AbstractIn this review, we account the recent advancements on cyclotriphosphazene (N3P3Cl6) based multi‐site coordination ligands for the synthesis of multi‐metallic architectures. In case of N3P3Cl6, Phosphorous centres with two chlorides use to be bonded with nitrogen [−P(Cl2)=N−]3 at alternative positions in six membered ring to provide a robust planar structure to anchor the ligand arms with wide flexibility. Cyclic P−N robust motif is renowned as the potential support for the design of multi‐site ligands originating from phosphorus centres by viable substitution of variable number of ligand units (1 to 6) consist of one or more coordination site(s) via geminal and/or non‐geminal modes. Despite these phosphorus centres are promising to support in ligand structure, occasionally involve in coordination with transition metals as well. Three of alternative nitrogen atoms in this heterocyclic core are supposed to function as Lewis base centres while the electron releasing substituents occurred to tether on phosphorus centres, also when the resultant ring size favours adequate interaction towards the transition metal ions with respect to skeletal flexibility. The ligand modes vary with respect to numbers and orientation of coordination sites, which is imperative to enrich the ligation ability. Moreover, several examples of cyclotriphosphazene core ligands consist of the spacer oxygen atom while tethering with exocyclic ligand units to incorporate more flexibility. In accordance with coordination sites on ligand framework and choice of metal, the skeletal ring nitrogen atom(s) of the cyclotriphosphazene can be involved in coordination with metal ions. Indeed, the coordination of ligands is reported to coordinate with main group, transition and lanthanide metal ions to form variety of metal clusters and porous coordination polymers. Subsequently, the geometry of homo or heterometallic complexes with respect to versatile coordination modes of metallic complexes tends to afford inherent chemical properties for specific application. In terms of exocyclic ligands, the heterocyclic units such as pyridyloxy‐, pyrazolyl‐, pyridylalkylamino‐, bipyridyl‐, 1,10‐phenanthroline, porphyrinato, hydrazide, hydrazone, Schiff's base and spirocyclic amine units attached to cyclotriphosphazene were exploited to form series of metal complexes. Similarly, the variety of metal clusters, polymeric network (1D to 3D) and porous coordination polymers have been reported on basis of cyclic core molecule substituted with ligand units, pyridyloxy‐, multi‐carboxylic, aromatic amines substituted pyridyloxy‐ and imidazole moiety involved to form variety of products.
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Jun 3, 2021
Sanjit K Mahato + 3
Open Access