Remarkably efficient hydrolysis of methylparathion catalyzed by [2-(2-pyridyl)phenyl-C,N]palladium(II) complexes

Article abstract of DOI:10.1021/ic0600578

The reaction of the palladium(II) acetate derivative [Pd(N∧C)(OAc)] ₂ (N∧C = (NC₅H₄-2-C₆H ₄(C²,N) or (2-(2-pyridyl)-phenyl-C,N)) with methylparathion and water in THF leads to the formation of [Pd(N∧C)(μ- SP(=O)(OCH₃)₂)]₂ (1), which reacts with PPh₃ in THF to afford mononuclear complex [Pd(N∧C)(SP(=O) (OCH₃)₂)(PPh₃)] (2). Compounds 1 and 2 have been characterized by ¹H, ¹³C, and ³¹P NMR spectroscopy; elemental analysis; and single-crystal X-ray diffraction. When dissolved in water, 1 serves as a precatalyst for the hydrolysis of methylparathion. Kinetic and spectroscopic studies suggest that compound 1 dissociates in aqueous solution to afford cationic diaqua complex [Pd(N∧C)(OH₂)₂]⁺ (A). At basic pH, A is converted into its deprotonated form [Pd(N∧C)(OH₂)(OH)] (B), which dimerizes to afford a dinuclear complex, presumably [Pd(N∧C)(μ-OH)] ₂ (C). At pH 7, the reaction is first order in substrate and first order in palladium catalyst A, with k₂ = 146 ± 9 M ^-1 s^-1 at 303 K. At more-basic pH, the reaction rate increases and shows an apparent half-order dependence in palladium catalyst. These observations suggest that the active form of the catalyst at basic pH is B, whose concentration is controlled by an equilibrium with inactive C. Analysis of the data obtained at pH 9 yields a dimer formation constant K_f = [C]/[B]² = (6.6 ± 5.6) × 10⁶ M^-1 and a second-order rate constant k₂ of (8.6 ± 3.6) × 10³ M^-1 s^-1 at 298 K. The pH dependence of the reaction rate as well as a spectroscopic titration indicates that the pK _a of A is in the 9.5-9.7 range. Determination of the activation parameters at both pH 7 and 9 suggests that catalysis occurs via an associative mechanism whose rate-determining step involves the substitution of a water ligand of A by a molecule of methylparathion at neutral pH and nucleophilic attack of the phosphorus center of methylparathion by a hydroxide ligand of B at basic pH.

Full text of DOI:10.1021/ic0600578

Inorg. Chem. 2006, 45, 5600−5606
Remarkably Efficient Hydrolysis of Methylparathion Catalyzed by
[2-(2-Pyridyl)phenyl-C,N]palladium(II) Complexes
Mieock Kim, Alexandre Picot, and Franc¸ois P. Gabba1ı*
Chemistry Department, Texas A&M UniVersity, 3255 TAMU, College Station, Texas 77843-3255
Received January 11, 2006
The reaction of the palladium(II) acetate derivative [Pd(N
phenyl-C,N)) with methylparathion and water in THF leads to the formation of [Pd(N
which reacts with PPh3 in THF to afford mononuclear complex [Pd(N C)(SP(
∧
C)(OAc)]2 (N
∧
C
)
(NC5H4-2-C6H4(C2,N) or (2-(2-pyridyl)-
C)( -SP( O)(OCH3)2)]2 (1),
dO)(OCH3)2)(PPh3)] (2). Compounds
∧
µ
d
∧
1 and 2 have been characterized by 1H, 13C, and 31P NMR spectroscopy; elemental analysis; and single-crystal
X-ray diffraction. When dissolved in water, 1 serves as a precatalyst for the hydrolysis of methylparathion. Kinetic
and spectroscopic studies suggest that compound 1 dissociates in aqueous solution to afford cationic diaqua
complex [Pd(N
which dimerizes to afford a dinuclear complex, presumably [Pd(N
order in substrate and first order in palladium catalyst A, with k2
∧
C)(OH2)2]+ (A). At basic pH, A is converted into its deprotonated form [Pd(N
C)( -OH)]2 (C). At pH 7, the reaction is first
146 ±
9 M- s- at 303 K. At more-basic pH,
∧C)(OH2)(OH)] (B),
∧
µ
1
1
)
the reaction rate increases and shows an apparent half-order dependence in palladium catalyst. These observations
suggest that the active form of the catalyst at basic pH is B, whose concentration is controlled by an equilibrium
with inactive C. Analysis of the data obtained at pH 9 yields a dimer formation constant Kf
)
[C]/[B]2
)
(6.6
103 M- s- at 298 K. The pH dependence of the
9.7 range. Determination
± 5.6)
1
1
1
×
106 M- and a second-order rate constant k2 of (8.6
±
3.6)
×
reaction rate as well as a spectroscopic titration indicates that the pKa of A is in the 9.5
−
of the activation parameters at both pH 7 and 9 suggests that catalysis occurs via an associative mechanism
whose rate-determining step involves the substitution of a water ligand of A by a molecule of methylparathion at
neutral pH and nucleophilic attack of the phosphorus center of methylparathion by a hydroxide ligand of B at basic
pH.
Introduction
exception concerns cyclometalated palladium and platinum
complexes.5-9 For example, palladium and platinum aryl
oxime complexes have been shown to greatly accelerate the
hydrolysis of substrates such as ethylparathion.5 According
to Ryabov, the activity of such catalysts can be explained
by invoking the coordination of ethylparathion at the metal
The study of catalytic systems that promote the hydrolytic
decomposition of organophosphorus neurotoxins continues
to generate interest.1 Thus far, the greatest hydrolytic
activities have been observed with enzymatic systems such
as the organophosphorus hydrolase (OPH) of pseudoma
diminuta. This metalloenzyme, which possesses a dinuclear
zinc active site, promotes the hydrolysis of a broad range of
organophosphorus compounds, including the thiophosphate
triester methylparathion.2 Despite many attempts, however,
efforts to discover metal-based catalysts that parallel the
activity of this enzyme for the hydrolysis of thiophosphate
triesters have remained largely unsuccessful.3,4 A noteworthy
(2) (a) Donarski, W. J.; Dumas, D. P.; Heitmeyer, D. P.; Lewis, V. E.;
Raushel, F. M. Biochemistry 1989, 28, 4650-4655. (b) Dumas, D.
