Inversion of axial chirality in coordinated bis-β-diketonato ligands

Abstract

Mononuclear and dinuclear ruthenium(III) complexes with bis-β-diketonato ligands (denoted by [Ru(acac)₂(L–LH)] and [Ru(acac)₂(L–L)Ru(acac)₂], respectively) were synthesized, where acac, L–LH⁻ and L–L²⁻ denote acetylacetonato, monoprotonated and unprotonated bis-β-diketonato ligands, respectively. The following three ligands were used as the bis-β-diketonato ligand (L–L²⁻): 1,2-diacetyl-1,2-dibenzoylethanato (denoted by dabe²⁻), 1,2-diacetyl-1,2-bis(3-methylbutanoyl)ethanato (baet²⁻) and 1,2-diacetyl-1,2-dipropanoylethanato (dpe²⁻). For the mononuclear and the meso-type dinuclear complexes, a pair of diastereomeric species were identified as Δ- (or Λ-) [Ru(acac)₂(R- or S-L–LH)] and [Δ-Ru(acac)₂(R- or S-L–L)Λ-Ru(acac)₂], respectively. The possibility of thermal inversion in coordinated L–LH⁻ (mononuclear) or L–L²⁻ (dinuclear) was pursued by monitoring the changes in the electronic circular dichroism or the ¹H NMR spectra. No inversion occurred for the dinuclear complexes, when their chloroform solutions were kept at 50 °C for ca. 100 h. In contrast, some of the mononuclear complexes underwent the inversion of axial chirality to give an equilibrium mixture under the same conditions. The reaction followed the first-order rate law and the overall first-order rate constants (k) of [Ru(acac)₂(L–LH)] were determined to be k = 0.13, 0.0048 and less than 0.001 h⁻¹ for L–LH⁻ = dabeH⁻, baetH⁻ and dpeH⁻, respectively. The results suggest that the main factor determining the barrier height of the internal rotation is not the steric but the electronic properties of the carbon–carbon bond connecting the two β-diketonato moieties.