Platinum(II) and gold(III) allenylidene complexes: Phosphorescence, self-assembled nanostructures and cytotoxicity

Article abstract of DOI:10.1002/chem.201301481

The bright side of triple double bonds: Two series of stable tridentate cyclometalated Pt^II and Au^III allenylidene complexes were found to exhibit many functionalities, such as long-lived excited states, self-assembly into supramolec

Full text of DOI:10.1002/chem.201301481

COMMUNICATION
DOI: 10.1002/chem.201301481
Platinum(II) and GoldACTHNUTRGNE(UNG III) Allenylidene Complexes: Phosphorescence,
Self-Assembled Nanostructures and Cytotoxicity
Xin-Shan Xiao,[a] Wai-Lun Kwong,[a] Xiangguo Guan,[a] Chen Yang,[a] Wei Lu,*[b] and
Chi-Ming Che*[a, c]
Luminescent metal–carbon complexes are well document-
ed to have enormous impact to the development of materi-
als science and photochemistry.[1] We and others have been
working on the synthesis of new phosphorescent organome-
tallic complexes containing acetylide[2] and N-heterocyclic
carbene (NHC)[3] ligands and their applications in high-per-
formance OLEDs, nonlinear optics, chemosensors, bio-
ACHTUNGTRENNUNGprobes and photocatalysts. Along this line of research, cu-
mulenic metal complexes with long-lived excited-states
could be a starting point of investigation on new classes of
phosphorescent metal–carbon bonded complexes.
Cumulene is a reactive functional group but its stability
can be enhanced upon coordination to a metal ion.[4] Metal
complexes with an allenylidene (the C3 homologue of cumu-
lene) ligand continue to attract much attention since the iso-
lation of first stable allenylidene complexes by Fischer and
Berke in 1976.[5] The s-donation from a-carbon (Ca) and p-
interaction between metal ion and the linear allenylidene
À
bridge strengthen the M Ca bond, and hence render the
metal ion an intriguing site for catalysis and the allenylidene
ligand a conductive wire for charge transport.[6] Although a
variety of [LnM=C=C=C(R’)R’’] complexes, in which M is
metal ion of Cr, W, Mn, Ru, Os, Ir, Ag, Pd or Pt, have been
prepared,[4,7] only two types of allenylidene complexes have
been reported to display phosphorescence. Slageren and co-
Scheme 1. Synthesis and mesomeric structures of the PtII, AuIII and AuI
allenylidene complexes. Reaction conditions: a) CH3OTf, CH2Cl2, and
then NH4PF6.
workers reported that [Cl
plexes are emissive in glassy solutions at 93 K.[8] Nazeerud-
din and co-workers claimed [(ppy)2A(Ph3P)Ir=C=C=C-
(OMe)N
(CH2)4]+ to be the first allenylidene complex that
ACHTUNGERTN(NUNG dppm)2Ru=C=C=C(R’)R’’] com-
emits in fluid solutions (with an emission quantum yield of
2.9% and a lifetime of 465 ns).[9] Herein, we describe two
PtII and two AuIII allenylidene complexes (Scheme 1) sup-
ported by tridentate cyclometalated ligands having mixed
N- and C-donor atoms, three of which show phosphores-
cence in solutions at room temperature. These metal–alleny-
lidene complexes exhibit many intriguing functionalities in-
cluding self-assembled nanostructures and biological activity.
The present work features the first report on AuIII allenyli-
dene complexes. Just recently, Hashmi and co-workers re-
ported the X-ray crystal structure of the first AuI allenyli-
dene complex supported by a NHC ligand.[10]
CTHUNGTRENNUNG
G
ACHTUNGTRENNUNG
[a] X.-S. Xiao, W.-L. Kwong, Dr. X. Guan, C. Yang, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Institute of Molecular Functional Materials
HKU-CAS Joint Laboratory on New Materials, and
Department of Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (P.R. China)
E-mail: cmche@hku.hk
[b] Prof. Dr. W. Lu
Department of Chemistry
Our approach to phosphorescent PtII and AuIII allenyli-
dene complexes is outlined in Scheme 1. First, the respective
acetylide precursors (1a/b for PtII and 3a/b for AuIII,
Scheme 1) were prepared by a CuI-catalyzed reaction of
[(C^N^N)PtCl] and [(C^N^C)AuCl] (both C^N^N and
C^N^C represent p-conjugated cyclometalated/oligopyridyl
ligands) with propargylic amine. Second, the heteroatom-
South University of Science and Technology of China
Shenzhen, Guangdong 518055 (P.R. China)
E-mail: luw@sustc.edu.cn
[c] Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen, Guangdong 518053 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301481.
