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

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 53; no. 5; pp. 2167 - 218
• Main Authors: Biswas, Mitrali, Dey, Sanchaita, Dhara, Suman, Panda, Sanjib, Lahiri, Goutam Kumar
• Format: Journal Article
• Published: 30.01.2024
• DOI: 10.1039/d3dt03730d
• ISSN: 1477-9226

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-CH&z.dbd;N-(CH 2 ) n -N&z.dbd;CH-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 (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) 2 Ru II (μ-L1′)Ru II (acac) 2 , 1a , while the corresponding anti -form (ethylene linker) of the metal-bound L1 in 2 ((acac) 2 Ru II (μ-L1)Ru II (acac) 2 ) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L′′)) via O 2 activation to generate Ru III Ru III -derived (acac) 2 Ru 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) 2 Ru III (μ-L)Ru III (acac) 2 ](ClO 4 ) 2 ([ 2 ](ClO 4 ) 2 -[ 5 ](ClO 4 ) 2 ) and [{(PPh 3 ) 2 (CO)(H)Ru II } 2 (μ-L)](ClO 4 ) 2 ([ 6 ](ClO 4 ) 2 -[ 8 ](ClO 4 ) 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 . Redox-induced cyclisation or oxygenation of bis(aldimine) derived ligands on a selective {Ru(acac) 2 } 2 platform was conceived based on the alkylene conformation.