(S)-4-tert-Butyl-2-phenyl-2-oxazoline derived palladacycles as efficient catalysts for the decomposition of P=S pesticides

Article abstract of DOI:10.1080/00958972.2010.491545

Three palladacycles with N, N-trans coordination, (S)-{2-[2-(4-tert-butyl) oxazolinyl]phenyl-C1, N}(4-R-pyridine)(aqua)npalladium(II) triflate (R = CF ₃, n = 1, 1; R = H, n = 0, 2; R = NMe2, n = 1, 3), have been prepared from the respective chloride precursors and were characterized with IR and 1H-NMR and elemental analyses. The crystal structures of 2 and a pincer complex (S, S)-{2, 6-bis[2-(4-iso-propyl)oxazolinyl]phenyl-N, C₁, N}bromo-palladium(II) hydrate (5) were determined. For methanolysis of P=S pesticides, it was found that ortho-palladated complexes 1-3 effectively catalyzed the reaction; however, the pincer complex 4 with triflate counterion was much less active. Relative to the methoxide-promoted background reaction at ^s_sPH 10.80, 1 mmol L^-1 of palladacycles 1-3 can accelerate the methanolysis by 8.7 × 10⁹, 1.5 × 10 ⁹, and 1.0 × 10⁸-fold, respectively. For racemic P=S pesticides, such as letophos and EPN, it proved that there is no clear chiral discrimination during the catalysis. The reaction mechanism of the catalytic methanolysis was discussed according to the experimental results.

Full text of DOI:10.1080/00958972.2010.491545

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(S)-4-tert-Butyl-2-phenyl-2-oxazoline
derived palladacycles as efficient
catalysts for the decomposition of P=S
pesticides
Zhong-Lin Lu a , Xiao-Sheng Yang a , Zhi-Fo Guo a & Rui-Yao Wang b
a College of Chemistry, Beijing Normal University , Beijing 100875,
China
b Department of Chemistry , Queen's University , Kingston, ON,
K7L 3N6, Canada
Published online: 15 Jun 2010.
To cite this article: Zhong-Lin Lu , Xiao-Sheng Yang , Zhi-Fo Guo & Rui-Yao Wang (2010)
(S)-4-tert-Butyl-2-phenyl-2-oxazoline derived palladacycles as efficient catalysts for the
decomposition of P=S pesticides, Journal of Coordination Chemistry, 63:14-16, 2659-2672, DOI:
10.1080/00958972.2010.491545
To link to this article: http://dx.doi.org/10.1080/00958972.2010.491545
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Journal of Coordination Chemistry
Vol. 63, Nos. 14–16, 20 July–20 August 2010, 2659–2672
(S )-4-tert-Butyl-2-phenyl-2-oxazoline derived palladacycles as
efficient catalysts for the decomposition of P^S pesticides#
ZHONG-LIN LU*y, XIAO-SHENG YANGy, ZHI-FO GUOy
and RUI-YAO WANGz
yCollege of Chemistry, Beijing Normal University, Beijing 100875, China
zDepartment of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada
(Received 27 January 2010; in final form 2 March 2010)
Three palladacycles with N,N-trans coordination, (S)-{2-[2-(4-tert-butyl)oxazolinyl]phenyl-
C1,N}(4-R-pyridine)(aqua)npalladium(II) triflate (R ¼ CF3, n ¼ 1, 1; R ¼ H, n ¼ 0, 2;
R ¼ NMe2, n ¼ 1, 3), have been prepared from the respective chloride precursors and were
1
characterized with IR and H-NMR and elemental analyses. The crystal structures of 2 and a
pincer complex (S,S)-{2,6-bis[2-(4-iso-propyl)oxazolinyl]phenyl-N,C1,N}bromo-palladium(II)
hydrate (5) were determined. For methanolysis of P¼S pesticides, it was found that ortho-
palladated complexes 1–3 effectively catalyzed the reaction; however, the pincer complex 4 with
triflate counterion was much less active. Relative to the methoxide-promoted background
reaction at sspH 10.80, 1 mmol Lꢀ1 of palladacycles 1–3 can accelerate the methanolysis by
8.7 ꢁ 109, 1.5 ꢁ 109, and 1.0 ꢁ 108-fold, respectively. For racemic P¼S pesticides, such as
letophos and EPN, it proved that there is no clear chiral discrimination during the catalysis.
The reaction mechanism of the catalytic methanolysis was discussed according to the
experimental results.
Keywords: Palladacycle; Methanolysis; Pesticide; Kinetic
1. Introduction
Thiophosphoric acid esters comprise a family of compounds that are usually used as
agricultural pesticides because of their high insecticidal and acaricidal properties but
much lower mammalian toxicity than their P¼O counterparts [1, 2]. With an annual
world production of hundred thousands of tons, their widespread use and environ-
mental accumulation are becoming an ecological and health threat. Finding green
catalytic processes for efficient degradation of these chemicals has spurred much
attention. Many efforts have been directed to catalytic or enzymatic hydrolysis and
oxidation of these chemicals [3–9], but their efficiency including reaction speed and
completeness of the degradation still needs to be greatly improved.
Recently, we demonstrated that ortho-palladated complex, Pd(dmba)(py)(OTf)
(dmba ¼ N,N-dimethylbenzylamine) [10], effectively catalyzed the methanolysis of
*Corresponding author. Email: luzl@bnu.edu.cn
#Dedicated to Prof. Rudi van Eldik on the occasion of his 65th birthday.
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.491545

