Unexpected Formation of a Cobalt(III) Phenoxazinylate Electron Reservoir

Ni(II) and Pd(II) complexes of a new redox-active pentadentate azo-appended 2-aminophenol ligand: Pd(II)-assisted intraligand cyclization forms a phenoxazinyl ring

Square planar complexes of Ni(II) and Pd(II) of a new redox-active pentadentate azo-appended 2-aminophenol ligand (H4L = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino-ortho-azobenzene) in three accessible redox levels [amidophenolate(2-), semiquinonate(1-) π radical, and quinone(0)] were synthesized. The coordinated HL(3-) ligand provides four donor sites [two N(iminophenolates), an N'(azo), and an O(phenolate)], while the phenolic OH group remains free in the three complexes. Cyclic voltammetry on complex [Ni(L)] 1 and its corresponding Pd(II) analogue [Pd(L)] 2 in CH2Cl2 displayed three redox responses (two oxidative at E1/2 = 0.06 V and Epa (anodic peak potential) = 0.80 V and one reductive at -0.77 V for 1 and at E1/2 = 0.08 V and Epa = 0.85 V and at -0.74 V for 2vs. Fc+/Fc). The chemical oxidation of 1 with AgSbF6 afforded [Ni(L)]SbF6·2CH2Cl2 (3·2CH2Cl2). Complex [Pd(L*)] 4, which is coordinated by a phenoxazinyl derivative of L(4-), was obtained via intraligand cyclization in the parent complex 2 under basic oxidizing conditions. The molecular structures of 1, 2, 3·2CH2Cl2 and 4 were elucidated through X-ray crystallography at 100 K. Characterization using 1H NMR, X-band EPR, and UV-VIS-NIR spectroscopy established that the complexes have [NiII{(LISQ)˙2-}] 1, [PdII{(LISQ)˙2-}] 2, [NiII{(LIBQ)-}]SbF6/1+SbF6-(3), and [PdII{(L*AP)˙2-}] 4 electronic states. Complexes 1, 2, and 4 possess paramagnetic St (total spin) = 1/2 ground-state, whereas 3 is diamagnetic (St = 0). Density functional theory (DFT) electronic structural calculations at the B3LYP level rationalized the observed experimental results. Time-dependent (TD)-DFT calculations allowed us to identify the nature of the observed absorption spectra.

Phenoxazinyl Zn(II) diradical complex formed via redox-driven cyclization of a 2-aminophenol-based N3O ligand. Isolation of the modified N3 ligand radical and its Ni(II) complex

Aerobic reaction between the pyridine-2-carboxamide-2-aminophenol N3O ligand (H3L1) and Zn(ClO4)2·6H2O in CH3CN affords an N3 phenoxazinylate coordinated Zn(II) complex; its diradical electronic structure [ZnII{(L1*)˙-}2] has been elucidated from redox, spectroscopic (UV-VIS and EPR), and magnetic measurements and DFT calculations. Isolation and characterization of the metal-assisted redox-driven modified N3 ligand as a radical cation (H2L1*)˙+ and its Ni(II)-diradical complex [NiII{(L1*)˙-}2] have also been achieved.

Metal Complexes of Redox Non-Innocent Ligand N,N'-Bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine

Redox non-innocent ligands react with metal precursors to form complexes where the oxidation states of the ligand and thus the metal atom cannot be easily defined. A well-known example of such ligands is bis(o-aminophenol) N,N'-bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine, previously developed by the Wieghardt group, which has a potentially tetradentate coordination mode and four distinct protonation states, whereas its electrochemical behavior allows for five distinct oxidation states. This rich redox chemistry, as well as the ability to coordinate to various transition metals, has been utilized in the syntheses of metal complexes with M2L, ML and ML2 stoichiometries, sometimes supported with other ligands. Different oxidation states of the ligand can adopt different coordination modes. For example, in the fully oxidized form, two N donors are sp2-hybridized, which makes the ligand planar, whereas in the fully reduced form, the sp3-hybridized N donors allow the formation of more flexible chelate structures. In general, the metal can be reduced during complexation, but redox processes of the isolated complexes typically occur on the ligand. Combination of this non-innocent ligand with redox-active transition metals may lead to complexes with interesting magnetic, electrochemical, photonic and catalytic properties.

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.

