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Type of Document	Dissertation
Author	Schofer, Susan Jessica
URN	etd-06042004-173118
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-06042004-173118
Title	The effect of ligand array on stereocontrol and molecular weight in metallocene-catalyzed alpha-olefin polymerization and (PNP)CrPh3 complexes as well-defined ethylene trimerization catalysts
Degree	PhD
Option	Chemistry
Advisory Committee	Advisor Name Title David Tirrell Committee Chair John Bercaw Committee Member Jonas Peters Committee Member Robert Grubbs Committee Member
Keywords	tetramethylaluminate stereocontrol in metallocene-catalyzed olefin poly scandocene zirconocene ethylene trimerization chromium catalyst
Date of Defense	2004-05-28
Availability	mixed
Abstract A series of neutral and cationic group 4 and neutral group 3 model complexes of the Me2Si(eta^5-C5H4)(eta^5-3-(CMe3)-C5H3)(tBuSp) ligand have been prepared: tBuSpZrCl2(1), tBuSpZr(CH2SiMe3)2 (8), tBuSpZrMe2 (9), [tBuSpZrH2]2 (10), tBuSpZrMeCl (11), tBuSpZrMe(CH2CMe3) (12), tBuSpZrMe(CH2SiMe3) (13), tBuSpScCl(THF) (14), tBuSpScCH(SiMe3)2 (17), [tBuSpScH] (18), and tBuSpTiCl2 (19). The kinetically preferred isomers of 12 and 13 have been identified, and in both cases the syn isomer is preferred. We have obtained solid state structures of [tBuSpZrH2]2 (10), tBuSpZrMeCl (11), tBuSpZrMe(CH2CMe3) (12), and tBuSpScCl(THF) (14). Complexes 12 and 13 isomerize to form predominantly the thermodynamically preferred anti isomers. The ratio of isomers observed for [tBuSpZrMe]+[MeB(C6F5)3]- (21) and [tBuSpZrCH2SiMe3]+[Me B(C6F5)3]- (22) suggests that the tBuSp ligand does not distinguish very well between different groups in the metallocene wedge ([delta]G° = 0.27-0.89 kcal/mol). The tBu group is not an effective stereodirector in the ground state structures and the remarkable stereodirecting capability of this ligand in propylene polymerizations must be reconciled by its effect on transition state energies. A series of scandocene tetramethylaluminate have been prepared, including Ind2Sc(mu-Me)2AlMe2 (1), Cp2Sc(mu-Me)2AlMe2 (2), Cp*CpSc(mu-Me)2AlMe2 (3), and meso-DpSc(mu-Me)2AlMe2 (4). These complexes display characteristic terminal and bridging methyl resonances in solution at room temperature, indicating static structures. Complexes 1-4 oligomerize 1-pentene to form a range of oligomers. The tetramethylaluminate complexes initiate oligomerization from a scandium-methyl species after dissociation of AlMe3, and the primary mechanism of chain transfer is beta-hydrogen elimination. Complexes 1-4 react with L donors (L = DMAP, THF, PMe3) to form (RnCp)2ScMe(L) compounds (5-9). To study the chromium-based ethylene trimerization catalyst involving the diphosphine ligand (o-MeO-C6H4)2PN(Me)P(o-MeO-C6H4)2 (PNPOMe (1)), we have synthesized (PNPOMe-d12)CrPh3 (11), (PNPSMe-d12)CrPh3, (12), and (PNPOMe-d12)CrPh2Cl (14) containing deuterated PNP ligands. Activation of 11 by protonation with H+(OEt2)2B[C6H3(CF3)2]4- in the presence of ethylene provides an active trimerization catalyst that gives similar selectivity and activity to 1-hexene as the originally reported system and represents the first examples of an active, well-defined trimerization catalyst based on chromium. The trimerization of a 1:1 mixture of C2H4 and C2D4 gives only C6D12, C6D8H4, C6D4H8, and C6H12, the 1-hexene isotopomers without H/D scrambling, which is consistent with a trimerization mechanism involving metallacyclic intermediates.
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