Redox properties and electron transfer in a triarylamine-substituted HS-Co 2+ /LS-Co 3+ 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)1 H 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 VT1 H NMR and VT/vis spectroscopy.