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) exh ibit an asymmetric doublet which can be resolved into two doublets, each co rresponding to high and low spin states in equilibrium. The quadrupole spli tting (DeltaE(Q)), in general, increases with the molecular weight of the a lkyl group in both the cases. Plots of magnetic moment (mu (e)ff) versus te mperature show that dimethyl-, diethyl-, di-n-propyl- and di-n-butyl-substi tuted dithiocarbamato complexes are equilibrium mixtures of high and low sp in states at room temperature, but increasingly adopt low spin at the liqui d nitrogen temperature. However, the di-i-propyl- and di-i-butyl-substitute d dithiocarbamato complexes exhibit primarily low spin state in the 77-350 K range, with a small contribution (< 15%) of high spin state. Fe-S stretch ing vibrations in far i.r. region also show spin equilibrium states. Thermo gravimetric studies show fast decomposition in the 200-300 degreesC range, yielding Fe(SCN)(3) as an intermediate product followed by slow decompositi on, leading finally to constant weight corresponding to Fe2O3 at ca. 650 de greesC. Mossbauer spectra of the final products of all the complexes exhibi t a six line spectrum with H-eff = 517 +/- 3 kOe corresponding to that of a lpha -Fe2O3 without any possibility of Fe2S3 as proposed in literature.