Four-electron oxidative formation of aryl diazenes using a tantalum redox-active ligand complex.

Transition-metal complexes capable of mediating multielectron transformations are critical components for a variety of small-molecule transformations. For example, the oxidation of C H bonds and the reduction of protons to H2 are both two-electron transformations. The oxidation of water to O2 is a four-electron process and the reduction of nitrogen to ammonia is an overall six-electron process. The design of metal complexes to promote or catalyze these multielectron reactions usually relies on one or more transition-metal ions capable of two-electron changes in a formal oxidation state. An alternative strategy is to incorporate redox-active ligands into the metal coordination sphere to supply reducing or oxidizing equivalents during a multielectron transformation. Herein, we report the use of a tridentate redox-active ligand, N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide ([ONO] ), coordinated to tantalum, to effect the fourelectron oxidative formation of aryl diazenes. In its reduced form, [ONO] is a planar, tridentate ligand that coordinates to transition metals in a meridional geometry. The organometallic synthon TaMe3Cl2 [7] has been used to prepare [ONO]TaMe2 (1), which was then converted into the bridging imido dimer {[ONO]Ta[m-N(p-tolyl)]L}2 (2a, L= NH2(p-tolyl); 2b, L= pyridine (py); Scheme 1). Oxidation of 2b resulted in the quantitative elimination of (p-tolyl)N=N(ptolyl). To the best of our knowledge, this is the first example of N=N double bond formation and organic diazene elimination from a tantalum(V) bridging imido dimer. Oxidation studies of the related complex [ONO]TaCl2 (4) with PhICl2 suggest that the redox-active ligand plays the pivotal role of collecting oxidizing equivalents within the tantalum coordination sphere. The work presented herein highlights a new strategy for the design of metal complexes capable of multielectron oxidation reactions. The bridging imido complexes 2a and 2b were prepared via dimethyl complex 1 (Scheme 1). Double deprotonation of H3[ONO ] with nBuLi (2 equiv) followed by treatment with TaCl2Me3 afforded 1 in 49% yield following recrystallization from pentane. The H and C NMR spectra of 1 showed diagnostic resonances for the [ONO] ligand. The methyl ligands of 1 resonated at d= 0.77 and 59.7 ppm in the H and C NMR spectra, respectively. The methyl ligands of 1 are susceptible to protonolysis by anilines, which results in the formation of bimetallic complexes with two bridging imido ligands. As shown in Scheme 1, benzene solutions of 1 heated to reflux with two equivalents of NH2(p-tolyl) resulted in the formation of