Kinetics of the SN1 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) n PPh2}­NCMe]­PF6, [CpFe­(PPh2Me)2NCMe]­PF6, and [CpFe­{Ph2P­(CH2) n PPh2}­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.