Oxygen vs. carbon co-ordination of α-keto-stabilized phosphorus ylides Ph3PC(H)COR (R = Me, Ph or OMe) to palladium(II) cationic complexes

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Abstract
The reactivity of several C,N-cyclometallated palladium(II) complexes [Pd(C 6 H 4 CH 2 NMe 2 -2)L(L ′)]ClO 4 and [Pd{R-C 6 H 4 CH(Me)NMe 2 -2 }L(L′)]ClO 4 [L = PPh 3 or pyridine (py), L′ = NCMe or thf (tetrahydrofuran); L = L′ = NCMe] towards α-keto-stabilized phosphorus ylides Ph 3 PC(H)COR (R = Me, Ph or OMe) has been studied. When L = PPh 3 , L′ = thf the ylide co-ordinates invariably through the carbonyl oxygen atom, and trans to the orthometallated C 6 H 4 group, a rather unusual mode for the soft palladium(II) centre. When L = py, L′ = thf the co-ordination mode of the ylide and its site varies as a function of the nucleophilic character of the ylidic carbon atom. Thus, Ph 3 PC(H)COMe (where the ylidic carbon is a poor nucleophile) co-ordinates through the carbonyl oxygen atom (trans to the C 6 H 4 group), while the better nucleophile Ph 3 PC(H)CO 2 Me co-ordinates through the ylidic carbon and trans to the NMe 2 group. When L = L′ = NCMe the less nucleophilic ylides Ph 3 C(H)COR (R = Me or Ph), give products in which a mixture of both O- and C-co-ordination modes are found, while Ph 3 PC(H)CO 2 Me gives exclusively C-co-ordination trans to the NMe 2 group. Infrared and NMR spectroscopies allow the unambiguous characterization of these products, and the crystal structure of [Pd(C 6 H 4 CH 2 NMe 2 -2)(py) {C(H)(CO 2 Me)PPh 3 }]ClO 4 has also been determined.
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