Tetra-, tetradeca- and octadecametallic clusters of Mn

Abstract

Reaction of equimolar amounts of MnBr₂·4H₂O and HL¹ ((3,5-dimethyl-1H-pyrazol-1-yl)methanol) in a basic MeCN solution leads to the formation of [Mn^II₄(L¹)₄Br₄(H₂O)₄] (1), whose metallic skeleton is a [Mn^II₄] tetrahedron and cluster core a [Mn^II₄(μ₃-O)₄] cubane. Replacing MnBr₂·4H₂O with Mn(O₂CMe)₂·4H₂O affords [Mn^III₂Mn^II₁₂O₂(L¹)₄(OAc)₂₂] (2) which is best described as a series of edge-sharing [Mn₄] tetrahedra that have self-assembled into a linear array in which each [Mn₂] pair is ‘twisted’ with respect to its neighbours in a corkscrew-like manner. Employment of the triangle [Mn^III₃O(OAc)₆(py)₃](ClO₄) as a reactant instead of a Mn^II salt results in the formation of [Mn^III₁₄Mn^II₄O₁₄(L¹)₄(HL¹)₂(OAc)₁₈(H₂O)₂] (3) whose core is comprised of three vertex-sharing [Mn^III₄] butterflies flanked on either side by one [Mn^III₄] cubane and one [Mn^III₂Mn^II₂] tetrahedron. Dc magnetic susceptibility and magnetisation measurements of polycrystalline samples of 1–3 reveal the predominance of antiferromagnetic exchange interactions. For [Mn₄] (1) this leads to a diamagnetic ground state, while for [Mn₁₈] (3) competing exchange interactions result in Single-Molecule Magnet (SMM) behaviour with U_eff = 22 K.