Coordination Driven Self-Assembly and Anticancer Activity of Molecular Rectangles Containing Octahedral Ruthenium Metal Centers

Metal Organic Polygons and Polyhedra: Instabilities and Remedies

The field of coordination chemistry has undergone rapid transformation from preparation of monometallic complexes to multimetallic complexes. So far numerous multimetallic coordination complexes have been synthesized. Multimetallic coordination complexes with well-defined architectures are often called as metal organic polygons and polyhedra (MOPs). In recent past, MOPs have received tremendous attention due to their potential applicability in various emerging fields. However, the field of coordination chemistry of MOPs often suffer set back due to the instability of coordination complexes particularly in aqueous environment-mostly by aqueous solvent and atmospheric moisture. Accordingly, the fate of the field does not rely only on the water solubilities of newly synthesized MOPs but very much dependent on their stabilities both in solution and solid state. The present review discusses several methodologies to prepare MOPs and investigates their stabilities under various circumstances. Considering the potential applicability of MOPs in sustainable way, several methodologies (remedies) to enhance the stabilities of MOPs are discussed here.

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)₃Mn(μ-SeR)₂Mn(CO)₃}₂(μ-L)₂] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

Metal-Coordinated Supramolecular Self-Assemblies for Cancer Theranostics

Metal-coordinated supramolecular nanoassemblies have recently attracted extensive attention as materials for cancer theranostics. Owing to their unique physicochemical properties, metal-coordinated supramolecular self-assemblies can bridge the boundary between traditional inorganic and organic materials. By tailoring the structural components of the metal ions and binding ligands, numerous multifunctional theranostic nanomedicines can be constructed. Metal-coordinated supramolecular nanoassemblies can modulate the tumor microenvironment (TME), thus facilitating the development of TME-responsive nanomedicines. More importantly, TME-responsive organic-inorganic hybrid nanomaterials can be constructed in vivo by exploiting the metal-coordinated self-assembly of a variety of functional ligands, which is a promising strategy for enhancing the tumor accumulation of theranostic molecules. In this review, recent advancements in the design and fabrication of metal-coordinated supramolecular nanomedicines for cancer theranostics are highlighted. These supramolecular compounds are classified according to the order in which the coordinated metal ions appear in the periodic table. Furthermore, the prospects and challenges of metal-coordinated supramolecular self-assemblies for both technical advances and clinical translation are discussed. In particular, the superiority of TME-responsive nanomedicines for in vivo coordinated self-assembly is elaborated, with an emphasis on strategies that enhance the accumulation of functional components in tumors for an ideal theranostic outcome.

Enantiopure Polyradical Tetrahedral Pd12 L6 Cages

The synthesis of cages with a polyradical framework remains a challenging task. Herein is reported an enantiomeric pair of quinoid-bridged polyradical tetrahedral palladium(II) cages that are stabilized by an unusual dianionic diradical form (dhbq^..2- ). These cages have been characterized by electron paramagnetic resonance and UV-visible spectroscopy, squid magnetometry and mass spectrometry. Single-crystal-derived X-ray investigations of the iso-structural cages built on fluoranilate linkers confirm the tetrahedral structure of the obtained radical cages. Theoretical calculations showed that the diradical state of the dhbq anions is more stable than the usual monoradical state. A weak ferromagnetic exchange between adjacent radical centers was observed in DFT studies.

Biomedical Applications of Metal Organic Polygons and Polyhedra (MOPs)

Since the discovery and structural characterization of metal organic polygons and polyhedra (MOPs), scientists have explored their potential in various applications like catalysis, separation, storage, and sensing. In recent years, scientists have explored the potential of supramolecular MOPs in biomedical application. Pioneering works by Ehrlich, Rosenberg, Lippard, Stang and others have demonstrated that MOPs have great potential as a novel class of metallo-therapeutics that can deliver cargoes (drugs and dyes) selectively. In this article, we document the progress made over the past two decades on the biomedical applications of MOPs and discuss the future prospects of this emerging field.