2660
Z.-L. Lu et al.
–
–
OTf
OTf
O
O
O
F3C
N
Pd
N
N
Pd
N
Me2N
N
H
Pd
N
OH2
OTf
OH2
2
1
3
O
O
O
O
–
OTf
H2O
N
Pd
OH2
4
N
N
Pd
Br
5
N
H
H
H
Scheme 1. Chemical structures of palladacycles.
phosphorothionates. The advantages of a catalytic alcoholysis method over hydrolysis
include greater substrate solubility in the alcohol medium, greatly increased reaction
rates probably attributable to a lower dielectric constant/polarity medium effect that
also gives enhanced Mxþ : substrate pre-equilibrium complexation and the fact that the
products are also neutral OP compounds that are non-inhibitory since they bind to the
metal catalyst with a similar strength as the starting OP materials [11, 12].
To further investigate the system and develop more efficient catalysts, we have
prepared palladacycles 1–3 derived from (S)-4-tert-butyl-2-phenyl-2-oxazoline ligands
as well as pincer palladacycles 4 and 5 (scheme 1) by considering that pincer complexes
are widely applied in catalysis [13–15]. The reason for incorporation of the chiral ligand
in palladacycles is to explore the possibility of asymmetric methanolysis since some
pesticides are racemic mixtures. Basicity effects of the co-coordinated pyridine ligands
in the catalysis were also investigated. We report here their synthesis, characterization,
crystal structures, and some kinetic studies of their catalytic methanolysis of certain
P¼S pesticides (scheme 2).
2. Experimental
2.1. Materials and methods
Methanol (99.8% anhydrous), sodium methoxide (0.5 mol dmꢀ3 solution in methanol)
and silver triflate (AgOTf) were purchased from Aldrich and used without
purification. HClO4 (70% aqueous solution) was purchased from BDH.
Fenitrothion (6, O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate), methyl-
parathion (7, O,O-dimethyl O-(4-nitrophenyl) phosphorothioate), letophos (8, O-4-
bromo-2,5-dichlorophenyl O-methyl phenylphosphonothioate), and EPN (9, O-ethyl
O-4-nitrophenylphenylphosphonothioate) were purchased from Chem. Service
Inc.; phoxim (10, ꢀ-[[(diethoxyphosphinthioyl)oxy]-imino]benzene-acetonitrile) and

Palladacycles
2661
Cl
S
S
H3CO P O
OCH3
S
CH3O P O
NO2
NO2
P O
Br
OCH3
CH3O
Cl
8
7
6
Cl
CN
S
S
S
N
P O
NO2
H3CH2CO P O N
CH3CH2O P O
CH3CH2O
Cl
OCH2CH3
CH3CH2O
9
10
11
Scheme 2. Chemical structures of pesticides investigated.
chlorpyrifos (11, O,O-diethyl-O-[3,5,6-trichloro-2-pyridyl]-phosphorothiate) are gifts
obtained from Beijing Yoloo Pesticide Company Ltd., China.
Caution: All the phosphorus substrates described herein are acetylcholinesterase
inhibitors; compounds 6–10 have oral LD50 values of 250, 6, 43, and 8, 82,300 mg kgꢀ1
,
respectively in rats [16].
1H- and 31P-NMR spectra were determined at 400 and 161.97 MHz using a Bruker
1
Avance-400 NMR spectrometer. In CDCl3, H-NMR spectra were referenced to the
CHCl3 peak at 7.26 ppm. 31P-{1H}-NMR spectra were referenced to an external
standard of 70% phosphoric acid in water, and upfield chemical shifts are negative.
Elemental analyses were performed on an Elementar Vario El instrument. IR spectra
were obtained as KBr pellets using a Nicolet 380 FT-IR spectrometer. Melting points
were measured on a Fisher–John melting point apparatus.
2.2. Preparation of palladacycles 1–4
2.2.1. General procedure for 1–3. To a solution of (S)-chloro{2-[2-(4-tert-butyl)oxazolinyl]-
phenyl-C,N}(4-R-pyridine)palladium(II) (R¼CF3, H, and NMe2) [17] (0.30 mmol) in
dichloromethane (20 mL) solid AgOTf (93.0 mg, 0.36 mmol, 1.2 eq) was added. The
mixture was stirred at room temperature for 3 h and then filtered through celite. The
celite was washed with another 20 mL of dichloromethane and the combined organics
stripped of solvent by rotary evaporation under reduced pressure. The solid residue
was recrystallized from a mixture of hexanes and dichloromethane to afford the desired
product.
(S)-{2-[2-(4-tert-butyl)oxazolinyl]phenyl-C1,N}(trifluoromethylpyridine)(aqua)-palladium(II)
triflate, 1. Yield: 75%, m.p.: 185ꢂC.
IR (KBr, cmꢀ1): 3420(br), 2967(m), 1629(vs), 1488(m), 1422(m), 1327(vs), 1253(vs),
1
1170(vs), 1145(vs), 1029(vs), 921(m), 840(m), 734(m), 679(m), and 637(m). H-NMR
0
0
(ppm, 400 MHz, CDCl3): ꢁ 9.18 (d, J ¼ 8.0 Hz, H2 and H6 on py), 7.75 (d, J ¼ 8.0 Hz,
0
0
H3 and H5 on py), 7.26 (d, J ¼ 8.0 Hz, H3 on phenyl), 7.10 and 6.99 (m, H4 and H5
on phenyl), 6.01 (d, J ¼ 8 Hz, H6 on phenyl), 4.73 (dd, J ¼ 9.0 Hz, 3.0 Hz, OCH),
4.64 (t, J ¼ 8.0 Hz, NCH), 4.08 (J ¼ 9.0 Hz, 3.0 Hz, OCH), 1.73 (br, aqua), and

