65016-01-5Relevant academic research and scientific papers
(η2-Olefin)(1,4-diaza-1,3-diene)dicarbonyliron complexes. 1. Preparation and dynamic NMR spectroscopic properties
Frühauf, Hans-Werner,Pein, Ingrid,Seils, Frank
, p. 1613 - 1620 (2008/10/08)
The reaction of (η4-diethyl muconate)(dad)Fe(CO) complexes (dad = 1,4-diaza-1,3-diene) with CO results in stepwise decoordination of the muconate ligand. The rates of the two consecutive displacement steps strongly depend on the properties of the respective dad ligands. Two examples of the intermediate (η2-muconate)(dad)Fe(CO)2 complexes were isolated. The complexity of their temperature-dependent NMR spectra precluded an analysis of the underlying dynamic processes. Photochemically, a series of analogous η2-monoolefin complexes has been prepared. At low temperatures, in the rigid limiting case, their spectra are consistent with a trigonal-bipyramidal arrangement where the olefin is in equatorial position, while the dad nitrogen atoms and the two carbonyl groups occupy axial/equatorial positions, resulting in overall C1 symmetry. A dynamic exchange process, observed in all cases, consists in rotation of the olefin about the bond to iron. With cis-disubstituted olefins, having local Cs symmetry when coordinated, simultaneous mutual exchange of the CO ligands, the dad halves, and the olefin halves is observed. This agrees with a Berry pseudorotation process, where olefin rotation occurs in the apical position of the intermediate square-pyramidal arrangement of overall Cs symmetry. The activation barriers depend (i) on the extent of π-back-bonding to the olefin, which, for a given olefin, is reduced when the more π-acidic dad ligands compete for the electron density at iron, and (ii) on the steric bulk of dad and olefin ligands. In cases where the olefin and the (dad)Fe(CO)2 fragments have no symmetry element in common, a second exchange process at higher temperatures is observed which again equilibrates the two inequivalent halves of the dad ligand. This exchange cannot be effected by the former mechanism. Of the possible explanations being discussed for this second dynamic exchange process, a reversible dissociation of the olefin is strongly favored.
