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Catalytically Generated Ferrocene-Containing Guanidines as Efficient Precursors for New Redox-Active Heterometallic Platinum(II) Complexes with Anticancer Activity Daniel Nieto,†,⊥ Sonia Bruña,†,⊥ Ana Ma González-Vadillo,† Josefina Perles,‡ Fernando Carrillo-Hermosilla,*,§ Antonio Antiñolo,§ José M. Padrón,*,∥ Gabriela B. Plata,∥ and Isabel Cuadrado*,† †

Departamento de Quı ́mica Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain Servicio Interdepartamental de Investigación (SIdI), Laboratorio de Difracción de Rayos X de Monocrystal, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain § Centro de Innovación en Quı ́mica Avanzada (ORFEO-CINQA), Departamento de Quı ́mica Inorgánica, Orgánica y Bioquı ́mica, Facultad de Ciencias y Tecnologı ́as Quı ́micas, Universidad de Castilla La Mancha, 13071 Ciudad Real, Spain ∥ BioLab, Instituto Universitario de Bio-Orgánica Antonio González (IUBO-AG), Centro de Investigaciones Biomédicas de Canarias (CIBICAN), Universidad de La Laguna, 38206 La Laguna, Spain ‡

S Supporting Information *

ABSTRACT: The potential of structurally new ferrocene-functionalized guanidines as redox-active precursors for the synthesis of heterometallic platinum(II)−guanidine complexes with anticancer activity was studied. To this end, an atom-economical catalytic approach was followed by using ZnEt2 to catalyze the addition of aminoferrocene and 4-ferrocenylaniline to N,N′-diisopropylcarbodiimide. Furthermore, reaction of a platinum(II) source with the newly obtained guanidines Fc−NC(NHiPr)2 (3) and Fc(1,4-C6H4)−NC(NHiPr)2 (4) provided access to the heterometallic complexes [PtCl2{Fc−NC(NHiPr)2}(DMSO)] (5), [PtCl2{Fc(1,4-C6H4)−NC(NHiPr)2}(DMSO)] (6), and [PtCl2{Fc(1,4-C6H4)−NC(NHiPr)2}2] (7). Electrochemical studies evidence the remarkable electronic effect played by the direct attachment of the guanidine group to the ferrocene moiety in 3, making its one-electron oxidation extremely easy. Guanidinebased Fe−Pt complexes 5 and 6 are active against all human cancer cell lines tested, with GI50 values in the range 1.4−2.6 μM, and are more cytotoxic than cisplatin in the resistant T-47D and WiDr cell lines.

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INTRODUCTION Molecules incorporating a guanidine-based functional group show impressive promise for a range of interesting applications in many areas of chemistry, such as organocatalysis, 1 coordination chemistry,2 and anion recognition.3,4 This is due in part to their ability to act as neutral, cationic, and anionic entities. Moreover, the guanidine moiety is an essential constituent for a number of biologically important molecules, and guanidines are also found in natural products and topselling pharmaceuticals.5 Consequently, there is an increasing interest focused on the discovery of new compounds with a guanidine core, suitable to develop active drugs, for example, as potential chemotherapeutic agents.6 The synthesis of guanidines has been intensively investigated by a variety of methods.1,7,8 Primary aliphatic amines can undergo direct addition to carbodiimides under rather forcing conditions. However, aromatic amines or secondary amines do not react with carbodiimides under the same (or even harsher) conditions. Among the direct methods for the synthesis of guanidines, the catalytic guanylation of amines with carbodiimides represents an attractive and convenient alternative to © 2015 American Chemical Society

traditional routes, as it has an atom economy of 100% and is a waste-free process.8 Recently, we reported the use of cheap and commercially available ZnEt2 (as a solution in hexane) as a very effective catalyst for the guanylation reaction of primary and secondary aromatic amines, including aromatic diamines and aliphatic, heterocyclic, and secondary cyclic amines, under milder conditions.8,9 In a different context, amino-functionalized ferrocenes, due to their rich and versatile chemistry, have played a significant role in areas such as asymmetric catalysis and NLO materials.10,11 Furthermore, several amino-ferrocenes have been investigated as sensors of anions,12,13 as redox mediators in electrochemical biosensing of DNA and proteins,14 and as ancillary ligands (with Pd and Pt centers) for catalytic olefin polymerizations.15 Likewise, positively charged amino-substituted ferrocenyl guests have proven to be capable of reaching high binding affinities with cucurbit[n]uril (CBn) hosts in aqueous medium, forming unusually stable inclusion comReceived: September 2, 2015 Published: November 2, 2015 5407

DOI: 10.1021/acs.organomet.5b00751 Organometallics 2015, 34, 5407−5417

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Organometallics plexes.16,17 Interestingly, the effects of aminoferrocene-based prodrugs have been recently investigated on human normal and cancer cells as well as bacterial cells.18 As a consequence of low toxicity, significant stability and lipophilicity, facile functionalization, and unique electrochemical behavior, the use of ferrocene-containing compounds for medicinal applications is a rapidly developing area of research.19 The ferrocenyl moiety has been incorporated into the structure of biologically active molecules such as anticancer, antimalaria, and antibiotic drugs, resulting in an increase of their activity. In particular, ferrocene derivatives are, along with ruthenium complexes, the most promising metal-based chemotherapy alternatives to platinum-based anticancer drugs.19 A number of neutral ferrocene derivatives as well as some cationic ferrocenium salts exhibit good cytotoxicities in vitro and tumor growth inhibition in vivo.19 Likewise, the cytotoxicity of some ferrocenyl derivatives was found to be enhanced by the incorporation of an additional cytotoxic transition metal in the same molecule.20−23 As part of our longstanding interest in the chemistry of ferrocene-containing compounds and macromolecules,24,25 we have recently investigated the ability of mono- and bisfunctionalized ferrocenylethylamines Fc(CH2)2NH2 and fc{(CH2)2NH2}2 (Fc = Fe(η5-C5H4)(η5-C5H5), fc = Fe(η5C5H4)2), to form heterometallic ferrocene- and platinum(II)containing compounds with anion sensing ability26 and antitumor activity.27 In an effort to expand this chemistry, we were particularly interested in studying the ability of ferrocenecontaining guanidines as nitrogen donor ligands to bind platinum(II) centers. Here we describe the first efficient atom-economical catalytic approach to ferrocene-based guanidines, which have been prepared starting from two different amino-functionalized ferrocenes: namely, aminoferrocene (1) and 4-ferrocenylaniline (2). Although a few examples of ferrocene-containing guanidine derivatives have been reported in relevant works in the past few years, by the groups of Molina and Tárraga,28 Siemeling29a and Rauf,29b,30 their syntheses are mainly based on multistep and stoichiometric reactions, with the use and production of undesirable mercury-based substances in some cases. In addition, we also report on structurally new heterometallic complexes where the electroactive ferrocenyl−guanidine ligand is covalently attached to the cytotoxic Pt(II) center, in order to try a synergistic biological effect between the guanidine moiety and the two active metal sites. The complete structural characterization and redox behavior are described. Moreover, the lipophilicity and cytotoxic activity of homo- and heterometallic ferrocene-based guanidines toward the panel of human solid tumor cell lines HBL-100 (breast), HeLa (cervix), SW1573 (nonsmall cell lung), T-47D (breast), and WiDr (colon) were evaluated in comparison to those of the established anticancer drug cisplatin.

