Spectroscopic evidence for argentophilicity in structurally characterized luminescent binuclear silver(I) complexes

Article abstract of DOI:10.1021/ja9904890

A spectroscopic and structural investigation of binuclear silver(I) complexes supported by aliphatic phosphine ligands, namely [Ag(PCy₃)(O₂CCF₃)]₂ (1), [Ag₂(μ-dcpm)₂]X₂ (X = CF₃SO₃, 2; PF₆, 3; dcpm = bis(dicyclohexylphosphino)methane), and [Ag₂(μ-dcpm)(μ-O₂CCF₃)₂] (4), is described. X-ray structural analyses of 1-4 reveal Ag-Ag separations of 3.095(1), 2.948 (av), 2.923 (av), and 2.8892(9) A, respectively. Due to the optical transparency of the phosphine ligands, the UV-vis absorption band at 261 nm in CH₃CN for 2 and 3 is assigned to a 4dσ* → 5pσ transition originating from Ag(I)-Ag(I) interactions. The argentophilic nature of this band is verified by the resonance Raman spectrum of 2 with 273.9 nm excitation, where virtually all of the Raman intensity appears in the Ag-Ag stretch fundamental (80 cm^-1) and overtone bands. Complexes 2 and 3 exhibit photoluminescence in the solid state at room temperature.

Full text of DOI:10.1021/ja9904890

2464
J. Am. Chem. Soc. 2000, 122, 2464-2468
Spectroscopic Evidence for Argentophilicity in Structurally
Characterized Luminescent Binuclear Silver(I) Complexes
Chi-Ming Che,* Man-Chung Tse, Michael C. W. Chan, Kung-Kai Cheung,
David Lee Phillips,* and King-Hung Leung
Contribution from the Department of Chemistry, The UniVersity of Hong Kong,
Pokfulam Road, Hong Kong SAR, China
ReceiVed February 16, 1999. ReVised Manuscript ReceiVed NoVember 22, 1999
Abstract: A spectroscopic and structural investigation of binuclear silver(I) complexes supported by aliphatic
phosphine ligands, namely [Ag(PCy3)(O2CCF3)]2 (1), [Ag2(µ-dcpm)2]X2 (X ) CF3SO3, 2; PF6, 3; dcpm )
bis(dicyclohexylphosphino)methane), and [Ag2(µ-dcpm)(µ-O2CCF3)2] (4), is described. X-ray structural analyses
of 1-4 reveal Ag-Ag separations of 3.095(1), 2.948 (av), 2.923 (av), and 2.8892(9) Å, respectively. Due to
the optical transparency of the phosphine ligands, the UV-vis absorption band at 261 nm in CH3CN for 2 and
3 is assigned to a 4dσ* f 5pσ transition originating from Ag(I)-Ag(I) interactions. The argentophilic nature
of this band is verified by the resonance Raman spectrum of 2 with 273.9 nm excitation, where virtually all
of the Raman intensity appears in the Ag-Ag stretch fundamental (80 cm-1) and overtone bands. Complexes
2 and 3 exhibit photoluminescence in the solid state at room temperature.
Introduction
For coordination compounds, accounts of ligand-unsupported
Ag-Ag contacts based on structural determinations are scarce.7,8
Short Ag(I)-Ag(I) separations (<3.4 Å, sum of van der Waals
radii9) in bi- and polynuclear complexes are typically maintained
by bridging ligands, but whether this constitutes the existence
of d10-d10 interactions is yet to be resolved.10,11 We now present
the first unequivocal spectroscopic evidence for argentophilicity
in binuclear silver(I) complexes bearing aliphatic phosphine
ligands.
