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Published on: July 27, 2022
Proton-Gated Pendant-Arm Decoordination Couples Hydration and Spin-State Switching in Fe(III) Complexes
Ai-Wen Ge1, Yu-Xiao Chen1, Xin Wang2
1Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Liangxiang Campus, Beijing Institute of Technology, Beijing102488, People's Republic of China.
Abstract:
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.
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