Accelerating water oxidation - a mixed Co/Fe polyoxometalate with improved turnover characteristics.

Water oxidation is a bottleneck reaction for the establishment of solar-to-fuel energy conversion systems. Earth-abundant metal-based polyoxometalates are promising heterogeneous water oxidation catalysts that can operate in a wide pH range. However, detailed structure-reactivity relationships are not yet comprehensively understood, hampering the design and synthesis of more effective polyoxometalate-based oxidation catalysts. Here we report the synthesis of an ordered, mixed-metal cobalt-iron Weakley archetype [CoII2(H2O)2FeIII2(CoIIW9O34)2]14- (Co2Fe2-WS), which unexpectedly highlights the strong influence of the central, coordinatively saturated metal ions on the catalytic water oxidation characteristics. The resulting species exhibits catalytic turnover frequencies which are up to 4× higher than those of the corresponding archetype tetracobalt-oxo species [CoII2(H2O)2CoII2(PW9O34)2]10- (Co4-WS). It is further striking that the system becomes catalytically inactive when one of the central positions is occupied by a WVI ion as demonstrated by [CoII2(H2O)2CoIIWVI(CoIIW9O34)2]12- (Co3W-WS). Importantly, this study demonstrates that coordinatively saturated metal ions in this central position, which at first glance appear insignificant, do not solely have a structural role but also impart a distinctive structural influence on the reactivity of the polyoxometalate. These results provide unique insights into the structure-reactivity relationships of polyoxometalates with improved catalytic performance characteristics.

2D Porphyrinic Metal-Organic Frameworks Featuring Rod-Shaped Secondary Building Units.

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called 'rod MOFs', which comprise infinitely extended 1D secondary building units (SBUs), represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2L-CuII, where H2 refers to the ligand's carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D MnII- or CoII-based rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties. Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-2 render them promising materials for future photocatalytic investigations.

Synthetic Approaches to Metallo-Supramolecular CoII Polygons and Potential Use for H2O Oxidation.

Metal-directed self-assembly has been applied to prepare supramolecular coordination polygons which adopt tetrahedral (1) or trigonal disklike topologies (2). In the solid state, 2 assembles into a stable halide-metal-organic material (Hal-MOM-2), which catalyzes H2O oxidation under photo- and electrocatalytic conditions, operating with a maximum TON = 78 and TOF = 1.26 s-1. DFT calculations attribute the activity to a CoIII-oxyl species. This study provides the first account of how CoII imine based supramolecules can be employed as H2O oxidation catalysts.

Nanoimprint Lithography-Directed Self-Assembly of Bimetallic Iron-M (M=Palladium, Platinum) Complexes for Magnetic Patterning.

Self-assembly of d8 metal polypyridine systems is a well-established approach for the creation of 1D organometallic assemblies but there are still challenges for the large-scale construction of nanostructured patterns from these building blocks. We describe herein the use of high-throughput nanoimprint lithography (NIL) to direct the self-assembly of the bimetallic complexes [4'-ferrocenyl-(2,2':6',2''-terpyridine)M(OAc)]+ (OAc)- (M=Pd or Pt; OAc=acetate). Uniform nanorods are fabricated from the molecular self-organization and evidenced by morphological characterization. More importantly, when top-down NIL is coupled with the bottom-up self-assembly of the organometallic building blocks, regular arrays of nanorods can be accessed and the patterns can be controlled by changing the lithographic stamp, where the mold imposes a confinement effect on the nanorod growth. In addition, patterns consisting of the products formed after pyrolysis are studied. The resulting arrays of ferromagnetic FeM alloy nanorods suggest promising potential for the scalable production of ordered magnetic arrays and fabrication of magnetic bit-patterned media.

Flexible Metal-Organic Frameworks for Light-Switchable CO2 Sorption Using an Auxiliary Ligand Strategy.

We report the synthesis and characterization of two photoactive metal-organic frameworks (MOFs), TCM-14 and TCM-15. The compounds were synthesized by incorporating 4,4'-azopyridine auxiliary ligands into pto-type scaffolds that are composed of dinuclear copper(II) "paddle-wheel"-based secondary building units and flexible, acetylene-extended, tritopic benzoate linkers. Room temperature CO2 sorption of the MOFs was studied, and UV-light irradiation is shown to result in reduced CO2 adsorption under static conditions. TCM-15 reveals a dynamic response leading to an instant desorption of up to 20% of CO2 upon incidence of UV light because of the occurrence of nonperiodic structural changes. Physicochemical and computational density functional theory studies were carried out to gain insight into the mechanism of the interaction of light with the frameworks.

A highly augmented, (12,3)-connected Zr-MOF containing hydrated coordination sites for the catalytic transformation of gaseous CO2 to cyclic carbonates.

