Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Rare Earths-The Answer to Everything

Rare earths, scandium, yttrium, and the fifteen lanthanoids from lanthanum to lutetium, are classified as critical metals because of their ubiquity in daily life. They are present in magnets in cars, especially electric cars; green electricity generating systems and computers; in steel manufacturing; in glass and light emission materials especially for safety lighting and lasers; in exhaust emission catalysts and supports; catalysts in artificial rubber production; in agriculture and animal husbandry; in health and especially cancer diagnosis and treatment; and in a variety of materials and electronic products essential to modern living. They have the potential to replace toxic chromates for corrosion inhibition, in magnetic refrigeration, a variety of new materials, and their role in agriculture may expand. This review examines their role in sustainability, the environment, recycling, corrosion inhibition, crop production, animal feedstocks, catalysis, health, and materials, as well as considering future uses.

Sensitization of lanthanide complexes through direct spin-forbidden singlet → triplet excitation

Ln^III complexes may display luminescence within the ultraviolet-visible-near-infrared spectral window and although they render bright emissions mainly due to the classical singlet-triplet-state-assisted ligand sensitization, which would be the photophysical parameters if they could be excited through direct spin-forbidden singlet → triplet transitions? Herein, we report the sensitization of Ln complexes through spin-forbidden S₀ → T transitions in a series of homobimetallic Eu^III, Tb^III, Er^III, and Yb^III complexes with halogen-substituted benzoate ligands. As halogens and Ln^III atomic numbers increase, intense singlet → triplet absorption/excitation bands and relative quantum yields up to 18% were achieved due to an increased spin-orbit coupling effect. Moreover, the near-UV-shifted excitation may enable application in luminescent solar concentrators where Yb^III near-infrared luminescence matches the maximum efficiency of the crystalline Si photovoltaic cell. Therefore, the spin-relaxed excitation channel provides new opportunities to improve the Ln^III complex luminescence and potential within the energy conversion field.

Luminescent lanthanide(III) complexes of DTPA-bis(amido-phenyl-terpyridine) for bioimaging and phototherapeutic applications

We report here a series of coordinatively-saturated and thermodynamically stable luminescent [Ln(dtntp)(H₂O)] [Ln(III) = Eu (1), Tb (2), Gd (3), Sm (4) and Dy (5)] complexes using an aminophenyl-terpyridine appended-DTPA (dtntp) chelating ligand as cell imaging and photocytotoxic agents. The N,N″-bisamide derivative of H₅DTPA named as dtntp is based on 4'-(4-aminophenyl)-2,2':6',2″-terpyridine conjugated to diethylenetriamine-N,N',N″-pentaacetic acid. The structure, physicochemical properties, detailed photophysical aspects, interaction with DNA and serum proteins, and photocytotoxicity were studied. The intrinsic luminescence of Eu(III) and Tb(III) complexes due to f → f transitions used to evaluate their cellular uptake and distribution in cancer cells. The solid-state structure of [Eu(dtntp)(DMF)] (1·DMF) shows a discrete mononuclear molecule with nine-coordinated {EuN₃O₆} distorted tricapped-trigonal prism (TTP) coordination geometry around the Eu(III). The {EuN₃O₆} core results from three nitrogen atoms and three carboxylate oxygen atoms, and two carbonyl oxygen atoms of the amide groups of dtntp ligand. The ninth coordination site is occupied by an oxygen atom of DMF as a solvent from crystallization. The designed probes have two aromatic pendant phenyl-terpyridine (Ph-tpy) moieties as photo-sensitizing antennae to impart the desirable optical properties for cellular imaging and photocytotoxicity. The photostability, coordinative saturation, and energetically rightly poised triplet states of dtntp ligand allow the efficient energy transfer (ET) from Ph-tpy to the emissive excited states of the Eu(III)/Tb(III), makes them luminescent cellular imaging probes. The Ln(III) complexes show significant binding tendency to DNA (K ~ 10⁴ M^-1), and serum proteins (BSA and HSA) (K ~ 10⁵ M^-1). The luminescent Eu(III) (1) and Tb(III) (2) complexes were utilized for cellular internalization and cytotoxicity studies due to their optimal photophysical properties. The cellular uptake studies using fluorescence imaging displayed intracellular (cytosolic and nuclear) localization in cancer cells. The complexes 1 and 2 displayed significant photocytotoxicity in HeLa cells. These results offer a modular design strategy with further scope to utilize appended N,N,N-donor tpy moiety for developing light-responsive luminescent Ln(III) bioprobes for theranostic applications.

