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 [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.

Tailoring of the Frontier Orbital Character in Co2+/3+ Complexes with Triarylamine Substituted Terpyridine Ligands

“…The assignment is corroborated by TD‐DFT analysis of [Co( 1 ) 2 ] 2+ , which predicts a single intense transition at λ = 463 nm, which actually exhibits ILCT character. Similar spectra have been recorded for a number of Co 2+ complexes of tpy or phen‐pyr scaffolds with appended Tara moieties . These latter transitions are expected to be susceptible towards oxidation events at the two redox centres Co 2+/3+ and Tara 0/+ in the complexes [Co( L ) 2 ](ClO 4 ) 2 ( L = 1 – 3 ).…”

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…A novel Co 2+ complex, 183 2+ , with phenanthroline‐based ligand is paramagnetic (Scheme ), however, the resonances of the 1 H NMR spectra were completely assigned by tracking the proton resonances from the Co 3+ to Co 2+ redox state by using 1 H‐ 1 H EXSY spectra (Figure ) . The absence of a charge‐transfer band for the electron transfer between the Co 2+/3+ and ligand redox centres in the UV/Vis/NIR region indicates a weak electronic coupling.…”

“…1 H-1 HE XSY NMR spectraof183 2 + ,which is partly oxidised to 183 3 + (Scheme59), in brackets are the chemical shifts in ppm, mixingt ime 100 ms. Adapted with permission from ref [266],. Copyright( 2017)t he Royal SocietyofC hemistry.Scheme60.…”

Spin Transition and Charge Transfer in Co2+/Co3+ Complexes of Meridional Ligands Holding Nearby Redox‐active Triarylamine

“…Recently, we reported a HS‐Co 2+ /LS‐Co 3+ redox couple with a triarylamine substituted phenanthroline‐pyridine ligand . The electronic and Coloumb interaction between the Tara 0/+ and Co 2+/3+ redox sites was weak at best and they were described as isolated redox sites.…”

“…The redox potentials of the Taras units were altered by using different substituents in para positions of the aryl moieties to equal the redox potential of the Co 2+/3+ redox couple and the Tara 0/+ redox couple. Co 2+ complexes of ligands 1 – 4 are exclusively HS complexes due to the weak ligand field strength of the phenanthroline‐pyridyl unit and the steric hindrance by bulky ortho substituent . The main aim of this work is the fine tuning and the photophysical properties of the intramolecular electron transfer equilibrium between the Tara 0/+ and the LS‐Co 3+ /HS‐Co 2+ redox sites in complexes of type [Co( L ) 2 ] 3+ with L = 1 – 4 .…”