Synthesis, crystal structure and magnetism of new salicylamidoxime-based hexanuclear manganese(iii) single-molecule magnets

Abstract

Salicylamidoxime was used to synthesize 13 new polynuclear Mn^III complexes. We present the crystallographic structures, the magnetic susceptibility and the magnetization measurements of eight of them (1–8) with the general formula [Mn₆O₂(H₂N-sao)₆(L)₂(solvent)_4–6] (L = carboxylate, chloride, 2-cyanophenolate; solvent = H₂O, MeOH, EtOH, py). These complexes consist of two trinuclear {Mn^III₃(μ₃-O)(H₂N-sao)₃}⁺ cationic units linked together via two oximate and two phenolate oxygen atoms. All behave as single-molecule magnets, with the spin ground state varying from 4 to 12 and anisotropy energy barriers from 24 to 86 K, the latter being as high as the present record barrier in the Mn₆ complexes. DFT calculations were performed to compute the exchange magnetic coupling constants J between the metallic ions and to provide an orbital interpretation of exchange. Our results are in line with previously reported results with the parent salicylaldoxime derivatives. The Mn–N–O–Mn torsion angle appears as the main parameter controlling the J values. The critical angle where the exchange coupling between two Mn^III switches from antiferromagnetic to ferromagnetic is 27°, less than the one found in related complexes with salicylaldoxime (30°). We propose a structural classification of the {Mn₆} complexes in four classes depending on the coordination of the axial carboxylate. The work points out the structural flexibility of such systems, their sensitivity to solvent effects and their ability to achieve high anisotropy energy barriers by simple desolvation.