Single-molecule magnets: Jahn–Teller isomerism and the origin of two magnetization relaxation processes in Mn 12 complexes

Abstract

Different crystallographic forms of the single molecule magnet [Mn 12O 12(O 2CR) 16(H 2O) 4] with a given R substituent have been isolated. X-ray structures are reported for two isomeric forms of the p-methylbenzoate complex, [Mn 12O 12(O 2CC 6H 4- p-Me) 16(H 2O) 4]·(HO 2CC 6H 4- p-Me) ( 2) and [Mn 12O 12(O 2CC 6H 4- p-Me) 16(H 2O) 4]·3H 2O ( 3). The Mn 12 molecules in complexes 2 and 3 are geometrical isomers differing in the positions of the H 2O and carboxylate ligands. In complex 2, one Mn III ion has an abnormal Jahn–Teller distortion axis oriented at an oxide ion, and thus 2 and 3 are also Jahn–Teller isomers. This reduces the symmetry of the core of complex 2 compared with that of complex 3. Complex 2 likely has a larger tunneling matrix element and this explains why this complex shows an out-of-phase ac peak ( χ M″) in the signal in the 2–3 K region, whereas complex 3 has its χ M″ peak in the 4–7 K range, i.e. the rate of tunneling of magnetization is greater in complex 2 than complex 3.