Platinum-Catalyzed Hydroamination of Ethylene: Study of the Catalyst Decomposition Mechanism

A study of the addition of nucleophilic reagents that are also strong Brønsted bases (Et₃N, pyridine, quinuclidine, MeO^–) to nBu₄P⁺[PtBr₃(C₂H₄)]⁻ (1) and trans-[PtBr₂(NHEt₂)­(C₂H₄)] (7) has provided key information on the deactivation of the hydroamination PtBr₂/Br^– catalyst, leading to metallic platinum. The addition of NEt₃ to 1 in CD₂Cl₂ is reversible and temperature dependent; the quantitative formation of the zwitterionic complex trans-[Pt⁽⁻⁾Br₂(NEt₃)­(CH₂CH₂N⁽⁺⁾Et₃)] (9) is observed only at low temperature, whereas slow deposition of metallic platinum occurs at room temperature. The addition of NEt₃ to 7 in CD₂Cl₂ is also reversible and temperature dependent, yielding trans-[Pt⁽⁻⁾Br₂(NHEt₂)­(CH₂CH₂N⁽⁺⁾Et₃)] (10) quantitatively at low temperature. At room temperature, this reaction led to the deposition of metallic platinum and to the formation of a complex identified as trans-[PtBr₂(NHEt₂)­(CH₂CHNEt₂)] (11). The carbyl ligand in 11 is shown by an X-ray structural study to be between the π-bonded enamine and the σ-bonded iminiumalkyl configurations. The addition of MeONa to 7 results in the formation of the same products 11 and Pt⁰. On the basis of these results, a mechanism for the base-induced decomposition of Pt^II(C₂H₄) complexes that involves Wacker-type β-H elimination followed by intermolecular hydride transfer, ligand rearrangements, and final deprotonation is proposed. Addition of more nucleophilic N-based ligands (pyridine, quinuclidine) to 7 ultimately leads to C₂H₄ and Et₂NH substitution rather than to metal reduction, even though evidence for a kinetically controlled nucleophilic addition to the coordinated ethylene is given by the quinuclidine system. From the reaction with pyridine, the complex cis-PtBr₂(py)₂ was isolated and structurally characterized.