[Fe₂L₃]⁴⁺ cylinders derived from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands: synthesis, structures and exploration of biological activity

Dicobalt(ii) helices kill colon cancer cells via enantiomer-specific mechanisms; DNA damage or microtubule disruption

Highly diastereoselective self-assembly reactions give both enantiomers (Λ and Δ) of anti-parallel triple-stranded bimetallic Co(ii) and Co(iii) cationic helices, without the need for resolution; the first such reaction for Co. The complexes are water soluble and stable, even in the case of Co(ii). Studies in a range of cancer and healthy cell lines indicate high activity and selectivity, and substantial differences between enantiomers. The oxidation state has little effect, and correspondingly, Co(iii) compounds are reduced to Co(ii) e.g. by glutathione. In HCT116 colon cancer cells the Λ enantiomer induces dose-dependent G2-M arrest in the cell cycle and disrupts microtubule architectures. This Co(ii) Λ enantiomer is ca. five times more potent than the isostructural Fe(ii) compound. Since the measured cellular uptakes are similar this implies a higher affinity of the Co system for the intracellular target(s); while the two systems are isostructural they have substantially different charge distributions as shown by calculated hydrophobicity maps. In contrast to the Λ enantiomer, Δ-Co(ii) induces G1 arrest in HCT116 cells, efficiently inhibits the topoisomerase I-catalyzed relaxation of supercoiled plasmid DNA, and, unlike the isostructural Fe(ii) system, causes DNA damage. It thus seems very likely that redox chemistry plays a role in the latter.

Interaction of dinuclear Co(III) cylinders with higher-order DNA structures

Alternative DNA structures play critical roles in fundamental biological processes linked to human diseases. Thus, targeting and stabilizing these structures by specific ligands could affect the progression of cancer and other diseases. Here, we describe, using methods of molecular biophysics, the interactions of two oxidatively locked [Co2L3]6+ cylinders, rac-2 and meso-1, with diverse alternative DNA structures, such as junctions, G quadruplexes, and bulges. This study was motivated by earlier results demonstrating that both Co(III) cylinders exhibit potent and selective activity against cancer cells, accumulate in the nucleus of cancer cells, and prove to be efficient DNA binders. The results show that the bigger cylinder rac-2 stabilizes all DNA structures, while the smaller cylinder meso-1 stabilizes just the Y-shaped three-way junctions. Collectively, the results of this study suggest that the stabilization of alternative DNA structures by Co(III) cylinders investigated in this work might contribute to the mechanism of their biological activity.

Synthesis and crystal structure of a PdII complex of ortho-xylylenebis(pyridyl-triazole)

A tetra-dentate ligand, namely, ortho-xylylenebis(pyridyl-triazole), o-xpt, was synthesized using the 'click' method and complexed with Pd(BF₄)₂. In the title com-plex, bis-{μ-1,1'-(o-xylylene)bis-[4-(pyridin-2-yl)triazole]-κN ³:N ^3'}dipalladi-um(II) tetra-kis-(tetrafluoridoborate)-di-methyl-formamide-diethyl ether (1/2/1), as the BF₄ salt, and including di-methyl-formamide and diethyl ether solvent mol-ecules, with stoichiometry [Pd₂(C₂₂H₁₈N₈)₂](BF₄)₄·2C₃H₇NO·C₄H₁₀O, the Pd complex and the disordered diethyl ether mol-ecule lie on inversion centers. The ligand coordinates to the Pd^II centers with square-planar geometry, forming a dimeric macrocycle. The Pd⋯Pd separation in the complex [Pd₂(o-xpt)₂]⁴⁺ cation is 3.6184 (4) Å. In the crystal, the complex mol-ecules are stacked along the b axis, with π-π inter-actions between the pyridyl-triazole ligands of two mol-ecules.

