxidations such as Pd(II) → Pd(III) → Pd(IV), while
the transiently formed Pd(III)−dimethyl intermediate can
undergo a chemical reaction to yield a Pd(II)−monomethyl
species on the CV time scale (vide infra). Interestingly, after one
CV cycle a reversible redox wave is observed at ∼−0.56 V, which
is assigned to a PdIV/III redox couple, as it is similar to that
observed during the reduction of 12+ (Figure S3). The CV of 12+
also suggests that the Pd(III)−dimethyl intermediate can be
stabilized upon reduction of 12+, yet further reduction to Pd(II)
leads to a decreased intensity for the reoxidations of Pd(II) to
Pd(III) and Pd(III) to Pd(IV).35 The observed limited stability
of the Pd(III) intermediate is likely due to the conformational
change of the N2S2 ligand. The oxidation of 1, with N2S2
binding in a κ2 conformation, generates a transient Pd(III)
species with a four- or five-coordinate metal center that is
expected to be less stable than a six-coordinate species. In
contrast, N2S2 adopts a κ4 conformation in 12+ and thus no
conformation change is needed upon reduction to generate a (κ4-
N2S2)PdIII species. If Pd(III) is further reduced to Pd(II), the
conformation change of N2S2 from κ4 to κ2 will lead to an
irreversible reoxidation to Pd(III).
For 2 and 3, both complexes display similar irreversible PdII/III
and quasi-reversible PdIII/IV waves (Table 1 and Figure S4 and S5
(Supporting Information)), indicating that the change in halogen
donors (Cl, Br) does not affect significantly the electrochemical
properties of the complexes. The additional features in the CVs
of 2 and 3 are likely due to side reactions of the Pd(III) species to
generate monohalide or monomethyl Pd(II) products that
exhibit high potential oxidation waves (e.g., [(N2S2)PdIICl]-
(OTf) (6-Cl), Figure S8 (Supporting Information)).35 The CV
of 4 shows an irreversible PdII/III wave at a potential much higher
than for any complex of 1−3, which is due to the decreased σ-
donor ability of a Cl vs methyl donor (Figure S6 (Supporting
Information)). These CVs suggest that, while the Pd(III)
oxidation state is accessible, it exhibits further reactivity on the
CV time scale and the formed side products lead to complex CV
profiles.
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Figure 2. Experimental and simulated EPR spectra of reaction mixtures of (N2S2)Pd complexes and redox reagents at 77 K: (a) 12+ + CoIICp2 (the
feature marked with an asterisk is from CoIICp2); (b) 2 + AcFc+ (the feature marked with an asterisk is from the hyperfine coupling to the 105Pd isotope
(abundance 22.3%, I = 5/2)); (c) 3 + AcFc+ (the simulated spectrum (blue) is a sum of the components of two Pd(III) species: 3+ (blue dashed line) and
species B (blue dotted line)); (d) 4 + NO+ (the features marked with an asterisk are due to the hyperfine coupling to the 105Pd isotope (22.3%, I = 5/2)).
Table 2. EPR Parameters for Pd(III) Species
g value
Superhyperfine Coupling (G)
species
g1
g2
g3
A1
A2
A3
1+
2+
2.097
2.143
2.097
2.045
1.988
2.038
a
ND
ACl = 25.0
AN1 = 15.0
A
A
N1 = 5.0
N2 = 13.0
3 + AcFc+
species A
2.135
2.048
2.020
AN1 = 15.0
ABr = 130.0
AN2 = 30.0
AN = 5.0
A
A
N1 = 5.0
N2 = 5.0
a
species B
2.177
2.093
2.086
2.084
2.014
2.005
ND
AN = 15.0
4+
a
Not determined.
Characterization of Pd(III) Intermediates. On the basis of
[(N2S2)PdIIIMeCl]+ (2+). 2+ was made by oxidation of 2 with 1
equiv of AcFc+ and trapped by rapid freezing at 77 K. The EPR
spectrum in 1/3 MeCN/PrCN (v/v) is more complex (Figure
2), and a reasonable simulation was obtained using the
parameters g1 = 2.143, g2 = 2.045 (ACl = 25.0 G, AN1 = 5.0 G,
AN2 = 13.0 G), and g3 = 2.038 (AN1 = 15.0 G) with superhyperfine
coupling to one Cl atom (35Cl, 37Cl, I = 3/2) and two 14N (I = 1)
atoms. The similarity of these g values to the literature values
implies that a Pd(III) intermediate has been produced.22
However, the g1 ≫ g2 ≈ g3 ordering of the g values is different
from that of most reported EPR spectra of mononuclear Pd(III)
the CV studies of (N2S2)Pd complexes, controlled-potential
electrolysis (CPE) was performed in an attempt to generate the
corresponding Pd(III) intermediates. Unfortunately, the limited
stability of the Pd(III) intermediates prevented their isolation.
