Gold(I) complexes incorporating emissive mercapto-pteridine ligands: Syntheses, X-ray structure, luminescence and preliminary cytotoxic evaluation

Article abstract of DOI:10.1002/ejic.201200147

The syntheses of six new mixed P/S-donor two-coordinate Au^I complexes are described. The complexes incorporate a pteridinyl ligand coordinated through a thiolate donor, and an ancillary tertiary phosphane (PPh₃ or PCy₃). The mercapto-pteridine ligands (L ¹-L³) differ in the nature of the substituents on the pteridine core. An X-ray crystal structure was obtained for one of the examples, [(L¹)Au(PPh₃)], revealing weak intermolecular interactions between two molecules of the complex: π-π contacts between aromatic rings appear to support an intermolecular Au-Au contact of approximately 3.05 A. All of the complexes are luminescent in solution, with emission arising from tuneable ligand-based excited states, and characterised as a perturbed fluorescence in nature. In this context, complexes of L³ displayed useful visible absorption and emission. Preliminary cytotoxicity assessments were conducted using the MTT assay, and the complexes each displayed impressive anti-proliferative activities (IC₅₀ < 5 μM) with respect to four different adenocarcinoma cell lines (MCF7, A549, PC3 and LOVO). For a given pteridine moiety, triphenylphosphane appeared to be the co-ligand of choice for enhancing biological activity. Copyright

Full text of DOI:10.1002/ejic.201200147

Job/Unit: I20147
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FULL PAPER
DOI: 10.1002/ejic.201200147
Gold(I) Complexes Incorporating Emissive Mercapto-Pteridine Ligands:
Syntheses, X-ray Structure, Luminescence and Preliminary Cytotoxic
Evaluation
Lucy A. Mullice,[a] Huw J. Mottram,[b] Andrew J. Hallett,[a] and Simon J. A. Pope*[a]
Keywords: Gold / Pteridine ligands / Coordination chemistry / Luminescence / Cytotoxicity
The syntheses of six new mixed P/S-donor two-coordinate
AuI complexes are described. The complexes incorporate a
pteridinyl ligand coordinated through a thiolate donor, and
emission arising from tuneable ligand-based excited states,
and characterised as a perturbed fluorescence in nature. In
this context, complexes of L3 displayed useful visible absorp-
an ancillary tertiary phosphane (PPh3 or PCy3). The mer- tion and emission. Preliminary cytotoxicity assessments were
capto-pteridine ligands (L1–L3) differ in the nature of the sub-
stituents on the pteridine core. An X-ray crystal structure was
obtained for one of the examples, [(L1)Au(PPh3)], revealing
weak intermolecular interactions between two molecules of
the complex: π–π contacts between aromatic rings appear to
support an intermolecular Au–Au contact of approximately
3.05 Å. All of the complexes are luminescent in solution, with
conducted using the MTT assay, and the complexes each dis-
played impressive anti-proliferative activities (IC50 Ͻ 5 μ
with respect to four different adenocarcinoma cell lines
(MCF7, A549, PC3 and LOVO). For a given pteridine moiety,
triphenylphosphane appeared to be the co-ligand of choice
for enhancing biological activity.
M)
implicated in genetic disorders.[4] Importantly, the single
pteridine unit can hydrogen-bond to a specific base oppo-
site an AP (apurinic/apyrimidinic) site.
Introduction
Pteridines, of which pterins are the most widely studied
variant, are bicyclic heterocyclic compounds comprised of a
fused pyrimidine and pyrazine ring. Pteridines are the core
structures of co-enzymatic constituents of biochemically
important systems, such as bio-pterin and folic acid.[1] Pter-
idines have also been synthesised for medicinal applications,
and the pteridine core is present in the lead anti-cancer drug
Methotrexate (an inhibitor of dihydrofolate reductase).[2]
Marques and co-workers have synthesised pteridine-sulfon-
amide conjugates to act as dual inhibitors of carbonic
anhydrases, which are also implicated in tumour growth.[3]
Although these compounds display inhibitory potential (al-
beit less than Methotrexate), millimolar concentrations
were required for good activity and attributed to the poor
transport across the cell membrane. In related work, several
pteridine analogues, conjugated (through a thiol unit) with
other heterocyclic units such as quinoline, benzimidazole,
isoxazole and uracil, have demonstrated efficient growth in-
hibition (ca. 60%) in cancer cell lines.[3] From an optical
perspective, fluorescent pteridine derivatives have allowed
investigation of single nucleotide polymorphisms, which are
A limited number of Au complexes have been reported
that incorporate biologically inspired ligand systems includ-
ing nucleobases adorned with thio functionality.[5] Some of
these compounds showed promising anti-arthritic activity
without worrying levels of associated toxicity, whilst the
complex of 6-mercaptopurine demonstrated anti-tumour
activity in murine PC6 plasmacytoma.[5g] Despite the pter-
idine moiety possessing diverse biological and medicinal
significance, their use in coordination chemistry is actually
limited to a handful of reports describing third- (Mn, Fe,
Ni, Cu), fourth- (Mo, Ru, Ag) and fifth-row (W, Re, Ir)
transition-metal complexes,[6,7] together with one example
of a (2:2, M/L) dimetallic PtII complex with 4-hydroxy-2-
mercaptopteridine;[8] the development of gold-based pterid-
ine agents with biological activity has not been reported.
