[Au(CN)]-Armed [FeFe] Square Complex Showing Unusual Spin-Crossover Behavior Due to a Symmetry-Breaking Phase Transition.

The incorporation of two different cyanide building blocks of [(Tp)Fe(CN)] and [Au(CN)] into one molecule afforded a novel hexanuclear [FeFeAu] complex (), in which the cyanide-bridged [FeFe] square was further grafted by two [Au(CN)] fragments as long arms in orientations. Complex undergoes a gradual spin crossover (SCO) ffrom low-spin (LS) to high-spin (HS) state for the Fe(II) centers upon desolvation. Remarkably, its desolvated phase () exhibits a reversible but atypical two-step (sharp-gradual) SCO behavior with considerable hysteresis (21 K). Variable-temperature single-crystal X-ray structural studies reveal that the hysteretic spin transition takes place synchronously with the concerted displacive motions of the molecules, representing another rare example including multistep and hysteretic spin transitions due to the synergetic SCO and structural phase transition.