Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

Abstract

We have determined the ultra-low temperature crystal structure of the archetypal single-molecule magnet (SMM) [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O·2MeCO₂H (1) at 2 K, by using a combination of single-crystal X-ray and single-crystal neutron diffraction. This is the first structural study of any SMM in the same temperature regime where slow magnetic relaxation occurs. We reveal an additional hydrogen bonding interaction between the {Mn₁₂} cluster and its solvent of crystallisation, which shows how the lattice solvent transmits disorder to the acetate ligands in the {Mn₁₂} complex. Unusual quantum properties observed in 1 have long been attributed to disorder. Hence, we studied the desolvation products of 1, in order to understand precisely the influence of lattice solvent on the structure of the cluster. We present two new axially symmetric structures corresponding to different levels of desolvation of 1, [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·4H₂O (2) and [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄] (3). In 2, removal of acetic acid of crystallisation largely resolves positional disorder in the affected acetate ligands, whereas removal of lattice water molecules further resolves the acetate ligand disorder in 3. Due to the absence of acetic acid of crystallisation, both 2 and 3 have true, unbroken S₄ symmetry, showing for the first time that it is possible to prepare fully axial Mn₁₂–acetate analogues from 1, via single-crystal to single-crystal transformations.