Chem. Eur. J. 2013, 19, 9457 – 9462
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9457

conjugated allenylidene ligand was introduced to the cyclo-
metalated metal complexes through methylation on the pro-
piolamide moiety of the respective acetylide precursors
(Scheme 1).[7c] A subsequent anion metathesis from OTfÀ to
PF6À precipitated the desired salts in 78–85% yields. The de-
tailed procedures for the synthesis are provided in the Sup-
porting Information. Complexes 2a/b are dark colored
solids whereas 4a/b are yellowish in color. All of these com-
plexes are stable towards air and moisture.
1
The H NMR spectra of the PtII complexes 2a/b and AuIII
complexes 4a/b in CD3CN each exhibit two triplet peaks
due to the two N-bound CH2 groups at 3.70/4.00, 3.62/3.94,
3.73/4.02 and 3.65/3.99 ppm, respectively, revealing that the
2
À
C
N
rotation in solutions. Compared with their corresponding
acetylide precursors 1a/b and 3a/b, an additional singlet
peak for the OCH3 group at 4.39, 4.32, 4.40 and 4.42 ppm
appears in the 1H NMR spectra of 2a/b and 4a/b, respective-
ly. The 13C NMR signal for Cb of the allenylidene ligand in
2a/b and 4a/b can be clearly identified at 90.54, 91.77, 88.80
and 85.16 ppm, respectively, which are shifted upfield from
that of 1a/b and 3a/b at 100.17, 100.08, 95.97, 92.90 ppm, re-
spectively. The IR spectra of complexes 2a/b and 4a/b all
show a sharp u
2146 cmÀ1 respectively; these u
to lower-energy from the u
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
acetylide precursors 1a/b and 3a/b at 2091, 2089, 2153 and
2149 cmÀ1, respectively. These spectroscopic data lend sup-
port to the allenylidenic character in complexes 2a/b and
4a/b. However, the extents of both the upfield shift of the
Figure 1. Perspective views of the cations of: a) PtII allenylidene complex
2a, and b) AuIII allenylidene complex 4a, determined with X-ray crystal-
lography, with hydrogen atoms and counter-ions omitted for clarity. The
Wiberg bond orders calculated based on the crystallographic parameters
are shown as bold numbers. Frontier orbitals of: c) 2a, and d) 4a, calcu-
lated based on the crystallographic geometries.
Cb signal and the lowering of the u
cant when compared with that found for the related com-
plexes [Br(PPh3)2M=C=C=C(OMe)N
(CH2)4]+ (M=Pd or
Pt)[7c] but comparable with that observed for complex
[(ppy)2A(Ph3P)Ir=C=C=C(OMe)N
(CH2)4]+.[9] Thus, the acety-
ACHTUNGERTN(NUNG C=C=C) are less signifi-
A
R
ACHTUNGTRENNUNG
C
E
U
of 2a and 4a, the complex cations are stacked in pairs in a
head-to-tail fashion, with an interplanar p–p separation of
around 3.4 ꢁ. For 2a, the intermetal distance of 3.29 ꢁ is
shorter than the sum of van der Waals radii of two Pt atoms
(3.44 ꢁ) and, together with the dark color of this complex in
solid state, reveals remarkable intermolecular Pt···Pt interac-
tions in the crystal structure.
The Wiberg bond orders in 2a and 4a are shown in
Figure 1 as bold numbers. A comparison in bond orders be-
tween the structures of 2a and 4a shows that the allenyli-
lide mesomeric structures have to be considered in the de-
scription of their overall bonding interactions as depicted in
Scheme 1.
