Molecular Wheels: New Mn12 Complexes as Single-Molecule Magnets

The preparation, structure and magnetic properties of three new wheel-shaped dodecanuclear manganese complexes, [Mn-12(Adea)(8)(CH3COO)(14)]center dot 7CH(3)CN (1 center dot 7CH(3)CN), [Mn-12(Edea)(8)(CH3CH2COO)(14)] (2) and [Mn-12(Edea)(8)(CH3COO)(2)-(CH3CH2COO)(12)] (3), are reported, where Adea(2-) and Edea(2-) are dianions of the N-allyl diethanolamine and the N-ethyl diethanolamine ligands, respectively. Each complex has six Mn(II) and six Mn(Ill) ions alternating in a wheel-shaped topology, with eight n-substituted diethanolamine dianions. All variable-temperature direct current (DC) magnetic susceptibility data were collected in 1, 0.1, or 0.01 T fields and in the 1.8-300 K temperature range. Heat capacity data, collected in applied fields of 0-9 T and in the 1.8-100 K temperature range, indicate the absence of a phase-transition due to long-range magnetic ordering for 1 and 3. Variable-temperature, variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges. All complexes show out-of-phase signals in the AC susceptibility measurements, collected in a 50-997 Hz frequency range and in a 1.8-4.6 K temperature range. Extrapolation to 0 K of the in-phase AC susceptibility data collected at 50 Hz indicates an S = 7 ground state for 1, 2, and 3. Magnetization hysteresis data were collected on a single crystal of 1 in the 0.27-0.9 K range and on single crystals of 2 and 3 in the 0.1-0.9 K temperature range. Discrete steps in the magnetization curves associated with resonant quantum tunneling of magnetization (QTM) confirm these complexes to be single-molecule magnets. The appearance of extra QTM resonances on the magnetic hysteresis of I is a result of a weak coupling between two Mn ions at opposite ends of the wheel, dividing the molecule into two ferromagnetic exchange-coupled S = 7/2 halves. The absence of these features on 2 and 3, which behave as rigid spin S = 7 units, is a consequence of different interatomic distances.