Luminescent μ-ethynediyl and μ-butadiynediyl binuclear gold(I) complexes: Observation of 3(ππ*) emissions from bridging Cn2- units

Article abstract of DOI:10.1021/ja001706w

The synthesis and X-ray structural and spectroscopic characterization for LAuC≡CAuL·4CHCl₃ and LAuC≡C-C≡CAuL·2CH₂Cl₂ (1·4CHCl₃ and 2·2CH₂Cl₂, respectively; L = PCy₃, tricyclohexylphosphine) are reported. The bridging C_n^2- units are structurally characterized as acetylene or diacetylene units, with C≡C distances of 1.19(1) and 1.199(8) A for 1·4CHCl₃ and 2·2CH₂Cl₂, respectively. An important consequence of bonding to Au(I) for the C_n^2- moieties is that the lowest-energy electronic excited states, which are essentially acetylenic ³(ππ*) in nature, acquire sufficient allowedness via Au spin-orbit coupling to appear prominently in both electronic absorption and emission spectra. The origin lines for both complexes are well-defined and are observed at 331 and 413 nm for 1 and 2, respectively. Sharp vibronic progressions corresponding to v(C≡C) are observed in both emission and absorption spectra. The acetylenic ³(ππ*) excited state of 2 has a long lifetime (τ₀ = 10.8 μs) in dichloromethane at room temperature and is a powerful reductant (E°[Au₂⁺/Au₂*] ≤ -1.85 V vs SSCE).

Full text of DOI:10.1021/ja001706w

J. Am. Chem. Soc. 2001, 123, 4985-4991
4985
Luminescent µ-Ethynediyl and µ-Butadiynediyl Binuclear Gold(I)
3
2-
Complexes: Observation of (ππ*) Emissions from Bridging Cn
Units
Chi-Ming Che,* Hsiu-Yi Chao, Vincent M. Miskowski, Yanqin Li, and Kung-Kai Cheung
Contribution from the Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road,
Hong Kong SAR, China
ReceiVed May 17, 2000
Abstract: The synthesis and X-ray structural and spectroscopic characterization for LAuCtCAuL‚4CHCl3
and LAuCtC-CtCAuL‚2CH2Cl2 (1‚4CHCl3 and 2‚2CH2Cl2, respectively; L ) PCy3, tricyclohexylphosphine)
2-
are reported. The bridging Cn units are structurally characterized as acetylene or diacetylene units, with
CtC distances of 1.19(1) and 1.199(8) Å for 1‚4CHCl3 and 2‚2CH2Cl2, respectively. An important consequence
of bonding to Au(I) for the Cn2- moieties is that the lowest-energy electronic excited states, which are essentially
acetylenic 3(ππ*) in nature, acquire sufficient allowedness via Au spin-orbit coupling to appear prominently
in both electronic absorption and emission spectra. The origin lines for both complexes are well-defined and
are observed at 331 and 413 nm for 1 and 2, respectively. Sharp vibronic progressions corresponding to V(CtC)
3
are observed in both emission and absorption spectra. The acetylenic (ππ*) excited state of 2 has a long
lifetime (τ0 ) 10.8 µs) in dichloromethane at room temperature and is a powerful reductant (E°[Au2+/Au2*]
e -1.85 V vs SSCE).
Introduction
excited states associated with M-Cn-M′ moieties. In this
regard, we envisioned that replacement of H+ by (R3P)Au+
would afford long-lived emissive triplet excited states for the
There is growing interest in the chemical and physical
properties of Cn-bridged multinuclear organometallic com-
plexes.1 Because of the anticipated electronic coupling between
metal ions through the Cn linker both in the ground and excited
states, this class of compounds has potential applications in
molecular-scale electronics and nonlinear optical materials.1
However, most studies in this area focus on synthetic and
structural aspects while spectroscopic investigations, especially
on the electronic transitions associated with the Cn2- chain, are
sparse. The electronic structure of (CO)5ReCtCRe(CO)5 was
reported by Beck and Trogler in 1990, and the lowest-energy
allowed absorption at 319 nm was assigned to a π(C22-) f
π*(CO) transition.2 A low-energy metal-to-metal charge-transfer
transition across a Cn bridge, which appears in the visible region,
has been independently observed by Gladysz3 and Bruce.4
Nevertheless, little is known about the spectroscopy and
2-
Cn units, from which their excited-state chemistry may be
developed. Because of the heavy atom effect, it is known that
coordination of gold(I) to unsaturated organic species can
enhance the lifetime and emission quantum yield of intraligand
triplet excited states.6
The first µ-ethynediyl digold(I) complexes were synthesized
by Cross and co-workers,7 and their photophysical properties
have been described in some instances.6b,8 However, these
luminescent complexes are supported by arylphosphine ancillary
ligands. Because the lowest triplet excited state of benzene lies
at 29470 cm-1,9 there is a significant chance that a benzenoid
state may be the lowest-energy excited state for these complexes.
To eliminate spectroscopic interference from ππ* states of aryl-
phosphines, and to maintain sterically congested environ-
ments around the gold centers to restrict oligomerization via
Au(I)‚‚‚Au(I) interactions, we have prepared C2- and C4-bridged
derivatives bearing tricyclohexylphosphine (PCy3) auxiliaries,
namely (Cy3P)AuCtCAu(PCy3) (1) and (Cy3P)AuCtC-Ct
CAu(PCy3) (2). The bulky PCy3 ligand has no interfering excited
states, and these compounds exhibit luminescence spectra that
can unequivocally be assigned to 3(ππ*) states of the bridging
Cn2- units. To our knowledge, this work provides a useful entry
to generate long-lived and emissive 3(ππ*) states of Cn2- chains
reactivity of the triplet excited states of the Cn chains.5
Our objective in this work is to design and synthesize new
model compounds for understanding the fundamentals of
intraligand, metal-to-ligand and ligand-to-metal charge-transfer
2-
* Corresponding author. Fax: +852 2857 1586. E-mail: cmche@hku.hk.
(1) (a) Beck, W.; Niemer, B.; Wieser, M. Angew. Chem., Int. Ed. Engl.
