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

By introduction of a new Fe(L 1 ) 2 spin-crossover (SCO) unit into the polynuclear system, a nano-scale Fe 4 (L 2 ) 4 molecular square architecture is designed through coordination-directed self-assembly strategy. Both the mononuclear Fe(L 1 ) 2 and tetranuclear Fe 4 (L 2 ) 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(L 1 ) 2 . By clear contrast, Fe 4 (L 2 ) 4 undergoes a gradual spin transition behavior with enlarged thermal hysteresis width and higher spin transition temperature.