Palladium(II) and platinum(II) complexes containing hydrophilic pyridinylimine-based ligands

Article abstract of DOI:10.1016/S0020-1693(03)00304-9

Reaction of the unsymmetrical hydrophilic pyridinylimine ligands 1a-e with either PdCl₂(C₆H₅CN)₂ or PtCl ₂(C₆H₅CN)₂ affords the square-planar d⁸ dichloride

Full text of DOI:10.1016/S0020-1693(03)00304-9

Inorganica Chimica Acta 355 (2003) 340Á346
/
www.elsevier.com/locate/ica
Palladium(II) and platinum(II) complexes containing hydrophilic
pyridinylimine-based ligands
Brian P. Buffin *, Evan B. Fonger, Abhijit Kundu
Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008, USA
Received 22 April 2003; accepted 3 May 2003
Abstract
Reaction of the unsymmetrical hydrophilic pyridinylimine ligands 1aÁ
/
e with either PdCl2(C6H5CN)2 or PtCl2(C6H5CN)2 affords
the square-planar d8 dichloride complexes 2aÁ
/
e and 3, respectively. Formation of these divalent species effectively stabilizes the
reactive imine bond towards hydrolysis. All of the Pd(II) and Pt(II) complexes were characterized by 1H and 13C NMR spectroscopy
and elemental analysis. The solid-state structure of 2a was established by X-ray crystallography and reveals relatively little
delocalization of the imine Cꢀ
/
N bond upon metallacycle formation.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Pyridinylimine ligands; Platinum(II) complexes; Palladium(II) complexes; Water-soluble complexes
1. Introduction
coordination and the rapid hydrolysis of imine-based
ligands in an aqueous environment are overcome.
Interest in the chemistry of water-soluble organome-
tallic complexes stems mainly from their potential
Recently we communicated our success at forming
hydrophilic pyridinylimine-based and 1,4-diazabuta-
diene ligands [20]. Coordination of these ligands to
low-valent Pt(0) and Pd(0) metal centers was shown to
stabilize the imine group towards hydrolysis. The ability
to form platinum and palladium chlorides with our
acidic a-diimine ligand has also been reported [21].
Herein we describe the preparation of divalent palla-
dium and platinum compounds with a variety of
hydrophilic pyridinylimine-based ligands and the struc-
ture of a Pd(II) chloride species as determined in the
solid state by X-ray crystallography. These water-
soluble complexes may find utility as late transition
metal polymerization catalyst precursors [22] or, in the
case of platinum-based compounds, as chemotherapeu-
application as recoverable ‘green’ catalysts [1Á4], their
/
role in the field of bioorganometallic chemistry [5,6], or
as biomedical diagnostic and therapeutic reagents [7].
The majority of research in this field has focused on the
use of hydrophilic tertiary phosphines as ancillary
ligands for the promotion of water-solubility [8]. Orga-
nometallic transition-metal complexes utilizing these
ligands have been found to catalyze a large variety of
chemical transformations in aqueous solution. While
catalysts employing nitrogen-containing ligands have
been shown to offer many advantages over related
phosphorous-based analogues in homogeneous systems
[9,10], the use of nitrogen ligands with ionic or polar
tic agents [23Á26].
/
substituents (e.g. Á
aqueous organometallic chemistry [11Á
[17Á19] has only recently started to develop as problems
of greater competition from water molecules for metal
/
SO3Na, Á
/
PO3Na2, Á
/
CO2H, Á
/
OH) in
/
16] and catalysis
/
2. Experimental
2.1. General details
Transition metal starting materials PdCl2(C6H5CN)2,
PdCl2(CH3CN)2, PdCl2(1,5-C8H12), and PtCl2(C6H5-
CN)2 were obtained from Strem Chemicals, Inc. (New-
* Corresponding author. Tel.: ꢀ
909.
E-mail address: brian.buffin@wmich.edu (B.P. Buffin).
/
1-269-3872 420; fax: ꢀ1-269-3872
/
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00304-9

B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á
/346
341
buryport, MA) and used without further purification.
Other reagents were obtained from commercial sources
and used as received unless otherwise noted. All of the
reactions and manipulations involving transition metal
compounds were performed under N2 using standard
inert atmosphere and Schlenk techniques [27,28]. Sol-
vents were dried and distilled by standard procedures.
1H and 13C{1H} NMR spectra were recorded at ambient
temperature on a JEOL Eclipse 400 NMR in DMSO-d6,
DMF-d7, or D2O at 400 and 100 MHz, respectively.
Chemical shifts are reported downfield in ppm (d)
relative to tetramethylsilane by reference to residual
protonated solvent signals with the following abbrevia-
tions: d, doublet; m, multiplet; q, quartet; s, singlet; t,
triplet; and br, broad signal. Elemental analyses of new
transition metal compounds were performed by Desert
Analytics, Tucson, AZ.
