Arene-Ruthenium(II) Complexes Containing 11H-Indeno[1,2-b]quinoxalin-11-one Derivatives and Tryptanthrin-6-oxime: Synthesis, Characterization, Cytotoxicity, and Catalytic Transfer Hydrogenation of Aryl Ketones.

A series of novel mono- and binuclear arene-ruthenium(II) complexes [(p-cym)Ru(L)Cl] containing 11H-indeno[1,2-b]quinoxalin-11-one derivatives or tryptanthrin-6-oxime were synthesized and characterized by X-ray crystallography, IR, NMR spectroscopy, cyclic voltammetry, and elemental analysis. Theoretical calculations invoking singlet state geometry optimization, solvation effects, and noncovalent interactions were done using density functional theory (DFT). DFT calculations were also applied to evaluate the electronic properties, and time-dependent DFT was applied to clarify experimental UV-vis results. Cytotoxicity for cancerous and noncancerous human cell lines was evaluated with cell viability MTT assay. Complexes demonstrated a moderate cytotoxic effect toward cancerous human cell line PANC-1. The catalytic activity of the complexes was evaluated in transfer hydrogenation of aryl ketones. All complexes exhibited good catalytic activity and functional group tolerance.

Introduction

Chemistry of half-sandwich arene–n class="Chemical">ruthenium(II) complexes has been intensively investigated in the recent years, due to their applications in different areas such as catalysis and pharmacology. Such complexes demonstrated high antiviral,[1−3] antibiotic,[3−6] and antitumor[7−17] activity.

Ruthenium(II) coordination compounds also have been extensively investigated for their catalytic activity in transfer n class="Chemical">hydrogenation reactions[18−27] and metathesis polymerization.[28−30]

Arene–n class="Chemical">ruthenium complexes have piano-stool type geometry with pseudo-octahedral symmetry near the ruthenium(II) atom. The arene fragment stabilizes the complex and protects the Ru(II) metal center from oxidation to ruthenium(III). Aromatic ligands are relatively inert in substitution reactions and can be considered as spectator ligands. The three remaining coordination sites opposite to the arene ligand can be used to introduce a wide variety of ligands with N-, O-, S- or P-donor atoms. In the past decade, there has been a surge of interest in nitrogen-donor ligands for use in catalysis and bidentate ligands have been very successful in this area.

Our group is involved in the research of n class="Chemical">polycyclic N-containing ligands—oximes of 11H-indeno[1,2-b]quinoxalin-11-one and its structural analogues.[31] 11H-Indeno[1,2-b]quinoxalin-11-one oxime, often denoted in the literature as IQ1, and its sodium salt, IQ1S, were reported as potent c-Jun N-terminal kinase (JNK3) inhibitors,[31,32] anti-inflammatory agents,[33−35] neuroprotectors,[36,37] and antitumor agents.[38−40] Tryptanthrin demonstrates a wide range of bioactivities,[41,42] including antibacterial,[43,44] antiviral,[45] anticancer,[46,47] and anti-inflammatory[48] activity. However, the coordination chemistry of such compounds is insufficiently explored. There are only few examples of nickel(II) and cobalt(II) complexes with 11H-indeno[1,2-b]quinoxalin-11-one oxime, which to the best of our knowledge has never been employed before in the synthesis of any noble metal complexes. Such oxime ligands as 11H-indeno[1,2-b]quinoxalin-11-one oxime in the presence of base (NEt3) can adopt up to eight different coordination modes, leading to Ni(II) and Co(II) complexes with nuclearities ranging from 1 to 8.[49,50] 11H-Indeno[1,2-b]quinoxalin-11-one oxime and its derivatives look promising in terms of stabilizing the complexes due to π–π stacking interactions and its coordinating ability. Herein, we report the first examples of arene–Ru(II) complexes with 11H-indeno[1,2-b]quinoxalin-11-one oximes and tryptanthrin-6-oxime, a derivative of natural alkaloid tryptanthrin. It was shown that such polycyclic oximes are capable of acting as chelating ligands, forming mono- and binuclear coordination compounds.

Results and Discussion

Synthesis of the Complexes

The ligands used in this work and their abbreviations are summn class="Chemical">arized in Scheme .

Scheme 1

Structures of Ligands HL–HL

Complexes 1, 2, and 4 were prepared in high yield by the reaction between the commercially available n class="Chemical">arene–ruthenium dimer [Ru(p-cym)Cl2]2 and the appropriate ligand in methanol/dimethylformamide (DMF) mixture (complexes 1, 4) or methanol (complexes 2, 3). It was shown that even in the absence of a base, deprotonation of the oxime group occurs, and the ligands (HL, HL, and HL) are coordinated in anionic form. It is worth noting that complex 3 could be obtained only in the presence of a base. Complex 3 was prepared by the reaction between [Ru(p-cym)Cl2]2 and HL in methanol in the presence of KOH. Ligands HL and HL form mononuclear complexes of the typical piano-stool type (Scheme ).

Scheme 2

Synthesis of Complexes 1–2

Ligands HL and HL contain an additional nitrogen atom; therefore, they can form both mono- and binuclen class="Chemical">ar complexes. Unexpectedly, the ratio of the reagents did not affect the product composition: at a ratio of Ru/HL 2:1, ligand HL forms a binuclear complex 3 of composition [Ru2(p-cym)2(L3)Cl3], whereas at a ratio of 1:1 instead of a mononuclear complex, the product contains the same complex 3 and the initial ligand (Scheme ).

Scheme 3

Synthesis of Complex 3

Similarly, reaction between HL and n class="Chemical">[Ru(p-cym)Cl2]2 is not affected by reactants ratios. Regardless of the metal–ligand ratio, only mononuclear complex of the composition [Ru(p-cym) (L4)Cl] was obtained (Scheme ). In this case, the formation of a dinuclear complex (via nitrogen donor atom in position 1 of the tetracyclic system) is probably hindered by the repulsion between the lone pair of the nitrogen atom in position 11 and bulky Ru(cym) fragment.

Scheme 4

Synthesis of Complex 4

All complexes are air-stable in the solid state; they n class="Chemical">are sparingly soluble in chloroform, acetonitrile, and DMSO and almost insoluble in water and alcohols. The complexes were fully characterized by elemental analysis, cyclic voltammetry, IR, NMR spectroscopy methods, and single-crystal X-ray diffraction.

