Hydrosulfide Adducts of Organo-Iridium Anticancer ... - ACS Publications

Feb 10, 2016 - Regional Centre of Advanced Technologies and Materials, Department of Inorganic Chemistry, Faculty of Science, Palacký. University, 17...
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Hydrosulfide Adducts of Organo-Iridium Anticancer Complexes Pavel Štarha,†,‡ Abraha Habtemariam,‡ Isolda Romero-Canelón,‡ Guy J. Clarkson,‡ and Peter J. Sadler*,‡ †

Regional Centre of Advanced Technologies and Materials, Department of Inorganic Chemistry, Faculty of Science, Palacký University, 17. listopadu 12, 77146 Olomouc, Czech Republic ‡ Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U. K. S Supporting Information *

ABSTRACT: Novel half-sandwich hydrosulfidoiridium(III) complexes [(η5-Cp*)Ir(phen)(SH)]PF6 (1), [(η5-Cp*)Ir(bpy)(SH)]PF6 (2), [(η5-Cpbiph)Ir(phen)(SH)]PF6 (3), and [(η5-Cpbiph)Ir(bpy)(SH)]PF6 (4) were prepared from the chlorido complexes by dechlorination and treatment with excess NaSH·xH2O; phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl, and Cpbiph = 1,2,3,4tetramethyl-5-biphenylcyclopentadienyl. Complexes 1−4 were characterized by various techniques including electrospray ionization mass spectrometry, NMR spectroscopy (δ(SH) ca. −2 ppm), and a single-crystal X-ray analysis. Complex [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) shows a typical piano-stool geometry with Ir−S bond length of 2.388(2) Å. Cpbiph complexes 3 (IC50 = 0.98 μM) and 4 (IC50 = 0.61 μM) showed significantly higher (p < 0.005) in vitro antiproliferative activity against A2780 human ovarian cancer cells, as compared with their Cp* analogues 1 (IC50 = 49.5 μM) and 2 (IC50 = 48.4 μM), and potency similar to the anticancer drug cisplatin. The complexes were relatively stable in aqueous solution toward hydrolysis and reactions with reduced glutathione (GSH), 9ethylguanine, or 9-methyladenine. Interestingly, GSH was readily oxidized to glutathione disulfide in the presence of Cpbiph complexes 3 and 4, as judged by 1H NMR, perhaps indicative of a possible redox-linked mechanism of action.

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(IC50 = 0.7 μM) is exceeded by that of [(η5-Cpbiph)Ir(phpy)(py)] (IC50 = 0.1 μM).2a The higher potency of pyridine complexes may be a consequence of slower intracellular inactivation by various biomolecules (e.g., glutathione) resulting in considerably slower hydrolysis of pyridine compared to chlorido complexes.2a,4c In the present study we focused on interactions of halfsandwich iridium(III) complexes with (hydro)sulfide. Sulfide is a natural cellular metabolite being present especially in iron− sulfur proteins, which play major roles in electron-transport pathways and therefore in the production of reactive oxygen species (ROS).5 We hypothesized that the high affinity of iridium for sulfur would have a major effect on its reactivity. Hydrosulfide is a common ligand in various types of transition metal complexes including iridium.6 However, only a few mononuclear organometallic hydrosulfidoiridium complexes, namely, [(η5-Cp*)Ir(SH)2(PR3)] (PR3 = PMe3 or PMPh3; MPh = 4-methylphenyl),7 [(η5-Cp*)Ir(H)(SH)(PR3)] (PR3 = PMe3, PPhMe2, or PPh3)7a,8 and [(η5-Cp*)Ir(X)(SH)(PMe3)] (X = Cl, Br, CS2, SC(S)HN-p-tol, or CF(CF3)2),7,8b,9 have been reported to date. We investigated half-sandwich hydrosulfidoiridium(III) complexes of the general formula [(η 5 -Cp x )Ir(N^N) (SH)]PF6 (1−4) containing bidentate N-donor ligands phen (1, 3) or 2,2′-bipyridine (bpy; 2, 4) and Cp* (1, 2) or Cpbiph (3, 4) aromatic rings (Figure 1). Complexes 1−4 were studied

INTRODUCTION Organometallic complexes of the platinum-group metals ruthenium, osmium, rhodium, and iridium have recently been shown to be promising anticancer agents.1 Current knowledge of these complexes indicates that their mechanism of action (MoA) as well as spectrum of sensitive tumors2 is different from conventional clinically-used anticancer platinum-based drugs.3 They may find clinical use for the treatment of Ptresistant cancers with fewer side effects and a broader spectrum of activity. Among these compounds, organometallic half-sandwich iridium(III) complexes containing bidentate N,N or C,N donor ligands are highly antiproliferative active agents acting in vitro through redox-mediated MoAs different from platinumbased drugs.2a,4 These complexes have interesting structure− activity relationships. For example, the inactive complex [(η5Cp*)Ir(phen)Cl]+ (IC50 > 100 μM against the A2780 human ovarian cancer cell line; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl and phen = 1,10-phenanthroline) acquires activity when (i) Cp* is replaced by extended cyclopentadienyl ligands Cpph (IC50 = 6.7 μM; Cpph = 1,2,3,4-tetramethyl-5-phenylcyclopentadienyl) or Cpbiph (IC50 = 0.7 μM; Cpbiph = 1,2,3,4tetramethyl-5-biphenylcyclopentadienyl)4a or when (ii) the N,N donor ligand (phen) is replaced by a C,N donor (phpy, 2-phenylpyridine) leading to an active electroneutral [(η5Cp*)Ir(phpy)Cl] complex having an IC50 of 10.8 μM.4b,c Moreover, replacement of the chlorido ligand by pyridine (py) or its derivatives enhances the biological potency as wellfor example, the high activity of [(η5-Cpbiph)Ir(phpy)Cl] complex © XXXX American Chemical Society

