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|Title:||Single selective inversion dynamic NMR applied to complex organometallic exchange|
|Authors:||Cramer, Anne Janice|
|Advisor:||Bain, Alex D.|
|Abstract:||<p>Organometallic species often participate in complex multi-exchange processes. The existing dynamic Nuclear Magnetic Resonance (NMR) spectroscopic methods are of limited use in complex systems. The classical line-shape method generally cannot provide mechanistic detail, and the actual data analysis may well be intractable if multi-exchange processes are occurring. Spin relaxation methods are more appealing for complex systems as they can also provide mechanistic information. The one-dimensional selective inversion relaxation experiment is inherently more precise than the corresponding two-dimensional method, but was thought to be impractial for multi-site systems as a separate experiment is required for each site. A one-dimensional selective inversion experiment in which all of the exchange processes are probed in a single experiment regardless of the number of sites was designed and tested for complex organometallic systems of the type M₂(CO)₆(μ-PPh₂)(μ₂-η¹η²-CCR) where M = Fe or Ru and R = -HC=CH₂ or −C≡CPrⁱ. Three potential exchange processes occur with these systems: trigonal carbonyl rotation on each metal atom and σ to π interconversion of the μ₂-η¹:η²-CCR ligand between the two metal atoms. The single selective inversion method successfully determined the temperature dependence of the chemical exchange rates for these processes in the chemical systems under study. In all cases, the line-shape data were in agreement with the single selective inversion values. The strengths and weaknesses of each of the dynamic NMR methods are complementary in complex exchanging systems and it appears that in many cases the optimal experiment is a combination of the available methods. A recommended combination strategy incorporating the single selective inversion methodology with both 2D-EXSY and line-shape analysis is outlined. Preliminary exchange studies on OS₃(CO)₉(μ-PPh₂)(μ₃-η¹:η²:η²-C≡C-t-Bu) with qualitative 2D-ESXY experiments permitted the detection of (1) a minor isomer in exchange with the major isomer at low temperature (233 K) and (2) both delocalized and localized carbonyl exchange processes at higher temperature (253 K). The chemical exchange process of the μ₂-η¹:η²-HC=CH₂ ligand in Fe₂(CO)₆(μ-PPh₂)(μ₂-η¹:η²-HC=CH₂) exhibited an interconversion barrier of 71 ± 6 kJ mol⁻¹. This was higher than expected, as most other vinyl and all acetylide complexes exhibit a barrier in the range of 42 to 55 kJ mol⁻¹. The use of two structural features to predict the barrier is proposed: (1) strength of the π interaction as determined by the difference in M-Cα and M-Cᵦ bond distances and (2) asymmetry in the electronic donation to each metal as determined through the metal to ligand distances. Application of these two criteria have so far allowed prediction of the barrier in all cases found in the literature to date.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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