Electroreduction of a Series of Alkylcobalamins: Mechanism of Stepwise Reductive Cleavage of the Co-C Bond
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- 作者：Ronald L. Birke ; Qingdong Huang ; Tudor Spataru ; and David K. Gosser ; Jr.
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：February 15, 2006
- 年：2006
- 卷：128
- 期：6
- 页码：1922 - 1936
- 全文大小：227K
- 年卷期：v.128,no.6(February 15, 2006)
- ISSN：1520-5126
文摘
The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperatureand solvent ratio vs a nonaqueous Ag/AgCl/Cl- reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over thetemperature range from 0 to -80 
C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8± 0.5 and 7.6 ± 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulationand curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 × 103s-1 give E ' = -1.466 V, k = 0.016 cm/s, and 
= 0.77 at 20 C for a quasi-reversible electrode process.Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) showsthat the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible stepfollowing the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants ofthese reactions. An electron coupling matrix element of 
kA = (4.7 ± 1.1) × 10-4 eV (~46 J/mol) is calculatedfor the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy forhomolytic cleavage of Me-Cbl is calculated as 31 ± 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-,n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentialswere correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations.
C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8± 0.5 and 7.6 ± 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulationand curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 × 103s-1 give E ' = -1.466 V, k = 0.016 cm/s, and = 0.77 at 20 C for a quasi-reversible electrode process.Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) showsthat the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible stepfollowing the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants ofthese reactions. An electron coupling matrix element of kA = (4.7 ± 1.1) × 10-4 eV (~46 J/mol) is calculatedfor the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy forhomolytic cleavage of Me-Cbl is calculated as 31 ± 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-,n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentialswere correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations.