Allylpalladium complexes of mixed-donor diphosphazane ligands bearing a stereogenic phosphorus centre: structure and stereodynamics

Abstract

(η³-Allyl)palladium complexes of general formula [Pd(η³-R'C₃H₄){η²-Ph₂PN(R)PPh(N₂C₃ HMe₂-3,5)}](PF₆) (3-6) and [Pd(η³-R'C₃H₄){η²-Ph₂P(S)N(CHMe₂)PPh(N ₂C₃HMe₂-3,5)}](PF₆) (7-8), containing pyrazolyl-substituted diphosphazane ligands bearing a stereogenic phosphorus centre, have been synthesised. NMR spectroscopic and X-ray diffraction studies showed that the diphosphazane ligand in [Pd(η³-R'C₃H₄){η²-Ph₂PN(R)PPh(N₂C₃ HMe₂-3,5)}](PF₆) [R' = Me or Ph; R = CHMe₂ (3 and 5) or (S)-CHMePh (4 and 6)] displayed unprecedented P,N-coordination instead of the hitherto observed P,P-coordination. These complexes existed in solution as mixtures of endo,syn,trans, exo,syn,trans, and endo,syn,cis diastereomers. In the solid state, only the endo,syn,trans isomer was observed for complex 5. Phase-sensitive 2-D NOESY and ROESY spectra showed that the complexes 3-5 underwent syn,anti isomerisation; the NMR spectroscopic data were consistent with a mechanism involving the opening of the η³-allyl group at the trans position with respect to the phosphorus centre. The isomerisation thus appeared to be electronically controlled. The complexes [Pd(η³-MeC₃H₄){η²-Ph₂P(S)N(CHMe₂)PPh(N ₂C₃HMe₂3,5)}](PF₆) (7) and [Pd(η³-PhC₃H₄){η²-Ph₂P(S)N(CHMe₂)PPh(N ₂C₃HMe₂-3,5)}](PF₆) (8), bearing the diphosphazane monosulfide Ph₂P(S)N(CHMe₂)PPh(N₂C₃HMe₂-3,5) (1c), displayed P,S-coordination and existed in solution as six and four isomers, respectively. Exchange was observed among four of the six isomers of 7 and among all the four isomers of 8 at 298 K, through the opening of the η³-allyl moiety, which appeared to be subject to both electronic and steric control.