2662
Z.-L. Lu et al.
0.97 (s, C(CH3)3). 19F-NMR (ppm, 376.45 MHz, CDCl3): ꢁ ꢀ67.73 (CF3-py), and
ꢀ81.1 (CF3SOꢀ3 ). Anal. Calcd for C20H22F6N2O5PdS (Fw: 622.87): C 38.57%,
H 3.56%, N 4.50%. Found: C 38.22%, H 3.53%, N 4.43%.
(S)-{2-[2-(4-tert-butyl)oxazolinyl]phenyl-C1,N}(pyridine)palladium(II) triflate, 2.
Yield: 79%, m.p.: 220ꢂC (decom.).
IR (KBr, cmꢀ1): 2961(m), 1625(vs), 1574(m), 1454(m), 1409(m), 1309(vs), 1235(vs),
1204(s), 1015(vs), 921(s), 736(m), 694(m), and 637(m). 01H-NMR (ppm, 400 MHz,
0
0
CDCl3): ꢁ 8.90 (d, J ¼ 8.0 Hz, H2 , H6 on py), 7.94 (m, H4 on py), 7.49 (t, J ¼ 6.0 Hz,
0
0
H3 , H5 on py), 7.23 (d, J ¼ 8.0 Hz, H3 on phenyl) and 7.07 (m, H4 on phenyl), 6.98
(m, H5 on phenyl), 6.10 (d, J ¼ 8.0 Hz, H6 on phenyl), 4.76 (dd, J ¼ 9.0 Hz, 3.0 Hz,
OCH), 4.58 (t, J ¼ 9.0 Hz, NCH), 4.17 (dd, J ¼ 9.0 Hz and 3.0 Hz, NCH), 1.04
19
(s, C(CH3)3). F-NMR (ppm, 376.45 MHz, CDCl3): ꢁ ꢀ81.1 (CF3SOꢀ3 ). Anal. Calcd
for C19H21F3N2O4PdS (Fw: 536.84): C 42.51%, H 3.94%, N 5.22%. Found: C 42.61%,
H 4.14%, N 5.11%.
(S)-{2-[2-(4-tert-butyl)oxazolinyl]phenyl-C1,N}(N,N-dimethylaminopyridine)-(aqua)
palladium(II), 3. Yield: 59%, m.p.: 210ꢂC (decom.).
IR (KBr, cmꢀ1): 3420(br), 2960(m), 1624(vs), 1541(s),(m), 1398(s), 1302(vs), 1266(vs),
1
1224(vs), 1207(vs), 1025(s), 921(m), 817(m), 735(m), 679(m), and 634(m). H-NMR
0
0
(ppm, 400 MHz, CDCl3): ꢁ 8.33 (d, J ¼ 6.5 Hz, H2 and H6 on py), 7.22 (m, H3 on
0
0
phenyl) and 7.02 (m, H4 and H5 on phenyl), 6.50 (d, J ¼ 6.5 Hz, H3 and H5 on py), 6.43
(m, H6 on phenyl), 4.70 (dd, J ¼ 8.0 Hz, 2.5 Hz, OCH), 4.55 (t, J ¼ 8.0 Hz, NCH), 4.12
(dd, J ¼ 8 Hz, 2.5 Hz, OCH), 3.05 (s, 6H), 1.92 (br, aqua), and 1.06 (s, 9H). Anal. Calcd
for C21H28F3N3O5PdS (Fw: 597.94): C 42.18%, H 4.72%, N 7.03%. Found: C 42.39%,
H 4.62%, N 7.15%.
(S,S)-{2,6-Bis[2-(40-iso-propyl)-oxazolinyl)]phenyl}aquapalladium(II) triflate (4)
Palladacycle 4 was prepared according to the literature method by using (S,S)-{2,
6-bis[2-(40-iso-propyl)-oxazolinyl)]phenyl}bromopalladium(II) hydrate (5) as starting
material and characterized by 1H-NMR and elemental analysis [18]. The crystal
structure of 5 has been determined.
2.3. X-ray crystal structure determination
Crystals of 2 and 5 suitable for diffraction measurements were prepared by slow
diffusion of n-hexanes into dichloromethane solutions of the corresponding pallada-
cycles. A single crystal of each was mounted on a glass fiber with grease and cooled to
ꢀ93ꢂC in a stream of nitrogen gas controlled with a Cryostream Controller 700. Data
collection was performed on a Bruker SMART CCD 1000 X-ray diffractometer with
˚
graphite-monochromated Mo-Kꢀ radiation (ꢂ ¼ 0.71073 A), operating at 50 kV and
30 mA over 2ꢃ ranges of 4.34–50.00ꢂ and 3.66–49.98ꢂ for 2 and 5, respectively. No
significant decay was observed during data collection.
Data were processed using the Bruker AXS Crystal Structure Analysis Package,
version 5.10 [19]. Neutral atom scattering factors were taken from Cromer and Waber
[20]. The raw intensity data were integrated using SAINT-Plus; absorption corrections
were applied using SADABS [21].
The structural data for the data collection and refinement for palladacycles 2 and 5
are given in table 1. Selected bond lengths, bond angles, and torsional angles for the two