Therapeutic agents based upon gold coordination com-
plexes (both AuI and AuIII) have been studied for many
years[9] and very recently reviewed;[9b] it is probable that
the biological and therapeutic activity of many gold species
derives from the highly thio- and selenophilic nature of gold
and ligand exchange.[9b,10] Targets relating to cancer, rheu-
matoid arthritis, AIDS and parasitic diseases are being ac-
tively pursued by using gold-based reagents.[9b] Auranofin
is probably the best known AuI therapeutic and has been
used as an anti-arthritic treatment in the past; the thiol-
carbohydrate ligand has been shown to exchange during
cell-based investigations.[11] An example of cellular uptake
[a] School of Chemistry, Main Building, Cardiff University,
Cardiff CF10 3AT, Cymru/Wales
Fax: +44-29-208-74030
E-mail: popesj@cardiff.ac.uk
[b] School of Pharmacy, Redwood Building, Cardiff University,
King Edward VII Avenue, Cardiff CF10 3NB, Cymru/Wales
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200147.
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FULL PAPER
has been shown with mixed alkyne/phosphane AuI com- tained by 31P NMR spectroscopy, which showed (for 96 h
plexes demonstrating antiproliferative behaviour, probably at room temperature) a resonance for the complex only, and
through interaction with proteins and enzymes;[12] targeted no evidence for either free or oxidised PPh3 (i.e. for these
uptake by mitochondria has been shown by exploiting lumi- species DMSO does not displace the P ligand). Warming of
nescent lipophilic (carbene)AuI complexes.[13,14]
the sample to 37 °C for 96 h produced similar results with
Ott and co-workers recently performed comparative over 99.5% of the integrated signal associated with the in-
studies on complexes of the type [ClAu(PR3)] (R = Me, Et, tact complex (Ͻ 0.5% associated with triphenylphosphane
tBu, Ph) together with related gold complexes incorporating oxide).
an emissive thionaphthalimide moiety, which has shown
anti-cancer activity by DNA intercalation.[15] The studies
showed that for [ClAu(PR3)] the nature (size and lipophil-
icity) of the alkyl/aryl substituents on the phosphorus atom
had a minor influence on the bio-activity of the complexes.
For the thionaphthalimide complexes, the substituent effect
was neglible, but the complexes demonstrated a general en-
hancement in activity compared to [ClAu(PEt3)], probably
due to improved uptake by nuclei.
The aim of this work was to investigate the coordination
chemistry of biologically active pteridine moieties with the
“Au(PR3)” core, and assess the resultant spectroscopic,
cytotoxic and anti-proliferative properties of these func-
tional complexes. This paper describes the synthesis and
structural characterisation of the first gold coordination
complexes of pteridine-derived ligands, together with the
cytotoxicological screening of these complexes against a
Scheme 1. General synthesis of the pteridinyl ligands and AuI com-
range of adenocarcinoma cell lines.
plexes (RЈ = Ph or Cy).
Results and Discussion
MALDI mass spectrometry showed peaks assigned to
the parent cations [M]+ and, for the PPh3 complexes, ad-
ditional common peaks at m/z = 721 and 1409. The former
is attributed to ligand re-distribution upon ionisation to
form the bis(phosphane) complex [Au(PPh3)2]+ and has
been observed in related species.[17a,17b] It should be noted
that there was no spectroscopic evidence for this species by
other techniques such as 31P{1H} NMR spectroscopy.[17c]
The latter peak at m/z = 1409 was assigned to [(AuPPh3)3-
S]+, in accordance with previous work, and attributed to
fragmentation and re-combination following ionisation.[18]
Elemental analyses quantitatively confirmed the formation
of 1:1 adducts.
