Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc^+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M^-1 s^-1 compared to the high-spin to low-spin redox couple [Co(bpy)₃]^3+/2+ at 0.27 M^-1 s^-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)₃]^3+/2+ redox shuttle (19.8 vs 18.0 mA/cm²). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

Charge Transfer Chromophores Derived from 3d-Row Transition Metal Complexes

A series of new charge transfer (CT) chromophores of "α-diimine-M^II-catecholate" type (where M is 3d-row transition metals-Cu, Ni, Co) were derived from 4,4'-di-tert-butyl-2,2'-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) in accordance with three modified synthetic approaches, which provide high yields of products. A square-planar molecular structure is inherent for monomeric [Cu^II(3,6-Cat)(bipy^tBu)]∙THF (1) and Ni^II(3,6-Cat)(bipy^tBu) (2) chromophores, while dimeric complex [Co^II(3,6-Cat)(bipy^tBu)]₂∙toluene (3) units two substantially distorted heteroleptic D-M^II-A (where D, M, A are donor, metal and acceptor, respectively) parts through a donation of oxygen atoms from catecholate dianions. Chromophores 1-3 undergo an effective photoinduced intramolecular charge transfer (λ = 500-715 nm, extinction coefficient up to 10⁴ M^-1·cm^-1) with a concomitant generation of a less polar excited species, the energy of which is a finely sensitive towards solvent polarity, ensuring a pronounced negative solvatochromic effect. Special attention was paid to energetic characteristics for CT and interacting HOMO/LUMO orbitals that were explored by a synergy of UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT study. The current work sheds light on the dependence of CT peculiarities on the nature of metal centers from various groups of the periodic law. Moreover, the "α-diimine-M^II-catecholate" CT chromophores on the base of "late" transition elements with differences in d-level's electronic structure were compared for the first time.

Mono- and Bis(iminoxolene)iridium Complexes: Synthesis and Covalency in π Bonding

N-(2,6-Diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Diso) reacts with the (cyclooctadiene)iridium chloride dimer to form a monoiminoxolene complex, (Diso)Ir(cod)Cl. Reaction of 2 equiv of the iminoquinone with chlorobis(cyclooctene)iridium dimer affords the bis-iminoxolene (Diso)₂IrCl. This five-coordinate complex adopts a distorted square pyramidal structure with an apical chloride ligand and undergoes halide exchange to form an air-stable iodide complex. (Diso)₂IrCl can be reduced by one electron to form neutral, square planar (Diso)₂Ir, while oxidation with PhICl₂ gives octahedral trans-(Diso)₂IrCl₂. The cis isomer can be prepared by air oxidation of (Diso)₂IrCl; cis/trans isomerization is not observed even on prolonged heating. Structural and spectroscopic features of the complexes are consistent with the presence of strong, covalent π bonding between the metal and the iminoxolene ligands, with structural data suggesting between 45 and 60% iridium character in the π bonding orbitals, depending on the ancillary ligands. The spectroscopic similarity of (Diso)₂Ir and (Diso)₂IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.

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.

Designing bimetallic zeolitic imidazolate frameworks (ZIFs) for aqueous catalysis: Co/Zn-ZIF-8 as a cyclic-durable catalyst for hydrogen peroxide oxidative decomposition of organic dyes in water

ZIF-8 is well known hybrid material that is self-assembled from inorganic and organic moieties. It has several potential applications due to its unique structure. One of these potential applications is in advanced oxidation processes (AOP) via a heterogeneous catalysis system. The use of modified ZIF-8/H2O2 for the destruction of the azo dye methyl orange (MO) is presented in this work to explore its efficacy. This work presents the bimetallic Co/Zn-ZIF-8 as an efficient catalyst to promote H2O2 oxidation of the MO dye. Co/Zn-ZIF-8 was synthesized through a hydrothermal process, and the pristine structure was confirmed using XRD, FTIR, and XPS. The Co/Zn-ZIF-8/H2O2 system successfully decolorized MO at the selected pH 6.5. It was found that more than 90% of MO (10 ppm) was degraded within only about 50 minutes. Proposed radical and redox mechanisms are presented for H2O2 decomposition where the redox mechanism is suggested to predominate via a Co(ii)/Co(iii) redox consecutive cyclic process.

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.

Magnetically bistable cobalt-dioxolene complexes with a tetradentate N-donor base

We describe a family of cobalt-dioxolene complexes exhibiting a high diversity of magnetic properties: from field-induced single-ion magnetism to thermally induced valence-tautomerism.

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.

A trinuclear nickel(II) Schiff base complex with phenoxido- and acetato-bridges: combined experimental and theoretical magneto-structural correlation

We describe the synthesis and characterization of a new trinuclear Ni(ii) Schiff base complex of formula [Ni3(L)2(NCS)2(OAc)2(CH3OH)2] (1) where HL is the 1 : 1 condensation product of 2-picolylamine and o-vanillin. The crystal structure of complex 1 shows that the two terminal Ni(ii) ions are connected to the central one through a phenoxido- and a syn-syn acetato bridge, giving rise to a very bent configuration in the Ni3-core. Magnetic susceptibility measurements show the presence of a weak antiferromagnetic coupling with J = -3.22(2) cm-1. We also report a magneto-structural correlation, performed with all the magnetically characterized Ni(ii) trimers with similar bridges, showing a linear dependence between the J value and the dihedral angle (θ) between the planes containing the Ni-O-Ni and the carboxylate bridges. The super-exchange interaction is investigated by extensive density functional calculations within the broken symmetry approximation which show good agreement with the experimental data. The analysis of the spin density distribution and the shape of the magnetically active single occupied molecular orbitals (SOMO) provide a mechanism of exchange coupling through the bridging groups.

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.

Photocatalytic and electrocatalytic hydrogen production using nickel complexes supported by hemilabile and non-innocent ligands

Nickel complexes with non-innocent ligands generated by one-electron reduction of octahedral Schiff base nickel(ii) complexes with hemilabile ligands exhibited excellent catalytic activities of over 5000 TONs through a metal-ligand cooperation mechanism for hydrogen evolution from water under visible light irradiation.

Tetrahedral nickel(ii) and cobalt(ii) bis-o-iminobenzosemiquinonates

The first examples of tetrahedral nickel(ii) and cobalt(ii) bis-o-iminobenzosemiquinonates.

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.