Self-assembly and molecular recognition of a luminescent gold rectangle

Article abstract of DOI:10.1021/ja0456508

A luminescent molecular rectangle [Au⁴(u-PAnP) ₂(u-bipy)₂](OTf)₄ (1·(OTf)₄) (PAnP = 9,10-bis-(diphenylphosphino)anthracene, bipy = 4,4′-bipyridine, X = NO₃^- or OTf^-), synthesized from the self-assembly of the molecular clip Au₂(u-PAnP)(OTf) ₂ and bipy, shows a large rectangular cavity of 7.921(3) x 16.76(3) A. The electronic absorption and emission spectroscopy, and electrochemistry of the metallacyclophane, have been studied. The 1⁴⁺ ions are self-assembled into 2D mosaic in the solid state via complementary edge-to-face interactions between the Ph groups. ¹H NMR titrations ratify the 1:1 complexation between 1⁴⁺ and various aromatic molecules. Comparing the structures of the inclusion complexes indicates an induced-fit mechanism operating in the binding. The emission of 1⁴⁺ is quenched upon the guest binding. The binding constants are determined by both ¹H NMR and fluorescence titrations. Solvophobic and ion-dipole effects are shown to be important in stabilizing the inclusion complexes.

Full text of DOI:10.1021/ja0456508

Published on Web 11/11/2004
Self-Assembly and Molecular Recognition of a Luminescent
Gold Rectangle
Ronger Lin,† John H. K. Yip,*,† Ke Zhang,† Lip Lin Koh,† Kwok-Yin Wong,‡ and
Kam Piu Ho‡
Contribution from the Department of Chemistry, The National UniVersity of Singapore,
10 Kent Ridge Crescent, 119260, Singapore, and Department of Applied Biology and
Chemical Technology, Hong Kong Polytechnic UniVersity, Hunghom, Kowloon,
Hong Kong SAR, People’s Republic of China
Received July 20, 2004; E-mail: chmyiphk@nus.edu.sg
Abstract: A luminescent molecular rectangle [Au4(µ-PAnP)2(µ-bipy)2](OTf)4 (1‚(OTf)4) (PAnP ) 9,10-bis-
(diphenylphosphino)anthracene, bipy ) 4,4′-bipyridine, X ) NO3 or OTf-), synthesized from the self-
-
assembly of the molecular “clip” Au2(µ-PAnP)(OTf)2 and bipy, shows a large rectangular cavity of 7.921(3)
× 16.76(3) Å. The electronic absorption and emission spectroscopy, and electrochemistry of the
metallacyclophane, have been studied. The 14+ ions are self-assembled into 2D mosaic in the solid state
via complementary edge-to-face interactions between the Ph groups. 1H NMR titrations ratify the 1:1
complexation between 14+ and various aromatic molecules. Comparing the structures of the inclusion
complexes indicates an induced-fit mechanism operating in the binding. The emission of 14+ is quenched
upon the guest binding. The binding constants are determined by both 1H NMR and fluorescence titrations.
Solvophobic and ion-dipole effects are shown to be important in stabilizing the inclusion complexes.
Introduction
of compounds is usually synthesized by the coordination-
directed self-assembly of transition metal and multitopic
A major facet of host-guest chemistry is to understand
molecular recognition, a multitude of processes and interactions
that lies at the heart of many biological phenomena.1 On the
practical side, the knowledge gleaned from the studies provides
leads to the design of artificial supramolecules that are applicable
in advanced technologies such as catalysis, chemical sensing,
molecular machinery, and electronics.2 Traditionally, the field
has been dominated by organic molecules, and the host-guest
chemistry of receptors such as crown ether,3 cryptand,4 and
cyclophanes5 has been studied extensively. Recently, there is a
burgeoning interest in the inorganic counterparts of cyclophanes,
often known as metallacyclophanes or metallacycles. This class
ligands.6 This synthetic methodology has been proved to be very
versatile and far-reaching, as based on it a myriad of molecular
triangles,7 squares,7d,8 rectangles,8y,9 pentagons,10 hexagons,11
marcocycles,12 polyhedrons, and cages13,14 have been created.
A major advantage of metallacyclophanes over the organic
cyclophanes is their modularity that allows control over and
fine-tuning of their dimensions, topology, electronic properties,
and binding selectivity. Furthermore, it is synthetically straight-
forward to introduce functional units such as catalytic,7m,15
luminescent,7c,d,8n-p,s,t,x,9f,l-n,11b,12e,16 or redox-active centers8r,16b,17
into the frameworks of the metallacyclophanes. These properties
of metallacyclophanes entail their potential applications in
advanced materials, and indeed significant progress of engineer-
† The National University of Singapore.
‡ Hong Kong Polytechnic University.
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J. AM. CHEM. SOC. 2004, 126, 15852-15869
10.1021/ja0456508 CCC: $27.50 © 2004 American Chemical Society

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
ing metallacyclophanes into luminescent and electrochemical
sensors, molecular sieves, and catalysts has been made in recent
years.15-18 Aside from the practical aspects, study of the
thermodynamics and kinetics of the self-assembly of metalla-
cyclophanes would provide important insights into the evolution
and emergence of complex structures.14,19
[M4(en)4(µ-bipy)4](NO3)8 (M ) PdII, PtII, en ) 1,2-diaminoeth-
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could exhibit shape selectivity and higher affinity for electron-
rich aromatics.21 The substrate-binding properties of metalla-
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(19) (a) Hasenknopf, B.; Lehn, J.-M.; Boumediene, N.; Dupont-Gervais, A.;
Van Dorsselaer, A.; Kneisel, B.; Fenske, D. Angew. Chem., Int. Ed. 1998,
37, 3265. (b) Levin, M. J.; Stang, P. J. J. Am. Chem. Soc. 2000, 122, 7428.
(c) Kersting, B.; Meyer, M.; Powers, R. E.; Raymond, K. N. J. Am. Chem.
Soc. 1996, 118, 7221.
(10) (a) Hasenknopf, B.; Lehn, J.-M.; Baum, G.; Kneisel, B. O.; Fenske, D.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1838. (b) Campos-Fernandez, C.
S.; Clerac, R.; Koomen, J. M.; Russell, D. H.; Dunbar, K. R. J. Am. Chem.
Soc. 2001, 123, 773.
(20) Maverick, A. W.; Klavetter, F. E. Inorg. Chem. 1984, 23, 4129.
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15853

A R T I C L E S
Lin et al.
Scheme 1. Synthesis of the Gold Rectangle
cycles of ZnII,22 RuII,23 PdII,8i,9j,24 PtII,8i,k,24,25 RhI,16d ReI,7d,8o,9e
and OsVI 26 were studied. The metallacycles form inclu-
sion complexes with guests as diverse as substituted ben-
zenes,7d,8b,c,11g,18d,21-23 anthracenes,9j,22 square planar PdII and
PtII, and metalloporphyrin complexes.8o,9e The binding invokes
π-π interactions between the aromatic rings of the hosts and
guests,9j,11g,22 but, in some cases, other interactions such as
hydrogen bonds,8y,26 metal-ligand coordination,8i,25b and metal-
metal bonds9j are also involved. Metallacyclophanes whose
luminescence is responsive to the binding of the substrate are
of special interest because of the potential applications of the
compounds in chemical sensing. Examples of this class of
compounds include the molecular rectangles [M4(CO)12(µ-bipy)-
(bipm)2] (M ) MnI or ReI),9d,e,f a RhI2-cyclophane,16d and
Described in this paper are the synthesis, spectroscopy,
electrochemistry, solid-state self-assembly, and host-guest
chemistry of a luminescent gold rectangle. The present work
stemmed from our recent studies of the ligand 9,10-bis-
(diphenylphosphino)anthracene (PAnP) (Scheme 1).12e The
ligand, containing a chromophoric anthracene, can impart
luminescence to its metal complexes, as exemplified by the gold
ring [Au3(µ-PAnP)3⊃ClO4](ClO4)2 which displays intense ππ*
fluorescence in solution.12e Another special feature of PAnP is
the adjustable bite distance between the two PPh2 donor groups,
which is an attribute of the flexible anthracenyl backbone. Our
study showed that the two P-Au-X units of the binuclear Au2-
(µ-PAnP)X2 (X ) Cl and Br) are syn-oriented and widely
separated (dAu-Au ) 9.154(2)Å).29 This conformation gives
the complex a U-shape geometry and an approximate C2V
symmetry, which invites the application of the [Au2(µ-PAnP)]2+
unit as a molecular “clip” in assembling the molecular rect-
angles. The idea of using a molecular “clip” in assembling
metallacycles was illustrated by the recent work of Stang in
which the organoplatinum [Pt2(µ-anth2-)(OTf)2] clip was used
in constructing molecular rectangles [Pt4(PEt3)8(µ-anth2-)2(µ-
L)2](PF6)4, cages, and trigonal prisms.9g,h,30 Rheingold and
Bosnich also demonstrated that a molecular cleft comprising
cofacial terpyridyl PdII complexes can combine with bipy to
form molecular rectangle and trigonal prisms with large
cavity.9j,31 The well-known propensity32 of gold(I) for linear
coordination geometry has been exploited in constructing
metallacycles and catenanes.33 As shown in the work of
Puddephatt, coupling the digold(I) complexes [Au2(µ-Ph2P(CH2)m-
PPh2)(CF3CO2)2] (n ) 1, 3, 5) with the ditopic diisocyanoben-
zene, bipy or bpe (L′) gives rise to the metallacycles [Au4(µ-
various ReI4,7d,8o,16e,18f ReI2PdII ,18d and RuII4 squares,23 and ReI3
2
and RuII triangles.7d,23
3
Stereoelectronic complementarity27 between binding partners
and preorganization of the binding site28 are pertinent to the
selectivity and strength of host-guest binding. In this regard,
molecular rectangles with extended π-surface should be able
to host planar aromatic compounds. Molecular rectangles8y,9 are
less common than molecular squares. Among the early reported
molecular rectangles are binuclear PdII complexes which can
be assembled into [2]catenanes.9a,b Other examples include [M4-
(CO)12(µ-bipy)(bipm)2] (M ) MnI and ReI),9d,e,f [Pt4(PEt3)8(µ-
anth2-)2(µ-L)]4+ (anth ) anthracene-1,8-diyl, L ) bipy, 1,4-
bis(4-ethynylpyridyl)benzene, 2,5-bis(4-ethynylpyridyl)furan or
bpe),9g,h and rectangles bearing cofacial terpyridyl PdII com-
plexes.9j
(21) Fujita, M.; Nagao, S.; Ilida, M.; Ogata, K.; Ogura, K. J. Am. Chem. Soc.
1993, 115, 1574.
(22) Houghton, M. A.; Bilyk, A.; Harding, M. M.; Turner, P.; Hambley, T. W.
J. Chem. Soc., Dalton Trans. 1997, 2725.
(23) Xu, D.; Hong, B. Angew. Chem., Int. Ed. 2000, 39, 1826.
(24) Whiteford, J. A.; Lu, C. V.; Stang, P. J. J. Am. Chem. Soc. 1997, 119,
2524.
(25) (a) Stang, P. J.; Chen, K.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 8793.
(b) Whiteford, J. A.; Stang, P. J.; Huang, S. D. Inorg. Chem. 1998, 37,
5595.
(26) (a) Jeong, K.-S.; Cho, Y. L.; Song, J. U.; Chang, H.-Y.; Choi, M.-G. J.
Am. Chem. Soc. 1998, 120, 10982. (b) Chang, S.-Y.; Jeong, K.-S. J. Org.
Chem. 2003, 68, 4014. (c) Jeong, K.-S.; Choi, J.-S.; Chang, S.-Y.; Chang,
H.-Y. Angew. Chem., Int. Ed. 2000, 39, 1692.
(27) Reference 5, p 61.
(28) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1986, 25, 1039.