Synthesis and Characterisation
The pteridine-based ligands (Scheme 1; L1–3) were syn-
thesised in a single step[16] by condensation of the appropri-
ate dione with 1 equiv. of 4,5-diamino-6-hydroxy-2-mer-
captopyrimidine hemisulfate hydrate in hot ethanol with a
trace of acetic acid. The desired ligands were obtained in
good yields as air-stable powders following filtration of the
reaction mixture (the spectroscopic characterisation is pro-
vided in the Experimental Section). The corresponding AuI
complexes were synthesised from the appropriate
[ClAu(PRЈ3)] precursor (PRЈ3 = PPh3 or PCy3) to give the
six target AuI agents, [(L)Au(PRЈ3)] (L = L1–3). The reaction
was allowed to proceed for either 24 h (for L1,2) or 48 h (for
L3) at room temperature in the absence of light. The result-
ant mixed-ligand AuI complexes were fully characterised by
using a comprehensive variety of spectroscopic techniques
X-ray Crystal Structure for the Complex
Single crystals of [(L2)Au(PPh3)] were obtained by vap-
and other analyses. Generally, ligand-based proton reso- our diffusion of diethyl ether into a concentrated chloro-
1
nances in the H NMR spectra showed downfield-shifted form solution of the complex. The parameters associated
resonances attributed to the coordination of the thiolate li- with the data collection are shown in Table S1 together with
1
gands. In the H NMR spectrum of [(L1)Au(PPh3)], two selected bond lengths and bond angles given in Table S2 in
narrow multiplets were observed at δ = 8.81 and 8.67 ppm the Supporting Information. Although the crystal was very
assigned to the two aromatic protons on the pyrazine ring, weakly diffracting, leading to a lower-quality data set, the
whilst the overlapping PPh3 resonances appeared between δ results are still sufficient for elucidating the key features of
= 7.50 and 7.80 ppm. 31P{1H} NMR spectroscopy revealed the structure. The structure showed that there are four
a single resonance for each PPh3 and PCy3 species at δ ≈ unique “[(L2)Au(PPh3)]” molecules per asymmetric unit
+38 and +58 ppm, respectively. Importantly, and with refer- (Figure 1). There is an intermolecular aurophilic Au–Au
ence to the cytotoxicity measurements discussed later, the contact in the crystalline form leading to dimerisation of
stability of the complexes in [D6]DMSO was also ascer- the complex.[19] Coordination geometry at AuI is approxi-
2
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Gold(I) Complexes Incorporating Emissive Mercapto-Pteridine Ligands
mately linear (S–Au–P 174–175°) with respect to the thio- (Table 1). The emission of the complexes can therefore be
late and phosphane donors. The Au–S [2.310(11) and moderately tuned through consideration of the pteridine li-
2.352(11) Å] and the Au–P bond lengths [2.226(12) and gand structure (Figure 2), whereby extending the conjuga-
2.274(11) Å] fall within the expected range observed for re- tion (cf. L3) bathochromically shifts the emission profile
lated examples.[19]
substantially into the visible region (ca. 485 nm). The series
of complexes show that more subtle tuning can be achieved
through the ancillary phosphane ligand. The corresponding
lifetimes of the luminescence are all very short (Ͻ 5 ns) in
aerated solution.
Figure 1. Single-crystal X-ray structure for [(L2)Au(PPh3)].
Electronic Spectra and Luminescence Data
The UV/Vis spectra of the ligands reveal absorptions tail-
ing into the visible region with a subtle bathochromic shift
in the longest-wavelength feature upon methylation (L2)
and conjugation (L3) of the pteridine core. These are as-
signed to both π–π* and weaker n–π* transitions. In solu-
tion, each ligand was luminescent in the visible region be-
Figure 2. Emission profiles for the complexes [(Ln)Au(PCy3)] with
L1 represented by the dashed (λex = 310 nm), L2 by the grey (λex
310 nm) and L3 by the black line (λex = 420 nm), respectively.
=
In the solid state the observations for the complexes are
tween 442 and 478 nm. The emission spectrum of L3 also very different, with most examples demonstrating emission
showed an additional weaker peak at 584 nm, which disap- profiles possessing vibronically structured features. In the
peared upon further dilution and is therefore attributed to case of [(L2)Au(PPh3)], where the X-ray structure revealed
aggregation in solution inducing excimer-type emission.
an Au–Au contact, there was no indication (350–800 nm) of
The absorption spectra of the complexes are dominated an aurophilic emission originating from such an interaction,
by ligand-centred transitions now including those of PPh3 although such a band could be hidden beneath the ligand-
at Ͻ 270 nm, which are subtly shifted from the correspond- centred transition. In general, the profiles of the solid-state
ing free-ligand values. LSR–MAu–CT bands are also ex- species are redshifted in comparison to the solution-state
pected for such species, but probably contribute to the low- data and are much more comparable to the spectra ob-
energy tail of the bands towards 400 nm. As expected from tained from [methanol/ethanol (1:1)] glasses at 77 K. For
the ligand structures, the absorption spectra associated with example, Figure 3 shows the total emission spectra obtained
L3 are the most redshifted, reflecting the extended, conju- for [(L3)Au(PRЈ3)], revealing a weak residual ligand fluores-
gated nature of the chromophore.
cence and highly structured lower-energy emission profiles.
