[{AuTl(C6cl5)2(toluene)} 2(dioxane)]: A striking structure that leads to a blue luminescence

Article abstract of DOI:10.1039/b304835g

The complex [{AuTl(C₆Cl₅)₂(toluene)} ₂(dioxane)] displays a structure with the thallium(I) center in an unprecedented trigonal planar environment, showing the shortest Au-Tl interaction ever found, a toluene molecule in a η⁶-mode and the disappearance of the Tl(I) inert pair, usually stereochemically active. These characteristics are responsible for its unusual luminescent behaviour.

Full text of DOI:10.1039/b304835g

[{AuTl(C6Cl5)2(toluene)}2(dioxane)]: A striking structure that leads to a
blue luminescence†
Eduardo J. Fernández,a Antonio Laguna,*b José María López-de-Luzuriaga,a M. Elena Olmosa and
Javier Péreza
a Departamento de Química, Universidad de La Rioja, Grupo de Síntesis Química de La Rioja, UA-CSIC,
Madre de Dios 51, E-26004 Logroño, Spain
b Departamento de Química Inorgánica-ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain.
E-mail: alaguna@posta.unizar.es; Fax: 34 976761187; Tel: 34976761185
Received (in Cambridge, UK) 30th April 2003, Accepted 30th May 2003
First published as an Advance Article on the web 18th June 2003
The complex [{AuTl(C6Cl5)2(toluene)}2(dioxane)] displays a
metal–metal interaction which is considered responsible for its
structure with the thallium(
I
) center in an unprecedented
luminescent behaviour. This white solid is obtained in high
5
trigonal planar environment, showing the shortest Au–Tl
yield by reacting a suspension of [AuTl(C6Cl5)2]n in toluene
interaction ever found, a toluene molecule in a h6-mode and
with dioxane. Its analytical and spectroscopic data are in
the “disappearance” of the Tl( ) inert pair, usually ster-
I
accordance with the proposed stoichiometry.7
eochemically active. These characteristics are responsible for
its unusual luminescent behaviour.
Single crystals of 1 suitable for X-ray diffraction studies were
obtained by slow diffusion of hexane in a saturated solution of
the complex in toluene, allowing its crystal structure, which
shows very interesting characteristics, to be unequivocally
established.‡ This structure consists of tetranuclear units
containing two [Au(C6Cl5)2]2 units, two Tl+ centres and a
dioxane molecule linked via unsupported Au…Tl and Tl…O
interactions with a toluene molecule interacting with each
Heterometal complexes displaying metal–metal interactions
without the presence of auxiliary bridging ligands are usually
built using several strategies that include hydrogen bonding, p-
staking or, more recently, by means of what is called
metallophilic attraction, a specific feature of closed-shell metals
consisting of oligomerization not accomplished by conven-
tional covalent or coordinative bonding. The most documented
example among these attractions is Aurophilicity, and, thus, this
phenomenon has been observed in a variety of mononuclear and
6
thallium centre in a h -mode (Figure 1). The complex contains
an inversion centre in the middle of the dioxane making each
half molecule of 1 equivalent. The first striking feature is the
Au–Tl distance of 2.8935(3) Å, which is the shortest one
described to date,4–6,8 being even shorter than the sum of Au–Tl
covalent radii (3.08 Å9 or 2.92 Å10), indicating the unprece-
dented strength of this interaction. The solvent molecule is only
weakly coordinated with a Tl–O distance of 2.827(4) Å, longer
than the Tl–O (THF) distances observed in other Tl complexes
