Revealing the diastereomeric nature of pincer terdentate nitrogen ligands 2,6-bis(arylaminomethyl)pyridine through coordination to palladium

Article abstract of DOI:10.1039/b201319c

Palladium complexes of formula [PdLCl]X (X = Cl 1, BF₄ 2) containing the pincer terdentate ligands L [L = 2,6-(ArNHCH₂)₂C₅H₃N (Ar = 4-MeC₆H₄ 1a and 2a, 4-MeOC₆H₄ 1b and 2b); L = 2,6-(4-MeC₆H₄- NMeCH₂)₂C₅H₃N 1c and 2c] have been synthesised as cis-trans diastereomeric mixtures. The chiral nature of both amine arms of the pincer ligands accounts for the mixtures of isomers. All the complexes show dynamic behavior assigned to conversion between diastereomers. A ΔG₂₇₃ of 68.9 kJ mol^-1 for 2a and a ΔG₂₇₃ of 65.8 kJ mol^-1 for 2c were found through complete DNMR line-shape analysis of the -CH₂- signals in the ¹H spectra. The cis-trans conversion requires the inversion of the pyramidal nitrogen of one of the coordinated amine arms. Two putative pathways have been proposed for such inversion: a) dissociation of one of the amine nitrogen atoms followed by inversion of the nitrogen atom and reformation of the nitrogen-palladium bond and b) deprotonation of one of the N-H bond leading to an amido intermediate which can reprotonate on the other side. Experimental evidence is given for both mechanisms. The structural characterization of 1a is described.

Full text of DOI:10.1039/b201319c

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Revealing the diastereomeric nature of pincer terdentate nitrogen
ligands 2,6-bis(arylaminomethyl)pyridine through coordination to
palladium†
Ana Arnáiz,a José V. Cuevas,a Gabriel García-Herbosa,*a Arancha Carbayo,a Juan A. Casaresb
and Enrique Gutierrez-Pueblac
a Departamento de Química, Facultad de Ciencias, Universidad de Burgos, 09001 Burgos, Spain.
E-mail: gherbosa@ubu.es
b Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid,
47005 Valladolid, Spain
c Instituto de Ciencia de Materiales, CSIC, Cantoblanco s/n, 28049 Madrid, Spain
Received 5th February 2002, Accepted 16th April 2002
First published as an Advance Article on the web 15th May 2002
Palladium complexes of formula [PdLCl]X (X = Cl 1, BF4 2) containing the pincer terdentate ligands L
[L = 2,6-(ArNHCH2)2C5H3N (Ar = 4-MeC6H4 1a and 2a, 4-MeOC6H4 1b and 2b); L = 2,6-(4-MeC6H4-
NMeCH2)2C5H3N 1c and 2c] have been synthesised as cis–trans diastereomeric mixtures. The chiral nature
of both amine arms of the pincer ligands accounts for the mixtures of isomers. All the complexes show
dynamic behavior assigned to conversion between diastereomers. A ∆G‡273 of 68.9 kJ molϪ1 for 2a and a ∆G‡
of 65.8 kJ molϪ1 for 2c were found through complete DNMR line-shape analysis of the –CH2– signals in the
273
1H spectra. The cis–trans conversion requires the inversion of the pyramidal nitrogen of one of the coordinated
amine arms. Two putative pathways have been proposed for such inversion: a) dissociation of one of the amine
nitrogen atoms followed by inversion of the nitrogen atom and reformation of the nitrogen–palladium bond
and b) deprotonation of one of the N–H bond leading to an amido intermediate which can reprotonate on
the other side. Experimental evidence is given for both mechanisms. The structural characterization of 1a is
described.
Introduction
Results and discussion
The interest in multidentate nitrogen-donor ligands and their
use in homogeneous catalysis has increased in the last few
years.1–3 There are several recent examples of processes
catalyzed by complexes bearing terdentate nitrogen-donor
ligands such as hydroalkoxylation,4 Suzuki coupling,5 polymer-
ization of ε-caprolactam,6 oxidation of benzylic alcohol to
benzaldehyde,7 hydrogen peroxide decomposition,8 olefin
polymerization,9–12 or alkene epoxidations.13 Complexes involv-
ing terdentate nitrogen-donor ligands show interesting reactiv-
ity such as CO insertion in Pd–Me1,14 bonds and alkene
insertion in acyl–palladium bonds.14,15 In some of these
processes the ligands display dynamic behavior16–18 that could
be interesting for the study of the catalytic properties of these
complexes, such as better stereoselective or chemoselective con-
trol of the reactions.19–21 Terdentate nitrogen ligands are also of
interest in the synthesis of rod-like extended materials22 and in
selective complexation reactions,23 useful for the separation of
actinides and lanthanides by solvent extraction.24
Synthesis of terdentate ligands
As shown in Scheme 1, the terdentate ligands 2,6-bis(aryl-
1
Here we report the synthesis, full characterization and H
NMR dynamic study in solution of palladium complexes with
terdentate ligands 2,6-bis(arylaminomethyl)pyridine. The
pyramidal nature of the nitrogen atoms of the amine arms of
these pincer ligands introduce stereochemical features onto
square-planar complexes which are absent in the related
terdentate pyridine-diimine ligands.17,25
Scheme 1 Synthesis of terdentate ligands. i) toluene, reflux; ii) NaBH4,
methanol; iii) 2.6 equiv. n-BuLi, THF; iv) 2.6 equiv. MeI.
