The properties of the [Mn12O12(O2CR)16(H2O)4] single-molecule magnets in truly axial symmetry:: [Mn12O12(O2CCH2Br)16(H2O)4]•4CH2Cl2

Detailed studies are reported of a Mn-12 single-molecule magnet (SMM) in truly axial (tetragonal) symmetry. The complex is [Mn12O12(O2CCH2Br) 16(H2O)(4)]center dot 4CH(2)Cl(2) (2 center dot 4CH(2)Cl(2) or Mn-12-BrAc), obtained by the standard carboxylate substitution method. The complex has an S = 10 ground state, typical of the Mn-12 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)]center dot 2MeCO(2)H center dot 4H(2)O (1 or Mn-12-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 Mn-55 NMR spectra on Mn-12-BrAc display much sharper signals than a single crystal of Mn-12-Ac, and this allows one Mn-III signal to show an almost baseline-resolved quintet from quadrupolar splitting (Mn-55, I = 5/2, 100%), allowing quadrupole coupling parameters (e(2)qQ) to be determined. In addition, it was found that crushing crystals of Mn-12-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 Mn-12-BrAc is far superior to Mn-12-Ac for the study of the intrinsic properties of the Mn-12 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.