P.; Durst, H. D.; Landis, W. G.; Raushel, F. M.; Wild, J. R. Arch.
Biochem. Biophys. 1990, 277, 155-159. (c) Caldwell, S. R.; New-
comb, J. R.; Schlecht, K. A.; Raushel, F. M. Biochemistry 1991, 30,
7438-7444.
(3) Ketellaar, J. A. A.; Gersmann, H. R.; Beck, M. Nature 1956, 177,
392-393. (b) Zeinali, M.; Torrents, A. EnViron. Sci. Technol. 1998,
32, 2338-2342. (c) Smolen, J. M.; Stone, A. T. EnViron. Sci. Technol.
1997, 31, 1664-1673. (d) Wan, H. B.; Wong, M. K.; Mok, C. Y.
Pestic. Sci. 1994, 42, 93-99.
(4) (a) Hartshorn, C. M.; Singh, A.; Chang, E. L. J. Mater. Chem. 2002,
12, 602-605. (b) Moss, R. A.; Morales-Rojas, H. Langmuir 2000,
16, 6485-6491. (c) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst,
D. H. J. Am. Chem. Soc. 1987, 109, 2800-2803. (d) Morrow, J. R.;
Trogler, W. C. Inorg. Chem. 1989, 28, 2330-2333. (e) Hay, R. W.;
Clifford, T.; Richens, D. T.; Lightfoot, P. Polyhedron 2000, 19, 1485-
1492. (f) Hendry, P.; Sargeson, A. M. Aust. J. Chem. 1986, 39, 1177-
1186.
* To whom correspondence should be addressed. E-mail:
gabbai@mail.chem.tamu.edu.
(1) (a) Morales-Rojas, H.; Moss, R. A. Chem. ReV. 2002, 102, 2497-
2521. (b) Yang, Y. C.; Baker, J. A.; Ward, J. R. Chem. ReV. 1992,
92, 1729-1743. (c) Iranzo, O.; Kovalevsky, A. Y.; Morrow, J. R.;
Richard, J. P. J. Am. Chem. Soc. 2003, 125, 1988-1993. (d) Deck,
K. M.; Tseng, T. A.; Burstyn, J. N. Inorg. Chem. 2002, 41, 669. (e)
Kuo, L. Y.; Perea, N. M. Inorg. Chem. 2000, 39, 2103-2106.
5600 Inorganic Chemistry, Vol. 45, No. 14, 2006
10.1021/ic0600578 CCC: $33.50
© 2006 American Chemical Society
Published on Web 06/14/2006

Efficient Hydrolysis of Methylparathion
124.3, 124.5, 129.0, 137.7, 140.1, 145.7, 147.7, 149.7, 163.3. 31P-
{1H} NMR (121.4 MHz, DMSO/D2O): δ 50.5. Solid-state 31P-
{1H} NMR: δ 35.9, 32.9, 29.3. Elemental Anal. Calcd for
C26H28N2O6P2Pd2S2: C, 38.87; H, 3.51; N, 3.49. Found: C, 39.25;
H, 3.60; N, 3.51.
center followed by an intramolecular nucleophilic attack by
the neighboring supernucleophilic oximate functionality.5 In
an effort to determine which factors control the activity of
such catalysts, we have studied phenyloxazoline palladium
(II) complexes and discovered that their activity in the
hydrolysis of methylparathion parallels those reported for
cyclometalated palladium and platinum aryl oxime com-
plexes.7,8 These results suggest that the presence of a
supernucleophilic functionality in the catalysts may in fact
not be needed to achieve high activity. To further explore
this hypothesis, we have decided to investigate the catalytic
activity of a simple ortho-metalated palladacycle that does
not possess any available nucleophilic groups. In this
contribution, we report on the remarkable catalytic activity
of palladium complexes bearing the 2-(2-pyridyl)-phenyl-
C,N ligand.
Synthesis of [Pd(N∧C)(SP(dO)(OCH3)2)(PPh3)] (2). To a
solution of complex 1 (40 mg, 50 µmol) in THF (10 mL) was added
a solution of triphenylphosphine (13 mg, 50 µmol) in THF (10
mL). After 20 h at room temperature, the colorless solution was
evaporated under vacuum. Slow recrystallization of the white
residue in acetone led to the formation of colorless needles that
were suitable for further X-ray analysis. Yield:45 mg, 85%. Mp:
1
3
281 °C. H NMR (300 MHz, CDCl3): δ 3.32 (d, JHP ) 7.2 Hz,
6H, OCH3), 6.47 (m, 1H), 6.57 (m, 1H), 6.94 (m, 1H), 7.28 (m,
2H), 7.37 (m, 6H), 7.43 (m, 3H), 7.55 (m, 1H), 7.78 (m, 1H), 7.83
(m, 7H). 13C NMR (75.4 MHz, CDCl3): δ 53.0, 119.2, 123.1, 124.3,
124.5, 129.0, 137.6, 140.1, 145.7, 147.7, 149.6, 163.2. 31P NMR
(121.4 MHz, THF): δ 43.1, 44.5. Elemental Anal. Calcd for C31H29-
NO3P2SPd: C, 56.07; H, 4.40; N, 2.11. Found: C, 55.89; H, 4.33;
N, 2.25.
