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-C_2nH_4n−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.