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

Abstract

An isostructural series of titanium, zirconium, and hafnium complexes, M[ap] 2L 2 (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 XeF 2, PhICl 2, and Br 2, 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 MX 2[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 Delta m s = 1 and Delta m s = 2 transitions for the antiferromagnetically coupled electrons.

A Redox‐Active Ligand as a Reservoir for Protons and Electrons: O2 Reduction at Zirconium(IV)

“…Thus, in the reaction the ligands serve as an electron reservoir to help effect multielectron reactivity at a non-redox-active metal. Similar reactions with redox-active ligands have been elucidated for oxidative addition, [12][13][14] reductive elimination, [15] and nitrene trans- fer [16,17] reactivity at d 0 complexes of the group 4 and 5 metals. In this report, we extend this approach beyond the utilization of the ligands as just electron reservoirs to develop a redox-active ligand that serves as both an electron and proton reservoir for O 2 reduction.…”

“…13 C NMR spectra were referenced to TMS by using the natural-abundance 13 C impurities of the solvent. All chemical shifts are reported by using the standard δ notation in parts per million; positive chemical shifts are to a higher frequency of the given reference.…”

Bis(acetylacetonato)bis(pyrazolato)ruthenate(iii) as a redox-active scorpionate ligand

“…Calcd (%) for C 38 H 46 N 8 O 8 Ru 3 ([Ru 2 Zn(acac) 4 ( pz) 4 ]-(C 6 H 6 )): C,45.18;H,4.59;N,11.10;Found: C,45.42;H,4.53;N,11.10 [Ru(acac) 2 ( pz)( pzH)] (0.425 g, 0.98 mmol), Ru 3 (CO) 12 (0.10 g, 0.16 mmol), and triethylamine (150 μl) were added to ethanol (50 ml). After the solution was kept at reflux for 14 h, it was cooled to rt and filtered.…”

Group IV Coordination Chemistry of a Tetradentate Redox‐Active Ligand in Two Oxidation States

“…Previous attempts to carry out halogen oxidation reactions with titanium complexes of redox-active ligands resulted in messy ligand disproportionation reactions. [10] We Attempts to generate odd-electron species through the addition of 0.5 or 1.5 equiv. of PhICl 2 resulted only in reduced yields of the two-electron oxidized products 4-6.…”

“…Whereas oxidized complexes of the form ZrX 2 (isq·) 2 could be obtained by halogen oxidative addition reactions, similar species were not accessible for titanium derivatives, where a titanium(III) oxidation state is viable. [10] In the titanium case, a valence tautomerization [12] could give Ti III and an iminoquinone ligand (iq), which would readily dissociate from the d 0 metal center. In contrast, the tetradentate [N 2 O 2 ox ] 2-ligand cannot dissociate, even if a valence tautomerization is established.…”