Ring-Opening Terpolymerisation of Elemental Sulfur Waste with Propylene Oxide and Carbon Disulfide via Lithium Catalysis.

Elemental sulfur, a waste product of the oil refinement process, represents a promising raw material for the synthesis of degradable polymers. We show that simple lithium alkoxides facilitate the polymerisation of elemental sulfur S8 with industrially relevant propylene oxide (PO) and CS2 (a base chemical sourced from waste S8 itself) to give poly(monothiocarbonate-alt-Sx) in which x can be controlled by the amount of supplied sulfur. The in situ generation of thiolate intermediates obtained by a rearrangement, which follows CS2 and PO incorporation, allows to combine S8 and epoxides into one polymer sequence that would otherwise not be possible. Mechanistic investigations reveal that alkyl oligosulfide intermediates from S8 ring opening and sulfur chain length equilibration represent the better nucleophiles for inserting the next PO if compared to the trithiocarbonates obtained from the competing CS2 addition, which causes the sequence selectivity. The polymers can be crosslinked in situ with multifunctional thiols to yield reprocessable and degradable networks. Our report demonstrates how mechanistic understanding allows to combine intrinsically incompatible building blocks for sulfur waste utilisation.

Easy Synthetic Access to High-Melting Sulfurated Copolymers and their Self-Assembling Diblock Copolymers from Phenylisothiocyanate and Oxetane.

Although sulfurated polymers promise unique properties, their controlled synthesis, particularly when it comes to complex and functional architectures, remains challenging. Here, we show that the copolymerization of oxetane and phenyl isothiocyanate selectively yields polythioimidocarbonates as a new class of sulfur containing polymers, with narrow molecular weight distributions (Mn=5-80 kg/mol with D≤1.2; Mn,max=124 kg/mol) and high melting points of up to 181 °C. The method tolerates different substituent patterns on both the oxetane and the isothiocyanate. Self-nucleation experiments reveal that π-stacking of phenyl substituents, the presence of unsubstituted polymer backbones, and the kinetically controlled linkage selectivity are key factors in maximising melting points. The increased tolerance to macro-chain transfer agents and the controlled propagation allows the synthesis of double crystalline and amphiphilic diblock copolymers, which can be assembled into micellar- and worm-like structures with amorphous cores in water. In contrast, crystallization driven self-assembly in ethanol gives cylindrical micelles or platelets.

Templation and Concentration Drive Conversion Between a FeII12L12 Pseudoicosahedron, a FeII4L4 Tetrahedron, and a FeII2L3 Helicate.

We report the construction of three structurally distinct self-assembled architectures: FeII12L12 pseudoicosahedron 1, FeII2L3 helicate 2, and FeII4L4 tetrahedron 3, formed from a single triazatriangulenium subcomponent A under different reaction conditions. Pseudoicosahedral capsule 1 is the largest formed through subcomponent self-assembly to date, with an outer-sphere diameter of 5.4 nm and a cavity volume of 15 nm3. The outcome of self-assembly depended upon concentration, where the formation of pseudoicosahedron 1 was favored at higher concentrations, while helicate 2 exclusively formed at lower concentrations. The conversion of pseudoicosahedron 1 or helicate 2 into tetrahedron 3 occurred following the addition of a CB11H12- or B12F122- template.

Heterocycle/Heteroallene Ring-Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers.

Heteroatom-containing polymers have strong potential as sustainable replacements for petrochemicals, show controllable monomer-polymer equilibria and properties spanning plastics, elastomers, fibres, resins, foams, coatings, adhesives, and self-assembled nanostructures. Their current and future applications span packaging, house-hold goods, clothing, automotive components, electronics, optical materials, sensors, and medical products. An interesting route to these polymers is the catalysed ring-opening copolymerisation (ROCOP) of heterocycles and heteroallenes. It is a living polymerization, occurs with high atom economy, and creates precise, new polymer structures inaccessible by traditional methods. In the last decade there has been a renaissance in research and increasing examples of commercial products made using ROCOP. It is better known in the production of polycarbonates and polyesters, but is also a powerful route to make N-, S-, and other heteroatom-containing polymers, including polyamides, polycarbamates, and polythioesters. This Review presents an overview of the different catalysts, monomer combinations, and polymer classes that can be accessed by heterocycle/heteroallene ROCOP.