Palladacycles
2663
Table 1. Experimental data for the X-ray diffraction study on 2 and 5.
Complex
2
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Space group
˚
Unit cell dimensions (A, )
C
536.84
180(2)
0.71073
P 2(1)2(1)2(1)
19H21F3N2O4PdS
C18H25BrN2O3Pd
503.71
180(2)
0.71073
P 2(1)2(1)2(1)
˚
ꢂ
a
b
c
ꢀ
ꢄ
ꢅ
10.0169(6)
10.5770(6)
20.1993(11)
90
90
90
2140.1(2), 4
1.666
1.018
14.4320(10)
5.9361(4)
11.1156(8)
90
90
90
952.27(11), 2
1.757
3.091
3
˚
Volume (A ), Z
Calculated density (g cmꢀ3
)
Absorption coefficient, (mmꢀ1
F(000)
)
1080
0.35 ꢁ 0.20 ꢁ 0.10
504
0.30 ꢁ 0.25 ꢁ 0.20
Crystal size (mm3)
ꢃ range for data collection (ꢂ)
Limiting indices
2.17–25.00
1.83–24.99
ꢀ17 ꢄ h ꢄ 16; ꢀ6 ꢄ k ꢄ 7;
ꢀ13 ꢄ l ꢄ 13
ꢀ11 ꢄ h ꢄ 11; ꢀ12 ꢄ k ꢄ 12;
ꢀ21 ꢄ l ꢄ 24
12,667
Reflections collected
5462
1665 [R(int) ¼ 0.0175]
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I 4 2ꢆ(I )]
3763 [R(int) ¼ 0.0178]
3763/2/365
1.000
1655/1/166
1.060
R1 ¼ 0.0186, wR2 ¼ 0.0489
R1 ¼ 0.0177, wR2 ¼ 0.0464
ꢀ3
˚
Largest difference peak and hole (e A
)
0.313 and ꢀ0.277
0.381 and ꢀ0.345
complexes are presented in table 2. ORTEP diagrams for 2 and 5 at the 50% probability
level are given in figures 1 and 2, respectively.
2.4. Kinetic methods
The CH3OHþ2 concentration was determined using a Denver Ultrabasic pH meter
s
s
calibrated with standard aqueous buffers (pH 4.00 and 10.00). Values of pH were
calculated by subtracting a correction constant of ꢀ2.24 from the experimental meter
reading as reported by Bosch et al. [22–24].
For designation of pH in non-aqueous solvents we use the forms recommended by
the IUPAC, Compendium of Analytical Nomenclature. Definitive Rules 1997 3rd Edn,
Blackwell, Oxford, UK (1998). The term sspH designates that ‘‘pH’’ is measured in, and
referenced to, the same solvent, which in this case is methanol. Since the autoprotolysis
constant of methanol is 10ꢀ16.77, neutral pH is 8.4 for methanol.
s
s
2.4.1. Kinetics on methanolysis of 6. The kinetics of methanolysis of fenitrothion 6
promoted by 1–4 were monitored by observing the rate of appearance of 3-methyl-4-
nitrophenol at 318 nm or 3-methyl-4-nitrophenolate at 404 nm using a Varian Cary 300
UV-Vis spectrophotometer. All reactions were followed to at least three half-times at
25 ꢃ 0.1ꢂC under buffered conditions. Buffers were prepared from N-methylmorpholine
(sspKa ¼ 8.23), trimethylamine (sspKa ¼ 9.60), triethylamine (sspKa ¼ 10.78), and 2,2,6,
6-tetramethylpiperidine (sspKa ¼ 11.86). The total [buffer] varied between 1 and

