Synthesis and characterization of zn(ii) and co(ii) pyrrole-aldiminato complexes and their catalytic activity in carbon dioxide/ carbon disulphide coupling with propylene oxide

Abstract

Pyrrole-aldimine ligands L1 – L4 were successfully prepared following the Schiff base condensation reaction using the respective primary amines with pyrrole-2-carboxaldehyde. The ligands were isolated as powders and the yields were in the range 75 % - 95%. The neutral ligands were then used in the preparation of their respective bis-(pyrrole-aldiminato) Zn(II) and Co(II) complexes, C1 – C4 and C5 – C8, respectively. The Zn(II) and Co(II) complexes were isolated as solids with varying consistencies and low yields in the range 30 % - 60 %. The ligands and their respective bis-(pyrrole-aldiminato) Zn(II) and Co(II) complexes were characterized using 1 H NMR, 13C NMR, 2D NMR, FT-IR and UV-Vis spectroscopy. Mass spectrometry and elemental analysis were also used. The bis-(pyrrole-aldiminato) Co(II) complexes were not characterized by NMR spectroscopy due to their paramagnetic nature. The catalytic activity of the Zn(II) and Co(II) complexes was evaluated in the coupling of CS2 and PO. C1 was used to establish the baseline reaction conditions for this coupling reaction, which were then applied in screening the activity of all 8 metal complexes. In the absence of a cocatalyst and solvent-less conditions, C1 was found to show no activity for the coupling of CS2 and PO at room temperature over 24 hours. The introduction of co-catalysts (TBAC, DMAP, [PPN]Cl and MeIm) under the same reaction conditions resulted in the formation of a mixture of 3 cyclic products, which were identified as 5-methyl-1,3-oxathiolane-2-thione, 4-methyl1,3-dithiolane-2-thione and 5-methyl-1,3-oxathiolan-2-one, with the highest conversion rate being 2.32 % for C1/TBAC. In the presence of MeOH, C1/MeIm resulted in the highest conversion rate and MeOH on its own resulted in the lowest conversion rate. These were found to be 61 % and 3.80 % respectively. Although a drastic improvement in catalytic activity was observed compared to when the reactions were carried out neat, a coupling product different to products obtained for neat reactions which was identified to be a methoxylated species of 4- methyl-1,3-dithiolane-2-thione was obtained as the major product. It was shown that after 6 hours, the selectivity for the methoxylated species of 4-methyl-1,3-dithiolane-2-thione is hampered as signals suggesting the presence of an additional cyclic product which could not be qualified were observed in the 1 H-NMR spectrum. The intensity of the additional signals was observed to increase with an increase in reaction time. The conversion rate was seen to increase with an increase in reaction time. The catalytic activity of all 8 metal complexes was then evaluated in the coupling of CO2/PO under mild conditions over 12 hours. C5/[PPN]Cl showed the highest activity, with a conversion rate of 1.05 % and C2/[PPN]Cl resulted in the lowest conversion rate of 0.27 %. This was found to be lower than that achieved with [PPN]Cl on its own and under the same reaction conditions, which was 0.38 %. Increasing the reaction iii time to 24 hours and increasing the pressure to 25 bar of CO2 resulted in a slight increase in conversion rate, however not by a significant margin. The addition of organic solvents was shown to impede the activity of C5/[PPN]Cl in this coupling process. The influence of ligand architecture on the catalytic activity of the Zn(II) and Co(II) complexes could not be established and this was attributed to their differences in degrees