Luminescent Gold(I) and Silver(I) Complexes of 2-(Diphenylphosphino)-1-methylimidazole (dpim): Characterization of a Three-Coordinate Au(I)-Ag(I) Dimer with a Short Metal-Metal Separation

Article abstract of DOI:10.1021/ic030200m

The Au(I) and Ag(I) closed-shell metal dimers of 2-(diphenylphosphino)-1-methylimidazole, dpim, were investigated. dpim formed the discreet binuclear species [Ag₂(dpim)₂(CH ₃CN)₂]²⁺ (1) when reacted w

Full text of DOI:10.1021/ic030200m

Inorg. Chem. 2003, 42, 8430−8438
Luminescent Gold(I) and Silver(I) Complexes of
2-(Diphenylphosphino)-1-methylimidazole (dpim): Characterization of a
Three-Coordinate Au(I)−Ag(I) Dimer with a Short Metal−Metal Separation
Vincent J. Catalano* and Stephen J. Horner
Department of Chemistry, UniVersity of NeVada at Reno, Reno, NeVada 89557
Received June 20, 2003
The Au(I) and Ag(I) closed-shell metal dimers of 2-(diphenylphosphino)-1-methylimidazole, dpim, were investigated.
dpim formed the discreet binuclear species [Ag2(dpim)2(CH CN)2]2+ (1) when reacted with appropriate Ag(I) salts.
3
Likewise, [Au2(dpim)2]2+ (3) and [AuAg(dpim)3]2+ (4) were produced via reactions with (tht)AuCl, tht is
tetrahydrothiophene, and Ag(I). Compound 3 exhibits an intense blue luminescence (λmax ) 483 nm) in the solid
state. However, upon initial formation of 3, a small impurity of Cl- was present giving rise to an orange emission
(λmax ) 548 nm). Attempts to form [Au2(dpim)2]Cl2 yielded only (dpim)AuCl (2), which is not visibly emissive. The
rare three-coordinate heterobimetallic complex [AuAg(dpim)3]2+ (4) exhibits intense luminescence in the solid-state
resembling that of 3. The crystal structures of 1−4 were determined, revealing strong intramolecular aurophilic and
argentophilic interactions in the dimeric compounds. Compound 1 has an Ag(I)−Ag(I) separation of 2.9932(9) Å,
while compound 3 has a Au(I)−Au(I) separation of 2.8174(10) Å. Compound 4 represents the first example of a
three-coordinate Au(I)−Ag(I) dimer and has a metal−metal separation of 2.8635(15) Å. The linear Au(I) monomer,
2, has no intermolecular Au(I)−Au(I) interactions, with the closest separation greater than 6.8 Å.
Introduction
mixed-donor ligands have been employed as bridging ligands
to hold two Au(I) ions in close proximity.
It is well-known that coordination compounds of gold(I)
often aggregate with Au(I)-Au(I) separations shorter than
the sum of their van der Waals radii.1-4 These interactions
have been termed aurophilic.5 The closed-shell (d10-d10)
interactions of monovalent gold have been widely docu-
mented both experimentally and theoretically for a variety
of mono- and multinuclear Au(I) compounds.6,7 While
intermolecular aurophilic interactions commonly form poly-
meric systems,1,6 the formation of discreet Au-Au dimers
is often utilized to facilitate our understanding of the forces
involved in aurophilic bonding.7,8 Because gold(I) has a
known affinity for phosphorus, and to a lesser extent
nitrogen,4-8 numerous diphosphine and phosphine-imine
Bis(diphenylphosphino)methane (dppm) and bis(dicyclo-
hexylphosphino)methane (dcpm) are representative of the
diphosphine ligands, while diphenylphosphinopyridine (dppy),
dimethylphosphinopyridine (dmpp), and 2-(diphenylphos-
phino)-1-benzylimidazole (BzImPh2P) are representative of
the mixed-donor species commonly used to prepare Au(I)
dimers.9-12 Even though the dppm and dcpm ligands are very
flexible and capable of separations of 2.4-3.6 Å,13 the
[(dppm)2Au2]2+ and [(dcpm)2Au2]2+ complexes contain Au-
(8) (a) Crespo, O.; Ferna´ndez, E. J.; Jones, P. G.; Laguna, A.; Lo´pez-de-
Luzuriaga, J. M.; Mend´ıa, A.; Monge, M.; Olmos, E. J. Chem. Soc.,
Chem. Commun. 1998, 2233-2234. (b) Wang, S.; Garzo´n, G., King,
C.; Wang, J.-C.; Fackler, J. P., Jr. Inorg. Chem. 1989, 28, 4623-
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Chem. Soc. 1988, 110, 3308-3310.
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Chem., Int. Ed. 1999, 38, 2783-2785. (b) Leung, K. H.; Phillips, D.
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2002, 41, 2054-2059. (c) Leung, K. H.; Phillips, D. L.; Tse, M.-C.;
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4803.
* To whom correspondence should be addressed. E-mail: vjc@unr.edu.
(1) Schmidbaur, H. Nature 2001, 413, 31-33.
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2001, 34, 14-19.
