The Properties of the [Mn_(12)O_(12)(O_2CR)_(16)(H_2O)_4] Single-Molecule Magnets in Truly Axial Symmetry:[Mn_(12)O_(12)(O_2CCH_2Br)_(16)(H_2O)_4] centre dot 4CH_2Cl_2

摘要

Detailed studies are reported of a Mn_(12)single-molecule magnet (SMM)in truly axial (tetragonal)symmetry.The complex is [Mn_(12)O_(12)(O_2CCH_2Br)_(16)(H_2O)_4] centre dot 4CH_2Cl_2 (2 centre dot 4CH_2CI_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 [Mn_(12)O_(12)(O_2CMe)_(16)(H_2O)_4] centre dot 2MeCO_2H centre dot 4H_2O (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 ~(55)Mn 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 (~(55)Mn,I =5/2,100%),allowing quadrupole coupling parameters (e~2qQ)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.