Mononuclear versus dinuclear palladacycles derived from 1,3-bis(N,N-dimethylaminomethyl)benzene: Structures and catalytic activity

Article abstract of DOI:10.1016/j.inoche.2010.03.035

Mononuclear and dinuclear palladacycles derived from 1,3-bis(N,N- dimethylaminomethyl)benzenes, [{Pd (Cl)}2,6-(Me₂NCH₂) ₂C₆H₃] (1) and [1-{Pd(H₂O)(Py)}-5- {Pd(OTf)(Py)-2,4-(Me₂NCH₂)₂C₆H ₂]-(OTf) (2), were synthesized and their structures were fully characterized. Complex 1 is a pincer complex with η³-mer NCN phenyl backbone, complex 2 is a bispalladium(II) complex with 1,2- and 4,5- two C,N-ortho phenyl backbone. Whereas the pincer complex 1 acted as a poor catalyst on methanolysis of fenitrothion, complex 2 demonstrated high catalytic activity in the same reactions, but there is no synergetic effect between two palladium ions. The results clearly indicate that a dissociable co-ligand in the palladacycle compounds significantly promotes the catalytic methanolysis.

Full text of DOI:10.1016/j.inoche.2010.03.035

Inorganic Chemistry Communications 13 (2010) 814–817
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Mononuclear versus dinuclear palladacycles derived from
1,3-bis(N,N-dimethylaminomethyl)benzene: Structures and catalytic activity
Bian-Bian Liu a, Xue-Rui Wang a, Zhi-Fo Guo a, Zhong-Lin Lu a,b,
⁎
a
College of Chemistry, Beijing Normal University, Beijing, 100875, China
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Mononuclear and dinuclear palladacycles derived from 1,3-bis(N,N-dimethylaminomethyl)benzenes, [{Pd
(Cl)}2,6-(Me2NCH2)2C6H3] (1) and [1-{Pd(H2O)(Py)}-5-{Pd(OTf)(Py)-2,4-(Me2NCH2)2C6H2]-(OTf) (2), were
synthesized and their structures were fully characterized. Complex 1 is a pincer complex with η3-mer NCN
phenyl backbone, complex 2 is a bispalladium(II) complex with 1,2- and 4,5- two C,N-ortho phenyl
backbone. Whereas the pincer complex 1 acted as a poor catalyst on methanolysis of fenitrothion, complex 2
demonstrated high catalytic activity in the same reactions, but there is no synergetic effect between two
palladium ions. The results clearly indicate that a dissociable co-ligand in the palladacycle compounds
significantly promotes the catalytic methanolysis.
Received 28 January 2010
Accepted 18 March 2010
Available online 25 March 2010
Keywords:
Palladacycle
Pincer complex
Methanolysis
Pesticide
© 2010 Elsevier B.V. All rights reserved.
Crystal structure
Introduction
a palladated complex with N,N-dimethylaminomethylphenyl ligand,
namely Pd(dmba)(py)(OTf), promotes the methanolysis of phos-
Palladacycles have received considerable attention for the past half
century, due to their facile syntheses and easy manipulation of their
electronic and steric properties, as well as their catalytic activities on
various chemical reactions. In fact, their applications have been
expanded in domains as diverse as biological chemistry, material
science, organic syntheses, and asymmetric catalysis [1–7]. The vast
majority of these compounds possess anionic four-electron (C,Y-
bidentate ) or six-electron (YCY′ tridentate) donor ligands (Y, Y′ could
be N, P, As, O, Se or S). Depending on their electronic and steric
properties, they may show different catalytic activity. The latter
palladacycles, namely pincer complexes, are usually more rigid as the
metal-carbon bond is assisted by the chelating arms. As a conse-
quence of the structural rigidity, pincer complexes have shown better
stability and special selectivity in some catalysis due to the fact that
three of the four coordination sites are occupied on the metal center.