Coordination of anionic forms of the ligands HL–HL with strong electron-withdrawing indeno[1,2-b]quinoxaline or n class="Chemical">tryptanthrin cores may be due to increased acidity of these ligands compared to the oximes of aliphatic or aromatic ketones, leading to increased concentration of their anionic forms. Thermochemical density functional theory (DFT) calculations were carried out in order to compare the thermodynamic stability of the complex 1 and its hypothetical form with the neutral coordinated HL ligand. Two reactions of formation of both of the complexes were considered

Free-energy and enthalpy changes were calculated at M062X 6-311+G(2d,p)/LANL2DZ level of theory, and solvent (n class="Chemical">DMF) effects were taken into account using SMD model. It was found that the formation of the complex with the anionic form is more thermodynamically favorable (ΔG1 = −94.9 kJ/mol; ΔG2 = −16.3 kJ/mol; ΔH1 = −97.6 kJ/mol; ΔH2 = −10.8 kJ/mol).

X-ray Crystal Structures

Complexes 1–4 crystallize in a monoclinic crystal system having a single formula unit as an asymmetric unit. Phase purity of the obtained products was confirmed by powder X-ray diffraction analysis (see the Supporting Information). In mononuclen class="Chemical">ar complexes 1, 2, and 4, ruthenium coordinates cymene molecule in a η6 fashion, which is considered to occupy three coordination sites of a quasi-octahedral arrangement. The other three sites are occupied by a chlorine atom and two nitrogen atoms of the deprotonated oximate ligands coordinated in a bidentate fashion forming a five-membered chelate ring (Figures –3). It should be noted that in most of the structurally characterized arene–ruthenium complexes with heterocyclic oximes, the N,N-coordinated ligands remain neutral[51−54] and so far only one example of deprotonated 2-pyridyl cyanoxime was reported.[51]

Figure 1

X-ray molecular structure of complex 1. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.101(1), Ru1–N2 2.147(1), Ru1–Cl1 2.4066(5), Ru1–C16 2.193(2), Ru1–C17 2.175(2), Ru1–C18 2.256(2), Ru1–C19 2.264(2), Ru1–C20 2.186(2), Ru1–C21 2.181(2).

Figure 3

X-ray molecular structure of complex 4. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms and DMF solvent molecules are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.094(2), Ru1–N2 2.124(2), Ru1–Cl1 2.4031(6), Ru1–C15 2.212(2), Ru1–C16 2.172(2), Ru1–C17 2.189(2), Ru1–C18 2.260(2), Ru1–C19 2.249(2), Ru1–C20 2.171(2).

X-ray molecular stn class="Chemical">ructure of complex 1. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.101(1), Ru1–N2 2.147(1), Ru1–Cl1 2.4066(5), Ru1–C16 2.193(2), Ru1–C17 2.175(2), Ru1–C18 2.256(2), Ru1–C19 2.264(2), Ru1–C20 2.186(2), Ru1–C21 2.181(2).

X-ray molecular stn class="Chemical">ructure of complex 2. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.079(1), Ru1–N2 2.159(1), Ru1–Cl1 2.4041(7), Ru1–C16 2.200(1), Ru1–C17 2.177(2), Ru1–C18 2.255(1), Ru1–C19 2.265(2), Ru1–C20 2.183(2), Ru1–C21 2.180(2).

X-ray molecular stn class="Chemical">ructure of complex 4. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms and DMF solvent molecules are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.094(2), Ru1–N2 2.124(2), Ru1–Cl1 2.4031(6), Ru1–C15 2.212(2), Ru1–C16 2.172(2), Ru1–C17 2.189(2), Ru1–C18 2.260(2), Ru1–C19 2.249(2), Ru1–C20 2.171(2).

The interatomic Ru–N distances involving n class="Chemical">oxime and heterocyclic nitrogen atoms are in the ranges of 2.08–2.10 and 2.12–2.16 Å and somewhat greater than the corresponding values typical for oxime–ruthenium complexes (1.95–2.04 and 1.95–2.08 Å), probably due to bulkiness of the ligands. The interatomic Ru–Cl distances of about 2.40 Å and Ru–C distances in the range 2.17–2.26 Å are close to typically found in arene–ruthenium complexes. All three complexes have chiral metal centers, but crystallize in a centrosymmetric space group, and thus, enantiomer resolution was not possible.

Compound 3 is a binuclear complex that crystallizes in a centn class="Chemical">rosymmetric monoclinic C2/c space group (Figure ). Arrangement of one of ruthenium atoms is similar to mononuclear complexes described above—it consists of two nitrogen atoms of deprotonated oxime ligand HL, chloride ion and p-cymene molecule, interatomic distances between ruthenium and donor atoms are also close to the ones observed for mononuclear complexes. The second ruthenium atom is achiral and coordinates one cymene molecule, two chloride ions, and a nitrogen atom in position 2 of ligand HL.

Figure 4

X-ray molecular structure of 3. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms and MeOH solvent molecules are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.096(2), Ru1–N2 2.124(2), Ru1–Cl1 2.387(2), Ru1–C15 2.207(5), Ru1–C16 2.146(6), Ru1–C17 2.182(4), Ru1–C18 2.238(3), Ru1–C19 2.230(3), Ru1–C20 2.189(4), Ru1–N4 2.134(2), Ru1–Cl2 2.425(1), Ru1–Cl3 2.412(1), Ru1–C25 2.203(4), Ru1–C26 2.179(5), Ru1–C27 2.153(5), Ru1–C28 2.187(5), Ru1–C29 2.169(4), Ru1–C30 2.183(3).

X-ray molecular stn class="Chemical">ructure of 3. Thermal ellipsoids are drawn at a 50% probability level. Hydrogen atoms and MeOH solvent molecules are omitted for clarity. Selected bond lengths [Å]: Ru1–N1 2.096(2), Ru1–N2 2.124(2), Ru1–Cl1 2.387(2), Ru1–C15 2.207(5), Ru1–C16 2.146(6), Ru1–C17 2.182(4), Ru1–C18 2.238(3), Ru1–C19 2.230(3), Ru1–C20 2.189(4), Ru1–N4 2.134(2), Ru1–Cl2 2.425(1), Ru1–Cl3 2.412(1), Ru1–C25 2.203(4), Ru1–C26 2.179(5), Ru1–C27 2.153(5), Ru1–C28 2.187(5), Ru1–C29 2.169(4), Ru1–C30 2.183(3).

