First observation of alkali metal ion induced trans-cis isomerization of palladium(II) phosphane complexes containing crown ether moieties

Article abstract of DOI:10.1002/anie.200351178

Ion switches: The addition of small metal ions (such as Li⁺ and Na⁺) to transpalladium(II) complexes of PPh₂-substituted benzo[15]crown-5 and benzo[18]-crown-6 results in the formation of 1:1 trans adducts. However, the addition of large alkali ions (such as K⁺, Rb⁺, and Cs⁺) enforces the complexes to undergo a trans-cis isomerization to enable formation of intramolecular sandwich adducts (see schematic representation).

Full text of DOI:10.1002/anie.200351178

Angewandte
Chemie
Metal-Ion Induced Isomerization
First Observation of Alkali Metal Ion Induced
Trans–Cis Isomerization of Palladium(ii)
Phosphane Complexes ContainingCrown
Ether Moieties**
Vivian Wing-Wah Yam,* Xiao-Xia Lu, and Chi-Chiu Ko
There has been growing interest in the design of allosteric
host molecules that can efficiently transmit the information
associated with the binding of an ion or molecule to another
[*] Prof. Dr. V. W.-W. Yam, Dr. X.-X. Lu, C.-C. Ko
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, Hong Kong (P. R. China)
Fax: (+852)2857-1586
E-mail: wwyam@hku.hk
[**] V.W.-W.Y. acknowledges support from The University of Hong Kong
and the University Grants Committee Area of Excellence Scheme
and the receipt of a Croucher Senior Research Fellowship (2000–
2001) from the Croucher Foundation. C.-C.K. acknowledges the
receipt of a Croucher Scholarship from the Croucher Foundation
and a Li Po Chun Postgraduate Scholarship from the Li Po Chun
Charitable Fund. The work has been supported by a grant from the
Research Grants Council of the Hong Kong SAR, China (Project
No. HKU7120/02P). Dr. K. Lau and Dr. K. K. Cheung are gratefully
acknowledged for developing the computer program for iterative fits
and for solving the X-ray crystal structures, respectively.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3385 – 3388
DOI: 10.1002/anie.200351178
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binding site through structural changes induced on bind-
ligand-to-metal charge-transfer (pp(X)!4d(Pd); X = Cl, Br,
I) transitions.
The addition of alkali-metal ions to a solution of 1 in
CH3CN/CH2Cl2 (9:1 v/v, 0.1 moldmꢀ3 nBu4NPF6) resulted in
changes in the UV/Vis spectrum with well-defined isosbestic
points (Figure 1). Similar spectral changes were not observed
ing.[1,2] Such allosteric binding behavior not only serves as a
means to control the binding or release of ions or molecules,
but also provides a new strategy for the design and
construction of molecular-switching devices. Although
trans–cis thermal and photoisomerization reactions are well-
known[3,4] and numerous works on photoinduced ion binding
have appeared in the literature,[3] there have been no reports
of metal-ion-induced or -assisted isomerization reactions
brought about by allosteric interactions.
in
a control experiment using a crown-free analogue
[PdCl2{PPh2{3,4-(OMe)2C6H3}}2] (5), which indicates that
these changes can be ascribed to the binding of the cations
In this regard, we were interested in
investigating the possibility of using
metal-ion complexation and allosteric
interactions to induce isomerization
processes. Herein, we report the unpre-
cedented trans–cis isomerization of
dichloropalladium(ii) phosphanylcrown
complexes that is induced by the binding
of alkali-metal ions, as evidenced by UV/
Vis, 1H NMR, and 31P NMR spectros-
copy as well as ESI-MS, and represents a
novel type of allosteric host that is the
first of its kind.
The reaction of [PdX2(PhCN)2] with
two equivalents of PPh2-benzo[15]-
crown-5 (PPh2(B15C5)) or PPh2-ben-
Figure 1. UV/Vis spectral changes of 1 upon addition of various concentrations of: a) NaClO4
and b) KPF6 in CH3CN/CH2Cl2 (9:1 v/v, 0.1 moldmꢀ3 nBu4NPF6). Insert: absorbance at 355 nm
+
( ) as a function of the concentration of Na or K+ ions with theoretical fits.
&
zo[18]crown-6 (PPh2(B18C6)) in benzene, by using a mod-
ification of literature procedures,[5] produced [PdX2-
{PPh2(B15C5)}2] (X = Cl, 1;[6] Br, 2; I, 3) and [PdCl2-
{PPh2(B18C6)}2] (4).[5] The identities of 1–4 have been
confirmed by 1H, 13C{1H}, and 31P{1H} NMR spectroscopic
analysis, positive-ion FAB mass spectrometry, and satisfac-
tory elemental analyses. The crystal structures of 1 and 2 have
also been established as having a trans configuration by X-ray
crystallographic analysis.[7]
Although the complexes exist as trans isomers in CD2Cl2,
dissolution of these complexes in mixed solution of CD3CN
and CD2Cl2 (9:1 v/v) results in an equilibrium between the cis
and trans isomers, with the trans form being the majority
species. For example, a solution of 1 in CD3CN/CD2Cl2
(9:1 v/v) shows two singlets in the 31P NMR spectra (one at
d = 24.02 ppm and one at d = 33.26 ppm) with an integral
ratio of 3.57:1 which are ascribed to the trans and cis isomers
of 1, respectively. This assignment is consistent with the
similar chemical shifts commonly observed in other related
[PdX2L2] systems (L = monodentate phosphane ligand).[8]
The assignment is further supported by an increase in the
intensity of the signal corresponding to the cis isomer at d =
33.26 upon irradiation at l = 256 nm of a solution of 1 at
ꢀ108C. This increase is commonly observed in the photo-
isomerization of trans-[PdX2L2] systems.[9]
to the polyether cavity. It is interesting to note that addition of
NaClO4 to a solution of 1 (6.4 ” 10ꢀ5 moldmꢀ3) in CH3CN/
CH2Cl2 (9:1 v/v, 0.1 moldmꢀ3 nBu4NPF6) resulted in an
increase in the absorbance and a small blue shift in the
absorption energy of the low-energy band, which reached
saturation at about 2.2 ” 10ꢀ3 moldmꢀ3 (Figure 1a). However,
a decrease in the absorbance and a small red shift of the band
were observed upon addition of KPF6, which reached
saturation at about 6.6 ” 10ꢀ5 moldmꢀ3 (Figure 1b). The
UV/Vis spectral changes upon addition of other alkali-metal
salts such as LiClO4 to a solution of 1 are similar to that of
NaClO4, whereas RbClO4 and CsClO4 showed similar
changes as KPF6. The inserts in Figure 1 show the titration
curves of 1 with NaClO4 and KPF6 together with their
