A two-co-ordinated gold(I) loop [Au(dpdo)]ClO4 [dpdo = 1,8-bis(diphenylphosphino)-3,6-dioxaoctane] as a luminescence light switch for substrate binding reactions

Article abstract of DOI:10.1039/a802464b

Reaction of [Au(tht)Cl] (tht = tetrahydrothiophene) with dpdo [dpdo = 1,8-bis(diphenylphosphino)-3,6-dioxaoctane] afforded [Au(dpdo)]⁺ 1 which was isolated as a perchlorate salt; binding of a phosphine moiety to 1 in solution triggered a strong photoluminescence.

Full text of DOI:10.1039/a802464b

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A two-co-ordinated gold(I) loop [Au(dpdo)]ClO4 [dpdo ؍ 1,8-
bis(diphenylphosphino)-3,6-dioxaoctane] as a luminescence light
switch for substrate binding reactions
Wing-Han Chan,a Thomas C. W. Makb and Chi-Ming Che*,†,a
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
b Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong
adopting a trans-chelating geometry. The Au atom is two-co-
Reaction of [Au(tht)Cl] (tht = tetrahydrothiophene) with dpdo
[dpdo = 1,8-bis(diphenylphosphino)-3,6-dioxaoctane] afforded
[Au(dpdo)]ϩ 1 which was isolated as a perchlorate salt; binding
of a phosphine moiety to 1 in solution triggered a strong
photoluminescence.
ordinated and adopts a nearly linear configuration [P(1)᎐Au(1)᎐
P(2) angle of 172.2(1)Њ]. The Au᎐O distances of 3.14 and 3.23 Å
indicate insignificant interaction between the Au and oxygen
atoms.
The 31P NMR spectrum of 1 shows a singlet at δ 35.2
(CD2Cl2, room temperature). Addition of stoichiometric
amounts of PPh3 or dppm gradually shifts the phosphorus
resonances to values closer to that of free PPh3 or dppm. Cool-
ing the solution to Ϫ70 ЊC leads to three singlets corresponding
to 1, species 2 or 3 and free PPh3 or dppm (Scheme 1). Attempts
to isolate the gold–phosphine species 2 by addition of diethyl
A ‘molecular light switch’ effect, i.e. the conversion of a non-
emissive metal complex to a strongly emissive one through
substrate binding reactions, is of increasing importance in
molecular recognition and sensory materials research. The
notable examples in this area are [Ru(bpy)2(dppz)]2ϩ [bpy =
2,2Ј-bipyridine, dppz = dipyrido(3,2-a:2Ј,3Ј-c)phenazine] for
DNA binding and a cyclodextrin derivative with a europium
aza crown for incorporation of aromatic hydrocarbons.1,2 In
these two cases, the external substrates (DNA and aromatic
hydrocarbons) bind to the molecular sensors through non-
covalent hydrophobic interactions. Our attention has focused
on mononuclear two-co-ordinate gold() complexes which are
known to be non-emissive but display strong photolumines-
cence upon interaction with added nucleophiles.3–6 In this
context, the gold() loop [Au(dpdo)]ClO4 [1]ClO4 [dpdo = 1,8-
bis(diphenylphosphino)-3,6-dioxaoctane] (Scheme 1) is of
interest. This complex is isostructural to [Au(PPh3)2]ϩ but the
chelating nature of the dpdo ligand imparts stability and dis-
favors phosphine ligand dissociation from the Au atom.4 We
herein report that photoluminescence is ‘switched on’ through
substrate binding reactions to cation 1. Among the nucleo-
philes studied, including SCNϪ and CNϪ, the light switching-on
effect is unique to phosphine ligands, suggesting that 1 is a
good sensor for phosphine detection even at 10Ϫ5 mol dmϪ3
concentration.
+
+
2+
P
P
P
P
dppm
Au
Au
Au
Ph3P Au
PPh3
O
P
P
P
P
P
P
2
1
3
PPh2
P
P = Ph2P
O
Scheme 1 Equilibria between cation 1 and PPh3 or dppm in solution
Reaction of [Au(tht)Cl] (tht = tetrahydrothiophene) with a
stoichiometric amount of dpdo in dichloromethane at room
temperature readily afforded 1, which was isolated as the per-
chlorate salt.‡ The structure of [1]ClO4 has been established by
an X-ray crystal analysis.§ As shown in Fig. 1, the complex
cation 1 displays a loop-like structure with the dpdo ligand
† E-Mail: CMCHE@HKUCC.HKU.HK
‡ A mixture of [Au(tht)Cl] (0.32 g, 1.0 mmol) and dpdo (0.89 g, 1.0
mmol) in dichloromethane (50 ml) was stirred at room temperature for
2 h. The solvent was removed in vacuo leaving a white residue. Meta-
thesis of the crude product with LiClO4 (0.11 g, 1.0 mmol) in methanol
(5 ml) afforded a colorless crystalline solid. Crystals of [1]ClO4 were
obtained by diffusion of diethyl ether into an acetonitrile solution (yield
0.51 g, 65%) (Found: C, 45.87; H, 4.15. Calc. for C30H32AuClO6P2: C,
46.02; H, 4.12%).
§ Crystal data for [1]ClO4: C30H32AuClO6P2, M = 782.9, monoclinic,
space group Cc, a = 15.605(1), b = 13.473(1), c = 15.962(1) Å, β =
117.12(1)Њ, U = 2987.0(2) Å3, Z = 4, µ(Mo-Kα) = 5.164 mmϪ1, no. of
unique reflections = 3567, no. of reflections with F > 4.0σ(F ) = 2888,
R = 0.033, RЈ = 0.038, T = 294 K. CCDC reference number 186/1024.
Fig. 1 A perspective drawing of complex cation 1. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles (Њ):
Au(1)᎐P(1) 2.328(6), Au(1)᎐P(2) 2.293(6), Au(1) ؒ ؒ ؒ O(1) 3.23,
Au(1) ؒ ؒ ؒ O(2) 3.14; P(1)᎐Au(1)᎐P(2) 172.2(1)
J. Chem. Soc., Dalton Trans., 1998, Pages 2275–2276
2275

