Six new Pd(II) and Ni(II) metal complexes of N, O-bidentate (L1, L2) and ONNO-tetradentate (L3) Schiff base ligands have been synthesized. The compounds were characterized via various physicochemical and spectroscopic techniques namely elemental analysis (CHN), FT-IR, 1H and 13C NMR as well as magnetic susceptibility measurement. All complexes showed diamagnetism indicating that they are square planar complexes. Catalytic performance of Pd(L1)2 and Pd(L2)2 were investigated for Heck cross-coupling reaction under optimum operating parameters, monitored using GC-FID for 6 h of reaction time in inert conditions. High catalytic activities of up to 90% were observed in the presence of triethylamine as base and DMA as solvent at 100oC with 1 mmol% catalyst loading. The mechanism of catalyzed Heck reaction is proposed to go through a series of conversion of Pd(0)/Pd(II).
A mononuclear and new tetranuclear metal complexes of Zn(II) with Schiff base ligands L1 and L2 respectively, were synthesised. L1 was obtained through the condensation of salicylaldehyde with ortho-phenylenediamine while L2 was the product of reaction between of ortho-vanillin with 2,4,6-trimethyl-m-phenylenediamine. The ligands and complexes were characterised via elemental analysis, melting point, IR and NMR spectroscopy. The shifting of v(C=N), v(C-OH) and v(O-CH3) infrared peaks upon coordination with Zn(II) indicated that these three moieties play a significant role in the complexation. It was found that L1 acted as tetradentate ligand, coordinating with Zn(II) centres through phenolic oxygen and imine nitrogen. The ligand L2 acted as a hexadentate ligand, bonded to metal via phenolic oxygen, imine nitrogen and methoxy oxygen, where four Zn(II) centres formed bridges to connect two ligands.
Chemical industries are greatly assisted by catalysts; and commonly used catalysts for C-C formation
reactions are mainly phosphine-based complexes which are air and moisture sensitive. New air stable
Schiff base Pd(II) complexes have been synthesised, characterised and screened for their catalytic potential.
This paper reports three ONNO Schiff bases, namely, L2C [2,2’-((1E,1’E)-((2,2-dimethylpropane-1,3-
diyl) bis (azanylylidene)) bis (methanylylidene)-) bis (4-chlorophenol)], L2M [2,2’-((1E,1’E)-((2,2-
dimethylpropane-1,3-diyl)bis(azanylyli-dene)) bis (methanylylidene)) bis (4-methylphenol)] and L2H
[2,2’-((1E,1’E) - ((2,2-dimethyl-propane-1,3-diyl) bis (azanylylidene)) bis (methanylylidene)) diphenol].
These were synthesised from 2,2-dimethyl-1,3-propanediamine and salicylaldehyde derivatives with Cl,
CH3 and H at the meta position, respectively. The compounds were reacted with palladium(II) acetate
yielding three palladium(II) complexes denoted as PdL2C, PdL2M and PdL2H. The structures of all
compounds were elucidated through elemental analysis, 1
H and 13C NMR, FTIR and melting point. The
complexes were screened for catalytic activities in Sonogashira coupling reaction between iodobenzene
and phenylacetylene in DMSO. PdL2H was found to be the most active catalyst with 87% iodobenzene
conversion after 12 hours of reaction.