Palladium complexes of heterobidentate ligands: Active catalysts for direct acylation of aryl halides with aldehydes via C(sp2)-H activation

• Synthesis of heterobidentate P-S donor ligands and their palladium complexes.
• Spectroscopic and X-ray structural characterization of the ligands and complexes.
• Catalytic activity of the complexes towards the C–H activation.
• Direct acylation of aliphatic aldehydes with aryl halides.

Heterobidentate P-S donor ligands [P-S = {2-(methylthio) phenyl}diphenylphosphine (a) and {2-((methylthio) methyl) phenyl}diphenylphosphine (b)], and their palladium complexes of the type [Pdη2-(P-S)Cl2] (1a, 1b) have been synthesised and characterised. Single crystal X-ray diffraction shows that in both the complexes, Pd occupies the centre of a slightly distorted square planar geometry. 1a forms a planner ring structure, whereas, the hexagonal ring of 1b bends from planarity to adjust any ring strain. Interesting differences between the complexes were observed in terms of the intermolecular forces. The catalytic activities of the synthesised complexes towards the direct acylation of aryl halides with aldehydes via C(sp2)-H activation were good to excellent. 1b shows better catalytic activity over 1a which may be attributed to the higher stability of the pentagonal ring of 1a. Aryl halides containing electron withdrawing group enhance the reaction, while electron donating substituent tend to retard the desired product formation. The difference in the bond lengths of Pd-P and Pd-S of the chelate complexes may impart hemilabile behaviour in the catalytic cycle by dissociating the weaker bond (Pd-S) to generate vacant coordination site at the metal centre and reassociate after the completion of the reaction.

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