Carbon–oxygen bond cleavage: a new reaction mode for amides bonded through oxygen to cobalt(III)

Azide ion competition experiments and oxygen-17 and -18 studies have been used to determine the extent of cleavage of the coordinated oxygen–carbon bond when (amide-O)pentaamminecobalt(III) complexes were reacted in aqueous base. Base hydrolysis in the presence of azide ion at 22°C (0.10 M NaOH, 1.0 M NaN3) of [(NH3)5CoOC(CH3)NH2]2(S2O6)3·3H2O and [(NH3)5CoOC(CH3)N(CH3)2](CF3SO3)3·H2O produced 12.5 and 13.0% azido complex respectively, as expected for tripositive complexes reacting solely by cobalt–ligand bond cleavage. However reaction of [(NH3)5CoOC(C6H5)NH2]2(S2O6)3·3H2O produced only 10.5% azido complex and this implied that 17% of the complex was reacting by carbon–oxygen bond cleavage. 17O NMR spectroscopy of the products of base hydrolysis of [(NH3)5CoOC(CH3)N(CH3)2](CF3SO3)3·H2O in 22 atom% H2 17O confirmed that it reacted largely by ligand substitution but the same experiment using [(NH3)5CoOC(CH2F)NH2]2(S2O6)3·3H2O was inconclusive because of secondary hydrolysis of the liberated amide. Oxygen-18 analysis of coordinated water has demonstrated conclusively that for complexes of primary or secondary amides with electron-withdrawing substituents carbon–oxygen bond cleavage in aqueous base is a significant reaction: [(NH3)5CoOC(C6H5)NH2]2(S2O6)3·3H2O, 19%; [(NH3)5CoOC(CH2F)NH2]2(S2O6)3·3H2O, 10%; and [(NH3)5Co(succinimido-O)](CF3SO3)2·H2O, 9%. The reaction was not detected when [(NH3)5CoOC(CH3)NH2]2(S2O6)3·3H2O and [(NH3)5CoOC(CH3)N(CH3)2](CF3SO3)3·H2O were dissolved in aqueous base as these amides are not electronically activated, nor was it detected for reaction of [(NH3)5CoOC(H)N(CH3)2](CF3SO3)3·H2O; this complex is activated towards addition by hydroxide ion (84% C–N cleavage) but lacks an ionisable proton on the amide nitrogen to facilitate proton transfer in the intermediate in the C–O bond rupture process.