Ruthenium Water Oxidation Catalysts based on Pentapyridyl Ligands.

Ruthenium complexes containing the pentapyridyl ligand 6,6''-(methoxy(pyridin-2-yl)methylene)di-2,2'-bipyridine (L-OMe) of general formula trans-[RuII (X)(L-OMe-κ-N5 )]n+ (X=Cl, n=1, trans-1+ ; X=H2 O, n=2, trans-22+ ) have been isolated and characterized in solution (by NMR and UV/Vis spectroscopy) and in the solid state by XRD. Both complexes undergo a series of substitution reactions at oxidation state RuII and RuIII when dissolved in aqueous triflic acid-trifluoroethanol solutions as monitored by UV/Vis spectroscopy, and the corresponding rate constants were determined. In particular, aqueous solutions of the RuIII -Cl complex trans-[RuIII (Cl)(L-OMe-κ-N5 )]2+ (trans-12+ ) generates a family of Ru aquo complexes, namely trans-[RuIII (H2 O)(L-OMe-κ-N5 )]3+ (trans-23+ ), [RuIII (H2 O)2 (L-OMe-κ-N4 )]3+ (trans-33+ ), and [RuIII (Cl)(H2 O)(L-OMe-κ-N4 )]2+ (trans-42+ ). Although complex trans-42+ is a powerful water oxidation catalyst, complex trans-23+ has only a moderate activity and trans-33+ shows no activity. A parallel study with related complexes containing the methyl-substituted ligand 6,6''-(1-pyridin-2-yl)ethane-1,1-diyl)di-2,2'-bipyridine (L-Me) was carried out. The behavior of all of these catalysts has been rationalized based on substitution kinetics, oxygen evolution kinetics, electrochemical properties, and density functional theory calculations. The best catalyst, trans-42+ , reaches turnover frequencies of 0.71 s-1 using CeIV as a sacrificial oxidant, with oxidative efficiencies above 95 %.

Ruthenium water oxidation catalysts containing the non-planar tetradentate ligand, biisoquinoline dicarboxylic acid (biqaH2).

Two ruthenium complexes containing the tetradentate ligand [1,1'-biisoquinoline]-3,3'-dicarboxylic acid, and 4-picoline or 6-bromoisoquinoline as axial ligands have been prepared. The complexes have been fully characterised and initial studies on their potential to function as molecular water oxidation catalysts have been performed. Both complexes catalyse the oxidation of water in acidic media with CeIV as a stoichiometric chemical oxidant, although turnover numbers and turnover frequencies are modest when compared with the closely related Ru-bda and Ru-pda analogues. Barriers for the water nucleophilic attack and intermolecular coupling pathways were obtained from density functional theory calculations and the crucial influence of the ligand framework in determining the most favourable reaction pathway was elucidated from a combined analysis of the theoretical and experimental results.

Supramolecular water oxidation with Ru-bda-based catalysts.

Extremely slow and extremely fast new water oxidation catalysts based on the Ru-bda (bda=2,2'-bipyridine-6,6'-dicarboxylate) systems are reported with turnover frequencies in the range of 1 and 900 cycles s(-1) , respectively. Detailed analyses of the main factors involved in the water oxidation reaction have been carried out and are based on a combination of reactivity tests, electrochemical experiments, and DFT calculations. These analyses give a convergent interpretation that generates a solid understanding of the main factors involved in the water oxidation reaction, which in turn allows the design of catalysts with very low energy barriers in all the steps involved in the water oxidation catalytic cycle. We show that for this type of system π-stacking interactions are the key factors that influence reactivity and by adequately controlling them we can generate exceptionally fast water oxidation catalysts.

A flow-system array for the discovery and scale up of inorganic clusters.

