Structure and luminescence properties of a well-known macrometallocyclic trinuclear Au(i) complex and its adduct with a perfluorinated fluorophore showing cooperative anisotropic supramolecular interactions

Article abstract of DOI:10.1039/c0dt00736f

The structure and luminescence properties of a well-known trinuclear Au(i) imidazolate complex are determined for the first time along with its interaction with the organic π acid perfluoronaphthalene in the solid state and solution.

Full text of DOI:10.1039/c0dt00736f

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Structure and luminescence properties of a well-known macrometallocyclic
trinuclear Au(I) complex and its adduct with a perfluorinated fluorophore
showing cooperative anisotropic supramolecular interactions†
Oussama Elbjeirami,‡a Maher D. Rashdan,§a Vladimir Nesterovb and Manal A. Rawashdeh-Omary*a
Received 28th June 2010, Accepted 18th August 2010
DOI: 10.1039/c0dt00736f
The structure and luminescence properties of a well-known
trinuclear Au(I) imidazolate complex are determined for the
first time along with its interaction with the organic p acid
perfluoronaphthalene in the solid state and solution.
solution complexation via fluorescence quenching similar to that
reported by Gabba¨ı and co-workers.8
Here we report the structure and corresponding luminescence
properties of 1 and its reaction product with perfluoronaphthalene,
C10F8 (Chart 1). Synthesis of 1 was performed according to
a published procedure.11 X-Ray quality single crystals were
obtained by slow evaporation from THF or C6F6 solution. On
the other hand, the synthesis of the binary p acid–base adduct was
accomplished by slow evaporation of a mixed solution composed
of a hexane solution of 1 and a dichloromethane solution of
Trinuclear Au(I) complexes have been studied since the 1970 s
and continue to receive current interest from multiple research
groups owing to their remarkable structural1 and photophysical2
properties. Interesting chemistries in these materials include p
acid–base interactions, aurophilic bonding, excited state Au–Au
covalent bonding, and stimulus-sensitive luminescence.1–4 Among
members of this class, the trinuclear Au(I) complexes [Au(m-
C2,N3-bzim)]3 (1, where bzim = 1-benzylimidazolate) and [Au(m-
C(OEt) N(p-C6H4CH3)]3 (2) have been subject to particularly
immense investigations since 1972 by Minghetti, Bonati, Burini,
Balch, Fackler and co-workers.1,2,5 While the structure5 and
luminescence properties2 of 2 have been determined, the same
is not true regarding 1 prior to this work as the compound is
usually isolated as a powder that is non-luminescent at room
temperature. Several papers by Burini, Fackler, and co-workers
showed that these complexes with Au(I) atoms bridged by the -
◦
C10F8. This mixture was kept at 25 C for several days to obtain
X-ray quality single crystals in 80% yield. Further synthetic and
characterization details are given in the ESI.†
Chart 1
N
C- carbeniate and imidazolate ligands form sandwich adducts
with a variety of electrophiles (Tl+, Ag+, an Hg(II) trimer, TCNQ,
C6F6, etc.) to produce supramolecular chains that exhibit versatile
optoelectronic properties influenced by the corresponding electro-
static interaction.6,7
1 crystallizes in the monoclinic space group C2/c. Fig. 1
shows that symmetric molecules of 1 associate as dimer-of-
trimer units with the shortest intermolecular Au . . . Au distance
Gabba¨ı and co-workers reported extensive studies regarding
stacking interactions of an electrophilic Hg(II) cyclic trinuclear
complex with organic fluorophores, resulting in arene-sensitized
phosphorescence with multiple orders of magnitude reduction in
radiative lifetime due to spin–orbit coupling enhancement.8 Mean-
while, a recent density-functional computational study suggested
that trinuclear Au(I) imidazolates such as 1 represent the strongest
nucleophiles among the class of cyclic trinuclear d10 complexes
known to date.9 Given that 2 forms a stacked adduct with the p-
acidic fluorophore F8-naphthalene (C10F8),10 an analogous study
for 1, being a stronger p base, is warranted. This study also includes
aDepartment of Chemistry and Physics, Texas Woman’s University, Denton,
Texas, 76204, USA. E-mail: momary@twu.edu
bDepartment of Chemistry, University of North Texas, Denton, TX 76203,
USA
† Electronic supplementary information (ESI) available: Further experi-
mental details. CCDC reference numbers 783172 and 783173. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00736f
‡ Present Address: King Fahd University of Petroleum & Minerals,
Dhahran, 31261, Kingdom of Saudi Arabia
§ Present Address: Department of Chemistry and Biochemistry, The
University of Texas at Arlington, Arlington, TX 76019, USA
Fig. 1 Crystal structure of 1 showing a dimer-of-trimer repeat unit.
