Osmium-promoted oxidative dimerization of aminoarenes to N-aryl-o-arenoquinone diimines (L). Synthesis, structure, electrochemistry, and reactivity of OSBr(2)L(2)

A family of o-arenoquinone diimine chelates, OsBr(2)L(2) (1, L = N-aryl-o-arenoquinone diimine), were isolated from the reaction of (NH4)(2)OsBr6 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 OsBr(2)L(2) (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 Sigma sigma(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)L(2)](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 OsBr(2)L(2) (R = H) are as follows: empirical formula C24H20Br2N4Os; crystal system triclinic, space group P (1) over bar; a = 10.464(6) Angstrom, b = 11.778(6) Angstrom, c = 12.595(10) Angstrom; alpha = 103.86(5)degrees, beta = 112.72(6)degrees, gamma = 107.79(4)degrees; V = 1245.3(10) Angstrom(3); Z = 2; R = 4.46%; R(w) = 5.21%.