Transition Metal Complexes with Appended Benzimidazole Groups for Sensing Dihydrogenphosphate.

Four new complexes [Ru(bpy)2(bbib)](PF6)2, [Ru(phen)2(bbib)](PF6)2, [Re(CO)3(bbib)(py)](PF6) and [Ir(ppy)2(bbib)](PF6) [where bbib = 4,4'-bis(benzimidazol-2-yl)-2,2'-bipyridine] have been prepared and their photophysical properties determined. Their behaviour has been studied with a variety of anions in acetonitrile, DMSO and 10% aquated DMSO. Acetate and dihydrogenphosphate demonstrate a redshift in the bbib ligand associated absorptions suggesting that the ligand is strongly interacting with these anions. The 3MLCT emissive state is sensitive to the introduction of small quantities of anion (sub-stoichiometric quantities) and significant quenching is typically observed with acetate, although this is less pronounced in the presence of water. The emissive behaviour with dihydrogenphosphate is variable, showing systematic changes as anion concentration increases with several distinct interactions evident . 1H NMR and 31P NMR titrations in a 10% D2O - D6-DMSO mixture suggest that with dihydrogenphosphate, the imidazole group able to act as both a proton acceptor and donor. It appears that all four complexes can form a {[complex]2-H2PO4} "dimer", a one-to-one species (which the X-ray crystallography study suggests is dimeric in the solid-state), and a complex with a combined bis(dihydrogenphosphate) complex anion. The speciation relies on complex equilibria dependent on several factors including the complex charge, the hydrophobicity of the associated ligands, and the solvent.

Rapid synthesis of hydrogen bond templated handcuff rotaxanes.

The rapid synthesis of hydrogen bond templated handcuff rotaxanes is described. The isolated rotaxanes were characterized by NMR and IR spectroscopies and high resolution mass spectrometry. This report represents a rare demonstration of preparing (2)handcuff [2]rotaxanes by covalently linking separate axles threaded through the rings of a bis-macrocycle by use of the copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction.

A pyridine-N-oxide catenane for cation recognition.

The rapid preparation of a pyridine-N-oxide containing [2]catenane is described. The [2]catenane was characterized by NMR spectroscopy, mass spectrometry and X-ray single crystal structure determination. 1H NMR titration experiments reveal the [2]catenane may be reversibly protonated, as well as an ability to bind lithium cations more strongly than sodium cations.

Solid-State [2+2] Photoreaction of Isostructural Cd(II) Metal Complexes and Solid-State Fluorescence.

A green method to synthesize cyclobutane derivatives has been developed over the past three decades in the form of solid-state [2+2] photochemical reactions. These solid-state reactions also play a major role in the structural transformation of hybrid materials. In this regard, crystal engineering has played a major role in designing photoreactive molecular systems. Here, we report three novel binuclear Cd(II) complexes with the molecular formula [Cd2(4spy)4L4], where 4spy = 4-styryl pyridine and L = p-toluate (1); 4-fluorobenzoate (2); and 3-fluorobenzoate (3). Although three different benzoates are used, all three complexes are isostructural, as corroborated through SCXRD experiments. Structural analysis also helped in identifying two potential photoreactions. These are both intra- and intermolecular in nature and are driven by the head-to-head (HH) and head-to-tail (HT) alignment of 4spy linkers within these metal complexes. 1H NMR spectroscopy studies showed evidence of a quantitative head-to-head photoreaction in all these three complexes, and SCXRD analysis of the recrystallization of the photoproducts also provided confirmation. TGA studies of these photoreactive complexes showed an increase in the thermal stability of the complexes due to the solid-state photoreaction. Photoluminescence studies of these complexes have been conducted, showing a blue shift in emission spectra across all three cases after the photoreaction.

Selective Transesterification to Control Copolymer Microstructure in the Ring-Opening Copolymerization of Lactide and ε-Caprolactone by Lanthanum Complexes.

