Synthesis, properties and crystal structures of diastereomeric areneruthenium(II) chiral Schiff-base complexes

Abstract

Diastereomers (S_Ru,S_c)-1a and (R_Ru,S_c)-1b, in a ratio of 85:15 and formulated as [Ru(η-MeC₆H₄Prⁱ-p)Cl(L^∗)], have been prepared by treating [{Ru(η-MeC₆H₄Prⁱ-p)Cl₂}₂] with the sodium salt of (S)-α-methylbenzylsalicylaldimine (HL^∗) in tetrahydrofuran at -70°C. The reaction of 1(1a+1b) with AgClO₄ in acetone followed by an addition of PPh₃ or 4-methylpyridine (4Me-py) leads to the formation of adducts [Ru(η-MeC₆H₄Prⁱ-p)(PPh₃)(L^∗)]ClO₄2[(S_Ru,S_c)2a, (F_Ru,S_c)2b] and [Ru(η-MeC₆H₄Prⁱ-p)(4Me-py)(L^∗)]ClO₄3[(S_Ru,S_c)3a, (R_Ru,S_c)3b] in the diastereomeric ratios (S_Ru,S_c) : (R_Ru,S_c) of 2 : 98 and 76 : 24, respectively. Complex 1 crystallises with equal numbers of 1a and 1b molecules in an asymmetric unit of monoclinic space group P2₁ with a= 10.854(1), b= 17.090(1), c= 12.808(4)Å, β= 110.51(1)°, and Z= 4. The structure was refined to R= 0.0552 and R'= 0.0530 with 2893 reflections having I ≥ 1.5σ(I). The absolute configurations of the chiral centres in the optically pure single crystal of the PPh₃ adduct have been obtained from an X-ray study. Crystals of formulation [Ru(η-MeC₆H₄Prⁱ-p)-(PPh₃)(L^∗)]₂[ClO₄][PF₆]·1.5 CHCl₃, obtained in presence of both ClO₄ and PF₆ anions, belong to the non-centric triclinic space group P1 with a= 10.852(2), b= 14.028(1), c= 15.950(2)Å, α= 91.51(1), β= 105.97(1), γ= 106.11(1)°, and Z=2. The final residuals were R= 0.0713, R'= 0.0752 with 7283 reflections having I≥2.5σ(I). The crystal structures of 1a,1b, and the PPh₃ adduct (2b,2b') consist of a ruthenium(II) centre bonded to a η-p-cymene, a bidentate chelating Schiff base, and a unidentate ligand (Cl or PPh₃). The chirooptical properties of the complexes have been studied using ¹H NMR and CD spectral data. The presence of a low-energy barrier for the intermediate involved in these reactions, showing both retention as well as inversion of the metal configuration, is discussed.