Synthesis, structure, and Mössbauer spectroscopic studies on the heat-induced solid-phase redox reactions of hexakis(urea-O)iron(III) peroxodisulfate

Abstract Anhydrous hexakis(urea-O)iron(III)]peroxydisulfate ([Fe(urea-O) 6 ] 2 (S 2 O 8 ) 3 (compound 1 ), and its deuterated form were prepared and characterized with single-crystal X-ray diffraction and spectroscopic (IR, Raman, UV, and Mössbauer) methods. Six crystallographically different urea ligands coordinate via their oxygen in a propeller-like arrangement to iron(III) forming a distorted octahedral complex cation. The octahedral arrangement of the complex cation and its packing with two crystallographically different persulfate anions is stabilized by extended intramolecular (N–H⋯O = C) and intermolecular (N–H⋯O–S) hydrogen bonds. The two types of peroxydisulfate anions form different kinds and numbers of hydrogen bonds with the neighboring [hexakis(urea-O) 6 iron(III)] 3+ cations. There are spectroscopically six kinds of urea and three kinds (2 + 1) of persulfate ions in compound 1 , thus to distinguish the overlapping bands belonging to internal and external vibrational modes, deuteration of compound 1 and low-temperature Raman measurements were also carried out, and the bands belonging to the vibrational modes of urea and persulfate ions have been assigned. The thermal decomposition of compound 1 was followed by TG-MS and DSC methods in oxidative and inert atmospheres as well. The decomposition starts at 130 °C in inert atmosphere with oxidation of a small part of urea (~ 1 molecule), which supports the heat demand of the transformation of the remaining urea into ammonia and biuret/isocyanate. The next step of decomposition is the oxidation of ammonia into N 2 along with the formation of SO 2 (from sulfite). The main solid product proved to be (NH 4 ) 3 Fe(SO 4 ) 3 in air. In inert atmosphere, some iron(II) compound also formed. The thermal decomposition of (NH 4 ) 3 Fe(SO 4 ) 3 via NH 4 Fe(SO 4 ) 2 formation resulted in α -Fe 2 O 3 . The decomposition pathway of NH 4 Fe(SO 4 ) 2 , however, depends on the experimental conditions. NH 4 Fe(SO 4 ) 2 transforms into Fe 2 (SO 4 ) 3 , N 2 , H 2 O, and SO 2 at 400 °C, thus the precursor of α -Fe 2 O 3 is Fe 2 (SO 4 ) 3 . Above 400 °C (at isotherm heating), however, the reduction of iron(III) centers was also observed. FeSO 4 formed in 27 and 75% at 420 and 490 °C, respectively. FeSO 4 also turns into α -Fe 2 O 3 and SO 2 on further heating. Graphical abstract