Electrochemical Synthesis of Poly(trisulfides).

With increasing interest in high sulfur content polymers, there is a need to develop new methods for their synthesis that feature improved safety and control of structure. In this report, electrochemically initiated ring-opening polymerization of norbornene-based cyclic trisulfide monomers delivered well-defined, linear poly(trisulfides), which were solution processable. Electrochemistry provided a controlled initiation step that obviates the need for hazardous chemical initiators. The high temperatures required for inverse vulcanization are also avoided resulting in an improved safety profile. Density functional theory calculations revealed a reversible "self-correcting" mechanism that ensures trisulfide linkages between monomer units. This control over sulfur rank is a new benchmark for high sulfur content polymers and creates opportunities to better understand the effects of sulfur rank on polymer properties. Thermogravimetric analysis coupled with mass spectrometry revealed the ability to recycle the polymer to the cyclic trisulfide monomer by thermal depolymerization. The featured poly(trisulfide) is an effective gold sorbent, with potential applications in mining and electronic waste recycling. A water-soluble poly(trisulfide) containing a carboxylic acid group was also produced and found to be effective in the binding and recovery of copper from aqueous media.

Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism.

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd2L4 metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 106 M-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.

Polymer networks of imine-crosslinked metal-organic cages: tuneable viscoelasticity and iodine adsorption.

The solution-state structure of an amine-functionalised Cu24L24 cage (MOP-15) is elucidated, enabling its direct covalent crosslinking into a series of highly tuneable organogels. These soft porous networks exhibit up to a ∼10-fold increase in capacity for iodine compared to the discrete cage precursor.

Assembly of a Heterometallic Cu(II)-Pd(II) Cage by Post-assembly Metal Insertion.

Porous structures based on multi-metallic motifs are receiving growing interest, but their general preparation still remains a challenge. Here, we report the self-assembly and structure of a CuII metal-organic cage (MOC) that is functionalized with free bis(pyrazolyl)methane sites. The homometallic Cu4L4 cage is isolated as a water-stable crystalline solid, and its formation is dependent on metal-ligand stoichiometry and the pre-organization of the Cu2 paddlewheel. We show by X-ray diffraction and SEM-EDX that PdII chloride can be quantitatively inserted into the free chelating sites of the MOC to yield a [Cu4(L(PdCl2))4] structure. Moreover, the solvent employed in the metalation dictates the solid-state isomerism of the heterometallic cage─a further handle to control the MOC's structural diversity and permanent porosity.

A europium metal-organic framework for dual Fe3+ ion and pH sensing.

Metal-organic frameworks (MOFs) with ratiometric sensing properties are desirable for many applications due to their intrinsic self-calibration. We report the re-assessment of the sensing properties of a MOF, originally reported as containing europium(III) and 2-hydroxyterephtalic acid, and having fluorescent ratiometric iron(III) sensing properties. Synchrotron single-crystal X-ray diffraction and proton nuclear magnetic resonance (1H NMR) spectroscopy revealed that the MOF is composed of 2-methoxyterephthalate, not 2-hydroxyterephthalate as originally reported. We found that the MOF exhibits a sensor turn-off response towards Fe3+ ion concentrations in the range 0.5-3.7 ppm (band 425 nm), and a turn-on response towards a decrease of pH from 5.4 to 3.0 (band 375 nm), both resulting from the addition of acidic Fe3+ salt solution to a MOF suspension. Thus, the ratiometric sensing properties and the originally proposed mechanism no longer apply; our work reveals a dynamic quenching mechanism for the fluorescence turn-off response due to the presence of Fe3+ ions, and a ligand protonation mechanism for the turn-on response to a decrease in pH. Our work highlights the importance of a thorough investigation of the structure of any newly synthesized MOF, and, in the case of potential sensors, their selectivity and any environmental effects on their sensing behavior.

Guest-Dependent Isomer Convergence of a Permanently Fluxional Coordination Cage.

A fluxional bis-monodentate ligand, based on the archetypal shape-shifting molecule bullvalene, self-assembles with M2+ (M=Pd2+ or Pt2+ ) to produce a highly complex ensemble of permanently fluxional coordination cages. Metal-mediated self-assembly selects for an M2 L4 architecture while maintaining shape-shifting ligand complexity. A second level of simplification is achieved with guest-exchange; the binding of halides within the M2 L4 cage mixture results in a convergence to a cage species with all four ligands present as the "B isomer". Within this confine, the reaction graph of the bullvalene is greatly restricted, but gives rise to a mixture of 38 possible diastereoisomers in rapid exchange. X-ray crystallography reveals a preference for an achiral form consisting of both ligand enantiomers. Through a combination of NMR spectroscopy and DFT calculations, we elucidate the restricted isomerisation pathway of the permanently fluxional M2 L4 assembly.

