om400493k?journalCode=orgnd7...32

Article pubs.acs.org/Organometallics

Di- and Trinuclear Ruthenium‑, Rhodium‑, and IridiumFunctionalized Pyridyl Aromatic Ethers: A New Class of Antiparasitic Agents Prinessa Chellan,† Kirkwood M. Land,‡ Ajit Shokar,‡ Aaron Au,‡ Seung Hwan An,‡ Dale Taylor,§ Peter J. Smith,§ Kelly Chibale,†,⊥ and Gregory S. Smith*,† †

Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa Department of Biological Sciences, University of the Pacific, Stockton, California 95211, United States § Division of Pharmacology, Department of Medicine, University of Cape Town, K45, OMB, Groote Schuur Hospital, Observatory, 7925, South Africa ⊥ Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Rondebosch 7701, South Africa ‡

S Supporting Information *

ABSTRACT: Two new di- and tripyridyl aromatic ether ligands (1 and 2) have been synthesized using the Williamson ether method. Functionalization of these ligands with either dichlorido(p-cymene)ruthenium(II), dichlorido(pentamethylcyclopentadienyl)rhodium(III), or dichlorido(pentamethylcyclopentadienyl)iridium(III) fragments yielded three di- and three trinuclear organometallic complexes (1a−c and 2a−c). Both ligands act as monodentate donors to each metal center via the pyridyl nitrogen atoms, and this coordination mode of the polyaryl ether ligands has been confirmed upon elucidation of the molecular structure of the dinuclear iridium complex 1c. All of the synthesized compounds were evaluated for inhibitory effects on the Plasmodium falciparum strain NF54 (chloroquine-sensitive) and found to have moderate to high antiplasmodial activities, with the trinuclear complexes displaying activities in the nanomolar range. The most active compounds were studied for their ability to inhibit formation of synthetic hemozoin in a cell-free medium. Against the Trichomonas vaginalis strain G3, these aryl ether compounds were only moderately active.

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INTRODUCTION

treatments, resistance and suspected resistance to these drugs have been reported in parts of Asia.5−9 The clinical success of cisplatin and other platinum-based drugs for the treatment of different cancers has had a profound effect on establishing the use of metal complexes in medicine. However, increasing drug resistance and the emergence of unwanted side effects to currently available therapies have bred a need for the development of new metal-based drugs with fewer side-effects and with different mechanisms of action. Among the transition metals, ruthenium appears to be the most promising candidate. The redox chemistry of ruthenium is rich and compatible with biological media, and the overall toxicity of ruthenium is lower than platinum, thus allowing higher doses of treatment. In addition, rhodium(III) and iridium(III) complexes (isoelectronic to ruthenium(II)) have also been identified as biologically relevant metals, and their complexes are gaining attention as alternatives to platinum-containing compounds.

Parasitic diseases have proven to be a major health problem particularly in developing countries, and there are only a few effective drugs currently available for their treatment. The frequent manifestation of resistance to these current therapies has posed a serious impediment to their use. This makes the discovery of new drug leads an urgent and challenging task.1 Malaria is among the deadliest and most widespread parasitic diseases found today. A recent World Health Organization (WHO) report estimated that 216 million cases of malaria infection were reported in 2010 with 655 000 resulting deaths mainly among African children.2 In fact, most of these malaria infections and deaths occurred in sub-Saharan Africa. The parasite strain Plasmodium falciparum is the most virulent strain and is responsible for the greatest number of malaria deaths.2 Currently, the standard first-line therapy for uncomplicated P. falciparum malaria infections is artemisinin-based combined therapy (ACT)3,4 which is a combination therapy that utilizes artemisinin or an artemisinin derivative with a second drug that has a separate parasite target. Despite the effectiveness of ACT © XXXX American Chemical Society

Received: May 31, 2013

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dx.doi.org/10.1021/om400493k | Organometallics XXXX, XXX, XXX−XXX

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studied a variety of non-quinoline-based ligands and their mono- and polynuclear complexes as antiparasitics.22−26 The results of these studies have identified several new potential chemotypes for the design of more potent antiparasitic agents. The use of metals such as rhodium, iridium, and ruthenium for the preparation of polynuclear complexes is attractive since these metals are stable in several oxidation states and possess an octahedral coordination geometry, thus having two additional binding sites as compared to Pt(II) metal-based drugs, giving rise to interesting metal complex architectures as well as physical or electronic properties.27−35 Poly(aryl ethers) are known to exhibit high thermal stability and have been studied as polymeric or dendritic supports in catalysis and biology.36−39 The study of molecules containing diaryl ether groups as pharmacological agents is pertinent since structure−activity analyses of several biologically important natural products have shown that these compounds contain diaryl ether functionalities.40 Some of these small-molecule diaryl ethers are proposed to act as tyrosine kinase inhibitors and have had some success for the treatment of metastatic non-small-cell lung cancer and pancreatic cancer.41 Mono- and polynuclear ruthenium-arene and pentamethylcyclopentadienyl rhodium and iridium pyridyl complexes have demonstrated promising pharmacological activities particularly as antiproliferative agents, making them viable candidates for further biological study.42−44 The concept of multinuclearity is an innovative strategy often resulting in improved biological activity with respect to mononuclear compounds, which could be attributed to favorable modulation of the stability, solubility, and/or lipophilicity of the organometallic scaffolds.45 Thus, the coupling of metalated pyridyl functionalities to a simple aryl ether scaffold may lead to an enhanced synergistic biological effect whereby the aryl ether framework would stabilize the metal moieties, allowing them to be delivered intact to their pharmacological target. In this paper, the synthesis and characterization of two new di- and tripyridyl aromatic ether ligands based on a triazine core, along with their Ru(II)-, Rh(III)-, and Ir(III)-functionalized organometallic complexes, are described. All of the compounds synthesized were evaluated as antiparasitic agents in vitro against the P. falciparum strains NF54 (chloroquinesensitive) and Dd2 (chloroquine-resistant) and the Trichomonas vaginalis strain G3. Studies into their physicochemical properties have also been initiated, and the preliminary results are also presented.

There are several reports on the study of metal complexes, including palladium, gold, and platinum, as antiplasmodial agents, and these examples have been the subject of recent review articles.3,10−13 Published accounts of platinum group metal (PGM) complexes as antiplasmodial agents consist largely of chloroquine or other aminoquinoline metal conjugates.3,10,11 Sánchez-Delgado et al. have reported that a [Ru(II)-chloroquine]2 dimer and mononuclear Rh(I) complex (Figure 1) proved active against P. falciparum strains in vitro.14

Figure 1. Structure of ruthenium and rhodium chloroquine metalconjugates and ferroquine.11,14,19

Consequent to the initial chloroquine metal conjugate work pioneered by Sánchez-Delgado et al. came the discovery of the chloroquine analogue ferroquine.15 This metallocene-containing complex has shown impressive antiplasmodial activity and is active against chloroquine-resistant strains.11,15−17 It has successfully completed phase I clinical trials and is expected to complete phase II clinical trials against uncomplicated malaria in the near future.18 Ruthenium-arene chloroquine conjugates have been extensively studied for their antiplasmodial activities.20,21 In contrast, there are fewer examples of antiplasmodial PGM complexes that are non-quinolone based. Recently, we have designed and Scheme 1a

a

Reaction conditions: (i) K2CO3/18-crown-6/reflux/THF/48 h; (ii) K2CO3/DMF/80 °C/16 h. B


