Mechanistic insight into the N=N bond-cleavage of Azo-compounds that was induced by an Al-Al-bonded compound [L ^2-Al ^II-Al ^IIL ^2-]

An α-diimine-stabilized Al-Al-bonded compound [L ^2-Al ^II-Al ^IIL ^2-] (L=[{(2,6-iPr ₂C ₆H ₃)NC(Me)} ₂]; 1) consists of dianionic α-diimine ligands and sub-valent Al ²⁺ ions and thus could potentially behave as a multielectron reductant. The reactions of compound 1 with azo-compounds afforded phenylimido-bridged products [L ^-Al ^III(μ ₂-NPh)(μ ₂-NAr)Al ^IIIL ^-] (2-4). During the reaction, the dianionic ligands and Al ²⁺ ions were oxidized into monoanions and Al ³⁺, respectively, whilst the [NAr] ^2- imides were produced by the four-electron reductive cleavage of the N=N double bond. Upon further reduction by Na, the monoanionic ligands in compound 2 were reduced to the dianion to give [(L ^2-) ₂Al ^III ₂(μ ₂-NPh) ₂Na ₂(thf) ₄] (5). Interestingly, when asymmetric azo-compounds were used, the asymmetric adducts were isolated as the only products (compounds 3 and 4). DFT calculations indicated that the reaction was quite feasible in the singlet electronic state, but the final product with the triplet-state monoanionic ligands could result from an exothermic singlet-to-triplet conversion during the reaction process.