(DMF-d7): d 161.3, 154.7, 150.6, 150.5, 150.1, 137.2,
128.8, 125.8, 124.4, 121.5, 121.1, 118.9.
2.2.3. Benzenesulfonic acid, 3-[(2-pyridinyl-(6-methyl)-
methylene)amino]-, sodium salt (1c)
Metanilic acid, sodium salt (1.00 g, 5.13 mmol) was
dissolved in dry methanol (8 ml). Formic acid (90%, 3
drops) was added as a catalyst, followed by the addition
of 6-methyl-2-pyridinecarboxaldehyde (0.62 g, 5.13
mmol). After stirring for 18 h at ambient temperature,
anhydrous MgSO4 was added to remove water. Stirring
was continued for another 3 h. The desired pyridineÁ
/
imine ligand 1c was obtained quantitatively after
separation of the MgSO4 desiccant and the removal of
1
methanol under reduced pressure. H NMR (DMSO-
d6): d 8.52 (s, 1H), 7.98 (d, 1H), 7.86 (t, 1H), 7.52 (d,
1H), 7.48 (s, 1H), 7.40 (m, 2H), 7.28 (d, 1H), 2.56 (s,
3H). 13C{1H} NMR (DMSO-d6): d 161.1, 158.2, 153.3,
150.0, 149.5, 137.3, 128.8, 125.1, 123.8, 120.8, 118.6,
118.4, 23.9.
2.2. Synthesis
2.2.1. Benzoic acid, 3-[(2-pyridinylmethylene)amino]
(1a)
2.2.4. Benzoic acid, 4-[(2-pyridinylmethylene)amino]
(1d)
3-Aminobenzoic acid (5.12 g, 37 mmol) was dissolved
in 250 ml of ethanol and 25 ml of benzene. 2-
Pyridinecarboxaldehyde (4.00 g, 37 mmol) was subse-
quently added in a dropwise fashion. The resulting
solution was heated until 1/3 of its original volume
remained. Precipitation of product was accomplished by
4-Aminobenzoic acid (2.74 g, 20 mmol) was dissolved
in dry methanol (15 ml). Formic acid (90%, 4 drops) was
added as a catalyst, followed by the dropwise addition
of 2-pyridinecarboxaldehyde (2.14 g, 20 mmol). The
reaction mixture was stirred at ambient temperature for
24 h. Product 1d, which precipitated from solution upon
cooling to ꢁ10 8C. The pale yellow solid was isolated
/
by filtration, washed with cold methanol, and dried
1
under vacuum. Yield 6.63 g (79%). H NMR (DMSO-
cooling to ꢁ10 8C, was isolated by filtration and
/
washed with cold methanol. Yield 3.90 g (86%). 1H
NMR (DMF-d7): d 12.8 (br, 1H), 8.80 (d, 1H), 8.66 (s,
1H), 8.25 (d, 1H), 8.12 (d, 2H), 8.06 (m, 1H), 7.61 (m,
1H), 7.47 (d, 2H). 13C{1H} NMR (DMSO-d6): d 170.5,
161.1, 154.6, 151.5, 150.5, 139.0, 137.7, 130.9, 126.2,
121.8, 120.5.
d6): d 13.1 (br, 1H), 8.74 (d, 1H), 8.62 (s, 1H), 8.17 (d,
1H), 7.97 (td, 1H), 7.87 (m, 1H), 7.82 (br s, 1H), 7.58-
7.53 (m, 3H). 13C{1H} NMR (DMSO-d6): d 167.5,
162.5, 154.4, 151.3, 150.3, 137.6, 132.6, 130.2, 127.9,
126.4, 125.8, 122.4, 122.0.
2.2.2. Benzenesulfonic acid, 3-[(2-
pyridinylmethylene)amino]-, sodium salt, (1b)
2.2.5. Benzenesulfonic acid, 4-[(2-
pyridinylmethylene)amino]-, sodium salt (1e)
Metanilic acid, sodium salt was prepared from the
reaction of metanilic acid and sodium hydroxide aqu-
eous solution followed by removal of the water under
reduced pressure. Metanilic acid, sodium salt (1.00 g,
5.13 mmol) was added to a solution of dry ethanol (70
ml) and benzene (10 ml). The resulting mixture was
heated to reflux, and 10 ml of anhydrous methanol was
added to facilitate complete dissolution of the acid salt.