aminomethyl)pyridine were prepared in two steps. The
condensation in toluene of ArNH2 (Ar = 4-MeC6H4, La;
4-MeOC6H4, Lb) with 2,6-bis(carbaldehyde)pyridine afforded
† Electronic supplementary information (ESI) available: input informa-
tion included in the computer simulation. See http://www.rsc.org/
suppdata/dt/b2/b201319c/
DOI: 10.1039/b201319c
J. Chem. Soc., Dalton Trans., 2002, 2581–2586
This journal is © The Royal Society of Chemistry 2002
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Table 1 Selected bond distances (Å) and angles (Њ) for complex 1a
the corresponding 2,6-bis(aryliminomethyl)pyridine. Reduction
of these terdentate imines with NaBH4 and further hydrolysis
afforded good yields of the terdentate amine ligands.
Upon treatment with n-BuLi and then with methyl iodide the
amine 2,6-bis(p-tolylaminomethyl)pyridine was converted into
the N-methylated product, Lc.
Pd(1)–N(1)
Pd(1)–N(2)
1.952(8)
2.068(9)
Pd(1)–N(3)
Pd(1)–Cl(1)
2.059(9)
2.295(2)
N(1)–Pd(1)–N(3)
N(3)–Pd(1)–N(2)
N(3)–Pd(1)–Cl(1)
80.8(4)
162.0(3)
97.2(2)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–Cl(1)
N(2)–Pd(1)–Cl(1)
81.2(4)
177.7(3)
100.9(2)
Synthesis of complexes [PdLCl]X {X ؍ Cl 1, BF4 2; L ؍ 2,6-
(ArNHCH2)2C5H3N (Ar ؍ 4-MeC6H4 1a and 2a, 4-MeOC6H4
1b and 2b); L ؍ 2,6-(4-MeC6H4NMeCH2)2C5H3N 1c and 2c}
closes the angles N(3)–Pd(1)–N(1) and N(1)–Pd(1)–N(2) at
about 81.0Њ, opening the angles N(3)–Pd(1)–Cl(1) and N(2)–
Pd(1)–Cl(1) at about 99.0Њ. In this structure, the p-tolyl groups
are oriented cis with regard to the coordination plane. There are
two hydrogen bonds between the Cl(2) and N(2) and N(3)
atoms, through the H(2) and H(3) hydrogen atoms, respectively.
Simple molecular models show that this arrangement is steri-
cally more constrained than trans, leading to close contact
between the aryl groups. In order to avoid this contact both
aminic nitrogen atoms undergo a distortion in the hybridization
leading to an intermediate situation between sp3 and sp2 with
more sp3 character. This geometric distortion, that is stronger in
the solid state for N(2), can be calculated by addition of the
values of the bonding angles around the nitrogen atoms. These
values are 342.0Њ for N(2) and 332.6Њ for N(3) both of them
between the theoretical 360Њ for sp2 hybridization and 328.2Њ for
sp3.
In spite of the square-planar geometry of the complexes here
described, the geometrical requirements of the terdentate
ligands and their symmetry (Cs or C2) supply interesting
stereochemical features. The molecular structure shown in
Fig. 1 corresponds to a chiral complex with C1 symmetry. The
lack of a plane of symmetry perpendicular to the coordination
plane is due to the non-planarity of the chelate rings. From
molecular models it is clear that both chelating rings have
to adopt the same conformation (i.e. δδ or λλ) to keep the
planarity of the pyridine ring. As a consequence of such a
requirement C(1) and C(7) lie on opposite sides of the coordin-
ation plane. The cis disposition of the aryl groups implies that
N(2) and N(3) must to have different R,S configurations. The
complex shown in Fig. 1 has Rδ,Sδ configuration and since the
space group P21/n is centric, both enantiomers Rδ,Sδ and
Rλ,Sλ are present in equal numbers in the unit cell.
As shown in Scheme 2, addition of a solution of the amine in
Scheme 2 Synthesis of complexes. i) terdentate ligand, CH2Cl2;
ii) TlBF4, methanol.
dichloromethane to
a solution of cis-[PdCl2(PhCN)2] in
dichloromethane led to substitution of the weakly coordinating
benzonitrile ligand by the chelating amine ligand in an almost
quantitative yield. Treatment of complexes 1a–c in methanol
with TlBF4 afforded the corresponding salts of tetrafluoro-
borate 2a–c. Complexes 1a and 1b were obtained as yellow
solids which are poorly soluble in acetone, dichloromethane
and chloroform, slightly soluble in methanol and ethanol and
soluble in dimethylsulfoxide. Replacement of N–H by N–Me
groups led to higher solubility of complex 1c when compared
with that of 1a and 1b. Replacement of the chloride counterion
by tetrafluoroborate led also to higher solubility of complexes
2a–c in common solvents when compared with that of
complexes 1a–c.
The simple structural behavior in the solid state contrasts
with the behavior in solution. Whereas only one conformer
(Rδ,Sδ) is expected for cis-1 in solution (the conformer Rδ,Sλ
is sterically forbidden as mentioned above), for trans-1 two
conformers can be expected (Rδ,Rδ and Rλ,Rλ) although
the barriers for their exchange should be very small and the
unobserved transformation very fast even at low temper-
atures.26,27 However, the cis–trans exchange can be followed by
dynamic NMR line-shape analysis as discussed below.