1993, 32, 923-949. (b) Lang, H. Angew. Chem., Int. Ed. Engl. 1994, 33,
547-550. (c) Bunz, U. H. F. Angew. Chem., Int. Ed. Engl. 1996, 32, 969-
971. (d) Bruce, M. I. Coord. Chem. ReV. 1997, 166, 91-119. (e) Paul. F.;
Lapinte, C. Coord. Chem. ReV. 1998, 178-180, 431-509.
(2) Heidrich, J.; Steimann, M.; Appel, M.; Beck, W.; Phillips, J. R.;
Trogler, W. C. Organometallics 1990, 9, 1296-1300.
(3) (a) Weng, W.; Bartik, T.; Gladysz, J. A. Angew. Chem., Int. Ed. Engl.
1994, 33, 2199-2202. (b) Brady, M.; Weng, W.; Zhou, Y.; Seyler, J. W.;
Amoroso, A. J.; Arif, A. M.; Bo¨hme, M.; Frenking, G.; Gladysz, J. A. J.
Am. Chem. Soc. 1997, 119, 775-788.
(6) (a) Li, D.; Hong, X.; Che, C. M.; Lo, W. C.; Peng, S. M. J. Chem.
Soc., Dalton Trans. 1993, 2929-2932. (b) Hong, X.; Cheung, K. K.; Guo,
C. X.; Che, C. M. J. Chem. Soc., Dalton Trans. 1994, 1867-1871.
(7) Cross, R. J.; Davison, M. F. J. Chem. Soc., Dalton Trans. 1986, 411-
414.
(8) Muller, T. E.; Choi, S. W. K.; Mingos, D. M. P.; Murphy, D.;
Williams, D. J.; Yam, V. W. W. J. Organomet. Chem. 1994, 484, 209-
224.
(4) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J. F.; Best, S. P.; Heath,
G. A. J. Am. Chem. Soc. 2000, 122, 1949-1962.
(5) Dembinski, R.; Bartik, T.; Bartik, B.; Jaeger, M.; Gladysz, J. A. J.
Am. Chem. Soc. 2000, 122, 810-822.
(9) Herzberg, G. Molecular Spectra and Molecular Structure: Electronic
Spectra of PolyAtomic Molecules, Vol. III; Krieger: Malabar, FL, 1991.
10.1021/ja001706w CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/03/2001

4986 J. Am. Chem. Soc., Vol. 123, No. 21, 2001
Che et al.
Table 1. Crystallographic Data for 1‚4CHCl3 and 2‚2CH2Cl2
1‚4CHCl3 2‚2CH2Cl2
Au2C38H66P2‚4CHCl3 Au2C40H66P2‚2CH2Cl2
3
in fluid solutions at ambient temperature. The (ππ*) excited
state of 2 has been shown to be a powerful reductant in solution
at room temperature.
formula
M (g/mol)
crystal system
space group
color of crystal
a (Å)
1456.33
monoclinic
P21/n (no. 14)
pale yellow
10.500(2)
18.464(3)
15.011(2)
90
100.12(2)
90
2864.9(8)
2
1172.71
triclinic
P1h (no. 2)
pale yellow
9.268(1)
9.985(2)
13.971(3)
104.81(2)
90.24(2)
109.30(2)
1174(1)
1
Experimental Section
Materials and Reagents. KAuCl4 and tricyclohexylphosphine were
obtained from Strem Chemicals. 2,2′-Thiodiethanol and lithium per-
chlorate were purchased from Aldrich Chemicals. (Caution! Perchlorate
salts are potentially explosiVe and should be handled with care and in
small amounts.) 1,4-Bis(trimethylsilyl)-1,3-butadiyne was obtained from
Lancaster Synthesis Ltd. The pyridinium quenchers were prepared
according to literature procedures.10 Au(PCy3)Cl was prepared from
chloro(thiodiglycol)gold(I) following a standard procedure for synthesis
of other phosphine derivatives.11 Details of solvent treatment for
photophysical studies have been provided earlier.12 The other solvents
used were of analytical grade.
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F(000)
1428
57.75
1.688
51.1
578
65.85
1.658
50.1
µ (cm-1
)
Dc (gcm-3
2θmax (°)
)
Synthesis. (Cy3P)AuCtCAuPCy3 (1). This complex was prepared
by modification of a literature method for related derivatives.7 Bubbling
C2H2 through a suspension of Au(PCy3)Cl (0.76 g, 1.49 mmol) and
NaOH (0.60 g, 1.49 mmol) in EtOH (100 mL) for 10 min produced a
clear colorless solution. This was evaporated to dryness, and the solid
residue was extracted with CH2Cl2 (50 mL). The volume of the filtrate
was reduced to 10 mL, and diethyl ether was added to afford a white
precipitate, which was collected and recrystallized by slow evaporation
of a chloroform solution to produce pale yellow crystals. Yield: 32%
(0.23 g). NMR (CDCl3): 1H δ 2.08-1.18 (m, Cy); 13C{1H} δ 147.7
(dd, 2JCP ) 122.6 Hz, 3JCP ) 19.0 Hz, CtC), 33.3 (d, 1JCP ) 27.5 Hz,
no. of unique refins
5102
4114
no. of reflns with I > 3σ(I) 4004
3717
226
0.029, 0.039
1.95
+0.95, -1.10
no. of parameters
262
b
Ra, Rw
0.043, 0.061
1.91
goodness-of-fit
residual F, e Å-3
+1.25, -1.11
2
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b Rw ) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]1/2
.
calculated by φ ) E/(Rstd - Rsmpl), where E is the area under the
corrected emission curve of the sample and Rstd and Rsmpl are the
corrected areas under the diffuse reflectance curves of the nonabsorbing
standard and the sample, respectively, at the excitation wavelength.
Emission quenching was performed by lifetime measurements, and the
quenching rate constant, kq, was deduced from the plot of 1/τ versus
[Q] according to the Stern-Volmer equation τ0/τ ) 1 + kqτ0[Q], where
τ0 and τ are the respective emission lifetimes in the absence and
presence of quencher Q.
