Synthesis, Structural Peculiarities, Theoretical Studies and Catalytic Properties of Organotin (IV) Carboxylate Complexes

Abstract

The prompt reduction of fossil fuel resources and their environmental effects has caused a serious threat to the humanity. Meanwhile, the exponential increase in the usage of fossil fuels by growth in world population, demands the exploration of new energy resources to overcome the energy crises. Biodiesel (mixture of alkyl monoesters) is indeed one of the appealing sources of energy. This research work mainly focused to develop new organic ligands and their complexation with organotin(IV) derivatives as catalysts for biodiesel production via transesterification. The synthesized ligands and their complexes have also been computationally studied to explore the structure properties relationship. Initially, a series of Schiff bases (SB1 to SB2) have been synthesized and characterized by various analytical techniques. However, it is noteworthy to mention here that no organotin(IV) complexes have been obtained from the Schiff bases probably due to the formation of Zwitter ion by the hydrogen of nearby hydroxyl group to nitrogen making a stale 5-membered stable ring. Therefore, we have preferred to synthesize various carboxylate ligands owing to their high donor/complexation capacity with organotin (IV). A series of carboxylate ligands (HL1 to HL10) have been synthesized via ring opening mechanism and were successfully characterized using various analytical techniques (FT-IR, 1H NMR, 13C NMR, Single and crystal X-ray diffraction). Form the experimental results, trigonal bipyramidal geometry and tetrahedral geometry were observed for triorganotin(IV) derivatives in both solution and solid state. In addition, trinuclear di-organotin(IV) complexes (C1-C4) were also synthesized and characterized with seven coordinated pentagonal bipyramidal geometry. The geometry, bond angle, and bond length were also studied computationally with density functional theory (DFT) and promisingly found highly aligned with the results obtained from the analytical techniques. All the synthesized complexes have been evaluated as homogeneous catalyst for the biodiesel production. The catalytic performance of the tested complexes was found as follow: triorganotin(IV) > diorganotin(IV) derivatives with maximum conversion efficiency of 92 % and 64% respectively. The order of catalytic performance can be explained on the basis of steric and Lewis acidic character around the tin(IV) centre. However, trinuclear di-organotin(IV) complexes were found less active (26 %) than tri/di- ix organotin(IV) that may be due to the high coordination number and therefore, Sn centre is less susceptible for nucleophilic attack. Moreover, the obtained castor oil biodiesel has been confirmed and quantified by FT-IR, NMR (1H and 13C) and GC-MS. Keeping in view the catalytic performance of the screened compounds, it is expected that they might be the potential future candidates for the expansion of catalyst systems for efficient biodiesel production from the various precursors (vegetables oils)