(2.74(3)–2.781(7) Å),4,6,11 but shorter than the Tl–O (acetone)
distances observed in [Au2Tl2(C6Cl5)4]·(CH3)2CNO (2.903(9)
and 2.968(9) Å).6 The coordination sphere of the Tl atoms is
polynuclear gold( ) species and has been described in recent
I
experimental and theoretical reviews.1 In this sense, while
aurophilic attractions can be considered the upper extreme of
metallophilic interactions in strength with values that can even
reach 46 kJ mol21, Tl(
I
)…Tl(
I
) interactions are among the
weaker ones, being estimated below 20 kJmol21 2
.
Recent
theoretical studies of these interactions conclude that, while
aurophilic attractions are enhanced by relativistic effects, these
effects weaken the inherent van der Waals attractions of the
6
completed by an unusual h -like p-arene contact to a toluene
interactions between s2 metal centres.3 By contrast, Au(
I)…Tl(
I)
molecule resulting in an nearly trigonal-planar environment for
each thallium centre (considering the third position occupied by
the centroid of the phenyl ring of the toluene) with a sum of
angles around the Tl atom of 355.2°. This implies that, very
surprisingly, the stereoactivity of the inert pair, usually
stereochemically active,12 is not apparent in this case. This fact,
together with the short Au–Tl distance makes us wonder about
the nature of this interaction. The Tl–C distances range from
interactions generated by acid–base reactions provide an
additional electrostatic force and thus, metallophilicity between
gold(
I
) and thallium( ) centres in extended linear chains with an
I
average metal–metal separation of 3.03 Å is estimated at about
276 kJmol21, of which 80% is due to ionic interaction and 20%
to dispersion attraction.4 This result is striking since these
interactions appear between centres in +1 oxidation state, and
these centres would normally be expected to repel each other. In
contrast, they produce aggregates with intermetallic separations
shorter than the sum of their van der Waals radii (3.62 Å).
Consequently, the rationalization of the bonding in these
species is still a challenge. In addition, many of these complexes
form highly luminescent extended arrangements in solid state
with potential applications as LEDs or selective VOCs
sensors.5
This work is part of our current interest in the chemistry of
heteronuclear gold–thallium systems made up by reaction of
pentahalophenyl gold(
C6F5, C6Cl5) with Tl(
I
) derivatives of the type Q[AuR2] (R =
I
) salts.4–6 The crystal structures of these
complexes involve a large variety of assemblies, as discrete
molecules,6 extended linear chains4,5,6 or two- or three-
dimensional networks6 with different coordination numbers and
geometries at thallium.
Here we describe the synthesis, structure and optical
properties of the tetranuclear complex [{AuTl(C6Cl5)2-
(toluene)}2(dioxane)] (1), which displays an unprecedented
Fig. 1 Crystal structure of complex 1. The thermal ellipsoids are drawn at
the 30% level; hydrogen atoms are omitted for clarity. Selected bond
distances [Å] and angles [°]: Au–Tl 2.8935(3), Au–C1 2.055(5), Au–C11
2.052(5), Tl–O 2.827(4), Tl–C31 3.411(5), Tl–C32 3.462(6), Tl–C33
3.455(6), Tl–C34 3.378(6), Tl–C35 3.291(6), Tl–C36 3.319(6), E( =
plane)1(C31–C36)–Tl 3.079(3), Au–Tl–O 105.5(1), Ct(centroid)1(C31–
36)–Tl–Au 139.6, Ct1–Tl–O 110.1.
† This work is dedicated to the memory of Dr Ma Teresa Pinillos.
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CHEM. COMMUN., 2003, 1760–1761
This journal is © The Royal Society of Chemistry 2003

