Stepwise Dehydrogenation of a Ru(III) Hexaamine Cage Complex to a Hexaimine Ru(II) Complex via Ru(IV) Intermediates

Article abstract of DOI:10.1021/ja00184a030

The sexidentate ligand sarcophagine (sar: 3,6,10,13,16,19-hexaazabicyclo<6.6.6>eicosane) when coordinated to ruthenium(III) (1) rapidly undergoes oxidative dehydrogenation to introduce an imine group into the cap portion of the ligand.Successive two-electron oxidations lead to a stable Ru(II) hexaimine complex, with all the imine groups in the cage caps.Each of the imine groups stabilizes the Ru(II) state by ca. 0.15 V leading to a stability of the Ru(II) hexaimine complex which is comparable to that of Ru(2,2'-bipyridine)3(2+).Ru(sar)(3+) disproportionates in aqueous solution to Ru(II)sar(2+) and a singly deprotonated Ru(IV) intermediate (λ_max(ε_max): 445 nm (7800 M^-1cm^-1)) which is converted into the Ru(II) imine product by both a base- and an acid-catalyzed pathway.Intermediate di- and triimine complexes were also observed en route to the hexaimine species.The kinetic and thermodynamic data for the disproportionation process imply that the secondary nitrogen in Ru(sar)(3+) is quite acidic (pK_a: 5-6) and that the Ru(IV) state is stabilized by >/= 2 V.Acid catalysis of the hydrogenation process from (3+) as well as a reversible hydration of Ru(imsar)(2+) below pH 2.5 are interpreted in terms of protonation at the metal center.