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

The first-row transition metal compounds, [M-II(L-1)(2))(ClO4)(2) (M = Ni (1); Co (2)), have been prepared by treatment of a neutral tetradentate ligand (L-1 = N-2,N-9-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-(t)butyl-tetra-zol-5-yl)-2,2'-bipyridine (L-2) with hydrated Fe-II(ClO4)(2) afforded a 8-coordinate Fe(II) compound, [Fe-II(L-2)(2)] (ClO4)(2) (3); however its reaction with hydrated Co-II(ClO4)(2) resulted in 6-coordinate [Co-II(L-2)(2)](ClO4)(2). 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 D value (56.2 cm(-1)) and a small E value (0.66 cm(-l)), indicating easy plane magnetic anisotropy. Consistent with the larger D value, an effective spin-reversal barrier of U-eff = 100 K (714 cm(-1)) 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.

Automated summary

The synthesis of first-row transition metal compounds with different coordination numbers suggests the potential of designing high coordination number compounds as single-molecule magnets. The magnetic studies reveal unique magnetic behaviors in some compounds, indicating their potential for practical applications.