Au2trien: a dinuclear gold(III) complex with unprecedented structural
features
Luigi Messori,a Francesco Abbate,a Pierluigi Orioli,*a Caterina Tempia and Giordana Marconb
a Department of Chemistry, University of Florence, Via G. Capponi 7, 50121 Florence, Italy.
E-mail: orioli@cerm.unifi.it
b CIRCMSB, Local Unit of Florence, University of Florence, Via G. Capponi 7, 50121 Florence, Italy
Received (in Cambridge, UK) 18th December 2001, Accepted 5th February 2002
First published as an Advance Article on the web 25th February 2002
The X-ray structure of a dinuclear gold(III) complex,
Au2trien, shows the presence of two square planar gold(III
centers bridged by a nitrogen donor, in a very unusual
fashion.
is a deprotonated amino nitrogen bridging the two gold(III)
)
centers. The two gold(III) coordination planes (Au1, N1, N4,
N7, Cl1) and (Au2, N7, Cl2, Cl3) form an angle of 76.6(2)°.
After completing the crystallographic determination, some
features of the solution behavior of Au2trien were analyzed by
electronic absorption and 1H NMR spectroscopies. Water
solutions of Au2trien are pale yellow and markedly acidic as
commonly found for gold(III) complexes with gold-coordinated
chlorides (indeed, the pH of freshly prepared mM solutions of
Au2trien falls in the range 2.5–3.0). The absorption spectra are
dominated by a main band, centered at 360 nm, that is
tentatively assigned as a N2 to gold(III) LMCT transition.6
Notably, the complex is stable in water, at low pH, for several
hours as suggested by the substantial stability of the visible and
1H NMR spectra. No significant spectroscopic changes were
observed over 3 h even after raising the temperature to 50 °C.
In contrast, the 1H NMR spectrum changes greatly by
increasing the pH and a complicated spectral pattern is observed
diagnostic of the presence of multiple species.§ Complex pH
dependent spectral patterns were previously reported by Sadler
for the simpler Audien compound.7 Studies are in progress to
elucidate the pH dependent equilibria of Au2trien. Eventually,
near physiological pH, gold partially precipitates as colloidal
gold. The yellow compound remaining in solution, at pH 7.4,
most likely corresponds to mononuclear Autrien as suggested
by ESI-MS measurements.¶
Today there is renewed interest in gold(III) complexes as
possible cytotoxic and antitumor drugs. Indeed, a number of
gold(III) complexes were recently described that possess
sufficient stability under physiological conditions and exhibit
relevant cytotoxic effects on cultured human tumor cell lines.1–3
In all cases only mononuclear gold(III) complexes were
considered.
Specifically, we have reported on a series of gold(III
)
polyamine complexes characterized by high stability at neutral
pH and by significant cytotoxic properties.4 Within this frame
the reaction of tetrachloroauric acid with the ligand triethylene-
tetramine (trien) has been carried out to obtain the homologue of
the well known diethylenetriamine gold(III) complex (Audien).5
The reaction,6 performed at low pH, led to the formation of a
gold(III) product with a yield of ca. 20%. This compound was
isolated and recrystallised from water as small pale yellow
crystals. To our surprise, elemental analysis data did not
confirm formation of the expected mononuclear Autrien
complex but rather indicated a 2+1 gold to ligand stoichio-
metry.† The compound [Au2(trien)Cl3]Cl2·2H2O (Au2trien)
was further characterized by crystallographic methods that
clearly revealed its dimetallic nature.‡ The structure of the
dication [Au2(trien)Cl3]2+ is shown in Fig. 1.
To our knowledge Au2trien is the first example of a water
soluble dinuclear gold(III) complex with N- and Cl-donors and
with asymmetric gold(III) centers. The crystal structure showing
a deprotonated amino group bridging two gold(III) centers is
unprecedented. Remarkably, the two gold(III) ions lower the
pKa of the bridging R2NH group to well below 3. Also, it is of
interest to observe that dinuclear Au2trien forms preferentially
with respect to mononuclear Autrien even in the presence of
excess ligand, suggesting that binding of gold(III) to trien is
cooperative and that formation of the compact dinuclear
complex is thermodynamically favored.
Notably this dinuclear gold(III) complex presents two square
planar gold(III) centers with different sets of donors. Au1 is
coordinated to three nitrogens and one chloride while Au2 is
coordinated to two nitrogens and two chlorides. The distance
between the two gold(III) ions is 3.46 Å. Most interestingly there
We thank Dr Samuele Ciattini for technical assistance in
collecting X-ray diffraction data and Cassa di Risparmio di
Firenze for the financial support.
Notes and references
† † Synthesis: A solution of trien·4HCl (1.36 g, 4.7 mmol) in water (2 ml)
was added slowly, with stirring, to a solution of HAuCl4·3H2O (0.70 g, 1.8
mmol) in the same solvent (2 ml). A yellow precipitate immediately formed.
The pH was adjusted to 1.7 by addition of sodium hydroxide. The mixture
was left stirring at 0 °C for 2 h. The light yellow precipitate was then filtered
off, washed twice with cold ethanol and dried over P4O10. The yield was ca.
20%. Found C 10.9%.
Fig. 1 Molecular structure of the dication [Au2(trien)Cl3]2+. All H omitted
for clarity. Selected bond distances (Å) and angles (°): Au1–N1 2.045(9),
Au1–N4 2.017(9), Au1–N7 2.060(9), Au1–Cl1 2.287(3), Au2–N7 2.074(8),
Au2–N10 2.024(9), Au2–Cl2 2.275(3), Au2–Cl3 2.282(3), N1–Au1–N4
84.2(4), N4–Au1–N7 85.9(4), N1–Au1–Cl1 95.2(3), N7–Au1–Cl1 95.0(2),
N4–Au1–Cl1 176.4(3), N1–Au1–N7 168.7(4), N7–Au2–N10 85.2(4),
N10–Au2–Cl3 90.5(3), N7–Au2–Cl2 91.7(2), Cl2–Au2–Cl3 92.6(1), N10–
Au2–Cl2 176.6(3), N7–Au2–Cl3 175.6(2).
‡ Crystallographic data: Slight yellow crystals of Au2trien were obtained
from slow evaporation of
a
water solution at room temperature.
11.504(4), b =
C6H21Cl5N4O2Au2, M 753.46, monoclinic, a
=
=
11.909(2), c = 13.031(4) Å, b = 100.18(2)°, V = 1757.2(13) Å3, T = rt,
space group P21/c, Z = 4, m = 37.98 mm21, 3628 reflections measured,
2806 unique (Rint = 0.05) which were used in all calculations. The final
agreement factor was R = 0.0393 for 2537 Fo > 4s(Fo) and 0.0431 for all
2806 data. Data were collected at 20 °C on a P4 Siemens installed on a
612
CHEM. COMMUN., 2002, 612–613
This journal is © The Royal Society of Chemistry 2002
Messori, Luigi
