Steric and electronic effects in stabilizing allyl-palladium complexes of "P-N-P" ligands, X₂PN(Me)PX₂ (X = OC₆H₅ or OC₆H₃Me₂-2,6)

Abstract

The chemistry of η³-allyl palladium complexes of the diphosphazane ligands, X₂PN(Me)PX₂ [X = OC₆H₅ (1) or OC₆H₃Me₂-2,6 (2)] has been investigated.The reactions of the phenoxy derivative, (PhO)₂PN(Me)P(OPh)₂ with [Pd(η³-1,3-R',R"-C₃H₃)(μ-Cl)]₂ (R' = R" = H or Me; R' = H, R" = Me) give exclusively the palladium dimer, [Pd₂{μ-(PhO)₂PN(Me)P(OPh)₂}₂Cl₂] (3); however, the analogous reaction with [Pd(η³-1,3-R',R"-C₃H₃)(μ-Cl)]₂ (R' = R" = Ph) gives the palladium dimer and the allyl palladium complex [Pd(η³-1,3-R',R"-C₃H₃)(1)](PF₆) (R' = R" = Ph) (4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with (allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(η³-1,3-R',R"-C₃H₃)(2)](PF₆) [R' = R" = H (5), Me (7) or Ph (8); R' = H, R" = Me (6)].Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution; the relatively less favourable isomer, anti-[Pd(η³-1-Me-C₃H₄)(2)](PF₆) (6b) and syn/anti-[Pd(η³-1,3-Me₂-C₃H₃)(2)](PF₆) (7b) are present to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phosphorus atoms in 2. The structures of four complexes (4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is observed in the solid state in each case.