From a mononuclear FeL₂ complex to a Fe₄L₄ molecular square: Designed assembly and spin-crossover property

By introduction of a new Fe(L¹)₂ spin-crossover (SCO) unit into the polynuclear system, a nano-scale Fe₄(L²)₄ molecular square architecture is designed through coordination-directed self-assembly strategy. Both the mononuclear Fe(L¹)₂ and tetranuclear Fe₄(L²)₄ complexes have been structurally confirmed by ¹H nuclear magnetic resonance (NMR), electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and temperature-dependent single crystal X-ray diffraction studies. Variable-temperature magnetic susceptibility measurements reveal the presence of an abrupt SCO behavior with a thermal hysteresis width of 4 K for Fe(L¹)₂. By clear contrast, Fe₄(L²)₄ undergoes a gradual spin transition behavior with enlarged thermal hysteresis width and higher spin transition temperature.