Mechanism Elucidation of the <i>cis–trans</i> Isomerization of an Azole Ruthenium–Nitrosyl Complex and Its Osmium Counterpart

Synthesis and X-ray diffraction structures of <i>cis</i> and <i>trans</i> isomers of ruthenium and osmium metal complexes of general formulas (<i>n</i>Bu₄N)­[<i>cis</i>-MCl₄(NO)­(Hind)], where M = Ru (<b>1</b>) and Os (<b>3</b>), and (<i>n</i>Bu₄N)­[<i>trans</i>-MCl₄(NO)­(Hind)], where M = Ru (<b>2</b>) and Os (<b>4</b>) and Hind = 1<i>H</i>-indazole are reported. Interconversion between <i>cis</i> and <i>trans</i> isomers at high temperatures (80–130 °C) has been observed and studied by NMR spectroscopy. Kinetic data indicate that isomerizations correspond to reversible first order reactions. The rates of isomerization reactions even at 110 °C are very low with rate constants of 10^–5 s^–1 and 10^–6 s^–1 for ruthenium and osmium complexes, respectively, and the estimated rate constants of isomerization at room temperature are of ca. 10^–10 s^–1. The activation parameters, which have been obtained from fitting the reaction rates at different temperatures to the Eyring equation for ruthenium [Δ<i>H</i>_{<i>cis‑trans</i>}^‡ <i>=</i> 122.8 ± 1.3; Δ<i><i>H</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 138.8 ± 1.0 kJ/mol; Δ<i><i>S</i></i>_{<i>cis‑trans</i>}^‡ <i>=</i> −18.7 ± 3.6; Δ<i><i>S</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 31.8 ± 2.7 J/(mol·K)] and osmium [Δ<i>H</i>_{<i>cis‑trans</i>}^‡ <i>=</i> 200.7 ± 0.7; Δ<i><i>H</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 168.2 ± 0.6 kJ/mol; Δ<i><i>S</i></i>_{<i>cis‑trans</i>}^‡ <i>=</i> 142.7 ± 8.9; Δ<i><i>S</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 85.9 ± 3.9 J/(mol·K)] reflect the inertness of these systems. The entropy of activation for the osmium complexes is highly positive and suggests the dissociative mechanism of isomerization. In the case of ruthenium, the activation entropy for the <i>cis</i> to <i>trans</i> isomerization is negative [−18.6 J/(mol·K)], while being positive [31.0 J/(mol·K)] for the <i>trans</i> to <i>cis</i> conversion. The thermodynamic parameters for <i>cis</i> to <i>trans</i> isomerization of [RuCl₄(NO)­(Hind)]⁻, viz. Δ<i><i>H</i>°</i> = 13.5 ± 1.5 kJ/mol and Δ<i>S</i>° = −5.2 ± 3.4 J/(mol·K) indicate the low difference between the energies of <i>cis</i> and <i>trans</i> isomers. The theoretical calculation has been carried out on isomerization of ruthenium complexes with DFT methods. The dissociative, associative, and intramolecular twist isomerization mechanisms have been considered. The value for the activation energy found for the dissociative mechanism is in good agreement with experimental activation enthalpy. Electrochemical investigation provides further evidence for higher reactivity of ruthenium complexes compared to that of osmium counterparts and shows that intramolecular electron transfer reactions do not affect the isomerization process. A dissociative mechanism of <i>cis</i>↔<i>trans</i> isomerization has been proposed for both ruthenium and osmium complexes

Striking Difference in Antiproliferative Activity of Ruthenium- and Osmium-Nitrosyl Complexes with Azole Heterocycles