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Scheme 2

Scheme 3

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antiparasitic and antitumoral agents.46−52 The new di- and tripyridyl aromatic ether ligands [3,5-bis(pyridin-4-ylmethoxy)phenyl]methanol (1) and 2,4,6-tris[4-(pyridin-4-ylmethoxy)phenyl][1,3,5]triazine (2) were prepared using two typical Williamson-ether synthesis protocols (Scheme 1).53 For ligand 1, the aromatic alcohol 3,5-di(hydroxyl)benzyl alcohol was reacted with 4-bromomethylpyridine hydrobromide in THF using 18-crown-6 as the phase transfer catalyst, together with the weak base K2CO3. The dipyridyl product (1) was isolated as an amorphous solid in 40% yield. The reaction conditions

RESULTS AND DISCUSSION

Synthesis and Characterization. The choice to prepare a simple dipyridyl ether ligand system is based on structure analyses of previously reported compounds that revealed simple diaryl ether derivatives to have biological activity.40,41 For the preparation of the tripyridyl ether ligands, a tris-hydroxyphenyltriazine scaffold was chosen since polyaryl-functionalized triazine derivatives and some of their metal complexes have been reported to exhibit in vitro cytotoxic effects as both C


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associated with the other protons of the now metalated pyridyl ether ligands was observed, confirming that coordination of the metal occurs only at the pyridyl nitrogen atoms and that there is no coordination to the nitrogen atoms of the triazine core. For the Ru(II) complexes 1a and 2a, the proton resonances corresponding to the aromatic protons of the p-cymene ring are observed downfield compared to the alkyl ether protons at 5.47 and 5.26 ppm, with the lower frequency resonance assigned to the protons ortho to the methyl substituent. The trinuclear complexes, 2a−c, exhibit low solubility in the chlorinated solvents DCM and chloroform and are insoluble in other common organic solvents such as methanol, ethanol, and acetone. The methyl protons of the isopropyl substituent of the arene ring are observed at 1.30 and 1.56 ppm for complexes 1a and 2a, respectively; the protons of the methyl substituent on the p-cymene ring resonate at ca. 2.10 ppm for both complexes. The rhodium- and iridium-functionalized complexes 1b,c and 2b,c display a singlet at approximately 1.55 ppm for the protons of the pentamethylcyclopentadienyl ring. Analysis of complexes 1a−c using 13C{1H} NMR spectroscopy reveals a high-frequency shift of the resonance assigned to the carbon ortho to nitrogen compared to the free ligand (1) to between 155.0 and 153.0 ppm. The resonances assigned to the alkyl ether carbons occur at approximately the same shift of between 67.5 and 67.8 ppm. The benzyl carbon also occurs at essentially the same resonance in the complexes 1a−c compared to the free ligand (1), confirming that complexation to the hydroxyl oxygen had not occurred. Singlets seen at 82.9 and 82.0 ppm are due to the resonances of the unsubstituted aromatic carbons of the p-cymene moieties of complex 1a. The isopropyl-substituted aromatic carbon and the methyl-substituted carbon resonate at 97.2 and 103.2 ppm, respectively. These resonances agree with similar mononuclear 4-pyridyl complexes.42 Due to the sparing solubility of complexes 2a−c in most common deuterated solvents, recording well-resolved 13C NMR spectra proved difficult, despite running extended overnight NMR experiments at elevated temperatures. Metalation of the pyridyl nitrogen atoms of the pyridyl rings was further confirmed for complexes 1a−c and 2a−c using IR spectroscopy. For all complexes a high-energy shift of the absorption band associated with the pyridyl CN vibration was observed compared to the corresponding free ligand. For the dinuclear complexes, an absorption band at approximately 1615 cm−1 is noted compared to ligand 1 (observed at 1597 cm−1), and for the trinuclear complexes the CNpyridyl vibration is observed between 1613 and 1623 cm−1. The C N vibration associated with the triazine functionality in complexes 2a−c shows no change compared to the free ligand (2) and is still observed at approximately 1606 cm−1. High-resolution ESI-MS data reveal a base peak for complexes 1a and 1b corresponding to a triply charged complex. The trinuclear complexes 2a−c also form adducts with either sodium (2a), methanol (2b), or water (2c). UV−Vis Spectrometry. The electronic properties of compounds 1, 2, 1a−c, and 2a−c were analyzed using UV− vis spectrometry. The UV−vis spectra for the dipyridyl (1 and 1a−c, Figure S1) and tripyridyl compounds (2 and 2a−c, Figure S2) are contained in the Supporting Information. Table 1 lists the energy absorptions observed. All of the compounds were analyzed at a concentration of 0.5 μM in dichloromethane (DCM) (dipyridyl compounds) or dimethylsulfoxide (DMSO) (tripyridyl compounds). Assignments of the absorption bands

used for the synthesis of 1 proved unsuccessful for the preparation of the tripyridyl ligand (2), and instead tris(4hydroxylphenyl)triazine was reacted with 4-bromomethylpyridine hydrobromide in DMF at 80 °C with K2CO3, yielding the tripyridyl ligand 2 as a yellow solid in 52% yield. Examination of the proton NMR spectra for ligands 1 and 2 gives convincing evidence for the formation of the aromatic ether functionalities. A singlet occurring at 5.18 and 5.21 ppm for 1 and 2, respectively, is assigned to the alkyl ether protons. This resonance shift is typical for aryl-alkyl ethers.53,54 For both ligands, the pyridyl protons, ortho and meta to nitrogen, resonate in the range 8.55−8.75 ppm and 7.40−7.43 ppm, respectively, as doublets as a result of through-bond coupling to each other. Coupling constants ranging between 6 and 9 Hz are observed and agree with a 3J ortho coupling for aromatic and heteraromatic protons. For 1, the benzyl protons resonate at 4.58 ppm as a singlet and relatively upfield to this shift, the hydroxyl proton resonates as a broad singlet at 4.22 ppm. The aromatic protons are observed in the expected region between 6.50 and 6.90 ppm. In the triazine-containing ligand 2, the protons of the benzene rings are observed as doublets at 8.65 and 7.12 ppm. The more downfield resonance is assigned to the protons that are in closer proximity to the triazine ring, as these protons would experience a stronger deshielding effect due to the triazine core. Analysis of 1 and 2 using 13C{1H} NMR spectroscopy reveals that the ortho and meta carbons of the pyridyl rings resonate at approximately 150.0 and 121.5 ppm. The alkyl ether carbons occur at 67.8 ppm for 1 and 67.9 ppm for 2. Further evidence confirming that etherification had occurred is garnered from the infrared analyses of 1 and 2. Both ligands show a strong absorption band in the region of 1130−1170 cm−1 as a consequence of the C−O ether bond vibration. For 1, the pyridyl imine CN stretching frequency is assigned to a strong absorption band at 1597 cm−1; a similar frequency is observed for 2. The triazine CN bond present in 2 vibrates at a higher frequency compared to the pyridyl imine and is assigned to an absorption band at 1607 cm−1. Electron-impact (EI) mass spectrometry analysis of 1 and 2 reveal that both compounds exhibit a base peak corresponding to the molecular ion peak, [M]+. Ligands 1 and 2 were reacted with the dimeric precursors dichlorido(p-cymene)ruthenium(II) dimer, dichlorido(pentamethylcyclopentadienyl)rhodium(III) dimer, and dichlorido(pentamethylcyclopentadienyl)iridium(III) dimer (Schemes 2 and 3). For complexes 1a−c and 2a, the appropriate ligands were reacted with the corresponding dimers in DCM at room temperature. Complexes 2b and 2c were prepared by reaction of the corresponding dimer and ligand 2 in refluxing methanol (Scheme 3). Complexes 1a−c and 2a−c were isolated as air-stable solids in moderate to high yields. When comparing the 1H NMR spectra of 1a−c and 2a−c with their corresponding free ligands (1 and 2), a downfield shift of the doublet associated with the pyridyl protons ortho to nitrogen to around 9.00 ppm is observed. This deshielding of the ortho proton resonances is characteristic of metal coordination to nitrogen and arises from increased backbonding between the metal and nitrogen, causing reduced electron density in the ortho carbon−hydrogen bond. This effect has been reported for other mono- and multinuclear Ru(II), Rh(III), and Ir(III) complexes containing monodentate 4-pyridyl ligands.42,44,55−60 A slight change in the resonances D


