Redox Non-Innocence and Isomer-Specific Oxidative Functionalization of Ruthenium-Coordinated β-Ketoiminate

This article deals with isomeric ruthenium complexes [Ru^III (L^R )₂ (acac)] (S=1/2) involving unsymmetric β-ketoiminates (AcNac) (L^R =R-AcNac, R=H (1), Cl (2), OMe (3); acac=acetylacetonate) [R=para-substituents (H, Cl, OMe) of N-bearing aryl group]. The isomeric identities of the complexes, cct (cis-cis-trans, blue, a), ctc (cis-trans-cis, green, b) and ccc (cis-cis-cis_, pink, c) with respect to oxygen (acac), oxygen (L) and nitrogen (L) donors, respectively, were authenticated by their single-crystal X-ray structures and spectroscopic/electrochemical features. One-electron reversible oxidation and reduction processes of 1-3 led to the electronic formulations of [Ru^III (L)(L^⋅ )(acac)]⁺ and [Ru^II (L)₂ (acac)]^- for 1⁺ -3⁺ (S=1) and 1^- -3^- (S=0), respectively. The triplet state of 1⁺ -3⁺ was corroborated by its forbidden weak half-field signal near g≈4.0 at 4 K, revealing the non-innocent feature of L. Interestingly, among the three isomeric forms (a-c in 1-3), the ctc (b in 2 b or 3 b) isomer selectively underwent oxidative functionalization at the central β-carbon (C-H→C=O) of one of the L ligands in air, leading to the formation of diamagnetic [Ru^II (L)(L')(acac)] (L'=diketoimine) in 4/4'. Mechanistic aspects of the oxygenation process of AcNac in 2 b were also explored via kinetic and theoretical studies.