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

Abstract

The first-row transition metal compounds, [M^II(L¹)₂](ClO₄)₂ (M = Ni (1); Co (2)), have been prepared by treatment of a neutral tetradentate ligand (L¹ = N²,N⁹-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-^tbutyl-tetrazol-5-yl)-2,2′-bipyridine (L²) with hydrated Fe^II(ClO₄)₂ afforded a 8-coordinate Fe(II) compound, [Fe^II(L²)₂](ClO₄)₂ (3); however its reaction with hydrated Co^II(ClO₄)₂ resulted in 6-coordinate [Co^II(L²)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⁻¹), indicating easy plane magnetic anisotropy. Consistent with the larger D value, an effective spin-reversal barrier of U_eff = 100 K (71.4 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.