The propensity for aggregation of formally closed-shell d10
metal ions in polynuclear coordination compounds and solid-
state lattices has long been recognized and exploited in, inter
alia, supramolecular assembly and the design of molecules with
rich photophysical properties.1 Model spectroscopic studies of
d10-d10 interactions in binuclear species, such as [Au2-
(dppm)2]2+ (dppm ) bis(diphenylphosphino)methane),2 have
established the occurrence of a low-energy ndσ* f (n+1)pσ
transition that is absent in the mononuclear two-coordinate
counterparts.3 This type of transition was originally observed
in the related d8-d8 systems for Rh(I) and Pt(II) species by
Gray and co-workers,4 and is considered to be reliable evidence
of weak metal-metal interaction in the ground state (and strong
interaction in the excited state). While aurophilic attraction
between gold(I) ions is widely acknowledged,5 the development
of silver-silver bonding interactions, or argentophilicity, re-
mains in its infancy. Reports by Jansen of extended silver(I)
aggregates in the ionic lattices of ternary silver(I) oxides and
halides appeared over 10 years ago.1a More recently, silver
nanoparticles with unique optical and electronic properties have
been investigated in the emerging field of nanoscale materials.6
Experimental Section
General Procedures. All reagents were obtained from commercial
sources and all solvents used for reactions were of analytical grade
and purified according to conventional methods. Details of solvent
treatment for photophysical studies, instrumentation, and emission
measurements have been provided earlier12 and are given in the
Supporting Information. [Ag2(µ-dmpm)2](PF6)2 (dmpm ) bis(dimeth-
ylphosphino)methane) was prepared by the literature method.13 [Ag-
(PR3)2]ClO4 (R ) Me, Cy) was synthesized by interaction of the
phosphine with AgClO4 and characterized by elemental analyses, 31P
NMR, and FAB mass spectroscopy. The resonance Raman apparatus
and methods have previously been described in detail,14 and a summary
is given here. The second anti-stokes Raman shifted line of the third
* Corresponding authors. Fax: +852 2857 1586. E-mail: cmche@hku.hk
or phillips@hku.hk.
(1) (a) Jansen, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 1098-1110.
(b) Pyykko¨, P. Chem. ReV. 1997, 97, 597-636. (c) Schmidbaur, H. Chem.
Soc. ReV. 1995, 24, 391-400. (d) Puddephatt, R. J. Chem. Commun. 1998,
1055-1062.
(7) (a) Kim, Y.; Seff, K. J. Am. Chem. Soc. 1978, 100, 175-180. (b)
Eastland, G. W.; Mazid, M. A.; Russell, D. R.; Symons, M. C. R. J. Chem.
Soc., Dalton Trans. 1980, 1682-1687. (c) Kappenstein, C.; Ouali, A.;
Guerin, M.; Cerna´k, J.; Chomic, J. Inorg. Chim. Acta 1988, 147, 189-
197. (d) Quiro´s, M. Acta Crystallogr. 1994, C50, 1236-1239.
(8) (a) Singh, K.; Long, J. R.; Stavropoulos, P. J. Am. Chem. Soc. 1997,
119, 2942-2943. (b) Omary, M. A.; Webb, T. R.; Assefa, Z.; Shankle, G.
E.; Patterson, H. H. Inorg. Chem. 1998, 37, 1380-1386.
(2) (a) Che, C. M.; Kwong, H. L.; Yam, V. W. W.; Cho, K. C. J. Chem.
Soc., Chem. Commun. 1989, 885-886. (b) King, C.; Wang, J. C.; Khan,
Md. N. I.; Fackler, J. P., Jr. Inorg. Chem. 1989, 28, 2145-2149.
(3) (a) Caspar, J. V. J. Am. Chem. Soc. 1985, 107, 6718-6719. (b)
Harvey, P. D.; Gray, H. B. J. Am. Chem. Soc. 1988, 110, 2145-2147.
(4) (a) Mann, K. R.; Gordon, J. G., II; Gray, H. B. J. Am. Chem. Soc.
1975, 97, 3553-3555. (b) Rice, S. F.; Gray, H. B. J. Am. Chem. Soc. 1983,
105, 4571-4575.
(9) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(10) Cotton, F. A.; Feng, X.; Matusz, M.; Poli, R. J. Am. Chem. Soc.
1988, 110, 7077-7083.
(11) (a) El-Bahraoui, J.; Molina, J.; Portal, D. J. Phys. Chem. A 1998,
102, 2443-2448. (b) Ferna´ndez, E. J.; Lo´pez-de-Luzuriaga, J. M.; Monge,
M.; Rodr´ıguez, M. A.; Crespo, O.; Gimeno, M. C.; Laguna, A.; Jones, P.
G. Inorg. Chem. 1998, 37, 6002-6006.
(12) Lai, S. W.; Chan, M, C. W.; Cheung, T. C.; Peng, S. M.; Che, C.
M. Inorg. Chem. 1999, 38, 4046-4055.
(13) Karsch, H. H.; Schubert, U. Z. Naturforsh. 1982, 37B, 186-189.
(14) Zheng, X.; Phillips, D. L. J. Chem. Phys. 1998, 108, 5772-5783.
