SYNTHESIS, SPECTRAL AND ELECTROCHEMICAL STUDIES OF HETEROBIMETALLIC COMPLEXES CONTAINING MOLYBDENUM (V) LINKED TO NICKEL AND COBALT SCHIFF BASE COMPLEXES

Abstract

In this thesis, the synthesis, characterization and electrochemical studies of monometallic and bimetallic complexes of schiff base ligands have been undertaken. Synthesis of bimetallic complexes initially involved preparation of appropriate tetradentate schiff base ligands, meta and para (TS1-TS4 and TS5-TS8) capable of accommodating two metal centres. This involved condensation of diamines with salicylaldehyde and 2,4- and 2,5-dihydroxybenzaldehyde that produced yellow solids, which were characterized using I.R and U.V analysis. Their formation was monitored by the υC=N bond occurring in the range of 1616 to 1636cm-1 and 1632 to 1654cm-1 for meta and para respectively. The preformed schiff base ligands were reacted with nickel chloride and cobalt acetate to afford monometallic complexes (TS9-TS24). They were characterized using F.T.I.R, U.V and cyclic voltammetric studies. The spectroscopic handle to monitor their formation was the shift of υC=N bond to lower frequencies in the range of 1612 to 1627cm-1 and 1608 to 1622cm-1 for nickel and cobalt meta complexes, respectively. For para substituted monometallic complexes, the shift was observed in the range of 1622 to 1629cm-1 and 1600 to 1612cm-1 for nickel and cobalt, respectively. This indicates a decrease in bond order as compared to the schiff bases due to the attachment of the metals at the tetradentate cavity of the schiff base. They all contain a broad absorption in the visible region, which can be assigned to a metal-to-ligand charge transfer (M.L.C.T) process due to the fact that there is a wide variation in λmax with the nature of linking group between the aromatic rings. This suggests the involvement of the polyethylene ligand in the transitions. Synthesis of bimetallic complexes (TS25-TS40) involved the addition of the appropriate monometallic complex to precursor molecule, Mo(O)Tp*Cl2, in a 1:1 molar ratio in presence of an excess triethylamine. They were characterized using F.T.I.R, U.V and Cyclic voltammetry studies. F.T.I.R studies indicated that the characteristic νC=N bond in all complexes underwent a hypsochromic (red shift) to higher frequencies. The υMo=O bond which was observed at the frequency of 960 cm-1 and 948 to 981cm-1 for nickel and cobalt meta, respectively acted as the spectroscopic handle to monitor the coordination of Mo(O)Tp* in the complex formation. For para bimetallic complexes, this bond was observed at 960cm-1 for nickel and 933 to 962cm-1 for cobalt. he electrochemical behaviour for all bimetallic complexes indicated that Mo(V) undergoes a one electron reduction reaction to Mo(IV) and a one electron oxidation to Mo(VI). The reduction potentials increased cathodically as the length of the polymethylene chain increases. In ACN, nickel and cobalt bimetallic complexes of type B reduced irreversibly at potentials between –0.572to –0.589V and –0.531 to –0.548V respectively while in DCM, the reduction was observed within –0.541 to –0.547V and –0.531 to –0.542V for nickel and cobalt respectively. Respective potential values for type A bimetallic nickel and cobalt complexes in ACN also reduced irreversibly within the ranges –0.561 to –0.590V and –0.551 to –0.559V while in DCM were –0.520 to –0.543V and –0.563 to –0.571V. This indicates that the electrochemical studies of bimetallic complexes are solvent dependent