(29) Zhang, K.; Prabhavathy, J.; Yip, J. H. K.; Koh, L. L.; Tan, G. K.; Vittal,
J. J. J. Am. Chem. Soc. 2003, 125, 8452.
(30) (a) Mukherjee, P. S.; Das, N.; Kryschenko, Y. K.; Arif, A. M.; Stang, P.
J. J. Am. Chem. Soc. 2004, 126, 2464. (b) Kryschenko, Y. K.; Seidel, S.
R.; Muddiman, D. C.; Nepomuceno, A. I.; Stang, P. J. J. Am. Chem. Soc.
2003, 125, 9647. (c) Das, N.; Mukherjee, P. S.; Arif, A. M.; Stang, P. J. J.
Am. Chem. Soc. 2003, 125, 13950. (d) Kuehl, C. J.; Yamamoto, T.; Seidel,
S. R.; Stang, P. J. Org. Lett. 2002, 4, 913.
(31) (a) Crowley, J. D.; Goshe, A. J.; Bosnich, B. J. Chem. Soc., Chem. Commun.
2003, 2824. (b) Goshe, A. J.; Crowley, J. D.; Bosnich, B. HelV. Chim.
Acta 2001, 84, 2971. (c) Goshe, A. J.; Bosnich, B. Synlett 2001, 941.
(32) Gold: Progress in Chemistry, Biochemistry, and Technology; Schmidbaur,
H., Ed.; Wiley: New York, 1999.
9
15854 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Chart 1
Ph2P(CH2)mPPh2)2(µ-L′)2](OTf)4.34 In view of these findings,
we envisioned that the molecular rectangle [Au4(µ-PAnP)2(µ-
bipy)2](OTf)4 (1‚(OTf)4) could be assembled from two Au2(µ-
PAnP)2+ clips and two bipy (Scheme 1). Analogous to the
cyclophanes containing 4,4′-bipyridinium groups35 such as
Stoddart’s cyclobis(paraquat-p-phenylene) ion36 (CBPQT4+), the
gold rectangle is a receptor for the various aromatic molecules
depicted in Chart 1. The complexation between 14+ and the
aromatic guests, which have different molecular dimensions,
substituents, and extent of π-conjugation, provides insights into
the factors which govern the stability of the host-guest
complexes.
the nonlinear least-squares regression on the data of NMR and
fluorescence titrations.
Physical Methods. UV-vis absorption and fluorescence spectra
were recorded on a Hewlett-Packard HP8452A diode array spectro-
photometer and a Perkin-Elmer LS50B spectrofluorophotometer,
respectively. The averaged optical path length for the emitting light is
1 cm. Luminescence quantum yield was referenced to anthracene in
chloroform (Φstd ) 0.27).38 1H and 31P{1H} NMR spectra were recorded
on a Bruker ACF 300 spectrometer. Elemental analyses were carried
out in the department of chemistry, National University of Singapore.
Electrospray ionization mass spectra (ESI-MS) were measured on a
Finnigan MAT 731 LCQ spectrometer. Solid-state reflectance spectra
were recorded on Shimadzu UV-2401 spectrophotometer equipped with
a solid-state sample holder. The samples were prepared by grounding
the crystals of the compounds in Nujol. Cyclic voltammetry was
performed in a conventional two-compartment electrochemical cell with
deaerated acetonitrile containing 0.1 M n-Bu4PF6 as electrolyte. The
glassy carbon disk working electrode (area 0.07 cm2) electrode was
treated by polishing with 0.05 µm alumina on a microcloth and then
sonicated for 5 min in deionized water followed by rinsing with
acetonitrile used in the electrochemical studies. An Ag/AgNO3 (0.1 M
in CH3CN) electrode was used as the reference electrode, whereas a
platinum wire was used as the counter electrode. The half-wave potential
(E1/2) values are the average of the cathodic and anodic peak potentials
for the oxidative and reductive waves of reversible couples.39a The
potential of the complex was always referred to the half-wave potential
of the ferrocenium/ferrocene (Cp2Fe+/0) couple as the internal reference.39b
Experimental Section
General Method. All of the syntheses were carried out in N2
atmosphere with standard Schlenck techniques. 9,10-Bis(diphenylphos-
phino)anthracene (PAnP)12e and Au2(µ-PAnP)Cl2 were synthesized
29
according to the reported methods. KAuCl4 was purchased from Oxkem.
Silver triflate, Me2S, and 4,4′-bipyridine were purchased from Aldrich
and used without being purified. All of the aromatic guests, obtained
from Aldrich, were purified by recrystallization from hot ethanol. The
solvents used were purified according to the literature procedures.37
The software program Kaleidagraph version 3.5 was used to carry out
(33) (a) McArdle, C. P.; Van, S.; Jennings, M. C.; Puddephatt, R. J. J. Am.
Chem. Soc. 2002, 124, 3959. (b) Hunks, W. J.; MacDonald, M.-A.;
Jennings, M. C.; Puddephatt, R. J. Organometallics 2000, 19, 5063. (c)
McArdle, C. P.; Irwin, M. J.; Jennings, M. C.; Puddephatt, R. J. Angew.
Chem., Int. Ed. 1999, 22, 3376. (d) Qin, Z.; Jennings, M. C.; Puddephatt,
R. J. Chem.-Eur. J. 2002, 8, 735. (e) McArdle, C. P.; Irwin, M. J.; Jennings,
M. C.; Vittal, J. J.; Puddephatt, R. J. Chem.-Eur. J. 2002, 8, 723. (f)
McArdle, C. P.; Vittal, J. J.; Puddephatt, R. J. Angew. Chem., Int. Ed. 2000,
39, 3819.
Determination of the Solubility of the Aromatic Guests. The
solubility of the aromatic guests was determined by the UV-vis
absorption spectrometric method. Saturated solutions of the aromatic
guests were then prepared by adding the large excess of the compounds
to volumetric flasks containing 5 mL of CH3CN until solids appear. 1
mL of saturated solution was pipetted out and diluted to 50 mL or 100
mL. UV-vis absorption spectra of the diluted solutions were measured
to give the absorbance of the π f π* bands. The concentrations of the
compound in the saturated solutions were then determined from the
extinction coefficients of the bands.
(34) (a) Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1998, 1055. (b) Irwin,
M. J.; Vittal, J. J.; Yap, G. P. A.; Puddephatt, R. J. J. Am. Chem. Soc.
1996, 118, 13101. (c) Irwin, M. J.; Puddephatt, R. J.; Rendina, L. M.; Vittal,
J. J. J. Chem. Soc., Chem. Commun. 1996, 1281. (d) Brandys, M.-C.;
Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 2000, 4601.
(e) Burchell, T. J.; Eisler, D. J.; Jennings, M. C.; Puddephatt, R. J. J. Chem.
Soc., Chem. Commun. 2003, 2228.
(35) Bu¨hner, M.; Geuder, W.; Gries, W.-K.; Hu¨nig, S.; Koch, M.; Poll, T. Angew.
Chem., Int. Ed. Engl. 1988, 27, 1553.
Modified Job’s Plot. CD3CN solutions of [G]t/{[G]t + [H]t} ratio
ranging from 0 to 1 were prepared ([G]t and [H]t are the total
concentration of the guest and host, respectively). The sum of [G]t and
[H]t was kept constant in all of the solutions. The stoichiometry of the
host-guest complexation was determined from the x-coordinate at the
(36) (a) Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart,
J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1547. (b)
Ashton, P. R.; Odell, B.; Reddington, M. V.; Slawin, A. M. Z.; Stoddart,
J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1550. (c)
Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.;
Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszkiewicz,
M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart,
J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193. (d)
Nielsen, M. B.; Jeppesen, J. O.; Lau, J.; Lomholt, C.; Damgaard, D.;
Jacobsen, J. P.; Becher, J.; Stoddart, J. F. J. Org. Chem. 2001, 66, 3559.
(37) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals,
2nd ed.; Pergamon Press: Oxford, 1980.
maximum in a modified Job’s plot in which the y-axis is [G]t(δG
-
(38) CRC Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC
Press: Boca Raton, FL, 1989.
(39) (a) Gritzner, G.; Ku˚ta, J. Pure Appl. Chem. 1982, 54, 1527. (b) Gagne´, R.
R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2855.
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15855

A R T I C L E S
Lin et al.
δGo) (δGo ) chemical shifts of the free guest protons) and the x-axis is
[G]t/{[G]t + [H]t}.40a
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
Au2(µ-PAnP)(NO3)2‚0.5Et2O
1
1H NMR Titrations. In the H NMR titrations, [G]t is fixed while
Au(1)-O(1B)
Au(1)-O(1A)
Au(1)-O(2A)
Au(1)-P(1)
2.11(2)
2.19(4)
2.43(3)
2.208(2)
2.092(7)
2.213(2)
3.375(6)
P(1)-Au(1)-O(1A)
P(1)-Au(1)-O(2A)
P(1)-Au(1)-(O1B)
O(1A)-Au(1)-O(1B)
P(2)-Au(2)-O(4)
162.5(1)
145.0(8)
178.8(8)
49.6(6)
[H]t is varied. The chemical shift δG of certain protons of the guest is
plotted against [H]t. The binding constant KNMR, and the chemical shift
difference (∆δG) between the free guest (δGo) and the complexed guest
(δGC), were obtained from nonlinear least-squares regression onto the
data of the eq 1.40
Au(2)-O(4)
Au(2)-P(2)
177.0(3)
Au(1)-Au(2)
∆δG
o
PAnP)Cl2 and 2 mol equiv of AgNO3 in CH2Cl2. The X-ray
crystal structure of the complex (Figure 1 and Table 1) shows
a U-shape geometry similar to that of Au2(µ-PAnP)Cl2.29 The
two gold atoms Au(1) and Au(2) are syn to each other and
separated by 9.197(3) Å. Au(2) is coordinated linearly to an O
δG ) δG
-
(B - B2 - 4[H] [G] )
(1)
x
t
t
(
)
2[G]t
1
B ) [H]t + [G]t +
(1.1)
KNMR
∆δH values for the host protons were also determined by similar
least-squares fittings. In the cases of weakly binding guests, the shifts
were very small and the ∆δH values were estimated as the largest
chemical shifts observed in the titrations.
Fluorescence Quenching Studies. The emission spectra of CH3-
CN solutions containing the same [H]t but different [G]t were recorded.
The intensity ratio IH/IH ratio (IH and IH are the emission intensities
of the host in the presence and absence of the guest, respectively) was
plotted against [G]t, and the binding constants KQ were obtained by
fitting by nonlinear least-squares regression with the eq 3 where kH‚G
and kH are constants related to the fluorescence intensities of the host-
guest complex and host, respectively.41
-
atom of the NO3 ion and a P atom of the PAnP (P(2)-Au-
(2)-O(4) ) 177.0(3)°). On the other hand, the NO3- ion bonded
to Au(1) is disordered. The Au-P distances are typical,29,44 and
the Au(2)-O(4) (2.092(7) Å) and Au(1)-O(1B) (2.11(2) Å)
distances are close to the ones observed in Au(PMe3)(NO3)
(Au-O ) 2.093(6) Å).44 The central anthracenyl backbone is
slightly curve toward the metal ions, and the dihedral angle be-
tween the two lateral rings is 15.5°. In the crystal, the molecules
aggregate into dimers, showing an intermolecular Au-Au dis-
tance of 3.375(3) Å. Similar Au-Au distances (∼2.9-3.5 Å),
observed widely in the solid-state aggregations of AuI com-
plexes, have been taken as the structural evidence for aurophilic
interactions.45 In addition, the two concaved anthracenyl rings
partially overlap, and the separation between their best planes
is 3.7 Å, which is typical for aromatic π-π interactions.46 It is
possible that the solid-state aggregation of the gold complex is
the result of cooperative actions of aurophilic and aromatic π-π
interactions. The 31P NMR spectrum (121.5 MHz, CDCl3) of
the complex shows a singlet at δ 20.31. This indicates that
o
o
kH‚G
KQ[G]t
(
)
IH
o ) 1 +
kH
(2)
1 + KQ[G]t
IH
In the quenching of the emission of the guests by the hosts, the
emission spectra of solutions containing the same [G]t but different
[H]t were recorded. The emission intensity of the guest (IG) at the wave-
length λ of 1‚(OTf)4 was measured. The data were best-fitted by a mod-
ified Beer’s Law (eq 3),42 where ꢀ is the extinction coefficient of the
-
complex reverts to C2V symmetry in solution with both NO3
o
ions adopting the same coordination mode.
host at the wavelength λ and IG is the emission intensity at [H]t ) 0.