Instead, chemical oxidation or reduction was employed to
generate Pd(III) species that were characterized in situ.
[(N2S2)PdIIIMe2]+ (1+). The formation of 1+ upon adding 1
equiv of Fc+ to 1 could not be detected by EPR. Alternatively, 1+
was generated by chemical reduction of 12+ with 1 equiv of
CoIICp2 and trapped by rapid freezing at 77 K. The EPR
spectrum in 1/3 MeCN/PrCN (v/v) reveals an axial signal with
g⊥ = 2.097 and g∥ = 1.988 (Figure 2), values similar to those
reported for [(tBuN4)PdIIIMe2]+,24 suggesting the presence of the
Pd(III)−dimethyl intermediate in an axially distorted octahedral
geometry. However, species 1+ is highly unstable and decays
within 1 min to generate EPR-silent species (vide infra).
1
2
complexes in a distorted-octahedral geometry with a (dz )
ground state.21 This suggests that the Pd(III) center could
adopt either a four-coordinate (square planar) or five-coordinate
(trigonal pyramidal or square pyramidal) geometry and a (dxy)1
1
2
2
or (dx −y ) ground state, although the geometry of the Pd(III)
species cannot be unambiguously determined. The proposed
geometries are also suggested by the observed superhyperfine
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Figure 3. (a) In situ UV−vis spectra of the oxidation of 4 (0.74 mM, black line) with 1 equiv of NO+ in MeCN at room temperature: (red line) initial
spectrum after adding NO+; (blue line) spectrum after 4 h. (b) Experimental and normalized calculated TD-DFT UV−vis spectrum of 4+.
Scheme 2. Oxidatively Induced Reactivity of 1 (Top) and Reductively Induced Reactivity of 12+ (Bottom)
coupling to Cl and/or N atoms in the equatorial plane and may
be the reason for the reduced stability of the Pd(III) species.
Further proof for the formation of a Pd(III) species was provided
by ESI-MS: a peak at m/z 429.9556 with the characteristic Pd
isotopic pattern was observed (calculated m/z for 2+ 429.9553;
Figure S9 (Supporting Information)),35 while no such peak was
observed in the ESI-MS of 2 itself, strongly suggesting the
formation of the Pd(III) species upon oxidation of 2 by AcFc+.
[(N2S2)PdIIIMeBr]+ (3+). Since 3 exhibits electrochemical
behavior similar to that of 2, chemical oxidation of 3 was
performed under the same conditions. ESI-MS of the oxidation
of 3 with 1 equiv of AcFc+ shows a peak at m/z 473.9027,
corresponding to 3+ (calculated m/z for 3+ 473.9046; Figure S10
(Supporting Information)),35 while such a peak was absent in the
ESI-MS of 3. The EPR spectrum of the frozen reaction mixture
reveals a complex pattern (Figure 2) that can be simulated by
employing two overlapping signals from two Pd(III) inter-
mediates in a 1/1 ratio: species A, in which Pd(III) is coupled to
one Br atom (79Br, 81Br, I = 3/2) and two 14N (I = 1) atoms, and
species B, where the Pd(III) center is coupled to one N atom
only. The parameters for species A, g1 = 2.135 (AN1 = 15.0 G), g2
= 2.048 (ABr = 130.0 G, AN1 = 5.0 G, AN2 = 5.0 G), and g3 = 2.020
(AN2 = 30.0 G), are comparable with those of 2+, except for the
coupling constant to Br is significantly larger, indicating that in
species A Pd(III) might adopt a coordination geometry similar to
that of 2+. For species B, the simulation yields a rhombic
spectrum with g1 = 2.177, g2 = 2.086 (AN = 15.0 G), and g3 =
2.014 (AN = 5.0 G) (Table 2), which is more in line with the
reported parameters for distorted-octahedral Pd(III) com-
plexes.22 Although more data are needed to unambiguously
assign the geometry of the Pd(III) centers in these species, these
results provide strong evidence that Pd(III) intermediates are
formed in the reaction mixture.
