Synthesis and Electrochemical Studies of Nickel Complexes with a Flexidentate Bipyridine-aza-crown Ether Ligand

Bipyridine ligands have been extensively employed in nickel catalysis, with ligand modifications focused on steric or electronic tuning. In this work, we explore modifications designed to modulate the coordination mode using a 2,2'-bipyridine derivative with an appended aza-crown ether macrocycle capable of flexidentate binding to nickel. A series of complexes varying in charge from neutral to dicationic demonstrates the flexibility of the macrocycle, with bipyridine-aza-crown ether denticity changing from к4 to к6 upon sequential abstraction of chloride ligands. The changes in binding mode can be reversed by addition of chloride ion. Comparisons between the macrocycle-containing ligand and an analogous ligand with a non-macrocyclic diethylamine donor provide insight into the role of the crown ether, including in electrochemical reductions probed via cyclic voltammetry.