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

We report the first examples of ruthenium complexes cis-[(N(4))Ru(III)Cl(2)](+) and cis-[(N(4))Ru(II)(OH(2))(2)](2+) supported by chiral tetradentate amine ligands (N(4)), together with a high-valent cis-dioxo complex cis-[(N(4))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)Ru(III)Cl(2)](ClO(4)) (1a), cis-[(Me(2)mcp)Ru(III)Cl(2)]ClO(4) (2a) and cis-[(pdp)Ru(III)Cl(2)](ClO(4)) (3a) (Me(2)mcp = N,N′-dimethyl-N,N′-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 cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)Ru(III)(O(2)CCF(3))(2)]ClO(4) (1b) and cis-[(pdp)Ru(III)(O(3)SCF(3))(2)]CF(3)SO(3) (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° = 1.11–1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with D(O–H) = 90.1–90.8 kcal mol(–1). The reaction of 1b or its (R,R)-mcp counterpart with excess (NH(4))(2)[Ce(IV)(NO(3))(6)] (CAN) in aqueous medium afforded cis-[(mcp)Ru(VI)(O)(2)](ClO(4))(2) (1e) or cis-[((R,R)-mcp)Ru(VI)(O)(2)](ClO(4))(2) (1e*), respectively, a strong oxidant with E(Ru(VI/V)) = 0.78 V (vs. Ag/AgNO(3)) 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 (t)BuOH/H(2)O (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)Ru(II)(OH(2))(2)](2+) (3c)-catalysed oxidation of saturated C–H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.