Three-Dimensional Metal–Organic Network Glasses from Bridging MF₆²– Anions and Their Dynamic Insights by Solid-State NMR

Glassy-state coordination polymers (CPs) are a new class of network-forming glasses. In this work, we constructed glass-forming CPs composed of both anionic and neutral ligands as network formers. With the use of hexafluoro anions (MF62–) and 1,3-bis(4-pyridyl)propane (bpp), two isostructural CP crystals, [Zn(SiF6)(bpp)2] (ZnSi) and [Zn(TiF6)(bpp)2] (ZnTi), were synthesized. Solid-state 19F NMR revealed rotational motion of MF62– with dissociation and re-formation of the Zn–F coordination bonds in both CP crystals, which reflects the thermodynamic parameters related to the glass formability. The mobility of SiF62– is larger than that of TiF62–, suggesting a higher glass formability of ZnSi. When mechanical ball milling was conducted, ZnSi completely changed into a glassy state, whereas ZnTi showed incomplete glass formation. Examination of the amorphous structures elucidated retention and partial destruction of the Zn–F coordination bonds in ball-milled ZnSi and ZnTi, respectively. These results provide the relationship between the ligand dynamics and glass formability of CPs.