Aurophilic interactions in cationic gold complexes with two isocyanide ligands. Polymorphic yellow and colorless forms of [(cyclohexyl isocyanide)2Au1](PF6) with distinct luminescence

Article abstract of DOI:10.1021/ja020902v

Crystallographic studies of yellow and colorless forms of [(C₆H₁₁NC)₂Au¹](PF₆) show that they are polymorphs with differing, but close, contacts between the gold atoms which form extended chains. In the colorless polymorph the gold cations form linear chains with a short Au···Au contact (3.1822(3) A) indicative of an aurophilic attraction. The structure of the yellow polymorph is more complicated with four independent cations forming kinked, slightly helical chains with very short Au···Au contacts of 2.9803(6), 2.9790(6), 2.9651(6), and 2.9643(6) A. However, in the related compound, [(CH₃NC)₂Au¹](PF₆) each cation is surrounded by six hexafluorophosphate ions and there is no close Au···Au contact despite the fact that the isocyanide ligand has less steric bulk. The crystalline colorless and yellow polymorphs are both luminescent at 298 K, λ_max: 424 nm (colorless) or 480 nm (yellow). Colorless solutions of the two polymorphs have identical absorption spectra and are nonluminescent at room temperature. Freezing solutions of [(C₆H₁₁NC)₂Au¹](PF₆) produces intense luminescence which varies depending upon the solvent involved. Each polymorph melts to give a colorless but luminescent liquid which reverts to the yellow polymorph upon cooling.

Full text of DOI:10.1021/ja020902v

Published on Web 01/04/2003
Aurophilic Interactions in Cationic Gold Complexes with Two
Isocyanide Ligands. Polymorphic Yellow and Colorless Forms
of [(Cyclohexyl Isocyanide)2AuI](PF6) with Distinct
Luminescence
Rochelle L. White-Morris, Marilyn M. Olmstead, and Alan L. Balch*
Contribution from the Department of Chemistry, UniVersity of California,
DaVis, California 95616
Received July 1, 2002 ; E-mail: albalch@ucdavis.edu
Abstract: Crystallographic studies of yellow and colorless forms of [(C6H11NC)2AuI](PF6) show that they
are polymorphs with differing, but close, contacts between the gold atoms which form extended chains. In
the colorless polymorph the gold cations form linear chains with a short Au‚‚‚Au contact (3.1822(3) Å)
indicative of an aurophilic attraction. The structure of the yellow polymorph is more complicated with four
independent cations forming kinked, slightly helical chains with very short Au‚‚‚Au contacts of 2.9803(6),
2.9790(6), 2.9651(6), and 2.9643(6) Å. However, in the related compound, [(CH3NC)2AuI](PF6), each cation
is surrounded by six hexafluorophosphate ions and there is no close Au‚‚‚Au contact despite the fact that
the isocyanide ligand has less steric bulk. The crystalline colorless and yellow polymorphs are both
luminescent at 298 K, λmax: 424 nm (colorless) or 480 nm (yellow). Colorless solutions of the two polymorphs
have identical absorption spectra and are nonluminescent at room temperature. Freezing solutions of
[(C6H11NC)2AuI](PF6) produces intense luminescence which varies depending upon the solvent involved.
Each polymorph melts to give a colorless but luminescent liquid which reverts to the yellow polymorph
upon cooling.
dimers,13 solvoluminescence,14 luminescence from [Au(CN)2]-
in a variety of environments,15 solvate dependent emission from
Introduction
In the solid state, uncharged, colorless Au(I) complexes show
a pronounced propensity to self-associate. Attractive aurophilic
interactions occur between closed shell, two-coordinate Au(I)
complexes when the Au‚‚‚Au separations are less than 3.6 Å1-3
and are important in determining the solid-state structures of
many of these complexes.4,5 Theoretical studies have shown that
this weakly bonding interaction is the result of correlation effects
that are enhanced by relativistic effects.6-9 Experimental studies
have shown that the strength of this attractive interaction is
comparable to hydrogen bonding: i.e. ca. 7-11 kcal/mol.10,11
Aurophilic attractions have been implicated in several lumi-
nescent features of gold(I) complexes,12 including emission from
the dimer, {Au(S2CN(C5H11)2}2,16 and a potassium ion sensor.17
NMR studies have shown that aurophilic attractions persist in
solution where they are favored by high concentrations and low
temperatures.18
Recently we found that the colorless, cationic gold carbene
complex, [Au{C(NHCH3)2}2](PF6), undergoes aggregation
through aurophilic attraction in the solid state and in frozen
solutions.19 In the crystalline state, the aurophilic attraction and
hydrogen bonding among ligand N-H groups and the hexafluo-
(13) For some examples see: King, C.; Wang, J.-C.; Khan, M. N. I.; Fackler,
J. P., Jr. Inorg. Chem. 1989, 28, 2145. Che, C.-M.; Kwong, H.-L.; Yam,
V. W.-W.; Cho, C.-K. J. Chem. Soc., Chem. Commun. 1989, 885. Fu, W.-
F.; Chan, K.-C.; Miskowski, V. M.; Che, C.-M. Angew. Chem., Int. Ed.
1999, 38, 2783. Che, C.-M.; Wong, W.-T,; Lai, T.-F.; Kwong, H.-L. J.
Chem. Soc., Chem. Commun. 1989, 243.
* Corresponding author.
(1) Jones, P. G. Gold Bull. 1986, 19, 46; 1983, 16, 114; 1981, 14, 159; 1981,
14, 102.
(2) Schmidbaur, H. Interdisip. Sci. ReV. 1992, 17, 213.
(3) Schmidbaur, H. Chem. Soc. ReV. 1995, 24, 391.
(4) Pathaneni, S. S.; Desiraju, G. R. J. Chem. Soc., Dalton Trans. 1993, 319.
