Proton-Gated Pendant-Arm Decoordination Couples Hydration and Spin-State Switching in Fe(III) Complexes.

pH-gated coordination-induced spin-state switching (CISSS) offers a chemically addressable route to manipulate Fe(III) spin energetics in solution, yet synchronizing spin-state response with hydration control in water remains underdeveloped. Here, we introduce a paired 1,4,7-triazacyclononane-based Fe(III) platform that operates at the coordination-decoordination boundary to coprogram ligand-field strength and inner-sphere water access. Protonation promotes pendant-arm dissociation and increases water accessibility in both complexes, while the bis-labile design additionally undergoes substantial low-spin/high-spin re-equilibration, thereby disentangling hydration-driven relaxivity enhancement from spin-state redistribution. Variable-pH magnetic measurements, structural analysis, and multilevel electronic-structure calculations support this proton-triggered reorganization of coordination, hydration, and spin-state energetics. These coupled changes translate into up to a 35-fold increase in r1 at 0.5 T and pronounced T1-weighted phantom contrast at 9.4 T under acidic conditions. More broadly, this work establishes a general aqueous Fe(III) design principle in which proton-triggered coordination lability synchronizes hydration and spin-state response to generate a functional relaxometric output.