2664
Z.-L. Lu et al.
ꢂ
ꢂ
˚
Table 2. Selected bond lengths (A), angles ( ), and torsion angles ( ) for 2 and 5.
2
5
Pd(1)–C(2)
Pd(1)–O(2)
Pd(1)–N(1)
Pd(1)–N(2)
C(2)–Pd(1)–N(1)
C(2)–Pd(1)–N(2)
N(1)–Pd(1)–N(2)
C(2)–Pd(1)–O(2)
N(2)–Pd(1)–O(2)
N(1)–Pd(1)–O(2)
1.978(2)
2.1789(18)
2.0301(17)
2.0386(18)
81.60(9)
92.69(10)
172.22(9)
175.72(9)
91.20(7)
Pd(1)–C(1)
Pd(1)–Br(1)
Pd(1)–N(1)
Pd(1)–N(1)#1
1.943(4)
2.5190(5)
2.068(2)
2.068(2)
78.89(7)
78.89(7)
157.77(13)
180.0
C(1)–Pd(1)–N(1)
C(1)–Pd(1)–N(1)#1
N(1)–Pd(1)–N(1)#1
C(1)–Pd(1)–Br(1)
N(1)–Pd(1)–Br(1)
101.11(7)
101.11(7)
94.36(7)
N(1)#1–Pd(1)–Br(1)
#1: ꢀx, ꢀyꢀ2, z.
Figure 1. ORTEP presentation of the molecular structure of 2. Displacement ellipsoids for non-H atoms are
shown at the 50% probability level and hydrogens are represented by circles of arbitrary size.
3 ꢁ 10ꢀ3 mol dmꢀ3 and the buffers were partially neutralized with 70% HClO4.
To avoid any chloride contamination from the glass electrode that might affect the
s
metal ion reactions duplicate solutions were prepared, one for pH measurements, and
s
the second portion being used for kinetics. In all cases, sspH values measured before and
after reaction were consistent within 0.1 unit.
The pseudo-first-order rate constants (kobs) were evaluated by fitting absorbance
versus time traces to a standard exponential model. Stock solutions of palladium
complexes 1–4 were prepared in pure methanol at 2 ꢁ 10ꢀ3 mol dmꢀ3. In order to
determine the kinetic order in [Pd], a series of kinetic runs was undertaken at sspH 10.80,
(triethylamine buffer) using 2.84 ꢁ 10ꢀ5 mol dmꢀ3 fenitrothion 6 in the presence of
varying [Pd] from (0.4 ꢁ 10ꢀ5 to 8 ꢁ 10ꢀ5 mol dmꢀ3. The pseudo-first-order rate
constants (kombasx) were obtained by extrapolating to [buffer] ¼ 0.0 mol dmꢀ3. The kobs

Palladacycles
2665
Figure 2. ORTEP presentation of the molecular structure of 5. Displacement ellipsoids for non-H atoms are
shown at the 50% probability level and hydrogens are represented by circles of arbitrary size.
0.3
0.2
0.1
0.0
0.0100
0.0075
0.0050
0.0025
0.0000
0.0
2.5
5.0
7.5
5
–3
[Pd] µ 10 (mol dm
)
Figure 3. A plot of kmax vs. [Pd] of palladacycles 1–3 for methanolysis of 6 [2.84 ꢁ 10ꢀ5 mol dmꢀ3] at 25ꢂC,
buffer triethylamine (1–3) ꢁ 10ꢀ3 mol dmꢀ3, extrapolated to zero [buffer], sspH 10.80. kmax of 1 (g) and 2 (H)
corresponds to left Y-axis and that of 3 (ꢅ) corresponds to right Y-axis. Maximal second-order rate constant
(km2 ax) for catalyzed reaction from the gradients of the plots.
versus [Pd] plots gave straight lines (figure 3), the gradients of which were taken as the
second-order catalytic rate constant (ko2bs) and listed in table 3.
s
The pH dependence of the catalytic rate constant was determined by obtaining the
s
k
rate constants for methanolysis of 2.84 ꢁ 10ꢀ5 mol dmꢀ3 of 6 between sspH 9.08 and
obs
max
12.2 in the presence of a single concentration of 1–3 (2.0 ꢁ 10ꢀ5 mol dmꢀ3). The ko2bs
max
second-order rate constants are obtained as k /(2 ꢁ 10ꢀ5 mol dmꢀ3) and are plotted in
obs
figure 4. The data of the kinetic experiments can be found in ‘‘Supplementary material’’,
named as suppl-kinetic-data.pdf.
2.4.2. Turnover experiments. To demonstrate that palladacycles were truly
catalytic, turnover experiments of the methanolysis of an excess of substrates 6–11
s
s
(scheme 3) relative to palladacycles were investigated in triethylamine buffer at pH

