Supramolecular Maleate Adducts of Copper(II) 12-Metallacrown-4: Magnetism, EPR, and Alcohol Sorption Properties

With maleate, [Cu₅(ahpha)₄]²⁺ gives two complexes, [Cu₅(ahpha)₄(DMF)₂](MalH)₂·2DMF (1) and [Cu₅(ahpha)₄(MalH)₂] (2) (ahpha^2– is 3-amino-3-hydroximinopropane hydroxamate; MalH₂ is maleic acid). Both 1 and 2 contain non-oligomerized [Cu₅(ahpha)₄]²⁺ metallacrown blocks; in 1, two MalH^– are bound to [Cu₅(ahpha)₄]²⁺ only via H-bonds, while in 2, they coordinate to Cu²⁺. The X-band EPR of 1 was observed at 20 K, 100 K, and 295 K. The EPR at 20 K was of an S = 1/2 ground state, while at room temperature anisotropic |ΔM_s| = 1 and |ΔM_s| = 2 resonances around 3200 G (g ≈ 2.19) and 1600 G (g ≈ 4.25) were observed for the Cu₅ system, due to partial occupation of an S = 3/2 state. The room temperature EPR of 1 was simulated as the weighted sum of the spectra of its various S = 1/2, 3/2, and 5/2 states. Its magnetic behavior was fitted with a χ_MT vs. T model using a spin Hamiltonian accounting for central–peripheral (J₁), peripheral–peripheral (J₂) Cu²⁺–Cu²⁺ and intercrown (zJ′) spin interactions, yielding J₁ = –156(4) cm^–1, J₂ = –63(1) cm^–1, and zJ′ = –4.5(3) cm^–1. For sorption of methanol and ethanol vapors at 293 K by a desolvated polycrystalline sample of 1, the shape of the adsorption and desorption isotherms indicated a “gates opening” phenomenon.