Experimental Section
General Considerations. Caution: Methylparathion is highly
toxic and should be handled in a well-Ventilated fume hood. All
glassware exposed to methylparathion should be decontaminated
with bleach. Solvents were dried by standard methods. All NMR
studies were carried out on Inova NMR spectrometers (300 or 500
MHz for 1H, 75.4 or 125.7 MHz for 13C, 121.4 MHz for 31P NMR).
Catalytic Hydrolysis of Methylparathion. In a typical experi-
ment, the cell was filled with 3.0 mL of a 1 × 10-2 M buffer
solution (HEPES, 6 < pH < 7; EPPS, 7.5 < pH < 8.5; CHES, 9
< pH < 10.5) and 0.139 mL of a stock solution of catalyst in
dioxane ([Pdtot] ) 2 × [1] ) 8.8 × 10-5 M). Following the addition
of 0.278 mL of a stock solution of methylparathion ([methylpar-
athion] ) 4.4 × 10-4 M in dioxane), the total volume of the solution
was adjusted to 3.5 mL by addition of water; the reaction was
followed by monitoring the release of p-nitrophenol, which features
an absorption band at 400 nm. The dependence of the reaction rate
on methylparathion was established by varying the methylparathion
concentration at constant catalyst concentration ([Pdtot] ) 3.5 ×
10-6 M). The dependence of the reaction rate on the catalyst was
established by varying the catalyst concentration at constant
methylparathion concentration (3.5 × 10-5 M).
H3PO4 (85%) was used as an external standard for the solution 31
P
NMR spectra. The proton and carbon signals of the deuterated
1
solvents were used as internal standards for the H and 13C NMR
spectra, respectively. Elemental analyses were performed by
Atlantic Microlab Inc., at Norcross, GA. Melting points were
measured on a Laboratory Devices Mel-Temp apparatus and were
not corrected. All UV-vis absorption spectra and spectrophoto-
metric measurements were recorded on a JASCO V530 UV-vis
spectrometer equipped with an automatic cell changer. For pH
titrations, a Radiometer PHM290 pH meter with VWR SympHony
electrode was used. The methylparathion solution was provided by
A/S CHEMINOVA, Lemvig, Denmark, as a gift. [Pd(OAc)(N∧C)]2
(N∧C ) (NC5H4-2-C6H4(C2,N) or (2-(2-pyridyl)-phenyl-C,N)) was
prepared in accordance with a literature procedure.10
Spectrometric Titration. Titrations were performed by addition
of a solution of NaOH (1.0 M) or a solution of HNO3 (1.0 M) to
an aqueous solution of compound 1 (4.4 × 10-5 M) containing
16% dioxane. The absorbance and pH of the solution were measured
after each addition of titrant.
Synthesis of [Pd(N∧C)(µ-SP(dO)(OCH3)2)]2 (1). A solution
of [Pd(OAc)(N∧C)]2 (320 mg, 0.5 mmol) in THF (10 mL) was
added to a solution of methylparathion (145 mg, 0.55 mmol) in
THF (20 mL). Following the addition of water (0.5 mL), the
resulting yellow solution was stirred overnight at 65 °C. Evaporation
of the solvents yielded a residue that was washed with ether and
dichloromethane and extracted with THF. The compound was
further purified by crystallization from DMF upon slow evaporation
Single-Crystal X-ray Analysis. X-ray data for 1 and 2 were
collected on a Bruker Smart-CCD diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). Specimens of
suitable size and quality were selected and mounted onto a glass
fiber with Apezion grease and run at 110 K. The structures were
solved by direct methods, which successfully located most of the
non-hydrogen atoms. Subsequent refinement on F2 using the
SHELXTL/PC package (version 6.1) allowed for location of the
remaining non-hydrogen atoms. Additional crystallographic details
are compiled in Table 1.
1
of the solvent. Yield: 297 mg, 74%. Mp: 281 °C. H NMR (500
3
MHz, DMSO): δ 3.65 (d, JHP ) 9.3 Hz, 12H, OCH3), 6.99 (m,
2H), 7.41 (m, 2H), 7.52 (m, 2H), 7.65 (m, 2H), 8.07 (m, 6H), 8.46
(m, 2H). 13C NMR (125.7 MHz, CDCl3): δ 53.0, 119.2, 123.1,
Results and Discussions
Synthesis. The reaction of the palladium(II) acetate
(5) Kazankov, G. M.; Sergeeva, V. S.; Efremenko, E. N.; Alexandrova,
L.; Varfolomeev, S. D.; Ryabov, A. D. Angew. Chem., Int. Ed. 2000,
39, 3117-3119.
10
derivative [Pd(N∧C)(OAc)]2 with methylparathion and
water in THF leads to the formation of [Pd(N∧C)(µ-SP(d
O)(OCH3)2)]2 (1), which can be obtained as a crystalline solid
(Scheme 1). This compound has been characterized by
elemental analysis, MAS 31P{1H} NMR spectroscopy, and
X-ray crystallography. The solid-state MAS 31P{1H} NMR
spectrum of 1 shows two different sets of signals, which most
likely correspond to the cis (29.3 and 35.9 ppm) and trans
isomers (32.9 ppm), as shown in Scheme 1.11 These 31P NMR
(6) Tao, J. C.; Jia, J.; Wang, X. W.; Zang, S. T. Chin. Chem. Lett. 2002,
13, 1170-1173.
(7) Kim, M.; Gabba¨ı., F. P. Dalton Trans. 2004, 3403-3407.
(8) Kim, M.; Liu, Q.; Gabba¨ı, F. P. Organometallics 2004, 23, 3379-
3387.
(9) For the methanolysis of phosphorothioate triesters mediated by
palladium complexes see: Lu, Z.-L.; Neverov, A. A.; Brown, R. S.
Org. Biomol. Chem. 2005, 3, 3379-3387.
(10) Gutierrez, M. A.; Newkome, G. R.; Selbin, J. J. Organomet. Chem.
1980, 202, 341-350.
Inorganic Chemistry, Vol. 45, No. 14, 2006 5601

Kim et al.
