Widely Separated Reduction Processes of abpy-Coupled Areneosmium(II) Reaction Centers (abpy = 2,2‘-Azobispyridine): Stabilization of the Radical Intermediate and of the Os0OsII State

Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks

This article demonstrates the stabilization of ground- and redox-induced metal-to-ligand charge transfer excited states on coordination of azo-coupled bmpd(L4) [bmpd = (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene; L4 = -N═N-] to the electron-rich {Ru(acac)₂} (acac = acetylacetonate) unit in mononuclear Ru^II(acac)₂(L4) (1) and diastereomeric dinuclear (acac)₂Ru^2.5(μ-L4^•-)Ru^2.5(acac)₂ [rac, ΔΔ/ΛΛ (2a)/meso, ΔΛ (2b)] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4^•- bridged isovalent higher analogue [(acac)₂Ru^III(μ-L4^•-)Ru^III(acac)₂]ClO₄ in diastereomeric forms, rac-[2a]ClO₄/meso-[2b]ClO₄. On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene)] delivered mononuclear Ru^II(acac)₂(L5) (3) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac)₂Ru^II(μ-L4)Ag^I(μ-L4)Ru^II(acac)₂]ClO₄ ([4]ClO₄) involving unreduced L4 via the reaction of 1 and AgClO₄ revealed the absence of any inner-sphere electron transfer (IET) as in precursor 1, which in turn reaffirmed an IET (at the interface of electron-rich Ru(acac)₂ and acceptor L4) mediated stabilization of 2. Structural authentication of the complexes with special reference to the tunable azo distance (N═N, N-N^•-, N-N^2-) of L and their spectro-electrochemical events in accessible redox states including the reversible electron reservoir feature of 2 → 2⁺/2⁺ → 2 were evaluated in conjunction with density functional theory/time-dependent density functional theory calculations. The varying extent of IET as a function of heteroaromatics appended to the azo group of L (L1 = abpy = 2,2'-azobipyridine, L2 = abbt = 2,2'-azobis(benzothiazole), L3 = abim = azobis(1-methylbenzimidazole), L4 and L5, Schemes 1 & 2) in the Ru(acac)₂-derived respective molecular setup has been addressed.

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap)₂(abbt)]ⁿ (1ⁿ) and [Ru(bpy)₂(abbt)]ⁿ (2ⁿ), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1⁺/2⁺ and 1²⁺/2²⁺, respectively, were made primarily based on their redox-sensitive azo (N═N) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt^•- was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1⁺, the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt⁰ in 2²⁺, along with the radical form in 2⁺ as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1⁺ and 2²⁺ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1-R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1⁺ and bpy-derived 2²⁺ as well as facile reduction of pap and abbt in 1⁺ and 2⁺, respectively.

Variable electronic structure and spin distribution in bis(2,2'-bipyridine)-metal complexes (M = Ru or Os) of 4,5-dioxido- and 4,5-diimido-pyrene

The odd-electron compounds [M(bpy)₂(L¹)](ClO₄) M = Ru ([1](ClO₄)) or Os ([2](ClO₄)), and the even-electron species [M(bpy)₂(H₂L²)](ClO₄)₂, M = Ru ([3](ClO₄)₂) or Os ([4](ClO₄)₂) were obtained from pyrene-4,5-dione, L¹, or 4,5-diaminopyrene, H₄L², and were characterised structurally, electrochemically and spectroscopically. Experimental and computational analysis (TD-DFT) revealed rather different electronic structures and spin distributions of the paramagnetic monocations 1⁺-4⁺. EPR investigations and electronic absorption studies exhibit increasing metal contributions to the singly occupied MO along the series 1⁺ < 3⁺ < 4⁺ < 2⁺, illustrated by g value and long-wavelength absorbance. In addition to variations of the metal (Ru,Os) and the donor atoms (O,NH) the extension of the π system of the semiquinone-type ligand has a large effect on the electronic structure of the paramagnetic cations.

Elemental mapping of half-sandwich azopyridine osmium arene complexes in cancer cells

Nanofocused synchrotron X-ray fluorescence and inductively coupled plasma-mass spectrometry provide insights into time-dependent ligand exchange reactions of organo-osmium anticancer complexes in cancer cells. Created with Biorender.com.

