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

Different crystallographic forms of the single molecule magnet [Mn12O12(O2C R)(16)(H2O)(4)] with a given R substituent have been isolated. X-ray struct ures are reported for two isomeric forms of the p-methylbenzoate complex, [ Mn12O12(O2CC6H4-pMe)(16)(H2O)(4)]. (HO2CC6H4-p-Me) (2) and [Mn12O12(O2CC6H4 -p-Me)(16)(H2O)(4)]. 3H(2)O (3). The Mn,, molecules in complexes 2 and 3 ar e geometrical isomers differing in the positions of the H2O and carboxylate ligands. In complex 2, one Mn-III ion has an abnormal Jahn-Teller distorti on axis oriented at an oxide ion, and thus 2 and 3 are also Jahn-Teller iso mers. 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 t his explains why this complex shows an out-of-phase ac peak (chi (M)") in t he signal in the 2-3 K region, whereas complex 3 has its chi (M)" peak in t he 4-7 K range, i.e. the rate of tunneling of magnetization is greater in c omplex 2 than complex 3. (C) 2001 Elsevier Science Ltd. All rights reserved .