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-Nt 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-Nt 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.

Heterotrimetallic Carbon Dioxide Copolymerization and Switchable Catalysts: Sodium is the Key to High Activity and Unusual Selectivity.

A challenge in polymer synthesis using CO2 is to precisely control CO2 placement in the backbone and chain end groups. Here, a new catalyst class delivers unusual selectivity and is self-switched between different polymerization cycles to construct specific sequences and desirable chain-end chemistries. The best catalyst is a trinuclear dizinc(II)sodium(I) complex and it functions without additives or co-catalysts. It shows excellent rates across different ring-opening (co)polymerization catalytic cycles and allows precise control of CO2 incorporation within polyesters and polyethers, thereby allowing access to new polymer chemistries without requiring esoteric monomers, multi-reactor processes or complex post-polymerization procedures. The structures, kinetics and mechanisms of the catalysts are investigated, providing evidence for intermediate speciation and uncovering the factors governing structure and composition and thereby guiding future catalyst design.

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-NtBu)]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.

Tris(2-pyridyl) Bismuthines: Coordination Chemistry, Reactivity, and Anion-Triggered Pyridyl Coupling.

A series of new tris(2-pyridyl) bismuthine ligands of the type [Bi(2-py')3] have been prepared, containing a range of substituents at various positions within their pyridyl rings (py'). They can act as intact ligands or, as a result of the low C-Bi bond energy, exhibit noninnocent reactivity in the presence of metal ions. Structural studies of Li+ and Ag+ complexes show that the coordination to metal ions using their pyridyl-N atoms and to anions using the Lewis acidity of their Bi(III) centers can be modified by the presence of substituents within the 2-pyridyl rings, especially at the 6- or 3-positions, which can block the donor-N or Lewis acid Bi sites. Electron withdrawing groups (like CF3 or Br) can also severely reduce their ability to act as ligands to metal ions by reducing the electron donating ability of the pyridyl-N atoms. Noninnocent character is found in the reactions with Cu+ and Cu2+, resulting in the coupling of pyridyl groups to form bipyridines, with the rate of this reaction being dependent on the anion present in the metal salts. This leads to the formation of Bi(III)/Cu(I) complexes containing hypervalent [X2Bi(2-R-py)]- (X = Cl, Br) anions. Alternatively, the tris(2-pyridyl) bismuthine ligands can act as 2-pyridyl transfer reagents, transferring 2-py groups to Au(I) and Fe(II).

Hydrogen-Bond-Assisted Symmetry Breaking in a Network of Chiral Metal-Organic Assemblies.

Herein we elucidate the interplay of chiral, chelate, solvent, and hydrogen-bonding information in the self-assembly of a series of new three-dimensional metal-organic architectures. Enantiopure ligands, each containing H-bond donors and acceptors, form different structures, depending on the ratio in which they are combined: enantiopure components form M4L4 assemblies, whereas racemic mixtures form M3L3 stacks. Chiral amplification within M3L3 enantiomers was observed when a 2:1 ratio of R and S subcomponent enantiomers was employed. Simply switching the solvent (from MeCN to MeOH) or chelating unit (from bidentate to tridentate) increased the diversity of structures that can be generated from these building blocks, leading to the selective formation of novel M2L2 and M3L2 assemblies. The addition of achiral ligand building blocks resulted in the formation of further structures: When an achiral subcomponent was combined with its R and S chiral congeners, a three-layer heteroleptic architecture was generated, with the achiral unit sitting at the top of the stack. When combined with the S enantiomer only, however, the achiral unit assembled in the center of the structure, thus demonstrating the selective placement of achiral units within chiral systems. Further sorting experiments revealed that combining R and S stereocenters within a single ligand led to diastereoselective product generation. These results show how geometric complementarity between different ligands impacts upon the degree of hydrogen-bonding within the assembly, stabilizing specific low-symmetry architectures from among many possible structural outcomes.

Tailoring the Binding Properties of Phosphazane Anion Receptors and Transporters.

