Designing the Macrocyclic Dimension in Main Group Chemistry

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Pre-arranged building block approach for the orthogonal synthesis of an unfolded tetrameric organic-inorganic phosphazane macrocycle

Inorganic macrocycles remain challenging synthetic targets due to the limited number of strategies reported for their syntheses. Among these species, large fully inorganic cyclodiphosphazane macrocycles have been experimentally and theoretically highlighted as promising candidates for supramolecular chemistry. In contrast, their hybrid organic-inorganic counterparts are lagging behind due to the lack of synthetic routes capable of controlling the size and topological arrangement (i.e., folded vs unfolded) of the target macrocycle, rendering the synthesis of differently sized macrocycles a tedious screening process. Herein, we report-as a proof-of-concept-the combination of pre-arranged building blocks and a two-step synthetic route to rationally enable access a large unfolded tetrameric macrocycle, which is not accessible via conventional synthetic strategies. The obtained macrocycle hybrid cyclodiphosphazane macrocycle, cis-[μ-P(μ-N^tBu)]₂(μ-p-OC₆H₄C(O)O)]₄[μ-P(μ-N^tBu)]₂ (4), displays an unfolded open-face cavity area of 110.1 Ų. Preliminary theoretical host-guest studies with the dication [MeNC₅H₄]₂²⁺ suggest compound 4 as a viable candidate for the synthesis of hybrid proto-rotaxanes species based on phosphazane building blocks.

Assembly of metallo-macrocycles through reductive C-C coupling of alkylnitriles by an Mg-Mg-bonded compound

Low-valent metal complexes have attracted much research interest owing to their novel reactivities toward small molecules. Herein the reactivity of the α-diimine-ligated, Mg-Mg-bonded compound [K(THF)₃]₂[LMg-MgL] (1, L = [(2,6-iPr₂C₆H₃)NC(Me)]₂^2-) with aliphatic nitriles has been studied. Complex 1 readily activates n-alkylnitriles (RCN; R = propyl, butyl, and pentyl) to afford the unique trinuclear magnesium metallo-macrocyclic complexes, [LMg(μ-{(NC-C(R)C(CH₂R)-NH})]₃[K₃(Solv)₆] (2-4: R = -(CH₂)_nCH₃, n = 2, 3, or 4; Solv = THF/DME), through a reductive deprotonation of the α-H of one nitrile molecule and C-C coupling between this α-carbon and the cyanide (CN) group of another nitrile, followed by a 1,3-H shift. The results demonstrate the possibility of assembling supramolecular architectures based on the α-diimine [LMg] fragment through small molecule activation.

A chiral phosphazane reagent strategy for the determination of enantiomeric excess of amines

Methods for measuring enantiomeric excess (ee) of organic molecules by NMR spectroscopy provide rapid analysis using a standard technique that is readily available. Commonly this is accomplished by chiral derivatisation of the detector molecule (producing a chiral derivatisation agent, CDA), which is reacted with the mixture of enantiomers under investigation. However, these CDAs have almost exclusively been based on carbon frameworks, which are generally costly and/or difficult to prepare. In this work, a methodology based on the readily prepared inorganic cyclodiphosph(iii)azane CDA ClP(μ-N^tBu)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be highly effective in the measurement of ee’s of chiral amines, involving in situ reaction of the chiral amines (R*NH₂) with the P–Cl bond of the CDA followed by quaternization of the phosphorus framework with methyl iodide. This results in sharp ³¹P NMR signals with distinct chemical shift differences between the diastereomers that are formed, which can be used to obtain the ee directly by integration. Spectroscopic, X-ray structural and DFT studies suggest that the NMR chemical shift differences between diastereomers is steric in origin, with the sharpness of these signals resulting from conformational locking of the bornyl group relative to the P₂N₂ ring induced by the presence of the P(v)-bonded amino group (R*NH). This study showcases cheap inorganic phosphazane CDAs as simple alternatives to organic variants for the rapid determination of ee.

The simple inorganic cyclodiphosph(iii)azane chiral derivatisation agent ClP(μ-^tBuN)₂POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be effective in the measurement of ee’s of chiral amines using ³¹P NMR spectroscopy.

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.

N-Bridged Acyclic Trimeric Poly-Cyclodiphosphazanes: Highly Tuneable Cyclodiphosphazane Building Blocks

We have synthesized a completely new family of acyclic trimeric cyclodiphosphazane compounds comprising NH, Nⁱ Pr, N^t Bu and NPh bridging groups. In addition, the first NH-bridged acyclic dimeric cyclophosphazane has been produced. The trimeric species display highly tuneable characteristics so that the distance between the terminal N(H)R moieties can be readily modulated by the steric bulk present in the bridging groups (ranging from ≈6 to ≈10 Å). Moreover, these species exhibit pronounced topological changes when a weak non-bonding NH⋅⋅⋅π aryl interaction is introduced. Finally, the NH-bridged chloride binding affinities have been calculated and benchmarked along with the existing experimental data available for monomeric cyclodiphosphazanes. Our results underscore these species as promising hydrogen bond donors for supramolecular host-guest applications.