2-Pyridinecarboxaldehyde (549 mg, 5.13 mmol) was
then added dropwise. The solution was heated until
approximately 1/2 of its original volume remained and
then cooled to ambient temperature to initiate product
precipitation. The resulting solid was isolated by filtra-
tion and washed with diethyl ether to afford 850 mg
Sulfanilic acid, sodium salt (1.95 g, 10 mmol) was
dissolved in dry methanol (10 ml). Formic acid (90%, 1
drop) was added as a catalyst, followed by the dropwise
addition of 2-pyridinecarboxaldehyde (1.07 g, 10 mmol).
The reaction mixture was stirred at ambient temperature
for 24 h. Product 1e, which precipitated from solution
upon standing, was isolated by filtration and washed
with cold methanol. Yield 0.36 g (12%). 1H NMR
(DMSO-d6): d 8.74 (d, 1H), 8.59 (s, 1H), 8.17 (d, 1H),
7.97 (t, 1H), 7.65 (d, 2H), 7.54 (m, 1H), 7.27 (d, 2H).
13C{1H} NMR (DMSO-d6): d 161.1, 153.9, 150.5,
149.8, 146.4, 137.1, 126.7, 125.8, 121.4, 120.4.
2.2.6. PdCl2(1a) (2a)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2(C6H5CN)2 (0.10 g, 0.26
1
(58%) of product 1b. H NMR (DMSO-d6): d 8.74 (d,
7.49
1H), 8.57 (s, 1H), 8.19 (d, 1H), 7.97 (td, 1H), 7.58Á
/
(m, 3H), 7.40 (t, 1H), 7.29 (m, 1H). 13C{1H} NMR
mmol) and 0.059 g (0.26 mmol) of the pyridineÁimine
/

342
B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á346
/
ligand 1a and flushed with nitrogen. Tetrahydrofuran
(THF, 40 ml) was added by syringe producing an
1H), 8.31 (t, 1H), 8.23 (d, 1H), 7.87Á7.74 (m, 3H), 7.41
/
(m, 2H), 3.10 (s, 3H). 13C{1H} NMR (DMF-d7): d
173.1, 166.4, 156.3, 149.8, 147.2, 140.5, 131.7, 128.1,
127.4, 126.1, 125.4, 120.3, 26.7. Anal. Calc. for
immediate color change from dark orange to yellowÁ
/
orange. The solution was sealed under nitrogen and
stirred for 18 h to complete transformation to the
desired product. Precipitation of the product was
accomplished by partial removal of THF under vacuum
C13H11Cl2N2NaO3PdS×/1/2MeOH: C, 32.98; H, 2.67;
N, 5.70. Found: C, 33.30; H, 2.61; N, 5.84%. The
presence of methyl alcohol was confirmed in the 1H
NMR spectrum.
and cooling to ꢁ10 8C. The resulting yellow solid was
/
isolated by filtration, washed with methyl alcohol and
diethyl ether, and dried under vacuum. Yield 0.074 g
(70%). Product 2a was also formed by reaction of ligand
1a with either PdCl2(CH3CN)2 or PdCl2(1,5-C8H12) in
an analogous manner with isolated yields of 57 and 53%,
2.2.9. PdCl2(1d) (2d)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2(C6H5CN)2 (0.050 g, 0.13
mmol) and 0.032 g (0.14 mmol) of the pyridineÁimine
/
1
respectively. H NMR (DMSO-d6): d 13.26 (br, 1H),
9.07 (d, 1H), 8.82 (s, 1H), 8.42 (td, 1H), 8.22 (d, 1H),
ligand 1d and flushed with nitrogen. THF (20 ml) was
added by syringe, and the resulting solution was sealed
under nitrogen and stirred for 48 h, during which time a
8.03Á
7.94 (m, 3H), 7.67 (m, 1H), 7.60 (t, 1H). 13C{1H}
/
NMR (DMSO-d6): d 173.9, 167.0, 156.3, 150.6, 147.6,
141.8, 131.5, 130.2, 129.8, 129.7, 129.2, 129.1, 125.2.
