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

A study of the addition of nucleophilic reagents that are also strong Bronsted bases (Et3N, pyridine, quinuclidine, MeO-) to nBu(4)P(+)[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 pi-bonded enamine and the sigma-bonded iminiumalkyl configurations. The addition of MeONa to 7 results in the formation of the same products 11 and Pt-0. On the basis of these results, a mechanism for the base-induced decomposition of Pt-II(C2H4) complexes that involves Wacker-type beta-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.