C−C and C−heteroatom bond coupling,31 has been used for the atom-economical synthesis of the ferrocenyl−guanidines. On the basis of our previous work on the synthesis of arylsubstituted guanidines,9a diethylzinc was used to catalyze the addition of N,N′-diisopropylcarbodiimide to FcNH2 (1) and Fc(1,4-C6H4)NH2 (2) under mild conditions and with high yields. In fact, the addition of a small amount (3 mol %) of ZnEt2 (1 M in hexane) to a deuterated toluene solution of the corresponding amino-ferrocene 1 or 2 and carbodiimide resulted in clean and quantitative conversion to the targeted guanidine Fc−NC(NHiPr)2 (3) or Fc(1,4-C6H4)−N C(NHiPr)2 (4), in 1−1.5 h at 50 °C (Scheme 1). Scheme 1. Catalytic Synthesis of Ferrocenyl-Containing Guanidines

Encouraged by the NMR-scale experiments, we next attempted the synthesis of ferrocenyl-containing guanidines on a preparative scale. The reactions proceeded with excellent yields, and the targeted ferrocene-containing guanidines 3 and 4 were obtained as crystalline orange solids. The characterization of ferrocene-containing guanidines was accomplished by elemental analysis, NMR and IR spectroscopy, and mass spectrometry. The 1H NMR spectra, in CDCl3, of 3 and 4 (see Figures S1 and S5 in the Supporting Information) reveal signals of the monofunctionalized ferrocenyl units from δ 4 to 4.6 ppm but do not show any signal corresponding to the NH protons of the guanidine moiety. However, the incorporation of the guanidine to the corresponding aminoferrocene 1 and 2 is proved by the isopropyl signals located around δ 1.2 and 2.8 ppm. In addition to the peaks of the isopropyl fragments in the 1H NMR, the 13C NMR spectra (in CDCl3) present the diagnostic peak of the central carbon of the guanidine moiety near δ 150 ppm. IR spectra of guanidines 3 and 4 display strong bands at 3300 cm−1 characteristic of the NH moieties and around 1600 cm−1 for the CN groups. FAB mass spectra show the corresponding molecular ions M+ at m/z 327.1 (3) and 404.1 (4), with agreement between the experimental and calculated isotopic patterns. The structures of ferrocene-containing guanidines 3 and 4 were determined by X-ray single-crystal diffraction. Single crystals were obtained by slow evaporation at room temperature, from a solution of 3 in CDCl3 and from a solution of 4 in CH2Cl2/n-hexane (10/1). A representation of the molecular structures of 3 and 4 is presented in Figure 1, and a summary of crystallographic data and data collection parameters is included in Table S1 in the Supporting Information. 2-(1-Ferrocenyl)-1,3-diisopropylguanidine (3) crystallizes in the monoclinic P21/c space group with one molecule per asymmetric unit. The ferrocene moiety presents an eclipsed

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RESULTS AND DISCUSSION Catalytic Formation of Ferrocene-Containing Guanidines. The first objective of this research was to study the ability of some amino-ferrocenes to undergo efficient atomeconomical guanylation reactions. Accordingly, the catalytic formation of organometallic guanidines was initially explored by in situ 1H NMR spectroscopy, in deuterated toluene, using aminoferrocene (1) and 4-ferrocenylaniline (2) as amine precursors. Diethylzinc, one of the simplest organometallic compounds with application in diverse catalyzed reactions for 5408


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Figure 1. Molecular structures of neutral ferrocenyl−guanidines 3 (A) and 4 (B). Hydrogen atoms have been omitted for clarity.

electronic properties of guanidine Fc−NC(NHiPr)2 (3) relative to guanidine Fc(1,4-C6H4)−NC(NHiPr)2 (4) might lead to different redox and biological properties for the corresponding Pt(II) complexes. The reaction between 2-(ferrocenyl)-1,3-diisopropylguanidine (3) and the platinum source cis-[PtCl2(DMSO)2] was carried out in a 2:1 molar ratio in refluxing toluene, as shown in Scheme 2. Purification was effected by column chromatography

configuration with an angle of 6.74(9)° between the two cyclopentadienyl rings. The angle between the substituted cyclopentadienyl ring and the plane containing the nitrogen atoms is 48.16(8)°. Guanidine 4 crystallizes in the trigonal R3̅ space group also with one molecule per asymmetric unit. In this case, the two cyclopentadienyl rings in the ferrocene unit are not eclipsed and are more parallel, with an angle of 2.1(5)° between them. The phenyl ring is almost parallel to the directly connected cyclopentadienyl ring (17.3(3)°), and the guanidine fragment is again very twisted, making an angle of 81.2(2)° with the phenyl ring. There are intermolecular hydrogen bonds N1′···H−N2 that form six-membered rings involving the molecules related by the 3̅ rotoinversion axis parallel to the [001] direction (Figure 2). Ferrocene- and Guanidine-Based Heterometallic Platinum(II) Complexes. In order to complement our investigations on ferrocene-functionalized guanidines, we sought to examine the coordination chemistry of electron-rich ferrocenyl−guanidines 3 and 4 toward the platinum(II) center. In particular, we anticipated that the differing steric and

Scheme 2. Synthesis of Platinum(II) Ferrocenyl−Guanidine Compounds 5−7

on silica gel, affording the heterometallic compound trans[PtCl2{Fc−NC(NHiPr)2}(DMSO)] (5), with only one ferrocenyl−guanidine 3 coordinated to the cytotoxic Pt(II) metal moiety, in 53% yield. We also attempted to prepare the related bis(ferrocenyl−guanidine) Pt(II) complex [PtCl2{Fc− NC(NHiPr)2}2]. Unfortunately, longer reaction times and an increase in the amount of ferrocenyl−guanidine 3 (from 2 to 4 equiv) did not enable the incorporation of a second

Figure 2. View along the c axis of the molecular packing of 4. 5409


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Figure 3. Molecular structures of heterometallic ferrocenyl−platinum(II) guanidine complexes 5 (left) and 6 (right). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for 5: Pt1A−Cl1 = 2.320(2), Pt1A−Cl2 = 2.297(2), Pt1A−N1 = 2.040(7), Pt1A−S1 = 2.213(2), N1−Pt1A−Cl1 = 82.1(2), Cl1−Pt1A−S1 = 90.18(8); S1−Pt1A−Cl2 = 93.80(8), Cl2−Pt1A−N1 = 87.8(2). Selected bond lengths (Å) and angles (deg) for 6: Pt1−Cl1 = 2.297(2), Pt1−Cl2 = 2.299(2), Pt1−N1 = 2.045(7), Pt1−S1 = 2.218(2); N1−Pt1−Cl1 = 82.1(2), Cl1−Pt1−S1 = 90.18(8), S1−Pt1−Cl2 = 93.80(8), Cl2−Pt1−N1 = 87.8(2), Pt2−Cl3 = 2.302(2), Pt2−Cl4 = 2.302(2), Pt2−N4 = 2.047(6), Pt2−S2 = 2.211(2); N1−Pt1−Cl1 = 88.1(2); Cl1−Pt1−S1 = 90.04(8), S1−Pt1−Cl2 = 93.44(9), Cl2−Pt1−N1 = 88.4(2); N4−Pt2−Cl3 = 88.1(2); Cl3−Pt2−S2 = 89.90(8), S2−Pt2−Cl4 = 93.63(8), Cl4−Pt1−N4 = 88.3(2).

