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Issue 40, 2012
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η1-Allylpalladium complexes with a tridentate PNP ligand with different phosphino groups

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Abstract

The iminodiphosphine 2-(PPh2)C6H4-1-CH[double bond, length as m-dash]NC6H4-2-(PPh2) (P–N–P′) is used for the preparation of the complexes [Pd(η1-CHR1–CH[double bond, length as m-dash]CR2R3)(P–N–P′)]BF4 [R1 = R2 = R3 = H: (1); R1 = R2 = Ph, R3 = H: (2); R1 = R3 = H, R2 = Ph: (3); R1 = H, R2 = R3 = Me: (4)]. The P–N–P′ tridentate coordination and the η1-allyl bonding mode in the solid are confirmed by the X-ray structural analysis of 1. In solution, the complexes 1 and 2 undergo an η1–η3–η1 rearrangement at 298 K interconverting the bonding site of the allyl group. A five-coordinate structure with the phosphine ligands in the axial position is proposed for the η3-allyl intermediate. For the dynamic process, a ΔG value of 53.8 kJ mol−1 is obtained from 1H NMR data of 2. In 3 and 4, the allyl ligand is rigidly bound to the metal through the less substituted terminus, in line with the higher free energy content of the corresponding isomers: [Pd(η1-CHPh–CH[double bond, length as m-dash]CH2)(P–N–P′)]+ +48.78 kJ mol−1; [Pd(η1-CMe2–CH[double bond, length as m-dash]CH2)(P–N–P′)]+ +69.35 kJ mol−1. The complexes react with secondary amines in the presence of fumaronitrile at different rates yielding allylamines and the palladium(0) derivative [Pd(η2-fn)(P–N–P′)] (5). On the basis of charge distribution on the allylic carbon atoms and of steric factors, the difference in rate and the regioselectivity in the amination of 1–3 are better rationalized by a mechanism with nucleophilic attack at the η3-intermediate rather than by an SN2 mechanism with nucleophilic attack at the Pd–CHR1 carbon atom. The high regioselectivity in the reaction of 4 with piperidine implies an SN2′ mechanism with nucleophilic attack at the CMe2 allyl carbon. A dynamic process occurs also for the 18-electron complex 5 consisting in a dissociation–association equilibrium of the olefin.

Graphical abstract: η1-Allylpalladium complexes with a tridentate PNP ligand with different phosphino groups

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Publication details

The article was received on 04 Apr 2012, accepted on 15 Aug 2012 and first published on 15 Aug 2012


Article type: Paper
DOI: 10.1039/C2DT30746D
Citation: Dalton Trans., 2012,41, 12490-12500
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    η1-Allylpalladium complexes with a tridentate PNP ligand with different phosphino groups

    B. Crociani, S. Antonaroli, P. Paoli and P. Rossi, Dalton Trans., 2012, 41, 12490
    DOI: 10.1039/C2DT30746D

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