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

Abstract

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 H3PO4, H2PO4−, HPO42− and PO43− species. These imido anions are typically generated using strong main group organometallic reagents such as nBuLi, Et2Zn, Me3Al and nBu2Mg, 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 (2Py) functionalized phosphonium salts and phosphoric triamides, we obtained the mono- and dianionic form of these imido ligands {[P(N2Py)2(NH2Py)2]−, [P(N2Py)2(NH2Py)]−, [PO(N2Py)(NH2Py)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)3PO]3− (R=tBu, cHex, iPr) were generated in a facile one pot reaction as their corresponding tri- and hexanuclear clusters of the type {Pd3[(NR)3PO](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[(NiPr)3PO]4(L)6} and cubic {Pd3[(NiPr)3PO]8(L)12} structures in the presence of suitable linker ligands (L2−). 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 [(Pd3X*)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.