Uncommon cis Configuration of a Metal–Metal Bridging Noninnocent Nindigo Ligand

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).

Indazole-Derived Mono-/Diruthenium and Heterotrinuclear Complexes: Switchable Binding Mode, Electronic Form, and Anion Sensing Events

The article deals with the newer classes of mononuclear: [(acac)₂Ru^III(H-Iz)(Iz^-)] 1, [(acac)₂Ru^III(H-Iz)₂]ClO₄ [1]ClO₄/[1']ClO₄, and [(bpy)₂Ru^II(H-Iz)(Iz^-)]ClO₄ [2]ClO₄, mixed-valent unsymmetric dinuclear: [(acac)₂Ru^III(μ-Iz^-)₂Ru^II(bpy)₂]ClO₄ [3]ClO₄, and heterotrinuclear: [(acac)₂Ru^III(μ-Iz^-)₂M^II(μ-Iz^-)₂Ru^III(acac)₂] (M = Co:4a, Ni:4b, Cu:4c, and Zn:4d) complexes (H-Iz = indazole, Iz^- = indazolate, acac = acetylacetonate, and bpy = 2,2'-bipyridine). Structural characterization of all the aforestated complexes established their molecular identities including varying binding modes (N_a and N_b donors and 1H-indazole versus 2H-indazole) of the heterocyclic H-Iz/Iz^- in the complexes. Unlike [1']ClO₄ containing two NH protons at the backface of H-Iz units, the corresponding [1]ClO₄ was found to be unstable due to the deprotonation of its positively charged quaternary nitrogen center, and this resulted in the eventual formation of the parent complex 1. A combination of experimental and density functional theory calculations indicated the redox noninnocent feature of Iz^- in the complexes along the redox chain. The absence of intervalence charge transfer transition in the near-infrared region of the (Iz^-)₂-bridged unsymmetric mixed-valent Ru^IIIRu^II state in [3]ClO₄ suggested negligible intramolecular electronic coupling corresponding to a class I setup (Robin and Day classification). Heterotrinuclear complexes (4a-4d) exhibited varying spin configurations due to spin-spin interactions between the terminal Ru(III) ions and the central M(II) ion. Though both [3]ClO₄ and 4a-4d displayed ligand (Iz^-/Iz^•)-based oxidation, reductions were preferentially taken place at the bpy and metal (Ru^III/Ru^II) centers, respectively. Unlike 1 or [2]ClO₄ containing one free NH proton at the backface of H-Iz, [1']ClO₄ with two H-Iz units could selectively and effectively recognize F^-, OAc^-, and CN^- among the tested anions: F^-, OAc^-, CN^-, Cl^-, Br^-, I^-, SCN^-, HSO₄^-, and Η₂PΟ₄^- in CH₃CN via intermolecular NH···anion hydrogen bonding interaction. The difference in the sensing feature between [1']ClO₄ and 1/[2]ClO₄ could be rationalized by their pK_a values of 8.4 and 11.3/10.8, respectively.

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.

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.

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.

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.

A Computational Study of the Protoisomerization of Indigo and Its Imine Derivatives

The protoisomerization (isomerization induced by protonation) mechanisms of indigo as well as indigo di- and monoimine derivatives were investigated using computational chemistry. Both density functional theory (M06-2X) and wave function theory (GMC-QDPT) methods were used to obtain reliable results. A solvation model (C-PCM with CH2Cl2 solvent) was employed to mimic the actual environment of the isomerization. The calculations reveal that the protoisomerizations of both the indigo and its imine derivatives are thermodynamically favorable. However, the energy barriers for rotating the double bonds in the derivatives are found to be lower than the one for indigo. More importantly, the imine derivatives undergo one-step isomerization, whereas a two-step process is predicted for indigo itself. The computational results provide insightful explanation for the different protoisomerization propensities of the parent indigo and its imine derivatives observed in experiment.

Protoisomerization of indigo di- and monoimines

Indigo di- and monoimines can be protonated to form stable salts in which the central C=C bond has isomerized from a trans to cis configuration. Deprotonation of these salts regenerates the neutral trans species. The protonation chemistry of indigo is also explored.

