Highly selective ion probe for Al3+ based on Au(i)...Au(i) interactions in a bis-alkynyl calix[4]arene Au(i) isocyanide scaffold

Article abstract of DOI:10.1039/c1cc12677f

A bis-alkynyl calix[4]arene Au(i) isocyanide complex has been synthesized and demonstrated to be a selective ion probe for Al³⁺ based on Au(i)...Au(i) interactions.

Full text of DOI:10.1039/c1cc12677f

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COMMUNICATION
Highly selective ion probe for Al3+ based on Au(I)Á Á ÁAu(I) interactions
in a bis-alkynyl calix[4]arene Au(I) isocyanide scaffoldw
Franky Ka-Wah Hau, Xiaoming He, Wai Han Lam and Vivian Wing-Wah Yam*
Received 6th May 2011, Accepted 21st June 2011
DOI: 10.1039/c1cc12677f
A bis-alkynyl calix[4]arene Au(I) isocyanide complex has been
synthesized and demonstrated to be a selective ion probe for
Al3+ based on Au(I)Á Á ÁAu(I) interactions.
of dinuclear gold(I) scaffolds that exhibit sensitive and selective
chemosensing properties.
Herein we report the design, synthesis and characterization of
an amide-containing calixarene bis-alkynyl-bridged Au(I) iso-
cyanide complex 1 and show that this receptor complex is
capable of serving as a selective luminescent ion probe for
Al3+ based on the switching on of the AuÁÁÁAu interactions.
The amide-free analogue 2 has been designed for the control
studies (Chart 1). The gold(I) alkynyl polymers were prepared by
reacting the respective alkyne with the gold(I) precursor. Both
complexes were then synthesized by the reaction of 2,6-dimethyl-
phenyl isocyanide with 0.5 equivalents of the respective gold(I)
alkynyl polymer. The identities of 1 and 2 have been confirmed
The development of chemosensors for the selective binding of
various biological and environmental relevant cations has
gained considerable attention in recent years with their great
potential applications.1,2 Aluminum, a group 3 metal, is the
most abundant metal on the crust of the earth and it is
commonly used in daily life. However, it is toxic and environ-
mentally harmful. For example, it is the potential culprit of
Parkinson’s disease3 and Alzheimer’s disease,4 and it also inhibits
plant growth on acid soils.5 Therefore, it is crucial for the
development of sensitive and selective Al3+ sensors. Compared
with other metal ions, limited examples of Al3+ chemosensors
have been reported so far6 and most of their signal transduction
is achieved by photoinduced electron transfer, photoinduced
1
by H NMR, ESI-MS and satisfactory elemental analysis.
The electronic absorption spectrum of complex 1 in
n
CH2Cl2–MeCN (1: 1 v/v, 0.1 M Bu4NPF6) solution at 298 K
shows intense high-energy absorption bands at ca. 260–280 nm,
and low-energy bands at ca. 300–330 nm (Fig. 1). With reference
to previous spectroscopic work on related alkynylgold(I)
complexes,7d,8,10d the high-energy bands are tentatively
assigned as the intraligand transitions of the coordinated
phenylalkynyl and the aryl isocyanide units. The lower-energy
absorption bands, which are absent in the free ligand, should
be assigned to arise from the alkynyl-to-aryl isocyanide ligand-
to-ligand charge transfer (LLCT) transition.
charge-transfer or Forster resonance energy transfer (FRET).
¨
The study of polynuclear gold(I) complexes, in particular, with
regard to the phenomenon of aurophilicity, has attracted
increasing attention in recent years.7–10 Amongst organogold(I)
complexes, gold(I) isocyanides and alkynyls are usually photo-
luminescent, and they have been used for developing cluster
compounds,11 mesogenic materials,12 polymorphic materials11,13
and rigid-rod polymers.14 Furthermore, due to the sterically
undemanding property of the isocyanide and alkynyl moieties,
gold(I) isocyanides and alkynyls are promising candidates for
developing chemosensors.15 Previous works by our group have
demonstrated the novel concept of the utilization of the on–off
switching of AuÁ ÁÁAu interactions for metal ion sensing with
dinuclear gold(I) complexes with bridging diphosphines and
crown ether-functionalized thiolate ligands.8c,g With our recent
success in the employment of calixarenes in chemosensing work15
together with their unique molecular structures and ease of
chemical transformations, it is envisaged that the calixarene
moiety would serve as an ideal building block for the construction
A structured emission band centered at about 448–478 nm
was observed in complex 1 (Fig. 2), with vibrational progessional
spacings of about 1300 cmÀ1, corresponding to the C C
vibrational modes of the aromatic rings. Given the close similarity
of the emission to that of other alkynylgold(I) complexes,7d,8,10d
an origin of triplet states arising from a metal-pertubed intraligand
Institute of Molecular Functional Materials (Areas of Excellence
Scheme, University Grants Committee, Hong Kong) and Department
of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong. E-mail: wwyam@hku.hk; Fax: +(852) 2857-1586;
Tel: +(852) 2859-2153
w Electronic supplementary information (ESI) available: Experimental
procedures and the computational details. See DOI: 10.1039/c1cc12677f
Chart 1 Structures of gold(I) isocyanide complexes 1 and 2.
c
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More interestingly, the emission showed a drop in intensity of
the green emission at ca. 455 nm with the concomitant formation
of a new low-energy orange-red emission band at 665 nm with an
isoemissive point of about 537 nm (Fig. 2). The binding process
can be monitored by naked eyes (Fig. 2 inset). The new low-
energy emission band at about 665 nm presumably results from
the switching on of the AuÁÁÁAu interaction upon the binding of