Structural characterization of [Pd{2,6-(4-MeC6H4NHCH2)2-
C5H3N}Cl]Cl 1a and stereochemistry of ligands and complexes
A single crystal of 1a suitable for X-ray diffraction was
obtained by layering a solution of the compound in ethanol
with n-hexane. The corresponding ORTEP diagram is shown in
Fig. 1. Selected bond lengths and angles are collected in Table 1.
The palladium atom has a distorted square-planar geometry.
The chelating ligand is acting as an N,N,N-chelate and this
Structure and dynamic behavior in solution of complexes 1 and 2
1
The H NMR spectra of complexes 1a and 1b in deuterated
dimethylsulfoxide show broad and complicated signals in the
methylene region between 4.5 and 5.5 ppm (the –CH2– region)
suggesting that we have two species in solution and that prob-
ably these species are in equilibrium. The highly coordinating
ability of the solvent can be responsible, at least in part, for
such less informative spectra. The chloride counterion has
coordinating ability and it can also account for the origin of the
uncertainty in interpreting the spectra. In order to gain more
information about the structure of the complexes in solution we
decided upon the substitution of the chloride counterion by
tetrafluoroborate and this met with success.
On one hand, the higher solubility of complexes 2a and 2b
allows the use of the less coordinating solvent acetone but on
Ϫ
the other, the BF4 counterion is not prone to participate in
Fig. 1 ORTEP34 drawing of [Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}-
Cl]Cl 1a. Only hydrogen atoms attached to N(2) and N(3) are shown for
clarity.
substitution processes whereas chloride can participate, as has
been reported in related cases.17 1H NMR spectra were recorded
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J. Chem. Soc., Dalton Trans., 2002, 2581–2586

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for 2a and 2b using both deuterated dimethylsulfoxide and
deuterated acetone. The 1H NMR spectra of the methylene
region for complex 2a in deuterated dimethylsulfoxide and
acetone are shown in Fig. 2.
consequence of a cis–trans transformation that is faster at
higher temperatures. Spectral simulation afforded the values of
the kinetic constants (major to minor isomer) at different tem-
peratures.28 The correlation between the temperature and the
kinetic constants kmajor–minor gave the activation parameters from
the Eyring representation as ∆H‡ = 46.5(2.5) kJ molϪ1, ∆S‡ =
Ϫ82(7) J molϪ1 KϪ1 and ∆G‡273 = 68.9(4.4) kJ molϪ1
.
The influence of DMSO-d6 on the exchange process has been
studied for complex 2a in CD3COCD3 at 20 ЊC. Addition of a
small amount of DMSO-d6 (5%) leads to an increase of the
exchange rate (k = 6 sϪ1). Further addition of DMSO-d6 does
not increase this value.
1
In light of the results obtained for 2 we suggest that the H
NMR spectra of 1 correspond to the same behavior, but their
low solubility in non-freezing solvents prevented us performing
similar dynamics studies for 1.
Attempts to grow crystals of 2 for X-ray diffraction studies
were unsuccessful.
When N-methylated complexes 1c and 2c were studied we
could see a similar behavior to that reported above for 1a and
1b and 2a and 2b. The higher solubility of complex 1c, com-
pared to 1a and 1b, allows the use of mixtures of deuterated
acetone with deuterated dimethylsulfoxide whereas for com-
plexes 1a and 1b pure DMSO-d6 had to be used in order to
dissolve the solids. Fig. 4 shows the variation of the resonances
of the methylenic protons with temperature for complex 2c.
Fig.
2
Methylene region of the 1H NMR spectra (400 MHz)
of [Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}Cl](BF4) (2a) at room
temperature in a) DMSO-d6 and b) CD3COCD3.
The spectra recorded in DMSO-d6 show a better resolution
for 2a and 2b than for 1a and 1b indicating that the chloride
counterion in complexes 1a and 1b is likely to be involved in
substitution reactions. The spectrum of 2a in deuterated acet-
one displays a pattern of four double doublets (sixteen signals)
assigned to two ABX(–CH2–NH–) systems. A COSY experi-
ment confirms that the external set of two double doublets
belong to one compound, and the internal set of two double
doublets belong to the other compound. The 1H NMR spectra
corresponds to a mixture of cis and trans isomers of 2a where
cis and trans refers to the disposition of the aryl groups with
regard to the coordination plane. Analysis of the integral values
gives an isomeric ratio of 1.5 : 1 but, with the NMR inform-
ation, it is not possible to distinguish which is the major isomer
(cis or trans).
1
Variable temperature H NMR spectra of complex 2a in a
mixture of deuterated acetone–dimethylsulfoxide are collected
in Fig. 3. The change of the signal shape can be explained as a
Fig. 4 Methylene region of the variable-temperature 1H NMR spectra
(400 MHz) of [Pd{2,6-(4-MeC6H4NMeCH2)2C5H3N}Cl](BF4) (2c) in
CD3COCD3 in the temperature range Ϫ20 to 50 ЊC.