The batch synthesis of inorganic clusters can be both time consuming and limited by a lack of reproducibility. Flow-system approaches, now common in organic synthesis, have not been utilized widely for the synthesis of clusters. Herein we combine an automated flow process with multiple batch crystallizations for the screening and scale up of syntheses of polyoxometalates and manganese-based single-molecule magnets. Scale up of the synthesis of these architectures was achieved by programming a multiple-pump reactor system to vary reaction conditions sequentially, and thus explore a larger parameter space in a shorter time than conventionally possible. Also, the potential for using the array as a discovery tool is demonstrated. Successful conditions for product isolation were identified easily from the array of reactions, and a direct route to 'scale up' was then immediately available simply by continuous application of these flow conditions. In all cases, large quantities of phase-pure material were obtained and the time taken for the discovery, repetition and scale up decreased.

Integrated 3D-printed reactionware for chemical synthesis and analysis.

Three-dimensional (3D) printing has the potential to transform science and technology by creating bespoke, low-cost appliances that previously required dedicated facilities to make. An attractive, but unexplored, application is to use a 3D printer to initiate chemical reactions by printing the reagents directly into a 3D reactionware matrix, and so put reactionware design, construction and operation under digital control. Here, using a low-cost 3D printer and open-source design software we produced reactionware for organic and inorganic synthesis, which included printed-in catalysts and other architectures with printed-in components for electrochemical and spectroscopic analysis. This enabled reactions to be monitored in situ so that different reactionware architectures could be screened for their efficacy for a given process, with a digital feedback mechanism for device optimization. Furthermore, solely by modifying reactionware architecture, reaction outcomes can be altered. Taken together, this approach constitutes a relatively cheap, automated and reconfigurable chemical discovery platform that makes techniques from chemical engineering accessible to typical synthetic laboratories.

Solution-phase monitoring of the structural evolution of a Molybdenum Blue nanoring.

The inorganic host-guest complex Na(22){[Mo(VI)(36)O(112)(H(2)O)(16)]⊂[Mo(VI)(130)Mo(V)(20)O(442)(OH)(10)(H(2)O)(61)]}·180H(2)O ≡ {Mo(36)}⊂{Mo(150)}, compound 1, has been isolated in its solid crystalline state via unconventional synthesis in a custom flow reactor. Carrying out the reaction under controlled flow conditions selected for the generation of {Mo(36)}⊂{Mo(150)} as the major product, allowing it to be reproducibly isolated in a moderate yield, as opposed to traditional "one-pot" batch syntheses that typically lead to crystallization of the {Mo(36)} and {Mo(150)} species separately. Structural and spectroscopic studies of compound 1 and the archetypal Molybdenum Blue (MB) wheel, {Mo(150)}, identified compound 1 as a likely intermediate in the {Mo(36)} templated synthesis of MB wheels. Further evidence illustrating the template effect of {Mo(36)} to MB wheel synthesis was indicated by an increase in the yield and rate of production of {Mo(150)} as a direct result of the addition of preformed {Mo(36)} to the reaction mixture. Dynamic light scattering (DLS) techniques were also used to corroborate the mechanism of formation of the MB wheels through observation of the individual cluster species in solution. DLS measurement of the reaction solutions from which {Mo(36)} and {Mo(150)} crystallized gave particle size distribution curves averaging 1.9 and 3.9 nm, consistent with the dimensions of the discrete clusters, which allowed the use of size as a possible distinguishing feature of these key species in the reduced acidified molybdate solutions and to observe the templation of the MB wheel by {Mo(36)} directly.

Osmotically driven crystal morphogenesis: a general approach to the fabrication of micrometer-scale tubular architectures based on polyoxometalates.