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Fig. 2 Portions of the packing of {[1][C10F8]}• crystals showing 1D columnar stacking along the a-axis via p acid–base interactions (left) and 2D sheet
structure across the bc planes via H ◊ ◊ ◊ F interactions (right). Further views of molecular structure and packing are available in the ESI.†
˚
being 3.3465(4) A, which is even shorter than the intramolecular
bonds and HCH (benzyl) ◊ ◊ ◊ F interactions with varying strength (2.50–
2.68 A). All of these H ◊ ◊ ◊ F interactions are spread in 2D
2
˚
˚
ligand-assisted aurophilic distances (3.453, 3.458, and 3.465 A).
The hexanuclear dimer-of-trimer units adopt a semi-prismatic
layers that cooperatively stabilize the supramolecular structure
along with the columnar p acid–base interactions. Precedents
of cooperative aurophilic/H-bonding interactions exist for other
Au(I) species.13
Crystals of 1 exhibit bright green luminescence even at room
temperature (Fig. 3, I). Cooling leads to enhancement in the
intensity and lifetime; the latter increases from t = 7.8 0.5 ms at
298 K to 23.0 1.5 ms at 77 K. The broad/structureless emission
profile, large Stokes’ shift (~6500 cm-1), and lifetime magnitude
are consistent with an Au-centered phosphorescent emission from
a largely distorted excimeric triplet state. The strong intertrimer
˚
˚
conformation with one long (3.558 A) and two short (3.346 A)
intertrimer aurophilic distances and torsion angles of ~17◦. The
intertrimer aurophilic interactions in 1 are stronger than those in
the reported structure of 2, which entails a chair conformation with
only two of the three Au atoms engaged in intertrimer aurophilic
5
˚
interaction (3.422 A). A prismatic structure is somewhat rare
among trinuclear d10 complexes and has been reported for poly-
morphs of [Au(m-C(OMe) NMe)]3,3a,12 for which an extended
columnar structure was obtained, whereas a more similar situation
to the one herein with discrete dimer-of-trimer units has been
reported for {[3,5-(i-Pr)2Tz]Au}3,4f as well as charge transfer
adducts of [Au(m-C(OEt) NMe)]3.3b
¯
The adduct 1·C10F8 crystallizes as a triclinic (P1) system.
An extended structure with two principal modes of anisotropic
supramolecular interaction (Fig. 2) exist: one-dimensional (1D) p
acid–base quadrupolar interactions manifest by alternating C10F8
and 1 planar moieties stacked perfectly on top of each other
in infinite columns along the a-axis, and two-dimensional (2D)
H ◊ ◊ ◊ F dipole–dipole interactions involving the F atoms of C10F8
and H atoms of both the benzyl and imidazolyl groups of 1 to
form extended planar sheets across the bc-planes. The centroids
˚
of the p-acidic C10F8 and p-basic 1 are separated by 3.506 A in
10
˚
the 1D stacks, very similar to that in 2·C10F8 (3.509 A) and
7b
˚
shorter than in 2·C6F6 (3.565 A). The shortest Au ◊ ◊ ◊ C (C10F8)
˚
distances are 3.395 and 3.436 A. These distances are close to the
˚
sum of the estimated van der Waals radii of Au and C (3.36 A).
The intramolecular Au ◊ ◊ ◊ Au distances become somewhat longer
˚
˚
upon adduct formation (average 3.472 A vs. 3.459 A in 1 alone).
