Preparation of a magnetic metal–organic square and metal–organic cubes using 4,5-bis(2-imidazolinyl)imidazolate: slow magnetization relaxation behavior in mixed-valent octamanganese(ii/iii) clusters

Abstract

Functional metal–organic squares (MOSs) and metal–organic cubes (MOCs) are important building units for zeolite-like metal–organic frameworks (ZMOFs), which are required to exhibit solid-state properties, such as dielectric, conductive, and magnetic properties. This work describes the preparation and magnetism of a tetracopper(II) macrocyclic complex [Cu^II₄(im-H₂bizn)₄(DMSO)₃(THF)](ClO₄)₄·8DMSO (1) (Him-H₂bizn = 4,5-bis(4,5-dihydro-1H-imidazol-2-yl)imidazole) as a MOS and octametallic clusters [Ni^II₈(im-H₂bizn)₁₂](ClO₄)₄·10MeOH·3H₂O (2) and [Mn^II₄Mn^III₄(im-H₂bizn)₁₂](ClO₄)₈·14MeOH (3) as MOCs. The Cu^II ion in 1 possesses a five-coordinated square pyramidal geometry, resulting in the formation of an M₄L₄-type square, which gives an estimated intramolecular antiferromagnetic interaction with an exchange coupling constant of J_Cu–Cu = −95 K. Meanwhile, 2 and 3 present six-coordinated octahedral geometries, giving M₈L₁₂-type cubes, of which 2 is a normal paramagnetic compound with intramolecular antiferromagnetic interactions, and where J_Ni–Ni = −32 K. The most notable compound 3 is a Mn^II₄Mn^III₄ mixed valence state compound, which exhibits a slow magnetization relaxation behavior similar to that of single-molecule magnets. This is attributed to the contribution of magnetic anisotropy caused by the Jahn–Teller effect of the Mn^III ions. Utilizing a modified Arrhenius plot to extract the values of the thermal barrier for magnetization reversal (E_a/k_B) and the pre-exponential factor (τ₀), the parameters for the relaxation behavior were estimated to be E_a/k_B = 6.38 K and τ₀ = 3.87 × 10⁻⁷ s. UV-vis spectroscopy and electrochemical measurements in solution were also carried out. Compound 3 will be expected to lead to a solid-state material in which the magnetic and dielectric properties of encapsulated small molecules cooperate with the slow magnetization relaxation properties of the MOC backbone.