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

Abstract

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.

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 ).…”

“…It is noted that the above‐cited complexes with ortho ‐appended O‐Aryl–N(Aryl) 2 exhibit a somewhat stabilised Co 3+ state in CH 3 CN solution, with potentials around 290 mV. By contrast, congeners with a phen‐pyr (phen = 1,10‐phenanthroline, pyr = pyridine) meridian substituting for the tpy‐scaffold of [Co( L ) 2 ](ClO 4 ) 2 (this work) exhibit even farther anodic shifts of the Co 2+/3+ couple by > 200 mV. Overall the Tara‐appended systems studied that far prove substantially harder to be oxidised than the parent [Co(tpy) 2 ] 2+ with its Co 2+/3+ couple located at E 0 1 ′ = –0.13 V .…”

“…Complexes [Co( L ) 2 ] 3+ ( L = 1 , 3 ) show a diamagnetic behaviour at ambient temperature revealing a d 6 LS configuration at cobalt, thus indicating metal‐based oxidation Co 2+ → Co 3+ . An alternative ligand‐centred oxidation, which was recently shown to prevail at high temperature for a very similar Tara‐decorated complex [Co 2+ ( 4 )( 4 + · )] 3+ [ 4 : N , N ‐bis(4‐methoxyphenyl)‐6‐(1,10‐phenanthrolin‐2‐yl)pyridin‐2‐amine], will instead yield open‐shell ligand‐radical complexes with d 7 HS configuration at cobalt . Aromatic proton resonances in [Co( L ) 2 ] 3+ ( L = 1 , 3 ) are found between 9.2 and 5.4 ppm (Figures S19, S20, Supporting Information) and exhibit the expected multiplicities.…”

“…Additionally, neutral Taras exhibit absorptions around λ = 300 nm , . A second set of significant but less intense absorptions in the vis region at around λ = 415 nm ( ε max = 8000 L mol –1 cm –1 ) is attributed to an intraligand charge transfer (ILCT) from the diaryl amine unit to the pyridine moieties , . Cobalt complexes deriving from native tpy lack of intense transitions in this spectral region , .…”

“…Previously, we have reported a HS‐Co 2+ /LS‐Co 3+ redox couple in combination with Tara‐substituted phenanthroline‐pyridyl based ligands . 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+ .…”