Unusual Magneto-Structural Features of the Halo-Substituted Materials [FeIII (5-X-salMeen)2 ]Y: a Cooperative [HS-HS]↔[HS-LS] Spin Transition

Abstract

X-ray structures of the halo-substituted complexes [Fe^III (5-X-salMeen)₂ ]ClO₄ (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N-H_amine ⋅⋅⋅O_perchlorate interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the Fe^III -salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C-H_imine ⋅⋅⋅O_phenolate interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and Mössbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]↔[HS-LS] with a broad plateau. In contrast, [Fe^III (5-Cl-salMeen)₂ ]BPh₄ ⋅2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.

Keywords: crystallographic phase transition; electronic and electrochemical processes; ferric spin crossover; noncovalent interactions; spin-transition plateau.