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.

A stable yellow derivative of cobyrinic acid heptamethyl ester, (5R,6R)-Coα,Coβ-dicyano-5,6-dihydro-5-hydroxy-heptamethylcob(III)yrinate-c,6-lactone (DCSYCbs), was prepared from dicyanocobyrinic acid heptamethyl ester (DCCbs). The C5 carbon is oxidized and the c side chain cyclized to form a lactone at C6; the 13 atom, 14 π-e(-) delocalized system of corrins is interrupted, giving a triazamethine system with four conjugated double bonds between N22 and N24 and an isolated double bond between N21 and C4. Stable yellow aquacyanocobyrinic acid heptamethyl ester (ACSYCbs) was prepared by driving off HCN with N(2) in a methanol/acetic acid solution. The electronic spectra of DCCbs and DCSYCbs appear similar except that the bands in DCSYCbs are shifted to shorter wavelengths and the γ-band is much less intense. The experimental spectra were adequately modeled using TD-DFT at the PBE1PBE/6-311G(d,p) level of theory. DCSYCbs crystallizes in the space group P2(1)2(1)2(1) (R(1) = 6.08%) with Z = 4, including one methanol solvent molecule and one water molecule per cobester. The addition of a hydroxyl group at C5 causes loss of the double bond between C5 and C6 and elongation of the C5-C6 bond. From a combination of two-dimensional (1)H TOCSY and ROESY NMR spectra and (1)H/(13)C HSQC and HMBC data, the complete (1)H and (13)C NMR assignments of DCSYCbs were possible, except for two of the ester methyl groups and the (13)C resonances of the two axial cyanide ligands. The latter were assigned using relative chemical shifts calculated by GIAO-DFT methods. The (59)Co resonance of DCCbs was observed at 4074 ppm while that of DCSYCbs is shifted downfield to 4298 ppm. Comparison with available (59)Co data of analogous systems suggests that the more π-conjugated corrin of DCCbs interacts more strongly with the metal than the less extensively conjugated macrocycle of DCSYCbs. As the strength of the interaction between Co(III) and an equatorial macrocycle increases, ν(CN) of axially coordinated CN(-) shifts to lower frequency; in DCSYCbs and DCCbs ν(CN) occurs at 2138 and 2123 cm(-1), respectively. Hence the corrin ligand in DCCbs interacts more strongly with the metal than the stable yellow corrin ligand, with its diminished conjugation. The UV-vis spectral data and DFT-calculated MOs are consistent with greater overlap between the corrin and the metal orbitals in DCCbs relative to DCSYCbs, which gives the metal in the former a softer, more covalent character.

Neopentylcobalamin in neutral solution at 25 "C is stable under nitrogen but is decomposed by 0, and by imidazole to give a cob(lii)alamin and by hydrogen-atom donors such as thiols and Pr'OH to give the cob(l1)alamin and neopentane. The reaction is ascribed to reversible homolytic fission of the Co-C bond to give a low steadystate concentration of the cob(1i)alamin and neopentyl radicals, which can then form neopentane by the abstraction of a hydrogen atom. Neopentylcobinamide is also decomposed by high concentrations of imidazole, while the purely five-co-ordinate neopentylcobalamin in acid and neopentylcobinamide in neutral solution are stable both in the presence and absence of 0,. Labilisation of the Co-C bond is ascribed to steric distortion around the coordinated C atom in the six-co-ordinate neopentyl complexes. it is suggested that neopentylcobalamin provides a model for the labilisation of the Co-C bond in the vitamin B,, coenzymes, and possible mechanismsfor the isomerase reactions are discussed.

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