Syntheses, Structural Characterization, and Host-Guest Chemistry of Ethynylcrown Ether Containing Polynuclear Gold(I) Complexes

Article abstract of DOI:10.1021/ic0499603

Two novel ethynylcrown ether containing di- and tetranuclear gold-(I) complexes have been synthesized and structurally characterized; their binding ability toward various metal ions has also been studied.

Full text of DOI:10.1021/ic0499603

Inorg. Chem. 2004, 43, 2225−2227
Syntheses, Structural Characterization, and Host−Guest Chemistry of
Ethynylcrown Ether Containing Polynuclear Gold(I) Complexes
Xiao-Xia Lu, Chi-Kwan Li, Eddie Chung-Chin Cheng, Nianyong Zhu, and Vivian Wing-Wah Yam*
Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug
DiscoVery and Synthesis, Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based
Materials Research, and Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road,
Hong Kong SAR, People’s Republic of China
Received January 8, 2004
a
Scheme 1
Two novel ethynylcrown ether containing di- and tetranuclear gold-
(I) complexes have been synthesized and structurally characterized;
their binding ability toward various metal ions has also been studied.
The blossoming of gold(I) chemistry in the past few
decades has been partially invoked by the presence of its
unique aurophilicity and the associated rich photolumines-
cence properties.1-3 Recently we reported a series of di-
a
nuclear gold(I) mercaptocrown ether containing complexes
as luminescence chemosensors toward alkali metal ions.4 The
Au‚‚‚Au interactions and a low-energy emission character-
istic of the presence of Au‚‚‚Au interactions were shown to
be switched on upon the encapsulation of metal ions of
appropriate sizes. With the recent enormous attention focused
on the chemistry of carbon-rich organometallics,5,6 it was
thought that the preferred linear two-coordinate geometry
and unique aurophilicity of gold(I) centers, together with the
sp hybridized carbon-rich alkynyls, could also be employed
for the design of luminescent chemosensors. Alkynyls should
serve as good candidates for the design of chemosensing
systems due to their simple linear geometry and rigidity, in
addition to the presence of a conjugated π system which is
usually involved in the origin of the luminescence. Herein
we report the design and syntheses of a novel class of crown
ether ligands of different cavity sizes that possess two and
four alkynyl units attached in a branched fashion. Bis- and
Reagents and conditions: (i) ICl, CHCl3; (ii) HCtCSiMe3, Et3N,
Pd(PPh3)2Cl2, CuI; (iii) Na2CO3, MeOH; (iv) Et3N, Au(PPh3)Cl, CuI, THF.
tetrakis-aurated complexes have also been successfully
isolated and structurally characterized, and these, to the best
of our knowledge, also represent the only examples of
disubstitution of ethynylgold(I) at the ortho positions of the
benzene moiety to date. Meta- and para-disubstituted ethynyl-
gold(I) aryls on the other hand are not uncommon.7 Their
binding behavior toward alkali metal ions has also been
investigated.
4,5-Diethynylbenzo-15-crown-5 [(HCtC)2B15C5-4,5] and
4,4′,5,5′-tetraethynyldibenzo-18-crown-6 [(HCtC)4DB18C6-
4,4′,5,5′] were obtained by iodination of the respective benzo
crowns using ICl in CHCl3, followed by Sonogashira cross-
coupling reaction and subsequent desilylation of the tri-
methylsilylalkynes (Scheme 1).8 Reaction of [(HCtC)2B15C5-
4,5] with 2 equiv of [Au(PPh3)Cl] in the presence of an
excess of triethylamine and a catalytic amount of CuI in thf
afforded [{(Ph3P)AuCtC}2B15C5-4,5] (1) as yellow crystals
after subsequent recrystallization from dichloromethane-n-
hexane. [{(Ph3P)AuCtC}4DB18C6-4,4′,5,5′] (2) was pre-
pared similarly to 1 except that [(HCtC)4DB18C6-4,4′,5,5′]
and 4 equiv of [Au(PPh3)Cl] were used instead. The identities
* Author to whom correspondence should be addressed. E-mail: wwyam@
hku.hk.
(1) (a) Schmidbaur, H. Gold Bull. 1990, 23, 11. (b) Schmidbaur, H. Chem.
Soc. ReV. 1995, 391.
(2) Pyykko¨, P. Chem. ReV. 1997, 97, 597.
(3) Yam, V. W.-W.; Lai, T.-F.; Che, C.-M. J. Chem. Soc., Dalton Trans.
1990, 3747.
(4) (a) Yam, V. W.-W.; Li, C.-K.; Chan, C.-L. Angew. Chem., Int. Ed.
1998, 37, 2857. (b) Yam, V. W.-W.; Chan, C.-L.; Li, C.-K.; Wong,
K. M.-C. Coord. Chem. ReV. 2001, 216-217, 173.
(5) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586.
(6) (a) Yam, V. W.-W. Acc. Chem. Res. 2002, 35, 555. (b) Yam, V. W.-
W. Chem. Commun. 2001, 789. (c) Yam, V. W.-W.; Lo, K. K.-W.;
Wong, K. M.-C. J. Organomet. Chem. 1999, 578, 3.
(7) See for example: (a) Vicente, J.; Chicote, M. T.; Alvarez-Falco´n, M.
M.; Abrisqueta, M.-D.; Herna´ndez, F. J.; Jones, P. G. Inorg. Chim.
Acta 2003, 347, 67. (b) Irwin, M. J.; Vittal, J. J.; Puddephatt, R. J.
Organometallics 1997, 16, 3541. (c) Puddephatt, R. J. Coord. Chem.
ReV. 2001, 216-217, 313 and references therein.
(8) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627.
10.1021/ic0499603 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/09/2004
Inorganic Chemistry, Vol. 43, No. 7, 2004 2225

