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.

Mechanistic Study of the Ru-Catalyzed Asymmetric Hydrogenation of Nonchelatable and Chelatabletert-Alkyl Ketones Using the Linear Tridentate sp3P/sp3NH/sp2N-Combined Ligand PN(H)N: RuNH- and RuNK-Involved Dual Catalytic Cycle

“…Chen reported that the p K a value of RuNH related to A-H was 14. , Assuming that A-H has the same level of acidity, p K a( A‑H ) = −log­(10 –15.5 × K eq ) = 14 is established, deducing K eq = 31.6. Substitution to a = ( K eq H + 1)/ K eq ( K eq K + 1) = 0.000 194 gives 163 K eq H + 162 = K eq K , indicating that K eq K > 162 and K eq K / K eq H > 163 as both of K eq H and K eq K are greater than 0: Most of A-K moves to B-K , and the easiness of the deprotonation is two orders higher than that of A-H .…”

Iron(II) Complexes with Scorpiand-Like Macrocyclic Polyamines: Kinetico-Mechanistic Aspects of Complex Formation and Oxidative Dehydrogenation of Coordinated Amines

“…It is interesting to note that the disproportionation step occurs with first order kinetics both in acetonitrile and water, although in the latter solvent it occurs with initial proton dissociation because this process is favored by the protic nature of the solvent. In the literature, there are examples of oxidative dehydrogenations for which the disproportionation occurs with both second ,, and first ,,,, order kinetics with respect to the metal complex, and there are even some cases in which the rate constants for all of the individual steps have been resolved . Therefore, the rate constants for the different processes involved in disproportionation appear to be delicately balanced for each particular complex.…”

The versatile ruthenium(II/III) tetraazamacrocycle complexes and their nitrosyl derivatives

“…In ruthenium complexes the initial step in the process is the pHdependent one-electron oxidation of Ru(II) to Ru(III) followed by a complex series of reactions which involve amine deprotonation and Ru(III) disproportionation into Ru(II) and Ru(IV), as occurs for [Ru III (en) 3 ] 3+ , [Ru III (tame) 2 ] 3+ and [Ru III (sar)] 3+ complexes whose reactions result in Ru(II)-imine complexes. These reactions are favored for ruthenium for its ability to attain higher oxidation states [152,154]. Thus, synthesis of unsaturated macrocyclic amine complexes can be designed through the oxidative dehydrogenation of the unsaturated macrocyclic parent complexes.…”