Synthesis, characterization of novel imino-pyridyl complexes and carbon carbon coupling reactions

Abstract

This study was based on developing new thiophene based imino-pyridyl complexes which could be tested for Heck coupling reactions. The ligands, (5-methylthiophene-2-pyridylmethyl)imine (L I), (5-methylthiophene- 2-pyridylethyl)imine (L2), (5-bromothiophene- 2-pyridylmethyl)imine (L3) and (5-bromothiophene-2-pyridylethyl)imine (L4) were successfully synthesized via Schiff base condensation of 5-methyl- 2-thiophenecarboxaldehyde and 5-bromo-2- thiophenecarboxaldehyde. The new ligands were isolated as light orange oils in good yields over 70% and characterized using FTIR spectroscopy, 'a and l3C NMR spectroscopy, and ESI-Mass spectroscopy. The ligands were observed to be soluble in common organic solvents, and not susceptible to air and moisture. The ligands (LI-L4) were used to prepare their corresponding palladium (II), nickel (II), and platinum (II) complexes, by reacting in a 1:1 ratio with PdCh(cod), PdClMe(cod), NiBr2(DME), and K2PtC4 giving complexes CI - Cl2 in moderate to high yields. The complexes were observed to be stable both in solid and solution and soluble in common organic solvents and were again fully characterized. The magnetic susceptibility for complexes C9 - cn were all positive and in the order 10-8, which indicates paramagnetism. Single crystal X-ray structures of complex C2 and C8 are reported for the first time. Complex C2 formed crystals in the monoclinic space group P2l1e and C8 formed crystals in the monoclinic space group P2]/n. The X-ray crystal structures showed that the coordination geometry around the palladium center is distorted square planar, and also revealed that the ligands were coordinated through nitrogen atoms in a bidentate mode. Imino-pyridyl Pd (II) and Ni (II) complexes were used as catalyst precursors for Heck coupling reactions. Complex Cl, C2, C3, C4, ClO and Cll proved to be very active and efficient catalyst precursors in Heck coupling reactions ofiodobenzene, 4-iodotoluene, and l-iodo-4-nitrobenzene with methyl acrylate to form Heck products, giving conversions as high as 93% without forming palladium black.