Size-control in the synthesis of oxo-bridged phosphazane macrocycles via a modular addition approach

Abstract

Inorganic macrocycles remain largely underdeveloped compared with their organic counterparts due to the challenges involved in their synthesis. Among them, cyclodiphosphazane macrocycles have shown to be promising candidates for supramolecular chemistry applications due to their ability to encapsulate small molecules or ions within their cavities. However, further developments have been handicapped by the lack of synthetic routes to high-order cyclodiphosphazane macrocycles. Moreover, current approaches allow little control over the size of the macrocycles formed. Here we report the synthesis of high-order oxygen-bridged phosphazane macrocycles via a “3 + n cyclisation” (n = 1 and 3). Using this method, an all-PIII high-order hexameric cyclodiphosphazane macrocycle was isolated, displaying a larger macrocyclic cavity than comparable organic crown-ethers. Our approach demonstrates that increasing building block complexity enables precise control over macrocycle size, which will not only generate future developments in both the phosphazane and main group chemistry but also in the fields of supramolecular chemistry.

Synergic Topological- and Size-Control on Phosphazane Chemistry: First Unfolded Hybrid Tetrameric Macrocycle

“…[35] Furthermore, there were no resonances recorded consistent with the formation of the aryl oxide cis di-substituted derivative [(HOC(O)C 6 H 4 O)P(μ-N t Bu)] 2 (2b, in Scheme 1), since no signals were observed at δ ~144 ppmwhere cis acyclic alkoxide or aryloxide species would have been expected. [31][32][33] Additionally, neither signals for the trans di-substituted (2c, in Scheme 1) monomer (i.e., mixed aryl oxide and alkoxide), nor the previously reported trans dimeric macrocycle (3a, Figure 3 bottom) were observed. Unfortunately, attempts to isolate analytically pure samples of 2a were unsuccessful.…”

“…At more than two-fold the average P-O bond distance observed in phosphazane species this further proves the difficulty of the cis-dimeric macrocycle 3b. [31][32][33] Alternatively, rotation around the P-O bond in int I could potentially locate the P-Cl and -Ph-OH terminal moieties close enough for the intramolecular nucleophilic attack required to form 3b to take place. Within this context, an energy scan for the rotation around the P-OC(O)PhOH bond in 2a was calculated.…”

“…[5][6][7][8][9][10][11][12][13][14] Cyclophosphazane species have been widely explored over the past decades due to their chemical versatility, [15] action as excellent neutral and/or anionic ligands for metal coordination [16][17][18][19][20][21][22][23] and building blocks for the construction of larger molecules, as well as utility in biological applications and supramolecular chemistry. [12,13,[24][25][26][27][28][29][30][31][32] Depending on the nature of the bifunctional linkers employed, their reaction with dichlorocyclodiphosphazane, [ClP(μ-N t Bu)] 2 (1), yields differently-sized fully inorganic or hybrid macrocycles. In this context, over the past two decades there has been particularinterest in the area of hybrid P III 2 N 2 macrocycles due to their dual inorganic-organic nature.…”

“…[33,34,39] Within the hybrid phosphazane macrocyclic family there is a large number of reported examples of condensation reactions comprising symmetric bifunctional organic linkers (L-L) to produce hybrid inorganic-organic macrocycles of general formula [(μ-L-L)P(μ-N t Bu)] n . [31,32,34,38] Among these species, the majority are dimeric species (n = 2). [31,32,34] In the case of trimeric P III 2 N 2 macrocycles (n = 3), only two examples have been isolated and fully characterised.…”

“…[31,32,34,38] Among these species, the majority are dimeric species (n = 2). [31,32,34] In the case of trimeric P III 2 N 2 macrocycles (n = 3), only two examples have been isolated and fully characterised. [34,39] Large folded tetrameric macrocycles (n = 4) have only been obtained from 1,4-diaminobenzene in moderate yields, [40] and with resorcinol as a minor product.…”