Complex 1c also shows broadening of the signals at high
temperatures (likely due to dynamic behavior), but the signals
were too broad to develop an accurate calculation. Again, the
Ϫ
substitution of the chloride counterion with BF4 led to more
informative spectra for 2c. At 253 K it is possible to see two
doublets at 4.98 and 5.10 ppm and an apparent triplet at 5.35
ppm. This triplet is actually two doublets with two signals over-
lapped. Upon heating the sample to 323 K the signals broaden
and finally resolve into two broad signals centered at 5.18 ppm.
The lineshape analysis carried out, under the same conditions
as for 2a, gave the activation parameters from the Eyring repre-
sentation as ∆H‡ = 59.8(3.8) kJ molϪ1, ∆S‡ = Ϫ22(12) J molϪ1
KϪ1 and ∆G‡ = 65.8(7.1) kJ molϪ1, (standard deviations in
273
parentheses).
In complexes with an N–H bond (1a and 1b and 2a and 2b)
two mechanisms, which are shown in Scheme 3, can be invoked
to explain the cis–trans isomerization. The first mechanism
involves dissociation of one of the amine nitrogens which may
be initiated by associative coordination of chloride or solvent in
complexes 1 or by solvent alone in complexes 2.29 The next steps
are the inversion of the nitrogen atom and the reformation of
the nitrogen–palladium bond. The second mechanism starts
with a cis isomer that loses a proton from a coordinated amine
Fig. 3 Methylene region of the 1H NMR spectra (300 MHz) of
[Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}Cl](BF4) (2a) in DMSO-d6–
CD3COCD3 (0.7 : 0.3) in the temperature range Ϫ17 to 90 ЊC. The N–H
signal was irradiated.
J. Chem. Soc., Dalton Trans., 2002, 2581–2586
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second involves deprotonation of one of the N–H bonds and
reprotonation on the other side with inversion of the nitrogen
atom.
Experimental
General
All manipulations were carried out using standard Schlenk
techniques. Solvents were dried and stored under nitrogen. The
starting compounds 2,6-pyridinedicarbaldehyde30 and cis-
[PdCl2(PhCN)2]31 were prepared according to the literature. All
other starting chemicals were used as commercially obtained.
1H NMR spectra were recorded on either Bruker ARX 300,
Bruker AC 80 or Varian Unity Inova-400 spectrometers. Chem-
ical shifts are reported in ppm relative to SiMe4 (TMS) as
external standard. Variable-temperature 1H NMR spectra were
obtained on Bruker ARX 300 and Varian Unity Inova-400
instruments, the variable-temperature units were calibrated
using methanol in methanol-d4 for low temperatures and ethyl-
ene glycol in DMSO-d6 for high temperatures. Kinetic data
1
were derived from band shape analysis of H NMR spectra
using a version of the standard DNMR program28 for [Pd{2,6-
(4-MeC6H4NHCH2)2C5H3N}Cl](BF4) 2a and a version of the
gNMR program32 for [Pd{2,6-(4-MeC6H4NMeCH2)2C5H3N}-
Cl](BF4) 2c.† Activation parameters were calculated from a
least-squares fit of the Arrhenius and Eyring plots. Quoted errors
are statistical errors on scattering of the rate constants around
the straight line only. IR spectra were recorded as KBr disks on
a Nicolet Impact 410 instrument, elemental analyses were made
on a Leco CHNS 932, GC analysis was on a Hewlett-Packard
serie II chromatograph and mass spectra were obtained on a
Hewlett-Packard 5791A with a mass selective detector.
Scheme 3
Synthesis
to form an amido complex. This spontaneous deprotonation
can be helped by the presence of water as the deuterated
solvents used always showed small amounts of water in the
spectra. After this deprotonation the nitrogen atom can be
reprotonated and, depending on the side of reprotonation, it
generates the cis or the trans isomer. It has been observed that
the addition of 0.6 equivalents of proton sponge to a solution
of 2a in DMSO-d6–CD3COCD3 (0.7 : 0.3) led to a seven-
fold increase of the exchange rate which supports the second
mechanism.
An interesting observation is that the chloride counterion
broadens the signals of the methylene groups compared to the
BF4Ϫ counterion. This fact can be related to a faster process in
complexes with chloride as counterion supporting the associ-
ative mechanism 1. The calculated values of the entropy of
activation (they are negative) support the first mechanism as
well, but mechanism 2 can not be ruled out for those complexes
containing N–H bonds. In the presence of base, mechanism 2
dominates over mechanism 1 but both pathways are involved in
these processes. Obviously, only mechanism 1 is possible for
complexes 1c and 2c.
2,6-Bis[{N-(4-methylphenyl)imino}methyl]pyridine,
2,6-(4-
MeC6H4NCH)2C5H3N. A 250 ml flask equipped with a stir bar
and a Dean–Stark glassware was charged with 2,6-pyridine-
dicarbaldehyde (2 g, 14.8 mmol), p-toluidine (p-methylaniline)
(3.19 g, 29.6 mmol) and a catalytic amount of p-toluenesulfonic
acid in toluene (50 cm3). It was refluxed for 2 h.33 The solution
was evaporated to dryness. The precipitate was recrystallized
in hexane and dried in vacuo yielding a white solid product.
