Synthesis of a fluorescent gold(I) complex with a thiosemicarbazone, [Au2(3-NO2-Hbtsc)4]Cl2 · 2CH3CN

Article abstract of DOI:10.1016/j.inoche.2008.09.023

Direct reaction of gold(I) chloride with 3-nitrobenzaldehyde thiosemicarbazone (3-NO₂-Hbtsc) in the presence of triphenylphosphine in acetonitrile has yielded ionic complex, [Au₂(3-NO₂-Hbtsc)₄]Cl₂ · 2CH₃CN (1). It exhibited an intense fluorescence band (λ_em) at 383 nm (λ_ex = 300 nm), and two bands of medium intensity at 413 and 436 nm (λ_ex = 350 nm). Complex 1 represents first example of ionic gold(I) complex with a thiosemicarbazone.

Full text of DOI:10.1016/j.inoche.2008.09.023

Inorganic Chemistry Communications 11 (2008) 1433–1435
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Inorganic Chemistry Communications
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Synthesis of a fluorescent gold(I) complex with a thiosemicarbazone,
[Au2(3-NO2-Hbtsc)4]Cl2 Á 2CH3CN
a
b
Tarlok S. Lobana a, , Sonia Khanna , Ray J. Butcher
*
a Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India
b Department of Chemistry, Howard University, Washington DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Direct reaction of gold(I) chloride with 3-nitrobenzaldehyde thiosemicarbazone (3-NO2-Hbtsc) in the
presence of triphenylphosphine in acetonitrile has yielded ionic complex, [Au2(3-NO2-
Hbtsc)4]Cl2 Á 2CH3CN (1). It exhibited an intense fluorescence band (kem) at 383 nm (kex = 300 nm), and
two bands of medium intensity at 413 and 436 nm (kex = 350 nm). Complex 1 represents first example
of ionic gold(I) complex with a thiosemicarbazone.
Received 2 September 2008
Accepted 27 September 2008
Available online 2 October 2008
Keywords:
Gold(I)
Ó 2008 Elsevier B.V. All rights reserved.
3-Nitrobenzaldehyde thiosemicarbazone
2D sheet structure
Fluorescence bands
The interest in gold(I) chemistry originates from its contribu-
tion to the field of medicine in the form of drugs for arthritis and
rheumatism [1–3], catalysis [4–6], photophysics (luminescence
and optical properties) [7] and in more recent nanotechnology
[8,9]. Gold(I) has a tendency to react readily with thio-ligands, such
as with heterocyclic thioamides in presence of tertiary phosphines
[10], and with thiosemicarbazones [11]. However, there is only one
NO2-Hbtsc) in CH3CN, in the presence of one mole of PPh3 did
not form the expected compound, [AuCl(3-NO2-Hbtsc)(PPh3)],
and instead
yielded
yellow
crystals
of
[Au2(3-NO2-
Hbtsc)4]Cl2 Á 2CH3CN (1). PPh3 did not coordinate to gold(I) atom,
and was separated along with crystals of 1 as white crystals of
[AuCl(PPh3)] [18a]. Reaction of gold(I) chloride with 3-NO2-Hbtsc
in 1:2 molar ratio in the absence of PPh3 in CH3CN yielded an insol-
uble compound, and addition of PPh3 was necessary to get the