Received: November 21, 2015

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DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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precipitate was removed by centrifugation (5000 rpm for 3 min) after 1 h of stirring at ambient temperature in the dark. The resulting solutions were degassed under the reduced pressure for 5 min and kept under nitrogen gas. An excess (14 mg, corresponding to ca. 0.15 mmol) of NaSH·xH2O was then added under a nitrogen atmosphere, leading to a color change from yellow to dark brown. The mixtures were stirred for 2 h under nitrogen, centrifuged (5000 rpm for 3 min), and an excess of NH4PF6 (1.0 mmol) was added. The solutions were evaporated slowly with nitrogen gas at ambient temperature until a precipitate formed. The crude products (1*, 2*, 3*, and 4*) were collected by filtration, washed with diethyl ether, and dried under vacuum for 15 min. They were suspended in 50 mL of chloroform and filtered, and the obtained solutions were evaporated to dryness, giving the final products 1−4 (Figure 1). [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) was isolated by analogous procedure as described above for the PF6− salt 1, by using NaBPh4 (1 mmol) instead of NH4PF6. [(η5-Cp*)Ir(phen)(SH)]PF6 (1). Yield: 40%. 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): δ = 9.22 (d, 2H, JHH = 4.6 Hz), 8.92 (d, 2H, JHH = 7.3 Hz), 8.36 (s, 2H), 8.17 (dd, 2H, JHH = 8.3, 5.5 Hz), 1.74 (s, 15H), −2.22 (s, 1H) ppm. 13C NMR (150 MHz, DMSO-d6): δ = 152.1, 145.8, 138.4, 130.6, 127.9, 127.2, 90.0, 8.1 ppm. CHN Anal. Calcd for C22H24N2SPF6Ir (685.7): C, 37.5; H, 3.5; N, 4.1; found: C, 37.5; H, 3.3; N, 3.9%. Electrospray ionization mass spectrometry (ESI-MS; MeOH): 507.1 (calcd 507.2 for {[(η5Cp*)Ir(phen)]−H}+; 5%), 525.2 (calcd 525.2 for [(η5-Cp*)Ir(phen)(OH)]+; 5%), 541.1 (calcd 541.1 for [(η5-Cp*)Ir(phen)(SH)]+; 100%) m/z. [(η5-Cp*)Ir(bpy)(SH)]PF6 (2). Yield: 35%. 1H NMR (400 MHz, DMSO-d6): δ = 8.79 (m, 4H), 8.29 (m, 2H), 7.81 (t, 2H, JHH = 6.2 Hz), 1.67 (s, 15H), −2.14 (s, 1H) ppm. CHN Anal. Calcd for C20H24N2SPF6Ir (661.7): C, 36.3; H, 3.6; N, 4.2. Found: C, 36.1; H, 3.1; N, 4.2%. ESI-MS (MeOH): 483.1 (calcd 483.2 for {[(η5Cp*)Ir(bpy)]−H}+; 10%), 501.1 (calcd 501.2 for [(η5-Cp*)Ir(bpy)(OH)]+; 15%), 517.1 (calcd 517.1 for [(η5-Cp*)Ir(bpy) (SH)]+; 100%) m/z. [(η5-Cpbiph)Ir(phen)(SH)]PF6 (3). Yield: 35%. 1H NMR (400 MHz, DMSO-d6): δ = 8.99 (d, 2H, JHH = 5.3 Hz), 8.94 (d, 2H, JHH = 8.0 Hz), 8.39 (s, 2H), 8.19 (dd, 2H, JHH = 8.3, 5.5 Hz), 7.75 (m, 4H), 7.50 (m, 4H), 7.41 (t, 1H, JHH = 7.3 Hz), 1.88 (s, 6H), 1.80 (s, 6H), −1.87 (s, 1H) ppm. Anal. Calcd for C33H30N2SPF6Ir (823.8): C, 48.1; H, 3.7; N, 3.4. Found: C, 48.3; H, 3.7; N, 3.7%. ESI-MS (MeOH): 645.2 (calcd 645.2 for {[(η5-Cpbiph)Ir(phen)]−H}+; 2%), 663.2 (calcd 663.2 for [(η5-Cpbiph)Ir(phen)(OH)]+; 5%), 679.2 (calcd 679.2 for [(η5Cpbiph)Ir(phen)(SH)]+; 100%) m/z. [(η5-Cpbiph)Ir(bpy)(SH)]PF6 (4). Yield: 30%. 1H NMR (400 MHz, DMSO-d6): δ = 8.84 (d, 2H, JHH = 8.0 Hz), 8.62 (d, 2H, JHH = 5.5 Hz), 8.30 (t, 2H, JHH = 7.8 Hz), 7.74 (m, 6H), 7.47 (m, 5H), 1.81 (s, 6H), 1.74 (s, 6H), −1.80 (s, 1H) ppm. Anal. Calcd for C31H30N2SPF6Ir (799.8): C, 46.6; H, 3.8; N, 3.5. Found: C, 46.1; H, 3.9; N, 3.8%. ESI-MS (MeOH): 507.1 (calcd 507.2 for {[(η5Cpbiph)Ir(bpy)]−H}+; 5%), 541.1 (calcd 541.1 for [(η5-Cpbiph)Ir(bpy)(SH)]+; 100%) m/z. Methods. X-ray Crystallography. Single crystals of [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) were grown from a saturated dichloromethane solution using a diffusion method with hexane. A suitable crystal was selected and mounted on a glass fiber with Fromblin oil and placed on an Xcalibur Gemini diffractometer with a Ruby CCD area detector. The crystal was kept at 150(2) K during data collection. Using Olex2,13 the structure was solved with the ShelXT14 structure solution program using Direct Methods and refined with the ShelXL15 refinement package using Least Squares minimization. X-ray crystallographic data for 1′ were deposited in the Cambridge Crystallographic Data Centre under the accession number CCDC 1426202. All atoms (except sulfur) of one of two complex cations (containing Ir2 atom) found in the asymmetric unit of 1′ were refined disordered over two closely related positions. The occupancy of Ir2 and coordinated phen was fixed at 80 and 20%, while it was fixed at 85 and 15% for Cp*. Both disordered models shared the same sulfur in the model. The methyl atoms of Cp* coordinated to Ir1 within the