the 13C NMR spectra present the corresponding signals at δ 43 ppm. The 195Pt NMR spectra of heterometallic guanidines 5−7 also provide convincing evidence for the coordination sphere of the mixed ferrocene and platinum complexes, since the platinum resonances are extremely sensitive to the coordination environment around the Pt atom, affected among others by the steric and electronic effects of the surrounding ligands.33 Thus, the single resonance detected in the 195Pt{1H} NMR spectrum of monoferrocenyl−guanidines (in CDCl3) appears at δ −2951 ppm (for 5) and at δ −2988 ppm (for 6) (Figures S11 and S16 in the Supporting Information), which are consistent with the values reported for a platinum nucleus surrounded by a N, Cl2, and SDMSO set of donor atoms.27,33,34 Concerning the bis(ferrocenyl−guanidine) complex 7, its 195Pt{1H} NMR spectrum (Figure S21 in the Supporting Information) reveals one resonance at δ −1887 ppm, and this value, which is considerably shifted to low field relative to that observed for bimetallic 5 and 6, agrees with a N2Cl2 set of donor atoms around the platinum center.27,33−35 Mass spectrometric studies of 5−7 confirmed the heterometallic proposed structures, showing the corresponding molecular ions M+ at m/z 671.1 (for 5), 747.0 (for 6), and 1072.2 (for 7). Their isotopic distributions are in agreement with the calculated values. In order to unambiguously identify the proposed heterometallic structures, single-crystal X-ray diffraction studies of the ferrocene- and guanidine-based Pt(II) complexes 5−7 were undertaken. Single crystals of 5 and 6 were obtained, at room temperature, from a solution of the corresponding compound in a mixture of diethyl ether/n-hexane (10/1). Likewise, crystals of trimetallic 7 suitable for X-ray diffraction were obtained by slow evaporation from a solution of the compound in CH2Cl2/ Et2O (10/1), at room temperature. The molecular structures of complexes 5−7 are illustrated in Figures 3 and 4, and data collection parameters are included in Table S1 in the Supporting Information. Complex 5 crystallizes in the orthorhombic Pbca space group with one molecule per asymmetric unit. The platinum atom is coordinated in 90% of the molecules in the crystal in a squareplanar geometry to two Cl atoms, one sulfur from the DMSO molecule, and nitrogen N1 from the guanidine ligand. The remaining 10% present another coordination environment for

organometallic guanidine to the platinum center. Consequently, one might assume that steric reasons were mostly responsible for the selectivity of this reaction. The synthesis of platinum complexes bearing ferrocenyl− guanidine 4, derived from 4-ferrocenylaniline 2, was also explored. The reaction was performed with a 1:2 molar ratio of platinum complex cis-[PtCl2(DMSO)2] to guanidine, in dry toluene at 100 °C. In this case, coordination of 4 to the Pt(II) center resulted in the displacement of one and two DMSO ligands from the coordination sphere of platinum, leading, after careful column chromatographic purification, to the neutral diand trimetallic compounds trans-[PtCl2{Fc(1,4-C6H4)−N C(NHiPr)2}(DMSO)] (6) and trans-[PtCl2{Fc(1,4-C6H4)− NC(NHiPr)2}2] (7) in 41 and 21% yields, respectively. Compounds 5−7 represent the first examples of Pt(II) complexes having organometallic guanidines as ligands.32 The new Pt(II)- and ferrocene-containing guanidines 5−7 are stable toward air and moisture in the solid state and soluble in solvents such as CH2Cl2, CH3CN, and DMSO. The stability of these heterometallic guanidines in DMSO solution has been checked by 1H NMR. We have to note that complexes 5−7 are considerably stable in DMSO-d6 solution at room temperature, since no decomposition products were observed over the course of 1 week. The structural identity of the novel heterometallic guanidinebased molecules 5−7 was straightforwardly established on the basis of elemental analysis, IR and multinuclear (1H, 13C, 195Pt) NMR spectroscopy, and mass spectrometry. Comparison of the IR spectra of 5−7 with those of the ferrocenyl−guanidine precursors 3 and 4 reveals similar bands for the NH moieties and for the CN groups. Furthermore, typical bands assignable to the SO bond are observed in the IR spectra of 5 and 6 at ∼1125 cm−1, demonstrating the coordination of the sulfur atom of the DMSO ligand to the platinum atom in the complexes. For these heterometallic guanidines the ν(Pt− S) and ν(Pt−Cl) stretches are also observed at 439 and 336 cm−1 for 5 and at 436 and 334 cm−1 for 6. Concerning the 1H NMR spectra of platinum complexes 5 and 6, the most characteristic feature is the presence of a singlet for the six DMSO protons at δ 3.41 and 3.34 ppm, respectively (Figures S9 and S14 in the Supporting Information). Likewise, 5410


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In comparison to the geometry of the noncoordinated ferrocenyl−guanidine 4, the two cyclopentadienyl rings in the ferrocene are almost eclipsed and form an angle between them of 2.0(5)°. The phenyl ring is almost parallel to the directly connected cyclopentadienyl ring (26.8(2)°), and the guanidine fragment is again very twisted, making an angle of 67.8(3)° with the phenyl ring. There are intermolecular interactions Cl1′···H−N2 and Cl1″···H−N3 between adjacent molecules to yield layers parallel to the (110) plane. Electrochemical Behavior of Homo- and Heterometallic Ferrocene-Containing Guanidines. The anodic electrochemistry of the ferrocene-containing guanidines 3 and 4 as well as of the corresponding platinum(II) complexes 5−7 was examined by cyclic voltammetry (CV) and square wave voltammetry (SWV), using acetonitrile and/or dichloromethane as solvent and tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) as the supporting electrolyte. Direct comparison of the voltammetric responses of ferrocenyl− guanidines 3−7 in CH3CN/n-Bu4NPF6 was prevented by the insolubility of trimetallic complex 4 in this medium. The recorded voltammetric potential data are given in Table 1.

