Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

Organic diradicals are uncommon species
that have been intensely
studied for their unique properties and potential applicability in
a diverse range of innovative fields. While there is a growing class
of stable and well-characterized organic diradicals, there has been
recent focus on how diradical character can be controlled or modulated
with external stimuli. Here we demonstrate that a diiron complex bridged
by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate
(TTFtt2–) undergoes a thermally induced Fe-centered
spin-crossover which yields significant diradical character on TTFtt2–. UV–vis–near-IR, Mössbauer,
NMR, and EPR spectroscopies with magnetometry, crystallography, and
advanced theoretical treatments suggest that this diradical character
arises from a shrinking TTFtt2– π-manifold
from the Fe­(II)-centered spin-crossover. The TTFtt2–-centered diradical is predicted to have a singlet ground state by
theory and variable temperature EPR. This unusual phenomenon demonstrates
that inorganic spin transitions can be used to modulate organic diradical
character.