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

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 Co2+/3+ centred oxidation to [Co3+(1)(2)](3+) (E degrees'(M2+/3+) = 284 mV vs. Fc/Fc(+)) followed by oxidation of the Tara(0/+) centres (E degrees'(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 [Co2+/3+(1)(2)](2+/3+) couple. The electron transfer by self-exchange in [Co2+/3+(1)(2)](2+/3+) was further investigated by variable temperature (VT) H-1 NMR spectroscopy. In addition, the resonances found in the paramagnetic proton NMR spectra were assigned by using COSY, T-1 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 H-1 NMR and VT/vis spectroscopy.