Systematic control of the spin crossover profile in dinuclear iron(iii) complexes via the bridging ligand redox-state

Switchable molecular materials that undergo multistep transitions are critical to advancing ternary data storage, complex logic systems, and highly sensitive colorimetric sensors. Dinuclear metal complexes capable of two-step spin crossover (SCO) interconversions of the form [LS–LS] ⇌ [LS–HS] ⇌ [HS–HS] (LS = low spin, HS = high spin) hold great promise for constructing such multistable systems. However, achieving reliable and predictable two-step SCO in discrete complexes remains a significant challenge. Here, we introduce a conceptual advance to overcome this limitation: the deliberate use of a mixed-valence bridging ligand to impose ligand field asymmetry, a strategy not previously proposed or demonstrated. We synthesized a family of dinuclear FeIII complexes in which the SCO behavior is systematically tuned by varying the oxidation state of a redox-active bis(dioxolene) bridging ligand between cat2−–cat2−, cat2−–SQ˙− and SQ˙−–SQ˙− (cat2− = catecholate, SQ˙− = semiquinonate), affording: [{FeIII(tpa)}2(theacat–cat)](PF6)2 (1), [{FeIII(tpa)}2(theacat–SQ)](PF6)3 (2), and [{FeIII(tpa)}2(theaSQ–SQ)](PF6)4 (3) (theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene, tpa = tris(2-pyridylmethyl)amine). Comprehensive structural, magnetic, spectroscopic, electrochemical, and computational analysis reveals three fundamentally different SCO profiles within a single dinuclear framework: 1 undergoes partial [LS–HS] ⇌ [HS–HS] interconversion, 2 exhibits two-step [LS–LS] ⇌ [LS–HS] ⇌ [HS–HS] SCO, while 3 undergoes an incomplete [LS–LS] ⇌ [LS–HS] transition. Electron paramagnetic resonance and near-infrared spectroscopies demonstrate the mixed-valence thea3˙− ligand in 2 has a localized cat2−–SQ˙− redox state, creating asymmetric ligand fields at the two FeIII centers. This electronic asymmetry is the key to enabling the two-step SCO observed in 2. These results establish a new and generalizable design strategy for achieving predictable, tunable two-step SCO through the use of stable mixed-valence bridging ligands with localized redox states.