Spin crossover in discrete polynuclear iron(ii) complexes

Area of Science:

• Coordination Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Iron(ii) spin crossover (SCO) materials are explored as molecular switches for applications in displays, sensors, and memory devices.
• Existing SCO materials are predominantly monometallic or polymeric, limiting the scope for enhanced properties.
• Recent efforts focus on discrete polynuclear SCO complexes to achieve improved switching characteristics and functionalities.

Purpose of the Study:

• To review ligand designs facilitating the self-assembly of discrete di- to poly-nuclear iron(ii) complexes.
• To analyze the impact of supramolecular architectures (helicate, cage, cube) on SCO activity and host-guest interactions.
• To investigate structure-property relationships, particularly concerning octahedral distortion parameters and SCO behavior.

Main Methods:

• Comprehensive literature review of ligand designs and resulting polynuclear iron(ii) SCO complexes over the past two decades.
• Analysis of structural data, including octahedral distortion parameters (Σ, CShM), for different supramolecular architectures.
• Categorization of complexes based on nuclearity (di-, tri-, tetra-, etc.) and ligand donor types (e.g., azoles).

Main Results:

• Identified diverse ligand designs enabling self-assembly of discrete polynuclear iron(ii) SCO complexes with helicate, cage, and cube architectures.
• Observed significant differences in octahedral distortion between structural types; grid iron(ii) centers are more distorted yet remain SCO-active.
• Detailed the composition of reviewed complexes: 127 total, 54% dinuclear, 31% tetranuclear; 93 unique ligands, 60 featuring azoles.

Conclusions:

• Ligand design is crucial for constructing discrete polynuclear iron(ii) SCO complexes with tailored supramolecular architectures and enhanced SCO properties.
• Structural variations, such as octahedral distortion, influence SCO activity, offering pathways for fine-tuning molecular switch performance.
• The focus on discrete polynuclear systems opens new avenues for developing advanced molecular devices and exploring host-guest chemistry.