A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines

Abstract

Complexes [Co(L)₂](ClO₄)₂ (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a–c) containing three redox active centres (a Co²⁺ 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) Co²⁺ and the diamagnetic low-spin (LS) Co³⁺ state, which was proven using ¹H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L)₂](ClO₄)₂, an equilibrium between the diamagnetic [Co³⁺(L)]³⁺ and paramagnetic [Co²⁺(L)(L⁺)]³⁺ state in [Co(L)₂]³⁺ (L = 1a–c) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between the [Co²⁺(L)₂]²⁺ and [Co³⁺(L)₂]³⁺ redox states mediated by [Co²⁺(L)(L⁺)]³⁺. Variable temperature vis-NIR spectroscopy of in situ generated [Co(L)₂]³⁺ revealed a temperature-dependent redox equilibrium between the [Co³⁺(L)₂]³⁺ and the [Co²⁺(L⁺)(L)]³⁺ states (L = 1a–c). Magnetic coupling between the HS-Co²⁺ ion and the Tara⁺ radical in [HS-Co²⁺(L⁺)(L)]³⁺ (L = 1a,c) was deduced from broad and undetectable lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co³⁺(L⁺)₂]⁵⁺ (L = 1a,c) leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.