X-ray Crystallography. Crystals of 1‚4CHCl3 and 2‚2CH2Cl2 were
grown by slow evaporation of a chloroform solution and by diffusion
of diethyl ether into a dichloromethane solution, respectively. Crystal
data and details of collection and refinement are listed in Table 1. For
1‚4CHCl3, diffraction experiments were performed at 301 K on a MAR
diffractometer with a 300 mm image plate detector using graphite
monochromatized Mo KR radiation (λ ) 0.71073 Å). For 2‚2CH2Cl2,
diffraction experiments were performed at 301 K on a Rigaku AFC7R
diffractometer (λ ) 0.71073 Å, ω-2θ scans). Details of structure
refinements are available in Supporting Information.
2
Cy), 30.6 (s, Cy), 27.2 (d, JCP ) 12.3 Hz, Cy), 26.0 (s, Cy); 31P{1H}
δ 58.3. Raman (cm-1): 2008 (s, CtC). FAB-MS: m/z 980 [M+
+
H]. Anal. Calcd for C38H66Au2P2‚3.5CHCl3: C, 35.80; H, 5.04%.
Found: C, 35.82; H, 5.17%.
(Cy3P)AuCtC-CtCAu(PCy3) (2). A methanolic solution (15 mL)
of 1,4-bis(trimethylsilyl)-1,3-butadiyne (0.02 g, 0.10 mmol) and an
excess of NaOH (0.08 g, 2.0 mmol) was stirred for 30 min. Au(PCy3)Cl
(0.10 g, 0.19 mmol) was then added, and the mixture was stirred for 3
h. The white solid was collected by filtration. Pale yellow crystals were
obtained by slow evaporation of diethyl ether into a dichloromethane
solution. Yield: 80% (0.08 g). NMR (CDCl3): 1H δ 2.08-1.18 (m,
Cy); 13C{1H} δ 123.8 (d, 2JCP ) 134.1 Hz, Au-CtC), 88.0 (d, 3JCP
)
1
25.8 Hz, Au-CtC), 33.2 (d, JCP ) 27.8 Hz, Cy), 30.6 (s, Cy), 27.1
2
(d, JCP ) 11.7 Hz, Cy), 25.9 (s, Cy); 31P{1H} δ 56.74. IR (cm-1):
2145 (vw, CtC), 2010 (vw, CtC). Raman (cm-1): 2150 (s, CtC),
2087 (m, CtC). FAB-MS: m/z 1004 [M+ + H]. Anal. Calcd for
C40H66Au2P2‚0.2CH2Cl2: C, 47.33; H, 6.56%. Found: C, 47.48; H,
6.70%.
[Au(PCy3)2]ClO4 (3). The synthesis was similar to that of [Au-
(PCy3)2]Cl.13 Tricyclohexylphospine (0.05 g, 0.18 mmol) was added
to a suspension of Au(PCy3)Cl (0.10 g, 0.19 mmol) in MeOH (20 mL).
After stirring for 3 h, a clear colorless solution was produced. Excess
LiClO4 was added, and slow evaporation to 5 mL produced colorless
crystals. Yield: 88% (0.14 g). NMR (CD3CN): 1H δ 2.08-1.18 (m,
Cy); 31P{1H} δ 65.41. IR (cm-1): 2928, 2852 (s, Cy), 1089 (s, br,
ClO4-). FAB-MS: m/z 757 [Au(PCy3)2+]. Anal. Calcd for C36H66-
AuClO4P2: C, 50.44; H, 7.77%. Found: C, 50.18; H, 7.94%.
Instrumentation. Details of physical instrumentation are available
in the Supporting Information. The solution emission quantum yields
were measured by the method of Demas and Crosby14 using quinine
sulfate in degassed 0.1 N sulfuric acid as standard (φr ) 0.546).
Emission quantum yields of powder samples were measured using the
method of Wrighton and co-workers15 with KBr as the standard and
Results
Syntheses and Characterization. The syntheses of µ2-Cn
binuclear metal complexes have already been subjected to
extensive studies1,3-5,16-20 notably by Gladysz and co-workers.
Many µ2-C2 and µ2-C4 binuclear derivatives incorporating the
late transition metal ions have been reported.1 Synthetic methods
for the former typically employ C2H2 or metal ethynyl species
as the C2 source.7,8,21 For µ2-C4 complexes, terminal alkyne
coupling reactions between two metal ethynyl moieties22 or the
(16) Peters, T. B.; Bohling, J. C.; Arif, A. M.; Gladysz, J. A. Organo-
metallics 1999, 18, 3261-3263.
(17) Sakurai, A.; Akita, M.; Moro-oka, Y. Organometallics 1999, 18,
3241-3244.
(10) Che, C. M.; Kwong, H. L.; Poon, C. K.; Yam, V. W. W. J. Chem.
Soc., Dalton Trans. 1990, 3215-3219.
(18) Yam, V. W. W.; Lau, V. C. Y.; Cheung, K. K. Organometallics
1996, 15, 1740-1744.
(11) Al-sa’ady, A. K.; Mcauliffe, C. A.; Parish, R. V.; Sandbank, J. A.
Inorg. Synth. 1985, 23, 191-194.
(19) Gil-Rubio, J.; Laubender, M.; Werner, H. Organometallics 1998,
17, 1202-1207.
(12) Lai, S. W.; Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C.
M. Inorg. Chem. 1999, 38, 4046-4055.
(20) Bruce, M. I.; Hall, B. C.; Kelly, B. D.; Low, P. J.; Skelton, B. W.;
White, A. H. J. Chem. Soc., Dalton Trans. 1999, 3719-3728.
(21) Muller, T. E.; Mingos, D. M. P.; Williams, D. J. J. Chem. Soc.,
Chem. Commun. 1994, 1787-1788.
(13) Muir, J. A.; Muir, M. M.; Pulgar, L. B.; Jones, P. G.; Sheldrick, G.
M. Acta Crystallogr. 1985, C41, 1174-1176.
(14) Demas, J. N. G.; Crosby, A. J. Phys. Chem. 1971, 75, 991-1024.
(15) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974,
78, 2229-2233.
(22) (a) Zhou, Y.; Seyler, J. W.; Weng, W.; Arif, A. M.; Gladysz, J. A.
J. Am. Chem. Soc. 1993, 115, 8509-8510. (b) Brady, M.; Weng, W.;
Gladysz, J. A. J. Chem. Soc., Chem. Commun. 1994, 2655-2656.

