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The Properties of the [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] Single-Molecule Magnets in Truly Axial Symmetry: [Mn 12 O 12 (O 2 CCH 2 Br) 16 (H 2 O) 4 ]·4CH 2 Cl 2

2006; American Chemical Society; Volume: 128; Issue: 21 Linguagem: Inglês

10.1021/ja060796n

1943-2984

N.E. Chakov, Sheng-Chiang Lee, Andrew Harter, P. L. Kuhns, A. P. Reyes, Stephen Hill, Naresh S. Dalal, Wolfgang Wernsdorfer, Khalil A. Abboud, George Christou,

Metal complexes synthesis and properties

Detailed studies are reported of a Mn12 single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn12O12(O2CCH2Br)16(H2O)4]·4CH2Cl2 (2·4CH2Cl2 or Mn12−BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn12 family, and displays frequency-dependent out-of-phase AC susceptibility signals and hysteresis in single-crystal magnetization vs applied DC field sweeps. Single-crystal high-frequency EPR spectra in frequencies up to 360 GHz exhibit narrow signals that are not overlapping multiplets, in contrast to [Mn12O12(O2CMe)16(H2O)4]·2MeCO2H·4H2O (1 or Mn12−Ac), which also crystallizes in an axial (tetragonal) space group but which now is recognized to consist of a mixture of six hydrogen-bonded isomers in the crystal and thus gives multiple, inhomogeneously broadened EPR signals. Similarly, single-crystal 55Mn NMR spectra on Mn12−BrAc display much sharper signals than a single crystal of Mn12−Ac, and this allows one MnIII signal to show an almost baseline-resolved quintet from quadrupolar splitting (55Mn, I = 5/2, 100%), allowing quadrupole coupling parameters (e2qQ) to be determined. In addition, it was found that crushing crystals of Mn12−BrAc into a microcrystalline powder causes severe broadening and shifts of the NMR resonances, emphasizing the superiority of single-crystal studies. The combined results establish that Mn12−BrAc is far superior to Mn12−Ac for the study of the intrinsic properties of the Mn12 family of SMMs in axial symmetry, and for the search for new phenomena such as quantum interference effects caused by higher-order (>2nd-order) transverse terms in the spin Hamiltonian.