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8430 Inorganic Chemistry, Vol. 42, No. 25, 2003
10.1021/ic030200m CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/29/2003

Luminescent Gold(I) and SilWer(I) Complexes
Au separations of 2.8-3.1 Å.9 The more rigid dmpp ligand
constrains the metal-metal separation, and [dmpp2Au2]-
(BF4)2 has a Au-Au separation of only 2.776 Å.14 More
recently, silver(I) complexes of similar ligands have been
shown to exhibit argentophilic bonding analogous to the Au-
(I) systems.15 Pyykko¨ and Laguna16 recently reported theo-
retical studies suggesting that systems containing heterome-
tallic Au-Ag interactions will have even shorter metal-
metal separations than their homometallic analogues. The
occurrence of Au(I)-Ag(I) dinuclear complexes is limited,
and they are of current interest.15
One important property of the Au(I)-Au(I) complexes is
that they are often intensely luminescent,2-4,17 making them
attractive with respect to possible application toward lumi-
nescent display devices18 and luminescent sensors.19 The
emission maxima of the dinuclear complexes is red-shifted
compared to their mononuclear analogues.11 The origin of
the emission from Au(I) dimers was previously thought to
be associated with the metal-localized 5dσ* f 6pσ transi-
tion.9 However, in a recent series of papers, Che and co-
workers argue that the visible emission is actually associated
with an exciplex formation where Au-L, Au-solvent, and
Au-counterion interactions play a significant role.9 Che
demonstrated that the emission of [(dcpm)2Au2]2+ depends
on the coordinating ability of the counterion, with the
emission maxima of 368 nm for the ClO4- salt and 530 nm
for the I- complex in the solid state.9a
Previously we have utilized larger mixed-donor phosphine
ligands to form metallocryptands20 that include three-
coordinate Au(I) species, which are also known to be
luminescent.21 The gold-metallocryptands are capable of
encapsulating other closed-shell metal ions to induce Au-M
interactions, where M ) Tl(I) or Hg(0), within the cages.20
These mixed-metal systems are highly luminescent20 and
provide a method by which compounds may be altered to
produce a wide range of emissive complexes. In an effort to
create more simple systems containing Au-M interactions,
the use of other mixed-donor ligands capable of bridging
must be considered. We report here a study including a
mononuclear Au(I) and the dinuclear Au(I)-Au(I), Ag(I)-
Ag(I), and Au(I)-Ag(I) complexes of 2-(diphenylphos-
phino)-1-methylimidazole (dpim).
Results
dpim was synthesized according to a literature procedure.22
Complexes 1-4 were synthesized in acetonitrile solution at
room temperature. All of the complexes reported here are
air-stable and with the exception of 3 soluble in common
organic solvents including acetonitrile, dichloromethane, and
acetone.
According to Scheme 1, the dimeric silver(I) complex
[Ag2(dpim)2(CH3CN)2](ClO4)2, 1, was prepared by reacting
1 equiv of Ag(ClO4)‚H2O with 1 equiv of dpim. Removal
of solvent and trituration of the residue with diethyl ether
afforded the colorless, microcrystalline solid. The monomeric
gold(I) complex (dpim)AuCl, 2, was formed by reacting 1
equiv of (tht)AuCl, where tht ) tetrahydrothiophene, with
1 equiv of dpim. The solvent volume was reduced over mild
heat, and the complex crystallized upon cooling to room
temperature. The dimeric gold(I) complexes [Au2(dpim)2]-
(ClO4)2‚2CH3CN, 3(ClO4), and [Au2(dpim)2](BF4)2‚2CH3CN,
3(BF4), were prepared by reacting 1 equiv of (tht)AuCl with
1 equiv of Ag(ClO4)‚H2O or AgBF4 respectively. After
removal of the AgCl precipitate, the complexes were
crystallized from the reaction mixtures via slow diffusion
of diethyl ether at -5 °C. In a preparation similar to 3, the
heterobimetallic complex [AuAg(dpim)3](ClO4)2, 4, was
formed by reacting 1 equiv of (tht)AuCl with 2 equiv of
Ag(ClO4)‚H2O and 3 equiv of dpim. After removal of AgCl
precipitate, the filtrate was evaporated to dryness. The residue
was triturated with diethyl ether to give 4 as a colorless,
microcrystalline solid.
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Zuccaccia, C. Inorg. Chim. Acta 2001, 323, 45-54. (c) Tejel, C.; Bravi,
R.; Ciriano, M. A.; Oro, L. A.; Bordonaba, M.; Graiff, C.; Tiripicchio,
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Galassi, R.; Pietroni, B. R.; Rafaiani, G. J. Organomet. Chem. 1996,
519, 161-167.
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39, 5299-5305. (c) Balch, A. L.; Catalano, V. J. Inorg. Chem. 1992,
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Chim. Acta 2002, 334, 376-384.
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Pyykko¨, P.; Runeberg, N. Eur. J. Inorg. Chem. 2002, 750-753.
(17) Forward, J. M.; Fackler, J. P., Jr.; Assefa, Z. In Optoelectronic
Properties of Inorganic Compounds; Roundhill, D. M., Fackler, J. P.,
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Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329-1330.
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39, 121-127. (b) Catalano, V. J.; Bennett, B. L.; Noll, B. C. J. Chem.
Soc., Chem. Commun. 2000, 1413-1414. (c) Catalano, V. J.; Bennett,
B. L.; Yson, R.; Noll, B. C. J. Am. Chem. Soc. 2000, 121, 10056-
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M. A.; Noll, B. C. J. Chem. Soc., Chem. Commun. 2001, 581-582.
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Chem. Soc. 1995, 117, 9103-9104.
The most striking feature of 3 is its solid-state lumines-
cence. As seen in Figure 1, crystalline complexes of 3 exhibit
two distinctly different emission spectra with different
excitation modes. Upon initial formation of 3, the crystals
exhibit a visibly orange luminescence (λmax ) 548 nm, λex
) 336 nm) when excited with a hand-held UV lamp.
However, upon crushing, heating, or recrystallization the
complex luminesces blue (λmax ) 483 nm, λex ) 368 nm).
(22) Jalil, M. A.; Yamada, T.; Fujinami, S.; Honjo, T.; Nishikawa, H.
Polyhedron 2001, 20, 627-633.
Inorganic Chemistry, Vol. 42, No. 25, 2003 8431

Catalano and Horner
Scheme 1
appears to have no effect upon the emission. The metrical
parameters from the crystal structures of 3(ClO4) or 3(BF4),
regardless of emission color, are identical (vide infra). This,
coupled with the ability to reproduce the formation of both
the “orange” and “blue” under various reaction conditions,
suggests that the origin of the contrasting emissions may be
an impurity from the initial synthesis of 3 (vide infra) and
is likely related to incomplete halide abstraction by Ag+ ion.