A series of novel lanthanum amido complexes, supported by ligands designed around the salan framework (salan = N,N'-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane) were synthesized and fully characterized in the solid and solution states. The ligands incorporate benzyl or 2-pyridyl substituents at each tertiary amine center. The complexes were investigated as catalysts in the ring-opening homopolymerization of lactide (LA) and ε-caprolactone (ε-CL) and copolymerization of equimolar amounts of LA and ε-CL at ambient temperature. Solvent (THF or toluene) and the number of 2-pyridyl groups in the complex were found to influence the reactivity of the catalysts in copolymerization reactions. In all cases, complete conversion of LA to PLA was observed. The use of THF, a coordinating solvent, suppressed ε-CL polymerization, while the presence of one or more 2-pyridyl groups promoted ε-CL polymerization. Each copolymer gave a monomodal trace in gel permeation chromatography-size-exclusion chromatography (GPC-SEC) experiments, indicative of copolymer formation over homopolymerization. Copolymer microstructure was found to be dependent on catalyst structure and reaction solvent, ranging from blocky to close to alternating. Experiments revealed rapid conversion of LA in the initial stages of the reaction, followed by incorporation of ε-CL into the copolymer by either transesterification or propagation reactions. Significantly, the mode of transesterification (TI or TII) that occurs is determined by the structure of the metal complex and the reaction solvent, leading to the possibility of controlling copolymer microstructure through catalyst design.

Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host-Guest Complexes.

A series of arylazopyrazole-loaded metal-organic frameworks were synthesized with the general formula Zn2(BDC)2(DABCO)(AAP)x (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo-[2.2.2]octane; AAP = arylazopyrazole guest). The empty framework adopts a large pore tetragonal structure. Upon occlusion of the E-AAP guests, the frameworks contract to form narrow pore tetragonal structures. The extent of framework contraction is dependent on guest shapes and pendant groups and ranges between 1.5 and 5.8%. When irradiated with 365 nm light, the framework expands due to the photoisomerization of E-AAP to Z-AAP. The proportion of Z-isomer at the photostationary state varies between 19 and 57% for the AAP guests studied and appears to be limited by the framework which inhibits further isomerization once fully expanded. Interestingly, confinement within the framework significantly extends the thermal half-life of the Z-AAP isomers to a maximum of approximately 56 years. This finding provides scope for the design of photoresponsive host-guest complexes with high stability of the metastable isomer for long-duration information or energy storage applications.

Preparation and Synthetic Applications of Phototropone.

An optimized multigram-scale route to phototropone (bicyclo[3.2.0]hepta-2,6-dien-7-one) is reported via the 4-π-photocyclization of tropone complexed to Lewis acid. Phototropone is a highly versatile molecular building block, and its conversion into 18 novel derivatives using standard transformations is demonstrated, allowing access to a variety of rigid bicyclic scaffolds.

Light-driven flagella-like motion of coordination compound single crystals.

Single crystals of coordination complexes that show mechanical motion under the influence of external stimuli are of great interest due to their applications in photoactuators, sensors and probes. The solid-state [2+2] cycloaddition reaction has been one of the most prominent chemical reactions for photoresponsive materials in recent years. However, a relatively limited number of compounds have been reported, and most of these compounds have only shown destructive photosalient effects. Here, we report two photoreactive Zn(II) metal complexes with a thiophene-based photoreactive linker, 2tpy (4-(2-(thiophen-2-yl)vinyl)pyridine). In addition, under photoirradiation these complexes showed flagella-like bending, first towards and subsequently away from the excitation light source. This is the first report of metal-complexes and the solid-state [2+2] cycloaddition reaction that presents flagella-like motion in single crystals.

Drug/bioactive eluting chitosan composite foams for osteochondral tissue engineering.

Joint defects associated with a variety of etiologies often extend deep into the subchondral bone leading to functional impairment and joint immobility, and it is a very challenging task to regenerate the bone-cartilage interface offering significant opportunities for biomaterial-based interventions to improve the quality of life of patients. Herein drug-/bioactive-loaded porous tissue scaffolds incorporating nano-hydroxyapatite (nHAp), chitosan (CS) and either hydroxypropyl methylcellulose (HPMC) or Bombyx mori silk fibroin (SF) are fabricated through freeze drying method as subchondral bone substitute. A combination of spectroscopy and microscopy (Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), and X-ray fluorescence (XRF) were used to analyze the structure of the porous biomaterials. The compressive mechanical properties of these scaffolds are biomimetic of cancellous bone tissues and capable of releasing drugs/bioactives (exemplified with triamcinolone acetonide, TA, or transforming growth factor-ß1, TGF-ß1, respectively) over a period of days. Mouse preosteoblast MC3T3-E1 cells were observed to adhere and proliferate on the tissue scaffolds as confirmed by the cell attachment, live-dead assay and alamarBlue™ assay. Interestingly, RT-qPCR analysis showed that the TA downregulated inflammatory biomarkers and upregulated the bone-specific biomarkers, suggesting such tissue scaffolds have long-term potential for clinical application.

