Factors influencing the nitrogen vs oxygen bonding mode of amides bound to pentaamminecobalt(III) and the kinetics and mechanism of rearrangement

Abstract

The preparation, characterization, properties, and rearrangements of a series of pentaamminecobalt(III) complexes of amides, RCONH2, are described (R = H, CH3, CF3, CH2Cl, CH2F, CH = CH2, C6H5, C6H4-4-F, C6H4-2-NO2). Some of the nitrogen-bonded amide complexes were synthesized by base-catalyzed hydration of [(NH3)5CoN = CR]3+; others, including those inaccessible by this route, were synthesized by linkage isomerization of the oxygen-bonded amide complexes, [(NH3)5CoOC-(NH2)R]3+, in coordinating but aprotic solvents containing noncoordinating base. The N-bonded amide products were isolated pure in both basic and acidic forms, [(NH3)5CoNHCOR]2+ and [(NH3)5CoNH = C(OH)R]3+. The former are thermodynamically stable, while the latter (pK'(a) < 4), although kinetically robust, are thermodynamically unstable with respect to the corresponding O-bonded linkage isomer and rearrange slowly in solution (t 1/2 hours, 25-degrees-C). The isomer equilibrium for amides as O- or N-bonding neutral ligands lies at least 100:1 to the side of the O-bonded isomer in sulfolane, in which neither ligand deprotonation nor solvolysis of either isomer could be detected. In coordinating solvents, [(NH3)5CoNH = C(OH)R]3+ also solvolyzes, at a rate comparable to that for competing N to O isomerization. For Me2SO these reactions have been identified by H-1 NMR measurements. The results require the isomer interconversion to be intramolecular. All the N-bonded amide complexes [(NH3)5CoNHCOR]2+ protonate at oxygen (in Me2SO-d6), producing [(NH3)5CoNH = C(OH)R]3+; the sole exception is the case R = CF3, which does not detectably protonate. The rate of N to O isomerization in sulfolane is dependent on the substituent R, but the rates span a range of only a factor of about 20. When the substituent can donate an electron pair (R = NH2, NHCH3, N(CH3)2, NHC6H5, OC2H5, OH), N to O isomerization is several orders of magnitude faster (t 1/2 seconds). The rate distinction between these two classes of isomerizing compounds is attributed to the different positions of the tautomeric equilibrium between N- and O-protonated forms of [(NH3)5CoNHCOR]2+ and the differences in reactivity between the tautomers. The solution structures for these tautomers in Me2SO are established by the H-1 NMR spectra. The O to N linkage isomerization was not observed in neutral aqueous solution because competing hydrolysis is faster. However the reaction can be forced in aprotic solvents in the presence of a noncoordinating base, and the propensity for this reaction is related to the ability of the O-coordinated neutral amide to dissociate a proton from the remote nitrogen (pK'(a) ca. 11, H2O, formamide-O, and acetamide-O). The mechanism is discussed and analogies are drawn with the Chapman rearrangement, which involves O to N migration of substituents on organic amides and imino esters. Factors that influence the interconversion of linkage isomers, including the site of protonation, isomer acidity, solvent, temperature, and amide substituents, are discussed and compared with related linkage isomeric complexes of pentaamminecobalt(III).