Polynuclear ruthenium organometallic complexes containing a 1,3,5-triazine ligand: synthesis, DNA interaction, and biological activity

It is now well established that ruthenium complexes are attractive alternatives to platinum-based anticancer agents. Most of the ruthenium compounds currently under investigation contain a single metal center. The synthesis of multinuclear analogues may provide access to novel complexes with enhanced biological activity. In this work, we have synthesized a set of three trinuclear complexes containing organometallic ruthenium fragments-(arene)RuCl-coordinated to a 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazine core [(Arene = benzene (2), p-cymene (1), or hexamethylbenzene (3)]. The interaction of the complexes with DNA was extensively studied using a variety of biophysical probes as well as by molecular docking. The complexes bind strongly to DNA with apparent binding constants ranging from 2.20 to 4.79 × 10⁴ M^-1. The binding constants from electronic absorption titrations were an order of magnitude greater. The mode of binding to the nucleic acid was not definitively determined, but the evidence pointed to some kind of non-specific electrostatic interaction. None of the complexes displayed any significant antimicrobial activity against the organisms that were studied and exhibited anticancer activity only at high (> 100 μM) concentration.

Self-Assembled BODIPY-Based Iridium Metallarectangles: Cytotoxicity and Propensity to Bind Biomolecules

A new 4-ethynylpyridine 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based ligand L, which was synthesized by means of the Sonogashira coupling method, was used to obtain two new [2+2] iridium-based metallarectangles, 3 and 4. Ligand L and metallarectangles 3 and 4 were fully characterized through various analytical techniques. The structure of rectangle 4 was further confirmed by single-crystal X-ray diffraction analysis, which showed the formation of an expected [2+2] supramolecule, in which the iridium metal centers were bridged with ligand L to form the desired metallarectangle 4. In the context of the growing biological interest in metallarectangles, rectangle 4 was found to be highly active against two types of cancer cells, with IC₅₀ values almost threefold superior to those of cisplatin. Both 3 and 4 showed dose-dependent abilities to bind bovine serum albumin and salmon sperm DNA; this indicated their tendency to interact with such biomolecules as a potential mode of action.

Self-Assembly of Discrete RuII8 Molecular Cages and Their in Vitro Anticancer Activity

Four new octanuclear Ru(II) cages (OC-1-OC-4) were synthesized from dinuclear p-cymene ruthenium(II) acceptors [Ru₂(μ-η⁴-C₂O₄)(CH₃OH)₂(η⁶-p-cymene)₂](O₃SCF₃)₂ (A₁), [Ru₂(μ-η⁴-C₆H₂O₄)(CH₃OH)₂(η⁶-p-cymene)₂](O₃SCF₃)₂ (A₂), [Ru₂(dhnq)(H₂O)₂(η⁶-p-cymene)₂](O₃SCF₃)₂ (A₃), and [Ru₂(dhtq)(H₂O)₂(η⁶-p-cymene)₂](O₃SCF₃)₂ (A₄) separately with a tetradentate pyridyl ligand (L₁) in methanol using coordination-driven self-assembly [L₁= N,N,N',N'-tetra(pyridin-4-yl)benzene-1,4-diamine]. The octanuclear cages are fully characterized by various spectroscopic techniques including single-crystal X-ray diffraction analysis of OC-4. The self-assembled cages show strong in vitro anticancer activity against human lung adenocarcinoma A549 and human cervical cancer HeLa cell lines as observed from the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Of all the octanuclear cages, OC-3 exhibits remarkable anticancer activity against both cancer cell lines and is more active than that reported for cisplatin. The excellent anticancer activity of OC-3 and OC-4 highlights the importance of the synergistic effects of the spacer component of the dinuclear p-cymene Ru(II) acceptor clips.

