NITROGEN-BONDED AND OXYGEN-BONDED URETHANE - HYDROLYSIS AND LINKAGE ISOMERIZATION OF [(NH3)5CONH2CO2C2H5]3+ AND [(NH3)5COOC(NH2)OCH2CH3]3+

Coordination of ethyl carbamate via N rather than O to the pentaamminecobalt(III) moiety results in activation toward ligand hydrolysis. The carbonyl oxygen of the carbamate anion and of its ethyl ester (urethane) are preferentially coordinated in kinetically controlled syntheses. Noncoordinating bases facilitate the thermal rearrangement of the O-bound urethane complex in solution to the N-bonded linkage isomer, which has been isolated as its unreactive deprotonated form [(NH3)5CoNHCO2C2H5]2+. Similarly, the N-bound carbamate complex, previously believed to be an intermediate in the acid-induced conversion of [(NH3)5CoNCO]2+ to [(NH3)6Co]3+ and CO2, has been detected for the first time in this reaction by quenching with strong base and chromatographic isolation as the unreactive ion [(NH3)5CoNHCO2]+. In acid solution, the N-coordinated ethyl carbamate complex protonates, giving the reactive entity [(NH3)5CoNH2CO2C2H5]3+. Its acidity constant (pK′a = 0.38, 25 °C, I = 1.0 M, NaClO4) has been determined kinetically, and this value and the specific rate of decay (k(obsd) = 1.21 × 10-2 s-1, 25 °C, H2O) are very similar to the corresponding numbers for the N-bonded carbamate species. The product distribution for the reaction of the [(NH3)5CoNH2CO2C2H5]3+ ion in water is acid-independent (0.1-1.2 M H+; I = 1.0 M, NaClO4, 25 °C), consistent with a single reactive species. The products are [(NH3)6Co]3+ (40%), [(NH3)5CoOC(NH2)OCH2CH3]3+ (36%), and [(NH3)5CoOH2]3+ (24%). These products arise through three parallel reactions: ligand degradation (CoN-C cleavage, kdeg = 4.84 × 10-3 s-1), intramolecular linkage isomerization (kNO = 4.36 × 10-3 s-1), and normal aquation (Co-O cleavage, kaq = 2.90 × 10-3 s-1). No O-bonded carbamate complex [(NH3)5CoOC(NH2)O]2+ was detected as a product, even for larger scale product distribution experiments. This observation excludes an elimination pathway to the N-bonded cyanate complex, and it also excludes a primary hydrolysis path of the N-bonded ester complex to the corresponding N-bonded acid or acidate, [(NH3)5CoNH2C02H]3+/ [(NH3)5CoNH2C02]2+, since the product distribution for reaction of the latter is acid-dependent, and it results in 2% O-bonded carbamate product at 1 M HCl04. Thus, the ester complex [(NH3)5CoNH2C02C2H5]3+ would appear to decay directly to [(NH3)6Co]3+, CO2, and ethanol, and the rate is many orders of magnitude greater than that for reaction of the free urethane ligand. The O-bonded ethyl carbamate complex forms only [(NH3)5CoOH]2+ in aqueous base and [(NH3)5CoOH2]3+ in aqueous acid (pH < 4; ks = 5.95 × 10-5 s-1, 1 M HClO4, 25 °C). However, at pH values above the pK′a of the N-bonded species but not so high as to induce base-catalyzed reaction, some O- to N-linkage isomerization (2%) is detectable (pH = 6.3, 0.1 M NaMES buffer, I = 1.0 M, NaClO4). In dimethyl sulfoxide, only solvolysis is observable. The N-bonded/O-bonded isomer equilibrium is pH-dependent owing to the different acidities of the N- and O-bonded ethyl carbamate complexes. Coordination through the carbonyl oxygen is preferred in aqueous acid (K′NO = 3570) and in nonaqueous solution, where the proton of the N-bonded isomer is not dissociated. The N-bonded isomer becomes substantially more stable when coordinated as an anionic ligand (pH = 6.3, K′NO(obsd) = K′NO[H+]/([H+] + K′a) = 0.004). © 1990, American Chemical Society. All rights reserved.