Ligand Dehydrogenation in Ruthenium-Amine Complexes: Reactivity of 1,2-Ethanediamine and 1,1,1-Tris(aminomethyl)ethane.

Abstract

The mechanisms of oxidative ligand dehydrogenation in high-valent ruthenium hexaamine complexes of bidentate 1,2-ethanediamine (en) and tridentate 1,1,1-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 deprotonated species [Ru(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(1im) = 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) >/= 2 x 10(7) M(-)(1) s(-)(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.