Multinuclear Solid-State NMR Studies of Supported Transition Metal Carbonyls
Walter, Thomas Henry
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https://hdl.handle.net/2142/70404
Description
Title
Multinuclear Solid-State NMR Studies of Supported Transition Metal Carbonyls
Author(s)
Walter, Thomas Henry
Issue Date
1988
Doctoral Committee Chair(s)
Oldfield, Eric
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Abstract
High field carbon-13 and oxygen-17 nuclear magnetic resonance (NMR) spectra have been obtained for a variety of transition metal carbonyl clusters, metal oxides, and oxide-supported metal carbonyl complexes. Magic-angle spinning (MAS) C-13 NMR spectra of polycrystalline metal carbonyl clusters yield both isotropic chemical shifts and chemical shift anisotropies (CSA's) for the carbonyl ligands, providing information on solid-state symmetry, J-coupling, and carbonyl exchange processes.
Carbon-13 MAS NMR results are also described for two supported complexes: $\rm Mo(CO)\sb6$ on ${\rm Al\sb2O\sb3}$ and ${\rm Os\sb3(CO)\sb }$ on ${\rm SiO\sb2}$. It is shown that CSA information can be used to study the mobilities of chemisorbed metal carbonyl fragments. Evidence is presented for very facile rotation of a ${\rm Mo(CO)\sb5}$(ads) species about the surface-Mo bond. In contrast, a ${\rm MO(CO)\sb3}$ (ads) species is shown to be rigid on the NMR time scale. Three surface species have been observed for the ${\rm Os\sb3(CO)\sb /SiO\sb2}$ system: physisorbed ${\rm Os\sb3(CO)\sb }$, a chemisorbed triosmium cluster, and mononuclear osmium (II) carbonyl fragments. Based on solution C-13 NMR results for a number of model compounds, the spectrum of the chemisorbed cluster is shown to provide strong evidence for a HOs$\sb3$(CO)$\sb $(OSi$\equiv)$ structure. Analysis of the CSA's of the carbonyl resonances for this cluster indicates that it is undergoing restricted rotation, the results being most consistent with a rotational jump model.
Oxygen-17 NMR spectra of a variety of O-17 labelled metal oxides and hydroxides have been obtained using cross polarization and dipolar decoupling. It is shown that differences in cross-relaxation rates may be used to edit spectra by selectively enhancing protonated oxygen resonances. When applied to high surface area metal oxides such as silica and alumina, this selectivity enables the observation of resonances from surface hydroxyl groups that are otherwise difficult to detect. The spectra yield nuclear quadrupole coupling constants, electric field gradient asymmetry parameters, and isotropic chemical shifts for each chemically distinct oxygen site. It is shown that, for a variety of aluminum oxides and hydroxides, the observed quadrupole coupling parameters are in agreement with the values calculated using a point charge model for the elements of the electric field gradient tensor at each oxygen site.
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