Synthesis, characterization and X-ray structure of N-2-pyridylmethylidene-1 -phenylethylamine - PdCl2 complexes

Article abstract of DOI:10.1016/S0022-328X(00)00193-5

The PdCl₂ complexes with imines derived from reactions of 2-pyridinealdehyde with (R)-, (S)- and racemic 1-phenylethylamine have been synthesized and characterized by various physico-chemical methods. The (R)-1 and (S)-1 palladium complexes obtained from the (R)- and (S)- ligands were optical antipodes and differed only by the optical rotation sign and opposite circular dichromism spectra. Two crystals I and II resulted from these complexes were studied by the X-ray analysis. The first structure turned out to be racemic (it had the centrosymmetric space group P2₁/c) while the second one (space group P2₁) contained the molecules of a single enantiomer. The cis chelate coordination of the organic ligand and almost square-planar coordination geometry was found for both crystals.

Full text of DOI:10.1016/S0022-328X(00)00193-5

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Journal of Organometallic Chemistry 608 (2000) 1–5
Synthesis, characterization and X-ray structure of
N-2-pyridylmethylidene-1-phenylethylamine–PdCl2 complexes
Anatoly Mishnev, Irina Iovel, Juris Popelis, Ilze Vosekalna, Edmunds Lukevics *
Lat6ian Institute of Organic Synthesis, 21 Aizkraukles St., LV-1006 Riga, Lat6ia
Received 12 March 2000; received in revised form 13 March 2000
Abstract
The PdCl2 complexes with imines derived from reactions of 2-pyridinealdehyde with (R)-, (S)- and racemic 1-phenylethylamine
have been synthesized and characterized by various physico-chemical methods. The (R)-1 and (S)-1 palladium complexes obtained
from the (R)- and (S)- ligands were optical antipodes and differed only by the optical rotation sign and opposite circular
dichromism spectra. Two crystals I and II resulted from these complexes were studied by the X-ray analysis. The first structure
turned out to be racemic (it had the centrosymmetric space group P21/c) while the second one (space group P21) contained the
molecules of a single enantiomer. The cis chelate coordination of the organic ligand and almost square-planar coordination
geometry was found for both crystals. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Optically active Schiff base; Palladium(II) complex; Structure
tion of aldehydes in the presence of RhCl3,
[Rh(COD)Cl]2, AlCl3 complexes with the optically ac-
tive N,N%-bi- and N,N%,N%%-tridentate chelating ligands
based on the pyridine framework) and taking into
account our prior interest in palladium(II) complex
catalysts [17] we are reporting herein the preparation
and investigation of PdCl2 complexes with PPEI. Re-
ports in recent literature on the studies concerning the
structures of complexes with the pairs of enantiomers
are scarce. In this work we have synthesized and stud-
ied the palladium complexes with (R)-, (S)- and
racemic ligands.
1. Introduction
Complexes of transition metals with optically active
ligands, including Schiff bases, have been used as chiral
inductors in a large number of asymmetric catalytic
processes [1–3]. Optically active (R)- and (S)-N-2-
pyridylmethylidene-1-phenylethylamine (named in Ref.
[1] as 2-pyridinal-1-phenylethylimine (PPEI)) was found
to be an efficient ligand of Rh(I) and Ir(I) complexes
for enantioselective hydrosilylation and hydrogen trans-
fer reduction, respectively [4–7]. The complexes of this
ligand with a number of transition metals were known
earlier: the tetragonal pyramidal compounds [M(h5-
C5H5)(PPEI)(CO)2]PF6 (M=V [8], Mo and W [9–11])
and the analogous cobalt derivatives [12]. The structure
of one of them was determined by X-ray analysis [11].
The structure of this ligand complex with one metal of
the platinum group was previously reported [6]: PPEI-
(COD)IrI, synthesized from (S)-PPEI, [Ir(COD)Cl]2
and NaI (COD=cis,cis-1,5-cyclooctadiene).
2. Results and discussion
2.1. Synthesis and characterization of the PPEI ligands
and the (R)-1, (S)-1 and racemic 1 complexes
The above named complexes were prepared from
K2[PdCl4] [18] and the corresponding PPEI [4] (Scheme
1).
In a continuation of our studies [13–16] in the area
of asymmetric catalysis (hydrosilylation and hydrogen
transfer reduction of ketones and trimethylsilylcyana-
Three prepared complexes and the (R)-, (S)-PPEI
ligands were investigated by means of IR, UV, CD,
1
MS, H-NMR, determination of optical rotation and
1
* Corresponding author. Fax: +371-7550338.
elemental analysis. The IR and H-NMR spectra are
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00193-5