Anal. Calc. for C13H10Cl2N2O2Pd: C, 38.69; H, 2.50; N,
6.94. Found: C, 38.44; H, 2.41; N, 6.80%.
yellowÁorange solid precipitated from the solution. The
/
solid was isolated by filtration, washed with diethyl
ether, and dried under vacuum. Yield 0.048 g (91%). 1H
NMR (DMF-d7): d 13.0 (br, 1H), 9.24 (d, 1H), 8.98 (s,
1H), 8.53 (td, 1H), 8.41 (m, 1H), 8.11 (d, 2H), 8.07 (m,
1H), 7.65 (d, 2H). 13C{1H} NMR (DMF-d7): d 173.2,
166.9, 156.2, 151.0, 150.6, 141.5, 131.1, 129.9, 129.6,
129.5, 124.6. Anal. Calc. for C13H10Cl2N2O2Pd+1/
3THF: C, 40.26; H, 2.99; N, 6.55. Found: C, 40.65; H,
2.74; N, 7.00%. The presence of THF was confirmed in
2.2.7. PdCl2(1b) (2b)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2(C6H5CN)2 (0.075 g, 0.20
mmol) and 0.055 g (0.19 mmol) of the pyridineÁimine
/
ligand 1b and flushed with nitrogen. Anhydrous methyl
alcohol (20 ml) was added by syringe, and the resulting
solution was sealed under nitrogen and stirred for 24 h
to complete transformation to the desired product. The
solution was subsequently filtered through Celite†, and
diethyl ether (20 ml) was added to initiate product
precipitation. The resulting yellow solid was isolated by
1
the H NMR spectrum.
2.2.10. PdCl2(1e) (2e)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2(C6H5CN)2 (0.100 g, 0.26
mmol) and 0.075 g (0.26 mmol) of the pyridineÁimine
/
filtration, washed with diethyl ether (4ꢂ
/
5 ml), and
ligand 1e and flushed with nitrogen. Anhydrous methyl
alcohol (25 ml) was added by syringe, and the resulting
solution was sealed under nitrogen and stirred for 20 h,
dried under vacuum. Yield 0.065 g (72%). 1H NMR
(DMF-d7): d 9.22 (d, 1H), 8.95 (s, 1H), 8.50 (td, 1H),
8.42 (d, 1H), 8.07 (1H, overlapped by DMF-d7 signal,
presence confirmed in D2O), 7.80 (m, 2H), 7.48 (m, 1H),
7.40 (m, 1H). 13C{1H} NMR (DMF-d7): d 173.0, 156.6,
150.5, 150.4, 147.2, 141.5, 129.9, 129.3, 127.4, 126.4,
125.7, 120.5. Anal. Calc. for C12H9Cl2N2NaO3PdS: C,
31.22; H, 1.97; N, 6.07. Found: C, 31.37; H, 2.35; N,
6.04%.
during which time a yellowÁorange solid precipitated
/
from the solution. The solid was isolated by filtration,
washed with diethyl ether, and dried under vacuum.
1
Yield 0.106 g (88%). H NMR (DMF-d7): d 9.22 (d,
1H), 8.92 (s, 1H), 8.51 (t, 1H), 8.40 (d, 1H), 8.06 (1H,
overlapped by DMF-d7 signal, presence confirmed in
D2O), 7.80 (d, 2H), 7.47 (d, 2H). 13C{1H} NMR (DMF-
d7): d 172.8, 156.5, 150.6, 149.8, 147.5, 141.6, 129.9,
2.2.8. PdCl2(1c) (2c)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PdCl2(C6H5CN)2 (0.100 g, 0.26
129.4,
126.0,
123.8.
Anal.
Calc.
for
C12H9Cl2N2NaO3PdS: C, 31.22; H, 1.97; N, 6.07.
Found: C, 31.19; H, 2.00; N, 5.88%.
mmol) and 0.078 g (0.26 mmol) of the pyridineÁimine
/
ligand 1c and flushed with nitrogen. Anhydrous methyl
alcohol (20 ml) was added by syringe, and the resulting
solution was sealed under nitrogen and stirred for 27 h
to complete transformation to the desired product. The
solution was subsequently filtered through Celite†, and
diethyl ether (30 ml) was added to initiate product
precipitation. The resulting yellow solid was isolated by
filtration, washed with THF, and dried under vacuum.
2.2.11. PtCl2(1a) (3)
A 125-ml Schlenk flask equipped with a magnetic stir
bar was charged with PtCl2(C6H5CN)2 (0.20 g, 0.42
mmol), and THF (25 ml) was added via syringe. A
separate flask was charged with 0.101 g (0.45 mmol) of
the pyridineÁimine ligand 1a and THF (10 ml). Trans-
/
ferring of the ligand solution by syringe to the
PtCl2(C6H5CN)2 solution under a positive flow of
nitrogen slowly resulted in a color change from yellow
1
Yield 0.075 g (60%). H NMR (DMF-d7): d 8.92 (s,

B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á
/346
343
to orange. The resulting solution was sealed under
nitrogen and stirred for 24 h to complete transformation
to the desired product. Upon further stirring, an orange
precipitate separated from solution. The resulting solid
was isolated by filtration, washed with pentane, and
dried under vacuum. Yield 0.17 g (82%). Crystalline
solid was obtained by slow evaporation of an acetone
Table 1
Crystallographic data for PdCl2(1a)×
/
DMSO
Formula
C13H10Cl2N2O2Pd×C2H6SO
/
Formula weight
Crystal system
Space group
481.67
triclinic
¯
P1
Unit cell dimensions
˚
a (A)
˚
8.154(4)
10.366(4)
11.736(3)
108.99(3)
95.78(3)
102.52(4)
899.78
2
1
solution of 3. H NMR (DMSO-d6): d 13.46 (br, 1H),
3
9.49 (d, 1H, JPtÁH
b (A)
˚
c (A)
3
75 Hz), 9.41 (s, 1H, JPtÁH
ꢃ
/
ꢃ78
/
Hz), 8.46 (t, 1H), 8.25 (d, 1H), 8.02 (m, 3H), 7.72 (m,
1H), 7.64 (t, 1H). 13C{1H} NMR (DMSO-d6): d 173.4,
166.5, 157.1, 149.0, 146.9, 140.7, 131.0, 129.9, 129.7,
129.5, 128.8, 128.6, 125.0. Anal. Calc. for
C13H10Cl2N2O2Pt: C, 31.72; H, 2.05; N, 5.69. Found:
C, 31.58; H, 2.05; N, 5.69%.