Synthesis of the Gold Rectangle. The gold rectangle [Au4-
(µ-PAnP)2(µ-bipy)2](NO3)4 (1‚(NO3)4) was synthesized in a high
yield of 82% by reacting the molecular “clip” Au2(µ-PAnP)-
(NO3)2 with 1 mol equiv of 4,4′-bipyridine (bipy) in CH2Cl2
for 5 h (Scheme 1). The product 1‚(NO3)4 was slowly precipi-
tated from the reacting solution as yellow solids. Metatheses
of 1‚(NO3)4 with NaOTf, LiClO4, and NaPF6 gives 1‚(OTf)4,
1‚(ClO4)4, and 1‚(PF6)4, respectively. While 1‚(NO3)4 is entirely
insoluble in CH2Cl2, CH3CN, and MeOH but dissolves fairly
well in a mixture of CH2Cl2/MeOH (8:2, v/v), the other three
compounds are soluble in CH3CN and a CH2Cl2/MeOH mixture
but sparingly soluble in CH2Cl2. The 1H NMR spectra of all of
the gold complexes are basically identical and will be discussed
later. Consistent with the rectangular structure of 14+, the 31P-
{1H} NMR spectra of the four complexes display a singlet at
similar chemical shift (δ 21.35-22.89).
o
IG
) ꢀ × [H]t
(3)
IG
X-ray Crystallography. The diffraction experiments were carried
out on a Bruker AXS SMART CCD 3-circle diffractometer with a
sealed tube at 23 °C using graphite-monochromated Mo KR radiation
(λ ) 0.71073 Å). The software used was as follows: SMART43a for
collecting frames of data, indexing reflection, and determination of
lattice parameters, SAINT43a for integration of intensity of reflections
and scaling, SADABS43b for empirical absorption correction, and
SHELXTL43c for space group determination, structure solution, and
2
least-squares refinements on |F| . Anisotropic thermal parameters were
refined for the rest of the non-hydrogen atoms. The hydrogen atoms
were placed in their ideal positions. A brief summary of crystal data
and experimental details are given in the Table S1, and the selected
bond length, angles, and structural parameters are given in Tables 1
and 2.
The ESI-MS spectra of 1‚(NO3)4, 1‚(OTf)4, 1‚(ClO4)4, and
1‚(PF6)4 (Figures S1-S20) measured in CH3CN display sig-
Results and Discussion
Structure of Au2(µ-PAnP)(NO3)2‚0.5Et2O. Complex Au2-
(µ-PAnP)(NO3)2 was synthesized from the reaction of Au2(µ-
(43) (a) SMART & SAINT Software Reference Manuals, Version 4.0; Siemens
Energy & Automation, Inc., Analytical Instrumentation: Madison, WI,
1996. (b) Sheldrick, G. M. SADABS: a Software for Empirical Absorption
Correction; University of Gottingen: Gottingen, Germany, 1996. (c)
SHELXTL Reference Manual, Version 5.03; Siemens Energy & Automation,
Inc., Analytical Instrumentation: Madison, WI, 1996.
(40) (a) Hirose, K. J. Inclusion Phenom. Macrocyclic Chem. 2001, 39, 193. (b)
Macomber, R. S. J. Chem. Educ. 1992, 69, 375.
(41) Connors, K. A. Binding Constants: The Measurement of Molecular
Complex Stability; Wiley-Interscience: New York, 1987; pp 24-28.
(42) Physical Chemistry: Principles and Applications in Biological Sciences;
Tinoco, I., Jr., Sauer, K., Wang, J. C., Eds.; Prentice Hall: New Jersey,
1995; pp 388-390.
(44) Mathieson, T.; Schier, A.; Schmidbaur, H. J. Chem. Soc., Dalton Trans.
2000, 3881.
(45) (a) Schmidbaur, H. Gold Bull. 1990, 23, 11. (b) Schmidbaur, H. Chem.
Soc. ReV. 1995, 24, 391.
(46) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
9
15856 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Table 2. Selected Bond Length (Å), Angles (deg), and Other Structural Parameters of 1 (OTf)4‚4.8H2O, (1⊃An)‚(OTf)4,
(1⊃Py)‚(OTf)4‚CH2Cl2, and (1⊃Br-An)‚(OTf)4‚Et2O
1 (OTf)4
‚
4.8H2O
(1
⊃An)
‚
(OTf)4
(1⊃Py)
‚
(OTf)4
‚
CH2Cl2
(1⊃Br−An)‚(OTf)4‚Et2O
Au-Aua (Å)
P-Pa (Å)
7.069(3)
6.375(3)
7.665(3)
7.921(3)
2.255(2)
2.044(1)
6.854(2)
6.351(3)
7.229(2)
7.341(4)
2.239(3)
2.052(3)
110.0(3)
178.5(3)
175.9(4)
177.5(3)
6.811(2)
6.342(2)
7.102(3)
7.187(3)
2.247(4)
2.063(1)
110.4(1)
177.7(3)
176.8(3)
178.7(2)
6.526(2), 6.604(2)
6.255(4), 6.256(4)
6.767(4), 6.910(4)
6.980(4), 6.981(4)
2.240(4), 2.246(4)
2.073(1), 2.071(1)
109.4(5), 109.5(5)
175.0(4), 178.5(5)
175.6(4), 176.4(4)
177.5(4)
3.401(4)
16.95
2.71
56.5, 49.9
40.0
13.2
25.9
21.4, 19.4
N-Na (Å)
γ-C-γ-Ca (Å)
Au(1)-P(1) (Å)
Au(1)-N(1) (Å)
C(6)-P(1)-Au(1) (deg)
P(1)-Au(1)-N(1) (deg)
Au(1)-N(1)-C(3) (deg)
N(1)-C(3)-C(3a) (deg)
db (Å)
108.7(3)
177.4(4)
173.4(4)
175.8(3)
3.645(2)
3.552(8)
X-Xc (Å)
16.76
21.69
43.7
22.3
16.8
16.76
10.42
43.3
30.5
10.5
12.4
14.7
17.05
3.60
47.4
31.4
8.0
θd (deg)
$e (deg)
φf (deg)
øg (deg)
τh (deg)
0.9
21.9
ꢀi (deg)
16.4
a Intrannular Au-Au, P-P, N-N, and γ-C-γ-C distances. b d ) interplanar distance between the 4,4′-bipyridine and the aromatic guest, defined as the
difference between b (the perpendicular distance between a carbon atom and the plane of the aromatic guest) and a (distance between the carbon atom in
the best plane of bipy). c X-X ) distance between the centroids of the two anthrancenyl rings in 14+ d θ ) twist angle between the two pyridyl rings of
.
the bipy. e $ ) tilt angle of the anthracenyl rings, defined as the angle between the normal of the best plane of the anthrancenyl ring and the long meridian
of the rectangle. f φ ) angle subtended by the two P-C bonds of the PAnP ligand. g ø ) angle subtended by the two Au-N bonds. h τ ) angle between
the long axis of the aromatic guest and the central axis of the bipy. i ꢀ ) dihedral angle between the lateral rings in the anthracenyl ring.
nificant peaks which can be attributed to the triply charged
species (14++NO3)3+ (m/z 751.39, calcd. 751.77), (14++OTf)3+
(m/z 780.37, calcd. 780.76), (14++ClO4)3+ (m/z 763.51 calcd.
764.09), and (14++PF6)3+ (m/z 779.18, calcd. 779.43), respec-
tively. The assignments are further confirmed by calculated
isotopic distributions. The most prominent peaks in the spectra
of 1‚(NO3)4, 1‚(OTf)4, and 1‚(ClO4)4 belong to the “half-
rectangle” [Au2(µ-PAnP)(bipy)+X]+ (X ) NO3-, OTf-, and
ClO4-). On the other hand, no “half-rectangle” peak is observed
in the ES-MS of 1‚(PF6)4. Instead, the largest peak of the
spectrum at m/z 1241.40 (calcd. 1241.6) is identified by
simulated isotopic distributions as doubly charged (14++2PF6)2+
(Figures S17, 18). These results lend support to the rectangular
structure of 14+ and suggest the stability of the gold complexes
in solutions. Elemental analyses of the compounds also agree
well with the proposed molecular formulas. The structure of
1‚(OTf)4 was confirmed by X-ray crystallography (vide infra).
Among all of these salts, 1‚(OTf)4 is the only one that can be
crystallized, and hence is the focus of the present study.
A simpler and nearly quantitative synthesis (yield ) 90%)
of 1‚(OTf)4 involves the reaction of Au2(µ-PAnP)(OTf)2 gener-
ated in situ from the reaction of (AuCl)2(µ-PAnP) and AgOTf
in CH2Cl2, with 1 mol equiv of bipy. Attempts to isolate Au2-
(µ-PAnP)(OTf)2 were unsuccessful as the compound slowly
decomposes in the solid state. Nonetheless, the 31P NMR
spectrum (121.5 MHz, CD2Cl2) of Au2(µ-PAnP)(OTf)2 prepared
in situ shows a singlet at δ 21.66, suggesting that the complex
could have a C2V “clip”-like structure similar to that of Au2(µ-
PAnP)(NO3)2. The self-assembly of 1‚(OTf)4 is completed
within 5 h at room temperature, as indicated by the precipitation
of the product from the solution.
That the reaction between the digold(I) clip and bipy gives
the molecular rectangle as the major product suggests that the
macrocylic 14+ ion is either thermodynamically or kinetically
stable. Apparently, entropy would favor the formation of discrete
rectangle over polymer. An additional driving force for the
formation of the gold rectangle could come from the insolubility
of the metallacyclophane in reacting solvent CH2Cl2, as the
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15857

A R T I C L E S
Lin et al.
distance between the centroids of the opposite anthracenyl rings
X-X, is 16.76(3) Å, and its width, taken as the distance between
the γ-carbon atoms (γ-C-γ-C) of the two opposite bipy, is
7.921(3) Å. The cavity of 14+ is significantly larger than that
of CBPQT4+ ion, which is also rectangular in shape (6.8 × 10.3
Å).36a
The 14+ ion is slightly bellied on its four sides. For the two
sides of the rectangle, the angle ø subtended by the two Au-N
bonds is 16.8°. The bulging is due to the C(anthracenyl)-P(1)-
Au(1) angle (108.7(3)°), the bending of the bipy (N(1)-C(3)-
C(3A) ) 175.8(3)°), and the small offset between the central
axis of the bipy and the Au-P bond (Au(1)-N(1)-C(3) )
173.4(4)°). The out-of-plane distortion of the central ring of
the anthracenyl backbone, indicated by the angle φ of 22.3°
subtended by the P-C(anthracenyl) bonds, is responsible for
the bulging at the two ends of the rectangle. The distortion
significantly shortens the Au-Au distance to 7.069(3) Å (cf.
9.154(2) Å in Au2(µ-PAnP)(NO3)2). The anthracenyl rings are
curved away from the Au atoms, and the dihedral ꢀ between
the two lateral rings of the anthracenyl backbone is 16.4°.
