Click Reaction Driven, Highly Fluorescent Dinuclear Organogold(I) Complex Exhibits a Dual Role: A Rare Au···H Interaction and an Antiproliferative Agent

Article abstract of DOI:10.1021/acs.organomet.9b00291

The metal-hydrogen (M···H) bonds in transition elements are well documented. However, the attractive Au^I···H bond interaction is rare and significant experimental evidence for it still remains a challenge. Herein, we exhibit a rare example of a

Full text of DOI:10.1021/acs.organomet.9b00291

Communication
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Click Reaction Driven, Highly Fluorescent Dinuclear Organogold(I)
Complex Exhibits a Dual Role: A Rare Au···H Interaction and an
Antiproliferative Agent
Sanjay K. Verma,† Shagufi Naz Ansari,† Pratibha Kumari,‡ and Shaikh M. Mobin*,†,‡,§
‡
§
†Discipline of Chemistry, Discipline of Biosciences and Bio-Medical Engineering, and Discipline of Metallurgy Engineering and
Materials Science, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore 453552, India
S
* Supporting Information
ABSTRACT: The metal−hydrogen (M···H) bonds in
transition elements are well documented. However, the
attractive AuI···H bond interaction is rare and significant
experimental evidence for it still remains a challenge. Herein,
we exhibit a rare example of a C−H···Au interaction in both
the solid state and solution by employing a single-crystal X-ray
diffraction study and NMR spectroscopy for the novel
dinuclear Au(I) complex 1, prepared by a click reaction
between organized and strained internal alkynes ((PPh3Au−
CC−)2−C6H4) as reacting partners. Furthermore, 1 was
found to be highly fluorescent in nature and revealed remarkable antiproliferative properties against skin melanoma cell lines
with IC50 values ranging from 3.12 to 8.5 μM.
ransition metal complex based M···H interactions have
ensuing diverse applicability,32 including anticancer activity.33
The noncovalent interactions are the primary driving forces for
the construction of exciting supramolecular architectures in
gold complexes. In particular, organometallic gold complexes
have drawn attention because of their aurophilic properties,34
appealing photophysical properties, and their ability to build a
supramolecular array.35 The catalytic chemistry of gold has
been strongly stimulated by the availability of 1,2,3-triazoles as
convenient ligands in catalysis.36 These triazole-containing
heterocyclic ligands have properties that are often congruent to
the class of phosphine ligands, and they stabilize highly unusual
and previously obscure reactive species.37 Conjugated
ethynylated materials and metal-containing acetylide com-
plexes have been investigated for their luminescent,38
conducting,39 supramolecular,40 and electron/energy transfer
properties.41 Gold(I) complexes are an important class of
metal complexes due to their luminescent nature, featuring a
triplet excited state emission, a relativistic property which
allows for the formation of weak Au···Au interactions.42
Recently, there has been tremendous interest in organogold
chemistry due to the unique AuI···H bonding interactions in
gold(I) complexes which can affect not only solid-state
properties such as crystal packing but also chemical reactions
and solution-state studies. Au(I) complexes have recently
gained attention because of their strong antiproliferative effects
due to their antimitochondrial activity, inhibition of the
enzyme TrxR, and increasing the formation of reactive oxygen
species (ROS) in cells.43,44 Currently non-platinum derivatives
T
attracted enormous interest in fundamental and applied
chemistry, such as in organometallic catalysts.1 In general, the
M···H hydrogen bonds involving N−H/O−H donor groups
with various transition-metal elements are well docu-
mented.2−11 In addition, aurophilic (Au/Au) interactions
have been well reported.12 However, the Au···H interaction
in gold is still rarely found and has been less explored in gold
chemistry and is present either due to the ligand−counterion
interaction or crystal packing rather than direct interactions
between Au and H atoms.13−17 However, M···H hydrogen
bonds in other transition-metal complexes have been well
investigated both structurally in the solid state and
spectroscopically in the solution state.4−9 However, to the
best of our knowledge there exist few reports of experimental
evidence of rare C−H···Au “hydrogen bond” interactions both
in the solid state and in the solution state.1,18
C−H···Au contacts are commonly complementary in nature
and are observed together with stronger noncovalent
interactions. The Au···H−C interactions are mainly observed
in the phosphine and thione complexes of Au(I), due to the
presence of an almost linear geometry around the gold atoms,
which influence the coordination site for the formation of
gold−hydrogen interactions. Generally, Au···H−C contacts
have been observed in the case of intermolecular Au(I)
complexes, whereas intramolecular Au···H−C interactions
restrict the supramolecular 1D-chain.19−29
Recently, organogold(I) 1,2,3-triazolate complexes have
gained much attention because of their fascinating luminescent
behavior in order to understand their photophysical and
photochemical properties30,31 and mechanisms and their
Received: May 2, 2019
© XXXX American Chemical Society
A
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are being intensely studied for their application in cancer
chemotherapy.43 To achieve a specific biomolecular target, the
metal complex is prepared through a strategic design of the
ligand.