Ruthenium nitrosyl complexes of the general formulas (cation)⁺[<i>cis</i>-RuCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (Hind) (<b>1c</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (Hpz) (<b>2c</b>), (cation)⁺ = (H₂bzim)⁺, Hazole = 1<i>H</i>-benzimidazole (Hbzim) (<b>3c</b>), (cation)⁺ = (H₂im)⁺, Hazole = 1<i>H</i>-imidazole (Him) (<b>4c</b>) and (cation)⁺[<i>trans</i>-RuCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>1t</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (<b>2t</b>), as well as osmium analogues of the general formulas (cation)⁺[<i>cis</i>-OsCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (<i>n</i>-Bu₄N)⁺, Hazole =1<i>H</i>-indazole (<b>5c</b>), 1<i>H</i>-pyrazole (<b>6c</b>), 1<i>H</i>-benzimidazole (<b>7c</b>), 1<i>H</i>-imidazole (<b>8c</b>), (cation)⁺ = Na⁺; Hazole =1<i>H</i>-indazole (<b>9c</b>), 1<i>H</i>-benzimidazole (<b>10c</b>), (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>11c</b>), (cation)⁺ = H₂pz⁺, Hazole = 1<i>H</i>-pyrazole (<b>12c</b>), (cation)⁺ = (H₂im)⁺, Hazole = 1<i>H</i>-imidazole (<b>13c</b>), and (cation)⁺[<i>trans</i>-OsCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = <i>n</i>-Bu₄N⁺, Hazole = 1<i>H</i>-indazole (<b>5t</b>), 1<i>H</i>-pyrazole (<b>6t</b>), (cation)⁺ = Na⁺, Hazole = 1<i>H</i>-indazole (<b>9t</b>), (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>11t</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (<b>12t</b>), have been synthesized. The compounds have been comprehensively characterized by elemental analysis, ESI mass spectrometry, spectroscopic techniques (IR, UV–vis, 1D and 2D NMR) and X-ray crystallography (<b>1c</b>·CHCl₃, <b>1t</b>·CHCl₃, <b>2t</b>, <b>3c</b>, <b>6c</b>, <b>6t</b>, <b>8c</b>). The antiproliferative activity of water-soluble compounds (<b>1c</b>, <b>1t</b>, <b>3c</b>, <b>4c</b> and <b>9c</b>, <b>9t</b>, <b>10c</b>, <b>11c</b>, <b>11t</b>, <b>12c</b>, <b>12t</b>, <b>13c</b>) in the human cancer cell lines A549 (nonsmall cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon adenocarcinoma) has been assayed. The effects of metal (Ru vs Os), cis/trans isomerism, and azole heterocycle identity on cytotoxic potency and cell line selectivity have been elucidated. Ruthenium complexes (<b>1c</b>, <b>1t</b>, <b>3c</b>, and <b>4c</b>) yielded IC₅₀ values in the low micromolar concentration range. In contrast to most pairs of analogous ruthenium and osmium complexes known, they turned out to be considerably more cytotoxic than chemically related osmium complexes (<b>9c</b>, <b>9t</b>, <b>10c</b>, <b>11c</b>, <b>11t</b>, <b>12c</b>, <b>12t</b>, <b>13c</b>). The IC₅₀ values of Os/Ru homologs differ by factors (Os/Ru) of up to ∼110 and ∼410 in CH1 and SW480 cells, respectively. ESI-MS studies revealed that ascorbic acid may activate the ruthenium complexes leading to hydrolysis of one M–Cl bond, whereas the osmium analogues tend to be inert. The interaction with myoglobin suggests nonselective adduct formation; i.e., proteins may act as carriers for these compounds

Ruthenium-Nitrosyl Complexes with Glycine, l‑Alanine, l‑Valine, l‑Proline, d‑Proline, l‑Serine, l‑Threonine, and l‑Tyrosine: Synthesis, X‑ray Diffraction Structures, Spectroscopic and Electrochemical Properties, and Antiproliferative Activity

The reactions of [Ru­(NO)­Cl₅]^2– with glycine (Gly), l-alanine (l-Ala), l-valine (l-Val), l-proline (l-Pro), d-proline (d-Pro), l-serine (l-Ser), l-threonine (l-Thr), and l-tyrosine (l-Tyr) in <i>n</i>-butanol or <i>n</i>-propanol afforded eight new complexes (<b>1</b>–<b>8</b>) of the general formula [RuCl₃(AA–H)­(NO)]⁻, where AA = Gly, l-Ala, l-Val, l-Pro, d-Pro, l-Ser, l-Thr, and l-Tyr, respectively. The compounds were characterized by elemental analysis, electrospray ionization mass spectrometry (ESI-MS), ¹H NMR, UV–visible and ATR IR spectroscopy, cyclic voltammetry, and X-ray crystallography. X-ray crystallography studies have revealed that in all cases the same isomer type (from three theoretically possible) was isolated, namely <i>mer</i>(Cl),<i>trans</i>(NO,O)-[RuCl₃(AA–H)­(NO)], as was also recently reported for osmium analogues with Gly, l-Pro, and d-Pro (see <i>Z. Anorg. Allg. Chem.</i> <b>2013</b>, <i>639</i>, 1590–1597). Compounds <b>1</b>, <b>4</b>, <b>5</b>, and <b>8</b> were investigated by ESI-MS with regard to their stability in aqueous solution and reactivity toward sodium ascorbate. In addition, cell culture experiments in three human cancer cell lines, namely, A549 (nonsmall cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon carcinoma), were performed, and the results are discussed in conjunction with the lipophilicity of compounds

