Electronic State of Iron Complexes in the Interlayer of Hydrotalcite-Like Materials

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Abstract
Infrared and Mössbauer spectroscopic studies were made on hydrotalcite-like materials, Mg6Al2(OH)16(NO3)2·4H2O, anion exchanged in aqueous solution of K3Fe(CN)6, K4Fe(CN)6·3H2O, or Na2Fe(CN)5NO·2H2O. The material anion-exchanged in aqueous solution of K3Fe(CN)6 gave infrared (IR) absorption bands at 2120 and 2040 cm-1 in the C≡N stretching region, suggesting that part of the ferrate(III) complex was reduced to ferrate(II) complex on the intercalation chiefly because pure K3Fe(III)(CN)6 and K4Fe(II)(CN)6·3H2O gives bands at 2120 and 2040 cm-1, respectively. On the intercalation of Fe(CN)64-, no change in the oxidation state of iron was observed. These features were confirmed by Mössbauer spectroscopy. In the IR spectra of material anion-exchanged in aqueous solution of Na2Fe(CN)5NO·2H2O, the intensity of bands due to N≡O (1940 cm-1) and C≡N stretching was much less than that observed for Na2Fe(CN)5NO·2H2O, indicating that most of N≡O ligand was eliminated during the intercalation. Four bands were observed in the C≡N stretching region: a band at 2143 cm-1 was assigned to C≡N groups in Fe(CN)5NO2-; a band at 2120 cm-1 was tentatively assigned to Fe(CN)5H2O2-; bands at 2040–2050 cm-1 were assigned to Fe(CN)5H2O3-.