Assembly and Covalent Cross-Linking of an Amine-Functionalised Metal-Organic Cage.

The incorporation of reactive functional groups onto the exterior of metal-organic cages (MOCs) opens up new opportunities to link their well-defined scaffolds into functional porous solids. Amine moieties offer access to a rich catalogue of covalent chemistry; however, they also tend to coordinate undesirably and interfere with MOC formation, particular in the case of Cu2 paddlewheel-based MOCs. We demonstrate that tuning the basicity of an aniline-functionalized ligand enables the self-assembly of a soluble, amine-functionalized Cu4L4 lantern cage (1). Importantly, we show control over the coordinative propensity of the exterior amine of the ligand, which enables us to isolate a crystalline, two-dimensional metal-organic framework composed entirely of MOC units (2). Furthermore, we show that the nucleophilicity of the exterior amine of 1 can be accessed in solution to generate a cross-linked cage polymer (3) via imine condensation.

Self-sorting of porous Cu4L2L'2 metal-organic cages composed of isomerisable ligands.

We report the self-sorting of a dynamic combinatorial library (DCL) of metal-organic cages composed of a rotationally isomerisable ligand. Convergence of the DCL occurs upon crystallisation and leads to low-symmetry Cu4L2L'2 cages that display differing porosities based on their overall shape and ligand configuration.

Linking metal-organic cages pairwise as a design approach for assembling multivariate crystalline materials.

Using metal-organic cages (MOCs) as preformed supermolecular building-blocks (SBBs) is a powerful strategy to design functional metal-organic frameworks (MOFs) with control over the pore architecture and connectivity. However, introducing chemical complexity into the network via this route is limited as most methodologies focus on only one type of MOC as the building-block. Herein we present the pairwise linking of MOCs as a design approach to introduce defined chemical complexity into porous materials. Our methodology exploits preferential Rh-aniline coordination and stoichiometric control to rationally link Cu4L4 and Rh4L4 MOCs into chemically complex, yet extremely well-defined crystalline solids. This strategy is expected to open up significant new possibilities to design bespoke multi-functional materials with atomistic control over the location and ordering of chemical functionalities.

A covalent deprotection strategy for assembling supramolecular coordination polymers from metal-organic cages.

A Cu4L4 metal-organic cage (MOC) composed of amine-protected ligands forms supramolecular coordination polymers (SCPs) upon covalent post-assembly deprotection. The amorphous SCPs form by virtue of aniline-copper coordination and possess a tunable porosity based on the rate of deprotection.

On/off porosity switching and post-assembly modifications of Cu4L4 metal-organic polyhedra.

Synthetic porous materials composed of metal-organic polyhedra (MOPs) have found application in topical areas such as gas storage, separation and catalysis. Control over their physical properties (e.g. porosity) has typically been achieved through ligand design or judicious choice of metal ions. Here, we demonstrate pore-size control and on/off porosity in Cu4L4 MOPs by exploiting their structural non-rigidity. We report an aldehyde-functionalised MOP (1) that can be isolated in five distinct solvatomorphs, each exhibiting different structural flexibility. When soaked in MeOH, two of these solvatomorphs undergo a rapid transformation to a thermodynamically favoured phase, whilst in acetone they template the crystallisation of an entirely new crystal packing. We support these findings by single and powder X-ray diffraction and rationalise the observed phase transformations by lattice energy calculations. Of the five solvatomorphs, three can be obtained as solvent-exchanged pseudo-polymorphs with distinct porosities in their activated form (SABET = 35-455 m2 g-1). Further control over the crystal packing of MOPs is achieved through covalent post-assembly modifications, which promote the crystallisation of isoreticular 2-D sheet-like structures.

Donor-Site-Directed Rational Assembly of Heteroleptic cis-[Pd2 L2 L'2 ] Coordination Cages from Picolyl Ligands.

