Coordination chemistry of iridium and platinum complexes as model homogeneous catalysts

[摘要] English: Hydroformylation for the production of aldehydes from alkenes, is a large and importanthomogeneously catalyzed industrial process. Most of these resulting aldehydes are hydrogenatedto alcohols, having applications in plasticizer alcohols, detergents, wood preservatives andsurfactants. Numerous phosphine ligands have been applied in these catalytic reactionssignifying that changes in the ligand environment induce different steric and electronic propertiesinto the catalyst system allowing to tune catalyst behaviour towards higher activity andselectivity.A series of diphosphinoamine (PNP) ligands with various substituents on the N-atom, inducingdifferent steric properties were synthesized and characterized (Scheme I). Single crystal X-raycrystallographic studies of the PNP ligands revealed that the P-N-P bond angle decreases as thesteric 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 toquantify different effects which could then be rationalized for the Rh(I) and Ir(I) pre-catalystssystems for use in olefin hydroformylation. The reason for using Pt(ID and Pd(ID was thereforeprimarily to gain information on the coordination mode of these ligands, rather than thenotoriously difficult to isolate and unstable Rh and Ir complexes. A total of three freePNP-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 theoptimized structure and the crystal data revealed small differences, illustrating that predictionscan be made in terms of ligand design in particular when solid state data is hard to obtain. Thecalculated structures indicated that the phenyl ring arrangement is affected by the steric bulk ofthe 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 couplingconstants as determined from the corresponding Pt-PNP complexes showing no significantdifference between electronic properties of the ligands.The hydroformylation of 1-octene was investigated utilizing Rh(I) and Ir(I) metal centres. Thelinearity of the aldehyde product increased with an increase in steric bulk of the ligand at theexpense of side product formation during the rhodium catalyzed hydroformylation catalysis of1-octene. A striking feature was that a 27 % improvement in the linear selectivity could beachieved by increasing the ON-sub angle of the N-substituent from 64 to 84 °. The parallelcompeting isomerisation of 1-octene varied from 63.7 % for a cone angle of 64 °, with a decreaseto 27.3 % observed for PNP-Dimprop, with cone angle of 86 °. The N-alkyl moiety of the PNPligandcan therefore be structurally fine-tuned towards efficient hydroformylation catalysts.Combining the PNP-ligands with PPh3 gives rise to a more superior system with higher reactivityand lower alkene loss through isomerisation.Iridium catalyzed hydroformylation did not yield the same trend of linear selectivity increasewith an increase in Bi;-sub of the ligand, but did show similar behaviour to the Rh analogue whenPPh3 was combined with the PNP-ligands.

[发布机构] University of the Free State