A new synthetic method for annulating hydroquinones to Fe 2 S 2 (CO) 6 moieties is reported. Piperidine catalyzed a multistep reaction between Fe 2 (μ-SH) 2 (CO) 6 and quinones to afford bridged adducts in 26-76% yields. The hydroquinone adducts undergo reversible two-electron reductions. In the presence of acetic acid, hydrogen is produced catalytically with these adducts at potentials more negative than that of the initial reversible reduction. Spectroscopic studies suggest the presence of intramolecular hydrogen bonding between the phenolic OH groups and the adjacent sulfur atoms. Computations, which are in good agreement with the electrochemical studies and spectroscopic data, indicate that the hydrogen bonding is most important in the reduced forms of the catalysts. This hydrogen bonding lowers the reduction potential for catalysis but also lowers the basicity and thereby the reactivity of the catalysts.

Oxalate, together with other ligands, plays an important role in the dissolution of iron(hdyr)oxides and the bio-availability of iron. The formation and properties of oxalate surface complexes on lepidocrocite were studied with a combination of infrared spectroscopy (IR), density functional theory (DFT) calculations, dissolution, and photochemical experiments. IR spectra measured as a function of time, concentration, and pH (50-200 µM oxalate, pH 3-7) showed that several surface complexes are formed at different rates and in different proportions. Measured spectra could be separated into three contributions described by Gaussian line shapes, with frequencies that agreed well with the theoretical frequencies of three different surface complexes: an outer-sphere complex (OS), an inner-sphere monodentate mononuclear complex (MM), and a bidentate mononuclear complex (BM) involving one O atom from each carboxylate group. At pH 6, OS was formed at the highest rate. The contribution of BM increased with decreasing pH. In dissolution experiments, lepidocrocite was dissolved at rates proportional to the surface concentration of BM, rather than to the total adsorbed concentration. Under UV-light (365 nm), BM was photolyzed at a higher rate than MM and OS. Although the comparison of measured spectra with calculated frequencies cannot exclude additional possible structures, the combined results allowed the assignment of three main structures with different reactivities consistent with experiments. The results illustrate the importance of the surface speciation of adsorbed ligands in dissolution and photochemical reactions.

1 presence of diverse ligands and over a broad pH range 2 . Low Fe(II) concentrations catalyze the dissolution of various Fe(III) (hydr)oxide minerals in the presence of diverse ligands and over a broad pH range. Environmental Science and Technology, 53(1), 98-107. Abstract 19 Dissolution of Fe(III) (hydr)oxide minerals by siderophores (i.e. Fe-specific, biogenic ligands) is 20 an important step in Fe acquisition in environments where Fe availability is low. The observed 21 co-exudation of reductants and ligands has raised the question of how redox reactions might 22 affect ligand-controlled (hydr)oxide dissolution and Fe acquisition. We examined this effect in 23 batch dissolution experiments using two structurally distinct ligands (desferrioxamine B (DFOB) 24 and N,N ′ -di(2-hydroxybenzyl)ethylene-diamine-N,N ′ -diacetic acid (HBED)) and four Fe(III) 25 (hydr)oxide minerals (lepidocrocite, 2-line ferrihydrite, goethite and hematite) over an 26 environmentally-relevant pH range (4 -8.5). The experiments were conducted under anaerobic 27 conditions with varying concentrations of (adsorbed) Fe(II) as the reductant. We observed a 28 catalytic effect of Fe(II) on ligand-controlled dissolution even at sub-micromolar Fe(II) 29 concentrations with up to a 13-fold increase in dissolution rate. The effect was larger for HBED 30 than for DFOB. It was observed for all four Fe(III) (hydr)oxide minerals, but it was most 31 pronounced for goethite in the presence of HBED. It was observed over the entire pH range 32 with the largest effect at pH 7 and 8.5, where Fe deficiency typically occurs. The occurrence of 33 this catalytic effect over a range of environmentally relevant conditions and at very low Fe(II) 34 concentrations suggests that redox-catalysed, ligand-controlled dissolution may be significant in 35 biological Fe acquisition and in redox transition zones. 36 Keywords: 37 Ligand-controlled dissolution, reductive dissolution, Fe(II), catalytic effect, synergism, Fe 38 acquisition, pH, Fe(hydr)oxides, DFOB, HBED, electron transfer, atom exchange 39 157 stirring bar. NaCl was applied as background electrolyte to a final concentration of 0.01 M. The 158 effect of the electrolyte and ionic strength was not further explored in this work. The pH was 159 buffered with 0.005 M of either PIPPS (pH 4 and pH 8.5), MES (pH 6), or MOPS (pH 7). Batch 160 dissolution and adsorption experiments were conducted at the same buffer concentration so 161 that potential effects from buffers on ligand and Fe(II) adsorption 39 could be accounted for in 162 parameterizing rate law equations of surface-controlled dissolution reactions. The pH was set 163 by adding HCl or NaOH, and remained constant at the set pH values (ΔpH = ±0.05) throughout 164 235 the subsample was filtered (0.1 µm) to remove Fe(III) precipitates and the sample was analyzed 236 by UV-vis photospectrometry. All adsorption experiments were carried out in duplicates. 237 238 Results and Discussion 239 Influence of Fe(II) concentration on the rate of ligand-controlled dissolut...

Dissolution of iron(III)phases is a key process in soils, surface waters and the ocean. Previous studies found that traces of Fe(II) can greatly increase ligand controlled dissolution rates at acidic pH, but the extent that this also occurs at circumneutral pH and what mechanisms are involved are not known. We addressed these questions with infrared spectroscopy and 57 Fe isotope exchange experiments with lepidocrocite (Lp) and 50 µM ethylenediaminetetraacetate (EDTA) at pH 6 and 7. Addition of 0.2-10 µM Fe(II) led to an acceleration of the dissolution rates by factors of 7-31. Similar effects were observed after irradiation with 365 nm UV light. The catalytic effect persisted under anoxic conditions, but ceased as soon as air or phenanthroline was introduced. Isotope exchange experiments showed that added 57 Fe remained in solution, or quickly reappeared in solution when EDTA was added after 57 Fe(II), suggesting that catalyzed dissolution occurred at or near the site of 57 Fe incorporation at the mineral surface. Infrared spectra indicated no change in the bulk, but changes in the spectra of adsorbed EDTA after addition of Fe(II) were observed. A kinetic model shows that the catalytic effect can be explained by electron transfer to surface Fe(III) sites and rapid detachment of Fe(III)EDTA due to the weaker bonds to reduced sites. We conclude that the catalytic effect of Fe(II) on dissolution of Fe(III)(hydr)oxides is likely important under circumneutral anoxic conditions and in sunlit environments.

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