A porous Zr-based MOF, [Zr6(BTEB)4(µ3-O)4(µ3-OH)4(H2O)4], which contains partially hydrated, 12-connected {Zr6} nodes and extended carboxylate ligands (BTEB3-), was synthesized and physicochemically characterised. The resulting (12,3)-connected, 3D framework adopts an uncommon llj topology with a large, solvent accessible void volume of ca. 79% of the unit cell volume. The porous structure facilitates the uptake of N2 and activated samples give rise to BET surface areas of >1000 m2 g-1. Furthermore, the porosity and accessibility of Lewis acidic Zr(iv) sites promote the catalytic transformation of gaseous CO2 to cyclic carbonates via cycloaddition reactions using epoxide reactants, whereby solvated MOFs exhibit higher catalytic performance than thermally treated samples.

Light-harvesting, 3rd generation RuII/CoII MOF with a large, tubular channel aperture.

A photoactive, hetero-metallic CoII/RuII-based metal-organic framework (MOF) with a large channel aperture, ca. 21 Å, is reported. The photophysical properties of the MOF are derived from the RuII nodes giving rise to emission centred at ca. 620 nm and relatively long triplet 3MLCT lifetimes. In addition to the optical attributes, the 1H-imidazo [4,5-f][1,10]-phenanthroline ligand imparts structural functionality to the MOF which is composed of alternating CoII- and RuII-based nodes of Δ and Λ helicity. The framework maintains its integrity upon activation and shows gas sorption behaviour that is characteristic of mesoporous materials promoting high CO2 sorption capacities and selectivities over N2.

Multimodal switching of a redox-active macrocycle.

Molecules that can exist in multiple states with the possibility of toggling between those states based on different stimuli have potential for use in molecular switching or sensing applications. Multimodal chemical or photochemical oxidative switching of an antioxidant-substituted resorcinarene macrocycle is reported. Intramolecular charge-transfer states, involving hemiquinhydrones are probed and these interactions are used to construct an oxidation-state-coupled molecular switching manifold that reports its switch-state conformation via striking variation in its electronic absorption spectra. The coupling of two different oxidation states with two different charge-transfer states within one macrocyclic scaffold delivers up to five different optical outputs. This molecular switching manifold exploits intramolecular coupling of multiple redox active substituents within a single molecule.

Near-saturated red emitters: four-coordinate copper(i) halide complexes containing 8-(diphenylphosphino)quinoline and 1-(diphenylphosphino)naphthalene ligands.

Recently, highly emissive neutral copper halide complexes have received much attention. Here, a series of four-coordinate mononuclear Cu(i) halide complexes, [CuX(dpqu)(dpna)] (dpqu = 8-(diphenylphosphino)quinoline, dpna = 1-(diphenylphosphino)naphthalene, X = I (1), Br (2) and Cl (3)), were synthesized, and their molecular structures and photophysical properties were investigated. These complexes exhibit near-saturated red emission in the solid state at room temperature and have peak emission wavelengths at 669-691 nm with microsecond lifetimes (τ = 0.46-1.80 µs). Small S1-T1 energy gaps in the solid state indicate that the emission occurs from a thermally activated excited singlet state at ambient temperature. The emission of the complexes 1-3 mainly originates from MLCT transition. The solution-processed devices of complex 1 exhibit stable red emission with a CIE(x, y) of (0.62, 0.38) for a doped device and (0.63, 0.37) for a non-doped device.

Cyclometalated Iridium(III) Carbene Phosphors for Highly Efficient Blue Organic Light-Emitting Diodes.

Five deep blue carbene-based iridium(III) phosphors were synthesized and characterized. Interestingly, one of them can be fabricated into deep blue, sky blue and white organic light-emitting diodes (OLEDs) through changing the host materials and exciton blocking layers. These deep and sky blue devices exhibit Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.186) and (0.152, 0.277) with external quantum efficiency (EQE) of 15.2% and 9.6%, respectively. The EQE of the deep blue device can be further improved up to 19.0% by choosing a host with suitable energy level of its lowest unoccupied molecular orbital (LUMO).

Bio-inspired synthetic approaches: from hierarchical, hybrid supramolecular assemblies to CaCO3-based microspheres.

A bio-inspired synthetic approach to unprecedented hybrid supramolecular assemblies [Ca(Me2hda)(H2O)3]·H2O (1) and [Ca(C12hda)(H2O)2]·H2O (2), that are stabilized by iminodiacetate-substituted organic ligands is reported. The results of the single-crystal X-ray analysis of 1 further allowed the use of electron microscopy to verify the supramolecular structure of the fibrous assemblies of 2 that incorporate extended alkyl-substituted ligand derivatives. 2 reveals interesting features that distinguish these soft structures from purely inorganic, brittle materials: meshes of nanobelts transform on solid supports to form homogeneous films covering extended, micro-sized areas. The use of the reported ligand system as a habit modifier for CaCO3 results in hierarchical calcite aggregates. The structure-influencing effects of the ligands and their supramolecular assemblies promote the formation of calcite disks that tessellate into hollow microspheres that contain distinctive openings.

Ultra-large supramolecular coordination cages composed of endohedral Archimedean and Platonic bodies.