New bis- and tetrakis-1,2,3-triazole derivatives: Synthesis, DNA cleavage, molecular docking, antimicrobial, antioxidant activity and acid dissociation constants

In this study, a series of bis- and tetrakis-1,2,3-triazole derivatives were synthesized using copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry in 73-95% yield. The bis- and tetrakis-1,2,3-triazoles exhibited significant DNA cleavage activity while the tetrakis-1,2,3-triazole analog 6g completely degraded the plasmid DNA. Molecular docking simulations suggest that compound 6g acts as minor groove binder of DNA by binding through several noncovalent interactions with base pairs. All bis- and tetrakis-1,2,3-triazole derivatives were screened for antibacterial activity against E. coli, B. cereus, S. aureus, P. aeruginosa, E. hirae, L. pneumophila subsp. pneumophila strains and antifungal activity against microfungus C. albicans and C. tropicalis strains. Compound 4d exhibited the best antibacterial activity among bis-1,2,3-triazoles against E. coli and E. hirae, while 6c exhibited the best antibacterial activity among tetrakis-1,2,3-triazoles against E. hirae. Furthermore, the best antifungal activity against C. albicans and C. tropicalis was reported for the compound 5, while 6d displayed the best antifungal activity against C. tropicalis and C. albicans. Reasonable iron chelating activities and DPPH radical scavenging abilities were found for some of the compounds. Finally, the acid dissociation constants (pK_a) of the bis-1,2,3-triazoles were also determined with the help of HYPERQUAD program using the data obtained from potentiometric titrations. The reported data here concludes that the bis- and tetrakis-1,2,3-triazoles are important cores that should be considered for further development of especially new anticancer agents acting through the DNA cleavage activity.

Cavity-Containing [Fe2L3]4+ Helicates: An Examination of Host-Guest Chemistry and Cytotoxicity

Two new di(2,2′-bipyridine) ligands, 2,6-bis([2,2′-bipyridin]-5-ylethynyl)pyridine (L1) and bis(4-([2,2′-bipyridin]-5-ylethynyl)phenyl)methane (L2) were synthesized and used to generate two metallosupramolecular [Fe₂(L)₃](BF₄)₄ cylinders. The ligands and cylinders were characterized using elemental analysis, electrospray ionization mass spectrometry, UV-vis, ¹H-, ¹³C and DOSY nuclear magnetic resonance (NMR) spectroscopies. The molecular structures of the [Fe₂(L)₃](BF₄)₄ cylinders were confirmed using X-ray crystallography. Both the [Fe₂(L1)₃](BF₄)₄ and [Fe₂(L2)₃](BF₄)₄ complexes crystallized as racemic (rac) mixtures of the ΔΔ (P) and ΛΛ (M) helicates. However, ¹H NMR spectra showed that in solution the larger [Fe₂(L2)₃](BF₄)₄ was a mixture of the rac-ΔΔ/ΛΛ and meso-ΔΛ isomers. The host-guest chemistry of the helicates, which both feature a central cavity, was examined with several small drug molecules. However, none of the potential guests were found to bind within the helicates. In vitro cytotoxicity assays demonstrated that both helicates were active against four cancer cell lines. The smaller [Fe₂(L1)₃](BF₄)₄ system displayed low μM activity against the HCT116 (IC₅₀ = 7.1 ± 0.5 μM) and NCI-H460 (IC₅₀ = 4.9 ± 0.4 μM) cancer cells. While the antiproliferative effects against all the cell lines examined were less than the well-known anticancer drug cisplatin, their modes of action would be expected to be very different.

Triazole-based, optically-pure metallosupramolecules; highly potent and selective anticancer compounds

Functionalised triazole aldehydes are used in the highly selective self-assembly of water-compatible, optically pure, low symmetry Fe(ii)- and Zn(ii)-based metallohelices. Sub-micromolar antiproliferative activity is observed against various cancerous cell lines, accompanied by excellent selectivity versus non-cancerous cells and potential for synergistic combinatorial therapy with cisplatin.

