Kinetics and Mechanism of the Demetallation of Macrocyclic Nickel(II) Complexes by Cyanide

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Abstract
The kinetics and the mechanism of the cyanide‐induced demetallation of a series of Ni2+ complexes with macrocyclic ligands of different ring size (12‐ to 14‐membered; see 1–4) and steric constraints was studied. Although the rates differ by almost five orders of magnitude when compared to each other under fixed experimental conditions (pH 10.5, [CN−]=10−2 M), all reactions proceed through the relatively rapid formation of cyano adducts [Ni(CN)nL] (n=1, 2), which then react with additional CN− or HCN to give the final products. Of paramount importance for the reaction rate is the geometry and configuration of the cyano adducts [Ni(CN)nL] (n=1,2). cis‐Dicyano derivatives with a folded macrocycle react faster than trans‐compounds. In the case of (1,4,8,11‐tetraazacyclotetradecane)nickel(2+) ([Ni (4)]2+), which gives a trans‐ dicyano adduct, the base‐catalyzed N‐inversion necessary to obtain the cis‐dicyano derivative becomes rate determining at high CN− concentrations.