Luminescent µ-Ethynyl Gold(I) Complexes
J. Am. Chem. Soc., Vol. 123, No. 21, 2001 4987
Figure 2. Perspective view of (Cy3P)AuCtC-CtCAu(PCy3)‚2CH2Cl2
(2‚2CH2Cl2). Selected bond distances (Å) and angles (deg): Au(1)-
P(1) 2.286(1), Au(1)-C(1) 2.012(6), C(1)-C(2) 1.199(8), C(2)-C(2*)
1.37(1), C(1)-C(2)-C(2*)178.7(9), P(1)-Au(1)-C(1) 179.4(2), Au(1)-
C(1)-C(2) 175.8(6), Au(1)-P(1)-C(3) 111.8(2), Au(1)-P(1)-C(9)
111.8(2), Au(1)-P(1)-C(15) 109.5(2).
Figure 1. Perspective view of (Cy3P)AuCtCAu(PCy3)‚4CHCl3 (1‚
4CHCl3), showing the C-H‚‚‚π interactions with CHCl3 molecules.
Selected bond distances (Å) and angles (deg): Au(1)-P(1) 2.287(2),
Au(1)-C(1) 2.000(7), C(1)-C(1*) 1.19(1), P(1)-Au(1)-C(1) 176.3(2),
Au(1)-C(1)-C(1*) 178.3(9), Au(1)-P(1)-C(2) 109.6(2), Au(1)-
P(1)-C(8) 113.5(3), Au(1)-P(1)-C(14) 112.3(3).
respectively) (Np ) naphthyl),21 there are cocrystallized CHCl3
molecules which are positioned with their C-H bonds directed
orthogonally toward the center of the ethynediyl unit in the
crystal lattice of 1‚4CHCl3. The distances of H(34) and H(35)
to the center of the ethynediyl bond are ca. 2.4 and 2.5 Å,
respectively. The H(34)-H(34*) and H(35)-H(35*) vectors are
inclined by 86 and 88° respectively to the C(1)tC(1*) bond
and constitute a pseudo-octahedral arrangement. Intermolecular
distances of H(34) and H(35) to C(1) and C(1*) range between
2.5 and 2.6 Å. The T-shaped intermolecular CH‚‚‚π interactions
in 1‚4CHCl3 are comparable in nature to those in (NpPh2P)-
Au-CtC-Au(PNpPh2)‚2CHCl3 (2.42 Å)8 and (Np2PhP)Au-
CtC-Au(PNp2Ph)‚6CHCl3 (2.50 and 2.58 Å, respectively).21
These interactions can be rationalized in terms of the acidity of
the C-H proton in CHCl3 and back-donation of electron density
from the gold atom to the CtC bond. Theoretical calculations
on (H3P)Au-CtC-Au(PH3)‚‚‚CHCl3 predicted interaction
distances of 2.17 and 2.01 Å and interaction energy in the range
of 25 kJ mol-1, values which are typical for classical hydrogen-
bonded systems.24
For each molecule of (Cy3P)AuCtC-CtCAu(PCy3) (2),
there are two cocrystallized CH2Cl2 molecules (Figure 2). Two
Au(PCy3) fragments are bridged by a nearly linear C4 unit, with
Au-C(1)-C(2) and C(1)-C(2)-C(2*) angles of 175.8(6) and
178.7(9)°, respectively. The Au-P and Au-C bond lengths are
2.286(1) and 2.012(6) Å, respectively, and are comparable to
those in ethynediyl digold(I) complexes.8,21,23 The C(1)-C(2)
and C(2)-C(2*) bond distances of the C4 unit are 1.199(8) and
1.37(1) Å, respectively, which are similar to those in the related
C4-bridged bimetallic complexes (SS,RR)-[{Re(η5-C5Me5)(NO)-
(PPh3)}2(µ-CtC-CtC)] (1.202(7), and 1.389(5) Å, respec-
tively),22a [{Fe(η5-C5Me5)(η2-dppe)}2(µ-CtC-CtC)][PF6]
(1.236(9) and 1.36(1) Å, respectively),25 [{Rh(PiPr3)2(CO)}2-
(µ-CtC-CtC)] (1.205(5) and 1.388(7) Å, respectively),26 and
[{Re(tBu2bpy)(CO)3}2(µ-CtC-CtC)] (1.19(2) and 1.43(4) Å,
respectively).18 There are likely to be CH‚‚‚π interactions
between the CH2Cl2 molecules and the C4 unit, as indicated by
the locations of the solvent molecules, but because of disorder
of the CH2Cl2 molecules in the X-ray refinement process, these
cannot be quantified.
reactions of metal chloride derivatives with 1,4-bis(trimethyl-
silyl)-1,3-butadiyne4,20 have been used. The ethynediylgold(I)
complex (Cy3P)AuCtCAu(PCy3) (1) was prepared using a
modification of the procedure reported by Cross and Davison.7
Thus, a suspension of Au(PCy3)Cl in ethanol reacts with ethyne
gas in the presence of NaOH to produce (Cy3P)AuCt
CAu(PCy3); the rapid reaction produces a clear solution from
which 1 is easily isolated. The Raman spectrum of 1 shows a
very strong peak at 2008 cm-1, which is characteristic of a CtC
stretch. Hence, the bridging C2 ligand of 1 can be confidently
classified as an ethynediyl unit.1a The 31P{1H} NMR spectrum
indicates a single environment for the phosphine ligands at 58.3
ppm, and this is slightly downfield from that for Au(PCy3)Cl
(55.21 ppm). Evidently, the electronic effect of the ethynediyl
group, as transmitted through Au(I), is not very different from
that for Cl. The 13C{1H} NMR spectrum shows a doublet of
doublets centered at 147.7 ppm for the ethynediyl unit, with
3
coupling to two phosphorus nuclei (2JCP ) 122.6 and JCP
)
19.0 Hz).
Complex 2 was synthesized by the reaction of 2 equiv of
Au(PCy3)Cl with 1,4-bis(trimethylsilyl)-1,3-butadiyne in the
presence of NaOH in MeOH. In the Raman spectrum of 2, the
two peaks at 2150 and 2087 cm-1 are attributed to V(CtC)
modes. The 13C{1H} NMR spectrum of 2 shows two doublets
centered at 88.0 and 123.8 ppm. The former is assigned to C(2)
3
with JCP ) 25.8 Hz, and the latter is assigned to C(1) with
2JCP ) 134.1 Hz (for nomenclature, see Figure 2).