A donor-site engineering approach facilitates the formation of heteroleptic [Pd2 L2 L'2 ]4+ cage structures through a favored cis-'in2 /out2 ' spatial configuration of the methyl groups of 5- and 3-substituted bis-monodentate picolyl ligands with flat acridone and bent phenothiazine backbones. The heteroleptic cages were confirmed by ESI-MS and 2D NMR experiments as well as DFT calculations, which pointed toward a cis-configuration being energetically favored. This was further supported by the synthesis and X-ray structure of a previously unreported cis-[Pd(2-picoline)4 ]2+ complex. The formation of homoleptic structures, however, was met with considerable steric hindrance at the PdII centers, as observed by the formation of [Pd2 L3 (solvent)2 ]4+ and [Pd2 L2 (solvent)4 ]4+ species when only one type of acridone-based ligand was offered. In contrast, bent phenothiazine ligands with outside-pointing methyl groups showed the ability to form interpenetrated double-cages, as revealed by X-ray crystallography. The general route presented herein enables the assembly of uniform cis-[Pd2 L2 L'2 ]4+ coordination cages, thus furthering the possibility to increase structural and functional complexity in supramolecular systems.

Hierarchical Assembly of an Interlocked M8 L16 Container.

The self-assembly of eight PdII cations and sixteen phenanthrene-derived bridging ligands with 60° bite angles yielded a novel M8 L16 metallosupramolecular architecture composed of two interlocked D4h -symmetric barrel-shaped containers. Mass spectrometry, NMR spectroscopy, and X-ray analysis revealed this self-assembled structure to be a very large "Hopf link" catenane featuring channel-like cavities, which are occupied by NO3- anions. The importance of the anions as catenation templates became imminent when we observed the nitrate-triggered structural rearrangement of a mixture of M3 L6 and M4 L8 assemblies formed in the presence of BF4- anions into the same interlocked molecule. Furthermore, the densely packed structure of the M8 L16 catenane was exploited in the preparation of a hexyloxy-functionalized analogue, which further self-assembled into vesicle-like aggregates in a reversible manner.

Integrative self-sorting of coordination cages based on 'naked' metal ions.

Coordination-driven self-assembly of metal ions and organic ligands has been extensively utilised over the past four decades to access a variety of nano-sized cage assemblies, with functions ranging from sensing and catalysis to drug delivery. Many of the reported examples, however, are highly symmetric architectures that contain one type of organic ligand carrying not more than a single functionality. This contrasts significantly with the level of structural and functional complexity encountered in biological macromolecular hosts, which are able to bind and chemically convert smaller molecules in their highly-decorated internal cavities. To address this disparity, rational approaches that facilitate heteroleptic assembly by regulating integrative self-sorting of metal ions and multiple ligand components have emerged. Among these, routes to access coordination cages from 'naked' metal cations that offer more than two coordination sites are still in early development, as the complexity of the self-sorted products in terms of composition and stereochemistry presents an entropic challenge. This feature article highlights recent progress in controlling integrative self-sorting of multi-component cage systems with a focus on structures composed of 'naked' metal cations and two different ligands. Once heteroleptic self-assembly strategies find a wider implementation in supramolecular design, the resultant interplay between tailored combinations of precisely positioned substituents promises enhanced functionality in nanoscale structures.

Morphological Control of Heteroleptic cis- and trans-Pd2 L2 L'2 Cages.

Control over the integrative self-sorting of metallo-supramolecular assemblies opens up possibilities for introducing increased complexity and function into a single self-assembled architecture. Herein, the relationship between the geometry of three ligand components and morphology of three self-sorted heteroleptic [Pd2 L2 L'2 ]4+ cages is examined. Pd-mediated assembly of two bis-monodentate pyridyl ligands with native bite angles of 75° and 120° affords a cis-[Pd2 L2 L'2 ]4+ cage while the same reaction with two ligands with bite angles of 75° and 60° gives an unprecedented, self-penetrating structural motif; a trans-[Pd2 (anti-L)2 L'2 ]4+ heteroleptic cage with a "doubly bridged figure eight" topology. Each heteroleptic assembly can be formed by cage-to-cage conversion of the homoleptic precursors and morphological control of [Pd2 L2 L'2 ] cages is achieved by selective ligand displacement transformations in a system of three ligands and at least six possible cage products.

X-ray crystallographic insights into post-synthetic metalation products in a metal-organic framework.

Post-synthetic modification of metal-organic frameworks (MOFs) facilitates a strategic transformation of potentially inert frameworks into functionalized materials, tailoring them for specific applications. In particular, the post-synthetic incorporation of transition-metal complexes within MOFs, a process known as 'metalation', is a particularly promising avenue towards functionalizing MOFs. Herein, we describe the post-synthetic metalation of a microporous MOF with various transition-metal nitrates. The parent framework, 1: , contains free-nitrogen donor chelation sites, which readily coordinate metal complexes in a single-crystal to single-crystal transformation which, remarkably, can be readily monitored by X-ray crystallography. The presence of an open void surrounding the chelation site in 1: prompted us to investigate the effect of the MOF pore environment on included metal complexes, particularly examining whether void space would induce changes in the coordination sphere of chelated complexes reminiscent of those found in the solution state. To test this hypothesis, we systematically metalated 1: with first-row transition-metal nitrates and elucidated the coordination environment of the respective transition-metal complexes using X-ray crystallography. Comparison of the coordination sphere parameters of coordinated transition-metal complexes in 1: against equivalent solid- and solution-state species suggests that the void space in 1: does not markedly influence the coordination sphere of chelated species but we show notably different post-synthetic metalation outcomes when different solvents are used.This article is part of the themed issue 'Coordination polymers and metal-organic frameworks: materials by design'.

