A Modular Approach to Inorganic Phosphazane Macrocycles

Abstract

These are not the final page numbers! Ü Ü Scheme 2. The hexameric P III /P V macrocycles (D) and trimeric all-P V macrocycles (E). Scheme 3. Left: Synthesis of the hexamers 3. Deprotonation (2), oxidation (3) and reaction with [ClP(m-NR)] 2 are done in situ without any further purification necessary. R' = t Bu (3-t Bu), neopentyl (3-CH 2 t Bu), R-1-(2-naphthyl)ethyl (3-R-naphEt) Right: Solid-state structure of 3-t Bu. Thermal ellipsoids are set at 50 % probability; Hydrogen atoms are omitted for clarity. Color code: yellow = S, purple = P, blue = N. The solid-state structure of 3-CH 2 tBu is shown in the Supporting Information, along with details of the data collections and refinements and selected bond lengths and angles for 3-t Bu and 3-CH 2 t Bu.

Synthesis of Unique Phosphazane Macrocycles via Steric Activation of C–N Bonds

“…In the case of inorganic macrocycles, ,,, it is well-established that their size is directly influenced by the nature and steric bulk of the bridging atoms or groups present within the macrocyclic backbone. For instance, tetrameric or pentameric P­(III) macrocycles are formed when small NH groups or oxygen atoms act as bridging atoms, respectively; ,, whereas dimeric macrocycles are favored when the steric bulk of the bridging groups is increased from NH to N i Pr or N t Bu groups. ,,− Selective formation of pentameric over tetrameric macrocyclic species can be achieved by halide templating during the condensation reactions leading to the formation of the desired macrocyclic arrangements (Figure a,b). ,,, Large mixed-valence macrocyclic arrangements (Figure c) can also be formed if the dimeric phosphazane [ClP­(μ-NR)] 2 reacts with its chalcogen-oxidized anionic counterparts [EP­(S)­(μ-NR)] 2 2– (E = S or Se) . Despite the increasing attention, their intrinsic bond lability has impeded phosphazane frameworks from becoming commonplace in technological applications.…”

“…24,25,28,29 Large mixed-valence macrocyclic arrangements (Figure 2c) can also be formed if the dimeric phosphazane [ClP(μ-NR)] 2 reacts with its chalcogen-oxidized anionic counterparts [EP(S)(μ-NR)] 2 2− (E = S or Se). 30 Despite the increasing attention, their intrinsic bond lability has impeded phosphazane frameworks from becoming commonplace in technological applications. Previously, we have demonstrated the enhancement of air and hydrolytic stability of cyclophosph(III/III)azane frameworks upon oxidation of the phosphorus centers.…”

“…31,32 This finding was also corroborated by the Stahl and Wright groups who reported similar conclusions in phosphazane metal complexes and macrocyclic arrangements, respectively. 30,33 Subsequent to our initial findings on acyclic phosph(III/ III)azanes, we investigated the stability of macrocyclic phosphazane frameworks of the type [{P(μ-N t Bu)} 2 (μ-N t Bu)] 2 (1) upon oxidation with selenium, which resulted in an unexpected C−N bond activation and cleavage process. 34 Herein, we demonstrate that this process can be exploited rationally, by judiciously fine-tuning steric factors to produce a series of macrocyclic compounds with varying degrees of bond cleavage.…”