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) 2 Ru III } 2 (μ-L 2- ) ( S = 1, 1 - trans / 1 - cis, green ). 1 displayed Ru(III)-based anisotropic EPR in CH 3 CN but without the resolution of the forbidden (Δ M 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) 2 Ru III (μ-L ' - )Ru II (acac) 2 ( 2 , S = 1/2, pink ) followed by bis(α-ketodiimine) (L ″ )-bridged isovalent (acac) 2 Ru II (μ-L ″ )Ru II (acac) 2 ( 3 , S = 0, red ). The role of O 2 toward 1 → 2 / 3 was corroborated by 18 O 2 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 (10 5 k s -1 ) of 1.8 (303 K), 3.5 (313 K), 7.7 (323 K), and 17.0 (333 K) and Δ H ⧧ /Δ S ⧧ /Δ G ⧧ 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 O 2 supported its valence tautomeric form ( VT 1 , Ru III -L 2- -Ru III ↔ Ru III -L •- -Ru II , S = 1), which in effect facilitated oxygen activation at the ligand backbone.