Influence of Organic Ligands on the Reduction of Polyhalogenated Alkanes by Iron(II)

Abstract

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 quantitativelyto 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'), deltaG(0'), 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.