3.291(6) to 3.462(6) Å, slightly longer than some of the Tl–C
range, which are assigned to transitions localized in the p-
orbitals of the perhalophenyl groups modified by the presence
of the gold centre.6 The initial optical properties of 1 are
recovered by evaporation of the solvent.
6
distances observed in the few examples of h -like p-arene
thallium complexes described to date that range from 3.14 to
3.49 Å.12,13
In addition to its striking structure, complex 1 also shows a
very interesting luminescent behaviour in solid state at room
temperature and at 77 K (Figure 2). Its excitation spectrum
shows a maximum at ca. 340 nm, associated with a maximum
blue emission band appearing at 442 nm at room temperature
and at 468 nm at 77 K. It is worth noting that although blue
luminescent materials are one of the components necessary for
full-colour displays, they are still scarce and, for instance, in
organometallic compounds, usually require appropriate choice
of substituents in organic emitters.14 Blue photoluminescence
from d10 metal complexes has also been observed in dinuclear
derivatives.15 The fluorescence lifetime, determined by the
phase-modulation technique in solid state at room temperature
fits a double-exponential decay with values of 1843.98 and
752.08 ± 0.05 ns (c2 = 0.327). This lifetime measurement
within the microsecond time scale seems to indicate that the
emission is probably phosphorescence. In this context, similar
values were found in the gold–thallium complex
[Au2Tl2(C6Cl5)4]·(CH3)2CNO, in which the excited state was
almost completely based on the thallium centres, in agreement
with the TD-DFT calculations carried out.6 Similarly, in
complex 1, the TD-DFT results also agree with this proposal
and, thus, the thallium centres are likely to be the responsible
emitter atoms for the luminescence in this case. On the other
hand, and in reference to this and our previous work, the
[Au(C6Cl5)2]2 units are likely to be in the origin of the
electronic transitions. In short, this transition can be considered
as a MMCT (Metal (gold) to Metal (thallium) Charge Transfer)
in origin. Interestingly, the blue emission contrasts with those
previously described for extended linear chains in the sense that
the reduced Au–Tl distance found in 1 should produce a shift of
the emission to lower energies if compared to those. In fact,
previous Fenske–Hall molecular orbital calculations8a indicate
that a reduced Au–Tl distance would produce a better
In short, this complex does not show the stereochemically
active inert pair and displays the shortest Au–Tl distance and the
higher emission reported for this kind of system, results that are
likely to be due to an unusually strong interaction between the
metal centres, which is currently under study.
This work was supported by the UR (API02/12), C.A.R.
(ANGI2001/28) and DGI (MCYT) (BQU2001-2409)
Notes and references
‡ Crystal data for 1: C21H12AuCl10OTl, M = 1036.14, crystal dimensions
¯
0.2 3 0.2 3 0.1 mm, triclinic, P1, a = 9.3482(1), b = 11.1221(1), c =
13.1833(2) Å, a = 78.009(1), b = 89.278(1)°, g = 81.670(1), V =
1326.42(3) Å3, T = 2100 °C, Z = 2, m(MoKa) = 12.615 mm21, 19605
measured reflections, 6281 independent reflections (Rint = 0.040), 309
refined parameters with R1 = 0.0363 and wR2 = 0.1055 for I > 2s(I),
Goodness-of-fit on F2 = 1.057. CCDC 204510. See http://www.rsc.org/
suppdata/cc/b3/b304835g/ for crystallographic data in CIF or other
electronic format.
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7 Complex 1 was prepared in high yield by adding 0.2 mmol (18 µL) of
dioxane to a suspension of [AuTl(C6Cl5)2]n (0.2 mmol, 0.18 g) in
toluene (20 mL). The suspension was stirred for 15 minutes, and the
solvent was evaporated to ca. 5 mL. Addition of n-hexane led the
precipitation of 1 as a white solid. Yield: 0.15 g (75%). 1H NMR (300
MHz, [D8]THF, 25 °C, TMS): d 7.3 (m, 10H; CH), 3.5 (s, 8H; CH2),
2.30 ppm (s, 6H; CH3); MS: (ES2): m/z (%): 695 (100) [Au(C6Cl5)2]2;
(ES+): 204 (100) Tl+; elemental analysis calcd for C42H24Au2Cl20O2Tl2
(%): C 24.34, H 1.17; found: C 24.75, H 0.79.
2
overlapping of the 5dz (Au) and 6s(Tl) orbitals reducing the
HOMO-LUMO gap and, consequently, the energy of the
emission. By contrast, in this case, complex 1 displays an
unprecedented blue emission for these Au–Tl systems. This
result is likely to be originated by the characteristics of this
interaction.
Finally, complex 1 loses its emissive properties in solution,
where the Au–Tl interaction is probably no longer present. In
addition, the electronic absorption spectra in acetone for 1 and
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for the gold( ) precursor NBu4[Au(C6Cl5)2] show similar
I
features with very low absorptions in the 385–324 nm spectral
9 G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refine-
ment, University of Göttingen, Germany, 1997.
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Fig. 2 Excitation and emission spectra of complex 1 in the solid state at
293K (upper lines) and at 77K (lower lines).
CHEM. COMMUN., 2003, 1760–1761
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