Apparently, the shortening of the Au-Au distance is to maintain
the linear metal coordination (P(1)-Au(1)-N(1) ) 177.4(4)°),
which is known to be the preferential geometry of AuI ion. The
two pyridyl rings of the bipy are not coplanar, showing a twist
angle θ of 21.69°. The Au-P and Au-N bond lengths are
normal. The two opposite anthracenyl rings are parallel and tilted
toward one end of the rectangle. The tilt angle $, defined as
the angle between the normal of the ring and long meridian
X-X of the rectangle, is 43.7°. The eight phenyl rings in 14+
can be grouped into two geometrically different sets: four
axially oriented phenyl rings (Phax) and four equatorially
oriented phenyl rings (Pheq) (Figure 2b). Possibly, the anthra-
cenyl rings are tilted to minimize the steric repulsion with the
Phax rings. The Phax rings at the two ends of 14+ are pointing
to opposite directions and are related by a C2 rotation. Similarly,
the four Pheq rings are equivalent. However, the Phax rings are
diastereotopic with the Pheq rings.
Figure 1. ORTEP diagram of Au2(µ-PAnP)(NO3)2‚0.5Et2O showing the
dimerization of the complex in the solid state via aurophilic attraction.
Hydrogen atoms and solvent molecules are omitted for clarity. Thermal
ellipsoids are shown at 50% probability.
precipitation would shift the equilibrium toward the formation
of the metallacyclophane. The syn-orientation of the two Au
atoms in the molecular clips Au2(µ-PAnP)(X)2 (X ) NO3- and
OTf-) would also favor the formation of the macrocylic
structure, as in the case of the metallacycle [Au4(µ-Ph2PCH2-
PPh2)2(µ-bipy)2](CF3CO2)4.34a-d As will be discussed later, it
appears that the marcocyclic 14+ is also stabilized kinetically:
once formed, the complex would be locked in the rectangular
structure and would not open up to form the polymer.
The self-assembly is sensitive to the anions: when AgClO4
or AgBF4 is used instead of AgCF3SO3, the reported ring-
12e
like compound [Au3(µ-PAnP)3⊃ClO4](ClO4)2
or [Au3(µ-
PAnP)3⊃BF4](BF4)247 is identified by NMR and X-ray crystal-
lography as the major product. It is possible that the tetrahedral
-
-
ClO4 and BF4 ions have a template effect that promotes the
ring formation. In fact, like many other metallacycles arising
from anion-templation,2d,10a,19a,48 the gold rings encapsulate an
anion in their cavities. 1‚(OTf)4 is infinitely stable in the solid
state but undergoes slow decomposition in solution under light.
Solution Structure of 1‚(OTf)4. Some metallacyclophanes
are known to dissociate or undergo dynamic exchanges with
other oligomeric or polymeric structures in solutions.7b,8h,s Of
particular relevance to the present study is the work of the
Puddephatt group which suggested the presence of equilibria
between tetragold(I) metallacyles [Au4(µ-PPh2(CH2)mPPh2)2(µ-
L′)2](CF3CO2)4 (m ) 3-6, L′ ) bipy, bpe, or 1,4-diisocy-
anobenzene) and coordination polymers [Au2(µ-PPh2(CH2)m-
PPh2)(µ-L′)]n(CF3CO2)2n,34 and other species arising from the
ring opening.34e However, it is believed that the gold rectangle
is stable and retains its macrocyclic structures in solution. First
of all, if 1‚(OTf)4 equilibrates with the polymer [Au2(µ-PAnP)-
(µ-bipy)]n(OTf)n, or other olgiomeric species in solution, one
X-ray Crystal Structure of 1‚(OTf)4‚4.8H2O. The crystals
of 1‚(OTf)4‚4.8H2O were obtained from slow diffusion of ether
into a CH2Cl2/MeOH (8:2, v/v) solution of the gold compound.
The X-ray crystal structure 1‚(OTf)4‚4.8H2O reveals a tetracation
1
4+ comprising two opposite [Au2[(µ-PAnP)]2+ units linked by
two bipy ligands (Figure 2 and Table 2, see scheme for Table
2 for the definitions of the structural parameters). The 14+ ion
has a rectangular shape and is fenced on its four sides by the
anthracenyl and pyridyl rings. The 14+ ion shows an approximate
C2h symmetry where the C2 axis bisects the two pyridyl rings
of the bipy and the center of inversion coincides with the center
of the rectangle. Four disordered OTf- ions are located around
the rim of each 14+ ion. In the center of the molecule is a large
cavity (Figure 2a). The length of the rectangle, taken as the
1
would expect significant broadening H and 31P NMR signals
and even the appearance of new peaks at low temperature.
However, the 31P{1H} NMR spectrum (CD3CN, 121 MHz)
(Figure S21) of the complex displays a singlet at δ 22.76 whose
line shape remains unchanged even at 243 K. On the other hand,
(47) Prabhavathy, J. M. Sc. Thesis, National University of Singapore.
(48) (a) Mann, S.; Huttner, G.; Zsolnai, L.; Heinze, K. Angew Chem., Int. Ed.
Engl. 1996, 35, 2808. (b) Hasenknopf, B.; Lehn, J.-M.; Boumediene, N.;
Dupont-Gervais, A.; Van Dorsselaer, A.; Kniesel, B.; Fenske, D. J. Am.
Chem. Soc. 1997, 119, 10956. (c) Fleming, J. S.; Mann, K. L. V.; Carraz,
C.-A.; Psillakis, E.; Jeffrey, J. C.; McCleverty, J. A.; Ward, M. D. Angew.
Chem., Int. Ed. 1998, 37, 1279. (d) Schnebeck, R.-D.; Freisinger, E.;
Lippert, B. Angew. Chem., Int. Ed. 1999, 38, 168.
1
the H NMR signals are slightly broadened as the temperature
is lowered from 298 to 243 K (Figure S21). Nonetheless, it can
be aptly explained by a fluxional process involving the flipping
of the anthracenyl rings (Scheme 2): In contrast to the X-ray
crystal structure of 14+ in which the two ends of the tilted
9
15858 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Figure 2. ORTEP diagrams of 1‚(OTf)4‚4.8H2O showing (a) the top view and (b) the side view of the 14+ ion. (b) Hydrogen atoms, anions, and H2O
molecules are omitted for clarity. Thermal ellipsoids are shown at 50% probability.
Scheme 2. Proposed Anthracenyl Ring Flipping of 14+
anthracenyl rings are nonequivalent (i.e., H2,3 * H6,7, H1,4
*
three species: 14+ (host), the aromatic guests, and the host-
guest complex. This indicates that 14+ does not dissociate or is
not in equilibrium with other species in solution. The similar
NMR spectroscopic properties displayed by 1‚(NO3)4, 1‚(ClO4)4,
and 1‚(PF6)4 also indicate that the rectangular structure of 14+
is not affected by its counterion.
H5,8), the room-temperature 1H NMR spectrum of the complex
shows two multiplets at δ 7.34 and 8.42 attributable to H1,4,5,8
and H2,3,6,7, respectively. The merge of signals signifies a fast
exchange between H2,3 h H6,7 and H1,4 h H5,8 in solution. The
exchange involves flipping of the anthracenyl rings which occurs
concomitantly with a swapping between the diastereotopic Phax
and Pheq (Phax h Pheq). Because the two exchanging partners
are identical, the P atoms should remain equivalent throughout
the process. In accord with this is the invariable singlet observed
in the VT 31P{1H} NMR measurements.
The fact that 14+ retains its macrocyclic structure in solution
could be due to kinetic stabilization. Recently, Stang et al. have
demonstrated that the molecular rectangles [Pt4(PEt3)8(µ-anth2-)2-
(µ-L)2](X)4 (L ) bipy, bpe, 1,4-bis(4-ethynylpyridyl)benzene,
2,5-bis(4-ethynylpyridyl)furan, X ) ClO4-, PF6-, BF4-) are
kinetically stable with respect to dissociation as the weakly
nucleophilic anions are unable to displace the dipyridyl ligands.9g
Another piece of evidence supporting the integrity of the 14+
ion in solution comes from our binding studies. As will be
mentioned later, 1H NMR and fluorescence titrations unambigu-
ously show that 14+ forms 1:1 complexes with various aromatic
molecules (Chart 1). The binding of all of the guests to 14+ can
be properly described by an equilibrium which involves only
-
Similarly, the OTf-, ClO4-, and PF6 ions in the gold
complexes, being poor electron donors, should not be able to
substitute the bipy in 14+. Furthermore, the anthracenyl rings,
oriented almost perpendicular to the plane of the rectangle,
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15859

A R T I C L E S
Lin et al.
Figure 3. UV-vis absorption (blue) and emission (red) spectra of 1‚(OTf)4.
Solvent: CH3CN, T ) 298 K. Excitation wavelength ) 420 nm, excitation
and emission slit widths ) 10 nm.
Figure 4. Cyclic voltammogram of 1‚(OTf)4. Solvent, CH3CN (0.1 M
n-Bu4NPF6); working electrode, glassy carbon; counter electrode, platinum
wire; reference electrode, Ag/AgNO3 (0.1 M in CH3CN).
would possibly shield the gold atoms from the attack of the
anion. These could be the reasons for the exceptional stability
of 14+ in comparison to other gold metallacycles.
mV) and -2.2 V (∆Ei ) 70 mV), respectively. As the CV
(Figures S22) of the free PAnP ligand also shows two similar
reversible couples at -1.9 and -2.2 V, these two couples can
be assigned to the reduction of PAnP liberated from 14+ after
decomposition. However, it is noted that the cathodic wave (c)
in the voltammogram of 14+ is much larger than its anodic
counterpart (c′). A differential pulse voltammetric study on wave
(c) shows that it is actually an overlap of two waves. Scanning
the potential of the working electrode to -1.9 V also leads to
deposition of gold metal on the electrode surface. We thus
attribute wave (c) to be an overlap of two reduction processes,
the reversible reduction of free PAnP and the irreversible
reduction of AuI species generated from the decomposition of
14+ as a result of reduction wave (a). The appearance of more
than one AuI reduction wave is commonly observed in poly-
nuclear AuI clusters.49 The couple (e,e′) at E1/2 of -2.3 V is
due to the bipy generated from the cathodic decomposition of
UV-Vis Absorption and Luminescence of 1‚(OTf)4. The
UV-vis absorption spectrum (Figure 3) of 1‚(OTf)4 displays a
very intense band at 270 nm (ꢀmax ) 1.4 × 105 M-1 cm-1),
1
which corresponds to the high-energy π f π* transition of
the anthracenyl rings. In addition, there is a moderately intense
absorption band in 320-480 nm which displays vibronic peaks
at 367, 390, 418, and 441 nm (λmax ) 441 nm, ꢀmax ) 2.6 ×
104 M-1 cm-1). The absorption band arises from the low energy
1π f π* transition of the anthracenyl ring. Similar absorption
bands are found in the spectra of other PAnP-containing
complexes such as [Au3(µ-PAnP)3⊃ClO4](ClO4)2.12e Irradiating
an aerated CH3CN solution of 1‚(OTf)4 at 420 nm gives an
emission maximized at 480 nm with quantum yield of 0.05
(Figure 3). The small Stoke shift indicates that the emission is
1π* f π fluorescence. [Au3(µ-PAnP)3⊃ClO4](ClO4)2 displays
emission of similar energy and band shape.12e
1
4+, as the free ligand also shows a reversible couple at the
same potential (Figure S23). A small irreversible wave attribut-
able to the oxidation of decomposed product from 14+ appears
in the reverse anodic scan. No oxidation wave is observed if
the initial voltammetric scan is made toward the anodic side.