From a photophysical perspective, the solution-state The latter show very similar peak positions irrespective of
emission spectra of the complexes appear to display per- RЈ, suggesting that these bands comprise significant spin-
turbed, pteridine-centred luminescence in each case orbit-assisted (cf. weak residual fluorescence) intra-ligand
Table 1. Absorption and emission data for the complexes.[a]
Complex
λabs [nm]
λem [nm] in CHCl3
λem [nm] in the solid[d]
λem [nm] in glass[e]
L1
288, 348
291, 350
284, 354
247, 305, 346
247, 306, 341
250, 295, 349, 384
306, 349
306, 342
298, 346, 385
442 (br.)
477
475, 584
–
–
–
–
–
–
L2
L3
[(L1)Au(PPh3)]
[(L2)Au(PPh3)]
[(L3)Au(PPh3)]
[(L1)Au(PCy3)]
[(L2)Au(PCy3)]
[(L3)Au(PCy3)]
415[b] (Ͻ 1 ns)
410[b] (3.3 ns)
488[c] (Ͻ 1 ns)
420[b] (Ͻ 1 ns)
415[b] (Ͻ 1 ns)
482[c] (Ͻ 1 ns)
424, 438
414, 430, 467, 509
416, 435
427, 461, 492
418, 472, 490
418, 436, 490
478, 501
465, 489
452, 478, 560, 608
439, 482, 494
427, 478, 500
480, 560, 608
[a] In aerated CHCl3. [b] λex = 310 nm. [c] λex = 420 nm. [d] 298 K. [e] 77 K, methanol/ethanol (1:1).
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FULL PAPER
triplet (3IL) excited-state character. It is likely therefore that Table 2. Cytotoxicity data for the complexes.[a]
the room-temperature solid-state emission also features
Compound
Adenocarcinoma cell line (IC50 [μm])
3
emissive AuI complexes characterised by IL emission.[20]
MCF 7 A549
PC3
LOVO
[(L1)Au(PPh3)]
[(L1)Au(PCy3)]
[(L2)Au(PPh3)]
[(L2)Au(PCy3)]
[(L3)Au(PPh3)]
[(L3)Au(PCy3)]
0.5
5
0.5
5
0.5
5
5
5
5
50
5
5
5
5
5
5
5
5
5
5
5
5
5
50
[a] MTT assay. Standard deviations for each measurement are re-
ported in the Supporting Information.
For cell line A549, [(L1)Au(PPh3)], [(L1)Au(PCy3)], [(L2)-
Au(PPh3)] and [(L3)Au(PPh3)] all showed a much higher
activity than Cisplatin (Ͼ 50 μm).[23] For complexes of L1,
the same IC50 values were observed irrespective of the phos-
phane co-ligand, whereas – in contrast – both [(L2)-
Au(PPh3)] and [(L3)Au(PPh3)] displayed ten times the ac-
tivity of their PCy3 analogues. Generally, all of the AuI
complexes are ten times less active in A549 cells than in
MCF7 cells. Finally, for the PC3 and LOVO cell lines, the
high anti-proliferativity (ca. 5 μm) appeared to be similar
for all the complexes. It is noteworthy that within the series
of complexes [(L1)Au(PCy3)] was unique in displaying simi-
larly advantageous biological activity (ca. 5 μm), irrespec-
tive of the cell line of study.
Figure 3. Emission spectra of [(L3)Au(PRЈ3)] [R = Ph (black) and
Cy (grey)] recorded on frozen glasses [methanol/ethanol (1:1)] at
77 K (λex = 410 nm).
Preliminary Cytotoxicity Evaluation
The complexes were assessed for anti-cancer activity by
MTT assay, providing the cytotoxicity profiles for four dif-
ferent cancer cell lines. The cell lines chosen for this study
were MCF7 (breast adenocarcinoma), A549 (lung adeno-
carcinoma), PC3 (prostate adenocarcinoma) and LOVO
(colon adenocarcinoma). For the MTT assay, the com-
pounds were initially dissolved and were stable in DMSO,
and doses of 0.1, 1, 10 and 100 μm were tested to analyse
the activity of different concentrations and then compared
to a control medium with no treatment. The approximate
IC50 values for these compounds are shown in Table 2.