Metallohelices emulate the properties of short cationic α-helical peptides

Naturally occurring peptides in many living systems perform antimicrobial and anticancer host defence roles, but their potential for clinical application is limited by low metabolic stability and relatively high costs of goods. Self-assembled helical metal complexes provide an attractive synthetic platform for non-peptidic architectures that can emulate some of the properties of short cationic α-helical peptides, with tuneable charge, shape, size and amphipathicity. Correspondingly there is a growing body of evidence demonstrating that these supramolecular architectures exhibit bioactivity that emulates that of the natural systems. We review that evidence in the context of synthetic advances in the area, driven by the potential for biomedical applications. We note some design considerations for new biologically-relevant metallohelices, and give our outlook on the future of these compounds as therapeutic peptidomimetics.

Covalent Post-Assembly Modification: A Synthetic Multipurpose Tool in Supramolecular Chemistry

The use of covalent post-assembly modification (PAM) in supramolecular chemistry has grown significantly in recent years, to the point where PAM is now a versatile synthesis tool for tuning, modulating, and expanding the functionality of self-assembled complexes and materials. PAM underpins supramolecular template-synthesis strategies, enables modular derivatization of supramolecular assemblies, permits the covalent 'locking' of unstable structures, and can trigger controlled structural transformations between different assembled morphologies. This Review discusses key examples of PAM spanning a range of material classes, including discrete supramolecular complexes, self-assembled soft nanostructures and hierarchically ordered polymeric and framework materials. As such, we hope to highlight how PAM has continued to evolve as a creative and functional addition to the synthetic chemist's toolbox for constructing bespoke self-assembled complexes and materials.

Glycoconjugated Metallohelices have Improved Nuclear Delivery and Suppress Tumour Growth In Vivo

Monosaccharides are added to the hydrophilic face of a self-assembled asymmetric FeII metallohelix, using CuAAC chemistry. The sixteen resulting architectures are water-stable and optically pure, and exhibit improved antiproliferative selectivity against colon cancer cells (HCT116 p53+/+ ) with respect to the non-cancerous ARPE-19 cell line. While the most selective compound is a glucose-appended enantiomer, its cellular entry is not mainly glucose transporter-mediated. Glucose conjugation nevertheless increases nuclear delivery ca 2.5-fold, and a non-destructive interaction with DNA is indicated. Addition of the glucose units affects the binding orientation of the metallohelix to naked DNA, but does not substantially alter the overall affinity. In a mouse model, the glucose conjugated compound was far better tolerated, and tumour growth delays for the parent compound (2.6 d) were improved to 4.3 d; performance as good as cisplatin but with the advantage of no weight loss in the subjects.

Ru(II) coordination compounds of N-N bidentate chelators with 1,2,3 triazole and isoquinoline subunits: Synthesis, spectroscopy and antimicrobial properties

Bidentate chelators 1-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline and 3-(1-benzyl-1,2,3-triazol-4-yl)isoquinoline were prepared from benzyl bromide and trimethylsilylethynylisoquinoline precursors using a tandem deprotection/substitution/CuAAC synthetic approach. Each chelator is capable of forming a stable 3:1 Ru(II) coordination compound, which forms as a geometric isomer mixture. These Ru(II) complexes possess unique MLCT absorbance signatures at 450/472 nm (1-isomer) and 367 nm (3-isomer) relative to their constituent chelating units. Minimum inhibitory concentration values as low as 0.4 μM are observed for Ru(II) complexes against representative Gram-positive bacteria Bacillus subtilis and Staphylococcus epidermidis. Comparing the MIC values of these isoquinoline compounds with analogous 2-(1-benzyl-1,2,3-triazol-4-yl)pyridine compounds shows a 2.5- to 40-fold improvement in potency. This study establishes that increased hydrophobicity introduced at the central chelating units of Ru(II) coordination compounds can be a useful means by which to optimize antimicrobial activity that is complimentary to the variation of peripheral substituent identity at the chelator's N1 triazole position.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Biomedical Applications of Metallosupramolecular Assemblies-Structural Aspects of the Anticancer Activity