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A. Mishne6 et al. / Journal of Organometallic Chemistry 608 (2000) 1–5
Table 3
Specific optical rotation of amines, imines and complexes 1
20
546
Compound
[h]
(°)
Conditions
(R)-PEA a
(R)-PPEI
(R)-1
(S)-PEA
(S)-PPEI
(S)-1
+35 [20]
− 55
+140
c=10, ethanol
c=10, acetone
c=1, chloroform
c=10, ethanol
c=10, acetone
c=1, chloroform
Scheme 1. Synthesis of complexes 1.
−35 [20]
+56 b
Table 1
−140
IR spectra in the 1490–1800 cm−1 region
a PEA-1-phenylethylamine.
a
Compound
Assignments (w, cm−1
)
b [h]25406=+55.7° (c=51.0, acetone) [4].
Pyridine ring
CꢀN
The CD spectra of both (R)- and (S)- complexes
shown in Fig. 1 confirm that these coordination com-
pounds are optical antipodes. All optically active bands
could also be followed up in the UV spectra, which
were identical for two enantiomers.
PPEI
1 b
Dw
1495 s
1498 m
+3
1570 s
1600 m
+30
1590 s
1623 w
+33
1650 s
1687 m, br s
+37
a According to [19]; s-strong, m-medium, w-weak, br – broad.
b The identical spectra of (R)-, (S)- and racemic 1 were found.
2.2. Molecular and crystal structure
1
listed in Tables 1 and 2. H-NMR spectra of the PPEI
Single crystals I and II, suitable for X-ray structure
determination were obtained from (R)-1 and (S)-1
complexes, respectively. The attempts to prepare a crys-
tal of a complex synthesized from racemic PPEI suit-
able for X-ray analysis were not successful. The
structure of the palladium complexes I and II are
shown in Figs. 2–4. The selected bond distances and
angles of the crystal I are given in Table 4. Contrary to
expectation the crystals afforded from two optical an-
tipodes in the same conditions were not antipodes. It
turned out that crystal I obtained from (R)-1 was
racemic (centrosymmetric space group P21/c), while
crystal II grown from (S)-1 contained the molecules of
one enantiomer (non-centrosymmetric space group
P21).
ligands were comparable with that given in [4] for the
(S)-isomer. The comparison of the spectra of the lig-
ands with the spectra of the complexes evidenced for
the coordination of Pd atom with N atoms of pyridine
and imine groups as expected.
It is worthy to note the changes in the sign and
optical rotation value observed in amine–imine-com-
plex consecutive transformations (Table 3). The PPEI
obtained by condensation, e.g. of (R)-(+)-PEA with
2-pyridinecarbaldehyde was levorotary while the com-
plexation with PdCl2 changed the sign of optical rota-
tion, and also increased the optical rotation value.
However, it is not a general rule for conversions of all
optically active amines in condensations and complexa-
tions, as was affirmed in Ref. [21]. The signs of optical
rotation of some primary amines remain unchanged
after the condensation with aldehyde [22].
The X-ray study of the crystals I and II confirmed
the cis chelate coordination of the ligand and almost
square-planar coordination geometry. In structure I the
Table 2
1H-NMR (360 MHz, CDCl3, TMS, 303 K; l in ppm, J in Hz)
Compound
PPEI
CH3
CHCH3
C6H5
Pyridine ring
CH=N
H-3
H-4
H-5
H-6
1.61
d
4.64
q
7.2–7.5
m
8.09
ddd
7.72
m
7.29
ddd
8.63
m
8.46
m
J=6.9
J=6.9
J1=7.7
J2=1.4
J3=1.0
J1=7.7
J2=1.7
J1=7.7
J2=4.9
J3=1.4
J1=4.9
J2=1.7
J3=1.0
JB1.0
1 a
1.85
d
5.99
q
7.3–7.5
m
7.90
m
8.09
m
7.64
ddd
9.23
m
8.03
m
J=6.9
J=6.9
J=7.6
J1=7.6
J2=1.5
J1=7.6
J2=5.6
J3=1.5
J1=5.6
J2=1.5
JB1.0
a The identical spectra of (R)-, (S)- and racemic 1 were found.