[(N2S2)PdIIICl2]+ (4+). Since 4 exhibits a higher oxidation
potential than 1−3, the stronger oxidant NOBF4 was used to
oxidize 4. Upon oxidation of 4 by 1 equiv of NO+, the initially
yellow solution turns red-purple, followed by a color change to
orange during the course of several hours. The EPR in 1/3
MeCN/PrCN (v/v) at 77 K reveals a Pd(III) signal with gx =
2.093, gy = 2.084, and gz = 2.005 (Figure 2 and Table 2), similar to
that of 1+, suggesting that in 4+ the Pd(III) center is in a
distorted-octahedral environment with the N2S2 ligand in a κ4
conformation. Monitoring of the oxidation reaction in MeCN at
room temperature by UV−vis spectroscopy reveals two bands at
552 and 368 nm that increase in intensity within the first 2 min,
followed by their decay during the next few hours and appearance
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Scheme 3. Proposed Mechanism for C−C Bond Formation
a
Scheme 4. Oxidative Reactivity Studies of 2 and 3
a
ppt = precipitate.
of a shoulder at 434 nm (Figure 3a). By comparing to other
reported mononuclear Pd(III) complexes,22,42 we tentatively
assign these bands to Cl to Pd and S to Pd ligand to metal charge
transfer (LMCT) transitions for 4+, while the shoulder of 434 nm
is likely an LMCT transition of the resulting product. Further
insight into the electronic properties of 4+ was obtained by
density functional theory (DFT) calculations. The geometry
optimized structure of 4+ reveals a Pd(III) center in a distorted-
out to investigate the oxidatively induced reactivity of organo-
metallic (N2S2)PdII complexes 1−3.
C−C Bond Formation. Oxidation of 1 with 1 equiv of Fc+ in
acetone-d6 at room temperature leads to the quantitative
formation of MeMe (49 2%) and 5 (98 1%) within 30
min (Scheme 2), as detected by NMR (Figure S12 (Supporting
Information)).35 In order to gain insight into the possible
mechanism of ethane formation, we first examined the reaction
of 1 with Fc+ in the presence of 2 equiv of TEMPO, a common
alkyl radical trapping agent, yet no Me-TEMPO product was
observed and the yield of ethane was not affected. Along with the
lack of formation of CH4, these results suggest that a methyl
radical pathway is unlikely.22 Thus, we propose the initial
formation of 1+, followed by methyl group transfer and
disproportionation of 1+ to [(N2S2)PdIVMe3]+ and a mono-
methyl Pd(II) product, the resulting [(N2S2)PdIVMe3]+ species
being responsible for the subsequent C−C bond formation
(Scheme 3). A similar nonradical methyl group transfer and C−
C bond formation reactivity was proposed by Sanford, Meyer, et
al. for the one-electron oxidation of the (tBu2bpy)PdIIMe2
complex to generate an observable [(tBu2bpy)PdIVMe3]+
intermediate.43,44 In addition, the recent isolation of [(Me3tacn)-
PdIVMe3]+ and [(MeN4)PdIVMe3]+ species upon aerobic
oxidation of the corresponding Pd(II) precursors and methyl
group transfer at Pd(III) or Pd(IV) centers provides further
support for the proposed mechanism (Scheme 3).13,25
2
octahedral geometry and the unpaired electron in a dz ground
state, in line with the observed EPR spectrum.35 In addition, the
TD-DFT calculated UV−vis spectrum and transitions of 4+
match well the experimental spectrum and proposed charge
transfer transitions (Figure 3b and Table S4 (Supporting
Information)) and thus provide strong support for the proposed
octahedral geometry of 4+ with the N2S2 ligand binding in a κ4
conformation. Since the Pd(III) intermediate decays quickly at
room temperature, low-temperature CPE of 4 (2.0 mM, 10 mL)
was performed in MeCN at −40 °C. After the charge equivalent
to a one-electron oxidation has passed, the solution turns purple,
indicating formation of 4+. In situ CV immediately after CPE at
−40 °C reveals a reversible PdIII/II redox wave at 0.39 V, followed
by another reduction at −0.11 V (Figure S6 (Supporting
Information)).35 Although the purple solution is stable at −35
°C for a few days, the isolation of the purple product was not
successful, an orange powder being obtained instead (vide infra).
