Single molecule magnets: High frequency electron paramagnetic resonance study of two isomeric forms of an Mn12 molecule

Different crystallographic forms of the single molecule magnet [Mn12O12(O2CR)16(H2O)4] (complex 1) with a given R substituent have been isolated. The two different isomeric forms of the p-methylbenzoate complex crystallize as [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅(HO2CC6H4-p-Me) (complex 2) and [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅3H2O (complex 3). In complex 2, one MnIII ion has an abnormal Jahn–Teller distortion axis oriented at an oxide ion, and thus 2 and 3 are 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 (χM″) in the signal in the 2–3 K region, whereas complex 3 has its χM″ peak in the 4–7 K range, i.e., the rate of tunneling of magnetization is greater in complex 2 than complex 3. High frequency electron paramagnetic resonance (HFEPR) experiments were performed on both isomers. Computed simulations of the experimental HFEPR data yield spin Hamiltonian parameters for both complexes.