(5) Grohmann, A.; Schmidbaur, H. In ComprehensiVe Organometallic Chem-
istry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier: Oxford,
U.K., 1995; Vol. 3, p 1.
(6) Pyykko¨, P.; Li, J.; Runeberg, N. Chem. Phys. Lett. 1994, 218, 133.
(7) Pyykko¨, P. Chem. ReV. 1997, 97, 597.
(8) Pyykko¨, P.; Runeberg, N.; Mendizabal, F. Chem. Euro. J. 1997, 3, 1451.
(9) Pyykko¨, P.; Mendizabal, F. Chem. Euro. J. 1997, 3, 1458.
(10) Schmidbaur, H.; Graf, W.; Mu¨ller, G. Angew. Chem., Int. Ed. Engl. 1988,
27, 417.
(14) Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A. L. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1179. Fung, E. Y.; Olmstead, M. M.; Vickery, J.
C.; Balch, A. L. Coord. Chem. ReV. 1998, 171, 151. Olmstead, M. M.;
Jiang, F.; Attar, S.; Balch, A. L. J. Am. Chem. Soc. 2001, 123, 3260.
(15) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler, J. P.,
Jr. J. Am. Chem. Soc. 2001, 123, 111237. Rawashdeh-Omary, M. A.;
Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 2000, 122, 10371.
Rawashdeh-Omary, M. A.; Omary, M. A.; Shankle, G. E.; Patterson, H.
H. J. Phys. Chem. B 2000, 104, 6143. Omary, M. A.; Patterson, H. H. J.
Am. Chem. Soc. 1998, 120, 7696.
(16) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.;
Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329.
(17) Yam, V. W. W.; Li, C. K.; Chan, C. L. Angew. Chem., Int. Ed. 1998, 37,
2857.
(11) Harwell, D. E.; Mortimer, M. D.; Knobler, C. B.; Anet, F. A. L.; Hawthorne,
M. F. J. Am. Chem. Soc. 1996, 118, 2679.
(12) Forward, J. M.; Fackler, J. P., Jr.; Assefa, Z. In Optoelectronic Properties
of Inorganic Compounds; Roundhill, D. M., Fackler, J. P., Jr., Eds.; Plenum
Press: New York, 1999; p 195.
(18) Balch, A. L.; Fung, E. Y.; Olmstead, M. M. J. Am. Chem. Soc. 1990, 112,
5181.
(19) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch, A. L.
J. Am. Chem. Soc. 2002, 124, 2327.
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J. AM. CHEM. SOC. 2003, 125, 1033-1040
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A R T I C L E S
White-Morris et al.
Table 1. Crystallographic Data for Compounds 1-3
rophosphate ions helps to overcome the inherent Coulombic
repulsion between cations to produce extended columns of
cations with an Au‚‚‚Au separation of only 3.1882(1) Å. These
crystals are strongly luminescent, but when dissolved in solution,
the salt is no longer luminescent. However, aggregation of these
cations occurs upon freezing of the solvents and produces some
notable variations in the luminescence that are readily detected
by the human eye. Thus, the frozen acetonitrile solution produces
a green-yellow luminescence, while in dimethyl sulfoxide and
pyridine the emission is different shades of blue, and with
acetone it is orange. However, in frozen dimethylformamide
no luminescence is observed from this complex. These variations
in luminescence have been ascribed to the formation of
aggregates of cations which self-associate through aurophilic
attractions. With other anions the nature of the self-association
differs,19 and discrete dimers of [Au{C(NHCH3)2)2]+ have been
isolated.20
However, such self-association through aurophilic interactions
appears to be lacking in some related salts. Specifically, a study
by Schmidbaur and co-workers on the colorless bis(isocyanide)-
gold(I) complexes, [(RNC)2AuI]X, with R ) methyl, phenyl,
and mesityl and X ) triflate or tetrafluoroborate, found that
supramolecular aggregation is observed only with the smallest
alkyl substituent.21 Moreover, the Au‚‚‚Au interaction in
[(CH3NC)2AuI](O3SCF3) is quite long, 3.624 Å. In view of the
unusual spectroscopic studies of the gold carbene complexes
and the lack of aurophilic attractions in the isocyanide gold
cations, we became interested in the observation of a yellow
gold complex which was reported to form during the preparation
of [(C6H11NC)2AuI](PF6).22
I
I
[(C H NC)2Au](PF )
[(MeNC)2Au](PF )
6
11
6
6
colorless polymorph
yellow polymorph
color/habit
formula
fw
a, Å
b, Å
colorless needle yellow plate
C14H22AuF6N2P C14H22AuF6N2P C4H6AuF6N2P
colorless block
560.27
6.3644(5)
16.9806(15)
16.7224(13)
90
92.693(3)
90
1805.2(3)
4
560.27
11.5235(7)
24.1416(15)
26.0516(16)
90
424.04
9.8097(7)
9.8097(7)
9.8097(7)
90
c, Å
R, deg
â, deg
90
90
90
90
γ, deg
V, Å3
7247.4(8)
16
orthorhombic
P212121
91(2)
0.710 73
2.054
8.264
943.99(12)
4
Z
cryst syst
space group
T, °C
monoclinic
P21/c
cubic
Pa
90(2)
0.710 73
2.984
15.809
0.036
0.075
91(2)
0.710 73
2.061
8.294
0.019
λ, Å
F, g/cm3
µ, cm-1
R1 (obsd data)a
wR2a (all data,
F2 refinement)
0.055
0.123
0.053
a R1 ) ∑[||Fo| - |Fc||]/∑|Fc|; wR2 ) ∑[w(F -F 2)2]/∑[w(F )2].
