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.
This work was supported by Spanish MINECO (MDM-2015-0538, CTQ2017-83486-P, and ENE2016-79608-C2-1-R). C.M.-G., N.M.P., J.C.-G., and M.B. thank the Spanish MINECO for a Ramón y Cajal Fellowship (RYC-2012-10894),Marie Skłodowska Curie Fellowship (H2020-MSCA-IF-2016-GF-749359-EnanSET), FPI Scholarship (CTQ2014-59209-P), and Juan de la Cierva Fellowship (FJCI-2016-30567),respectively.