Activation of Nitrogen-Rich Substrates by Low-Valent, Redox-Active Aluminum Species

Reactivity of low-valent AlII species containing redox-active
dpp-dad or dpp-Bian ligand (La = dpp-dad = [(2,6-iPr2C6H3)­NC­(CH3)]2; Lb = dpp-Bian = 1,2-[(2,6-iPr2C6H3)­NC]2C10H6) toward nitrogen-rich substrates is described. Reactions
of dialumanes [L2–Al–AlL2–] (La, 1a, or Lb, 1b) with azides (Me3SiN3, BenzylN3) or azobenzene at room temperature lead to the facile elimination
of dinitrogen (in the case of azides) and formation of dimeric imido-complexes
[L•–Al­(μ2-NR)2AlL•–] (2a–2d). During the reaction, the initially dianionic ligands (L2–) and AlII ions are oxidized into monoanions (L•–) and AlIII in the products, respectively, while the [NR]2– imides are produced by a four-electron reductive
cleavage of the NN double bond of azides or azo compounds.
The monoanionic α-diimine ligands in 2 can be reduced
again by Na, yielding analogous products with (La)̇•–/(La)2– or (La)2–/(La)2– ligands (3, 4a, and 4b).
Treatment of 1a with benzyl azide at −30 °C
in the presence of Na results in [(La)2–Al­(μ2-N3Benzyl)2Al­(La)2–]·[Na­(THF)]2 (5), with unprecedented bridging triazene groups, while reaction with
trimethylsilyldiazomethane (Me3SiCHN2) or diphenyldiazomethane
(Ph2CN2) affords dinuclear complexes with doubly
reduced diazo dianions as the bridges (6a and 6b). All the products were structurally and spectroscopically characterized.
The results clearly demonstrate the unique reactivity of α-diimine-ligated
dialumanes 1a and 1b, which can activate
nitrogen-rich molecules by cooperation of metal and the redox ligand.