Chemistry and optoelectronic properties of stacked supramolecular entities of trinuclear gold(I) complexes sandwiching small organic acids [9]

Full text of DOI:10.1021/ja016279g

J. Am. Chem. Soc. 2001, 123, 9689-9691
Chart 1. Structure of the Trinuclear Compounds Studied
9689
Chemistry and Optoelectronic Properties of Stacked
Supramolecular Entities of Trinuclear Gold(I)
Complexes Sandwiching Small Organic Acids
Manal A. Rawashdeh-Omary, Mohammad A. Omary,† and
John P. Fackler, Jr.*
Department of Chemistry, Texas A&M UniVersity
College Station, Texas 77843
cyanides may directly bind to the “exposed” linear 2-coordinate
Au(I) centers.9 However, DFT calculations that we have carried
out8 clearly show that the donor regions in the trinuclear Au(I)
compounds are located at the center of the nine-membered ring
and that they extend to regions in space above and below the
ring plane, as opposed to being localized on the Au atoms. Indeed,
we show here that a charge-transfer complex with a columnar
structure does form when a small tetracyano-substituted organic
acceptor reacts with 1. Previous attempts for the reaction with
cyano-substituted organic acceptors have lead to the rupture of
the trinuclear Au(I) unit and formation of mononuclear cations.10
We also show here that the liquid organic acid C6F6 forms a
stacked complex with 2 and quenches its luminescence, thus
suggesting a potential sensing action.
A saturated solution containing 7.0 mg (0.034 mmol) of 7,7,8,8-
tetracyanoquinodimethane (TCNQ) dissolved in 1 mL of CH2-
Cl2 was prepared and warmed in a water bath until dissolution.
A 35 mg (0.033 mmol) sample of 1 was added to the light-yellow
CH2Cl2 solution, and an intense green color was observed
immediately. Crystallization from CH2Cl2/ether produced dark
crystals that were analyzed by X-ray crystallography and scanning
electron microscopy (SEM).
The product, 3, of the reaction of 1 with TCNQ was
characterized by elemental analysis11 and X-ray crystallogra-
phy12,13 as [1]2TCNQ. The crystal structure of 3 is shown in Figure
1. The TCNQ molecule is sandwiched between two units of 1
from each side, in a face-to-face manner so that a molecule of 3
is best represented by the formula (π-1)(µ-TCNQ)(π-1). The
cyanide groups are clearly not coordinated to the gold atoms. The
distance between the centroid of TCNQ to the centroid of the
Au3 unit is 3.964 Å. The packing of 3 shows a stacked linear-
chain structure, as shown in Figure 1, with a repeat pattern of
‚‚‚(Au3)(Au3)(µ-TCNQ)(Au3)(Au3)(µ-TCNQ)‚‚‚ The stacking in
3 is similar to the previously reported7,8 sandwich adducts of 1
with Tl+, Ag+, and Hg3(µ-C6F4)3, all of which also show a
‚‚‚BBABBA‚‚‚ infinite chain pattern with intermolecular auro-
philic bonding between four out of the six Au atoms in adjacent
Au3 units. A striking difference in 3, however, is the two very
short intermolecular Au‚‚‚Au distances of 3.152 Å (identical for
the two aurophilic bonds). The intermolecular Au‚‚‚Au distance
in 3 is even shorter than the intramolecular distances in the
compound, which were found to be 3.475, 3.471, and 3.534 Å.
The adjacent Au3 units in 3, as well as in the previously reported
adducts of 1 with Tl+, Ag+, and Hg3(µ-C6F4)3, form a chair-type
structure as opposed to the face-to-face (nearly eclipsed) pattern
reported in Balch’s studies9 of the nitro-9-fluorenones adducts
with the trinuclear Au(I) alkyl-substituted carbeniate complexes.
Rossana Galassi, Bianca R. Pietroni, and Alfredo Burini*
Dipartimento di Scienze Chimiche
UniVersity of Camerino, Via S. Agostino
1-062032- Camerino, Italy
ReceiVed May 25, 2001
Extended linear-chain inorganic compounds have special
chemical and physical properties.1,2 This has led to new develop-
ments in fields such as supramolecular chemistry, acid-base
chemistry, luminescent materials, and various optoelectronic
applications. Among the recent examples are the developments
of a vapochromic light-emitting diode from linear-chain Pt(II)/
Pd(II) complexes,3 a luminescent switch consisting of an Au(I)
dithiocarbamate complex that possesses a luminescent linear-chain
form only in the presence of vapors of organic solvents,4 mixed-
metal (Ag/Au) compounds that exhibit different colors and
emissions when different organic solvents are introduced or
removed,5 and the discovery of a new phenomenon, solvolumi-
nescence,6 in a trinuclear Au(I) complex whose extended-chain
structure is responsible for storage of energy and release of it as
long-lived orange phosphorescence upon contact with solvent.