Structure-activity relationships for osmium(II) arene phenylazopyridine anticancer complexes functionalised with alkoxy and glycolic substituents

Twenty-four novel organometallic osmium(II) phenylazopyridine (AZPY) complexes have been synthesised and characterised; [Os(η⁶-arene)(5-RO-AZPY)X]Y, where arene = p-cym or bip, AZPY is functionalized with an alkoxyl (O-R, R = Me, Et, ⁿPr, ⁱPr, ⁿBu) or glycolic (O-{CH₂CH₂O}_nR*, n = 1-4, R* = H, Me, or Et) substituent on the pyridyl ring para to the azo-bond, X is a monodentate halido ligand (Cl, Br or I), and Y is a counter-anion (PF₆^-, CF₃SO₃^- or IO₃^-). X-ray crystal structures of two complexes confirmed their 'half-sandwich' structures. Aqueous solubility depended on X, the AZPY substituents, arene, and Y. Iodido complexes are highly stable in water (X = I ⋙ Br > Cl), and exhibit the highest antiproliferative activity against A2780 (ovarian), MCF-7 (breast), SUNE1 (nasopharyngeal), and OE19 (oesophageal) cancer cells, some attaining nanomolar potency and good cancer-cell selectivity. Their activity and distinctive mechanism of action is discussed in relation to hydrophobicity (RP-HPLC capacity factor and Log P_o/w), cellular accumulation, electrochemical reduction (activation of azo bond), cell cycle analysis, apoptosis and induction of reactive oxygen species (ROS). Two complexes show ca. 4× higher activity than cisplatin in the National Cancer Institute (NCI) 60-cell line five-dose screen. The COMPARE algorithm of their datasets reveals a strong correlation with one another, as well as anticancer agents olivomycin, phyllanthoside, bouvardin and gamitrinib, but only a weak correlation with cisplatin, indicative of a different mechanism of action.

In Vivo Selectivity and Localization of Reactive Oxygen Species (ROS) Induction by Osmium Anticancer Complexes That Circumvent Platinum Resistance

Platinum drugs are widely used for cancer treatment. Other precious metals are promising, but their clinical progress depends on achieving different mechanisms of action to overcome Pt-resistance. Here, we evaluate 13 organo-Os complexes: 16-electron sulfonyl-diamine catalysts [(η6-arene)Os( N, N')], and 18-electron phenylazopyridine complexes [(η6-arene)Os( N, N')Cl/I]+ (arene = p-cymene, biphenyl, or terphenyl). Their antiproliferative activity does not depend on p21 or p53 status, unlike cisplatin, and their selective potency toward cancer cells involves the generation of reactive oxygen species. Evidence of such a mechanism of action has been found both in vitro and in vivo. This work appears to provide the first study of osmium complexes in the zebrafish model, which has been shown to closely model toxicity in humans. A fluorescent osmium complex, derived from a lead compound, was employed to confirm internalization of the complex, visualize in vivo distribution, and confirm colocalization with reactive oxygen species generated in zebrafish.

Redox Noninnocent Azo-Aromatic Pincers and Their Iron Complexes. Isolation, Characterization, and Catalytic Alcohol Oxidation

The new redox-noninnocent azoaromatic pincers 2-(arylazo)-1,10-phenanthroline (L¹) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L²) are reported. The ligand L¹ is a tridentate pincer having NNN donor atoms, whereas L² is tetradentate having two azo-N donors and two N-donor atoms from the 1,10-phenanthroline moiety. Reaction of FeCl₂ with L¹ or L² produced the pentacoordinated mixed-ligand Fe(II) complexes FeL¹Cl₂ (1) and FeL²Cl₂ (2), respectively. Homoleptic octahedral Fe(II) complexes, mer-[Fe(L¹)₂](ClO₄)₂ [3](ClO₄)₂ and mer-[Fe(L²)₂](ClO₄)₂ [4](ClO₄)₂, have been synthesized from the reaction of hydrated Fe(ClO₄)₂ and L¹ or L². The ligand L², although having four donor sites available for coordination, binds the iron center in a tridentate fashion with one uncoordinated pendant azo function. Molecular and electronic structures of the isolated complexes have been scrutinized thoroughly by various spectroscopic techniques, single-crystal X-ray crystallography, and density functional theory. Beyond mere characterization, complexes 1 and 2 were successfully used as catalysts for the aerobic oxidation of primary and secondary benzylic alcohols. A wide variety of substituted benzyl alcohols were found to be converted to the corresponding carbonyl compounds in high yields, catalyzed by complex 1. Several control reactions were carried out to understand the mechanism of this alcohol oxidation reactions.

