Slow magnetic relaxation in high-coordinate Co(II) and Fe(II) compounds bearing neutral tetradentate ligands.

The first-row transition metal compounds, [M(L1)](ClO) (M = Ni (1); Co (2)), have been prepared by treatment of a neutral tetradentate ligand (L1 = ,-dibutyl-1,10-phenanthroline-2,9-dicarboxamide) with metal perchlorate salts in MeOH. Both compounds have been structurally characterized by X-ray crystallography and it was found that the coordination numbers are 6 and 7, respectively. The reaction of 6,6'-bis(2-butyl-tetrazol-5-yl)-2,2'-bipyridine (L2) with hydrated Fe(ClO) afforded a 8-coordinate Fe(II) compound, [Fe(L2)](ClO) (3); however its reaction with hydrated Co(ClO) resulted in 6-coordinate [Co(L2)](ClO). It is interesting to observe field-induced slow magnetic relaxation in the 7-coordinate Co(II) compound 2 and 8-coordinate Fe(II) compound 3, which further supports the validity of designing high coordination number compounds as single-molecule magnets. Direct current magnetic studies demonstrate that 2 has a very large positive value (56.2 cm) and a small value (0.66 cm), indicating easy plane magnetic anisotropy. Consistent with the larger value, an effective spin-reversal barrier of = 100 K (71.4 cm) is obtained, which is the highest value reported for 7-coordinate Co(II) complexes with a pentagonal bipyramidal geometry. In contrast, 8-coordinate Fe(II) compound 3 exhibits uniaxial magnetic anisotropy.