2
2
x
o
c
o
Results
The two forms of [(C6H11NC)2AuI](PF6) (colorless and
yellow) were synthesized using previously reported methods.22
Yellow crystals suitable for X-ray diffraction were taken directly
from the reaction mixture. The colorless polymorph was
obtained by dissolving the yellow form in hot 1-propanol and
allowing the solution to slowly cool to room temperature and
to gradually evaporate. Crystallographic data for the two
polymorphs are given in Table 1. These data indicate that the
yellow and colorless forms of [(C6H11NC)2AuI](PF6) are true
polymorphs (different crystalline forms of the same molecule
or in this case salt),23,24 and neither involves the inclusion of
any solvent molecules into the solid.
Crystal Structure of the Colorless Polymorph,
[(C6H11NC)2AuI](PF6). A perspective view of a portion of the
structure of the colorless polymorph of [(C6H11NC)2AuI](PF6)
is shown in Figure 1. Selected bond lengths and angles are given
in the figure caption. The asymmetric unit of the colorless
polymorph consists of two half-cations with the gold atoms
located at centers of symmetry and one anion in a general
position. Since each gold atom is located on a center of
symmetry, the two isocyanide ligands coordinate gold in a linear
fashion. In the cation containing Au1, the isocyano group is
Figure 1. View of a chain of cations within the colorless polymorph of
[(C6H11NC)2AuI](PF6). The Au1‚‚‚Au2 distance is 3.1822(3) Å (1/2 of a).
Anions are deleted for clarity. Selected distances: Au1-C1, 1.973(2); Au2-
C8, 1.975(2); C1-N1, 1.134(3); C8-N2, 1.136(3) Å. Selected angles:
Au1-Au2-Au1′, Au2-Au1-Au2′, 180; Au1-C1-N1, 178.3(2); Au2-
C8-N2, 177.6(2)°.
located in an axial site on the cyclohexyl ring, while in the cation
containing Au2, the isocyano group resides in an equatorial
position in the cyclohexyl ring. The gold cations aggregate into
an infinite linear chain along the crystallographic a axis. Within
these chains, the Au‚‚‚Au contact (3.1822(3) Å) is indicative
of an aurophilic attraction between cations. Adjacent cations
alternate in orientation and form a semi-staggered array with a
dihedral C1-Au1-Au2-C8 angle of 65.9°.
Figure 2 shows a stereoscopic view of two chains of the
cations and allows the reader to see the location of the anions.
The closest approach of a fluorine atom of a hexafluorophos-
phate ion to a gold atom is 3.070 Å for Au1‚‚‚F4. This distance
is similar to the sum of the van der Waals radii for Au and F
(3.13 Å).
(20) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Balch, A. L. Inorg. Chem.
2002, 41, 2313.
(21) Schneider, W.; Sladek, A.; Bauer, A.; Angermaier, K.; Schmidbaur, H. Z.
Naturforsch. B 1997, 52, 53.
(22) Parks, J. E.; Balch, A. L. J. Organomet. Chem. 1974, 71, 453.
(23) Dunitz, J. D.; Bernstein, J. Acc. Chem. Res. 1995, 28, 193.
(24) Braga, D.; Grepioni, F. Chem. Soc. ReV. 2000, 29, 229.
Crystal Structure of the Yellow Polymorph,
[(C6H11NC)2AuI](PF6). Figure 3 shows a drawing of a portion
of the structure of the yellow polymorph of [(C6H11NC)2AuI]-
(PF6). This structure is more complicated than that of the
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Aurophilic Interactions in Cationic Gold Complexes
A R T I C L E S
Figure 2. Stereoview of a two chains of gold ions and the hexafluorophosphate ions in the colorless polymorph of [(C6H11NC)2AuI](PF6). Atom colors are
as follows: gold, orange; fluorine, green; phosphorus, red; carbon, gray; hydrogen, purple; nitrogen, blue.
anions in the asymmetric unit. Each of the four individual cations
involves a central gold ion which is coordinated in a linear
fashion by the two isocyanide ligands. The isocyano groups in
the cations containing Au1 and Au3 are in axial positions in
the cyclohexyl rings, while the isocyano groups in the cations
containing Au2 and Au4 are in equatorial positions of the
cyclohexyl rings. These positions are probably determined by
crystal packing forces. The cations form infinite chains through
aurophilic interactions. Within these kinked chains, the cations
stack with the ligands in semi-staggered fashions. Although
bulky cyclohexyl groups are present, the longest Au‚‚‚Au
distance (2.9803(6) Å) is on the short end of aurophilic
attractions and is much shorter than the corresponding distances
in the colorless polymorph (3.1822(3) Å) and in [Au-
{C(NHCH3)2}2](PF6)‚0.5acetone (3.1882(1) Å).19 The other
three Au‚‚‚Au distances (2.9790(6), 2.9651(6), and 2.9643(6)
Å) approach the Au‚‚‚Au distance in metallic gold (2.889(1)
Å). The chain of cations is bent, with Au‚‚‚Au‚‚‚Au angles of
173.313(18), 152.129(17), 169.652(18), and 153.637(18)°. All
eight independent cyclohexyl rings adopt the chair conformation.
The anions are nestled in space between the staggered
cyclohexyl isocyanide ligands. Three of the four hexafluoro-
phosphate anions are disordered with two sets of alternate
positions for the fluorine atoms. Figure 4 shows the positioning
of the most prevalent of the hexafluorophosphate sites within
the kinked chains of gold atoms. The closest approach that a
hexafluorophosphate ion makes with the cation involves a
fluorine to gold contact (Au3‚‚‚F7, 3.152 Å) whose magnitude
is close to the sum of the van der Waals radii (3.13 Å) for these
two atoms. The other close anion/cation interactions involve
short F‚‚‚H contacts.
The yellow polymorph of [(C6H11NC)2AuI](PF6) crystallizes
in the chiral space group P212121. As a consequence, the chains
of gold atoms form a helical ribbon as shown in part B of Figure
3. However, the structure itself was refined as a racemic twin
with a Flack twin parameter of 0.5. Consequently due to the
twinning, the crystal consists of a racemic mixture of the two
enantiomers of the helical chains.