a (8)
b (8)
g (8)
3
V (A )
˚
Z
Dcalc (g cmꢁ3
)
1.778
1.46
0.71073
m (mmꢁ1
)
˚
l (A)
Crystal dimensions (mm3)
T (K)
0.37ꢂ
295
/0.35ꢂ0.19
/
2.3. Structure determination
Scan method
F000
v/2u
480 e
6948
5731
221
X-ray quality crystals of 2a were grown from a
DMSO solution upon standing at ambient temperature.
Total no. of reflections
No. of reflections [I ꢀ3s(I))]
Structural parameters
Refinement method
/
An orange prism, approximately 0.37 mmꢂ
/
0.35 mmꢂ
/
0.19 mm in size, was chosen by size, habit, and polarized
light microscopy and mounted on a glass fiber. Intensity
data were collected at ambient temperature on an
full-matrix least-squares on F2
a
R1 [Foꢀ
/
4s(Fo)]
0.0369
0.0469, 0.1061
1.076
R1 a, wR2 (all data)
GOF c
b
EnrafÁNonius CAD-4 diffractometer equipped with
/
graphite monochromated Mo Ka radiation. The unit
cell was determined and refined from 25 reflections with
a
R1ꢃ
/
ajjFojꢁ
wR2ꢃ
[a[w(Fo2ꢁ
Goodness-of-fitꢃ
/
jFcjj/ajFoj.
b
c
/
/
Fc2)2]/a[w(Fo2)2]]1/2
.
/
[a[w(Fo2ꢁ
/
Fc2)2]/(Nobs
ꢁ , based on
Nparam)]1/2
/
1B
/
2u B258. Reflection data were corrected for Lor-
/
entz-polarization effects but not for absorption. The
structure was solved by automated Patterson methods
all data.
and subsequent difference Fourier techniques (DIRDIF
-
DMSO-d6 show a characteristic imine proton resonance
at approximately 8.6 ppm as a sharp singlet. Protons in
the 6-position of the pyridine ring are observed as
doublets slightly downfield from the imine signal (d
92) [29] and refined with SHELXL-97 [30]. Non-hydrogen
atoms were refined with anisotropic thermal parameters.
Hydrogen atoms were placed in idealized positions on
parent atoms in the final refinement. The largest
8.7Á8.8). An imine resonance at 161 ppm is also
/
observed in the 13C NMR spectra of these compounds.
Not surprisingly, addition of the ligands to water
resulted in their immediate hydrolysis as evidenced by
the appearance of aldehyde signals in the 1H NMR
spectra.
unassigned peak in the final difference map was 1.55 e
ꢁ3
˚
˚
located 0.73 A from the Pd atom. Details of the
A
crystal parameters, data collection, and structure refine-
ment are given in Table 1.
Dichloropalladium(L) complexes 2aÁ
/
e (Fig. 1) were
3. Results and discussion
synthesized in good yields (60Á91%) by substitution of
/
weakly coordinating benzonitrile ligands from
PdCl2(C6H5CN)2. The reactions proceed steadily at
ambient temperature in either THF or methanol solvent
but can be accelerated by increasing the reaction
Reaction of substituted anilines with either 2-pyridi-
necarboxaldehyde or 6-methyl-2-pyridinecarboxalde-
hyde affords the hydrophilic pyridineÁ
/
imine ligands
1aÁe as depicted in Fig. 1. These condensation reactions
/
temperature to 50 8C. PdCl2(COD) (CODꢃ(Z,Z)-1,5-
/
were accomplished either by refluxing a 1:1 mixture of
the reagents in ethanol, with benzene (approximately
10%, v/v) added to promote azeotropic removal of water
byproduct (1a and 1b), or at ambient temperature in
anhydrous methanol with a catalytic amount of formic
cyclooctadiene) and PdCl2(CH3CN)2 were also em-
ployed for the formation of 2a, with little difference in
reactivity noted between any of the three Pd precursors.