(5) Schmidbaur, H. Gold Bull. 1990, 23, 11-21.
(6) For recent examples, see: (a) Quaroni, L.; Chumanov, G. J. Am.
Chem. Soc. 1999, 121, 10642-10643. (b) Markovich, G.; Collier, C. P.;
Henrichs, S. E.; Remacle, F.; Levine, R. D.; Heath, J. R. Acc. Chem. Res.
1999, 32, 415-423.
10.1021/ja9904890 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/02/2000

Argentophilicity in Luminescent SilVer(I) Complexes
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2465
Table 1. Crystal Data
1
2
3
4
formula
fw
crystal system
space group
color
crystal size, mm
a, Å
b, Å
C40H66O4Ag2F6P2
1002.63
C52H92O6Ag2F6P4S2
1331.04
monoclinic
P21/c
colorless
0.35 × 0.25 × 0.15
22.873(3)
16.540(3)
17.502(4)
110.05(2)
6220(2)
C50H92Ag2F12P6
1322.84
C29H46O4Ag2F6P2
850.35
monoclinic
C2/c (No. 15)
colorless
0.40 × 0.20 × 0.10
27.074(3)
9.649(2)
18.948(3)
119.34(2)
4314(1)
monoclinic
P21/a (No. 14)
colorless
0.25 × 0.20 × 0.08
18.572(4)
14.758(3)
22.858(4)
108.14(2)
5953(1)
monoclinic
P2/n (No. 13)
colorless
0.30 × 0.25 × 0.15
13.205(3)
9.109(3)
14.327(3)
92.76(2)
1721.4(6)
301
c, Å
â, deg
V, Å3
T, K
301
298
301
Z
4
4
4
2
Dc (g cm-3
µ, cm-1
)
1.543
10.43
2064
1.421
8.59
2768
1.476
8.87
2736
1.640
12.91
860
F(000)
2θmax, deg
52
44
51
50
b
R,a Rw
0.067, 0.091
+1.05, -0.85
0.055, 0.084
+1.38, -1.54
0.066, 0.108
+1.07, -1.04
0.033, 0.042
+0.76, -0.41
residual F, e Å-3
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
harmonic of a Nd:YAG laser provided the excitation frequency for the
resonance Raman experiment. The laser beam was loosely focused onto
the sample using ∼130° backscattering geometry and reflective optics
were used to collect the Raman scattered light and image it through a
depolarizer and entrance slit of a 0.5 m spectrometer. The grating of
the spectrometer dispersed the Raman light onto a liquid nitrogen cooled
CCD detector. Accumulations of about 1 min were acquired from the
CCD and about 30 of these scans were added up to find the resonance
Raman spectrum. Known acetonitrile solvent bands and Hg lamp
emission lines were used to calibrate the Raman shifts of the resonance
Raman spectrum. Appropriately scaled solvent and quartz cell back-
ground spectra (accumulated simultaneously with the sample spectra
on a different part of the CCD) were subtracted to remove the solvent
bands, the Rayleigh line, and the quartz cell background signal from
the resonance Raman spectrum.
Syntheses. (a) [Ag(PCy3)(O2CCF3)]2 (1). A CH2Cl2 solution (30
mL) of Ag(O2CCF3) (0.10 g, 0.45 mmol) and tricyclohexylphosphine
(0.13 g, 0.45 mmol) was stirred at room temperature for 6 h. A white
solid was afforded upon precipitation by n-hexane. Recrystallization
from a mixture of CH2Cl2/n-hexane yielded colorless rods. Yield: 0.19
g, 85%. Mp 177-178 °C. MS-FAB (+ve, m/z): 1003 (M+), 501 (M+/
2). 31P{1H} NMR (202 MHz, CDCl3): δ 30.4 (dm, J ) 580 Hz). FT-
IR (KBr, cm-1): 2988s, 1712s, 1450m, 1418m, 1021s, 950m, 761m,
691s, 600m, 555s. Anal. Calcd for C40H66O4Ag2F6P2: C, 47.92; H, 6.63.
