Supramolecular effects on anion-dependent spin-state switching properties in heteroleptic iron(II) complexes

We describe the preparations, characterizations, and spin-state properties of heteroleptic Fe(II) complexes containing 2,2'-bi-1,4,5,6-tetrahydropyrimidine (H(2)bip) and 2-(aminomethyl)pyridine (pic): [(H(2)bip)(2)Fe(pic)]X(2) (X = Br (1), BPh(4) (2)) and [(H(2)bip)(1.75)Fe(pic)(1.25)](BPh(4))(2) (3). The ditopic H(2)bip ligand serves as an anion binding group while pic is intended to adjust the Fe(II) ion's ligand field close to the thermal spin-crossover region. The solid state magnetic behavior of each complex salt is found to be heavily influenced by anion and solvate molecules, and is correlated with the first coordination sphere molecular structure, intermolecular interactions, and solvate-induced packing effects. Anion-dependent spin-state switching is observed in dichloromethane solution for samples of 2 treated with (n)Bu(4)NBr, albeit at significantly lower temperatures than what would be expected based on ligand field considerations alone. The origins of this behavior are discussed: circumstantial evidence points to unintended effects of anion-mediated complex pairing in solution as a significant contributor to the lower-than-expected operating temperatures.