The Pt dichloride complex 3 (Fig. 1) was synthesized
from PtCl2(C6H5CN)2 by reaction with the pyridineÁ
/
acid (1cÁ
light yellow solids in moderate yield. H and 13C NMR
data are consistent with pyridineÁimine ligand forma-
tion. The 1H NMR spectra for these compounds in
/
e) [31,32]. All ligands were isolated as air-stable
1
imine ligand 1a in methanol at ambient temperature.
All of the transition metal dichloride complexes were
isolated as air-stable orange solids. Diagnostic of
/

344
B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á346
/
Fig. 1. Synthetic scheme for the preparation of hydrophilic pyridinylimine ligands and divalent transition metal complexes.
chelating ligand coordination in these complexes is the
1
downfield shift in the H NMR spectra of the imine
hydrolysis after 1Á2 h, as evidenced by the appearance
/
1
of aldehyde signals in the H NMR spectra, several of
the Pt(0) species showed only traces of decomposition
even after several days in water. It is likely that a higher
level of metallacycle conjugation and a greater level of
electronic donation into the p-acidic ligands in the
zerovalent species relative to the divalent chlorides are
responsible for the observed stability difference.
proton signal by 0.2Á0.4 ppm upon coordination to the
/
Pd(II) center and by approximately 0.8 ppm when
coordinated to Pt(II). Similar resonance shifts have
been observed in related divalent compounds [33Á
while larger chemical shift differences are found when
pyridineÁimine type ligands are coordinated to Pd(0) or
Pt(0) [20]. Imine signals in the 13C NMR spectra also
12.1 ppm upon
/36],
/
Complexes 2a, 2d and 3, which contain ÁCO2H
/
functionalized ligands, are water insoluble. Previous
work in our group has shown that W(0) carbonyl
complexes with these ligands can be transferred into
an aqueous phase by deprotonation of the acidic form of
the ligand with base [37]. Attempts to generate water-
soluble anionic forms of the Pd(II) and Pt(II) complexes
through addition of NaOH to aqueous suspensions of
the solids resulted in their immediate dissolution.
However, all three complexes decomposed in less than
5 min under these alkaline conditions. It is likely that
nucleophilic hydroxide ions either react at the unsatu-
rated imine carbon or compete with the imine nitrogen
atom for metal coordination. Given that the free ligands
are readily hydrolyzed in aqueous solution, displace-
ment of the imine arm from the metal center could easily
lead to complex breakdown. Work is currently under-
way to fully identify the decomposition products.
experience a downfield shift of 10.9Á
/
metal coordination. 195Pt satellites in the 1H NMR
spectrum of complex 3 on both the imine proton signal
(3JPtÁH ca. 78 Hz) and on the ortho proton of the
pyridine ring (3JPtÁH ca. 75 Hz) indicates that the
chelating nitrogen ligand retains coordination to plati-
num in solution on the NMR time scale.
Products 2aÁe and 3 are moderately soluble in polar
/
organic solvents such as methanol, ethanol, DMSO and
DMF, but are insoluble in non-polar hydrocarbon
solvents. Anionic complexes 2b, 2c and 2e are soluble
in water, and the resulting neutral aqueous solutions
were found to be stable for several hours at ambient
temperature. Thus, the reactive imine functionality has
been effectively stabilized towards hydrolysis by coordi-
nation to Pd(II). Related Pd(0) and Pt(0) olefin com-
plexes also stabilize these ligands in aqueous solution,
and for a longer duration [20]. For example, while the
Pd(II) complexes start to decompose through imine
The molecular structure of 2a and the associated
atom-numbering scheme are depicted in Fig. 2, with

B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á
/346
345
Fig. 2. ORTEP representation of the molecular structure of complex 2a with 50% probability ellipsoids (H atoms and DMSO solvent molecule
omitted for clarity)
˚
selected bond lengths and angles given in Table 2. The
geometry around the d8 Pd center in 2a is a distorted
relatively short Cꢀ
C(6)ÁN(2), when compared to the C(7)Á
length of 1.438(3) A and the C(5)Á
/
N bond length of 1.289(3) A for
/
/
N(2) bond
N(1) length of
1.357(3) A in the pyridine ring, is consistent with a
lack of imine double-bond delocalization in 2a. All of
the Pd-N and PdÁCl bond lengths and angles are very
˚
/
square plane, where the relatively small N(1)Á
/
PdÁN(2)
/
˚
bond angle of 80.51(8)8 is a result of chelating ligand
steric constraints. Consistent with a planar geometry are
the bond angles around the Pd atom which sum to
360.06(12)8. A mean plane analysis of the five atoms
comprising the transition metal square plane reveals a
/
similar to those previously reported in related systems
[34,38].
