Tris(2,2'-azobispyridine) complexes of copper(II): X-ray structures, reactivities, and the radical nonradical bis(ligand) analogues

Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2'-azobis(benzothiazole), L1-L3; bmpd: (E)-1,2-bis(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N•-) versus nonradical (N═N0) states of L on selective OsII-platforms in structurally/spectroscopically characterized monomeric [1]ClO4-[6]ClO4 and [1](ClO4)2-[2](ClO4)2/[7](ClO4)2-[8](ClO4)2, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os(bpy)2} facilitated the stabilization of both nonradical ([1](ClO4)2-[2](ClO4)2) and radical ([1]ClO4-[2]ClO4) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]ClO4. In contrast, {Os(pap)2} generated radical states of L1-L3 in [4]ClO4-[6]ClO4, respectively, without any alteration of the redox state of OsII and azo (N═N0) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7](ClO4)2 or [8](ClO4)2 irrespective of the nature of the metal fragment {Os(bpy)2} or {Os(pap)2}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.

Redox Triggers Guest Release and Uptake Across a Series of Azopyridine-Based Metal-Organic Capsules

Precise control over guest release and recapture using external stimuli is a valuable goal, potentially enabling new modes of chemical purification. Including redox moieties within the ligand cores of molecular capsules to trigger the release and uptake of guests has proved effective, but this technique is limited to certain capsules and guests. Herein, the construction of a series of novel metal-organic capsules from ditopic, tritopic, and tetratopic ligands is demonstrated, all of which contain redox-active azo groups coordinated to FeII centers. Compared to their iminopyridine-based analogs, this new class of azopyridine-based capsules possesses larger cavities, capable of encapsulating more voluminous guests. Upon reduction of the capsules, their guests are released and may then be re-encapsulated when the capsules are regenerated by oxidation. Since the redox centers are on the ligand arms, they are modular and can be attached to a variety of ligand cores to afford varying and predictable architectures. This method thus shows promise as a generalized approach for designing redox-controlled guest release and uptake systems.

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.

A multi-technique approach to unveil the redox behaviour and potentiality of homoleptic CuI complexes based on substituted bipyridine ligands in oxygenation reactions

The effect of differently substituted 2,2'-bipyridine ligands (i.e. 6,6'-dimethyl-2,2'-bipyridine, 5,5'-dimethyl-2,2'-bipyridine, 6,6'-dimethoxy-2,2'-bipyridine and 2,2'-bipyridine) on the reversible oxidation of the resulting Cu^I homoleptic complexes is investigated by means of a multi-technique approach (electronic and vibrational spectroscopies, DFT, electrochemistry). Among the four tested complexes, [Cu^I(6,6'-dimethyl-2,2'-bipyridine)₂] (PF₆) shows a peculiar behavior when oxidized with an organic peroxide (i.e. tert-butyl hydroperoxide, tBuOOH). The simultaneous use of UV-Vis-NIR and Raman spectroscopy methods and cyclovoltammetry, supported by DFT based calculations, allowed identifying (i) the change in the oxidation state of the copper ion and (ii) some peculiar modification in the local structure of the metal leading to the formation of a [Cu^IIOH]⁺ species. The latter, being able to oxidize a model molecule (i.e. cyclohexene) and showing the restoration of the original Cu^I complex and the formation of cyclohexanone, confirms the potential of these simple homoleptic Cu^I complexes as model catalysts for partial oxygenation reactions.

Polymerisation of styrene using pincer type amine functionalized azo aromatic complexes of Co(II) as catalysts

In the present report, three mononuclear azo-aromatic complexes of Co(II), 1-3, and an imine-based Co(II) complex, 4, were synthesized through a reaction of respective amine-functionalized pincer-like ligands, HL1-4, with CoCl₂·6H₂O in the ligand-to-metal ratio of 1 : 1. All the complexes, 1-4, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory (DFT) calculations. Complexes 1-4 were explored for the catalytic styrene polymerisation reaction separately in the presence of modified methyl aluminoxane (MMAO). All the complexes, 1-4, are indeed active for the polymerisation of styrene under mild conditions at room temperature upon activation with MMAO. Among the azo-aromatic complexes 1-3, complex 3 is the most efficient. The activity of the imine complex 4 is poor compared to those of the azo-aromatic complexes 1-3. The weight average molecular weight (M_w) of the isolated polystyrene ranges from 32.9 to 144.0 kg mol^-1, with a polydispersity index (Đ) in the range of 1.1-1.8. Microstructural analysis of the isolated polymer from complexes 1-4 was carried out by ¹³C NMR spectroscopy, infrared spectroscopy, and powder X-ray diffraction studies. Their thermal properties were scrutinized by differential scanning calorimetry and thermogravimetric analysis. These studies have shown the atactic and amorphous nature of the polymers. The mechanical strength of the polymers was measured by a nanoindentation technique which has shown the good plastic/soft nature of the polymers.

