Synthesis, structure and catalytic activity of ruthenium diaminodiphosphine complexes

The reaction of N, N′-bis[o-(diphenylphosphino)benzylidene]-1,2-diaminoethane (L²) with one equivalent of RuCl₂(PPh₃)₃ in dichloromethane at room temperature gave trans-RuCl₂(PPh₃)(κ³-L²) 2 in high yield. When refluxed in toluene in air, 2 was converted quantitatively to trans-RuCl₂(κ⁴-L²) 3. When treated with one equivalent of hydrogen peroxide in chloroform, 2 was oxidized to trans-RuCl₂(PPh₃)(κ³-L ³) 4, in which the pendant phosphine has been oxidized to a phosphine oxide. Complex 4 can be further oxidized with another equivalent of hydrogen peroxide to trans-RuCl₂(PPh₃)(κ³-L⁴) 5, in which the amino group trans to PPh₃ has been oxidized to an imino group. When treated with excess hydrogen peroxide in ethanol, 3 was oxidized to trans-RuCl₂(κ⁴-L⁵) 6, in which the diamino moiety [-N(H)CH₂CH₂N(H)-] has been oxidized to a conjugated diimino moiety (-N=CHCH=N-). The solid-state structures of 2, 4, 5 and 6 were ascertained by X-ray crystallography. Catalytic studies show that 2 is an effective catalyst for the oxidation of alkanes, alkenes and alcohols with air or tert-butyl hydroperoxide. Experimental evidence suggests that free radicals are probably involved in the catalytic oxidation processes.