A. Mishne6 et al. / Journal of Organometallic Chemistry 608 (2000) 1–5
3
Fig. 1. CD spectra of (R)-1 and (S)-1 complexes.
,
antipodes differing only by the optical rotation sign and
opposite circular dichromism spectra. However,
the X-ray analysis of crystals I and II obtained from
these two complexes by their recrystallization under the
same conditions from chloroform–hexane–1,4-dioxane
at ambient temperature has revealed that one of them
was racemic while another contained the molecules of
a single enantiomer. It shows that optical antipodes
may have different tendencies to racemization. Possi-
bly, easy racemization of some complexes is a reason
for their low enantiomeric selectivity in catalytic pro-
cesses.
PdꢁCl1 (2.286(2) A) distance was longer than PdꢁCl2
,
,
(2.270(2) A). In turn PdꢁN1 (2.014(5) A) was slightly
,
shorter than PdꢁN2 (2.032(4) A). The smallest bond
angle at Pd atom was N1ꢁPdꢁN2=80.7(2)°. The sum
of bond angles around Pd atom was just 360°. Pyridine
plane formed a dihedral angle of 6.1° with the coordi-
nation plane. The phenyl ring was turned out from the
coordination plane by a dihedral angle of 74.8°.
In crystal the molecular fragments of complex 1
formed stacks with PdꢁPd intermolecular distances of
4.376 A. Channels between the stacks were filled with
1,4-dioxane molecules fixed close to each other by two
electron-rich phenyl groups from the adjacent stacks
(Fig. 3). Possibly, the solvate molecules stabilized the
centrosymmetric structure of the crystal I.
,
In the structure of the crystal II three independent
molecules (a, b, c) have been found in the asymmetric
unit cell (Fig. 4). Insufficient accuracy of refinement for
crystal II, resulting from the poor quality of the crystals
used, does not permit the discussion of its geometry in
detail. However, it is worth noting that all three
molecules had the same absolute configuration with
respect to the asymmetric atom C(7) and differed in the
orientation of the phenyl rings. The solvate molecules
were absent in the structure of crystal II.
Thus, two (R)-1 and (S)-1 palladium complexes syn-
thesized from the known PPEI ligand were optical
Fig. 2. Molecular structure of the crystal I.

4
A. Mishne6 et al. / Journal of Organometallic Chemistry 608 (2000) 1–5
Table 4
Selected geometric data for crystal I
,
Bond distances (A)
PdꢁN1
PdꢁN2
2.014(5)
2.032(4)
PdꢁCl2
PdꢁCl1
2.270(2)
2.286(2)
Bond angles (°)
N1ꢁPdꢁN2
N1ꢁPdꢁCl2
N2ꢁPdꢁCl2
N1ꢁPdꢁCl1
N2ꢁPdꢁCl1
80.7(2)
176.3(1)
95.9(1)
93.1(1)
173.4(1)
C5ꢁN1ꢁPd
C1ꢁN1ꢁPd
C6ꢁN2ꢁPd
C7ꢁN2ꢁPd
Cl1ꢁPdꢁCl2
129.2(4)
113.2(3)
113.0(4)
125.8(3)
90.3(8)
spectroscopy in CH2Cl2. The spectra were measured on
a Dichrograph J700 (Jasco) within the wavelength
range of 200–500 nm at 22–25°C. The Dichrograph
was calibrated by the standards of epiandrosterone and
10-camphorsulphonic acid. To record the spectra
quartz cells of Helma were used: 0.01 cm for the
spectral range 230–500 nm and 0.005 cm for the range
200–350 nm. The concentration of the solutions was
10−2–10−3 M. The molar ellipticity [q] was measured
as degree cm2 dmol−1. The UV spectra were recorded
on the spectrophotometer Lambda2-S (Perkin–Elmer)
within the wavelength range of 200–600 nm; the same
quartz cells as for recording of CD spectra were used.
Fig. 3. Diagram of the crystal packing for I (1.1/2 1,4-dioxane
solvate).
3.3. X-ray crystallographic study
Two single crystals I and II obtained from (R)-1 and
(S)-1, respectively, were studied by the X-ray analysis.
The data were collected on a Syntex P21 automatic
diffractometer (MoꢁKa radiation, graphite monochro-
mator) by q/2q method. The cell constants were ob-
tained from the least-squares refinement on the setting
angles of 20 reflections in the range of 20B2qB25°.
No absorbtion correction was used. Crystallographic
data for both structures are given in Table 5. The
structures were solved by a direct method using the
program SHELXS-86 [23] and refined by the full-matrix
least-squares method on F2 (SHELXL-93) [24] anisotrop-
ically for non-hydrogen atoms. H atoms were con-
strained to ride on the adjacent non-H atoms.
Fig. 4. Molecular structure of the crystal II.
3. Experimental
3.1. General methods and materials
The following instruments were used to characterize
the compounds: Perkin–Elmer 580 B IR spectrometer;
Bruker AM-360 (360 MHz) NMR spectrometer, CDCl3
as a solvent, TMS as an internal standard; Kratos
MS-25 GC–MS (70 eV) spectrometer and MS-50 (70
eV) mass spectrometer. All starting chemicals were
purchased from Fluka or Merck and were of commer-
cial grade.
3.4. K2[PdCl4] and PPEI (R-, S- and racemic)
K2[PdCl4] and PPEI (R-, S- and racemic) were ob-
tained using the slightly modified methods described in
Refs. [4,18]. Mass spectra of PPEI: MS (70 eV), m/z
(%): 210 (12) [M+], 195 (51) [M+−CH3], 133 (10)
[M+−C6H5], 105 (100) [C5H4NꢁCHꢀN+], 92 (18), 79
(22), 78 (15) [C5H4N+], 77 (28) [C6H+5 ], 65 (12), 51 (21),
39 (13), 28 (22). For other data, see Tables 1–3.
3.2. CD and UV study
Two enantiomeric (R)-1 and (S)-1 complexes were
investigated by means of circular dichromism (CD)

