57Fe NMR Study of Ligand Effects in Cyclopentadienyliron Complexes

The first systematic 57 Fe 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 (C 5 H 4 Y)Fe(CO)(PPh 3 )(Me), Y = Me, SiMe 3, NEt 2, I, and Ph. The crystal structure of (C 5 H 4 I)Fe(CO)(PPh 3 )(Me) ( 32 ) was determined. For the series CpFe(CO) 2 R (type I ), 57 Fe 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 57 Fe chemical shift which increases with larger cone angles (θ). 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 57 Fe resonance to higher frequencies. In the complexes (C 5 H 4 Y)Fe(CO)(PPh 3 )(Me), Y = Me, H, SiMe 3, NEt 2, I, Ph, COOMe, and CO i Pr (type IV ), again electronic effects are dominant, whereby electron acceptor substituents (e.g. Y = CO i Pr, 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 IV complexes are also apparent from the correlation of δ( 57 Fe) with the electronic substituent parameter σ I . The 57 Fe 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 (σ para ). The 1 J ( 57 Fe, 13 C) coupling constants and longitudinal relaxation times T 1 of 57 Fe in selected complexes have been determined and ligand effects are discussed.