Osmium-promoted oxidative dimerization of aminoarenes to N-aryl-o-arenoquinone diimines (L). Synthesis, structure, electrochemistry, and reactivity of OsBr₂L₂

Abstract

A family of o-arenoquinone diimine chelates, OsBr₂L₂ (1, L=N-aryl-o-arenoquinone diimine), were isolated from the reaction of (NH₄)₂OsBr₆ and primary aromatic amines, Ar-NH₂. These complexes constitute the first examples of osmium compounds of the o-benzoquinone diimine family. The above oxidative dimerization reaction of Ar-NH₂ was authenticated by the X-ray structure determination of a representative example. The X-ray structure of OsBr₂L₂(R =H) revealed a cis geometry with respect to two coordinated bromides. The bond length trends within the chelate as well as the aromatic rings are consistent with a diimine character of the ligand bound to osmium in its bivalent oxidation state. The complexes showed multiple transitions in near-IR and visible regions. These are electroactive and undergo multiple electron transfer. Three successive voltammetric responses, two oxidative and one reductive, were observed. The potential range was +1.7 to -0.8 V vs SCE in CH₃CN solution (Pt electrode). The potentials of the above responses for differently substituted compounds depend on the Hammett Σσ_p parameters of the substitution on L. The EPR spectrum of the electrogenerated 1-electron-oxidized complex 1a⁺ is described. The electrode potentials of 1 are compared with those of related systems. A qualitative correlation between Os-N bond distances and Os(III)/Os(II) formal potentials for the Os(II) compounds of three different N,N donors is noted. The synthesis of a mixed-ligand tris chelate, [Os(bpy)L₂](ClO₄)₂.H₂O, is described, involving the reaction of 1 equiv of 1 with 2 equiv of Ag(bpy)₂⁺. The redox properties of the tris complex are reported. Crystal data for OsBr₂L₂ (R=H) are as follows: empirical formula C₂₄H₂₀Br₂N₄Os; crystal system triclinic, space group P↑ a=10.464(6) Å, b=11.778(6) Å, c=12.595(10) Å α=103.86(5)°, β=112.72(6)°, γ=107.79(4)°; V = 1245.3(10) ų; Z=2; R=4.46%; R_w=5.21%.