IONIZATION OF ETA-6-DIPHENYLMETHANE)(ETA-5-CYCLOPENTADIENYL)IRON CATIONS IN H2O-ME2SO AND METHANOL-ME2SO MIXTURES - A KINETIC, NMR, AND EHMO STUDY

Rates of proton abstraction from the diphenylmethanes [(PhCH2Ph)FeCp], 8, and [(PhCH2-Ph)(FeCp)2]2+, 9, by OH- and CH3O- and of reprotonati on of the conjugate carbon bases (C-8, C-9) by the solvent have been m easured at 25-degrees-C in various aqueous and methanolic dimethyl sul foxide solutions. From these data, the pK(a) values of 8 and 9 in thes e mixtures have been determined. Comparison of the results obtained wi th those for 4-nitro- and (4,4'-dinitrodiphenyl)-methanes (5 and 6) sh ows that the thermodynamic acidifying influence of a FeCp+ moiety is s imilar to that of a p-NO2 group. The kinetic effects of the two substi tuents are different, however, the FeCp+ compounds having greater rate s of proton abstraction but the nitro compounds giving carbanions with a lower susceptibility to reprotonation by the solvent. This indicate s that the FeCp+ group exerts a greater -1 effect but a lower -M effec t than the NO2 group. A determination of the intrinsic reactivities of the various compounds confirms this conclusion, but it also shows cle arly that the resonance effect of a FeCp+ moiety is very substantial. H-1 and C-13 NMR data for the ionization of 8 and 9 provide additional evidence for this proposal, which is also supported by molecular orbi tal calculations at the extended Huckel level.