Tuning the Spin-Crossover Properties of FeII4L6 Cages via Ligand Modifications in the First and Second Coordination Spheres

The effects of modifications to the first and second coordination spheres of the metal centers in FeII-based cages on the spin-crossover properties is not well understood. Here, we report a family of FeII4L6 spin-crossover cages based on 2,2’-pyridylbenzimidazoles where subtle modifications to the coordination motif (first coordination sphere) and the linker type (second coordination sphere) of the organic ligand lowered the spin crossover temperature in CD3CN to a range between 430 K and 244 K. Comparing pairs of cages, CH3 substituents on either the coordination motif or phenylene linker lowered the spin-crossover temperature by 48 K, 91 K or 186 K, attributed to predominantly first, second coordination sphere effects and a combination of both, respectively. This study provides first insight into how ligand modifications in the first and second coordination sphere can be harnessed on the path towards the improved rational design of spin-crossover cages.