Synthesis of functionalized spirocyclic oxetanes through Paternò-Büchi reactions of cyclic ketones and maleic acid derivatives.

A telescoped three-step sequence to functionalised spirocyclic oxetanes is reported, involving Paternò-Büchi reactions between maleic acid derivatives and cyclic ketones. p-Xylene suppresses the competing alkene dimerization that has plagued previous work, allowing access to 35 novel spirocyclic oxetanes that cannot be prepared using existing methodologies, and which represent versatile intermediates for further elaboration.

Electrochemically Enhanced Delivery of Pemetrexed from Electroactive Hydrogels.

Electroactive hydrogels based on derivatives of polyethyleneglycol (PEG), chitosan and polypyrrole were prepared via a combination of photopolymerization and oxidative chemical polymerization, and optionally doped with anions (e.g., lignin, drugs, etc.). The products were analyzed with a variety of techniques, including: FT-IR, UV-Vis, 1H NMR (solution state), 13C NMR (solid state), XRD, TGA, SEM, swelling ratios and rheology. The conductive gels swell ca. 8 times less than the non-conductive gels due to the presence of the interpenetrating network (IPN) of polypyrrole and lignin. A rheological study showed that the non-conductive gels are soft (G' 0.35 kPa, G″ 0.02 kPa) with properties analogous to brain tissue, whereas the conductive gels are significantly stronger (G' 30 kPa, G″ 19 kPa) analogous to breast tissue due to the presence of the IPN of polypyrrole and lignin. The potential of these biomaterials to be used for biomedical applications was validated in vitro by cell culture studies (assessing adhesion and proliferation of fibroblasts) and drug delivery studies (electrochemically loading the FDA-approved chemotherapeutic pemetrexed and measuring passive and stimulated release); indeed, the application of electrical stimulus enhanced the release of PEM from gels by ca. 10-15% relative to the passive release control experiment for each application of electrical stimulation over a short period analogous to the duration of stimulation applied for electrochemotherapy. It is foreseeable that such materials could be integrated in electrochemotherapeutic medical devices, e.g., electrode arrays or plates currently used in the clinic.

Unified Approach to Diverse Fused Fragments via Catalytic Dehydrative Cyclization.

A range of highly functionalized polycyclic fragments have been synthesized, employing a catalytic dehydrative cyclization. A range of nucleophiles are shown to be successful, with the reaction producing numerous high value motifs.

Efficient solid-state photoswitching of methoxyazobenzene in a metal-organic framework for thermal energy storage.

Efficient photoswitching in the solid-state remains rare, yet is highly desirable for the design of functional solid materials. In particular, for molecular solar thermal energy storage materials high conversion to the metastable isomer is crucial to achieve high energy density. Herein, we report that 4-methoxyazobenzene (MOAB) can be occluded into the pores of a metal-organic framework Zn2(BDC)2(DABCO), where BDC = 1,4-benzenedicarboxylate and DABCO = 1,4-diazabicyclo[2.2.2]octane. The occluded MOAB guest molecules show near-quantitative E → Z photoisomerization under irradiation with 365 nm light. The energy stored within the metastable Z-MOAB molecules can be retrieved as heat during thermally-driven relaxation to the ground-state E-isomer. The energy density of the composite is 101 J g-1 and the half-life of the Z-isomer is 6 days when stored in the dark at ambient temperature.

A structural investigation of organic battery anode materials by NMR crystallography.

