Hochfeld-NMR-Spektroskopie. Analyse der 1H-NMR-Spektren von η1.η2-Alkenyl-und η3-Cycloalkenyl-η5 -cyclopeiitadienylnickelkomplexen / High Field NMR Spectroscopy. : 1H NMR Spectra Analysis of η 1,η2-Alkenyl-and η3-Cycloalkenyl-η5 -cyclopentadienylnickel Complexes

Abstract

The 400 and 500 MHz 1H NMR spectra of a series of η1,η2-alkenyl- and η3-cycloalkenyl- η5-cyclopentadienylnickel complexes were analysed with the help of special NMR techniques like double resonance, automated spectra analysis, and nuclear Overhauser measurements. Ligands containing a 4-pentenyl unit adopt a certain conformation when complexed as bidentate to a transition metal (Ni, Ru). These η1,η2-alkenyl complexes form a rigid ring in which the β-H atoms point away from the metal. This conformation is found independent of whether the chain is substituted with alkyl or cyclopropyl groups or if the 4-pentenylrest is part of a cyclo-4-octenyl ring. The structure of cyclic η3-alkenyl complexes can also be characterised with typical NMR parameters. The three carbon atoms of the η3-allyl group as well as their adjacent carbon atoms are in plane; in the greater rings (C6−C8) the other carbon atoms point away from nickel. The nuclear Overhauser difference spectroscopy has been found to be an extremely sensitive tool for stereospecific assignments: in particular, very small intensity changes (below 1%) even of hidden protons can be detected. A procedure for the application of the automated spectra analysis is introduced. Based on these new experimental facilities a general strategy is proposed for the analysis of the 1H NMR spectra of complex molecules.