Enantiopure Polyradical Tetrahedral Pd12 L6 Cages

Squarate-Tethered Enantiomeric Imido-Pd(II) Cages for Recognition and Separation of Chiral Organic Molecules

Chiral coordination cages have emerged as an efficient platform for enantioselective processes via host-guest interactions. Here, we report an enantiomeric pair of tetrahedral cages of formula [(Pd3[PO(N(*CH(CH3)Ph)3])4(C4O4)6] supported by chiral tris(imido)phosphate trianions and squarate (C4O4)2- linkers. These cages exhibit unusual coordination isomerism for Pd(II)-linker bonds compared with the other Pd(II) cages of this family. Further, they were employed for the recognition and separation of small chiral molecules containing various functionalities. High enantioselectivities of 67 and 41 were found in the case of R-4-hydroxydihydrofuran-2(3H)-one and S-epichlorohydrin, recognized by the R-isomer of the cage. Chiral separation studies showed remarkable enantiomeric excess values of 93 and 85% for S-epichlorohydrin and R-4-benzyl-2-oxazolidinone, respectively, from their racemic mixtures. These studies showcase the potential of coordination cages for enantioselective applications.

Polyanionic Imido-P(V) Ligands: From Transition Metal Complexes to Coordination Driven Self-Assemblies

The chemistry of the imido-anions of the main group elements has been studied for more than three decades. The imido (NR)^- group is isoelectronic to the oxo (=O) group and can coordinate with metal ions through its lone pairs of electrons. The polyimido-P(V) anions are well explored as they resemble the phosphorus oxo moieties such as H₃ PO₄ , H₂ PO₄ ^- , HPO₄ ^2- and PO₄ ^3- species. These imido anions are typically generated using strong main group organometallic reagents such as ⁿ BuLi, Et₂ Zn, Me₃ Al and ⁿ Bu₂ Mg, etc. As a result, their coordination chemistry has been restricted to reactions in anhydrous aprotic solvents for a few main group metal ions. This account presents our findings on using certain soft transition metal such Ag(I) and Pd (II) for isolating these imido-P(V) anions as their corresponding self-assembled clusters and cages. Using the various salts of Ag(I) ions in reaction with 2-pyridyl (² Py) functionalized phosphonium salts and phosphoric triamides, we obtained the mono- and dianionic form of these imido ligands {[P(N² Py)₂ (NH² Py)₂ ]^- , [P(N² Py)₂ (NH² Py)]^- , [PO(N² Py)(NH² Py)₂ ]^2- } and derived interesting examples of tri, penta, hepta and octanuclear Ag(I) clusters. Interestingly, by using the salts of Pd (II) ions, the elusive imido-phosphate trianions of the type [(RN)₃ PO]^3- (R=^t Bu, ^c Hex, ⁱ Pr) were generated in a facile one pot reaction as their corresponding tri- and hexanuclear clusters of the type {Pd₃ [(NR)₃ PO](OAc)₃ }_n (n=1 or 2). These trianions acts as a cis-coordinated hexadentate ligand for a trinuclear Pd (II) cluster and serve as the polyhedral building units for constructing hitherto unknown family of neutral cages in tetrahedral {Pd₃ [(Nⁱ Pr)₃ PO]₄ (L)₆ } and cubic {Pd₃ [(Nⁱ Pr)₃ PO]₈ (L)₁₂ } structures in the presence of suitable linker ligands (L^2- ). These cages show interesting host-guest chemistry and post-assembly reactions. Remarkably, by employing chiral tris(imido)phosphate trianions, enantiopure chiral cages of the type [(Pd₃ X*)₄ (L)₆ ], ([X*]^3- =RRR- or SSS-[PO(N(*CH(CH₃ )Ph)₃ ]^3- ), were synthesized and used for the chiral-recognition and enantio-separation of small racemic guest molecules. Some of these chiral cages were also shown to exhibit polyradical framework structures. In future, these and other similar types of cages are envisioned as potential molecular vessels for performing the reactions in their confined environment. The enantiomeric cages can be probed for asymmetric catalysis and the separation of a range of small chiral molecules.