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Kinetics and mechanism of decarboxylation of cis-carbonato- and bicarbonato(3,3′,3′′-triaminotripropylamine)cobalt(III) ions

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Abstract

The acid-catalysed hydrolysis of [Co(trpn)(CO3)]+ and [Co(trpn)(HCO3)]2+ ions (trpn = 3,3′,3′′-triaminotripropylamine) have been studied spectrophotometrically in aqueous 1.0m HClO4/NaClO4. For the carbonato complex, [HClO4] = 0.02–0.25m and T = 20–35°C; for the bicarbonato complex, [HClO4] = 0.025–0.30m and T = 25°C. Both complexes hydrolyse to form the same cis-diaqua species. The rate law for the hydrolysis is d(ln[CoIII])/dt = k0+k1[H3O+]. The values of the rate constants (25°C), ΔH‡ (kJmol−1) and ΔS‡ (Jmol−1K−1)are:[Co(trpn)(CO3)]+,k0 = (1.7±0.6)× 10−4s−1, ΔH‡ 0 = 57±21, Δ S‡ 0 = −126±75; k1 = (1.0 ±0.1)×10−2m−1s−1, ΔHDagger;1 = 62±8, Δ S‡1 = −75 ±21, and for [Co(trpn)(HCO3)]2+, k0 = (2.9±0.7)× 10−4s−1, and k1 = (7.8±1.0)×10−2m−1s−1. The carbonato complex exhibits a deuterium isotope effect with k1D/k1H = 1.9, consistent with a rapid pre-equilibrium protonation, followed by rate-controlling ring-opening. The rate constants k0 and k1 (25°C and μ = 1.0m) for the ring-opening decarboxylation of the two systems studied lie within the experimental error. The results are compared with o ther related systems and the factors which influence the ring-opening decarboxylation (steric hindrance, ring strain, electron-donor ability of the amines) are discussed. The k1 path is interpreted in terms of concerted ring-opening and bond-making in the highly unstable aquabicarbonato intermediate.

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References

  1. Ligand abbreviations: trpn = 3,3′,3″-triaminotripropylamine; tren = 2,22,2″-triaminotriethylamine; trien = 1,8-diamino-3,6-diazaoctane; Me2trien = 3,6-dimethyl-1,8-diamino-3,6-diazaoctane; en = 1,2-diaminoethane; pn = 1,2-diaminopropane; tn = 1,3-diaminopropane; bmen = N, N2-dimethyl-1,2-diaminoethane; tme = 2,3-diamino-2,3-dimethylbutane; py = pyridine.

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  1. Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, Alexandria, 21321, Egypt

    Salah S. Massoud

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Massoud, S.S. Kinetics and mechanism of decarboxylation of cis-carbonato- and bicarbonato(3,3′,3′′-triaminotripropylamine)cobalt(III) ions. Transition Metal Chemistry 22, 304–308 (1997). https://doi.org/10.1023/A:1018485012656

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