Ambient temperature anion-dependent spin state switching observed in “mostly low spin” heteroleptic iron(ii) diimine complexes

Abstract

We report the synthesis, characterization, and representative anion-binding studies on a series of heteroleptic Fe^II diimine complexes that have been designed to show anion-triggered spin state switching at ambient temperatures. Starting with [(H₂bip)₂FeBr₂] (1), ligand substitution affords [(H₂bip)₂Fe(NN)]²⁺ complexes, where (NN) = pipi (2), bpy (3), and phen (4). In the solid state, the tetraphenylborate and bromide salts of the complexes display different thermally induced spin crossover properties, with spin transition temperatures above 395 K. The solid state magnetic properties depend on the Fe^II ligand field parameters and anion–cation interactions as well as solvent and packing effects. In dichloromethane solution, the weakly bound tetraphenylborate salts show spin state diminution, amplified chemical shift responses toward the diamagnetic ¹H NMR spectral window, and visible colour changes upon titration of bromide anions (as ⁿBu₄N⁺ salts). Both ¹H NMR- and electronic absorption-monitored titration studies unequivocally link anion binding to a high spin → low spin transition for the minority species in solution. Further, the complexes show pronounced air stability, in contrast to the homoleptic parent complex [Fe(H₂bip)₃]²⁺. Tuning the Fe^II ligand field by judicious ligand substitution toward “mostly low spin” species thus increases both the environmental stability and operating temperature of the complexes, and provides evidence that spin state switching may serve as a viable signalling event for chemical sensing in solution.