On the other hand, this rigidity may reduce the catalytic activity for
some other reactions where two free coordination sites are needed
during the catalytic process [8–10].
phorothionates, demonstrating great catalytic efficacy and specificity
toward these P=S derivatives [14]. It is believed that the catalytic
methanolysis with palladacycle involves a pre-equilibrium binding of
the P=S to form a transient complex that undergoes intramolecular
attack of the coordinated methoxide on phosphorus atom. However,
whether pyridine is dissociated or not in the catalytic process is not
clear. In order to get deep insight of the catalytic mechanism and to
further develop more efficient catalysts, we have synthesized a
mononuclear and a dinuclear Palladacycles 1 and 2, both derived from
1,3-bis(N,N-dimethylaminomethyl)benzenes, and characterized their
structures and catalytic property. As expected, they exhibited
significantly different catalytic activity in the methanolysis of
phosphorothionate triesters, due to their different coordination
modes (Scheme 1).
Compound 1 was obtained by following the literature method [15]
and its structure was confirmed by the FTIR, 1H NMR spectroscopy,
and elemental analysis. Palladacycle 2 was synthesized from [1,5-
{PdCl(C5H5N)}2-2,4-(Me2NCH2)2C6H2], which was prepared accord-
ing to literature [16], by the reaction with silver triflate in
dichloromethane at room temperature with a yield of 80% [17].
The presence of water molecule in the complex 2 can be inferred
Phosphorothionate triesters are commonly used as agricultural
pesticides. Since their widespread use and persistence, the environ-
mental accumulation of these toxic chemicals present an ecological
and health threat. It has been noticed that C,N-ortho cyclometalated
palladium and platinium complexes can effectively catalyze the
hydrolysis of these materials [11–13]. We have reported before that
from the strong broad peak around 3380 cm−1 in IR spectrum. Its 1
H
NMR spectrum shows the characteristics of 1,2,4,5-tetrasubstituted
benzene ring with two singlet peaks at 6.57 (H3) and 4.56 (H6) ppm,
respectively, which are consistent with those in its chloride precursor
[16] and the closely related [1,5-{PdCl(PPh3)}2-2,4-(Et2NCH2)C6H2]
[18]. The signals from two pyridine rings appear at 8.58, 7.69, and
7.16 ppm in a integral ratio of 4:2:4 corresponding to the H3 or H6
⁎
Corresponding author. College of Chemistry, Beijing Normal University, Beijing,
100875, China.
E-mail address: luzl@bnu.edu.cn (Z.-L. Lu).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.03.035

B.-B. Liu et al. / Inorganic Chemistry Communications 13 (2010) 814–817
815
Single crystals suitable for diffraction measurements were pre-
pared by diffusing n-hexanes into the dichloromethane solution of the
complexes at room temperature. General views of palladacycles 1 and
2 are shown in Figs. 1 and 2, respectively, with the most important
bond lengths and angles given in the figure captions. Data collection
details are given in reference [20].
For complex 1, there are four different molecules in the crystal,
with only slight differences in bond lengths and bond angles. Only one
representative structure is shown in Fig. 1. It can be seen that the
geometry around the palladium atom is square-planar, and the metal
is bound to the η3-mer NCN ligand and the chloro ligand. The five
membered metacycles are puckered in the same direction, with Pd
(1)-N(1)-C(7)-C(2) and Pd(1)-N(2)-C(10)-C(6) torsion angles of
28.17° and 57.97°, respectively. The Pd–N and Pd–C bond lengths
are similar to those generally encountered in NCN pincer palladium(II)
complexes [10,21,22], but they are shorter than those in the C,N-ortho
coordinated palladacycles [14,19].
The crystal structure of 2 contains two independent molecules
with slight differences, of which only one is shown in Fig. 2. Non-
coordinated triflate is in a distorted form. The two molecules are
connected into dimer by hydrogen bonding between the coordinated
water molecules and non-coordinated triflate counter ions. It can be
seen that each palladium atom displays the expected square-plannar
coordination geometry, as a result of being bound to the carbon atom
of the phenyl, the two nitrogen atoms from pyridine and amine
moiety which are positioned trans to each other, and the oxygen atom
from triflate for Pd(1) or water molecule for Pd(2).
Scheme 1.
proton on the phenyl backbone. The great up-field shift of H6 can be
attributed to the double shielding effects from two pyridine rings,
which are positioned trans to the amine moieties. In the C,N-ortho
mononuclear palladacycles, H6 normally appears around 6.0 ppm
[19]. The up-field shift of H6 also indicates that N,N-trans configura-
tion in the coordination sphere of palladium atom in the solid states
(see following) is still kept in solution.
