Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand.

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)Pd(CN)]·G (furpy: -(pyridin-4-yl)furan-2-carboxamide, G = HO,EtOH (·HO,Et), and HO (·HO)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for ·HO,Et and an abrupt single-step SCO for ·HO with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for ·HO,Et characterized by a unique Fe center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, ·HO shows a triclinic symmetry with two inequivalent Fe centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.