Figure 3. (A) View of a chain of cations within the yellow polymorph of
[(C6H11NC)2AuI](PF6). The Au‚‚‚Au distances are as follows: Au1-Au2,
2.9803(6); Au1-Au4′, 2.9790(6); Au2-Au3, 2.9651(6); and Au3-Au4,
2.9643(6) Å. Anions are deleted for clarity. Other selected distances: Au1-
C1, 1.974(9); Au1-C8, 2.007(9); Au2-C15, 1.993(9); Au2-C22, 1.972(10);
Au3-C29, 1.965(11); Au3-C36, 1.985(11); Au4-C50, 1.961(10); Au4-
C43, 1.986(9); C1-N1, 1.136(13); C8-N2, 1.108(13); C15-N3, 1.123-
(11); C22-N4, 1.140(12); C29-N5, 1.145(14); C36-N6, 1.129(14); C43-
N7, 1.131(12); C50-N8, 1.173(12) Å. Selected angles: Au4′-Au1-Au2,
173.313(18); Au1-Au2-Au3, 152.129(17); Au2-Au3-Au4, 169.652(18);
Au1′-Au4-Au3, 153.637(18); C1-Au1-C8 177.8(4); C15-Au2-C22,
176.8(4); C29-Au3-C36, 179.8(5); C43-Au4-C50, 178.0(4); Au1-C1-
N1, 176.0(10); Au1-C8-N2, 171.4(9); Au2-C15-N3, 175.2(9); Au2-
C22-N4, 174.9(10); Au3-C29-N5, 178.7(11); Au3-C36-N6, 174.2(11);
Au4-C43-N7, 175.6(9); Au4-C50-N8, 177.1(9). (B) View down a chain
of gold ions (with all other atoms deleted) which emphasizes the helical
nature of the chain.
Crystal Structure of Colorless [(CH3NC)2AuI](PF6). To see
whether the hexafluorophosphate ion is responsible in some
fashion for the close approach of the Au(I) centers in the
polymorphs of [(C6H11NC)2Au](PF6), the structure of [(CH3-
colorless polymorph, since it contains four cations and four
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A R T I C L E S
White-Morris et al.
Figure 4. Stereoview of a chain of cations and the hexafluorophosphate ions in the yellow polymorph of [(C6H11NC)2AuI](PF6). Atom colors are as
follows: gold, orange; fluorine, green; phosphorus, red; carbon, gray; hydrogen, purple; nitrogen, blue.
Figure 5. View of the unit cell of [(CH3NC)2AuI](PF6). The shortest gold/gold separation is 6.9365(5) Å, and the closest Au/F contact is 3.411(4) Å.
Selected distances: Au1-C1, 1.987(12); C1-N1, 1.128(16) Å. Selected angles: C1-Au-C′, 180; Au-C1-N1, 180°.
NC)2AuI](PF6) was determined to provide a comparison with
the previously obtained structure of [(CH3NC)2AuI](O3SCF3).21
A view of the unit cell of [(CH3NC)2AuI](PF6) is shown in
Figure 5. Each cation has crystallographic 3h symmetry with the
crystallographic axis passing through the CNCAuCNC portion
of the cation. Dimensions within the cation are entirely normal.
The closest approach of two neighboring AuI centers is
6.9365(5) Å, which is much longer than the closest Au‚‚‚Au
contact (3.624 Å) in [(CH3NC)2AuI](O3SCF3) and far too long
for any aurophilic interaction. Moreover, in [(CH3NC)2AuI](PF6)
each cation is surrounded by six hexafluorophosphate ions with
the shortest Au/F contact being 3.411 (4) Å, which is longer
than the sum of the van der Waals radii (3.13 Å) of gold and
fluorine. Thus, the hexafluorophosphate ion alone does not
promote aurophilic interactions.
polymorphs. Indeed these spectra are also virtually identical
to the spectrum reported for [(EtNC)2AuI](ClO4) earlier.25
The absorption and magnetic circular dichroism spectra for
[(EtNC)2AuI](ClO4) have been interpreted as resulting from
metal-to-ligand charge-transfer states that involve excitation
from the filled gold dz orbital to a π* orbital of the ligand.25
2
Since the identity of the alkyl substituent on the isocyanide has
little effect on the π* orbital, which is localized within the
isocyano function, the absorption spectra of the different
alkylisocyanide complexes of the type [(RNC)2AuI]+ are similar.
Thus, the absorption spectrum of [(CH3NC)2AuI](PF6) is also
similar to that of [(C6H11NC)2AuI](PF6) with corresponding
absorption maxima at 246 and 214 nm. At room temperature,
solutions of [(C6H11NC)2AuI](PF6) or [(CH3NC)2AuI](PF6) are
nonluminescent.
Absorption and Emission Spectra of [(C6H11NC)2AuI](PF6)
and [(CH3NC)2AuI](PF6). As seen in part A of Figure 6, the
absorption spectra for acetonitrile solutions of the two forms
of [(C6H11NC)2AuI](PF6) are identical, with prominent λmax at
244 and 216 nm. Thus, the absorption data show that the
differences between the two forms do not persist in solution
and are consistent with the proposition that these salts are truly
However, each of the two crystalline polymorphs displays a
distinct luminescence spectrum. Relevant data are shown in parts
B and C of Figure 6. Visually, the colorless polymorph produces
a bluish-white emission. Its emission spectrum at 298 K displays
a maximum at 424 nm with an excitation maximum at 353 nm.
(25) Chastain, S. K.; Mason, W. R. Inorg. Chem. 1982, 21, 3717.