Figure 1. Synthesis route and general structural formula of [(η5Cpx)Ir(N^N)(SH)]PF6 (1−4); N^N = 1,10-phenanthroline (phen; for 1 and 3) or 2,2′-bipyridine (bpy; for 2 and 4), and Cpx = 1,2,3,4,5pentamethylcyclopentadienyl (Cp*; for 1 and 2) or 1,2,3,4tetramethyl-5-biphenylcyclopentadienyl (Cpbiph; for 3 and 4).

for their cytotoxicity toward A2780 human ovarian cancer cells and in model reactions that might be relevant to their MoA, including hydrolysis and interactions with the biomolecules reduced glutathione (GSH), 9-ethylguanine (9EtG), and 9methyladenine (9MeA), and the results are discussed in relation to previously studied chlorido and pyridine complexes.2a,4a,e,10 To the best of our knowledge, this appears to be the first work dealing with antiproliferative activity and related mechanistic processes of half-sandwich transition metal hydrosulfido complexes.

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EXPERIMENTAL SECTION

Materials. IrCl3·nH2O (99%) was purchased from Precious Metals Online. 1,2,3,4,5-Pentamethylcyclopentadiene (95%), 2,3,4,5-tetramethyl-2-cyclopentanone (95%), 4-bromobiphenyl (98%), butyllithium solution (1.6 M in hexanes), 1,10-phenanthroline (≥99%), 2,2′-bipyridine (≥99%), silver trifluoromethanesulfonate (≥99%; silver triflate, AgOTf), sodium hydrosulfide hydrate (≥60% of NaSH; NaSH·xH2O), ammonium hexafluorophosphate (≥95%; NH4PF6), sodium tetraphenylborate (≥99.5%; NaBPh4), sodium chloride (>99.999%; NaCl), reduced glutathione (≥98.0%), 9-ethylguanine (≥98%), and 9-methyladenine (97%) were purchased from SigmaAldrich. Solvents of laboratory grade were used for syntheses without further purification, while solvents of HPLC grade were used for RPHPLC experiments with an addition of trifluoroacetic acid (Htfa). Roswell Park Memorial Institute (RPMI-1640) medium, fetal bovine serum, glutamine, penicillin/streptomycin mixture, trypsin, and phosphate-buffered saline (PBS) were purchased from PAA Laboratories GmbH. The starting Ir(III) dimers [(η5-Cp*)IrCl2]2 11 and [(η5-Cpbiph)IrCl2]2,4a and the chlorido complexes [(η5-Cp*)Ir(phen)Cl]Cl and [(η5-Cp*)Ir(bpy)Cl]Cl,12 were prepared according to reported methods, while [(η5-Cpbiph)Ir(phen)Cl]Cl and [(η5-Cpbiph)Ir(bpy)Cl]Cl were obtained by a modification of the reported syntheses,4a as described in Supporting Information. Synthesis. The starting chlorido complexes [(η5-Cp*)Ir(phen)Cl]Cl, [(η5-Cp*)Ir(bpy)Cl]Cl, [(η5-Cpbiph)Ir(phen)Cl]Cl, and [(η5Cpbiph)Ir(bpy)Cl]Cl (0.1 mmol) were dissolved in methanol (5 mL), and 2 mol equiv of silver triflate was added to the solutions. AgCl B

DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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of complexes and interacting biomolecules was 500 μM. The presence of methanol or dimethyl sulfoxide ensured the solubility of complexes, and 20% of methanol or dimethyl sulfoxide was used with respect to low solubility of the studied complexes in water. The mixtures were monitored by 1H NMR spectroscopy at multiple time points over 48 h (298 K) using a Bruker DPX-400 NMR spectrometer. NaCl (4 mol equiv) was added to the 20% d4-MeOD/80% D2O (for 1 and 2) or 20% DMSO-d6/80% D2O (for 3 and 4), and 1H NMR spectra were recorded at multiple time points over the next 24 h. The spectra were calibrated against the residual signal of D2O (4.75 ppm). Similar ESIMS experiments were performed on the solutions in nondeuterated solvents.