In complexes 1 and 2 the heterocyclic n class="Chemical">oxime ligands are involved in π–π stacking with intermolecular distances 3.379(3) and 3.384(2) Å correspondingly (Figures S1 and S2). In compound 4, the molecules of ligand HL cannot align in parallel planes due to the presence of lattice DMF molecules. In this case, each of HL molecules is involved in four CH···N short contacts with interatomic H···N distances of 2.658 and 2.741 Å (Figure S3).

In order to provide a quantum mechanical foundation to the experimental observations, the analysis of the non-covalent interaction (NCI) isosurfaces was performed both on monomers and dimers of compounds 1, 2, and 4 (Figure S4). The intramolecular interactions that are mainly responsible for the stabilization of the monomers (blue areas) involve hydrogen atoms, which are involved in attractive interactions with the two oxygen atoms, carbon atoms, and with the chlorine atom. Concerning dimers, the main contribution to the stabilization of 1 and 2 is a wide π–π interaction between two ligands; the case of 4 is rather different, by involving four punctual but more attractive interactions between hydrogen atoms of one ligand and nitrogen atoms of the other ligand molecule (Figure S4).

Thermodynamic preference of N,N-bidentate over N,O-bidentate coordination mode of the ligand L was confirmed by DFT calculations. Both in the gas phase and n class="Chemical">DMF N,N-coordination of the deprotonated ligand L with the formation of a five-membered ring is about 19 kJ/mol more favorable (Figure S5).

Two possible orientations of the cymene ligand—one with isopropyl group close to the n class="Chemical">oxime fragment and the other with the opposite orientation—methyl group facing the oxime were evaluated by DFT calculations (Figure S6). It was found that the first orientation is about 8 kJ/mol more stable than the second one consistent with the X-ray crystal structures. Despite the proximity of the bulky isopropyl group and the oxime fragment, this form is more stable probably due to the repulsion of the isopropyl group and the aromatic part of the HL1 ligand in the structure with the opposite cymene orientation.

Spectroscopic Characterization

The IR spectra of free neutral ligands display broad bands in the range 3200–3250 cm–1 due to OH stretching vibrations and bands in the range 1640–1630 cm–1 due to C=N stretching vibrations. Upon coordination OH stretching bands disappear, whereas C=N stretching bands shift to 1625–1600 cm–1, in accordance with the deprotonation of the ligands and their coordination to the n class="Chemical">metal center in N,N′-bidentate chelating fashion.

The 1H NMR spectra of complexes 1–4 recorded in n class="Chemical">DMSO-d6 display all of the expected signals of the coordinated p-cymene and N,N′-ligand. The resonances of the N,N′-ligand atoms are shifted upfield with respect to those of uncoordinated ligands, confirming its coordination to the ruthenium(II) center. The chirality of ruthenium center in the complexes induces significant changes to the NMR signals of the p-cymene moiety in complexes 1–4. The 1H NMR spectra of 1, 2, and 4 exhibit a doublet of doublets for the methyl groups in the isopropyl moiety, and four doublets attributable to the protons of p-cymene ring in the range of 5.5–6.3 ppm, which is typical of ruthenium–arene systems with an asymmetric ruthenium center.[55,56] The 1H NMR spectra of binuclear complex 3 exhibit two doublets of doublets for the methyl groups in the isopropyl moiety, and eight doublets attributable to the p-cymene protons in the range of 5.3–6.2 ppm. There is no signal of the oxime proton in the spectra of all complexes, which confirms the coordination of the ligand in the anionic form. All complexes demonstrate resonances in the range typical of related compounds and in accordance with the existence of only one species in solution.

UV–vis spectrum of complex 1 (Figure S7) in n class="Chemical">acetonitrile demonstrates a complex shape that can be deconvoluted into five Gaussian bands (Figure S8). In order to gain insight into the origin of the absorption bands, time-dependent DFT calculations were carried out (Figure S9). The main features of the singlet excited states showing the highest values of oscillator strength (f) together with the experimental positions of deconvoluted Gaussian bands are given in Table . Because of the complexity of the transition nature in terms of MOs elementary transitions, the natural transition orbital (NTO) analysis was also carried out. As it can be seen from the transition diagrams, all of the absorption bands are of mixed character and correspond to both local and charge transfer excitations. Although the nature of the longer absorption transitions (500–260 nm) is charge transfer, calculations performed by employing long-range corrected hybrid functionals (viz. CAM-B3LYP and ω-B97X[D]) did not show a significant improvement in the match with the experimental values with respect to the B3LYP functional; on the contrary, for the short wavelength region transitions (200–230 nm), ω-B97X[D] demonstrated a good performance.

Table 1

TDA–DFT (B3LYP 6-311+G(2d,p)/LANLDZ IEFPCM) Calculated Singlet Excited States and Experimental Absorption Maxima in the UV–Vis Spectrum of Complex 1

Values calculated using ω-B97X[D] 6-311+G(2d,p)/LANLDZ IEFPCM method.

Cyclic Voltammetry Studies

To study the redox properties of mononuclear and binuclen class="Chemical">ar complexes 1 and 3 in solution, cyclic voltammograms were recorded at room temperature and a scan rate of 0.1 V/s using a 0.1 M solution of Bu4NPF6 in DMF (Figures S10 and S11). The corresponding values E1/2 of the redox potentials are shown in Table . In the positive region, an anodic peak was detected at +1.30 and +1.37 V (versus Ag/AgCl) for complexes 1 and 3, respectively. These irreversible processes apparently correspond to the oxidation of Ru(II) to Ru(III). This is consistent with the fact that the highest occupied molecular orbital in 1 is partially localized on the ruthenium atom (8,5%, Table S1). Sufficiently high oxidation potentials are consistent with the π-acceptor properties of heterocyclic oxime and p-cymene ligands. Ru-centered oxidation in the region from +1.0 to +1.5 V (vs Ag/AgCl) was also found for similar p-cymene complexes of Ru(II).[57,58] The irreversibility of this process is probably due to the instability of the p-cymene–Ru(III) species and the removing of the p-cymene fragment. This is evidenced by the elongation of Ru–C bonds and a loss in energy during the transition from Ru(II) complex to Ru(III) complex according to DFT calculations.

