Syntheses and functional studies of chiral metal-organic cages and supramolecular assemblies derived from imido and amido P(V) scaffolds

Abstract

Chiral Metal-organic supramolecular architectures have received much attention due to their unique and fascinating structures and material applications. These have been useful in various fields of research such as chiral recognition and separation, asymmetric catalysis, nonlinear optical, biomedical research and enzyme mimetics. This presentation focuses on the use of chiral phosphonium and phosphoramide ligands towards the formation of various discrete chiral hydrogen-bonded frameworks and metal-organic cages and their applications in chiral recognition and separation. A facile deprotonation protocol involving enantiopure phosphoramide ligands and Pd(II) ions generated the trinuclear complex of the chiral tris(imido)phosphate trianions {Pd3[PO(N(*CH(CH3)Ph)3]}3+. This in-situ generated imido-Pd3 species readily self-assembles in the presence of oxalate dianions to yield an enantiomeric pair of tetrahedral cages of formula [(Pd3XRRR)4(C2O4)6] and [(Pd3XSSS)4(C2O4)6].Further, these cages were shown to exhibit moderate enantioselective separation capabilities for epichlorohydrin, β-butyrolactone, 3-methyl- and 3-ethyl cyclopentanone via crystallization inclusion method. Single crystal X-ray analysis of ± β-butyrolactone@±1 system showed that guest recognition predominantly takes place at the extrinsic cavities of the cage. Further by utilizing the same PBUs with chloranilate dianions, we have synthesized iso-structural chiral tetrahedral cages [(Pd3XRRR)4(C6O4Cl2)6] and [(Pd3XSSS)4(C6O4Cl2)6]. Due to the presence of quinoid portals in these tetrahedral cages, they exhibit considerable enantioselective separation capabilities towards a series of styrene epoxides via the crystallization inclusion method. The highest enantioseparation of 80% has been observed for the R (+) Fluoro styrene oxide compound using the cage [(Pd3XRRR)4(C6O4Cl2)6]. Use of other anilate ligands such as Dihydroxy benzoquinone (DHBQ) and bromanilate anions resulted in the formation of elusive polyradical cages ([(Pd3XRRR or SSS)4(L)6] L= C6O4H2, C6O4Br2). The Triplet and singlet diradical behaviour of these metal-organic cages were characterized by variable temperature EPR and SQUID measurements as well as by computational studies. To establish the dianionic diradical nature of these anilate ligands in these spin cages, we have synthesized the model compounds of these anions with chiral aminophosphonium salts PX4 (X= RRR or SSS-[NH*CH (CH3)Ph]. The X-ray structural analysis of the salts of these model compounds showed the stable nature of these elusive dianionic diradicals. Also, the model compound studies confirm the triplet nature of these dianionic diradicals in the DHBQ derived cages ([(Pd3XRRR or SSS)4(C6O4H2)6].