Oxygen versus Nitrogen Bonding of Carboxamides to Pentaammineruthenium(II/III).
Reversible linkage isomerizations are identified for monodentate carboxamides on pentaammineruthenium(II) and -(III). The equilibrium between O-bonded and N-bonded amides is pH and oxidation-state dependent. When both O- and N-bound amides are neutral uncharged ligands (pH < 7, Ru(II); pH < 2, Ru(III)), the O-bonded isomers are thermodynamically more stable for both oxidation states. They are, however, inherently unstable, solvolyzing in coordinating solvents with loss of amide ligand (Ru(II), t(1/2) < 1 s; Ru(III), t(1/2) < 2 h; 25 degrees C, H(2)O) or isomerizing to deprotonated N-bonded isomers in nonacidic solutions. Cyclovoltammetry in base produces reversible Ru(III/II) couples for the substitution-inert deprotonated form, [(NH(3))(5)RuNHCOR](2+/+), which protonates for Ru(II) in acidic and neutral solutions (pK(a) approximately 7) and isomerizes to the O-bonded amide. Following oxidation to Ru(III), isomerization of the O-bonded amides (pK(a) >/= 10) back to the N-bonded amides (pK(a) </= 2) is driven by selective deprotonation to [(NH(3))(5)Ru(III)NHCOR](2+). In water (pH 6.2, 25 degrees C) the O-bonded amides on Ru(III) isomerize ( approximately 50%) in parallel with aquation (R = H, k(obs) = k(ON) + k(aq) = 8.1 x 10(-)(4) s(-)(1)), and both processes are catalyzed by base and Ru(II). In strong acid, Ru(III) complexes of uncharged N-bound amides are thermodynamically unstable but kinetically robust (t(1/2) > 6 days, 18 degrees C, H(2)O) as the stable iminol tautomer, [(NH(3))(5)RuNH=C(OH)R](3+). This has to tautomerize to [(NH(3))(5)RuNH(2)COR](3+) before N- to O-isomerization which is much slower than subsequent solvolysis of the O-bonded isomer and hence undetectable in coordinating solvents. The substitution lability of O-bonded amides in coordinating solvents, tautomerization of N-bonded amides on Ru(III), and catalysis by Ru(II) and base complicate measurements of the rates for linkage isomerizations.
Fairlie
David P.
DP
Department of Chemistry, Stanford University, Stanford, California 94305.
Ilan
Yigal
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Taube
Henry
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