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

We report the first examples of ruthenium complexes cis-[(N4())Ru(III)Cl(2)](+) and cis-[(N-4)Ru-II(OH2)(2)](2+) supported by chiral tetradentate amine ligands (N-4), together with a high-valent cis-dioxo complex cis[(N4())Ru(VI)(O)(2)](2+) supported by the chiral N-4 ligand mcp (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl) cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)(RuCl2)-Cl-III](ClO4) (1a), cis[(Me(2)mcp)(RuCl2)-Cl-III] ClO4 (2a) and cis-[(pdp)(RuCl2)-Cl-III](ClO4) (3a) (Me(2)mcp = N,N'-dimethyl-N,N'-bis((6-methylpyridin-2-yl) methyl)cyclohexane-1,2-diamine, pdp = 1,10-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-a configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp) Ru-III(O2CCF3)(2)]ClO4 (1b) and cis-[(pdp)Ru-III(O3SCF3)(2)]CF3SO3 (3c'). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp) Ru-VI(O)(2)](2+) and cis-[(pdp) Ru-VI(O)(2)](2+) complexes are strong oxidants with E degrees = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with DO-H = 90.1-90.8 kcal mol(-1). The reaction of 1b or its (R,R)-mcp counterpart with excess (NH4)(2)[Ce-IV(NO3)(6)] (CAN) in aqueous medium afforded cis-[(mcp) Ru-VI(O)(2)](ClO4)(2) (1e) or cis-[((R,R)-mcp) Ru-VI(O)(2)](ClO4)(2) (1e*), respectively, a strong oxidant with E(Ru-VI/V) = 0.78 V (vs. Ag/AgNO3) in acetonitrile solution. Complex 1e 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 1e or 1e* in (BuOH)-Bu-t/H2O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp) RuII(OH2)(2)](2+) (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.