Abstract: Coordination driven self-assembly of chosen metal ions and suitable bi/polydentate ligands under appropriate reaction condition is a reliable strategy to construct MmLn type discrete complexes. Combination of palladium(II) ions with selected ligands results PdmLn type single or multicavity coordination cages. Structural and functional aspects of such cages are explored.1 In the seminar, we will discuss the influence of counter anions on structural behaviour of a set of reported isomeric Pd2L4 cages, generated from Pd(NO3)2 and regioisomeric 3-pyridine appended urea functionalized bidentate ligands (L1-L3).2 Variations in the counter anions revealed that even a specific achiral anion induced helicity to form helical Pd2L4 architecture.3 These ligands were further modified by changing spacer, resulting in regioisomeric bidentate ligands L4–L6, which lengthened and widened the Pd2L4 cavity to accommodate fitting organic anions in the endohedral space. Linear extension of ligands L1-L3 resulted a set of three regioisomeric tridentate ligands L7-L9 to target double-decker shaped (D) Pd3L4 type double cavity cages where two identical Pd2L4 units are conjoined in a linear fashion. Ligand L7 and its corresponding Pd3L4 complex were prepared and preliminary result showed unsatisfactory NMR spectrum but unusual hour-glass(H) architecture was characterized by SCXRD.4 A careful evaluation of the results obtained in the present work, however, revealed that both D-type and H-type Pd3L4 cages exist in solution. Complexation of palladium(II) with L8 resulted both D- and H-type Pd3L4 cages in solution, as opposed to L9, which only formed D-type Pd3L4 cage in solution and solid state.5 Similarly, we attempted to prepare isomeric Pd5L4 type quadruple cavity cages, which consist of a core Pd3L4 cage (D and/or H type) that is conjoined with two units of smaller Pd2L4 cages resulted from three regioisomeric ester/urea functionalized pentadentate ligands L10-L12. Furthermore, the difference in reactivity due to endohedral and exohedral dispositions of (H)urea atoms in Pd2L4 and Pd3L4 type cages as catalytic sites for efficient Michael addition reaction in water will also be presented.
References:
1. Sharma, S.; Sarkar, M.; Chand, D. K. Chem. Commun. 2023, 59, 535-554.
2. Dasary, H.; Jagan, R.; Chand, D. K. Inorg. Chem. 2018, 57, 12222-12231.
3. Sarkar, M.; Dasary, H.; Chand D. K. J. Organomet. Chem. 2021, Article 950 .
4. Dasary, H. PhD thesis, IIT Madras, 2019
5. Dasary, H.; Sarkar, M.; Chand D. K. Chem. Commun. 2022, 58, 8480-8483. (Cover page)