Molecular Spin Crossover in Slow Motion: Light-Induced Spin-State Transitions in Trigonal Prismatic Iron(II) Complexes

A straightforward access is provided to iron­(II) complexes showing exceedingly slow spin-state interconversion by utilizing trigonal-prismatic directing ligands (<b>L</b>^{<i><b>n</b></i>}) of the extended-tripod type. A detailed analysis of the interrelations between complex structure (X-ray diffraction, density functional theory) and electronic character (SQUID magnetometry, Mössbauer spectroscopy, UV/vis spectroscopy) of the iron­(II) center in mononuclear complexes [<b>FeL</b>^{<i><b>n</b></i>}] reveals spin crossover to occur along a coupled breathing/torsion reaction coordinate, shuttling the complex between the octahedral low-spin state and the trigonal-prismatic high-spin state along Bailar’s trigonal twist pathway. We associate both the long spin-state lifetimes in the millisecond domain close to room temperature and the substantial barriers against thermal scrambling (<i>E</i>_a ≈ 33 kJ mol^–1, from Arrhenius analysis) with stereochemical constraints. In particular, the topology of the κ⁶N ligands controls the temporary and structural dynamics during spin crossover