Thermal monomerization unlocks 3/2 ↔ 5/2 spin crossover in a kinetically trapped high-spin Fe(III) dimer

We report a novel iron(III) coordination system featuring a thermally controlled dimer-to-monomer transformation accompanied by a pronounced change in the spin state. At room temperature, the reaction of an in situ-generated tridentate ligand HL (obtained by condensation of 3-methoxysalicylaldehyde and N-benzylethylenediamine) with Fe(NCSe)₃ afforded a kinetically favored dimeric complex [Fe₂(L)₂(OMe)₂(NCSe)₂]·MeOH (2) where the methoxy groups bridge the metal centers, and NCSe^- is directly bound to the Fe(III) center, stabilizing a high-spin configuration. Upon gentle heating, this dimer undergoes dissociation to form a monomeric complex [Fe(L)₂]NCSe (1), where NCSe^- acts as a counterion and two tridentate ligands bind to Fe(III), resulting in an appropriate ligand field strength that enables spin crossover (SCO) behavior. This work exemplifies the interplay between nuclearity and spin states, highlighting how kinetic trapping can stabilize high-spin states and how thermal activation leads to a thermodynamically stable monomer with SCO properties. Theoretical calculations were also performed to rationalize the experimental findings and to evaluate exchange parameters. Our findings offer new insights into the design of switchable spin-state materials through controlled structural transformation.