A diamagnetic iron complex and its twisted sister - structural evidence on partial spin state change in a crystalline iron complex

We report here the syntheses of a diamagnetic Fe complex [Fe(HL)₂] (1), prepared by reacting a redox non-innocent ligand precursor N,N'-bis(3,5-di-tert-butyl-2-hydroxy-phenyl)-1,2-phenylenediamine (H₄L) with FeCl₃, and its phenoxazine derivative [Fe(L')₂] (2), which was obtained via intra-ligand cyclisation of the parent complex. Magnetic measurements, accompanied by spectroscopic, structural and computational analyses show that 1 can be viewed as a rather unusual Fe(III) complex with a diamagnetic ground state in the studied temperature range due to a strong antiferromagnetic coupling between the low-spin Fe(III) ion and a radical ligand. For a paramagnetic high-spin Fe(II) complex 2 it was found that, when crystalline, it undergoes a thermally induced process where 25% of the molecules in the material change to a diamagnetic low-spin ground state below 100 K. Single crystal X-ray studies conducted at 95 K afforded detailed structural evidence for this partial change of spin state of 2 showing the existence of crystallographically distinct molecules in a 3 : 1 ratio which exist in high- and low-spin states, respectively. Also, the magnetic behaviour of 2 was found to be related with the crystallinity of the material as demonstrated by near-IR radiation to unpaired electrons conversion ability of amorphous sample of 2.

A Sterically Hindered Derivative of 2,1,3-Benzotelluradiazole: A Way to the First Structurally Characterised Monomeric Tellurium-Nitrogen Radical Anion

Interaction of the tetradentate redox-active 6,6'-[1,2-phenylenebis(azanediyl)]bis(2,4-di-tert-butylphenol) (H₄ L) with TeCl₄ leads to neutral diamagnetic compound TeL (1) in high yield. The molecule of 1 has a nearly planar TeN₂ O₂ fragment, which suggests the formulation of 1 as Te^II L^2- , in agreement with the results of DFT calculations and QTAIM and NBO analyses. Reduction of 1 with one equivalent of [CoCp₂ ] leads to quantitative formation of the paramagnetic salt [CoCp₂ ]⁺ [1]^.- , which was characterised by single-crystal XRD. The solution EPR spectrum of [CoCp₂ ]⁺ [1]^.- at room temperature features a quintet due to splitting on two equivalent ¹⁴ N nuclei. Below 150 K it turns into a broad singlet line with two weak satellites due to the splitting on the ¹²⁵ Te nucleus. Two-component relativistic DFT calculations perfectly reproduce the a(¹⁴ N) HFI constants and A_∥ (¹²⁵ Te) value responsible for the low-temperature satellite splitting. Calculations predict that the additional electron in 1^.- is localised mainly on L, while the spin density is delocalised over the whole molecule with significant localisation on the Te atom (≥30 %). All these data suggest that 1^.- can be regarded as the first example of a structurally characterised monomeric tellurium-nitrogen radical anion.

Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands

New soluble and intensely near-IR-absorbing transition metal (Ti, Zr, V, Ni) complexes were synthesized using a redox non-innocent N,N'-bis(3,5-di-tertbutyl-2-hydroxy-phenyl) -1,2-phenylenediamine (H4L) as a ligand precursor. In all the complexes, ([Ti(Lox)2, [Zr(Lox)2], [V(Lsq1)(HLox)] and [Ni(HLox)2], two organic molecules coordinate to the metal center as tri- or tetradentate ligands. The solid-state structures of the complexes were determined using single crystal XRD, and the compounds were further characterized with Electrospray Ionisation Mass Spectrometry (ESI-MS). Thermoanalytical measurements indicated the thermal stabilities of the complexes. All compounds absorb strongly in the near-IR region and show very interesting magnetic and electrochemical properties. Moreover, it was shown that the V and Ni complexes can also convert absorbed near-IR photons to (un)paired electrons, which indicates great promise in photovoltaic applications.

Stabilization of different redox levels of a tridentate benzoxazole amidophenoxide ligand when bound to Co(iii) or V(v)

The electronic structure of Co and V complexes of a tridentate benzoxazole-containing aminophenol ligand NNOH2 were characterized by both experimental and theoretical methods.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Structural snapshots of the rearrangement of the bis(di-tert-butyl-aminophenyl)amine pincer ligand in the presence of transition metal ions

The bis(di-tert-butyl-aminophenyl)amine ligand H3LN,N,N was reacted with Mn(ii), Co(ii), Fe(iii) and Cu(ii) salts in air. The ligand undergoes oxidative transformations, which involve intra and intermolecular C-N and N-N bond formations. A rare aromatic C-N bond cleavage leading to a C-O bond has also been observed.