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Single-Crystal X-ray Structural Analysis of Compound 1c. Crystals of complex 1c suitable for single-crystal X-ray diffraction analysis were grown from a mixture of methanol and hexane. Complex 1c crystallized in monoclinic space group Pn with one molecule of methanol and three molecules of water in the asymmetric unit. The molecular structure of 1c validates the proposed structures of 1a−c put forth by the spectroscopic data and characterizations discussed earlier. Figure 2 shows the complex structure of 1c·CH3OH·3H2O with the solvent molecules omitted, and the crystal data are listed in the reference section.63 Table 2 lists selected bond lengths and angles for the molecular structure of 1c·CH3OH·3H2O.

Table 1. Spectral Data for Compounds 1, 2, 1a−c, and 2a−c Using UV−Vis Spectrometry λmax (nm)a

compound 1 1a 1b 1c 2 2a 2b 2c a

358 (π−π*) 343 (π−π*) 336 (π−π*) 323 (π−π*) 345 (π−π*) 343 (π−π*) 345 (π−π*) 368 (π−π*)

383−471 (MLCT, d−πligand) 369−469 (MLCT, dmetal−πligand) 415−465 (MLCT, dmetal−πligand) 409 (π−π*) 405 (MLCT, dmetal−πligand) 482 (LMCT, πligand− dmetal) 410 (MLCT, dmetal−πligand)

Table 2. Selected Bond Lengths and Bond Angles Observed for Complex 1c

374 (MLCT, dmetal−πligand) 446 (LMCT, πligand− dmetal)

Bond Lengths (Å) Ir1−N1 Ir1−C1 Ir1−C2 Ir1−C3 Ir1−C4 Ir1−C5 Ir1−Cl1 Ir1−Cl2 N1−C11 N1−C15 O1−C16 O1−C17

Transitions are given in parentheses.

were made based on similar complexes reported in the literature.58,61,62 All of the metal complexes exhibit a hyperchromic shift in the absorption bands compared to their corresponding metal-free ligands. In the visible region, the absorption bands are fairly broad, which is characteristic of highly conjugated aromatic systems. Ligand 1 shows a broad Soret band in the UV region at 345 nm and is attributed to the intraligand π−π* transitions of the aromatic and heteroaromatic rings. This intraligand transition shows a hypsochromic shift in the complexes (1a−c). Compared to the corresponding dinuclear complexes, the absorption spectra of 1 are much less intense. The complexes 1a−c are intensely colored, and the metal to ligand and ligand to metal charge transfer (MLCT and LMCT) bands thus absorb strongly in the visible region, while the free ligand is a pale beige color and shows no absorption in the visible region. The tripyridyl-functionalized triazine ligand 2 is a bright yellow solid and shows a strongly absorbing Soret band in the visible region at 409 nm due to the intraligand π−π* transitions. The MLCT bands for the complexes 2a−c absorb strongly in this region and most likely mask the intraligand absorptions in the visible region. Complexes 2a and 2c show shoulders to their MLCT bands at 482 and 446 nm, respectively, and these absorptions are most likely due to ligand to metal charge transfers (LMCT).

2.095(12) 2.122(14) 2.120(17) 2.146(14) 2.158(11) 2.120(17) 2.408(4) 2.405(4) 1.303(19) 1.343(18) 1.408(18) 1.381(16) Bond Angles

Ccentroid−Ir1−Cl1 Ccentroid−Ir1−Cl2

124.8(5) 128.7(5)

Ir2−N2 Ir2−C30 Ir2−C31 Ir2−C32 Ir2−C33 Ir2−C34 Ir2−Cl3 Ir2−Cl4 N2−C27 N2−C28 O3−C24 O3−C19 (deg) Ccentroid−Ir2−Cl3 Ccentroid−Ir2−Cl4

2.067(11) 2.090(18) 2.171(17) 2.238(19) 2.141(19) 2.11(3) 2.418(7) 2.419(5) 1.25(2) 1.43(2) 1.429(19) 1.399(18) 124.5(5) 128.6(6)

Each iridium metal center adopts a typical “piano-stool” conformation with the two chlorido ligands and the pyridyl ring of the aryl ether ligand as the three legs. Comparison of the Ir− C bond lengths between the metal and each bonded carbon of the pentamethylcyclopentadienyl ring reveals them to be similar, indicating that the ring is symmetrically bonded to iridium. The mean Ir−Cl, Ir−C, and Ir−N bond distances with their standard uncertainties are 2.413(4), 2.142(1), and 2.08(1) Å, respectively. These values agree with those observed for similar complexes in the literature55,59,64,65 as well as the expected length calculated from the covalent radii of iridium,

Figure 2. Molecular structure of 1c with hydrogen atoms and solvent molecules omitted. E


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ether compounds are good candidates for in vitro biological testing. Precipitation of these compounds from the various in vitro assays would be unlikely, and thus the activities observed for these compounds are a true reflection of their in vitro activity since all of the compounds should remain in solution. In Vitro Antiparasitic Activity against P. falciparum. The in vitro antiplasmodial activity of all compounds was ascertained against the chloroquine-sensitive (CQS) NF54 P. falciparum strain (Table 4). Compounds exhibiting good

chlorine (Ir−Cl: 2.40 Å), carbon (Ir−C: 2.09 Å), and nitrogen (Ir−N: 2.09 Å).66 In the coordinated pyridyl ether ligand, the C−N bond lengths in the pyridyl ring are found to be between 1.25 and 1.40 Å and the alkyl C−O bond lengths of the ether functionalities between 1.41 and 1.43 Å, comparing favorably to the interatomic distances typical of pyridines and ethers (C N: 1.34 Å and C−O: 1.43 Å).67 In 1c, the bond angles observed for Cl−Ir−Cl and N−Ir−Cl are all close to 90°, and the bond angles formed between the bonded carbons of the Cp* ligand, iridium, and the chloride ligand vary from 90° to 150°. This has also been observed for the molecular structures of similar Cp*Ir(III) pyridyl complexes.55,59,64,65 Turbidimetric Solubility Studies. The solubility of a compound in aqueous-based media is an important property, as it has a bearing on a compound’s bioavailability since it impacts on absorption. Solubility also validates the in vitro assay data obtained. In general, on the basis of their solubility range, compounds can be classified into three different categories: (i) 60 μg/mL (soluble). If a compound displays solubility greater than 60 μg/mL, then good absorption is likely. With respect to metal complexes, often polynuclear complexes display poor solubility in aqueous-based in vitro assays and are sometimes screened as suspensions in the assay, thus casting doubt on the in vitro activities observed. One method used to validate aqueous solubility during early drug discovery is the turbidimetric (kinetic) solubility assay.68−71 This assay was used to evaluate the solubility of compounds 1, 2, 1a−c, and 2a−c and the [(p-cymene)RuCl 2 ] 2 , [Cp*RhCl2]2, and [Cp*IrCl2]2 dimers in phosphate-buffered saline at pH 7.4 (Table 3). The drugs reserpine and

