Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. Wits, Johannesburg, 2050 South Africa.
Inorg Chem. 2011 Sep 19;50(18):8719-27. doi: 10.1021/ic200288b. Epub 2011 Aug 18.
Equilibrium constants (log K) for the substitution of coordinated H(2)O in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, ACCbs) and stable yellow aquacyanocobyrinic acid heptamethyl ester (stable yellow aquacyanocobester, ACSYCbs), in which oxidation of the C5 carbon of the corrin interrupts the normal delocalized system of corrins, by ligands with soft (CN(-), SO(3)(2-), and S(2)O(3)(2-)) and hard (NO(2)(-) and N(3)(-)) donors have been determined. The ligands with a harder donor atom (N in N(3)(-) and NO(2)(-)) produce ΔH values that are more negative in their reactions with ACSYCbs than with ACCbs. If the donor atom is softer (C in CN(-) and S in SO(3)(2-)), then ΔH is less positive, or more negative, for reactions with ACCbs than with ACSYCbs. The softer metal in ACCbs has a preference for softer ligands and the harder metal in ACSYCbs for the harder ligands. A kinetics study in which CN(-) substitutes H(2)O on Co(III) shows that ACCbs is more labile than ACSYCbs; the second-order rate constant k(II) is between 4.6 (at 5 °C) and 2.6 (at 35 °C) times larger. ΔH(‡) for the reaction of CN(-) with ACCbs is smaller by some 12 kJ mol(-1) than that for the reaction with ACSYCbs, consistent with an earlier transition state in which bonding between the softer metal of ACCbs and the ligand is greater than that of ACSYCbs with its harder metal. This difference in ΔH(‡) makes ACCbs over 100 times more labile, although the effect is masked by a ΔS(‡) value that is over 30 J K(-1) mol(-1) more negative. There is a significant increase in the inertness of Co(III) upon a decrease in the extent of conjugation of the corrin ligand. Modifying the electronic structure of the equatorial ligand in the cobalt corrins can modify the thermodynamics and kinetics of its reactions with exogenous ligands.
平衡常数（log K）测定了取代配位 H(2)O 的反应，配位 H(2)O 位于水合氰钴胺酸七甲基酯（aquacyanocobyrinic acid heptamethyl ester，ACCbs）和稳定的黄色水合氰钴胺酸七甲基酯（stable yellow aquacyanocobyrinic acid heptamethyl ester，ACSYCbs）中，这两种物质中钴卟啉的 C5 位发生了氧化，从而破坏了钴卟啉的离域体系。反应中涉及的配体包括软配体（CN(-)、SO(3)(2-) 和 S(2)O(3)(2-)）和硬配体（NO(2)(-) 和 N(3)(-)）。与 ACSYCbs 相比，含更硬供体原子（N 位于 N(3)(-)和 NO(2)(-)中）的配体与 ACSYCbs 反应时，ΔH 值更负。如果供体原子较软（C 位于 CN(-)中，S 位于 SO(3)(2-)中），则与 ACCbs 反应的ΔH 为正值或负值，且比与 ACSYCbs 反应的ΔH 更小。在 ACCbs 中，较软的金属更喜欢软的配体，而在 ACSYCbs 中，较硬的金属则更喜欢硬的配体。动力学研究表明，CN(-)取代 Co(III)上的 H(2)O，ACCbs 比 ACSYCbs 更不稳定；二级反应速率常数 k(II)在 5°C 时为 4.6 倍，在 35°C 时为 2.6 倍。与 ACSYCbs 相比，CN(-)与 ACCbs 反应的ΔH(‡)小约 12 kJ mol(-1)，这与较早的过渡态一致，在该过渡态中，ACCbs 中较软金属与配体之间的键合强于 ACSYCbs 中较硬金属与配体之间的键合。尽管由于ΔS(‡)值负向超过 30 J K(-1) mol(-1)而掩盖了这种影响，但这种ΔH(‡)的差异使 ACCbs 的活泼性提高了 100 多倍。当钴卟啉配体的共轭程度降低时，Co(III)的惰性显著增加。修饰钴卟啉中轴向配体的电子结构可以改变其与外源配体的反应热力学和动力学。
