Dicyanoaurate(I) salts with 1-alkyl-3-methylimidazolium: Luminescent properties and room-temperature liquid forming

Article abstract of DOI:10.1039/b515869a

Room-temperature ionic liquids containing dicyanoaurate(I) anions were prepared utilizing 1-alkyl-3-methylimidazolium cations, and display luminescence possibly due to the oligomerization of the significant fraction of Au(CN) ₂ anions in the li

Full text of DOI:10.1039/b515869a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Dicyanoaurate(I) salts with 1-alkyl-3-methylimidazolium: luminescent
properties and room-temperature liquid forming{
Yukihiro Yoshida,*a Junichi Fujii,a Gunzi Saito,*a Takaaki Hiramatsub and Naoki Satob
Received 8th November 2005, Accepted 5th January 2006
First published as an Advance Article on the web 19th January 2006
DOI: 10.1039/b515869a
Room-temperature ionic liquids containing dicyanoaurate(I)
anions were prepared utilizing 1-alkyl-3-methylimidazolium
cations, and display luminescence possibly due to the oligo-
merization of the significant fraction of Au(CN)2 anions in
the liquids.
Scheme 1 Molecular structures of HMI (R = H, 1), EMI (R = Et, 2),
BMI (R = n-butyl, 3), C6MI (R = hexyl, 4) and C8MI (R = octyl, 5).
Gold(I) systems have been the subject of intense studies aimed at
understanding fundamental chemistry issues such as supramole-
cular chemistry,1 as well as their potential applicability such as
negligible volatility and extremely high concentration (>1 M). In
optical- or bio-sensors, and photocatalysts.2 It is currently well-
this study, we prepared five Au(CN)2 salts of 1-alkyl-3-methyl-
imidazolium cations with different alkyl chain lengths: proton
established that gold(I) complexes aggregate through the attractive
(HMI, 1), ethyl (EMI, 2), n-butyl (BMI, 3), hexyl (C6MI, 4) and
interactions between closed-shell gold(I) centers (so-called
octyl (C8MI, 5) (Scheme 1), and characterized their thermal,
‘‘aurophilicity’’ or ‘‘aurophilic bonding’’) with separations less
3
luminescent and structural properties.
˚
than 3.6 A. Experimental studies have shown that the attractive
interaction is comparable to hydrogen bonding (7–11 kcal mol21).4
Recent reports of [(C6H11NC)2Au](PF6) polymorphs with different
luminescences,5 and K[Au(CN)2] solutions with tunable lumines-
cence depending upon temperature, concentration and solvent6
have particularly stimulated the fundamental studies associated
with the structure–luminescence relationship. For Au(CN)2 salts,
which are among the most stable two-coordinated gold(I)
complexes, thermal studies have revealed that the luminescence
energy of crystalline M[Au(CN)2] (M = K and Tl) increases as the
Salts 1–5 were prepared by the metathesis of each 1-alkyl-3-
methylimidazolium chloride and K[Au(CN)2] in dry acetone.
While 1 and 2 were purified by recrystallization from ethyl acetate,
salts 3–5 were purified by washing with acetone and dichloro-
methane–H2O.{ No trace of Cl and K was detected via energy-
dispersive X-ray spectroscopy (EDS).
Several characteristic values of 1–5 are summarized in Table 1.
At room temperature (RT), salts 1 and 2 have a colorless
crystalline form, and show a melting event at 103 and 64 uC,
respectively. On the other hand, salts 3–5 with expanded alkyl
chains are pale yellow viscous liquids at RT, i.e., these are the first
RT liquids containing Au(CN)2 anions. Their quenched phases
are glass forming, and the glass transition temperatures measured
upon heating lie in the range between 270 and 259 uC. The
absence of crystallization and melting events in 5 is closely
related to the further expanded chain, which leads to increased
molecular reorientation energy for crystallization. All the salts
prepared in this study except for 1 decompose at significantly high
temperatures (300–340 uC), providing a wide liquid range of more
than 270 uC.
7
8
…
Au Au distance elongates. With heterocyclic organic cations,
on the other hand, no obvious relationship has been observed for
the Au(CN)2 salts with infinite anionic chains, presumably due to
the presence of hydrogen bonds between the ligand amino group
(or water molecule included) and the Au(CN)2 anion. The
hydrogen bonding insures the close proximity of the cations to
the anions, and helps to minimize the Coulombic repulsion
between the anions. Then the weakened hydrogen bonding can
…
spur a wide variety of Au Au separations and assembly patterns
via varied aurophilic attractions.
