Synthesis, Molecular and Electronic Structures of Six-Coordinate Transition Metal (Mn, Fe, Co, Ni, Cu, and Zn) Complexes with Redox-Active 9-Hydroxyphenoxazin-1-one Ligands

Abstract

A series of pseudo-octahedral metal (M = Mn, Fe, Co, Ni, Cu, Zn) complexes 4 of a new redox-active ligand, 2,4,6,8-tetra(tert-butyl)-9-hydroxyphenoxazin-1-one 3, have been synthesized, and their molecular structures determined with help of X-ray crystallography. The effective magnetic moments of complexes 4 (M = Mn, Fe, Co, and Ni) measured in the solid state and toluene solution point to the stabilization of their high-spin electronic ground states. Detailed information on the electronic structure of the complexes and their redox-isomeric forms has been obtained using density functional theory (DFT) B3LYP*/6-311++G(d,p) calculations. The energy disfavored low-spin structures of manganese, iron, and cobalt complexes have been located, and based on the computed geometries and distribution of spin densities identified as Mn(IV)[(Cat-N-SQ)](2), Fe(II)[Cat-N-BQ)](2), and Co(II)[Cat-N-BQ)](2) compounds, respectively. It has been shown that stabilization of the high-spin structures of complexes 4 (M = Mn, Fe, Co) is caused by the rigidity of the molecular framework of ligands 3 that sterically inhibits interconversions between the redox-isomeric forms of the complexes. The calculations performed on complex 4 (M = Co) predict that a suitable structural modification that might provide for stabilization of the low-spin electromeric forms and create conditions for the valence tautomeric rearrangement via stabilization of the low-spin electromer and narrowing energy gap between the low-spin ground state tautomer and the minimal energy crossing point on the intersection of the potential energy surfaces of the interconverting structures consists in the replacement of an oxygen in the oxazine ring by a bulkier sulfur atom.

Rational Design of Electronically Labile Dinuclear Fe and Co complexes with 1,10‐Phenanthroline‐5,6‐Diimine: A DFT study

“…The density functional theory (DFT) calculations were performed using the Gaussian 09 program package with the standard 6‐311++G(d,p) basis set including diffuse and polarization functions at all atoms. As it was shown in the previously published works, depending on the central ion, the type of the magnetic bistability mechanism resulting in switching the magnetic characteristics, and the presence of charge of the complex, the best agreement with the experiment is achieved when using hybrid functionals UB3LYP* or UTPSSh . Due to the electroneutrality of the modeled compounds, all the calculations were performed with the use of modified UB3LYP functional—UB3LYP* .…”

Pseudo-octahedral nickel(ii) complexes of strongly absorbing benzannulated pincer-type amido ligands: ligand-based redox and non-Aufbau electronic behaviour

“…For example, redox-active biquinone O ^ N ^ O ligands have yielded transition metal complexes of unusual oxidation states, 5 often boasting impressive absorption profiles. 6 While these tridentate ligands contain central amido donors, their redox-active nature centres on the catecholate/semiquinonate couple. In the context of MCCs, nitrogen-based amido lone pairs can be oxidized to give metal-coordinated aminyl radicals and represent what is by now a common example of redox ‘non-innocence’.…”

Unexpected Formation of a Cobalt(III) Phenoxazinylate Electron Reservoir

“…When complexed to metals, the most commonly observed ligand oxidation state is 2– (H 4 L → L 2– + 4H + + 2e – , Scheme left), which can be written with both closed shell2 and diradical electron configurations (only one shown). To the best of our knowledge, however, similar adducts with middle 3d metals (Mn, Fe, Co) have not received similar attention, despite the intriguing combination of this ligand with redox‐rich metals 1,6–8. Towards this goal, we report here an unprecedented ligand transformation of L 2– whereby one of the phenolate/phenoxyl arms has cyclized to form a mixed phenolate/phenoxazinyl radical species, which we denote L′ 2– (L 2– → L′ 2– + H + + e – , Scheme right).…”