The process of osmotically driven crystal morphogenesis of polyoxometalate (POM)-based crystals is investigated, whereby the transformation results in the growth of micrometer-scale tubes 10-100 µm in diameter and many thousands of micrometers long. This process initiates when the crystals are immersed in aqueous solutions containing large cations and is governed by the solubility of the parent POM crystal. Evidence is presented that indicates the process is general to all types of POMs, with solubility of the parent crystal being the deciding parameter. A modular approach is adopted since different POM precursor crystals can form tubular architectures with a range of large cationic species, producing an ion-exchanged material that combines the large added cations and the large POM-based anions. It is also shown that the process of morphogenesis is electrostatically driven by the aggregation of anionic metal oxides with the dissolved cations. This leads to the formation of a semi-permeable membrane around the crystal. The osmotically driven ingress of water leads to an increase in pressure, and ultimately rupture of the membrane occurs, allowing a saturated solution of the POM to escape and leading to the formation of a "self-growing" microtube in the presence of the cation. It is demonstrated that the growth process is sustained by the osmotic pressure within the membrane surrounding the parent crystal, as tube growth ceases whenever this pressure is relieved. Not only is the potential of the modular approach revealed by the fact that the microtubes retain the properties of their component parts, but it is also possible to control the direction of growth and tube diameter. In addition, the solubility limits of tube growth are explored and translated into a predictive methodology for the fabrication of tubular architectures with predefined physical properties, opening the way for real applications.

Fine tuning reactivity: synthesis and isolation of 1,2,3,12b-tetrahydroimidazo[1,2-f]phenanthridines.

A facile route for the synthesis and isolation of 1,2,3,12b-tetrahydroimidazo[1,2-f]phenanthridines (TIPs) has been developed. The heterocycle is a reactive intermediate in the three-step cascade synthesis of 2,3-dihydro-1H-imidazo[1,2-f]phenanthridinium cations (DIPs), a biologically active DNA intercalating framework; however, the intermediate has previously only been characterized in situ. Derivatization of the structure at the imidazo-N position controls the reactivity of the intermediate with respect to electronic potential and pK(a) allowing isolation of a selection of TIP structures. Correlations between these parameters and reaction outcome have been made, and other influences such as steric and solvent effects have also been investigated.

A new C-C bond forming annulation reaction leading to pH switchable heterocycles.

A C-C bond forming reaction resulting from the alpha-addition of carbon based nucleophiles to N-bromoethyl phenanthridinium leads to the formation of 2,3-dihydro-12H-pyrrolo[1,2-f]phenanthridine-based derivatives which undergo reversible ring-opening/closing under pH control.

Realization of a "lockable" molecular switch via pH- and redox-modulated cyclization.

A switchable organic system involving four distinct states that can be interconverted by use of both pH and redox chemistry as control parameters has been developed. The key molecules involved in this system are the phenanthridine-based heterocycles 1-isobutyl-1,2,3,12b-tetrahydroimidazo[1,2-f]phenanthridine (TIP) and 5-[2-(isobutylamino)ethyl]phenanthridinium (AEP). These two states are interchangeable via pH control, and in addition they can also be further manipulated by oxidation or reduction to convert them to their "pH-inert" forms: 1-isobutyl-2,3-dihydro-1H-imidazo[1,2-f]phenanthridinium (DIP) and 5-[2-(isobutylamino)ethyl]-5,6-dihydrophenanthridine (AEDP), respectively. UV and (1)H NMR experiments carried out in a biphasic dichloromethane (DCM)/water solution were used for in situ structure determination. The results showed that the pH-modulated cyclization and phase-transfer process between the TIP and AEP states was essentially quantitative and repeatable without any significant loss in activity and that reduction or oxidation could be used to lock out these states against such acid-base-induced changes.

A general and efficient five-step one-pot procedure leading to nitrogen-bridgehead heterocycles containing an imidazole ring.

A very simple annulation reaction was designed, allowing an imidazole moiety to be fused onto a range of pyridine-based derivatives. The methodology consists of an activation step via the formation of a pyridinium salt to increase the electrophilicity of the pyridine ring, followed by a cascade reaction triggered by a nucleophilic attack of the iminium moiety. Depending on the pyridinium salt, it is possible to obtain functionalized imidazole moieties.