Synthesis, characterization and thermal investigation of M[M(C 2O 4) 3]· xH 2O ( x=4 for M=Cr(III); x=2 for M=Sb(III) and x=9 for M=La(III))

Abstract

The complexes, M[M(C 2O 4) 3]· xH 2 O, where x=4 for M=Cr(III), x=2 for M=Sb(III) and x=9 for M=La(III) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR and electronic spectral data, conductivity measurement and powder X-ray diffraction (XRD) studies. The chromium(III)tris(oxalato)chromate(III)tetrahydrate (COT), Cr[Cr(C 2 O 4) 3]·4H 2O, released water in a stepwise fashion. Removal of the last trace of water was accompanied by a partial decomposition of the oxalate group. Thermal investigation using TG, DTG and DTA techniques in air produced Cr 2O 3 at ∼858°C through the intermediate formation of Cr 2O 3 and CrC 2O 4 at around 460°C. While DSC study in nitrogen up to 670°C produced a mixture of Cr 2O 3 and CrC 2O 4. In antimony(III)tris(oxalato)antimonate(III)dihydrate (AOD), Sb[Sb(C 2O 4) 3]·3H 2O the dehydration took place during the decomposition of precursor at 170–290°C and finally at ca. 610°C Sb 2 O 5 along with trace amounts of Sb 2O 4 were produced. Trace amount of Sb 2O 3 and Sb along with Sb 2O is proposed as the end product at 670°C of AOD in nitrogen. The oxide La 2O 3 is formed at ∼838°C from the study with TG, DTG and DTA in air of lanthanum(III)tris(oxalato)lanthanum(III)nonahydrate (LON), La[La(C 2O 4) 3]·9H 2O. Intermediate dioxycarbonate, La 2O 2CO 3 was generated at ∼526°C prior to its decomposition to lanthanum oxide in air; whereas in N 2 the formation of La 2(CO 3) 3 at ∼651°C was proposed. The thermal parameters have been evaluated for each step of the dehydration and decomposition of COT, AOD and LON using five non-mechanistic equations i.e. Flynn and Wall, Freeman and Carroll, Modified Freeman and Carroll, Coats–Redfern and MacCallum–Tanner equations. Kinetic parameters, such as, E *, k o, Δ H *, Δ S * etc. were also supplemented by DSC studies in nitrogen for all the three complexes. Some of the intermediate species have been identified by analytical and powder XRD studies. Tentative schemes has been proposed for the decomposition of all three compounds in air and nitrogen.