Platinum-Catalyzed Ethylene Hydroamination with Aniline: Synthesis, Characterization, and Studies of Intermediates

Starting from either K 2 PtCl 4 or K[PtCl 3 (C 2 H 4 )]·H 2 O (Zeise’s salt), complexes ( n Bu 4 P) 2 [PtBr 4 ] ( 1 ), n Bu 4 P[PtBr 3 (C 2 H 4 )] ( 2 ), n Bu 4 P[PtBr 3 (PhNH 2 )] ( 3 ), trans -[PtBr 2 (C 2 H 4 )(PhNH 2 )] ( 4 ), cis -[PtBr 2 (C 2 H 4 )(PhNH 2 )] ( 5 ), and cis -[PtBr 2 (PhNH 2 ) 2 ] ( 6 ) have been obtained by efficient one-pot procedures. All have been fully characterized by microanalysis (C, H, N), multinuclear NMR spectrometry ( 1 H, 13 C, 195 Pt), UV−visible spectroscopy, and single-crystal X-ray diffraction. Compound 1 slowly loses Br − in solution to yield ( n Bu 4 P) 2 [Pt 2 Br 6 ] ( 1′ ), which has also been characterized crystallographically. The relative stability of the various compounds has been probed experimentally by NMR studies in several solvents and computationally by gas-phase geometry optimizations followed by C-PCM calculations of the solvation effects in dichloromethane and aniline. The calculations also included the bis(ethylene) complexes [PtBr 2 (C 2 H 4 ) 2 ] in the trans (two different conformations 7 and 7′ ) and cis ( 8 ) configurations. The solution experiments gave no evidence for a nucleophilic attack of aniline onto coordinated ethylene under mild conditions ( T up to 68 °C), setting a lower limit of 29 kcal mol −1 for the activation barrier of this process. Therefore, the relative energies computed for the other compounds suggest that all ethylene-containing complexes ( 2, 4, 5, 7, and 8 ) are viable candidates for the nucleophilic addition step of the PtBr 2 -catalyzed ethylene hydroamination by aniline. Use of the isolated complex 2, 4, or 5 in combination with n Bu 4 Br as precatalysts for the ethylene hydroamination by aniline yields similar catalytic activities.