Geometry-Driven Iminosemiquinone Radical to Cu(II) Electron Transfer and Stabilization of an Elusive Five-Coordinate Cu(I) Complex: Synthesis, Characterization, and Reactivity with KO2

Azo-oximate metal-carbonyl to metallocarboxylic acid via the intermediate Ir(III) radical congener: quest for co-ligand driven stability of open- and closed-shell complexes

The redox non-innocent behavior of the diaryl-azo-oxime ligand L^NOH1 has been accentuated via the synthesis of metastable anion radical complexes of type trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 (CO is trans to azo group of the ligand) by the oxidative coordination reaction of 1 with Vaska's complex. The stereochemical role of co-ligands vis-à-vis the interplay of π-bonding has been found to be decisive in controlling the aptitude of the coordinated redox non-innocent ligand to accept or reject an electron. This has been clarified via the isolation of quite a few complexes as well as the failure to synthesize some others. The oxidized analogues of type trans-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺2+ (CO and azo group of the ligand are trans) as well as its cis isomer cis-[Ir(L^NO-)Cl(CO)(PPh₃)₂]⁺3+ (CO and azo group of the ligand are cis) have been structurally characterized but the radical anion congener of the latter could not be synthesized. Furthermore, the closed shell complexes [Ir(L^NO-)Cl₂(PPh₃)₂] 4 and [Ir(L^NO-)₂Cl(PPh₃)] 5 have been well characterized by diffraction as well as spectral techniques but their corresponding azo anion radical complexes could not be isolated and this is attributed to the trans influence of ancillary ligands. The anion radical complexes trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 may be rapidly transformed to the metallocarboxylic acids trans-[Ir(L^NO-)Cl(CO₂H)(PPh₃)₂] 6via a proton-coupled electron transfer (PCET) process, thereby demonstrating the role of odd electron over the coordinated ligand framework to trigger metal-mediated carbonyl to carboxylic acid functionalization. Complexes 6 are further stabilized via intramolecular -CO₂H⋯ON- (carboxylic acid⋯oximato) H-bonding. The optoelectronic properties as well as the origin of transitions in the complexes were analyzed by TD-DFT and theoretical analysis, which further disclose that the odd electron in trans-[Ir(L^NO˙-)Cl(CO)(PPh₃)₂] 2 is primarily azo-oxime centric with very low contribution from the iridium center.

Ni(II) complexes of a new tetradentate NN'N''O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H₃L² in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N' and N'' (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L²)] 1 in CH₂Cl₂ established a three-membered electron-transfer series (oxidative response at E_1/2 = 0.57 V and reductive response at -0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L²)]^z (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF₆ affords [Ni(L²)](SbF₆) (2) and reduction of 1 with cobaltocene yields [Co(η⁵-C₅H₅)₂][Ni(L²)] (3). The molecular structures of 1·CH₃CN, 2·0.5CH₂Cl₂ and 3·C₆H₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹H NMR, X-band EPR (g_av = 2.006 (solid); 2.008 (CH₂Cl₂-C₆H₅CH₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni^II{(L²)˙^2-}], [Ni^II{(L²)^-}]⁺/1⁺ and [Ni^II{(L²)^3-}]^-/1^- electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic S_t (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state S_t = 0. Interestingly, the variable-temperature (2-300 K) magnetic measurement reveals that 1 with the S_t = 1/2 ground state attains the antiferromagnetic S_t = 0 state at a very low temperature, due to weak noncovalent interactions via π-π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800-1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.

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.

Complexes of Metal Halides with Unreduced o-(Imino)quinones

A series of complexes of metal halides with unreduced quinone-type ligands have been synthesized and characterized in detail. The 3,6-di-tert-butyl-o-benzoquinone (1) and 4,6-di-tert-butyl-N-aryl-substituted o-iminobenzoquinones (2-5) (aryl is 2,6-dimethylphenyl in 2, 2-methyl-6-ethylphenyl in 3, 2,6-diethylphenyl in 4, and 2,6-diisopropylphenyl in 5) were used to obtain the molecular complexes with metal 12 group halides as well as with indium(III) iodide. The molecular structures of five complexes, bearing an unreduced form of redox-active ligand, have been established by single-crystal X-ray analysis. The spectral data, electrochemical measurements, and DFT calculations indicate the significant transformations of the molecular orbitals of 1-5 upon complexation with Lewis acids. The reduction potentials of o-(imino)quinones in complexes with metal halides shift into the anodic region versus uncoordinated ones. The choice of metal halide allows varying the shift magnitude up to 1.7 V in 2·CdI₂. The change of the oxidizing ability of the 1-5 upon coordination with Lewis acids enables the oxidation of mercury and ferrocene, infeasible for free ligands.

Synthesis, crystallographic and spectroscopic characterization, and theoretical elucidation of an elusive aminyl radical containing a CuII-aminyl-iminosemiquinone complex

An elusive aminyl radical and an iminosemiquinone radical-coordinated square pyramidal Cu(ii) complex (1) have been isolated by the reaction between the noninnocent ligand H₄L_Py^(AP) and Cu(ClO₄)₂·6H₂O in the presence of Et₃N and air as the sole oxidant. The geometry and electronic structure of the complex were concluded by X-ray crystallography, magnetic and EPR measurements, and density functional theory (DFT) calculations. This work reports the first crystallographic example of the two different types of radicals co-existing in a stable complex.

