Unusual and Tunable Negative Linear Compressibility in the Metal–Organic Framework MFM-133(M) (M = Zr, Hf)

High-pressure single-crystal
X-ray structural analyses of isostructural
MFM-133­(M) (M = Zr, Hf) of flu topology and incorporating
the tetracarboxylate ligand TCHB4– [H4TCHB = 3,3′,5,5′-tetrakis­(4-carboxyphenyl)-2,2′,4,4′,6,6′-hexamethyl-1,1′-biphenyl]
and {M6(μ3-OH)8(OH)8(COO)8} clusters confirm negative linear compressibility
(NLC) behavior along the c axis. This occurs via
a three-dimensional wine-rack NLC mechanism leading to distortion
of the octahedral cage toward a more elongated polyhedron under static
compression. Despite the isomorphous nature of these two structures,
MFM-133­(Hf) shows a higher degree of NLC than the Zr­(IV) analogue.
Thus, for the first time, we demonstrate here that the NLC property
can be effectively tuned in a framework material by simply varying
the inorganic component of the frameworks without changing the network
topology and structure.