Single crystals of the Au(I) complex 1 were obtained by slow
evaporation of a dichloromethane solution at room temper-
ature. The single-crystal X-ray diffraction technique has been
employed to better understand the influence of substituents on
the crystal packing features in the solid-state structure of
complex 1.
1 crystallizes in the monoclinic crystal system with space
group P21/n (Tables S1 and S2). X-ray diffraction analysis of
the crystal confirmed the formation of dinuclear Au(I)
complex 1 (Figure 1). The asymmetric unit contains a half
Herein we report the designed synthesis, characterization,
and photophysical properties of the dinuclear Au(I) complex
1, in which the Au(I) units are attached with abnormal
mesoionic carbenes of 1,2,3-triazoles. 1 displays a red shift and
stronger fluorescence with a high quantum yield probably due
to greater perturbation of Au(I) on the π system. Additionally,
single-crystal studies of Au(I) complex 1 reveal that it shows
rare noncovalent C−H···Au(I) intramolecular interactions.
Au(I) complex 1 was synthesized by the reaction of
AuCl(PPh3) with 1,4-diethynylbenzene in sodium ethoxide
in ethanol solution, resulting in the bis product 1,4-C6H4(C
CAu(PPh3))2 in 82% yield (Scheme 1).45 1,4-C6H4(C
Scheme 1. Schematic Representation of the Synthesis of 1
Figure 1. Perspective view of 1 showing Au(I)···H(1)−C(1)
intramolecular interactions.
formula unit of the Au(I) complex 1. The coordination
geometry around the Au(I) atom is found to be linear with the
C5−Au1−P1 bond angle measuring about 174.08(3)°, an
Au1−C5 bond length of 2.052(11) Å, and Au1−P1 bond
length of 2.266(3) Å, falling within the range previously
described for gold complexes.
The C4−C5−Au1 and Au1−C5−N3 bond angles are
132.80(9) and 123.40(9)°, respectively. The dihedral angle
between the NHC plane {N1N2N3C4C5} and the phenyl ring
of the benzonitrile unit {C7C8C9C10C11C12} is found to be
76.08° (Figure S8).48 A closer look at the packing features of 1
reveals the presence of rare C−H···Au(I) intramolecular
interactions. The intramolecular interaction C(1)−H(1)···
Au(1), at a distance of 2.638(12) Å, involves the donor
carbon atom of the phenyl group and the gold(I) acceptor
atom (Figure 1).
