Anion-assisted trans-cis isomerization of palladium(II) phosphine complexes containing acetanilide functionalities through hydrogen bonding interactions

Article abstract of DOI:10.1039/b418202b

The anion-assisted shift of trans-cis isomerization equilibrium of a palladium(II) complex containing acetanilide functionalities brought about by allosteric hydrogen bonding interactions has been established by UV/Vis, ¹H NMR, ³¹P N

Full text of DOI:10.1039/b418202b

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Anion-assisted trans–cis isomerization of palladium(II) phosphine
complexes containing acetanilide functionalities through hydrogen
bonding interactions{
Xiao-Xia Lu, Hau-San Tang, Chi-Chiu Ko, Jenny Ka-Yan Wong, Nianyong Zhu and Vivian Wing-Wah Yam*
Received (in Cambridge, UK) 3rd December 2004, Accepted 19th January 2005
First published as an Advance Article on the web 26th January 2005
DOI: 10.1039/b418202b
Upon dissolution of 1 in CDCl3, an equilibrium mixture of the
cis and trans isomers with the trans form being the major species
(trans:cis # 4:1) was observed, as revealed by the observation of
two singlets in the 31P NMR spectrum at d 22.98 and 32.94 ppm,
assigned as the trans and cis isomers, respectively. Similar chemical
shifts have been observed in other related trans- and cis-
dichloropalladium(II) phosphines.6 This is in contrast to the
related trans-[PdCl2(Ph2PB15C5)2]3 and complex 2, where only the
trans isomer was observed in CDCl3. The observation of both
trans and cis isomers in 1 but not in 2 may be attributed to the
stabilization of the cis isomer of 1 by the formation of
intramolecular hydrogen bonds between the –NH and the –CLO
moieties on the adjacent acetanilides.
The anion-assisted shift of trans–cis isomerization equilibrium
of a palladium(II) complex containing acetanilide functionalities
brought about by allosteric hydrogen bonding interactions has
been established by UV/Vis, 1H NMR, 31P NMR and ESI-MS
studies.
The phenomenon of trans–cis isomerization in square-planar d8
metal complexes, such as those of palladium(II) and platinum(II), is
well known and has been extensively studied.1,2 Most of these
studies were directed towards trans–cis isomerization reactions that
were induced either thermally or by photochemical means.
Corresponding studies involving ion-induced or assisted isomer-
ization reaction brought about by allosteric interactions were not
known. It was only recently that we reported the first system where
the binding of alkali metal ions in a sandwich fashion between the
bis(crown) unit of a crown ether-containing palladium(II) complex
resulted in the trans–cis isomerization of the palladium complex.3
As an extension of our work on the use of allosteric interactions
to provide the driving force for trans–cis isomerization
reactions, together with the growing importance of the role of
anions in biological, chemical and environmental processes,4
we are interested in investigating the possible extension of
using anion complexation and allosteric interactions to
promote the isomerization processes. Herein, we report the
The electronic absorption spectrum of 1 in CHCl3 showed an
intense high-energy band at 260 nm, assigned as metal-perturbed
intraligand (IL) transition of the phosphine ligand. With
reference to previous spectroscopic studies on dihalopalladium(II)
phosphines,7 the moderately intense low-energy absorption
band at 354 nm, with absorption coefficient of the order of
104 dm3 mol21 cm21, is assigned as a ligand-to-metal charge-
transfer (pp(Cl) A 4d(Pd)) transition.
Addition of tetra-n-butylammonium chloride (nBu4NCl) to a
solution of 1 in CHCl3 resulted in changes in the UV/Vis
absorption spectrum with well-defined isosbestic points (Fig. 1).
synthesis and anion-assisted trans–cis isomerization of
a
palladium(II) complex through the formation of hydrogen bonding
interactions.
The complexes, [PdCl2(Ph2PC6H4NRCOMe)2] (R 5 H, 1; Me,
2), were prepared by the reaction of [PdCl2(PhCN)2] with two
equivalents of Ph2PC6H4NHCOMe and Ph2PC6H4NMeCOMe
(see ESI{ for detailed experimental procedures), respectively, in
benzene, using a modified procedure.5 Subsequent recrystallization
from slow diffusion of diethyl ether vapor into chloroform
solutions of complexes 1 and 2, respectively, yielded 1 and 2 as pale
yellow crystals. Both 1 and 2 were characterized by 1H and
31P{1H} NMR spectroscopy, positive-ion FAB mass spectrometry
and gave satisfactory elemental analyses. The trans disposition of
the two phosphines in the solid state was confirmed by X-ray
crystallographic determination of 1 (see ESI{ for perspective
drawing and crystal structure data of 1{).