second complex cation were refined using DELU restraints. No hydrogens were located on both the sulfur atoms (coordinated to Ir1 and Ir2) but were included in the formula for completeness. The crystal data and structure refinements are given in Table S1. The graphics were drawn, and additional structural calculations were performed by DIAMOND16 and Mercury17 software. NMR Spectroscopy. 1H and 13C NMR spectroscopy and 1H−1H gsCOSY, 1H−1H gs-NOESY, 1H−13C gs-HMQC, and 1H−13C gsHMBC two-dimensional correlation experiments (gs = gradient selected, COSY = correlation spectroscopy, NOESY = nuclear Overhauser effect spectroscopy, HMQC = heteronuclear multiple quantum coherence, HMBC = heteronuclear multiple bond coherence) were obtained in 5 mm NMR tubes at 298 K using DMSO-d6 or CDCl3 solutions on Bruker DPX-400 or AVIII 600 NMR spectrometers. Spectra were calibrated according to residual signals of solvent at 2.50 ppm (1H NMR) and 39.52 ppm (13C NMR) for DMSO-d6 and 7.26 ppm (1H NMR) for CDCl3. The splitting of proton resonances in the reported 1H spectra is defined as s = singlet, d = doublet, t = triplet, dd = doublet of doublets and m = multiplet. Mass Spectrometry. ESI-MS was performed by either a Bruker Esquire 2000 ion trap spectrometer (in positive ionization mode (ESI +) in the range of either 50−500 or 400−1000 m/z on the methanol solutions of 1−4 or respective intermediates) or a Bruker MaXis with Proxeon Easy-nLC and Dionex RS3000 HPLC high-resolution mass spectrometer (for 1). Data were processed with Bruker Data Analysis 3.3. Elemental Analysis. Elemental analyses were performed using CE440 elemental analyzer by Exeter Analytical (U.K.) Ltd. High-Performance Liquid Chromatography. Agilent 1200 system with a VDW and 100 μL loop was used for HPLC purity checking, using an Agilent Zorbax Eclipse Plus C18, 250 × 4.6 mm with a 5 μm pore size. Mobile phase used was H2O 0.1% Htfa/MeCN 0.1% Htfa at gradients of t = 0 min 10% B, t = 30 min 80% B, t = 40 min 80% B, t = 41 min 10% B, and t = 55 min 10% B over a 55 min period. Flow rate was 1 mL min−1, and the detection wavelength was set at 254 nm with the reference wavelength at either 360 or 510 nm. Sample injections were half the loop volume (50 μL) with needle washes of MeOH and H2O between injections. In Vitro Cell Growth Inhibition. The A2780 ovarian carcinoma human cancer cell line was purchased from European Collection of Cell Cultures (ECACC) and cultured in RPMI-1640 supplemented with 10% of fetal calf serum, 1% of 2 mM glutamine, and 1% penicillin/streptomycin. The A2780 cells were cultured in 75 cm2 culture flasks as adherent monolayers split repeatedly with 0.25% v/v trypsin when 80−90% confluence was reached. The cells were grown and maintained at 310 K and 5% CO2 in a humidified incubator, seeded into 96-well culture plate (5000 cells/well), and preincubated in drug-free medium for 48 h at 310 K. Stock solutions of 1−4 were prepared in 5% v/v DMSO and a mixture of 0.9% w/v saline and cell culture medium (1:1 v/v). Various concentrations of the tested compounds, prepared from the stock solutions by dilution with cell culture medium, were added to the wells of the culture plates. The supernatants were removed after 24 h of drug exposure, and the cells were washed with drug-free PBS followed by 72 h of recovery in drug-free medium at 310 K. The cell viability was evaluated by the sulforhodamine B (SRB) assay using a BioRad iMark microplate reader with a 470 nm filter. The data were expressed as the percentage of cell viability, compared to the untreated controls. The data from the cancer cells were acquired from two independent sets of experiments conducted in triplicates, using cells from different passages. The resulting IC50 values (μM; concentration causing 50% of cell growth inhibition) were calculated from dose−response curves, and the results are presented as arithmetic mean ± standard deviation (SD). The significance of the differences between the obtained results (p < 0.05, 0.01, and 0.005 levels were checked) was assessed by the ANOVA analysis (QC Expert 3.2, Statistical software, TriloByte Ltd.). Hydrolysis and Interaction with Biomolecules (GSH, 9EtG, 9MeA). Complexes were dissolved in 100 μL of d4-MeOD (1 and 2) or DMSO-d6 (3 and 4) and 400 μL of D2O or solutions of either GSH, 9EtG, or 9MeA in 400 μL of D2O were added. The final concentration

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RESULTS Syntheses. Complexes 1−4 (Figure 1) were prepared according to a general synthetic procedure. [(η5-Cpx)Ir(N^N)Cl]Cl complexes were dissolved in methanol and dechlorinated with 2 mol equiv of silver triflate providing the [(η5Cpx)Ir(N^N)(OTf)]+ species detectable in ESI+ mass spectra at 657.1 m/z (for 1; Figures S1 and S2), 633.1 m/z (for 2), 795.2 m/z (for 3), or 771.1 m/z (for 4); Cpx = Cp* (1, 2) or Cpbiph (3, 4), and N^N = phen (1, 3) or bpy (2, 4). Subsequent addition of NaSH·xH2O led to a color change from yellow to dark brown. ESI-MS monitoring of the reaction progress clearly showed that (i) the formation of the resulting complexes 1−4 was, contrary to the immediate color change, gradual and complete in not less than 2 h (Figure S1) and that (ii) an excess of NaSH·xH2O is required for complete replacement of the coordinated OTf by hydrosulfide (Figure S2). In particular, the formation of the hydrosulfido complexes was evident from the gradual decrease of the intensity of the peaks for [(η5Cpx)Ir(N^N)(OTf)]+ and simultaneous increase in peaks for [(η5-Cpx)Ir(N^N)(SH)]+ (Figure S1). The crude products (1*−4*) were isolated as PF6− salts, and their 1H NMR spectra in DMSO-d6 contained several Cp* signals (for 1 and 2), several sets of Cpbiph signals (for 3 and 4), but only one SH 1H NMR signal at ca. −2 ppm (Figure S3). However, 1*−4* were only partially soluble in deuterated chloroform, and the 1H NMR spectra showed only one peak for Cp* (for 1 and 2) or one set of signals for Cpbiph (for 3 and 4), as well as one SH peak, as observed for the CDCl3 solutions of 1*−4* after removal of insoluble species (Figure S3). This indicated that dissolution of the crude products in chloroform and following isolation of the soluble species could be used for purification of the crude products. After purification, 1−4 showed one Cp* signal (1, 2) or one set of signals of Cpbiph (3, 4) in their 1H NMR spectra recorded on DMSO-d6 solutions of solid recovered from the chloroform solutions of the crude products (Figures S3 and S4). General Characterization. 1H NMR spectra of 1−4 contained all the C−H signals detected for the starting chlorido complexes. Moreover, a signal for coordinated hydrosulfide was detected at high field (ca. −2 ppm). The SH signal did not show any cross peaks in 1H−1H gs-COSY, 1 H−13C gs-HMQC, and 1H−13C gs-HMBC spectra, but in the 1 H−1H gs-NOESY spectrum of the representative complex [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) had a cross-peak with Cp* hydrogens (Figure 2). 1H NMR spectra of all the complexes 1− 4 are shown in Figure S4. ESI+ mass spectra contained a dominant peak for [(η5-Cpx)Ir(N^N)(SH)]+ with m/z values (see Experimental Section), and isotopic distribution fitting the calculated values well (Figure S5 and S6). Complexes 1−4 were also characterized by elemental analysis, and their purity was determined by HPLC. However, C

