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 [Co(1)](ClO/BF) was prepared and the order of the oxidation steps was analysed by cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. Oxidation of [Co(1)] to [Co(1)] proceeds in two steps. The first step is the Co centred oxidation to [Co(1)] (E°'(M) = 284 mV vs. Fc/Fc) followed by oxidation of the Tara centres (E°'(Tara) = 531 mV). Both kinds of oxidation processes were independently investigated in the analogous complexes [Zn(1)](ClO) and [Co(2)](BF) 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(1)] couple. The electron transfer by self-exchange in [Co(1)] was further investigated by variable temperature (VT) H NMR spectroscopy. In addition, the resonances found in the paramagnetic proton NMR spectra were assigned by using COSY, T and EXSY measurements in combination with the Co-N distances obtained from X-ray analysis. [Co(1)] is found in the HS state. In contrast, the Fe species [Fe(1)](ClO) is a spincrossover system. The SCO was analysed in solution by VT H NMR and VT/vis spectroscopy.

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

Phenanthroline‐pyridyl bases containing a triarylamine (Tara) 1–4 and their respective Co2+ ([Co(L)2](ClO4)2, L = 1–4) and Co3+ complexes ([Co(L)2](ClO4)3–n(BF4)n, L = 2, 3) were prepared. The structure of [Co(3)2](ClO4)2 in the solid state was determined by single‐crystal X‐ray diffractometry. The oxidation processes of [Co(L)2](ClO4)2 (L = 1–4) were investigated by cyclic voltammetry. The first oxidation step in [Co(L)2](ClO4)2 (L = 1–3) is chemically reversible and cobalt‐centered. This process is followed by the irreversible oxidation of the Tara units at higher potentials. In case of [Co(4)2](ClO4)2 the first oxidation is Tara‐centered and electrochemically reversible. The electronic coupling between the Co3+ ion and Tara in in‐situ prepared [Co(L)2]3+ (L = 2, 3) was investigated by UV/Vis/NIR spectroelectrochemistry. In spectra of in situ prepared [Co(L)2]3+ (L = 2, 3) ligand‐to‐metal charge transfer (LMCT) bands were found between 600–1250 nm, which were assigned to a charge transfer from the Tara to the Co3+ ion. Physical parameters (wavenumber ν̃max, extinction εmax, and full‐width at half‐maximum Δν1/2) of the charge transfer (CT) bands were obtained by deconvolution of the UV/Vis/NIR spectra. In addition analogous Co3+ complexes [Co(L)2]3+ (L = 2, 3) were isolated as the mixed BF4–/ClO4– salts. These compounds exhibit the same CT bands as observed in the in situ oxidation experiments.

The X-factor: Exchange of pyridine (X = N) for phenyl (X = CH) substituents renders the coordination number of Fe2+ complexes spin-state dependent. Slow spin-state exchange emerges through kinetic and thermodynamic stabilization of the N6(+2) high-spin state.

Tara‐decorated terpyridines 1–3 and their 1:2 Co2+ [Co(L)2](ClO4)2 (L = 1–3) and Co3+ complexes [Co(L)2](ClO4)2(SbCl6) (L = 1,3) are reported. The solid‐state complex metrics of [Co(3)2]2+ and [Co(1)2]3+ indicate an assignment as spin‐quartet Co2+ or spin‐singlet Co3+, which is in accord with DFT and 1H NMR studies. Vis‐NIR spectra of 1[Co3+(L)2]3+ show characteristic Tara→Co3+ charge‐transfer bands, giving no direct indication of L‐based oxidation events, irrespective of the solvent. Theoretical studies reveal no charge transfer induced spin transition in [Co3+(L)2]3+, although a substantial decrease of the HOMO‐LUMO gap prevails in [Co3+(1)2]3+ as compared with [Co(tpy)2]3+. The destabilisation of the HOMO is not sufficient to allow an equilibration among closed‐ and open‐shell formulations. The computed singlet‐triplet and singlet‐quintet splitting in [Co(1)2]3+ renders these open‐shell formulations to reside only moderately above the ground state. Design criteria are derived from this analysis aiming at a further stabilisation of the open‐shell species through implementation of more strongly electron releasing Tara substituents.