Table 2

Redox Potentialsa (V, vs Ag/AgCl) for Complexes 1, 3, and HL

compound	Epox	Epred
1	+1.30	–1.06, −2.06
3	+1.37	–0.85, −1.82
HL1b		–1.80, −2.21

Determined in 0.1 M Bu4NPF6 in DMF at room temperature and a scan rate of 0.1 V/s.

In CH3CN.

Determined in 0.1 M Bu4NPF6 in n class="Chemical">DMF at room temperature and a scan rate of 0.1 V/s.

A number of irreversible processes were detected in the negative region both for complexes 1 and 3, probably related to the reduction of the heterocyclic oxime ligand. This is consistent with the composition of lowest unoccupied moleculn class="Chemical">ar orbital in 1, which mainly consists of the oxime ligand orbitals (53%, Table S1). Similar irreversible reduction processes shifted to a more cathodic region were also found in the cyclic voltammogram of the starting HL ligand in acetonitrile (Figure S12).

Catalytic Activity Studies

We have tested the catalytic activity of complexes for the selective hydrogenation of n class="Chemical">aryl ketones using 2-propanol as both the reducing agent and solvent in the presence of NaOH as the base. Reaction between isopropyl alcohol and acetophenone was chosen as a model reaction (Scheme ).

Scheme 5

Ru-Catalyzed Transfer Hydrogenation of Aryl Ketones

Reduction of acetophenone did not go to completion (Table ), which can be explained by the reversibility of n class="Chemical">hydrogen transfer. Conversion versus time plots for complexes 1–4 are shown in Figure . Complex 1 demonstrated the best catalytic activity, providing 93% conversion after 1 h. Other complexes (2–4) gave ∼50% conversions after 1–2 h, and nearly full conversion for catalysts 2 and 3 was achieved in 6 h.

Table 3

Transfer Hydrogenation of Acetophenone Catalyzed by Complexes 1—4

bomplex	yield, %a	TON	TOF, h–1
1	93	18	18
2	95	19	5
3	93	18	6
4	69	13	2

Reactions were carried out in 2-propanol(4.0 mL), in the presence of NaOH (0.1 mmol), acetophenone (1 mmol), and catalyst (0.05 mmol) at 82 °C during 6 h [Ru]/substrate/NaOH molar ratio = 1/20/2. Hydrogenated products were determined by gas chromatography using phenetole as internal standard. TON = turnover number = mol of product/mol of pre-catalyst. TOF = turnover frequency = mol of product/mol of pre-catalyst/time.

Figure 5

Conversion versus reaction time for acetophenone transfer hydrogenation.

Conversion versus reaction time for acetophenone transfer n class="Chemical">hydrogenation.

Reactions were carried out in n class="Chemical">2-propanol(4.0 mL), in the presence of NaOH (0.1 mmol), acetophenone (1 mmol), and catalyst (0.05 mmol) at 82 °C during 6 h [Ru]/substrate/NaOH molar ratio = 1/20/2. Hydrogenated products were determined by gas chromatography using phenetole as internal standard. TON = turnover number = mol of product/mol of pre-catalyst. TOF = turnover frequency = mol of product/mol of pre-catalyst/time.

Encouraged by the potential of complex 1, we were interested to test this complex for transfer hydrogenation of substituted n class="Chemical">aryl ketones (Scheme ), and complex 1 was found to exhibit good catalytic activity and functional groups tolerance (Table ).

Table 4

Transfer Hydrogenation of Aryl Ketones Catalyzed by Complex 1

Hydrogenated products were determined by gas chromatography using phenetole as internal standard.

Hydrogenated products were determined by gas chromatography using n class="Chemical">phenetole as internal standard.

Conversion versus time plots for the five ketones (Table , entries 2–6) n class="Chemical">are shown in Figure . It was shown that ketones with electron-donating groups reacted faster. It might be due to the increase of the electron density on the carbon atom of the carbonyl group, which favorably affects the coordination of the carbonyl group to ruthenium(II) center. In turn, electron-withdrawing substituents (Table , entries 6, 7) reduce the electron density on the carbonyl group of the substrate, which makes coordination to the ruthenium(II) more difficult, which results in the decrease of the reaction rate.

Figure 6

Conversion versus time plots (entries 2–6 in Table ).

High chemoselectivity of the reduction process was noted: nitro groups (Table , entries 6, 7) remain unchanged under the hydrogenation conditions. For ketone with the double C=C bond conjugated to the carbonyl group (Table , entry 8), only ketone moiety undergoes transformation. Moreover, high yields can also be achieved in case of 4-bromooacetophenone hydrogenation (Table , entry 5), with no evidence of dehydrobromination or ring reduction. It is worth noting that a heteroaromatic ketone can also be reduced (Table , entry 10). Pyridine moiety can potentially bind to ruthenium center and inhibit the catalytic reaction; however, 2-acetylpyridine (Table , entry 10) was rapidly hydrogenated.

Cytotoxicity

Pancreatic cancer is one of the most aggressive and deadly types of n class="Disease">cancer. The increasing resistance of the pancreatic cancer to effective chemotherapy has become an urgent problem.[59] Therefore, the obtained complexes were screened at different concentrations for their ability to affect the cell viability of human adenocarcinoma cell line PANC-1 over noncancerous human retinal pigment epithelial cell line ARPE-19. The cytotoxic activity of the complexes was analyzed by cell viability MTT assay.

