Synthesis, reactivities, and structural studies on high-valent ruthenium oxo complexes. Ruthenium(IV), ruthenium(V), and ruthenium(VI) oxo complexes of tertiary amine ligands

The synthesis and characterization of trans-[RuVI(L)O2]2+, trans-[RuV(L)O2]+, and trans-[RuIV(L)O(X)]n+ complexes are described [L = 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), 15-TMC (1,4,8,12-tetramethyl-1,4,8,12-tetraazapentadecane), 16-TMC (1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane), (TMEA)2 (bis(N,N,N′,N′-tetramethyl-1,2-diaminoethane)); n = 1, X = Cl, N3, NCO; n = 2, X = CH3CN]. All trans-Ru(VI)-dioxo complexes are diamagnetic whereas the molar magnetic susceptibilities for trans-[RuV(L)O2]+ and trans-[RuIV(L)O(X)]n+ systems are 1.94 and 2.70-2.95 μB, respectively. In CH3CN, trans-[RuVI(L)O2]2+ and trans-[RuV(L)O2]+ exhibit vibronic-structured dxy → dπ* (dπ* = dxz, dyz) transition bands centered at ∼380 and 430 nm, respectively. The X-ray structure of trans-[RuIV(14-TMC)O(NCO)]ClO4 has been determined: C15H32N5O2RuClO4, Mr 514.97, orthorhombic, space group Pnma, a = 12.507 (2) Å, b = 10.646 (1) Å, c = 15.681 (2) Å, V = 2088.1 Å3, Z = 4, dcalcd = 1.638 g cm-3, μ(Mo Kα) = 9.14 cm-1, m = 1943. The Ru=O bond distances of trans-[RuIV(14-TMC)O(X)]n+ are all 1.765 Å, which are longer than those for trans-[RuVI(L)O2]2+ (1.70-1.71 Å). The E1/2 value for the trans-[RuV(L)O(X)]2+/trans-[RuIV(L)O(X)] + couple lies between 0.70 and 1.10 V vs. the Cp2Fe+/0 couple and decreases in the order X = Cl > NCO > N3. All trans-[RuIV(L)O(X)]n+ complexes are active electrocatalysts for the oxidation of benzyl alcohol to benzaldehyde. The E1/2 values for the trans-[RuVI(L)O2]2+/trans-[Ru IV(L)O(OH2)]2+ and trans-[RuVI-(L)O2]2+/trans-[Ru V(L)O2]+ couples are insensitive to the macrocyclic hole size of L. Replacement of the σ-saturated tertiary amine by the π-aromatic pyridine or 2,2′-bipyridine weakens the Ru=O bond and increases the oxidation potential of the Ru=O complexes. Studies on the reactions of trans-[RuVI(L)O2]2+ and trans-[RuVI(TMP)O2] (H2TMP = 5,10,15,20-tetramesitylporphyrin) with organic substrates indicated that the former system prefers attack on an activated C-H bond.