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

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.     

Ruthenium hydride complexes of chiral and achiral diphosphazane ligands and asymmetric transfer hydrogenation reactions

The half-sandwhich ruthenium chloro complexes bearing chelated diphosphazane ligands, [(η⁵-Cp)RuCl{κ²-P,P-(RO)₂PN(Me)P(OR)₂}] [R = C₆H₃Me₂-2,6] (1) and [(η⁵-Cp^∗)RuCl{κ²-P,P-X₂PN(R)PYY′}] [R = Me, X = Y = Y′ = OC₆H₅ (2); R = CHMe₂, X₂ = C₂₀H₁₂O₂, Y = Y′ = OC₆H₅ (3) or OC₆H₄^tBu-4 (4)] have been prepared by the reaction of CpRu(PPh₃)₂Cl with (RO)₂PN(Me)P(OR)₂ [R = C₆H₃Me₂-2,6 (L¹)] or by the reaction of [Cp^∗RuCl₂]_n with X₂PN(R)PYY′ in the presence of zinc dust. Among the four diastereomers (two enantiomeric pairs) possible for the “chiral at metal” complexes 3 and 4, only two diastereomers (one enantiomeric pair) are formed in these reactions. The complexes 1, 2, 4 and [(η⁵-Cp)RuCl{κ²-P,P-Ph₂PN((S)-^∗CHMePh)PPhY}] [Y = Ph (5) or N₂C₃HMe₂-3,5 (S_CS_PR_Ru)-(6)] react with NaOMe to give the corresponding hydride complexes [(η⁵-Cp)RuH{κ²-P,P-(RO)₂PN(Me)P(OR)₂}] (7), [(η⁵-Cp^∗)RuH{κ²-P,P′-X₂PN(R)PY₂}] [R = Me, X = Y = OC₆H₅ (8); R = CHMe₂, X₂ = C₂₀H₁₂O₂, Y = OC₆H₄^tBu-4 (9)] and [(η⁵-Cp)RuH{κ²-P,P-Ph₂PN((S)-^∗CHMePh)PPhY}][Y = Ph (10) or N₂C₃HMe₂-3,5 (S_CS_PR_Ru)-(11a) and (S_CS_PS_Ru)-(11b)]. Only one enantiomeric pair of the hydride 9 is obtained from the chloro precursor 4 that bears sterically bulky substituents at the phosphorus centers. On the other hand, the optically pure trichiral complex 6 that bears sterically less bulky substituents at the phosphorus gives a mixture of two diastereomers (11a and 11b). Protonation of complex 7 using different acids (HX) gives a mixture of [(η⁵-Cp)Ru(η²-H₂){κ²-P,P-(RO)₂PN(Me)P(OR)₂}]X (12a) and [(η⁵-Cp)Ru(H)₂{κ²-P,P-(RO)₂PN(Me)P(OR)₂}]X (12b) of which 12a is the major product independent of the acid used; the dihydrogen nature of 12a is established by T₁ measurements and also by synthesizing the deuteride analogue 7-D followed by protonation to obtain the D-H isotopomer. Preliminary investigations on asymmetric transfer hydrogenation of 2-acetonaphthone in the presence of a series of chiral diphosphazane ligands show that diphosphazanes in which the phosphorus centers are strong π-acceptor in character and bear sterically bulky substituents impart moderate levels of enantioselectivity. Attempts to identify the hydride intermediate involved in the asymmetric transfer hydrogenation by a model reaction suggests that a complex of the type, [Ru(H)(Cl){κ²-P,P-X₂PN(R)PY₂}(solvent)₂] could be the active species in this transformation.