Scheme 1
Scheme 2
Table 1. Crystal Data, Data Collection, and Structure Refinement for 1
and 2
1
2
formula
Mr
C
26H28N2O6P2Pd2S2 C31H29NO3P2PdS
803.36 663.95
cryst size (mm3)
cryst system
space group
a (Å)
0.32 × 0.35 × 0.42 0.15 × 0.32 × 0.20
orthorhombic
Pbca
triclinic
P1h
7.8374(16)
19.451(4)
19.600(4)
9.0616(18)
17.793(4)
19.315(4)
112.91(3)
96.39(3)
94.58(3)
2824.9(10)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
disorder. Instead, the atom located trans from the sulfur atom
with the longest Pd-S bond was assigned to carbon because
phenyl exerts a greater trans effect than pyridine. Accord-
ingly, the atom trans from the sulfur atom with the shortest
Pd-S bond was assigned to nitrogen. The resulting Pd-
C(1) (2.049(11) Å) and Pd-N (2.060(11) Å) bonds are
comparable to those observed for [Pd(N∧C)(µ-Cl)]2.12
To assess the lability of the palladium sulfur bridges in 1,
its reaction with triphenylphosphine has been studied. In
THF, this reaction afforded mononuclear complex [Pd-
(N∧C)(SP(dO)(OCH3)2)(PPh3)] (2) (Scheme 2). The forma-
tion of 2 is confirmed by the appearance of two new 31P
NMR signals at 43.1 and 44.5 ppm that correspond to the
phosphorus atom of the dimethylthiophosphate and tri-
phenylphosphine ligand, respectively. Compound 2 crystal-
lizes in the triclinic space group P1h, with two independent
molecules in the asymmetric unit (Figure 2, Table 1). In both
of these molecules, the palladium center adopts a square
planar environment, with the triphenylphosphine and the
carbon of the phenylpyridine ligand in a cis arrangement.
This observation agrees with the documented tendency of
phosphines to avoid coordinating trans to carbon donor
2987.9(10)
4
1.786
1.493
1600
Z
F
calcd (g cm-3
)
1.561
0.878
1352
163(2)
µ (Mo K) (mm-1
F(000) (e)
T (K)
)
163(2)
scan mode
hkl ranges
ω
ω
-8 e h e 8,
-22 e k e 14,
-20 e l e 22
12 111
-11 e h e 5,
-22 e k e 23,
-23 e l e 25
17 862
no. of measured reflns
no. of unique reflns [Rint
]
2281 [0.0780]
12 514 [0.0368]
12 514
SADABS
0.330042
707
0.0548, 0.1028
0.942, -0.887
no. of reflns used for refinement 2281
abs corr SADABS
min/Tmax
T
0.790290
181
0.0767, 0.1510
2.129, -0.690
no. of refined params
R1a, wR2b [I > 2σ(I)]
F
fin (max/min) (e Å-3
)
a R1 ) ∑(Fo - Fc)/∑Fo. b wR2 ) {[∑w(Fo2 - Fc )2]/[∑w(Fo )2]}1/2; w
2
2
2
2
) 1/[σ2(Fo ) + (ap)2 + bp]; p ) (Fo2 + 2Fc )/3; a ) 0.02 (1), 0.0394 (2);
b ) 50 (1), 3.4354 (2).
chemical shifts are comparable to that of 29 ppm measured
for [Pd{C,N-(C6H4C(CH3)dNOH)-2}(µ-SP(dO)(OCH3)2)]2,
which retains its dimeric structure in solution.7 Compound
1 crystallizes in P1h triclinic space group in the form of a
centrosymmetrical dimer (Figure 1, Table 1). The two square
planar palladium moieties (∑(angle at Pd) ) 360.2(2)°) are
linked by the sulfur atoms of the bridging dimethylthiophos-
phate ligands. The Pd-S and Pd-S(0A) distances of 2.418-
(3) and 2.467(3) Å are comparable to those observed in
related complexes such as (R,S)-di-µ-(dimethylthiophosphate-
S,S)-bis{2-[2-(4-carbomethoxy)-oxazolinyl]-phenyl-C,N}-
dipalladium(II) (average Pd-S ) 2.40 Å) which also features
bridging thiophosphate groups.8 The remaining coordination
sites of the palladium center are occupied by the phenylpy-
ridine carbon and nitrogen donor atoms. The quality of the
X-ray measurement did not allow us to directly determine
the identity of the phenylpyridine atoms bound to palladium
nor did it allow us to unambiguously rule out a possible
Figure 1. ORTEP view of one of the two independent molecules in the
structure of 1 (50% probability level ellipsoid). Selected bond lengths (Å)
and angles (deg): Pd-C(1) 2.049(11), Pd-N 2.060(11), Pd-S 2.418(3),
Pd-S#1 2.467(3), S-Pd#1 2.467(3); C(1)-Pd-N 80.9(5), C(1)-Pd-S
96.7(3), N-Pd-S 175.5(3), C(1)-Pd-S#1 177.3(3), N-Pd-S#1 98.6(3),
S-Pd-S#1 83.98(11), P-S-Pd 101.75(17), P-S-Pd#1 97.72(16), Pd-
S-Pd#1 96.02(11).
(11) For other examples of cis/trans isomerism in palladium complexes,
see: (a) Schildbach, D.; Arroyo, M.; Lehmen, K.; Mart´ın-Barrios, S.;
Sierra, L.; Villafan˜e, F.; Strohmann, C. Organometallics 2004, 23,
3228-3238. (b) Hockless, D. C. R.; Gugger, P. A.; Leung, P.-H.;
Mayadunne, R. C.; Pabel, M.; Wild, S. B Tetrahedron 1997, 53, 4083-
4093.
