Structure of a MOF-derived molecular glass

This work was was result of a collaboration with Kyoto university, particularly with Dr. Daiki Umeyama and Prof. Satoshi Horike.

Most of the interest in metal-organic frameworks (MOFs) has focussed on those that are ordered and crystalline. Moreover, MOFs typically don’t melt when heated, instead decomposing, which means that they can be difficult to form into the shapes needed for useful applications. Glass, in contrast, is is incredibly useful because we can form it into very elaborate shapes without introducing any internal boundaries.

In previous work, Prof Horike’s group in Kyoto showed that a class of MOFs based on metal cations connected by phosphate anions and nitrogen containing ligand molecules (such as triazole) with very promising proton conductivity also melted at relatively modest temperatures (~150C). What’s more, some of these zinc MOFs could be made into glasses (i.e. noncrystalline solids) by cooling them very rapidly. This was amongst the very first times that anyone had managed to make a glass from a molten MOF.

In this paper we examined the structure of one of these MOF glasses (Zn(H₂PO₄)2(HTr)₂, HTr = 1,2,4-triazole) using X-ray total scattering (pair distribution function) and X-ray spectroscopy. We found that our X-ray data could be explained if the glass was formed from isolated molecular units, rather than the extended framework found in the crystal. This ‘2D-0D’ transition is very unusual, also helps explain why this material melts and forms a glass so easily, relative to other MOFs and framework compounds more generally.