Unlike 3, the emission spectra of crystals of 1, 2, and 4
are much less complicated. The emission spectrum of 1
consists of a broad, featureless band at 442 nm (λex 355 nm)
that tails out to 600 nm. Complexes 2 and 4 also exhibit
broad, featureless emission bands that tail out beyond 600
nm. However, the emission maxima are at 493 nm (λex
)
335 nm) and 490 nm (λex ) 360 nm) respectively, and the
excitation spectra are not identical. The emission and
excitation spectra of crystalline 4 are presented in Figure 2.
Visibly, 1 and 2 luminesce blue-white, while 4 is yellow,
when illuminated with a hand-held lamp (λex ) 366 nm).
Figure 1. Normalized solid-state excitation (left) and emission (right)
spectra of 3-orange (solid) and 3-blue (dashed), at room temperature.
The emission intensity of the “blue” crystals is much higher
than that of the “orange” crystals, while the luminescence
from “blue” powder is significantly more intense to the eye
than the “blue” crystals. Upon crushing or heating, the intense
blue emission swamps the weaker orange signal making it
appear (incorrectly) that this mechanical action has converted
the orange form into the blue form. Once “orange” 3 is
purified by recrystallization to form the “blue” form, the
“orange” form becomes inaccessible. Thus far, the “orange”
form can only be reproduced by repeating the initial
synthesis. The formation of the two species with vastly
different colored emissions is independent of solvents of
crystallization, as both forms have been prepared from
acetonitrile, propionitrile, benzonitrile, and even 1,2-dichoro-
ethane. Changing from perchlorate to tetrafluoroborate anions
In solution the emission maxima are significantly blue-
shifted. Complexes 1 and 4 display a single, featureless
emission band at room temperature at 370 nm (λex ) 294
nm) and 354 nm (λex ) 290 nm), respectively. The emission
spectra of 2 and 3 consist of broad emission bands in the
UV region that extend beyond 550 nm. Complex 2 emits at
376 nm with a secondary band at 443 nm (λex ) 294 nm).
Complexes 3(ClO4) and 3(BF4) emit at 360 nm with a
secondary band at 431 nm (λex ) 290 nm). These emission
spectra are similar to that of free dpim ligand under similar
conditions. An acetonitrile solution of dpim emits at 360 and
493 nm (λex ) 296 nm); however, the lower energy band
dominates the spectrum. A concentration dependence study
of the emission for 3 in acetonitrile shows that as the
8432 Inorganic Chemistry, Vol. 42, No. 25, 2003

Luminescent Gold(I) and SilWer(I) Complexes
Figure 2. Solid-state excitation (left) and emission (right) spectra of
[AuAg(dpim)3](ClO4)2, 4.
concentration increases (from 10-6 to 10-4 M incrementally)
the intensity of the high energy emission band at 360 nm is
decreased. At the same time a low-energy (431 nm) band
appears and increases in intensity while red-shifting to 493
nm. There is also a dramatic decrease in the overall intensity
of emission as the concentration is increased suggesting an
intermolecular process at higher concentrations.
Figure 3. Thermal ellipsoid plot (40%) of the cationic portion of 1. Only
the crystallographically unique portion and Au(1A) are numbered. Hydrogen
atoms are removed for clarity.
The electronic absorption spectra of 1-4 in acetonitrile
solution are nearly identical to that of the free dpim. dpim
exhibits strong π-π* bands at 233 and 252 nm. Complexes
1-4 exhibit intense absorptions in the ranges of 230-232
nm and 248-252 nm.
The 31P{1H} NMR spectroscopy is consistent with the
formulations presented in Scheme 1. The spectra of 2-4 are
comprised of sharp single resonances at 12.1, 18.9, and 17.5
ppm, respectively, with no evidence of 107Ag or 109Ag
coupling in 4. All attempts to determine the extent of Cl-
ion contamination in the initial preparation of 3 by NMR
spectroscopy were thwarted by low solubility in acetonitrile
and the complete lack of solubility in chloroform. This
resulted in spectra with no definition from the baseline of
any resonance except that associated with 3. The 31P{1H}
NMR spectrum of 1 consists of a broad multiplet at -4.9
ppm, arising from the complicated coupling to 107Ag (I )
1/2, 51.8% abundance) and 109Ag (I ) 1/2, 48.2% abun-
dance).
Crystals of 1 suitable for X-ray diffraction analysis were
grown by slow diffusion of diethyl ether into a saturated
acetonitrile solution of the complex at room temperature. A
thermal ellipsoid plot of 1 is presented in Figure 3 with
selected bond distances and angles presented in Table 1. The
colorless complex crystallizes in the monoclinic space group
P21/c with an inversion center midway between the silver
atoms, making only half of the complex crystallographically
unique. The asymmetric unit also contain one ClO4- anion,
one acetonitrile molecule bound the silver, and one and one-
half acetonitrile solvates. The two silver atoms are bridged
by two dpim ligands in a head to tail fashion. Each silver is
three coordinate, bound to a phosphorus atom of one ligand,
an imine nitrogen of the second ligand, and to an acetonitrile
molecule. The N(2a)-Ag(1)-P(1), N(2a)-Ag(1)-N(3), and
Figure 4. X-ray structural drawing of complex 2. Thermal ellipsoids are
drawn at 40%.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1
Ag(1)‚‚‚Ag(1A)
Ag(1)-P(1)
Ag(1)-N(2A)
Ag(1)-N(3)
2.9932(9)
2.3807(14)
2.198(4)
P(1)-C(1)
P(1)-C(5)
P(1)-C(11)
1.809(6)
1.831(5)
1.814(6)
2.506(6)
P(1)-Ag(1)-N(2A)
P(1)-Ag(1)-N(3)
N(3)-Ag(1)-N(2A)
157.54(12) P(1)-Ag(1)-Ag(1A) 83.94(4)
115.38(16) N(3)-Ag(1)-Ag(1A) 104.53(17)
86.75(19) C(1)-P(1)-C(11)
406.4(2)
N(2A)-Ag(1)-Ag(1A) 86.62(12)
P(1)-Ag(1)-N(3) angles are 157.54(12), 86.75(19), and
115.38(16)° respectively. The Ag-Ag separation is 2.9932-
(9) Å.
