Coordination chemistry of iridium and platinum complexes as model homogeneous catalysts

English: Hydroformylation for the production of aldehydes from alkenes, is a large and important homogeneously catalyzed industrial process. Most of these resulting aldehydes are hydrogenated to alcohols, having applications in plasticizer alcohols, detergents, wood preservatives and surfactants. Numerous phosphine ligands have been applied in these catalytic reactions signifying that changes in the ligand environment induce different steric and electronic properties into the catalyst system allowing to "tune" catalyst behaviour towards higher activity and selectivity. A series of diphosphinoamine (PNP) ligands with various substituents on the N-atom, inducing different steric properties were synthesized and characterized (Scheme I). Single crystal X-ray crystallographic studies of the PNP ligands revealed that the P-N-P bond angle decreases as the steric bulk of the alkyl moiety increases. See scheme in PDF full text. The synthesis and coordination of the PNP-ligands to Pt(ID and Pd(ID served as models to quantify different effects which could then be rationalized for the Rh(I) and Ir(I) pre-catalysts systems for use in olefin hydroformylation. The reason for using Pt(ID and Pd(ID was therefore primarily to gain information on the coordination mode of these ligands, rather than the notoriously difficult to isolate and unstable Rh and Ir complexes. A total of three free PNP-ligands, four [Pt(PNP-alkyl)2] and one [Pd(PNP-alkyl)2] solid state crystal structures were solved, which provided excellent structural fundamentals from which the catalysis could be pursued. The study was also supplemented with theoretical chemistry. The comparison between the optimized structure and the crystal data revealed small differences, illustrating that predictions can be made in terms of ligand design in particular when solid state data is hard to obtain. The calculated structures indicated that the phenyl ring arrangement is affected by the steric bulk of the nitrogen-coordinated alkyl moiety which could ultimately affect the catalytic selectivity. The steric demand of the ligands was defined by the Effective Tolman-based N-substituent steric effect (ON-sub). The electron donating ability was evaluated through the first order Pt-P coupling constants as determined from the corresponding Pt-PNP complexes showing no significant difference between electronic properties of the ligands. The hydroformylation of 1-octene was investigated utilizing Rh(I) and Ir(I) metal centres. The linearity of the aldehyde product increased with an increase in steric bulk of the ligand at the expense of side product formation during the rhodium catalyzed hydroformylation catalysis of 1-octene. A striking feature was that a 27 % improvement in the linear selectivity could be achieved by increasing the ON-sub angle of the N-substituent from 64 to 84 °. The parallel competing isomerisation of 1-octene varied from 63.7 % for a cone angle of 64 °, with a decrease to 27.3 % observed for PNP-Dimprop, with cone angle of 86 °. The N-alkyl moiety of the PNPligand can therefore be structurally fine-tuned towards efficient hydroformylation catalysts. Combining the PNP-ligands with PPh3 gives rise to a more superior system with higher reactivity and lower alkene loss through isomerisation. Iridium catalyzed hydroformylation did not yield the same trend of linear selectivity increase with an increase in Bi;-sub of the ligand, but did show similar behaviour to the Rh analogue when PPh3 was combined with the PNP-ligands.