COMMUNICATION
Table 1. Photophysical Data of Complexes 1 and 2
absorption λ/nm
emission λ/nm
(τ0/µs)
a
complex
(ꢀ/dm3 mol-1 cm-1
)
medium (T/ K)
1
268 (41990)
276sh (33910)
320 (22230)
334 (22750)
268 (68465)
276sh (56195)
317 (42280)
335 (30150)
solid (298)
solid (77)
CH2Cl2 (298)
CH2Cl2 (77)
solid (298)
solid (77)
530 (<0.1)
535 (<0.1)
530 (1.2)
500, 538 (4.7)b
555 (<0.1)
555 (<0.1)
560 (1.7)
2
CH2Cl2 (298)
CH2Cl2 (77)
535 (<0.1)
a Measured in CH2Cl2 at 298 K. b Vibronically structured, with progres-
sional spacings of ca. 1400 cm-1
.
absorption bands at 268-276 nm are tentatively assigned to
the intraligand transition characteristic of triphenylphosphine,
since similar absorption bands are also observed in the
mononuclear [Au(PPh3)(CtCPh)] and [Au(PPh3)Cl].9,10 The
lower energy absorption bands at 317-335 nm are assigned
as π f π* intraligand transitions of the alkynyl ligands. This
is supported by the observation of similar bands in the
mononuclear [Au(PPh3)(CtCPh)] and [Au(PPh3)(CtCC6H4-
OMe-p)], as well as the dinuclear [Au2(dppb)(CtCPh)2].9,10
With reference to the electronic absorption studies of the
related alkynylgold(I) phosphine complexes,9,10 the low
energy absorption tails extending to ca. 400-500 nm may
be characteristic of the alkynylgold(I) systems. Excitation
of solid samples and dichloromethane solutions of 1 and 2
at λ > 350 nm at both room temperature and 77 K produced
long-lived intense luminescence at ca. 530-560 nm. The
relatively large Stokes shift together with the observed
luminescence lifetimes in the microsecond range are sug-
gestive of emission with triplet parentage. An origin tenta-
tively assigned to states arising from σ(Au-P) f π*(CtC)
or metal-perturbed intraligand π f π*(CtC) transition is
suggested. In view of the lack of intra- and intermolecular
Au‚‚‚Au interactions, as found in the X-ray crystal structures,
the slight red shift of the emission energy from 1 to 2 may
be ascribed to the different electronic effects of the tetra-
ynyl versus the di-ynyl ligands.
Figure 1. Perspective drawings of (a) [{(Ph3P)AuCtC}2B15C5-4,5] (1)
and (b) [{(Ph3)PAuCtC}4DB18C6-4,4′,5,5′] (2) with atomic numbering
scheme. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids
are shown at the 30% probability level.
of both 1 and 2 have been confirmed by satisfactory
elemental analyses, 1H NMR, 31P{1H} NMR, IR, and positive
FAB-MS. Their solid-state structures have also been deter-
mined by X-ray crystallography.
The perspective drawings of 1 and 2 are shown in Figure
1. The molecular structure of 1 is essentially planar except
for the two triphenylphosphine units which are arranged in
a typical trigonal pyramidal fashion. The angle of 178.9°
for C(1)-Au(1)-P(1) is close to linearity and is typical of
sp hybridization for the Au(I) and acetylenic carbon. The
CtC bond distance of 1.187(9) Å is also typical of gold(I)
alkynyl systems. The shortest Au‚‚‚Au contact is 7.499 Å,
which indicates the absence of both inter- and intramolecular
Au‚‚‚Au interactions. Complex 2 adopts a “chair” conforma-
tion with the 4,4′,5,5′-tetraethynyldibenzo-18-crown-6 unit
at the center, and linked to the four Au(PPh3) units via the
acetylenic carbons on the two sides of the ethynylcrown
ether, with two -P-Au-CtC- units on the same side of
the complex pointing up and the other two pointing down.
The two adjacent -P-Au-CtC- units on the same side
of the complex deviate only slightly from coplanarity. The
respective P(1)-Au(1)-C(37) and Au(1)-C(41)-C(44)
angles of 176.9° and 174.0° are almost linear, with slight
deviations from the ideal 180°. All other bond angles and
bond lengths are normal and comparable to that of 1.
Both complexes 1 and 2 exhibit intense absorptions at ca.
268-335 nm with tails extending to ca. 400 nm for the
former and ca. 500 nm for the latter (Table 1). The intense
The change in the UV-vis absorption spectra upon
addition of alkali metal ions to a solution of 1 or 2 in CH2-
n
Cl2-MeOH (1:1 v/v, 0.1 M Bu4NPF6) was monitored.
Addition of Na+ to 1 (Figure 2) and K+ to 2 gave rise to
observable UV-vis absorption spectral changes. For in-
stance, clean isosbestic points at 290, 314, and 355 nm for
1 were observed in Figure 2, indicating that the reaction is
clean and probably involves only two absorbing species
present in equilibrium in the solution. The binding constants
(Ks) of the ethynylcrown ether containing complexes for
metal ions at 298 K are determined by using the equation
derived for a 1:1 complexation stoichiometry.11,12 The close
(9) Li, D.; Hong, X.; Che, C. M.; Lo, W. C.; Peng, S. M. J. Chem. Soc.,
Dalton Trans. 1993, 2929.
(10) (a) Yam, V. W.-W.; Choi, S. W.-K. J. Chem. Soc., Dalton Trans.
1996, 4227. (b) Yam, V. W.-W.; Choi, S. W.-K.; Cheung, K.-K.
Organometallics 1996, 15, 1734. (c) Yam, V. W.-W.; Cheung, K.-L.;
Yuan, L.-H.; Wong, K. M.-C.; Cheung, K.-K. Chem. Commun. 2000,
16, 1513.
(11) Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552.
(12) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.; Cheung,
K.-K. Inorg. Chem. 2001, 40, 571.
2226 Inorganic Chemistry, Vol. 43, No. 7, 2004