X-ray-quality crystals of 2 form upon cooling a warm,
saturated acetonitrile solution of 2 to 0 °C. Figure 4 shows
a thermal ellipsoid drawing of 2 with selected bond distances
and angles presented in Table 2. The colorless complex
crystallizes in the orthorhombic space group P212121. The
two-coordinate Au(I) atoms are nearly linear with the angle
P(1)-Au(1)-Cl(1) of 178.8(2)° and Au(1)-P(1) and Au-
Inorganic Chemistry, Vol. 42, No. 25, 2003 8433

Catalano and Horner
Figure 5. Thermal ellipsoid plot (40%) of the cationic portion of 3(ClO4)2.
Only the crystallographically unique portion and Au(1A) are labeled.
Hydrogen atoms removed for clarity.
Figure 6. Packing diagram of 3(ClO4)2 arbitrarily oriented to emphasize
the perchlorate-Au(I) contacts within the lattice.
Table 2. Bond Lengths (Å) and Angles (deg) for 2
Au(1)-P(1)
Au(1)-Cl(1)
P(1)-C(11)
2.227(6)
2.293(7)
1.75(2)
P(1)-C(1)
P(1)-C(5)
Au‚‚‚Au
1.80(2)
1.803(19)
6.83
P(1)-Au(1)-Cl(1)
C(11)-P(1)-C(1)
C(11)-P(1)-C(5)
C(1)-P(1)-C(5)
178.8(2)
C(11)-P(1)-Au(1)
C(1)-P(1)-Au(1)
C(5)-P(1)-Au(1)
114.0(9)
115.5(7)
112.6(8)
105.3(11)
105.1(10)
103.2(10)
Table 3. Bond Lengths (Å) and Angles (deg) for 3(ClO4)‚2CH3CN
Au(1)‚‚‚Au(1A)
Au(1)-P(1)
Au(1)-N(1)
2.8261(11)
2.238(3)
2.090(11)
P(1)-C(5)
P(1)-C(11)
P(1)-C(1)
1.802(14)
1.812(15)
1.835(14)
P(1)-Au(1)-N(1)
N(1)-Au(1)-Au(1A)
P(1)-Au(1)-Au(1A)
178.0(4)
88.1(4)
90.89(11)
C(5)-P(1)-C(11)
C(5)-P(1)-C(1)
C(11)-P(1)-C(1)
109.7(7)
105.7(6)
107.8(7)
(1)-Cl(1) separations of 2.227(6) and 2.293(7) Å, respec-
tively. The Au(1)-P(1)-C(1), Au(1)-P(1)-C(5), and Au-
(1)-P(1)-C(11) angles are expanded from the ideal tetrahedral
angles (109.5°) to 115.5(7), 112.6(8), and 114.0(9)°, respec-
tively.
Figure 7. X-ray structural drawing of the cationic portion of 4 emphasizing
the 3-fold symmetry. Hydrogen atoms have been removed and carbon atoms
drawn as open circles for clarity. Only the crystallographically unique portion
is labeled.
crystallize in the monoclinic P21/n space group with the Au-
Au vector residing on a crystallographic inversion center,
making only half of the complex crystallographically unique.
The two Au atoms are bridged by two dpim ligands in a
head to tail fashion, with each Au coordinated to the
phosphorus atom of one ligand and the imidazole nitrogen
of the second ligand. The N-Au-P angles are nearly linear
and range from 177.9(3) to 178.0(4)°. The Au-N and Au-P
separations range from 2.083(9) to 2.090(11) Å and 2.235-
(3) to 2.238(3) Å, respectively, while the aurophilic Au-
Au separations are short and range from 2.8174(10) to
2.8261(11) Å. The anions and solvent molecules show a
slight attraction toward the Au atoms with one solvent and
two anion contacts per Au dimer. As shown in Figure 6, the
perchlorate anion weakly interacts with the Au center with
the shortest Au-O separation being 3.22(7) Å.
Colorless crystals of both 3(ClO4) and 3(BF4) are obtained
in two ways. The first method is capable of producing either
the “orange” or “blue” form while the second method yields
exclusively the “blue” form. X-ray-quality crystals can be
grown by slow diffusion of diethyl ether into the acetonitrile
filtrate of the initial reaction mixture at -5 °C. The thermal
ellipsoid plot of the cationic portion of the identical structures
of 3(ClO4) and 3(BF4) is presented in Figure 5 with selected
bond distances and angles in Tables 3and 4. The crystals
obtained visibly luminesce orange, and only a few are
suitable for X-ray analysis. These crystals can be recrystal-
lized from acetonitrile/diethyl ether at either -5 °C or room
temperature. This procedure nearly quantitatively yields
crystals that visibly luminesce blue, and the bulk of the
crystals are X-ray quality. The four species, 3(ClO4) (“or-
ange” and “blue”) and 3(BF4) (“orange” and “blue”), all
8434 Inorganic Chemistry, Vol. 42, No. 25, 2003

Luminescent Gold(I) and SilWer(I) Complexes
For example, the phosphine-bridged [Ag3(dppn)3]3+, where
dppn is 2,9-bis(diphenylphosphino)-1,8-naphthyridine, that
contains an Ag-Ag separation of 3.145(2) Å20a is intensely
luminescent with a broad emission band at 550 nm whereas
1 emits at 442 nm (λexi ) 335 nm).26 Additionally, this
species also contains a solvent molecule (DMSO) coordinated
to the Ag similar to the acetonitrile bound to Ag in 1.20a
The monomeric gold(I) complex (2) can be considered
analogous to the ((BzIm)Ph2P)AuCl complex reported by
Burini and co-workers.12 The Au-P and Au-Cl bond lengths
of 2.227(6) and 2.293(7) Å respectively, are not significantly
different from the (BzIm)Ph2P derivative (2.303(4) and
2.239(3) Å).12 However, there are two significant differences
that may be considered the results of crystal packing forces.