Dinuclear Ruthenium Complex Based on a π-Extended Bridging Ligand with Redox-Active Tetrathiafulvalene and 1,10-Phenanthroline Units

The synthesis of a π-extended bridging ligand with both redox-active tetrathiafulvalene (TTF) and 1,10-phenanthroline (phen) units, namely, bis(1,10-phenanthro[5,6-b])tetrathiafulvalene (BPTTF), was realized via a self-coupling reaction. Using this ligand and Ru(tbbpy)2Cl2 (tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), the dinuclear ruthenium(II) compound [{Ru(tbbpy)2}2(BPTTF)](PF6)4 (1) has been obtained by microwave-assisted synthesis. Structural characterization of 1 revealed a crossed arrangement of the TTF moieties on adjacent dimers within the crystal structure. The optical and redox properties of 1 were investigated using electrochemical, spectroelectrochemical, electron paramagnetic resonance (EPR), and absorption spectroscopic studies combined with theoretical calculations. One exhibits a rich electrochemical behavior owing to the multiple redox-active centers. Interestingly, both the ligand BPTTF and the ruthenium compound 1 are EPR-active in the solid state owing to intramolecular charge-transfer processes. The results demonstrate that the TTF-annulated bis(phen) ligand is a promising bridging ligand to construct oligomeric or polymeric metal complexes with multiple redox-active centers.

Impact of {Os(pap)2} in fine-tuning the binding modes and non-innocent potential of deprotonated 2,2'-bipyridine-3,3'-diol

The reaction of ctc-Os(II)(pap)2Cl2 (pap = 2-phenylazopyridine, ctc = cis-trans-cis with respect to chlorides and pyridine/azo nitrogens of pap, respectively) and ambidentate 2,2'-bipyridine-3,3'-diol (H2L) leads to the simultaneous formation of isomeric [Os(II)(pap)2(HL(-))](+) ((2+)/(3+)), seven-membered chelate containing Os(II)(pap)2(L(2-)) (4) and diastereomeric [{Os(II)(pap)2}2(μ-L(2-))](2+) (5a(2+) (meso, ΔΛ)/5b(2+) (rac, ΔΔ/ΛΛ)). The reaction of 2,2'-biphenol (H2L') and ctc-Os(II)(pap)2Cl2 yields Os(II)(pap)2(L'(2-)) (6), an analogue of 4. The identities of the newly designed complexes have been established by different analytical, spectroscopic and X-ray diffraction techniques. (1)H-NMR spectra of the complexes and single crystal X-ray structures of selective derivatives [2]ClO4, [3]ClO4, [5a](ClO4)2, and 6 establish the retention of the tc-configuration of the precursor {Os(pap)2}. In isomeric 2(+) and 3(+), monodeprotonated HL(-) is linked to the {Os(II)(pap)2} fragment through N,N and N,O(-) donors, resulting in nearly planar five- and six-membered chelates with O-HO(-) and O-HN hydrogen bonds at its back face, respectively. The O(-),O(-) donating L'(2-) extends a severely twisted seven-membered chelate with the {Os(pap)2} unit in 6. The N,O(-)/O(-),N donors of deprotonated L(2-) bridge the two {Os(II)(pap)2} units in a symmetric fashion in 5a(2+), forming two moderately twisted six-membered chelates. Though the deprotonation of the O-HN hydrogen bond in (+) by another unit of {Os(II)(pap)2} generates a diastereomeric mixture of 5a(2+) and 5b(2+), attempts to deprotonate the relatively stronger O-H···O(-) hydrogen bond in 2(+) have failed. The isomeric 2(+)/3(+), seven-membered chelate containing 4/6 and diastereomeric 5a(2+)/5b(2+) exhibit distinctive (1)H-NMR and absorption spectra as well as electrochemical responses. The pap (N[double bond, length as m-dash]N) based two successive reductions and the participation of HL(-), L(2-), L'(2-) in the oxidation processes of the respective complexes have been revealed using EPR and DFT calculated MOs and Mulliken spin density plots at the intermediate paramagnetic states.

