Three-Dimensional Metal-Organic Network Glasses from Bridging MF 6 2- 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 (MF 6 2- ) and 1,3-bis(4-pyridyl)propane (bpp), two isostructural CP crystals, [Zn(SiF 6 )(bpp) 2 ] ( ZnSi ) and [Zn(TiF 6 )(bpp) 2 ] ( ZnTi ), were synthesized. Solid-state 19 F NMR revealed rotational motion of MF 6 2- 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 SiF 6 2- is larger than that of TiF 6 2- , 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.