Substituent and solvent effects in the insertion and isomerization of olefins by platinum (bis-phosphine) complexes:: An ab initio study of the Pt(PR3)2H(propene)+ model systems

Traditional ab initio (HF, MP2) and density functional theory (DFT) calculations are applied on the cationic Pt(PR3)(2)(H)(propene)(+) complexes (R = H, F, CH3) in order to study the insertion of propene in the Pt-H bond. In general, insertion and beta-elimination barriers tend to be small. Insertion barriers of about 10 kJ/mol are found for R = H and 3-9 kJ/mol for R = CH3, and an almost negligible insertion barrier appears for R = F (2-4 kJ/mol). Since propene insertion can generate both linear propyl and branched isopropyl complexes, it is possible to study the distribution between these two complexes, which is of importance for catalyst selectivity. It turns out that linear complexes are favored over branched ones, in agreement with available experimental data. The energy gap between the two forms decreases in the order R = Me > R = H > R = F. Another point of interest is the double-bond isomerization of propene which can arise from isopropyl complexes. Two pathways are identified: a direct exchange of beta-hydrogens in the beta-agostic isopropyl complex ("isopropyl rock") and a process in which isomerization occurs by association and dissociation of a coordinating solvent molecule (acetonitrile was used in the present study). Which one of these two processes dominates seems to depend on the nature of the solvent and the substituents on the phosphines. Even though very few experimental data are available, a satisfying agreement is found between optimized geometries and X-ray data of a related compound as well as between computed and experimental product distributions. The calculated "isopropyl rock" barriers are also in accord with recent NMR measurements from which the barrier could be determined. Finally, a crude estimate of the isomerization rate seems to agree with the theoretical predictions.