A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines

Abstract

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.

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

“…As the experiment rules out an open‐shell formulation of the oxidation product, massive secondary electron reorganisation necessarily takes place upon initial oxidation. This pathway supports the previously observed mediator effect of the Taras concerning the Co 2+/3+ redox process . As an alternative, a HS to LS SCO step may be invoked prior to oxidation, equilibrating the energy levels of the ligand and metal donors.…”

“…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 ).…”

“…Intriguingly, however, the HOMO‐LUMO gap of 2.4 eV in [Co( 1 ) 2 ] 3+ is already way smaller than the value of 4.2 eV found in parent [Co(tpy) 2 ] 3+ ; this reduction in the formal singlet‐triplet promotion energy mainly refers to high‐lying Tara donor levels. It is noted that the HOMO‐LUMO gap is further decreased to only 1.5 eV in the related system [Co( 5 ) 2 ] 3+ , which has been recently shown to underlie a thermal equilibration among ligand‐ and metal‐centred oxidation . In keeping with this, both triplet and quintet open‐shell formulations of [Co( 1 ) 2 ] 3+ are only moderately destabilised with respect to the closed‐shell singlet.…”

“…This pathway supports the previously observed mediator effect of the Taras concerning the Co 2+/3+ redox process. [39][40][41] As an alternative, a HS to LS SCO step may be invoked prior to oxidation, equilibrating the energy levels of the ligand and metal donors. Such way, post-oxidative electron rearrangements will be minimised.…”

“…Previously, we have reported a HS-Co 2+ /LS-Co 3+ redox couple in combination with Tara-substituted phenanthrolinepyridyl based ligands. [39][40][41] The investigations revealed both a light-induced charge transfer from the LS-Co 3+ to the Tara unit and an intramolecular redox equilibrium between 1 [LS-Co 3+ (L) 2 ] 3+ and 5 [HS-Co 2+ (L)(L +· )] 3+ . The CTIST behaviour of these complexes was strongly influenced by their environment and their structures.…”