the Al3+ ion, which gives rise to a reduced HOMO–LUMO
energy gap. The excitation spectra of 1 in the absence and in the
presence of Al3+ ion revealed that the low-energy and the high-
energy bands were derived from different excited state origin
(Fig. S1 in ESIw). The 665-nm emission band is derived from an
excitation band at about 375 nm, which coincides with the new
absorption shoulder formed at about 350 nm in the UV-Vis
spectra resulting from the switching on of the Au(I)ÁÁÁAu(I)
interaction upon Al3+ ion-binding.
Fig. 1 Electronic absorption spectral traces of 1 (1.24 Â 10À5 M) in
CH2Cl2–MeCN (1 : 1 v/v, 0.1
M
nBu4NPF6) upon addition of
To examine the selectivity of 1 toward various metal ions, the
luminescence response upon addition of different metal ions was
investigated. A drastic spectral change is shown by Al3+ binding
(Fig. 3), indicating the high selectivity of 1 toward Al3+. While
In3+ shows a similar emission enhancement, the change is to a
much lesser extent. Competition experiments of Al3+ binding
with other metal ions have also been performed and interferences
were low (Fig. S2 in ESIw). This result suggests that 1 can be used
as a selective chemosensor for the detection of Al3+. A detection
Al(ClO4)3 at 298 K. Upper Inset: Plot of absorbance (’) at 310 nm
as a function of Al3+ concentration and its theoretical fit (
) for the
1 : 1 binding of complex 1 with Al3+. Lower Inset: Job’s plot for
the binding of complex 1 with Al3+ showing 1 : 1 stoichiometry. The
absorbance (K) at 370 nm was plotted as a function of the molar ratio
wM , where wM = [Al3+]/([1] + [Al3+])
limit of 1.2 Â 10À7 M is found for Al3+
.
Due to the small size, high charge density and oxophilicity of
Al3+, the binding site would possibly involve the carbonyl oxygen
atoms. To verify this hypothesis, the amide free-analogue 2 was
synthesized and its interaction with Al3+ studied. The emission
spectrum of 2 showed essentially no spectral changes even in the
presence of a large excess of Al3+ salt (Fig. S3 in ESIw). This
indicates that the amide carbonyl oxygen is crucial for the binding
of Al3+. The binding of the Al3+ ion to the carbonyl oxygen of
the amide could also be rationalized by the fact the higher electron
density located on the oxygen of amide through the delocalization
of the nitrogen lone pair on the carbonyl oxygen is favourable for
the binding of the electron-deficient Al3+
.
Computational studies have been performed by using
local density functionals of the Slater type to investigate the
molecular structure of complex 1 and its Al3+ ion-bound
adduct (1ÁAl3+) (see the computational details in ESIw). The
optimized structure of 1 reveals a cone conformation of the
Fig. 2 Emission spectral changes of 1 (1.24 Â 10À4 M) upon addition
of various concentrations of Al(ClO4)3 in CH2Cl2–MeCN (1 : 1 v/v,
0.1M nBu4NPF6) upon excitation wavelength at 345 nm. Inset:
Photograph showing the luminescence colour change upon the addition
of Al3+ to induce the switching on of AuÁ Á ÁAu interactions.
transition, mixed with an alkynyl-to-aryl isocyanide LLCT
character is tentatively assigned.
Upon addition of Al3+, the absorption band at ca. 306 nm
showed a drop in intensity, with a concomitant growth of a
higher energy band at ca. 264 nm and and a new low-energy
shoulder at ca. 350 nm (Fig. 1). Three well-defined isosbestic
points at ca. 254, 281 and 345 nm were observed, indicative of
a clean conversion of 1 to a new chemical species, probably the
ion-bound adduct. A binding constant, log Ks, of 4.6 Æ 0.04
was obtained from a nonlinear least-squares fit according to a
1 : 1 binding stoichiometry (Fig. 1 upper inset). The close
agreement of the experimental data to the theoretical fit is
supportive of a 1 : 1 complexation stoichiometry. Furthermore,
this 1 : 1 binding mode has been confirmed by the Job’s method
of continuous variation16 (Fig. 1 lower inset).
Fig. 3 Responses of 1 (1.6 Â 10À4 M) in CH2Cl2–MeCN (1 : 1 v/v,
0.1 M nBu4NPF6) upon addition of 3 equiv of different metal ions.
Excitation was at their respective isosbestic wavelength, and the
emission was monitored at 665 nm.
c
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1
binding mode has further been supported by H NMR spectro-
scopy of 1 in the presence of Al3+ in d6-acetone. A downfield
shift of the amide proton and neighbouring proton resonances
was observed as a result of the electron-withdrawing effect of the
cation-binding. Nevertheless, a slight upfield shift of the hydroxy
proton resonances was found. It may be due to the removal
of the intramolecular hydrogen bonding interactions between
the ether oxygen and the OH groups upon Al3+ ion binding
(Fig. S4 in ESIw).
In conclusion, a novel calix[4]arene alkynylgold(I) isocyanide
complex has been designed and synthesized, and demonstrated
to show selective binding towards Al3+. The unique mode of
signal transduction by the switching on of AuÁ Á ÁAu interactions
to give a visual luminescence change from green to orange-red
color has been demonstrated.
V.W.-W.Y. acknowledges support from the University Grants
Committee Areas of Excellence Scheme (AoE/P-03/08) and
the Research Grants Council of Hong Kong Special Adminis-
trative Region, China (HKU 7060/09P). F.K.-W.H acknowledges
the receipt of a Postgraduate Studentship administered by The
University of Hong Kong. We also thank the Computer Centre at
the University of Hong Kong for providing the computational
resources.
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8780 Chem. Commun., 2011, 47, 8778–8780
This journal is The Royal Society of Chemistry 2011