Polybis(4-azabenzimidazolato)-iron(II) and -cobalt(II). 3-D single diamond-like framework materials which exhibit spin canting and ferromagnetic ordering at low temperatures

Abstract

The direct reaction between ferrocene and an excess of molten 4-azabenzimidazole yielded amber-green crystals of polybis(4-azabenzimidazolato)iron(II), 1. Single crystal X-ray diffraction studies revealed tetrahedrally coordinated iron ions linked by single imidazolate bridges and forming a 3-D single diamond-like (diamondoid) extended lattice. Magnetization studies show the effects of antiferromagnetic exchange above a critical temperature, T_c = 21 K, and long-range ferromagnetic ordering below this temperature, behavior that classifies 1 as a molecule-based magnet. Magnetic hysteresis studies at 4.8 K yield a coercive field, H_coer = 80 G and a remnant magnetization, M_rem = 2100 cm³ G mol⁻¹. The isomorphous polybis(4-azabenzimidazolato)cobalt(II), 2, has also been studied. It too is a low temperature molecule-based magnet. For 2, T_c = 11 K and from magnetic hysteresis studies at 10 K, H_coer = 400 G and M_rem = 22 cm³ G mol⁻¹.