Two [1,2,4-(Me₃C)₃C₅H₂]₂CeH Molecules are Involved in Hydrogenation of Pyridine to Piperidine as Shown by Experiments and Computations

Abstract

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me₃C)₃C₅H₂]₂CeH, abbreviated as Cp′₂CeH or [Ce]′-H, is reported. The reaction proceeds from Cp′₂Ce­(2-pyridyl), isolated from the reaction of pyridine with Cp′₂CeH, to Cp′₂Ce­(4,5,6-trihydropyridyl), and then to Cp′₂Ce­(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp′₂Ce­(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H₂ and temperature. The dependence of the rate on the H₂ pressure is associated with the formation of Cp′₂CeH, which increases the rate of the first and/or second additions of H₂ but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H₂ with a reasonably high energy, Δ<i>G</i>^⧧ = 20.5 kcal mol^–1, on Cp′₂Ce­(2-pyridyl), leading to Cp′₂CeH­(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp′₂CeH to the C⁴C⁵ bond, followed by hydrogenolysis, giving Cp′₂Ce­(4,5,6-trihydropyridyl) and regenerating Cp′₂CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp′₂Ce­(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp′₂CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp′₂Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C⁴C⁵ double bond. In the presence of H₂, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp′₂Ce­(2-pyridyl) and piperidine. The structures of Cp′₂CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature ¹H NMR spectroscopy