1,3-Dialkylimidazolium cations (Scheme 1), which have received
much attention due to their ability to form stable liquid states
(so-called ‘‘ionic liquids’’9), are particularly prone to reducing
interionic interactions. Considering the fact that [Au(CN)2]n
oligomers larger than dimers can display exciplex emission, a
possible liquid involving Au(CN)2 anions might possess great
potential for photochemical applications in connection with their
Fig. 1 shows a portion of the crystal structure of 1.{ The almost
linear Au(CN)2 anions associate to form a dimer in an eclipsed
Table 1 Several characteristic values of 1–5a
Cation Tg/uC Tc/uC Tm/uC Td/uC db/g cm23 Cb/mol cm23
e
e
e
e
1
2
3
4
5
a
HMI
EMI
BMI
103
64
ca. 160 2.537c
ca. 340 2.261c
ca. 320 1.87d
ca. 300 1.65d
ca. 300 1.59d
7.64 6 1023
6.28 6 1023
4.82 6 1023
3.96 6 1023
3.58 6 1023
aDivision of Chemistry, Graduate School of Science, Kyoto University,
Sakyo-ku, Kyoto, 606-8502, Japan.
259
25
11
13
C6MI 270 217
C8MI 261
e
e
E-mail: yoshiday@kuchem.kyoto-u.ac.jp; saito@kuchem.kyoto-u.ac.jp;
Fax: +81 75 753 4036; Tel: +81 75 753 4036
Tg = glass transition temperature, Tc = crystallization temperature,
Tm = melting point, Td = decomposition temperature, d = density,
C = molar concentration. At 20 uC. By structural refinement.
By pycnometer. Not observed.
bInstitute for Chemical Research, Kyoto University, Uji, 611-0011,
Japan
b
c
d
e
{ Electronic supplementary information (ESI) available: syntheses,
characterizations and structural details. See DOI: 10.1039/b515869a
724 | J. Mater. Chem., 2006, 16, 724–727
This journal is ß The Royal Society of Chemistry 2006

View Article Online
…
…
Au Au Au angle of 170.93u. Additionally, there is an interionic
…
C–H N contact between the cation and one of the cyano groups
…
in the anion. The C2 N12 distance of 3.26 A is comparable to
˚
10
the corresponding vdW distance (3.25 A ). Similar anionic
˚
chains have been crystallographically observed for Au(CN)2
…
salts with several heterocyclic cations, but their Au Au distances
˚
are significantly shorter than the vdW distance; i.e., 3.25 A
for 2-methylimidazolium,8a 3.0969(3)
for piperidinium,8c
˚
A
8c
3.0795(4) A for pyrrolidinium, 3.0886(4) A for diphenylhydra-
˚
˚
Fig. 1 One-dimensional infinite chain composed of HMI cations and
Au(CN)2 dimers in 1. The 50% probability thermal ellipsoids are shown.
zinium,8c and 3.247(2) A for 4-amide-3,5-diethyl-1,2,4-triazolium.
8e
˚
All of these salts include the significant hydrogen bonding-type
interactions between amino groups in cation (or water solvent) and
…
…
Dotted lines represent short interionic contacts of C2–H N11 (3.25 A),
˚
…
…
˚
N1–H N12 (2.83 A) or Au Au (3.553 A).
˚
Au(CN)2 anion. It then seems that the reduced cation anion
…
interactions in 2 give rise to an expansion of the Au Au
…
separation, which leads to the rather low melting point.