For molecule 1, the palladium atoms deviate from the coordination
plane by 0.010 and 0.005 . The dihedral angle between the planes
{C(6)Pd(1)N(1)} and {N(3)Pd(1)O(1)} is 5.23°, while the dihedral
angle between the planes {C(4)Pd(2)N(2)} and {N(4)Pd(2)O(4)}is
4.46°, indicating that the coordination sphere has a slightly distorted
square planar configuration. The two pyridine rings are not par-
allel to each other but with the dihedral angle of 35.44º. The Pd–C,
Pd–O, and Pd–N (amine and pyridne) bond lengths are slightly
shorter than those values reported for mononuclear palladacycles of
N,N-dimethylbenzylamine ligand and aryl oxime [14,19].
To evaluate the catalytic activity of 1 and 2, methanolysis of pes-
ticide fenitrothion (O,O-dimethyl O-(3-methyl-4-nitrophenyl)phos-
phorothioate) (3) with the assistance of the two catalysts, were
investigated (see Scheme 2). The reactions were monitored by ob-
serving the rate of appearance of 3-methyl-4-nitrophenol at 320 nm
Fig. 1. Molecular structure of complex 1 showing the atom numbering. Displacement
parameters are shown at the 50% level. Selected bond distances (Å) and bond angles
(°): Pd(1)–C(1) 1.922(4); Pd(1)–N(1) 2.103(3); Pd(1)–N(2) 2.102(3); Pd(1)–Cl(1)
2.4243(10); C(1)–Pd(1)–N(2) 81.59(14); C(1)–Pd(1)–N(1) 81.63(13); N(1)–Pd(1)–
N(2) 163.21(11); C(1)–Pd(1)–Cl(1) 178.94(11); N(1)–Pd(1)–Cl(1) 99.31(8); N(2)–
Pd(1)–Cl(1) 97.47(9).
Fig. 2. Molecular structure of complex 2 showing the atom numbering. Displacement parameters are shown at the 50% level. Selected bond lengths (Å) and bond angles (°): Pd(1)–
C(6) 1.942(8); Pd(1)–N(1) 2.058(6); Pd(1)–N(3) 2.018(7); Pd(1)–O(1) 2.171(6); Pd(2)–C(4) 1.956(8); Pd(2)–N(2) 2.038(6); Pd(2)–N(4) 2.013(7); Pd(2)–O(1 W) 2.172(7); C(6)–
Pd(1)–N(3) 93.5(3); C(6)–Pd(1)–N(1) 82.6(3); N(3)–Pd(1)–N(1) 175.1(3); C(6)–Pd(1)–O(1 W) 174.0(3); N(3)–Pd(1)–O(1W) 90.9(2); N(1)–Pd(1)–O(1W) 93.3(2); C(4)–Pd(2)–N(4)
95.6(3); C(4)–Pd(2)–N(2) 81.3(3); N(4)–Pd(2)–N(2 ) 175.9(3); C(4)–Pd(2)–O(1W) 176.7(3); N(4)–Pd(2)–O(1W) 84.2(3); N(2)–Pd(2)–O(1W) 99.0(3).

816
B.-B. Liu et al. / Inorganic Chemistry Communications 13 (2010) 814–817
Scheme 2.
clearly indicates that two palladium ions work independently, and
there is no synergetic effect in its catalytic process. Compared to the
methoxide-catalyzed background reaction which is calculated to be
7.7×10−10 s−1 at sspH 10.80 [24], 1×10−3 mol dm−3 of 2 afforded an
dramatic acceleration of 4.4×109 folds.
obs
The pseudo-first order rate constants (k ) determined after
max
extrapolation to [buffer]=0 for the methanolysis of 3 promoted by
1.0×10−5 M solutions of 2 at sspH values between 8.22 and 12.23 were
studied. The corresponding second-order rate constants, ko2bs, were
obtained from kmobasx/[Pd] at each sspH and plotted in Fig. 4. The
appearance of the plot suggests a catalytic process where a basic form
of the catalyst is active. A kinetic sspKa of 10.73 0.06 was obtained by
fitting the curve according to a standard single ionization model,
which is also very close to that of mononuclear palladacycle (10.85
0.10) [14].
