Emission Energy Correlates with Inverse of Gold-Gold Distance for Various [Au(SCN)2]- Salts

Full text of DOI:10.1021/ja037012f

Published on Web 12/13/2003
Emission Energy Correlates with Inverse of Gold-Gold Distance for Various
[Au(SCN)2]- Salts
Nathan L. Coker, Jeanette A. Krause Bauer, and R. C. Elder*
Department of Chemistry, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0172
Received July 1, 2003; E-mail: Richard.Elder@uc.edu
Luminescence of gold(I) compounds in the solid state has
received considerable study.1,2 The aurophillic interaction to form
gold-gold dimers, trimers, and infinite chains is known to be
intimately involved as a source of the observed emissions; however,
generally, the luminescence energy is not correlated with the gold-
gold distance.3,4 This could well result from the changes which occur
in ligating atom or ligand type as well as changes in gold-gold
distance in many of the studies.
We have synthesized a series of salts of the bis(thiocyanato)-
aurate(I) anion, including the following counterions: K+, Rb+, Cs+,
Me4N+, [(n-Bu)4N]+, and Ph4P+. Generally, the compounds have
been synthesized by reaction of 2 equiv of the thiocyanate salt of
the desired cation with chlorogold(I)tetrahydrothiophene5 in aceto-
nitrile. The metal chloride formed is nearly insoluble and may be
filtered off. X-ray quality crystals were generally obtained by
Figure 1. Ortep drawing of {K[Au(SCN)2]}n dimeric portion.
dissolution in acetonitrile followed by ether vapor diffusion at 4
°C. Each of these materials was characterized by negative ion FT-
ICR MS which showed the bis(thiocyanato)aurate ion as the
principal peak. The compounds were characterized by IR, Raman,
and UV-visible spectroscopies. Representative spectra for the K+
species are shown in the Supporting Information. All compounds
have similar spectra, except for the tetraphenylphosphonium salt
where the benzene moieties make obvious contributions. The nature
of the cation was determined by the syntheses and confirmed by
proper fits to atomic number and ion structure in the crystal
structures.
The single-crystal X-ray structures of each salt have been
determined.6 The structures fall into three classes. The phosphonium
salt contains isolated monomers with an Au-S-C angle of 101.2°
and a pure trans configuration of the thiocyanate ligands. The alkali
salts are nearly isostructural, consisting of infinite linear chains with
alternating short and long gold-gold bond distances along the chain.
The [Me4N]+ salt forms a kinked chain of trimers joined at a shared
gold atom as the kink. They have the pattern of short-short/long-
long Au-Au bonds. Finally, the [(n-Bu)4N]+ salt contains isolated
dimers with relatively short, 3.07 Å Au-Au distances. Figure 1
shows, as an example, the structure of the short Au-Au dimer
portion of the K+ salt.
Table 1. Au-Au Bond Distances (Å), Excitation and Emission
a
Wavelengths (nm), and Energy (cm-1
)
Au−Au (Å)
excitation
emission
energy
K+
3.0064(5)
3.0430(5)
3.0700(8)
3.0821(4)
3.1144(5)
3.1794(2)
3.2654(2)
3.2128(11)
3.2399(11)
320
516
19 379
[(n-Bu)4N]+
360
320
539
506
18 552
19 762
Rb+
Me4N+
Cs+
325
320
320
635
670
710
15 748
14 925
14 084
Na)
a X-ray data for the Rb+ and Cs+ salts were collected at 170 K; all others
were collected at 150 K.
All of the materials with gold-gold bonds emit as polycrystalline
specimens at 77 K with λmax ranging from 506 to 670 nm. Table 1
lists the observed gold-gold distances and the energy of the
emission maximum for each structure. Note that both the short and
long Au-Au distances are listed for the infinite chain structures.
Figure 2 shows the shift in emission versus the short gold-gold
interactions.7 Fackler, Schmidbaur, and co-workers8 suggested that
emission should correlate inversely with distance. Figure 3 shows
a plot of emission energy (cm-1) versus the reciprocal of the short
Au-Au distances (Å-1). There appears to be a relatively strong
correlation between the two. Interestingly, the point for Rb+, which
by eye appears to give the poorest correlation, has little effect on
the results sought. The important aspect is the slope of the line,
Figure 2. Emission spectra of oligomeric salts of [Au(SCN)2]- at 77 K.
i.e., the shift in wavelength as a function of bond length, and
inclusion of the Rb+ point has little bearing on the slope.9 Length
appears not to be a factor as the dimeric [(n-Bu)4N]+ salt falls close
to the line of the infinite chain alkali salts. Also, the trimeric
[Me4N]+ salt with two adjacent 3.18 Å short distances falls on the
same line. Finally, cation size is not determinative of the emission
9
12
J. AM. CHEM. SOC. 2004, 126, 12-13
10.1021/ja037012f CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
(2) Lee, Y.-A.; McGrath, J. E.; Lachicotte, R. J.; Eisenberg, R. J. Am. Chem.