Anticancer potency studies of coordination driven self-assembled arene–Ru-based metalla-bowls

New tetranuclear cationic metalla-bowls 5–7 with the general formula [Ru4(p-cymene)4(N∩N)2(OO∩OO)2]4+ (N∩N=2,6-bis(N-(4-pyridyl carbamoyl)pyridine, OO∩OO=2,5-dihydroxy-1,4-benzoquinonato (5), OO∩OO=5,8-dioxydo-1,4-naphthaquinonato (6), OO∩OO=hoxonato (7)) were prepared by the reaction of the respective dinuclear ruthenium complexes 2–4 with a bispyridine amide donor ligand 1 in methanol in the presence of AgO3SCF3.These new molecular metalla-bowls were fully characterized by analytical techniques including elemental analysis as well as 1H and 13C NMR and HR-ESI-MS spectroscopy. The structure of metalla-bowl 6 was determined from X-ray crystal diffraction data. A UV/visible study was also carried out for the entire suite of new complexes. As with recent studies of similar arene–Ru complexes, the inhibition of cell growth by metalla-bowls was established against SK-hep-1 (liver cancer), AGS (gastric cancer), and HCT-15 (colorectal cancer) human cancer cell lines. Inhibition of cell growth by 6 was found to be considerably stronger against all cancer cell lines than the anticancer drugs, doxorubicin and cisplatin. In particular, in colorectal cancer cells, expression of the cancer suppressor genes APC and p53 was increased following exposure to 6.

Supramolecular architectures with pyridine-amide based ligands: discrete molecular assemblies and their applications

This perspective offers an overview on the recent progress achieved in the design of discrete molecular assemblies utilizing assorted pyridine-amide based ligands.

Biomedical and biochemical applications of self-assembled metallacycles and metallacages

Metal ions and metal complexes with organic molecules are ubiquitous in nature. Bulk metal ions of Na, K, Mg, and Ca constitute as much as 1% of human body weight. The remaining trace ions, most commonly of Fe, Ni, Cu, Mn, Zn, Co, Mo, and V, make up ∼0.01% by weight, but their importance in biological processes cannot be overstated. Although nature is limited to the use of bioavailable metal ions, many rarer transition metals can elicit novel biological responses when they interact with biomolecules. For this reason, metal-biomolecule complexes are of interest in medicinal applications. A well-known example is cisplatin, which contains Pt, rare in nature, but highly effective in this context as an anticancer drug in the form of cis-Pt(NH3)2Cl2 and analogous Pt(II) complexes. This and other examples have led to strong interest in discovering new metalloanticancer drugs. In this Account, we describe recent developments in this area, particularly, using coordination-driven self-assembly to form tunable supramolecular coordination complexes (SCCs) with biomedical applications. Coordination-driven self-assembly describes the spontaneous formation of metal-ligand bonds in solution, transforming molecular building blocks into single, 2D metallacycles, or 3D metallacages depending on the directionality of the precursors used. Such SCCs have well-defined internal cavities and simple pre- or post-self-assembly functionalizations. They are highly tunable both spatially and electronically. Metal ions are necessary structural elements for the directional bonding approach, which can be exploited to provide biological activity to an SCC, particularly for Pt- and Ru-based structures. Since these two metals are not only among the most commonly used for coordination-driven self-assembly but are also the basis for a number of small molecule anticancer agents, researchers have evaluated a growing number of SCCs for their antitumor properties. The biological application of SCCs is still an emergent field of study, but the examples discussed in this Account confirm that supramolecular scaffolds have relevance to a wide variety of biochemical and biomedical targets. SCCs can serve as anticancer agents, act as selective sensors for biologically important analytes, or interact with DNA and proteins. The myriad of possible SCCs and their almost limitless modularity and tunability without significant synthetic penalty suggests that the biological applications of such species will continue along this already promising path.

Anticancer potency and multidrug-resistant studies of self-assembled arene-ruthenium metallarectangles

A suite of three tetraruthenium metallacycles have been obtained from [2+2] self-assemblies between N,N'-Di-(4-pyridyl)-1,4,5,8-naphthalenetetracarbo-xydiimide (4) and one of the three dinuclear arene ruthenium clips, (η(6)-p-iPrC6H4Me)2Ru2(OO∩OO)][OTf]2 (OO∩OO = oxalate 1, 2,5-dioxydo-1,4-benzoquinonato (dobq) 2, 5,8-dihydroxy-1,4-naphthaquinonato (donq) 3; OTf = triflate). All complexes were isolated in good yield (>85 %) as triflate salts and were fully characterized by using (1)H NMR and UV/Vis spectroscopies, and high-resolution electrospray mass spectrometry. A single crystal of the metallarectangle 5 was suitable for X-ray diffraction structural characterization. The biological activities of the metallacycles were determined by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays, establishing their in vitro anticancer properties. Our results show that for the AGC (gastric cancer) cell lines, the cytotoxicity of (donq)-containing SCC 7 exceeds that of cisplatin, which was used as a control. For HCT15 (colon cancer) cell lines, the cytotoxicity is comparable to both cisplatin and doxorubicin. An in vivo hollow fiber model was used to show growth-inhibitory activity against HCT15 and image-based cytometry experiments indicated that 7 induced apoptosis as the mode of cell death. Complex 7 also showed significant antitumor activity for multidrug-resistant HCT15/CLO2 cell lines, for which doxorubicin was ineffective.

