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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
journal contribution
posted on 2017-06-22, 17:35 authored by Henri Brunner, Hikaru Kitamura, Takashi TsunoThe 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.