The P-Au-Cl bond angle of 1 (178.8(2) °) is much closer
to the value reported for Ph3PAuCl (179.68°)27 than to that
of (BzIm)Ph2PAuCl (175.1(3)°).12 The aurophilic Au-Au
intermolecular separations in (BzIm)Ph2PAuCl are 3.03(2)
Å,12 whereas compound 2 exhibits no intermolecular Au-
Au interactions, with the closest separation being 6.83 Å.
Complex 2 exhibits an intense solid-state emission at 493
nm (λex ) 335 nm) at room temperature. This closely relates
to the solid-state emission of Ph3PAuCl (λmax ) 491 nm, λex
) 330 nm),11 also at room temperature, suggesting that the
emission is likely to be intraligand (π-π*) in nature.
The dimeric gold(I) complex (3) has an Au-Au separation
of ∼2.82 Å for both the perchlorate and tetrafluoroborate
salts. This separation is consistent with other gold(I) dimers,
including [dmpp2Au2]2+ (2.776 Å),14 [(dppm)2Au2]2+ (2.982
Å),11 and various [(dcpm)2Au2]2+ dimers (2.9063(9)-3.0132-
(6) Å).9 The closest anion-Au contacts in 3(ClO4) are ∼3.22
Å, which agree with those seen in [Au2(dcpm)2](ClO4)2
(3.36(2) Å).9a
The solution-state absorption and emission properties of
complex 3 are independent of anion. The electronic absorp-
tion spectrum of 3 in acetonitrile (6 × 10-6 M) shows only
absorptions similar to that of the free dpim ligand. This is
in sharp contrast to both the [Au2(dppm)2](BF4)211 and [Au2-
(dcpm)2](ClO4)29a dimers, which display intense absorptions
at 293 and 277 nm, respectively. These absorptions have been
assigned as dσ* f pσ transitions.9,11 For [Au2(dppm)2](BF4)2,
this absorption closely resembles the excitation spectrum in
solution, giving rise to an emission at 593 nm (λex ) 293
nm).11 Complex 3 exhibits a much higher energy emission
at 360 nm (λex ) 290 nm) in acetonitrile at room temperature.
The differences in solution electronic absorption and emis-
sion can be explained by considering the different donor
abilities of dppm versus dpim. Assuming dpim to be a weaker
donor to gold(I) than dppm, the Au-Au interactions would
be weakened such that the dσ* f pσ transition is much
higher in energy for 3 versus [Au2(dppm)2](BF4)2. The
absorption band associated with this transition will then
overlap with the π-π* transitions of the ligand. The solution
state emission of 3 shows two bands, similar to the ligand
Table 4. Bond Lengths (Å) and Angles (deg) for 3(BF4)‚2CH3CN
Au(1)‚‚‚Au(1A)
Au(1)-P(1)
Au(1)-N(1)
2.8174(10)
2.235(3)
2.083(9)
P(1)-C(5)
P(1)-C(11)
P(1)-C(1)
1.814(12)
1.808(12)
1.812(11)
P(1)-Au(1)-N(1)
N(1)-Au(1)-Au(1A)
P(1)-Au(1)-Au(1A)
177.9(3)
88.3(3)
90.91(9)
C(5)-P(1)-C(11)
C(5)-P(1)-C(1)
C(11)-P(1)-C(1)
108.8(5)
106.0(6)
107.2(6)
Table 5. Bond Lengths (Å) and Angles (deg) for 4
Au(1)‚‚‚Ag(1)
Au(1)-P(1)
Ag(1)-N(1)
2.8635(15)
2.374(2)
2.236(8)
P(1)-C(5)
P(1)-C(1)
P(1)-C(11)
1.809(10)
1.821(10)
1.812(7)
P(1)-Au(1)-P(1A)
N(1)-Ag(1)-Au(1)
P(1)-Au(1)-Ag(1)
119.979(3)
86.3(2)
90.84(7)
C(5)-P(1)-C(11)
C(5)-P(1)-C(1)
C(11)-P(1)-C(1)
105.8(4)
103.7(5)
108.6(4)
Colorless, X-ray-quality crystals of 4 were obtained by
slow diffusion of diethyl ether into a saturated acetonitrile
solution at room temperature. The structural drawing of 4 is
represented in Figure 7, with selected bond distances and
angles in Table 5. The colorless complex crystallizes in the
cubic space group Pa-3, with the Au-Ag bond lying on a
3-fold rotational axis, making only 1/3 of the molecule
unique. The Au-Ag separation is 2.8635(15) Å. Due to the
high crystallographic symmetry of the molecule, the three
Au-P distances are equivalent (2.374(2) Å) as are the three
Ag-N bond distances (2.236(8) Å). The P-Au-P angles
are nearly ideally trigonal planar at 119.979(3)°, as are the
N-Ag-N angles of 119.59(5)°. The P-Au-Ag and N-Ag-
Au bond angles are 90.84(7) and 86.3(2)°, respectively.
