How Does a Coordinated Radical Ligand Affect the Spin Crossover Properties in an Octahedral Iron(II) Complex?

The influence of a coordinated π‐radical on the spin crossover properties of an octahedral iron(II) complex was investigated by preparing and isolating the iron(II) complex containing the tetradentate N,N′‐dimethyl‐2,11‐diaza[3.3](2,6)pyridinophane and the radical anion of N,N′‐diphenyl‐acenaphtene‐1,2‐diimine as ligands. This spin crossover complex was obtained by a reduction of the corresponding low‐spin iron(II) complex with the neutral diimine ligand, demonstrating that the reduction of the strong π‐acceptor ligand is accompanied by a decrease in the ligand field strength. Characterization of the iron(II) radical complex by structural, magnetochemical, and spectroscopic methods revealed that spin crossover equilibrium occurs above 240 K between an S=1/2 ground state and an S=3/2 excited spin state. The possible origins of the fast spin interconversion observed for this complex are discussed. Redox‐controlled changes in a coordinated ligand have been demonstrated to lead to a temperature‐induced spin crossover in an octahedral iron(II) complex. The influence of the resulting coordinated π‐radical on the spin crossover properties of the octahedral iron(II) ion is discussed. The structural and electronic differences between both oxidation states of the redox couple are presented.