Geometric Complementarity in Assembly and Guest Recognition of a Bent Heteroleptic cis-[Pd2LA2LB2] Coordination Cage.

Due to the inherent difficulties in achieving a defined and exclusive formation of multicomponent assemblies against entropic predisposition, we present the rational assembly of a heteroleptic [Pd2LA2LB2]4+ coordination cage achieved through the geometric complementarity of two carefully designed ligands, LA and LB. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [Pd2LA4]4+ strained helical assembly and a [Pd4LB8]8+ box-like structure, both of which were characterized by X-ray analysis. Combined, however, the two ligands could be used to cleanly assemble a cis-[Pd2LA2LB2]4+ cage with a bent architecture. The same self-sorted product was also obtained by a quantitative cage-to-cage transformation upon mixing of the two homoleptic cages revealing the [Pd2LA2LB2]4+ assembly as the thermodynamic minimum. The structure of the heteroleptic cage was examined by ESI-MS, COSY, DOSY, and NOESY methods, the latter of which pointed toward a cis-conformation of ligands in the assembly. Indeed, DFT calculations revealed that the angular ligands and strict Pd(II) geometry strongly favor the cis-[Pd2LA2LB2]4+ species. The robust nature of the cis-[Pd2LA2LB2]4+ cage allowed us to probe the accessibility of its cavity, which could be utilized for shape recognition toward stereoisomeric guests. The ability to directly combine two different backbones in a controlled manner provides a powerful strategy for increasing complexity in the family of [Pd2L4] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.

X-ray Crystallography in Open-Framework Materials.

Open-framework materials, such as metal-organic frameworks (MOFs) and coordination polymers have been widely investigated for their gas adsorption and separation properties. However, recent studies have demonstrated that their highly crystalline structures can be used to periodically organize guest molecules and non-structural metal compounds either within their pore voids or by anchoring to their framework architecture. Accordingly, the open framework can act as a matrix for isolating and elucidating the structures of these moieties by X-ray diffraction. This concept has broad scope for development as an analytical tool where obtaining single crystals of a target molecule presents a significant challenge and it additionally offers potential for obtaining insights into chemically reactive species that can be stabilized within the pore network. However, the technique does have limitations and as yet a general experimental method has not been realized. Herein we focus on recent examples in which framework materials have been utilized as a scaffold for ordering molecules for analysis by diffraction methods and canvass areas for future exploration.

Probing post-synthetic metallation in metal-organic frameworks: insights from X-ray crystallography.

For post-synthetic metallation (PSMet) of a metal­organic framework (MOF, [Mn3(L2)(L')] = 1) we show a solvent dependency for the form of the metallated product. PSMet of 1 with MnCl2 in ethanol leads to a single metal entity per coordinating site while acetonitrile yields a remarkably complex multiply metallated product; thus, determining the extent of metallation requires an intimate understanding of the chemistry occurring in the MOF pores.

Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks.

Post-synthetic metallation is employed strategically to imbue metal-organic frameworks (MOFs) with enhanced performance characteristics. However, obtaining precise structural information for metal-centred reactions that take place within the pores of these materials has remained an elusive goal, because of issues with high symmetry in certain MOFs, lower initial crystallinity for some chemically robust MOFs, and the reduction in crystallinity that can result from carrying out post-synthetic reactions on parent crystals. Here, we report a new three-dimensional MOF possessing pore cavities that are lined with vacant di-pyrazole groups poised for post-synthetic metallation. These metallations occur quantitatively without appreciable loss of crystallinity, thereby enabling examination of the products by single-crystal X-ray diffraction. To illustrate the potential of this platform to garner fundamental insight into metal-catalysed reactions in porous solids we use single-crystal X-ray diffraction studies to structurally elucidate the reaction products of consecutive oxidative addition and methyl migration steps that occur within the pores of the Rh-metallated MOF, 1·[Rh(CO)2][Rh(CO)2Cl2].