Coordination and Organometallic Chemistry of Relevance to the Rhodium-Based Catalyst for Ethylene Hydroamination
Résumé
The RhCl3·3H2O/PPh3/nBu4PI catalytic system for the hydroamination of ethylene by aniline is shown to be thermally stable by a recycle experiment and by a kinetic profile study. The hypothesis of the reduction under catalytic conditions to a RhI species is supported by the observation of a high catalytic activity for complex [RhI(PPh3)2]2. New solution equilibrium studies on [RhX(PPh3)2]2 (X = Cl, I) in the presence of ligands of relevance to the catalytic reaction (PPh3, C2H4, PhNH2, X–, and the model Et2NH amine) are reported. Complex [RhCl(PPh3)2]2 shows broadening of the 31P NMR signal upon addition of PhNH2, indicating rapid equilibrium with a less thermodynamically stable adduct. The reaction with Et2NH gives extensive conversion into cis-RhCl(PPh3)2(NHEt2), which is however in equilibrium with the starting material and free Et2NH. Excess NHEt2 yields a H-bonded adduct cis-RhCl(PPh3)2(Et2NH)···NHEt2, in equilibrium with the precursors, as shown by IR spectroscopy. The iodide analogue [RhI(PPh3)2]2 shows less pronounced reactions (no change with PhNH2, less extensive addition of Et2NH with formation of cis-RhI(PPh3)2(NHEt2), less extensive reaction of the latter with additional Et2NH to yield cis-RhI(PPh3)2(Et2NH)···NHEt2. The two [RhX(PPh3)2]2 compounds do not show any evidence for addition of the corresponding X– to yield a putative [RhX2(PPh3)2]− adduct. The product of C2H4 addition to [RhI(PPh3)2]2, trans-RhI(PPh3)2(C2H4), has been characterized in solution. Treatment of the RhCl3·3H2O/PPh3/nBu4PI/PhNH2 mixture under catalytic conditions yields mostly [RhCl(PPh3)2]2, and no significant halide exchange, demonstrating that the promoting effect of iodide must take place at the level of high energy catalytic intermediates. The equilibria have also been investigated at the computational level by DFT with treatment at the full QM level including solvation effects. The calculations confirm that the bridge splitting reaction is slightly less favorable for the iodido derivative. Overall, the study confirms the active role of rhodium(I) species in ethylene hydroamination catalyzed by RhCl3·3H2O/PPh3/nBu4PI and suggest that the catalyst resting state is [RhCl(PPh3)2]2 or its C2H4 adduct, RhCl(PPh3)2(C2H4), under high ethylene pressure.
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