Two-dimensional frameworks formed by pentagonal bipyramidal cobalt(ii) ions and hexacyanometallates: antiferromagnetic ordering, metamagnetism and slow magnetic relaxation

Abstract

We herein report the syntheses, structures, and magnetic properties of two isostructural two-dimensional (2D) coordination polymers based on a pentagonal bipyramidal Co^II unit [Co(TODA)]²⁺ and two hexacyanometallates, namely [M^III(CN)₆]₂[Co^II(TODA)]₃·9H₂O (M = Cr (1), Co (2), TODA = 1,4,10-trioxa-7,13-diazacyclopentadecane). Structure analyses show that both complexes have 2D honeycomb structures where the [Co(TODA)]²⁺ units are bridged by the [M^III(CN)₆]³⁻ groups through three cyano groups in the facial positions. Magnetic investigation reveals ferromagnetic coupling between the Cr^III and Co^II centres through cyanides in 1. Due to the antiferromagnetic interaction between the layers, compound 1 exhibits an antiferromagnetic ordering below 11.4 K, and shows a metamagnetic phase transition under an external dc field. Due to the disorder of the TODA ligands, compound 1 shows a spin glass behavior, which leads to slow magnetic relaxation in 1. A butterfly-shaped hysteresis loop at 1.8 K can be observed with a coercive field of 720 Oe, which is quite large for cyano-bridged Cr–Co molecular magnets. For compound 2 containing the diamagnetic [Co^III(CN)₆]³⁻ unit, field-induced slow magnetic relaxation was also verified, which makes compound 2 a rare example of an SIM assembled in a 2D network. An easy-plane magnetic anisotropy with a positive D value (29.9 cm⁻¹ by PHI and 26.5 cm⁻¹ by Anisofit2.0) was deduced for hepta-coordinated Co^II centers. These results show the efficiency of the strategy of combining cyanometallates and pentagonal bipyramidal precursors for novel molecular magnetic materials.