Photo-physical, theoretical and photo-cytotoxic evaluation of a new class of lanthanide(iii)–curcumin/diketone complexes for PDT application

Improved ISC in La(iii) complex of curcumin, on activation with visible light, has resulted in high yield of ¹O₂ in HeLa/MCF-7 cells, leading to the oxidative stress which was responsible for remarkable caspase 3/7-dependent apoptotic photocytotoxicity.

Synthesis, characterization, DNA/BSA interactions and in vitro cytotoxicity study of palladium(II) complexes of hispolon derivatives

Thirteen novel palladium(II) complexes of the general formula [Pd(bipy)(O,O'-dkt)](PF₆), (where bipy is 2,2'-bipyridine and O,O'-dkt is β-diketonate ligand hispolon or its derivative) have been prepared through a metal-ligand coordination method that involves spontaneous formation of the corresponding diketonate scaffold. The obtained palladium(II) complexes have been characterized by NMR spectroscopy, ESI-mass spectrometry as well as elemental analysis. The cytotoxicity analysis indicates that most of the obtained palladium(II) complexes show promising growth inhibition in three human cancer cell lines. Flow cytometry analysis shows complex 3e could promote intracellular reactive oxygen species (ROS) accumulation and lead cancer cell death. And the suppression of ROS accumulation and the rescue of cell viability in HeLa cells by N-acetyl-L-cysteine (NAC) suggest the possible link between the increase in ROS generation and cytotoxicity of complex 3e. Flow cytometry analysis also reveal that complex 3e cause cell cycle arrest in the G2/M phase and collapse of the mitochondrial membrane potential, promote the generation of ROS and lead to tumor cell apoptosis. The interactions of complex 3e with calf thymus DNA (CT-DNA) have been evaluated by UV-Vis spectroscopy, fluorescence quenching experiments and viscosity measurements, which reveal that the complex interact with CT-DNA through minor groove binding and/or electrostatic interactions. Further, the results of fluorescence titration and site marker competitive experiment on bovine serum albumin (BSA) suggest that complex 3e can quench the fluorescence of BSA via a static quenching process and bind to BSA in Sudlow's site II.

Lanthanide complexes with phenanthroline-based ligands: insights into cell death mechanisms obtained by microscopy techniques

The biological properties of four lanthanide complexes with phenanthroline derivatives in ovarian cancer cells.

Complexes of lanthanides(iii) with mixed 2,2′-bipyridyl and 5,7-dibromo-8-quinolinoline chelating ligands as a new class of promising anti-cancer agents

MeOMBrQ-Ho induced HeLa cell apoptosis was mediated by inhibition of telomerase activity and dysfunction of mitochondria. Remarkably, MeOMBrQ-Ho obviously inhibited HeLa xenograft tumor growth in vivo.

Click Chemistry-Assisted Synthesis of a β-d-Galactose-Targeted SiO2@RC Shell-Core Structure as a Nanoplatform for Metal-Based Complex Delivery

A facile reversed-phase microemulsion method was used to synthesize shell-core nanospheres of SiO₂@RCs (SiO₂-encapsuled rare-earth metal complexes). β-d-Galactose was then grafted onto the surfaces of the nanospheres through the copper(I)-catalyzed azide-alkyne cycloaddition click reaction for targeted delivery. The chemical characteristics and surface profiles of the nanocarriers were investigated by Fourier transform infrared spectroscopy, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy. A high-efficiency microwave synthesis method was applied to prepare five complex cores by the reaction of different rare-earth metal salts with two isomeric ligands, o-CPA (2-chlorophenoxyacetic acid) and m-CPA (3-chlorophenoxyacetic acid). The crystal structures of the five synthesized RC cores were confirmed through X-ray diffraction, which revealed the formulas of five RCs, [Dy( o-CPA)₃(H₂O)]·H₂O RC₁, [Ho( o-CPA)₃(H₂O)]·H₂O RC₂, 2[Er( m-CPA)₃(H₂O)]·3H₂O RC₃, 2[Gd( m-CPA)₃(H₂O)]·3H₂O RC₄, and [Ce₂( m-CPA)₆(H₂O)₃]·2H₂O RC₅. An in vitro cell study revealed that all RCs exhibited certain anticancer activities. RC₂, in particular, showed the strongest cytotoxicity against HepG2 cells. The enhanced cell permeability and drug retention considerably improved the cytotoxicity of all SiO₂@RC₂-gal relative to that of RC₂. The selective uptake of the β-d-galactose-conjugated nanospheres by HepG2 cells through mechanisms mediated by cell surface receptors resulted in fewer side effects on extrahepatic tissues. Our contribution provides a novel design concept of a target SiO₂@RCs-gal nanocarrier for delivering affordable antitumor complexes in cancer therapy.

