1H NMR Investigation of Paramagnetic Chromium(III) Olefin Polymerization Catalysts: Experimental Results, Shift Assignment and Prediction by Quantum Chemical Calculations
详细信息 查看全文
- 作者：Pablo Ferná ; ndez ; Hans Pritzkow ; Jorge J. Carbó ; Peter Hofmann ; Markus Enders
- 刊名：Organometallics
- 出版年：2007
- 出版时间：August 27, 2007
- 年：2007
- 卷：26
- 期：18
- 页码：4402 - 4412
- 全文大小：313K
- 年卷期：v.26,no.18(August 27, 2007)
- ISSN：1520-6041
文摘
The new paramagnetic quinolyl-functionalized Cp chromium(III) complexes 4-8, which serve asprecursors for highly active olefin polymerization catalysts, have been synthesized and were investigatedtogether with the known complexes 1-3 by 1H NMR. Full assignment of the observed NMR signals inthese systems was achieved by comparison of the different spectra and by spin unrestricted densityfunctional calculations (UB3LYP level) of the Fermi contact term at the hydrogen atoms. All the geometrieswere optimized using the experimentally determined solid-state molecular structures as starting points.We obtained a very good correlation (r2 = 0.97) between calculated Fermi contact spin densities at theUB3LYP/6-311G(d) level and experimental paramagnetic 1H NMR shifts. Polarization basis set functionsat heavy elements are required to reproduce experimental results, whereas polarization functions athydrogen atoms and/or diffuse functions do not improve the results. The good correlation betweencalculated and experimental results indicates that the experimental hyperfine shifts are dominated byFermi contact interactions. Nevertheless, we were able to identify and estimate non-negligible dipolarcontributions to the chemical shifts for some protons. It is possible to predict 1H NMR shifts of similarorganometallic compounds and to obtain structural information of in situ generated paramagnetic species,which play a key role in several catalytic transformations with paramagnetic catalysts. Additionally,calculations provided us with detailed information of the spin density distribution along the molecularsystems.