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(N2C3HMe2-3,5)}](PF6) (3−6) and [Pd(η3-R′C3H4){η2-Ph2P(S)N(CHMe2)PPh(N2C3HMe2-3,5)}](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(N2C3HMe2-3,5)}](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(N2C3HMe23,5)}](PF6) (7) and [Pd(η3-PhC3H4){η2-Ph2P(S)N(CHMe2)PPh(N2C3HMe2-3,5)}](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. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)