To assist in the spectra interpretation of 1 and 2, the bis-
(phosphine) complex [Au(PCy3)2]ClO4 (3) was prepared. The
31P{1H} NMR spectrum of 3 shows a single peak at 65.41 ppm.
Crystal Structures. Figure 1 shows the perspective view of
1‚4CHCl3, which has a crystallographic rotation center located
at the center of the ethynediyl unit. The gold atoms reside in a
virtually linear two-coordinate environment, and are bridged by
a C22- ligand with P-Au-C and Au-C-C angles of 176.3(2)
and 178.3(9)°, respectively. The CtC bond length is 1.19(1)
Å; this bond length, as well as those of the Au-C (2.000(7) Å)
and Au-P (2.287(2) Å) bonds, are almost identical to those in
the PPh3 analogue, (Ph3P)Au-CtC-Au(PPh3)‚2C6H6 (1.19(2),
2.00(1), and 2.280(3) Å, respectively).23 As observed for
(NpPh2P)Au-CtC-Au(PNpPh2)‚2CHCl3 (1.222(16), 1.983(8),
and 2.277(2) Å, respectively)8 and (Np2PhP)Au-CtC-
Au(PNp2Ph)‚6CHCl3 (1.225(34), 1.986(17), and 2.289(5) Å,
(24) Fan, M. F.; Lin, Z.; McGrady, J. E.; Mingos, D. M. P. J. Chem.
Soc., Perkin Trans. 2 1996, 563-568.
(25) Narvor, N. L.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995,
117, 7129-7138.
(23) Bruce, M. I.; Grundy, K. R.; Liddell, M. J.; Snow, M. R.; Tiekink,
R. T. J. Organomet. Chem. 1988, 344, C49-C52.
(26) Gevert, O.; Wolf, J.; Werner, H. Organometallics 1996, 15, 2806-
2809.

4988 J. Am. Chem. Soc., Vol. 123, No. 21, 2001
Che et al.
Figure 6. Emission spectrum of (Cy3P)AuCtC-CtCAu(PCy3) (2)
in degassed CH2Cl2 solution at 298 K with excitation wavelength at
364 nm.
Figure 3. Normalized emission spectra of (Cy3P)AuCtCAu(PCy3)‚
4CHCl3 (1‚4CHCl3) in the solid state at 298 K (dashed line) and 77 K
(solid line) with excitation wavelength at 280 nm.
except that it is red-shifted by ∼6000 cm-1 (the progression
frequency ∼2100 cm-1). Complex 1‚4CHCl3 exhibits, in most
media, two broad emissions centered at λmax 415 and 530 nm
which are completely dominant for the room-temperature solid
and in alcoholic glass (Figures 3 and 4).
It is noteworthy that we have been able to observe the sharp-
line emissions of 1 in n-butyronitrile glass and 2 in fluid
solutions. Luminescence excitation spectra for these emissions,
as illustrated in Figure 5 for 1, are in excellent agreement with
absorption spectra. Absorption spectra of 1 and 2 in CH2Cl2
are shown in Figures 7 and 8, and the data are collected in Table
2. The absorption spectrum of 3 (see Supporting Information)
is in good agreement with those reported by Mason and co-
workers for analogous Au(I) trialkylphosphine complexes.27
Thus, even at very high concentrations (∼10-2 mol dm-3), no
bands could be detected at lower energies relative to the 251
nm band, and absorbance is negligible beyond 265 nm. In
contrast, 1 and 2 exhibit well-defined absorption bands extending
to much longer wavelengths (>260 nm). Indeed, the absorption
bands above 260 nm are absent from the spectrum of 3. The
lowest-energy feature in the room-temperature spectrum of 1
in CH2Cl2 (Figure 7) is a weak (ꢀ ≈ 50 dm3 mol-1 cm-1),
narrow absorption band at 331 nm that corresponds extremely
well to the 330 nm origin line in both the 77 K n-butyronitrile
emission and excitation spectra (Figure 5). Evidently these
features represent the origin lines of an electronic transition that
is electronic-dipole allowed, albeit weakly; the negligible Stokes
shift is a consequence of the narrow line-width.
Figure 4. Emission spectrum of (Cy3P)AuCtCAu(PCy3) (1) in
MeOH/EtOH (4:1 v/v) at 77 K with excitation wavelength at 280 nm.
The excitation spectrum of complex 1 (Figure 5) at 77 K is
better resolved than the room-temperature absorption spectrum.
A band at 313 nm and a shoulder at 298 nm have appropriate
intensities relative to the origin line and thus correspond to one
and two quanta of the excited-state progression frequency; this
assignment hence implies a very substantial decrease of the
vibrational frequency in the excited state, to ∼1700 cm-1. Two
additional low-energy bands at 302 and 288 nm are resolved in
the excitation spectrum. These are more intense than the features
discussed above (ꢀ ≈ 330 dm3 mol-1 cm-1 for the 303 nm band,
Figure 7), and their positions do not fit into a vibrational
progression based upon the 330 nm origin. We therefore attribute
the 302 nm feature to another electronic origin. The separation
between the 302 and 288 nm bands of ∼1600 cm-1 suggests
that this state distorts along the same vibrational coordinate as
the lowest-energy state and is therefore electronically related.
Figure 5. Emission (solid line) and excitation (dashed line) spectra
of (Cy3P)AuCtCAu(PCy3) (1) in glassy n-butyronitrile at 77 K. The
emission wavelength was monitored at 381 nm.
Electronic Emission and Absorption Spectra. Emission
spectra of 1 and 2 are presented in Figures 3-6, and the data
are summarized in Table 2. For solutions of 2 in moderately
polar solvents (CH2Cl2 and THF) and for microcrystalline solids
of 1‚4CHCl3 at 77 K and 2‚2CH2Cl2 at room temperature, well-
resolved vibronic structures are shown in the emission spectrum.