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Table 1. Selected Bond Lengths (Å) and Angles (deg) for [(η-Cp*)Ir(phen)(SH)]BPh4 (1′) bond lengthsa (Å) Ir−N Ir−Cb

Ir−Cgc Ir−S

2.097(7) 2.076(6) 2.176(10) 2.176(9) 2.162(10) 2.151(11) 2.170(11) 1.8070(4) 2.388(2)

bond anglesa (deg) N−Ir−N N−Ir−Cg N−Ir−S S−Ir−Cg

77.4(2) 130.8(2) 131.4(2) 86.2(2) 86.5(2) 127.24(6)

a

Data are given only for [(η5-Cp*)Ir(phen)(SH)]+ cation with no disorder (i.e., Ir1). bAromatic carbon atoms of Cp* ring. cCg = centroid of Cp* aromatic ring. Figure 2. 1H−1H gs-NOESY spectrum of [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) dissolved in DMSO-d6 showing an SH/Cp* cross-peak indicative of their close proximity.

The complex cation adopts a typical piano-stool geometry (Figure 3). The Ir−C bond lengths for the Cp* ring ranged from 2.151(11) to 2.176(10) Å (Table 1). The distance between Ir and Cp* centroid is 1.807 Å. The phen ligand is chelated through its nitrogen atoms, and the inner coordination sphere is complemented by the sulfur atom of hydrosulfide (see Table 1 for Ir−N and Ir−S bond lengths). The dihedral angle of the phen ring and ring carbons of Cp* is 54.9(3)°. The Ir···B distance between [(η 5 -Cp*)Ir(phen)(SH)] + and BPh 4 − counteranion is 7.530(8) Å (Figure S9). The crystal structure involves several noncovalent contacts of the C−H···C and C··· C type, as well as π−π stacking between the central rings of two adjacent phen ligands with a centroid−centroid distance of 3.4369 Å (Figure S10). Cancer Cell Growth Inhibition Assays. Complexes 1−4 showed contrasting antiproliferative activity against A2780 human ovarian cancer cells. In particular, complexes 1 and 2 were moderately active with IC50 values of 49.5 ± 0.8 and 48.4 ± 0.8 μM, respectively (Figure 4). Complexes 3 and 4, differing from 1 and 2 by the presence of a biphenyl substituent on the Cp ring, exhibited significantly higher (p < 0.005) potency with submicromolar IC50 values of 0.98 ± 0.03 (for 3) and 0.61 ± 0.02 (for 4) μM (Figure 4), exceeding the anticancer drug cisplatin (IC50 1.2 ± 0.2 μM). Interestingly, complex 4

use of common HPLC conditions (CH3CN/Htfa) for elution of such complexes from a reverse-phase column led to partial decomposition, and several peaks were observed (Figure S7). These were assignable by ESI-MS to the tfa-containing species. In the case of 1, [(η5-Cp*)Ir(phen)(tfa)]+ (621.1 m/z at tR = 7. 7 32 , 9 . 03 4, 12 . 49 6, an d 1 3. 3 97 m i n ) , { [ (η 5 Cp*)2Ir2(phen)2(SH)(tfa)]+O−H}+ (1174.8 m/z at tR = 7.732 min), {[(η 5-Cp*) 2 Ir 2(phen)2 (SH)(tfa)]+S+O−H} + (1208.9 m/z at t R = 12.496 min), and {[(η 5 Cp*)2Ir2(phen)2(SH)(tfa)]+S+2O−H}+ (1224.9 m/z at tR = 13.397 min) were detected in the mass spectra (Figure S7). Similar ESI-MS experiments (without HPLC) revealed tfacontaining species [(η 5 -Cp x )Ir(N^N)(tfa)] + and {[(η 5 Cpx)2Ir2(N^N)2(SH)(tfa)]−H}+, when performed on the MeCN/H2O solution (1:1, v/v) with addition of 0.1% Htfa (Figure S8). X-ray Structure of [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′). A single-crystal X-ray structural analysis showed that the asymmetric unit of 1′ contains two complex cations [(η5Cp*)Ir(phen)(SH)]+ and two BPh4− counterions. The parameters of one complex cation with no disorder (Figure 3, Table 1) are discussed below, while those of the second cation disordered over two positions (see Experimental Section) are given in Supporting Information (Figure S9, Table S2).

Figure 4. Antiproliferative activity of complexes 1−4 against A2780 human ovarian carcinoma cell line. Experiments included 24 h of drug exposure and 72 h of cell recovery in drug-free medium. Data are expressed as IC50 ± SD (μM), and the significant differences (*** p < 0.005) between the IC50 values are depicted (green linesa comparison of the complexes containing the same N-donor ligand; red linea comparison of the complexes containing the same Cpx ligand).