˚
maximum deviation from ideality of 0.0775(9) A for
N(2). In addition, a least-squares analysis of the 11
atoms that include the pyridine ring and the metal
chelate ring finds a maximum deviation of only
4. Conclusion
˚
0.2465(13) A for Cl(1). The aryl ring of the pyridinyli-
Hydrophilic pyridinylimine ligands form monomeric
square-planar Pd(II) or Pt(II) dichlorides in which
coordination is through the chelating nitrogen atoms.
When the hydrophilic ligand contains a charged sub-
stituent, the resulting metal complex is water-soluble,
and the reactive imine bond is effectively stabilized
towards hydrolysis for several hours at neutral pH and
ambient temperature. Under alkaline conditions, all of
the complexes decompose in a relatively short amount of
time, which may limit their usefulness as catalyst
precursors. The solid-state structure of 2a reveals very
mine ligand containing C(7)ÁC(12) is tilted out-of-plane
/
with respect to the metal chelate plane with a calculated
torsion angle of 57.89(9)8 between the two planes.
Terminal phenyl rings that contain bulkier non-hydro-
gen substituents in the ortho-position(s) tend to adopt a
nearly perpendicular position relative to the metal
chelation plane in related compounds [33,34,38]. The
Table 2
˚
Selected bond lengths (A) and bond angles (8) for PdCl2(1a)
little delocalization of the imine Cꢀ/N bond upon
Bond lengths
metallacycle formation. Our inability to readily synthe-
size the sodium salt derivatives of 2a and 2d with anionic
ligands may be a result of carboxylate competition for
metal coordination. The reaction chemistry of these
metal complexes is currently under investigation.
Pd(1)Á
Pd(1)Á
Pd(1)Á
Pd(1)Á
/
Cl(1)
Cl(2)
N(1)
N(2)
2.2858(7) N(2)Á
/
C(7)
1.438(3)
1.453(3)
1.487(4)
1.204(4)
1.318(3)
/
2.2769(7) C(5)Á
/
C(6)
C(13)
/
2.029(2)
2.040(2)
1.357(3)
1.289(3)
C(9)Á
/
/
C(13)Á
/O(1)
N(1)Á/C(5)
C(13)Á
/O(2)
N(2)Á/C(6)
Bond angles
N(1)ÁPd(1)Á
Cl(1)ÁPd(1)Á
N(1)ÁPd(1)Á
N(2)ÁPd(1)Á
N(1)ÁPd(1)Á
N(2)ÁPd(1)Á
/
/
N(2)
Cl(2)
80.51(8) C(5)Á
90.78(3) C(6)Á
93.40(6) C(1)Á
95.37(6) C(7)Á
175.81(6) O(1)Á
171.40(7)
/
N(1)Á
N(2)Á
N(1)Á
N(2)Á
C(13)Á
/
Pd(1)
Pd(1)
Pd(1)
Pd(1)
112.70(14)
113.4(2)
128.6(2)
/
/
/
/
5. Supplementary material
/
/
Cl(1)
Cl(2)
Cl(2)
Cl(1)
/
/
/
/
/
/
129.18(15)
122.9(3)
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 208783 for 2a. Copies of this
/
/
/
/
O(2)
/
/

346
B.P. Buffin et al. / Inorganica Chimica Acta 355 (2003) 340Á346
/
[14] P.K. Baker, A.E. Jenkins, A.J. Lavery, D.J. Muldoon, A.
Shawcross, J. Chem. Soc., Dalton Trans. (1995) 1525.
[15] W.A. Herrmann, W.R. Thiel, J.G. Kuchler, Chem. Ber. 123
(1990) 1953.
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: ꢀ44-1223-336-033; e-mail: deposit@
/
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
[16] W.A. Herrmann, W.R. Thiel, J.G. Kuchlere, J. Behm, E.
Herdtweck, Chem. Ber. 123 (1990) 1963.
[17] R.A. Sheldon, I.W.C.E. Arends, G.-J. Ten Brink, A. Dijksman,
Acc. Chem. Res. 35 (2002) 774.
Acknowledgements
[18] A. Held, F.M. Bauers, S. Mecking, Chem. Commun. (2000) 301.
[19] G.-J. ten Brink, I.W.C.E. Arends, R.A. Sheldon, Adv. Synth.
Catal. 344 (2002) 355.
Gratitude is expressed to Dr. Marc W. Perkovic for
his help with the X-ray crystal structure analysis.