The X-ray crystal structures of 2a and 4a have been de-
termined.[11] Perspective views of their complex cations are
shown in Figure 1a and b, respectively; detailed crystal data
are provided in the Supporting Information. Both the Pt
atom in 2a and Au atom in 4a adopt a distorted square-
À
planar geometry. The Pt1 C17 length of 1.925(8) ꢁ in 2a is
À
À
À
À
shorter than the Pt C
(ꢀCb) and Pt C
N
denic character is more significant in 2a (Pt Ca 1.23, Ca Cb
À
À
À
1.957(9)–1.98(1) ꢁ and 1.982–2.016 ꢁ, respectively, found in
2.51 and Cb Cg 1.22) than that in 4a (Au Ca 1.03, Ca Cb
the related [(C^N^N)PtCꢀCR][2b] and [(C^N^N)Pt-
2.61 and Cb Cg 1.19). This is consistent with the stronger p-
À
(NHC)]+[3c] complexes. The Au1 C18 length of 1.956(6) ꢁ
back-donating capability of PtII than AuIII. The bond orders
À
À
À
À
in 4a is only slightly shorter than the Au C
aA
of Cg N and Cg O in 2a (1.54 and 1.53) and 4a (1.57 and
1.38) are larger than a single bond, indicating that the acety-
lide mesomeric structures (Scheme 1) weigh in the bonding
structures of these complexes, in line with the NMR spectro-
scopy and IR characterizations.
CaACHTUNGTRENNUNG(NHC) length of 1.967–2.008 ꢁ and 1.967–2.017 ꢁ, re-
spectively, found in the related [(C^N^C)AuCꢀCR][2d] and
[(C^N^C)Au
(NHC)]+[3a] complexes. The C17 C18 length of
À
À
1.22(1) ꢁ in 2a and C18 C19 length of 1.208(8) ꢁ in 4a are
slightly longer than the Ca ꢀCb bond generally observed for
PtII (typically 1.18 ꢁ)[2b] and AuIII (typically 1.19 ꢁ)[2d] acety-
lide complexes. These bond lengths reveal allenylidenic
character in complexes 2a and 4a. In the crystal structures
The electrochemical redox potentials of 1a/b, 2a/b, 3a/b
and 4a/b were measured using 0.1m nBu4NPF6 as support-
ing electrolyte in acetonitrile (for PtII complexes) or DMF
(for AuIII complexes) and reported versus the Fc+/Fc
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Platinum(II) and GoldACTHNUTRGNEU(GN III) Allenylidene Complexes
COMMUNICATION
couple. The oxidation (irreversible anode peak)/reduction
(quasi-reversible) of 0.66/À1.61 V for 2a and 0.79/À1.71 V
for 2b are anodically shifted from those of 1a (0.49/
À1.73 V) and 1b (0.46/À1.84 V); this is consistent with an
increased p-back-bonding donation from Pt to the allenyli-
dene moiety. The electrochemical reduction of the AuIII
complexes 3a/b and 4a/b are irreversible, but a similar shift-
ing trend in cathode peak potentials has also been observed
from the Au–acetylide (À1.83 V for 3a and À1.96 V for 3b)
to Au–allenylidene (À1.62 V for 4a and À1.59 V for 4b)
complexes.
tions in solution. As for 4a and 4b, their absorption spectra
display characteristic features as that of other AuIII com-
plexes having the same cyclometalating ligands, which are
mainly attributed to intraACTHNUTRGENNGlU iCAHUTGTNRENgUNG and charge transfer (ILCT) tran-
sitions.[12] The frontier molecular orbitals (FMO) calculated
based on crystallographic geometries of 2a and 4a are
shown in Figure 1c and d, respectively. The electron density
in HOMO and LUMO of 2a are mainly localized on the Pt
d-orbital/phenyl of C^N^N and bipyridyl of C^N^N, respec-
tively, revealing a mixture of MLCT/ILCT transitions to ac-
count for the lowest-energy absorption of 2a. The electron
density in HOMO and LUMO of 4a are mainly localized
on the C^N^C and allenylidene ligand, respectively, reveal-
ing a LLCT transition to be responsible for the lowest-
energy absorption of 4a.
The electronic absorption and emission data of complexes
2a/b and 4a/b together with their acetylide precursors 1a/b,
and 3a/b are listed in the Supporting Information and the
representative spectra for 2a and 4b are shown in Figure 2.