Inner-sphere electron transfer at the ruthenium-azo interface

Metal complexes exhibiting multiple reversible redox states have drawn continuing research interest due to their electron reservoir features. In this context, the present article described ruthenium-acac complexes (acac=acetylacetonate) incorporating redox-active...

Azide-Alkyne "Click" Reaction in Water Using Parts-Per-Million Amine-Functionalized Azoaromatic Cu(I) Complex as Catalyst: Effect of the Amine Side Arm

A series of Cu(II) complexes, 1-4 and 6, were synthesized through a reaction of amine-functionalized pincer-like ligands, HL^1,2, L^a,b, and a bidentate ligand L¹ with CuCl₂·2H₂O. The chemical reduction of complex 1 using 1 equiv of sodium l-ascorbate resulted in a dimeric Cu(I) complex 5 in excellent yield. All of the complexes, 1-6, were thoroughly characterized using various physicochemical characterization techniques, single-crystal X-ray structure determination, and density functional theory calculations. Ligands HL^1,2 and L^a,b behaved as tridentated donors by the coordination of the amine side arm in their respective Cu(II) complexes, and the amine side arm remained as a pendant in Cu(I) complexes. All of these complexes (1-6) were explored for copper(I)-catalyzed 1,3-dipolar azide-alkyne cycloaddition (CuAAC) reaction at room temperature in water under air. Complex 5 directly served as an active catalyst; however, complexes 1-4 and 6 required 1 equiv of sodium l-ascorbate to generate their corresponding active Cu(I) catalyst. It has been observed that azo-based ligand-containing Cu(I)-complexes are air-stable and were highly efficient for the CuAAC reaction. The amine side arm in the ligand backbone has a dramatic role in accelerating the reaction rate. Mechanistic investigations showed that the alkyne C-H deprotonation was the rate-limiting step and the pendant amine side arm intramolecularly served as a base for Cu-coordinated alkyne deprotonation, leading to the azide-alkyne 2 + 3 cycloaddition reaction. Thus, variation of the amine side arm in complexes 1-4 and use of the most basic diisopropyl amine moiety in complex 4 has resulted in an unique amine-functionalized azoaromatic Cu(I) system for CuAAC reaction upon sodium l-ascorbate reduction. The complex 4 has shown excellent catalysis at its low parts-per-million level loading in water. The catalytic protocol was versatile and exhibited very good functional group tolerance. It was also employed efficiently to synthesize a number of useful functional triazoles having medicinal, catalytic, and targeting properties.

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.

Photo-/Electroinduced Irreversible Isomerization of 2,2'-Azobispyridine Ligands in Arene Ruthenium(II) Complexes

Novel arene Ru^II complexes containing 2,2'-azobispyridine ligands were synthesized and characterized by using ¹ H and ¹³ C NMR spectroscopy, UV/vis spectroscopy, electrochemistry, DFT calculations and single-crystal X-ray diffraction. Z-configured complexes featuring unprecedented seven-membered chelate rings involving the nitrogen atom of both pyridines were isolated and were shown to undergo irreversible isomerization to the corresponding E-configured five-membered chelate complexes in response to light or electrochemical stimulus.

Coordination polymers of Ag(i) and Hg(i) ions with 2,2'-azobispyridine: synthesis, characterization and enhancement of conductivity in the presence of Cu(ii) ions

The cationic coordination polymers (CPs) of the types [Hg₂(abpy)₂]_n[PF₆]_2n (1) and [Ag(abpy)]_n[PF₆]_n (2) (abpy = 2,2'-azobispyridine) were synthesized and characterized. Experimentation using the crystals confirmed that 1 and 2 are conductors of electricity. The relative conductivity of 1 is 62 times greater than that of 2. The conductivity of 1 increases 70 fold when it reacts with Cu²⁺ ions.

Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate and aldimine anion radicals to oxidovanadium(iv)

o-Benzosemiquinonate, o-iminobenzosemiquinonate and hitherto unknown aldimine anion radical complexes of oxidovanadium(iv) are reported.

RhCl(PPh3)3-mediated C–H oxyfunctionalization of pyrrolido-functionalized bisazoaromatic pincers: a combined experimental and theoretical scrutiny of redox-active and spectroscopic properties

Non-trivial coordination mode of symmetrical NNN ligands with Rh(iii) leads to redox-active NNO-scaffolds via C(sp²)–H oxyfunctionalization at rt, opening an opportunity to juxtapose different redox-active domains.