A Hydrogen-Bonded Cyanide-Bridged [Co2Fe2] Square Complex Exhibiting a Three-Step Spin Transition

Co-crystallization of a cyanide-bridged tetranuclear complex [Co2Fe2] with 4-cyanophenol (CP) gave a hydrogen bonding donor-acceptor system, [Co2Fe2(bpy*)(4)(CN)(6)(tp*)(2)]( PF6)(2).2CP.8BN (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.