Realistic Modelling of Organometallic Reactivity in Solution: Computational Studies on the Mechanism of Methanolysis of Palladium-Acyl Bonds

Lead Research Organisation: University of St Andrews
Department Name: Chemistry


Methoxycarbonylation is a process that converts cheap, widely available feedstocks (alkenes, carbon monoxide and methanol) into commercially-important intermediates for the chemicals industry. This process uses Pd-based catalysts and the best example is the reaction of the simplest alkene, ethene, to give the intermediate methyl methacrylate, which is used in the synthesis of plastics. More recently, alkenes such as vinyl acetate have been shown to undergo methoxycarbonylation to generate intermediates that are themselves useful as green solvents (low-volatility/biodegradable) or as monomers for the formation of biodegradable polymers. These have the potential to replace traditional materials such as polystyrene or polythene. Methoxycarbonylation of butadiene promises a new route to adipic acid, one of the co-monomers involved in the manufacture of nylon. As yet the methoxycarbonylation of vinyl acetate and butadiene have not been optimised and greater insight into these reactions is required before effective industrial processes are in place.A key issue that remains to be solved in the methoxycarbonylation reaction is the detailed mechanism by which the products are released - the so-called methanolysis step. There a several possibilities for this process, however, it is extremely difficult to obtain information on this from experiment as the reaction itself is incredibly fast. In these circumstances the use of computational modelling comes into its own, as this can readily provide information on the energies of the species involved in reactivity. The methanolysis reaction is, however, very complicated and is strongly dependent on the precise nature of the reacting species and the nature of the solvent being used. To obtain reliable modelling data these factors must be taken into account, a fact that makes the task of modelling these systems very challenging.This proposal seeks to use high level computational modelling to assess the mechanism of the methanolysis on the simplest methoxycarbonylation system - ethene/CO/MeOH - and the most effective Pd catalysts. Our approach will be to employ hybrid calculations where the catalyst and reacting molecules are dealt with at a high level of theory (density functional theory) but the solvent molecules (many 10s or hundreds) are treated at a lower level of theory based on classical force fields. Through this approach the effect of the solvent on the reactivity at the Pd catalyst will be taken into account and we aim to provide extremely reliable data to define the preferred mechanism. We will test our approach by comparing with an alternative catalyst which displays a different reactivity, thus giving a stringent test of our modelling approach.Once we have defined the correct way to treat these complicated reactions - as well as the mechanism by which methanolysis occurs - we will be in a position to tackle the new reactions of vinyl acetate and butadiene. We hope to provide sufficient insight into these processes that experimental chemists will be able to design new improved catalysts for more efficient methoxycarbonylation of these feedstocks on an industrial scale.
Description Many industrial chemical processes are homogeneous, in that the catalyst (often a transition metal complex), the feedstocks and products are all dissolved in solution. Understanding how such processes work, i.e. the mechanism of reaction, is important for the future design of new, more efficient and selective catalysts. One effective way to provide such insight is to use computational modelling. However, standard modelling approaches usually ignore the role played by solvent, or only treat this in a very approximate fashion. This project set out to design a computational approach that would properly include solvent molecules in the modelling of catalytic processes.In order to incorporate solvent effects on reactivity, large numbers (hundreds) of solvent molecules need to be included in the calculation. We therefore employed hybrid methods, where the catalyst and reacting molecules are dealt with at a high level of theory (density functional theory, DFT) but the solvent molecules are treated at a lower level of theory based on molecular mechanics (MM). Another challenging issue is the conformational flexibility of these large models, i.e. the number of different arrangements in space that the different molecules can adopt. To tackle this we employed molecular dynamics (MD) simulations in order to ensure a proper coverage of the conformational space. The combination of DFT/MM calculations and MD calculations meant that our initial targeted system, the methoxycarbonylation of simple alkenes at Pd catalysts, represented too great a technical challenge for the exploration of our approach. Instead we identified a number of well-defined chemical reactions occurring at transition metal centres for which experimental data on the energetic barriers to reaction were available. These were then used to benchmark our approach. In particular these reactions involved the cleavage of ionic bonds and hence the involvement of charged species. Modelling these processes in the gas-phase is completely unrealistic and the inclusion of solvent is essential. Given the many important processes that involve the participation of charged species, our work has the potential to have a significant impact on the modelling of real chemical processes in solution. To date the results from our studies are very promising. With a simple (but difficult to model) chemical process such as H2O/Cl- substitution at [Pt(C2H4)Cl3]- we correctly reproduce the selectivity for the trans position and also obtain good absolute values for the activation energies (trans: 19 kcal/mol; cis: 3 kcal/mol). This is extended to the successive H2O/Cl- substitution at cis-[Pt(Cl)2(NH3)2] (with activation barriers of 19 kcal/mol and 23 kcal/mol respectively). We have also considered the Monsanto Reaction, an important industrial process for the production of acetic acid. Here we correctly reproduce the expected "SN2" mechanism, unlike recent computational reports where this has been discounted. A further benefit of our approach is the ability to describe the interaction between solvent and solute molecules correctly. Such interactions have a strong contribution from dispersion effects that are poorly modelled with standard DFT calculations. Thus the phosphine dissociation energy from trans-[Pd(Cl)(Ph)(PPh3)2] is correctly modelled when both the reactant and the dissociated fragments are incorporated into a box containing several hundred benzene molecules. The PDRA employed (at Heriot Watt) on this project (at Her brought the necessary expertise to the project, but required training in QM methods for gas-phase calculations and resulted two papers. We acted as advisors on experimental aspects of the project as originally envisaged.