Heterometallic Titanium–Organic Frameworks by Metal-Induced Dynamic Topological Transformations

Reticular chemistry has boosted the
design of thousands of metal
and covalent organic frameworks for unlimited chemical compositions,
structures, and sizable porosities. The ability to generate porous
materials at will on the basis of geometrical design concepts is responsible
for the rapid growth of the field and the increasing number of applications
derived. Despite their promising features, the synthesis of targeted
homo- and heterometallic titanium–organic frameworks amenable
to these principles is relentlessly limited by the high reactivity
of this metal in solution that impedes the controlled assembly of
titanium molecular clusters. We describe an unprecedented methodology
for the synthesis of heterometallic titanium frameworks by metal-exchange
reactions of MOF crystals at temperatures below those conventionally
used in solvothermal synthesis. The combination of hard (titanium)
and soft (calcium) metals in the heterometallic nodes of MUV-10­(Ca)
enables controlled metal exchange in soft positions for the generation
of heterometallic secondary building units (SBUs) with variable nuclearity,
controlled by the metal incorporated. The structural information encoded
in the newly formed SBUs drives an MOF-to-MOF conversion into bipartite
nets compatible with the connectivity of the organic linker originally
present in the crystal. Our simulations show that this transformation
has a thermodynamic origin and is controlled by the terminations of
the (111) surfaces of the crystal. The reaction of MUV-10­(Ca) with
first-row transition metals permits the production of crystals of
MUV-101­(Fe,Co,Ni,Zn) and MUV-102­(Cu), heterometallic titanium MOFs
isostructural with archetypical solids such as MIL-100 and HKUST.
In comparison to de novo synthesis, this metal-induced topological
transformation provides control over the formation of hierarchical
micro-/mesopore structures at different reaction times and enables
the formation of heterometallic titanium MOFs not accessible under
solvothermal conditions at high temperature, thus opening the door
for the isolation of additional titanium heterometallic phases not
linked exclusively to trimesate linkers.