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°′(M2+/3+) = 284 mV vs. Fc/Fc+) followed by oxidation of the Tara0/+ 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 [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) 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.