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.

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.

The noninnocent ligand H2LAP(Ph) contained a bulky phenyl substituent at the ortho position to the aniline moiety. The ligand reacted with 0.5 equiv of CuCl2·2H2O in the presence of Et3N under air and provided the corresponding Cu(II)-bis(imonosemiquinone) complex (1). The complex upon oxidation by a stoichiometric amount of ferrocenium hexafluorophosphate (FcPF6) yielded the four-coordinate [Cu(II)-(imonosemiquinone)(iminoquinone)]PF6 complex (3), while the oxidation by an equivalent amount of CuCl2·2H2O produced the five-coordinate Cu(I)-bis(iminoquinone)Cl complex (2). Thus, a ligand-based oxidation followed by ligand-to-metal electron-transfer was realized for the latter oxidation process. Removal of the Cl- ion from complex 2 rendered the four-coordinate complex 4. The oxidation state of both Cu(I) and iminoquinone moieties remained unaltered upon the change in the coordination number. All the complexes were characterized by X-ray crystallography. Complexes 2, 3, and 4 were diamagnetic with an St = 0 ground state as evident by electron paramagnetic resonance (EPR) and 1H NMR measurements. The UV-vis-NIR spectra of all the complexes were dominated by charge-transfer transitions. Two oxidations and two reductions waves were noticed in the cyclic voltammogram (CV) of complex 1. Complex 2 and complex 3 underwent one oxidation and three reductions. Unlike complex 3, which experienced ligand-based oxidation, in complex 2 the oxidation was metal-centered [oxidation of Cu(I)-to-Cu(II)]. UV-vis-NIR spectral changes during the fixed-potential coulometric one-electron oxidation and thereafter EPR analysis consolidated the metal-based oxidation in complex 2. Complex 2 was air stable; however, it oxidized KO2 to oxygen molecule, and complex 1 was formed in due course as evident by UV-vis-NIR spectral changes and EPR measurements. Time dependent density functional theory calculations have been incorporated to assign the transitions that appeared in the UV-vis-NIR spectra of the complexes.

Monoradical-containing four-coordinate Co(iii) complexes: homolytic S-S and Se-Se bond cleavage and catalytic isocyanate to urea conversion under sunlight.

Four-coordinate, monoradical-containing Co(iii) complexes participated in the non-innocent ligand driven homolytic cleavage of S-S and Se-Se bonds and catalyzed the conversion of RNCO (R = phenyl and naphthyl) to the corresponding urea derivatives (TON 480) in dry CH2Cl2 under sunlight stimulus.

Dioxygen Reactivity of an Iron Complex of 2-Aminophenol-Appended Ligand: Crystallographic Evidence of the Aromatic Ring Cleavage Product of the 2-Aminophenol Unit.

2-Aminophenol appended pentadentate ligand H2GanAP was synthesized by mixing equimolar amounts of 2-[bis(2-pyridylmethyl)aminomethyl]aniline (A) and 3,5-di-tert-butyl catechol in hexane in the presence of Et3N under air. The ligand reacted with Fe(ClO4)2·6H2O or Fe(ClO4)3·6H2O in the presence of tetrabutylammonium perchlorate, and Et3N under air and provided a µ2 oxo-bridged dinuclear iron complex (1). X-ray single-crystal analysis of complex 1 revealed the presence of a furan derivative, resulting from the oxidative aromatic C-C bond cleavage product of 2-aminophenol derivative, in the coordination sphere of each iron center. Mechanistic investigation for the formation of complex 1 established that in the absence of molecular oxygen no oxidation of the appended 2-amidophenolate unit took place. An iron(III)-amidophenolate complex, formed initially, further reacted with molecular oxygen and caused oxidative aromatic C-C bond cleavage via a putative alkylperoxo species.