Reactivity Studies of (N2S2)Pd Complexes. In light of the
recently reported reductive elimination of ethane and MeCl from
isolated Pd(III) complexes and the proposed role of Pd(III)
intermediates in reductive elimination reactions,22,24,43 we set
To further probe the mechanism of the reaction, a crossover
experiment was performed using a 1/1 mixture of 1 and
(N2S2)PdII(CD3)2 (1-d6) in the presence of 2 equiv of Fc+ (1
equiv Fc+ per Pd), which leads to rapid formation of 5,
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Scheme 5. Possible Mechanisms for Reaction of 2 or 3 with AcFc+ (X = Cl, Br)
[(N2S2)PdII(CD3)]+ (5-d3), CH3CH3, and CH3CD3 in 43 9,
57 9, 12 2, and 24 3% yields, respectively (Figure S13
(Supporting Information)). Given the typical yield of ∼50%
ethane upon oxidation of 1, the yield of CD3CD3 can be
estimated as ∼14 6%, suggesting a ∼ 1/2/1 ratio of CH3CH3,
CH3CD3, and CD3CD3. This ethane isotopologue ratio is similar
to that observed for the one-electron oxidation of the
(tBu2bpy)PdIIMe2 complex, further supporting the methyl
group transfer mechanism.43 Interestingly, this result is in
contrast to a mechanism involving a rapid rearrangement of the
methyl groups before reductive elimination, which should give a
1/1/1 ratio of CH3CH3, CH3CD3, and CD3CD3, as observed for
the aerobic oxidation of (RN4)PdIIMe2 systems.24,25
The involvement of Pd(IV) species during C−C bond
formation is also suggested by the reaction of 1 with 10 equiv
of MeI, which gives almost stoichiometric amounts of (N2S2)-
PdIIMeI and ethane in 70 h (Scheme 2 and Figure S14
(Supporting Information)), suggesting an oxidative addition step
followed by ethane elimination. Moreover, the reaction of 1 with
10 equiv of CD3I gives a less than 1/2 ratio of CH3CH3 and
CH3CD3 and the corresponding monomethyl Pd(II) products
(Figure S15 (Supporting Information)). Since a rapid reductive
elimination step should lead to only CH3CD3, the presence of
both ethane isotopologues suggests that the transient Pd(IV)
intermediate can undergo a rearrangement of the equatorial CH3
and axial CD3 groups before ethane elimination,24 likely due to
the reduced rate of reductive elimination from a six-coordinate
(N2S2)PdIVMe2(CD3)I transient species and in line with the
observed longer reaction times (∼70 h) for this reaction
(Scheme 2).13,45,46
C−X Bond Formation (X = Cl, Br). Oxidation of 2 by 1 equiv
of AcFc+ in acetone-d6 generates 32 3% MeCl within 2 h, as
well as 12+, 5, and [(N2S2)PdIICl]+ (6-Cl) in 14 3%, 31 3%,
and 32 3% yields, respectively (Scheme 4, top). The reaction of
2 with 1 equiv of Fc+ gives a similar result, yet it takes 8 h to reach
completion.35 In addition, formation of 4 was observed by H
1
NMR in the early stages of the reaction but disappeared at the
completion of the reaction along with formation of a yellow
precipitate identified as 4. Since the Pd(III) intermediate 2+ was
already observed by EPR and ESI-MS, several possible
mechanisms were proposed for the observed reactivity: (a) a
radical pathway, (b) a methyl group transfer/disproportionation
step, and (c) direct reductive elimination from the Pd(III)
species (Scheme 5). The formation of small amounts of 12+
during the reaction is most likely due to the subsequent oxidation
of the monomethyl Pd(II) product 5, followed by methyl group
transfer and disproportionation.27
Addition of TEMPO during the oxidation reaction does not
lead to formation of TEMPO-Me and does not affect the yield of
MeCl, suggesting that a mechanism involving methyl radicals is
unlikely. Monitoring of the reaction of 2 with AcFc+ by 1H NMR
at −20 °C reveals the formation of a significant amount of 5
(∼60% yield), followed by its decay and an increase of the
amount of all products during the course of several hours (Figure
S23a (Supporting Information)),35 suggesting a possible Cl
radical pathway. However, the absence of any Cl trapped
products by using halogen radical trapping agents (2,3-dimethyl-
1
1,3-butadiene, styrene, 1-hexene, etc.) in GC-MS or H NMR
makes the Cl radical pathway less likely.47−49 Moreover, the
reaction of 2 with 1 equiv of 13CH3I produces only 13CH3Cl and
(N2S2)PdIIMeI (Figure S18 (Supporting Information)),
suggesting that the Pd(IV) oxidation state is accessible, a fast
and selective reductive elimination of 13CH3Cl occurs from a
[(N2S2)PdIV(13CH3)MeCl]+ intermediate, and no scrambling
occurs between the axial and equatorial methyl groups due to the
high reactivity of the transient Pd(IV) species. Overall, these
results suggest a mechanism involving methyl group transfer and
disproportionation to give a [(N2S2)PdIVMe2Cl]+ species,
followed by reductive elimination (Scheme 5, path B).
