Synthesis of a diruthenium μ-η 4 -α-diimine complex via dehydrogenative coupling of cyclic amines and its role in dehydrogenative oxidation of pyrrolidine.

The reaction of [Cp ‡ Ru(μ-H) 4 RuCp ‡ ] (1: Cp ‡ = 1,2,4-tri- tert -butylcyclopentadienyl) with cyclic amines at 180 °C afforded a μ-η 4 -α-diimine complex, [(Cp ‡ Ru) 2 (μ-η 4 -C 2 n H 4 n -4 N 2 )] (5a-c: n = 4, 5, 6), via dehydrogenative coupling of two cyclic amine molecules. An intermediate μ-η 2 -1-pyrroline complex, [{Cp ‡ Ru(μ-H)} 2 (μ-η 2 -C 4 H 7 N)] (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-η 2 :η 2 -C 4 H 6 N)(μ-H)] (3a). Complex 3a was transformed into 5a via the incorporation of 1-pyrroline, which was formed by the reaction of 2a with H 2 . DFT calculations on the model complexes supported by C 5 H 5 groups at the B3LYP level suggested that the μ-η 4 -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.