A DFT study of the catalytic hydrocyanation of ethylene with nickel complexes

[摘要] ENGLISH ABSTRACT: DFT calculations employing the B3LYP functional were done to investigate the mechanism for the Ni-catalyzed hydrocyanation of ethylene as proposed by Tolman. Although this reaction is an important industrial process, its mechanism has never been studied computationally, apart from calculations pertaining to ligand tailoring.This study comprises a detailed configurational analysis of each step of the reaction cycle, charge decomposition analysis of pertinent species and analysis of the activation barriers involved at each step. A model ligand, PH3, is employed, due to its electronic similarity to the experimental ligand most widely used, P(O-o-tolyl)3, and its small size, which makes it amenable for calculations at this level.It was found that oxidative addition of HCN to the precursor complex (ethylene)NiL2 (L=PH3) can take place in one step and that it is the rate-determining step in the gas phase. The resulting adduct has H+ (which becomes a hydride) and CN- coordinated in the cis configuration. Ligand dissociation yields three configurations of (ethylene)-NiHCNL, of which only two can participate in the catalytic cycle. It is shown that this is because migration-insertion of ethylene into the Ni-H bond takes place before, or concomitant with, association of a second ethylene molecule, contrary to expectation. This path therefore requires that ethylene and hydrogen are coordinated in the cis configuration, something only possible for two of the three isomers of (ethylene)NiHCNL. The calculations support the mechanism of associative reductive elimination and shows that elimination can only take place if the ethyl and cyanide groups are in the cis configuration.Analysis of the energetic profile of the reaction shows that entropy effects play a very important role in the propagation of the cycle, at least in the gas phase.Preliminary work on the effect of Lewis acids the catalytic cycle is presented, with structural and energetic analysis.An important general conclusion is that the standard way of representing the energy profile of reactions where intermolecular transitions (as opposed to intramolecular transitions only) take place can be misleading. It will be argued that the implicit assumption that two species which are minimum energy structures on distinct potential energy surfaces will also be an energy minimum on one potential energy surface skews the energy profile of the reaction. The consequence of this is that care must be taken in representing energy profiles for reactions where more than one distinct species participates.

[发布机构] Stellenbosch University