Overall, the p acid–base stacking interactions are similar to those
reported for the adduct of the carbeniate analogue 2 with the
same fluorophore and with C6F6. However, the same cannot
be said regarding the H ◊ ◊ ◊ F interactions, as only intra-stack
Htolyl ◊ ◊ ◊ F interactions were described7b,10 in those adducts of 2. In
contrast, 1·C10F8 exhibits both intra-stack and inter-stack H ◊ ◊ ◊ F
interactions, including HPh(benzyl) ◊ ◊ ◊ F and Himidazolyl ◊ ◊ ◊ F hydrogen
Fig. 3 Normalized photoluminescence excitation (left) and emission
(right) spectra for crystals of 1 (I), crystals of 1·C10F8 (II), and deaerated
THF frozen solutions of C10F8 and 1 : 1 molar mixture of 1 and C10F8 (III).
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interactions in the crystal structure of 1 support this assignment.
The aurophilic association in the crystal imparts a significant red
shift in the excitation bands for the solid (lexc ~ 390 nm) compared
to the electronic absorption bands in dilute solution, which occur
at lmax ~ 250 nm in dichloromethane, assignable to metal-to-ligand
charge transfer (MLCT) in monomeric 1 units.6 The solid powder
of 1 is non-luminescent at room temperature but exhibits weak
luminescence at 77 K with the same emission and excitation profile
as in the crystals (Fig. S8†), suggesting the same crystallographic
form for the microcrystalline powder.
Finally, in order to investigate possible charge transfer adducts
in fluid solution, we monitored the fluorescence spectrum of dilute
solutions of C10F8 in CH2Cl2 upon titration with 1. Gradual
quenching of arene fluorescence was observed upon incremental
addition of 1. Stern–Volmer analysis16 yields a KSV value of
374 5 M-1 (Fig. 4). The quenching data suggest static rather than
dynamic quenching, as the lifetime remains constant during the
titration experiment (Fig. 4). These data suggest the formation of a
ground-state complex in solution, i.e. [1·C10F8], with an association
constant equalling KSV. This complex undergoes photoexcitation
as an integral binary unit, which is substantiated by the appearance
of the CT excitation band in the 1 : 1 molar admixture of 1 and
C10F8 in dichloromethane (albeit that was for a frozen solution
while the fluorescence quenching is for a fluid solution). The
solution quenching results here are qualitatively similar to those
for naphthalene quenching by fluorinated Hg3 and Ag3 p-acidic
complexes that have been reported by Gabba¨ı et al.8 and Dias
et al.,4h respectively. Solution quenching was not reported for C10F8
quenching by the p-basic Au3 complex 2 in the preceding study by
Fackler et al.2
The Au-centered green unstructured phosphorescence of 1
crystals changes qualitatively in the 1·C10F8 solid adduct, which
exhibits bright yellow emission with a structured profile. The
vibronic structure becomes more resolved upon cooling to 77 K.
These features are characteristic of the triplet state emission
of the C10F8 arene, which alone exhibits the same emission
only at cryogenic temperatures. There is no significant shift in
the phosphorescence energy of 1·C10F8 crystals vs. uncomplexed
C10F8 (Fig. 3). At ambient temperature, the yellow organic-based
luminescence is very bright for 1·C10F8 but undetectable for solid
octafluoronaphthalene, similar to the observation made for the
analogous 2·C10F8 adduct.10 Lifetime measurements verify that
the yellow luminescence of 1·C10F8 is phosphorescence with t =
1.8 0.05 ms at 298 K, also within the same order of magnitude
obtained for the analogous 2·C10F8 adduct. The luminescence
intensity and lifetime both increase upon cooling to 77 K (t =
3.8 0.08 ms). This is expected because of the corresponding
nonradiative depopulation of the emitting T1 state of the organic
p acid. The excitation bands for solid 1·C10F8 are red-shifted from
the absorption bands of the C10F8 and 1 isolated components,
suggesting a charge-transfer excitation route promoted by the
cooperative electrostatic attraction forces discussed above. Such
a route also has been invoked in literature precedents with Au3,10
Hg3,8 and Ag34f triplet sensitizers. The photoluminescence spectra
of frozen 2-Me-THF solutions containing equimolar amounts of
1 and C10F8 exhibit only a yellow emission with a structured
profile, assigned to C10F8 monomer phosphorescence with t =
12.5 0.2 ms compared to 0.4 s for the pure C10F8 (see Fig. 3).