Discussion
The dpim ligand allows the facile formation of gold(I) and
silver(I) homo- and heterobimetallic complexes possessing
interesting structural and photophysical properties. Complex
1 is a discreet dimer with an Ag-Ag separation of 2.9932-
(9) Å. This separation is shorter than that of [Ag2(dppy)2]-
(NO3)2 (3.146 Å),25 where dppy ) bis(diphenylphosphino)-
pyridine. The two complexes have very similar metal to
ligand (Ag-P and Ag-N) bond distances of 2.3807(14) and
2.198(4) Å in 1 and 2.378(2) and 2.234(6) Å in the dppy
complex, respectively.22,25 While 1 and the dppy complex
are structurally similar, they are strikingly different than the
[Ag2(dpim)2](NO3)2 complex recently reported by Nishikawa
and co-workers.22 [Ag2(dpim)2](NO3)2 consists of Ag-Ag
dimers connected by Ag-ONO2 interactions to form a
polymeric chain. Although the Ag-P and Ag-N separations
(2.391(3) and 2.214(9) Å)22 are similar to 1, the Ag-Ag
separation (2.918(3) Å)22 is slightly shorter than that found
in 1. The 31P{1H} NMR spectrum of 1 shows a broad
multiplet at -4.9 ppm, consistent with the dppy and nitrate
polymer complexes.22,25 There were no luminescent proper-
ties reported for the dppy or nitrate polymer complex;
therefore, no comparisons can be made. However, complexes
containing Ag-Ag interactions are known to be luminescent.
(23) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmer, F. J. Organometallics 1996, 15, 1518-1520.
(24) Uson, R.; Laguna, A.; Laguna, M. Inorg. Synth. 1989, 26, 85-91.
(25) Liu, H. F.; Liu, W.; Zhang, P.; Huang, M. S.; Zhen, L. X. Xiamen
Daxue Xuebao Ziran Kexueban 1992, 31, 57-60.
(26) Uang, R.-H.; Chan, C.-K.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Chem. Commun. 1994, 2561-2562.
(27) Baenziger, N. C.; Bennett, W. E.; Soboroff, D. M. Acta Crystallogr.
B 1976, 32, 962-963.
Inorganic Chemistry, Vol. 42, No. 25, 2003 8435

Catalano and Horner
in solution. This suggests that in the absence of the solid-
state anion-cation exciplex interactions the intraligand bands
predominate the solution spectra.
into the stable and highly crystalline Au(dpim)Cl (2). Further,
the formation of 2 can be followed by 31P NMR spectroscopy
where successive addition of tetrabutylammonium chloride
shifts the resonance of 3 toward that of 2. Attempts to directly
synthesize [Au2(dpim)2]Cl2 independently have only yielded
2. The red-shifted luminescence of the possible [Au2(dpim)2]-
Cl2 versus 3 is consistent with red-shifted luminescence
displayed by [Au2(dcpm)2]I2 versus [Au2(dcpm)2](BF4)2
Perhaps the most striking feature of this work is the
unusual blue-orange emissions observed for 3, independent
-
-
of anion (BF4 /ClO4 ) or solvents of crystallization. The
“orange” and “blue” forms have identical crystal structures,
suggesting that the origin of the difference in luminescent
properties lies in a minor impurity not visible by X-ray
diffraction rather than a conformational change. An undetect-
able impurity within the crystal structure is possible if there
is a small amount of impurity that resides in the same position
as an anion or solvent molecule. This impurity could include
a minor amount of acid (from ligand purification), water
(from wet solvents), or Cl- (from incomplete halide abstrac-
tion), all of which were explored. Solutions of 3 (“blue”
form) in acetonitrile were intentionally doped with varying
concentrations of acid and water, and crystals were grown
from each solution. Each attempt produced only “blue”
crystals. Likewise, addition of small amounts of triethylamine
produced similar results.
-
reported by Che, as is the similar emission of either the BF4
-
or ClO4 salts.9a Additionally, the relative increase of the
low-energy emission band upon concentration of the aceto-
nitrile solution of 3 is consistent with the exciplex mechanism
proposed by Che and co-workers.9 The relative intensities
of these disparate emissions, “blue” powder > “blue” crystal
> “orange” crystal, are also in line with the emissive
properties of [Au2(dppm)2](BF4)2.11
Three-coordinate gold(I) complexes have been shown to
exhibit intense luminescent properties.21 However, to the best
of our knowledge, complex 4 is the first structurally
characterized three-coordinate Au-Ag dimer and one of only
a handful of Au-Ag heterobimetallic dimers to be structur-
ally characterized.15 Interestingly, attempts to synthesize
[AuAg(dpim)2]2+ resulted only in the formation of 4. The
hypothetical [AuAg(dpim)2]2+ was expected to be analogous
to AgAu(MTP)2. However, the reaction resembles that seen
by Burini, in the [Ag2((BzIm)Ph2P)3]2+ complex.12 The 31P-
{1H} NMR spectrum is insensitive to ligand:Ag:Au ratios
(2:2:1 or 3:2:1) with the product consistently exhibiting a
single sharp resonance at 17.5 ppm, suggesting that there is
no equilibrium present between 4 and [AuAg(dpim)2]2+.
The increase of emission intensity upon grinding is not
well-understood, but it does not appear to be related to a
phase change because powders formed by the evaporation
of solutions of 3 are equally emissive. Recently, Eisenberg
and Lee28 reported the reversible luminescence tribo-
chromism from a structurally related gold(I) thiouracilate
dimer whose unique emissive properties can be related to
two different structural motifs interconverted by protonation
of the uracilate ligand. Likewise, complex 3 also possess an
imine base within the imidazole framework; however, it
appears unresponsive to protonation, suggesting a different
mechanism is responsible for its emission. Fackler and co-
workers29 reported a similar change in emission intensity
upon grinding for their linear one-dimensional chain com-
pound [(TPA)2Au][Au(CN)2], where TPA is 1,3,5-triaza-7-
phosphaadamantine. Single crystals of this material are
nonemissive but become strongly photoluminescent after
grinding. This property was attributed to the generation of
lattice defects near the surface. A similar mechanism may
be present in this work. Heating samples of 3 to eject the
acetonitrile molecules of solvation could also disrupt the
lattice producing similar defects.