Table 4. In Vitro IC50a Data for Compounds 1, 2, 1a−c, and 2a−c against the P. falciparum Strains NF54 (CQS) and Dd2 (CQR) compound 1 2 1a 1b 1c 2a 2b 2c [(p-cymene) RuCl2]2 [Cp*RhCl2]2 [Cp*IrCl2]2 ferroquine chloroquine diphosphate

compound 1 2 1a 1b 1c 2a 2b 2c [(p-cymene) RuCl2]2 [Cp*RhCl2]2 [Cp*IrCl2]2 reserpine hydrocortisone

determined turbidimetric solubility (μg/mL)

>200 >200 160−200 80−120 160−200 40−80 80−120 80−120 >200

>64.27 >126.14 149.35−186.70 75.15−112.73 178.89−223.61 61.97−123.94 124.62−186.94 146.06−219.09 >122.48

>200 >200 20−40 >200

>103.62 >159.34 12.17−24.34 >72.49

NF54b (μM)

Dd2b (μM)

0 0 2 2 2 3 3 3 2

36.36 0.39 10.68 13.67 56.79 0.24 0.23 1.64 16.80

(3.91) (0.06) (1.41) (1.51) (9.95) (0.01) (0.03) (0.25) (2.94)

not tested 0.86 (0.07) not tested not tested not tested 0.64 (0.03) 0.44 (0.06) not tested not tested

2 2 1

20.90 59.40 0.03 0.02

(0.87) (19.45) (0.01) (0.01)

not tested not tested not tested 0.133

resistance indexc 2.22

2.67 1.89

6.65

a

Concentration inhibiting 50% of parasite growth. bStandard error of the mean given in parentheses. cResistance index (RI) = IC50 Dd2/ IC50 NF54.

Table 3. Turbidimetric Solubility Data for Compounds 1, 2, 1a−c, and 2a−c determined turbidimetric solubility (μM)

no. of metal moieties

activities against NF54 were then screened for inhibitory effects on the chloroquine-resistant (CQR) P. falciparum strain Dd2. The ruthenium, rhodium, and iridium dimers [(pcymene)RuCl2]2, [Cp*RhCl2]2, and [Cp*IrCl2]2 and the organometallic chloroquine analogue ferroquine were also screened for activity. Chloroquine diphosphate was used as a control for the in vitro experiments. With respect to the metal-free ligands (1 and 2), the tripyridyl ether ligand (2) was the most active (IC50 = 0.39 μM). The significantly greater activity observed for 2 compared to 1 may be attributed to the central triazine core. The incorporation of a triazine moiety into potential antimalarials has been previously reported.48,52 Some substituted s-triazines have shown in vitro activities against D6 (CQS),48 W2 (CQR),48 3D7 (CQS),52 and K1 (CQR)52 P. falciparum strains similar to the IC50 value determined for 2. Substituted striazine compounds are believed to target dihydrofolate reductase (DHFR) in the P. falciparum parasite.48,72 DHFR is an enzyme that catalyzes the nicotinamide adenine dinucleotide phosphate (NADPH)-dependent reduction of dihydrofolate to tetrahydrofolate. Functionalization of ligand 2 with ruthenium (2a) and rhodium (2b) led to an enhancement of the in vitro antiplasmodial activity [IC50 = 0.24 μM (2a) and 0.23 μM (2b)]. The iridium derivative 2c (IC50 = 1.64 μM) was less active, but inspection of all compounds tested reveals that ligand 2 and its trinuclear organometallic complexes (2a−c) displayed significantly greater activity. While the other compounds tested displayed only moderate activities, a trend in activities could be observed. The presence

hydrocortisone were used as controls for the solubility assays. In micromolar concentration, most of the compounds were soluble in PBS buffer up to concentrations above 100 μM. Converting this data to μg/mL shows that all of the compounds had solubility greater than 60 μg/mL. Complex 2a showed the lowest solubility in the buffer. However, its solubility range is still higher than 60 μg/mL. The free ligands (1 and 2), the trinuclear rhodium and iridium ether complexes 2b and 2c, and the ruthenium, rhodium, and iridium dimers display no turbidity up to the highest concentration tested. Overall, the data ascertained implies that the di- and tripyridyl F


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complexes, but against the Dd2 strain, it seems that the rhodium complexes may have better activity. Screening of the other ruthenium (1a) and rhodium (1b) complexes against the Dd2 strain must be carried out to confirm if complexes of rhodium are better than ruthenium derivatives against this CQR strain. Their resistance index (RI) was determined on the basis of the IC50 values observed for each P. falciparum strain. The RI value is an important implement when analyzing potential drug candidates and provides information on whether or not the compound is susceptible to exactly the same mechanism of resistance as chloroquine that is expressed in the Dd2 strain. A low RI value may suggest that the compound is likely to exhibit activity against resistant parasite strains.73,74 Chloroquine has a relatively high RI value (6.65), and compounds 2 and 2b have a lower RI value (2.22 and 1.89, respectively), suggesting that these compounds are likely to be active against chloroquineresistant strains of the parasite. β-Hematin Inhibition Studies. The mechanism of hemozoin formation is currently a target for antiplasmodial drug discovery. In the life cycle of the P. falciparum parasite, degradation of the infected host’s hemoglobin to provide essential amino acids for parasite growth and nutrition is a vital step.75 A side product of this hemoglobin digestion is free heme, which is toxic to the parasite. To circumvent these effects, the parasite initiates a detoxification mechanism and removes the threat by conversion of the free heme into a crystalline solid known as hemozoin (also known as malaria pigment), which is nontoxic to the parasite. Clinically used drugs amodiaquine and chloroquine are believed to inhibit the formation of hemozoin.76 The ability of a potential drug to inhibit formation of hemozoin can be measured using the NP40-mediated β-hematin (synthetic hemozoin) inhibition assay. NP-40 is a low-cost, lipophilic detergent that can mediate the formation of synthetic hemozoin (β-hematin). The NP-40mediated assay seeks to mimic the conditions of the acidic food vacuole in the parasite to give a better measure of hemozoin formation. From the compounds screened for in vitro antiplasmodial activity against the NF54 CQS P. falciparum strain, those compounds showing cytotoxicities of approximately 10 μM or less (2, 1a, and 2a−c) were assayed for β-hematin inhibition (Table 5). The dimers [(p-cymene)RuCl2]2, [Cp*RhCl2]2, and [Cp*IrCl2]2 were also assayed for comparison. This screening revealed that the tripyridyl ether triazine ligand 2 was the most active compound (IC50 = 9.93 μM). Complex 2c, the iridiumfunctionalized tripyridyl ether complex, was the least active (IC50 = 64.64 μM). The dimers did not show any activity highlighting the importance of the interaction between the metal and ligand. The triruthenium and trirhodium pyridyl ether complexes (2a and 2b) exhibited the best in vitro parasite growth inhibition against the NF54 strain. These complexes were found to have β-hematin inhibitory activities [IC50 = 20.72 μM (2a) and 20.56 μM (2b)] comparable to chloroquine (IC50 = 18.43 μM). Overall, the best inhibitor of β-hematin formation was the ligand 2, which showed better inhibition than ferroquine and chloroquine. All of the compounds screened using the NP-40-mediated assay are polyaryl in nature, making them capable of intermolecular π−π interactions. Thus, it may be possible that these compounds inhibit formation of hemozoin through π−π stacking with hematin. Most of the compounds screened

