NMR parameters as steric probes for arylcyclopentadienyl iron complexes

Abstract

The complexes [(η 5-C 5H 4Me)Fe(CO)(L)I] (LPPh 3, P( p-C 6H 4Me) 3, P( p-C 6H 4F) 3) have been synthesised and characterised by IR and NMR spectroscopy. The difference in chemical shift separation between the cyclopentadienyl ring proton resonances flanking the methyl substituents, Δ(H2–H5), was found to be invariant with change in the para phosphine ligand substituent, suggesting that the measure is relatively free of electronic influences. A series of aryl substituted cyclopentadienyl complexes, [(η 5-C 5H 4Ar)Fe(CO)(L)I] (Ar  Ph, pC̄ 6H 4Me, p-C 6H 4 tBu; LP(OMe) 3, PMe 2Ph, P(O iPr) 3, P(O- o-tol) 3, PPh 3, P(CH 2Ph) 3) was also synthesised and characterised by IR and NMR spectroscopy. Again the parameter δ(H2–H5) was found to be invariant with change in the substituent on the cyclopentadienyl ring, suggesting that changes at a distance do not influence this steric measure. NOE different spectra recorded on [(η 5-C 5H 4- p-C 6H 4Me)Fe(CO)(L)I] (LP(OMe) 3, P(O iPr) 3) indicate that the substituted cyclopentadienyl ring rotates freely about the Fe-ring centroid axis relative to the ligand set. NOE spectral data for LP(OMe) 3 also reveal that the p-tolyl and P(OMe) 3 protons are close enough in space to give rise to a positive nuclear Overhauser effect. The relative size of the aryl ring is shown, by NMR spectroscopy, to lie between that of a methyl and a tert-butyl group.