2666
Z.-L. Lu et al.
Table 3. Second-order rate constants (ko2bs) from concentration dependence at sspH
10.80, and the maximum second-order rate constants from fits to equation (2) of the
sspH/k2 kinetic data for methanolysis of
6 catalyzed by palladacycles 1–3 at
[Pd] ¼ 2.0 ꢁ 10ꢀ5 mol dmꢀ3, [6] ¼ 2.84 ꢁ 10ꢀ5 mol dmꢀ3, and 25ꢂC.
Palladacycles
ko2bs (molꢀ1 dm3 sꢀ1
)
km2 ax (molꢀ1 dm3 sꢀ1
)
pKa
1
2
3
(6.7 ꢃ 0.2) ꢁ 103
(0.080 ꢃ 0.001) ꢁ 103
7.2 ꢁ 10ꢀ4
(7.5 ꢃ 0.8) ꢁ 103
(4.2 ꢃ 0.2) ꢁ 103
(0.36 ꢃ 0.11) ꢁ 103
10.78 ꢃ 0.08
11.00 ꢃ 0.03
11.0 ꢃ 0.2
(1.15 ꢃ 0.04) ꢁ 103
MeOꢀ
4.5
3.5
2.5
1.5
0.5
9
10
11
pH
12
s
s
Figure 4. Plot of log ko2bs vs. sspH for methanolysis of 6 at [6] ¼ 2.84 ꢁ 10ꢀ5 mol dmꢀ3, catalyzed by
palladacycles 1 (g), 2 (H), 3 (ꢅ), ([Pd] ¼ 2.0 ꢁ 10ꢀ5 mol dmꢀ3) at 25ꢂC. k2max and kinetic spKa values were
s
obtained according to the literature method [10].
O
O
N
Pd
N
N
Pd
Cl
N
OTf
2
AgOTf/H2O
dichloromethane
O
OTf-
O
N
R
N
Pd
N
Pd
R
OH2
Cl
R = CF3 1; NMe2
Scheme 3. Preparation of palladacycles.
3
10.80 using a Varian Cary 300 UV-Vis spectrophotometer and confirmed by 31P-NMR
spectroscopy.
In most cases, intensity versus time data adhered to good first-order behavior for at
least three half-times, and standard fitting of the data to an exponential model yielded
the pseudo-first-order rate constants given in table 4 along with the turnover numbers.

Palladacycles
2667
Table 4. Turnover conditional rate constants for methanolysis of different phosphorothioate esters
catalyzed by 1 (8.2 ꢁ 10ꢀ6 mol dmꢀ3) at 1 mmol Lꢀ1 TEA, pH 10.80, 25ꢂC.
s
s
31P-NMR (ppm)
Phosphorothioate esters/(mol dmꢀ3
)
TON
kobs (sꢀ1
)
ko2bs (dm3 molꢀ1 sꢀ1
)
Substrate Product
6
7
8
9
10
11
1.45 ꢁ 1ꢀ0ꢀ4 4
4.8 ꢁ 10ꢀ4
1.6 ꢁ 10
17.7
58.5
19.5
19.5
58.5
19.5
9.6 ꢁ 10ꢀ4
4.0 ꢁ 10ꢀ3
1.02 ꢁ 10ꢀ2
1.03 ꢁ 1ꢀ0ꢀ3 2
2.2 ꢁ 10
117
487
1243
1256
268
66.05
65.95
85.37
88.85
68.45
61.16
73.11
73.14
88.57
90.92
69.54
69.54
1.6 ꢁ 10ꢀ4
4.8 ꢁ 10ꢀ4
1.6 ꢁ 10ꢀ4
2.8 ꢁ 10ꢀ4
34.1
TEA, tetraethylammonium chloride and TON, turnover numbers.
3. Results and discussion
3.1. Syntheses of 1–3
Syntheses of the palladacycles were straightforward, exchange of counterion from
chloride to triflate with silver triflate gave good yields of the desired compounds
(scheme 3). Three new palladacycles were characterized by IR, 1H-NMR, and elemental
analysis. Palladacycles containing aqua and triflate ligands or counterion have attracted
much attention due to their high catalytic activities and rich structure variety [25–29].
The presence of water in 1 and 3 were confirmed by IR spectra, which showed a
broad peak at 3400 cmꢀ1. The C¼N stretching bands at 1629, 1625, and 1624 cmꢀ1 for
1–3, respectively, redshift compared to 1654 cmꢀ1 of the free oxazoline ligand
supporting coordination of the oxazoline ring.
The upfield shift of H6 results from shielding of pyridine ring, which is at the trans
position to nitrogen in the palladacycle ligand and non-planar to phenyl ring [30]. H6 of
the palladacycle ring is shifted upfield to ꢁ 6.01, 6.10, and 6.43 ppm, respectively, for
1–3. Thus, 1H-NMR spectra of 1–3 show typical features of N,N-trans conformation in
solution. For the three complexes, CH2 of the oxazoline moiety shows two double–
double splittings with spin–spin coupling constants of 8.0–9.0 and 2.5–3.0 Hz, which
indicates conformation of oxazoline ring in ꢂ(S) configuration [31].
3.2. Description of crystal structures of 2 and 5
The structure of 2 (figure 1) is similar to its chloride precursor [17]. The palladium
displays the expected square planar coordination with pyridine and oxazoline nitrogens
trans to one another. The dihedral angles between pyridine ring and palladacycle ring is
75.75ꢂ. The dihedral angle between the planes {C(1)Pd(1)N(1)} and {N(2)Pd(1)(O1)} is
ꢂ
˚
5.61 , with a maximum displacement of Pd of 0.049 A. The Pd–C and Pd–N bond
lengths in 2 fall within the range of values reported for other cyclopalladated derivatives
of N,N-dimethylbenzylamine and oxazoline [10, 31].
For 5 (figure 2), the palladium is bound to two mutually trans oxazoline moieties, the
phenyl carbon (via C(1)) and bromide. The geometry around palladium is a slightly
distorted square-planar coordination sphere with C(1)–Pd–Br(1) and N(1)–Pd–N(1)#
angles being 180ꢂ and 157.77(13)ꢂ, respectively. The five-membered metallacycles are

