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

Abstract

The complexes [Ru^II(tacn)(bipy)(OH₂)]²⁺1 and [Ru^IV(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 , 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 RuO and Ru–OH₂ moieties. The Ru^IVO 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 Ru^II to Ru^III and Ru^III to Ru^IV. Complex 2 has been found to be a competent oxidant for alkene epoxidation. Preliminary-kinetic studies revealed the rate law, rate =k₂[Ru^IV][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.