Indeed, extremely sharp progressions are observed in a single
frequency ∼2000 cm-1 which is assigned to the symmetric
V(CtC) mode. The emission spectrum of 2 in CH2Cl2 (Figure
6) is very similar to that of 1 in n-butyronitrile glass (77 K)
(27) (a) Savas, M. M.; Mason, W. R. Inorg. Chem. 1987, 26, 301-307.
(b) Jaw, H. R. C.; Mason, W. R. Inorg. Chem. 1989, 28, 4370-4373. (c)
Chastain, S. K.; Mason, W. R. Inorg. Chem. 1982, 21, 3717-3721.

Luminescent µ-Ethynyl Gold(I) Complexes
Table 2. Photophysical Data for Complexes 1-3
J. Am. Chem. Soc., Vol. 123, No. 21, 2001 4989
abs/nm (ꢀmax/dm3 mol-1 cm-1
)
λ
em/nm (τ/µs)a
φem
a
complex
medium (T/K)
CH2Cl2 (298)
λ
1
239 (47260), 256 (55020),
270 (6840), 283 (3500),
303 (330), 315 (sh, 120), 331 (50)
solid (298)
solid (77)
424 (0.28), 534 (0.33)
0.058
322 (2.9), 335, 344, 359,
369, 388, 399, 416, 432, 514 (6.7)
417 (1.4), 521 (5.5)
330 (4.6), 354, 381, 412 (1.2)
417 (10.8), 457, 506, 566, 643
MeOH/EtOH (4:1 v/v) (77)
n-butyronitrile (77)
CH2Cl2 (298)
2
245 (77930), 252 (sh, 27350),
264 (119880), 291 (1770), 309 (1410),
328 (580), 339 (sh, 130), 361 (80),
381 (40), 413 (20)
246 (49100), 256 (sh, 53800),
270 (103340), 295 (2700), 313 (2320),
334 (1150), 359 (80), 368 (90),
386 (50), 418 (40), 434 (20)
0.21
0.025
0.17
THF (298)
419 (2.4), 461, 511, 573, 650
417 (25), 457, 507, 566, 643
solid (298)
3
CH2Cl2 (298)
239 (2200), 251 (1810)
a Errors for τ and φem are estimated to be (5% and 10%, respectively.
to red-shifted [5d(Au)] f [6p(Au), π*(phosphine)] transitions.
In addition, a weak, vibronically structured absorption system
is found that is very similar to that of 1 but, like the emission,
red-shifted by ∼6000 cm-1. As before, there is good zero-zero
overlap between the absorption and emission spectra. We once
again infer two electronic origins at 413 and 361 nm for 2. The
only band in the absorption spectrum of 2 that does not have
an obvious blue-shifted analogue in the spectrum of 1 appears
at 328 nm (ꢀ ≈ 580 dm3 mol-1 cm-1). It is possible that the
corresponding band for 1 is obscured by Au d f p transitions.
Reactivity and Photoredox Properties of the Butadiynediyl
3(ππ*) State of Complex 2. Although 1 is nonemissive in
solution at room temperature, 2 displays a long-lived acetylenic
3(ππ*) emission in dichloromethane and tetrahydrofuran solu-
tions (Table 2). Like the emissions of aromatic organic
molecules, the emission lifetime of 2 is affected by complex
concentration. A linear relationship (R ) 0.93) can be obtained
by plotting 1/τ versus [2] in dichloromethane; the self-quenching
rate constant (kq ) 8.2 × 107 dm3 mol-1 s-1) and lifetime at
infinite dilution (τ0 ) 10.8 µs) are determined from the slope
and reciprocal intercept of the plot (see Supporting Information).
However, even in concentrated solutions of 2 (up to ∼10-3 mol
dm-3) in dichloromethane, there is no change in the emission
peak maxima, and no additional low-energy band attributable
to excimer emission is observed. The solvent effect on the
photoluminescence of complex 2 has been examined in two
solvents due to its general low solubility. Changing the solvent
from dichloromethane to tetrahydrofuran decreases both the
lifetime and quantum yield (Table 2).
Figure 7. Absorption spectrum of (Cy3P)AuCtCAu(PCy3) (1) in
CH2Cl2 solution at 298 K.
The acetylenic 3(ππ*) excited state of 2 is a powerful
photoreductant. The emission of 2 in dichloromethane/aceto-
nitrile (1:1 v/v) solution is oxidatively quenched by a series of
pyridinium acceptors (A+) of various reduction potentials.28 The
quenching rate constants are summarized in Table 3. As shown
in Table 3, the rate constants decrease with the E°[A+/A] values
suggesting an electron-transfer mechanism for the quenching
reactions (eq 1).
Figure 8. Absorption spectrum of (Cy3P)AuCtC-CtCAu(PCy3) (2)
in CH2Cl2 solution at 298 K.
At higher energies, four absorption bands of significantly
greater intensity are observed. The two bands of 1 at 239 and
256 nm initially appear to correspond to the two bands of 3 at
239 and 251 nm, but the extinction coefficients of the bands of
1 are too high for such a correlation. We suggest instead that
the two bands at 270 and 283 nm for 1 correlate to the lowest-
energy bands of 3. The very intense bands of 1 at 239 and 256
nm can then be correlated to bands in 3 and related complexes27
that appear at λ < 210 nm.
[(Cy3P)Au-CtC-CtC-Au(PCy3)] + A+f
[(Cy3P)Au-CtC-CtC-Au(PCy3)]+ + A (1)
Notably, even with 2,6-dimethyl-4-methoxy-N-methylpyridin-
The absorption spectrum of 2 (Figure 8) is similar to that of
1, but with additional complexities. Bands at 245, 264, 291,
and 309 nm are similar to the bands of 1 that we have assigned
(28) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984,
106, 3027-3029.

4990 J. Am. Chem. Soc., Vol. 123, No. 21, 2001
Che et al.
