Octacyanidorhenate(V) Ion as an Efficient Linker for Hysteretic Two‐Step Iron(II) Spin Crossover Switchable by Temperature, Light, and Pressure

A two-step hysteretic FeII spin crossover (SCO) effect was achieved in programmed layered CsFe (3-CNpy) 2 Re (CN) 8 ⋅H2 O (1) (3-CNpy=3-cyanopyridine) assembly consisting of cyanido-bridged FeII -ReV square grid sheets bonded by Cs+ ions. The presence of two non-equivalent FeII sites and the conjunction of 2D bimetallic coordination network with non-covalent interlayer interactions involving Cs+, ReV (CN) 8 3- ions, and 3-CNpy ligands, leads to the occurrence of two steps of thermal SCO with strong cooperativity giving a double thermal hysteresis loop. The resulting spin-transition phenomenon could be tuned by an external pressure giving the room-temperature range of SCO, as well as by visible-light irradiation, inducing an efficient recovery of the high-spin FeII state at low temperatures. We prove that octacyanidorhenate (V) ion is an outstanding metalloligand for induction of a cooperative multistep, multiswitchable FeII SCO effect.