Figure 3. X-ray crystal structure of one of the cations in the unit cell of [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′); hydrogen atoms are omitted for clarity, and thermal ellipsoids are drawn at 50% probability. Different labels than in CIF file (CCDC 1426202) were used for some atoms in the drawing: N1 in the drawing = N101 in the CIF file; N10 = N112; S1 = S003. D

DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry containing N,N-chelated bpy showed significantly higher activity (p < 0.005) than 3 containing phen (Figure 4). Studies of Aqueous Chemistry. 1H NMR data revealed that complexes 1 and 2 are unstable in aqueous solution (20% d4-MeOD/80% D2O, Figure S11). Several new signals for Cp* appeared at higher field (1.24−1.73 ppm for 1 and 1.24−1.68 ppm for 2) than for Cp* of 1 (1.76 ppm) and 2 (1.69 ppm). The intensities of the new signals increased with time, and a ratio of the integrals for the starting complex and new species were 10:19 (1) and 10:6 (2) after 48 h. Surprisingly, the increase in intensity of the new Cp* signals continued even after addition of 4 mol equiv of NaCl and reached ratios of 10:30 (for 1) and ca. 10:11 (2) after a further 24 h (Figure S11). The ESI+ mass spectra showed a new peak at 556.2 (for 1) and 532.2 (for 2) m/z, whose masses correspond to {[(η5Cp*)Ir(N^N)(SH)]−H+O}+. Interestingly, the peaks of [(η5Cp*)Ir(N^N)(OH)]+, detected at 525.2 (1) and 501.1 (2) m/ z in the ESI+ mass spectra of 1 and 2 dissolved in MeOH without addition of water, disappeared when the complexes were dissolved in 20% MeOH/80% H2O. Complexes 3 and 4 were poorly soluble in the mixed solvent (20% d4-MeOD/80% D2O). To enhance the solubility, 20% DMSO-d6/80% D2O was used. The 1H NMR data indicated that complexes 3 and 4 did not undergo any changes under these conditions since no new signals were detected even after 48 h. Regarding ESI-MS, the results obtained for Cpbiph complexes 3 and 4 differ from the Cp* (1 and 2). A weak peak for {[(η5-Cpbiph)Ir(N^N)(SH)]+H+O}+ detected in the spectra of 3 and 4 differed from {[(η5-Cp*)Ir(N^N)(SH)]−H +O}+ found in the spectra of 1 and 2. Moreover, in contrast to [(η5-Cp*)Ir(N^N)(OH)]+ detected for 1 and 2 in MeOH but not in 20% MeOH/80% H2O, similar peaks of [(η5Cpbiph)Ir(N^N)(OH)]+ appeared in the spectra of 3 (663.2 m/z) and 4 (639.2 m/z) dissolved in 20% MeOH/80% H2O with the same intensity as in the case of MeOH solutions. Interaction with Biomolecules (GSH, 9EtG, 9MeA). 1H NMR spectra of 1 and 2 mixed with GSH contained only the same signals as detected in 20% d4-MeOD/80% D2O solutions without GSH (see above) together with signals for free GSH. This indicated no interaction between GSH and the Cp*containing complexes 1 and 2 or products of their decomposition in aqueous solutions. In contrast, the 1H NMR spectra of 3 and 4 after reaction with GSH gradually changed with time (Figure 5) and showed oxidation of GSH to glutathione disulfide (GSSG).18 This was indicated by the decrease in intensity of Cys-α CH and Cys-β CH2 peaks for free reduced glutathione (GSH) at 4.46 and 2.85 ppm, respectively and the subsequent appearance of new signals for Cys-β CH2 at 3.21 and 2.88 ppm. All the GSH in the reactions with 3 and 4 was converted into GSSG after 12 h. Formation of a new pair of CH3 signals for Cpbiph was observed as well for both complexes 3 and 4 after reaction with GSH. The intensity of the peaks for {[(η5-Cpbiph)Ir(N^N)(SH)]+H+O}+ species was markedly higher as compared with experiments performed without GSH. The ESI-MS spectra of 1 and 2 contain signals of both GSH and GSSG in the ratio similar to the spectrum of free GSH, indicating that no GSSG is formed. Remarkably, the increased level of the GSSG peaks was detected in the ESI-MS spectra after 48 h of standing at ambient temperature (Figure S12), which is consistent with the results of the 1H NMR studies discussed above. Complexes 1−4 did not show any binding to model nucleobases 9EtG and 9MeA: no new peaks were detected in

Figure 5. 1H NMR spectra at various times for the reactions of [(η5Cpbiph)Ir(phen)(SH)]PF6 (3; left) or [(η5-Cp*)Ir(bpy)(SH)]PF6 (4; right) with 1 mol equiv of GSH in 20% d4-MeOD/80% D2O solutions. (●) GSH; (■) GSSG; (▲) CH3 of Cpbiph.

the 1H NMR or ESI+ MS spectra of the equimolar mixtures of 1−4 in the presence of either 9EtG or 9MeA even after 24 h.

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DISCUSSION The starting chloridoiridium(III) complexes [(η5-Cp*)Ir(N^N)Cl]+ were prepared as described previously12 or, in the case of Cpbiph complexes, by an adapted protocol,4a which dramatically shortened the preparation time of [(η5-Cpbiph)Ir(phen)Cl]+ and [(η5-Cpbiph)Ir(bpy)Cl]+ from 10 and 16 h, respectively, to 1 min by using microwave synthesis (Supporting Information). Chlorido complexes were subsequently dechlorinated by AgOTf, and the resulting [(η5Cpx)Ir(N^N)(OTf)]+ species reacted with an excess of sodium hydrosulfide hydrate. A similar synthetic strategy using dechlorinated organo-iridium(III) complexes was recently used for the preparation of half-sandwich pyridine complexes.2a,19 ESI-MS control of the reaction progress clearly showed that formation of the resulting complexes 1−4 is gradual and requires 2 h of stirring at ambient temperature (Figures S1 and S2). Although the crude products consist of several species, as judged by 1H NMR in DMSO-d6 (Figure S3), a simple purification using recovery of the solid from the filtered chloroform solutions of the crude products was found to be sufficient to give final products of acceptable purity. The C−H signals detected in 1H NMR spectra of 1−4 showed only small upfield shifts as compared with the starting chlorido complexes, up to 0.20 and 0.12 ppm for N^N ligands and aromatic hydrogen atoms of Cpbiph, respectively. Similarly the CH3 peaks for Cp* and Cpbiph were only slightly shifted downfield by 0.02−0.03 ppm. The δ values for the hydrosulfido ligand, detected at −2.22 (1), −2.14 (2), −1.87 (3), and −1.80 E

DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (4) ppm, differed markedly between Cp* (1, 2) and Cpbiph (3, 4) complexes. Moreover, phen complexes (1, 3) had slightly lower 1H NMR δ SH values than their bpy-containing analogues (2, 4). Complex [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′) was characterized crystallographically and exhibits the piano-stool geometry (Figure 3) typical of analogous half-sandwich iridium(III) complexes, such as [(η5-Cp*)Ir(phen)Cl]CF3SO3,20 [(η5Cp*)Ir(phpy)Cl],21 or [(η5-Cpph)Ir(phen)Cl]PF6.4a The Ir− N bonds between the Ir(III) and phen were comparable for 1′ (Table 1) and for [(η5-Cp*)Ir(phen)Cl]CF3SO3 (2.110 and 2.123 Å) as well as for the phen Cp* complex [(η5Cp*)Ir(phen)(SDmp)]BArF4·C6H6 containing S-donor 2,6bis(2,4,6-trimethylphenyl)phenylthiolate (BArF4 = tetrakis(3,5bis(trifluoromethyl)phenyl)borate; 2.099 and 2.108 Å).22 The CCDC database also contains several mononuclear hydrosulfidoiridium(III) complexes, namely, [(η5-Cp*)Ir(SH)2(PMe3)2],7a [(η5-Cp*)Ir(PMe3)(SH) (SC(S)HN-ptol),8b trans-[Ir(Cl)(H)(SH)(CO)(PPh3)2],23 cis-[Ir(H) (SH)(PMe3)4]PF6,24 and trans,mer-[IrCl2(SH)(PMe2Ph)3]25 involving in all cases a combination of SH− ligand and phosphinebased P-donor ligand. The Ir−S bond lengths ranged from 2.370 to 2.429 Å (average of 2.397 Å) for these complexes, which is consistent with the bond length found for 1′ (Table 1). The structure reported here for 1′ therefore represents a new structural type of half-sandwich iridium(III) complex, whose inner coordination sphere contains Cpx, a bidentate Ndonor and coordinated SH− ligands. Because no hydrogen was located on the coordinated sulfur atom (see Experimental Section), we sought to confirm the coordination of SH in the crystal structure of 1′ by comparison of its Ir−S bond length with the CCDC deposited hydrosulfidoiridium (see above) and with the sulfido complexes. However, no structures of sulfidoiridium complexes containing a terminal S2− anion have been deposited within CCDC to date. The 18 deposited sulfido complexes involving a bridging sulfide have shorter average Ir−S bond lengths of 2.351 Å (2.240−2.462 Å range) than the hydrosulfido complex 1′. The complexes studied here showed moderate (1, 2) or high (3, 4) in vitro antiproliferative activity against A2780 ovarian cancer cells, depending on the type of Cpx ring (Figure 4). In particular, complexes [(η5-Cpbiph)Ir(N^N)(SH)]PF6 (3, 4) showed significantly higher potency than their Cp* analogues, that is, [(η5-Cp*)Ir(N^N)(SH)]PF6 (1, 2). An effect of Cpx ring substituents on antiproliferative activity of half-sandwich organo-iridium(III) complexes has been recently reported for [(η5-Cpx)Ir(N^N)Cl]+ and [(η5-Cpx)Ir(phpy)Cl] complexes, where N^N = phen (see Introduction) or bpy.4a−c For bpy complexes, IC50 values of >100.0, 15.9, and 0.6 μM were reported for [(η5-Cp*)Ir(bpy)Cl]+, [(η5-Cpph)Ir(bpy)Cl]+, and [(η5-Cpbiph)Ir(bpy)Cl]+ complexes, respectively, against the A2780 cell line.4a In the case of [(η5-Cpx)Ir(phpy)Cl] complexes containing the C,N-donor ligand phpy, their potency similarly increases with the extent of Cpx substitution, with IC50 values of 10.8, 2.1, and 0.7 μM (A2780 cells) for complexes containing Cp*, Cp ph , and Cp biph ligands, respectively.4b,c It is also known, for half-sandwich iridium(III) complexes, that the replacement of the chlorido ligand in [(η5Cpx)Ir(phpy)Cl] by pyridine leads to higher antiproliferative activity, as reported for [(η5-Cpph)Ir(phpy)Cl] (IC50 = 2.1 μM) versus [(η5-Cpph)Ir(phpy)(py)] (IC50 = 1.0 μM) or [(η5Cpbiph)Ir(phpy)Cl] versus [(η5-Cpbiph)Ir(phpy)(py)] (see Introduction) pairs (activity against the A2780 cell line).2a,4b,c,e