5602 Inorganic Chemistry, Vol. 45, No. 14, 2006

Efficient Hydrolysis of Methylparathion
the fact that the extinction coefficient of p-nitrophenol
decreases with the pH. Kinetic studies carried out at pH 7
indicate that the reaction rate follows second-order kinetics
with a first-order dependence in both substrate and palladium
catalyst (Figure 3). Similar rate laws have been observed
for other palladium catalysts such as phenyloxazoline pal-
ladium (II) complexes and cyclometalated palladium aryl
oxime complexes.5,7,8 Unlike in previous palladium catalysts,
however, we note that the catalyst generated by dissolution
of 1 in water is active at neutral or even slightly acidic pH.
The k2 value of 146 ( 9 M-1 s-1 measured at pH 7 and 303
K (Table 2) is the highest ever recorded for a synthetic
catalyst at neutral pH, which is a significant achievement.
In fact, the k2 value obtained for this catalysts at neutral pH
is comparable to those previously reported for palladium
complexes in the 8.5-9.5 pH range.5 Under basic conditions,
the observed rate of the reaction increases substantially. This
increase, which occurs over a narrow range,16 is accompanied
by a change in the order of the reaction rate. Indeed, at pH
9, the rate follows an apparent half-order dependence in
palladium catalyst (Figure 3). The half-order dependence can
be rationalized by invoking a catalytically active mono-
nuclear palladium complex whose concentration is controlled
by an equilibrium with a catalytically inactive dimer.17 As
the catalyst concentration decreases and passes below 1.0
× 10-7 M, the initial rate of the reaction ceases to follow an
apparent half-order dependence in palladium catalyst. Such
behavior has been previously observed and rationalized.18
At low catalyst concentrations, the concentration in the
inactive dimer becomes negligible, which is the reason the
apparent half-order dependence does not hold any longer.
We note in passing that the palladium catalyst in 3.5 × 10-6
M concentration accelerates the reaction by a factor of about
5000-6000 at pH 7 and 20 °C and about 1200 at pH 9 and
25 °C when compared to the uncatalyzed hydrolysis of
parathion at k1 ) (6.2 ( 0.2) × 10-8 s-1 at pH 7.4 and 20
°C and k1 ) (3.5 ( 0.2) × 10-6 s-1 at pH 9 and 25 °C.19
Figure 2. ORTEP view of one of the two independent molecules in the
structure of 2 (50% probability level ellipsoid). Selected bond lengths (Å)
and angles (deg), with the corresponding metrical parameters of the second
independent molecule given in brackets. Pd(1)-C(1) 2.024(5) [Pd(2)-C(18)
2.026(5)], Pd(1)-N(1) 2.089(4) [Pd(2)-N(2) 2.080(4)], Pd(1)-P(2)
2.2612(15) [Pd(2)-P(2) 2.2548(15)], Pd(1)-S(1) 2.4139(14) [Pd(2)-S(2)
2.4427(14)]; N(1)-Pd(1)-P(2) 173.68(11) [N(2)-Pd(2)-P(2) 173.97(11)],
C(1)-Pd(1)-S(1) 172.67(13) [C(18)-Pd(2)-S(2) 172.95(14)], N(1)-Pd-
(1)-S(1) 93.51(12) [N(2)-Pd(2)-S(2) 91.81(11)], P(2)-Pd(1)-S(1) 90.50(6)
[P(4)-Pd(2)-S(2) 94.21(5)].
ligands.13 The resulting Pd-P (2.261(15) and 2.255(15)
Å)11a,13,14 and Pd-S (2.414(14) and 2.443(14) Å)5,6,15 dis-
tances fall within the expected ranges for these types of
linkages. When dissolved in neat THF, 31P NMR measure-
ments also suggest dissociation of 1 and formation of the
mononuclear derivative [Pd(N∧C)(SP(dO)(OCH3)2)(THF)]
(3) (Scheme 2). The 31P NMR resonance of the dimethylth-
iophosphate ligand in 3 appears at 50.5 ppm. This chemical
shift is comparable to that measured for the dimethylthio-
phosphate ligand in 2. It is also comparable to that measured
at 53.6 ppm for the monomeric complex [Pd{C,N-(C6H4C-
(CH3)dNOH)-2}(SP(dO)(OCH3)2)(py)] that we previously
reported.7
Catalytic Studies
Complex 1 serves as a precatalyst for the hydrolysis of
methylparathion, which can be conveniently followed by
monitoring the release of p-nitrophenol using UV-vis
spectroscopy. In the 6-7.5 pH range, the activity of the
catalyst remains essentially constant with HEPES, EPPS, or
CHES as a buffer. Although the catalyst may be active below
pH 6, studying the reaction at acidic pH is complicated by
The low solubility of compound 1 in water prevented a
straightforward characterization of the resulting aqueous
palladium species by NMR spectroscopy. Nonetheless, the
system could be satisfactorily modeled (vide infra) by
considering a palladium diaqua catalyst, presumably [Pd-
(N∧C)(OH2)2]+ (A), in equilibrium with its deprotonated
Figure 3. kobs for the hydrolysis of methylparathion catalyzed by 1 (a) at pH 7 as a function of [Pdtot] and (b) at pH 9 as a function of [Pdtot
initial rate/[So], [Pdtot] ) total concentration in palladium, and [So]) initial methylparathion concentration ) 3.5 × 10-5 M).
] )
1/2. (kobs
Inorganic Chemistry, Vol. 45, No. 14, 2006 5603

Kim et al.