Figure 4. Molecular structure of trimetallic complex 7 with the atoms in the asymmetric unit labeled. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt1−Cl1 = 2.308(2), Pt1−N1 = 2.044(6), N1−Pt1−Cl1= 90.6(2), Cl1−Pt1− N1= 89.4(2) ((a) −x, −y + 1, −z).

the metal center, and the Pt atom displays a distorted-squareplanar geometry. The two types of molecules are randomly distributed within the crystal. On comparison of the geometry of the organometallic guanidine ligand 3 with that seen in the crystal structure of compound 5, the ferrocenyl moiety no longer presents an eclipsed configuration and the angle between the two cyclopentadienyl rings decreases to 3.6(2)°. The angle between the substituted η5-C5H4 ring and the plane containing the nitrogen atoms, however, increases to 78.9(4)°. There are intramolecular N3−H···Cl bonds, but no supramolecular hydrogen bonds, and molecules are packed in the crystal only by van der Waals forces. Heterodimetallic complex 6 crystallizes in the monoclinic P21/c space group with two chemically identical molecules per asymmetric unit. In both molecules, the platinum atom is coordinated in a square-planar geometry to two Cl atoms, one sulfur from the DMSO molecule, and nitrogen N1 from the guanidine ligand. In one of the molecules there are two alternative positions for one of the nitrogen atoms in the ligand, N3A and N3B, with similar probabilities of occupation: 52 and 48%, respectively. In comparison to the disposition of the two cyclopentadienyl rings in the ferrocene unit in the structure of 4, the rings in 6 are almost eclipsed for the two molecules, and in both of them they are more parallel, with angles of 0.6(6) and 1.4(7)°. The phenyl ring is almost parallel to the η5-C5H4 cyclopentadienyl ring (35.1(4) and 28.2(4)°), and the guanidine moiety is again very twisted, making an angle with the phenyl ring of 67.8(4)° in one of the molecules in the asymmetric unit for the plane defined by N1, N2, and N3A, 72.2(4)° for the plane defined by N1, N2, and N3B, and 74.9(3)° for the other molecule. There are intermolecular hydrogen bonds O1′···H−N5 and O2′···H−N2 between adjacent molecules that form layers parallel to the (101) plane and also intramolecular NH···Cl interactions in both molecules. Concerning 7, this guanidine-based trimetallic complex crystallizes in the orthorhombic Pbca space group with half of a molecule per asymmetric unit. The platinum atom is located at the inversion center, and the coordination environment displays a square-planar geometry, where the metal atom is coordinated to two crystallographically equivalent Cl1 chlorine atoms and two N1 nitrogen atoms from the guanidine ligand.

Table 1. Electrochemical Data Obtained for FerroceneFunctionalized Guanidines and Their Pt(II) Complexes compd

E1/2 (V)a

ΔEp (mV)

3 4 5 6 7

0.058 0.379b 0.310 0.384 0.382

68 90 63 63 65

a

Half-wave potentials measured against SCE in CH3CN solution. bIn CH2Cl2. Scan rate: 0.1 V s−1. Under our conditions, the ferrocene redox couple [Fe(η5-C5H5)2]0/+ is +0.462 and the decamethylferrocene redox couple [Fe(η5-C5Me5)2]0/+ is −0.056 V vs SCE in CH2Cl2/ 0.1 M n-Bu4NPF6.

Figure 5 compares the CV response, in the potential region between 0 and +0.7 V vs SCE, of 2-(ferrocenyl)-1,3diisopropylguanidine (3) with that of the corresponding

Figure 5. Comparative CV responses, in CH3CN containing 0.1 nBu4NPF6, of ferrocene-containing neutral guanidine 3 and heterometallic Fe−Pt complexes 5 and 7. Scan rate: 0.1 V s−1. 5411


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heterometallic complexes 6 and 7 show additional quasireversible oxidation processes, at Ep = +1.120 V (for 6) and Ep = +0.890 and +1.120 V (for 7), probably related to the oxidation of the guanidine moiety (Figures S28, S32, and S33 in the Supporting Information). Similar oxidation waves have been noted in the oxidative electrochemistry of other nitrogencontaining ferrocene derivatives.11a,28,40 Antiproliferative Studies of Ferrocenyl−Guanidines and Heterometallic Ferrocene−Platinum(II) Complexes. The quest for alternative drugs to the well-known platinumbased anticancer metallodrugs such as cisplatin, carboplatin, and oxaliplatin has stimulated the search for cytotoxic compounds based on both platinum and other transitionmetal centers that are able to overcome the limitations of cisplatin.41 Likewise, as mentioned in the Introduction, heterometallic molecules bearing ferrocenyl moieties linked to a second cytotoxic metal such as Pt(II),20,27 Pd(II),21 Au(I),22 and Ru(II),23 among other metals, have been reported with the aim to achieve a synergistic effect between the two active metals. Often, the antiproliferative properties of the majority of heterometallic compounds improved with respect to those of the corresponding monometallic precursors. In this context, we have recently evaluated the antiproliferative activity of the heterometallic compound cis-[PtCl2{Fc(CH2)2NH2}2] (8) shown in Chart 1, bearing two redox-active

heterodimetallic complex 5 and of the trimetallic (2Fe−Pt) compound 7. The anodic electrochemical behavior of guanidine-based compounds 3−7 is dominated by the presence of the wave corresponding to the ferrocene/ferrocenium redox couple. The voltammetric features, ipc/ipa being essentially equal to 1, the peak to peak separation values (ΔEp) being about 63− 70 mV (in CH3CN), and Ep being independent of the scan rate, show that oxidation of the ferrocenyl moiety in guanidine-based compounds 3−7 is chemically and electrochemically reversible.36,37 From Figure 5 and Table 1 it becomes patently obvious that guanidine 3 exhibits a distinctive voltammetric behavior. Clearly, the direct attachment of the guanidine group to the cyclopentadienyl ring significantly increases the electronic density in the ferrocene nucleus, causing a striking effect on the observed half-wave redox potential (E1/2). The extremely high electron-rich nature of this ferrocenyl−guanidine results in a very low half-wave potential, E1/2 = 0.058 V vs SCE (E1/2 = −0.404 V vs ferrocene/ferrocenium), indicating that oxidation of the ferrocene moiety becomes thermodynamically facilitated. In particular, the oxidation of the ferrocenyl moiety in this guanidine 3 is easier than that in 1-aminoferrocene precursor 1 (E1/2 = +0.140 V vs SCE in CH2Cl2/n-Bu4NPF6). In contrast, the ferrocenyl unit in guanidine 4 is oxidized at a more positive potential: E1/2 = +0.379 vs SCE. In this molecule, the electrondonating capability of the CN3 guanidine moiety is attenuated by the phenyl ring linking the ferrocene nucleus to the guanidine. Indeed, to the best of our knowledge, 2-(ferrocenyl)-1,3diisopropylguanidine 3 exhibits one of the lowest FeII/FeIII E1/2 values for monosubstituted ferrocene derivatives reported so far, and it is almost as electron rich as decamethylferrocene, Fe(η5-C5Me5)2 (see Table 1).38,39 This finding is in agreement with the studies recently reported by Siemeling and co-workers, who have prepared bis(guanidines) of the type Fe{(η5-C5H4)− NC(NRR′)2}2 by condensation of 1,1′-diaminoferrocene with a series of chloroformamidinium salts.29a In this case, the two guanidine units linked to the ferrocene moiety afforded E1/2 values down to −0.90 V vs the ferrocenium/ferrocene couple.29a Figure 5 also shows that, in comparison to the free neutral ferrocenyl−guanidine 3, in the heterodimetallic Fe−Pt complex 5 the oxidation of the ferrocenyl moiety shifts to more positive values to a remarkable extent (by 0.25 V), while the reversibility of the redox processes was maintained. Therefore, coordination of the nitrogen-donor ligand 3 to the platinum atom significantly reduces the electron density at the ferrocenyl moiety and, consequently, it is more difficult to oxidize. Despite its greater number of electroactive organometallic units, the CV and SWV of the heterotrimetallic (2Fe−Pt) complex 7 in the potential region between 0 and +0.80 V vs SCE (Figure 5 and Figure S27 in the Supporting Information) also showed a single reversible oxidation process, at E1/2 = +0.382 V vs SCE, attributed to the Fe2+/Fe3+ redox system. This finding reveals that the two ferrocenyl units in compound 7 behave as independent redox moieties because their reversible oxidation to the positively charged ferrocenium form takes place at roughly the same potential. Consequently, in this trimetallic molecule the two ferrocenyl−guanidine moieties are spatially well separated and their degree of electronic communication is negligible. Finally, it is worth mentioning that when the potential scan is extended up to +1.4 V, the CVs of guanidine 4 and of its