The electrochemistry of 1‚(OTf)4 is drastically different from
that reported for the CBPQT4+, and the bipy-containing
Electrochemistry of 1‚(OTf)4. The room-temperature cyclic
voltammogram (CV) of a CH3CN solution of 1‚(OTf)4 is shown
in Figure 4. Upon reduction, an irreversible peak (a) appears at
about -1.2 V versus Cp2Fe+/0. This is followed by a small
quasi-reversible couple (b,b′) at E1/2 of -1.38 V (∆Ei ≈ 80
mV). The irreversible peak (a) is likely a metal-centered
reduction as most AuI-phosphine compounds show a metal-
centered irreversible reduction at similar potential in acetoni-
trile.49 Holding the potential of the working electrode at -1.2
V also leads to slow deposition of gold metal on the electrode
surface which can be observed with the naked eye. The
reversible couple (b,b′) is attributed to redox reaction centered
at the bipy in 14+ by comparison with the voltammogram of
the Pt-rectangle [Pt4(PEt3)8(µ-anth2-)2(µ-bipy)2]4+ (which has
the same charge as 14+ and also shows a bipy-centered reduction
at -1.44 V).50 The irreversible nature of wave (a) indicates that
rectangles [Pt4(PEt3)8(µ-anth2-)2(µ-bipy)2](PF6)4 and [Re4-
50
(CO)12(µ-bipy)(bipm)2],51 which are dominated by ligand-
centered redox processes. The organic cyclophane CBPQT4+
exhibits two quasi-reversible two-electron couples at E1/2 ) 0.59
V and -1.01 V (vs Cp2Fe+/0) which correspond to bipyridinium-
centered two-electron reductions Similarly, the CV of [Pt4-
(PEt3)8(µ-anth2-)2(µ-bipy)2](PF6)4 and [Re4(CO)12(µ-bipy)-
(bipm)2] shows reversible reductions localized in the heterocyclic
ligands. No metal-centered reduction is observed. On the
contrary, the CV of 1‚(OTf)4 is dominated by the irreversible
metal-centered reductions and the reduction of products arising
from cathodic decomposition. This could be related to the early
onset of the irreversible AuI reduction to Au0 at -1.2 V, which
1
4+ decomposes upon reduction. Further scanning the potential
toward the cathodic side leads to the appearance of two small
reversible couples (c,c′) and (d,d′) at E1/2 of -1.9 V (∆Ei ) 70
(50) Kaim, W.; Schwederski, B.; Dogan, A.; Fiedler, J.; Kuehl, C. J.; Stang, P.
J. Inorg. Chem. 2002, 41, 4025.
(51) Hartmann, H.; Berger, S.; Winter, R.; Fiedler, J.; Kaim, W. Inorg. Chem.
2000, 39, 4977.
(49) Rakhimov, R. D.; Butin, K. P.; Grandberg K. I. J. Organomet. Chem. 1994,
464, 253.
9
15860 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Figure 5. (a) Ball-and-stick and (b) space-filling representations of the X-ray crystal structure of (1⊃An)‚(OTf)4, and ball-and-stick representations of the
X-ray crystal structures of (c) (1⊃Py)‚(OTf)4‚CH2Cl2, and (d) (1⊃Br-An)‚(OTf)4‚Et2O. Hydrogen atoms and anions are omitted. Color scheme: guest
carbon atoms (red), Au (orange), P (purple), guest protons (blue), bipy proton (green).
precedes the bipy-centered reduction at -1.38 V. Our observa-
tion is also consistent with the electrochemical properties of
most AuI-compounds reported in the literature.49
rings.52 It is noted that the structures of the inclusion complexes
of CBPQT4+ and hydroquinols show similar C-H‚‚‚π between
the guest and the p-phenylene rings of the host.36 The distances
(2.8-2.9 Å) between the interacting hydrogen atoms and the
p-phenylene ring are close to the distance between the H3,7 and
the anthracenyl rings (2.928 Å). The HR and Hâ of the bipy are
close to the rings of the An.
X-ray Crystal Structures of the Inclusion Complexes.
Complexation between 14+ and the aromatic guests was first
hinted by the remarkable increase in the solubility of some of
the polycyclic molecules such as An and Py in the presence of
the gold rectangle, and was further confirmed by the NMR and
the luminescence studies (vide infra). Orange crystals of three
inclusion complexes (1⊃An)‚(OTf)4, (1⊃Py)‚(OTf)4‚CH2Cl2,
and (1⊃Br-An)‚(OTf)4‚Et2O were obtained by slow diffusion
of ether into CH2Cl2/MeOH (8:2, v/v) solutions containing
1‚(OTf) and excess guests; their X-ray crystal structures are
shown in Figure 5 (Table 2). The aromatic guests are interposed
between the two bipy ligands of the gold rectangle. As
commonly observed in the π-π stacking of aromatic molecules,
the overlap of the aromatic rings of the guests and bipy is
staggered.36 The structures of the compounds confirm the 1:1
host-guest stoichiometry of the complexation.
The An molecule in (1⊃An)‚(OTf)4 is intercalated between
from the two bipy ligands. The orientation of the guest in the
cavity is centrosymmetric with the center of the molecule co-
inciding with the center of inversion of the host. As a result,
the complex shows an approximate C2h symmetry. Furthermore,
the An is oriented with its long C2 axis close to the N-N vector
of the bipy (Figure 5a and Table 2). The small angle τ of 12.4°
between the axes results in extensive overlap of the aromatic
rings of the An and the bipy. The H1,5 of the guest are protruded
slightly out of the periphery of 14+, while the rest of its hydrogen
atoms are close to the bipy rings. Notably, the H3 and H7 at the
two ends of the guest are found directly pointed toward the lat-
eral rings of the anthracenyl unit. The calculated distance be-
tween the H3,7 and the centroids of the lateral rings is 2.928 Å,
and the distance between centroids of the lateral ring and the
ring containing the interacting hydrogen atoms is 5.115 Å. These
structural features suggest the presence of weak C-H‚‚‚π or
edge-to-face interactions between the guest and the anthracenyl
The bound Py in (1⊃Py)‚(OTf)4‚CH2Cl2 is disordered equally
over two positions (Figure 5c and Table 2). The C2 axis of the
rectangle passes through the midpoint between the C5a and C5a1
of the Py, which also coincides with the center of the rectangle.
The two disordered positions of the Py are generated by a 180°
rotation of the guest around the C2 axis. The guest is located
slightly closer to one end of the rectangle than the other. The
long C2 axis of the guest is almost in line with the horizontal
N-N axis of the bipy, showing a small angle τ of 0.9°. This
leads to extensive overlaps between of the Py and bipy rings.
The H9,10 of the Py are protruded out of the cavity of 14+, while
the rest of the guest protons are close to the bipy. Notably, the
H2 and H3 are directed toward the lateral ring of the anthracenyl
backbone. The calculated distance between the centroids of the
lateral rings and H2 and H3 are 2.673 and 3.189 Å, respectively,
and the distance between the centroids of the lateral ring and
the axial ring of the Py bearing the two protons is 4.678 Å.
These structural features compare favorably with the ones
reported for edge-to-face interactions,52 suggesting the presence
of the hydrogen bonding between the host and the guest.
The Br-An in (1⊃Br-An)‚(OTf)4‚Et2O is disordered over
two sites A (55% occupancy) and B (45% occupancy), which
are related by 180° rotation around the long axis of the molecule
(Figure 5d and Table 2). The position of the guest is asymmetric,
being closer to one end of the host than the other. The long
(52) (a) Bacon, G. E.; Curry, N. A.; Wilson, S. A. Proc. R. Soc. London, Ser.
A 1964, 279, 98. (b) Kelbe, G.; Diederich, F. Philos. Trans. R. Soc. London,
Ser. A 1993, 345, 37. (c) Oikawa, S.; Tsuda, M.; Kato, H.; Urabe, T. Acta
Crystallogr., Sect. B 1985, 41, 437. (d) Bong, D. T.; Ghadiri, M. R. Angew.
Chem., Int. Ed. 2001, 40, 2163. (e) Dance, I.; Scudder, M. Chem.-Eur. J.
1996, 5, 482.
9
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A R T I C L E S
Lin et al.
axis of the Br-An and the N-N vector of the bipy is separated
by an angle τ of 25.9°. In site A, the H1, H2, H5, and H6 of the
guest are protruded out of the rim of the 14+ ion, while the H4
and H7 are near to the pyridyl rings. The Br atom and its H10
are projected out of the cavity. The terminal protons of the guest
are involved in edge-to-face interactions with the anthracenyl
rings as the H3-C3-C4-H4 edge of the guest is directed toward
one of the lateral rings of the anthracenyl backbone. The
distances between the H3 and H4 and the centroid of the lateral
ring are 3.419 and 2.766 Å, respectively. The centroids of the
lateral ring and the ring containing the interacting H atoms are
separated by 4.846 Å. These structural parameters compare
favorably with the aromatic compounds that exhibit similar
edge-to-face interactions.52 The HR and Hâ of the bipy are close
to the π-rings of the guest. The distances between the Br atom
and the HR and Hâ of the bipy (3.598-3.841 Å) are too long
for C-H‚‚‚Br hydrogen bonding.
13.2° (cf. ø ) 16.8° for the free 14+). In addition, the bipy in
the complexes are less twisted as the twist angles θ for the Br-
An- (2.71°), Py- (3.60°), and An-(10.42°) complexes are
significant smaller than that for the free host (θ ) 21.69°).
Similarly, the binding of 1,4-dimethoxybenzene to the CBPQT4+
ion reduces the twist angle of the bipy units from 19° to 4°.36a,b
Apparently, the flattening of the bipy would improve the overlap
with the aromatic guests and thus enhance the π-π interactions.
Analogous to the width of the 14+ ion, the decrease in the twist
angle θ observed in the three complexes is in line with the
increase in the -∆GNMR. This observation lends support to the
induced-fit mechanism.
Similar to the free host ($ ) 43.7°), the anthracenyl rings in
the An- and Py-complexes are tilted toward the same direction
with tilt angles $ of 43.3° (An-complex) and 47.4° (Py-
complex). In addition, the rings are slightly curved with the
dihedral angles ꢀ of 14.7° (An-complex) and 21.9° (Py-
complex). The binding of the guests leads to significant
shortening of the intrannular P-P distances. To sustain such a
short P-P distance, the central rings in the anthracenyl units
are distorted from planarity and the bow angles φ in the An-
(30.5°) and Py-complexes (31.4°) are markedly larger than that
in the free host (22.3°). The 14+ ion in the Br-An complex
shows a different conformation in which the anthracenyl rings
are titled in opposite directions (Figure 5d). Moreover, the two
ends of the rectangle are nonequivalent, having slightly different
intraannular Au-Au (6.526(2), 6.604(2) Å), and N-N (6.767-
(4), 6.910(4) Å) distances. As a result, the symmetry of the 14+
ion descends to Cs where the plane of reflection bisects the
anthracenyl rings. The intrannular P-P distance (6.255(4) Å)
is the shortest among the four gold compounds and is maintained
by severe distortion of the central ring of the anthracenyl
backbone (φ ) 40°).
Solid-State Self-Assembly. The crystal structure of 1‚(OTf)4‚
4.8H2O reveals linear stacking of the 14+ ions along the c-axis,
which results in open channels (Figure 6). Similar open channels
have been observed in the crystals of [Pt4(dppp)4(µ-bipy)4]8+
(dppp ) 1,3-bis(diphenylphosphino)propane)8e,q and CBPQT4+.36a
The distance between two adjacent Au atoms in a column is
10.308 Å. Sandwiched between two 14+ ions are four OTf-
ions whose CF3-groups are pointed toward the center of the
rectangle, and the oxygen atoms of the SO3-groups are near
the R-H of the pyridyl rings. There are on average 4.8 H2O
molecules per 14+ ion, two of them are found between two
stacking 14+ ions, and the remaining are located in the interstitial
space. The water molecules could be originated from the
solvents used in crystal growing, such as MeOH.
