The Anisotropic Responses of a Flexible Metal–Organic Framework Constructed from Asymmetric Flexible Linkers and Heptanuclear Zinc Carboxylate Secondary Building Units

A new
porous and flexible metal–organic framework (MOF)
has been synthesized from the flexible asymmetric linker N-(4-carboxyphenyl)­succinamate (CSA) and heptanuclear zinc oxo-clusters
of formula [Zn7O2(carboxylate)10DMF2] involving two coordinated terminal DMF ligands. The structural
response of this MOF to the removal or exchange of its guest molecules
has been probed using a combination of experimental and computational
approaches. The topology of the material, involving double linker
connections in the a and b directions
and single linker connections along the c axis, is
shown to be key in the material’s anisotropic response. The a and b directions remain locked during
guest removal, whereas the c axis linker undergoes
large changes significantly reducing the material’s void space.
The changes to the c axis linker involve a combination
of a hinge motion on the linker’s rigid side and conformational
rearrangements on its flexible end, which were probed in detail during
this process despite the presence of crystallographic disorder along
this axis, which prevented accurate characterization by experimental
methods alone. Although inactive during guest removal, the flexible
ends of the a and b axis linkers
are observed to play a prominent role during DMF to DMSO solvent exchange,
facilitating the exchange reaction arising in the cluster.