fashion and the gold centers are connected through an Au Au
˚
Visually, colorless crystals 1 and 2 display an intense blue
emission. Their emission and excitation spectra at RT are shown in
Fig. 3(a) and (b), respectively. For 1 with the dimeric structure,
there is a symmetrical emission band with a maximum at 443 nm,
while the excitation profile shows a maximum at 325 nm. The
emission spectrum of 2 with the infinite chain structure also shows
a symmetrical band at a shorter wavelength (401 nm) than those
for salts involving the Au(CN)2 chain structure, i.e., 436 nm for
piperidinium,8c 451 nm for pyrrolidinium,8c and 459 nm for
4-amide-3,5-diethyl-1,2,4-triazolium.8e Only the diphenylhydrazi-
nium salt has a comparable emission energy (400 nm).8c It is then
likely that the increased emission energy is associated with the
contact of 3.553 A, which lies in the range where aurophilic
attractions occur.3 Each cyano group in an Au(CN)2 anion is
˚
hydrogen-bonded to a cation with the distance being 3.25 A for
10
N11 C2 (the sum of van der Waals (vdW) radii is 3.25 A ) or
…
˚
10
…
˚
˚
2.83 A for N12 N1 (the sum of vdW radii is 3.10 A ). As a
consequence, a pattern of alternating cations and anions produces
a one-dimensional infinite chain, which runs diagonally along
the unit cell. The closest distance between the gold(I) ions in
˚
different chains is greater than 4.9 A. The structure resembles
those of [2,29-(1,2-ethanediyl)bis(4,5-dihydro-1H-imidazolinium)]-
[Au(CN)2]2 and [2,29-(1,4-phenylene)bis(1,4,5,6-tetrahydropyr-
imidinium)][Au(CN)2]2,8d in which each Au(CN)2 dimer with an
…
expanded Au Au separation. These emission spectra remain
…
˚
Au Au distance of 3.33 and 4.22 A, respectively, is connected
unchanged when the excitation wavelength is altered.
with each other through multiple hydrogen-bonding with bis-
…
The observation that the emission energy of dimeric 1 (443 nm)
is significantly low in comparison with that of polymeric 2
amidinium cations. The relatively long Au Au distance in the
three dimeric structures is in good conformity with a database
analysis that shows longer contacts for the eclipsed orientation
than for the staggered one,11 and is associated with Coulombic
repulsion between negatively charged cyano groups.
Crystallographic study of 2 reveals that the almost linear
Au(CN)2 anions aggregate into a one-dimensional infinite chain
along the a-axis via aurophilic interactions, in contrast to dimeric
…
˚
form 1, with an Au Au distance of 3.347 A (Fig. 2).{ The
uniformly spaced neighboring anions have a staggered orientation
with a dihedral angle of 66.3u, and the chain is slightly bent with an
Fig. 3 Emission (right) and excitation (left) spectra of 1 (a, lex = 330 nm,
lem = 440 nm), 2 (b, lex = 350 nm, lem = 400 nm), 3 (c, lex = 350 nm,
Fig. 2 One-dimensional infinite Au(CN)2 chain in 2. The 50%
probability thermal ellipsoids are shown. Dotted lines represent short
…
…
˚
˚
lem = 470 nm), 4 (d, lex = 320 nm, lem = 450 nm) and 5 (e, lex
=
interionic contacts of C2 N12 (3.26 A) or Au Au (3.347 A). EMI
cations connected with upper and lower Au(CN)2 anions in the figure are
omitted for clarity.
330 nm, lem = 450 nm) at RT, and frozen 5 after immersion in liquid
nitrogen (f, lex = 340 nm, lem = 440 nm).
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 724–727 | 725

View Article Online
(401 nm) contrasts markedly with the expectation that the dimeric
[Au(CN)2]2 has a larger HOMO–LUMO gap than that of the
polymeric [Au(CN)2]‘6a,b This peculiar feature might be related to
the formation of side-by-side infinite chains composed of Au(CN)2
dimers via HMI cations through hydrogen bonds. A similar
result was obtained for [2,29-(1,2-ethanediyl)bis(4,5-dihydro-1H-
imidazolinium)][Au(CN)2]2, which contains adjacent Au(CN)2
luminescent at RT regardless of their aggregation forms strongly
suggests that a significant fraction of Au(CN)2 anions assemble
to form ion pairs and/or ion aggregation in the liquids.
Understanding the oligomeric species should permit the lumines-
cence feature seen in this work to be developed into a non-volatile
luminescent liquid.
This work was in part supported by a COE Research
on Elements Science (No. 12CE2005) and a Grant-in-Aid
(21st Century COE program on Kyoto University Alliance for
Chemistry) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. The authors also acknowledge the
financial support from the Grant-in-Aid for Scientific Research
(No. 15205019) by JSPS.
…
˚
dimers with an Au Au distance of 3.33 A connected through
the dications, and displays an emission band with a maximum at
430 nm.8d
Liquids 3–5 also display an emission at RT (blue–green for 3,
and blue for 4 and 5), but their intensities are significantly weak in
comparison with those of crystals 1 and 2, by a factor of 102–103.
A previous report on luminescent RT ionic liquid was limited to
the (CF3SO2)2N salt of the PAMAM dendrimer,12 whereas there
have been several luminescent behavior studies of pyrene and its
derivatives13 or lanthanide compounds14 dissolved in non-
luminescent ionic liquids. Fig. 3(c)–(e) show the emission and
excitation spectra of the three kinds of liquids at RT. In contrast to
the spectra of crystals 1 and 2, there are two distinct emissions,
one with a maximum at 376–390 nm and the other at around
450–470 nm. The relative intensities of these two emission bands
depend on the excitation wavelength. The spectra consisting of two
distinct bands resemble those obtained for K[Au(CN)2] in
methanol frozen solution at 77 K,6c and [(C6H11NC)2Au](PF6)5
and [{C(NHMe)2}2Au](PF6)?0.5(acetone)15 frozen solutions of a
variety of organic solvents at 77 K. Systematic variation in
concentration of the K[Au(CN)2] solution reveals that the high
energy band is readily assigned to oligomeric species including a
small number of Au(CN)2 anions such as dimer, trimer and
tetramer, since the band definitely weakens as the concentration
increases. In contrast, the low energy band might originate from
the larger [Au(CN)2]n oligomers, since extended Hu¨ckel calcula-
tions predict that the HOMO–LUMO gap of [Au(CN)2]n
oligomers decreases as the n value increases.6a,b Upon freezing,
salts 3–5 display intense luminescence with a maximum at
440–455 nm (Fig. 3(f) for 5), which is comparable to the
wavelength of the low energy band observed at RT. This result
provides us with further confirmation that the low energy bands
originate from the extended chain structures. For BMI-based ionic
liquids formed with PF6, BF4, (CF3SO2)2N and so on, Watanabe
and coworkers reported that the molar conductivity estimated
from impedance measurements is significantly low in comparison
with that estimated on the basis of pulsed-field-gradient spin-
echo NMR measurements, indicating ion association and/or
ion pairs of a significant fraction of oppositely charged ions.16
At present, the number of Au(CN)2 units in the oligomeric
species in 3–5 is unclear. However, it is entirely reasonable that
there are more than two distinct species in the ion associations,
and the smaller oligomers disappear upon freezing. The similarity
in the RT emission and excitation spectra seen for 3–5 suggests
that the oligomeric species found in the liquids are similar to
each other.
Notes and references
¯
{ Crystal data for 1: C6H7N4Au1. M = 332.12, triclinic, space group P1,
˚
a = 6.915(1), b = 7.014(2), c = 10.590(2) A, a = 100.18(2), b = 97.31(2),
c = 117.66(2)u, V = 434.7(2) A , Z = 2, dcalc = 2.538 g cm23, m(MoKa) =
3
˚
16.9 mm21, 2219 measured, 1355 independent reflections, R = 0.056 (F2 ¢
4s(F2)), wR = 0.195 (all data). Crystal data for 2: C8H11N4Au1. M =
360.17, monoclinic, space group P21/c, a = 8.370(2), b = 19.900(4), c =
6.672(2) A, b = 107.84(2)u, V = 1057.9(4) A , Z = 4, dcalc = 2.261 g cm23
,
3
˚
˚
m(MoKa) = 13.8 mm21, 2426 measured, 1690 independent reflections, R =
0.043 (F2 ¢ 4s(F2)), wR = 0.131 (all data). CCDC reference numbers
280734 and 280735. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b515869a
1 (a) M. Meln´ınk and R. V. Parish, Coord. Chem. Rev., 1986, 70, 157; (b)
H. Schmidbaur, Chem. Soc. Rev., 1995, 391.
2 (a) R. Blonder, S. Levi, G. Tao, I. Ben-Dov and I. Willner, J. Am.
Chem. Soc., 1997, 119, 10467; (b) M. A. Mansour, W. B. Connick,
R. J. Lachicotte, H. J. Gysling and R. Eisenberg, J. Am. Chem. Soc.,
1998, 120, 1329; (c) M. O. Wolf and M. A. Fox, J. Am. Chem. Soc.,
1995, 117, 1845.
3 (a) P. Pyykko¨, Chem. Rev., 1997, 97, 597; (b) H. Schmidbaur, Gold Bull.,
2000, 33, 3.
4 (a) H. Schmidbaur, W. Graf and G. Mu¨ller, Angew. Chem., Int. Ed.
