Pyridine derivatives are used as ligands for a wide range of metal complexes. Ligands in which the pyridine groups are part of a macrocyclic structure are particularly interesting because they provide a chemically stable environment. Most members of this ligand class contain one or two pyridine fragments within the macrocyclic system, while molecules with three pyridine fragments have been less well explored. These ligands can be tuned to accommodate metals of different sizes by modifying the number of atoms in the linker between the pyridine rings. The ring size of [2.1.1]-(2,6)-pyridinophanes is best suited for coordinating d-block metal atoms.

Andrei N. Vedernikov, University of Maryland, College Park, USA, and colleagues have developed a new ligand of this type, 1,7-dioxa-[2.1.1]-(2,6)-pyridinophane (pictured). In this compound, two methylene-bridges have been replaced by oxygen atoms, which leads to an electron-poor ligand system. The ligand was prepared in a four-step synthesis starting from 2-chloro-6-methylpyridine in an overall yield of 11 %. The researchers also prepared the palladium(II)dichloro complex and the copper(II)chloro complex of the ligand—from PdCl₂(MeCN)₂ in acetonitrile and CuCl in dichloromethane, respectively.

Both complexes have a κ²–N,N-coordination. The copper center is coordinated by the nitrogen atoms of two equivalent pyridine fragments, while the palladium center has an asymmetrical coordination mode. The team tested the catalytic properties of the copper complex in aziridation reactions. Olefin substrates were converted to aziridines in high yields ranging from 78 to 98 % after short reaction times of 5 to 15 minutes. Due to its electron-poor nature, complexes of the ligand could be well-suited for applications under strongly oxidizing conditions.

• An Electron-Poor Dioxa-[2.1.1]-(2,6)-pyridinophane Ligand and Its Application in Cu-Catalyzed Olefin Aziridination,
  Fan Yang, Jiaheng Ruan, Peter Y. Zavalij, Andrei N. Vedernikov,
  Inorg. Chem. 2019.
  https://doi.org/10.1021/acs.inorgchem.9b02618