Pleasingly, strong anti-proliferative effects were observed
for all complexes. Generally, for a given pteridine ligand the
complexes incorporating PPh3 co-ligands (Ͻ 1 μm) ap-
peared more active than their PCy3 analogues (5 μm) in
MCF7 cells. Although accurate comparison is not possible,
the PPh3 complexes did display comparable activity with
that reported for Cisplatin (IC50 = 2.0 μm)[15c] and Aurano-
fin (IC50 = 1.1 μm)[15c] in these cells. In comparison with
[ClAu(PPh3)] (IC50 = 2.6 μm),[21] the [(Ln)Au(PPh3)] com-
plexes displayed much higher biological activity and there-
fore indicate the additive cytotoxic role of the pteridine li-
gand compared to chloride. These compounds also demon-
strate comparable or improved activity to the naphthal-
imide-derived AuI complexes reported by Ott and co-
workers (IC50 = 1.3 μm[15a] and 3.7 μm for PEt3 and PPh3
derivatives, respectively).[15c] These results show that from
an anti-proliferative (MCF7) perspective, a more favourable
hydrophilic/lipophilic balance[22] is apparently provided by
the complexes incorporating a coordinated triphenylphos-
phane.
Conclusions
We have described the synthesis and characterisation of
neutral, two-coordinate AuI complexes incorporating a
thiol-derived pteridine unit and a tertiary phosphane co-
ligand. The complexes are soluble in a wide range of sol-
vents and retain luminescence arising from ligand-centred
excited states. All of the complexes display impressive anti-
proliferative activities (IC50 Ͻ 5 μm) with respect to four
different adenocarcinoma cell lines. The results also showed
that, in general, for a given pteridine moiety, triphenylphos-
phane was the co-ligand of choice for enhancing biological
activity, suggesting that the choice of ancillary phosphane
should not be ignored. Future work will focus upon a wider
range of pteridine derivatives and an elucidation of the bio-
logical mode of action (by the use of the fluorescent vari-
ants of the thiol-appended pteridines) as a function of both
coordinated ligands. Exploitation of the emissive character
in optical studies by using cells will allow consideration of
the mechanistic aspects of the cytotoxic activity.
Experimental Section
General: All photophysical data were obtained with a JobinYvon-
Horiba Fluorolog spectrometer fitted with a JY TBX picosecond
photodetection module. For the emission lifetimes 295 and 459 nm
NanoLEDs (operating at 1 MHz) were utilised. All lifetimes were
obtained by using the JY-Horiba FluoroHub single-photon-count-
ing module and fitted by using the provided DAS v6 software by
reconvolution of the detector response to a scattering sample. IR
spectra were recorded with an ATR-equipped Varian 7000 FT-IR
4
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Gold(I) Complexes Incorporating Emissive Mercapto-Pteridine Ligands
spectrometer. LR MS data were obtained with a Bruker MicroTOF
(10 mL) and AcOH (0.5 mL) for 16 h. The resultant precipitate was
LC. UV/Vis data were recorded as solutions with a Jasco 570 spec- filtered, washed with EtOH and dried in vacuo to give the product
trophotometer. 1H and 31P{1H} NMR spectra were recorded either as a beige solid. Yield 125 mg, 56%. 1H NMR (250 MHz, [D6]-
3
3
with an NMR-FT Bruker 250, 400 or 500 MHz spectrometer and
DMSO): δH = 8.72 (d, JHH = 2.0 Hz, 1 H, ArH), 8.61 (d, JHH =
obtained by using CDCl3, or [D6]DMSO solutions. 1H NMR 2.0 Hz, 1 H, ArH) ppm. IR (solid): ν = 3032, 1691 (CO), 1542,
˜
chemical shifts (δ) were determined relative to internal TMS and
are given in ppm. Elemental analyses were conducted by Medac
Ltd (UK).
1384, 1221, 1196, 1052, 876 cm–1. UV/Vis [DMSO/CHCl3
(0.5:99.5)]: λmax (ε) = 288 (9480), 348 (3230 mol–1 dm3 cm–1) nm.
MS (EI): m/z = 180.1 [M]+.
Synthesis of L2:[28] Prepared similarly from 2,3-butanedione
(104 mg, 1.2 mmol). Product isolated as a beige solid. Yield
190 mg, 63%. 1H NMR (250 MHz, [D6]DMSO): δH = 2.57 (s, 3 H,
Crystallography
Data Collection and Processing: Diffraction data for [(L2)Au(PPh3)]
was collected with a Nonius KappaCCD by using graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å) at 150 K. A table con-
taining the data collection and crystal parameters are included in
the Supporting Information, together with selected bond lengths
and angles.
CH ), 2.54 (s, 3 H, CH ) ppm. IR (solid): ν = 3186, 1695 (CO),
˜
3
3
1542, 1354, 1271, 1128, 840 cm–1. UV/Vis [DMSO/CHCl3
(0.5:99.5)]: λmax (ε) = 291 (28880), 350 (10380 mol–1 dm3 cm–1) nm.
MS (AP+): m/z = 209.0 [M + H]+ and 250.1 [M + MeCN + H]+.
