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

Several single-molecule magnets with the composition [Mn12O12(O2CR)(16)(H2O )(x)] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility sign als. one in the 4-7 K region and the other in the 2-3 K region. New Mn-12 c omplexes were prepared and structurally characterized, and the origin of th e two magnetization relaxation processes was systematically examined. Diffe rent crystallographic forms of a Mn-12 complex with a given R substituent e xist where the two forms have different compositions of solvent molecules o f crystallization and this results in two different arrangements of bound H 2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn1 2O12(O2CEt)(16)(H2O)(3)]. 4H(2)O (space group P (1) over bar) (complex 2a) has been reported previously. The more prevalent needle-form of ]Mn12O12(O2 CEt)(16)(H2O)(3)] (complex. 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degreesC has a = 16.462(7) Angstrom, b = 22.401(9) A ngstrom, c = 20.766(9) Angstrom, beta = 103.85(2)degrees, and Z = 4. The ar rangements of H2O and carboxylate ligands on the Mn-12 molecule are differe nt in the two crystal forms. The complex [Mn12O12-(O2CC6H4-p-Cl)(16)(H2O)(4 )]. 8CH(2)Cl(2) (5) crystallizes in the monoclinic space group C2/c, which at -172 degreesC has a = 29.697(9) Angstrom, b = 17.708(4) Angstrom, c = 30 .204(8) Angstrom, beta = 102.12(2)degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) Angstrom, b = 18.2288(2) Angstrom, c 26.5882(4 ) Angstrom, beta 125.8359(2)degrees, and Z = 4. [Mn12O12(O2CC6H4-P-Me)(16)( H2O)(4)].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) Angstrom, b = 32.2371(4) Angstrom, c = 29.8738( 6) Angstrom, beta = 99.2650(10)degrees, and Z = 8. The Mn-12 molecules in c omplexes 6 and 7 differ in their arrangements of the four bound H2O ligands . Complex 6 exhibits an out-of-phase ac peak (chi (M)") in the 2-3 K region , whereas the hydrate complex 7 has a chi (M)" signal in the 4-7 K region. In addition, however, in complex 6, one Mn-III ion has an abnormal Jahn-Tel ler distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Tel ler isomers. This reduces the symmetry of the core of complex 6 compared wi th complex 7. Thus: complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi (M)" signal in the 2-3 K reg ion, whereas complex 7 has its. chi (M)" peak in the 4-7 K region, i.e., th e rate of tunneling of magnetization is greater in complex 6 than complex 7 . Detailed H-1 NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p methyl-benzoate Mn-12 complex es and confirm the structural integrity of the (Mn12O12) complexes upon dis solution. In solution there is rapid ligand exchange and no evidence for th e different isomeric forms of Mn-12 complexes seen in the solid state.