The (Dioximate)NiII/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.