Synthesis and functional studies of transition metal complexes derived from amido and imido P(V) ligands

Abstract

Imido-analogues of common phosphorus oxo-anions have been the topic of immense research interest in main group chemistry and are employed as ligands in coordination chemistry. Earlier reported procedures to access these imido-P(V) anions involve the use of highly reactive main-group metal reagents in reaction with a phosphonium salt like [(RNH)4P]Cl or phosphoramides such as [(RNH)3P=E] (E = NSiMe3, O, S or Se) or with preformed iminophosphoranes. However due to the highly reactive nature of these metal precursors as well as to the presence of residual metal-alkyl/aryl/halide/silylamide bonds in these complexes, the imido ligand chemistry has largely been limited to main-group metal ions in anhydrous aprotic and non-polar solvents. In this effort, we have developed new synthetic routes in which salts of Cu(I), Ag(I) were shown to be efficient in deprotonating phosphonium cation such as [(NH2Py)4P]+ or the phosphoric triamide, [(NH2Py)3PO] (2Py = 2-pyridyl) and leads to the formation of imido-anions analogous to H2PO4− and HPO42− ions in CH3OH/H2O medium. Employing the salts of Pd(II) ions in deprotonation reactions, an efficient and facile route to access the highly basic trianion, [(RN)3PO]3‒ analogous to PO43− ion, as its tri- or hexa-nuclear Pd(II) complexes was developed. In addition to their structural novelty, these imido-Pd(II) clusters serve as excellent synthons for supramolecular assemblies and as efficient catalysts for organic transformations. In related systems, peripherally functionalized multi-site amido-P(V) ligands have been synthesized and employed in generating several novel functional materials. For example, coordination polymers of composition [ZnL(HCO2)]∞ and [CuL2(H2O)2]∞ were synthesized from an in-situ generated ligand [PO2(NH4Py)2]− (L¯). These polymers exhibit a high preferential adsorption of water vapours over alcohols, having industrial implication in the purification of bioethanol.