As it is shown in Figure , complexes 1–3 demonstrate moderate cytotoxic effect toward n class="CellLine">PANC-1 but practically no cytotoxic effect ARPE-19 cell lines. Complex 2 at the concentrations of 30 and 90 μM decreased cell viability for PANC-1 cell line to 95 and 70%, respectively. Complex 4 at the concentrations 0.3, 30, and 90 μM decreased cell viability for PANC-1 cell line to 55, 40, and 55%, respectively. It can be concluded that complexes 2 and 4 have mild cytotoxic activity for cancer human cell lines. Nevertheless, the cytotoxicity of the studied complexes is significantly inferior to the cytotoxicity of drugs widely used in clinical practice for the chemotherapeutical treatment of pancreatic adeonocarcinoma, that is, fluorouracil (IC50 3.85 μM) and gemcitabine (IC50 27 nM).[60] It is interesting to note that at low concentrations of the complexes viability of the cells increased (relative to control) despite the known cytotoxic potential of arene–ruthenium core. This is probably due to the JKN3-inhibitive activity of the oxime ligands discovered recently,[31] which limit the cell apoptosis process.

Figure 7

Cell viability MTT assay results for complexes 1–4 against PANC-1 (top) and ARPE-19 (bottom) cell lines (values are shown as the mean ± SEM of four experiments, the concentrations of the complexes on the horizontal axis are given in μM).

Cell viability MTT assay results for complexes 1–4 against n class="CellLine">PANC-1 (top) and ARPE-19 (bottom) cell lines (values are shown as the mean ± SEM of four experiments, the concentrations of the complexes on the horizontal axis are given in μM).

Conclusions

Ruthenium(II)–n class="Chemical">arene complexes containing 11H-indeno[1,2-b]quinoxalin-11-one derivatives and tryptanthrin-6-oxime were prepared and their catalytic, biological, and electrochemical properties were evaluated. The crystal structures of all complexes display a typical piano-stool geometry with ligands N,N′-coordinated to the ruthenium in each case.

Our research also shows that n class="Chemical">arene–ruthenium(II) complexes with 11H-indeno[1,2-b]quinoxalin-11-one derivatives and tryptanthrin-6-oxime are effective catalysts for the transfer hydrogenation reaction. These complexes tolerate a range of functional groups and catalyze the reaction, giving target alcohols in high yields. Ruthenium complexes with tryptanthrin-6-oxime and 6H-indeno[1,2-b]pyrido[3,2-e]pyrazin-6-one oxime demonstrated mild cytotoxic activity for human adenocarcinoma cell line PANC-1.

Experimental Section

Materials and Methods

The dimer [Ru(p-cym)Cl2]2 was purchased from Alfa Aesn class="Chemical">ar. The ligands 11H-indeno[1,2-b]quinoxalin-11-one oxime (HL), 6-(hydroxyimino)indolo[2,1-b]quinazolin-12(6H)-one (tryptanthrin-6-oxime, HL), 10H-indeno[1,2-b]pyrido[3,4-e]pyrazin-10-one oxime (HL), and 6H-indeno[1,2-b]pyrido[3,2-e]pyrazin-6-oneoxime (HL) were prepared as previously described.[36] All other materials were obtained from commercial sources and were used as received.

IR spectra were recorded on an Agilent Cary 630 FTIR spectrometer quipped with a diamond attenuated total reflectance tool from 700 to 3500 cm–1. NMR spectn class="Chemical">roscopic data were recorded with a Brucker AVANCE III 400 spectrometer (400.13 MHz for 1H and 100.61 MHz for 13C) and were referenced to residual solvent signal.

Chromato-mass spectrometric analysis was carried out using Agilent 7890A gas chromatograph with Agilient 5975C mass-selective detector with quadn class="Chemical">rupole mass-analyzer (electron impact ionization energy 70 eV). The injector temperature was maintained at 300 °C, and the injection volume was 1 μL (split 40:1). The instrument was equipped with a HP-5 ms capillary column of 30 m length, 0.25 mm i.d., and 0.25 μm film thickness. The carrier gas was helium at a constant flow rate of 1 mL/min. The GC oven program started at 79 °C (1.0 min hold) ramped up to 300 °C (heating rate 13 °C/min) for 10 min, total chromatogram time was 28 min. Transfer line temperature was 300 °C, MS Source—230 °C and MS Quadrouple—150 °C. The electron energy was 70 eV. From 0 to 4 min, the MS was switched off (solvent delay). Data analysis and instrument control was carried out with MSD 5975C software.

Cyclic Voltammetry

Cyclic voltammograms were recorded with a 797 VA Computrace system (Metrohm, Switzerland). All measurements were carried out at room temperature with a conventional three-electrode configuration consisting of glassy n class="Chemical">carbon working electrode, platinum auxiliary electrode, and Ag/AgCl/KCl reference electrode. The solvent (DMF or CH3CN) was deoxygenated before use. The solution of tetra-n-butylammonium hexafluorophosphate Bu4NPF6 (0.1 M) was used as a supporting electrolyte. The concentrations of the compounds were 10–3 M.

X-ray Crystallography

Single-crystal X-ray diffraction (XRD) data for the compounds were collected by a Bruker Apex DUO diffractometer using the n class="Chemical">graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were applied with the use of the SADABS program.[61] The crystal structures were solved and refined by means of the SHELXT[62] and SHELXL[63] programs using OLEX2 GUI.[64] Atomic thermal displacement parameters for nonhydrogen atoms except for some solvate molecules were refined anisotropically. The positions of H atoms were calculated corresponding to their geometrical conditions and refined using the riding model. In 3, O atoms of one of solvate methanol molecules was disordered over three positions with restrained C–O distances (DFIX) and occupancy of 0.33. H atoms of hydroxyl group of methanol molecules were not located but included in the formula of 3. The crystallographic data and details of the structure refinements are summarized in Table . Crystallographic data for the structures 1–4 have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as Supporting Information nos. 1974048–1974051. Copies of the data can be obtained free of charge on application to CCDC (http://www.ccdc.cam.ac.uk/data_request/cif).

