NMR-based solution structures and mechanisms of metallocene olefin polymerization catalysis

The dihydrogen activation by cationic rhodium diphosphine complexes is the rate and enantiodetermining step in the catalytic asymmetric hydrogenation. The addition of H₂ to [Ir(bisphosphine)(COD)]+ complexes is examined herein as a model for stereocontrol and dynamic processes related to catalytic hydrogenation. For the model dihydride, [IrH₂(Me-DuPhos)(COD)]BF ₄ (2–10), quantitative analysis of the 2D NOE data using 2-Dimensional Conformer Population Analysis (2D CPA) establishes the absolute stereochemistry and the three dimensional structures of predominant conformers.; Reaction of rac-(S)-1-[(R)-1′,2-(bisdiphenylphosphino)ferrocenyl]ethyl-1-aza-2,3-benzo-15-crown-5, rac-3-1, with [Rh(NBD)₂]OTf yields [Rh(NBD)( rac-3-1)]OTf which is a catalyst precursor for asymmetric hydrogenation. The three dimensional solution structure of [Rh(NBD)( rac-3-1)]OTf has been determined by NOESY experiments and analysis using the two-dimensional conformer population analysis algorithm (2DCPA). The NOESY data reveal a rapid, pairwise chemical exchange between vinyl protons. However, the lability of the aza crown ether may limit the ability of these catalysts to control selectivity via secondary interactions.; Dynamic NMR studies of the contact ion-pairs [(Me₄Cp) ₂ZrMe][MeB(C₆F₅)₃] (4-1a) and [rac-C₂H₄(1-indenyl)₂ZrMe][MeB(C ₆F₅)₃] (4-2a) reveal a diversity of complex exchange processes. These exchange processes include rapid permutation of the Zr-Me and B-Me groups of 4-1a and 4-2a, exchange between diastereotopic groups of the cyclopentadienyl and indenyl ligands, and inter-ion exchange in mixtures of contact ion-pairs such as 4-1a and 4-2a. Exchange in the ansa-metallocene 4-2a is dominated by different processes (borane association and dissociation) than those (primarily ion-separation) at non-ansa metallocene 4-1a. Detailed examination of the kinetics of exchange as a function of metallocene concentration, in the presence of additives with different polarity, and over a wide temperature range provides experimental support for the dissociation of the contact ion-pair 4-1a into solvent-separated ion-pairs. In contrast, there is no direct evidence for such a process at 4-2a . Comparison of observed rates for insertion of 1-hexene into the Zr-Me bond of 4-2a with upper limits placed on the rate of separation of 4-2a into solvent-separated ion-pairs suggests that ion-pair separation does not precede alkene insertion.