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 degrees C (0.10 M NaOH, 1.0 M NaN3) of [(NH3)(5)CoOC(CH3)NH2](2)(S2O6)(3). 3H(2)O and [(NH3)(5)CooC(CH3N(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)(5)CoOC(C6H5)NH2](2)(S2O6)(3) . 3H(2)O produced only 10.5% azido complex and this implied that 17% of the complex was reacting by carbon-oxygen bond cleavage. O-17 NMR spectroscopy of the products of base hydrolysis of [NH3)(5)CoOC(CH3)N(CH3)(2)](CF3SO3)(3) . H2O in 22 atom% H-2 O-17 confirmed that it reacted largely by ligand substitution but the same experiment using [(NH3)(5)CoOC(CH2F)NH2](2)(S2O6)(3) . 3H(2)O 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)(5)CoOC(C6H5)NH2](2)(S2O6)(3) . 3H(2)O, 19%; [(NH3)(5)CoOC(CH2F)NH2](2)(S2O6)3 . 3H(2)O, 10%; and [(NH3)(5)Co(succinimido-O)](CF3SO3)(2) . H2O, 9%. The reaction was not detected when [(NH3)(5)CoOC(CH3)NH2](2)(S2O6)(3) . 3H(2)O and [(NH3)(5)CoOC(CH3N(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)(5)CoOC(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. (C) 1999 Elsevier Science S.A. All rights reserved.