Influence of organic ligands on the reduction of polyhalogenated alkanes by iron(II)

Experimental work demonstrates that polyhalogenated alkanes (PHAs) are rapidly reduced in aqueous solutions containing Fe^II complexes with organic ligands that possess either catechol or organothiol Lewis base groups in their structure and are representative of extracellular ligands and metal-complexing moieties within humic substances (tiron, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid, thioglycolic acid, and 2,3-dimercaptosuccinic acid). In solutions containing Fe^II-tiron complexes, 1,1,1-trichloroethane (1,1,1-TCA) is reduced quantitatively to acetaldehyde, a product previously reported for reactions with Cr^II, but not with Fe-based reductants. Observed pseudo-first-order rate constants for 1,1,1-TCA reduction by Fe^II-organic complexes (k′_obs) generally increase with increasing pH and ligand concentration when Fe ^II concentration is fixed. For the Fe^II-tiron system, k′_obS is linearly correlated with the concentration of the 1:2 Fe^II-tiron complex (FeL₂^6-), and kinetic trends can be described by k′_obs = k_FeL2^6- [FeL₂^6-], where k_Fel2^6- is the bimolecular rate constant for PHA reaction with the 1:2 Fe^II-tiron complex. Comparing reaction rates for 14 polyhalogenated ethanes and methanes reveals linear free energy relationships (LFERs) with molecular descriptors for PHA reduction (D_R-X′, ΔG⁰′, and E _LUMO), with the strongest correlation being obtained using carbon-halogen bond dissociation energies, D_R-X′. The collective experimental results are consistent with a dissociative one-electron transfer process occurring during the rate-limiting step.