The T1 emission of C10F8 is drastically enhanced because the
three gold centers per molecule in 1·C10F8 provide a significant
external heavy-atom effect with substantial spin–orbit coupling
(x5d = 5100 cm-1).14
The excitation spectra for the single crystal form of {[1][C10F8]}•
or a frozen solution of a 1 : 1 molar admixture of 1 and C10F8
reveal a red-shifted feature that covers the spectral range of ~340
to 400 nm (Fig. 3). This excitation band is assignable to a charge
transfer (CT) transition in the ground state adduct because it
appears at lower energy than the absorption bands for dilute
solutions of 1 and of C10F8, which represent monomers of these
species (see ESI†). This CT assignment is consistent with the
strongly nucleophilic character of the trinuclear Au(I) imidazolate
complex, which acts as a p-base7c,10 that donates electron density
to the p-acid C10F8. The luminescence excitation spectra suggest
that this CT transition represents an additional excitation route
that leads to the yellow phosphorescence of the crystalline adduct.
However, while significant, this CT route is somewhat minor to the
conventional route that governs the excitation mechanism of the
free arene C10F8 (i.e., S0→Sn followed by intersystem crossing to
T1)15 based on analysis of relative intensities of excitation bands.
Fig. 4 Stern–Volmer plots for fluorescence quenching of C10F8 by 1.
To conclude, this work reports an uncommon prismatic dimer-
of-trimer aurophilic association for a trinuclear Au(I) imida-
zolate complex whose crystal structure has alluded researchers
investigating this complex since the 1970s. Metal-centered green
phosphorescence typical of aurophilic systems is exhibited only
by single crystals of the trinuclear complex, whereas the non-
crystalline material isolated in the synthesis does not exhibit
detectable luminescence. Upon complexation of the electron-rich
complex with perfluoronaphthalene, the aurophilic interactions
are replaced by cooperative p acid–base quadrupolar and H ◊ ◊ ◊ F
dipolar interactions, which in turn lead to yellow arene-centered
monomer phosphorescence in the solid state or frozen solution
and arene fluorescence quenching in solution.
This work was supported by a DARPA Young Investigator
Award (W71B7J-7078-B368) and ACS-PRF (# 49511-UNI3) to
M.A.R.-O., and a departmental Welch Foundation grant. Further
experimental details are available as ESI.†
Notes and references
1 A. Burini, A. A. Mohamed and J. P. Fackler, Jr., Comments Inorg.
Chem., 2003, 24, 253.
2 M. A. Omary, A. A. Mohamed, M. A. Rawashdeh-Omary and J. P.
Fackler, Coord. Chem. Rev., 2005, 249, 1372.
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3 (a) J. C. Vickery, M. M. Olmstead, E. Y. Fung and A. L. Balch, Angew.
Chem., Int. Ed. Engl., 1997, 36, 1179; (b) M. M. Olmstead, F. Jiang,
S. Attar and A. L. Balch, J. Am. Chem. Soc., 2001, 123, 3260; (c) A.
Hayashi, M. M. Olmstead, S. Attar and A. L. Balch, J. Am. Chem.
Soc., 2002, 124, 5791.
Omary, M. A. Rawashdeh-Omary, B. R. Pietroni and R. J. Staples, J.
Am. Chem. Soc., 2000, 122, 11264.
8 (a) O. Elbjeirami, C. N. Burress, F. P. Gabba¨ı and M. A. Omary, J.
Phys. Chem. C, 2007, 111, 9522; (b) C. N. Burress, M. I. Bodine, O.
Elbjeirami, J. H. Reibenspies, M. A. Omary and F. P. Gabba¨ı, Inorg.
Chem., 2007, 46, 1388.
9 S. M. Tekarli, T. R. Cundari and M. A. Omary, J. Am. Chem. Soc.,
2008, 130, 1669.
10 A. A. Mohammad, M. A. Rawashdeh-Omary, M. A. Omary and J. P.
Fackler, Jr., Dalton Trans., 2005, 2597.
