The Electrochemical Evaluation of the Metal–Carbon Bond Energies (−ΔGBF) of Alkylated Iron and Cobalt Porphyrins [(por)M–R]
✍ Scribed by Aimin Qiu; Donald T. Sawyer
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 267 KB
- Volume
- 01
- Category
- Article
- ISSN
- 1088-4246
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✦ Synopsis
The electron-transfer oxidation-reduction chemistry for the alkyl derivatives of iron and cobalt porphyrins [(por)M III -R] has been characterized on the basis of cyclic voltammetric and controlled-potential-electrolysis measurements. The electrogenerated anions of iron and cobalt porphyrins[(por)M -and (por -•)M -] are strong nucleophiles that react with alkyl halides (RX) via a nucleophilic displacement process to form metal-carbon bonds [(por)M-R and (por -•)M-R]. The difference in the reduction potentials for RX and (por)M II provides an approximate measure of the (por)M-R bond-formation free energy (-∆G BF ). The -∆G BF values for iron porphyrins (14-35 kcal mol -1 ) and for cobalt porphyrins (20-38 kcal mol -1 ) depend on the electron density of the porphyrin ring (OEP > TPP > Cl 8 TPP > F 20 TPP) and the structure of the alkyl group (1°> 2°> 3°). Thus, the apparent metal-carbon bond energy (-∆G BF ) for (OEP)Fe III -Bu-n is 28 ± 2 kcal mol -1 , and for [(MeO) 4 TPP]Co III -Bu-n is 36 ± 2 kcal mol -1 . The (por -•)M -dianions react with carbon dioxide in an electrocatalysed reduction cycle to give CO and CO 3 2-via the apparent transient formation of a metal-carbon bond [(por -•)M-C(O)O -; -∆ G BF ≥ 12 kcal mol -1 for iron porphyrins].