Trihaloacetic acids: an investigation of steric and inductive ligand effects on the synthesis of [Mn12O12(O2CCX3)16(H2O)4] single-molecule magnets

Abstract

The preparation of three new single-molecule magnets (SMM), [Mn₁₂O₁₂(O₂CCX₃)₁₆(H₂O)₄], where X = F, Cl or Br, was attempted. Reaction of [Mn₁₂O₁₂(O₂C^tBu)₁₆(H₂O)₄] (1) with the carboxylic acid CF₃COOH yielded small black needle-like crystals of complex [Mn₁₂O₁₂(O₂CCF₃)₁₆(H₂O)₄]·2.5H₂O (2). Frequency-dependent out-of-phase ac signals in the range 4–7 K were observed, which indicates that complex 2 functions as an SMM. On the contrary, the use of the bulkier trihaloacetic acids CCl₃COOH and CBr₃COOH is shown to disrupt the formation of the [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄] single-molecule magnets, giving rise to the formation of low nuclearity clusters. On the basis of these results and by comparison with the stability of previously described Mn₁₂ complexes, an interplay of steric and inductive effects is proposed as the critical factor in determining the thermal stability of these complexes.