11 F. Bonati, A. Burini, B. R. Pietroni and B. Bovio, J. Organomet. Chem.,
1989, 375, 147.
12 R. L. White-Morris, M. M. Olmstead, S. Attar and A. L. Balch, Inorg.
Chem., 2005, 44, 5021.
13 (a) C. Hollatz, A. Schier and H. Schmidbaur, J. Am. Chem. Soc., 1997,
119, 8115; (b) C. Hollatz, A. Schier, J. Riede and H. Schmidbaur, J.
Chem. Soc., Dalton Trans., 1999, 111; (c) W. J. Hunks, M. C. Jennings
and R. J. Puddephatt, Inorg. Chem., 2000, 39, 2699; (d) L. Hao, M. A.
Mansour, R. J. Lachicote, H. J. Gysling and R. Eisenberg, Inorg. Chem.,
2000, 39, 5520; (e) J. J. Guy, P. G. Jones, M. J. Mays and G. M. Sheldrick,
J. Chem. Soc., Dalton Trans., 1977, 8; (f) B. Ahrens, P. G. Jones and
A. K. Fischer, Eur. J. Inorg. Chem., 1999, 1103; (g) F. Mendizabal, P.
Pyykko¨ and N. Runeberg, Chem. Phys. Lett., 2003, 370, 733.
14 J. S. Griffith, Theory of Transition Metal Ions, Cambridge University
Press, Cambridge, 1964.
4 (a) M. Enomoto, A. Kishimura and T. Aida, J. Am. Chem. Soc., 2001,
123, 5608; (b) G. Yang and R. G. Raptis, Inorg. Chem., 2003, 42, 261;
(c) A. Kishimura, T. Yamashita and T. Aida, J. Am. Chem. Soc., 2005,
127, 179; (d) H. V. R. Dias, H. V. K. Diyabalanage, M. G. Eldabaja,
O. Elbjeirami, M. A. Rawashdeh-Omary and M. A. Omary, J. Am.
Chem. Soc., 2005, 127, 7489; (e) M. A. Omary, M. A. Rawashdeh-
Omary, M. W. A. Gonser, O. Elbjeirami, T. Grimes, T. R. Cundari,
H. V. K. Diyabalanage, C. S. Palehepitiya Gamage and H. V. R.
Dias, Inorg. Chem., 2005, 44, 8200; (f) C. Yang, M. Messerschmidt,
C. Coppens and M. A. Omary, Inorg. Chem., 2006, 45, 6592; (g) T.
Grimes, M. A. Omary, H. V. R. Dias and T. R. Cundari, J. Phys. Chem.
A, 2006, 110, 5823; (h) M. A. Omary, O. Elbjeirami, C. S. Palehepitiya
Gamage, K. M. Sherman and H. V. R. Dias, Inorg. Chem., 2009, 48,
1784.
5 (a) G. Minghetti and F. Bonati, Angew. Chem., Int. Ed. Engl., 1972, 11,
429; (b) G. Minghetti and F. Bonati, Inorg. Chem., 1974, 13, 1600; A.
L. Balch, M. M. Olmstead and J. C. Vickery, Inorg. Chem., 1999, 38,
3494.
6 J. P. Fackler, Inorg. Chem., 2002, 41, 6959.
7 (a) A. Burini, R. Bravi, J. P. Fackler, Jr., R. Galassi, T. A. Grant, M.
A. Omary, B. R. Peitroni and R. J. Staples, Inorg. Chem., 2000, 39,
3158; (b) M. A. Rawashdeh-Omary, M. A. Omary, J. P. Fackler, Jr, R.
Galassi, B. R. Pietroni and A. Burini, J. Am. Chem. Soc., 2001, 123,
9689; (c) A. Burini, J. P. Fackler, Jr, R. Galassi, T. A. Grant, M. A.
15 S. P. McGlynn, T. Azumi and M. Kinoshita, Molecular Spectroscopy of
the Triplet State, Prentice-Hall, Englewood Cliffs, New Jersey, 1969.
16 J. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer/Plenum,
New York, NY, 1999.
9468 | Dalton Trans., 2010, 39, 9465–9468
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