Conversion of the “orange” form to the “blue” form
undoubtedly involves a different mechanismsmost likely the
inclusion of an impurity. Treatment of an acetonitrile solution
of 3 with a methanolic slurry of potassium chloride followed
by evaporation yielded a solid residue that luminesces orange,
suggesting that Cl- is indeed a contaminant of 3 in the
“orange” crystals. However, upon dissolution in acetonitrile
and crystallization by addition of diethyl ether only the
monomeric complex 2 is obtained. This is in agreement with
the observation that during the recrystallization of the original
“orange” form, the purported [Au2(dpim)2]Cl2 dissociated
The Au-Ag separation of 2.8635(15) Å is between that
of AgAu(MTP)2 (2.9124(13) Å)15 and the [AuAg(dppy)2]2+
complex (2.820(1) Å).10 AgAu(MTP)2 was reported to be
luminescent in the solid state at 77 K (λem ) 420 nm, λex
)
335 nm).15 Compound 4 is also luminescent in the solid state
at room temperature with an emission at 490 nm (λex ) 360
nm). The emission is surprisingly similar to 3, as 4 appears
yellow to the naked eye while 3 appears blue. However, the
emission intensity of 3 is much higher than that of 4.
Conclusion
The dpim ligand allows the facile formation of gold(I) and
silver(I) homo- and heterobimetallic complexes. The com-
plexes exhibit luminescent properties similar to those reported
for other various d10-d10 binuclear complexes. dpim also
favors a three-coordinate Au(I)-Ag(I) heterobimetallic
complex that is unprecedented in the literature.
For the intensely luminescent Au(I)-Au(I) dimer, 3(ClO4)2,
the close proximity of the anion and the high probability of
emission dependence upon coordinating versus noncoordi-
nating anions closely resemble the [Au2(dcpm)2]2+ complexes
reported by Che and co-workers.9a These results appear to
support Che’s conclusion that the luminescent behavior of
Au(I)-Au(I) dimers arises from exciplex formation with
anions or solvent. The unusual orange/blue emission of 3 is
likely a result of a Cl- partially occupying the position where
traditionally noncoordinating anions typically reside.
(28) Lee, Y.-A.; Eisenberg, R. J. Am. Chem. Soc. 2003, 125, 7778-7779.
(29) Assefa, Z.; Omary, M. A.; McBurnett, B. G.; Mohamed, A. A.;
Patterson, H. H.; Staples, R. J.; Fackler, J. P., Jr. Inorg. Chem. 2002,
41, 6274-6280.
8436 Inorganic Chemistry, Vol. 42, No. 25, 2003

Luminescent Gold(I) and SilWer(I) Complexes
Table 6. Crystallographic Data for 1-4
1‚3CH3CN
2
3(ClO4)2‚2CH3CN
3(BF4)2‚2CH3CN
4
formula
fw
C40H44Ag2Cl2O8N9P2
559.66
C16H15AuClN2P
498.69
C36H36Au2Cl2O8N6P2
608.72
C36H36Au2B2F8N8P2
632.15
C48H45AgAuB2F8N6P3
1277.27
a, Å
b, Å
c, Å
â, deg
V, Å3
space group
Z
13.4304(10)
16.6652(18)
11.737(2)
109.693(10)
2473.4(6)
P21/c
9.8042(13)
12.3697(17)
13.267(2)
90
1608.9(4)
P212121
9.148(4)
9.071(3)
21.535(2)
21.535(2)
21.535(2)
90
9987.0(16)
Pa-3
8
18.191(3)
14.1585(19)
90.867(17)
2355.9(11)
P21/n
18.202(3)
14.122(4)
90.855(16)
2331.4(10)
P21/n
4
4
4
4
D
calc, g/cm3
1.503
2.059
1.716
1.801
1.699
cryst size, mm3
µ(Mo KR), mm-1
λ, Å
0.24 × 0.50 × 0.40
0.10 × 0.48 × 0.12
0.46 × 0.50 × 0.50
0.42 × 0.66 × 0.42
0.42 × 0.44 × 0.44
3.673
9.403
3.934
6.424
3.494
0.710 73
298(2)
0.710 73
298(2)
0.710 73
298(2)
0.710 73
298(2)
0.710 73
298(2)
temp, K
transm factors
R1, wR2 (I g 2σ(I))
0.48-0.65
0.0511, 0.1329
0.16-0.41
0.0575, 0.1353
0.39-0.46
0.0611, 0.1541
0.18-0.53
0.0555, 0.1149
0.28-0.42
0.0487, 0.1196
Experimental Section
was stirred for 5 min. The suspension was filtered through
a pad of Celite to remove the precipitate. The colorless filtrate
was collected and layered with diethyl ether. Orange
luminescing colorless crystals formed from the solution at
-5 °C. The crystals were collected via vacuum filtration,
and washed with diethyl ether. Yield: 0.6700 g (87%). Blue
luminescing crystals are prepared by dissolution of the
material in a minimum amount of acetonitrile and precipita-
All solvents were purified with a Grubbs apparatus.23
dpim22 and (tht)AuCl24 were prepared from literature pro-
1
cedures. H chemical shifts are reported relative to TMS,
and 31P{1H} chemical shifts are reported relative to 85% H3-
PO4. Combustion analysis was performed by Desert Ana-
lytics, Tucson, AZ. UV-vis spectra were obtained using a
Hewlett-Packard 8453 diode array spectrometer (1 cm path-
length cells). Emission data were recorded using a Spex
Fluoromax steady-state fluorometer.
1
tion with diethyl ether. H NMR (CD3CN, 25 °C): δ 7.60
ppm (m), δ 3.32 ppm (s). 31P{1H} NMR (CD3CN/CH3CN,
25 °C): δ 19.0 ppm (s). Anal. Calcd for C32H30Au2-
Cl2O8P2N4: C, 34.15; H, 2.69; N, 4.98. Found: C, 34.15;
H, 2.69; N, 4.96.