CAu(PPh3))2 undergoes a Cu(I)-catalyzed click reaction with
4-(azidomethyl)benzonitrile in the presence of sodium
ascorbate and copper sulfate in tert-butyl alcohol and water
mixture under reflux conditions for 4−5 h and the resulting
product obtained is the dinuclear Au(I) complex 1 (Scheme
1).46,47 1 was characterized by mass spectrometry, 1H and 13
C
NMR, and a single-crystal X-ray diffraction study. The Au(I)
complex 1 is well soluble in dimethylformamide and slightly
soluble in dichloromethane and chloroform. The progress of
the reaction was monitored by TLC, and the formation of 1
was authenticated by 1H, 13C, and 31P NMR spectroscopy. The
Au(I) complex 1 was characterized by FTIR spectroscopy. The
band at ∼3100−3000 cm−1 was due to the presence of
A recent report by Abu Bakar et al.1 has thoroughly detailed
C−H···Au interactions in simple diphosphine ligand based
hexagold clusters, whereas in our case we have designed the
gold(I) complex 1 by activating the challenging internal
strained alkyne unit by employing a click reaction between the
dimeric {(PPh3Au−CC−)2−C6H4} and 4-(azidomethyl)-
benzonitrile, which resulted in this rare C−H···Au noncovalent
interaction. The bond distances for C−H···Au at 2.638(12) Å
(present work) and 2.62 Å (reported) are in agreement.
The packing diagram of 1 reveals that the coordination
environment of Au(I) is extended further by two different
intermolecular C−H···π interactions between the triazole ring
and methylene (H6A) proton having distances of 2.748 Å and
another intermolecular C−H···π interaction involving the H6B
proton of the same methylene group and one of the phenyl
ring of the PPh3 group having a distance of 3.130 Å (Table
S3). It has been observed that neighboring molecules are
linked together through these interactions along the a axis and
c axis, yielding a one-dimensional polymeric chain (Figure 2
and Figure S9).
1
aromatic C−H stretching vibrations. The H NMR spectrum
of 1 shows the absence of ethynyl C−H protons of 1,4-
diethynylbenzene after substitution with gold which was
otherwise observed at δ 5.78 ppm for its parent compound.
The resonance for the central aryl C−H protons (δ 8.70−8.62
ppm) was shifted significantly downfield in comparison to their
analogous resonance (δ 7.37 ppm) in complex 1 (Figures S1−
S4). Similarly, the resonance for the carbon atoms of the
central phenyl ring was also shifted downfield (δ 131.1−133.1
ppm) in comparison to their corresponding resonances in the
free phenyl ring (Figure S5). The resonances for the
characteristic carbene carbon atoms were observed at δ
141.8 ppm in the 13C{1H} NMR spectra of complex 1. 1
was also characterized by a 31P NMR spectrum (Figure S6).
The LC-MS spectrum of gold(I) complex 1 has a peak at m/z
1358.97, which corresponds to [M]+ (Figure S7) .
After having witnessed the presence of strong C−H···Au(I)
intramolecular interactions in the solid state via single-crystal
X-ray studies, we broadened the scope by exploring the
B
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and NaNO3 as an acceptor, resulting in the downfield shift of
the H-2 proton. Gold(I) complex 1 isolated from the above
1
experiment is found to be stable, as shown in the H NMR
spectrum recorded after 48 h in the DMSO-d6 solution (Figure
S4).
Further, the electronic absorption spectral data of the Au(I)
complex 1 were recorded in CH2Cl2 solution. The absorption
bands of 1 were observed at 220 and 280 nm and were
attributed to the π → π* and n → π* transitions for the ligand
moiety, occurring due to the heteroaromatic moiety (Figure
4). The fluorescence emission data of the 1 were recorded in
Figure 2. Supramolecular C−H···π interaction of 1 along between a
and c-axis.
presence of the same C−H···Au(I) interactions in the solution
state by employing NMR spectroscopy.
1
The H NMR signals of C−H units of the central phenyl
ring appear at considerably downfield regions, indicating the
presence of hydrogen-bonding interactions in 1 (Figure 3). 31
P
Figure 4. UV−visible and fluorescence spectrum of complex 1 at
room temperature in 10−5 M CH2Cl2 solution.
CH2Cl2 solution. The emission spectrum of 1 exhibited the
highest intensity bands at 327 and 439 nm upon excitation at
λex 280 nm (Figure 4). Moreover, the Stokes shifts of Au(I)
complex 1 were found to be 47 and 159 nm. The quantum
yield of 1 was found to be 0.541.