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suggest that the emission comes from a three-co-ordinated
gold–phosphine species 2 that is in equilibrium with 1 and PPh3
(Scheme 1). As expected, the emission intensity increases with
further addition of PPh3 but reaches a plateau value at high
PPh3 concentrations (Fig. 2 and insert). The estimated equi-
librium constant of the PPh3-binding reaction is not large
(32 3 dm3 molϪ1), and hence the dramatic photolumines-
cence enhancement is likely due to a high emission quantum
yield of the three-co-ordinated gold() phosphine species.
Enhancement of emission has also been observed with dppm
(ca. 10 molar equivalents). However, the emission maximum is
red-shifted to 610 nm (τ = 10.8 µs). Presumably, it is due to the
bonding interaction between adjacent AuP3 units (Scheme 1).
Other nucleophiles X (X = Et2S, SCNϪ or CNϪ) (≈1000 molar
equivalents), do not lead to any notable ‘molecular light switch’
effects. This may be attributed to low formation constants and/
or low emission quantum yields of the three-co-ordinated
AuP2X species.
Table 1 Photophysical data for degassed solutions of [1]ClO4 * in the
presence of different substrates measured at room temperature
Substrate
λmaxem/nm
(excited at
328 nm)
(concentration/
mol dmϪ3
)
Solvent
τ/µs
—
—
CH3CN
DMF
CH3CN
CH3CN
CH3CN
CH3CN
DMF
—
—
—
4.7
10.8
—
—
—
468
510
610
—
—
474
PPh3 (10Ϫ5–10Ϫ1
)
dppm (10Ϫ4
Et2S (р10Ϫ2
NaSCN (р10Ϫ2
NaCN (р10Ϫ2
)
)
)
)
* Concentration of [1]ClO4 in acetonitrile ≈ 1 × 10Ϫ5 mol dmϪ3
.
In this work, we have shown that photoluminescence is trig-
gered through binding of phosphine substrates to the co-
ordinatively unsaturated AuI. Compared with [Au(PPh3)2]ϩ, the
structure of 1 is more robust and hence the complex can be
exploited as a sensitive spectroscopic probe for compounds
with a phosphine moiety.
Acknowledgements
We acknowledge support from The University of Hong Kong,
the Hong Kong Research Grants Council and the Croucher
Foundation.
References
1 A. E. Friedman, J. C. Chambron, J. P. Sauvage, N. J. Turro and J. K.
Barton, J. Am. Chem. Soc., 1990, 112, 4960; R. M. Hartshorn and
J. K. Barton, J. Am. Chem. Soc., 1992, 114, 5919; Y. Jenkins and J. K.
Barton, J. Am. Chem. Soc., 1992, 114, 8736.
2 M. A. Mortellaro and D. G. Nocera, J. Am. Chem. Soc., 1996, 118,
7414.
3 J. C. Vickery, M. M. Olmstead, E. Y. Fung and A. L. Balch, Angew.
Chem., Int. Ed. Engl., 1997, 36, 1179.
4 M. A. Mansour, W. B. Connick, R. J. Lachicotte, H. J. Gysling and
R. Eisenberg, J. Am. Chem. Soc., 1998, 120, 1329.
Fig. 2 Room temperature emission spectra of [1]ClO4 in the presence
of PPh3 in degassed acetonitrile solution. Molar ratio of PPh3 :1 = X:1.
Insert: the graph shows a plot of the emission intensity of 1 ϩ PPh3 vs.
the molar concentration of PPh3 :1 (concentration of [1]ClO4 = 10Ϫ5
mol dmϪ3
)
ether to an acetonitrile solution were unsuccessful and the start-
ing complex [1]ClO4 was recovered. Other nucleophiles such as
Et2S, SCNϪ and CNϪ with concentrations 100 times higher
than that of PPh3 do not show any notable effect on the 31P
NMR spectrum of [1]ClO4 in acetonitrile.
5 C. King, M. N. I. Khan, R. J. Staples and J. P. Fackler, jun., Inorg.
Chem., 1992, 31, 3236.
6 C. M. Che, H. L. Kwong and C. K. Poon, J. Chem. Soc., Dalton
Trans., 1990, 3215; S. J. Shieh, D. Li, S. M. Peng and C. M. Che,
J. Chem. Soc., Dalton Trans., 1993, 195; R. H. Uang, C. K. Chan,
S. M. Peng and C. M. Che, J. Chem. Soc., Chem. Commun., 1994,
2561; B. C. Tsang, W. C. Lo, C. M. Che and S. M. Peng, Chem.
Commun., 1996, 181.
The photophysical data of [1]ClO4, and its solutions in the
presence of different substrates, are listed in Table 1. The com-
plex is non-emissive in the solid state and in degassed
acetonitrile solution. However, addition of PPh3 (у10Ϫ5 mol
dmϪ3
)
immediately triggers an intense yellow photo-
luminescence at 510 nm with a long lifetime (τ = 4.7 µs). We
Received 31st March 1998; Communication 8/02464B
2276
J. Chem. Soc., Dalton Trans., 1998, Pages 2275–2276