Fe^III Spin-Crossover Complexes with Photoisomerizable Ligands: Experimental and Theoretical Studies on the Ligand-Driven Light-Induced Spin Change Effect

Graphical Abstract

[Fe(salten)(3-azpy)]BPh₄ (1) and [Fe(salten)(4-azpy)]BPh₄ (2) have been synthesized and characterized [salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate), azpy = phenylazopyridine]. Both exhibit a low to high spin transition in the solid state and are predominantly high spin in solution. cis–trans isomerization of coordinated azpy leaves their spin equilibria almost unaffected.

Abstract

Conflicting results have been reported with respect to the photoinduced switching of the magnetic properties of [Fe^III(salten)]⁺ complexes [salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate)] coordinated by photoisomerizable ligands. In order to address this problem, two Fe^III complexes [Fe(salten)(3-azpy)]BPh₄ (1) and [Fe(salten)(4-azpy)]BPh₄ (2) have been synthesized and characterized by various physicochemical methods (azpy = phenylazopyridine). Both 1 and 2 exhibit a low spin (S = 1/2) to high spin (HS, S = 5/2) transition in the solid state. In solution at room temperature both complexes are predominantly HS. Upon exposure to 310 (trans → cis) and 440 nm radiation (cis → trans) the free and coordinated 3- and 4-azpy ligands undergo a reversible cis–trans isomerization. For 2 a corresponding reduction of the HS fraction 2 % is observed, whereas in 1 no effect is observed. Extensive DFT calculations, which employ different functionals and basis sets, explain this experimental result. The consequences of these findings with respect to the design of spin-switchable iron(III) complexes with photoactive ligands are discussed.