1
(3.66 g, 79%). H NMR (80 MHz, CDCl ): δ 8.68 (s, 2H, N᎐
᎐
3
CH), 8.26 (2H, m, py H3–5), 7.88 (1H, m, py H4), 7.23 (8H, m,
Ar), 2.37 (6H, s, Me); IR (KBr): νC᎐N = 1624 cmϪ1; Found:
᎐
C, 80.12; H, 6.05; N, 13.09. C21H19N3 requires C, 80.48; H, 6.11;
N, 13.41%.
2,6-Bis[{N-(4-methoxyphenyl)imino}methyl]pyridine, 2,6-(4-
MeOC6H4NCH)2C5H3N. The synthesis was carried out accord-
ing to the procedure followed above for 2,6-(4-MeC6H4NCH)2-
C H N. 1H NMR (80 MHz, CDCl ): δ 8.70 (s, 2H, N᎐CH), 8.25
᎐
5
3
3
(2H, m, py H3–5), 7.89 (1H, m, py H4), 7.16 (8H, m, Ar), 3.64
(6H, s, MeO); IR (KBr): νC᎐N = 1625 cmϪ1; Found: C, 72.79;
᎐
H, 5.85; N, 11.88. C21H19N3O2 requires C, 73.03; H, 5.54;
N, 12.17%.
Conclusion
Chloro-palladium square-planar complexes containing pincer
terdentate pyridine-diamine ligands 2,6-bis(arylaminomethyl)-
pyridine have been synthesised and characterized as cis–trans
diastereomeric mixtures. The geometry depends on the arrange-
ment of the aryl groups with regard to the coordination plane.
Both diastereomers are chiral. The chirality in the cis isomers
arises from the non-planarity of the chelate rings. The mixtures
of isomers show dynamic behavior due to cis–trans conversion.
Two mechanisms are possible for such conversion. The first
involves nitrogen dissociation, which may be initiated by chlor-
ide or solvent, subsequent inversion of the uncoordinated
nitrogen and reformation of the nitrogen–palladium bond. The
2,6-Bis[{N-(4-methylphenyl)amino}methyl]pyridine, 2,6-(4-
MeC6H4NHCH2)2C5H3N, La. To a stirred solution of 2,6-(4-
MeC6H4NCH)2C5H3N (2.44 g, 9.86 mmol) in methanol
(60 cm3) at 40–50 ЊC was slowly added NaBH4 (0.93 g, 24.6
mmol).33 The solution was then refluxed for 1.5 h and, after
cooling, water (50 cm3) was added. The methanol was removed
in a rota-vapor. The organic product was extracted with diethyl
ether (3 × 30 cm3) and dried over anhydrous MgSO4. The fil-
tered solution was evaporated to dryness. The residue was
recrystallized from dichloromethane–hexane (1 : 1) and dried in
vacuo yielding a pale yellow solid product (2.65 g, 84.76%). 1H
NMR (80 MHz, CDCl3): δ 7.57 (m, 1H, py H4), 7.19 (m, 2H, py
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J. Chem. Soc., Dalton Trans., 2002, 2581–2586

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H3–5), 6.79 (m, 8H, Ar), 4.43 (br, 6H, –CH2 ϩ NH), 2.24 (s, 6H,
Me); 13C{1H} NMR (52.3 MHz, CDCl3): δ 158.4, 145.8, 136.9,
129.6, 126.7, 119.7, 113.2, 49.6, 20.2; IR (KBr): ν(N–H): 3351
cmϪ1; Found: C, 79.21; H, 7.16; N, 13.24. C21H23N3 requires C,
79.46; H, 7.30; N, 13.27%.
formed instantaneously. After filtering the solution through
Celite the methanol was evaporated to dryness. The precipi-
tate was recrystallized from dichloromethane–hexane (1 : 1).
The resulting precipitate was filtered off, washed with hexane
(2 × 10 cm3) and diethyl ether (2 × 15 cm3) and dried in vacuo
1
yielding 2a as a yellow solid (77.6 mg, 98%). H NMR (399.9
MHz, CD3COCD3, rt):‡ δ 8.35 (t, 1H, JH –H 7.9 Hz, py H4),
4
3–5
2,6-Bis[{N-(4-methoxyphenyl)amino}methyl]pyridine, 2,6-(4-
MeOC6H4NHCH2)2C5H3N, Lb. The synthesis was carried out
according to the procedure followed above for 2,6-(4-MeC6-
H4NHCH2)2C5H3N. Yield 84% of a pale yellow solid. 1H NMR
(80 MHz, CDCl3): δ 7.58 (m, 1H, py H4), 7.19 (m, 2H, py H3–5),
6.70 (m, 8H, Ar), 4.41 (a, 6H, –CH2), 3.97 (br, 2H, NH), 2.24 (s,
6H, Me); 13C{1H} NMR (52.3 MHz, CDCl3,): δ 158.5, 152.4,
8.29 (t, 1HЈ, JH –H 7.9 Hz, py H4), 7.98 (br, 2H ϩ 2HЈ, NH),
4
3–5
7.84 (d, 2H, JH –H 7.9 Hz, py H3–5), 7.77 (d, 2HЈ, JH –H
4
3–5
4
3–5
7.9 Hz, py H3–5), 7.37–7.10 (m, 8H ϩ 8HЈ, Ar), 5.46 (dd, 2H,
JH
2HЈ, JH
17.73 Hz, JCH H –NH 6.93 Hz, CHAHB), 5.23 (dd,
A–HB
B
17.60 HzA, JCH H –NH 6.45 Hz, CHAHB), 5.01
A–HB
A
B
(dd, 2HЈ, JH
17.60 Hz, JCH H –NH 4.71 Hz, CHAHB),
A–HB
B
142.3, 137.0, 119.9, 115.0, 114.4, 55.8, 50.3; IR (KBr): ν(N–H)
:
4.85 (dd, 2H, JH
17.73 Hz, JCAH H –NH 1.72 Hz, CHAHB),
A–HB
A
3388 cmϪ1; Found: C, 71.90; H, 6.37; N, 11.01. C21H23N3O2
requires C, 72.18; H, 6.63; N, 12.03%.