Table 3. Bimolecular Rate Constants for the Oxidative Quenching of 2 by Pyridinium Acceptors in Degassed Dichlomethane/Acetonitrile (1:1
v/v) (0.1 mol dm-3 nBu4NPF6) at 298 K
E° (A+/A)
kq
kq′
c
quenchera
(vs SSCE)b
(dm3 mol-1 s-1
)
(dm3 mol-1 s-1
)
ln kq′
N,N′-dimethyl-4,4′-pyridinium
4-cyano-N-methylpyridinium
4-methoxycarbonyl-N-methylpyridinium
4-amido-N-ethylpyridinium
3-amido-N-benzylpyridinium
3-amido-N-methylpyridinium
2,6-dimethyl-N-methylpyridinium
2,4,6-trimethyl-N-methylpyridinium
2,6-dimethyl-4-methoxy-N-methylpyridinium
-0.45
-0.67
-0.78
-0.93
-1.07
-1.14
-1.52
-1.67
-1.88
5.37 × 109
4.77 × 109
4.28 × 109
2.56 × 109
2.03 × 109
1.64 × 109
2.76 × 109
1.10 × 109
4.75 × 108
9.72 × 109
7.92 × 109
6.65 × 109
3.25 × 109
2.44 × 109
1.90 × 109
3.58 × 109
1.21 × 109
4.95 × 108
23.00
22.79
22.62
21.90
21.62
21.37
22.00
20.91
20.02
a All quenchers are hexafluorophosphate salts. b From ref 28. c (1/kq′) ) (1/kq) - (1/kd) where kd is the diffusion-limited rate constant, taken to
be 1.2 × 1010 dm3 mol-1 s-1. Errors for kq and kq′ are estimated to be (5%.
ium hexafluorophosphate which has a reduction potential of
of 3 can be assigned to [5d(Au)] f [6p(Au), π*(phosphine)]
transitions that have singlet-triplet parentage, but spin-orbit
mixing is large.27 These two transitions can be correlated to
the absorption bands at 270 and 283 nm for 1, and 291 and
309 nm for 2. This assignment is made by considering the
extinction coefficients of these transitions. In contrast, the
[5d(Au)] f [6p(Au), π*(phosphine)] transitions with singlet-
singlet parentage, which are an order of magnitude more intense,
are located at <210 nm,27 and the highly intense bands at 239
and 256 nm for 1, and at 245 and 264 nm for 2, can be ascribed
to this type of transition. These assignments imply that all
[5d(Au)] f [6p(Au), π*(phosphine)] transitions for 1 are red-
shifted by ∼5000 cm-1 compared to those for 3. For 2, red-
shifts of ∼3000 and ∼1000 cm-1 for the singlet-triplet and
singlet-singlet parentage transitions, respectively, are apparent.
In principle, such an effect may arise from two possible
-1.87 V,29 the quenching rate constant just remains high (4.95
× 108 mol-1 dm3 s-1). Because the range of the quenching rate
+
constants is not wide, the excited-state potential of E°[Au2
/
Au2*] (Au2 ) 2) cannot be accurately determined by applying
ln kq′ versus E°[A+/A] according to the Marcus equation.30
However, E°[Au2+/Au2*] can be estimated by the equation:
E°[Au2+/Au2*] ) E°[Au2+/Au2] - E0-0, where E0-0 is
calculated spectroscopically (2.97 eV), and E°[Au2+/Au2] is
obtained from cyclic voltammetry. Complex 2 is electrochemi-
cally inactive toward reduction but undergoes irreversible
oxidation at 1.12 V. Thus E°[Au2+/Au2] is estimated to be e1.12
V vs SSCE (sodium chloride saturated calomel electrode),31 and
E°[Au2+/Au2*] is calculated at e-1.85 V vs SSCE. Flash
photolysis experiments with excitation at 355 nm of a dichlo-
romethane/acetonitrile (1:1 v/v) solution containing complex 2
(∼10-3 mol dm-3) and MV2+ (methyl viologen) (∼10-2 mol
dm-3) revealed a signal corresponding to the formation of MV+
(λmax ) 390 and 600 nm) 2 µs after the laser flash. However,
2-
mechanisms. First, π- (or σ-) donation of Cn can destabilize
the 5d(Au) orbitals, but this is unlikely since Mason27 has
emphasized that all available evidence indicates ligand interac-
tions with 5d(Au) orbitals to be weak. Moreover, the pattern of
d f p transitions would be different from 3 if this mechanism
is operating. For example, acetylenic π-donation would specif-
complex
2
is unstable upon rendering photo-
irradiation, thus making photochemical reactivity studies dif-
ficult.
2
2
2
ically destabilize dxz,yz, but not dz and dxy,x -y . A more
2-
Discussion
reasonable possibility is acetylenic π-acceptance: if π*(Cn
)
is higher in energy than px,y(Au), this will stabilize all of the
observed 5d f 6p excitations, as the low-lying ones all terminate
px,y(Au).24 The π* orbital of acetylene is extremely high in
energy;34 while the spin- and dipole-allowed π f π* excitation
Ethynediyl-bridged complexes LnM-C2-MLn with bond
distances consistent with C2 bond orders ranging from three
(M-C bond order of one) to two (M-C bond order of two)32
to one (M-C bond order of three)33 have been reported. The
vast majority, including complex 1 of the present study,
correspond to the first case1 and may be regarded as “dimetalated
hydrocarbons”.1a
(1Σg f Σu+) is buried under Rydberg transitions.9
+
1
2-
(2) The second spectacular consequence of the bridging Cn
ligands is the observation of a near-UV electronic emission that
exhibits a very sharp single vibronic progression (V(CtC) ≈
2000 cm-1). There is a corresponding weak absorption band
with an identical origin energy but a dramatically reduced
progression frequency of ∼1700 cm-1, plus an overlapping
progression with similar vibronic structure but a higher energy
origin (by ∼3000 cm-1). One explanation is that these features
are due to [5d(Au)] f [π*(C2)] transitions. The sharpness of
the vibronic structure is inconsistent with such metal-to-ligand
charge transfer (MLCT) assignment because authentic MLCT
transitions typically show substantial distortions along metal-
ligand coordinates. Indeed, there is an obvious assignment for
these bands. It has been known for many years that electron
correlation effects result in very large splitting of the six excited
states that result from the one electron π f π* excitation of a
π4π*0 linear molecule.34,35 A qualitative state diagram is given
There are several significant spectroscopic consequences of
2-
the bridging Cn ligands:
(1) The [5d(Au)] f [6p(Au), π*(phosphine)] transitions,
which are well-resolved for complexes 1 and 2, are strongly
red-shifted from those of the bis(phosphine) derivative 3.