Synthesis of L3:[29] Prepared similarly from 1,2-acenaphthylene-
Structure Analysis and Refinement: The structure was solved by di-
rect methods using SHELX-97 and was completed by iterative cy-
cles of ΔF syntheses and full-matrix least-squares refinement. All
non-H atoms were refined anisotropically, and difference Fourier
syntheses were employed in positioning idealised hydrogen atoms
and were allowed to ride on their parent C atoms. All refinements
were against F2 and used SHELXS-97.[24]
dione (225 mg, 1.2 mmol). Product isolated as a dark brown solid.
1
Yield 260 mg, 71%. H NMR (400 MHz, [D6]DMSO): δH = 8.37
3
3
(d, JHH = 10 Hz, 2 H, ArH), 8.32 (d, JHH = 8 Hz, 2 H, ArH),
8.00–7.94 (m, 2 H ArH) ppm. IR (solid): ν = 3415, 1709 (CO),
˜
1541, 1476, 1378, 1310, 1253, 125, 1038, 766 cm–1. UV/Vis [DMSO/
CHCl3
(0.5:99.5)]:
λmax
(ε)
=
284
(11620),
354
(16450 mol–1 dm3 cm–1) nm. MS (EI): m/z = 304.1 [M]+.
CCDC-802052 contains the supplementary crystallographic data
for this paper {[(L2)Au(PPh3)]}. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of the Complexes
Synthesis of [(L1)Au(PPh3)]: To a solution of L1 (18 mg, 0.10 mmol)
and NaOH (4.4 mg, 0.11 mmol) in methanol (10 mL) was added
[ClAu(PPh3)] (50 mg, 0.10 mmol). The mixture was stirred in the
absence of light at room temperature in an inert atmosphere for
24 h. Water (30 mL) was added, and the resultant precipitate was
filtered. The product was dissolved in CH2Cl2 and added dropwise
to cold diethyl ether, filtered and dried in vacuo to give a cream-
coloured solid. Yield 42 mg, 66%. 1H NMR (250 MHz, CDCl3):
δH = 8.81–8.10 (m, 1 H, ArH), 8.67–8.66 (m, 1 H, ArH), 7.57–7.47
Cytotoxicity
Cytotoxicity Assessment via MTT Assay: The cytotoxicity of the
complexes was assessed by using the colourimetic and quantitative
MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium brom-
ide] assay, first reported by Mosmann et al.[25] Quantification was
achieved by using a multi-well scanning spectrophotometer and re-
ported as an IC50 value.[21]
(m, 15 H, PPh3) ppm. 31P{1H} NMR (202.4 MHz, CDCl3): δP
=
Method for Cytotoxicity Analysis: Anti-tumour evaluation in
MCF7, LOVO, A549 and PC3 cell lines was performed by an MTT
assay. Compounds were prepared as 0.1–100 mm stock solutions in
DMSO and stored at –20 °C. Cells were seeded into 96-well micro-
titre plates at a density of 5ϫ103 cells per well and allowed 24 h
to adhere. Decimal compound dilutions were prepared in medium
immediately prior to each assay (final concentration 0.1–100 μm).
Experimental medium was DMEM + 10% FCS (PC3 and LOVO)
or RPMI + 10% heat-inactivated FCS (A549 and MCF7). Follow-
ing 96 h compound exposure at 37 °C, 5% CO2, MTT reagent
(Sigma Aldrich) was added to each well (final concentration
0.5 mgmL–1). Incubation at 37 °C for 4 h allowed reduction of
MTT by viable cells to an insoluble formazan product. MTT was
removed and formazan solubilised by addition of 10% Triton X-
100 in PBS. Absorbance was read with a Tecan Sunrise spectropho-
tometer at 540 nm as a measure of cell viability; thus, inhibition
relative to control was determined (IC50).
+37.5 ppm. IR (solid): ν = 3381, 1697 (CO) 1514, 1470, 1381, 1215,
˜
1098, 745 cm–1. UV/Vis (CHCl3): λmax (ε) = 247 (20630), 305
(16890), 346 (4740 mol–1 dm3 cm–1) nm. MS (MALDI): m/z = 638.0
[M]+, 721.1 [Au(PPh3)2]+, 1097.0 [M
+
Au(PPh3)]+, 1409.0
[(AuPPh3)3S]+, 1555.0 [M – H + 2 AuPPh3]+. C24H18AuN4OPS·
0.5CH2Cl2 (680.9): calcd. C 43.22, H 2.81, N 8.23; found C 43.31,
H 2.64, N 7.53.
Synthesis of [(L2)Au(PPh3)]: Prepared similarly from L2 (25 mg,
0.10 mmol). Product isolated as a beige powder. Yield 53 mg, 79%.
