Characterization of an Intermediate Phase in a Two-stepped Electron Transfer-coupled Spin Transition

Introduction Cyanide-bridged multi-nuclear complexes have potential to show specific molecular functions due to electronic and magnetic interactions between metal ions. We have previously reported a novel cyanide-bridged molecular square of [Co2Fe2(CN)6(tp*)2(dtbbpy)4](PF6)2 ·2MeOH (1) (tp* = hydrotris(3,5-dimethylpyrazol-1yl)borate, dtbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine). In 1, Fe and Co ions are alternately bridged by cyanide ions, forming macrocyclic tetranuclear cores. Magnetic susceptibility measurements confirmed that 1 exhibits two-step electron transfer-coupled spin transitions (ETCST) centred at T1/2 = 275 and 310 K in the solid state. In the X-ray diffraction study using the conventional Xray source (Mo-Kα), the Fe and Co ions are the low-spin (LS) Fe(III) and high-spin (HS) Co(II) ions in the hightemperature (HT) phase ([FeLS2CoHS2]) at 330 K, while a low-temperature (LT) phase ([FeLS2CoLS2]) with LS Fe(II) and Co(III) ions was dominant below 260 K. At 298 K one dinuclear unit of [(tp*)Fe(CN)2-(μ-CN)Co(dtbbpy)2] was observed as the asymmetric unit, and the average coordination bond length about the Co ions were in the middle of the typical bond lengths for LS Co(III) and HS Co(II) ions in the IM phase at 298 K. It remained unclear whether [FeLS2CoLS2] and [FeLS2CoHS2] are positionally disordered or not, and weak super lattice reflections originating from long-range order could be missed in the X-ray diffraction data using conventional X-ray source. We, therefore, collected a single crystal X-ray diffraction data of the IM phase at 298 K using synchrotron radiation.