Table 5

Crystallographic Data of the Complexes 1–4

compound	1	2	3	4
empirical formula	C25H22ClN3ORu	C25H22ClN3O2Ru	C35.5H41Cl3N4O2.5Ru2	C25.5H24.5ClN4.5O1.5Ru
formula weight	516.97	532.97	872.23	554.51
temperature/K	200(2)	298(2)	298(2)	150(2)
crystal system	monoclinic	monoclinic	monoclinic	monoclinic
space group	P21/n	P21/n	C2/c	P21/c
a/Å	15.3365(10)	15.601(4)	34.0106(13)	18.414(2)
b/Å	8.0892(6)	8.0342(18)	12.5712(5)	16.071(2)
c/Å	17.0781(14)	17.108(4)	20.0181(8)	7.6248(11)
β/deg	93.968(2)	93.210(7)	123.3950(10)	97.453(4)
volume/Å3	2113.6(3)	2140.9(8)	7145.7(5)	2237.4(5)
Z	4	4	8	4
ρcalc/g/cm3	1.625	1.654	1.619	1.646
μ/mm–1	0.891	0.886	1.109	0.852
F(000)	1048.0	1080.0	3516.0	1128.0
crystal size/mm3	0.15 × 0.1 × 0.08	0.3 × 0.25 × 0.25	0.3 × 0.2 × 0.1	0.33 × 0.17 × 0.05
2Θ range for data collection/deg	3.452 to 57.452	3.438 to 57.468	3.544 to 51.466	3.376 to 57.46
index ranges	–20 ≤ h ≤ 13, –10 ≤ k ≤ 10, –23 ≤ l ≤ 23	–21 ≤ h ≤ 21, –10 ≤ k ≤ 10, –23 ≤ l ≤ 23	–38 ≤ h ≤ 41, –10 ≤ k ≤ 15, –24 ≤ l ≤ 23	–23 ≤ h ≤ 24, –21 ≤ k ≤ 21, –7 ≤ l ≤ 10
reflections collected	28,954	31,935	26,579	15,991
independent reflections	5460 [Rint = 0.0304, Rsigma = 0.0221]	5550 [Rint = 0.0243, Rsigma = 0.0142]	6810 [Rint = 0.0351, Rsigma = 0.0354]	5678 [Rint = 0.0336, Rsigma = 0.0442]
restraints/parameters	0/283	0/292	4/426	12/330
goodness-of-fit on F2	1.026	1.069	1.020	1.023
final R indexes [I >≥ 2σ (I)]	R1 = 0.0224, wR2 = 0.0540	R1 = 0.0190, wR2 = 0.0508	R1 = 0.0313, wR2 = 0.0724	R1 = 0.0296, wR2 = 0.0597
final R indexes [all data]	R1 = 0.0273, wR2 = 0.0565	R1 = 0.0211, wR2 = 0.0525	R1 = 0.0447, wR2 = 0.0788	R1 = 0.0424, wR2 = 0.0639
largest diff. peak/hole/e Å–3	0.58/–0.34	0.36/–0.43	0.78/–0.40	0.51/–0.78

Catalytic Activity Determination

A typical transfer hydrogenation reaction was cn class="Chemical">arried out under air as follows: to 0.05 mmol of catalyst precursor contained in the reaction flask were added isopropyl alcohol (4 mL), NaOH (0.2 mmol) and acetophenone (1 mmol), in that order. The reaction mixture was stirred for 6 h at 82 °C. After that, reaction mass was analyzed by GC and GC–MS. The performed blank experiments confirmed that no products of reaction were obtained unless the catalyst and NaOH were added.

Cytotoxicity Determination

PANC-1 (n class="Disease">pancreatic adenocarcinoma) were obtained from Russian Cell Culture Collection (Institute of Cytology RAS). ARPE-19 (ATCC CRL-2302 retinal pigment epithelial) cells were obtained from the American Type Culture Collection (ATCC) (Manassas, VA). PANC-1 and ARPE-19 cells were cultured in DMEM-F12 medium supplemented with 10% fetal bovine serum, 100 units/mL penicillin, and 100 μg/mL streptomycin, at 37 °C in a humidified atmosphere containing 95% air and 5% CO2. The cytotoxicity of tested compounds was studied using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay as described previously.[65] MTT-formazan crystals formed by metabolically active cells were dissolved in dimethyl sulfoxide, and absorbance was measured at 540 and 690 nm in Varioskan LUX Multimode Microplate Reader (Thermo Scientific, USA). Values measured at 540 nm were subtracted for background values at 690 nm, and the data were shown as a percent of control untreated samples.

Computational Chemistry Details

The calculations were performed using Gaussian 09 package.[66] Experimental X-ray structures of complex 1 was used as a starting point for DFT geometry optimizations. Singlet state geometry optimizations were carried out at the DFT level of theory employing the three-parameter hybrid B3LYP functional[67−70] and 6-311+G(2d,p) basis set[71−74] for the first and second-row atoms, while LanL2DZ[75,76] was used for the ruthenium atoms. Solvation effects were taken into account using the IEFPCM model[77] and acetonitrile as solvent. Frequency calculations were performed for the optimized geometries in order to establish the nature of the stationary points. Lack of imaginary vibration modes for the optimized structures indicate that the stationary points found corresponded to minima on the potential energy surface. Time-dependent DFT calculations with Tamm–Dancoff approximation[78] were carried out using the same basis sets and B3LYP,[67−70] CAM-B3LYP,[79] and ω-B97X[D][80] functionals. The nature of electronic transitions was analyzed using the NTOs approach.[81] The energies of the coordination isomers of complex 1 were compared after geometry optimization and calculation of vibrational frequencies using M062X[82] functional and 6-311+G(2d,p)/LanL2DZ[75,76] basis sets; solvation effects in dimethylformamide were taken into account using the SMD model.[83]

NCI Analysis by means of the Reduced Density Gradient[84−88] was Performed on the Density of the Optimized Structures Using a Homemade Code

Synthesis and Characterization

[Ru(p-cym) (L1)Cl] (1)