2.28 (s, 6H, Me), 2.26 (s, 6HЈ, Me); IR B(KBr): ν(N–H) 3426, ν(B–F)
1084 cmϪ1; Found: C, 45.55; H, 4.36; N, 7.62. PdC21H23N3-
ClBF4 requires C, 46.19; H, 4.25; N, 7.69%.
2,6-Bis[{N-(4-methylphenyl)-N-methylamino}methyl]pyridine,
2,6-(4-MeC6H4NMeCH2)2C5H3N, Lc. To a stirred solution of
2,6-(4-MeC6H4NHCH2)2C5H3N (3.6 g, 11.3 mmol) at Ϫ25 ЊC in
dry THF (40 cm3) were added 29.33 mmol of n-BuLi. The solu-
tion was allowed to warm to room temperature and stirred for
an additional hour. On cooling the solution to 0 ЊC 4.0 g (28.25
mmol) of MeI was added and the solution was allowed to warm
to room temperature again. The solution was evaporated to
dryness and the crude product was purified by flash column
chromatography (AcOEt–hexane, 1 : 5) to give 2.6 g of pale
brown solid (68%). 1H NMR (200 MHz, CDCl3): δ 7.49 (t, 1H,
py H4), 7.02 (m, 2H, py H3–5), 6.85 (m, 8H, Ar), 4.63 (s,
4H, –CH2), 3.10 (s, 6H, N–Me), 2.26 (s, 6H, Me); 13C NMR
(52.3 MHz, CDCl3): δ 159.6, 147.4, 137.6, 129.9, 126.0, 119.1,
112.6, 59.3, 39.4, 20.5; m/z 345 (Mϩ, 11.6), 226 (100), 134 (14.1),
121 (22.5), 120 (19.7) 107 (11); Found: C, 79.61; H, 7.86; N,
11.94. C23H27N3 requires C, 79.96; H, 7.88; N, 12.16 %.
[Pd{2,6-(4-MeOC6H4NHCH2)2C5H3N}Cl](BF4) 2b. The syn-
thesis was carried out according to the procedure followed
1
above for 2a. Yield 79% of a yellow solid. H NMR (399.9
MHz, CD3COCD3, rt):‡ δ 8.35 (t, 1H, JH –H 7.9 Hz, py H4),
4
3–5
8.28 (t, 1HЈ, JH –H 7.9 Hz, py H4), 8.02 (br, 2HЈ, NH), 7.96
4
3–5
(br, 2H, NH), 7.83 (d, 2H, JH –H 7.9 Hz, py H3–5), 7.76 (d,
4
3–5
2HЈ, JH –H 7.9 Hz, py H3–5), 7.46–6.83 (m, 8H ϩ 8HЈ, Ar),
4
3–5
5.47 (dd, 2H, JH
5.23 (dd, 2HЈ, JH
17.8 Hz, JCH H –NH 6.6 Hz, CHAHB),
A–HB
A
B
17.70 Hz, JCH H –NH 5.5 Hz, CHAHB),
A–HB
A
B
4.99 (dd, 2HЈ, JH
17.70 Hz, JCH H –NH 4.00 Hz, CHA-
A–HB
A
B
HB), 4.85 (d, 2H, JH
17.80 Hz, CHAHB), 3.78 (s, 6H,
A–HB
MeO), 3.76 (s, 6HЈ, MeO); IR (KBr): ν(N–H) 3426, ν(B–F) 1083
cmϪ1; Found: C, 43.47; H, 4.26; N, 7.31. PdC21H23N3O2ClBF4
requires C, 43.63; H, 4.01; N, 7.27%.
[Pd{2,6-(4-MeC6H4NMeCH2)2C5H3N}Cl]Cl 1c. To a solution
of cis-[PdCl2(PhCN)2] (333 mg, 0.87 mmol) in dichloromethane
(20 cm3) was slowly added 2,6-(4-MeC6H4NMeCH2)2C5H3N
(300 mg, 0.87 mmol). The mixture was stirred at 20 ЊC for
4 h at which point hexane was added to induce precipitation.