Au(I) complexes are known for their unusual photo-
luminescence due to the σ-donating property of the anionic
carbons, which effectively raises the energy of the d−d states,
diminishing their deactivating effect. Thus, the combination of
the σ-donating cabon atom of the triazole moiety and Au atom
makes it interesting to explore the photoluminescence
properties of the Au−triazole unit. The photoluminescence
spectrum of 1 reveals the presence of two electronic transitions
at 327 and 439 nm attributed to π* → π and π* → n
transitions for the ligand moiety (Figure S10) upon excitation
at λex 280 nm. The lowest energy absorption for all of the
complexes has a metal to ligand charge transfer (MLCT) band
which shows an appreciable red shift in some of the unusual
complexes. In 1 both the emission spectrum and the
photoluminescence spectrum show a similar pattern of the
MLCT band, which splits into two partially resolved bands.49
The excellent photophysical properties of 1 prompted us to
explore the biological activities of 1. Thus, to determine
whether 1 inhibited the proliferation of cancerous cells, (i) a
skin melanoma cell line (A375), (ii) a prostate cancer cell lines
(DU145), (iii) a cervical cancer cell line (HeLa), and (iv) a
breast cancer cell line (MCF-7) were explored. In order to
check the cytotoxicity of 1, a normal human embryonic kidney
cell line (HEK) was also employed for comparative study.50
The average cell viability decreased with an increase in the
concentrations of the compounds. The concentration-depend-
ent inhibitory effects of 1 on the growth of A375, DU145,
HeLa, MCF-7, and HEK after 24 h incubation were depicted
as the IC50 values 3.12 0.35, 8.5 0.81, 8.5 0.79, 4.68
0.54, and 30 0.23 μM, respectively (Table S4 and Figure 5).
Figure 3. Comparison of 1H NMR spectra of the organogold(I)
complex 1 added to a 1.0 mM D2O solution of NaNO3 in DMSO-d6
solution. The arrow indicates the downfield shift of the H-2 proton
signals. The other unlabeled signals are due to the P−Ph protons.
NMR spectrum also suggest the presence of a P−Ph moiety in
the gold(I) complex 1. After the addition of 5−30 μL (1.0
mM) of NaNO3 solution in D2O, in the DMSO-d6 solution of
1 shows a downfield shift of the H-2 proton (δ 8.62 to δ 10.02
ppm). The Au···H interactions basically involve the H-2
proton of 1 and NaNO3 due to the electronic communication
between them. In such interactions C−H behaves as a donor
C
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right (Figure 6), as the cells were treated with 1 for 2 h,
specifies an increase in the fluorescence emission intensity,
Figure 6. Flow cytometry analysis (FACS) for reactive oxygen species
(ROS) generation by compound 1 performed using a DCFDA probe.
The generation of ROS causes a right shift in the histogram (green) in
comparison to cells the alone (red) and cells treated with DCFDA
(blue). A greater shift suggests a greater production of DCF and thus
a higher production of ROS.
resulting from the generation of DCF by the ROS-mediated
pathway by oxidation of DCFDA. These reactive oxygen
species may cause cell apoptosis.
Later, ROS generation was studied by confocal micros-
copy.52 HeLa cells treated with 1 and the DCFDA probe show
strong green fluorescence distributed throughout the cells. A
control experiment (without 1) did not show any green
fluorescence, suggesting no ROS generation (Figure S12).
In conclusion, we have presented here a synthetic strategy
for the organometallic dinuclear 1,2,3-triazole-based Au(I)
complex 1. 1 was well characterized by various standard
spectroscopic techniques such as FTIR, LC-MS, UV−vis,
fluorescence, and 1H, 13C, and 31P NMR. 1 has been
authenticated by a single-crystal X-ray diffraction study. For
the first time a rare and strong intramolecular Au···H−C
interaction of 2.638 Å in 1,2,3-triazole-based Au(I) complex
Figure 5. Cell viability studies of 1 on cancerous and normal cell
lines: (a) skin melanoma cell line (A375) and prostate cancer cell line
(DU145); (b) cervical cancer cell line (HeLa), breast cancer cell line
(MCF-7), and human embryonic kidney cell line (HEK). These cells
were exposed to different concentrations of 1 for 24 h, and then cell
viability was measured by an MTT assay.