COMMUNICATION
In summary, two novel crown ethers that are functionalized
with two and four ethynyl units have been successfully
synthesized. Incorporation of Ph3PAu units to these ligands
can give the respective di- and tetranuclear gold(I) com-
plexes, all of which have been structurally characterized and
shown to exhibit rich photoluminescence properties. The
binding ability toward various metal ions has also been
studied. Such a class of complexes may serve as model
systems for the design and development of molecular
chemosensors as well as for the construction of molecular
assemblies and architecture.
Figure 2. The electronic absorption spectral traces of [{(Ph3P)AuCtC}2-
B15C5-4,5] (1) in CH2Cl2-MeOH (1:1 v/v) upon addition of NaClO4 at
298 K.
agreement of the experimental data to the theoretical fits is
supportive of a 1:1 complexation stoichiometry. It is found
that the log Ks values of 1 for Na+ and 2 for K+ ions are 3.2
and 5.1, respectively. Studies of the binding behavior of 1
toward K+ and 2 toward Cs+ have also been pursued, but
only very little spectral changes have been observed, making
the determination of the corresponding binding constants
unreliable and difficult. Such findings signify the 1:1 binding
ability of alkali metal ions toward crown ether moieties of
appropriate cavity sizes and suggest the preferential binding
of Na+ ions to 1 and K+ to 2. The ion-binding processes
were also investigated by emission spectrophotometric
methods, albeit the spectroscopic changes upon addition of
ions were too small to be determined. Nevertheless, uptake
of alkali metal cations by the crown moieties of 1 and 2 has
been further supported by the positive ESI-MS experiments.
For 1 and 2, 1:1 adducts such as [1‚Na]+ and [2‚K]+ could
be observed.
Acknowledgment. V.W.-W.Y. acknowledges support
from the University Grants Committee Area of Excellence
Scheme, The University of Hong Kong Foundation for
Education Development and Research Limited, C.-K.L. the
receipt of a postgraduate studentship, and E.C.-C.C. the
receipt of university postdoctoral fellowships from The
University of Hong Kong. The work has been supported by
a CERG Grant from the Research Grants Council of the
Hong Kong Special Administrative Region, China (Project
No. HKU 7097/01P).
Supporting Information Available: Characterization data of
ligands and complexes 1 and 2 and the X-ray crystallographic
determination data of 1 and 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0499603
Inorganic Chemistry, Vol. 43, No. 7, 2004 2227