Improved cooperativity of spin-labile iron(III) centers by self-assembly in solution.

Supramolecular principles have been applied for improving the spin crossover activity of metal centers due to cooperative effects in solution. Thus, incorporation of alkyloxy tails at the phenyl group of Fe(sal2trien) 2a provides amphiphilic complexes Fe(sal-OR2trien) 2b-d (b, R = C6H13; c, R = C8H17; d, R = C18H37) comprising an apolar group for supramolecular organization and a polar headgroup with potential spin crossover activity due to the presence of a spin-labile iron(III) center. Self-assembly of these complexes in solution resulted in the formation of microsize and submicrosize particles when the alkyl chain was long enough (2d) but not with shorter chains (2a-c). Solutions of 2d showed enhanced spin crossover activity as compared to complexes 2a-c, both in terms of transition temperature and steepness of the transition. This observation has been correlated to an improved cooperativity of the metal centers in 2d due to self-assembly, thus facilitating a tandem spin transition.