Sub lattice driven spin state ordering and coordination elasticity in Fe(II) 1,3,4-thiadiazole complexes

Area of Science:

• Coordination Chemistry
• Materials Science
• Solid-State Chemistry

Background:

• Designing responsive spin-crossover (SCO) materials requires understanding co-ligand effects on iron(II) spin-state energetics.
• Investigating how ligand structure and intermolecular interactions influence SCO behavior is crucial for materials development.

Purpose of the Study:

• Synthesize and characterize a novel unsymmetrical ligand, 1-(5-phenyl-1,3,4-thiadiazol-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine (LPh-TDA).
• Explore the spin-crossover properties of its iron(II) complexes with varying co-ligands (NCS-, NCSe-, NCBH3-).
• Determine the interplay between ligand-field strength, crystal packing, and SCO behavior.

Main Methods:

• Synthesis of the LPh-TDA ligand and its iron(II) complexes.
• Variable-temperature single-crystal X-ray diffraction.
• SQUID magnetometry and 57Fe Mössbauer spectroscopy.
• Density functional theory (DFT) calculations.

Main Results:

• Complex C1 ([Fe(LPh-TDA)(NCS)2]·H2O) exhibits packing-induced sublattice spin-state ordering with two distinct Fe(II) sites, one transitioning to low-spin (LS) and the other remaining high-spin (HS).
• Complex C2 ([Fe(LPh-TDA)(NCSe)2]·H2O) remains HS across the temperature range.
• Complex C3 ([Fe(LPh-TDA)(NCBH3)2]·H2O) shows a complete, one-step SCO with a transition temperature (T1/2) of 153 K.
• DFT calculations confirm that intermolecular packing effects dominate over intrinsic ligand-field trends in C1.

Conclusions:

• Crystal packing significantly influences spin-crossover behavior in Fe(II) complexes, overriding intrinsic ligand-field effects.
• Solid-state organization plays a dominant role in dictating SCO properties, even when co-ligand electronic effects are engineered.
• This highlights the importance of considering crystal structure in the design of responsive spin-crossover materials.