Chart 1. Aminoethylferrocene-Based Heterometallic Pt(II) Complex with Antiproliferative Activity

2-(ferrocenyl)ethylamine ligands covalently attached to the cytotoxic Pt(II) center. Interestingly, we observed that trimetallic 8 was active against all human solid tumor cell lines tested (Table 2) with GI50 values in the range 1.7−2.3 μM. More importantly, the compound showed stronger activity against the colon cancer cell line in comparison to the standard anticancer drug cisplatin. In addition, the mechanism of action was concluded to be different from that of cisplatin.27 On the basis of the promising cytotoxic properties of amino− ferrocene−Pt(II) compound 8, we became interested in exploring the biological properties of the structurally related heterometallic derivatives prepared in this work, containing ferrocenyl−guanidine ligands instead of ferrocenyl−amines. In accord with the National Cancer Institute protocol,42 only compounds soluble in DMSO at 40 mM were tested. From the set of ferrocenyl−guanidines 3−7, only trimetallic compound 7 was insoluble under such conditions. Therefore, the antitumor activities of compounds 3−6 were evaluated against the human solid tumor cell lines HBL-100, HeLa, SW1573, T-47D, and WiDr. The results are given in Table 2. The standard anticancer agent cisplatin was used for comparison. In addition to the antitumor activity, the lipophilicity (C log P) of this series of ferrocene-based guanidines was evaluated by 5412


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Table 2. Lipophilicity (C log P) and Antiproliferative Activity (GI50) of Compounds 3−8 and Cisplatin against Human Solid Tumor Cell Lines GI50a compd

C log P

HBL-100 (breast)

3 4 5 6 7 8b cisplatin

2.82 4.44 3.48 4.15 10.1 4.09 −1.68

>100 10 ± 2.6 1.4 ± 0.1 2.0 ± 0.1 nd 2.0 ± 0.5 1.9 ± 0.6

HeLa (cervix) >100 6.6 ± 1.6 ± 2.1 ± nd 1.7 ± 2.0 ±

1.9 0.3 0.2 0.2 0.3

SW1573 (lung) >100 5.3 ± 1.5 ± 2.1 ± nd 2.0 ± 3.0 ±

1.2 0.2 0.2 0.7 0.4

T-47D (breast)

WiDr (colon)

>100 8.1 ± 0.9 2.6 ± 0.3 2.4 ± 0.3 nd nd 15 ± 2.3

>100 1.7 ± 1.2 1.9 ± 0.4 2.1 ± 0.3 nd 2.3 ± 0.5 26 ± 5.3

a Values expressed as GI50 are given in μM ± standard deviation and are means of two to five experiments. nd = not determined due to low solubility in DMSO. bFrom ref 27.

Figure 6. Cell cycle phase distribution of untreated cells (C) and cells treated (drug dose in μM) for 24 h with cisplatin (CDDP) and complexes 5 and 6.

in silico calculations on the basis of their chemical structure. C log P values were calculated to correlate lipophilicity with antitumor activity and are also shown in Table 2. C log P values for all active compounds are in the range 3.5−4.8, in good agreement with Lipinski’s rule of five.43 From the results on the antiproliferative activity we can infer some preliminary structure−activity relationships. It is interesting to note that neutral guanidine 4, derived from 4ferrocenylaniline (2), was active against all cell lines, while the related guanidine 3, derived from 1-aminoferrocene (1), was inactive. The C log P values are not enough to explain the marked differences observed in the biological activity. Interestingly, heterobimetallic derivatives 5 and 6 were both active against all cell lines and were overall more active than ferrocenylguanidine 4, with GI50 values in the range 1.4−2.6 μM. From these results we speculate that the preferred mechanism of action of compounds 5 and 6 is related to the platinum atom, while the contribution of ligand 4 does not produce an additive effect. We cannot discard the notion hat compound 4 exerts its activity by a mechanism different from

that of platinum complexes 5 and 6. Being active on its own, ligand 4 appears as an interesting candidate to be studied further in cisplatin-resistant cancer cell lines. In comparison to cisplatin, compounds 5 and 6 showed a superior activity profile in the more drug resistant T-47D and WiDr cell lines. This is a relevant result, taking into account that complexes 5 and 6 are trans-Pt(II) isomers as opposed to cisplatin, which is a cis-Pt(II) compound. In addition, the GI50 values of 5 and 6 are comparable to those obtained for the trimetallic complex 8, which shows a cis configuration. Thus, the geometrical configuration of the heterometallic platinum(II) complexes seems to be irrelevant for the biological activity. This could be attributed to a mechanism of action different from that of cisplatin, as anticipated for complex 8.27 Recently, a new class of potential anticancer drugs based on guanidine Pt(II) complexes has been reported.32a The antiproliferative activity after 96 h of drug exposure did not improve on that of the standard anticancer drug cisplatin. Thus, our results are a remarkable improvement on the results reported earlier. 5413


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Organometallics

mechanism of action of the novel ferrocenyl−guanidine−Pt(II) complexes might contribute to overcome the three main limitations of cisplatin: toxicity, tumor resistance, and poor oral bioavailability.

