Group IV Imino-Semiquinone Complexes Obtained by Oxidative Addition of Halogens

An isostructural series of titanium, zirconium, and hafnium complexes, M[ap]2L2 (M = Ti, Zr, Hf; L = THF, pyridine), of the redox-active 4,6-di-tert-butyl-2-tert-butylamidophenolate ligand ([ap]2−) have been prepared. The zirconium and hafnium derivatives react readily with halogen oxidants such as XeF2, PhICl2, and Br2, leading to products in which one-electron oxidation of each [ap]2− ligand accompanies halide addition to the metal center. Iodine proved to be too weak of an oxidant to yield the corresponding oxidative addition product, and under no conditions could halogen oxidative addition products be obtained for titanium. According to X-ray crystallographic studies, the zirconium and hafnium oxidation products are best formulated as MX2[isq·]2 ([isq·]− = 4,6-di-tert-butyl-2-tert-butylimino-semiquinonate; M = Zr, Hf; X = F, Cl, Br) species, in which the molecule is symmetric with each redox-active ligand in the semiquinone oxidation state. Temperature-dependent magnetization measurements suggest a singlet (S = 0) ground-state for the diradical complexes with a thermally accessible triplet (S = 1) excited state. Solution electron paramagnetic resonance (EPR) spectra are consistent with this assignment, showing both Δms = 1 and Δms = 2 transitions for the antiferromagnetically coupled electrons.