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 (Ln) 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 [FeLn] 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
(Ea ≈ 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.