Self-assembly of ambidentate pyridyl-carboxylate ligands with octahedral ruthenium metal centers: self-selection for a single-linkage isomer and anticancer-potency studies

The synthesis of six new [2+2] metallarectangles through the coordination-driven self-assembly of octahedral Ru(II)-based acceptors with ambidentate pyridyl-carboxylate donors is described. These molecular rectangles are fully characterized by (1)H NMR spectroscopy, high-resolution electrospray mass spectrometry, and single-crystal X-ray diffraction. In each case, despite the possible formation of multiple isomers, based on the relative orientation of the pyridyl and carboxylate groups (head-to-head versus head-to-tail), evidence for the formation of a single preferred ensemble (head-to-tail) was found in the (1)H NMR spectra. Furthermore, the cytotoxicities of all of the rectangles were established against A549 (lung), AGS (gastric), HCT-15 (colon), and SK hep 1 (liver) human cancer cell lines. The cytotoxicities of rectangles that contained the 5,8-dihydroxy-1,4-naphthaquinonato bridging moiety between the Ru centers (9-11) were particularly high against AGS cancer cells, with IC50 values that were comparable to that of reference drug cisplatin.

Self-assembled metalla-rectangles bearing azodipyridyl ligands: synthesis, characterization and antitumor activity

Sixteen arene-Ru based molecular-rectangles were self-assembled in high yields by the equimolar mixing of arene-Ru acceptors (Aa-Ad) with various azopyridyl ligands (1,2-di(pyridyl-4yl)diazene (L1), 1,2-bis(pyridin-4-ylmethylene)hydrazine (L2), 1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine (L3), 1,2-bis(pyridin-4-ylmethylene)hydrazine (L4)) in nitromethane-methanol solutions. These new molecular-rectangles were fully characterized by a host of analytical techniques including elemental analysis, (1)H and (13)C NMR and HR-ESI-MS. The solid-state structures of two molecular-rectangles (1b and 4d) were determined by single crystal X-ray diffraction data. UV-visible and fluorescence studies were also carried out for the entire suite of rectangles. As with recent studies of similar arene-Ru complexes, the anti-proliferative activities of these complexes were evaluated against SK-hep-1 (liver cancer) and A-549 (lung cancer) human cancer lines. Additionally, the cellular pharmacology and intracellular localizations in AGS (gastric cancer) human cancer cells were determined for selected complexes (1c, 1d and 4c) by apoptosis and fluorescence microscopy studies. These studies confirm that arene-Ru molecular-rectangles inhibit cell cycle progression to the G0 phase, in contrast to that of cisplatin which arrests cell growth in the G2 phase.

Photophysical and computational investigations of bis(phosphine) organoplatinum(II) metallacycles

A series of endohedral and exohedral amine-functionalized ligands were synthesized and used in the construction of supramolecular D(2h) rhomboids and a D(6h) hexagon. These supramolecular polygons were obtained via self-assembly of 120° dipyridyl donors with 180° or 120° diplatinum precursors when combined in 1:1 ratios. Steady-state absorption and emission spectra were collected for each ligand and metallacycle. Density functional theory (DFT) and time-dependent DFT calculations were employed to probe the nature of the observed optical transitions for the rhomboids. The emissive properties of these bis(phosphine) organoplatinum metallacycles arise from ligand-centered transitions involving π-type molecular orbitals with modest contributions from metal-based atomic orbitals. The D(2h) rhomboid self-assembled from 2,6-bis(4-pyridylethynyl)aniline and a 60° organoplatinum(II) acceptor has a low-energy excited state in the visible region and emits above 500 nm, properties which greatly differ from those of the parent 2,6-bis(4-pyridylethynyl)aniline ligand.