Upon excitation beyond 400 nm, PtII complexes 2a and
2b in diluted and degassed CH2Cl2 solutions show strong
yellow-green emission with lmax (quantum yield, lifetime) of
542 nm (34%, 3.9 ms) and 530 nm (63%, 4.5 ms), respective-
ly. Based on the T1 optimized geometries, the TDDFT ap-
proach associated with the polarized continuum model
(PCM) in CH2Cl2 was used to calculate the emission proper-
ties. To better describe the charge-transfer character of the
electronic excitation of 2a and 2b, the density functional
wB97X-D,[13] which includes 100% long-range exact ex-
change, 22% of short-range exact exchange, the B97 corre-
lation density functional,[14] and empirical dispersion correc-
tions, was employed for the TDDFT calculations. The calcu-
lated triplet emissions of 2a and 2b in solutions are in the
520–525 nm region; this is in good agreement with the emis-
sion data from experiments. These emissions can be attribut-
3
ed to MLCT/ILCT excited states with reference to previous
works on cyclometalated PtII complexes.[15] Complex 2a ex-
hibits an emission maximum at 683 nm whereas the emission
of 2b is markedly red-shifted to the NIR region (lmax
=
785 nm). On the other hand, the AuIII complex 4b having
extended C^N^C ligand gives dual emission bands in dilut-
ed, degassed CH2Cl2 solution upon excitation at 385 nm; the
weak band at around 450 nm and the vibronically structured
band at 525 nm (with a quantum yield of 3.3% and a life-
time of 111 ms) are ascribed to excited states having singlet
and triplet ILCT origin, respectively.[12] Although 4a is
weakly emissive in solutions, it shows an intense bright-
yellow emission at lmax of 540 nm (with a lifetime of 5.8 ms)
upon excitation at 350 nm in solid state at 298 K. The blue-
shift emission of 4a with respect to 2a is due to the higher
p* (LUMO) energy of allenylidene than that of bipyridyl
moiety of C^N^N.
The combination of a planar cationic structure and intri-
guing phosphorescent properties renders these metal alleny-
lidene complexes to potentially act as efficient switched-on
probes for DNA molecules in aqueous solutions. Thus, an
emission titration was performed with solutions of 2a and
2b (2 mm) in Tris-buffered saline/DMSO (3 mL, 99:1 v/v) by
aliquots of a calf thymus DNA (ctDNA) stock solution
(37 mm). As depicted in Figure 3a, upon increasing the
ctDNA concentration from 0 to 3.6ꢂ10À4 m in intervals of
Figure 2. Absorption and normalized emission spectra of: a) 2a, and
b) 4b in different matrixes.
To differentiate the absorption of allenylidene moiety from
those of the cyclometalated ligand, a model complex, the
heteroleptically coordinated allenylidene AuI complex (6)
was prepared from its acetylide precursor (5; Scheme 1). A
CH2Cl2 solution of 6 shows a distinct lowest-energy absorp-
tion band at 278 nm (see the Supporting Information for a
spectrum) and a structureless emission at lmax =416 nm, re-
vealing that ligand-centered transitions of the allenylidene
moiety itself do not occur at spectral region beyond 300 nm.
Solutions of complexes 2a and 2b in CH2Cl2 are yellow with
a modest low-energy absorption band at lmax =400 nm.
When the concentration of 2a in CH2Cl2 is increased
beyond 2ꢂ10À5 m, an additional distinct absorption band
emerges at lmax =440 nm; this band is tentatively ascribed to
a MMLCT transition derived from molecular aggregates
through Pt···Pt interactions. The absence of MMLCT absorp-
tion band in the case of 2b is attributed to the two bulky
tert-butyl groups disfavoring intermolecular stacking interac-
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W. Lu, C.-M. Che et al.
Figure 3. a) Emission traces of 2a (2 mm) with increasing concentration of calf thymus DNA (ctDNA) in Tris-HCl (5 mm) buffer and NaCl (50 mm) at
pH 7.2. Inset: response curve of I/Io against [ctDNA]/ACTHNUGTRNEUNG[2a]. b) Image of 2a (20 mm in buffer) in the presence of ctDNA on a transilluminator. Enhanced
emission of 2a was observed with increasing ctDNA concentration. c) Cellular imaging of 2a and 2b in viable HeLa cells, observed under fluorescence
microscope with 100ꢂ oil immersed objective (excitation: 450 nm). Lower panel shows the enlarged images revealing the subcellular localization of 2a
and 2b. d) Cytotoxicity profiles of complex 2a, 2b, 4a and 4b towards HeLa cells as determined by MTT assay; cisplatin was used as a control.
