Noninnocence of Indigo: Dehydroindigo Anions as Bridging Electron-Donor Ligands in Diruthenium Complexes

Tetracoordinated 15-Electron Ruthenium(I) in a Discrete Triruthenium Framework

This paper highlights the unique case of a tetracoordinated Ru(I) (15-electron) component in a structurally characterized discrete triruthenium setup, [(acac)2RuIIIL1(μ-RuI)L1RuII (acac)2](ClO4)2 ([3](ClO4)2, where acac = acetylacetonate; S = 1), which was formed along with the monomeric [(acac)2RuIII(L1)] ([1]ClO4; S = 1/2) and dimeric [{(acac)2RuIII}2(μ-L1)](ClO4)2 ([2](ClO4)2; S = 1) counterparts upon interaction of {Ru(acac)2} and L1 = 3,3'-dipyridin-2-yl-1,1'-bis(imidazo[1,5-a]pyridinyl).

Bidirectional noninnocence of hinge-like deprotonated bis-lawsone on selective ruthenium platform: a function of varying ancillary ligands

The present work aimed to obtain discrete heavier metal complexes of unperturbed deprotonated bis-lawsone (hinge-like H₂L = 2,2'-bis(3-hydroxy-1,4-napthoquinone). This is primarily due to its limited examples with lighter metal ions (Co, Zn, and Ga) and the fact that our earlier approach with the osmium ion facilitated its functionalisation. Herein, we demonstrated the successful synthesis and structural characterisation of L^2--derived diruthenium [(bpy)₂Ru^II(μ-L^2-)Ru^II(bpy)₂](ClO₄)₂ [1](ClO₄)₂ (S = 0), (acac)₂Ru^III(μ-L^2-)Ru^III(acac)₂2 (S = 1) and monoruthenium (pap)₂Ru(L^2-) 3 (S = 0) derivatives (bpy = 2,2'-bipyridine, acac = acetylacetonate, and pap = 2-phenylazopyridine). The crystal structures established that (i) O,O^-/O,O^- donating five-membered bis-bidentate and O^-,O^- donating seven-membered bidentate chelating modes of deprotonated L^2- in rac (ΔΔ/ΛΛ) diastereomeric [1](ClO₄)₂, 2 and 3, respectively. (ii) The L^2- bridging unit in [1](ClO₄)₂, 2 and 3 underwent twisting its two naphthoquinone rings with respect to the ring connecting C-C bond by 73.01°, 62.15° and 59.12°, respectively. (iii) Intermolecular π-π interactions (∼3.5 Å) between the neighbouring molecules. The paramagnetic complex 2 (S = 1) with two non-interacting Ru(III) (S = 1/2) ions exhibited weak antiferromagnetic coupling only at very low temperatures. In agreement with the magnetic results, 2 displayed typical Ru^III-based anisotropic EPR in CH₃CN (<g>/Δg: 2.314/0.564) but without any forbidden g_1/2 signal at 120 K. The complexes exhibited multiple redox processes in CH₃CN in the experimental potential window of ± 2.0 V versus SCE. The analysis of the redox steps via a combined experimental and theoretical (DFT/TD-DFT) approach revealed the involvement of L^2- to varying extents in both the oxidative and reductive processes as a consequence of its bidirectional redox non-innocent feature. The mixing of the frontier orbitals of the metal ion and L^2- due to their closeness in energy indeed led to the resonating electronic form in certain redox states instead of any precise electronic structural state.

On the non-innocence and reactive versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Slightly modified ligand designs in diruthenium setups have major impacts on the reactivity/stability of coordination complexes. The 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione bridge is also potentially redox non-innocent.

Strong magnetic coupling of spins in Fe(ii) dimers with differently charged thioindigo ligands

A first coordination {[2.2.2]cryptand(K+)}2{FeII(TI˙-)(TI2-)}2·2C6H4Cl2 (1) complex of iron(ii) containing radical anions and dianions of thioindigo (TI) was obtained. The complex has two high-spin FeII centers bound by two oxygen atoms, and the TI˙- radical anions are coordinated to each FeII. As a result, the 4-spin system consisting of TI˙- (S = 1/2)-FeII (S = 2)-FeII (S = 2)-TI˙- (S = 1/2) coupled spins is formed within a dimer with strong FeII-FeII (J = -51.1 cm-1) and weaker FeII-TI˙- interactions of (J = -35.4 cm-1).

