Cobalt-carbon bond energies

Investigations of the cobalt-carbon bond energies in organometallic cobalt porphyrins continue to attract interest, originally because of their similarity to coenzyme B 2, and more recently because of their role in the catalysis of free radical... 

Cobalt-Carbon Bond Homolysis Studies with coenzyme Bi2 model compounds and TEMPO as a radical trap have allowed the determination of rate constants and activation parameters, which in turn led to estimates of the Co-C bond dissociation energies (BDEs). The key role of the homolytic Co-C cleaving step in these reactions was established by the following observations (Co11) is produced addition of external... 

In summary, the electrochemical results indicate that the alkyl-metal bond-formation free energies range from 54 to 146 kJ mol-1 for iron porphyrins and from 84 to 159 kJ mol-1 for cobalt porphyrins. The maximum bond energies are for primary alkyl groups bonded to [(MeO)4TPP]Con and (OEP)Fen porphyrins. The porphyrin dianions [(porT)nFe and (porr)nCo ] facilitate the reduction of C02 to CO via the transient formation of a metal-carbon bond [(por7)M—C(0)0- — AGBF > 50 kJ mol-1 for iron porphyrins]. Thus, iron and cobalt porphyrins are especially effective electrocatalysts for the reduction of C02 ... 

The electronic absorption spectrum of the complex shows an intense band at 295 nm (e = 18,000), but ligand field (LF) bands were not observed at lower energy. The intense band was assigned as a benzyl-to-cobalt CT transition, and it was proposed that the photoreaction proceeds through population of such a CT state leading to homolytic cleavage of the cobalt-carbon bond. 

The thermal decomposition of cobalt carbonate fitted the Avrami-Erofeev equation with n = 1.37 at 493 K and 2.54 at 543 K [74]. These data were later reanalyzed by Engberg [75] who allowed for an initial reaction, 0.02 and found , = 95 kJ mol, which is close to the Co-O bond dissociation energy (91 kJ mol ). 

Free Energy Profile for Cobalt-Carbon Bond Cleavage and H-atom Transfer Steps 1487... 

Determination of Cobalt-Carbon Bond Dissociation Energies... 

COBALT CARBON BOND ENERGIES. The cobalt-carbon bond dissociation energy has been reported (18) for a series of base adducts of alkyl-substituted bis (dimethylglyoximato) cobalt(II),... 

We have rewritten Equation 8 as cobalt-carbon bond formation and expressed it as the sum of a constant energy process (W), an adduct dissociation reaction and an adduct formation reaction to produce the desired reaction, Equation 13. Steps 11 and 12 are the type reaction treated by ECW leading respectively to Equations 14 and 15. 

Table II. Cobalt-Carbon Bond Dis sociation Energies, -AH, for R-Co(DH)2 B Adducts...

Drago, R. S., Wong, N., Bilgrien, C., Vogel, C. (1987). E and C parameters from Hammett substituent constants and use of E and C to understand cobalt-carbon bond energies. Inorg. Chem. 26, 9-14. 

Qiu, A. and D.T. Sawyer (1997). The electrochemical evaluation of the metal-carbon bond energies (—AGbf) of alkylated iron and cobalt porphyrins [(por)-M-R]. J. Porphyrins Phthalocyanines 1, 125-134. 

The strength of cobalt-carbon bonds has been extensively investigated because of the presence of a Co-C bond in coenzyme Bj. Reported cobalt-alkyl bond dissociation energies range from 17 to 37 kcal/mol in complexes of the type Co(N -macrocyde)R, with and without axial bases. Much of the variation in Co-C bond energy appears to result from steric factors. Indeed, structural data reveal a steric influence on the length of the Co-C bond. ... 

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