We have been studying trinuclear Au(I) compounds with
aromatic-substituted imidazolate, 1, and carbeniate, 2, bridging
ligands (Chart 1). These compounds are colorless and do not form
extended-chain structures. However, we have recently reported
that they can produce brightly colored complexes by sandwiching
naked Tl+ and Ag+ ions to form linear-chain complexes with
fascinating luminescence properties such as luminescence thermo-
chromism.7 More recent results have demonstrated that the
electron-rich trinuclear Au(I) complexes can interact with the
neutral inorganic Lewis acid Hg3(µ-C6F4)3 to produce infinite
linear-chain complexes.8 We have then focused our efforts on
studying the reactivity of 1 and 2 with small organic Lewis acids
and electron acceptors. Balch and co-workers have recently
demonstrated that trinuclear Au(I) compounds with alkyl-
substituted carbeniate bridging ligands can interact with the large
organic acceptors nitro-9-fluorenones.9 It was suggested that
cyano-substituted acceptors should be avoided because the
† Permanent address: Department of Chemistry, University of North Texas,
Denton, TX 76209.
(1) Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum Press:
New York, 1982; Vols. 1-3.
(2) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 846.
(3) Kunugi, Y.; Mann, K. R.; Miller, L. L.; Exstrom, C. L. J. Am. Chem.
Soc. 1998, 120, 589.
(4) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.;
Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329.
(5) Laguna, A. Personal communications.
(6) Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A. L. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1179.
(7) (a) Burini, A.; Bravi, R.; Fackler, J. P., Jr.; Galassi, R.; Grant, T. A.;
Omary, M. A.; Pietroni, B. R.; Staples, R. J. Inorg. Chem. 2000, 39, 3158.
(b) Burini, A.; Fackler, J. P., Jr.; Galassi, R.; Pietroni, B. R.; Staples, R. J.
Chem. Commun. 1998, 95.
(10) Jiang, F.; Olmstead, M. M.; Balch, A. L. J. Chem. Soc., Dalton Trans.
2000, 4098.
(11) Anal. Calcd (found) for C72H58N16Au6: C, 37.13 (36.60); H, 2.51
(2.34); N, 9.62 (9.25).
(12) Crystallographic data were obtained using a Siemens SMART CCD
diffractometer, Mo KR radiation (λ ) 0.71069 Å), T) 110(2) K.
(13) Crystal data for 3: triclinic, space group P-1, a ) 11.581(5) Å, b )
11.688(5) Å, c ) 13.572(5) Å, R )106.249(5)°, â ) 107.005(5)°, γ )94.076-
(5)°, V ) 1663.0(12) Å3, Z ) 2, Fcalc) 2.326 g cm-3, 7716 data, R1 ) 0.0579
(all data).
(8) Burini, A.; Fackler, J. P., Jr.; Galassi, R.; Grant, T. A.; Omary, M. A.;
Rawashdeh-Omary, M. A.; Pietroni, B. R.; Staples, R. J. J. Am. Chem. Soc.
2000, 122, 11264.
(9) Olmstead, M. M.; Jiang, F.; Attar, S.; Balch, A. L. J. Am. Chem. Soc.
2001, 123, 3260.
10.1021/ja016279g CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/07/2001

9690 J. Am. Chem. Soc., Vol. 123, No. 39, 2001
Communications to the Editor
Figure 1. Crystal structure of 3 showing a stereoview of the columnar
structure formed by the repeat unit [1]2TCNQ. The benzyl groups are
omitted for clarity.
Figure 3. Crystal structure of 4 showing a stereoview of the columnar
structure formed by interleaving units of 1 and C6F6.
conducting material suggests that these crystals are noninsulators,
because the charge from the electron beam accumulates on the
surface of insulating materials, which precludes the observation
of a clear SEM image for an insulator.15 Conducting or semi-
conducting materials, on the other hand, can diffuse the charge
and, thus, allow for the observation of a clear SEM image. We
have attempted to carry out preliminary resistivity measurements
for a large crystal of 3 that we were able to grow from a saturated
CH2Cl2 solution containing 1 and TCNQ at 4 °C. However, the
resistivity of the crystals at ambient temperature was too high to
be measured with the setup we had access to, which can only
measure resistivities for conducting materials. The apparent optical
band gap of 3 (the absorption edge) is about 1.4 eV, which is
within the band gap region for semiconductors, not conductors.
