Recent aspects of the structure and reactivity of cyclophosphazenes

Abstract

This review describes the reactivity and structural aspects of cyclophosphazenes. The reactions of halogenocyclophosphazenes with a variety of monofunctional and difunctional reagents are described. The mechanisms of the reactions involved are discussed in light of kinetic evidence and product analysis. Reactions involving main-group and transition-metal reagents and halogenocyclophosphazenes are also described. The chemistry of hydridophosphazenes containing a P-H bond is described in more detail. Some of the rearrangement reactions including cis-trans isomerism and tautomerism which are common to some cyclophosphazene derivatives have been discussed. The coordination chemistry involving cyclophosphazenes as ligands via endocydic and/or exocyclic coordination sites resulting in several types of derivatives including metallocenyl cyclophosphazenes, metal dusters and carboranyl cyclophosphazenes has been critically evaluated. The structural aspects of cyclophosphazenes have been reviewed with particular emphasis on multinuclear NMR and single crystal X-ray methods. Bond angle-bond length variations in X-ray structures of cyclophosphazenes have been explained based on the theories of bonding prevalent in these molecules.

Phosphazenes as scaffolds for the construction of multi-site coordination ligands

“…The bond length differences observed are consistent with the bonding model of Craig and Paddock. [1][2][3] Accordingly when the lone pair on the ring nitrogen is involved in coordination to the metal ion it is not available for p-bonding within the ring with a consequent increase in the corresponding P-N bond length. The coordination environment around the metal in most instances also deviates from regular geometry to accommodate the ligation mode of the pyrazolylcyclophosphazene ligand.…”

“…The substitution of P-X bonds in [NPX 2 ] n (X = Cl or F; n = 3 or 4) by many nucleophilic reagents has been studied in detail as this forms the principal route to synthesise a variety of organocyclophosphazenes containing exocyclic P-N, P-O, P-S or P-C bonds. 1,2 The sequential substitution of the P-X bonds from [NPX 2 ] n by nucleophiles has also been of great interest in view of the stereo-and regioselectivities that are involved in these reactions. 3 A second reason of interest in these compounds stems from the close relationship that exists between these ring systems with the high molecular weight polyphosphazenes, the largest family of inorganic polymers known.…”

Silver Complexes of Cyclic Hexachlorotriphosphazene

“…[2,5] It is known that the P 3 N 3 Y 6 molecules (Y = alkoxy, alkyl, NH 2 , NHR, NR 2 , etc.) are versatile ligands for transition metals, for example, P 3 N 3 -(NMe 2 ) 4 (NHCH 2 CH 2 NH)·PdCl 2 , [6] P 3 N 3 (OPh) 4 (Pz) 2 ·PdCl 2 , [6] and P 3 N 3 (OPh) 4 (Pz) 2 ·Mo(CO) 3 [1] (Pz = dimethylpyrazolyl). Some covalently bound complexes in which a transitionmetal fragment substitutes one or two halogen atoms in P 3 N 3 X 6 (X = halogen), are also known, for example, [(CO) 2 CpFe]P 3 N 3 F 4 , [7] N 3 P 3 Cl 5 [Cr(CO) 3 Cp], [8] and N 3 P 3 Cl 4 (Cp)[Mo(CO) 3 Cp].…”

“…Cyclic phosphazenes of the general formula (PNY 2 ) n (n = 3-7) are prominent examples of inorganic ring systems [1][2][3][4] with applications in polymer chemistry. [2,5] It is known that the P 3 N 3 Y 6 molecules (Y = alkoxy, alkyl, NH 2 , NHR, NR 2 , etc.)…”

Synthesis and Characterization of Side Group–Modified Cyclotetraphosphazene Derivatives

“…[13][14][15][16][17][18][19][20] Cyclotetraphosphazenes, which contain a flexible eight-membered ring rather than the rigid planar six-membered ring present in their trimer analogues, might form complexes for efficient DNA cleavage. 21 Recently, we reported a series of polydentate cyclotriphosphazene ligands, 22,23 which showed good nuclease activity with hydrolytic cleavage mechanism. To obtain more insight into the selective recognition and efficient cleavage of DNA by different metal complexes of cyclotetraphosphazenes, we synthesized two eight-armed, star-branched cyclotetraphosphazenes.…”