Oxygen versus Nitrogen Bonding of Carboxamides to Pentaammineruthenium(II/III)

Abstract

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.