same crystalline product 1. Here, PPh3 did not bind, and was sepa-
rated out from the mother liquor as free ligand. The latter proce-
dure enhanced the yield from 34% to 70%.
In this communication, we report first example of an ionic
gold(I)–thiosemicarbazone complex, [Au2(3-NO2-Hbtsc)4]Cl2 Á
2CH3CN (1), prepared using a new synthetic route involving direct
reaction of gold(I) chloride with a ligand (see Scheme 1).
neutral gold(I)–thiosemicarbazone complex, [AuCl(g
1-S-L)(PEt3)]
(HL = Vit. K3 thiosemicarbazone) [11], though there are more re-
ports on gold(III)–thiosemicarbazone chemistry [12–15].
In gold(I) chemistry, usually the precursors such as [AuX(tht)],
(X = Cl, Br) (tht = tetrahydrothiophene) [16a,16b], [AuCl(SMe2)]
(SMe2 = dimethylsulfide) [16c] or [AuCl(PPh3)] [17,18a] are reacted
with the ligands for preparing their complexes, as there are no re-
ports of direct reactions of gold(I) halides with the ligands. Here,
direct reactions of gold(I) chloride with N, S donor ligands have
been investigated. In a typical case, a direct reaction of gold(I) chlo-
ride with benzaldehyde thiosemicarbazone (R1 = Ph, R2 = H; Hbtsc)
in CH3CN in the presence of PPh3 did not yield any crystalline prod-
uct and during crystallization in different combinations of solvents
(CH3CN, CH2Cl2 or CH3OH), the compound decomposed. A similar
behavior was observed for salicylaldehyde thiosemicarbazone
(R1 = 2-OH-C6H4, R2 = H; H2stsc) with a hydroxyl group at ortho po-
sition of phenyl ring as well as for 2-acetylpyridine thiosemicarba-
zone (R1 = py, R2 = Me; Hapytsc).
IR spectroscopy of complex 1 confirmed the presence of
(C–N), (C@C), (C@N) vibrational modes. The thioamide
bands due to
(C@S) at 870 and 820 cmÀ1 show marginal shifts
m(N-
H),
m
m
m
m
relative to the free ligand. Complex 1 is insoluble in CDCl3 and
other organic solvents. The 1H NMR spectrum of complex 1
showed a downfield shift in the diagnostic signal of –N2H proton
at d 11.94 ppm vis-à-vis that in free ligand (d 11.51 ppm), which
supports the coordination of neutral thiosemicarbazone ligand.
Only one signal is observed due to –N1H2 protons at
d
7.78 ppm while other signal is merged in the multiplet of signals
due to the ring protons in the region, d 7.44–7.73 ppm. Complex
1 exhibited an intense fluorescence band (kem) at 383 nm upon
excitation at kex = 300 nm, and two bands of medium intensity
at 413 and 436 nm (red light emission) were observed on exci-
tation at kex = 350 nm (Fig. 1). Fluorescence may be attributed
Further, direct 1:1 molar reaction of gold(I) chloride with 3-
nitrobenzaldehyde thiosemicarbazone (R1 = 3NO2-C6H4, R2 = H; 3-
* Corresponding author. Fax: +91 183 2 258820.
E-mail address: tarlokslobana@yahoo.co.in (T.S. Lobana).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2008.09.023

