Redox properties and electron transfer in a triarylamine-substituted HS-Co2+/LS-Co3+ redox couple

Abstract

A new tridentate phenanthroline-pyridyl-based ligand 1 containing a redox active Tara (triaryl amine) unit has been developed (1 = 4-((6-(1,10-phenanthrolin-2-yl)pyridin-2-yl)oxy)-N,N-di-p-tolylaniline). The complex [Co²⁺(1)₂](ClO₄/BF₄)₂ was prepared and the order of the oxidation steps was analysed by cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. Oxidation of [Co²⁺(1)₂]²⁺ to [Co³⁺(1⁺)₂]⁵⁺ proceeds in two steps. The first step is the Co^2+/3+ centred oxidation to [Co³⁺(1)₂]³⁺ (E°′(M^2+/3+) = 284 mV vs. Fc/Fc⁺) followed by oxidation of the Tara^0/+ centres (E°′(Tara) = 531 mV). Both kinds of oxidation processes were independently investigated in the analogous complexes [Zn(1)₂](ClO₄)₂ and [Co(2)₂](BF₄)₂ allowing an assignment of changes in the electronic spectra to the redox states (2 = 2-(6-phenoxypyridin-2-yl)-1,10-phenanthroline). Although spectroelectrochemistry did not indicate substantial coupling between the redox centres the Tara unit is an efficient mediator for the self-exchange in the [Co^2+/3+(1)₂]^2+/3+ couple. The electron transfer by self-exchange in [Co^2+/3+(1)₂]^2+/3+ was further investigated by variable temperature (VT) ¹H NMR spectroscopy. In addition, the resonances found in the paramagnetic proton NMR spectra were assigned by using COSY, T₁ and EXSY measurements in combination with the Co–N distances obtained from X-ray analysis. [Co(1)₂]²⁺ is found in the HS state. In contrast, the Fe²⁺ species [Fe(1)₂](ClO₄)₂ is a spincrossover system. The SCO was analysed in solution by VT ¹H NMR and VT/vis spectroscopy.