Remarkable Pressure Responses of Metal–Organic Frameworks: Proton Transfer and Linker Coiling in Zinc Alkyl Gates

Metal–organic frameworks demonstrate
a wide variety of behavior
in their response to pressure, which can be classified in a rather
limited list of categories, including anomalous elastic behavior (e.g.,
negative linear compressibility, NLC), transitions between crystalline
phases, and amorphization. Very few of these mechanisms involve bond
rearrangement. Here, we report two novel piezo-mechanical responses
of metal–organic frameworks, observed under moderate pressure
in two materials of the zinc alkyl gate (ZAG) family. Both materials
exhibit NLC at high pressure, due to a structural transition involving
a reversible proton transfer between an included water molecule and
the linker’s phosphonate group. In addition, the 6-carbon alkyl
chain of ZAG-6 exhibits a coiling transition under pressure. These
phenomena are revealed by combining high-pressure single-crystal X-ray
crystallography and quantum mechanical calculations. They represent
novel pressure responses for metal–organic frameworks, and
pressure-induced proton transfer is a very rare phenomenon in materials
in general.