A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines. Issue 37 (4th September 2018)

• Record Type: Journal Article
• Authors: Schnaubelt, Linda; Petzold, Holm; Dmitrieva, Evgenia; Rosenkranz, Marco; Lang, Heinrich
• Is Part Of: Dalton transactions. Volume 47:Issue 37(2018)
• Page Start: 13180
• Page End: 13189
• Publication Date: 2018-09-04
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c8dt02538j
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Synthesis of Co 2+ complexes bearing redox-active triaryl amines, exhibiting thermochromic redox equilibrium after one-electron oxidation. Abstract : Complexes [Co(L )2 ](ClO4 )2 (L = o -substituted 2-(pyridine-2-yl)-1, 10-phenanthrolines1a–c ) containing three redox active centres (a Co 2+ ion and two triaryl amine (Tara) units) have been synthesised. The order of oxidation steps in [Co(L )2 ](ClO4 )2 (L =1a–c ) was determined using cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. In acetonitrile solutions, at room temperature, the first oxidation is Co-centred followed by the Tara oxidation at more anodic potentials. The order of oxidation is inverted in solutions of the less polar solvent dichloromethane. The Co 3+/2+ -centred redox event leads to a spin transition between the paramagnetic high-spin (HS) Co 2+ and the diamagnetic low-spin (LS) Co 3+ state, which was proven using 1 H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L )2 ](ClO4 )2, an equilibrium between the diamagnetic [Co 3+ (L )] 3+ and paramagnetic [Co 2+ (L )(L + )] 3+ state in [Co(L )2 ] 3+ (L =1a–c ) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between the [Co 2+ (L )2 ] 2+ and [Co 3+ (L )2 ] 3+ redox states mediated by [Co 2+ (L )(L + )] 3+ . Variable temperature vis-NIR spectroscopy of in situ generated [Co(L )2 ] 3+ revealed a temperature-dependent redox equilibrium between the [Co 3+ (L )2 ] 3+ and the [Co 2+ (L + )(L )] 3+ states (L =1a–c ). Magnetic coupling between the HS-Co 2+ ion and the Tara + radical in [HS-Co 2+ (L + )(L )] 3+ (L =1a, c ) was deduced from broad and undetectable lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co 3+ (L + )2 ] 5+ (L =1a, c ) leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.