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.