The binding and sensing of anions is an important cross-disciplinary field, which impacts broad areas such as biology, supramolecular chemistry and catalysis. To date, however, this area has been dominated by organic architectures which function as H-bonding, anion receptor molecules. Inorganic anion receptors have largely been based on Lewis acidic metals, with very few examples of H-bonding counterparts of organic systems having been systematically studied. This paper develops strategies for enhancing the anion binding properties of phosphazanes of the type [(RNH)(E)P(µ-N tBu)]2 (E = O, S, Se) which are bench-stable, H-bond receptors that can be regarded as inorganic analogues of squaramides (a key class of organic anion receptor). The distinct advantages of these inorganic receptors over organic counterparts is the ease by which their functionality and electronic character can be altered (by means of the R group, chalcogenide, or metal present). Se substitution at the P centers, the presence of electron-withdrawing R groups, and metal coordination to the soft donor centers can be used to modulate and enhance anion binding. The water stability and superior anion binding properties of the seleno-phosph(V)azanes give them applications as synthetic anion transporters through phospholipid layers.

A Tris(3-pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages.

The systematic assembly of supramolecular arrangements is a persistent challenge in modern coordination chemistry, especially where further aspects of complexity are concerned, as in the case of large molecular mixed-metal arrangements. One targeted approach to such heterometallic complexes is to engineer metal-based donor ligands of the correct geometry to build 3D arrangements upon coordination to other metals. This simple idea has, however, only rarely been applied to main group metal-based ligand systems. Here, we show that the new, bench-stable tris(3-pyridyl)stannane ligand PhSn(3-Py)3 (3-Py=3-pyridyl) provides simple access to a range of heterometallic SnIV /transition metal complexes, and that the presence of weakly coordinating counter anions can be used to build discrete molecular arrangements involving anion encapsulation. This work therefore provides a building strategy in this area, which parallels that of supramolecular transition metal chemistry.

Fluorometric Recognition of Nucleotides within a Water-Soluble Tetrahedral Capsule.

The design of aqueous probes and binders for complex, biologically relevant anions presents a key challenge in supramolecular chemistry. Herein, a tetrahedral assembly with cationic faces and corners is reported that is capable of discriminating between anionic and neutral guests in water. Electrostatic repulsion between subcomponents can be overcome by the addition of an anionic template, or generating a robust covalent framework by incorporating tris(2-aminoethyl)amine (TREN). The resultant TREN-capped, water-soluble, fluorescent cage binds mono- and poly-phosphoric esters, including nucleotides. Its covalent skeleton renders it stable at micromolar concentrations in water, enabling the fluorometric detection of biologically relevant guests in an aqueous environment. Selective supramolecular encapsulants, such as 1, could enable new sensing applications, such as recognition of toxins and drugs, under biological conditions.

Guest Binding via N-H⋠⋠⋠π Bonding and Kinetic Entrapment by an Inorganic Macrocycle.

Modern supramolecular chemistry is overwhelmingly based on non-covalent interactions involving organic architectures. However, the question of what happens when you depart from this area to the supramolecular chemistry of structures based on non-carbon frameworks remains largely unanswered, and is an area that potentially provides new directions in molecular activation, host-guest chemistry, and biomimetic chemistry. In this work, we explore the unusual host-guest chemistry of the pentameric macrocycle [{P(µ-Nt Bu}2 NH]5 with a range of anionic and neutral guests. The polar coordination site of this host promotes new modes of guest encapsulation via hydrogen bonding with the π systems of the unsaturated C≡C and C≡N bonds of acetylenes and nitriles as well as with the PCO- anion. Halide guests can be kinetically locked within the structure by oxidation of the phosphorus periphery by oxidation to PV . Our study underscores the future promise of p-block macrocyclic chemistry.

Designing the Macrocyclic Dimension in Main Group Chemistry.

Outside the confines and well-established domain of organic chemistry, the systematic building of large macromolecular arrangements based on non-carbon elements represents a significant and exciting challenge. Our aim in the past two decades has been to develop robust synthetic methods to construct new types of main group architectures in a methodical way, principles of design that parallel those used in the organic arena. This Concept article addresses the fundamental thermodynamic and kinetic problems involved in the design and synthesis of main group macrocycles and looks to future developments of macromolecules in this area, as well as new applications in coordination chemistry.

Flexible Bonding of the Phosph(V)azane Dianions [S(E)P(µ-NtBu)]22.