Photosensitized samarium(iii) and erbium(iii) complexes of planar N,N-donor heterocyclic bases: crystal structures and evaluation of biological activity

A series of Sm(iii) and Er(iii) complexes of N,N-donor heterocyclic bases were studied for their crystal structures, luminescence properties, binding with biomolecules and photo-induced DNA damage activity.

Mitochondrial selectivity and remarkable photocytotoxicity of a ferrocenyl neodymium(iii) complex of terpyridine and curcumin in cancer cells

A mixed-ligand neodymium(iii) complex of ferrocene appended terpyridine and curcumin targets the mitochondria and shows remarkable visible-light induced cytotoxicity in HeLa and MCF-7 cancer cells while being much less toxic in dark and to MCF-10A normal cells.

Ruthenium(ii) complexes with dppz: from molecular photoswitch to biological applications

The present article describes the recent advances in biological applications of the Ru-dppz systems in DNA binding, cellular imaging, anticancer drugs, phototherapy, protein aggregation detecting and chemosensors.

Targeted photoresponsive TiO2–coumarin nanoconjugate for efficient combination therapy in MDA-MB-231 breast cancer cells: synergic effect of photodynamic therapy (PDT) and anticancer drug chlorambucil

We have developed for the first time an excellent targeted metallic single component nanoparticle system for combination of PDT and chemotherapy.

Luminescent europium and terbium complexes of dipyridoquinoxaline and dipyridophenazine ligands as photosensitizing antennae: structures and biological perspectives

Luminescent europium and terbium complexes of quinoxaline and phenazine ligands were studied for their structures, luminescence properties, interaction with DNA, and photo-induced DNA cleavage activity.

Enhancing the photocytotoxic potential of curcumin on terpyridyl lanthanide(III) complex formation

Lanthanide(III) complexes [Ln(R-tpy)(cur)(NO3)2] (Ln = La(III) in 1, 2; Gd(III) in 5, 6) and [Ln(R-tpy)(scur)(NO3)2] (Ln = La(III) in 3, 4; Gd(III) in 7, 8), where R-tpy is 4′-phenyl-2,2′:6′,2′′-terpyridine (ph-tpy in 1, 3, 5, 7), 4′-(1-pyrenyl)-2,2′:6′,2′′-terpyridine (py-tpy in 2, 4, 6, 8), Hcur is curcumin (in 1, 2, 5, 6) and Hscur is diglucosylcurcumin (in 3, 4, 7, 8), were prepared and their DNA photocleavage activity and photocytotoxicity studied. Complexes [La(ph-tpy)(cur)(NO3)2] (1) and [Gd(ph-tpy)(cur)(NO3)2] (5) were structurally characterized. The complexes in aqueous-DMF showed an absorption band near 430 nm and an emission band near 515 nm when excited at 420 nm. The complexes are moderate binders to calf-thymus DNA. They cleave plasmid supercoiled DNA to its nicked circular form in UV-A (365 nm) and visible light (454 nm) via (1)O2 and ˙OH pathways. The complexes are remarkably photocytotoxic in HeLa cells in visible light (λ = 400–700 nm) and are non-toxic in the dark. FACScan analysis of the HeLa cells treated with 2 and 4 showed cell death via an apoptotic pathway. Nuclear localization of 1–4 is evidenced from confocal imaging on HeLa cells. The hydrolytic instability of curcumin gets significantly reduced upon binding to the lanthanide ions while retaining its photocytotoxic potential.