Acknowledgment is made to the Donors of The
Petroleum Research Fund, administered by the Amer-
ican Chemical Society, for partial support of this
research (PRF no./ 32973-GB3) and to Western Michi-
gan University.
[20] A. Kundu, B.P. Buffin, Organometallics 20 (2001) 3635.
[21] B.P. Buffin, A. Kundu, Inorg. Chem. Commun. 6 (2003) 680.
[22] S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000)
1169.
[23] E. Wong, C.M. Giandomenico, Chem. Rev. 99 (1999) 2451.
[24] G. Natile, M. Coluccia, Coord. Chem. Rev. 216Á217 (2001) 383.
/
[25] G. Cavigiolio, L. Benedetto, E. Boccaleri, D. Colangelo, I. Viano,
D. Osella, Inorg. Chim. Acta 305 (2000) 61.
[26] H. Brunner, M. Schmidt, H. Schonenberger, Inorg. Chim. Acta
¨
123 (1986) 201.
References
[27] D.F. Shriver, The Manipulation of Air-Sensitive Compounds,
McGraw-Hill, New York, 1969.
[1] P.T. Anastas, R.L. Lankey (Eds.), Advancing Sustainability
Through Green Chemistry and Engineering, American Chemical
Society, Washington, DC, 2002.
[28] R.J. Angelici, Synthesis and Technique in Inorganic Chemistry,
2nd ed., W.B. Saunders, Philadelphia, PA, 1977.
[29] P.T. Beurskens, G. Admiraal, G. Beurskens, W.P. Bosman, S.
Garcia-Granda, R.O. Gould, J.M.M. Smits, C. Smykalla, The
DIRDIF Program System, Technical Report of the Crystallogra-
phy Laboratory, University of Nijmegen, Nijmegen, The Nether-
lands, 1992.
[2] P.T. Anastas, M.M. Kirchhoff, Acc. Chem. Res. 35 (2002) 686.
[3] P.T. Anastas, L.B. Bartlett, M.M. Kirchhoff, T.C. Williamson,
Catal. Today 55 (2000) 11.
[4] J.N. Armor, Appl. Catal. A: Gen. 189 (1999) 153.
[5] G. Jaouen, R.H. Fish, J. Organomet. Chem. 668 (2003) 1.
[6] G. Jaouen, A. Vessie´res, I.S. Butler, Acc. Chem. Res. 26 (1993)
361.
[30] G.M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structures, University of Gottingen, Germany, 1997.
¨
[31] K. Yang, R.J. Lachicotte, R. Eisenberg, Organometallics 16
(1997) 5234.
[7] K.V. Katti, H. Gali, C.J. Smith, D.E. Berning, Acc. Chem. Res.
32 (1999) 9.
[8] B. Cornils, W.A. Herrmann (Eds.), Aqueous-Phase Organome-
[32] H. tom Dieck, M. Svoboda, T. Greiser, Z. Naturforsch., Teil B:
Anorg. Chem., Org. Chem. 36B (1981) 823.
tallic Catalysis, WileyÁ
[9] F. Fache, E. Schulz, M.L. Tommasino, M. Lemaire, Chem. Rev.
100 (2000) 2159.
/
VCH, Weinheim, 1998.
[33] T.V. Laine, U. Piironen, K. Lappalainen, M. Klinga, E. Aitola,
M. Leskela, J. Organomet. Chem. 606 (2000) 112.
¨
[34] T.V. Laine, M. Klinga, M. Leskela, Eur. J. Inorg. Chem. (1999)
¨
959.
[10] A. Togni, L.M. Venanzi, Angew. Chem., Int. Ed. Engl. 33 (1994)
497.
[35] K. Kawakami, T. Ohara, G. Matsubayashi, T. Tanaka, Bull.
Chem. Soc. Jpn. 48 (1975) 1440.
[11] C. Borriello, M.L. Ferrara, I. Orabona, A. Panunzi, F. Ruffo, J.
Chem. Soc., Dalton Trans. (2000) 2545.
[36] B. Crociani, F. Di Bianca, R. Bertani, L. Zanotto, Inorg. Chim.
Acta 141 (1988) 253.
[37] B.P. Buffin, Unpublished results.
[12] M.L. Ferrara, F. Giordano, I. Orabona, A. Panunzi, F. Ruffo,
Eur. J. Inorg. Chem. (1999) 1939.
[13] M.L. Ferrara, I. Orabona, F. Ruffo, M. Funicello, A. Panunzi,
Organometallics 17 (1998) 3832.
[38] A. Bacchi, M. Carcelli, C. Pelizzi, G. Pelizzi, P. Pelagatti, S.
Ugolotti, Eur. J. Inorg. Chem. (2002) 2179.