cis -Oxoruthenium complexes supported by chiral tetradentate amine (N 4 ) ligands for hydrocarbon oxidations

• Published in: Chemical science (Cambridge) 2018, Vol.9 (10), p.2803-2816
• Main Authors: Tse, Chun-Wai, Liu, Yungen, Wai-Shan Chow, Toby, Ma, Chaoqun, Yip, Wing-Ping, Chang, Xiao-Yong, Low, Kam-Hung, Huang, Jie-Sheng, Che, Chi-Ming
• Format: Article
• Language: English
• Subjects: Acetonitrile; Alcohols; Aldehydes; Alkenes; Chemical bonds; Chemistry; Crystal structure; Cyclohexane; Diols; Ethane; Hydrocarbons; Ketones; Ligands; Oxidation; Oxidizing agents; Ruthenium
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c7sc05224c

We report the first examples of ruthenium complexes -[(N )Ru Cl ] and -[(N )Ru (OH ) ] supported by chiral tetradentate amine ligands (N ), together with a high-valent -dioxo complex -[(N )Ru (O) ] supported by the chiral N ligand mcp (mcp = , '-dimethyl- , '-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of -[(mcp)Ru Cl ](ClO ) ( ), -[(Me mcp)Ru Cl ]ClO ( ) and -[(pdp)Ru Cl ](ClO ) ( ) (Me mcp = , '-dimethyl- , '-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1'-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a -α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for -[(mcp)Ru (O CCF ) ]ClO ( ) and -[(pdp)Ru (O SCF ) ]CF SO ( ). Electrochemical analyses showed that the chemically/electrochemically generated -[(mcp)Ru (O) ] and -[(pdp)Ru (O) ] complexes are strong oxidants with ° = 1.11-1.13 V SCE (at pH 1) and strong H-atom abstractors with = 90.1-90.8 kcal mol . The reaction of or its ( , )-mcp counterpart with excess (NH ) [Ce (NO ) ] (CAN) in aqueous medium afforded -[(mcp)Ru (O) ](ClO ) ( ) or -[(( , )-mcp)Ru (O) ](ClO ) ( ), respectively, a strong oxidant with (Ru ) = 0.78 V ( Ag/AgNO ) in acetonitrile solution. Complex oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by or in BuOH/H O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using . The -[(pdp)Ru (OH ) ] ( )-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.