Structural characterization, magnetic properties, and electrospray mass spectrometry of two Jahn−Teller isomers of the single-molecule magnet [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄]

Abstract

The syntheses and characterization of two new, highly soluble, single-molecule magnets [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄]·2CF₃COOH·4H₂O (1) and [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄]·CF₃COOH·7H₂O (2) are reported. Compound 1 was isolated from the reaction of [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄] with trifluoroacetic acid in CH₂Cl₂. Compound 1 crystallizes in the tetragonal space group I4^- (No. 82) with unit cell parameters a = b = 18.128(3) Å, c = 13.048(3) Å, V = 4287.9(19) ų, Z = 2 and is isostructural to [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄]. Compound 2 was prepared from the reaction of [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄] with neat trifluoroacetic acid, and crystallizes in the monoclinic space group P2₁/n (No. 14) with unit cell parameters a = 15.221(8) Å, b = 21.870(12) Å, c = 27.217(15) Å, β = 90.53(1)°, V = 9060(9) ų, and Z = 4. The dc magnetic susceptibility measurements in the 2−300 K temperature range support a high-spin ground state. The magnetization data collected in the 1−7 T field range from 1.8 to 4.0 K were best fit to the parameters S = 10, g = 2.15, D = −0.65 cm^-1, and E = 0 cm^-1 for 1 and S = 10, g = 1.87, D = −0.34 cm^-1, and E = −0.10 cm^-1 for 2. The ac susceptibility data for compound 1 reveal out-of-phase (χ_m") signals in the 4−7 K temperature range, whereas the χ_m" signals for compound 2 appear below temperatures of 4 K. This variation in blocking temperatures is a consequence of the two different crystallographic forms of compounds 1 and 2. Compound 1 exhibits the same structural geometry and distortions found in [Mn₁₂O₁₂(CF₃COO)₁₆(H₂O)₄], while compound 2 is of lower molecular symmetry with two Jahn−Teller axes of distortion being oriented along oxide ligands. This different structural arrangement facilitates a different tunneling pathway that leads to a lower effective barrier for magnetization reorientation for compound 2. The substitution of the acetate ligands by trifluoroacetic acid was monitored by mass spectrometry, which is a convenient tool for judging completion of the substitution process.