Preclinical Anticancer Activity of an Electron-Deficient Organoruthenium(II) Complex

Ruthenium compounds have been shown to be promising alternatives to platinum(II) drugs. However, their clinical success depends on achieving mechanisms of action that overcome Pt-resistance mechanisms. Electron-deficient organoruthenium complexes are an understudied class of compounds that exhibit unusual reactivity in solution and might offer novel anticancer mechanisms of action. Here, we evaluate the in vitro and in vivo anticancer properties of the electron-deficient organoruthenium complex [(p-cymene)Ru(maleonitriledithiolate)]. This compound is found to be highly cytotoxic: 5 to 60 times more potent than cisplatin towards ovarian (A2780 and A2780cisR), colon (HCT116 p53+/+ and HCT116 p53-/-), and non-small cell lung H460 cancer cell lines. It shows no cross-resistance and is equally cytotoxic to both A2780 and A2780cisR cell lines. Furthermore, unlike cisplatin, the remarkable in vitro antiproliferative activity of this compound appears to be p53-independent. In vivo evaluation in the hollow-fibre assay across a panel of cancer cell types and subcutaneous H460 non-small cell lung cancer xenograft model hints at the activity of the complex. Although the impressive in vitro data are not fully corroborated by the in vivo follow-up, this work is the first preclinical study of electron-deficient half-sandwich complexes and highlights their promise as anticancer drug candidates.

Flavanthrone – a new ligand with accessible radical anion and dianion states: preparation of zwitterionic {(Cp2V)2(flavanthrone)} and {(Cp2V)2(chloranil)} complexes

Molecular structure, optical and magnetic properties of the radical anions and dianions of flavanthrone dye are presented. Flavanthrone and chloranil coordinate two vanadocenes forming zwitter-ionic {(Cp₂V⁺)₂(Flavanthrone²⁻)} and {(Cp₂V⁺)₂(QCl₄²⁻)}.

Chelate rings of different sizes with non-innocent ligands

Redox-active unsaturated chelate ligands can be realised with different ring sizes of the resulting metallacycles. An overview is presented, starting from an exposition of non-innocent behaviour and chelate effects. A systematic approach is used to describe the most familiar situation, the metal complexes of 1,4-hetero-1,3-dienes in established forms (e.g. o-quinone, α-dithiolene, and α-diimine ligands) and with less common combinations of O, S, and N heteroatoms. The different steric and electronic conditions in six-membered chelate ring systems derived from the β-diketonate structure will be discussed with examples of substituted and π extended ligands, including 9-oxidophenalenyl, formazanate, and anions derived from indigo or 9,10-anthraquinone. Four-membered chelate rings existing in at least two ligand-based oxidation states are available through steric and electronic stabilisation in amidinate or triazenide complexes. Three-membered and seven-membered chelate ring situations are discussed briefly as further alternatives.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Optical and magnetic properties of trans-indigo˙− radical anions. Magnetic coupling between trans-indigo˙− (S = 1/2) mediated by intermolecular hydrogen N–H⋯OC bonds

Reduction of trans-indigo yields salt {cryptand[2.2.2](K⁺)}₆{trans-indigo}₇·5.5C₆H₄Cl₂ (1). The trans-indigo˙⁻ radical anions are bonded by intermolecular hydrogen N–H⋯OC bonds of 2.11–2.17 Å.

Synthesis, Photophysical, Electrochemical, and Halochromic Properties of peri-Naphthoindigo

A facile synthesis of peri-naphthoindigo (PNI) was reported for the first time from simple precursor. Installation of a chromophore at the peri-position of naphthalene is very unique in terms of synthetic challenges and properties. PNI exists in monoenol form, undergoes halochromism in acidic medium, and displays a wide and strong absorption band (ε = 33390 M^-1cm^-1) with maxima at 632 nm (chloroform). The dye undergoes oxidation and reduction at +0.30 and -0.58 V (vs Fc/Fc⁺), respectively, in chloroform.

Coordination Complexes of Titanium(IV) and Indium(III) Phthalocyanines with Carbonyl-Containing Dyes: The Formation of Singly Bonded Anionic Squarylium Dimers

Reduction methods for the preparation of coordination complexes of titanium(IV) and indium(III) phthalocyanines (Pc) with organic dyes such as indigo, thioindigo, and squarylium dye III (SQ) have been developed, which allow one to obtain crystalline {cryptand(K⁺ )}{(cis-indigo-O,O)^2- Ti^IV (Pc^2- )}(Cl^- )⋅C₆ H₄ Cl₂ (1), {cryptand(K⁺ )}{(cis-thioindigo-O,O)^2- In^III (Pc^2- )}^- ⋅C₆ H₄ Cl₂ (2), and {cryptand(K⁺ )}{[(SQ)₂ -O,O]^2- In^III (Pc^2- )}^- ⋅3.5 C₆ H₄ Cl₂ (3) complexes. The formation of these complexes is accompanied by the reduction of the starting dyes to the anionic state. Transition of trans-indigo or trans-thioindigo to the cis conformation in 1 and 2 provides coordination of both carbonyl oxygen atoms of the dye to Ti^IV Pc or In^III Pc. SQ is reduced to the radical anion state and forms unusual diamagnetic singly bonded (SQ^- )₂ dimers in 3. These dimers have two closely positioned carbonyl oxygen atoms coordinated to In^III Pc. Dianionic Pc^2- macrocycles have been found in 1-3. The complexes contain two chromophore molecules at one metal center. However, their optical spectra are defined mainly by absorption bands of the metal phthalocyanines.