Engl., 1988, 27, 417; (b) D. E. Harwell, M. D. Mortimer, C. B. Knobler,
F. A. L. Anet and M. F. Hawthorne, J. Am. Chem. Soc., 1996, 118,
2679.
5 R. L. White-Morris, M. M. Olmstead and A. L. Balch, J. Am. Chem.
Soc., 2003, 125, 1033.
6 (a) M. A. Omary and H. H. Patterson, J. Am. Chem. Soc., 1998, 120,
7696; (b) M. A. Rawashdeh-Omary, M. A. Omary and H. H. Patterson,
J. Am. Chem. Soc., 2000, 122, 10371; (c) M. A. Rawashdeh-Omary,
M. A. Omary, H. H. Patterson and J. P. Fackler, J. Am. Chem. Soc.,
2001, 123, 11237.
7 (a) N. Nagasundaram, G. Roper, J. Biscoe, J. W. Chai, H. H. Patterson,
N. Blom and A. Ludi, Inorg. Chem., 1986, 25, 2947; (b) P. Fischer,
A. Ludi, H. H. Patterson and A. W. Hewat, Inorg. Chem., 1994,
33, 62.
8 (a) A. H. Schwellnus, L. Denner and C. A. Boeyens, Polyhedron, 1990,
9, 975; (b) R. E. Cramer, D. W. Smith and W. VanDoorne, Inorg.
Chem., 1998, 37, 5895; (c) M. Stender, M. M. Olmstead, A. L. Balch,
D. Rios and S. Attar, Dalton Trans., 2003, 4282; (d) C. Paraschiv,
S. Ferlay, M. W. Hosseini, V. Bulach and J.-M. Planeix, Chem.
Commun., 2004, 2270; (e) W. Dong, Y.-Q. Sun, B. Yu, H.-B. Zhou,
H.-B. Song, Z.-Q. Liu, Q.-M. Wang, D.-Z. Liao, Z.-H. Jiang, S.-P. Yan
and P. Cheng, New J. Chem., 2004, 28, 1347.
9 (a) M. C. Buzzeo, R. G. Evans and R. G. Compton, ChemPhysChem,
2004, 5, 1106; (b) C. Chiappe and D. Pieraccini, J. Phys. Org. Chem.,
2005, 18, 275.
In conclusion, we prepared five dicyanoaurate(I) salts with a
wide variation in their mode of aggregation such as dimer, infinite
chain and liquid forming. The 1-alkyl-3-methylimidazolium
10 A. Bondi, J. Phys. Chem., 1964, 68, 441.
11 S. S. Pathaneni and G. R. Desiraju, J. Chem. Soc., Dalton Trans., 1993,
319.
12 J. -F. Huang, H. Luo, C. Liang, I. -W. Sun, G. A. Baker and S. Dai,
J. Am. Chem. Soc., 2005, 127, 12784.
…
cations are prone to elongate the Au Au separation, and the
expansion of the alkyl chain in the cations gives rise to the
stabilization of the liquid state. The finding that all present salts are
13 For examples: (a) P. Bonhoˆte, A. -P. Dias, M. Armand,
N. Papageorgiou, K. Kalyanasundaram and M. Gra¨tzel, Inorg.
726 | J. Mater. Chem., 2006, 16, 724–727
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Chem., 1996, 35, 1168; (b) R. Karmakar and A. Samanta, Chem. Phys.
Lett., 2003, 376, 638; (c) G. A. Baker, S. N. Baker and T. M. McCleskey,
Chem. Commun., 2003, 2932.
E. Beurer, K. Driesen, R. V. Deun, K. V. Hecke, L. V. Meervelt and
K. Binnemans, Chem. Commun., 2005, 4354.
15 R. L. White-Morris, M. M. Olmstead, F. Jiang, D. S. Tinti and
A. L. Balch, J. Am. Chem. Soc., 2002, 124, 2327.
16 H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and
M. Watanabe, J. Phys. Chem. B, 2004, 108, 16593.
14 For examples: (a) E. Guillet, D. Imbert, R. Scopelliti and J.-C. G. Bu¨nzli,
Chem. Mater., 2004, 16, 4063; (b) K. Driesen, P. Nockemann and
K. Binnemans, Chem. Phys. Lett., 2004, 395, 306; (c) P. Nockemann,
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 724–727 | 727