The (Dioximate)Ni^II/I₂ System: Ligand Oxidation and Binding Modes of Triiodide Species

Reinvestigation of (o-benzoquinonedioximate)₂Ni/I₂ systems demonstrated that the reaction itself and also the crystallization conditions dramatically affect the identity of generated species. Crystallization (CHCl₃, 20-25 °C) of the nickel(II) dioximate complex [Ni(bqoxH)₂] (bqoxH₂ = o-benzoquinonedioxime) with I₂ in the 1:(1-10) molar ratios of the reactants led to several (o-benzoquinonedioximate)₂Ni derivatives and/or iodine adducts [Ni(I)(bqoxH)(bqoxH₂)]·³/₂I₂, [Ni(I₃)(bqoxH)(bqoxH₂)]·[Ni(bqoxH)₂], and [Ni(I₃)(bqox^•-)(bqoxH₂)]·I₂; the latter one, featuring the anion-radical bqox^•- ligand, is derived from the formal (-2H⁺/1e^-)-oxidation of bqoxH₂. In these three adducts, various types of noncovalent interactions were identified experimentally and their existence was supported theoretically. The [Ni(I₃)(bqox^•-)(bqoxH₂)]·I₂ adduct exhibits simultaneous semicoordination and coordination patterns of the triiodide ligand; this is the first recognition of the semicoordination of any polyiodide ligand to a metal center. The semicoordination noncovalent contact Ni···I₃ (3.7011(10) Å) is substantially longer that the Ni-I₃ coordination bond (2.8476(9) Å), and the difference in energies between these two types of linkages is 8-12 kcal/mol.