Density functional theory study on surface-enhanced Raman scattering of 4,4'-azopyridine on silver

Surface-enhanced Raman scattering (SERS) of 4,4'-azopyridine (AZPY) on silver foil substrate was measured under 1064nm excitation lines. Density-functional theory (DFT) methods were used to calculate the structure and vibrational spectra of models such as Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY complexes with B3LYP/6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set. The Raman bands of AZPY were identified on the ground of analog computation of potential energy distribution. The calculated spectra of Ag(4)-AZPY and Ag(6)-AZPY models were much approximated to the experimental results than that of Ag-AZPY model. The DFT results showed that the angles between two pyridyl rings keep 0 degrees from AZPY to Ag-AZPY, Ag(4)-AZPY and Ag(6)-AZPY model. The energy gaps between the HOMO and LUMO changed from 363 to 1140nm for AZPY-Ag complexes according to the DFT results. An conclusion was conceived that chemical enhancement mechanism may play an important role in the SERS of AZPY on silver substrate.

Surface-enhanced Raman spectroscopy and density functional theory study on 4,4'-bipyridine molecule

The molecular geometry and vibrational frequencies of 4,4'-bipyridine (BPE) in the ground state were calculated using density functional theory (DFT) methods (B3LYP) with 6-31++G(d,p) basis set. The optimized geometric bond lengths and bond angles are obtained by DFT employing the hybrid of Beckes non-local three parameter exchange and correlation functional and Lee-Yang-Parr correlation functional. Fourier transform infrared (FT-IR), Fourier transform Raman (FT-Raman) and near-infrared surface-enhanced Raman scattering (NIR-SERS) spectra of BPE on the silver foil substrate have been recorded. All FT-IR, FT-Raman and NIR-SERS band were assigned on the basis of the B3LYP/6-31++G(d,p) method. The vibrational frequencies obtained by DFT(3LYP) are in good agreement with observed results. The NIR-SERS of BPE excited by 1064nm laser line is little difference with that excited by visible laser line. This phenomenon is result to the increase of the contribution of CHEM enhancement effect. Surface selection rules derived from the electromagnetic enhancement model were employed to infer the orientations of BPE on the silver foil substrate surface. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPE, and to the dihedral angle were concluded.

The Metal−NO Interaction in the Redox Systems [Cl5Os(NO)]n-, n = 1−3, and cis-[(bpy)2ClOs(NO)]2+/+: Calculations, Structural, Electrochemical, and Spectroscopic Results

Experimental and computational results for the two-step redox system [Cl5Os(NO)]n- (n = 1-3) are reported and discussed in comparison to the related one-step redox systems [Cl5Ru(NO)]n- and [Cl5Ir(NO)]n- (n = 1, 2). The osmium system exhibits remarkably low oxidation and reduction potentials. The structure of the precursor (PPh4)2[Cl5Os(NO)] is established as an {MNO}6 species with almost linear OsNO arrangement at 178.1 degrees. Density-functional theory (DFT) calculations confirm this result, and a comparison of structures calculated for several oxidation states reveals an increased labilization of the trans-positioned M-Cl bond on reduction in the order M = Ir < Os < Ru. Accordingly, the intact reduced form [Cl5Os(NO)]3- could not be observed in fluid solution even on electrolysis at -70 degrees C in n-butyronitrile solution, as confirmed both by DFT calculations and by comparison with the electron paramagnetic resonance and infrared spectroelectrochemically characterized redox pairs cis-[(bpy)2ClOs(NO)]2+/+ and [(CN)5Os(NO)]2-/3-. The DFT calculations indicate that the oxidation of [Cl5Os(NO)]2- occurs largely on the metal, the highest occupied molecular orbital (HOMO) of the precursor being composed of Os 5d (58%) and Cl(eq) 3p orbitals (41%). As for the related [(CN)5Os(NO)]2-, the reduction is largely NO centered, the lowest unoccupied molecular orbital (LUMO) of [Cl5Os(NO)]2- has 61% pi*(NO) character with significant 5d Os contributions (34%). A rather large degree of metal-NO back-donation is estimated to occur in the {OsNO}7 configuration of [Cl5Os(NO)]3- which leads to an unusual low value of 1513 cm(-1) calculated for nu(NO), signifying contributions from an Os(III)(NO-) formulation. Detailed analyses of the conformational dependence of the g anisotropy suggest that the different reduced species reported previously for [Cl5Os(NO)]3- in AgCl host lattices may be distinct in terms of eclipsed or staggered conformations of the bent NO. axial ligand relative to the Os(II)Cl4 equatorial plane. The staggered form is calculated to be more stable by 105 cm(-1). The weak absorptions of [Cl5Os(NO)]2- at 573, 495, and 437 nm are assigned as MLCT/LLCT transitions to the doubly degenerate pi*(NO) LUMO. The oxidized form [Cl5Os(NO)]- contains Os(III) in an {OsNO}5 configuration with a spin density of 0.711 on Os. In all three states of [Cl5Os(NO)]n-, the N bonded form is vastly preferred over the NO-side-on bonded alternative.