The second adduct we report here was synthesized by suspend-
ing a 30 mg sample of 2 (0.028 mmol) in an excess of C6F6 (0.1
mL). Methylene chloride was added dropwise and, after each
addition, the mixture was stirred and warmed in a water bath
until all the solid dissolved. The saturated solution was placed in
the refrigerator (4 °C) for crystallization. Colorless single crystals
with a high optical purity started to form within a few hours of
cooling. The crystals of the product, 4, were characterized by
elemental analysis,16 X-ray crystallography,12,17 and photolumi-
nescence spectroscopy. A thermogravimetric analysis for crystals
of 4 indicated a weight loss of 14.7% between 73 and 178 °C,
corresponding to a loss of one equivalent of C6F6 (theoretically
14.8% in 2‚C6F6). On melting of 4, the sample first lost
crystallinity at about 160 °C and then decomposed at 180 °C,
the exact decomposition point of 2, which was tested simulta-
neously.
Figure 2. Visible absorption spectrum of 3 in CH2Cl2 solution at ambient
temperature. The inset shows an SEM image of crystals of 3 taken without
coating with a conducting film.
The shortened intermolecular Au-Au distances in 3 may be
ascribed to charge-transfer from the electron-rich Au center to
the known electron acceptor TCNQ; a partial oxidation of the
Au(I) atoms leads to a shortening of Au-Au distances. In the
limiting case, upon complete oxidation to Au(II), a gold-gold
single bond forms.14 The charge-transfer assignment is further
supported by the dark intense color of 3 (appears black, although
thin crystals and films are dark green), in contrast to the colorless
crystals of 1 and the light-orange crystals of TCNQ. The color
change was the basis of the charge-transfer assignment in ref 9.
The charge-transfer adduct forming between 1 and TCNQ is
believed to remain intact in solution. Figure 2 shows the
absorption spectrum of 3 in a CH2Cl2 solution. The spectrum
appears to show a small amount of anionic TCNQ-, εmax 43,300
M-1 cm-1, when it is compared to absorption peaks below 400
nm for 1 and TCNQ alone. Because its absorption was near 900
nm, it was desired to obtain some data to study the conductivity
of 3. A scanning-electron microscope (SEM) image for crystals
of 3 is shown in the inset of Figure 2. The observation of a clear
SEM image for crystals of 3 that were not coated with a
The crystal structure of 4 shows a columnar stack consisting
of alternating C6F6 and 2 molecules (Figure 3). The C6F6 molecule
is sandwiched between two units of 2, one from each side, in a
face-to-face manner so that a molecule of 4 is best represented
(15) Scanning Electron Microscopy and X-ray Microanalysis; Goldstein,
J. I., et al., Eds.; Plenum Press: New York, 1992.
(16) Anal. Calcd (found) for C36H36O3N3F6Au3: C, 34.22 (33.90); H, 2.87
(2.72); N, 3.33 (3.30); F, 9.02 (9.08).
(14) For example, see: (a) Carlson, T. F.; Fackler, J. P. Jr. J. Organomet.
Chem. 2000, 596, 237. (b) Yam, V. W. W.; Choi, S. W. K.; Cheung, K. K.
Chem. Commun. 1996, 1173.
(17) Crystal data for 4: monoclinic, space group P21/c, a ) 7.3364(12)
Å, b ) 35.272(6) Å, c ) 14.356(2) Å, â ) 99.804(3)°, V ) 3660.7(10) Å3,
Z ) 4, Fcalc ) 2.293 g cm-3, 8978 data, R1 ) 0.0481 (all data).

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 39, 2001 9691
by the formula (π-2)0.5(µ-C6F6)(π-2)0.5. The distance between the
centroid of C6F6 to the centroid of the Au3 unit is 3.565 Å. The
packing of 4 shows a stacked linear-chain structure (Figure 3)
with a repeat pattern of ‚‚‚(Au3)(µ-C6F6)(Au3)(µ-C6F6)‚‚‚. This
is the first example in which the trinuclear Au(I) compounds
shown in Chart 1 exist without intermolecular Au-Au aurophilic
bonds. The crystal structure of 2 by itself shows a dimeric
structure with intermolecular Au-Au bonds.18 Therefore, the C6F6
Lewis acid disrupts the intermolecular aurophilic bonding in 2,
and the resulting linear-chain adduct is stabilized by acid-base
interactions. This result is analogous but opposite to the interac-
tions, reported by Gabba¨ı and co-workers, between the Lewis acid
Hg3(µ-C6F4)3 and benzene,19 in which benzene acts as a Lewis
base coordinating in a µ-6 manner to six Hg centers, three from
each side. The two Au3 units interacting with hexaflurobenzene
in 4 are eclipsed with respect to each other (nearly D3h), whereas
the two Hg3 units in Hg3(µ-C6F4)3‚benzene19a are nearly staggered
(D3d).