Substituent directed selectivity in anion recognition by a new class of simple osmium-pyrazole derived receptors

The present article deals with the structurally, spectroscopically and electrochemically characterised osmium-bipyridyl derived complexes [(bpy)2Os(II)(HL1)Cl]ClO4 [1]ClO4 and [(bpy)2Os(II)(HL2)Cl]ClO4 [2]ClO4 incorporating neutral and monodentate pyrazole derivatives (HL) with one free NH function (bpy = 2,2'-bipyridine, HL1 = pyrazole, HL2 = 3,5-dimethylpyrazole). The crystal structures of [1]ClO4 and [2]ClO4 reveal intramolecular hydrogen bonding interactions between the free NH proton of HL and the equatorially placed Cl(-) ligand (N-HCl) with donor-acceptor distances of 3.114(7) Å and 3.153(6) Å as well as intermolecular hydrogen bonding interactions between the NH proton and one of the oxygen atoms of ClO4(-) (N-HO) with donor-acceptor distances of 2.870(10) Å and 3.024(8) Å, respectively. The effect of hydrogen bonding interactions has translated into the less acidic nature of the NH proton of the coordinated HL with estimated pKa > 12. 1(+) and 2(+) exhibit reversible Os(II)/(III) and irreversible Os(III)/(IV) processes in CH3CN within ± 2.0 V versus SCE. The effect of 3,5-dimethyl substituted HL2 on 2(+) has been reflected in the appreciable lowering (40 mV) of the Os(II/III) potential, along with the further decrease in the acidity of the NH proton (pKa > 13.0) with regard to HL1 coordinated 1(+) (pKa: ∼ 12.3). The electronic spectral features of Os(ii) (1(+)/2(+)) and electrochemically generated Os(III) (1(2+)/2(2+)) derived complexes have been analysed by TD-DFT calculations. The efficacy of the 1(+) and 2(+) encompassing free NH proton towards the anion recognition process has been evaluated by different experimental investigations using a wide variety of anions. It however establishes that receptor 1(+) can recognise both F(-) and OAc(-) in acetonitrile solution, while 2(+) is exclusively selective for the F(-) ion.

Cyclometalated Osmium-Amine Electronic Communication through the p-Oligophenylene Wire

A series of bis-tridentate cyclometalated osmium complexes with a redox-active triarylamine substituent have been prepared, where the amine substituent is separated from the osmium ion by a p-oligophenylene wire of various lengths. X-ray crystallographic data of complexes 3(PF6) and 4(PF6) with three or four repeating phenyl units between the osmium ion and the amine substituent are presented. These complexes show two consecutive anodic redox couples between +0.1 and +0.9 V vs Ag/AgCl, with the potential splitting in the range of 300-390 mV. A combined experimental and theoretical study suggests that, in the one-electron-oxidized state, the odd electron is delocalized for short congeners and localized on the osmium component for long congeners. The electronic coupling parameter (Vab) was estimated by the Marcus-Hush analysis. The distance dependence plot of ln(Vab) versus the osmium-amine geometrical distance (Rab) gives a negative linear relationship with a decay slope of -0.19 Å(-1), which is slightly steeper with respect to the previously reported ruthenium-amine series with the same molecular wire. DFT calculations with the long-range-corrected UCAM-B3LYP functional gave more reasonable results for the osmium complexes with respect to those with UB3LYP.

Recognition of fractional non-innocent feature of osmium coordinated 2,2′-biimidazole or 2,2′-bis(4,5-dimethylimidazole) and their interactions with anions

The non-innocent feature of osmium coordinated doubly deprotonated 2,2′-biimidazole derivatives in symmetric dimeric complexes leads to a mixed electronic structural configuration.

Counter-ligand control of the electronic structure in dinuclear copper-tetrakisguanidine complexes

Decision-making counter-ligands: a bridging redox-active ligand in a dinuclear copper complex could be either neutral (complex type [Cu^II-GFA-Cu^II]) or dicationic (complex type [Cu^I-GFA-Cu^I]), depending on the nature of the counter-ligands X.