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

• Published in: Inorganic chemistry Vol. 36; no. 13; pp. 2804 - 2815
• Main Authors: Bernhard, Paul, Bull, Daryl J, Bürgi, Hans-Beat, Osvath, Peter, Raselli, Andrea, Sargeson, Alan M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 18.06.1997
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic961021q

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, [RuIII(tame)2]3+ undergoes rapid initial deprotonation (pK III = 10.3). This is followed by a pH-dependent disproportionation step involving either [RuIII(tame)2-H+]2+ + [RuIII(tame)2]3+ (k 1d = 8300 M-1 s-1) or two singly deprotonated [RuIII(tame)2-H+]2+ ions (k 2d = 3900 M-1 s-1). The products are [RuII(tame)2]2+ and either the singly deprotonated species [RuIV(tame)2-H+]3+ (pK IV = 8.2) or the doubly deprotonated [RuIV(tame)2-2H+]2+. These Ru(IV) complexes undergo spontaneous dehydrogenation to give the imine [RuII(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 [RuIII(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 [RuIV(en)3-H+]3+ (pK IV = 8.9) and its deprotonated counterpart, [RuIV(en)3-2H+]2+, undergo dehydrogenation to give [RuII(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 [RuIII(sar)]3+ ion are reexamined (k 1d = 4 × 107 M-1 s-1, k 2d ≥ 2 × 107 M-1 s-1, pK IV = 2.0, k 1im = 17 s-1, k 2im = 5 × 10-4 s-1 at 25 °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.