Table 2. Temperature Dependence of the Rate Constant for the Hydrolysis of Methylparathion by 1 and the Dimer Formation Constant of 1
pH 9 pH 7
a
T (°C)
k
1.5 (M-1/2 s-1
)
Kf (M-1
)
k2 (M-1 s-1
)
T (°C)
k2 (M-1 s-1
)
25
30
38
45
55
2.35 ( 0.07
2.43 ( 0.13
2.63 ( 0.11
2.86 ( 0.08
3.09 ( 0.26
(6.6 ( 5.6) × 106
(0.5 ( 10.8) × 106
(1.0 ( 1.4) × 106
(1.2 ( 1.4) × 106
(1.5 ( 4.9) × 106
(8.57 ( 3.65) × 103
(1.00 ( 0.64) × 104
(1.18 ( 0.85) × 104
(1.42 ( 0.79) × 104
(1.67 ( 2.81) × 104
20
30
35
44
52
(1.00 ( 0.07) × 102
(1.46 ( 0.10) × 102
(1.71 ( 0.12) × 102
(2.29 ( 0.15) × 102
(2.79 ( 0.19) × 102
1/2
a k1.5 was obtained experimentally by dividing the initial rate by [S] and [Pdtot
] .
form [Pd(N∧C)(OH2)(OH)] (B) (eq 1), which dimerizes to
afford an inactive dinuclear complex, presumably [Pd(N∧C)-
kobs is equal to the initial rate divided by the initial
methylparathion concentration [So] and is dependent on the
total concentration in palladium [Pdtot] and Kf, the formation
constant of C, as defined in eq 5.17,20,22 A plot of the kobs as
a function of [Pdtot]1/2 followed by a least-squares linear fit
and graphical analysis affords a second-order rate constant
of (8.6 ( 3.6) × 103 M-1 s-1 and a Kf value of (6.6 ( 5.6)
× 106 M-1 at 298 K (Table 1). The kinetic data obtained by
this method indicate that the second-order rate constant
observed at pH 9 is 2 orders of magnitude larger than that
at pH 7. Although the errors affecting the parameters
determined by this method are often neglected, a rigorous
error propagation calculation indicates that, at least in the
present case, the uncertainties on both k2 and Kf are
considerable.
20
(µ-OH)]2 (C) (eq 2). To confirm the dissociation of 1 in
water, we recorded the 31P NMR spectrum of 1 (1.43 × 10-5
M) in water at pH 7 using CHES (10 mM) as a buffer. These
experiments allowed for the detection of the dimethylthio-
phosphate anion at δ 57.121 thus confirming that the
dimethylthiophosphate anion is liberated by 1 in water at
neutral pH. A simple derivation (eq 3-7) shows that the
concentrations of three palladium species involved in this
model can be expressed as a function of the total concentra-
tion in palladium [Pdtot], the proton concentration [H+], the
acidity constant of the diaqua complex A (Ka), and the
formation constant of dinuclear complex C (Kf) (eq 7-9).
Ka
+
+ y
(1)
[Pd(N∧C)(OH )(OH)]
[Pd(N∧C)(OH)2)2
2
kobs ) initial rate/[SO] ) k2[B] ) k2(-1/2 + (1/4 +
2Kf[Pdtot])1/2)/2Kf (10)
B
A
Kf
2[Pd(N∧C)(OH )(OH)]
[Pd(N∧C)(µ-OH)]
y\z 2H2O +
2
2
B
C
To further evaluate the model, we have studied the pH
dependence of kobs at 298 K between pH 6 and 10.5. Because
both A and B are brought to coexist in this pH range, the
data obtained in these experiments was manually fitted to
eq 11, which accounts for the contribution of catalysts A
and B to the overall rate of the reaction. In eq 11, [A] and
[B] are expressed as a function of Ka and Kf as per eq 7 and
8, respectively. The fit presented in Figure 4 was obtained
manually with Ka ) 1 × 10-9.7 M, Kf ) 6.6 × 106 M-1, a
second-order rate constant at neutral pH (k2(neutral)) of 140
(2)
[Pdtot] ) [A] + [B] + 2[C]
Ka ) [H+][B]/[A]
(3)
(4)
(5)
Kf ) [C]/[B]2
[Pdtot] ) [A] + Ka[A]/[H+] + 2KfKa2[A]2/[H+]2 (6)
[A] )
[H+]2{-1-K [H+]+ (1+K /[H+])2 +8[Pd ]K K 2/[H+]2}
x
a
a
tot
f a
(16) Changes in kobs over a pH range of 1 are not unprecedented and have
been observed in the hydrolysis of phosphatediesters catalyzed by
copper(II) complexes; see: Scarpellini, M.; Neves, A.; Horner, R.;
Bortoluzzi, A. J.; Szpoganics, B.; Zucco, C.; Silva, R. A. N.; Drago,
V.; Mangrich, A. S.; Ortiz, W. A.; Passos, W. A. C.; Oliveira, M. C.
B.; Terenzi, H. Inorg. Chem. 2003, 42, 8353-8365.
(17) Balzarek, C.; Weakley, T. J.; Kuo, L. Y.; Tyler, D. R. Organometallics
2000, 19, 2927.
4KfKa2
(7)
(8)
Ka[A]
[H+]
[B] )
(18) Deck, K. M.; Tseng, T. A.; Burstyn, J. N. Inorg. Chem. 2002, 41,
669-677.
KfKa2[A]2
[H+]2
(19) The accelerations are equal to k2[Pd]/k1 at pH 7 and k1.5[Pdtot]
1/2/k1 at
[C] )
(9)
pH 9. The k1 value for the uncatalyzed hydroysis of methylparathion
at pH 9 has been measured independently in the corresponding buffer.
The value of k1 at pH 7 was derived from the published half-time
values (130 days at 20 °C and pH 7.4); see: Freed, V. H.; Chiou, C.
T.; Schmedding, D. W. J. Agric. Food Chem. 1979, 27, 706-708.
(20) Sanchez, G.; Garcia, J.; Meseguer, D.; Serrano, J. L.; Garcia, L.; Perez,
J.; Lopez, G. Dalton Trans. 2003, 4709-4717.
Assuming that, at pH 9, the concentration in A is negligible,
the data obtained at this pH can be fitted to eq 10, where
(12) Constable, E. C.; Thompson, A. M. W. C.; Leese, T. A.; Reese, D.
