Magnetic moment and Mossbauer spectral studies of spin-crossover in tris(N,N′-dialkyldithiocarbamato)iron(III) complexes and their thermal decomposition

Room temperature Mossbauer spectra of tris(N,N'-dialkyldithiocarbamato)iron(III) complexes [(R2NCS2)(3)Fe] (R = Me, Et, n-Pr, i-Pr, n-Bu and i-Bu) exhibit an asymmetric doublet which can be resolved into two doublets, each corresponding to high and low spin states in equilibrium. The quadrupole splitting (DeltaE(Q)), in general, increases with the molecular weight of the alkyl group in both the cases. Plots of magnetic moment (mu (e)ff) versus temperature show that dimethyl-, diethyl-, di-n-propyl- and di-n-butyl-substituted dithiocarbamato complexes are equilibrium mixtures of high and low spin states at room temperature, but increasingly adopt low spin at the liquid nitrogen temperature. However, the di-i-propyl- and di-i-butyl-substituted dithiocarbamato complexes exhibit primarily low spin state in the 77-350 K range, with a small contribution (< 15%) of high spin state. Fe-S stretching vibrations in far i.r. region also show spin equilibrium states. Thermogravimetric studies show fast decomposition in the 200-300 degreesC range, yielding Fe(SCN)(3) as an intermediate product followed by slow decomposition, leading finally to constant weight corresponding to Fe2O3 at ca. 650 degreesC. Mossbauer spectra of the final products of all the complexes exhibit a six line spectrum with H-eff = 517 +/- 3 kOe corresponding to that of alpha -Fe2O3 without any possibility of Fe2S3 as proposed in literature.