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Aurophilic Interactions in Cationic Gold Complexes
A R T I C L E S
Figure 6. (A) The absorption spectra of acetonitrile solutions of the yellow
(lower) and the colorless (upper, offset by 0.4 absorbance units) polymorphs
of [(C6H11NC)2AuI](PF6). The emission (right side) and excitation (left side)
spectra of (B) the colorless and (C) the yellow polymorphs of [(C6H11-
NC)2AuI](PF6).
The yellow polymorph visually produces a yellow-green
luminescence. The emission spectrum shows emission at a
longer wavelength (480 nm) than the colorless polymorph and
an excitation profile with a peak at 394 nm. The yellow
polymorph, as expected from its color, has an excitation
maximum at lower energy (394 nm) than does the colorless
polymorph (353 nm). Notice, however, that for each of the two
polymorphs the onset of the excitation profile occurs at a longer
wavelength than that seen in the absorption spectrum (part A
of Figure 6). Consequently, the luminescence must result from
the extended Au‚‚‚Au interactions between cations in their solid-
state supramolecular structures. The emission lifetimes of the
two polymorphs are similar. The yellow polymorph shows a
long-lived emission with a lifetime of 0.73 µs, while the lifetime
for the emission from the colorless polymorph is 0.55 µs. These
lifetimes suggest that the emission is due to phosphorescence.
In contrast to the behavior of the polymorphs of [(C6H11-
NC)2AuI](PF6), crystalline [(CH3NC)2AuI](PF6), which lacks
aurophilic interactions, is not luminescent. Hence, it provides
further confirmation of the connection between Au‚‚‚Au interac-
tions and the luminescence of these gold(I) complexes.
The absorption spectra of both crystalline polymorphs have
been examined in KBr wafers. The colorless polymorph displays
an absorption maxima at 340 nm, which is similar to the
excitation maxima at 353 nm seen in Figure 5. The yellow
polymorph has an absorption maxima at 400 nm, which
corresponds to the excitation maximum at 394 nm. The
absorption maxima at 340 nm in the white polymorph and at
394 nm in the yellow polymorph are new features of the solid
which do not have counterparts in the solution spectra shown
Figure 7. Emission (right) and excitation (left) spectra of frozen 6.0 mM
solutions of [(C6H11NC)2AuI](PF6) in various solvents at 77 K. The
wavelength used to monitor the excitation profile is given below each
excitation spectrum, and the wavelength used for excitation is shown below
each emission spectrum.
in Figure 6. These additional features are believed to originate
in the extended Au‚‚‚Au interactions and probably involve
2
promotion of an electron from the filled 5dz band to the empty
6pz band where the z axis is collinear with the Au‚‚‚Au stacking
direction.
While both polymorphs of [(C6H11NC)2AuI](PF6) dissolve in
a variety of organic solvents to give colorless, nonluminescent
solutions, freezing these solutions does produce strong lumi-
nescence. Moreover, the emission spectra obtained from these
frozen solutions vary as the solvent is changed. Visually the
effect is not as striking as it is in the case of [Au{C(NHCH3)2}2]-
(PF6), but the emissions from these frozen solutions do show
significant alterations, as seen in spectra shown in Figures 7
and 8.
Figure 7 presents emission and excitation spectra for 6.0 mM
solutions of [(C6H11NC)2AuI](PF6) in a variety of solvents which
have been frozen at 77 K. The emission spectra from the frozen
chloroform and dichloromethane solutions are similar with
9
J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003 1037

A R T I C L E S
White-Morris et al.
Infrared Spectroscopy. The infrared spectra of the two
polymorphs, each dispersed in a KBr wafer, have been
examined. As might be expected, the two spectra are extremely
similar. However, there are differences in the isocyanide
stretching region. The colorless polymorph shows two bands
in the ν(CN) region at 2273 and 2256 cm-1, while the yellow
polymorph shows a single band at 2257 cm-1
.
Thermal Behavior of the Polymorphs of [(C6H11NC)2AuI]-
(PF6). Heating the crystalline colorless polymorph results in
melting over the range 115-120 °C to give a colorless melt.
This melt is luminescent and produces a bluish-white emission
that is visually similar to that from the crystalline colorless
polymorph. Cooling of this melt, however, produces a lumi-
nescent yellow solid. The emission and excitation spectra of
this solid are virtually identical to that of the yellow polymorph.
Similar heating of the yellow polymorph reveals melting over
the range 110-115 °C to produce a colorless melt which also
produces a bluish-white emission. Cooling this melt produces
a yellow solid with an emission and excitation spectrum
indicative of the re-formation of the yellow polymorph. Since
the colorless polymorph is transformed into the yellow poly-
morph in these experiments, the yellow polymorph must be the
thermodynamically more stable form.
Discussion
The results presented here demonstrate that [(C6H11NC)2AuI]-
(PF6) exists in two polymorphic forms each of which shows
strong aurophilic attractions among the cations. The strong
aurophilic attractions seen in both polymorphs contrast markedly
with the structures of three other members of the [(RNC)2AuI]X
family, [(CH3NC)2AuI](PF6), [(C6H5NC)2AuI](BF4), and [2,4,6-
(CH3)3C6H2NC)2AuI](BF4), which have no aurophilic interac-
tions, and [(CH3NC)2AuI](O3SCF3), which has only a weak
interaction between cations (Au‚‚‚Au distances of 3.611 and
3.624 Å).21 Although both forms of [(C6H11NC)2AuI](PF6)
involve the formation of extended chains of cations, the Au‚‚‚Au
separation in the yellow polymorph is significantly shorter than
it is in the white polymorph. Additionally, the chain of cations
in the yellow polymorph is slightly kinked into a helix, while
it is crystallographically required to be linear in the white
polymorph. To our knowledge, this represents the first case of
polymorphism in gold(I) chain complexes where the polymorphs
differ in the nature of the aurophilic attraction. However, since
aurophilic interactions are relatively weak and can span such a
wide range of Au‚‚‚Au distances, we suspect that further
example of polymorphism involving varying degrees of auro-
philic attraction will be found.