1H NMR (500 MHz, CDCl3): δH = 9.25 (s, 1 H, NH or OH), 7.63–
7.59 (m, 6 H, PPh3), 7.49–7.44 (m, 9 H, PPh3), 2.63 (s, 3 H, CH3),
2.61 (s, 3 H, CH3) ppm. 31P{1H} NMR (202.4 MHz, CDCl3): δP
=
+37.8 ppm. IR (solid): ν = 3478, 1674 (CO), 1517, 1476, 1380,
˜
1212, 1907, 954, 749 cm–1. UV/Vis (CHCl3) λmax (ε) = 247 (21000),
306 (19600), 341 (6600 mol–1 dm3 cm–1) nm. MS (MALDI): m/z =
666.1 [M]+, 721.1 [Au(PPh3)2]+, 1125.1 [M + Au(PPh3)]+, 1409.1
[(AuPPh3)3S]+ and 1584.2 [M + 2 AuPPh3]+. C26H23AuN4OPS
(667.5): calcd. C 46.78, H 3.47, N 8.39; found C 45.96, H 3.30, N
8.12.
Materials and Procedures: 4,5-Diamino-6-hydroxy-2-mercaptopyr-
imidine hemisulfate hydrate was obtained from the Aldrich Chemi-
cal Company. Hydrogen tetrachloroaurate(III) was provided on
loan by Johnson Matthey plc. All reagents were used as received.
The gold precursor materials were obtained according to literature
procedures.[26]
Synthesis of [(L3)Au(PPh3)]: Prepared similarly from L3 (30 mg,
0.1 mmol) but stirred for 48 h. Product isolated as a light brown
powder. Reproducible elemental analyses were not obtainable for
this compound, possibly due to incomplete combustion. Yield
49 mg, 65%. 1H NMR (250 MHz, CDCl3): δH = 8.44–8.36 (m, 2
H, ArH), 8.10–8.02 (m, 2 H, ArH), 7.79–7.74 (m, 2 H, ArH), 7.62–
7.57 (m, 6 H, PPh3), 7.54–7.44 (m, 9 H, PPh3) ppm. 31P{1H} NMR
Synthesis of the Ligands
Synthesis of L1:[16a,27] 4,5-Diamino-6-hydroxy-2-mercaptopyrimid-
ine hemisulfate hydrate (250 mg, 1.2 mmol) and ethane-1,2-dione
(70 mg, 1.2 mmol) were heated at reflux in a mixture of EtOH
(121.44 MHz, CDCl ): δ = +36.0 ppm. IR (solid): ν = 3198, 2923,
˜
3
P
Eur. J. Inorg. Chem. 0000, 0–0
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/KAP1
Date: 02-05-12 16:58:00
Pages: 8
L. A. Mullice, H. J. Mottram, A. J. Hallett, S. J. A. Pope
FULL PAPER
2839, 1665 (CO), 1630, 1515, 1433, 1303, 1252, 1100, 749 cm–1.
UV/Vis (CHCl3): λmax (ε) = 250 (24500), 295 (15000), 349 (16100),
384 (14100 mol–1 dm3 cm–1) nm. MS (MALDI): m/z = 721.1
[Au(PPh3)2]+, 762.1 [M]+, 1221.1 [M + AuPPh3]+, 1409.1 [(AuPPh3)3-
S]+.
[2]
[3]
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[4]
[5]
Synthesis of [(L1)Au(PCy3)]: Prepared similarly from L1 (18 mg,
0.1 mmol), NaOH (4.2 mg, 0.11 mmol) and [ClAu(PCy3)] (50 mg,
0.1 mmol) to give a cream-coloured solid. Yield 50 mg, 78%. 1H
NMR (250 MHz, CDCl3): δH = 9.35 (s, 1 H, NH or OH), 8.66 (s,
1 H, ArH), 8.46 (s, 1 H, ArH), 2.01–1.25 (m, 33 H, PCy3) ppm.
31P{1H} NMR (202.4 MHz, CDCl3): δP = +57.9 ppm. IR (solid):
ν = 2915, 2843, 1673 (CO), 1519, 1383, 1221, 950 cm–1. UV/Vis
˜
(CHCl3): λmax (ε) = 306 (22700), 349 (6300 mol–1 dm3 cm–1) nm. MS
(MALDI): m/z = 656.2 [M]+, 657.2 [M + H]+, 757.4 [Au(PCy3)2]+,
1133.3 [M – H + Au(PCy3)]+, 1463.5 [(AuPCy3)3S]+, 1609.5 [M –
2 H + 2 AuPPh3]+. C24H36AuN4OPS (656.6): calcd. C 43.90, H
5.53, N 8.53; found C 43.30, H 5.49, N 7.93.
