Oxygen vs. carbon co-ordination of α-keto-stabilized phosphorus ylides Ph3PC(H)COR (R = Me, Ph or OMe) to palladium(II) cationic complexes

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.