Molecular and Electronic Structures of Ruthenium Complexes Containing an ONS-Coordinated Open-Shell π Radical and an...

The coordination chemistry of 2,4-di-tert-butyl-6-[(2-mercaptophenyl)amino]phenol (L_ONSH₃), which was isolated as a diaryl disulfide form, (L_ONSH₂)₂, with a Ru ion is disclosed. It was established that the trianionic L_ONS^3- is redox-noninnocent and undergoes oxidation to either a closed-shell singlet (CSS), L_ONS^-, or an open-shell π-radical state, L_ONS^•2-, and the reactivities of the [Ru^II(L_ONS^•2-)] and [Ru^II(L_ONS^-)] states are different. The reaction of (L_ONSH₂)₂ with [Ru(PPh₃)₃Cl₂] in toluene in the presence of PPh₃ affords a ruthenium complex of the type trans-[Ru(L_ONS)(PPh₃)₂Cl] (1), while the similar reaction with [Ru(PPh₃)₃(H)(CO)Cl] yields a L_ONS^•2- complex of ruthenium(II) of the type trans-[Ru^II(L_ONS^•2-)(PPh₃)₂(CO)] (2). 1 is a resonance hybrid of the [Ru^II(L_ONS^-)Cl] and [Ru^III(L_ONS^•2-)Cl] states. It is established that 2 incorporating an open-shell π-radical state, [Ru^II(L_ONS^•2-)(CO)], reacts with an in situ generated superoxide ion and promotes an oxidative aromatic ring cleavage reaction, yielding a α-N-arylimino-ω-ketocarboxylate (L_NS^2-) complex of the type [Ru^II(L_NS^2-)(PPh₃)(CO)]₂ (4), while 1 having a CSS state, [Ru^II(L_ONS^-)Cl], is inert in similar conditions. Notably, 2 does not react with O₂ molecule but reacts with KO₂ in the presence of excess PPh₃, affording 4. The redox reaction of (L_ONSH₂)₂ with [Ru(PPh₃)₃Cl₂] in ethanol in air is different, leading to the oxidation of L_ONS to a quinone sulfoxide derivative (L_ONSO⁰) as in cis-[Ru^II(L_ONSO⁰)(PPh₃)Cl₂] (3), via 1 as an intermediate. The molecular and electronic structures of 1-4 were established by single-crystal X-ray crystallography, electron paramagnetic resonance spectroscopy, electrochemical measurements, and density functional theory calculations. 1⁺ is a resonance hybrid of [Ru^III(L_ONS^-)(PPh₃)₂Cl ↔ Ru^IV(L_ONS^•2-)(PPh₃)₂Cl]⁺ states, 2^- is a L_ONS^3- complex of ruthenium(II), [Ru^II(L_ONS^3-)(PPh₃)₂(CO)]^-, and 2⁺ is a ruthenium(II) complex of L_ONS^- of the type [Ru^II(L_ONS^-)(PPh₃)₂(CO)]⁺, where 35% diradical character of the L_ONS^- ligand was predicted.