2668
Z.-L. Lu et al.
symmetrically puckered in the same direction, with Pd(1)–N(1)–C(5)–C(2) torsion angle
of 0.5(3)ꢂ. The bond lengths and angles (table 2) are in good agreement with those in
(S,S)-{2,6-bis[2-(4-iso-propyl)oxazolinyl]phenyl-N,C1,N}chloropalladium(II) hydrate
[25] and (S,S)-{2,6-bis[2-(4-iso-propyl)oxazolinyl]phenyl-N,C1,N}aquapalladium(II)
triflate [18]. The bromide is hydrogen-bonded to water with water–oxygen to bromide
˚
distance of 3.468(2) A.
3.3. Methanolysis of 6 catalyzed by 1–4
3.3.1. Preliminary experiments. Before we studied the concentration and pH depen-
dences of the catalytic methanolysis of P¼S pesticides in buffered solution (scheme 4),
s
s
we first screened the activities of the above-mentioned palladacycles at pH 10.80,
[6] ¼ 2.84 ꢁ 10ꢀ5 mol Lꢀ1 and [Pd] ¼ 2 ꢁ 10ꢀ5 mol Lꢀ1. It was found that kobs with 2 is
2.25 minꢀ1, while that of 4 is 0.0228 minꢀ1, i.e., about 100 times slower than that of 2.
Thus, the pincer system is not highly proficient at promoting methanolysis of 6, and
further kinetic work was focused on the ortho-palladated compounds 1–3. Second, we
tried to use palladacycles 1–3 for asymmetric methanolysis of chiral thiophosphates. It
was immediately found that the absorbance versus time traces for methanolysis of the
racemic thiophosphates, letophos 8 and EPN 9, promoted by 2 are strictly first order,
showing no evidence of two independent first-order curves with different rate constants,
and therefore no evidence of kinetically significant chiral discrimination under the
experimental conditions.
s
s
3.3.2. Dependence of the kinetic values for methanolysis of 6 on 1–3 and pH. On the
basis of our previous studies [10], the concentration dependences on palladacycles 1–3
for the methanolysis of fenitrothion were measured at sspH 10.80 (triethylamine buffer)
obs
max
with the k
values being plotted in figure 3. The plots are linear with no upward or
downward curvature, indicative of kinetically active monomeric 1–3 without evidence
of any appreciable amount of dimer at these low concentrations. The linearity of the
concentration dependences is different from their chloride precursors, which showed
downward curvature at higher concentration due to inhibition of chloride [17].
O
N
Pd
N
S
S
R„
OMe
R
P
R
O CH
+
3
R
P
R
O Ar
ArOH
1
1
CH OH, RT
3
2
2
O
R
„„
N
R
N:
„„: H, CF , N(CH )
3
3 2
N
i
Pr
R : alkoxyl; R : alkoxyl or phenyl; Ar: substituted aryl; R
i
t
„
: - Pr/ Bu;
1
2
Scheme 4. Catalytic methanolysis of pesticides with palladacycles.

