Metal–ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(ii) platform. Cyclisation versus oxygenation

Abstract

This article addresses the impact of metal–ligand redox cooperativity on the functionalisation of coordinated ligands. It demonstrates the structure–reactivity correlation of bis(aldimine) derived bis-bidentate L (Py-CHN–(CH₂)_n–NCH-Py, with n = 2 (L1), 3 (L2), 4 (L3)) as a function of the conformation (syn/anti) of its alkylene linker as well as the overall structural form (cis/trans) of (acac)₂Ru^II(μ-L)Ru^II(acac)₂ complex moieties (1–5) possessing an electron-rich acetylacetonate (acac) co-ligand. A systematic variation of the bridging alkylene unit of L in Ru^II/Ru^II-derived 1–5 led to the following reactivity/redox events, which were validated through structural, spectroscopic, electrochemical and theoretical evaluations: (i) Cyclisation of the ethylene linked (syn conformation) bis-aldimine unit of L1 via C–C coupling yielded pyrazine bridged (acac)₂Ru^II(μ-L1′)Ru^II(acac)₂, 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)₂Ru^II(μ-L1)Ru^II(acac)₂) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L′′)) via O₂ activation to generate Ru^IIIRu^III-derived (acac)₂Ru^III(μ-L1′′²⁻)Ru^III(acac)₂ (2a). (ii) Consequently, propylene and butylene linked L2 and L3 bridged between two {Ru(acac)₂} units in 3 and 4/5 underwent oxygenation of L to L′′ to yield diruthenium(III) complexes 3a and 4a/5a, respectively. (iii) In contrast, analogous L bridged oxidised [(acac)₂Ru^III(μ-L)Ru^III(acac)₂](ClO₄)₂ ([2](ClO₄)₂–[5](ClO₄)₂) and [{(PPh₃)₂(CO)(H)Ru^II}₂(μ-L)](ClO₄)₂ ([6](ClO₄)₂–[8](ClO₄)₂) involving electron poor co-ligands failed to undergo the oxygenation of L irrespective of its n value, reemphasising the effective role of redox interplay between Ru^II and L particularly in the presence of an electron-rich acac co-ligand in the functionalisation of the latter in 1a–5a.