Covalent and ionic bonding in bi- and tricyclic Group 15 amides: equidistant P-I and As-I bonds and fluxional cations

A series of trivalent Group 15 bis(tert-butylamido)cyclodiphosph(iii)azane element bi- and tricycles of the formulae {[(tBuNP)2(tBuN)2]ElX}, El = P, As, Sb, Bi, where X = Ph, OPh, OtBu, N3, hexamethyldisilylamide (HMDS), OTf, was synthesized from the corresponding chlorides via salt elimination. The ensuing compounds were studied spectroscopically and X-ray crystallographically with a particular focus on the length of the El-X bond. While the Group 15 element to phenyl and HMDS were of normal lengths and completely covalent, those to azide appeared to be partly ionic. The {[(tBuNP)2(tBuN)2]ElI} showed El-I bonds that were substantially longer than the typical element iodide bonds, suggesting a very high degree of polarity and bordering on ionic bonding. Finally, the triflate [(tBuNP)2(tBuN)2]P] + [SO3CF3]- proved to be an ion pair in the solid state. The antimony analog, however, showed a long covalent Sb-O bond in the solid state, although it appears to dissociate into ions in solution. The phosphonium triflate salt is fluxional and exhibits a previously unseen highly symmetrical structure in solution. The bonding trends from completely covalent to completely ionic are discussed in terms steric restrictions and the delocalization of charge in either the cation or the anion.

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.

Aluminum containing molecular bowls and pyridinophanes: use of pyridine modules to access different molecular topologies

Molecular topologies varying from simple complexes to pyridinophanes (neutral and cationic) and to bicyclic pyridinophane containing organoaluminum (Al-Me) species were synthesized by varying the relative stoichiometry of bis(trimethylsilyl)-N,N'-2,6-diaminopyridine (bap) and the reactive partner (AlMe₃). The ultimate goal of these reactions was to systematically design cyclic structures containing group 13 elements. To highlight the reaction potential of these shapes, the bowl-shaped pyridinophane was reacted with the Lewis acid, B(C₆F₅)₃, to generate a stable cationic derivative. An unprecedented bicyclic pyridinophane, [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂, was obtained from the reaction of bap with AlH₃·NMe₂Et. The formation of [2,6-(Me₃SiN)₂C₅H₃N]₃Al₂ is in contrast to the known reaction between BH₃·SMe₂ and bap that afforded the syn-tetraazadibora[3.3](2,6)pyridinophane. Quantum chemical calculations have been performed to rationalize the preference for the formation of B-pyridinophane and Al-bicyclic pyridinophane and can be attributed to the nature of B-N and Al-N bonds.

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.

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

Herein we describe that oxidation reactions of the dimeric cyclophosphazanes, [{P(μ-NR)}₂(μ-NR)]₂, R = ^tBu (1), to produce a series of diagonally dioxidized products P₄(μ-N ^tBu)₆E₂ [E = O (2), S (3), and Se (4)] and tetraoxidized frameworks. The latter display an unexpected C-N bond activation and cleavage to produce a series of novel phosphazane macrocyclic arrangements containing newly formed N-H bonds. Macromolecules P₄(μ-N ^tBu)₄(μ-NH)₂O₄ (5) and P₄(μ-N ^tBu)₃(μ-NH)₃E₄, E = S (6) and Se (7), dicleaved and tricleaved products, respectively, are rare examples of dimeric macrocycles containing NH bridging groups. Our theoretical and experimental studies illustrate that the extent to which these C-N bonds are cleaved can be controlled by modification of steric parameters in their synthesis, by adjusting either the steric bulk of the substituents in the parent framework or the size of the chalcogen element introduced during the oxidation process. Our findings represent new synthetic pathways for the synthesis of otherwise-elusive macrocycle arrangements within the phosphazane family.

Schiff-base -ate derivatives with main group metals: generation of a tripodal aluminate metalloligand

A new type of -ate derivatives is described, where the alkali metal exerts a strong influence on the final outcome.

Orthogonality in main group compounds: a direct one-step synthesis of air- and moisture-stable cyclophosphazanes by mechanochemistry

Mechanochemistry has been established to be an environmentally-friendly way of conducting reactions in a solvent-free manner.

Formation and selection of the macrocycle [{(tBuN)P(μ-NtBu)}2(μ-Se)2{P(μ-NtBu)}2]3

The Se-bridged P^III/P^V phosphazane macrocycle [{(^tBuN)P^V(μ-N^tBu)}₂(μ-Se)₂{P^III(μ-N^tBu)}₂]₃ is obtained selectively using [Se(^tBuN)P(μ-N^tBu)]₂²⁻ as a nucleophilic component.