1434
T.S. Lobana et al. / Inorganic Chemistry Communications 11 (2008) 1433–1435
O2N
4
H
S
N2
1
N3
C2
C
1
NH2
H
2AuCl, PPh3
CH3CN
Fig. 2a. Structure of monomeric unit of complex 1 with atomic numbering scheme.
Selected bond lengths (Å) and angles (°): Au–S(1A), 2.279(5), Au–S(1B) 2.278(4),
S(1A)–C(1A) 1.722(17), S(1B)–C(1B) 1.697(18), N(2A)–N(3A) 1.411(19), S(1B)–Au(-
S(1A) 172.70(16), C(1A)–S(1A)–Au 106.8(6), C(1B)–S(1B)–Au 108.3(7).
+2
Au
S
S
R
S
S
R
R
2Cl-
R
Au
1
Scheme 1.
Fig. 2b. Six membered cavity in the center of 1.
mation of complex 1 by direct reaction of gold(I) chloride with the
ligand in the presence of PPh3, which however did not coordinate
to the metal center. Other ligands, Hbtsc, H2stsc and Hapytsc, did
not form similar products. The fluorescence is attributed to the
presence of weak AuÁ Á ÁAu contacts.
Fig. 1. Fluorescence spectra of complex 1 (Pink curve, kex = 300 nm, kem = 383 nm;
blue curve, kex = 350 nm, kem = 413 nm and 436 nm). (For interpretation to colours
in this figure, the reader is referred to the web version of this paper.).
1: To AuCl (0.025 g, 0.11 mmol) suspended in acetonitrile
(15 mL) was added solid 3-NO2-Hbtsc ligand (0.021 g, 0.11 mmol),
followed by stirring for 2 h, resulting in separation of pale yellow
solid. It was followed by addition of solid PPh3 (0.028 g,
0.11 mmol) (in situ) resulting in a clear solution, which was filtered
and kept for crystallization. The slow evaporation of solution re-
sulted in the formation of yellow colored crystals of 1 along with
white crystals of [AuCl(PPh3)]. The yield was 34%. Reaction of
gold(I) chloride with 3-NO2-Hbtsc in 1:2 molar ratio in the absence
of PPh3 in CH3CN yielded an insoluble compound, and addition of
PPh3 was necessary to get the same crystalline compound 1. This
increased the yield to 70%.
to LMCT from ligand to gold(I) atom [RS ? Au(I) excited state]
[18b].
In complex 1, gold(I) atom is coordinated linearly to two sulfur
atoms of two thiosemicarbazone ligands at Au–S distances of
2.278(4) and 2.279(5) Å, shorter than 2.319(1) Å in [AuCl(g
1-S-
L)(PEt3)] (HL = Vit K3 thiosemicarbazone) [11]. The C–S bond dis-
tance of 1.697(18) and 1.722(17) Å is comparable to the bond dis-
tance of 1.718(6) Å in [AuCl(g
1-S-L)(PEt3)] [11]. The S–Au–S bond
angle is slightly bent with an angle of 172.70(16)°. No intramolec-
ular interactions are observed in the molecular structure of 1 (see
Fig. 2a).
Data for 1: M.p. 220–222 °C, yield: 34%. CHN analysis for
C16H16AuClN8S2: C, 28.21; H, 2.35; N, 16.46%; Found: C, 28.17; H,
Two monomeric molecules are linked to each other via AuÁ Á ÁAu
(3.698 Å) and SÁ Á ÁS interactions (3.477 Å) to form an associated di-
mer like structure with a six membered cavity in the center
(Fig. 2b). The SÁ Á ÁS interactions (3.477 Å) are smaller, while AuÁ Á ÁAu
interactions (3.698 Å) are longer than the sum of van der Walls ra-
dius of sulfur and gold atoms respectively [19]. Chloride anion
forms hydrogen bond with imino hydrogen atoms (–N2HÁ Á ÁCl) to
form a H-bonded dimer. The amino hydrogen of one thiosemicar-
bazone ligand is engaged in hydrogen bonding with another chlo-
ride anion (ClÁ Á ÁHN1H) resulting in 2D structure.
2.36; N, 16.91%. Main IR peaks (KBr, cmÀ1):
m
(N–H) 3443b, 3256b
(C–H) 3050 m; (C@N) + d
(C@S) 870s, 820s (thioamide
(C–N) 998 m, 949s. 1H NMR (CDCl3, dmso-d6, d ppm):
7.44–7.73 (m, Ph + –N1H2), 7.78 (sb, –N1H2), 11.94 (s, –N2H). Elec-
(–NH2–), 3142 m
NH2 + (C@C) 1585s, 1525b, 1479s;
moiety);
m
(–NAH);
m
m
m
m
m
tronic absorption data (dmso, kmax/nm,
e ) 325
/L molÀ1 cmÀ1
(1.31 Â 104) Fluorescence data (dmso, kex = 300 nm, kem = 383 nm;
kex = 350 nm, kem = 413, 436 nm).
Crystallographic data for 1: C18H19AuClS2O4, M = 721.96, mono-
The chloride anion is thus involved in hydrogen bonding with
both imino and amino hydrogen atoms to form a 2D sheet like
structure (Fig. 3). Acetonitrile molecules present as a solvent of
crystallization form hydrogen bond with an amino hydrogen atom
(HN1HÁ Á ÁNCCH3).
clinic, a = 14.5869(4), b = 10.9442(2), c = 15.9379(5) Å,
a = 90,
b = 105.802(3),
c
= 90°, V = 2448.20(11) Å3, T = 200(2) K, Space
group P1 21/c 1,
q
calcd = 1.959 g cmÀ3
,
Z = 4,
l(MoKa) =
14.294 mmÀ1, 10565 reflections measured on a Oxford Diffraction
Gemini diffractometer unique 4345 (Rint = 0.0300). The final R1
It may be concluded that the presence of nitro group at 3-posi-
tion of the phenyl ring in 3-NO2-Hbtsc ligand has stabilized the for-
0.0772 was for 3798 reflections [I > 2r (I)] and wR2 was 0.2030.
Crystallographic data for the structural analysis in CIF format has

T.S. Lobana et al. / Inorganic Chemistry Communications 11 (2008) 1433–1435
1435
Fig. 3. 2D sheet network of complex 1.
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Acknowledgement
Financial support from CSIR, New Delhi to one of us (Sonia) is
gratefully acknowledged.
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