In conclusion, mononuclear and dinuclear palladacycles 1 and 2
with 1,3-bis(N,N-dimethylamino)pheneyl backbone have been pre-
pared and structurally characterized. Complex 1 is a pincer complex,
which is nearly nonactive in the catalytic methanolysis of feni-
trothion. Complex 2 is a two C,N-ortho coordinated bipalladium
complex, which can greatly catalyze the methanolysis reaction of
pesticide with each metal ion working independently. The results
suggest that a dissociable co-ligand in the coordination sphere of
palladium atom is favorable for the catalysis, therefore, pincer
palladacyles may not be good choices for the catalytic methanolysis
of phosphorothinates. Further work on the modification of the
structure of co-ligand pyridine with different substituents, theoretical
calculation of reaction intermediates, and screening of the activities
on different pesticides are currently under investigation in our
laboratory.
Fig. 3. A plot of kmobasx vs. [2] for the methanolysis of 3 (2.84×10−5 mol dm−3) at 25 °C,
buffer triethylamine (1–3)×10−3 mol dm−3, extrapolated to zero [buffer], sspH 10.80.
The second order rate constant (km2 ax) for catalyzed reaction obtained from the gradient
of the plot is (3.26 0.08)×103 dm3 mol−1 s−1
.
using a Cary300 UV/visible spectrophotometer [23]. Under the condi-
tions of sspH 10.80 ( triethylamine buffer), [3]=2.84×10−5 mol dm−3
and [Pd]=2×10−5 mol dm−3, the kobs of 2 is 2.041 min−1, whereas
the kobs of 1 is only 0.0011 min−1 at the otherwise same reaction con-
ditions. In other words, that same reaction catalyzed by 1 is about 1855
times slower than that of 2 per palladium atom. This discovery
provides direct evidence that the pincer system is not as proficient
at promoting the methanolysis of fenitrothion, although pincer
complexes are good catalysts in the carbon–carbon coupling reac-
tions [8–10].
The kmobasx vs. concentration plot for the methanolysis of fenitrothion
3 catalyzed by varying [2] at sspH 10.80 exhibits a good linearity
(Fig. 3). The gradient, defined as second-order rate constant ko2bs, is
(3.26 0.08)×103 dm3 mol−1 s−1. It is evident that ko2bs of 2 is almost
double that of mononuclear complex [Pd(dmba)(Py)(OTf)], which
has a ko2bs =1.88×103 dm3 under the same conditions [14]. This
Acknowledgements
The authors gratefully acknowledge the financial assistance from
Beijing Normal University (107026), Beijing Municipal Commission of
Education, and the Scientific Research Foundation for the Returned
Overseas Chinese Scholars (213006), Program for New Century
Excellent Talents at Universities, The Ministry of Education of
China, and Dr. Ruiyao Wang for his assistance in the determination
of the X-ray crystal structures of compounds 1 and 2.
Appendix A. Supplementary material
Crystallographic data for the structure analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
No. 720256 and 720257, for compounds 1 and 2, respectively. Copies
of this information may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-
336033; e-mail: deposit@ccdc.ac.uk or http://www.cccdc.cam.ac.uk).
Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2010.03.035. The data analyses
and refinement were following the references [25–26].
References
Fig. 4. Plot of logko2bs vs sspH for the methanolysis of fenitrothion (2.84×10−5 mol dm−3
)
catalyzed by 2 (1.0×10−5 mol dm−3) at 25 °C. The line through the data computed from
the fit according literature method gives a km2 ax of (5.3 0.6)×103 dm3 mol−1 s−1, and
a kinetic sspKa of 10.73 0.06.
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694 (m), 636 (s). 1H NMR (ppm, 400 MHz, CDCl3): δ 8.59 (d, J=8.0 Hz, H2 , H6
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phenyl), 4.57 (s, H6 on phenyl), 3.85 (s, 4H, CH2), 2.82 (s, 12H, NMe2). 19F NMR
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:
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3.45, N 6.38%. Found: C 33.30, H 3.75, N 5.95%.
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