Found: C, 47.85; H, 6.54.
components yielded a white crystalline solid. The crude product was
recrystallized by vapor diffusion of diethyl ether into a dichloromethane
solution to afford colorless crystals. Yield: 0.21 g, 73%. Mp 222-
223 °C. MS-FAB (+ve, m/z): 850 (M+). 31P{1H} NMR (202 MHz,
CDCl3): δ 35.0 (dm, J ) 660 Hz). FT-IR (KBr, cm-1): 2990s, 1701s,
1469m, 1420m, 1010s, 930s, 752m, 603m, 543s. Anal. Calcd for
C29H46O4Ag2F6P2: C, 40.96; H, 5.45. Found: C, 40.79; H, 5.59.
X-ray Crystallography. Crystal data and details of collection and
refinement are summarized in Table 1. For 1/3 respectively, a total of
4085/10929 unique reflections were collected on a MAR diffractometer
with a 300 nm image plate detector [λ(Mo KR) ) 0.71073 Å]. The
structures were solved by direct methods (SIR9215), expanded using
Fourier techniques, and refined by full-matrix least squares using the
TeXsan16 software package for 3012/6074 reflections with I > 3σ(I)
and 244/599 parameters. For 2/4 respectively, a total of 8069/3248
unique reflections were collected on a Nonius CAD4/Rigaku AFC7R
diffractometer [λ(Mo KR) ) 0.71073 Å, ω - 2θ scans]. The structures
were solved by Patterson17/direct15 methods, expanded using Fourier
techniques, and refined by full-matrix least squares using the TeXsan16
software package for 5838/2267 absorption-corrected (transmission
0.91-1.00/0.85-1.00) reflections with I > 3σ(I) and 649/195 param-
eters.
Results and Discussion
(b) [Ag2(µ-dcpm)2](CF3SO3)2 (2). Bis(dicyclohexylphosphino)-
methane (0.16 g, 0.40 mmol) and a suspension of Ag(CF3SO3) (0.10
g, 0.39 mmol) in dichloromethane (30 mL) was stirred for 6 h. A white
solid was afforded upon precipitation by diethyl ether. Recrystallization
by diffusion of diethyl ether into an acetonitrile solution gave colorless
crystals. Yield: 0.23 g, 90%. Mp 210-212 °C. MS-FAB (+ve, m/z):
1033 (M+). 31P{1H} NMR (202 MHz, CDCl3): δ 33.9 (dm, J ) 660
Hz). FT-IR (KBr, cm-1): 2989s, 1480m, 1423m, 1008s, 948s, 725m,
633m, 501s. Anal. Calcd for C52H92O6Ag2F6P4S2: C, 46.92; H, 6.97.
Found: C, 46.98; H, 6.89.
(c) [Ag2(µ-dcpm)2](PF6)2 (3). A suspension of AgPF6 (0.10 g, 0.40
mmol) in dichloromethane (40 mL) and bis(dicyclohexylphosphino)-
methane (0.16 g, 0.40 mmol) was stirred at 40 °C for 5 h. Upon
concentration and addition of diethyl ether, a white crystalline solid
was obtained. Recrystallization by diffusion of diethyl ether into a
dichloromethane solution afforded colorless crystals. Yield: 0.24 g,
92%. Mp 236-237 °C. MS-FAB (+ve, m/z): 1033 (M+). 31P{1H}
NMR (202 MHz, CDCl3): δ 36.3 (dm, J ) 690 Hz). FT-IR (KBr,
cm-1): 3001s, 1475m, 1401m, 1004vs, 977s, 745m, 622m. Anal. Calcd
for C50H92Ag2F12P6: C, 45.40; H, 7.01. Found: C, 45.56; H, 6.95.
(d) [Ag2(µ-dcpm)(µ-O2CCF3)2] (4). A suspension of Ag(O2CCF3)
(0.15 g, 0.68 mmol) and bis(dicyclohexylphosphino)methane (0.14 g,
0.34 mmol) in dichloromethane was stirred for 5 h. Removal of volatile
Synthesis and Structural Characterization. Reaction of
stoichiometric amounts of Ag(O2CCF3) with tricyclohexylphos-
phine (PCy3) in CH2Cl2 afforded colorless crystals of [Ag-
(PCy3)(O2CCF3)]2 (1, 85%) upon recrystallization from CH2Cl2/
n-hexane. Similarly, treatment of AgX (X ) O3SCF3 and PF6)
with bis(dicyclohexylphosphino)methane (dcpm) in CH2Cl2 gave
[Ag2(µ-dcpm)2]X2 as colorless solids [X ) CF3SO3 (2, 90%),
PF6 (3, 87%)] which can be recrystallized from CH3CN/Et2O
for 2 and CH2Cl2/Et2O for 3, respectively. The bis(trifluoro-
acetate) species [Ag2(µ-dcpm)(µ-O2CCF3)2] (4, 73%) was
formed by interaction of 2 molar equiv of Ag(O2CCF3) with
dcpm. All four complexes displayed distinctive doublets of
multiplets in their 31P{1H} NMR spectra at δ 30-36 in CDCl3.