We have also compared the inhibitory effects of 1 with those of
the complex [μ-(1,4-phenylenedi-2,1-ethynediyl)]bis-
(triphenylphosphine)digold (A) on A375 and HEK cells. It
was found that 1 shows more antiproliferative activity in
comparison to (PPh3Au−CC−)2−C6H4 (A) on A375 and
HEK cells after 24 h incubation having IC50 values of 8.5
1
was identified by both H NMR techniques in the solution
0.51 and >30
0.46 mM, respectively (Figure S11).
state and single-crystal X-ray diffraction studies in the solid
state. This novel gold(I) compound reveals remarkable
antiproliferative properties against skin melanoma cell lines
(A375), prostate cancer cell lines (DU145), cervical cancer cell
lines (HeLa), and breast cancer cell lines (MCF-7) with IC50
values ranging from 3.12 to 8.5 μM. Moreover, thorough
localization of 1 was evident from confocal imaging with green
emission produced by conversion of DCFDA to DCF due to
ROS production inside cells. Induction of apoptosis via ROS
generation was further evidenced by flow cytometry studies
using DCFDA.
Interestingly, after the click reaction, the introduction of
heteroatoms such as 1,2,3-traizole moiety and the nitrile group
enhance the inhibitory effects. Further, an in vitro cytotoxicity
assay confirmed that the inhibitory effect of 1 on normal cells
is significantly lower in comparison to that the selected tumor
cell lines. 1 may act as a potential chemotherapeutic agent
against cancer.
Furthermore, reactive oxygen species (ROS) generated by
complex 1 were examined in live HeLa cells by a 2′,7′-
dichlorofluorescein diacetate (DCFDA) assay. DCFDA is a
fluorescence probe which can easily permeate live cells. It
measures peroxyl, hydroxyl, and ROS activity within the live
cells. Once DCFDA enters inside the cells, it is deacetylated by
cellular esterases to a nonfluorescent probe, which is later
oxidized by ROS within cells into the highly fluorescent 2′,7′-
dichlorofluorescein (DCF).51 The amount of DCFDA oxidized
to DCF is directly proportional to the generated fluorescence
signal. DCF has an emission at 529 nm which can be detected
by fluorescence spectroscopy or quantified by flow cytometry
analysis. A significant shift in the emission band toward the
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00291.
Experimental details, characterization data, crystallo-
graphic data, and cell viability studies (PDF)
D
DOI: 10.1021/acs.organomet.9b00291
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Communication
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Accession Codes
CCDC 1573150 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Anthracene Derived Dinuclear Gold(I) Diacetylide Complexes:
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AUTHOR INFORMATION
Corresponding Author
*E-mail for S.M.M.: xray@iiti.ac.in.
■
̈
̈
̈
(15) Koskinen, L.; Jaaskelainen, S.; Kalenius, E.; Hirva, P.; Haukka,
M. Role of C−H···Au and Aurophilic Supramolecular Interactions in
Gold−Thione Complexes. Cryst. Growth Des. 2014, 14 (4), 1989−
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ORCID
Shaikh M. Mobin: 0000-0003-1940-3822
Notes
The authors declare no competing financial interest.
(16) Martínez, A. Do Anionic Gold Clusters Modify Conventional
Hydrogen Bonds? The Interaction of Anionic Aun (n = 2−4) with the
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ACKNOWLEDGMENTS
■
The authors are grateful to the Sophisticated Instrumentation
Centre (SIC), IIT Indore, for providing characterization
facilities. S.K.V. is grateful to the SERB for providing a
National Postdoctoral Fellowship. S.N.A. and P.K. thank the
MHRD, New Delhi, for fellowships. S.M.M. thanks the SERB-
DST (Project No, EMR/2016/001113), New Delhi, Govern-
ment of India, and IIT Indore for financial support.
(17) Baukova, T. V.; Kuz’mina, L. G.; Oleinikova, N. A.;
Lemenovskii, D. A.; Blumenfel’d, A. L. New Organogold(I)
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