Washing the residue with hexane (2 × 10 cm3) and diethyl ether
(2 × 15 cm3) and drying in vacuo yielded 1c as a yellow solid
(310 mg, 71%). 1H NMR (399.9 MHz, CDCl3, rt):‡ δ 8.20–6.05
(broad and unresolved, 11H ϩ 11HЈ, H4, H3–5, Ar), 5.04 (br,
4H ϩ 4HЈ, –CH2), 3.22 (s, 6H, NMe), 3.16 (s, 6HЈ, NMe). 2.25
(s, 6HЈ, Me), 2.18 (s, 6H, Me); Found: C, 52.64; H, 5.23; N,
8.08. C23H27Cl2N3Pd requires C, 52.84; H, 5.21; N, 8.04%.
[Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}Cl]Cl 1a. To a solution
of cis-[PdCl2(PhCN)2] (180 mg, 0.47 mmol) in dichloromethane
(20 cm3) was slowly added the terdentate ligand 2,6-(4-
MeC6H4NHCH2)2C5H3N (148 mg, 0.47 mmol). The mixture
was stirred at 20 ЊC for 2 h and then hexane was added to
induce precipitation. Washing the residue with hexane (2 × 10
cm3) and diethyl ether (2 × 15 cm3) and drying in vacuo yielded
1
1a as a yellow solid (198 mg, 85%). H NMR (300.13 MHz,
DMSO-d6, rt):‡ δ 9.02 (br, 2HЈ, NH), 8.94 (d, 2H, JCH H –NH
A
B
6.05 Hz, NH), 8.25 (m, 1H ϩ 1HЈ, py H4), 7.68 (m, 2H ϩ 2HЈ,
py H3–5), 7.13 (m, 8H ϩ 8HЈ, Ar), 5.13 (dd, 2H, JH
17.37
A–HB
Hz, JCH H –NH 6.05 Hz, CHAHB), 4.83 (br, 4HЈ, CHAHB),
B
4.62 (d, A2H, JH
17.37 Hz, CHAHB), 2.22 (s, 6H ϩ 6HЈ,
[Pd{2,6-(4-MeC6H4NMeCH2)2C5H3N}Cl](BF4) 2c. To a solu-
tion of 1c (150 mg, 0.28 mmol) in acetone (75 cm3) was added
TlBF4 (83.5 mg, 0.28 mmol). A white precipitate (TlCl) was
formed instantaneously. After filtering the solution though
Celite and concentration of the filtrate hexane was added to
induce precipitation. The resulting precipitate was filtered off,
washed with hexane and dried in vacuo yielding 2c as a yellow
A–HB
Me); IR (KBr): ν(NH) = 3436 cmϪ1; Found: C, 50.71; H, 4.75; N,
8.60. PdC21H23N3Cl2 requires C, 50.98; H, 4.69; N, 8.49%.
[Pd{2,6-(4-MeOC6H4NHCH2)2C5H3N}Cl]Cl 1b. The syn-
thesis was carried out according to the procedure followed
1
above for 1a. Yield 88% of a yellow solid. H NMR (300.13
1
solid (99 mg, 62%). H NMR (399.9 MHz, CD3COCD3, 253
MHz, DMSO-d6, rt):‡ δ 9.18 (br, 2HЈ, NH), 9.04 (d, 2H,
K):‡ δ 8.34 (t, 1H, JH –H 8.0 Hz, py H4), 8.31 (t, 1HЈ, JH –H
4
3–5
4
3–5
JCH H –NH 6.31 Hz, NH), 8.22 (m, 1H ϩ 1HЈ, py H4), 7.66
8.1 Hz, py H4), 7.87 (d, 2H, JH –H 8.0 Hz, py H3–5), 7.79 (d,
B
(m,A2H ϩ 2HЈ, py H3–5), 7.06 (m, 8HЈ, Ar), 7.03 (m, 8H, Ar),
4
3–5
2HЈ, JH –H 8.1 Hz, py H3–5), 7.77–7.22 (m, 8H ϩ 8HЈ, Ar),
5.36 (d, 2H, JH 17.52 Hz, CHAHB), 5.31 (d, 2HЈ, JH
4
3–5
5.08 (dd, 2H, JH
17.56 Hz, JCH H –NH 6.31 Hz, CHAHB),
A–HB
4.83 (br, 4HЈ, CHAHB), 4.60 (d, 2H, AJHB
17.56 Hz, CHAHB),
A–HB
A–HB
A–HB
17.40 Hz, CHAHB), 5.10 (d, 2H, JH
17.52 Hz, CHAHB),
A–HB
3.69 (s, 6H ϩ 6HЈ, MeO); IR (KBr): ν(N–H) 3456 cmϪ1; Found: C,
47.49; H, 4.77; N, 7.96. C21H23Cl2N3O2Pd requires C, 47.88; H,
4.40; N, 7.98%.
4.98 (d, 2HЈ, JH
17.40 Hz, CHAHB), 3.38 (s, 6H, NMe),
A–HB
3.29 (s, 6HЈ, NMe), 2.30 (s, 6H, Me), 2.29 (s, 6HЈ, Me); IR
(KBr): ν(B–F) 1056 cmϪ1; Found: C, 48.04; H, 4.83; N, 7.22.
PdC23H27N3ClBF4 requires C, 48.11; H, 4.74; N, 7.32%.
[Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}Cl](BF4) 2a. To a solu-
tion of 1a (72 mg, 0.145 mmol) in methanol (50 cm3) was added
TlBF4 (42.4 mg, 0.145 mmol). A white precipitate (TlCl) was
Crystal structure of [Pd{2,6-(4-MeC6H4NHCH2)2C5H3N}Cl]Cl
1a
The molecular structure of 1a was determined by a single-
crystal X-ray diffraction study. Structural and refinement
details are given in Table 2.
1
‡ The H NMR spectra of complexes 1 and 2 correspond to the pres-
ence of two species as discussed in the text. Protons are reported as H
(major isomer) and HЈ(minor isomer).
