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A strategy for modulating the coordination mode and redox properties of nickel complexes is introduced, using a 2,2'-bipyridine derivative featuring a flexidentate and cation-responsive aza-crown ether. The bipyridine-aza-crown ether ligand can reversibly change denticity from к4 to к6 by the addition/removal of chloride ion or acetonitrile. A key impact of the crown ether pendant is that the electrochemical response of a nickel complex is tunable by addition of redox-inactive ions. Redox-switchable cation binding occurs after the first reduction, with strong Na+ binding induced upon reduction from nickel(II) to nickel(I). Even though Na+ has a low charge and is a weak Lewis acid, large anodic shifts (ranging from 72 to 236 mV) are observed for the subsequent metal- and ligand-centered reductions. Even larger anodic shifts (160 to 428 mV) are observed in the presence of Ca2+ ions. The potential shifts of some features can be reversed by cation removal via the addition of free crown ether. A mechanism for changes in Ni primary coordination sphere under different electrochemical conditions is proposed, providing insight into how intersecting design features of flexidenticity and cation-crown interactions can introduce reversibly tunable properties of nickel complexes relevant to a range of catalytic reactions. 

An acid-promoted dearomative rearrangement of O-arylhydroxylamines affords 2-aminocyclohexadien-1-ones, which can in turn be reductively quenched for the synthesis of trans-aminoalcohols on a cyclohexadiene core. This method serves as an efficient entry to the pharmaceutically relevant 1-arylcyclohexylamine scaffold in two steps (one purification) from commercially available or readily prepared 2-arylphenols. 

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.