G. F.; Tocher, D. A. Inorg. Chim. Acta 1991, 182, 93-100.
(13) Vicente, J.; Arcas, A.; Bautista, D. Organometallics 1997, 16, 2127-
2138.
(14) Albert, J.; Cadena, J. M.; Granell, J. R.; Solans, X.; Font-Bardia, M.
Tetrahedron: Asymmetry 2000, 11, 1943-1955.
(15) (a) Qin, Y.; Selvaratnam, S.; Vittal, J. J.; Leung, P. Organometallics
2002, 21 (24), 5301-5306. (b) Narayan, S.; Jain, V. K.; Butcher, R.
J. J. Organomet. Chem. 1997, 549, 73-80.
(21) This chemical 31P shift is essentially identical to that previously
reported for dimethylthiophosphate salts. Mahajna, M.; Quistad, G.
B.; Casida, J. E. Chem. Res. Toxicol. 1997, 10, 64-69. Jamer, P. C.
J.; Roelen, H. C. P. F.; van den Elst, H.; van der Marel, G. A.; van
Boom, J. H. Tetrahedron Lett. 1989, 30, 6757-6760.
(22) (a) Deal, K. A.; Burstyn, J. N. Inorg. Chem. 1996, 35, 2792-2798.
(b) Young, M. J.; Wahnon, D.; Hynes, R. C.; Chin, J. J. Am. Chem.
Soc. 1995, 117, 9441-9447.
5604 Inorganic Chemistry, Vol. 45, No. 14, 2006

Efficient Hydrolysis of Methylparathion
change observed in the absorbance of the sample as a
function of pH is consistent with the formation of B and C
by deprotonation of A. The theoretical absorbance (Abs) can
be approximated using eq 12, where [A], [B], and [C] are
expressed as a function of Ka and Kf (eq 7-9) and where
ꢀA, ꢀB, and ꢀC are the extinction coefficients of A, B, and C,
respectively. ꢀA was derived from the absorbance at pH 6-7,
where A is the predominant species. Because B and C coexist
above pH 9.5, a direct determination of their respective
extinction coefficients is complicated. We monitored the
absorbance of a solution of 1 at pH 10 in the 2.01 × 10-5
<
[Pdtot] < 1.13 × 10-4 range and observed an essentially linear
variation of the absorbance. One might expect that the
equilibrium between B and C would result in a nonlinear
dependence of the absorbance as a function of [Pdtot].
However, the large equilibrium constant Kf of 6.6 × 106 M-1
as derived from the aforementioned kinetic studies (Figure
4) indicates that C is the major species ([C]/[B] ) 7.8 for
[Pdtot] ) 2.01 × 10-5; [C]/[B] ) 19 for [Pdtot] ) 1.13 ×
10-4) and therefore the major contributor to the observed
absorbance. Increasing the level of dilution to better study
the equilibrium between B and C was not possible because
of the limited magnitude of the extinction coefficients. For
these reasons, the extinction coefficient of B and C cannot
be independently determined and were instead derived by
the following method.
Figure 4. pH dependence of the hydrolysis of methylparathion catalyzed
by 1 ([Pdtot] ) 1.75 × 10-6 M, [methylparathion] ) 3.5 × 10-5 M). The
fit (dotted line) was obtained using eq 11 with pKa ) 9.7, Kf ) 6.6 × 106
M-1, k2(neutral) ) 140 M-1 s-1, and k2(basic) ) 10.5 × 103 M-1 s-1
.
Abs ) [A]ꢀA + [B]ꢀB + [C]ꢀC
(12)
Figure 5. Spectrophotometric titration curve of 1 in aqueous solution with
16% dioxane at 240 nm and 25 °C ([1] ) 4.4 × 10-5 M). The fit (dotted
Using Kf ) 6.6 × 106 M-1, ꢀA ) 18 000 ( 200 M-1 cm-1,
fitting of the spectrophotometric data to eq 12 affords pKa
) 9.5 ( 0.1 and ꢀB ) 12 000 ( 6000 M-1 cm-1, ꢀc ) 14 000
( 500 M-1 cm-1. The pKa value determined by this method
is almost identical to that determined by analyzing the pH
dependence of the initial reaction rate. The value of ꢀB is
affected by a large error, because B remains a minor species
at all pH levels.
line) was obtained using eq 12 with pKa ) 9.5, Kf ) 6.6 × 106 M-1, ꢀA
)
18 000 M-1 cm-1, ꢀB ) 12 000 M-1 cm-1, and ꢀc ) 14 000 M-1 cm-1
.
M-1 s-1, and a second-order rate constant at basic pH (k2(basic)
)
of 10.5 × 103 M-1 s-1. We note that the values of k2(neutral)
and k2(basic) are similar, within error, to those determined by
analyzing the initial rates at pH 7 and pH 9, respectively,
whereas the value of Kf is identical to that determined by
studying the dependence of the initial rate as a function of
[Pdtot]1/2 (Table 1). This fit also suggests that the pKa of the
cationic diaqua complex A is close to 9.7. This pKa value is
intermediate between that usually observed for cationic
cyclometalated palladium monoaqua complexes23,24 and that
recently reported for the cationic cyclopalladated diaqua
arylphosphinate complex [Pd{C,P-(4-MeC6H3-2-(OPiPr2))}-
(OH2)]+ (pKa ) 10.9).25
To get additional mechanistic information, the rate of the
reaction was studied at different temperatures (Table 1). The
k2 values obtained at pH 7 and pH 9 (Table 1) were analyzed
using the Eyring equation, which afforded the following
activation parameters: ∆Hq ) 22.9 ( 0.15 kJ mol-1, ∆Sq
) -129 ( 0.5 J K-1 mol-1 at pH 7; ∆Hq ) 15.8 ( 4.2 kJ
mol-1 and ∆Sq ) -117 ( 13 J K-1 mol-1 at pH 9. The data
collected at pH 9 were also used to calculate Kf at each
temperature. A van’t Hoff analysis of the Kf values indicates
that the formation of C is endothermic (∆Hf0 ) 21.3 ( 4.2
kJ mol-1). Comparable thermodynamic parameters have been
calculated for the dimerization of platinum complexes26 or
copper hydroxide complexes.22b
kobs ) initial rate/[S] ) [A]k2(neutral) + [B]k2(basic) (11)
The pKa of cationic diaqua complex A was also measured
by a spectrophotometric titration. To this end, the absorbance
of a solution of 1 as a function of pH in water/dioxane was
measured at 240 nm between pH 6 and 10.5 (Figure 5). The
The very negative value of the entropy of activation
measured at neutral and basic pH indicates that the associa-
tion of the catalyst with the substrate is involved in the rate-
determining step of the reaction (Scheme 3). It is well-known
that ligand-substitution reactions in square planar palladium-
(23) Raybov, D.; Kazankov, G. M.; Yatsimirsky, A. K.; Kuz’mina, L. G.;
Burtseva, O. Y.; Dvortsova, N. V.; Polyako, V. A. Inorg. Chem. 1992,
31, 3083-3090.