HL (24.7 mg, 0.1 mmol) was dissolved in DMF (1.5 mL), n class="Chemical">[Ru(p-cym)Cl2]2 (30.6 mg, 0.05 mmol) was dissolved in MeOH (1.5 mL), and the resulting solution was added to the initial one. After 24 h, red crystals formed were filtered off and washed twice with MeOH. Yield 43 mg (83%). 1H NMR (400 MHz, DMSO-d6): δ 0.99 (d, 3H, Me-iPr, J = 8 Hz), 1.05 (d, 3H, Me-iPr, J = 8 Hz), 2.15 (s, 3H, Me-Ar), 2.64 (m, 1H, CH-Ar), 2.73 (s, Me-iPr), 5.76 (d, 1H, CH (p-cym), J = 4 Hz), 5.99 (d, 1H, CH (p-cym), J = 4 Hz), 6.03 (d, 1H, CH (p-cym), J = 8 Hz), 6.08 (d, 1H, CH (p-cym), J = 8 Hz), 7.53 (t, 1H, H2-L1, J = 8 Hz), 7.65 (t, 1H, H3-L1, J = 8 Hz), 7.93 (t, 1H, H1-L1, J = 4 Hz), 8.02 (t, 1H, H4-L1, J = 8 Hz), 8.08 (d, 1H, H9-L1, J = 8 Hz), 8.18 (d, 1H, H8-L1, J = 8 Hz), 8.33 (d, 1H, H10-L1, J = 2 Hz), 8.40 (d, 1H, H7-L1, J = 8 Hz) ppm. 13C NMR (100 MHz, CDCl3): δ 22.1 (Me-Ar), 22.5 (Me-iPr), 24.1 (Me-iPr), 31.2 (CH-iPr), 80.9 (p-cym), 81.3 (p-cym), 83.7 (p-cym), 84.1 (p-cym), 89.5 (p-cym), 103.7 (p-cym), 123.0 (L1), 123.4 (L1), 124.1 (L1), 126.3 (L1) 128.0 (L1), 128.4 (L1), 128.9 (L1), 130.3 (L1), 131.3 (L1), 132.0 (L1), 134.4 (L1), 140.2 (L1), 141.6 (L1), 148.0 (L1), 157.4 (L1) ppm. IR: 3075 (w), 3059 (w), 3030 (w), 2963 (w), 2867 (w), 1604 (w), 1596 (m), 1497 (s), 1468 (s), 1447 (m), 1420 (m), 1383 (m), 1356 (m), 1327 (m), 1303 (m), 1244 (s), 1212 (m), 1199 (m), 1164 (m), 1140 (m), 1124 (s), 1092 (m), 1068 (m), 1052 (m), 1034 (m), 1012 (m), 970 (m), 946 (m), 871 (s), 794 (s), 765 (s), 749 (s), 725 (s). Found, %: C, 58.40; H, 4.41; N, 8.32. C25H22N3OClRu. Calcd, %: C, 58.08; H, 4.29; N, 8.13.

[Ru(p-cym) (L2)Cl] (2)

HL (53 mg, 0.2 mmol) was dissolved in MeOH (1.5 mL). n class="Chemical">[Ru(p-cym)Cl2]2 (61.2 mg, 0.1 mmol) was dissolved in MeOH (1.5 mL), and the resulting solution was added to the initial one. After 24 h, red crystals formed were filtered off and washed twice with MeOH. Yield 80 mg (75%). 1H NMR (400 MHz, CDCl3): δ 1.08 (d, 3H, Me-iPr, J = 8 Hz), 1.11 (d, 3H, Me-iPr, J = 8 Hz), 2.16 (s, 3H, Me-Ar), 2.70 (m, 1H, CH-Ar), 5.74 (d, 1H, CH (p-cym), J = 8 Hz), 5.925 (d, 1H, CH (p-cym), J = 4 Hz), 5.98 (d, 1H, CH (p-cym), J = 8 Hz), 6.03 (d, 1H, CH (p-cym), J = 8 Hz), 7.45 (t, 1H, H-8, J = 8 Hz), 7.52 (td, 1H, H-9, J = 8 Hz, J = 1.2 Hz), 7.65 (td, 1H, H-2, J = 8 Hz, J = 2.4 Hz), 7.98 (d, 1H, H-4, J = 8 Hz), 8.02 (t, 1H, H-3, J = 4H), 8.03 (dd, 1H, H-7, J = 8 Hz, J = 0.8 Hz), 8.37 (d, 1H, H-1, J = 4 Hz), 8.39 (d, 1H, H-10, J = 4 Hz) ppm. 13C NMR (100 MHz, CDCl3): δ 19.2 (Me-Ar), 22.1 (Me-iPr), 22.6 (Me-iPr), 31.2 (CH-iPr), 83.1 (p-cym), 83.4 (p-cym), 83.9 (p-cym), 88.6 (p-cym), 102.5 (p-cym), 103.7 (p-cym), 116.9 (L2), 117.7 (L2), 120.2 (L2), 121.7 (L2) 122.9 (L2), 125.8 (L2), 127.4 (L2), 128.1 (L2), 128.2 (L2), 128.9 (L2), 136.2 (L2), 137.9 (L2) 146.3 (L2), 157.0 (L2), 157.6 (L2) ppm. IR: 3083 (w), 3062 (w), 3033 (w), 2971 (w), 2873 (w), 1692 (s), 1625 (m), 1593 (m), 1561 (m), 1540 (w), 1495 (s), 1460 (s), 1441 (s), 1404 (m), 1356 (m), 1330 (m), 1316 (m), 1303 (m), 1242 (s), 1191 (m), 1159 (m), 1146 (m), 1124 (m), 1111 (m), 1095 (m), 1055 (m), 1039 (m), 1026 (m), 1007 (m), 940 (m), 868 (m), 807 (s), 794 (m), 759 (s), 743 (s), 727 (m), 709 (m), 687 (s), 674 (m). Found, %: C, 56.58; H, 4.22; N, 7.95. C25H22N3O2ClRu. Calcd, %: C, 56.34; H, 4.16; N, 7.88.

