Mesogenic palladium complexes with pincer ligands derived from dipicolinic acid

Area of Science:

• Coordination chemistry
• Materials science
• Supramolecular chemistry

Background:

• Pyridine-2,6-dicarboxylic acids and their thio-analogues serve as versatile pincer ligands.
• Palladium complexes with these ligands are of interest for their structural and electronic properties.
• The incorporation of mesogenic units can lead to liquid crystalline materials.

Purpose of the Study:

• To synthesize and characterize novel palladium complexes featuring O,N,O and S,N,S pincer ligands.
• To investigate the influence of ligand type (O,N,O vs. S,N,S) and ancillary ligands on the thermal and mesomorphic properties of the complexes.
• To explore the potential of these complexes as new thermotropic liquid crystalline materials.

Main Methods:

• Synthesis of 4-substituted pyridine-2,6-dicarboxylic acids ((E)-dipicH2) and 4-substituted pyridine-2,6-bis(thiocarboxylic) acids ((E)-pdtcH2).
• Complexation of these ligands with palladium.
• Characterization of the resulting palladium complexes using various analytical techniques.
• Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) for thermal and mesomorphic property analysis.

Main Results:

• Palladium complexes with both O,N,O and S,N,S pincer ligands were successfully synthesized.
• Thermotropic mesomorphism, specifically columnar phases, was observed in certain O,N,O-pincer complexes bearing specific ancillary ligands (L5, n=12).
• S,N,S-pincer complexes exhibited significantly lower melting points and broader mesophase ranges compared to their O,N,O counterparts.
• The mesomorphic behavior observed in an O,N,O derivative was not retained in its S,N,S analogue.
• Alkylsulfanyl compounds showed lower transition temperatures and wider mesophase ranges than their alkoxy analogues.

Conclusions:

• The choice between O,N,O and S,N,S pincer ligands significantly impacts the thermal and mesomorphic properties of palladium complexes.
• The S,N,S pincer ligands can induce or modify liquid crystalline behavior, offering a route to novel materials.
• Alkylsulfanyl chains provide advantages over alkoxy chains in terms of lower transition temperatures and wider mesophase ranges for these types of complexes.