Iron(III) Complexes of a Hexadentate Thioether-Appended 2-Aminophenol Ligand: Redox-Driven Spin State Switchover

A green complex [Fe(L³)] (1), supported by the deprotonated form of a hexadentate noninnocent redox-active thioether-appended 2-aminophenolate ligand (H₄L³ = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)ethane), has been synthesized and structurally characterized at 100(2) K and 298(2) K. In CH₂Cl₂, 1 displays two oxidative and a reductive one-electron redox processes at E_1/2 values of -0.52 and 0.20 V, and -0.85 V versus the Fc⁺/Fc redox couple, respectively. The one-electron oxidized 1⁺ and one-electron reduced 1^- forms, isolated as a blackish-blue solid 1(PF₆)·CH₂Cl₂ (2) and a gray solid [Co(η⁵-C₅H₅)₂]1·DMF (3), have been structurally characterized at 100(2) K. Structural parameters at 100 K of the ligand backbone and metrical oxidation state values unambiguously establish the electronic states as [Fe^III{(L^AP_O,N)^2-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] (1) (two tridentate halves are electronically asymmetric-ligand mixed-valency), [Fe^III{(L^ISQ_O,N)^•-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}]⁺ (1⁺), and [Fe^III{(L^AP_O,N)^2-}{(L^AP_O,N)^2-}{(L_S,S)⁰}]^- (1^-) [dianionic 2-amidophenolate(2-) (L^AP_O,N)^2- and monoanionic 2-iminobenzosemiquinonate(1-) π-radical (S_rad = 1/2) (L^ISQ)^•- redox level]. Mössbauer spectral data of 1 at 295, 200, and 80 K reveal that it has a major low-spin (ls)-Fe(III) and a minor ls-Fe(II) component (redox isomers), and at 7 K, the major component exists exclusively. Thus, in 1, the occurrence of a thermally driven valence-tautomeric (VT) equilibrium (asymmetric) [Fe^III{(L^AP_O,N)^2-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] ⇌ (symmetric) [Fe^II{(L^ISQ_O,N)^•-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] (80-295 K) is implicated. Mössbauer spectral parameters unequivocally establish that 1⁺ is a ls-Fe(III) complex. In contrast, the monoanion 1^- contains a high-spin (hs)-Fe(III) center (S_Fe = 5/2), as is deduced from its Mössbauer and EPR spectra. Complexes 1-3 possess total spin ground states S_t = 0, 1/2, and 5/2, respectively, based on ¹H NMR and EPR spectra, the variable-temperature (2-300 K) magnetic behavior of 2, and the μ_eff value of 3 at 300 K. Broken-symmetry density functional theory (DFT) calculations at the B3LYP-level of theory reveal that the unpaired electron in 1⁺/2 is due to the (L^ISQ)^•- redox level [ls-Fe(III) (S_Fe = 1/2) is strongly antiferromagnetically coupled to one of the (L^ISQ)^•- radicals (S_rad = 1/2)], and 1^-/3 is a hs-Fe(III) complex, supported by (L³)^4- with two-halves in the (L^AP)^2- redox level. Complex 1 can have either a symmetric or asymmetric electronic state. As per DFT calculation, the former state is stabilized by -3.9 kcal/mol over the latter (DFT usually stabilizes electronically symmetric structure). Time-dependent (TD)-DFT calculations shed light on the origin of observed UV-vis-NIR spectral absorptions for 1-3 and corroborate the results of spectroelectrochemical experiments (300-1100 nm) on 1 (CH₂Cl₂; 298 K). Variable-temperature (218-298 K; CH₂Cl₂) absorption spectral (400-1000 nm) studies on 1 justify the presence of VT equilibrium in the solution-state.

Where is the unpaired electron density? A combined experimental and theoretical finding on the geometric and electronic structures of the Co(iii) and Mn(iv) complexes of the unsymmetrical non-innocent pincer ONS ligand

The geometry and electronic structures of the Co and Mn complexes of the pincer H3LONS ligand composed of both hard and soft donor atoms at the coordinating sites are reported.

