Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material

Area of Science:

• Coordination Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Spin crossover (SCO) materials exhibit distinct high-spin (HS) and low-spin (LS) states, crucial for molecular switches and sensors.
• Hofmann-type frameworks offer tunable properties through ligand functionalization and guest molecule inclusion.
• Controlling SCO behavior, particularly achieving multistep transitions, remains a challenge in materials design.

Purpose of the Study:

• To synthesize and characterize a novel 1,2,4-triazole ligand, thiome, for creating functionalized 2D Hofmann-type coordination polymers.
• To investigate the spin transition properties of the resulting [FeII Pd(CN)4(thiome)2]·2H2O framework.
• To explore the influence of guest molecules and external pressure on the spin crossover phenomenon.

Main Methods:

• Synthesis of the 4-functionalized 1,2,4-triazole ligand (thiome).
• Preparation of the 2D Hofmann-type coordination polymer [FeII Pd(CN)4(thiome)2]·2H2O.
• Variable-temperature magnetic susceptibility measurements.
• Single-crystal X-ray diffraction under varying pressure conditions.

Main Results:

• The [FeII Pd(CN)4(thiome)2]·2H2O framework initially shows inhibited spin transition due to steric effects.
• Reversible removal of guest water molecules induces an abrupt, hysteretic one-step spin transition.
• Application of hydrostatic pressure (0–0.68 GPa) leads to a two-step spin transition at ambient temperature.

Conclusions:

• The steric bulk of the thiome ligand and guest water molecules influences the spin transition behavior.
• Guest removal can unlock spin crossover in otherwise inhibited systems, demonstrating control via guest-host interactions.
• Hydrostatic pressure provides a pathway to achieve multistep spin transitions in Hofmann-type materials, highlighting the role of lattice flexibility.