[6]
[7]
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Synthesis of [(L2)Au(PCy3)]: Prepared similarly from L2 (24 mg,
0.1 mmol) to give a cream-coloured solid. Yield 37 mg, 55%. 1H
NMR (250 MHz, CDCl3): δH = 9.27 (s, 1 H, NH or OH), 2.58 (s,
3 H, CH3), 2.54 (s, 3 H, CH3), 2.06–1.22 (m, 33 H, PCy3) ppm.
31P{1H} NMR (101.2 MHz, CDCl3): δP = +57.9 ppm. IR (solid):
ν = 3477, 2924, 2849, 1670 (CO), 1524, 1435, 1350, 1251, 1153,
˜
954, 821 cm–1. UV/Vis (CHCl3): λmax (ε) = 306 (22800), 342
(7300 mol–1 dm3 cm–1) nm. MS (MALDI): m/z = 684.1 [M]+, 685.2
[M + H]+, 757.4 [Au(PCy3)2]+, 1161.4 [M + Au(PCy3)]+, 1463.5
[(AuPCy3)3S]+, 1637.6 [M – H + 2 AuPPh3]+. C26H41AuN4OPS·
CH2Cl2 (770.6): calcd. C 42.08, H 5.62, N 7.27; found C 42.72, H
5.53, N 7.55.
[8]
[9]
Synthesis of [(L3)Au(PCy3)]: Prepared similarly from L3 (30 mg,
0.1 mmol). The solvent was removed in vacuo, the crude product
dissolved in CH2Cl2, filtered through Celite, and the solvent re-
moved in vacuo to give the product as a light brown solid. Yield
[10]
[11]
[12]
[13]
1
62 mg, 81%. H NMR (400 MHz, CDCl3): δH = 9.54 (s, 1 H, NH
or OH), 8.40 (d, 3JHH = 7.0 Hz, 1 H, ArH), 8.28 (d, 3JHH = 7.0 Hz,
3
3
1 H, ArH), 8.09 (d, JHH = 7.0 Hz, 1 H, ArH), 8.02 (d, JHH
=
7.0 Hz, 1 H, ArH), 2.03–1.29 (m, 33 H, PCy3) ppm. 31P{1H} NMR
(202.4 MHz, CDCl3): δP = +58.4 ppm. IR (solid): ν = 2917, 2845,
˜
[14]
[15]
1665 (CO), 1618, 1507, 1441, 1307, 1260, 1120, 1033, 971, 825,
772 cm–1. UV/Vis (CHCl3): λmax (ε) = 298 (14200), 346 (18100), 385
(14600 mol–1 dm3 cm–1) nm. MS (MALDI): m/z = 780.2 [M]+, 781.2
[M + H]+, 757.4 [Au(PCy3)2]+, 1257.4 [M + Au(PCy3)]+, 1463.5
[(AuPCy3)3S]+, 1734.9 [M
+ 2
AuPCy3]+. C34H41AuN4OPS·
1.5CH2Cl2 (909.1): calcd. C 46.90, H 4.88, N 6.16; found C 46.46,
H 5.00, N 6.66.
Supporting Information (see footnote on the first page of this arti-
cle): Diffraction data for the structure of [(L2)Au(PPh3)], dose re-
sponse cytotoxicity data for the cell line screening.
[16]
[17]
Acknowledgments
We thank Cardiff University and Engineering and Physical Sci-
ences Research Council (EPSRC) UK for financial support and are
indebted to Prof. Chris McGuigan for assistance with the biological
cellular studies. Dr. B. M. Kariuki is acknowledged for obtaining
crystallographic data. We are very grateful to Johnson Matthey
PLC for the generous loan of hydrogen tetrachloroaurate(III). We
also acknowledge the efforts of the staff of the EPSRC National
Mass Spectrometry Service (University of Swansea) UK.
[18]
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Pages: 8
Gold(I) Complexes Incorporating Emissive Mercapto-Pteridine Ligands
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Received: February 13, 2012
Published Online: ᭿
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www.eurjic.org
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Job/Unit: I20147
/KAP1
Date: 02-05-12 16:58:00
Pages: 8
L. A. Mullice, H. J. Mottram, A. J. Hallett, S. J. A. Pope
FULL PAPER
Gold(I) Complexes
The syntheses and characterisation of neu-
tral, two-coordinate AuI complexes in-
corporating a thiol-derived pteridine unit
are described. The complexes possess lumi-
nescence arising from ligand-centred ex-
cited states. All of the complexes display
L. A. Mullice, H. J. Mottram,
A. J. Hallett, S. J. A. Pope* .............. 1–8
Gold(I) Complexes Incorporating Emissive
Mercapto-Pteridine Ligands: Syntheses, X-
ray Structure, Luminescence and Prelimi-
nary Cytotoxic Evaluation
notable anti-proliferative activities (IC50
Ͻ
5 μm) with respect to four different adeno-
carcinoma cell lines.
Keywords: Gold / Pteridine ligands / Coor-
dination chemistry / Luminescence / Cyto-
toxicity
8
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