Crystals of 4 form rather easily in the presence of excess C6F6,
whereas the trinuclear Au(I) compound 2 by itself is hard to
crystallize, and its crystal structure18 was published several years
after the original synthesis of this type of compound was
reported.20 Furthermore, 2 exhibits a blue photoluminescence in
the solid state at ambient temperature, whereas the large single
crystals of 4 are not luminescent at ambient temperature. We have
noticed that when 4 is immersed in a solvent that does not dissolve
2, such as diethyl ether, the compound loses its crystallinity, and
the resulting powder exhibits the blue photoluminescence,
characteristic of 2, indicating that C6F6 is liberated from 4. In
contrast, when solid 2 is suspended in C6F6, its blue luminescence
starts to quench with time. These results have prompted us to
study the interaction of these complexes with vapors of organic
compounds. Figure 4 shows a representative example from our
ongoing investigation in this effort. A solid sample of 2 was placed
at a fixed position in a closed chamber, and its luminescence
spectra were acquired as a function of time in the presence of
C6F6 vapors. The vapors were produced as a result of the vapor
pressure at ambient temperature (293 K) and pressure (1 atm) of
a liquid sample of neat C6F6 in a small beaker that is placed at
the bottom of the closed chamber. The quenching of the
luminescence of 2 was first detected after only several minutes
and continued gradually until the next day (Figure 4). A control
experiment in the absence of C6F6 vapors was carried out and
showed no quenching in the luminescence of 2 with time. A
detailed photophysical study of changes in the luminescence
Figure 4. Emission spectra of 2 versus exposure time to C6F6 vapor at
ambient temperature and pressure. The exposure time was (top-to-bottom)
0, 51, 65, 98, 148, 208, 1322, 1439, and 1462 min, respectively.
behavior of 2 and 4 in the presence of a variety of volatile organic
molecules will be published elsewhere.
The evidence we have gathered thus far suggests that, unlike
3, 4 dissociates in solution into its component trinuclear Au(I)
compound and organic Lewis acid. The electronic absorption
spectra for solutions of 4 show the same absorption peaks as those
for 2 and C6F6. The 19F NMR spectra for even nearly saturated
solutions of 4 in CDCl3 show one peak at the same chemical
shift (-168 ppm) as that observed for C6F6 alone in CDCl3.
In conclusion, this paper reports the interaction of two electron-
rich trinuclear gold compounds with two small organic electron
acceptors to form acid-base adducts. The resulting compounds
have extended-chain structures, which can form regardless of the
presence or absence of aurophilic bonds. The benzylimidazolate
derivative, 1, forms a 2:1 charge-transfer complex with the organic
electron acceptor TCNQ despite the presence of cyanide groups,
which do not coordinate to the Au atoms. The carbeniate
derivative, 2, forms a 1:1 adduct in which the C6F6 molecule
disrupts the aurophilic bonding that is present in 2 alone.
Preliminary results have been presented to illustrate potential
optoelectronic applications for the compounds studied, specifically
as candidates for new semiconducting materials and sensors for
the detection of volatile organic compounds.
Acknowledgment. The financial support of this project has been
provided by the Robert A. Welch Foundation of Houston, TX, and the
University of Camerino, Italy. The purchase of the CCD X-ray diffrac-
tometer was made possible by a Grant from the NSF (CHE-98 07975).
We also thank Professors Joel Miller and Kim Dunbar for thoughts about
TCNQ.
(18) Tiripicchio, A.; Camellini, M. T.; Minghetti, G. J. Organomet. Chem.
1979, 171, 399.
(19) Tsunoda, M.; Gabba¨ı, F. P. J. Am. Chem. Soc. 2000, 122, 8335.
(20) (a) Minghetti, G.; Bonati, F. Angew. Chem., Int. Ed. Engl. 1972, 11,
429. (b) The structure of the related carbeniate CH3, CH3 derivative was
determined in 1997 (ref 6), many years after its original synthesis: Parks, J.
E.; Balch, A. L. J. Organomet. Chem. 1974, 71, 453.
Supporting Information Available: X-ray crystallographic files for
3 and 4 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA016279G