Striking Difference in Antiproliferative Activity of Ruthenium- and Osmium-Nitrosyl Complexes with Azole Heterocycles

Ruthenium nitrosyl complexes of the general formulas (cation)⁺[<i>cis</i>-RuCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (Hind) (<b>1c</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (Hpz) (<b>2c</b>), (cation)⁺ = (H₂bzim)⁺, Hazole = 1<i>H</i>-benzimidazole (Hbzim) (<b>3c</b>), (cation)⁺ = (H₂im)⁺, Hazole = 1<i>H</i>-imidazole (Him) (<b>4c</b>) and (cation)⁺[<i>trans</i>-RuCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>1t</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (<b>2t</b>), as well as osmium analogues of the general formulas (cation)⁺[<i>cis</i>-OsCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = (<i>n</i>-Bu₄N)⁺, Hazole =1<i>H</i>-indazole (<b>5c</b>), 1<i>H</i>-pyrazole (<b>6c</b>), 1<i>H</i>-benzimidazole (<b>7c</b>), 1<i>H</i>-imidazole (<b>8c</b>), (cation)⁺ = Na⁺; Hazole =1<i>H</i>-indazole (<b>9c</b>), 1<i>H</i>-benzimidazole (<b>10c</b>), (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>11c</b>), (cation)⁺ = H₂pz⁺, Hazole = 1<i>H</i>-pyrazole (<b>12c</b>), (cation)⁺ = (H₂im)⁺, Hazole = 1<i>H</i>-imidazole (<b>13c</b>), and (cation)⁺[<i>trans</i>-OsCl₄(NO)­(Hazole)]⁻, where (cation)⁺ = <i>n</i>-Bu₄N⁺, Hazole = 1<i>H</i>-indazole (<b>5t</b>), 1<i>H</i>-pyrazole (<b>6t</b>), (cation)⁺ = Na⁺, Hazole = 1<i>H</i>-indazole (<b>9t</b>), (cation)⁺ = (H₂ind)⁺, Hazole = 1<i>H</i>-indazole (<b>11t</b>), (cation)⁺ = (H₂pz)⁺, Hazole = 1<i>H</i>-pyrazole (<b>12t</b>), have been synthesized. The compounds have been comprehensively characterized by elemental analysis, ESI mass spectrometry, spectroscopic techniques (IR, UV–vis, 1D and 2D NMR) and X-ray crystallography (<b>1c</b>·CHCl₃, <b>1t</b>·CHCl₃, <b>2t</b>, <b>3c</b>, <b>6c</b>, <b>6t</b>, <b>8c</b>). The antiproliferative activity of water-soluble compounds (<b>1c</b>, <b>1t</b>, <b>3c</b>, <b>4c</b> and <b>9c</b>, <b>9t</b>, <b>10c</b>, <b>11c</b>, <b>11t</b>, <b>12c</b>, <b>12t</b>, <b>13c</b>) in the human cancer cell lines A549 (nonsmall cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon adenocarcinoma) has been assayed. The effects of metal (Ru vs Os), cis/trans isomerism, and azole heterocycle identity on cytotoxic potency and cell line selectivity have been elucidated. Ruthenium complexes (<b>1c</b>, <b>1t</b>, <b>3c</b>, and <b>4c</b>) yielded IC₅₀ values in the low micromolar concentration range. In contrast to most pairs of analogous ruthenium and osmium complexes known, they turned out to be considerably more cytotoxic than chemically related osmium complexes (<b>9c</b>, <b>9t</b>, <b>10c</b>, <b>11c</b>, <b>11t</b>, <b>12c</b>, <b>12t</b>, <b>13c</b>). The IC₅₀ values of Os/Ru homologs differ by factors (Os/Ru) of up to ∼110 and ∼410 in CH1 and SW480 cells, respectively. ESI-MS studies revealed that ascorbic acid may activate the ruthenium complexes leading to hydrolysis of one M–Cl bond, whereas the osmium analogues tend to be inert. The interaction with myoglobin suggests nonselective adduct formation; i.e., proteins may act as carriers for these compounds