Among all of the structural features of the inclusion com-
plexes, the one of particular importance is the interplanar
distance d between the bipy and the guest, as it is related to the
strength of the guest binding. Because the pyridyl rings in the
bipy are not coplanar, the interplanar distance is defined as d
) b - a where b is the perpendicular distance between the
R-carbon atom in the bipy and the plane of the guest, and a is
the perpendicular distance between the carbon atom and the
best plane of the bipy (Scheme for Table 2). It is found that the
interplanar distances in the three complexes fall into the range
of π-π interactions (∼3.5-3.7 Å) and follow the order of Br-
An (d ) 3.401(4) Å) < Py (d ) 3.552(8) Å) < An (d ) 3.645-
(2) Å). This indicates that the Br-An binds most strongly to
1
4+, followed by the Py and then the An. It is corroborated by
the correlation between the d and the free energy of binding
-∆GNMR determined by 1H NMR titration (vide infra), that are
19.7 ( 0.4, 18.2 ( 0.1, and 17.2 ( 0.2 kJ mol-1 for the binding
of the Br-An, Py, and An, respectively. Comparing the
dimensions of the 14+ ions in the complexes and the free 14+
ion in 1‚(OTf)4‚4.8H2O shows that the guest binding leads to
the shortening of the width of the rectangle: the intraannular
Au-Au (6.526(2)-6.854(2)Å), N-N (6.767(4)-7.341(4) Å),
and γ-C-γ-C ) (6.980(4)-7.341(4) Å) distances in the three
inclusion complexes are markedly shorter than the ones in the
free host: Au-Au ) 7.069(3) Å, N-N ) 7.665(3) Å, and
γ-C-γ-C ) 7.921(3) Å. Again the intraannular Au-Au, N-N,
and γ-C-γ-C distances in the three complexes follow the
order: Br-An < Py < An. These findings indicate that the
guest binding induces contraction of the molecular rectangle,
whose extent is proportional to the binding strength of the guests.
These observations point to an induced-fit mechanism53 in which
the binding of the guest causes the host to undergo conforma-
tional changes which in turn enhance its π-π interactions with
the guest. The conformation changes involve mainly the
contraction of the bite distance of PAnP, which is expected not
to be energy demanding as the ligand is highly flexible.12d,29
The complexation causes other structural changes in 14+ as
well. First, the 14+ ions in the inclusion complexes are less
bulged than the free 14+, showing a smaller angle ø of 10.5-
The solid-state packing of the 14+ ions reveals intriguing
tectonic supramolecular arrangement: the 14+ ions, paneled on
the ab-plane, form a 2D mosaic in which each ion is surrounded
at its corners by four neighboring ions (Figure 6). The packing
leads to highly porous crystal with a large interstitial space,
which resembles an open channel, located in the center of four
adjacent stacking columns of the 14+ ions. Viewing down the
c-axis, one would see the two Phax rings at the “head” of the
14+ ion are projected upward, whereas the two Phax rings are
pointing down at the “tail”. The equatorial Ph rings at the head
are tilted upward, while the ones at the tail are tilted downward.
The axial and equatorial Ph rings in the head and tail are labeled
Phaxh, Pheqh, Phaxt, and Pheqt. In the panel, the head and tail of
(53) (a) Kasai, K.; Aoyagi, M.; Fujita, M. J. Am. Chem. Soc. 2000, 122, 2140.
(b) Fujita, M.; Nagao, S.; Ogura, K. J. Am. Chem. Soc. 1995, 117, 1649.
(c) Hamilton, A. D.; Van Engen, D. J. Am. Chem. Soc. 1987, 109, 5035.
(d) Sijbesma, R. P.; Wijmenga, S. S.; Nolte, R. J. M. J. Am. Chem. Soc.
1992, 114, 9807. (e) Koshland, D. E., Jr. Angew. Chem., Int. Ed. Engl.
1995, 33, 2375.
9
15862 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Figure 6. Crystal packing diagram of 1‚(OTf)4‚4.8H2O showing the open channels and the 2D mosaic on the ab-plane. Hydrogen atoms, anions, and H2O
molecules are omitted. Inset shows the complementary edge-to-face Ph-Ph interactions between adjacent 14+ ions. Color scheme: see Figure 5.
Table 3. Structural Parameters for the Edge-to-Face Ph-Ph Interactions
complexes
1 (OTf)4
‚
4.8H2O
(1
‚An)
‚(OTf)4
(1‚Py)
‚
(OTf)4
‚
CH2Cl2
(1‚Br−An)‚(OTf)4‚Et2O
i (Å)
j (Å)
k (Å)
4.988
3.496, 3.279
6.025
4.900
3.484, 3.279
5.996
4.954
3.438, 3.395
5.962
5.078, 5.316
3.312, 3.493, 3.518, 3.629
5.995
dihedral angle
87.8
89.0
89.4
88.1, 89.9
between Phax and Pheq
dihedral angle between
Phaxh and Phaxt or Pheqh and Pheq
0.0
0.0
0.0
7.9, 5.4
t
a rectangle are next to the tails and the heads of its neighboring
molecules, respectively. This head-to-tail orientation allows
complementary edge-to-face interactions of the Ph rings between
is 4.988 Å. The intermolecular P-P distance (k) between the
two interacting PPh2 groups is 6.025 Å. As demonstrated by
the comprehensive surveys of Dance and Scudder, edge-to-face
Ph-Ph interactions, also known as “phenyl embrace”, widely
occur in the crystals of metal complexes containing PPh3 or
diphosphines, such as PPh2CH2PPh2 as ligands, and have a
determining effect on the solid-state packing of the com-
plexes.52e,54 The values of i and k observed in 1‚(OTf)4‚4.8H2O
fall into the average values of i (5 Å) and k (e7 Å) obtained
from the surveys. In addition, the dihedral angle (87.8°) and
the i value (4.988 Å) of the gold complex compare favorably
h
adjacent molecules: the Phax rings of a rectangle are faced
t
directly to the edges of the Pheq rings of two molecules in the
front. Conversely, the edges of Pheqh rings of the rectangle are
t
directed toward the faces of Phax rings of the two preceding
t
molecules (inset in Figure 6). These complementary Phaxh-Pheq
and Phax -Pheqh interactions occur at every corner in the mosaic.
t
The H2 and H3 in the Pheqt/Pheq ring are directed toward the
h
centroid of the Phaxh/Phax with the calculated H‚‚‚centroid
t
distances j of 3.496 and 3.279 Å (Table 3, for the definitions
of structural parameters, see Scheme for Table 3). The interact-
ing Ph rings are almost perpendicular to each other, showing a
dihedral angle of 87.8°. The distance (i) between their centroids
(54) (a) Dance, I.; Scudder, M. J. Chem. Soc., Chem. Commun. 1995, 1039. (b)
Dance, I.; Scudder, M. J. Chem. Soc., Dalton Trans. 2000, 1579. (c) Dance,
I.; Scudder, M. J. Chem. Soc., Dalton Trans. 2000, 1587. (d) Scudder, M.;
Dance, I. J. Chem. Soc., Dalton Trans. 1998, 3167. (e) Dance, I.; Scudder,
M. J. Chem. Soc., Dalton Trans. 1998, 1341.
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15863

A R T I C L E S
Lin et al.
Figure 7. 1H NMR spectral change upon addition of 1‚(OTf)4 (concentration ) 0.00-5.06 mM) to a CD3CN solution of An (concentration ) 2.52 mM).
The 1‚(OTf)4 and An protons are labeled in blue and red, respectively. The top and bottom spectra belong to the free An and the free 1‚(OTf)4, respectively.
The shifts of the host protons increase as the [H]t decreases because when the [G]t is fixed the percentage of occupied host increases as [H]t is lowered.
with the corresponding parameters between a pair of benzene
molecules engaging in edge-to-face interactions (dihedral angle
) 86.5°, i ) 5.025 Å).52a-c It is believed that the complementary
edge-to-face Ph-Ph interactions are a major driving force for
the self-assembly of 14+ in the solid state.
Reminiscent of the free host, the 14+ ions in the solids of
An- and Py-complexes form linear channels and assembled into
2D mosaic via the complementary interactions of the nearly
perpendicular Phaxh-Pheqt and Phaxt-Pheqh rings (Figures S24,
25). The i, j, and k distances are comparable with the ones
observed in 1‚(OTf)4‚4.8H2O (Table 3). The channels are
occupied by the guests, and the separations of 14+ ions in the
columns and the positions of anions are similar to those in 1‚
(OTf)4‚4.8H2O. The CH2Cl2 molecules in (1⊃Py)‚(OTf)4‚CH2-
Cl2 are sandwiched between the stacking 14+ ions.
On the other hand, the 14+ ions in the Br-An-complex form
zigzag-like columns (Figure S26). Sandwiched between two
stacking gold rectangles are one CH2Cl2 molecule and four OTf-
ions. While the 14+ ions in the Br-An-complex self-assemble
into a 2D mosaic similar to those observed in the other three
complexes, there are some subtle differences. Unlike the other
three gold complexes, the anthracenyl rings in the Br-An-com-
plex are tilted toward each other (Figure 5d). As a consequence,
the four Phax rings at both ends of the rectangle are pointing
upward, while the four Pheq rings are tilted downward. It is
apparent that if all of the 14+ ions in the panel have the same
orientation, complementary Phax-Pheq interactions as observed
in the other three compounds would not be possible. It is
therefore intriguing to find that each rectangle in the Br-An-
complex is surrounded by four “flipped-over” rectangles whose
orientation is different from that of the surrounded ion by a
180° rotation around the long central axis (Figure S27). As such,
the four upward pointed Phax rings of the surrounded rectangle
now face directly to the four upward tilted Pheq of its neighbors.
Conversely, the downward pointed Phax rings of the four
adjacent molecules face to the edges of the downward tilted
Pheq rings of the central molecule. The interacting Ph rings are
nearly perpendicular to each other, and the i, j, and k distances
are close to those found in the other three compounds (Table
3). The special arrangement of the 14+ ions in the crystal of the
Br-An-complex underscores the important role played by the
complementary edge-to-face Phax-Pheq interactions in the solid-
state self-assembly of the gold complex.
1H NMR Titrations. The complexation between 14+ and the
guest was investigated by monitoring chemical shifts δG of
certain guest protons as a function of the total concentration of
the host [H]t. Figure 7 shows the spectroscopic changes observed
in the titration of 1‚(OTf)4 and An. Invariably, the complexation
9
15864 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Table 4. Binding Constants and Free Energy of Binding for the
Aromatic Guests Determined by 1H NMR and Fluorescence
Titrations
Table 5. Chemical Shifts (δ) of the Free Host and Guest Protons
and the Change in the Chemical Shifts (∆δ) Induced by the
Complexation
1
1
1
1
guest
K
NMR (M-
)
−∆
G
NMR (kJ mol-
)
K
Q (M-
)
−∆G )
Q (kJ mol-
Dmb
50 ( 7
9.7 ( 0.4
a
a
Bip
Nap
Phen
An
126 ( 10
128 ( 52
527 ( 26
1029 ( 71
12.0 ( 0.2
12.0 ( 1.0
15.5 ( 0.1
17.2 ( 0.2
17.9 ( 0.1
17.4 ( 0.1
19.7 ( 0.4
19.4 ( 0.3
18.2 ( 0.1
118 ( 31
463 ( 29
909 ( 103
b
11.8 ( 0.7
16.9 ( 0.3
MeO-An 1353 ( 70
NC-An
Br-An
1137 ( 32
2858 ( 508
b
2277 ( 160
2208 ( 223
1376 ( 40
19.2 ( 0.2
19.1 ( 0.3
17.9 ( 0.1
CO2H-An 2559 ( 350
Py
1564 ( 34
a No quenching. b Cannot be determined as the emissions of the guests
and the host overlap.
moves all of the guest protons, and the HR and Hâ of bipy upfield
but the H1,4,5,8 and H2,3,6,7 of the anthracenyl rings in the host
downfield (Figures S28-36). That all of the guest protons and
the HR and Hâ of the bipy are shifted upfield indicates the
participation of the bipy in the guest binding. It is in accordance
with the inclusion geometries exhibited by the three complexes.