According to Mason and co-workers, the 239 and 251 nm band
(29) Because the reduction of this compound is irreversible, the reduction
potential (E1/2) is estimated from the cathodic peak potential by E1/2 ) Epc
+ 0.04 V. Note that the peak-to-peak separation of the Cp2Fe+/0 couple is
80 mV under this condition.
(30) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten,
D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815-
4824.
(31) Complex 2 does not show any reductive couples even at potential
of -2 V vs SSCE. It shows an irreversible oxidative wave at 1.12 V vs
SSCE.
(32) Neithamer, D. R.; LaPointe, R. E.; Wheeler, R. A.; Richeson, D.
S.; Van Duyne, G. D.; Wolczanski, P. T. J. Am. Chem. Soc. 1989, 111,
9056-9072.
(33) Caulton, K. G.; Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.;
Lobkovsky, E. B.; Xue, Z. Organometallics 1992, 11, 321-326.
(34) Buenker, R. J.; Peyerimhoff, S. D. Chem. ReV. 1974, 74, 127-188.
(35) (a) Ross, I. G. Trans. Faraday Soc. 1952, 48, 973-991. (b) Kammer,
W. E. Chem. Phys. Lett. 1970, 6, 529-532. (c) Ditchfield, R.; Bene, J. D.;
Pople, J. A. J. Am. Chem. Soc. 1972, 94, 4806-4811.

Luminescent µ-Ethynyl Gold(I) Complexes
J. Am. Chem. Soc., Vol. 123, No. 21, 2001 4991
displays broad emission in room-temperature solid-state and 77
K alcoholic glasses (Figures 3 and 4, respectively). We
tentatively conjecture that these emissions involve exciplex
formation. One possible mechanism which is consistent with
these observations is the formation of strong hydrogen bonds
between the C2 unit in the excited state and the acidic H-atoms
of solvent molecules. As discussed earlier, chloroform molecules
have been located around the C2 moiety in the crystal lattice of
1‚4CHCl3, and the formation of strong hydrogen bonds is likely
to be a thermally activated process, since it intrinsically involves
movement of CHCl3 molecules within the crystal lattice. For
the acetylenic 3(ππ*) excited state of complex 2, both the solid-
state and solution emission spectra display a vibronic V(CtC)
spacing. We suspect that conjugation across the C42- chain may
result in extremely low frequencies of other vibronic features
other than V(CtC) that might not be resolved.
Figure 9. Qualitative π f π* state diagram for linear C2H2, neglecting
the very high-lying Σu state.
1
+
in Figure 9. The only spin- and electronic dipole-allowed
transition from the ground state occurs to the 1Σu+ state, which
lies at approximately double the energy of the cluster of excited
states that are shown. A very high-level theoretical calculation
has been reported on the lowest few relaxed excited states
resulting from the closely spaced manifold of vertical excited
states shown in Figure 9; the lowest of these was calculated at
3.49 eV (355 nm) with 3Σu+ parentage, while the lowest relaxed
We have demonstrated that the strategy of employing two
2-
Au(PCy3)+ termini for the Cn bridge has indeed afforded a
long-lived and emissive acetylenic 3(ππ*) excited state in
solution for complex 2. This excited state exhibits the lifetime,
emission quantum yield, and excited-state reduction potential
which are comparable to the prolific 3[dσ*pσ] state of
[Pt2(P2O5H2)4].4-37 Indeed, photoinduced electron-transfer reac-
tion involving the Au-C4-Au chain has been demonstrated.
3
state of ∆u parentage appears at 4.18 eV (297 nm).36
The good energetic match with the low-energy transitions of
3
1 strongly suggests that they are (π f π*) transitions. Hence
the ∼2000 cm-1 progression frequency in the emission spectra
of 1 (Figure 3) and 2 (Figure 6) is assigned to the V(CtC)
stretch. The ∼1700 cm-1 progression frequency in the excitation
spectra of 1 (Figure 5) is assigned to the V(CtC) stretch in the
3(π f π*) excited state. There is a large change in vibrational
frequency from the ground to the excited state, as is expected
because of the large Franck-Condon factor (implying large
distortion) indicated by the emission spectra. To the best of our
knowledge, the direct experimental observation of transitions
of this type for acetylenes is unprecedented in the literature,
but Au(I) spin-orbit coupling effects are expected to strongly
enhance their intensity even if there is no distinct π-bonding
3
2-
We envisage that the (ππ*) excited state of the Cn chains,
which is “switched on” through coordination to the “heavy
proton equivalent” Au(PCy3)+, will be readily manipulated to
yield desirable photophysical and photochemical properties, and
this will provide the impetus for future study.
Acknowledgment. We acknowledge financial support from
the Research Grants Council of the Hong Kong SAR, China
[HKU 7298/99P], The University of Hong Kong, and the Hong
Kong University Foundation.
2-
between Au(I) and the C2 unit. Importantly, the very simple
vibronic structure and the sharp V(CtC) progressions can be
rationalized by this ligand-localized assignment. The red-shift
for the analogous bands in 2 is anticipated for the conjugated
C42- bridging even though there is little experimental evidence
for the corresponding hydrocarbon.9
Another point worth addressing is that according to calcula-
tions,35b,36 the low-lying triplet states of C2H2 are strongly bent,
both cis- and trans-isomers are predicted to be minima. While
there is no evidence for vibronic involvement of any mode other
than V(CtC) in the 77 K solid emission of 1, this complex
Supporting Information Available: Details of physical
measurements, instrumentation; tables listing details of crystal
data and structure refinement, atomic coordinates, calculated
coordinates, isotropic and anisotropic displacement parameters,
and bond length and angles for 1‚4CHCl3 and 2‚2CH2Cl2; UV-
vis absorption spectrum of 3; a plot of 1/τ vs [2] (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA001706W
(36) Wetmore, R. W.; Schaefer, H. F., III. J. Chem. Phys. 1978, 69,
1648-1654.
(37) Roundhill, D. M.; Gray, H. B.; Che, C. M. Acc. Chem. Res. 1989,
22, 55-61.