Direct observation of 57Fe chemical shifts in diamagnetic organoiron complexes

Abstract

57Fe NMR spectra of 35 organoiron complexes predominantly of the type [Fe(CO) 3(diene)] have been measured by direct detection at 2.9 MHz and at natural isotope abundance (2.2%). The 57Fe chemical shift range so far observed is 3000 ppm, corresponding to about 9 KHz. The majority of the resonances are at higher frequency than that of Fe(CO) 5, which is proposed as a secondary standard. The 57Fe resonance frequencies are also reported as ratios relative to the 73Ge frequency of GeCl 4. The 57Fe shielding is discussed qualitatively in terms of charge distribution in the complexes, very large deshielding effects being observed for cationic olefinic ligands. For η 4-diene complexes, the shielding also depends upon ligand geometry and decreases with increasing ring size or CCC bond angle. Two types of ferrocenylcarbenium ions may be distinguished, indicated by high- or low-frequency shifts relative to ferrocene. For a number of [Fe(CO) 3(olefin)] complexes 1 J( 57Fe, 13C) coupling constants involving the carbonyl carbon are also reported.