Interligand Charge Transfer in a Complex of Deprotonated cis-Indigo Dianions and Tin(II) Phthalocyanine Radical Anions with Cp*IrIII

A diamagnetic complex, {(cis-indigo-N,N)^2-(Cp*Ir^III)} (1), in which deprotonated cis-indigo dianions coordinate an iridium center through two nitrogen atoms was obtained. By employment of the ability of the iridium center in 1 to coordinate an additional ligand, the complex [(Bu₄N⁺)₂{[Sn^II(Pc^•3-)](cis-indigo-N,N)^2-Cp*Ir^III}^•-₂·0.5(H₂Indigo)·2.5C₆H₄C_l2 (2), which has two functional ligands coordinating an Ir^III center, was obtained. This complex has a magnetic moment of 1.71 μ_B at 300 K, in accordance with an S = 1/2 spin state. The spin density is mainly delocalized over the Pc^•3- macrocycle and partially on (cis-indigo-N,N)^2-. Due to an effective π-π interaction, a thermally activated charge transfer from [Sn^II(Pc^•3-)]^•- to (cis-indigo-N,N)^2- is observed, with an estimated Gibbs energy (-ΔG°) of 9.27 ± 0.18 kJ/mol. The deprotonation of indigo associated with the coordination of Ir^III by the indigo releases H⁺ ions, which protonate noncoordinating indigo molecules to produce leuco cis-indigo (H₂Indigo). One H₂indigo links two (cis-indigo-N,N)^2- dianions in 2 to produce strong N-H···O═C and O-H···O═C hydrogen-bonding interactions.

Coordination-driven fast self-assembly of a charge-transfer hydrogel with reversible photochromism

Highly selective coordination-driven self-assembly of charge transfer hydrogel was obtained by simply mixing two-phase solution, once be irradiated by simulated sun light, will generate organic radicals in gel state, displaying reversible photochromism.

Recent advances in triphenylamine-based electrochromic derivatives and polymers

Triphenylamine-containing electrochromic materials with great potential applications in low energy-consumption displays, light-adapting mirrors in vehicles, and smart windows have experienced an exponential growth of research interests. In this review, the newly developed triphenylamine-based derivatives and polymers are reviewed and elaborated.

Mixed valency in ligand-bridged diruthenium frameworks: divergences and perspectives

Emerging fundamental issues involving intramolecular electron transfer at the mixed valent diruthenium frameworks and its application prospects have been highlighted.

Anti-inflammatory activity of electron-deficient organometallics

We report an evaluation of the cytotoxicity of a series of electron-deficient (16-electron) half-sandwich precious metal complexes of ruthenium, osmium and iridium ([Os/Ru(η⁶-p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)] (1/2), [Ir(η⁵-pentamethylcyclopentadiene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolato)] (3), [Os/Ru(η⁶-p-cymene)(benzene-1,2-dithiolato)] (4/5) and [Ir(η⁵-pentamethylcyclopentadiene)(benzene-1,2-dithiolato)] (6)) towards RAW 264.7 murine macrophages and MRC-5 fibroblast cells. Complexes 3 and 6 were found to be non-cytotoxic. The anti-inflammatory activity of 1-6 was evaluated in both cell lines after nitric oxide (NO) production and inflammation response induced by bacterial endotoxin lipopolysaccharide (LPS) as the stimulus. All metal complexes were shown to exhibit dose-dependent inhibitory effects on LPS-induced NO production on both cell lines. Remarkably, the two iridium complexes 3 and 6 trigger a full anti-inflammatory response against LPS-induced NO production, which opens up new avenues for the development of non-cytotoxic anti-inflammatory drug candidates with distinct structures and solution chemistry from that of organic drugs, and as such with potential novel mechanisms of action.

cis-Thioindigo (TI) - a new ligand with accessible radical anion and dianion states. Strong magnetic coupling in the {[TI-(μ2-O),(μ-O)]Cp*Cr}2 dimers

Reaction of decamethylchromocene (Cp*₂Cr) with thioindigo (TI) yields a coordination complex {[TI-(μ₂-O), (μ-O)]Cp*Cr}₂·C₆H₁₄ (1) in which one Cp* ligand in Cp*₂Cr is substituted by TI. TI adopts cis-conformation in 1 allowing the coordination of both carbonyl groups to chromium. Additionally, one oxygen atom of TI becomes a μ₂-bridge for two chromium atoms to form {[TI-(μ₂-O), (μ-O)]Cp*Cr}₂ dimers with a CrCr distance of 3.12 Å. According to magnetic data, diamagnetic TI^2- dianions and two Cr³⁺ atoms with a high S = 3/2 spin state are present in a dimer allowing strong antiferromagnetic coupling between two Cr³⁺ spins with an exchange interaction of -35.4 K and the decrease of molar magnetic susceptibility below 140 K. Paramagnetic TI˙^- radical anions with the S = 1/2 spin state have also been obtained and studied in crystalline {cryptand[2,2,2](Na⁺)}(TI˙^-) (2) salt showing that both radical anion and dianion states are accessible for TI.