A combined experimental and theoretical study on a single, unsupported oxo-bridged Mn(III,III) dimer coordinated to two iminobenzosemiquinone π-radical anions

Ligand H2LAP comprises a non-innocent 2-aminophenol unit and an innocent bis(pyridin-2-ylmethyl)amine unit. The ligand, upon reaction with an equivalent amount of Mn(ClO4)2·6H2O in the presence of Et3N under air in MeOH, provided a mono(oxo)-bridged dinuclear Mn2 complex ({[(LISQ)MnIII-O-MnIII(LISQ)][(ClO4)]2}; 1). X-ray crystal structure analysis of complex 1 revealed that in the dicationic unit, the physical oxidation state of each Mn ion was +III and the 2-aminophenol unit of ligand H2LAP was in its one-electron oxidized iminobenzosemiquinone form. 1H-NMR measurement of complex 1 confirmed that the complex acquired a diamagnetic ground state (St = 0). Thus, antiferromagnetic couplings among the paramagnetic centers were realized. The UV-Vis-NIR spectrum of complex 1 was consisted of ligand-to-metal charge-transfer transitions in the visible region, while ligand-to-metal and metal-to-ligand charge-transfer transitions were noticed in the near-infrared region due to the presence of iminobenzosemiquinone radical units. The cyclic voltammogram of the complex showed three one-electron oxidation waves and two one-electron reduction waves. While the first two oxidation processes were metal-based, the two successive reductions were ligand-centered. DFT-based theoretical studies confirmed the assignment.

36-Nuclearity Organophosphonate-Functionalized Polyoxomolybdates: Synthesis, Characterization and Selective Catalytic Oxidation of Sulfides

The crown-shaped 36-molybdate cluster organophosphonate-functionalized polyoxomolybdates with the highest nuclearity in organophosphonate-based polyoxometalate chemistry, (NH₄ )₁₉ Na₇ H₁₀ [Cu(H₂ O)TeMo₆ O₂₁ {N(CH₂ PO₃ )₃ }]₆ ⋅31 H₂ O, has been reported for the first time. The synthesized 36-molybdate cluster was characterized by routine techniques and tested as a heterogeneous catalyst for selective oxidation of sulfides with impressive catalytic and selective performances after heat treatment. High efficiency (TON=15333) was achieved in the selective oxidation of sulfides to sulfoxides, caused by the synergic effect between copper and polyoxomolybdates and the generation of the cuprous species during the heat treatment.

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.

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

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.

[CuII{(LISQ)˙-}2] (H2L: thioether-appended o-aminophenol ligand) monocation triggers change in donor site from N2O2 to N2O(2)S and valence-tautomerism

Using a potentially tridentate o-aminophenol-based redox-active ligand H₂L¹ (2-[2-(benzylthio)phenylamino]-4,6-di-tert-butylphenol) in its deprotonated form, [Cu(L¹)₂] has been synthesized and crystallized as [Cu^II(L¹)₂]·CH₂Cl₂ (1·CH₂Cl₂). A cyclic voltammetry experiment (in CH₂Cl₂; V vs. SCE (saturated calomel electrode)) on 1·CH₂Cl₂ exhibits two oxidative (E = 0.20 V (peak-to-peak separation, ΔE_p = 100 mV) and E = 0.90 V (ΔE_p = 140 mV)) and two reductive (E = -0.52 V (ΔE_p = 110 mV) and E = -0.92 V (ΔE_p = 120 mV)) responses. Upon oxidation using a stoichiometric amount of [Fe^III(η⁵-C₅H₅)₂](PF₆), 1·CH₂Cl₂ yielded [Cu(L¹)₂](PF₆) (2). Structural analysis (100 K) reveals that 1·CH₂Cl₂ is a four-coordinate bis(iminosemiquinonato)copper(ii) complex (CuN₂O₂ coordination), and that the thioethers remain uncoordinated. The twisted geometry of 1 (distorted tetrahedral) results in considerable changes in the electronic structure, compared to well-known square-planar analogues. Crystallographic analysis of 2 both at 100 K and at 293 K reveals that it is effectively a four-coordinate complex with a CuN₂OS coordination; however, a substantial interaction with the other phenolate O is observed. The metal-ligand bond distances and metric parameters associated with the o-aminophenolate rings indicate a valence-tautomeric (VT) equilibrium involving monocationic (iminosemiquinonato)(iminoquinone)copper(ii) and bis(iminoquinone)copper(i). Complex 1·CH₂Cl₂ is a three-spin system and a magnetic study (4-300 K) established that it has a S = 1/2 ground-state, owing to the strong antiferromagnetic coupling between the unpaired spin of the copper(ii) and the iminosemiquinonate(1-) π-radical anion. Electron paramagnetic resonance (EPR) spectral studies corroborate this result. Complex 2 is diamagnetic and the existence of VT in 2 was probed using variable-temperature (248-328 K) ¹H NMR and EPR (100-298 K) spectral measurements and X-ray photoelectron spectroscopic studies at 298 K. Remarkably, modification of the well-studied 2-anilino-4,6-di-tert-butylphenol by incorporation of a benzylthioether arm leads to the occurrence of VT in 2. The electronic structure of 1·CH₂Cl₂ and 2 has been assigned using density functional theory (DFT) calculations at the B3LYP-D3 level of theory. Time-dependent (TD)-DFT calculations have been performed to elucidate the origin of the observed UV-VIS-NIR absorptions.