Fe-57 NMR study of ligand effects in cyclopentadienyliron complexes

The first systematic Fe-57 NMR study of ligand effects in cyclopentadienyliron complexes is presented, Four series encompassing a total of 35 compounds have been studied. Among the compounds are five new ring-substituted complexes with the formula (C5H4Y)Fe(CO)(PPh(3))(Me), Y = Me, SiMe(3), NEt(2), I, and Ph. The crystal structure of (C5H4I)Fe(CO)(PPh(3))(Me) (32) was determined. For the series CpFe(CO)(2)R (type I), Fe-57 shielding was found to decrease with the bulkiness of the alkyl ligand R and to correlate with the CO-insertion rate of the complex. In type III complexes CpFe(CO)(L)(COMe), L = PR(3), and CO, it is again the steric requirement of the ligand L that dominates the Fe-57 chemical shift which increases with larger cone angles (theta). In contrast, a strong electronic effect was found for type II complexes CpFe(CO)(PPh(3))X, X = H, Me, and I. Ligands with higher electronegativity induce a shift of the Fe-57 resonance to higher frequencies. In the complexes (C5H4Y)Fe(CO)(PPh(3))(Me), Y = Me, H, SiMe(3), NEt(2), I, Ph, COOMe, and (COPr)-Pr-i (type IV), again electronic effects are dominant, whereby electron acceptor substituents (e.g. Y = (COPr)-Pr-i, COOMe) cause a deshielding of the iron nucleus, relative to complexes with electron donor substituents (e.g. Y = Me, H, SiMe(3)). Dominant electronic ligand effects in type TV complexes are also apparent from the correlation of delta(Fe-57) with the electronic substituent parameter sigma(1). The Fe-57 chemical shift is shown to be very sensitive to changes in the ligand sphere, and the effects are discussed in terms of the paramagnetic shielding constant (sigma(para)). The (1)J(Fe-57, C-13) coupling constants and longitudinal relaxation times T-1 of Fe-57 in selected complexes have been determined and ligand effects are discussed.