A closely related case of polymorphism in gold complexes
involves the exceptionally interesting studies of Bowmaker and
co-workers on {(C6H11)3P}2AuBr.26 This compound crystallizes
as three polymorphs which differ not in the approach of one
gold center to another but in the approach of the bromide ion
to the gold center. Thus in the R form, the cation is linear and
the bromide ion is far from the gold at a distance of 3.764(4)
Å. In the â form there are two molecules: one with a Au-Br
distance of 2.894(1) Å and a slightly bent P-Au-P (162.06(9)°)
portion; the other with a shorter Au-Br distance of 2.842(1) Å
and a somewhat greater bend (157.71°) to the P-Au-P portion.
Figure 8. Emission (right) and excitation (left) spectra of frozen 0.60 mM
solutions of [(C6H11NC)2AuI](PF6) in various solvents at 77 K. The
wavelength used to monitor the excitation profile is given below each
excitation spectrum, and the wavelength used for excitation is shown below
each emission spectrum.
emission maxima at 380 and ca. 450 nm. The emission spectra
from frozen acetone and acetonitrile solutions are also similar
with a strong emission near 470 nm and a much weaker shoulder
near 410 nm. The spectra obtained from frozen methanol
solutions are quite different with emissions at 535 and 482 nm.
The relative intensities of these two emission bands depend on
the excitation wavelength with excitation at longer wavelengths
producing enhanced emission at 535 nm.
Dilution of the solutions produces significant changes in the
luminescence from some solutions. Figure 8 presents emission
and excitation spectra for frozen 0.60 mM solutions of [(C6H11-
NC)2AuI](PF6) at 77 K. The spectra obtained from chloroform
and dichloromethane solutions are grossly similar to those shown
in Figure 7. However, there are small changes in the band
positions and in their relative intensities. The emission spectrum
from the frozen acetonitrile solution is similar to that of the 6.0
mM solution seen in Figure 7 except for the increased intensity
of the short wavelength band which is now at 392 nm. While
the emission spectrum obtained from a 6.0 mM solution of
[(C6H11NC)2AuI](PF6) in acetone resembled that of the corre-
sponding spectrum obtained from frozen acetonitrile solution,
no emission was detected from the frozen 0.60 mM acetone
solution of [(C6H11NC)2AuI](PF6). The emission from the frozen
methanol solution seen in Figure 8 is markedly different from
the spectra obtained from the more concentrated solution seen
in Figure 7. The long wavelength emission at 535 nm is absent
from the spectrum seen in Figure 8. The spectrum now shows
a band at 476 nm which is similar to that seen in Figure 7.
(26) Bowmaker, G. A.; Brown, C. L.; Hart, R. D.; Healy, P. C.; Rickard, C. E.
F.; White, A. H. J. Chem. Soc., Dalton Trans. 1999, 881.
9
1038 J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003

Aurophilic Interactions in Cationic Gold Complexes
A R T I C L E S
In the γ polymorph the Au-Br distance is shorter yet (2.777(1)
Å) and the P-Au-P portion is further bent (147.5(1)°). In
contrast, although the colorless and yellow polymorphs of
[(C6H11NC)2AuI](PF6) show significant differences in the
environment of the gold ions, there is no bending of the
C-Au-C portion of the cation that occurs as a result of the
aurophilic attraction.
The structural variations seen in the polymorphs of [(C6H11-
NC)2AuI](PF6) and {(C6H11)3P}2AuBr are related to the concept
of bond stretch isomers: molecules that “differ only in the length
of one or several bonds”.27 Several examples of bond stretch
isomers have been identified,28 and theoretical analyses of the
phenomenon have been presented.29 With regard to the varia-
tions seen in the Au‚‚‚Au contacts in the yellow and colorless
polymorphs of [(C6H11NC)2AuI](PF6), these do not constitute
bond stretch isomers in the strict sense of the definition, since
the Au‚‚‚Au interactions do not exist within a molecule or
molecular ion but rather involve interionic interactions that do
not persist in dilute solution. Nevertheless, significant differences
must exist in the interionic interactions in the yellow and
colorless polymorphs.
Two colorless polymorphs of {(C6H5)3As}AuCl have been
reported.30 These differ in the torsional angles of the phenyl
groups and in molecular packing but do not have close Au‚‚‚
Au contacts. The shortest Au‚‚‚Au separations are 5.916(1) Å
for the needle modification or 6.913(1) Å for the prismatic
modification. Both forms are luminescent, but the spectra of
the two forms show only minor differences.31 The luminescence
arises from the triphenylarsine ligand π-π states not from the
metal centers.
Usually, the aurophilic interactions between monomeric, two-
coordinate gold(I) complexes lead to simple dimerization rather
than the formation of extended chains in which each gold center
interacts with two neighbors. However, in the solid state
{(Me2S)AuCl}n forms a bent chain structure with an Au‚‚‚Au
distance of 3.226(1) Å and an Au‚‚‚Au‚‚‚Au angle of 168.1
(1)°.32 Colorless {(2-picoline)AuCl}n forms a similar chain
with an Au‚‚‚Au distance of 3.1960(4) Å and an Au‚‚‚Au‚‚‚Au
angle of 165.079(12)°.33 These chains, which involve neutral
molecules, both have Au‚‚‚Au separations that are longer than
those in either the colorless or the yellow polymorphs of
[(C6H11NC)2AuI](PF6). In the salt (C12H14N2)[AuI2]2, the [AuI2]-
ions self-associate into extended chains along a crystallographic
42 screw axis with an Au‚‚‚Au distance of 3.3767(3) Å.34 As
noted above [Au{C(NHCH3)2}2](PF6) also forms linear columns
along a crystallographic screw axis with a separation of
(3.1882(1) Å).19 In [(3-picoline)2Au][AuCl2], where cations
and anions associate into zigzagged chains, the Au‚‚‚Au
separation is somewhat shorter, 3.1538(12) Å.31 Only in
[(tetrahydrothiophene)2Au][AuI2] are the Au‚‚‚Au distances
(2.967(2) and 2.980(2) Å)35 as short as those found in the yellow
polymorph of [(C6H11NC)2AuI](PF6).
