Multi-titanium hydrides can selectively snip the strong structural bonds of stable organic molecules called pyridines, RIKEN researchers have shown1. This discovery could guide designing catalysts for applications in multiple branches of industrial chemistry, from oil refining to the synthesis of functional organic molecules.
Pyridines are stable aromatic molecules characterized by a ring consisting of one nitrogen atom and five carbon atoms. They are a common structural motif in complex organic molecules such as pharmaceuticals. They are also a component of crude oil that needs to be removed during refining.
"The removal of nitrogen-containing impurities such as pyridines from crude oil is an important industrial process in petroleum refining," notes Zhaomin Hou of the RIKEN Organometallic Chemistry Laboratory and the RIKEN Advanced Catalysis Research Group.
This process typically requires multicomponent solid catalysts and proceeds under harsh conditions of up to 500°C of heat and 200 atmospheres of pressure.
A cleaner, milder way to conduct this process could be possible through using multi-metallic polyhydride complexes, Hou's work suggests.
Previously, Hou and his team had shown that these well-defined clusters of titanium and hydrogen atoms can activate and transform inert small molecules such as N2, as well as stable aromatic compounds including benzene and pyridine.
"These findings motivated us to carry out a detailed investigation of the reactions of titanium polyhydride complexes with pyridines," says Hou.
The team found that the titanium polyhydride complex could snip the nitrogen atom out of the pyridine molecular skeleton with high selectivity.
"We found that aromatic N-heterocycles such as pyridine can undergo simultaneous cleavage of both C-N and C−C bonds, as well as reorganize the resulting hydrocarbon fragments," says Hou.
A variety of pyridine fragment complexes can result depending on the reaction temperature. But using mild heating of 160°C pushed the reaction toward a single common product (Fig. 1).
Synergistic cooperation between the three titanium atoms in the titanium polyhydride complex is the key to breaking apart the stable pyridine ring and selectively excising the nitrogen atom from the structure, the researchers showed.
"This work underscores the unique capability of multinuclear titanium polyhydride clusters to mediate skeletal cleavage and reorganization of aromatic N-heterocycles," says Hou. "This offers a new strategy for heterocycle-to-hydrocarbon molecular editing."
Titanium hydride clusters have potential in numerous areas of industrial chemistry. "In addition to developing selective and efficient platforms for aromatic skeletal transformation, we will investigate strategies to selectively and efficiently activate and assemble inert small molecules such as N2, CO2 and CO, together with organic substrates into functional organic molecules," says Hou.