A. Mishne6 et al. / Journal of Organometallic Chemistry 608 (2000) 1–5
5
Table 5
Crystal data and structure refinement for I and II
4. Supplementary material
The crystallographic data (excluding structure fac-
tors) for the structures reported in this paper were
deposited with the Cambridge Crystallographic Center.
The deposition numbers are CCDC 114914 and 114915.
The copies of these data can be obtained free of charge
from: The Director, CCDC, 12 Union Road, Cam-
bridge, CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail:
teched@chemcrys.cam.ac.uk or www: http://www.ccdc.
cam.ac.uk).
I
II
Empirical formula
C
14H14N2PdCl2·
3C14H14N2PdCl2
121,4-dioxane
431.62
Formula weight
Crystal system
Color
Space group
Crystal size (mm)
1162.83
Monoclinic
Light orange
P21/c
Monoclinic
Dark orange
P21
0.25×0.30×0.75 Shapeless
,
a (A)
14.795(9)
7.627(4)
16.409(10)
111.41(5)
1723.8
4
1.663
47.14
2535
14.608(3)
8.495(2)
18.765(4)
103.34(3)
2265.8(4)
6
1.704
50.14
3523
,
b (A)
,
c (A)
i (°)
References
3
,
V (A )
Z
Dcalc (Mg m−3
2qmax (°)
)
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with J\2|(J)
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ml) was treated with a slight excess of (R)-PPEI (0.22 g,
1.05 mmol) in 10 ml 1,4-dioxane. The product precipi-
tated after 8 h stirring at ambient temperature. The
precipitate was filtered off, washed with water, then
acetone and hexane, and dried in a vacuum-excitator.
Complex (R)-1 (0.33 g, 85%) was prepared as a micro-
crystalline yellow substance (m.p. 186–188°C (decom-
position)), the further investigations revealed that this
compound was stable in air and water. The complexes
(S)-1 and racemic 1 obtained in this manner had the
m.p. 184–185 and 189–201°C, respectively. The at-
tempts to obtain the MS (EI, FAB) spectra of PdCl2–
PPEI complexes were not successful; decomposition of
complex molecules took place. The characteristics of
the complexes synthesized are given in Tables 1–3. The
results of the elemental analysis for all three complexes
were found in good agreement with the calculated data.
Single orange crystals I and II, suitable for the X-ray
diffraction analysis were obtained by recrystallization
of (R)-1 and (S)-1 complexes from 1:1:0.1 chloroform–
hexane–1,4-dioxane for 10 h at ambient temperature.
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