Non-Porous Iron(II)-Based Sensor: Crystallographic Insights into a Cycle of Colorful Guest-Induced Topotactic Transformations

• Published in: Angewandte Chemie International Edition 2016-11, Vol.55 (48), p.15067-15071
• Main Authors: Rodríguez-Jiménez, Santiago, Feltham, Humphrey L. C., Brooker, Sally
• Format: Article
• Language: English
• Subjects: chemical sensor; crystal engineering; reversible guest exchange; single-crystal-to-single-crystal transformation; spin crossover
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201608813

Materials capable of sensing volatile guests at room temperature by an easily monitored set of outputs are of great appeal for development as chemical sensors of small volatile organics and toxic gases. Herein the dinuclear iron(II) complex, [FeII2(L)2(CH3CN)4](BF4)4⋅2 CH3CN (1) [L=4‐(4‐methylphenyl)‐3‐(3‐pyridazinyl)‐5‐pyridyl‐4H‐1,2,4‐triazole], is shown to undergo reversible single‐crystal‐to‐single‐crystal (SCSC) transformations upon exposure to vapors of different guests: 1 (MeCN)⇌2 (EtOH)→3 (H2O)⇌1 (MeCN). Whilst 1 and 2 remain dimetallic, SCSC to 3 involves conversion to a 1D polymeric chain (due to a change in L bridging mode), which, remarkably, can undergo SCSC de‐polymerization, reforming dimetallic 1. Additionally, SC‐XRD studies of two ordered transient forms, 1TF3 and 2TF3, confirm that guest exchange occurs by diffusion of the new guests into the non‐porous lattices as the old guests leave. These reversible SCSC events also induce color and magnetic responses. Indeed dark red 1 is spin crossover active (T1/2↓ 356 K; T1/2↑ 369 K), whilst orange 2 and yellow 3 remain high spin. Three guests, three colours: Reversible three way‐guest sensing by a diiron(II) complex (blue sphere Fe) results in interconversion of dark red low spin 1 (red box MeCN) with high spin orange 2 (orange triangle EtOH) and yellow 3 (yellow circle H2O). Remarkably these guest exchanges are reversible single‐crystal‐to‐single‐crystal (SCSC) transformations. Crystallography also reveals that SCSC transformation 1⇌3 involves a polymerization⇌depolymerization.