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Toxicity of Iron Nanoparticles

“Stabilization or Oxidation of Nanoscale Zerovalent Iron at Environmentally Relevant Exposure Changes Bioavailability and Toxicity in Medaka Fish” Chen, P-J; Tan, S-W; Wu, W-L. Environ. Sci. Technol. 2012, ASAP. DOI: 10.1021/es3006783

We’ve posted before on iron-catalyzed reactions (see here for a recent post) as greener alternatives to more traditional platinum group catalyzed reactions. However, even iron has toxicity concerns as described in this paper from National Taiwan University on the toxicity in medaka fish of  zerovalent iron (nZVI) nanoparticles (NPs). This is particularly pertinent research in light of the increased usage of iron(0) nanomaterials in remediation.

The study investigates the effects of four different iron dosing ‘solutions’ on the molecular, cellular and organismal health of medaka larvae: (i) carboxymethylcellulose stabilized nZVI (CMC-nZVI), (ii) non-stabilized nZVI (nZVI), (iii) magnetite NPs (nFe3O4), and (iv) soluble Fe(II).

They first characterize the dosing solutions. The sizes of their nanoparticles are 75 nm, 25-75 nm, and 27 nm for CMC-nZVI, nZVI, and nFe3O4 respectively. The zeta potentials were measured to show, not surprisingly, that the CMC-stabilized particles are much more stable to aggregation than the non-stabilized nZVI.

Interestingly, of the four iron dosing solutions, CMC-nZVI has the most significant impact on the level of dissolved oxygen, decreasing it to zero where it remained for 12 hours. Furthermore, this aerobic oxidation of CMC-nZVI leads to a release of 45 mg/L of soluble Fe(II) in 10 min from an initial concentration of 100 mg/L CMC-nZVI as well as an increase in reactive oxygen species (ROS). In contrast, nZVI and nFe3O4 are 20 – 40 % aggregated within 10 min and release less than 20 mg/L of Fe(II) during this time. Only nZVI induces the production of ROS with nFe3O4 and soluble Fe(II) showing no increase in ROS relative to the control. The following figure details these findings for CMC-nZVI; analogous graphs are found in the supplementary information for the other solutions.

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Iron-catalyzed intermolecular cycloaddition

“Iron-catalyzed Intermolecular [2π-2π] Cycloaddition” Russell, S. K.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2011, 133, 8858-8861. DOI: 10.1021/ja202992p

As the cost of precious metals increases dramatically along with concerns over the toxicity of 2nd and 3rd row metals, chemists are increasingly turning to employing earth-abundant metals in catalysis, especially iron.

In their recent contribution in the area of base metal catalysis, the Chirik group at Princeton reports an intermolecular, iron-catalyzed cycloaddition reaction. In addition to building on the intramolecular version of the reaction they had previously reported, the current contribution is also notable for their isolation of a catalytically competent intermediate.

The chemistry starts with their remarkable iron bis(dinitrogen) complex 1 (or a related bridging diiron dinitrogen complex), a formally zero-valent compound with an electronic structure better described as a dianionic bis(imino)pyridine ligand bound to an intermediate spin iron(II) ion.

The redox non-innocence of the supporting ligand enables the iron center to do two electron chemistry (required for oxidative addition and reductive elimination), reactions usually reserved for 2nd and 3rd row transition metals. In Chirik’s system, iron generally stays in the preferred ferrous oxidation state, while the ligand undergoes two electron reactions cycling between a neutral donor and a dianionic form during catalysis.

The bond-making and bond-breaking events still occur at the metal center (as for more traditional organometallic reactions, think Pd(0)/Pd(II) chemistry), the trick is that the accompanying redox changes occur at the ligand.

The bis(dinitrogen) complex 1 can catalyze the intermolecular cycloaddition of 1,3-butadiene with ethylene to form vinylcyclobutane. By introducing a methyl group into the butadiene substrate (isoprene), the 1,4 addition product is formed instead.

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