(15) SIR92: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi,
A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(16) TeXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation: The Woodlands, TX, 1985 and 1992.
(17) PATTY: Beurskens, P. T.; Admiraal, G.; Beursken, G.; Bosman,
W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The
DIRDIF program system, Technical Report of the Crystallography Labora-
tory, University of Nijmegen, The Netherlands, 1992.

2466 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Che et al.
Figure 1. Perspective view of [Ag(PCy3)(O2CCF3)]2, 1 (40% thermal
ellipsoids). Selected bond distances (Å) and angles (deg): Ag(1)-
Ag(1*) 3.095(1), Ag(1)-O(1) 2.173(5), Ag(1)-O(2*) 2.509(6), Ag(1)-
P(1) 2.377(2), O(1)-C(19) 1.202(9); Ag(1*)-Ag(1)-O(1) 85.4(1),
Ag(1*)-Ag(1)-P(1) 110.57(5), P(1)-Ag(1)-O(1) 160.7(2), Ag(1)-
O(1)-C(19) 123.7(5).
Figure 2. Perspective view of cation 1 in [Ag2(µ-dcpm)2](PF6)2, 3
(40% thermal ellipsoids). Selected bond distances (Å) and angles
(deg): Ag(1)-Ag(1*) 2.938(1), Ag(1)-P(1) 2.405(2), Ag(1)-P(2*)
2.405(2); Ag(1*)-Ag(1)-P(1) 92.89(5), Ag(1*)-Ag(1)-P(2*) 90.69(5),
P(1)-Ag(1)-P(2*) 172.37(8), Ag(1)-P(1)-C(1) 111.9(2), P(1)-
C(1)-P(2) 114.5(4).
Compounds 1-4 have been characterized by X-ray crystal-
lography. Complex 1 crystallizes in a dimeric form, with two
[(Cy3P)Ag(O2CCF3)] units (P-Ag-O ) 160.7(2)°) held to-
gether by an Ag-Ag contact of 3.095(1) Å (Figure 1). The
PCy3 and trifluoroacetate ligands are in a syn configuration,
with Ag-O(acetate) distances of 2.173(5) and 2.509(6) Å. The
crystal structure of the phosphine-bridged derivative [Ag2(µ-
dcpm)(µ-O2CCF3)2] (4, Figure 3) also reveals short and long
Ag-O(acetate) separations (2.191(3) and 2.446(4) Å respec-
tively), plus a silver-silver distance of 2.8892(9) Å.
The molecular structures of 2 and 3 (Figure 2) contain
Ag2P4C2 cores that adopt chair conformations. The silver-silver
separations of 2.936(1) and 2.960(1) Å in 2 and 2.907(1) and
2.938(1) Å in 3, plus those in 1 and 4, are typical for polynuclear
Ag(I) derivatives with proposed d10-d10 interactions.7,8,18
A
slightly longer Ag-Ag distance of 3.041(2) Å was reported for
the analogous complex [Ag2(µ-dmpm)2](PF6)2.13 Close cation-
anion contacts are evident in the crystal lattices of 2 (e.g. Ag-
(1)‚‚‚O(3) 2.692(7) Å) and 3 (e.g. Ag(1)‚‚‚F(2′) 2.64(5) Å), but
intriguingly, such interactions are not apparent in the X-ray
structure19 of the gold(I) congener [Au2(µ-dcpm)2](ClO4)2
(closest Au‚‚‚OClO3 3.36(2) Å).
Spectroscopic Characterization. Previous assignments of
the metal-centered 4dσ* f 5pσ transition in polynuclear silver-
(I) compounds with bridging arylphosphine ligands are prob-
lematic because the intraligand transitions of free arylphosphines
Figure 3. Perspective view of [Ag2(µ-dcpm)(µ-O2CCF3)2], 4 (40%
thermal ellipsoids). Selected bond distances (Å) and angles (deg):
Ag(1)-Ag(1*) 2.8892(9), Ag(1)-O(1) 2.191(3), Ag(1)-O(2) 2.446(4),
Ag(1)-P(1) 2.354(1), O(1)-C(14) 1.240(6); Ag(1*)-Ag(1)-O(1)
90.0(1), Ag(1*)-Ag(1)-O(2) 74.4(1), Ag(1*)-Ag(1)-P(1) 88.96(3),
P(1)-Ag(1)-O(1) 152.3(1), P(1)-Ag(1)-O(2) 113.5(1).