{ Electronic supplementary information (ESI) available: experimental
details of Ph2PC6H4NRCOMe (R 5 H and Me), characterization of 1 and
2, and crystallographic data of 1. See http://www.rsc.org/suppdata/cc/b4/
b418202b/
Fig. 1 UV/Vis spectral changes of 1 upon addition of nBu4NCl in
CHCl3. The inset shows the absorbance at 354 nm (&) as a function of the
concentration of Cl2 ions with theoretical fit (—).
*wwyam@hku.hk
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Similar spectral changes were not observed in a control experiment
using the N-methylated analogue, 2. Replacement of nBu4NCl
with nBu4NPF6 did not cause any spectral changes of 1, indicating
that the changes were not attributed to ionic strength effect. It is
likely that the spectral changes observed are a result of the binding
of chloride ions to the amide groups present in 1 via the formation
of hydrogen bonds. The inset in Fig. 1 shows the titration curve of
1 upon addition of nBu4NCl, with its theoretical fit to the equation
supporting the formation of a 1:1 adduct,8 giving a logK value of
3.39.
isomerization of square-planar palladium(II) complexes is well
known and the mechanism well studied.2 In most cases, the trans
isomer is usually the more thermodynamically favored product,
however, the equilibrium of the trans and cis isomer is shifted in
the case of 1 due to the preferential stabilization of the cis isomer
by the formation of hydrogen bonds between the two –NH amide
groups and the Cl2 ion (Scheme 1). Thus the formation of two
intermolecular hydrogen bonds between the palladium(II) complex
and the Cl2 ion results in the cis isomer being the more
thermodynamically favored product with a trans:cis ratio # 1:3.
Attempts to study other anions containing hydrogen bond donors,
such as H2PO42 in the form of its nBu4N+ salt, were unsuccessful,
because they also bind to 2, resulting in an increase of the cis
isomer due to hydrogen bonding to the carbonyl group of the
N-methylated amide group. It is interesting to note that the system
is reversible on addition of DMSO-d6 to a solution of 1 and
nBu4NOTf in CDCl3, resulting in a decrease in the cis isomer and
an increase in the trans isomer. The presence of DMSO-d6 results
in the breaking of the intermolecular hydrogen bonds between the
complex and guest, leading to the formation of the more
thermodynamically stable trans isomer in the absence of the
hydrogen bonded complex:guest adduct.
1
Both H and 31P NMR studies were undertaken to rationalize
1
these findings. The H NMR spectrum of complex 1 in CDCl3
showed two NH signals at d 7.6 and 8.3 ppm, assigned by NOESY
experiment as the trans and cis isomer, respectively. The cis amide
proton resonance appears at a more downfield region, probably
due to the presence of intramolecular hydrogen bonding between
the two adjacent amide functionalities. Similar to the UV/Vis
n
titration studies, addition of Bu4NCl to a CDCl3 solution of 1
caused a change, where the amide proton resonance of both the
trans and cis isomers shifted downfield initially. This is probably as
a result of the formation of hydrogen bonds between the –NH
n
amide groups and the chloride ion. Further addition of Bu4NCl
resulted in the conversion of the hydrogen-bonded chloride adduct
of the trans complex to the corresponding cis complex, as
evidenced by the growth in the intensity of the cis amide proton.
The greater stability of the hydrogen-bonded chloride-bound cis
adduct would favor cis product formation. No further attempts
were made to quantify the changes based on the integral ratios
of the amide protons since they were usually very broad. Since
the cis isomer is preferentially stabilized via hydrogen bond
formation between the complex and the guest, addition of other
guests that are capable of hydrogen bonding to the amide
functionalities should lead to similar observations. Indeed,
Scheme 1 Schematic representation of the reversible trans–cis isomeriza-
tion of 1 in the presence of Cl2 or OTf2 ions and DMSO.
