A Pd(II) complex of a β-cyclodextrin-based polydentate ligand: an efficient catalyst for the Suzuki reaction in aqueous media

Abstract

Molecular assembly has become a promising strategy for designing new polydentate ligands. But very often such ligands and their complexes are sparingly soluble in aqueous phase due to their intrinsic hydrophobic character. Pd(II) complexes are good homogeneous catalysts but their poor solubility in aqueous phase may limit their catalytic efficacy in the universal green solvent water. However, solubility related challenges especially in aqueous phase can be mitigated through the formation of inclusion complexes by exploiting the hydrophobic nature of the β-cyclodextrin (β-CD) cavity. Hence, an ionic liquid ChCD (1) was synthesized from β-CD and Choline bromide (ChBr). Next a supramolecular N, N^′, O-tridentate ligand 1⊂2 (3) was synthesized by the inclusion of 2,6-diaminopyridine (2) in the hydrophobic β-CD cavity of the ionic liquid ChCD (1) and was well characterized by elemental analysis, UV-visible, FTIR, ¹H NMR spectroscopy, etc. The stoichiometry of the inclusion complex 1⊂2 (3) was found to be 1:1 based on UV-visible spectrophotometric study. A new air stable, highly water soluble Pd²⁺-complex [κ³-N, N^′, O-Pd(1⊂2)H₂O]OAc (4) was then synthesized from the supramolecular ligand (3) with 1:1 stoichiometry and used as a catalyst for Suzuki cross-coupling reactions in water at ambient temperature with good to excellent yields. The catalyst can be removed and recycled. Additionally, the use of non-toxic solvent water makes the methodology green, sustainable, and economically viable.

Keywords:

• β-Cyclodextrin
• supramolecular palladium(II) complex
• aqueous medium
• reusable catalyst
• Suzuki coupling

Acknowledgement

The authors are grateful to the Departmental Special Assistance Scheme under the University Grants Commission, New Delhi (SAP-DRS-III, NO.540/12/DRS/2013) for financial support and SAIF, NEHU, Guwahati, India for ¹H NMR, and CDRI, Lucknow, India for elemental analysis.