The variation in Au‚‚‚Au interactions within these polymorphs
and within the general class of cations, [(isocyanide)2AuI]+, is
remarkable and reflects the generally weak nature of the
aurophilic attraction. A different but related form of aggregation
is seen in the crystalline forms of {Au(S2CN(C5H11)2}2.16 This
dimer crystallizes as an orange, luminescent form from dimethyl
sulfoxide. In this form, which is a solvate containing dimethyl
sulfoxide, the dimers (with an intramolecular Au‚‚‚Au distance
of 2.7690(7) Å) associate through further intermolecular Au‚‚
‚Au contacts with a separation of 2.9617(7) Å into extended
chains. In contrast, when crystallized from n-propanol/benzene
{Au(S2CN(C5H11)2}2 forms colorless, solvent-free crystals in
which the shortest intermolecular Au‚‚‚Au contact is over 8 Å
long.
Somewhat related phenomena occur in crystalline salts of the
anion [Pt(CN)4]2- which self-associates to form extended
columns with Pt‚‚‚Pt interactions. In numerous salts of this anion
in which different cations and varying amounts of water are
present, the Pt‚‚‚Pt separations cover the wide range from 3.09
to 3.71 Å.36 However, we are unaware of any examples of
polymorphism in [Pt(CN)4]2- salts. Polymorphism does occur
in platinum(II) polypyridine complexes where metal-metal
interactions and π-π stacking of the aromatic ligands influence
the absorption and luminescence of the complexes. For example,
(2,2′-bipyridine)PtIICl2 crystallizes in a yellow, monomeric
form37 and a red form which contains linear chains of platinum
ions with a spacing of 3.402(1) Å.38 Additionally, [(2,2′,6′,2:-
terpyridine)PtII(CtC-CtCH)](O3SCF3) forms a dark-green
polymorph with an extended chain structure with a Pt‚‚‚Pt
separation of 3.388 Å and a red form which consists of a chain
of dimers with an intradimer Pt‚‚‚Pt separation of 3.394 Å and
an interdimer Pt‚‚‚Pt separation of 3.648 Å.39
The luminescence obtained from [(C6H11NC)2AuI](PF6),
whether in the solid state, in the melt, or in frozen solutions,
results from the formation of aggregated species that self-
associate through aurophilic attractions. In solution, where the
ions are dispersed, [(C6H11NC)2AuI](PF6) is nonluminescent.
Additionally, crystalline [(CH3NC)2AuI](PF6) (which has no
aurophilic interactions) and its solutions are nonluminescent.
However, both crystalline forms of [(C6H11NC)2AuI](PF6),
which do show strong aurophilic interactions, are luminescent.
Moreover, as was the case with the [Au{C(NHMe)2}2]+ ion,
the luminescence is dependent upon the structure of the solid
state and the nature of the aurophilic interactions present therein.
Within the chains of gold atoms which are present in the
polymorphs of [(C6H11NC)2AuI](PF6), overlap of the occupied
(27) Parkin, G.; Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1462.
(28) Parkin, G. Acc. Chem. Res. 1992, 25, 455. Cotton, F. A.; Daniels, L. M.;
Jordan, G. T. Chem. Commun. 1997, 421. McGrady, J. E. Angew. Chem.,
Int. Ed. 2000, 39, 3077. Cle´rac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar,
K. R.; Murillo, C. A.; Wang, X. P. Inorg. Chem. 2001, 40, 1256.
(29) Stohrer, W.-D.; Hoffmann, R. J. Am. Chem. Soc. 1972, 94, 779. Stohrer,
W.-D.; Hoffmann, R. J. Am. Chem. Soc. 1972, 94, 1661. Jean, Y.; Lledos,
A.; Burdett, J. K.; Hoffmann, R. J. Am. Chem. Soc. 1988, 110, 4506.
Rohmer, M.-M.; Be´nard, M. J. Am. Chem. Soc. 1998, 120, 9372. Rohmer,
M.-M.; Be´nard, M. Chem. Soc. ReV. 2001, 30, 340
(30) Weissbart, B.; Larson, L. J.; Olmstead, M. M.; Nash, C. P.; Tinti, D. S.
Inorg. Chem. 1995, 34, 393.
(31) Larson, L. J.; McCauley, E. M.; Weissbart, B.; Tinti, D. S. J. Phys. Chem.
1995, 99, 7218.
2
gold 5dz orbitals (where z is the axis along the Au‚‚‚Au‚‚‚Au
chain) produces a filled band of orbitals, while overlap of the
empty gold 6pz orbitals produces a corresponding band of
(35) Ahrland, S.; Nore´n, B.; Oskarsson, A° . Inorg. Chem. 1985, 24, 1330.
(36) Williams, J. M.; Schultz, A. J.; Underhill, A. E.; Carnero, K. In Extended
Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press: New York,
1982; Vol. 1, 73.
(37) Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton Trans. 1974, 1002.
(38) Bielli, E.; Gidney, P. M.; Gillard, R. D.; Heaton, B. T. J. Chem. Soc., Dalton
Trans. 1974, 2133. Connick, W. B.; Henling, L. M.; Marsh, R. E.; Gray,
H. B. Inorg. Chem. 1996, 35, 6261.
(39) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. J. Am. Chem. Soc. 2002, 124,
6506.