Interestingly, no ethane formation was observed upon
thermolysis or photolysis of an acetone solution of 12+, as
expected for a rigid six-coordinate Pd(IV) species.23 In contrast,
the reduction of 12+ by 1.1 equiv of CoIICp2 can promote C−C
bond formation and produces 38 1% ethane and 80 6% of 5
(Scheme 2, bottom),35 confirming that the PdIII−dimethyl
species 1+ is a key intermediate during the proposed ethane
elimination mechanism.
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Scheme 6. Crossover Experiment between 2 and 3-13C
Scheme 7. Oxidation Reaction of 4 with 1 equiv of NOBF4
By comparison, oxidation of 3 with 1 equiv of AcFc+ gives 34
3% MeBr, 55 5% [(N2S2)PdIIBr]+ (6-Br), and 32 3% of 5,
respectively (Scheme 4, bottom; Figure S19 (Supporting
Information)), yet no 12+ or any precipitate was observed at
the end of the reaction of 2 h. Moreover, the time-dependent
than one mechanism, involving either an initial methyl group
transfer/disproportionation from a Pd(III) center followed by
C−Br bond formation or an SN2 reductive elimination from a
Pd(IV)−dimethyl intermediate.
To test whether a direct reductive elimination from a Pd(III)
center is involved during the oxidative reactivity of 2 and 3
(Scheme 5, path C), a crossover experiment was conducted by
reacting a 1/1 mixture of 2 and (N2S2)PdII(13CH3)Br (3-13C)
with AcFc+. A concerted reductive elimination is expected to
generate only the nonscrambled products MeCl and 13CH3Br,
respectively. However, the oxidation reactions generated CH3Br,
13CH3Br, 5, and [(N2S2)PdII(13CH3)]+ (5-13C) in a ∼1/1/1/1
ratio, as well as 6-Cl/Br and a yellow precipitate corresponding
to 4 (Scheme 6), while a negligible amount of MeCl (∼1%) was
produced after 4 h. The formation of the two isotopomers of
MeBr argues against a concerted reductive elimination from a
Pd(III) center and supports a methyl group transfer mechanism
as the preferred pathway for C−Br bond formation. The different
yields of MeCl and MeBr are proposed to be due to (a) the
slightly higher oxidation potential (80 mV) of PdII/III of 2 vs that
of 3 (although their oxidation by AcFc+ is rapid at room
temperature, as observed by NMR),35 (b) the differential
reactivity of [(N2S2)PdIVMe2Cl]+ and [(N2S2)PdIVMe2Br]+
species toward concerted reductive elimination;, (c) the more
rapid reaction of Br− vs Cl− with 12+ to give MeBr through an SN2
reductive elimination, or (d) a combination of the preceding.
On the basis of all these studies, we propose that the observed
oxidatively induced C−halide bond formation from 2 or 3 most
likely involves initial formation of Pd(III) species, followed by
methyl group transfer and disproportionation to form [(N2S2)-
PdIVMe2X]+ (X = Cl, Br) intermediates that selectively eliminate
MeCl or MeBr. However, a halogen radical pathway, especially
for 3, cannot be unambiguously excluded, yet such a radical
mechanism can generate an appreciable amount of 5, which can
be oxidized to yield 12+ that can undergo SN2 reductive
elimination with Br− (or Cl−) to yield MeBr (and to a lesser
extent MeCl). In addition, an SN2 reductive elimination from a
1
low-temperature H NMR studies of this reaction reveal a
behavior different from that of 2. All products form with similar
rates, while the amount of starting material decreases. While 12+
was generated at the beginning of the reaction, its concentration
remained constant for several hours at −20 °C and then slowly
decayed within 14 h at room temperature, along with an increase
of all product yields. Overall, these results indicate that different
mechanisms might be involved during the oxidative reactivity of
3.
A radical mechanism is also unlikely in this case, as no
TEMPO-Me product was observed when the oxidation reaction
was performed in the presence of TEMPO, and no Br-containing
adducts were obtained in the presence of various alkenes (2,3-
dimethyl-1,3-butadiene, 1-hexene, cyclohexene, styrene, etc.) as
halogen radical traps.47−49 However, the possibility of Br radical
cannot be excluded completely, due to a possible rapid reduction
of the Br radical to the Br− anion. Moreover, the intermediacy of
12+ leading to product formation suggests that 12+ could react
with Br− through an SN2 mechanism to produce MeBr and 5
(Scheme 5, bottom). Indeed, this has been confirmed by reacting
12+ with 1.8 equiv of tetrabutylammonium bromide (Bu4N+Br−),
which eliminates MeBr and 3 within 5 min (Figure S22
(Supporting Information)). By comparison, the reaction of 12+
with 1.8 equiv of tetrabutylammonium chloride (Bu4N+Cl−)
produces MeCl at a much slower rate than a similar reaction with
Bu4N+Br−.
