Reversible linkage isomerization of pentaamminecobalt(III) complexes of urea and its N-methyl derivatives

• Published in: Inorganica Chimica Acta Vol. 150; no. 1; pp. 81 - 100
• Main Authors: Fairlie, D.P., Jackson, W.G.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.1988
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(00)87628-8

The (NH 3) 5CoOC(NH 2) 2 3+ ion is consumed in water according to the rate law k(obs.) = k 1 + k 2[OH −], where k 1 = 4.0 × 10 −5 s −1 and k 2 = 14.2 M −1 s −1 (0–0.1 M [OH −];μ = 1.1 M, NaClO 4, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH 3) 5CoOH 2 3+ is observed, but in 0.1–1.0 M [OH −], 7% of the directly formed products is the urea- N complex (NH 3) 5CoNHCONH 2 2+ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea- N species. These results are interpreted in terms of an urea- O to urea- N linkage isomerization reaction competing with hydrolysis for both spontaneous ( k 1) and base-catalyzed ( k 2) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (p K′ a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k 2[OH −]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k 1 + k 2[OH −] = ( k s + k ON) + ( k OH + k′ ON) [OH −] for the reactions of the O-bonded isomers, where k s, k OH are the specific rates for hydrolysis, and k ON, k′ ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; k ON = 1.3 × 10 −5 s −1 and k′ ON = 1.1 M −1 s −1 (μ = 1.0 M, NaClO 4, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N, N-dimethylurea and N-phenylurea, but not for the N, N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH 2 capture ( versus H 2O), while −NHR and −NR 2 do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea- O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H 2O, Me 2SO) is discussed. The urea- N complexes have been isolated as their deprotonated forms, [(NH 3) 5CoNHCONRR′](ClO 4) 2· xH 2O (R,R′ = H, CH 3). They are kinetically inert in neutral to basic solution but in acid they protonate (H 2O, p K′ a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H 2O and Me 2SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form k NO(obs.) = ( k + k NO)[H +]/( K′ a + [H +]) and the active species in the reaction is the protonated form; k, k NO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me 2SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH 3) 5CoNH 2CONH 2 3+, diastereotopic exo-NH 2 protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. 13C and 1H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea- N/urea- O complex equilibrium is governed by the relation K′ NO(obs.) = K′ NO[H +]/[H +]( K′ a), where K′ NO is the equilibrium constant = [(NH 3 5Co(urea- O) 3+]/[(NH 3) 5Co(urea- N) 3+]. Values for K′ NO(= k NO/ k ON = 260 and p K′ a ≈ 3 for the NH 2CONH 2 system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′ NO(obs.) = 2.6 × 10 −2) and instability in acid solution (e.g. pH 1, K′ NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH 3) 5Co(III) are contrasted with the result for the analogous Rh(III)NH 2CONH 2 system K′ NO ≈ 1).