Defects play a crucial role in the chemistry of metal-organic frameworks (MOFs), with many recent studies highlighting the presence of defects, especially ligand vacancies, in many important families of MOFs. The presence of defects alone has been shown to drastically improve sorption properties [1,2], catalytic activity  and ionic conductivity . For many functional materials, however, it is not just the presence of defects but their interactions and ordering that determine their properties.
We have investigated the effects of defect incorporation in the canonical UiO-66 family of MOFs, and shown that at high concentrations defects are accommodated through the correlation of defects. Just as for oxide frameworks, this ordering leads to a diversity of structures. We have focussed on two examples of defect correlation: the nano-reo phase, where defects are accommodated by a nano-domain phase in which correlated ligand absences lead to metal cluster absences  the hcp phase, where ligand absence defects order to form new layered phase. Making use of a wide-range of complementary techniques (including pair-distribution function analysis, in situ diffraction of MOF growth and transmission electron microscopy) we were able to determine not only the structures of these new phases but also the key control parameters for synthesis. These are found to be the concentrations of defect-promoting modulator and added water, and the reaction time and temperature.
The control over structure provided by defects allowed us to tune the physical and chemical properties of the UiO family of MOFs. By varying the concentration of defects we were able to tune the very large isotropic negative thermal expansion of nano-reo UiO-66(Hf) , and the introduction of chemical anisotropy into hcp UiO-67(Hf) meant it was possible to delaminate it into nanosheets. This provides proof of principle of the use of defects as a design element in MOF materials.
 H. Wu et al., J. Am. Chem. Soc. 2013, 135, 10525.
 F. Vermoortele et al., J. Am. Chem. Soc. 2013, 135, 11465.
 J. Taylor et al., Chem. Mater. 2015, 27, 2286.
 M.J. Cliffe et al., Nat. Commun. 2014, 5, 4176.
 M.J. Cliffe et al., Phys. Chem. Chem. Phys. 2015, 17, 11586.
 M. J. Cliffe et al., J. Am. Chem. Soc. 2017, 139, 5397.
Prof. Henry LaPierre (firstname.lastname@example.org)