theoretical fits to the equation for the formation of a
1:1 adduct.[10] Stoichiometry studies indicate that 1 forms
1:1 complexes with both Na+ and K+ ions under the con-
ditions studied. Addition of K+ ions (up to a concentration of
3 ” 10ꢀ4 moldmꢀ3) to 4 gave a similar UV/Vis spectral change
as that for 1 upon the addition of Li+ and Na+ ions, whereas
addition of Cs+ ions to 4 gave a similar spectral change as that
observed for 1 upon addition of K+ ions, that is, a decrease in
the absorbance and a small red shift in the low-energy
absorption band, which reached saturation at about
7.9 ” 10ꢀ5 moldmꢀ3.
The electronic absorption spectra of solutions of 1–4 in
CH3CN showed high-energy bands at about 270–300 nm
which are assigned as intraligand (IL) and metal-perturbed IL
transitions of the phosphane ligand. The low-energy absorp-
tion bands at about 350–430 nm (with extinction coefficients
of the order of 104 dm3 molꢀ1 cmꢀ1), which are strongly
dependent on the nature of the halides (labs: 1(354 nm)
< 2(374 nm) < 3(426 nm)), were tentatively assigned as
Both 1H and 31P NMR studies were undertaken to ration-
alize these findings. Coordination of alkali-metal ions to both
1 and 4 was signified by downfield shifts of the crown ether
protons. Interestingly, signals corresponding to the protons of
cis-1 initially shifted downfield upon titration with NaClO4 at
low concentrations, which gradually disappeared altogether
upon addition of a large excess of NaClO4 (ca. 500-fold),
while addition of even a small amount of KPF6 (< 1-fold)
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resulted in the growth of proton signals corresponding to the
cis isomer. A more noticeable change could be observed in
the 31P NMR titration experiments of 1 with NaClO4 and
KPF6. Figure 2 shows the changes in the 31P NMR spectra of a
solution of 1 in CD3CN/CD2Cl2 (9:1 v/v) upon complexation
of Na+ (Figure 2a) and K+ ions (Figure 2b). Addition of
binding modes in 1 would favor the conversion into the
cis configuration, in which the two crown ether rings within
the same molecule of 1 would bind to one metal ion. On the
other hand, the binding of Li+ and Na+ ions into the cavity of
benzo[15]crown-5 from solutions at high ion concentrations
would favor the formation of the trans isomer as a conse-
quence of electrostatic interactions arising from the binding
of two metal ions to a molecule of 1. Similarly, addition of
K+ ions, which could fit into the cavity of benzo[18]crown-6,
to a solution of 4 would give rise to the formation of the
trans isomer at high concentrations of K+ ions, while Cs+ ions
should favor the formation of the cis form for a sandwich-type
of binding, even at low concentrations.
The logK values obtained from the UV/Vis spectropho-
tometric methods at low to moderate concentrations of Na+
and K+ ions, that is, under the conditions for formation of a
1:1 adduct, are 3.88 ꢁ 0.02 for 1 with Na+ and 4.36 ꢁ 0.03 for 4
with K+ ions. The close agreement of the experimental data
with the theoretical fits confirms the formation of
a
Figure 2. 31P NMR (162 MHz) spectral changes of 1
1:1 adduct. However, a conversion of the trans-1 isomer into
the cis-1 form occurred in the case of the binding of K+ ions to
1, as indicated by 31P NMR spectroscopic studies (Figure 2b).
The presence of both the trans-1 and cis-1 isomers would
require a description involving at least four species in
equilibrium[11] (Scheme 2), assuming that only 1:1 1·M ad-
(1.0”10ꢀ3 moldmꢀ3) upon addition of a) NaClO4 and b) KPF6 in
CD3CN/CD2Cl2 (9:1 v/v) at 298 K, with signals corresponding to the
~
*
cis ( ) and trans ( ) isomers indicated.
Na+ ions initially gave rise to a slight downfield shift of the
31P NMR signal, which is typical for the binding of a metal ion
to the crown cavity. However, the addition of an excess of
Na+ ions (ca. 500-fold) resulted in the 31P signal correspond-
ing to the cis isomer at d = 33.26 disappearing, and leaving
behind only the signal corresponding to the trans isomer. The
addition of K+ ions to 1, in contrast, immediately gave rise to
an increase in the population of the cis isomer, with the
cis form becoming the predominant species as the K+ con-
centration increased until saturation was reached at a [K+]:[1]
ratio of approximately unity. Similar findings were not
observed in the control experiment with the crown-free
analogue 5 under the same conditions, which supports the
importance of the crown moieties in the specific association of
the metal ions. We believe that the addition of K+ ions, which
are too large to fit into the cavity size of the benzo[15]crown-5
moiety in 1 resulted in the trans to cis isomerization of the
palladium(ii) complex, while addition of Na+ ions in large
excess would favor the formation of the trans isomer
(Scheme 1). In general, benzo[15]crown-5 derivatives tend
to form 2:1 (crown ether ring/metal ions) sandwich-type
complexes with metal ions that are slightly greater in size than
the crown ether cavity, that is, K+, Rb+, and Cs+, with K+ ions
giving the best fit. Such intramolecular sandwich or tweezer
Scheme 2. Proposed binding equilibria between 1 and K+ ions.
ducts are formed under the conditions studied.[12] Detailed
treatment of the 31P NMR data according to Scheme 2 gave
K1, K2, and K3 values of 0.38 ꢁ 0.01, 1.01 ” 106 ꢁ 25000 and
2.55 ” 105 ꢁ 6300mꢀ1, respectively. The close resemblance of
the experimental data to the theoretical fits confirmed the
complexation stoichiometry of the K+ ions to 1 to be 1:1 and,
thus, confirmed the validity of Scheme 2. An overall equili-
brium constant logK of 5.59 ꢁ 0.02 was obtained for the
process M+trans-1Qcis-1·M, where K = K1K2. This result is in
close agreement with a logK value of 5.26 ꢁ 0.1 determined
by UV/Vis spectrophotometry.
Results from the ESI mass spectrometric measurements
provided direct evidence for supporting this hypothesis. In the
case of 1, only the {1·K}+ adduct was observed, irrespective of
the concentration of K+ ions used, while for 4, only {4·K}+ was
Scheme 1. Schematic representation of the trans–cis isomerization in the presence of different metal ions.
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3387

