A Flexidentate Bipyridine-Aza-Crown Ether Ligand Enables Cation-Tunable Nickel Redox Properties

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 κ⁴ to κ⁶ 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 electrochemical features. 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.