(18) For example, see: (a) Zank, J.; Schier, A.; Schmidbaur, H. J. Chem.
Soc., Dalton Trans. 1999, 415-420. (b) Villanneau, R.; Proust, A.; Robert,
F.; Gouzerh, P. Chem. Commun. 1998, 1491-1492. (c) Ren, T.; Lin, C.;
Amalberti, P.; Macikenas, D.; Protasiewicz, J. D.; Baum, J. C.; Gibson, T.
L. Inorg. Chem. Commun. 1998, 1, 23-26. (d) Chen, X. M.; Mak, T. C.
W. J. Chem. Soc., Dalton Trans. 1991, 1219-1222 and references therein.
(e) Tsuda, T.; Ohba, S.; Takahashi, M.; Ito, M. Acta Crystallogr. 1989,
C45, 887-890. (f) Papasergio, R. I.; Raston, C. L.; White, A. H. J. Chem.
Soc., Chem. Commun. 1984, 612-613. (g) Ho, D. M.; Bau, R. Inorg. Chem.
1983, 22, 4073-4079. (h) Chivers, T.; Parrez, M.; Schatte, G. Angew.
Chem., Int. Ed. 1999, 38, 2217-2219. (i) Robinson, F.; Zaworotko, M. J.
J. Chem. Soc., Chem. Commun. 1995, 2413-2414.
usually occur in a similar energy region. In this work, the
aliphatic PCy3 and dcpm ligands are optically transparent in
the UV region λ g 250 nm. It is therefore feasible to probe the
4dσ* f 5pσ transition originating from Ag(I)-Ag(I) interac-
tions in the ground and excited states. In CH2Cl2, an intense
UV-vis absorption band is observed for 2 and 3 at λmax 266
nm (ꢀ ) 2.2 × 104 dm3 mol-1 cm-1), and for 4 at λmax 243 nm
(ꢀ ) 1.3 × 104 dm3 mol-1 cm-1; see Supporting Information).
(19) Fu, W. F.; Chan, K. C.; Miskowski, V. M.; Che, C. M. Angew.
Chem., Int. Ed. 1999, 38, 2783-2785.

Argentophilicity in Luminescent SilVer(I) Complexes
J. Am. Chem. Soc., Vol. 122, No. 11, 2000 2467
fundamentals of the Ag-Ag stretch was measured. A value of
0.40 was obtained, which approaches the theoretical value of
0.33 for a ndσ* f (n+1)pσ transition. The observed ν(Ag2) at
80 cm-1 provides experimental support for that proposed for
[Ag2(dmpm)2](PF6)2 (76 cm-1 23
)
and Tl[Ag(CN)2] (∼88 cm-1 8b
)
using nonresonant Raman techniques. The ν(Au2) value for the
analogous Au2(dcpm)22+ species (88 cm-1 20
) is comparable to
ν(Ag2) for 2 despite the higher atomic mass of gold. This is
consistent with the fact that gold(I) has a greater tendency to
undergo homoatomic attraction. The energy of the 4dσ* f 5pσ
transition for 2 and 3 also accounts for the difficulty in locating
the corresponding transition in previous spectroscopic studies
of polynuclear silver(I) compounds,24 since the bridging or
ancillary ligands in most instances absorb in a similar UV region.
We note that the 4dσ* f 5pσ transition for [Ag2(µ-dmpm)2]2+
appears at 256 nm (ꢀ ) 1.3 × 104 dm3 mol-1 cm-1) in CH3-
CN; this minor blue shift compared to 2 and 3 is in accordance
with a slightly longer Ag-Ag separation in [Ag2(µ-dmpm)2]-
(PF6)2 (3.041(2) Å).13
The force constant F(Ag2) estimated from the 80 cm-1 ν(Ag2)
value for 2 assuming a pure Ag2 stretch mode is 0.203 mdyn
Å-1 25
For nine binuclear Ag2 compounds, Harvey and co-
.
