Single-molecule magnets:: Jahn-Teller isomerism and the origin of two magnetization relaxation processes in Mn12 complexes

Different crystallographic forms of the single molecule magnet [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)] with a given R substituent have been isolated. X-ray structures are reported for two isomeric forms of the p-methylbenzoate complex, [Mn(12)O(12)(O(2)CC(6)H(4)-pMe)(16)(H(2)O)(4)]. (HO(2)CC(6)H(4)-p-Me) (2) and [Mn(12)O(12)(O(2)CC(6)H(4)-p-Me)(16)(H(2)O)(4)]. 3H(2)O (3). The Mn,, molecules in complexes 2 and 3 are geometrical isomers differing in the positions of the H(2)O 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 (chi (M)") in the signal in the 2-3 K region, whereas complex 3 has its chi (M)" peak in the 4-7 K range, i.e. the rate of tunneling of magnetization is greater in complex 2 than complex 3. (C) 2001 Elsevier Science Ltd. All rights reserved.