The higher activity of pyridine complexes as compared with their chlorido analogues can be related, from a mechanistic point of view, to slower hydrolysis (equilibrium reached after 4 h with ∼60% hydrolyzed for pyridine complex, while the chlorido analogue reached equilibrium in 100 μM), while Cpbiph complexes 3 (IC50 = 0.98 μM) and 4 (IC50 = 0.61 μM) showed activity of the same order as previously reported for analogous chlorido complexes (IC50 = 0.7 and 0.6 μM, respectively, for phen and bpy complexes).4a It was of interest to investigate whether and/or how these results correlate with reactions of anticancer metallodrugs relevant to their mechanism of action, such as hydrolysis and interaction with biomolecules. The Cp* (1 and 2) and Cpbiph (3 and 4) complexes showed different behavior in aqueous media. In the case of 1 and 2, we detected several new peaks for Cp* in the 1H NMR spectra, whose intensity increased in time (Figure S11). We assume that this is connected with decomposition and not with hydrolysis of 1 and 2 because (i) an addition of 4 mol equiv of NaCl, thought to be sufficient to suppress hydrolysis,2a,4a,b,g did not lead to a decrease of intensity of any of the new signals (Figure S11), (ii) peaks of [(η5-Cp*)Ir(N^N)(OH)]+ detected in the ESI+ mass spectra of 1 and 2 dissolved in MeOH (without water) showed markedly lower intensity in 20% MeOH/80% H2O, and (iii) a new peak was detected in the ESI+ mass spectra at 556.2 (1) and 532.2 (2) m/z with mass corresponding to {[(η5-Cp*)Ir(N^N)(SH)]−H+O}+, which indirectly appeared on oxidation of complexes 1 and 2 under the experimental conditions used resulting, most likely, in sulfenato complexes, as reported previously for half-sandwich RuII and IrIII thiolato complexes.18,26,27 The 1H NMR data indicated that complexes 3 and 4 are relatively stable in aqueous media: no new signals were detected even after 48 h. As in the case of 1 and 2, we obtained evidence of the slow oxidation of 3 and 4 connected with a formation of {[(η5Cpbiph)Ir(N^N)(SH)]+H+O}+ species detected in the ESI+ mass spectra. Moreover, the relevant changes in the intensity of the GSH and GSSG peaks in the ESI-MS spectra of 3 and 4 connected with the increased level of the GSSG peaks indirectly showed that GSH is oxidized on interaction with the Cpbiph complexes (Figure S12). Interaction of 1−4 with abundant intracellular thiol GSH differed for Cp* complexes 1 and 2, as compared with Cpbiph complexes 3 and 4. Complexes 1 and 2 did not react with GSH; that is, the presence of the SH ligand hinders interaction with GSH, in contrast to the chlorido analogues, which readily form [(η5-Cpx)Ir(N^N)(SG−)]+ species.2a The lack of reaction of 1 and 2 with GSH might contribute to the higher activity of these complexes as compared with their chlorido analogues. Similarly complexes 3 and 4 did not form Ir−SG adducts. Intriguingly, they promoted oxidation of GSH to GSSG, as judged by 1H NMR studies (Figure 5).18 This appears to be the first F

DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

observation of the catalytic oxidation of GSH to GSSG by halfsandwich iridium(III) complexes. A similar behavior was reported recently for half-sandwich ruthenium(II) complexes [(η6-ar)Ru(en) (SR)]+ containing isopropylthiolate or thiophenylate (ar = hexamethylbenzene or p-cymene).18,26 These ruthenium(II) complexes oxidize GSH to GSSG and involve formation of sulfenatoruthenium(II) species and H2O2. Such a mechanism may also be possible for Cpbiph complexes 3 and 4, since a peak for {[(η5-Cpbiph)Ir(N^N)(SH)]+H+O}+ was detected.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ̌ P.S. was supported by the National Program of Sustainability I (LO1305) of the Ministry of Education, Youth and Sports of the Czech Republic. We thank the ERC (Grant No. 247450), Science City (AWM/ERDF), and EPSRC (Grant No. EP/ F034210/1) for support, and members of EC COST Action CM1105 for stimulating discussions. We also thank Dr. I. Prokes for acquiring 13C NMR and NOESY data and Dr. L. Song for performing HR-MS experiments.

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CONCLUSIONS The hydrosulfidoiridium(III) complexes 1−4 of general formula [(η5-Cpx)Ir(N^N)(SH)]PF6 represent a new structural type of half-sandwich iridium(III) cyclopentadienyl complexes, whose inner coordination sphere contains a bidentate N-donor ligand and hydrosulfide anion; Cpx = Cp* (for 1 and 2) or Cpbiph (for 3 and 4), N^N = phen (for 1 and 3) or bpy (for 2 and 4). A 1H NMR SH signal was detected at ca. −2 ppm in the spectra of 1−4, and its cross-peak with Cp* hydrogen atoms was present in the 1H−1H gs-NOESY spectrum of the representative complex [(η5-Cp*)Ir(phen)(SH)]BPh4 (1′). Xray structural analysis showed that 1′ adopts a piano-stool geometry. We have also shown for the first time that [(η5Cpx)Ir(N^N)(SH)]+ complexes can exhibit antiproliferative activity against the A2780 human ovarian cancer cells. The complexes under investigation are oxidized to, most likely, sulfenato species, in aqueous solution as suggested by ESI-MS. Complexes 1−4 did not bind to 9EtG and 9MeA. Only Cpbiph complexes 3 and 4 reacted with GSH; however, they did not form Ir−SG adducts as recently reported for analogous chloridoiridium(III) cyclopentadienyl complexes, but unusually promoted oxidation of GSH to GSSG. This indirectly suggests a possible redox-mediated MoA for these and related organoiridium complexes. There is already evidence for their involvement in mitochondrial dysfunction and production of reactive oxygen species (ROS).10a,b Hydrosulfide complexes could be formed in cancer cells by abstraction of sulfide from the prevalent ferredoxin (iron−sulfur) proteins. Such an abstraction might itself lead to perturbations of the redox state of cancer cells. The possible role of (hydro)sulfide complexes in the mechanism of action of organo-iridium and possible other organometallic complexes is therefore worthy of further investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02697. Synthesis of [(η5-Cpbiph)Ir(phen)Cl]Cl and [(η5-Cpbiph)Ir(bpy)Cl]Cl, details of X-ray crystallographic data, bond lengths, and angles, ESI-MS and 1H NMR control of the synthesis and characterization of the studied complexes, HPLC analysis, X-ray crystal structure showing disorder and π−π stacking, 1H NMR studies of aqueous chemistry. (PDF) X-ray crystallographic data in CIF format. (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G

DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02697 Inorg. Chem. XXXX, XXX, XXX−XXX