workers26 observed a good linear correlation between the Ag2
bond distance and ln F(Ag2) [r(Ag2) ) -0.284 ln F(Ag2) +
2.53]. Our values for 2 fit very closely to the curve of the Harvey
correlation, and provide further support for assignment of the
resonance Raman bands of 2 to a Ag-Ag stretch. In addition,
a resonance Raman intensity analysis study of 2 shows that the
resonance Raman and absorption data can only be simulated
accurately using a high excited-state frequency of ca. 180 cm-1
and a bond length change of ca. 0.20 Å.27 The apparent change
to a substantially larger frequency reflects the expected Ag-
Ag single bonded excited state for a ndσ* f (n+1)pσ transition.
This is in agreement with previous studies by Omary and
Patterson,28 who proposed Ag-Ag bonded excited states to
account for the solid-state photoluminescence of [Ag(CN)2]-
systems.
At 298 K, compounds 2 and 3 exhibit photoluminescence in
the solid state at λmax 420 and 417 nm (lifetime ∼0.5 µs, Eex
270 nm), respectively.29 The particularly large Stokes shifts of
the solid emission maxima from the 4dσ* f 5pσ absorption
band in acetonitrile (ca. 14500 cm-1) indicate that the electroni-
cally excited [dσ*, pσ] state may not be responsible for the
luminescence. We have observed in our laboratory that changes
in metal coordination, in both ground and excited states, can
dramatically alter the emissions of related Au(I)19 and Cu(I)30
systems, while recent studies by Catalano and co-workers also
Figure 4. UV-vis absorption (top) and resonance Raman (bottom)
spectra for [Ag2(µ-dcpm)2](CF3SO3)2 (2) in acetonitrile (solvent
subtraction artifacts marked by +).
In contrast, the absorption spectra of 1 (monomeric in solution)
and mononuclear [Ag(PR3)2]ClO4 (R ) Me, Cy) show ꢀ values
below 102 at λ g 250 nm. The high ꢀ values and profiles of the
prominent absorptions are similar to the 5dσ* f 6pσ transitions
of Au2(P-P)22+ (P-P ) dppm,2 dcpm20), which exhibit weak
Au(I)-Au(I) interactions in the ground state. Significantly, the
intense band for 2 and 3 displays a slight blue shift to 261 nm
in acetonitrile (ꢀ ) 1.7 × 104 dm3 mol-1 cm-1).21 We suggest
that weak solvent (CH3CN)‚‚‚Ag contacts can disrupt the
intramolecular Ag-Ag interaction, which would consequently
affect the 4dσ* f 5pσ transition. Previous reports have
demonstrated that coordination of N-donor molecules, including
acetonitrile, to the [Cu2(µ-dppm)2]2+ moiety results in very long
Cu‚‚‚Cu interactions (>3.7 Å).22
The 261 nm UV band for 2 in CH3CN has been examined
using resonance Raman spectroscopy, which confirms that it is
metal-centered in nature. Figure 4 shows the 261 nm absorption
and a resonance Raman spectrum obtained with 273.9 nm
excitation: virtually all of the Raman intensity appears in the
Ag-Ag stretch fundamental and overtone bands. It is evident
that the transition is predominantly localized on the Ag-Ag
bond. The depolarization ratio for the Stokes and anti-Stokes
(23) Perreault, D.; Drouin, M.; Michel, A.; Miskowski, V. M.; Schaefer,
W. P.; Harvey, P. D. Inorg. Chem. 1992, 31, 695-702.
(24) (a) Assefa, Z.; Patterson, H. H. Inorg. Chem. 1994, 33, 6194-6200.
(b) Ford, P. C.; Vogler, A. Acc. Chem. Res. 1993, 26, 220-226. (c) Che,
C. M.; Yip, H. K.; Li, D.; Peng, S. M.; Lee, G. H.; Wang, Y. M.; Liu, S.
T. J. Chem. Soc., Chem. Commun. 1991, 1615-1617. (d) Henary, M.; Zink,
J. I. Inorg. Chem. 1991, 30, 3111-3112. (e) Vogler, A.; Kunkely, H. Chem.
Phys. Lett. 1989, 158, 74-76.
(25) F(Ag2) ) µ[2πcν(Ag2)]2; µ ) reduced mass.
(26) (a) Perreault, D.; Drouin, M.; Michel, A.; Harvey, P. D. Inorg. Chem.
