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

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-P^III 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.

The Coordination Chemistry of the N-Donor-Substituted Phosphazanes

Phosph(III)azanes, featuring the heterocyclobutane P₂ N₂ ring, have now been established as building blocks in main-group coordination and supramolecular compounds. Previous studies have largely involved their use as neutral P-donor ligands or as anionic N-donor ligands, derived from deprotonation of amido-phosphazanes [RNHP(μ-NR)]₂ . The use of neutral amido-phosphazanes themselves as chelating, H-bond donors in anion receptors has also been an area of recent interest because of the ease by which the proton acidity and anion binding constants can be modulated, by the incorporation of electron-withdrawing exo- and endo-cyclic groups (R) and by the coordination of transition metals to the ring P atoms. We observed recently that the effect of P,N-chelation of metal atoms to the P atoms of cis-[(2-py)NHP(μ-N^t Bu)]₂ (2-py=2-pyridyl) not only pre-organises the N-H functionality for optimum H-bonding to anions but also results in a large increase in anion binding constants, well above those for traditional organic receptors like squaramides and ureas. Here, we report a broader investigation of ligand chemistry of [(2-py)NHP(μ-^t NBu)]₂ (and of the new quinolyl derivative [(8-Qu)NHP(μ-N^t Bu)]₂ (8-Qu=8-quinolyl). The additional N-donor functionality of the heterocyclic substituents and its position has a marked effect on the anion and metal coordination chemistry of both species, leading to novel structural behaviour and reactivity compared to unfunctionalized counterparts.

Charge-assisted phosph(v)azane anion receptors

Coordination of Cu(i) or Pd(ii) to seleno-cyclodiphosph(v)azanes of the type [RNH(Se)P(μ-NtBu)]2 results in positively charged anion receptor units which have increased anion affinity over the neutral seleno-phosph(v)azanes, due to the increase in electrostatic interactions between the receptor and the guest anions. The same effect is produced by replacement of one of the P[double bond, length as m-dash]Se units by a P-Me+ unit.

Conformational Control in Main Group Phosphazane Anion Receptors and Transporters

Anion binding by receptor molecules is a central field of modern chemistry which impacts areas of catalysis as well as biological and materials chemistry. As binding often requires high chemical stability under aerobic and aqueous conditions for practical applications, carbon-based anion receptors have dominated this field, with main group element analogues receiving far less attention. The recent observation that the air- and moisture-stable amino-cyclophosph(V)azanes of the type [RN(E)P(μ-NR)]₂ (E = O, S, Se) can exhibit halide binding that is competitive with topologically related organic receptors (such as squaramides and thioureas) has motivated us here to explore how the binding properties of phosphazane receptors can be enhanced further. Coordination of transition metals by the two P,N metal coordination sites of the phosph(III)azane dimer [(2-py)NHP(μ-N^tBu)]₂ not only activates the receptor for anion binding (by fixing the optimum exo-exo conformation and polarizing the endocyclic N-H substituents) but also stabilizes the P₂N₂ ring to hydrolysis and oxidation. We show how the binding properties of these receptors can be modulated by the coordinated metal fragments and that they can bind chloride 1 to 2 orders of magnitude stronger than the related squaramides and thioureas. These features can be utilized in anion transport through phospholipid bilayers under aqueous conditions for which transport can be improved by 1 order of magnitude compared to the previous best phosphazane and thiourea transporters. This study demonstrates how careful design of inorganic systems can result in potent supramolecular functionality, beyond that observed for organic counterparts.

Catechol and 1,2,4,5-tetrahydroxybenzene functionalized cyclodiphosphazane ligands: synthesis, structural studies, and transition metal complexes

This paper describes the syntheses of two novel cyclodiphosphazane derivatives and their coordination chemistry with Cu^I, Ru^II, Rh^I, Pd^II and Au^I is also described.