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

The syntheses and characterization of two new, highly soluble, single-molecule magnets [Mn12O12(CF3COO)(16)-(H2O)(4)].2CF(3)COOH.4H(2)O (1) and [Mn12O12(CF3COO)(16)(H2O)(4)].CF3COOH.7H(2)O (2) are reported. Compound 1 was isolated from the reaction of [Mn12O12(CH3COO)(16)(H2O)(4)] with trifluoroacetic acid in CH2Cl2. Compound 1 crystallizes in the tetragonal space group l (4) over bar (No. 82) with unit cell parameters a = b = 18.128(3) Angstrom, c = 13.048(3) Angstrom, V = 4287.9(19) Angstrom(3), Z = 2 and is isostructural to [Mn12O12(CH3COO)(16)(H2O)(4)]. Compound 2 was prepared from the reaction of [Mn12O12(CH3COO)(16)(H2O)(4)] with neat trifluoroacetic acid, and crystallizes in the monoclinic space group P2(1)/n (No. 14) with unit cell parameters a = 15.221(8) Angstrom, b = 21.870(12) Angstrom, c = 27.217(15) Angstrom, beta = 90.53(1)degrees, V = 9060(9) Angstrom(3), 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 (chi(m)") signals in the 4-7 K temperature range, whereas the chi(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(CH3COO)(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.