Related Experiment Video
Updated: Aug 25, 2026
Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks
Published on: June 9, 2023
Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)]: structural and spin-state phase transitions and
Brian O Patrick1, William M Reiff, Victor Sánchez
1Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1.
Abstract:
Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees 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 alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma 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 alpha-, beta-, and gamma-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 gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature Mössbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) 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 T(c) approximately 6.5 K.
Related Concept Videos
03:02Colors and Magnetism
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.
02:42Valence Bond Theory
02:46Crystal Field Theory - Tetrahedral and Square Planar Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
02:57Coordination Number and Geometry
02:34Structural Isomerism
Isomers are different chemical species that have the same chemical formula. Structural isomerism of coordination compounds can be divided into two subcategories, the linkage isomers and coordination-sphere isomers.
Linkage isomers occur when the coordination compound contains a ligand that can bind to the transition metal center through two different atoms. For example, the CN− ligand can bind through the carbon atom or through the nitrogen atom. Similarly, SCN− can be...
02:58Crystal Field Theory - Octahedral Complexes
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...