Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

Effective and Selective Ru(II)-Arene Complexes Containing 4,4'-Substituted 2,2' Bipyridine Ligands Targeting Human Urinary Bladder Cancer Cells

Cisplatin-based chemotherapy is a common regimen for bladder cancer, a life-threatening cancer with more than 500,000 new cases worldwide annually. Like many other metallodrugs, cisplatin causes severe side effects for its general toxicity. Organoruthenium is known for its structural stability, good anticancer activity, and possible low general toxicity. Here, we have prepared and characterized a series of water-soluble ruthenium-arene complexes with N,N'-chelating ligands: [Ru(II)-η6-arene-(4,4'-(X)2-2,2'-bipyridine)Cl]Cl (arene = p-cymene, X = C4H9 (1), COOH (2), COOCH3 (3), COOC2H5 (4); arene = benzene, X = C4H9 (5), COOCH3 (6), COOC2H5 (7)). These complexes are carefully characterized using single-crystal X-ray diffraction, UV-vis, IR, 1H NMR, and MALDI-TOF MS spectroscopy. Their DFT-calculated structural and thermodynamic properties are consistent with the experimental observations. Biophysicochemical studies of complex interaction with CTDNA and BSA supported by molecular docking simulations reveal suitable properties of 1-7 as anticancer agents. Cytotoxicities of 1-7 are evaluated on healthy human MCF-10a-breast epithelial and African green monkey Vero cells, and carcinoma human HepG-2-hepatic, T24-bladder, and EAhy-926-endothelial cells. All complexes exhibit much higher cytotoxicity for T24 than cisplatin. Particularly, 1 and 2 are also highly selective toward T24. Fluorescence imaging and flow cytometry demonstrate that 1 and 2 penetrate T24 cell membrane and induce early apoptosis at their respective IC50 concentrations, which ultimately lead to cell death. Statistical analysis suggests that the order of importance for T24 cell antiproliferation is protein binding, Log p, Ru-Cl bond length, while DNA binding is the least important. This study is the first to report the anti-bladder cancer efficacy of Ru-arene-2,2'-bipyridine complexes, and may provide insights for rational design of organoruthenium drugs in the enduring search for new chemotherapeutic agents.

cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds

We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2–7) of the general formula [Y] fac-[RuX(DMSO–S)₃(O–O)]ⁿ (where O–O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO–O or Cl^–; n = −1/0/+1 depending on the nature and charge of X and O–O; when present, Y = K⁺ or PF₆^–) were efficiently prepared from the well-known cis-[RuCl₂(DMSO)₄] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2–7 afforded the target [Ru(L1)₂(O–O)]^0/+ complex together with the undesired (and unexpected) [Ru(L1)₃]²⁺ species. Nevertheless, we found that the formation of [Ru(L1)₃]²⁺can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)₂(O–O)]^0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100–200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)₂(O–O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)₃]²⁺ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O–O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O–O in [Ru(L1)₂(O–O)]^0/+ by L2, selectively affording [Ru(L1)₂(L2)]²⁺, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF₆.

cis-Locked Ru(II)-DMSO complexes bearing a symmetrical chelating anion, such as [K] fac-[RuCl(DMSO−S)₃(η²-mal)] (2), are suitable precursors for the two-step selective preparation of bis-heteroleptic polypyridyl compounds of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands).

Optical Properties of Isolated and Covalent Organic Framework-Embedded Ruthenium Complexes

Heterogenization of RuL₃ complexes on a support with proper anchor points provides a route toward design of green catalysts. In this paper, Ru(II) polypyridyl complexes are investigated with the aim to unravel the influence on the photocatalytic properties of varying nitrogen content in the ligands and of embedding the complex in a triazine-based covalent organic framework. To provide fundamental insight into the electronic mechanisms underlying this behavior, a computational study is performed. Both the ground and excited state properties of isolated and anchored ruthenium complexes are theoretically investigated by means of density functional theory and time-dependent density functional theory. Varying the ligands among 2,2′-bipyridine, 2,2′-bipyrimidine, and 2,2′-bipyrazine allows us to tune to a certain extent the optical gaps and the metal to ligand charge transfer excitations. Heterogenization of the complex within a CTF support has a significant effect on the nature and energy of the electronic transitions. The allowed transitions are significantly red-shifted toward the near IR region and involve transitions from states localized on the CTF toward ligands attached to the ruthenium. The study shows how variations in ligands and anchoring on proper supports allows us to increase the range of wavelengths that may be exploited for photocatalysis.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Influence of the Steric Bulk and Solvent on the Photoreactivity of Ruthenium Polypyridyl Complexes Coordinated to l-Proline