The design and development of metallosupramolecular systems has resulted in construction of a myriad of fascinating structures with highly diverse properties and potential applications. Assessment of the biomedical applications of metallosupramolecular assemblies is an emerging field of research that stems from the recently demonstrated promising results on such systems. After the pioneering works of Therrien and coworkers on organometallic Ru-cages with promising anticancer properties, this topic has evolved to the more recent studies on bioactivity of supramolecular coordination complexes built from different metal ions and various multidentate ligands. Sufficient amount of data on the anticancer activity of metallosupramolecules has already been reported and allows outlining some general tendencies in the structural aspects of the biological activity. The main structural properties of the complexes that can be readily modified to enhance their activity are the size, the shape and charge of the formed complexes. Moreover, the intrinsic properties of the building components could predetermine some of the main characteristics of the overall supramolecular complex, such as its optical properties, chemical reactivity, solubility, etc., and could, thereby, define the areas of its biomedical applications. The unique structural property of most of the metallosupramolecular assemblies, however, is the presence of a discrete cavity that renders a whole range of additional applications resulting from specific host-guest interactions. The encapsulations of small bioactive or fluorescent molecules have been employed for delivery or recognition purposes in many examples. On the other hand, metallosupramolecules have been imbedded into target-specific polymeric nanoparticles that resulted in a successful combination of their therapeutic and diagnostic properties, making them promising for theranostic application in cancer treatment. The aim of this review paper is to mark out some key tendencies in the reported metallosupramolecular structures in relation with their biological activity and potential areas of biomedical application. In this way, a useful set of guidelines can be delineated to help synthetic chemists broaden the application areas of their supramolecular systems by few structural changes.

To Loop or Not to Loop: Influence of Hinge Flexibility on Self-Assembly Outcomes for Acridine-Based Triazolylpyridine Chelates with Zinc(II), Iron(II), and Copper(II)

Coordination-driven self-assembly has been established as an effective strategy for the efficient construction of intricate architectures in both natural and artificial systems, for applications ranging from gene regulation to metal-organic frameworks. Central to these systems is the need for carefully designed organic ligands, generally with rigid components, that can undergo self-assembly with metal ions in a predictable manner. Herein, we report the synthesis and study of three novel organic ligands that feature 3,6-disubstituted acridine as a rigid spacer connected to two 2-(1,2,3-triazol-4-yl)pyridine "click" chelates through hinges of the same length but differing flexibility. The flexibility of these "three-atom" hinges was modulated by i) moving from secondary to tertiary amide functional groups and ii) replacing an sp² amide carbon with an sp³ methylene carbon. In an effort to understand the role of hinge flexibility in directing self-assembly into mononuclear loops or dinuclear cylinders, the impact of these changes on self-assembly outcomes with zinc(II), iron(II), and copper(II) ions is described.

Antiproliferative properties of iron supramolecular cylinders

The use of metallohelicates as potential antiproliferative agents is mostly exemplified by one sole family of supramolecular compounds that is based on bis-iminopyridine ligands. In the present investigation, two other types of metallocylinders have been selected and their potential DNA-binding and cytotoxic properties have been investigated. Hence, two new neutral iron(III) metallosupramolecular compounds have been prepared from bis-β-diketone ligands, and a known cationic iron(II) helicate from bis-pyrazole ligands has been used for comparison purposes. DNA-interaction experiments and cell studies reveal remarkable biological properties for one of the neutral iron cylinders and the positively charged, pyrazole-based helicate, as illustrated by their antiproliferative behaviours, which are far better than those of two well-known compounds, i.e. the most studied metallohelicate in the field and cisplatin.

Au(III) and Pt(II) Complexes of a Novel and Versatile 1,4-Disubstituted 1,2,3-Triazole-Based Ligand Possessing Diverse Secondary and Tertiary Coordinating Groups

