Kinetics of the S_N1 Dissociation of Ligands L (Nitriles, Phosphines) in the Complexes [CpFe(P-P)L]PF₆ with Variable Chelate Ring Size. A Surprising Bimolecular Substitution in the Nonchelate Complex [CpFe(PPh₂Me)₂L]PF₆

The complexes [CpFe­{Ph₂P­(CH₂)_nPPh₂}­NCMe]­PF₆, [CpFe­(PPh₂Me)₂NCMe]­PF₆, and [CpFe­{Ph₂P­(CH₂)_nPPh₂}­PPh₂(OR)]­PF₆, R = Me, Et, and iPr, with chelate ring sizes between 4 and 7 were synthesized and characterized by spectroscopy and X-ray analysis. In these complexes, the monodentate ligands acetonitrile and PPh₂(OR) tend to dissociate. The kinetics of the ligand exchanges MeCN/P­(OMe)₃ and PPh₂(OR)/P­(OMe)₃ was measured. In the acetonitrile series, the first-order reaction of the five-membered chelate complex [CpFe­(dppe)­NCMe]­PF₆ had a half-life of 549 min in CDCl₃ at 293 K. The other chelate complexes [CpFe­(P-P)­NCMe]­PF₆ and the nonchelate analogue [CpFe­(PPh₂Me)₂NCMe]­PF₆ reacted faster by factors of 20–50. The PPh₂(OR) series revealed a dramatic difference between the complexes [CpFe­(P-P)­PPh₂(OR)]­PF₆ with five- and six-membered chelate rings. The PPh₂(OR)/P­(OMe)₃ exchange in the dppp complex (six-membered chelate ring) was 500 times faster than in the dppe complex (five-membered chelate ring). This is due to the increase of the P–Fe–P angle in the dppp chelate ring, which diminishes the binding pocket of the PPh₂(OR) ligand. In the nonchelate complex [CpFe­(PPh₂Me)₂NCMe]­PF₆, a novel and unexpected bimolecular PPh₂Me/PPh₂(OMe) substitution was observed.