Similarly to 2, the reaction of 3 with 1 equiv of 13CH3I
produces only 13CH3Br and (N2S2)PdIIMeI (Scheme 4, bottom,
and Figure S20 (Supporting Information)), implying that
[(N2S2)PdIVMe2Br]+ could be a reaction intermediate (Scheme
5).35 Overall, these mechanistic studies suggest that the
oxidatively induced MeBr formation from 3 can occur by more
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PdIII−Me intermediate cannot be completely ruled out either,
although such a reaction was not observed in any of our previous
studies on organometallic Pd(III) systems.22−25
upon oxidation of Pd(II) to Pd(III) is proposed to lead to the
partial ligand oxidation; however, detailed mechanistic studies
are needed for a better understanding of this ligand oxidation
reactivity.
Partial Ligand Oxidation of [(N2S2)PdIIICl2]+ (4+). Character-
ization of the one-electron-oxidation product of 4 by EPR, low
temperature UV−vis, and CV strongly suggests the formation of
the red-purple Pd(III) species 4+ (vide supra). Unfortunately,
attempts to isolate pure 4+ were unsuccessful and led to the
formation of an orange solid. Diffusion of diethyl ether into an
MeCN solution of the orange powder yielded orange crystals
whose X-ray characterization reveals a unit cell that contains
[(N2S2)PdIICl]+ (6-Cl) and (N2S2)PdIICl2 (4), along with a
small amount of the new complex (N2S2-O)PdIICl2 (7; Scheme
7), in which one of the S atoms of N2S2 has been oxidized to a
sulfoxide (the sulfoxide O atom exhibits a 14% occupancy in the
crystal structure).35 The pure complex [6-Cl](OTf) was
synthesized independently, and its X-ray structure reveals an
uncommon κ3 conformation for the N2S2 ligand, which binds to
the Pd(II) center through one N and two S donors, and the
chloride completes the square-planar geometry (Figure 4), in
Comparison of Electronic Properties of Analogous
N2S2 and tBuN4 Pd(III) Complexes. The results above show
that the Pd(III) complexes supported by the N- and S-donor
N2S2 ligand exhibit limited stability, although the Pd(IV)
complex [(N2S2)PdIVMe2](PF6)2 is uncommonly stable. By
comparison, recent reports from our laboratory have shown that
the analogous Pd(III) complexes of the all-N macrocyclic ligand
tBuN4 are extremely stable, although the corresponding Pd(IV)
species could not be isolated.21,22,24 In order to address whether
the differential stability of the Pd(III) complexes is due to the
axial donor atom (N vs S) or just steric effects, density functional
theory (DFT) calculations were employed to evaluate the
electronic properties of the geometry optimized PdIIIMe2
cationic complexes stabilized by N2S2 or tBuN4, which most
likely exhibit the same geometry of the Pd(III) centers. By
comparison, the geometry of the Pd(III) centers in 2 and 3 is not
clear at this point, either a five- or six-coordinate geometry being
possible. Interestingly, the atomic contribution to the frontier
molecular orbitals is similar for N2S2 vs tBuN4 Pd(III) complexes
(Table 3 and Tables S1−S6 (Supporting Information)), with the
N2S2 complexes exhibiting slightly more covalent metal−ligand
interactions and suggesting that the change in axial donors has
only a minor effect. Thus, the steric effect of the N-tBu group is
most likely responsible for the dramatic stabilization of the
distorted-octahedral Pd(III) complexes, while the absence of
such groups in the N2S2 ligand supports an octahedral geometry
preferred by Pd(IV) complexes. Moreover, a similar steric effect
R
Figure 4. X-ray crystal structure of the cation of [6-Cl](OTf) with 50%
thermal ellipsoids. Average bond distances (Å): Pd1−N1 = 2.030, Pd1−
Cl1 = 2.292, Pd1−S1 = 2.319, Pd1−S2 = 2.328.
was observed for the Pd complexes of N4 ligands (R = Me,
iPr),23,25 which can stabilize both Pd(III) and Pd(IV) oxidation
states. Use of the N2S2 ligand thus further highlights the
paramount role of ligand sterics on Pd(III) complex stability that
seems to trump even the change in axial donor atoms.
