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

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)(2)Ru-II(mu-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)(2)Ru-II(mu-L1 ')Ru-II(acac)(2), 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)(2)Ru-II(mu-L1)Ru-II(acac)(2)) led to oxygenation at the ligand backbone (bis-aldimine (L) -> bis(carboxamido) (L '')) via O-2 activation to generate (RuRuIII)-Ru-III-derived (acac)(2)Ru-III(mu-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)(2)Ru-III(mu-L)Ru-III(acac)(2)](ClO4)(2) ([2](ClO4)(2)-[5](ClO4)(2)) and [{(PPh3)(2)(CO)(H)Ru-II}(2)(mu-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.