Oxidation of the PIII dianion [S-P(µ-NtBu)]22- (1) with elemental sulphur, selenium and tellurium gives the PV dianions [(S)(E)P(µ-NtBu)]22- (E = S (6 a), Se (6 b), Te (6 c)). Although 6 c proves to be too unstable, the S,S-dianion 6 a and ambidentate S,Se-dianion 6 b are readily transferred intact to main group and transition metal elements, producing a range of new cage and coordination compounds. While their coordination characteristics are in many ways similar to closely-related isoelectronic phosph(V)azane anions [(E)(RN=)P(µ-NtBu)]22- , the sterically unhindered nature of 6 introduces an expanded range of coordination modes, that is, facial S,S- and Se,Se-bonding as well as side-on S,Se-coordination. All of these bonding modes are observed for the amibidentate S,Se dianion 6 b.

Postfunctionalization of Tris(pyridyl) Aluminate Ligands: Chirality, Coordination, and Supramolecular Chemistry.

Postfunctionalization of the aluminate anion [EtAl(6-Me-2-py)3 ]- (1) (2-py=2-pyridyl) with alkoxide ligands can be achieved by the selective reactions of the lithium salt 1 Li with alcohols in the appropriate stoichiometry. This method can be used to introduce 3- and 4-py functionality in the form of 3- and 4-alkoxymethylpyridyl groups, while maintaining the integrity of the aluminate framework, thereby giving entry to new supramolecular chemistry. Chirality can be introduced either by using a chiral alcohol as a reactant or by the stepwise reaction of 1 Li with two different nonchiral alcohols. The latter route has allowed the synthesis of a rare example of a chiral-at-aluminium aluminate.

Different Selectivities in the Insertions into C(sp2 )-H Bonds: Benzofulvenes by Dual Gold Catalysis Competition Experiments.

An unprecedented, often almost quantitative access to tricyclic aromatic compounds by dual gold catalysis was developed. This synthetic route expands the scope of benzofulvene derivatives through a C(sp2 )-H bond insertion in easily available starting materials. The insertion takes place with an exclusive chemoselectivity with respect to the competing aromatic C-H positions. A bidirectional synthesis with two competing ortho-aryl C-H bonds in the selectivity-determining step also shows perfect selectivity; this result is explained by a computational investigation of the two conceivable intermediates. The intramolecular competition of two non-equivalent aryl C-H bonds with a benzylic methyl group also showed perfect selectivity.

How Changing the Bridgehead Can Affect the Properties of Tripodal Ligands.

Although a multitude of studies have explored the coordination chemistry of classical tripodal ligands containing a range of main-group bridgehead atoms or groups, it is not clear how periodic trends affect ligand character and reactivity within a single ligand family. A case in point is the extensive family of neutral tris-2-pyridyl ligands E(2-py)3 (E=C-R, N, P), which are closely related to archetypal tris-pyrazolyl borates. With the 6-methyl substituted ligands E(6-Me-2-py)3 (E=As, Sb, Bi) in hand, the effects of bridgehead modification alone on descending a single group in the periodic table were assessed. The primary influence on coordination behaviour is the increasing Lewis acidity (electropositivity) of the bridgehead atom as Group 15 is descended, which not only modulates the electron density on the pyridyl donor groups but also introduces the potential for anion selective coordination behaviour.

A Modular Approach to Inorganic Phosphazane Macrocycles.

Inorganic macrocycles, based on non-carbon backbones, present exciting synthetic challenges in the systematic assembly of inorganic molecules, as well as new avenues in host-guest and supramolecular chemistry. Here we demonstrate a new high-yielding modular approach to a broad range of trimeric and hexameric S- and Se-bridged inorganic macrocycles based on cyclophosphazane frameworks, using the building blocks [S=(H)P(µ-NR)]2 . The method involves the in situ generation of the key intermediate [E....._ (S....._ )P(µ-NR)]22- (E=S, Se) dianion, which can be reacted with electrophilic [ClP(µ-NR)]2 to give PIII /PV hexameric rings or reacted with I2 to give trimeric PV variants. Important issues which are highlighted in this work are the competitive bridging ability of S versus Se in these systems and the synthesis of the first air-stable and chiral inorganic macrocycles.

Improved access to polythioesters by heterobimetallic aluminium catalysis.

Bimetallic Al(III) catalysis mediates thioanhydride/epoxide copolymerisation at greatly improved rates and monomer tolerance than analogous Cr(III) catalysis. Moving to sulfurated monomers furthermore generally improves rates and selectivites.