of the Cp*IrCl2 moieties (1c and 2c) seems to have a detrimental effect on inhibitory activities. Both these complexes were less active than the corresponding free ligand; complex 1c is 0.64 times less active than 1, and complex 2c is 0.24 times less active than 2. For the dinuclear complexes (1a−c), it is clear that activity decreased with a change in metal moiety in the order (p-cymene)RuCl2 > Cp*RhCl2 > Cp*IrCl2. Functionalization of the free ligands (1 and 2) with either the (p-cymene)RuCl2 (1a and 2a) or Cp*RhCl2 (1b and 2b) moieties led to a strong increase in antiplasmodial activity, suggesting that these metal moieties play a role in activity. Comparison of the activities of the dimers, [(p-cymene)RuCl2]2, [Cp*RhCl2]2, and [Cp*IrCl2]2, to the corresponding ether complex reveals that the dimers are less active than the di(1a−c) and trinuclear (2a−c) complexes. Coupled with the fact that the complexes (1a−c and 2a−c) are more active than the free ligands, this suggests that there is a cooperative effect between metal moiety and ligand on antiplasmodial activity. Both the ligands (1 and 2) and the dimers were only moderately active on their own, but conjugation of the different metal moieties onto the corresponding ligand led to a beneficial increase in activity. Based on the in vitro data determined on the NF54 CQS strain several key trends for the design of better organometallic antimalarials can be discerned. The incorporation of a triazine moiety into the complex structure (2a−c) has a profound effect on activity. The trinuclear complexes display better activities compared to the dinuclear derivatives, thus proposing that an increase in metal moieties leads to an increase in activity in vitro. All of the complexes showed better activity than the corresponding free ligand. Since the triazine-containing complexes 2a and 2b along with their free ligand 2 showed enhanced activities compared to the other complexes screened against CQS NF54 strain, they were screened for in vitro antiplasmodial activity against the chloroquine-resistant P. falciparum strain Dd2 (Table 5). All three compounds were Table 5. IC50 Data of Compounds Screened for β-Hematin Inhibitory Activity Using NP-40-Mediated β-Hematin Assaya compound

IC50 (μM)

95% confidence interval

2 1a 2a 2b 2c [(p-cymene)RuCl2]2 [Cp*RhCl2]2 [Cp*IrCl2]2 ferroquine chloroquine amodiaquine

9.93 37.16 20.72 20.56 64.64 no inhibition no inhibition no inhibition 14.51 18.43 6.83

9.61−10.25 36.30−38.04 19.46−22.07 18.75−22.55 61.70−67.72

13.72−15.34 17.56−19.34 6.57−7.10

a Only compounds showing in vitro cytoxic values of approximately 10 μM or less were screened.

less active against the Dd2 strain compared to the NF54 strain. The complexes 2a (IC50 = 0.64 μM) and 2b (IC50 = 0.44 μM) were slightly more active than the free ligand 2 (IC50 = 0.86 μM), providing further evidence that functionalization of 2 with the metal moieties enhances antiplasmodial activity. Complex 2b was more active than complex 2a. Against the NF54 strain, the ruthenium complexes were more active than the rhodium G


Organometallics

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Figure 3. Graphical representation of the percentage inhibition data ascertained for compounds 2, 1a−c, and 2a−2c against the T. vaginalis strain G3.

using this assay were able to inhibit β-hematin formation in a lipidic atmosphere to some degree. Should these compounds be able to enter the acidic food vacuole of the parasite intact, then they may very likely inhibit parasite growth through inhibition of hemozoin. It should be noted that a direct correlation between the activities observed for the β-hematin inhibition assays and the activities obtained for the compounds in the parasite is not possible, as the β-hematin inhibition assays are carried out in a parasite-free medium. In Vitro Antiparasitic Activity against T. vaginalis. T. vaginalis is a flagellated facultative anaerobic protozoan that causes the sexually transmitted infection trichomoniasis in humans and is reported to infect more than 170 million people worldwide, mainly in industrialized countries.77 Infection can have serious implications such as premature birth and increased risk of acquiring HIV, cervical cancer, and aggressive prostate cancer.77−80 Metronidazole is the current FDA-approved treatment,81 but approximately 5% of all cases reported display resistance to this compound, and this percentage is increasing.82 There is therefore a need to find alternative forms of treatment due to this increase in resistance. The effect on parasite viability of a selection of the compounds was evaluated on the T. vaginalis strain G3 (Figure 3). Cells were inoculated with 50 and 25 μM doses of compounds. All of the compounds were able to influence parasite viability in a dose-dependent manner. At 50 μM, the best percent inhibitions were observed for complexes 1b (92.4%) and 2c (89.4%). Moderate inhibitory effects were observed for complexes 1c, 2a, and 2c (81.8%, 65.2%, and 83.3%, respectively). The tripyridyl ether ligand 2 demonstrated only 72.7% inhibition, while its rhodium and iridium complexes (2b and 2c) displayed much better inhibitions. The ruthenium derivative 2a showed decreased inhibition (65.2%) compared to 2. The dinuclear rhodium and iridium complexes (1b and 1c) also displayed better percent inhibitions (92.4% and 81.8%, respectively) than the ruthenium analogue (1a, 63.6%). When comparing the dinuclear (1a−c) and trinuclear (2a− c) complexes, it can be discerned that with respect to the type of metal moiety the dinuclear complexes display similar inhibitory effects to their trinuclear analogues. This seems to imply that there is no increase in activity with an increase in the number of metal moieties. Furthermore, incorporation of the triazine core does not appear to result in enhanced inhibitory effects, as the dinuclear and trinuclear complexes show similar

activity, as evidenced by their percent inhibitions. At the lower concentration of 25 μM, a decrease in the percent inhibitions is observed for all of the compounds. Complexes 1a and 2a display the highest decrease in activity (17.1% and 14.3%, respectively), while complex 2b showed the slightest change in percent inhibition. The rhodium complexes 1b and 2b have similar inhibitions at 25 μM (72.3% and 78.7%), and the same trend can be seen for the iridium complexes 1c and 2c. Out of all the pyridyl ether complexes, the rhodium complexes (1b and 2b) have shown the best in vitro activity against G3. None of the compounds were as active as the FDA-approved treatment for T. vaginalis infections, metronidazole. This drug displays 100% inhibition at both concentrations. Nevertheless, these complexes represent a new chemotype for the inhibition of the T. vaginalis strain G3. Modification of the ligand structure and/or the metal moieties may increase the activity of these compounds. For example, preparation of ligands that are able to chelate to the metal in a bidentate manner may lead to the formation of complexes that are more stable in the biological medium. Modification of the arene or cyclopentadienyl ligands to give added water solubility to the metal moieties could also enhance activity.