The upfield shifts are attributable to the influence of the diatropic
ring currents of the aromatic rings. Similar upfield shifts have
been observed in the binding of aromatic guests to the CBPQT4+
ion.36
Except the HR and Hâ, which are slightly broadened, the
signals of all of the guest and host protons remain sharp
throughout the titrations. This implies that the guest exchanges
rapidly between the cavity of 14+ and the solution in the NMR
time scale. Thus, the observed chemical shifts of the guest
protons (δG) are the weighted average of the chemical shifts of
C
the complexed guest δG and the free guest δGo. The stoichi-
ometry of the inclusion complexes was established to be 1:1
by Job’s plots (Figures S37-46). The binding constants KNMR
and the chemical shift differences ∆δG (∆δG ) δGC - δGo) are
obtained from the least-squares fit (Figures S47-56) to eq 1,40
where [G]t is the total concentration of the guest.
∆δG
o
δG ) δG
-
(B - B2 - 4[H] [G] )
(1)
x
t
t
(
)
2[G]t
1
B ) [H]t + [G]t +
(1.1)
KNMR
The binding constants KNMR of the aromatic guests differ
greatly, ranging from 50 ( 7 M-1 for Dmb to 2858 ( 508
M-1 for Br-An. The KNMR and the free energy of the binding
-∆GNMR are listed in Table 4.
Chemical Shift Differences Induced by Binding. Other
parameters extracted from the least-squares fit are the chemical
shifts differences of the guest (∆δG) and host (∆δH) protons
induced by the complexation (Table 5). It is noted that the ∆δG
values roughly correlate with the binding free energy -∆GNMR
;
that is, the weakly binding Dmb, Bip, and Nap (-∆GNMR
)
a Cannot be determined as the signals are masked.
9.7-12.0 kJ mol-1) show ∆δG values (-0.16 to -0.76 ppm)
that are markedly smaller than those observed for the Phen,
An, 9-substituted anthracenes, and Py (-∆GNMR ) 15.5-19.7
kJ mol-1, ∆δG ) -0.65 to -1.68 ppm). The same trend is
observed in the ∆δH for the HR and Hâ: for example, the binding
of Dmb, Biph, and Nap induces upfield shifts of the Hâ (∆δH
) -0.07 to -0. 11 ppm) that are remarkably smaller than those
elicited by the binding of the other guests (∆δH ) -0.62 to
-1.47 ppm). As discussed in the previous section, the increase
in -∆GNMR reduces the interplanar distance between the bipy
ligands and the guest. It is reasonable to expect that the decrease
in separation would lead to stronger mutual shielding which in
turn gives rise to the larger ∆δG and ∆δH values observed.
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15865

A R T I C L E S
Lin et al.
The ∆δG values of the protons in a guest are dependent on
the inclusion geometry, the distance between the protons and
the bipy, and the intrinsic susceptibility of the protons.36c,55 In
general, the ∆δG values of the protons in the three inclusion
complexes are in accordance with their X-ray crystal structures,
suggesting that the solid-state inclusion geometries could
represent the time-averaged orientation of the bound guests in
solution.
First, all of the protons in the An show similar large ∆δG
values of -1.15 to -1.40 ppm (Table 5), consistent with the
extensive overlap between the aromatic rings of An and bipy
observed in the structure of the An-complex. Although the H1,5
and H4,8, H9 and H10, and H2,6 and H3,7 are nonequivalent in
the solid-state structure, only three sets of mulitplets corre-
sponding to H1,4,5,8, H9,10, and H2,3,6,7 are found in the solution
NMR spectrum of 14+ and An mixture (Figure 7). The
coalescence of the signals is due to rapid rotation of the guest
inside the host and/or fast exchange between the bound and
free guest molecules. Notably, the ∆δG value for the H2,3,6,7
(-1.40 ppm) is slightly larger than the ∆δG values of the other
protons (-1.15 ppm). This could be due to the edge-to-face
interactions between the anthracenyl rings and the protons, or
the shielding of the terminal protons by the diatropic current of
the anthracenyl rings. Similarly, the hydroquinols protons
involved in edge-to-face interactions with the p-phenylene rings
of CBPQT4+ are more upfield shifted than the other protons.36c
The complexation between Py and the gold rectangle leads
to a large upfield shift of the apical H2,7 (∆δG ) -1.68 ppm)
(Table 5). It can be aptly accounted by the solid-state orientation
of the guest where the H7 is covered by two pyridyl rings and
the H2 is engaged in edge-to-face interactions with the anthra-
cenyl ring.36c The H1,3,6,8 shows a smaller ∆δG value as they
Figure 8. Emission spectral change upon addition of An to a CH3CN
solution of 1‚(OTf)4. Excitation wavelength ) 420 nm, excitation and
emission slit width ) 10 nm. Inset shows the emission titration curve and
the least-squares fit using the eq 2. The emission intensity of the host is
monitored at 480 nm.
-1.47 ppm) are more affected than the HR (∆δH ) -0.55 to
-0.80 ppm) (Table 5). It is reasonable, as the Hâ, being closer
than HR to the central position, would be more shielded by the
π-rings of the guests. Unlike the bipy protons, the protons of
the anthracenyl rings are downfield shifted upon the complex-
ation. And invariably, the H2,3,6,7 are more affected (∆δH
)
0.01-0.21 ppm) than the H1,4,5,8 (∆δH ) 0.00-0.10 ppm) at
the ends of the ring. The downfield shifts could be due to the
distortion of the anthracenyl ring caused by the binding. Another
possible cause is the interactions of the rings with the apical pro-
tons of the guests. Similarly, downfield shifts of the p-phenylene
rings of the CBPQT4+ ion upon binding of hydroquinols were
also accounted for by edge-to-face interactions between the
protons of the hydroquinols and the p-phenylene rings.36c
Fluorescence Quenching of the Host. The emission of 14+
at 480 nm is quenched by Nap, Phen, An, Br-An, CO2H-An,
MeO-An, NC-An, and Py (Figures S57-60). However, the
fluorescence intensity is not diminished even in the presence
of a large excess of Dmb and Bip. Figure 8 shows the change
of the emission intensity of 14+ upon addition of An. The
binding constants KQ for the guests were determined from the
least-squares fits to the eq 241 for 1:1 host-guest complexation
(Figures S61-65).
are not so close to the bipy and anthracenyl rings. The H4,5,9,10
,
which are protruded out of the rim of the rectangle, show the
smallest ∆δG value of -0.65 ppm.
The ∆δG values for the Br-An protons can also be correlated
with the solid-state inclusion geometry of the complex (Table
5). The H10, which is far from the cavity, is less shielded (∆δG
) -0.84 ppm) than the other protons. The ∆δG values for H1,8
and H2,7 cannot be determined as they are masked by the other
signals. The H4,5 (-0.97 ppm) and H3,6 (-1.10 ppm) show
similar large ∆δG values, which are reasonable as these protons
are close to the bipy and involved in the edge-to-face interactions
with the anthracenyl rings. Similarly, the terminal H2,7 and H3,6
of NC-An and CO2H-An are more shielded than the H10.
The methoxyl protons in Dmb (∆δG ) -0.16 ppm) and
MeO-An (∆δG ) -0.39 ppm) are much less affected by the
binding than the aromatic protons of the compounds (∆δG
)
-0.32 ppm for Dmb and ∆δG ) -1.20 and -1.28 ppm for
MeO-An) (Table 5). The small ∆δ values displayed by the
methoxyl protons reflect their long separations from the π-rings
of the bipy. In view of the structures of the three inclusion
complexes, one would expect the binding involves mainly the
aromatic rings of Dmb and MeO-An, and their methoxyl
groups could be projected out of the rims of the 14+ ion.
The binding also moves the resonances of the Hâ and HR of
the bipy to higher field. In all cases, the Hâ (∆δH) -0.87 to
kH‚G
1 +
KQ[G]t
(
)
IH
kH
)
(2)
o
1 + KQ[G]t
IH
The KQ values for MeO-An and NC-An are not determined,
as the emission of the guests overlaps with that of 14+. The KQ
and the binding free energy -∆GQ are listed in Table 4. The
results show that the KQ values are close to the corresponding
(55) (a) Odashima, K.; Itai, A.; Iitaka, Y.; Arata, Y.; Koga, K. Tetrahedron
Lett. 1980, 21, 4347. (b) Diederich, F.; Griebel, D. J. Am. Chem. Soc. 1984,
106, 8037. (c) Cowart, M. D.; Sucholeiki, I.; Bukownik, R. R.; Wilcox, C.
S. J. Am. Chem. Soc. 1988, 110, 6204.
K
NMR values determined from 1H NMR titrations. This leads to
the conclusion that the quenching only occurs when the guest
9
15866 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
emission of 14+, which would facilitate energy transfer from
the excited state of 14+ to the CT excited state. The CT excited
state, believed to be nonemitting, would relax to the ground
state via radiationless decay. The absence of similar CT
absorption in the spectra of the mixtures of 1‚(OTf)4 and excess
Dmb and Bip could account for the fact that the two compounds
do not quench the emission.
Apparent Quenching of the Guests. The emission intensity
of all of the guests is reduced upon addition of 1‚(OTf)4 to the
CH3CN solutions of the compounds. The decrease in the
emission intensity of the guests is found to obey the Beer’s
o
Law, showing linear plots of log IGo/IG versus [H]t (IG and IG
are the emission intensity of the guests in the presence and
absence of 14+, respectively). Furthermore, the gradients of the
plots are equal to the extinction coefficients of 14+ (Figures
S72-76). In other words, the 14+ ion, acting like an emission
filter, attenuates the fluorescence intensity of the guests by
absorbing the emitted light. The filter effect is due to the
extensive overlap between the intense low-energy π f π*
absorption band of 14+, which ranges from 320 to 480 nm, and
the fluorescence of the guests. Nonetheless, it does not neces-
sarily rule out quenching of the guests by processes such as
intramolecular energy transfer to the CT excited state. It is
because the absorption of the host is so dominating that it is
virtually impossible to extract the binding constants from the
titrations.
Stability of the Inclusion Complexes. The structural and
spectroscopic results clearly show that the complexation between
14+ and the guests is predominantly resulted from the interac-
tions between the aromatic rings of the host and the guests. In
addition, the edge-to-face interactions could contribute to the
stability of the inclusion complexes containing the “long” guests
such as An and Py. In fact, the binding of the guests to 14+
belongs to a class of host-guest complexation which involves
receptors with permanent charge and neutral aromatic mol-
ecules.57 A multitude of secondary interactions including van
der Waal, charge-transfer (CT), solvophobic effect,58 and
electrostatic interactions have been invoked as the driving forces
of this class of complexation.
The common way to probe the driving forces of the
complexation is to correlate the free energy of binding with
different molecular properties of the guests and other physical
quantities.58,59 CT interactions have been suggested to be
responsible for the complexation between CBPQT4+ and
electron-rich aromatic molecules.36a,b,60 It is supported by the
correlation between the free energy of binding and the electron-
donating ability of the guests.60 Furthermore, for the complex-
ation between CBPQT4+ and tetrathiafulvalene and its deriva-
tives (TTF), the free energy of binding is increased as the
reduction potential of TTF is decreased.60c However, the plot
Figure 9. UV-vis absorption spectra of CH3CN solutions of 1‚(OTf)4
(concentration ) 2.0 mM, black) and Py (concentration ) 8.0 mM, blue),
the sum of the two spectra (red), and the mixture of 1‚(OTf)4 and Py of the
same concentrations (orange).
binds to 14+. In other words, the emission is not quenched by
bimolecular (Stern-Volmer) reactions between 1‚(OTf)4 and
the aromatic molecules.