Pseudo electron-deficient organometallics: limited reactivity towards electron-donating ligands

This work presents the unusual reactivity of a family of electron-deficient half-sandwich metal complexes.

Strong direct exchange coupling and single-molecule magnetism in indigo-bridged lanthanide dimers

The first lanthanide–indigo complexes are described, including a dysprosium single-molecule magnet and one of the strongest exchange coupling constants measured in a gadolinium compound.

A structurally characterised redox pair involving an indigo radical: indigo based redox activity in complexes with one or two [Ru(bpy)2] fragments

Various radical intermediates of the non-innocently behaving dehydroindigo ligand were characterized by EPR, spectroelectrochemistry and structure analysis.

cis-Conformation of indigo in the coordination complex (indigo-O,O)(Cp*CrIICl)

The interaction of decamethylchromocene (Cp*₂Cr) with indigo in the presence of a Cl^- source yields the coordination complex (indigo-O,O)(Cp*Cr^IICl) (1) in which one Cp* ligand at chromium is substituted by indigo. Indigo adopts an unusual cis-conformation in 1, allowing the coordination of both indigo carbonyl groups to one Cr^II center. Complex 1 contains Cr^II with an S = 1 spin state and indigo⁰. At the same time, calculations show that an excited ionic state is positioned close to the neutral ground state, providing the appearance of intense low-energy NIR bands in the spectrum of 1 at 820 and 1002 nm attributed to metal-to-ligand charge transfer.

Multistate Redox Switching and Near-Infrared Electrochromism Based on a Star-Shaped Triruthenium Complex with a Triarylamine Core

A star-shaped cyclometalated triruthenium complex 2(PF₆)_n (n = 3 and 4) with a triarylamine core was synthesized, which functions as a molecular switch with five well-separated redox states in both solution and film states. The single-crystal X-ray structure of 2(PF₆)₃ is presented. This complex displays four consecutive one-electron redox waves at +0.082, +0.31, +0.74, and +1.07 V vs Ag/AgCl. In each redox state, it shows significantly different NIR absorptions with λ_max of 1590 nm for 2⁴⁺, 1400 nm for 2⁵⁺, 1060 nm for 2⁶⁺, and 740 nm for 2⁷⁺, respectively. Complex 2⁴⁺ shows a single-line EPR signal at g = 2.060, while other redox states are all EPR inactive. The spin density distributions and NIR absorptions in different redox states were rationalized by DFT and TDDFT calculations. A vinyl-substituted triruthenium analogous 3(PF₆)₄ was prepared, which was successfully polymerized on ITO glass electrode surfaces by reductive electropolymerization. The obtained poly-3ⁿ⁺/ITO film was characterized by FTIR, AFM, and SEM analysis. It shows four well-defined redox couples and reversible multistate NIR electrochromism. In particular, a contrast ratio (ΔT%) up to 63% was achieved at the optic telecommunication wavelength (1550 nm).

Localized Mixed-Valence and Redox Activity within a Triazole-Bridged Dinucleating Ligand upon Coordination to Palladium

The new dinucleating redox-active ligand (L^H4 ), bearing two redox-active NNO-binding pockets linked by a 1,2,3-triazole unit, is synthetically readily accessible. Coordination to two equivalents of Pd^II resulted in the formation of paramagnetic (S=1/2 ) dinuclear Pd complexes with a κ² -N,N'-bridging triazole and a single bridging chlorido or azido ligand. A combined spectroscopic, spectroelectrochemical, and computational study confirmed Robin-Day Class II mixed-valence within the redox-active ligand, with little influence of the secondary bridging anionic ligand. Intervalence charge transfer was observed between the two ligand binding pockets. Selective one-electron oxidation allowed for isolation of the corresponding cationic ligand-based diradical species. SQUID (super-conducting quantum interference device) measurements of these compounds revealed weak anti-ferromagnetic spin coupling between the two ligand-centered radicals and an overall singlet ground state in the solid state, which is supported by DFT calculations. The rigid and conjugated dinucleating redox-active ligand framework thus allows for efficient electronic communication between the two binding pockets.