Soc. 2002, 124, 10662-10663.
(3) Forward, J. M.; Bohmann, D.; Fackler, J. P., Jr.; Staples, R. J. Inorg.
Chem. 1995, 34, 6330-6336.
(4) van Zyl, W. E.; Lo´pez-de-Luzuriaga, J. M.; Mohamed, A. A.; Staples, R.
J.; Fackler, J. P., Jr. Inorg. Chem. 2002, 41, 4579-4589.
(5) LeBlanc, J. D. Thiol Complexes of Gold(I): Structure and Chemistry of
the Gold Based Antiarthritis Drugs. Ph.D. Dissertation, McMaster
University, Hamilton, Canada, 1996.
(6) {K[Au(SCN)2]}n, 1, X-ray data were collected at 150 K on a SMART
1K CCD (Mo KR). Structure solutions were obtained by standard Patterson
and difference Fourier methods and refined on F2. a ) 13.1181(10) Å, b
) 6.0494(5) Å, c ) 17.7732(14) Å, R ) 90°, â ) 110.3870(10)°, γ )
90°, V ) 1322.07(18) Å3, monoclinic C2/c, Z ) 4, θmin/max ) 3.31 /29.45°,
1257 [I > 2σ(I)] unique observed reflections were used to refine 74
variable parameters and gave a final R factor of 0.0376. {Rb[Au(SCN)2]}n,
2, X-ray data were collected at 173 K on a Kappa goniostat with a
SMART6000 CCD detector at ChemMatCARS using synchrotron radia-
tion tuned to Ag KR radiation, λ ) 0.55940 Å. Structure solutions were
obtained by direct methods and difference Fourier methods and refined
on F2. a ) 13.0814(2) Å, b ) 6.1965(6) Å, c ) 17.6736(2) Å, R ) 90°,
â ) 108.536(4)°, γ ) 90°, V ) 1358.3(3) Å3, monoclinic C2/c, Z ) 4,
θmin/max ) 2.59 /21.92°, 1335 [I > 2σ(I)] unique observed reflections were
used to refine 74 variable parameters and gave a final R factor of 0.0376.
{Cs[Au(SCN)2]}n, 3, X-ray data were collected at 173 K on a Kappa
goniostat with a SMART6000 CCD detector at ChemMatCARS using
synchrotron radiation tuned to Ag KR radiation, λ ) 0.55940 Å. Structure
solutions were obtained by direct methods and difference Fourier methods
and refined on F2. a ) 13.1550(1) Å, b ) 6.4417(9) Å, c ) 17.790(2) Å,
R ) 90°, â ) 107.229(6)°, γ ) 90°, V ) 1439.9(3) Å3, monoclinic C2/c,
Z ) 4, θmin/max ) 2.80/16.26°, 649 [I > 2σ(I)] unique observed reflections
were used to refine 74 variable parameters and gave a final R factor of
0.0309. {[Me4N][Au(SCN)2]}n, 4, X-ray data were collected at 150 K on
a SMART 1K CCD (Mo KR). Structure solutions were obtained by direct
methods and difference Fourier methods and refined on F2. a ) 12.6423-
(3) Å, b ) 9.6723(2) Å, c ) 19.0040(6) Å, R ) 90°, â ) 105.357(1)°,
γ ) 90°, V ) 2240.84(10) Å3, monoclinic P21/c, Z ) 4, θmin/max ) 1.67/
28.28°, 4710 [I > 2σ(I)] unique observed reflections were used to refine
220 variable parameters and gave a final R factor of 0.0410. [(n-Bu)4N]2-
[Au2(SCN)4], 5, X-ray data were collected at 150 K on a SMART 1K
CCD (Mo KR). Structure solutions were obtained by direct methods and
difference Fourier methods and refined on F2. a )12.512(3) Å, b )
28.964(8) Å, c ) 12.898(3) Å, R ) 90°, â ) 93.014(2)°, γ ) 90°, V )
4667.9(19) Å3, monoclinic P21/n, Z ) 4, θmin/max ) 2.62/26.37°, 5151 [I
> 2σ(I)] unique observed reflections were used to refine 427 variable
parameters and gave a final R factor of 0.0410. [Ph4P][Au(SCN)2], 6,
X-ray data were collected at 292 K on a SMART 1K CCD (Mo KR).
