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

Conflicting results have been reported with respect to the photoinduced switching of the magnetic properties of [FeIII(salten)]+ complexes [salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate)] coordinated by photoisomerizable ligands. In order to address this problem, two FeIII 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 cistrans 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.