Structural Study of the Conformational Isomers of 4-Benzylideneacetone- and 4-DieneFeCO2L Complexes by Inverse TripleResonance Iron-57 NMR

Abstract

For the first time, 2D(31P, 57Fe){1H}-triple-resonance NMR spectroscopy was used at low temperatures (213 and 179 K) to study the conformers of (η4-benzylideneacetone)- and (η4-diene)Fe0(CO)2L complexes (L = phosphanes, phosphites). In some cases not only the 57Fe chemical shifts and the 1J(57Fe, 31P) coupling constants of the major isomer but also those of a less populated isomer were determined. Based on earlier 31P and 13C NMR and molecular orbital studies of the complexes, the trends of the 57Fe chemical shifts are discussed in terms of the paramagnetic shielding constant. For both series investigated the apical conformers (compared with the basal) exhibit the more shielded iron nuclei and the larger 57Fe, 31P coupling constants indicating stronger coordination of the phosphorus ligands in the apical position.