Nickel(II) Complex of a Hexadentate Ligand with Two o-Iminosemiquinonato(1-) π-Radical Units and Its Monocation and Dication

Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H4L(3) = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)-ethane, in CH3OH with Ni(II)(O2CCH3)2·4H2O and Et3N afforded isolation of a reddish-brown crystalline solid [Ni(L(3))] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [Fe(III)(η(5)-C5H5)2][PF6] and AgBF4 in CH2Cl2 led to the isolation of one-electron oxidized species [1](1+) as purple [1][PF6]·CH2Cl2 and two-electron oxidized species [1](2+) as dark purple [1][BF4]2·CH2Cl2, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [Ni(II){(L(ISQ)O,N)(•-)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}] 1, [Ni(II){(L(IBQ)O,N)(0)}{(L(ISQ)O,N)(•-)}{(LS,S)(0)}][PF6]·CH2Cl2, and [Ni(II){(L(IBQ)O,N)(0)}{(L(IBQ)O,N)(0)}{(LS,S)(0)}][BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate(1-) π-radical (Srad = 1/2) (L(ISQ))(•-) and neutral o-iminoquinone (L(IBQ))(0) redox-levels. Complexes 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2-300 K) magnetic behavior of 1 and [1][PF6]·CH2Cl2, and a μeff value of [1][BF4]2·CH2Cl2 at 300 K. Both 1 and [1][PF6]·CH2Cl2 exhibit ferromagnetic exchange-coupling between the two electrons of Ni(II) and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1](1+), and [1](2+). The observed UV-vis-NIR absorptions for 1, [1][PF6]·CH2Cl2, and [1][BF4]2·CH2Cl2 have been assigned, based on time-dependent (TD)-DFT calculations.

Evaluation of the coordination preferences and catalytic pathways of heteroaxial cobalt oximes towards hydrogen generation

Three new heteroaxial cobalt oxime catalysts, namely [CoIII(prdioxH)(4tBupy)(Cl)]PF6 (1), [CoIII(prdioxH)(4Pyrpy)(Cl)]PF6 (2), and [CoIII(prdioxH)(4Bzpy)(Cl)]PF6 (3) have been studied. These species contain chloro and substituted tert-butyl/pyrrolidine/benzoyl-pyridino ligands axially coordinated to a trivalent cobalt ion bound to the N4-oxime macrocycle (2E,2'E,3E,3'E)-3,3'-(propane-1,3-diylbis(azanylylidene))bis(butan-2-one)dioxime, abbreviated (prdioxH)- in its monoprotonated form. Emphasis was given to the spectroscopic investigation of the coordination preferences and spin configurations among the different 3d6 CoIII, 3d7 CoII, and 3d8 CoI oxidation states of the metal, and to the catalytic proton reduction with an evaluation of the pathways for the generation of H2via CoIII-H- or CoII-H- intermediates by mono and bimetallic routes. The strong field imposed by the (prdioxH)- ligand precludes the existence of high-spin configurations, and 6-coordinate geometry is favored by the LSCoIII species. Species 1 and 3 show a split CoIII/CoII electrochemical wave associated with partial chemical conversion to a [CoIII(prdioxH)Cl2] species, whereas 2 shows a single event. The reduction of these CoIII complexes yields LSCoII and LSCoI species in which the pyridine acts as the dominant axial ligand. In the presence of protons, the catalytically active CoI species generates a CoIII-H- hydride species that reacts heterolytically with another proton to generate dihydrogen. The intermediacy of a trifluoroacetate-bound CoIII/CoII couple in the catalytic mechanism is proposed. These results allow for a generalization of the behavior of heteroaxial cobalt macrocycles and serve as guidelines for the development of new catalysts based on macrocyclic frameworks.

A singlet ground state for a cobalt(II)-anilinosalen radical complex

The cobalt(II) anilinosalen complex [Co(II)(L)] was prepared and subsequently oxidized by one electron. The resulting cation comprises a square planar low spin Co(II) ion anti-ferromagnetically exchange coupled to an anilinyl radical.

Modulation of electronic and redox properties in phenolate-rich cobalt(iii) complexes and their implications for catalytic proton reduction

The redox, spectroscopic and catalytic activity of a series of cobalt complexes with phenolate-rich environments was investigated. The complex [Co^III(L^Cl)MeOH] shows considerable proton reduction in MeCN:HOAc with TON = 10.8.