Conjugated alkali metal dicarboxylates have recently received attention for applications as organic anode materials in lithium- and sodium-ion batteries. In order to understand and optimise these materials, it is important to be able to characterise both the long-range and local aspects of the crystal structure, which may change during battery cycling. Furthermore, some materials can display polymorphism or hydration behaviour. NMR crystallography, which combines long-range crystallographic information from diffraction with local information from solid-state NMR via interpretation aided by DFT calculations, is one such approach, but this has not yet been widely applied to conjugated dicarboxylates. In this work, we evaluate the application of NMR crystallography for a set of model lithium and sodium dicarboxylate salts. We investigate the effect of different DFT geometry optimisation strategies and find that the calculated NMR parameters are not systematically affected by the choice of optimisation method, although the inclusion of dispersion correction schemes is important to accurately reproduce the experimental unit cell parameters. We also observe hydration behaviour for two of the sodium salts and provide insight into the structure of an as-yet uncharacterised structure of sodium naphthalenedicarboxylate. This highlights the importance of sample preparation and characterisation for organic sodium-ion battery anode materials in particular.

Effect of Transition Metal Substitution on the Flexibility and Thermal Properties of MOF-Based Solid-Solid Phase Change Materials.

A series of azobenzene-loaded metal-organic frameworks were synthesized with the general formula M2(BDC)2(DABCO)(AB)x (M = Zn, Co, Ni, and Cu; BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo[2.2.2]octane; and AB = azobenzene), herein named M-1⊃ABx. Upon occlusion of AB, each framework undergoes guest-induced breathing, whereby the pores contract around the AB molecules forming a narrow-pore (np) framework. The loading level of the framework is found to be very sensitive to the synthetic protocol and although the stable loading level is close to M-1⊃AB1.0, higher loading levels can be achieved for the Zn, Co, and Ni frameworks prior to vacuum treatment, with a maximum composition for the Zn framework of Zn-1⊃AB1.3. The degree of pore contraction upon loading is modulated by the inherent flexibility of the metal-carboxylate paddlewheel unit in the framework, with the Zn-1⊃AB1.0 showing the biggest contraction of 6.2% and the more rigid Cu-1⊃AB1.0 contracting by only 1.7%. Upon heating, each composite shows a temperature-induced phase transition to an open-pore (op) framework, and the enthalpy and onset temperatures of the phase transition are affected by the framework flexibility. For all composites, UV irradiation causes trans → cis isomerization of the occluded AB molecules. The population of cis-AB at the photostationary state and the thermal stability of the occluded cis-AB molecules are also found to correlate with the flexibility of the framework. Over a full heating-cooling cycle between 0 and 200 °C, the energy stored within the metastable cis-AB molecules is released as heat, with a maximum energy density of 28.9 J g-1 for Zn-1⊃AB1.0. These findings suggest that controlled confinement of photoswitches within flexible frameworks is a potential strategy for the development of solid-solid phase change materials for energy storage.

Solid-state nuclear magnetic resonance study of polymorphism in tris(8-hydroxyquinolinate)aluminium.

Tris(8-hydroxyquinolinate)aluminium (Alq3 ) is a metal-organic coordination complex, which is a widely used electroluminescent material in organic light-emitting diode technology. Crystalline Alq3 is known to occur in five polymorphic forms (denoted α, ß, γ, δ, and ε), although the structures of some of these polymorphs have been the subject of considerable debate. In particular, the structure of α-Alq3 , which is a model for the local structure in amorphous films used in devices, is highly complex and has never been conclusively solved. In this work, we use solid-state nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations to investigate the local structure of four Alq3 samples. We find that the first structure proposed for α-Alq3 is inconsistent with all of the samples studied, and DFT calculations further suggest that this structure is energetically unfavourable. Instead, samples containing the meridional (mer) isomeric form are found to contain local structures consistent with ε-Alq3 , and a sample containing the facial (fac) isomeric form is consistent with a mixture of γ-Alq3 and δ-Alq3 . We also investigate the influence of different strategies for dispersion correction in DFT geometry optimisations. We find that a recently proposed modified semiempirical dispersion correction scheme gives good agreement with experiment. Furthermore, the DFT calculations also show that distinction between mer and fac isomers on the basis of ηQ that has been assumed in previous work is not always justified.

Silicon photosensitisation using molecular layers.

