Probing Organometallic Reactions With 19F NMR
Hawrelak, Eric James
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This dissertation explores fundamental aspects of the reaction of group 4 metallocenes with methylaluminoxane (MAO) that lead to active Ziegler-Natta olefin polymerization catalysts. A novel experimental approach is described, in which a unique spectroscopic probe (a fluorinated substituent) is attached to the metallocene ancillary ligands and the metallocene/MAO mixtures are analyzed using 19F NMR spectroscopy. Group 4 metallocene dimethides bearing pentafluorophenyl (C6F5) substituents were synthesized and treated with MAO in benzene-d6. 19F NMR spectroscopic analysis demonstrated reversible methide transfer to form "cation-like" methylmetallocenium methylaluminates. A series of quantitative titration studies showed that fewer than 10% of the aluminum centers in MAO actually participate in the methide transfer process. A systematic study of metallocene substituent effects suggested that MAO contains active centers of extremely high but varying Lewis acidity. Activation of group 4 metallocene dichlorides using MAO was also analyzed using 19F NMR. Initial Cl/CH3 exchange was followed by Cl transfer to aluminum, whereas "normal" subsequent transfer of CH3 from Al to the methylmetallocenium cation was apparently inhibited by the abstracted chloride. Additional studies showed that the 19F NMR probe is sensitive to the interactions of Zr-Cl bonds with simple alkylaluminum species such as Me3Al, Me2AlCl, MeAlCl2, and Et3Al. However, the method was arguably less useful than 1H NMR spectroscopy in following the metathesis of Zr-Cl and Al-R (R = Me, Et) bonds. New methods of preparing methylhalometallocenes were investigated. The reactions of eleven metallocene dimethyls with triphenylmethyl chloride were highly selective (> 95%) with the five most electron-deficient metallocenes studied. Two other examples showed good selectivity on an NMR scale but could not be isolated from the 1,1,1-triphenylethane byproduct. Reactions of dimethylmetallocenes with benzyl bromide were also selective for formation of the corresponding methylbromo-metallocenes, however the reactions were too slow to be of practical value. The observation of long initation periods and the analysis of organic byproduct distributions suggested that these halogenation reactions may proceed by a radical chain mechanism rather than simple sigma bond metathesis. To demonstrate "proof of concept" in the use of 19F NMR to analyze the reactions of paramagnetic metallocenes, the coordination of CO and CN- to C6F5-substituted chromocenes were analyzed. Whereas CO coordinates readily to chromocene, cyanide coordinates effectively to 1,1'-bis(pentafluorophenyl)chromocene. This observation is interpreted in terms of the electron-withdrawing effect of the C6F5 substituent, which should strengthen bonding to sigma-donor ligands (CN-) and weaken bonding to pi-acceptors (CO).
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