line with the atom connectivity obtained from the isolated orange
mixture. A similar ligand conformation was observed for the
recently reported dinuclear Pd(II) complexes [(N2S2)-
CONCLUSION
PdIIMe]2 and [(N2S2)PdII(MeCN)]24+, yet no metal−metal
2+
■
interactions were observed for [6-Cl](OTf) in the solid state.27
By comparison, the N2S2 ligand in 4 and 7 is found in the more
common κ2 conformation. The conformational change of N2S2
In summary, reported herein is the characterization of a series of
organometallic (N2S2)PdII complexes that exhibit oxidatively
induced ethane, MeCl, or MeBr elimination reactivity. We have
Table 3. Comparison of the β Lowest Unoccupied Molecular Orbitals (LUMOs) of [(N2S2)PdIIIMe2]+ and [(tBuN4)PdIIIMe2]+
Complexes
3352
dx.doi.org/10.1021/om400286j | Organometallics 2013, 32, 3343−3353

Organometallics
Article
(13) Khusnutdinova, J. R.; Qu, F.; Zhang, Y.; Rath, N. P.; Mirica, L. M.
Organometallics 2012, 31, 4627.
(14) Chuang, G. J.; Wang, W.; Lee, E.; Ritter, T. J. Am. Chem. Soc. 2011,
shown that the Pd(III) oxidation state is accessible on the basis of
the redox properties of these complexes and that the oxidative
reactivity of these Pd(II) complexes involves Pd(III) inter-
mediates. EPR, UV−vis, ESI-MS, and low-temperature CV
provide direct evidence for the presence of Pd(III) intermediates.
Mechanistic studies suggest that C−C bond formation occurs
most likely through a methyl group transfer and formation of a
transient [(N2S2)PdIVMe3]+ intermediate. In contrast, C−halide
bond formation may involve the formation of [(N2S2)-
PdIVMe2X]+ intermediates, yet a halogen radical pathway cannot
be completely excluded. The isolated PdIV−dimethyl complex
reveals a N2S2 ligand that adopts a tetradentate κ4 conformation,
while in the PdII−monochloro complex N2S2 adopts a tridentate
κ3 conformation, both showing that the S-donor atoms interact
with the metal center in different oxidation states. Moreover, the
Pd complexes described herein reveal the ability of the N2S2
ligand to adopt various conformations and denticities when
binding to metal centers in various oxidation states and ligand
field strengths. The coordination versatility of N2S2 is currently
being explored for other transition-metal ions.
133, 1760.
(15) Powers, D. C.; Benitez, D.; Tkatchouk, E.; Goddard, W. A., III;
Ritter, T. J. Am. Chem. Soc. 2010, 132, 14092.
(16) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302.
(17) Cotton, F. A.; Koshevoy, I. O.; Lahuerta, P.; Murillo, C. A.; Sanau,
M.; Ubeda, M. A.; Zhao, Q. J. Am. Chem. Soc. 2006, 128, 13674.
(18) Cotton, F. A.; Gu, J. D.; Murillo, C. A.; Timmons, D. J. J. Am.
Chem. Soc. 1998, 120, 13280.
(19) Blake, A. J.; Gordon, L. M.; Holder, A. J.; Hyde, T. I.; Reid, G.;
Schroder, M. J. Chem. Soc., Chem. Commun. 1988, 1452.
̈
(20) Blake, A. J.; Holder, A. J.; Hyde, T. I.; Schroder, M. J. Chem. Soc.,
̈
Chem. Commun. 1987, 987.
(21) Mirica, L. M.; Khusnutdinova, J. R. Coord. Chem. Rev. 2013, 299.
(22) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc.
2010, 132, 7303.
(23) Tang, F.; Qu, F.; Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M.
Dalton Trans. 2012, 41, 14046.
(24) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc.
2012, 134, 2414.
(25) Tang, F.; Zhang, Y.; Rath, N. P.; Mirica, L. M. Organometallics
2012, 31, 6690.
(26) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. Angew. Chem., Int.
Ed. 2011, 50, 5532.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving cyclic voltammograms, ESI-
MS spectra, EPR simulation details, 1H NMR spectra, computa-
tional details, and X-ray crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) Luo, J.; Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. Chem.
Commun. 2012, 48, 1532.
(28) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(29) Foley, S. R.; Stockland, R. A., Jr.; Shen, H.; Jordan, R. F. J. Am.
Chem. Soc. 2003, 125, 4350.
