MOSSBAUER SPECTRAL STUDIES OF SPIN CROSSOVER IN IRON COMPLEXES

Abstract

Studies on the spin crossover in iron complexes have been of immense interest during last few decades. Many chemically and biologically significant transformations are accompanied by a change in the spin state of iron as FeS6 moiety has been known to act as model system in many enzymes. Iron(ll) and iron(lll) complexes with suitable ligands containing S, N, and O donor atoms are well known for their spin crossover behaviour. Several tris(/V,/V-dialkyldithiocarbamato) iron(lll) complexes [(R'R"NCS2)3Fe] and its mixed ligand complexes, have shown high spin (6A1g) *-* low spin (2T2g) behaviour. Spin crossover in iron(lll) complexes with dithiocarbamate ligands has been investigated by various techniques such as variable temperature magnetic moment, electronic, i.r., Mossbauer spectroscopy, NMR, EPR and X-ray crystal structure. Many workers have attempted to correlate spin crossover with the nature of substituent. Thermal decomposition behaviour of metal dithiocarbamates has also been a subject matter of interest because of wide spread industrial applications such as vulcanization accelerators, effective heat stabilizers and reprocessing of polymers etc. Hence their thermal decomposition studies are likely to provide an insight into the mechanistic aspects. The present work deals with the synthesis of twenty five iron(lll) dialkyldithiocarbamates with different alkyl substituents and six mixed ligand complexes. Besides characterization, spin crossover studies, were also carried out by variable temperature magnetic moment and Mossbauer spectral measurements down to 77 K. Further thermal decomposition studies were carried out and an attempt was made to identify the intermediate and final products by Mossbauer spectral studies of heated products. The work embodied in this thesis has been (ii) presented in five chapters with details as follows: Chapter I introduces the principle and applications of Mossbauer spectroscopy. Literature on various aspects of Mossbauer spectroscopy in different disciplines of science and technology including chemistry has been reviewed. Principles of magnetic measurements, thermal decomposition and spin crossover were also discussed. This is followed by a statement about the motivation and objectives of the work. Chapter II brings out the details of instrumentation regarding the Mossbauer spectroscopy and cryostat used for low temperature measurements. Salient features of vibrating sample magnetometer and thermogravimetric analysis system have been included in brief. Instrumental details of other technique are also given. Chapter III summarizes the spin crossover studies of twenty five tris(A/,/Vdialkyldithiocarbamato) iron(lll) complexes with symmetric (R'=R"= CH3, C2H5, n- C3H7, /-C3H7, n-C4H9, ^C4H9, n-C6H13, n-C8H17> C6H5, CH2C6H5, C6H„S CH2=CH-CH2), unsymmetric (R'= CH3, R"= n-C4H9, C6H5 and C6H„; R' = C2H5, R"= i-C3H7, a?-C4H9> C6H5 and C6H„; R' = CH2CH2OH, R" = CH3> C2H5, n-C3H7, A7-C4H9) and ring (N(CH2)4 and N(CH2)5) substituents. Far i.r. spectra show characteristic v(Fe-S) bands corresponding to high and low spin states in region 208- 285 cm'1and 311-380 cm"1 respectively. Another band at ~ 1500 cm"1 corresponds to N-^C with partial double bond character and at -980 cm'1 corresponds to C-^S exhibiting chelating character. Single crystal X-ray structure determination of tris(A/,/Vdiallyldithiocarbamato) iron(lll) showed FeS6 core in distorted octahedral geometry with space group C2/c. Variable temperature magnetic moment and Mossbauer spectral studies have shown that 4 complexes undergo spin crossover, 6A1g^2T2g whereas others undergo spin transitions 6Alg^4T1g (5 complexes) or (iii) 4T1g*=r2T2g (11 complexes). Chapter IV deals with the thermal decomposition studies of all the complexes and five transition metal (M = Co, Ni, Cu, Zn and Cd) complexes with A/-ethyl,AAhydroxyethyldithiocarbamate ligand. The kinetic and thermodynamic parameters were evaluated by Freeman and Carroll's method. On the basis of thermogravimetric studies the complexes can be divided into two groups; volatile (5 complexes) and non volatile (20 complexes). Mossbauer studies of the heated products suggest that in some cases intermediate products such as ferric thiocyanate or FeS2 are formed. In all cases, however, decomposition process varies with the nature of alkyl group and oc-Fe203 is formed as an end product at ~ 700 °C. Chapter V includes studies on the mixed ligand complexes of the type {NC2H5,/ V-/7-C4H9NCS2)2LFe and (A/-C2H5,/V-n-C4H9NCS2)L2Fe where L = phen, oxine and acac by far i.r., variable temperature magnetic moment and Mossbauer spectral studies. Phen and oxine substituted complexes exhibit spin crossover (6A1g-»2T2g) behaviour whereas acac substituted complexes exhibit spin transition (4T1g-»2T2g).