A novel 1,4-disubstituted 1,2,3-triazole-based ligand, 2-(4-(pyridin-2-yl)-1 H-1,2,3-triazol-1-yl)propane-1,3-diamine (ptpd), which possesses pyridyl and diamino secondary and tertiary coordinating groups was synthesized in excellent yield. The reactivity of 2-(1-phenyl-1 H-1,2,3-triazol-4-yl)pyridine (ptp), di- tert-butyl (2-azidopropane-1,3-diyl)dicarbamate (Boc₂-ptpd), and ptpd·3HCl was investigated with Au(III) and Pt(II) precursors. Analysis including X-ray crystal structures of [Au(III)Cl₃(ptp)] (1), [Au(III)Cl₂(ptpd)][Au(I)Cl₂][OH]{[NaAuCl₄·2H₂O]} n (3), and [Pt(II)Cl₂(ptpdH₂)][PtCl₄] (4) revealed that ptpd (i) serves as a monodentate ligand for Au(III) coordinating to the metal center via the pyridine nitrogen only, (ii) preferentially coordinates Au(III) via the bidentate diamino group over the monodentate pyridine group, (iii) can coordinate Pt(II) in a bidentate fashion via the pyridyl nitrogen and the triazole N-3, and (iv) can bridge two Pt(II) centers through bidentate chelation at the diamino group and bidentate chelation via the pyridyl nitrogen and the triazole N-3. ptpd represents a versatile ligand template for the development of mixed metal complexes.

Computational investigations of click-derived 1,2,3-triazoles as keystone ligands for complexation with transition metals: a review

In recent years, metal complexes of organo 1,2,3-triazole click-derived ligands have attracted significant attention as catalysts in many chemical transformations and also as biological and pharmaceutical active agents. Regarding the important applications of these metal-organo 1,2,3-triazole-based complexes, in this review, we focused on the recently reported investigations of the structural, electronic, and spectroscopic aspects of the complexation process in transition metal complexes of 1,2,3-triazole-based click ligands. In line with this, the coordination properties of these triazole-based click ligands with transition metals were studied via several quantum chemistry calculations. Moreover, considering the complexation process, we have presented comparative discussions between the computational results and the available experimental data.

Chiral metallosupramolecular architectures

Over the past few decades, supramolecular chirality in discrete metallosupramolecular architectures has received considerable attention. In this review, a comprehensive summary of discrete, chiral coordination-driven structures, including helices, metallacycles, metallocages, etc., is presented. Although chirality can be introduced prior to, during or even after the coordination self-assembly process, this review puts major emphasis on the more recent development of metallosupramolecular architectures from chiral components, where chirality arises from the enantiopure or racemic scaffolds (bridging or auxiliary ligand). Special attention will be paid to homochiral metallo-assemblies using achiral components where chirality is obtained as a consequence of the twisting of the ligands. Additionally, the potential applications of homochiral metallosupramolecular architectures are also discussed. We hope that this review will be of interest to researchers attempting to design new elaborate homochiral metallosupramolecular architectures with even greater complexity and potential for functions such as chiral recognition, enantiomer separation, asymmetric catalysis, nonlinear sensors, and devices.

Functional metal complexes from CuAAC “click” bidentate and tridentate pyridyl-1,2,3-triazole ligands

This Frontiers article examines the use of “click” complexes for the development of catalysts, anti-cancer and anti-bacterial agents and emissive materials.

Self-Assembly with 2,6-Bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine: Silver(I) and Iron(II) Complexes

A new "click" ligand, 2,6-bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine (L) featuring a tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (tripy) pocket and two pyridyl (py) units was synthesized in modest yield (42%) using the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The coordination chemistry of the ligand with silver(I) and iron(II) ions was examined using a battery of solution (¹H and DOSY (diffusion ordered spectroscopy) nuclear magnetic resonance (NMR), infrared and absorption spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)), and solid state (X-ray crystallography, elemental analysis) techniques. When treated with silver(I) ions, the ligand forms discrete [Ag(L)]⁺ (X-, where X- = BF₄-, NO₃- or SbF₆-) complexes in dimethyl sulfoxide (DMSO) solution but these complexes crystallize as coordination polymers. The addition of [Fe(H₂O)₆](BF₄)₂ to an acetonitrile solution of the ligand forms the expected monomeric octahedral [Fe(L)₂]2+ complex and treatment of the iron(II) complex with AgBF₄ generates a heterometallic linear coordination polymer.

