Theoretical design and development of catalyst systems

Wave function based ab initio and nonlocal density functional methods have been employed to impart a unique insight into the competing mechanisms operating in the carbonylation of alkyl-palladium(H) complexes of bidentate ligands. The theoretical methods employed were established by two benchmark studies. Geometries of physically meaningful accuracy were obtained using second-order Moller-Plesset (MP2) methods or nonlocal density functional theory (DFT) with a small-core pseudopotential on palladium and double-c basis sets with polarisation functions on the ligands. Reliable reaction energies required a higher level of correlation (such as CCSD(T)) together with large basis sets incorporating diffuse functions and polarisation functions. For every system investigated, the lowest energy pathway proceeded via novel five-coordinate intermediates and transition structures. This is in contrast to the generally assumed four-coordinate pathways and has important ramifications in the context of rationalising experimentally observed CO/olefin copolymensation activities. The competing carbonylation mechanisms for the model neutral and cationic palladium(fl) systems Pd(N-0)(CH3)(PH3) + CO - Pd(N-0)(COCH3)(PH3) (N-O = NHCHCOO (1), NHCHCHO (21)) have been investigated in detail. Despite marked differences in the lowest energy mechanisms, variation in the overall energetics for the neutral and cationic systems was found to be less than 15 kJ/mol. Furthermore, it has been unequivocally demonstrated that differences in CO/ethylene copolymerisation activity of neutral and cationic palladium(II) complexes of bidentate N-O ligand can be attributed to the activation energy of ethylene insertion (a variation of 55.4 kJ/mol). This is the first theoretical investigation to reproduce the distinct CO/ethylene copolymerisation rates of cationic and neutral complexes, thereby allowing clear elucidation of this important phenomenon. Ligand influences have been assessed by modelling the carbonylation reaction for a range of complexes: Pd(X-Y)(CH3)(L) (X-Y = NHCHCHO, L = PF3 (41), P(CH3)3 (51); X-Y = NHCHCHNH, L = PH3 (61), CH3 (71)). With the exception of the neutral diimine complex (71) variations in the activation energy for the methyl migration step are surprisingly small. Values ranged from 39.5 kJ/mol for 41 to 55.6 kJ/mol for 1. Conversely, it was shown that differences in carbonylation reactivity typically arise in the ligand substitution and isomerisation steps. The high migration barrier associated with 71 (67.5 kJ/mol) is due to a reduction in cy-donation by the migrating methyl group and an increase in metal-carbonyl it back-donation. The study provides the most definitive theoretical evidence to date for the participation of five-coordinate species in the migratory insertion reactions of alkyl-palladium(H) complexes. In particular, a novel transition structure has been identified which accounts for the isomerisation of square-pyramidal d 8complexes. The integral role proposed for five-coordinate species in the carbonyl and olefin migratory insertion reactions has led to the formulation of novel rationale to account for several ambiguous experimental trends. In summary, all of the theoretical results presented exhibit outstanding agreement with all available experimental structural and kinetic data. This study clearly demonstrates that computational chemistry is now mature and can make a significant contribution at the leading edge of catalyst design and development.