Dillen, Jan L. M.
Raubenheimer, Helgard G.
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.
AFRIKAANSE OPSOMMING: Die meganisme, soos deur Tolman voorgestel, van die Ni-gekataliseerde hidrosianering van etileen word ondersoek met behulp van Kohn-Sham elektrondigtheidsteorie (density finctional theory, DFT) berekeninge waarin die B3LYP-funksionaal gebruik word. Alhoewel die reaksie ‘n belangrike proses is in die industrie, is die volle me-ganisme nog nooit met behulp van berekeninge bestudeer nie. Daar is egter wel al werk gedoen aangaande sekere aspekte van die reaksie, byvoorbeeld ligandontwerp.
Hierdie studie behels ‘n noukeurige konfigurasionele analise van elke stap van die reaksie-siklus, ladingsverdelingsanalise (charge decomposition analysis, CDA) van sekere belangrike spesies asook die analise van die energiestappe betrokke by elke stap. Fosfien is gekies as ‘n modelligand, omdat dit elektronies ooreenstem met P(O-o-toliel)3, die ligand wat meestal in eksperimentele werk gebruik is. Die klein grootte van fosfien maak dit ook geskik vir be-rekeninge op hierdie vlak.
Daar is bevind dat die oksidatiewe addisie van HCN aan die voorgangerkompleks (etileen)-NiL2 (L=PH3) in een stap kan plaasvind en in die gasfase snelheidsbepalend is. Die adduk (ettileen)NiHCNL2 bevat H+ (wat ‘n hidried word) en CN- in die cis-posisie relatief tot mekaar. Liganddissosiasie lewer drie isomere van (etileen)NiHCNL. Daar is bevind dat slegs twee van dié isomere aan die katalitiese reaksie kan deelneem, omdat die migrasie-inplasing (migration-insertion) van etileen in die Ni-H-binding voor, of saam met, die assosiasie van ‘n tweede etileen-molekule plaasvind. Dit is slegs moontlik indien waterstof en etileen cis teenoor mekaar is, wat geld vir twee van die isomere. Die meganisme van assosiatiewe reduktiewe eliminasie word deur die berekeninge gerugsteun. Voorts blyk dit vanuit die berekeninge dat die etiel- en sianiedgroepe cis teenoor mekaar moet wees voordat reduktiewe eliminasie van propionitriel kan plaasvind.
Analise van die energetiese profiel van die reaksie toon dat entropie-effekte ‘n belangrike rol speel in die voortsetting van die reaksie in die gasfase.
Die invloed van Lewissure op die katalitiese siklus word, met behulp van strukturele en energetiese analise bespreek.
‘n Belangrike algemene gevolgtrekking is dat die standaardvoorstelling van die energetiese profiel van reaksies waarin intermolekulêre oorgange (teenoor slegs intramolekulêre oorgange) voorkom, misleidend kan wees. Dit word gestel dat die implisiete aanname dat twee spesies wat minumum-energiestrukture verteenwoordig op twee verskillende potensiële energie-oppervlaktes ook ‘n minimum-energiestruktuur voorstel op een potensiële energie-oppervlakte, die energieprofiel skeeftrek. Gevolgtrekkings vanuit hierdie energieprofiele van reaksies waar meer as een onderskeibare spesie deelneem, moet dus met omsigtigheid gemaak word.
iii, 166 leaves : ill.
Theses -- Chemistry
Dissertations -- Chemistry
Doctoral Degrees (Chemistry and Polymer Science)