Dehydrogenative Coupling of 4‑Substituted Pyridines Catalyzed by a Trinuclear Complex of Ruthenium and Cobalt

The dehydrogenative coupling of 4-substituted pyridines catalyzed by a heterometallic trinuclear complex composed of Ru and Co, (Cp*Ru)₂(Cp*Co)­(μ-H)₃(μ₃-H) (1, Cp* = η⁵-C₅Me₅), was investigated. When the pyridine substrate contains an electron-donating group at the 4-position, complex 1 showed a high catalytic activity compared to di- and triruthenium complexes (Cp*Ru)₂(μ-H)₄ (4) and (Cp*Ru)₃(μ-H)₃(μ₃-H)₂ (5). The catalytic activity of 1 was also remarkably higher than the congeners of other group 9 metals, Ru₂Rh (2) and Ru₂Ir analogues (3). The distinctive reactivity of 1 was attributed to a paramagnetic intermediate, (Cp*Ru)₂{(dmbpy)­Co}­(μ-H)­(μ₃-H)₂ (12, dmbpy = 4,4′-dimethyl-2,2′-bipyridine), which was formed by the reaction of 1 with 4-picoline accompanied by the dissociation of the Cp* at the Co atom. The reaction of 12 with unsubstituted pyridine resulted in the elimination of 4,4′-dimethyl-2,2′-bipyridine, indicating that the Co atom in 12 acts as a dissociation site. In contrast to the reaction of 1 with 4-picoline, the reaction of 2 and 3 with 4-picoline afforded the corresponding μ₃-pyridyl complexes (Cp*Ru)₂(Cp*M)­(μ-H)₃­(μ₃-η²(||)-C₅H₃NCH₃) (15, M = Rh; 16, M = Ir). 4-(Trifluoromethyl)­pyridine was not dimerized by 1; however, a similar μ₃-pyridyl complex, (Cp*Ru)₂(Cp*Co)­(μ-H)₃­(μ₃-η²(||)-C₅H₃NCF₃) (13), was obtained. The stability of the μ₃-pyridyl complex is probably one of the reasons for the low catalytic activity of 2 and 3 in the coupling reaction.