Title

On the understanding of metallocene structure and dynamics via solid-state NMR.

Date of Award

2005

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

Chemistry, Physical.

Rights

CC BY-NC-ND 4.0

Abstract

This thesis focuses on the use of solid-state NMR spectroscopy to study the metal nuclei in various metallocenes and metallocenium cations, including, Cp*2Al+, Cp*2B+, Cp* 2BMe, Cp2Be, Cp*2Be, (C5Me4H) 2Be, CP2Mg, Cp2ZrCl2, Cp*2ZrCl 2, Cp2ZrBr2, (C5Me3H 2)2ZrBr2, (Me3Si-C5H 4)2ZrBr2, O(Me2SiC5H 4)2ZrBr2, (O(Me2Si) 2C5H3)2ZrBr2, (Me2 ClSi-C5H4)ZrCl3, and Cp2ZrMe 2. Since few studies of this nature have been reported, much of the initial material involves examining the correlation between anisotropic quadrupolar and chemical shielding interaction tensors with structural and/or dynamic features of the metallocenes. Detailed quantum mechanical calculations are also employed in order to strengthen our understanding of these correlations and the origins of anisotropic NMR interactions. Notable properties which are specific to each metallocene are investigated in detail, examples include, the very small electric field gradient (EFG) and large chemical shielding anisotropy in Cp*2Al+, sigmatropic rearrangement of the eta1-Cp* ring in Cp*2B+, and the dynamic 'slip' structure of Cp2Be. In particular, strong correlations are observed between the spherical symmetry around the metal nuclei and the magnitude of the quadrupolar interaction, as well as between Cp' ring hapticity and the average nuclear magnetic shielding. Part of the reason why many metallocenes have not been studied by solid-state metal NMR is the inherently low sensitivity of the nuclei of interest, which arises from low natural abundance, low magnetogyric ratios and/or large anisotropic interactions. An attempt is made to overcome these obstacles by combining preparatory pulse sequences (double frequency sweeps (DFS), rotor-assisted population transfer (RAPT) and cross polarization (CP)) with a multiple-pulse method called the "Carr-Purcell Meiboom-Gill" (QCPMG) pulse sequence. These combined pulse sequences are tested on a variety of standard samples and result in signal enhancements of an order of magnitude or more depending on nuclear relaxation properties. These signal enhancement techniques make possible the solid-state NMR characterization of CP2Mg and Cp2ZrCl 2, which are important compounds in the fields of metalorganic chemical vapor deposition and olefin polymerization catalysis, respectively. The solid-state 91Zr NMR study of a series of zirconocenes is also presented, wherein an attempt is made at understanding the correlation between structure and measured EFG parameters. In particular, brominated zirconocenes which mimic the conformation of species before (Cp2ZrBr2, (C5Me3H2)2ZrBr2, (Me 3Si-C5H4)2ZrBr2) and after adsorption (O(Me2SiC5H4)2ZrBr 2, (O(Me2Si)2C5H3) 2ZrBr2) to surface materials show promise in the characterization of industrially relevant systems. The 91Zr quadrupole coupling constant is shown to be very sensitive to geometrical changes and serves as an invaluable probe for the characterization of zirconocenes.Dept. of Chemistry and Biochemistry. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2005 .H86. Source: Dissertation Abstracts International, Volume: 66-11, Section: B, page: 5995. Thesis (Ph.D.)--University of Windsor (Canada), 2005.