(32) Jones, P. G.; Lautner, J. Acta Crystallogr. 1988, C44, 2089.
(33) Jones, P. G.; Ahrens, B. Z. Naturforsch. B 1998, 53, 653.
(34) Tang, Z.; Litvinchuk, A. P.; Lee, H.-G.; Guloy, A. M. Inorg. Chem. 1998,
37, 4752.
9
J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003 1039

A R T I C L E S
White-Morris et al.
Experimental Section
unoccupied orbitals. Excitation of an electron from the filled
5dz band to the empty 6pz band strengthens the bonding along
2
Preparation of the Polymorphs of [(C6H11NC)2AuI](PF6). Crystals
of the yellow and colorless polymorphs of [(C6H11NC)2AuI](PF6) were
obtained by the procedures reported previously.22 A 2 equiv amount
of cyclohexyl isocyanide was added to an aqueous solution of hydrogen
tetrachloroaurate(III) hydrate. After stirring and subsequent filtration,
a saturated aqueous solution of ammonium hexafluorophosphate was
added. A colorless oil and solid formed immediately. However, upon
standing in the aqueous solution, this colorless solid was transformed
into a bright yellow, crystalline solid. This easily reproducible process
begins when small droplets of oil form on the side of the flask as well
as on the surface of the aqueous solution. The formation of this oil
was followed by the formation of patches of the yellow solid. This
process takes approximately 10 min to complete. Crystals of the yellow
polymorph were taken directly from the reaction mixture. Crystals of
the colorless polymorph were obtained through dissolving the yellow
polymorph in hot 1-propanol followed by cooling.
X-ray Crystallography and Data Collection. The crystals were
removed from the glass tubes in which they were grown together with
a small amount of mother liquor and immediately coated with a
hydrocarbon oil on the microscope slide. A suitable crystal of each
compound was mounted on a glass fiber with silicone grease and placed
in the cold stream of a Bruker SMART CCD with graphite monochro-
mated Mo KR radiation at 90(2) K. Check reflections were stable
throughout data collection. Crystal data are given in Table 1.
The structures were solved by direct methods and refined using all
data (based on F2) using the software of SHELXTL 5.1. A semiem-
pirical method utilizing equivalents was employed to correct for
absorption.43 Hydrogen atoms were added geometrically and refined
with a riding model.
these chains by removing what is effectively an antibonding
electron from the 5dz band and transferring that electron to the
2
bonding 6pz band. Emission results from the reverse process,
transfer of an excited electron from the 6pz band back to the
2
5dz band. Similar considerations have been made for related
gold isocyanide complexes.40,41 Additionally, there may be some
mixing of the gold 6pz orbitals with the isocyanide π orbitals,
since this mixing is symmetry allowed.
The variations in the luminescence obtained from frozen
solutions of [(C6H11NC)2AuI](PF6) are likely the result of the
formation of a variety of structurally different aggregates that
form upon freezing of the solvent. The variations seen in the
spectra of solutions with different concentrations of [(C6H11-
NC)2AuI](PF6) indicate that high concentrations of the salt favor
the observation of luminescence and that the solutions eventually
can become nonluminescent if they are diluted enough. During
the freezing process, the concentration of the solute in the
unfrozen solvent increases as the solvent itself crystallizes.42
Under these conditions aggregation of the solute is favored by
freezing. The emission bands in the 430-480 nm range in
Figures 7 and 8 can be assigned to the formation of extended
chain structures which may differ from solvent to solvent in
the Au‚‚‚Au separations, Au‚‚‚Au‚‚‚Au angles, and/or the
relative orientations of the isocyanide ligands. The emission at
shorter wavelength (ca. 380 nm) may be due to dimeric species,
since we have observed that dimeric forms of [Au{C(NHMe)2}2]+
emit at shorter wavelengths than do salts that consist of extended
chains of these cations.19,20 With the emission seen from the
frozen methanol solutions of [(C6H11NC)2AuI](PF6), the band
at 482 nm may originate in a columnar structure very similar
to that of the yellow polymorph. This frozen solution actually
acquires a yellow color upon freezing and the excitation profile
for such a frozen solution is similar to that of the yellow
polymorph. The emission band seen at 535 nm in the frozen
methanol solution may arise from aggregates of [(C6H11-
NC)2AuI](PF6) with even shorter separations than those found
in the two polymorphs reported here. These aggregates may
include trimers, tetramers, pentamers, etc., as well as interacting
chains in which the individual gold centers may have three close
Au‚‚‚Au contacts. Further work on the properties of solutions
of [(C6H11NC)2AuI](PF6) and related gold(I) complexes are in
progress.
Physical Measurements. Electronic absorption spectra were re-
corded using a Hewlett-Packard 8450A diode array spectrophotometer.
Fluorescence excitation and emission spectra were recorded on a Perkin-
Elmer LS50B luminescence spectrophotometer.
Acknowledgment. We thank Prof. D. S. Tinti for measuring
the emission lifetimes, Dr. H. M. Lee for a suggestion, and the
Donors to the Petroleum Research Fund (Grant 37056) for
support. The Bruker SMART 1000 diffractometer was funded
in part by NSF Instrumentation Grant CHE-9808259.
Supporting Information Available: X-ray crystallographic
files for the colorless and yellow polymorphs of [(C6H11NC)2AuI]-
(PF6), and for [(CH3NC)2AuI](PF6) (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA020902V
(40) Harvey, P. D. Coord. Chem. ReV. 2001, 219-221, 17.
(41) Xiao, H.; Chenug, K.-K.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1996,
3699.
(43) Sheldrick, G. M. SADABS 2.0, based on a method of Blessing, R. H. Acta
Crystallogr. 1995, A51, 33.
(42) Pincock, R. E. Acc. Chem. Res. 1969, 2, 97.
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1040 J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003