Structure solutions were obtained by direct methods and difference Fourier
methods and refined on F2. a ) 16.699(2) Å, b ) 7.3184(9) Å, c ) 22.002-
(3) Å, R ) 90°, â ) 111.913(3)°, γ ) 90°, V ) 2494.6(5) Å3, monoclinic
C2/c, Z ) 4, θmin/max ) 2.63/28.29°, 1854 [I > 2σ(I)] unique observed
reflections were used to refine 148 variable parameters and gave a final
R factor of 0.0519.
Figure 3. Energy (cm-1) versus 1/d (Å-1) of the short Au-Au interaction
n- 7
of [Au(SCN)2]n
.
energy, which would be expected to have the trend K+, Rb+, Cs+,
Me4N+, and [(n-Bu)4N]+ in contrast to the observed order.
A remarkable series of conclusions can be drawn, however. First,
given the isolated dimers in the [(n-Bu)4N]+ salt, a single gold-
gold pair must serve as the emissive source. Second, the trimeric
group of gold atoms of equal distance behaves as though a single
pair is the emissive source. Third, extending the chain to infinity,
by a series of short/long links again suggests that a single gold-
gold pair serves as the emissive source.
Further studies of the K+ salt, in collaboration with others,10 at
temperatures approaching 4 K indicate a second emission band with
a shorter lifetime arises at a shorter wavelength. A detailed report
will follow.
Acknowledgment. We thank Professors John Fackler Jr. of
Texas A&M University; Howard Patterson of the University of
Maine, Orono; Mohammad Omary of the University of North
Texas, Denton; and Thomas Ridgway, University of Cincinnati,
Cincinnati, for helpful discussions. SMART 1K data were collected
through the Ohio Crystallographic Consortium, funded by the Ohio
Board of Regents 1995 Investment Fund (CAP-075). J.A.K.B.
thanks Dr. Alan Pinkerton (Department of Chemistry, University
of Toledo) for use of the SMART6000. Synchrotron data were
collected at ChemMatCARS during commissioning of the beamline,
courtesy of J. Viccaro and the University of Chicago. J.A.K.B. and
N.C. thank D. Cookson and T. Graber for support. Use of the
Advanced Photon Source was supported by the U.S. Department
of Energy, Basic Energy Sciences, Office of Science, under Contract
No. W-31-109-Eng-38. Use of the ChemMatCARS Sector 15 was
supported by the National Science Foundation/Department of
Energy under Grant Nos. CHE9522232 and CHE0087817 and by
the Illinois Board of Higher Education.
(7) The excitation wavelengths used for the emission of Na+, K+, Rb+, Cs+,
NH4+, Me4N+, and (n-Bu)4N+ are 320, 320, 320, 320, 360, 325, and 360
nm, respectively.
(8) Assefa, Z.; McBurnett, B.; Staples, R.; Fackler, J. P., Jr.; Assmann, B.;
Angermaier, K.; Schmidbaur, H. Inorg. Chem. 1995, 34, 75-83. Figures
with plots of distance or reciprocal distance are of essentially equal
correlation merit (distance plot R2 ) 0.8563, reciprocal distance R2
0.8464).
)
(9) The R2 value for the five-point correlation of Figure 3 is 0.85 with a slope
of -24.0 ( 1.85 K cm-1/Å, with the results for the Rb+ appearing to be
an outlier. With this last point removed, the R2 for the four-point correlation
is 0.986 with a slope of -22.3 ( 1.85 K cm-1/Å. Statistically, the slopes
are equivalent, although by means of a t test one can reject the Rb+ datum
at the 99.5% confidence level. Using statistical tests to reject data with
such a small set of points is problematic, but with or without the Rb+
datum there appears to be a clear linear relationship between the inverse
of metal center distances and the emission energies for the compounds
studied.
Supporting Information Available: X-ray crystallographic data
in text format, figures showing the structure of each complex and
packing diagram, and tables for all complexes, spectroscopic, and
photophysical data (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(10) Arvapally, R. K.; Coker, N. L.; Hettiarachchi, S. R.; Elder, R. C.; Patterson,
H. H.; Omary, M. A. Abstracts of Papers, 225th National Meeting of the
American Chemical Society, New Orleans, LA, March 23-27, 2003;
American Chemical Society: Washington, DC, 2003; INOR-570.
References
(1) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J.
P., Jr. J. Am. Chem. Soc. 2001, 123, 11237-11247.
JA037012F
9
J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004 13