A computational study of rhodium pincer complexes with classical and nonclassical hydride centres as catalysts for the hydroamination of ethylene with ammonia.
The catalytic hydroamination of ethylene with ammonia was investigated by means of density functional theory (DFT) calculations. An initial computational screening of key reaction steps (C-N bond formation, N-H bond cleavage), which are assumed to be part of a catalytic cycle, was carried out for complexes with the [M(L)]-complex fragment (M=Rh, Ir; L=NCN, PCP; NCN=2,5-bis(dimethylaminomethyl)benzene, PCP=2,5-bis- (dimethylphosphanylmethyl)benzene). Based on the evaluation of activation barriers, this screening showed the rhodium compound with the NCN ligand to be the most promising catalyst system. A detailed investigation was carried out starting with the hypothetical catalyst precursor [Rh(NCN)(H)(2)(H(2))] (1). A variety of activation pathways to yield the catalytically active species [Rh(NCN)(H)(NH(2))] (5), as well as [Rh(NCN)(C(2)H(5))(NH(2))] (17), were identified. With 5 and 17 several closed catalytic cycles could be calculated. One of the calculated cycles is favoured kinetically and bond-forming events have activation barriers low enough to be put into practice. The calculations also show that for experimental realisation the synthesis of 1 is not necessary, as the synthesis of 17 would establish an active catalyst directly without the need for activation. Oligomerisation of ethylene would be possible in principle and would be expected as a competitive side reaction. Accordingly not only ethylamine would be observed in an experimental system, as amines with longer carbon chains also can be formed.
Uhe
Andreas
A
Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany.
Hölscher
Markus
M
Leitner
Walter
W