4.0ꢂ10À5 m, an emission band with lmax at approximately
580 nm for 2a/565 nm for 2b developed and finally reached
17-fold for 2a/fivefold for 2b enhancement in intensity. The
emission enhancement could be visualized with the naked
eye (Figure 3b). We thus further investigated the applica-
tions of these complexes in cell imaging. HeLa cells (15ꢂ
104) were treated with 2a or 2b (20 mm) for 2 h at 378C. Ob-
servation under a fluorescence microscope equipped with
450 nm excitation filter (Figure 3c) revealed living cell mor-
phologies with 2a predominately localized in the nucleus
whereas 2b mainly accumulated in the cytosolic region. The
bulky tert-butyl groups in 2b may impede the deep interca-
lation of 2b into DNA and hence entry into the nucleus.
The in vitro cytotoxicity of complexes 2a, 2b, 4a and 4b to-
wards HeLa cells were evaluated by using the MTT assays.
As depicted in Figure 3d, complexes 2a and 4a with IC50
(48 h incubation) values of 5.8 and 9.0 mm, respectively, are
more cytotoxic than cisplatin (IC50 =19 mm in the control ex-
periment).
bly of nanostructures.[16] Figure 4 depicts the morphologies
of the nanostructures obtained with 2a in solvent mixtures
of CH3CN/H2O at various volumetric ratios. Nanorods (typi-
cally 200 nm wide and several micrometers long) and nanor-
ings (typically 400 nm in outer diameter and 50 nm in inner
diameter) were observed by electron microscopy as domi-
nant morphologies in the 95:5 and 80:20 volumetric ratios of
CH3CN/H2O, respectively. Some intermediate morphologies,
such as bent nanorods and semicircles, were also detected
when the volumetric ratio of CH3CN/H2O was 90:10 or
85:15 (see the Supporting Information for micrographs).
Thus, increasing the water fraction in the solvent mixture fa-
cilitated the rod-to-ring transformation process. Interesting-
ly, all the rods and rings were crystalline in nature, showing
a typical d-spacing of 3.4 ꢁ along the rod longitude and the
ring periphery in their SAED patterns. This is evidence that
metallophilic and ligand p–p interactions play a key role in
the self-assembled formation of the intriguing nanostruc-
tures.
The metal···metal and p–p interactions of these allenyli-
dene complexes could act as the driving force for self-assem-
The nanostructures formed by complexes 2b and 4b were
also studied for comparison (see the Supporting Information
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Platinum(II) and GoldACTHNUTRGNEU(GN III) Allenylidene Complexes
COMMUNICATION
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diluted acetonitrile solutions of the metal allenylidene com-
plexes on a silicon wafer or copper grid. TEM micrographs
of 2b showed uniformly spread nanospheres with average
diameter of 200 nm. The absence of nanofibers with 2b is
attributed to the tert-butyl groups, which disfavor extensive
one-dimensional packing of the PtII cations. In addition, the
SEM micrographs of 4b demonstrated an entangled fibrillar
network, providing evidence for potential self-assembly be-
havior of the AuIII allenylidene complexes in solutions.
In summary, new PtII and AuIII allenylidene complexes
have been prepared and found to display promising proper-
ties, such as prominent phosphorescence, self-assembling ca-
pability (into nanostructures), and cellular imaging and effi-
cient cytotoxic activities. The applications of metal allenyli-
dene complexes could be as rich as that of metal acetylide
complexes, but allenylidene ligands would provide a new di-
mension for modification of the electronic structures and
properties of the metal complexes. We thus envisage that lu-
minescent metal–allenylidene complexes with long-lived ex-
cited states provide an entry to new classes of functional
molecular materials and to supramolecular science with a
bright future.
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Keywords: allenylidene · gold · luminescence · platinum ·
self-assembly
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Received: April 18, 2013
Published online: June 18, 2013
9462
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Chem. Eur. J. 2013, 19, 9457 – 9462