Syntheses of novel monomeric 1,4,7-trimethyl-1,4,7-triazacyclononane ruthenium complexes. Reactivities and structure of sterically encumbered cationic monoaquaruthenium(II) and monooxoruthenium(IV) complexes

The complexes [RuII(tacn)(bipy)(OH2)]2+ 1 and [RuIV(tacn)(bipy)O]2+ 2(tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, bipy = 2,2′-bipyridine) have been synthesised and their crystal structures determined: 1, monoclinic, space group C2/c, a = 34.093(4), b = 10.430(1), c = 17.369(2) Å, β = 118.54(1)°and Z = 8; 2, triclinic, space group 1̄, a = 10.795(1), b = 11.076(2), c = 11.248(1) Å, a = 110.41(2), β = 90.66(1), γ = 94.11(1)°and Z = 2. The structures reveal that tacn acts as a facial chelating ligand with bipy nearly perpendicular to the Ru=O and Ru-OH2 moieties. The RuIV=O and Ru-OH2 distances are 1.815(6) and 2.168(3) Å respectively. In aqueous solution both complexes display two reversible proton-coupled one-electron redox couples corresponding to the oxidation of RuII to RuIII and RuIII to RuIV. Complex 2 has been found to be a competent oxidant for alkene epoxidation. Preliminary-kinetic studies revealed the rate law, rate = k2[RuIV][alkene] where k2 at 299 K are (1.30 ± 0.11) × 10-3 and (2.52 ± 0.20) × 10-4 dm3 mol-1 s-1 for the epoxidation of styrene and norbornene respectively. The activation parameters have been measured and are discussed.