We next analyzed possible cell cycle disturbances as a consequence of the antiproliferative effect of complexes 5 and 6. We examined changes in the cell cycle profiles of HBL-100, HeLa, SW1573, and T-47D cells exposed to complexes 5 and 6 using flow cytometry. For comparison purposes, cisplatin was used as a reference drug. Cells were exposed to each agent at two different concentrations, which were chosen on the basis of the GI50 values and the cell line sensitivity. The results are shown in Figure 6. The treatment of cells for 24 h with complexes 5 and 6 produced two types of overall effects. In HBL-100, a sensitive cell line, compounds 5 and 6 did not show a significant alteration of the cell cycle but induced considerable cell death (6−12%). In contrast, in HeLa, SW1573, and T-47D cells death was not observed; instead, subtle accumulation of cells in S or G2/M phase occurred. The possibility of G2/M intervention of the new drugs can be discarded on the basis of inspection of the obtained data. The results indicate that the mechanism of antiproliferative activity of the new compound does not affect the cell cycle to a large extent. These results are similar to previously reported results for the trimetallic compound 8.27 However, the reference drug cisplatin produces a clear S-phase arrest, even at higher doses. Our results clearly demonstrate that the new ferrocenyl−guanidine−Pt(II) complexes induce growth inhibition in cancer cells by a mechanism different from that of cisplatin.

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EXPERIMENTAL SECTION

General Procedures and Equipment. All reactions were performed using standard Schlenk and glovebox techniques under an atmosphere of dry nitrogen or argon. Solvents were dried by standard procedures over the appropriate drying agents and distilled immediately prior to use. The platinum precursor cis-[PtCl2(DMSO)2] was prepared following literature methods.44 Silica gel (70−230 mesh; Aldrich) was used for column chromatographic purifications. Diethylzinc (1 M in hexane) was purchased from Sigma-Aldrich. Infrared spectra were recorded on a PerkinElmer 100 FT-IR spectrometer. Elemental analyses were performed in a LECO CHNS-932 elemental analyzer. 1H, 13C, and 195Pt NMR spectra, as well as two-dimensional spectra, were recorded on Bruker AMX-300, Bruker DRX-500, and Varian FT-400 spectrometers. Chemical shifts were reported in parts per million (δ) with reference to residual solvent resonances for 1H and 13C NMR (CDCl3: 1H, δ 7.27 ppm; 13 C, δ 77.0 ppm). 195Pt NMR spectra were referenced externally using 1.0 M Na2PtCl6 in D2O. The electrospray ionization (ESI) mass spectra were recorded on a QSTAR (Applied Biosystems) spectrometer, using methanol as the ionizing phase, while FAB mass spectra were obtained by using a VG AutoSpec (Waters) mass spectrometer with m-nitrobenzyl alcohol (m-NBA) as the matrix. Samples were prepared in dichloromethane solutions. Electrochemical Measurements. Cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments were recorded on a Bioanalytical Systems BASCV-50W potentiostat. CH2Cl2 and CH3CN (SDS, spectrograde) for electrochemical measurements were freshly distilled from calcium hydride under argon. The supporting electrolyte used was tetra-n-butylammonium hexafluorophosphate, n-Bu4NPF6 (Alfa Aesar), which was purified by recrystallization from ethanol and dried under vacuum at 60 °C. The supporting electrolyte concentration was 0.1 M. A conventional three-electrode cell connected to an atmosphere of prepurified nitrogen was used. The counter electrode was a coiled Pt wire, and the reference electrode was a BAS saturated calomel electrode (SCE). All cyclic voltammetric experiments were performed using either a platinum-disk working electrode (A = 0.020 cm2) or a carbon-disk working electrode (A = 0.070 cm2) (Bioanalytical Systems). The working electrodes were polished on a Buehler polishing cloth with Metadi II diamond paste for about 3 min followed by sonication in absolute ethanol, rinsed thoroughly with purified water and acetone, and allowed to dry. All potentials were referenced to the SCE electrode. Solutions were, typically, 10−3 M or 10−4 M in the redox-active species. The solutions for the electrochemical experiments were purged with nitrogen and kept under an inert atmosphere throughout the measurements. Square wave voltammetry (SWV) was performed using frequencies of 10 Hz. X-ray Crystal Structure Determination. Ferrocenyl−guanidines 3 and 4 and their corresponding heterometallic Pt(II) complexes 5−7 were structurally characterized by single-crystal X-ray diffraction. Suitable orange crystals of 3−7 were coated with mineral oil and mounted on a MiTeGen MicroMount. The samples were transferred to a Bruker D8 KAPPA series II diffractometer with APEX II areadetector system equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Collection and data details for all of the crystals can be found in the Supporting Information. After data collection and integration with the Bruker SAINT software package,45 absorption corrections (SADABS)46 were applied to the collected data as well as corrections for Lorentz and polarization effects. The software package Bruker SHELXTL47 was used for space group determination, structure solution, and refinement. The space group determination was based on a check of the Laue symmetry, and systematic absences were confirmed using the structure solution. The structures were solved by direct methods (SHELXS-97), completed with different Fourier syntheses, and refined with full-matrix least-squares using SHELXL-

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CONCLUDING REMARKS In summary, we have developed a new straightforward and atom-economical route to ferrocene-containing guanidines via guanylation of amino−ferrocenes FcNH2 (1) and Fc(1,4C6H4)NH2 (2) with N,N′-diisopropylcarbodiimide, in the presence of commercial ZnEt2 as catalyst. To the best of our knowledge, this is the first catalytic synthesis of ferrocene-based guanidines and opens up a very attractive route to the preparation of structurally diverse organometallic guanidines. The presence of an electron-rich ferrocenyl substituent makes guanidines 3 and 4 excellent basic ligands, and thus, we have explored their coordination chemistry. From these studies, new heterometallic complexes bearing ferrocenyl units coordinated to platinum(II) via the guanidine moiety have been prepared, which constitute the first reported complexes that contain both a cytotoxic Pt(II) center and organometallic guanidine ligands. Electrochemical studies revealed the electronic effect played by the direct attachment of the CN3 guanidine moiety to the ferrocene nucleus, making the oxidation of ferrocenyl− guanidine 3 extremely easy in thermodynamic terms. Interestingly, guanidine 3 is one of the most electron rich monosubstituted ferrocenes reported so far. The novel ferocenyl−guanidines 3−6 have been evaluated for their cytotoxicity against a representative panel of human solid tumor cell lines. The Pt(II)−ferrocenyl--guanidine complexes 5 and 6 exhibited cytotoxic activity surpassing that of cisplatin in T-47D and WiDr. The results of the biological studies reinforce the findings reported in the literature that heterometallic compounds are advantageous as anticancer agents in comparison to the clinical anticancer drugs cisplatin and carboplatin. Further research on the chemistry of 3 and 4 and new related ferrocene-containing guanidines obtained via ZnEt2-catalyzed reactions, along with experiments aimed to explore their ability to electrochemically sense anionic and/or cationic guests, are in progress in our laboratories. Likewise, ongoing studies on the 5414