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)₂(l-prol)]PF₆ ([1a]PF₆; bpy = 2,2′-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF₆ ([2a]PF₆ and [2b]PF₆; dmbpy = 6,6′-dimethyl-2,2′-bipyridine), and Λ-[Ru(dmbpy)₂(l-prol)]PF₆ ([3a]PF₆). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF₆ and [2b]PF₆. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF₆, [2a]PF₆, [2b]PF₆, and [3a]PF₆ was studied in water (H₂O) and acetonitrile (MeCN) using UV–vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and ¹H NMR spectroscopy. In H₂O, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF₆ was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF₆) or four ([3a]PF₆) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF₆ was photostable in PBS, [2a]PF₆ quantitatively isomerized to [2b]PF₆ upon light irradiation. In pure MeCN, [2a]PF₆ and [3a]PF₆ showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF₆ was photostable. Finally, in H₂O–MeCN mixtures, [3a]PF₆ showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed.

Three ruthenium polypyridyl l-proline complexes with increasing strain (R, R′ = H or Me) were synthesized and their photoreactivities studied in phosphate-buffered saline, pure acetonitrile (MeCN), and water−MeCN mixtures. Depending on the number of methyl groups, on the presence of air, and on the nature of the solvent, either photoisomerization, photooxidation of l-proline, selective photosubstitution, or nonselective photosubstitution was observed.

Synthesis and Electrochemical and Photophysical Characterization of New 4,4'-π-Conjugated 2,2'-Bipyridines that are End-Capped with Cyanoacrylic Acid/Ester Groups

Two new functionalized 4,4'-disubstituted 2,2'-bipyridines that were end-capped with cyanoacrylic acid or cyanoacrylic acid ester anchoring groups, which might allow their efficient functionalization on TiO2 or other metal-oxide semiconductor surfaces, have been synthesized and characterized by electrochemical, photophysical, and spectroscopic measurements. The electrochemical and photophysical properties of these 4,4'-disubstituted 2,2'-bipyridines with extended π systems, in particular their LUMO energies, make them promising candidates to build up inorganic-organic hybrid photosensitizers for the sensitization of metal-oxide semiconductors (e.g., TiO2 nanoparticles and/or nanotubes).

An efficient route to asymmetrically diconjugated tris(heteroleptic) complexes of Ru(ii)

A highly efficient and versatile route to the preparation of tris(heteroleptic) Ru(ii) polypyridyl complexes is described which permits access to two or more independently conjugatable termini in the final structure.

mer and fac isomerism in tris chelate diimine metal complexes

In this perspective, we highlight the issue of meridional (mer) and facial (fac) orientation of asymmetrical diimines in tris-chelate transition metal complexes. Diimine ligands have long been the workhorse of coordination chemistry, and whilst there are now good strategies to isolate materials where the inherent metal centered chirality is under almost complete control, and systematic methodologies to isolate heteroleptic complexes, the conceptually simple geometrical isomerism has not been widely investigated. In systems where the two donor atoms are significantly different in terms of the σ-donor and π-accepting ability, the fac isomer is likely to be the thermodynamic product. For the diimine complexes with two trigonal planar nitrogen atoms there is much more subtlety to the system, and external factors such as the solvent, lattice packing and the various steric considerations play a delicate role in determining the observed and isolable product. In this article we discuss the possibilities to control the isomeric ratio in labile systems, consider the opportunities to separate inert complexes and discuss the observed differences in their spectroscopic properties. Finally we report on the ligand orientation in supramolecular systems where facial coordination leads to simple regular structures such as helicates and tetrahedra, but the ability of the ligand system to adopt a mer orientation enables self-assembled structures of considerable beauty and complexity.