Fe(ii)-Complexation of tripodal hexapeptide ligands with three bidentate triazolylpyridines: induction of metal-centred chirality by peptide macrocyclization

Cyclic and open-chain hexapeptides with three common triazolyl-pyridine residues afforded two Fe(ii) complexes with distinct metal-centred chirality.

Oxidatively Locked [Co₂L₃]6+ Cylinders Derived from Bis(bidentate) 2-Pyridyl-1,2,3-triazole "Click" Ligands: Synthesis, Stability, and Antimicrobial Studies

A small family of [Co₂(Lpytrz)₃]6+ cylinders was synthesised from bis(bidentate) 2-pyridyl-1,2,3-triazole "click" ligands (Lpytrz) through an "assembly-followed-by-oxidation" method. The cylinders were characterised using ¹H, 13C, and DOSY NMR, IR, and UV-Vis spectroscopies, along with electrospray ionisation mass spectrometry (ESMS). Stability studies were conducted in dimethyl sulfoxide (DMSO) and D₂O. In contrast to similar, previously studied, [Fe₂(Lpytrz)₃]4+ helicates the more kinetically inert [Co₂(Lpytrz)₃]6+ systems proved stable (over a period of days) when exposed to DMSO and were even more stable in D₂O. The triply stranded [Co₂(Lpytrz)₃]6+ systems and the corresponding "free" ligands were tested for antimicrobial activity in vitro against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) microorganisms. Agar-based disk diffusion and Mueller-Hinton broth micro-dilution assays showed that the [Co₂(Lpytrz)₃]6+ cylinders were not active against either strain of bacteria. It is presumed that a high charge of the [Co₂(Lpytrz)₃]6+ cylinders is preventing them from crossing the bacterial cell membranes, rendering the compounds biologically inactive.

Intramolecular Electron Transfers Thwart Bistability in a Pentanuclear Iron Complex

With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(μ-bpp)3}2Mn(III)Mn(II)2(μ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(μ-bpp)3}2Fe(III)Fe(II)2(μ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(μ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

Enhanced kinetic stability of [Pd2L4]4+ cages through ligand substitution

[Pd₂(tripy)₄]⁴⁺ cage architectures (where tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine) were made more kinetically robust in the presence of range of nucleophiles by the addition of amino groups in either the 2-(2A-tripy) or 3-(3A-tripy) positions of the tripy ligands' terminal pyridines, with the [Pd₂(2A-tripy)₄]⁴⁺ cage proving the most stable.

Crystal structure of tris-(μ-bis-{4-[(pyridin-2-yl-methyl-idene)amino]-phen-yl}methane-κ(4) N,N':N'',N''')dizinc tetra-kis-(tetra-fluorido-borate) aceto-nitrile tris-olvate

The asymmetric unit of the title compound, [Zn2(C25H20N4)3](BF4)4·3CH3CN, consists of one dinuclear Zn(II) complex cation with a triple-helical [Zn2 L 3](4+) motif (L is bis-{4-[(pyridin-2-yl-methyl-idene)amino]-phen-yl}methane), four BF4 (-) anions and three CH3CN solvent mol-ecules. The Zn⋯Zn separation is 11.3893 (14) Å and the ligands wrap around the two Zn(II) atoms, forming a triple helix as defined by the Zn-N-N-Zn torsion angles of 104.05 (18), 99.06 (19) and 101.40 (19)°. The Zn-N(pyrid-yl) distances in the octahedral ZnN6 coordination sphere are in the range 2.128 (5)-2.190 (5) Å and the Zn-N(imine) distances are in the range 2.157 (5)-2.277 (5) Å.

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.