Anticancer Activity of Self-Assembled Molecular Rectangles via Arene-Ruthenium Acceptors and a New Unsymmetrical Amide Ligand

Two new and large molecular rectangles 4 and 5 were synthesized from two different arene-ruthenium [Ru(2)(μ-η(4)-C(2)O(4))(MeOH)(2)(η(6)-p-Pr(i)C(6)H(4)Me)(2)][O(3)SCF(3)](2) (2), and [Ru(2) (p-cymene)(2) (donq) (OH(2))(2)] [O(3)SCF(3)](2) (donq = 5,8-dioxydo-1,4-naphthaquinonato) (3) acceptors and a new unsymmetrical N-(4-(pyridin-4-ylethynyl)phenyl) isonicotinamide (1) donor ligand. X-ray crystallography of 4 confirmed a molecular rectangle. The (1)H NMR spectra of both rectangles 4 and 5 showed a mixture of two structural, head-to-tail (HTL) and head-to-head (HTH) type, isomers in a 1:1 ratio. The cytotoxicities of both rectangles have been established against Colo320 (colorectal cancer), A549 (lung cancer), MCF-7(breast cancer) and H1299 (lung cancer) human cancer cell lines. The cytotoxicity of rectangle 5 was found to be considerably stronger against all cancer cell lines than that of the reference drug cisplatin.

Coordination-driven self-assembly of ruthenium-based molecular-rectangles: synthesis, characterization, photo-physical and anticancer potency studies

A suite of eight cationic, tetra-metallic molecular rectangles (1-8) was generated via coordination-driven self-assembly using four dicarboxylate-bridged arene-Ru precursors (A1-A4) with one of two dipyridyl ligands (D1, D2). The high-yielding (84-92%) rectangles were characterized by (1)H NMR and HR-ESI-MS to support their structural assignments. The molecular structure of 5 was determined by single crystal X-ray analysis, which indicated that two D2 ligands bridge two A1 acceptors to form a rectangular construct. The photophysical properties of these metalla-rectangles and their molecular precursors were also investigated, as well as an MTT assay to evaluate the in vitro cytotoxicities relative to two chemotherapeutic agents, cisplatin and doxorubicin. MTT assays were conducted using SK-hep-1 (liver cancer) and HCT-15 (colon cancer) human cancer cell lines. Compounds 3, 4, 7 and 8 showed significant activity, with IC(50) values comparable to those of cisplatin and doxorubicin.

A unique non-catenane interlocked self-assembled supramolecular architecture and its photophysical properties

A novel, interlocked, self-assembled (M(2)L(2))(2) molecular architecture was constructed from an arene-Ru acceptor and a 1,4-di(pyridin-4-yl)buta-1,3-diyne donor. Two M(2)L(2) units, with cavities of ~7.21 Å, spontaneously interlock, with one unit encapsulating a twin in a non-catenane fashion. The dimeric host-guest complex thus formed is unique among two-dimensional self-assemblies and is stabilized by π-π interactions between the M(2)L(2) units.

Self-assembly of Cationic, Hetero- or Homo-nuclear Ru(II) Macrocyclic Rectangles and their Photophysical, Electrochemical and Biological Studies

A series of supramolecular rectangles, including two mixed-metal Ru/Pt complexes, have been formed by the coordination-driven self-assembly of a range of arene-Ru "molecular clip" acceptors (1a-1d) with rigid dipyridyl-based ligands (2a-2d) over the course of 10 hours in solution. The isolated products were characterized by multinuclear NMR ((1)H and (13)C or (31)P), HR-ESI-MS and an X-ray diffraction study to support the ascribed two-component rectangular structures. The rectangles were further characterized by UV-Vis and fluorescence studies. The redox behaviors of rectangles 3ca and 3da were also determined using cyclic voltammetry. Additionally, the antitumor activities of the suite of rectangles were determined against various human cancer cell lines and significant activity was shown by complexes 3ca, 3da, 3cb, 3cc and 3cd, with IC(50) values as low as 2.65 μM.