Thermal decomposition of some homobinuclear dihalide-bridged iron(III) complexes

Abstract

Thermal studies by TG and DTG on some homobinuclear dihalide-bridged iron(Ill) complexes of the general type [Fe(S2CNR2)2X]2~-X~) were carried out in air and nitrogen atmospheres. The apparent activation energies were determined by graphical methods and the TTN temperatures were calculated from the TG profiles. Finally, a possible mechanism of the decomposition is suggested on the basis of the pyrolysis and mass spectral data.In previous papers, results have been reported on the reactions of molecular halogens (X~) with the square-pyramidal complexes [Fe(S2CNR2)2X] (X = CI, Br, I) [1][2][3]. The salient feature of these reactions is the effect of the alkyl substituent in the dithiocarbamato ligand on the composition of the products obtained [4,5]. In this respect, a general mechanism accounting for the variety of products formed has been proposed [3].In this paper, we focus our attention on the binuclear iron(Ill) complexes formulated [2] as [Fe(S2CNR2)2X]2(/~-X~), where R = Me, Et, pri; X' = Br, I. M6ssbauer and magnetic measurements on this very interesting class of compounds, as well as an X-ray crystal structure determination [6] of another derivative of this series containing the pyrrolidinyldithiocarbamato ligand, have established the presence of the molecular halogen in the form of a bridging unit mediating the formation of weakly coupled dimers with antiferromagnetic interaction. In particular, the molecular halogen interacts weakly in an end-on fashion with the halide ligand of the mononuclear complexes, forming an extended tetratomic bridging unit, {X-X'-X'-X} 2-, between the two paramagnetic centres. Consequently, the binuclear complexes have been proved [7] to be potential carriers of activated molecular halogens and therefore very reactive in halogentransfer reactions.As an extension of our previous work [8][9][10] on thermal studies of metal diothiocarbamate complexes, a thermal investigation of the dihalide-bridged

Mass spectral and thermogravimetric studies of some iron(III) binuclear complexes with bridging thiuram disulfide ligands

“…The halide ligand X-is also evolved, whereas the possible intermediate [X'Fe(R2dtc)] 2 can decompose to give halogen, X' 2 and FeS as final residue. Also, dithio-oxamide may be a possible product [7]. Conclusively, this mechanism is analogous to that proposed for the dihalide-bridged iron(III) complexes, [Fe(R2dtc)2(/~-X'2), of the same stoichiometry [7], where the molecular halogen bridging unit, X' 2, was primarily evolved.…”

“…Finally, the calculated apparent activation energy, E*, of 205.8 kJ/mol, derived from Piloyan's TG method [I I] for the complex Fez(Me2dtc)z(tds~ is fairly close to those reported for the dihalide-bridged iron(llI) complexes [7], since the metal is the same in every case. Unfortunately, this method cannot be applied to the pyrrolidyl derivatives, because the obtained plot does not fit a straight line.…”

“…This mechanism involves the elimination of the bridging thiuram disulfide ligand, which decomposes further to yield carbon disulfide and either dithiocarbamate salt or sulfenamide [7][8][9][10]. The halide ligand X-is also evolved, whereas the possible intermediate [X'Fe(R2dtc)] 2 can decompose to give halogen, X' 2 and FeS as final residue.…”

“…In this case. the dimeric compounds studied in the present paper are formed.Continuing our interest in the thermal investigation of metal dithiocarbamate complexes [7][8][9][10], we report in this paper results concerning mass spectral and thermogravimetric studies of complexes Fe2(R2dtc)z(tds~ The E* values for the main decomposition stage, derived from the Piloyan method [11], TTN calculations [12], and a proposed mechanism of the decomposition are also reported and discussed. …”

“…In particular, the proposed mechanism [5] which accounts for the synthesis of these compounds involves as a first step the formation of the corresponding dihalide-bridged dimers, [Fe(R2dtc)zX]2(/I-X'2). Analogous iron(Ill) complexes have already been isolated and their structures were established on the basis of spectroscopic [1,3, 4] and X-ray crystal structure [6] determination methods, as well as the results of thermoanalytical studies [7]. Depending on the redox capacities of both the halogens and the dithiocarbamato ligands, an intramolecular electron transfer could take place resulting in the oxidation of half of the dithiocarbamate groups into thiuram disulfide.…”