1993, 32, 1903-1912. (b) Harvey, P. D. Coord. Chem. ReV. 1996, 153,
175-198.
(27) See supporting information for full details.
(28) (a) Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 1998, 120,
7696-7705. (b) Omary, M. A.; Patterson, H. H. Inorg. Chem. 1998, 37,
1060-1066.
(20) Leung, K. H.; Phillips, D. L.; Tse, M. C.; Che, C. M.; Miskowski,
V. M. J. Am. Chem. Soc. 1999, 121, 4799-4803.
(21) The perchlorate salt [Ag2(µ-dcpm)2](ClO4)2 was also prepared and
displays identical UV-vis absorption data.
(29) Complexes 2 and 3 display very weak emissions (φ e 10-4) at λmax
380 (max) and ∼590 (broad) nm in acetonitrile at 298 K.
(30) Details will be published in due course: Mao, Z.; Che, C. M.;
Phillips, D. L.
(22) (a) D´ıez, J.; Gamasa, M. P.; Gimeno, J.; Tiripicchio, A.; Tiripicchio
Camellini, M. J. Chem. Soc., Dalton Trans. 1987, 1275-1278. (b) Li, D.;
Che, C. M.; Wong, W. T.; Shieh, S. J.; Peng, S. M. J. Chem. Soc., Dalton
Trans. 1993, 653-654.
(31) Catalano, V. J.; Kar, H. M.; Garnas, J. Angew. Chem., Int. Ed. 1999,
38, 1979-1982. Also see ref 24d.

2468 J. Am. Chem. Soc., Vol. 122, No. 11, 2000
Che et al.
demonstrated similar findings for tetranuclear Ag(I) clusters.31
In this report, close metal-anion contacts are observed in the
crystal lattices of 2 and 3 (see above), and we therefore suggest
that weak metal-anion interactions may play a role in the solid
state emissions.
Concluding Remarks. Our current studies on [M2(µ-
dcpm)2]2+ (M ) Ag, Au19,20) afford a rare opportunity to directly
compare Ag-Ag and Au-Au metallophilic bonding. Using the
Raman-observed ν(M2) values [M ) Ag (80 cm-1), Au (88
cm-1)], the force constant assuming a pure M2 stretch mode is
calculated25 to be 0.203 mdyn Å-1 for F(Ag2) and 0.449 mdyn
Å-1 for F(Au2). These results may therefore serve as a useful
indicator for comparing the bond strengths of Ag-Ag versus
Au-Au interactions.
It is also pertinent to compare the ndσ* f (n+1)pσ transition
energies of the [Ag2(dcpm)2]2+ and [Au2(dcpm)2]2+ complexes.
The blue shift from 277 nm for Au2 to 261 nm for Ag2 is 2213
cm-1, which approaches the corresponding value of 2200 cm-1
for the isoelectronic d10 [M2(dppm)3] (M ) Pd, Pt) pair.3b This
similarity is interesting and can be qualitatively ascribed to the
difference in the nd f (n+1)p energy gap of the free metal
ions from second to third row transition metals. Quantitatively,
this difference in energy gap is considerably more pronounced
in the free metal ions (17200 cm-1 for Ag/Au1a). It is well
documented that Au(I) species exhibit metal-metal interactions
for both ground and excited states, and this is likely to be the
situation for the Ag(I) system in view of the small energy
difference between the ndσ* f (n+1)pσ transitions. Our results
for the Ag(I) system suggest that ndσ* f (n+1)pσ excitation
produces a silver-silver bonded excited state like for the Au-
(I) congener.20 In summary, this article highlights the first
spectroscopic verification for the existence of Ag(I)-Ag(I)
attractive interactions in coordination compounds.
Acknowledgment. We are grateful to The University of
Hong Kong, the Croucher Foundation of Hong Kong, and the
Research Grants Council of the Hong Kong SAR, China [HKU
7298/99P and HKU 974/94P]. Dr. C. K. Chan is acknowledged
for preliminary results in this work, and Dr. V. M. Miskowski
is thanked for helpful discussions.
Supporting Information Available: Details of solvent
treatment for photophysical studies, instrumentation, and emis-
sion measurements; ORTEP diagrams and tables of crystal data,
atomic coordinates, calculated hydrogen coordinates, anisotropic
displacement parameters, and bond distances and angles for
1-4; UV-vis absorption spectra for 1 and 4; resonance Raman
intensity analysis study for 2 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA9904890