Quest for Highly Connected Metal–Organic Framework Platforms: Rare-Earth Polynuclear Clusters Versatility Meets Net Topology Needs

Gaining
control over the assembly of highly porous rare-earth (RE)
based metal–organic frameworks (MOFs) remains challenging.
Here we report the latest discoveries on our continuous quest for
highly connected nets. The topological exploration based on the noncompatibility
of a 12-connected RE polynuclear carboxylate-based cluster, points
of extension matching the 12 vertices of the cuboctahedron (cuo), with 3-connected organic ligands led to the discovery
of two fascinating and highly connected minimal edge-transitive nets, pek and aea. The reduced symmetry of the employed
triangular tricarboxylate ligand, as compared to the prototype highly
symmetrical 1,3,5-benzene­(tris)­benzoic acid guided the concurrent
occurrence of nonanuclear [RE9(μ3-OH)12(μ3-O)2(O2C−)12] and hexanuclear [RE6(OH)8(O2C−)8] carboxylate-based clusters as 12-connected
and 8-connected molecular building blocks in the structure of a 3-periodic pek-MOF based on a novel (3,8,12)-c trinodal net. The use
of a tricarboxylate ligand with modified angles between carboxylate
moieties led to the formation of a second MOF containing solely nonanuclear
clusters and exhibiting once more a novel and a highly connected (3,12,12)-c
trinodal net with aea topology. Notably, it is the first
time that RE-MOFs with double six-membered ring (d6R) secondary building
units are isolated, representing therefore a critical step forward
toward the design of novel and highly coordinated materials using
the supermolecular building layer approach while considering the d6Rs
as building pillars. Lastly, the potential of these new MOFs for gas
separation/storage was investigated by performing gas adsorption studies
of various probe gas molecules over a wide range of pressures. Noticeably, pek-MOF-1 showed excellent volumetric CO2 and CH4 uptakes at high pressures.