부산대학교 도서관

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(ll)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 °C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated α → β and β → γ at 151 and 133 K, respectively. On warming, the γ → β and β → α t transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature β ↔ γ transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and Mössbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the α-, β-, and γ-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the γ-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall α → γ process involves a spin-crossover transition such that at least 1/6 of the iron sites in the γ-phase correspond to S = 0. This is supported by the low-temperature Mössbauer spectra of the γ-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(Il) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and Mössbauer techniques, occurs at Tc ∼ 6.5 K. [ABSTRACT FROM AUTHOR]