J. Chem. Soc., Dalton Trans., 2002, 2581–2586
2585

View Article Online
Table 2 Crystal data and structure refinement for 1a
9 G. J. P. Britovsek, V. C. Gibson, S. Mastroianni, D. C. H. Oakes,
C. Redshaw, G. A. Solan, A. J. P. White and D. J. Williams,
Eur. J. Inorg. Chem., 2001, 431.
Empirical formula
C21H23Cl2N3Pd
FW
T /K
λ/Å
Crystal system
Space group
a/Å
b/Å
c/Å
494.73
143
0.71073
Monoclinic
P21/n
13.731(2)
9.709 (1)
17.230(2)
10 G. J. P. Britovsek, M. Bruce, V. C. Gibson, B. S. Kimberley,
P. J. Maddox, S. Mastroianni, S. J. McTavish, C. Redshaw,
G. A. Solan, S. Stromberg, A. J. P. White and D. J. Williams,
J. Am. Chem. Soc., 1999, 121, 8728.
11 R. F. Chen, C. T. Qian, Y. Li and F. L. Zou, Chin. J. Org. Chem.,
2000, 20, 712.
12 D. M. Dawson, D. A. Walker, M. Thornton-Pett and M. Bochmann,
J. Chem. Soc., Dalton Trans., 2000, 459.
β/Њ
110.188(2)
2156.0(5)
4
1.524
11.19
5914
3279 (Rint = 0.0747)
R1 = 0.0747, wR2 = 0.1487
R1 = 0.1421, wR2 = 0.1773
13 J. Y. Chen and L. K. Woo, J. Organomet. Chem., 2000, 601, 57.
14 R. E. Rulke, V. E. Kaasjager, D. Kliphuis, C. J. Elsevier, P. van
Leeuwen, K. Vrieze and K. Goubitz, Organometallics, 1996, 15, 668.
15 J. H. Groen, A. de Zwart, M. J. M. Vlaar, J. M. Ernsting,
P. van Leeuwen, K. Vrieze, H. Kooijman, W. J. J. Smeets, A. L. Spek,
P. H. M. Budzelaar, Q. Xiang and R. P. Thummel, Eur. J. Inorg.
Chem., 1998, 1129.
16 R. E. Rulke, J. M. Ernsting, A. L. Spek, C. J. Elsevier,
P. van Leeuwen and K. Vrieze, Inorg. Chem., 1993, 32, 5769.
17 J. H. Groen, P. W. N. M. van Leeuwen and K. Vrieze, J. Chem. Soc.,
Dalton Trans., 1998, 113.
V/Å3
Z
ρcalc/g cmϪ3
µ/cmϪ1
Reflections collected
Independent reflections
Final R indices [I > 2σ(I )]
R indices (all data)
CCDC reference number 179002.
See http://www.rsc.org/suppdata/dt/b2/b201319c/ for crystal-
lographic data in CIF or other electronic format.
18 R. E. Rulke, J. G. P. Delis, A. M. Groot, C. J. Elsevier,
P. van Leeuwen, K. Vrieze, K. Goubitz and H. Schenk,
J. Organomet. Chem., 1996, 508, 109.
19 M. Alvarez, N. Lugan and R. Mathieu, J. Chem. Soc., Dalton Trans.,
1994, 2755.
20 P. Bhattacharyya, J. Parr and A. M. Z. Slawin, J. Chem. Soc.,
Dalton Trans., 1998, 3609.
21 M. Ahmad, S. D. Perera, B. L. Shaw and M. Thornton Pett,
J. Chem. Soc., Dalton Trans., 1997, 2607.
Acknowledgements
The authors gratefully acknowledge the Spanish Dirección
General de Enseñanza Superior (PB97–0470-C02) and the
Junta de Castilla y León (BU08/99) for financial support. We
also acknowledge the Servicios Centrales de Apoyo a la Investi-
gación (SCAI) de la Universidad de Burgos for technical
support.
22 J. M. Benech, C. Piguet, G. Bernardinellli, J. C. G. Bunzli and
G. Hopfgartner, J. Chem. Soc., Dalton Trans., 2001, 684.
23 P. B. Iveson, C. Riviere, D. Guillaneux, M. Nierlich, P. Thuery,
M. Ephritikhine and C. Madic, Chem. Commun., 2001, 1512.
24 M. G. B. Drew, M. J. Hudson, P. B. Iveson, C. Madic and
M. L. Russell, J. Chem. Soc., Dalton Trans., 2000, 2711.
25 S. Nuckel and P. Burger, Organometallics, 2001, 20, 4345.
26 A. von Zelewsky, Schereochemistry of Coordination Compounds,
Wiley, Chichester, 1995.
27 C. J. Hawkins and J. A. Palmer, Coord. Chem. Rev., 1982, 44, 1.
28 D. S. Stephenson and G. Binsch, DNMR5, Program 365, Quantum
Chemistry Program Exchange, Indiana University, USA, 1995.
29 H. F. Haarman, F. R. Bregman, J. M. Ernsting, N. Veldman,
A. L. Spek and K. Vrieze, Organometallics, 1997, 16, 54.
30 N. W. Alcock, P. Moore and C. Pierpoint, J. Chem. Soc., Dalton
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