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CONCLUSIONS Two new pyridyl ether ligands have been synthesized and metalated with ruthenium, rhodium, and iridium precursors to give the corresponding di- and trinuclear complexes. All of the compounds were isolated as air- and moisture-stable solids and were characterized using a variety of analytical and spectroscopic techniques. The molecular structure of 1c shows that iridium adopts a typical “piano-stool” conformation. The molecular structure confirms the monodentate coordination of the pyridyl ether ligands to each metal put forth by the spectroscopic and spectrometric characterizations of complexes 1a−c and 2a−c. All of the complexes and their corresponding free ligand were found to exhibit moderate to high antiplasmodial activity against the NF54 CQS P. falciparum strain. The trinuclear pyridyl ether complexes (2a−c) and their free ligand (2) demonstrated significantly higher activities than the other complexes tested, and this was attributed to the incorporation of a central triazine moiety into their structures. The trinuclear complexes display better activities compared to the dinuclear derivatives, thus proposing that an increase in metal moieties leads to an increase in activity in vitro. All of the H


Organometallics

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stirring. Upon cooling to room temperature, the reaction mixture was diluted with ethyl acetate (100 cm3) and washed with deionized water (3 × 50 cm3) and brine (1 × 50 cm3). The organic layer was then dried over magnesium sulfate and filtered, and the solvent evaporated under reduced pressure to afford the product (2) as a bright yellow, amorphous solid (0.092 g, 52%). Mp: 253−254 °C. 1H NMR (acetone-d6/δ/ppm): 8.71 (d, 3J(H−H): 9.19 Hz, 6H, py H), 8.65 (d, 3 J(H−H): 5.99 Hz, 6H, Ar H), 7.41 (d, 3J(H−H): 4.80 Hz, 6H, py H), 7.12 (d, 3J(H−H): 8.80 Hz, 6H, Ar H), 5.21 (s, 6H, CH2). 13C NMR (acetone-d6/δ/ppm): 158.1 (tz C), 149.8 (py Ca), 141.57 (Ar C), 134.0 (Ar C), 128.3 (Ar C), 123.3 (Ar C), 121.5 (py C), 115.12 (Ar C), 67.9 (CH2). IR: ν/cm−1 1607(CNtriazine stretch), 1592 (C Npyridyl stretch), 1131 (O−CH2 stretch). Anal. Found: C, 72.07; H, 5.10; N, 12.52. Calcd for C39H30O3N6·0.5C4H8O2: C, 72.29; H, 5.08; N, 12.45. EI-MS: m/z 630.21 [M]+, 100%. General Synthetic Method for Dinuclear Ru(II), Rh(III), and Ir(III) Pyridyl Ether Complexes. The ligand [3,5-bis(pyridin-4ylmethoxy)phenyl]methanol (1) (1 molar equiv) was dissolved in DCM (15 cm3), the appropriate ruthenium, rhodium, or iridium dimer (1.1 molar equiv) was added, and the reaction solution was stirred for 16 h at room temperature. The reaction solvent was then evaporated under reduced pressure to approximately a third of its original volume. The product was then precipitated from solution by addition of diethyl ether, isolated by vacuum filtration, washed with diethyl ether, and dried. Synthesis of Complex 1a. [3,5-Bis(pyridin-4-ylmethoxy)phenyl]methanol (1) (0.040 g, 0.12 mmol) was reacted with dichlorido(pcymene)ruthenium(II) dimer (0.083 g, 0.14 mmol). The product (1a) was isolated as dark red, amorphous solid (0.093 g, 80%). Mp: 141 °C, decomposition without melting. 1H NMR (CDCl3-d1/δ/ppm): 8.93 (d, 3J(H−H): 5.11 Hz, 4H, py H), 7.26 (d, 3J(H−H): 4.39 Hz,4H, py H), 6.54 (s, 2H, Ar H), 6.26 (s, 1H, Ar H), 5.44 (d, 3J(H−H): 5.99 Hz, 4H, Ar H), 5.25(d, 3J(H−H): 5.59 Hz, 4H, Ar H), 5.00 (s, 4H, OCH2), 4.53 (s, 2H, ArCH2), 4.46 (br s, 1H, OH), 2.00−2.90 (m, 2H, ArCH), 2.03 (s, 6H, ArCH3), 1.30 (d, 3J(H−H): 6.79 Hz, 12H, ArCH3). 13C NMR (acetone-d6/δ/ppm): 159.9 (Ar C), 154.5 (py C), 148.7 (py C), 144.8 (Ar C), 122.2 (py C), 106.2 (Ar C), 103.2 (ArCH3), 100.8 (Ar C), 97.2 (Ar C), 82.9 (Ar C), 82.0 (Ar C), 67.5 (OCH2), 64.1 (ArCH2), 30.6 (ArCH), 22.2 (ArCH3), 18.2 (ArCH3). IR: ν/cm−1 3435 (O−H stretch), 1598 (CN stretch), 1160 (O− CH2 stretch). Anal. Found: C 47.92; H, 5.40; N, 2.46. Calcd for C39H46Cl4N2O3Ru2·0.5CH2Cl2: C, 47.93; H, 4.74; N, 2.87. EI-MS: m/ z 312.01 [M + 3H]3+, 100%. Synthesis of Complex 1b. [3,5-Bis(pyridin-4-ylmethoxy)phenyl]methanol (1) (0.047 g, 0.15 mmol) was reacted with dichlorido(pentamethylcyclopentadienyl)rhodium(III) dimer (0.10 g, 0.16 mmol). The product (1b) was isolated as a bright orange, amorphous solid (0.11 g, 80%). Mp: 228−230 °C. 1H NMR (CDCl3-d1/δ/ppm): 8.91 (br s, 4H, py H), 7.38 (d, 3J(H−H): 6.39 Hz, 4H, py H), 6.57 (s, 2H, Ar H), 6.33 (s, 1H, Ar H), 5.07 (s, 4H, OCH2), 4.58 (s, 2H, ArCH2), 2.61 (br s, 1H, OH), 1.58 (s, 30H, Cp* H). 13C NMR (acetone-d6/δ/ppm): 159.2 (Ar C), 153.1 (py C), 148.7 (py C), 144.8 (Ar C), 122.9 (py C), 106.6 (Ar C), 101.3 (Ar C), 94.1 (Cp* C), 67.8 (OCH2), 64.5 (ArCH2), 8.8 (Cp* CH3). IR: ν/cm−1 3443 (O−H stretch), 1615 (CN stretch), 1152 (O−CH2 stretch). Anal. Found: C, 47.13; H, 5.42; N, 2.04. Calcd for C39H48Cl4N2O3Rh2·CH2Cl2· 0.5(C2H5)2O: C, 47.48; H, 5.21; N, 2.63. ESI-MS: m/z 351.01 [M + 3H + 3CH3CN]3+, 100%. Synthesis of 1c. [3,5-Bis(pyridin-4-ylmethoxy)phenyl]methanol (1) (0.037 g, 0.12 mmol) was reacted with dichlorido(pentamethylcyclopentadienyl)iridium(III) dimer (0.10 g, 0.13 mmol). The product (1c) was isolated as a bright yellow, amorphous solid (0.086 g, 67%). Mp: 195 °C, decomposition without melting. 1H NMR (CDCl3-d1/δ/ ppm): 8.89 (d, 3J(H−H): 5.60 Hz, 4H, py H), 7.38 (d, 3J(H−H): 4.80 Hz, 4H, py H), 6.59 (s, 2H, Ar H), 6.36 (s, 1H, Ar H), 5.12 (s, 4H, OCH2), 4.59 (s, 2H, ArCH2), 2.39 (br s, 1H, OH), 1.54 (s, 30H, Cp* H). 13C NMR (acetone-d6/δ/ppm): 159.2 (Ar C), 153.2 (py C), 149.1 (py C), 144.6 (Ar C), 123.0 (py C), 106.7 (Ar C), 101.3 (Ar C), 85.8 (Cp* C), 67.6 (OCH2), 64.6 (ArCH2), 8.5 (Cp* CH3). IR: ν/cm−1 3458 (O−H stretch), 1615 (CN stretch), 1152 (O−CH2 stretch).

complexes showed better activity than the corresponding free ligand. The most active compounds were assayed for inhibition of βhematin formation and found to inhibit formation of synthetic hemozoin to some degree. This activity may be a consequence of their polyaryl structure. If these compounds are able to enter the acidic food vacuole intact, then they may inhibit parasite growth through inhibition of hemozoin. Against the T. vaginalis strain G3, all compounds decreased parasite viability in a dosedependent manner.