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Organometallics 97 minimizing w(Fo2 − Fc2)2.48,49 Weighted R factors (Rw) and all goodness of fit values (S) are based on F2; conventional R factors (R) are based on F. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atom positions were calculated geometrically and were allowed to ride on their parent carbon or nitrogen atoms with fixed isotropic U values. All scattering factors and anomalous dispersion factors are contained in the SHELXTL 6.10 program library. The crystal structures of ferrocenyl−guanidines 3 and 4 and Pt(II) complexes 5−7 have been deposited at the Cambridge Crystallographic Data Centre with deposition numbers CCDC 1400396−1400400. Biology. All starting materials were commercially available research-grade chemicals and were used without further purification. RPMI 1640 medium was purchased from Flow Laboratories (Irvine, U.K.), fetal calf serum (FCS) was from Gibco (Grand Island, NY, USA), trichloroacetic acid (TCA) and glutamine were from Merck (Darmstadt, Germany), and penicillin G, streptomycin, DMSO and sulforhodamine B (SRB) were from Sigma (St. Louis, MO, USA). Cells, Culture, and Plating. The human solid tumor cell lines HBL100, HeLa, SW1573, and WiDr were used in this study. These cell lines were a kind gift from Prof. G. J. Peters (VU Medical Centre, Amsterdam, The Netherlands). Cells were maintained in 25 cm2 culture flasks in RPMI 1640 supplemented with 5% heat inactivated fetal calf serum and 2 mM L-glutamine in a 37 °C, 5% CO2, 95% humidified air incubator. Exponentially growing cells were trypsinized and resuspended in antibiotic-containing medium (100 units of penicillin G and 0.1 mg of streptomycin per mL). Single-cell suspensions displaying >97% viability by trypan blue dye exclusion were subsequently counted. After counting, dilutions were made to give the appropriate cell densities for inoculation onto well microtiter plates. Cells were inoculated in a volume of 100 μL per well at densities of 10000 (SW1573 and HBL-100), 15000 (HeLa), and 20000 (WiDr) cells per well, on the basis of their doubling times. Chemosensitivity Testing. Compounds were initially dissolved in DMSO at 400 times the desired final maximum test concentration. Control cells were exposed to an equivalent concentration of DMSO (0.25% v/v, negative control). Each agent was tested in triplicate at different dilutions in the range of 1−100 μM. The drug treatment was started on day 1 after plating. Drug incubation times were 48 h, after which time cells were precipitated with 25 mL ice-cold trichloroacetic acid (TCA, 50% w/v) and fixed for 60 min at 4 °C. Then the sulforhodamine B (SRB) assay was performed.50 The optical density (OD) of each well was measured at 492 nm, using BioTek’s PowerWave XS Absorbance Microplate Reader. Values were corrected for background OD from wells containing only medium. Cell Cycle Studies. Cells were seeded in six well plates at a density of (2.5−5) × 105 cells/well. After 24 h the products were added to the respective well and incubated for an additional period of 24 h. Cells were trypsinized, harvested, transferred to test tubes (12 × 75 mm), and centrifuged at 1500 rpm for 10 min. The supernatant was discarded, and the cell pellets were resuspended in 200 mL of cold PBS and fixed by the addition of 1 mL ice-cold 70% EtOH. Fixed cells were incubated overnight at 20 °C, after which time they were centrifuged at 1500 rpm for 10 min. The cell pellets were resuspended in 500 mL of phosphate-buffered saline (PBS), and 5 mL of DNasefree RNase solution (10 mg/mL) was added. The mixture was incubated at 37 °C for 30 min. Finally, 5 mL of propidium iodide (PI; 0.5 mg/mL) was added. Flow cytometric determination of DNA content (20000 cells/sample) was analyzed on an Accuri C6 Flow Cytometer (Becton Dickinson, San José, CA, USA). The fractions of the cells in G0/G1, S, and G2/M phases were analyzed with BD Accuri C6 software. General Procedure for Guanylation Reactions of 1-Aminoferrocene (1) and 4-Ferrocenylaniline (2) at NMR Tube Scale. Catalytic reactions were performed on a small scale in an NMR tube fitted with a concentric Teflon valve. Reactions were performed in the NMR spectrometer with 1 × 10−4 mol of the corresponding ferrocenylamine (1 or 2), 1 × 10−4 mol of carbodiimide, and 3 mol % of catalyst (ZnEt2) in deuterated toluene under nitrogen. Conversion of the starting material to ferrocenyl−guanidine product

