Synthesis of a diruthenium μ-η4-α-diimine complex via dehydrogenative coupling of cyclic amines and its role in dehydrogenative oxidation of pyrrolidine.

Abstract

The reaction of [Cp‡Ru(μ-H)4RuCp‡] (1: Cp‡ = 1,2,4-tri-tert-butylcyclopentadienyl) with cyclic amines at 180 °C afforded a μ-η4-α-diimine complex, [(Cp‡Ru)2(μ-η4-C2nH4n-4N2)] (5a-c: n = 4, 5, 6), via dehydrogenative coupling of two cyclic amine molecules. An intermediate μ-η2-1-pyrroline complex, [{Cp‡Ru(μ-H)}2(μ-η2-C4H7N)] (2a), was synthesized by the photoreaction of 1 with pyrrolidine and 5a was shown to be formed via the disproportionation of 2a upon thermolysis yielding 1 and a μ-imidoyl complex, [(Cp‡Ru)2(μ-η2:η2-C4H6N)(μ-H)] (3a). Complex 3a was transformed into 5avia the incorporation of 1-pyrroline, which was formed by the reaction of 2a with H2. DFT calculations on the model complexes supported by C5H5 groups at the B3LYP level suggested that the μ-η4-α-diimine ligand is formed via the insertion of a terminal cyclic aminocarbene ligand into the Ru-C bond of the μ-imidoyl group followed by the elimination of hydrogen. Although 5a was inert under an Ar atmosphere, it catalyzed the dehydrogenative oxidation of pyrrolidine under an atmosphere of hydrogen to yield γ-butyrolactam. An active species possessing a terminal cyclic aminocarbene ligand was generated via the heterolytic activation of hydrogen at the Ru-N bond followed by C-C bond cleavage.