Ligand dehydrogenation in ruthenium-amine complexes: Reactivity of 1,2-ethanediamine and 1,1,1-tris(aminomethyl)ethane

The mechanisms of oxidative ligand dehydrogenation in high-valent ruthenium hexaamine complexes of bidentate 1,2-ethanediamine (en) and tridentate 1,1,l-tris(aminomethyl)ethane (tame) are elucidated in detail. In basic aqueous solution, [Ru-III(tame)(2)](3+) undergoes rapid initial deprotonation (pK(III) = 10.3) This is followed by a pH-dependent disproportionation step involving either [Ru-III(tame)(2)-H+](2+) [Ru-III(tame)(2)](3+) (k(1d) = 8300 M-1 s(-1)) or two singly deprotonated [Ru-III(tame)(2)-H+](2+) ions (k(2d) = 3900 M-1 s(-1)). The products are: [Ru-II(tame)(2)](2+) and either the singly deproronated species [R-IV(tame)(2)-H+](3+) (pK(IV) = 8.2) or the doubly deprotonated [Ru-IV(tame)(2)-2H(+)](2+). These Ru(IV) complexes undergo spontaneous dehydrogenation to give the imine [Ru-II(imtame)(tame)](2+) (imtame = 1,1-bis(aminomethyl)-1-(iminomethyl)ethane), with first-order rate constants of k(1/m) = 320 s(-1) and k(2im) = 1.1 s(-1), respectively, In the [Ru-III(en)(3)](3+) system, the initial deprotonation (pK(III)= 10.4) is followed by the corresponding disproportionation reactions (k(1d) = 9000 M-1 s(-1), k(2d) = 3800 M-1 s(-1)). The complex [Ru-IV-(en)(3)-H+](3+) (pK(IV) = 8.9) and its deprotonated counterpart, [Ru-IV(en)(3)-2H(+)](2+), undergo dehydrogenation to give [Ru-II(imen)(en)(2)](2+) (imen = 2-aminoethanimine) with first-order rate constants of k(1im) = 600 s(-1) and k(2im) = 1.0 s(-1), respectively. In the light of this analysis, the disproportionation and ligand oxidation of the [Ru-III(sar)](3+) ion are reexamined (k(1d) = 4 x 10(7) M-1 s(-1), k(2d) greater than or equal to 2 x 10(7) M-1, pK(IV) = 2.0, k(1im) = 17 s(-1), k(2im) = 5 x 10(-4) s(-1) at 25 degrees C). While the disproportionation to Ru(II) and Ru(IV) has been recognized in such systems, the complexity of the paths has not been realized previously; the Surprising variation in the rates of the intramolecular redox reaction (from days to milliseconds) is now dissected and understood. Other facets of the intramolecular redox reaction are also analyzed.