was determined by integration of the product resonances relative to the substrate peaks in the 1H NMR spectrum. Preparative-Scale Synthesis of the Ferrocenyl−Guanidines 3 and 4. In a glovebox, a solution of ZnEt2 in hexanes (0.045 mmol) was added to a solution of the corresponding ferrocenylamine (1 or 2; 1.5 mmol) in toluene (10 mL) in a Schlenk tube. N,N′Diisopropylcarbodiimide (1.5 mmol) was then added to the above reaction mixture. The Schlenk tube was taken outside the glovebox, and the reaction was carried out at 50 °C for 1 h (for 4) or 1.5 h (for 3). The solution was concentrated under reduced pressure, and diethyl ether (for 3) or hexane (for 4) was added and placed in a refrigerator at −30 °C for 16 h. After filtration the guanidine products were obtained as orange microcrystalline solids. Fc−NC(NHiPr)2 (3). Yield: 92% (450 mg, 1.37 mmol). Anal. Calcd for C17H25FeN3·1/2H2O: C, 60.72; H, 7.79; N, 12.50. Found: C, 60.45; H, 7.81; N, 12.18 .1H NMR (CDCl3, 300 MHz, ppm): δ 1.25 (s, 12H, CH3), 3.82 (m, 2H, − CH−), 4.01 (m, 4H, C5H4), 4.17 (s, 5H, C5H5). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 23.5 (CH3), 42.8 (−CH−), 61.5, 64.3 (C5H4), 68.2 (C5H5), 106.1 (ipso-Fc), 150.8 (−NC−). IR (KBr, cm−1): ν(N−H) 3313, ν(C−H) 3093−2870, ν(CN) 1595. MS (FAB): m/z 327.1 [M+]. Fc(C6H4)−NC(NHiPr)2 (4). Yield: 89% (538 mg, 1.33 mmol). Anal. Calcd for C23H29FeN3·1/2H2O: C, 66.99; H, 7.33; N, 10.19. Found: C, 67.32; H, 7.27; N, 10.05. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.19 (d, J = 6.2 Hz, 12H, CH3), 3.78 (m, 2H, − CH−), 4.02 (s, 5H, C5H5), 4.26, 4.60 (s, 4H, C5H4), 6.82, 7.38 (AA′BB′ system, J = 8.3 Hz, 4H, C6H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 23.5 (CH3), 43.5 (−CH−), 66.1, 68.6 (C5H4), 69.6 (C5H5), 86.2 (ipso-Fc), 123.6, 127.1 (C6H4), 131.9 (ipso-Ph-Fc), 148.1 (ipso-Ph-NC), 150.4 (−NC−). IR (KBr, cm−1): ν(N−H) 3293, ν(C−H) 3079−2926, ν(CN) 1597, ν(CC) 1454. MS (FAB): m/z 404.1 [M+]. Synthesis of trans-[PtCl2{Fc−NC(NHiPr)2}(DMSO)] (5). In a 25 mL, two-necked, round-bottomed flask equipped with a gas inlet and an Allihn condenser topped with a bubbler, cis-[PtCl2(DMSO)2] (74 mg, 0.176 mmol) and 2-(ferrocenyl)-1,3-diisopropylguanidine (3; 115 mg, 0.352 mmol) were dispersed in 12 mL of dry toluene. The reaction mixture was stirred and refluxed for 7 h. During the course of the reaction, the solid slowly dissolved to afford a clear orange solution. After it was cooled to room temperature, the reaction mixture was stirred for an additional 12 h. Subsequently, the solvent was removed under vacuum and the obtained solid was purified by column chromatography on silica gel (2 cm × 20 cm). A major orange band was eluted with n-hexane/Et2O (1/10). Slow removal of the solvent gave orange crystals of heterometallic compound 5. Yield: 53% (63 mg, 0.094 mmol). Anal. Calcd for C19H31FeN3Cl2PtSO: C, 33.99; H, 4.65; N, 6.26; S, 4.78. Found: C, 34.11; H, 4.74; N, 6.08; S, 4.64. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.28 (d, J = 6.4 Hz, 12H, CH3), 3.41 (s, 6H, CH3-DMSO), 3.58 (m, 2H, −CH−), 4.11, 4.44 (m, 4H, C5H4), 4.53 (s, 5H, C5H5), 5.23 (br, 2H, NH). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 23.8 (CH3), 43.4 (CH3-DMSO), 47.6 (−CH−), 65.5, 65.7 (C5H4), 70.0 (C5H5), 103.1 (ipso-Fc), 159.3 (−NC−). 195Pt{1H} NMR (CDCl3, 64 MHz, ppm): δ −2951. IR (KBr, cm−1): ν(N−H) 3380−3334, ν(C−H) 2976−2916, ν(CN) 1609, ν(SO) 1124, ν(Pt−S) 439, ν(Pt−Cl) 336. MS (ESI): m/z 671.1 [M+]. Synthesis of trans-[PtCl2{Fc(1,4-C6H4)−NC(NHCH(CH3)2)2}(DMSO)] (6) and trans-[PtCl2{Fc(C6H4)−NC(NHiPr)2}2] (7). cis[PtCl2(DMSO)2] (84 mg, 0.198 mmol) and 4 (160 mg, 0.397 mmol) were dispersed in 15 mL of dry toluene. The reaction mixture was refluxed for 7 h and then was stirred for 12 h at room temperature. During the course of the reaction, the solid slowly dissolved to afford a clear yellow solution, and subsequently, the cooling of the mixture afforded a yellow precipitate. The solid was purified by two successive columns on silica gel (2 cm × 20 cm). From the first column, compound 6 was obtained, in the first fraction, on elution with CH2Cl2/Et2O (10/1). The remaining fractions were subjected to a second column, where a major orange band was eluted with n-hexane/ Et2O, to obtain compound 7. Slow elimination of the solvent of both fractions gave orange crystals of the corresponding compound. 5415


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Organometallics Complex 6. Yield: 41% (60 mg, 0.080 mmol). Anal. Calcd for C25H35FeN3Cl2PtSO: C, 40.17; H, 4.72; N, 5.62; S, 4.29. Found: C, 39.89; H, 4.80; N, 5.49; S, 4.21. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.24 (d, J = 6.4 Hz, 12H, CH3), 3.34 (s, 6H, CH3-DMSO), 3.61 (m, 2H, −CH−), 4.06 (s, 5H, C5H5), 4.32, 4.62 (s, 4H, C5H4), 4.69 (br, 2H, NH), 7.27, 7.42 (AA′BB′ system, J = 8.3 Hz, 4H, C6H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 23.7 (CH3), 43.6 (CH3-DMSO), 47.5 (−CH−), 66.5, 69.0 (C5H4), 69.6 (C5H5), 84.9 (ipso-Fc), 127.1, 127.4 (C6H4), 137.2 (ipso-Ph-Fc), 142.7 (ipso-Ph-NC), 158.0 (−NC−). 195Pt{1H} NMR (CDCl3, 64 MHz, ppm): δ −2988. IR (KBr, cm−1): ν(N−H) 3340, ν(C−H) 2971−2922, ν(CN) 1583, ν(CC) 1522, ν(SO) 1126, ν(Pt−S) 436, ν(Pt−Cl) 334. MS (FAB): m/z 747.0 [M+]. Complex 7. Yield: 21% (45 mg, 0.042 mmol). Anal. Calcd for C46H58Fe2N6Cl2Pt: C, 51.51; H, 5.45; N, 7.83. Found: C, 51.13; H, 5.66; N, 7.65. 1H NMR (CDCl3, 300 MHz, ppm): δ 1.15 (d, J = 6.4 Hz, 24H, CH3), 3.53 (m, 4H, −CH−), 4.04 (s, 10H, C5H5), 4.32, 4.65 (s, 8H, C5H4), 4.88 (br, 4H, NH), 7.35, 7.43 (AA′BB′ system, J = 8.5 Hz, 8H, C6H4). 13C{1H} NMR (CDCl3, 75 MHz, ppm): δ 23.7 (CH3), 47.3 (−CH−), 66.3, 68.9 (C5H4), 69.6 (C5H5), 85.1 (ipso-Fc), 126.5, 127.0 (C6H4), 135.8 (ipso-Ph-Fc), 144.4 (ipso-Ph−NC), 157.6 (−NC−). 195Pt{1H} NMR (CDCl3, 64 MHz, ppm): δ −1887. IR (KBr, cm−1): ν(N−H) 3317, ν(C−H) 3093−2852, ν(CN) 1589, ν(CC) 1524, ν(Pt−Cl) 318. MS (FAB): m/z 1072.2 [M+].

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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.organomet.5b00751. Supplementary figures referenced in the text, structural characterization data (IR, NMR, and mass spectra), X-ray crystallographic data, additional CVs and SWVs for 3−7 (PDF) X-ray crystallographic data for 3−7, and synthesis of 1 and 2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for F.C.-H.: [email protected]. *E-mail for J.M.P.: [email protected]. *E-mail for I.C.: [email protected]. Author Contributions ⊥

These authors contributed equally to this publication.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Carmen Navarro-Ranninger for valuable assistance and support. The authors are grateful to the Spanish Ministerio de Economıá y Competitividad (MINECO) (projects CTQ2012-30728 and CTQ2014-51912-REDC), the Junta de Comunidades de Castilla La Mancha (project 861 PEII-2014-041-P), the Instituto de Salud Carlos III (PI11/ 00840), the EU Research Potential (FP7-REGPOT-2012CT2012-31637-IMBRAIN), the European Regional Development Fund (FEDER), and the Spanish Ministerio de Educación (Programa Campus de Excelencia Internacional CEI10/00018) for the generous support of this work.

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REFERENCES

(1) (a) Ishikawa, T.; Kumamoto, T. Synthesis 2006, 2006, 737−752. (b) Superbases for Organic Synthesis: Guanidines, Amidines, 5416


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