Preparation of [(dpim)2Ag2(CH3CN)2](ClO4)2, 1. A 50
mL Erlenmeyer flask was charged with 20 mL acetonitrile,
a stir bar, 191 mg (0.72 mmol) of 2-(diphenylphosphino)-
1-methylimidazole (dpim), and 149 mg (0.72 mmol) of silver
perchlorate monohydrate. The solution was stirred at room
temperature for 10 min. Solvent was removed via rotary
evaporation and the residue triturated with diethyl ether to
give colorless crystals. The crystals were collected via
vacuum filtration and washed with diethyl ether. Yield:
0.2555 g (75%) 1H NMR (CD3CN, 25 °C): δ 7.53 ppm (m),
δ 7.14 (s), δ 3.32 ppm (s). 31P{1H} NMR (CD3CN/CH3CN,
25 °C): δ -4.9 ppm (m).
Preparation of Au(dpim)Cl, 2. To a stirred solution of
65.4 mg (0.25 mmol) of dpim in acetonitrile (10 mL) was
added a 10 mL acetonitrile solution of 78.4 mg (0.25 mmol)
of (tht)AuCl. The resulting solution was stirred for 15 min.
The volume of solvent was reduced (∼10 mL) over mild
heat. Colorless crystals of 2 form upon cooling to 0 °C. The
crystals were then collected via vacuum filtration and washed
with diethyl ether. Yield: 82.8 mg (68%). 1H NMR (CDCl3,
25 °C): δ 7.60 ppm (m), δ 7.17 (s), δ 3.91 ppm (s). 31P-
{1H} NMR (CDCl3, 25 °C): δ 12.1 ppm (s). Anal. Calcd
for C16H15AuClPN2: C, 38.53; H, 3.03; N, 5.62. Found: C,
38.79; H, 3.08; N, 5.65.
Preparation of [(dpim)2Au2](BF4)2, 3(BF4)2. The prepa-
ration of 3(BF4)2 was similar to that of 3(ClO4)2 using 54.3
mg (0.20 mmol) of dpim, 65.4 mg (0.20 mmol) of (tht)-
AuCl, and 39.8 mg (0.20 mmol) of AgBF4. Yield: 104 mg
(81%).
Preparation of [(dpim)3AuAg](BF4)2, 4. A 50 mL
Erlenmeyer flask was charged with 20 mL of acetonitrile, a
stir bar, 139 mg (0.52 mmol) of dpim, and 55.9 mg (0.17
mmol) of (tht)AuCl. The solution was stirred at room
temperature for approximately 10 min. A 10 mL acetonitrile
solution of 72.1 mg (0.35 mmol) of silver tetrafluoroborate
was added dropwise. A white precipitate forms upon
addition, and the suspension was stirred for 5 min. The
suspension was filtered through a pad of Celite. The colorless
filtrate was collected, and the solvent removed via rotary
evaporation. The oily residue was triturated with diethyl ether
to give a colorless microcrystalline solid. The solid was
collected via vacuum filtration and washed with diethyl ether.
Yield: 0.2157 g (95%).
1H NMR (CD3CN, 25 °C): δ 7.50 ppm (m), δ 7.23 ppm
(broad s), δ 3.17 ppm (s). 31P{1H} NMR (CD3CN/CH3CN,
25 °C): δ 17.5 ppm (s).
X-ray Crystallography. Crystal data for 1-4 are pre-
sented in Table 6. Suitable crystals of 1-4 were coated with
epoxy cement and mounted on a Siemens P4 diffractometer.
Unit cell parameters were determined by least-squares
analysis of 44 reflections with 7.36 < 2θ < 25.08° for 1, 39
reflections with 10.09 < 2θ < 25.00° for 2, 43 reflections
with 9.66 < 2θ < 25.03° for 3(ClO4)2, 25 reflections with
Preparation of [(dpim)2Au2](ClO4)2, 3(ClO4)2. A 125 mL
Erlenmeyer flask was charged with 40 mL of acetonitrile, a
stir bar, 365 mg (1.4 mmol) of 2-(diphenylphosphino)-1-
methyl imidazole, and 438 mg (1.4 mmol) of (tht)AuCl. The
solution was stirred at room temperature for approximately
10 min. A 20 mL acetonitrile solution of 284 mg (1.4 mmol)
of silver perchlorate monohydrate was added dropwise. A
white precipitate forms upon addition, and the suspension
Inorganic Chemistry, Vol. 42, No. 25, 2003 8437

Catalano and Horner
10.41 <2θ < 24.98° for 3(BF4)2, and 43 reflections with
4.625 < 2θ < 25.195° for 4. A total of 5398 reflections
were collected in the range of 2.02° < θ < 25.00°, yielding
4350 unique reflections (Rint ) 0.027) for 1. For 2, a total
of 2955 reflections were collected in the range of 2.25 < θ
< 22.49°, yielding 2111 unique reflections (Rint ) 0.0898).
For 3(ClO4)2, a total of 5276 reflections were collected in
the range of 2.24 < θ < 25.00°, yielding 4108 unique
reflections (Rint ) 0.0512), while a total of 5324 reflections
were collected in the range of 2.24 < θ < 25.00°, yielding
4152 unique reflections (Rint ) 0.0752) for 3(BF4)2. A total
of 7753 reflections were collected in the range of 1.89 < θ
< 22.50°, yielding 2182 unique reflections (Rint ) 0.0745).
Calculations were performed using the Siemens SHELX-
TL version 5.10 system of programs, refining on F2. The
structures were solved by direct methods. Common with
room-temperature crystal structures the fluorine and oxygen
atoms of the BF4 and ClO4 anions were positionally
disordered around the central atoms. Simple models of this
disorder produced satisfactory refinement.
-
-
Acknowledgment is made to the National Science Foun-
dation (Grant CHE-0091180) and to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for their generous financial support of this
research.
Supporting Information Available: Complete X-ray crystal-
lographic data for 1-4 (CIF format). This material is available free
of charge via the Internet at http://pubs.acs.org.
IC030200M
8438 Inorganic Chemistry, Vol. 42, No. 25, 2003