Herein, three dinuclear iron(II) helicates bearing the molecular formula [Fe2 (L1)3 ](ClO4 )4 ⋅ 2CH3 OH ⋅ 3H2 O (complex 1), [Fe2 (L2)3 ](ClO4 )4 ⋅ 6CH3 CN (complex 2), and [Fe2 (L3)3 ](ClO4 )4 ⋅ 0.5H2 O (complex 3) have been synthesized using imidazole and pyridine-imine-based ligands having fluorene moiety in the backbone. A change in the ligand field strength by terminal modulation led to a change in the spin-transition behaviour from incomplete, multi-step to complete, around room temperature in the solid state. Spin transition behaviour has also been observed in the solution phase characterized using variable temperature 1 H nuclear magnetic resonance spectroscopy (Evans method) and correlated using UV-visible spectroscopy. Fitting the NMR data using the ideal solution model yielded the transition temperature in the order T1/2 (1)<T1/2 (2)<T1/2 (3), which indicates an increase in the ligand field strength from complexes 1 to 3. This study accentuates the interplay between the ligand field strength, crystal packing, and supramolecular interactions in fine-tuning the spin transition behaviour effectively.
Keywords: non-covalent interactions; spin crossover; supramolecular assembly; triple helicates.
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