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 [Mn12O12(O2CR)16(H2O)4] with a given R substituent have been isolated. X-ray structures are reported for two isomeric forms of the p-methylbenzoate complex, [Mn12O12(O2CC6H4-p-Me)16(H2O)4]·(HO2CC6H4-p-Me) (2) and [Mn12O12(O2CC6H4-p-Me)16(H2O)4]·3H2O (3). The Mn12 molecules in complexes 2 and 3 are geometrical isomers differing in the positions of the H2O and carboxylate ligands. In complex 2, one MnIII 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 (M ) in the signal in the 2–3 K region, whereas complex 3 has its M peak in the 4–7 K range, i.e. the rate of tunneling of magnetization is greater in complex 2 than complex 3. © 2001 Elsevie