Communications
observed at low to moderate concentrations of K+ ions, but
[11] See supporting information for detailed derivation and fitting
procedure.
both the {4·K}+ and {4·K2PF6}+ ions were observed in the
positive ESI-mass spectra at high concentrations of K+ ions.
Other alkali-metal ions were also examined: Only {1·M}+ and
{4·M}+ adducts were observed at low to moderate concen-
trations of Li+ and Na+ ions, however, at high metal-ion
concentrations, a small amount of the 1:2 bound adducts
{1·M2ClO4}+ and {4·M2ClO4}+ adducts were also observed. In
the case of Cs+, however, only {1·Cs}+ and {4·Cs}+ appeared
irrespective of the concentration of Cs+ ions used. Similarly,
for Rb+ ions, only the {1·Rb}+ adduct was observed with 1
while both {4·Rb}+ and {4·Rb2ClO4}+ ions were found with 4
at high concentrations of Rb+ ions. In addition, 1 was found to
be the most selective towards the binding of K+ ions, while a
selective preference for the binding of Cs+ ions was observed
for 4. A likely explanation for this unprecedented ion-induced
or -assisted isomerization reaction is that metal ions which are
too large to fit into the size of the crown ether cavity tend to
form intramolecular sandwich/tweezer complexes with the
bis(crown) unit and this provides the driving force for the
isomerization of the trans to the cis isomer.
[12] Although ESI-MS could not differentiate between the formation
of a 1:1 complex from a 2:2 complex, we believe that under the
experimental conditions employed, in which the concentration
of 1 is about 6 ” 10ꢀ5 moldmꢀ3, the formation of a 2:2 complex is
unlikely. In addition, the formation of a 2:2 complex might also
be entropically disadvantageous.
In summary, the present bis(crown)-containing [PdX2L2]
system represents the first demonstration of a metal-ion-
induced switching of the trans–cis isomerization, which may
find interesting applications in the design of ion-controlled
switching devices.
Received: February 13, 2003
Revised: May 26, 2003 [Z51178]
Keywords: alkali metals · crown compounds ·
.
host–guest systems · isomerization · palladium
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