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EXPERIMENTAL SECTION

General Methods. Pentamethylcyclopentadiene, α-phellandrene, 4-bromomethylpyridine hydrobromide, 3,5-dihydroxybenzyl alcohol, and 4-cyanophenol were purchased from Sigma-Aldrich and used without further purification. Ruthenium trichloride trihydrate, rhodium trichloride trihydrate, and iridium trichloride trihydrate were kindly donated by AngloAmerican Platinum Limited. All solvents used were analytical grade and dried over molecular sieves. All reactions were carried out in air unless otherwise stated. The precursors, 2,4,6-tris(phydroxyphenyl)triazine, 83 dichlorido(p-cymene)ruthenium(II) dimer, 84 dichlorido(pentamethylcyclopentadienyl)rhodium(III) dimer,85 and dichlorido(pentamethylcyclopentadienyl)iridium(III) dimer,85 were synthesized using literature methods. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity XR400 MHz (1H at 399.95 MHz, 13C at 100.58 MHz), Varian Mercury XR300 (1H at 300.08 MHz, 13C at 75.46 MHz), or Bruker Biospin GmbH (1H at 400.22 MHz, 13C at 100.65 MHz) spectrometer at ambient temperature. Chemical shifts for 1H and 13C{1H} NMR shifts are reported using tetramethylsilane (TMS) as the internal standard, and 31P{1H} NMR spectra were measured relative to H3PO4 as the external standard. Infrared (IR) absorptions were measured on a Perkin-Elmer Spectrum 100 FT-IR spectrometer using a Universal Diamond Attenuated Total Reflection (ATR) accessory. Microanalyses for C, H, and N were carried out using a Thermo Flash 1112 Series CHNS-O analyzer, and melting points were determined using a Büchi B-540 melting point apparatus. Mass spectrometry determinations were carried out on all new compounds using either electron impact (EI) on a JEOL GC Matell instrument or electrospray ionization (ESI) on a Waters API Quattro Micro instrument in the positive mode. Synthesis of [3,5-Bis(pyridin-4-ylmethoxy)phenyl]methanol (1). To a solution of 3,5-dihydroxybenzyl alcohol (0.066 g, 0.47 mmol) in acetone (30 cm3) was added potassium carbonate (0.39 g, 2.82 mmol) and 18-crown-6 (0.025 g, 0.094 mmol). The mixture was stirred at room temperature for 15 min before 4-bromomethylpyridine hydrobromide (0.25 g, 0.99 mmol) was added, and the reaction mixture heated to reflux for 48 h. The reaction solvent was then evaporated under reduced pressure, and the crude brown residue was extracted with DCM (50 cm3). The organic layer was washed with deionized water (4 × 20 cm3) and brine (1 × 20 cm3), dried over magnesium sulfate, filtered, and the solvent evaporated under reduced pressure to afford the product (1) as a brown, amorphous solid (0.0618 g, 41%). Mp: 194−196 °C. 1H NMR (acetone-d6/δ/ppm): 8.58 (d, 3J(H−H): 6.39 Hz, 4H, py H), 7.42 (d, 3J(H−H): 5.59 Hz, 4H, py H), 6.90 (s, 2H, Ar H), 6.59 (s, 1H, Ar H), 5.18 (s, 4H, OCH2), 4.58 (s, 2H, ARCH2), 4.22 (br s, 1H, OH). 13C NMR (acetone-d6/δ/ppm): 159.5 (Ar C), 149.7 (py C), 146.3 (py C), 145.5 (Ar C), 121.45 (py C), 105.5 (Ar C), 100.4 (Ar C), 67.8 (OCH2), 63.4 (ArCH2). IR: ν/cm−1 3184 (O−H stretch), 1597 (CN stretch), 1166 (O−CH2 stretch). Anal. Found: C, 67.89; H, 5.73; N, 8.03. Calcd for C19H18O3N2·0.5CH3OH: C, 67.44; H, 5.36; N, 8.28. EI-MS: m/z 322.11 [M]+, 100% Synthesis of 2,4,6-Tris[4-(pyridin-4-ylmethoxy)phenyl][1,3,5]triazine (2). 2,4,6-Tris(4-hydroxyphenyl)triazine (0.10 g, 0.28 mmol) and potassium carbonate (0.23 g, 1.7 mmol) were added to DMF (30 cm3) and stirred for 5 min at room temperature. 4Bromomethylpyridine hydrobromide (0.23 g, 0.92 mmol) was then added, and the reaction mixture was heated to 80 °C for 18 h with I


Organometallics

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Anal. Found C 39.14; H, 4.49; N, 1.80. Calcd for C39H48Cl4N2O3Ir2· 2CH2Cl2·(C2H5)2O: C, 39.65; H, 4.52; N, 2.05. ESI-MS: m/z 1083.2 ([M − Cl]+, 100%). Synthesis of Trinuclear Ru(II), Rh(III), and Ir(III) Pyridyl Ether Complexes. Synthesis of Complex 2a. 2,4,6-Tris[4-(pyridin-4ylmethoxy)phenyl][1,3,5]triazine (2) (0.068 g, 0.11 mmol) was added to a stirred solution of dichlorido(p-cymene)ruthenium(II) dimer (0.10 g, 0.17 mmol) in DCM (15 cm3). The reaction was allowed to stir at room temperature for 16 h, at which time the volume of the solvent was reduced to approximately 5 cm3 and an excess of diethyl ether was added;, precipitating the product (2a). The product (2a) was isolated as a yellow-brown, amorphous solid by filtration under reduced pressure, washed with diethyl ether, and dried (0.077 g, 46%). Mp: 207−211 °C, decomposition without melting. 1H NMR (CDCl3-d1/δ/ppm): 9.02 (br s, 6H, py H), 8.53 (br s, 6H, Ar H), 7.36 (br s, 6H, py H), 6.95 (br s, 6H, Ar H), 5.48 (br s, 6H, Ar H), 5.27 (br s, 6H, Ar H), 5.12 (br s, 6H, O CH2), 3.01 (m, 3H, Ar CH), 2.12 (s, 9H, Ar CH3), 1.56 (br s, 18H, Ar CH3). IR: ν/cm−1 1623 (CNpyridyl stretch), 1607 (CNtriazine stretch), 1149 (O−CH2 stretch). Anal. Found: C, 52.11; H, 5.03; N, 4.62. Calcd for C69H72Cl6N6O3Ru3· 2H2O·CH3OH: C, 51.98; H, 5.05; N, 5.19. ESI-MS: m/z 270.6 ([M + 3Na+ + 3H+]6+, 100%). Synthesis of Complex 2b. 2,4,6-Tris[4-(pyridin-4-ylmethoxy)phenyl][1,3,5]triazine (2) (0.061 g, 0.097 mmol) was suspended in a stirred solution of dichlorido(pentamethylcyclopentadienyl)rhodium(III) dimer (0.10 g, 0.16 mmol) in methanol (20 cm3). The reaction mixture was stirred at reflux for 16 h and then cooled to room temperature. The product (2b) was isolated as a dark orange, amorphous solid by filtration under reduced pressure, washed with methanol (10 cm3) and diethyl ether (10 cm3), and dried (0.11 g, 71%). Mp: 267 °C, decomposition without melting. 1H NMR (CDCl3-d1/δ/ppm): 9.02 (d, 3J(H−H): 6.00 Hz, 6H, py H), 8.66 (d, 3J(H−H): 8.79 Hz, 6H, Ar H), 7.47 (d, 3J(H−H): 6.40 Hz, 6H, py H), 7.06 (d, 3J(H−H): 8.80 Hz, 6H, Ar H), 5.23 (s, 6H, O CH2), 1.56 (s, 45H, Cp* CH3). IR (atr, cm−1): ν 1613 (w, CNpyridyl), 1606 (w, CNtriazine), 1146 (s, −C−O−CH2−). Anal. Found: C, 52.00; H, 4.72; N, 5.10. Calcd for C69H75Cl6N6O3Rh3·CH3OH: C, 52.13; H, 4.75; N, 5.29. ESI-MS: m/z 810.2 ([M + 2H + 2CH3OH]2+, 100%). Synthesis of Complex 2c. 2,4,6-Tris[4-(pyridin-4-ylmethoxy)phenyl][1,3,5]triazine (2) (0.050 g, 0.079 mmol) was suspended in a stirred solution of dichlorido(pentamethylcyclopentadienyl)iridium(III) dimer (0.10 g, 0.13 mmol) in methanol (20 cm3). The reaction mixture was stirred at reflux for 16 h and then cooled to room temperature. The product (2c) was isolated as a yellow, amorphous solid by filtration under reduced pressure, washed with methanol (10 cm3) and diethyl ether (10 cm3), and dried (0.131 g, 91%). Mp: 257 °C, decomposition without melting. 1H NMR (CDCl3-d1/δ/ppm): 8.99 (d, 3J(H−H): 6.40 Hz, 6H, py H), 8.65 (d, 3J(H−H): 8.80 Hz, 6H, Ar H), 7.45 (d, 3J(H−H): 4.00 Hz, 6H, py H), 7.04 (d, 3J(H−H): 8.80 Hz, 6H, Ar H), 5.24 (s, 6H, O CH2) 1.56 (s, 45H, Cp* CH3). IR: ν/cm−1 1619 (CNpyridyl stretch), 1606 (CNtriazine stretch), 1146 (O−CH2 stretch). Anal. Found: C, 45.10; H, 4.31; N, 4.81. Calcd for C69H75Cl6N6O3Ir3: C, 45.40; H, 4.14; N, 4.60. ESI-MS: m/z 441.1 ([M − 4Cl− + 4H2O]4+, 100%). X-ray Structure Analysis. Single-crystal X-ray diffraction data were collected on a Bruker KAPPA APEX II DUO diffractometer using graphite-monochromated Mo Kα radiation (χ = 0.71073 Å). Data collection was carried out at 173(2) K. Temperature was controlled by an Oxford Cryostream cooling system (Oxford Cryostat). Cell refinement and data reduction were performed using the program SAINT.86 The data were scaled and absorption correction was performed using SADABS.2 The structure was solved by direct methods using SHELXS-9787 and refined by full-matrix least-squares methods based on F2 using SHELXL-9787 and using the graphics interface program X-Seed.88,89 The programs X-Seed and POV-Ray90 were both used to prepare molecular graphic images. There are three water and one methanol molecule in the asymmetric unit, and all were refined with half site occupancy factors. One water oxygen atom was refined anisotropically, and all other non-hydrogen atoms of the solvents were refined with isotropic displacement parameters. The