n
addition of Bu4NOTf increases the population of the cis isomer
As indicated by 31P NMR studies, a shift of the equilibrium
from trans to cis occurred upon the addition of Cl2 ions. In fact,
the presence of both the trans-1 and cis-1 isomers would require
a description involving at least four species in equilibrium
(Scheme 2), assuming that only 1:1 1?Cl2 adducts are
formed under the conditions studied. Detailed treatment3 of
the 31P NMR data according to Scheme 2 gave K1, K2 and
K3 values of 0.250 ¡ 0.004, 2410 ¡ 144 and 514 ¡ 111,
respectively. The overall equilibrium constant, log K of 3.38,
where K 5 K1K2, is in close agreement with the log K value
determined by UV/Vis spectrophotometry. Negative ESI mass
spectrometric measurements provided further supporting evidence
for the 1:1 adduct formation. In the case of 1, the {1?Cl}2 adduct
was observed, irrespective of the concentration of Cl2 ions added,
while for 2, no {2?Cl}2 was observed in the negative ESI-mass
spectrum.
with a log K value of 2.61.
A more noticeable change was observed in the 31P NMR
titration experiments of 1 with nBu4NCl (Fig. 2). Addition of Cl2
ions led to an increase in the population of the cis isomer at
d 32.94 ppm and a decrease of the trans isomer at d 22.98 ppm.
Similar findings were not observed in a control experiment with 2
under the same conditions, thus the presence of the –NH amide
groups promotes the formation of the cis isomer. The trans–cis
Fig. 2 31P NMR spectral changes of 1 (3.64 6 1024 mol dm23) upon
addition of nBu4NCl in CDCl3 at 298 K, with the signal corresponding to
the cis (N) and trans (D) isomers indicated.
Scheme 2 Proposed binding equilibria between 1 and Cl2 ions.
Chem. Commun., 2005, 1572–1574 | 1573
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Hong Kong, P. R. China. E-mail: wwyam@hku.hk;
Fax: (852)2857-1586; Tel: (852)2859-2153
In summary, the remarkable shift of isomerization equilibrium
brought about by allosteric interactions involving hydrogen bond
1
formation has been established by UV/Vis, H NMR, 31P NMR
Notes and references
and ESI-MS studies. The present trans–cis isomerization of the
square-planar palladium(II) complex that is promoted via the
binding of chloride or trifluoromethanesulfonate ions to amide
functionalities through hydrogen bonding, reversible on addition
of DMSO, represents another interesting demonstration of
principle in manoeuvring and shifting the thermodynamic stability
of the isomeric products via allosteric interactions.
{ Selected crystal data for 1, C72H108Cl4N4O2P2Pd, Mr 5 1371.76, crystal
size 0.40 6 0.25 6 0.20 mm, monoclinic P21/n, a 5 13.641(3),
3
˚
˚
b 5 18.267(4), c 5 15.156(3) A, b 5 92.42(3)u, V 5 3773.2(14) A , Z 5 2,
Dc 5 1.207 g cm23, m(Mo-Ka) 5 0.473 mm21, F(000) 5 1456, T 5
301(2) K, 16043 data (5926 unique, Rint 5 0.0472, 1.86 , h , 25.35u),
conventional R 5 0.0427 [I . 2s(I)], S 5 0.922 for 394 data. Residual
electron density extremes were 0.457 and 20.511 e A23. CCDC 258649. See
˚
http://www.rsc.org/suppdata/cc/b4/b418202b/ for crystallographic data in
.cif or other electronic format.
V. W.-W. Y. acknowledges support from the University
Development Fund of The University of Hong Kong, The
University of Hong Kong Foundation for Educational
Development and Research Limited, and the University Grants
Committee of the Hong Kong Special Administrative Region,
China (Project No. AoE/P-10/01). H.-S. T. acknowledges the
receipt of a Postgraduate Studentship, and C.-C. K. and J. K.-Y.
W. the receipt of University Postdoctoral Fellowships, both
administered by The University of Hong Kong. This work has
been supported by a grant from the Research Grants Council of
the Hong Kong SAR, China (Project No. HKU7120/02P).
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Xiao-Xia Lu, Hau-San Tang, Chi-Chiu Ko, Jenny Ka-Yan Wong,
Nianyong Zhu and Vivian Wing-Wah Yam*
Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based
Materials Research, Department of Chemistry, and Open Laboratory of
Chemical Biology of the Institute of Molecular Technology for Drug
Discovery and Synthesis, The University of Hong Kong, Pokfulam Road,
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1574 | Chem. Commun., 2005, 1572–1574
This journal is ß The Royal Society of Chemistry 2005