Biologically active [Pd2L4](4+) quadruply-stranded helicates: stability and cytotoxicity

There is emerging interest in the anti-proliferative effects of metallosupramolecular systems due to the different size and shape of these metallo-architectures compared to traditional small molecule drugs. Palladium(II)-containing systems are the most abundant class of metallosupramolecular complexes, yet their biological activity has hardly been examined. Here a small series of [Pd2(L)4](BF4)4 quadruply-stranded, dipalladium(II) architectures were screened for their cytotoxic effects against three cancer cell lines and one non-malignant line. The helicates exhibited a range of cytotoxic properties, with the most cytotoxic complex [Pd2(hextrz)4](BF4)4 possessing low micromolar IC50 values against all of the cell lines tested, while the other helicates displayed moderate or no cytotoxicity. Against the MDA-MB-231 cell line, which is resistant to platinum-based drugs, [Pd2(hextrz)4](BF4)4 was 7-fold more active than cisplatin. Preliminary mechanistic studies indicate that the [Pd2(hextrz)4](BF4)4 helicate does not induce cell death in the same way as clinically used metal complexes such as cisplatin. Rather than interacting with DNA, the helicate appears to disrupt the cell membrane. These studies represent the first biological characterisation of quadruply-stranded helicate architectures, and provide insight into the design requirements for the development of biologically active and stable palladium(II)-containing metallosupramolecular architectures.

Comparison of inverse and regular 2-pyridyl-1,2,3-triazole "click" complexes: structures, stability, electrochemical, and photophysical properties

Two inverse 2-pyridyl-1,2,3-triazole "click" ligands, 2-(4-phenyl-1H-1,2,3-triazol-1-yl)pyridine and 2-(4-benzyl-1H-1,2,3-triazol-1-yl)pyridine, and their palladium(II), platinum(II), rhenium(I), and ruthenium(II) complexes have been synthesized in good to excellent yields. The properties of these inverse "click" complexes have been compared to the isomeric regular compounds using a variety of techniques. X-ray crystallographic analysis shows that the regular and inverse complexes are structurally very similar. However, the chemical and physical properties of the isomers are quite different. Ligand exchange studies and density functional theory (DFT) calculations indicate that metal complexes of the regular 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine (R = phenyl, benzyl) ligands are more stable than those formed with the inverse 2-(4-R-1H-1,2,3-triazol-1-yl)pyridine (R = phenyl, benzyl) "click" chelators. Additionally, the bis-2,2'-bipyridine (bpy) ruthenium(II) complexes of the "click" chelators have been shown to have short excited state lifetimes, which in the inverse triazole case, resulted in ejection of the 2-pyridyl-1,2,3-triazole ligand from the complex. Under identical conditions, the isomeric regular 2-pyridyl-1,2,3-triazole ruthenium(II) bpy complexes are photochemically inert. The absorption spectra of the inverse rhenium(I) and platinum(II) complexes are red-shifted compared to the regular compounds. It is shown that conjugation between the substituent group R and triazolyl unit has a negligible effect on the photophysical properties of the complexes. The inverse rhenium(I) complexes have large Stokes shifts, long metal-to-ligand charge transfer (MLCT) excited state lifetimes, and respectable quantum yields which are relatively solvent insensitive.

Chloride triggered reversible switching from a metallosupramolecular [Pd2L4]4+ cage to a [Pd2L2Cl4] metallo-macrocycle with release of endo- and exo-hedrally bound guests

A metallosupramolecular [Pd₂L₄]⁴⁺ cage can be cleanly, and reversibly, converted into a [Pd₂L₂Cl₄] metallo-macrocycle upon addition or removal of chloride ions.

Non-equilibrium cobalt(iii) "click" capsules

Cobalt(iii) tetrahedral capsules have been prepared using an assembly-followed-by-oxidation protocol from a cobalt(ii) precursor and a readily derivatizable pyridyl-triazole ligand system. Experiments designed to probe the constitutional dynamics show that these architectures are in a non-equilibrium state. A preliminary investigation into the host-guest chemistry of a water-soluble derivative shows it can bind and differentiate a range of different neutral organic molecules. The stability of this ensemble also permits the study of guest-binding at high salt concentrations.

Systematic study of the synthesis and coordination of 2-(1,2,3-triazol-4-yl)-pyridine to Fe(ii), Ni(ii) and Zn(ii); ion-induced folding into helicates, mesocates and larger architectures, and application to magnetism and self-selection

The 2-pyridyl–1,2,3-triazole chelate was found to display unique coordination properties.