carbon atoms from C30 to C39 on the main molecule were restrained geometrically to a reasonable shape and were refined isotropically. All other non-hydrogen atoms on the main molecule were refined anisotropically. All hydrogen atoms, except those of the solvent molecule and the hydroxyl hydrogen H2 on O2, were placed in idealized positions and refined with geometrical constraints. The hydrogen atom H2 was located in the difference electron density map and refined with the distance of O2−H2 set to 0.97 Å. The structure was refined to an R factor of 0.0563. The highest peak is 3.01 e/Å3, 1.01 Å from IR1; the deepest hole is −1.95 e/Å3, 0.78 Å from IR2. P. falciparum in Vitro Assay. The test samples were tested in triplicate on one occasion against the chloroquine-sensitive NF54 strain and the chloroquine-resistant Dd2 strain of P. falciparum. Continuous in vitro cultures of asexual erythrocyte stages of P. falciparum were maintained using a modified method.91 Quantitative assessment of antiplasmodial activity in vitro was determined via the parasite lactate dehydrogenase assay using a modified method.92 The test samples were prepared to a 20 mg/cm3 stock solution in 100% DMSO and sonicated to enhance solubility. Samples were tested as a suspension if not completely dissolved. Stock solutions were stored at −20 °C. Further dilutions were prepared on the day of the experiment. Chloroquine (CQ) was used as the reference drug in all experiments. A full dose−response was performed for all compounds to determine the concentration inhibiting 50% of parasite growth (IC50 value). Test samples were tested at a starting concentration of 100 μg/cm3, which was then serially diluted 2-fold in complete medium to give 10 concentrations, with the lowest concentration being 0.2 μg/cm3. The same dilution technique was used for all samples. CQ was tested at a starting concentration of 100 ng/cm3 against the CQR strain and 1000 ng/cm3 against the CQS strain. The highest concentration of solvent to which the parasites were exposed had no measurable effect on the parasite viability (data not shown). The IC50 values were obtained using a nonlinear dose−response curve fitting analysis via Graph Pad Prism v.4.0 software. T. vaginalis in Vitro Assay. Cultures of T. vaginalis G3 strain were grown in 5 cm3 complete TYM Diamond’s media in a 37 °C incubator for 24 h. One hundred millimolar stocks of the compounds were made by dissolving in DMSO and screened against the G3 stain of T. vaginalis. Cells untreated and inoculated with 5 μL of DMSO were used as controls. Five microliters of 100 mM stocks of compound library was inoculated for a final concentration of 100 μM. Results were calculated based on counts utilizing a hemocytometer after 24 h. Detergent-Mediated Assay for β-Hematin Inhibitors. The βhematin formation assay method described by Carter et al.93 was modified for manual liquid delivery. Stock solutions of the test compounds were prepared at 10, 2, and 0.4 mM by dissolving each sample in DMSO with sonication. Test compounds were delivered to a 96-well plate in triplicate from 0 to 500 μM (final concentration) with a total DMSO volume of 10 μL in each well. Deionized H2O (70 μL) and NP-40 (20 μL; 30.55 μM) were then added. A 25 mM hematin stock solution was prepared by sonicating hemin in DMSO, for complete dissolution, and then suspending 177.76 μL of this in a 2 M acetate buffer (pH 4.8). The homogeneous suspension (100 μL) was then added to the wells to give final buffer and hematin concentrations of 1 M and 100 μM, respectively. The plate was covered and incubated at 37 °C for 16 h in a water bath. Analysis of the assay was carried out using the pyridine-ferrichrome method developed by Ncokazi and Egan.94 A solution of 50% (v/v) pyridine, 30% (v/v) H2O, 20% (v/v) acetone, and 0.2 M HEPES buffer (pH 7.4) was prepared, and 32 μL was added to each well to give a final pyridine concentration of ±5% (v/v). Acetone (60 μL) was then added to assist with hematin dispersion. The UV−vis absorbance of the plate wells was read on a SpectraMax plate reader. Sigmoidal dose−response curves were fitted to the absorbance data using GraphPad Prism v3.02. J


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ASSOCIATED CONTENT

S Supporting Information *

The UV−vis spectra for complexes 1a−c and 2a−c are available and can be accessed free of charge via the Internet at http://pubs.acs.org. CCDC 934523 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/ retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; e-mail: [email protected]. uk].

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AUTHOR INFORMATION

Corresponding Author

*Tel: +27-21-6505279. Fax: +27-21-6505195. E-mail: Gregory. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the University of Cape Town (UCT), the National Research Foundation (NRF) of South Africa, and Sasol South Africa is gratefully acknowledged. AngloAmerican Platinum Limited is kindly acknowledged for the donation of metal salts.

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REFERENCES

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