Hydroxamate Titanium–Organic Frameworks and the Effect of Siderophore-Type Linkers over Their Photocatalytic Activity

The
chemistry of metal–organic frameworks (MOFs) relies
on the controlled linking of organic molecules and inorganic secondary
building units to assemble an unlimited number of reticular frameworks.
However, the design of porous solids with chemical stability still
remains limited to carboxylate or azolate groups. There is a timely
opportunity to develop new synthetic platforms that make use of unexplored
metal binding groups to produce metal–linker joints with hydrolytic
stability. Living organisms use siderophores (iron carriers in Greek) to effectively assimilate iron in soluble form. These
compounds make use of hard oxo donors as hydroxamate or catecholate
groups to coordinate metal Lewis acids such as iron, aluminum, or
titanium to form metal complexes very stable in water. Inspired by
the chemistry of these microorganisms, we report the first hydroxamate
MOF prepared by direct synthesis. MUV-11 (MUV = materials of Universidad
de Valencia) is a crystalline, porous material (close to 800 m2·g–1) that combines photoactivity with
good chemical stability in acid conditions. By using a high-throughput
approach, we also demonstrate that this new chemistry is compatible
with the formation of single-crystalline phases for multiple titanium
salts, thus expanding the scope of accessible precursors. Titanium
frameworks are regarded as promising materials for photocatalytic
applications. Our photoelectrochemical and catalytic tests suggest
important differences for MUV-11. Compared to other Ti-MOFs, changes
in the photoelectrochemical and photocatalytic activity have been
rationalized with computational modeling, revealing how the chemistry
of siderophores can introduce changes to the electronic structure
of the frontier orbitals, relevant to the photocatalytic activity
of these solids.