Silicon photosensitisation via energy transfer from molecular dye layers is a promising area of research for excitonic silicon photovoltaics. We present the synthesis and photophysical characterisation of vinyl and allyl terminated Si(111) surfaces decorated with perylene molecules. The functionalised silicon surfaces together with Langmuir-Blodgett (LB) films based on perylene derivatives were studied using a wide range of steady-state and time resolved spectroscopic techniques. Fluorescence lifetime quenching experiments performed on the perylene modified monolayers revealed energy transfer efficiencies to silicon of up to 90 per cent. We present a simple model to account for the near field interaction of a dipole emitter with the silicon surface and distinguish between the 'true' FRET region (<5 nm) and a different process, photon tunnelling, occurring for distances between 10-50 nm. The requirements for a future ultra-thin crystalline solar cell paradigm include efficient surface passivation and keeping a close distance between the emitter dipole and the surface. These are discussed in the context of existing limitations and questions raised about the finer details of the emitter-silicon interaction.

Crystal and mol-ecular structures of a binuclear mixed ligand complex of silver(I) with thio-cyanate and 1H-1,2,4-triazole-5(4H)-thione.

The complete mol-ecule of the binuclear title complex, bis-[µ-1H-1,2,4-triazole-5(4H)-thione-κ2 S:S]bis-{(thio-cyanato-κS)[1H-1,2,4-triazole-5(4H)-thione-κS]silver(I)}, [Ag2(SCN)2(C2H3N3S)4], is generated by crystallographic inversion symmetry. The independent triazole-3-thione ligands employ the exocyclic-S atoms exclusively in coordination. One acts as a terminal S-ligand and the other in a bidentate (µ2) bridging mode to provide a link between two AgI centres. Each AgI atom is also coordinated by a terminal S-bound thio-cyanate ligand, resulting in a distorted AgS4 tetra-hedral coordination geometry. An intra-molecular N-H⋯S(thio-cyanate) hydrogen bond is noted. In the crystal, amine-N-H⋯S(thione), N-H⋯N(triazol-yl) and N-H⋯N(thio-cyanate) hydrogen bonds give rise to a three-dimensional architecture. The packing is consolidated by triazolyl-C-H⋯S(thio-cyanate), triazolyl-C-H⋯N(thiocyanate) and S⋯S [3.2463 (9) Å] inter-actions as well as face-to-face π-π stacking between the independent triazolyl rings [inter-centroid separation = 3.4444 (15) Å]. An analysis of the calculated Hirshfeld surfaces shows the three major contributors are due to N⋯H/H⋯N, S⋯H/H⋯S and C⋯H/H⋯C contacts, at 35.8, 19.4 and 12.7%, respectively; H⋯H contacts contribute only 7.6% to the overall surface.

Palladium-Catalyzed Synthesis of α-Carbonyl-α'-(hetero)aryl Sulfoxonium Ylides: Scope and Insight into the Mechanism.

Despite recent advances, a general method for the synthesis of α-carbonyl-α'-(hetero)aryl sulfoxonium ylides is needed to benefit more greatly from the potential safety advantages offered by these compounds over the parent diazo compounds. Herein, we report the palladium-catalyzed cross-coupling of aryl bromides and triflates with α-carbonyl sulfoxonium ylides. We also report the use of this method for the modification of an active pharmaceutical ingredient and for the synthesis of a key precursor of antagonists of the neurokinin-1 receptor. In addition, the mechanism of the reaction was inferred from several observations. Thus, the oxidative addition complex [(XPhos)PhPdBr] and its dimer were observed by 31P{1H} NMR, and these complexes were shown to be catalytically and kinetically competent. Moreover, a complex resulting from the transmetalation of [(XPhos)ArPdBr] (Ar = p-CF3-C6H4) with a model sulfoxonium ylide was observed by mass spectrometry. Finally, the partial rate law suggests that the transmetalation and the subsequent deprotonation are rate-determining in the catalytic cycle.

4-π-Photocyclization of 1,2-Dihydropyridazines: An Approach to Bicyclic 1,2-Diazetidines with Rich Synthetic Potential.

The 4-π-photocyclization of a range of 1,2-dihydropyridazines is described, generating bicyclic 1,2-diazetidines in high yields on multigram scale. The key bicyclic 1,2-diazetidines are versatile synthetic intermediates and were easily converted into a range of novel derivatives, including functionalized 1,2-diazetidines, cyclobutenes, cyclobutanes, and 1,3-dienes.