Palladacycles
2669
The second-order rate constants (ko2bs) are evaluated as the gradients of the linear plots
and are listed in table 3. The activities of palladacycles 1–3 varied with basicities of the
co-ligand pyridines. Palladacycle 1, which contains the electron withdrawing
4-trifluoromethylpyridine ligand, is the most active complex in the series. The k2 of 1
is 5.8 and 83.8 times higher than that of palladacycles 2 and 3, respectively.
The pseudo-first-order rate constants (kombasx) for methanolysis of 6 promoted by a
2.0 ꢁ 10ꢀ5 mol dmꢀ3 solution of 1–3 at different sspH values between sspH 9.08 and 12.22
were determined. The corresponding second-order rate constants for 1–3 are obtained
as kombasx/(2 ꢁ 10ꢀ5 mol dmꢀ3) and are plotted in figure 4. The appearance of the plot
suggests a catalytic process where a basic form of the catalyst is active. Following the
literature method [10], the km2 ax and kinetic pKa value can be obtained and are listed
s
s
in table 3.
Relative to the background reaction, each palladacycle 1–3 is much more active than
methoxide ion for promoting methanolysis of fenitrothion 6. This can be discussed from
two aspects. First, we compared the catalytic efficiency, i.e., k2 value of the reaction. It
can be seen that the k2 values of 1–3 are 9.3 ꢁ 106, 1.6 ꢁ 106, and 0.11 ꢁ 106-fold higher
than that of methoxide which is 7.2 ꢁ 10ꢀ4 mol dmꢀ3 sꢀ1 [32]. Secondly, we compared
the reaction half-times at sspH 10.80 in the presence or absence of the palladacycles. The
computed half-life times in the presence of 1 mmol dmꢀ3 of 1–3 are 0.10, 0.60, and 8.7 s,
respectively, compared to the background methoxide-promoted reaction which has a
half-life time of 9.0 ꢁ 108 s, the accelerations achieved by palladacycles 1–3 are
9.0 ꢁ 109, 1.5 ꢁ 109, and 1.0 ꢁ 108-fold, respectively.
3.4. Turnover experiments
The methanolysis of six P¼S pesticides (shown in scheme 2) catalyzed by 1 under
turnover conditions was investigated spectroscopically under buffered conditions
(1–2 mmol Lꢀ1 triethylamine, pH 10.80, and ambient temperature) in methanol. The
s
s
conditional rate constants determined are given in table 4. Complete methanolysis
1
of these thiophosphates under turnover conditions has been confirmed by H- and
31P-NMR. Palladacycles 1–3 are very effective catalytic systems for decomposition of
various pesticides, capable of forming non-toxic, methoxylated products under
turnover conditions with good first-order kinetics.
3.5. Postulated reaction mechanism
A proposed mechanism for the reaction which is consistent with the experimental data
obtained and previous studies [10, 17, 33] is given in scheme 5. The results suggest that
palladacycles 1–3, when introduced into methanol solution, exist predominantly as
mono-methanol complex 11. The catalytically active form is generated by the ionization
of a Pd-bound methanol to form 12 having a macroscopic sspKa of ꢆ10.90. The required
substitution of pyridine with P¼S substrate to form the highly catalytically active species
13 can be deduced from the different activities of palladacycles 1–3. While palladacycle 1,
containing the weakly coordinated trifluoromethylpyridine ligand, showed higher activ-
ity due to the ready dissociation of the weakly coordinating pyridine, palladacycle 3,
bearing a strongly coordinated 4-(N,N-dimethylamino)pyridine gave somewhat
poorer activity, and the non-dissociable pincer 4 gave even poorer activity.

2670
Z.-L. Lu et al.
O
O
-SP(OR)2(OCH3)
CH3O-
O
R
N
Pd
O
N
Pd
O
N
S
+Py
H3CO
RO
R
N
Pd
O
N
P
-
-
OR
12
CH3
CH3
CH3
H
S
15
11
- Py
RO
RO
P
+-OCH3
OAr
O
ArO-
Pd
S
S
Pd
N
ArO
P
RO
-
P
O
O
RO
OR
CH3
CH3
RO
13
14
Scheme 5. Postulated possible reaction mechanism.
The required substitution of pyridine with the P¼S-containing substrate to form the
kinetically competent species is suggested to arise from a typical bimolecular association
of the pesticide with the palladacycle to form a five-coordinate associative complex or
transient (not shown) from which expulsion of the pyridine occurs to give 13. Once
formed, 13 reacts via intramolecular delivery of the cis-coordinated methoxide to the
adjacent P¼S unit to form 14. Whether the P¼S–OAr cleavage reaction from 13 to form
14 involves a stepwise mechanism, through a five-coordinate phosphorane intermediate,
or concerted displacement of the leaving group from P cannot be ascertained on the basis
of the available evidence. Turnover of the catalyst requires displacement of the P¼S
product from 15 to regenerate 12, again probably through an associative process with
methoxide.
4. Conclusion
Three new palladacycles 1–3 derived from (S)-4-tert-butyl-2-phenyl-2-oxazoline have
been synthesized and characterized with IR, 1H-NMR, and elemental analysis.
Complex 2 and a pincer complex 5 were also characterized by X-ray single crystal
structure analyses. Palladacycles 1–3 have been successfully applied in the catalytic
methanolysis of P¼S pesticides. The most effective catalysts for methanolysis of
thiophosphates are the ortho-palladacycles containing weakly coordinating mono-
dentate ligands. The pincer palladacycles and the ortho-palladacycles containing
strongly binding monodentate ligands are far less active. There is no chiral
discrimination during the methanolysis under the experimental conditions. To achieve
asymmetric methanolysis by imputing chiral nitrogen ligands in the palladacycle system
and to create reusable polymer-bound or silica-supported catalyst are under
investigation.

Palladacycles
2671
Supplementary material
CCDC-710463 (for 2) and CCDC-763345 (for 5) contain the supplementary crystal-
lographic data for this article. These data can be obtained free of charge from the
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif
Acknowledgments
The authors thank Beijing Normal University (107026), Beijing Municipal Commission
of Education, Program for New Century Excellent Talents in Universities (no.
15770703), Specialized Research Fund for the Doctoral Program of Higher Education
(no. 273911), and the Scientific Research Foundation for the Returned Overseas
Chinese Scholars (no. 213006), Ministry of Education, China for the financial
assistance.
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