(30) Rulke, R. E.; Ernsting, J. M.; Spek, A. L.; Elsevier, C. J.; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(31) Moriguchi, T.; Kitamura, S.; Sakata, K.; Tsuge, A. Polyhedron
2001, 20, 2315.
(32) Constable, E. C.; King, A. C.; Raithby, P. R. Polyhedron 1998, 17,
4275.
(33) SADABS; Bruker Analytical X-Ray, Madison, WI, 2008.
(34) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(35) See the Supporting Information.
(36) de Graaf, W.; Boersma, J.; Smeets, W. J. J.; Spek, A. L.; van Koten,
G. Organometallics 1989, 8, 2907.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for L.M.M.: mirica@wustl.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Chemistry at Washington
University for startup funds and the DOE Catalysis Science
Program (DE-FG02-11ER16254) for support. We also thank Dr.
Julia R. Khusnutdinova for help with low-temperature electro-
chemical studies, Ying Zhang for ESI-MS analyses, Jason Schultz
for EPR measurements, and Dr. Xia Ge for low-temperature 1H
NMR experiments.
(37) Steffen, W. L.; Palenik, G. J. Inorg. Chem. 1976, 15, 2432.
(38) Stephen, E.; Blake, A. J.; Carter, E.; Collison, D.; Davies, E. S.;
Edge, R.; Lewis, W.; Murphy, D. M.; Wilson, C.; Gould, R. O.; Holder,
A. J.; McMaster, J.; Schroder, M. Inorg. Chem. 2012, 51, 1450.
̈
(39) Bennett, M. A.; Canty, A. J.; Felixberger, J. K.; Rendina, L. M.;
Sunderland, C.; Willis, A. C. Inorg. Chem. 1993, 32, 1951.
(40) Canty, A. J.; Jin, H.; Skelton, B. W.; White, A. H. Inorg. Chem.
1998, 37, 3975.
REFERENCES
■
(41) Zhao, X. D.; Dong, V. M. Angew. Chem., Int. Ed. 2011, 50, 932.
(42) Stephen, E.; Blake, A. J.; Carter, E.; Collison, D.; Davies, E. S.;
Edge, R.; Lewis, W.; Murphy, D. M.; Wilson, C.; Gould, R. O.; Holder,
(1) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling Reactions;
Wiley-VCH: Weinheim, New York, 1998.
(2) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147.
(3) Xu, L.-M.; Li, B.-J.; Yang, Z.; Shi, Z.-J. Chem. Soc. Rev. 2010, 39, 712.
(4) Sehnal, P.; Taylor, R. J. K.; Fairlamb, I. J. S. Chem. Rev. 2010, 110,
824.
(5) Henry, P. M. Palladium Catalyzed Oxidation of Hydrocarbons; D.
Reidel: Boston, MA, 1980.
(6) Negishi, E. Handbook of Organopalladium Chemistry for Organic
Synthesis; Wiley; Hoboken, NJ, 2002.
(7) van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding
the Art; Kluwer Academic: Dordrecht, The Netherlands, 2004.
(8) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science Books: Sausalito, CA, 2010.
(9) Canty, A. J. Dalton Trans. 2009, 10409.
A. J.; McMaster, J.; Schroder, M. Inorg. Chem. 2012, 51, 1450.
̈
(43) Lanci, M. P.; Remy, M. S.; Kaminsky, W.; Mayer, J. M.; Sanford,
M. S. J. Am. Chem. Soc. 2009, 131, 15618.
(44) Liu, G.-X.; Puddephatt, R. J. Inorg. Chim. Acta 1996, 251, 319.
(45) Byers, P. K.; Canty, A. J.; Skelton, B. W.; White, A. H.
Organometallics 1990, 9, 826.
(46) Racowski, J. M.; Sanford, M. S. Top. Organomet. Chem. 2011, 35,
61.
(47) Lin, T.-P.; Gabbai, F. P. J. Am. Chem. Soc. 2012, 134, 12230.
(48) Cook, T. R.; McCarthy, B. D.; Lutterman, D. A.; Nocera, D. G.
Inorg. Chem. 2012, 51, 5152.
(49) Teets, T. S.; Cook, T. R.; McCarthy, B. D.; Nocera, D. G. Inorg.
Chem. 2011, 50, 5223.
(10) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J. Q. Angew. Chem., Int.
Ed. 2009, 48, 5094.
(11) Muniz, K. Angew. Chem., Int. Ed. 2009, 48, 9412.
(12) Powers, D. C.; Ritter, T. Top. Organomet. Chem. 2011, 35, 129.
3353
dx.doi.org/10.1021/om400286j | Organometallics 2013, 32, 3343−3353