Probing the electronic structure of [Ru(L1)2]Z (z = 0, 1+ and 2+) (H2L1: a tridentate 2-aminophenol derivative) complexes in three ligand redox levels

Aerobic reaction between [RuII(DMSO)4Cl2], a redox-active 2-aminophenol-based ligand (H2L1: 2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) and Et3N in MeOH under refluxing conditions afforded a purple complex [Ru(L1)2] (S = 0). Structural analysis reveals that the tridentate ligand coordinates in a mer conformation providing a distorted octahedral RuN2O2S2 coordination. Cyclic voltammetry on 1 in CH2Cl2 reveals the accessability of the monocation, dication and monoanion forms. Reddish purple monocation [Ru(L1)2](PF6)·CH2Cl2 ([1OX1](PF6)·CH2Cl2; S = 1/2) and green dication [Ru(L1)2](BF4)2·H2O ([1OX2](BF4)2·H2O; S = 0) have been isolated through the chemical oxidation of 1 in CH2Cl2 by [FeIII(η5-C5H5)2](PF6) and AgBF4, respectively. A structural analysis of the single crystals of the monocation and the dication with the compositions [1OX1](PF6)·CH2Cl2·H2O (2) and [1OX2](BF4)2·1.7H2O (3), respectively, has been done. Metrical (metal-ligand and ligand backbone) parameters, values of metrical oxidation states of coordinated ligands, 1H NMR spectra of 1 and [1OX2](BF4)2·H2O, EPR spectra of [1OX1](PF6)·CH2Cl2, X-ray photoelectron and UV-VIS-NIR spectra of 1-3, spin population analysis from broken-symmetry (BS) density functional theory (DFT) calculations and quasi-restricted orbital (QRO) analysis have allowed us to assign the electronic structure of the complexes. The complexes exhibit highly covalent metal-ligand interactions. The electronic states of 1, [1OX1]1+ and [1OX2]2+ are best described as [RuII{(LISQ)˙-}2] ↔ [RuIII{(LAP)2-}{(LISQ)˙-}] (S = 0), [RuIII{(LISQ)˙-}2]1+ (S = 1/2) and [RuII{(LIBQ)0}2]2+ ↔ [RuIII{(LISQ)˙-}{(LIBQ)0}]2+ (S = 0), respectively. Notably, all redox processes are ligand-centred. Absorption spectral properties have been rationalized based on time-dependent (TD)-DFT calculations. For 1, the appearance of an IVCT band at 1100 nm supports its Class II-III (borderline) ligand-based mixed-valence character.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Ancillary ligand electro-activity effects towards phenyl acetylene homocoupling reaction by a nickel(ii) complex of a non-innocent O-amino phenol ligand: a mechanistic insight

A new Ni(ii) complex, was synthesized from the reaction of a non-innocent o-aminophenol ligand, and Ni(OAc)2. The crystal structure of NiIIL2 NIS (in which, IS stands for iminosemiquinone radical ligand with cyanide (shown by N in NIS) substituent on phenolate rings) exhibits the square planar environment of Ni(ii). The complex has been crystalized in the monoclinic system and Ni(ii) was surrounded by two oxygen and two nitrogen atoms of two ligands. Variable-temperature magnetic susceptibility measurement for crystalline samples of complex shows the effective magnetic moment per molecule (μ eff) of near zero and the diamagnetic nature of the complex (S = 0) which emphasize that strong antiferromagnetic coupling prevailed between the two unpaired electrons of LNIS ligands and Ni(ii) high spin electrons. The complex is EPR silent which confirms the diamagnetic character of the Ni(ii) complex. Electrochemical measurement (CV) indicates the redox-active character of ligand and metal. NiIIL2 NIS complex proved to be effective for free metal- or base counterpart homocoupling of phenyl acetylene at room temperature. To the best of our knowledge, this is the first example of using Ni(ii) complex without using any reducing agent due to the promotion ancillary effect of non-innocent o-aminophenol ligand which acts as an "electron reservoir" and can reversibly accept and donate electrons in the catalytic cycle. The theoretical calculation confirms the magnetostructure, electronic spectrum and confirmed the suggested mechanism of phenyl acetylene homocoupling with emphasis on the role of non-innocent ligand electro-activity and the effect of ligand substituent on the efficiency and stability of the complex.

Tuning of the redox potential and catalytic activity of a new Cu(ii) complex byo-iminobenzosemiquinone as an electron-reservoir ligand

The synthesis and characterization of a new Cu(ii) complex, LNIS2Cu^II(L^NIS=o-iminobenzosemiquinone), are reported.