Sequential Oxygenation of Bis(β-diketiminate) on a Selective Diruthenium Platform

This article demonstrated the redox-noninnocent phenylene-linked bis(β-diketiminate) (L^2-)-bridged first example of isomeric diruthenium(III)-acac species (acac = acetylacetonate) and its ability to activate dioxygen. The coordination of deprotonated L^2- to the {Ru(acac)₂} in bis(bidentate) mode led to isomeric {(acac)₂Ru^III}₂(μ-L^2-) (S = 1, 1-trans/1-cis, green). 1 displayed Ru(III)-based anisotropic EPR in CH₃CN but without the resolution of the forbidden (ΔM_s = 2) g_1/2 signal at 77 K. 1-cis, however, slowly transformed to the energetically favored 1-trans form. 1 underwent two-step oxygenation at the C_β sites of L^2- to form the β-diketiminate/α-ketodiimine (L'^-)-bridged mixed valent (acac)₂Ru^III(μ-L'^-)Ru^II(acac)₂ (2, S = 1/2, pink) followed by bis(α-ketodiimine) (L″)-bridged isovalent (acac)₂Ru^II(μ-L″)Ru^II(acac)₂ (3, S = 0, red). The role of O₂ toward 1 → 2/3 was corroborated by ¹⁸O₂ labeling experiment. Redox steps of 1-3 varied as a function of isomeric identity, bridge, and metal oxidation state. The calculated MOs and Mulliken spin densities attributed to the noninnocence of L^2-, L'^-, and L″ in the respective complexes. Spectrophotometric monitoring of 1 → 2 revealed pseudo-first-order rate constants (10⁵k s^-1) of 1.8 (303 K), 3.5 (313 K), 7.7 (323 K), and 17.0 (333 K) and ΔH^⧧/ΔS^⧧/ΔG^⧧ of 14.3 kcal mol^-1/-33.1 cal mol^-1 K^-1/24.2 kcal mol^-1 (298 K), respectively. Moreover, characterization of the short-lived blue intermediate obtained during the conversion of 1 → 2/3 upon exposure to O₂ supported its valence tautomeric form (1, Ru^III-L^2--Ru^III ↔ Ru^III-L^•--Ru^II, S = 1), which in effect facilitated oxygen activation at the ligand backbone.