(24) Kurzeev, S. A.; Kazankov, G. M.; Ryabov, A. D. Inorg. Chim. Acta
2000, 305, 1-6.
(25) Ogo, S.; Takebe, Y.; Uehara, K.; Yamazaki, T.; Nakai, H.; Watanabe,
Y.; Fukuzumi, S. Organometallics 2006, 25, 331-338.
(26) Sartori, D. A.; Einerwold, J. G.; Abbott, E. H. Inorg. Chim. Acta 1998,
268, 311-312.
Inorganic Chemistry, Vol. 45, No. 14, 2006 5605

Kim et al.
hydrolysis catalysts,30 may in fact be applicable to other
palladium-catalyzed hydrolysis reactions that we have previ-
ously reported but not studied in detail.8
Scheme 3
In summary, simple cyclopalladated complexes based on
the 2-(2-pyridyl)-phenyl-C,N ligand yield remarkably active
catalysts for the hydrolysis of methylparathion. The data that
we have gathered indicate the formation of a palladium(II)
diaqua complex (A) that is moderately active at acidic and
neutral pH. At pH 7, the reaction follows a second-order
rate law with a first-order dependence in palladium catalyst
and methylparathion. Deprotonation of this complex affords
a palladium(II) aqua hydroxo complex (B) whose activity
at pH 9 is much greater than that observed for A at neutral
pH. This increase is somewhat tempered by the fact that
catalyst B is in equilibrium with a nonactive dinuclear species
(C). This unproductive equilibrium becomes negligible at
low concentrations, where the catalyst exists mostly in the
dissociated active form. Finally, these results indicate that
the presence of a supernucleophilic functionality is not
necessary for achieving high catalytic activity.5 In fact, the
second-order rate constant observed at pH 9 with 1 as a
precatalyst is 1 to 2 orders of magnitude larger than those
reported for palladium and platinum aryl oxime complex-
es.5
(II) complexes often proceed by associative mechanisms,27
which are also characterized by very negative activation
entropies.28 By analogy, we propose that the rate-determining
step of the reaction at neutral pH may in fact involve the
substitution of a water ligand by a molecule of methylpar-
athion, which would proceed via a five-coordinate intermedi-
ate.29 This ligand-substitution reaction would be followed
by rapid hydrolysis of the methylparathion and release of
p-nitrophenol and dimethylthiophosphate. At basic pH, the
experimental observations can be reconciled on the basis of
a mechanism in which the catalyst-substrate interaction
involves nucleophilic attack of the phosphorus center by a
hydroxide ligand. Taking into account the thiophilic character
of palladium, this event may in fact be concomitant with
coordination of the sulfur atom to the palladium center. Such
a mechanism, which has been proposed for zinc-containing
Acknowledgment. Support from the U.S. Army Medical
Research and Materiel Command, Robert A. Welch Founda-
tion (Grant A-1423), and the Korea Science and Engineering
Foundation (Grant to M.K.) is gratefully acknowledged. We
thank A/S CHEMINOVA for providing us with methylpar-
athion. We thank the anonymous reviewers and Don Da-
rensbourg for their help with this paper.
(27) For example, see: Frankcombe, K. E.; Cavell, K. J.; Yates, B. F.;
Knott, R. B. J. Phys. Chem. 1996, 100, 18363-18370. Basolo, F.;
Gray, H. B.; Pearson, R. G. J. Am. Chem. Soc. 1960, 82, 4200-4203.
Goddard, J. B.; Basolo, F. Inorg. Chem. 1968, 7, 936-943.
(28) Frey, U.; Elmroth, S.; Moullet, B.; Elding, L. I.; Merbach, A. E. Inorg.
Chem. 1991, 30, 5033-5037. Groen, J. H.; Delis, J. G. P.; van
Leeuwen, P. W. N. M.; Vrieze, K. Organometallics 1997, 16, 68-
77. Minniti, D. Inorg. Chem. 1994, 33, 2631-2634. Kawataka, F.;
Kayaki, Y.; Shimizu, I.; Yamamoto, A. Organometallics 1994, 13,
3517-3524.
Supporting Information Available: Calculation details and
X-ray crystallographic data for complexes 1 and 2 in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0600578
(30) Goldberg, D. P.; diTargiani, R. C.; Namuswe, F.; Minnihan, E. C.;
Chang, S.; Zakharov, L. N.; Rheingold, A. L. Inorg. Chem. 2005, 44,
7559-7569.
(29) (a) Zhong, H. A.; Widenhoefer, R. A. Inorg. Chem. 1997, 36, 2610-
1616. (b) Cooper, J.; Ziegler, T. Inorg. Chem. 2002, 41, 6614-6622.
5606 Inorganic Chemistry, Vol. 45, No. 14, 2006