Possible Quenching Mechanism. The X-ray crystal struc-
tures of the inclusion complexes clearly show that the com-
plexation alters the conformation of 14+. Particularly, the central
ring in the anthracenyl undergoes severe out-of-plane distortion
upon the binding. It is possible that the distortion affects the
emission quantum yield (Φem) of the anthracenyl rings. Another
plausible quenching mechanism is energy transfer from the
anthracenyl ππ* excited state to a nonemitting charge-transfer
(CT) excited state which arises from the interactions between
the bipy and the guest. Previous studies showed that the
fluorescence of anthracene56 and hydroquinols36c is quenched
upon their binding to the CBPQT4+ ion. The UV-vis absorption
spectra of the inclusion complexes exhibit low-energy charge-
transfer absorption bands ascribable to the electronic transition
from the π-electron-rich guests to the π-electron-deficient
bipyridinium in CBPQT4+ 36,56
The quenching is attributed to
.
fast energy transfer from the emitting ππ* excited state of the
guest to the low-lying CT excited state, which returns to the
ground state via radiationless decay. While the UV-vis absorp-
tion spectra of CH3CN solutions which contain 1‚(OTf)4 and
excess guests show no distinct new absorption band, a weak
absorption tailing from 500 to 800 nm is observed in the spectra
of the mixtures of 1‚(OTf)4 and Nap, Phen, An, Br-An, CO2H-
An (Figures S66-70), and Py. No such absorption tail is found
in the sum of the spectra of 1‚(OTf)4 and the guests recorded
at the same concentrations. Figure 9 shows the absorption
spectra of the mixture of 1‚(OTf)4 and Py, and the sum of the
spectra of the two compounds. Similar absorption spanning
∼500-700 nm is observed in the solid-state UV-vis reflectance
spectra of the crystals of the An-, Br-An-, and Py-complexes
(Figure S71). The solids of 1‚(OTf)4 do not display such an
absorption tail which could be part of a CT band that overlaps
with the absorption of the host and the guest. Notably, there is
large overlap between the supposed CT absorption and the
(57) (a) Schneider, H.-J.; Mohammad-Ali, A. K. Receptors for Organic Guest
Molecules. In ComprehensiVe Supramolecular Chemistry; Lehn J.-M., Chair
Ed.; Atwood, J. L., Davis, J. E. D., MacNicol, D. D., Vo¨gtle, F., Exec.
Eds.; Pergamon: Oxford, UK, 1996; Vol. 3, Chapter 3. (b) Diederich, F.
Angew. Chem., Int. Ed. Engl. 1988, 27, 362.
(58) (a) Schneider, H.-J.; Kramer, R.; Simova, S.; Schneider, U. J. Am. Chem.
Soc. 1988, 110, 6442. (b) Ferguson, S. B.; Sandford, E. M.; Seward, E.
M.; Diederich, F. J. Am. Chem. Soc. 1991, 113, 5410. (c) Mirzoian, A.;
Kaifer, A. E. J. Org. Chem. 1995, 60, 8093. (d) Liu, T.; Schneider, H.-J.
Angew. Chem., Int. Ed. 2002, 41, 1368.
(59) (a) Schneider, H.-J.; Juneja, R. K.; Simova, S. Chem. Ber. 1989, 122, 1211.
(b) Canceill, J.; Cesario, M.; Collet, A.; Guilhem, J.; Lacombe, L.; Lozach,
B.; Pascard, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1246. (c) Smithrud,
D.; Diederich, F. J. Am. Chem. Soc. 1990, 112, 339.
(56) Ballardini, R.; Balzani, V.; Dehaen, W.; Dell’Erba, A. E.; Raymo, F. M.;
Stoddart, J. F.; Venturi, M. Eur. J. Org. Chem. 2000, 591.
9
J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15867

A R T I C L E S
Lin et al.
of -∆GNMR versus the reported reduction potentials of the
guests61 measured in CH3CN (Guest+ + 1e- f Guest0, E vs
SCE) (Figure 10a and Table 6) shows no correlation, implying
that the CT interactions are not the primary driving force for
the guest binding to 14+. This conclusion does not contradict
with the observation of CT absorption bands in the solutions
and solids of the inclusion complexes. In fact, studies showed
that the CT interactions between some aromatic donors and
acceptors can be very weak even though the mixtures of the
compounds display CT bands in the UV-vis spectra.62
As the guests are transferred from a solvent cage to the cavity
of 14+ upon complexation, their interactions with the solvent
could affect the binding strength. The affinity of the guests for
the solvent can be gauged by their solubility, which can be
determined by the UV-vis spectrometric method (Table 6). The
complexation also leads to the displacement of solvent molecules
from the cavity of 14+, which also requires free energy.
Assuming this free energy is similar for the binding of all of
the guests, one would expect a correlation between the solubility
of the guests and the -∆GNMR if the solvophobic effect is one
of the main driving forces of the complexation.58 Our analysis
shows that the correlation of -∆GNMR and the solubility of the
guests is reasonably good (R ) 0.93) (Figure 10b). In general,
the sparingly soluble polycyclic An, substituted An, Phen, and
Py, bind more strongly to 14+ than the more soluble Dmb, Bip,
and Nap. This indicates that the solvophobic effect could be
partly responsible for the stability of the inclusion complexes
of the large aromatic molecules.
van der Waals interactions consist of dispersive forces and
attractions that are due to dipole induced by permanent charges.
Schneider et al. have shown that the binding of neutral aromatic
guests to charged cyclophanes that contain a π-surface is
markedly stronger than that to analogous cyclophanes which
are neutral.63 The increase in the binding strength has been
attributed to the ion-dipole interactions between the charges
in the cyclophane and the π-electrons of the guests. Notably, a
study of the complexation between CBPQT4+ and disubstituted
biphenyls and benzenes shows a good correlation between the
binding free energy and the molecular polarizability of the
guests.64 The averaged molecular polarizability (RM) of Nap,
Phen, An, Br-An, and Py was reported.65 On the other hand,
the RM of Dmb, Bip, MeO-An, NC-An, and CO2H-An can
be estimated using Miller’s empirical equation66 Rm ) (4/N)
[∑aτa]2, where N is the total of number of electrons in the
molecule and τa is the parametrized polarizability of individual
atoms in the molecule (Table 6). As expected from their higher
number of π-electrons, the large aromatic molecules like Py
and An, are more polarizable than the smaller and less
(60) (a) Goodnow, T. T.; Reddington, M. V.; Stoddart, J. F.; Kaifer, A. E. J.
Am. Chem. Soc. 1991, 113, 4335. (b) Bernardo, A. R.; Stoddart, J. F.; Kaifer,
A. F. J. Am. Chem. Soc. 1992, 114, 10624. (c) Nielsen, M. B.; Jeppesen,
J. O.; Lau, J.; Lomholt, C.; Damgaard, D.; Jacobsen, J. P.; Becher, J.;
Stoddart, J. F. J. Org. Chem. 2001, 66, 3559.
(61) CRC Handbook Series in Organic Electrochemistry; Meites, L., Zuman,
P., Eds.; CRC Press: Cleveland, OH, 1976-1993; Vols. 1-6.
(62) Diederich, F.; Philip, D.; Seiler, P. J. Chem. Soc., Chem. Commun. 1994,
205.
Figure 10. Plots of -∆GNMR versus (a) oxidation potentials, (b) solubility,
and (c) molecular polarizability of the guests.
(63) (a) Schneider, H.-J.; Blatter, T.; Simova, S.; Theis, I. J. Chem. Soc., Chem.
Commun. 1989, 580. (b) Schneider, H.-J.; Blatter, T. Angew. Chem., Int.
Ed. Engl. 1988, 27, 1163.
conjugated Dmb, Bip, and Nap. The plot of -∆GNMR versus
the RM of the guests shows a reasonably good correlation with
R ) 0.97 (Figure 10c). This suggests that the higher binding
affinity exhibited by the large aromatic molecules is partly due
to the increased ion-dipole interactions.
(64) Castro, R.; Berardi, M. J.; Cordova, E.; Ochoa de Olza, M.; Kaifer, A. E.;
Evanseck, J. D. J. Am. Chem. Soc. 1996, 118, 10257.
(65) CRC Handbook of Chemistry and Physics, 84th ed.; Lide, D. R., Chair
Ed.; CRC Press: Cleveland, OH, 2003.
(66) Miller, K. J. J. Am. Chem. Soc. 1990, 112, 8533.
9
15868 J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004

Self-Assembly of a Luminescent Gold Rectangle
A R T I C L E S
Table 6. Binding Free Energy, Solubility, Molecular Polarizability,
and Oxidation Potential of the Guests
show the presence of π-π interactions between the guests and
the bipy and edge-to-face or C-H‚‚‚π bonding between the
guests and the anthracenyl backbones of the host. Comparing
the structures of the inclusion complexes suggests the mani-
festation of an induced-fit mechanism in the complexation. The
fluorescence of the gold rectangle is quenched upon the guest
binding. It has been argued that the quenching is due to either
structural changes of the gold rectangle triggered by the guest
binding or fast energy transfer from the emitting excited state
of the gold rectangle to the nonemitting CT excited state. Finally,
the higher binding affinity exhibited by the polycyclic aromatic
guests is related to the solvophobic effect, ion-dipole interac-
tions, and the edge-to-face interactions.
molecular
1
guests
−∆GNMR (kJ mol-
)
solubility (M) polarizability
R
M (Å3) E (V vs SCE)c
Dmb
Bip
Nap
Phen
9.7 ( 0.4
12.0 ( 0.2
12.0 ( 1.0
15.5 ( 0.1
17.2 ( 0.2
17.9 ( 0.1
17.4 ( 0.1
19.7 ( 0.4
19.4 ( 0.3
18.2 ( 0.1
2.420
1.030
1.602
0.310
0.020
0.150
0.033
0.085
0.031
0.082
146.0b
1.34
1.91
1.54
1.50
1.09
1.05
1.56
1.33
199.0b
175.9a
247.0a
259.3a
267.2b
264.3b
283.2a
264.9b
282.2a
An
MeO-An
NC-An
Br-An
CO2H-An
Py
1.16
a Reference 65. b The molecular polarizability of the compounds is
calculated from the empirical formula Rm ) (4/N)[∑aτa]2, where N is the
total of number of electrons in the molecule and τa is the parametrized
polarizability of individual atoms in the molecule.66 c All of the potentials
cited are half-wave potentials measured in CH3CN, except for that for Bip
which is the peak potential (ref 61).
Acknowledgment. This work is dedicated to Professor
Sunney I. Chan on the occasion of his retirement. We thank
Ms. Tan Geok Kheng for her assistance in the determinations
of the X-ray crystal structures. We also thank the National
University of Singapore for financial support.
Conclusions
Supporting Information Available: Synthesis and charac-
terizations, X-ray crystallography data, ESI-MS, calculated
isotopic distributions, cyclic voltamograms of the free ligands,
packing of the An and Py complexes, 1H NMR titrations,
modified Job’s plots, titration curve fittings, fluorescence
titrations and curve fittings, charge-transfer absorption spectra,
Beer’s plots of the absorption of the guest emission by the host,
and CIF files of Au2(µ-PAnP)(NO3)2‚0.5Et2O, 1‚(OTf)4‚4.8H2O,
(1⊃An)‚(OTf)4, (1⊃Py)‚(OTf)4‚CH2Cl2, and (1⊃Br-An) (OTf)4‚
Et2O. This material is available free of charge via the Internet
at http://pubs.acs.org.
To summarize, we have demonstrated the facile synthesis of
a kinetically stable nanoscopic luminescent gold rectangle by
the self-assembly between the di-gold(I) clip Au2(µ-PAnP)(X)2
(X ) NO3 or OTf-) and 4,4-bipyridine. The molecular
-
recognitions of the gold rectangles in the solid state and solution
have been studied in detail. It has been shown that the molecular
rectangles assemble into 2D mosaic in the solid state via the
complementary edge-to-face Ph-Ph interactions in the solid
state. 1H NMR and fluorescence titrations, along with the
structural results, showed that the gold rectangle is a receptor
for aromatic molecules of different sizes and electronic proper-
ties. The X-ray crystal structures of the inclusion complexes
JA0456508
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J. AM. CHEM. SOC. VOL. 126, NO. 48, 2004 15869