A Hydrogen-Bonded Cyanide-Bridged [Co2 Fe2 ] Square Complex Exhibiting a Three-Step Spin Transition.

Co-crystallization of a cyanide-bridged tetranuclear complex [Co2 Fe2 ] with 4-cyanophenol (CP) gave a hydrogen bonding donor-acceptor system, [Co2 Fe2 (bpy*)4 (CN)6 (tp*)2 ](PF6 )2 ⋅2 CP⋅8 BN (1). 1 exhibited a three-step phase transition between HT, IM1, IM2, and LT phases upon temperature variation. Variable temperature magnetic measurements and structural analyses revealed that the three-step spin transition is caused by electron-transfer-coupled spin transitions (ETCSTs) accompanied with alteration of the hydrogen bonding interactions.