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

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 Mossbauer 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