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

Abstract

Room temperature Mössbauer spectra of tris(N,N′-dialkyldithiocarbamato)iron(III) complexes [(R₂NCS₂)₃Fe] (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 (ΔE _Q ), in general, increases with the molecular weight of the alkyl group in both the cases. Plots of magnetic moment (μ_eff) 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 °C range, yielding Fe(SCN)₃ as an intermediate product followed by slow decomposition, leading finally to constant weight corresponding to Fe₂O₃ at ca. 650 °C. Mössbauer spectra of the final products of all the complexes exhibit a six line spectrum with H _eff = 517 ± 3 kOe corresponding to that of α-Fe₂O₃ without any possibility of Fe₂S₃ as proposed in literature.