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

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Schnaubelt, Linda ; Petzold, Holm ; Hörner, Gerald ; Rüffer, Tobias ; Klein, Niels ; Lang, Heinrich:
Tailoring of the Frontier Orbital Character in Co²⁺/³⁺ Complexes with Triarylamine Substituted Terpyridine Ligands.
In: European Journal of Inorganic Chemistry. Bd. 2019 (2019) Heft 7 . - S. 988-1001.
ISSN 1099-0682
DOI: https://doi.org/10.1002/ejic.201801433

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Abstract

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.