[Ru2(p-cym)2(L3)Cl2]·1.5MeOH (3)

HL (24.8 mg, 0.1 mmol) was suspended in MeOH (5 mL) and 1 mL of 0.1 M solution n class="Chemical">KOH in MeOH was added. Reaction mixture was stirred for 1 h to afford pale yellow precipitate of potassium salt. [Ru(p-cym)Cl2]2 (30.6 mg, 0.1 mmol) was dissolved in MeOH (2 mL), and solution was added to the initial suspension. The resulting red crystals were filtered, washed with MeOH, and dried at room temperature. Yield 47 mg (55%). 1H NMR (400 MHz, CDCl3): δ 1.08 (dd, 6H, Me-iPr (assymm. p-cym), J1 = 24 Hz, J2 = 4 Hz), 1.41 (dd, 6H, Me-iPr (symm. p-cym), J1 = 16 Hz, J2 = 8 Hz), 2.20 (s, 3H, Me-Ar (assymm. p-cym)), 2.34 (s, 3H, Me-Ar (symm. p-cym)), 2.72 (m, 1H, CH-Ar (assymm. p-cym)), 3.10 (m, 1H, CH-Ar (symm. p-cym)), 5.30 (d, 1H, CH (symm. p-cym), J = 4 Hz), 5.40 (d, 1H, CH (symm. p-cym), J = 8 Hz), 5.55 (d, 1H, CH (symm. p-cym), J = 4 Hz), 5.58 (d, 1H, CH (symm. p-cym), J = 8 Hz), 5.79 (d, 1H, CH (assymm. p-cym), J = 8 Hz), 5.96 (d, 1H, CH (assymm. p-cym), 8 Hz), 6.02 (d, 1H, CH (assymm. p-cym), 4 Hz), 6.19 (d, 1H, CH (assymm. p-cym), 8 Hz), 7.51 (t, 1H, H8-L3, J = 8 Hz), 7.67 (t, 1H, H7-L3, J = 8 Hz), 8.10 (d, 1H, H4-L3, J = 4 Hz), 8.23 (t, 2H, H9-L3, J = 8 Hz), 9.23 (d, 1H, H3-L3, J = 8 Hz), 10.10 (s, 1H, H1-L3) ppm. 13C NMR (100 MHz, DMSO-d6): δ 18.3 (Me-Ar), 18.9 (Me-Ar), 22.9 (Me-iPr), 22.2 (Me-iPr), 22.3 (Me-iPr), 30.4 (CH-iPr) 31.1 (CH-iPr), 83.3 (p-cym), 84.1 (p-cym), 85.2 (p-cym), 85.9 (p-cym), 86.8 (p-cym), 90.0 (p-cym), 100.5 (p-cym), 103.3 (p-cym), 103.7 (L3), 106.8 (L3), 121.9 (L3), 123.2 (L3), 124.9 (L3), 128.3 (L3), 132.5 (L3), 134.0 (L3), 144.4 (L3), 146.7 (L3), 146.9 (L3), 148.8 (L3), 151.8 (L3), 157.9 (L3) ppm. IR: 3051 (w), 2955 (m), 2926 (w), 2873 (w), 1604 (m), 1593 (w), 1561 (w), 1497 (s), 1473 (s), 1412 (s), 1394 (s), 1359 (m), 1316 (s), 1250 (s), 1204 (m), 1154 (s), 1132 (m), 1116 (m), 1068 (m), 1028 (m), 978 (m), 874 (m), 826 (s), 802 (s), 773 (s), 757 (s), 730 (s). Found, %: C, 49.11; H, 4.54; N, 6.43. C35.5H41Cl3N4O2.5Ru2. Calcd, %: C, 48.90; H, 4.74; N, 6.42.

[Ru(p-cym)2(L4)Cl2]·0.5DMF (4)

HL (24.8 mg, 0.1 mmol) was suspended in DMF (2.5 mL). n class="Chemical">[Ru(p-cym)Cl2]2 (30.6 mg, 0.05 mmol) and was dissolved in MeOH (2.5 mL), and the resulting solution was added to the initial suspension. The mixture was stirred until the ligand was completely dissolved. The resulting red precipitate was filtered, washed with MeOH, and dried at room temperature. Yield 42 mg (81%). 1H NMR (400 MHz, DMSO-d6): δ 1.005 (d, 3H, Me-iPr, J = 4 Hz), 1.075 (d, 3H, Me-iPr, J = 4 Hz), 2.15 (s, 3H, Me-Ar), 2.68 (m, 1H, CH-Ar), 5.805 (d, 1H, CH (p-cym), J = 4 Hz), 6.03 (d, 1H, CH (p-cym), J = 8 Hz), 6.045 (d, 1H, CH (p-cym), J = 4 Hz), 6.105 (d, 1H, CH (p-cym), J = 4 Hz), 7.57 (t, 1H, H8-L3, J = 8 Hz), 7.70 (t, 1H, H3-L3, J = 8 Hz), 8.015 (dd, 1H, H9-L3, J = 12 Hz, J = 4 Hz), 8.09 (d, 1H, H7-L3, J = 8 Hz), 8.26 (d, 1H, H10-L3, J = 8 Hz), 8.795 (dd, 1H, H4-L3, J = 10 Hz, J = 1.6 Hz), 9.195 (dd, 1H, H2-L3, J = 4 Hz, J = 1.6 Hz) ppm. 13C NMR (100 MHz, CDCl3): δ 21.9 (Me-Ar), 24.1 (Me-iPr), 30.9 (Me-iPr), 31.2 (CH-iPr), 83.6 (p-cym), 83.9 (p-cym), 85.9 (p-cym), 87.0 (p-cym), 89.3 (p-cym), 100.6 (p-cym), 122.8 (L4), 124.2 (L4), 124.6 (L4), 124.6 (L4) 126.0 (L4), 126.2 (L4), 127.3 (L4), 128.9 (L4), 132.9 (L4), 133.9 (L4), 136.3 (L4), 138.7 (L4), 139.7 (L4), 156.4 (L4) ppm. IR: 3056 (w), 2971 (w), 2923 (w), 2873 (w), 1679 (s), 1620 (m), 1604 (m), 1569 (m), 1511 (m), 1495 (s), 1476 (s), 1460 (s), 1417 (s), 1380 (m), 1359 (m), 1308 (s), 1282 (m), 1242 (s), 1204 (m), 1175 (m), 1154 (m), 1149 (m), 1111 (m), 1076 (m), 1036 (m), 1007 (m), 996 (m), 959 (m), 911 (m), 890 (m), 876 (m), 820 (m), 797 (m), 775 (s), 762 (s), 738 (m), 690 (m). Found, %: C, 55.03; H, 4.48; N, 11.53. C25.5H24.5ClN4.5O1.5Ru. Calcd, %: C, 55.23; H, 4.45; N, 11.37.