Pioneered by Lehn, Cram, Peterson and Breslow, supramolecular chemistry concepts have evolved providing fundamental knowledge of the relationships between the structures and reactivities of organized molecules. A particular fascinating class of metallo-supramolecular molecules are hollow coordination cages that provide cavities of molecular dimensions promoting applications in diverse areas including catalysis, enzyme mimetics and material science. Here we report the synthesis of coordination cages with exceptional cross-sectional diameters that are composed of multiple sub-cages providing numerous distinctive binding sites through labile coordination solvent molecules. The building principles, involving Archimedean and Platonic bodies, renders these supramolecular keplerates as a class of cages whose composition and topological aspects compare to characteristics of edge-transitive {Cu2} MOFs with A3X4 stoichiometry. The nature of the cavities in these double-shell metal-organic polyhedra and their inner/outer binding sites provide perspectives for post-synthetic functionalizations, separations and catalysis. Transmission electron microscopy studies demonstrate that single molecules are experimentally accessible.

Luminescence Color Tuning by Regulating Electrostatic Interaction in Light-Emitting Devices and Two-Photon Excited Information Decryption.

It is well-known that the variation of noncovalent interactions of luminophores, such as π-π interaction, metal-to-metal interaction, and hydrogen-bonding interaction, can regulate their emission colors. Electrostatic interaction is also an important noncovalent interaction. However, very few examples of luminescence color tuning induced by electrostatic interaction were reported. Herein, a series of Zn(II)-bis(terpyridine) complexes (Zn-AcO, Zn-BF4, Zn-ClO4, and Zn-PF6) containing different anionic counterions were reported, which exhibit counterion-dependent emission colors from green-yellow to orange-red (549 to 622 nm) in CH2Cl2 solution. More importantly, it was found that the excited states of these Zn(II) complexes can be regulated by changing the electrostatic interaction between Zn2+ and counterions. On the basis of this controllable excited state, white light emission has been achieved by a single molecule, and a white light-emitting device has been fabricated. Moreover, a novel type of data decryption system with Zn-PF6 as the optical recording medium has been developed by the two-photon excitation technique. Our results suggest that rationally controlled excited states of these Zn(II) complexes by regulating electrostatic interaction have promising applications in various optoelectronic fields, such as light-emitting devices, information recording, security protection, and so on.

Anion-directed supramolecular chemistry modulating the magnetic properties of nanoscopic Mn coordination clusters: from polynuclear high-spin complexes to SMMs.

We report a supramolecular approach to mixed-valent Mn coordination clusters that demonstrates how halide ions can be applied to influence the assembly of distinct tetranuclear building units to produce a number of related Mn coordination clusters with dense core structures that derive from a cuboctahedral arrangement of Mn ions. In all compounds the alignment of the Jahn-Teller axes of the MnIII centers coincides with the positions of the stabilizing chloride ligands. Thus, the relative chloride concentrations in the reaction mixtures allow us to modify the symmetry and magnetic anisotropy of this basic core structures resulting in isotropic polynuclear high-spin complexes at high Cl- concentrations and Single-Molecule Magnets at lower relative Cl- concentrations.

Towards multifunctional lanthanide-based metal-organic frameworks.

We report the synthesis, structure and physicochemical attributes of a new holmium(III)-based metal-organic framework whose 3D network structure gives rise to porosity; the reported structure-type can be varied using a range of different lanthanide ions to tune the photophysical properties and produce ligand-sensitised near-infrared (NIR) and visible light emitters.

Dual-Emissive Cyclometalated Iridium(III) Polypyridine Complexes as Ratiometric Biological Probes and Organelle-Selective Bioimaging Reagents.

In this Article, we present a series of cyclometalated iridium(III) polypyridine complexes of the formula [Ir(N^C)2(N^N)](PF6) that showed dual emission under ambient conditions. The structures of the cyclometalating and diimine ligands were changed systematically to investigate the effects of the substituents on the dual-emission properties of the complexes. On the basis of the photophysical data, the high-energy (HE) and low-energy (LE) emission features of the complexes were assigned to triplet intraligand ((3)IL) and triplet charge-transfer ((3)CT) excited states, respectively. Time-dependent density functional theory (TD-DFT) calculations supported these assignments and indicated that the dual emission resulted from the interruption of the communication between the higher-lying (3)IL and the lower-lying (3)CT states by a triplet amine-to-ligand charge-transfer ((3)NLCT) state. Also, the avidin-binding properties of the biotin complexes were studied by emission titrations, and the results showed that the dual-emissive complexes can be utilized as ratiometric probes for avidin. Additionally, all the complexes exhibited efficient cellular uptake by live HeLa cells. The MTT and Annexin V assays confirmed that no cell death and early apoptosis occurred during the cell imaging experiments. Interestingly, laser-scanning confocal microscopy revealed that the complexes were selectively localized on the cell membrane, mitochondria, or both, depending on the nature of the substituents of the ligands. The results of this work will contribute to the future development of dual-emissive transition metal complexes as ratiometric probes and organelle-selective bioimaging reagents.

Introduction of luminescent rhenium(I), ruthenium(II), iridium(III) and rhodium(III) systems into rhodamine-tethered ligands for the construction of bichromophoric chemosensors.

Several classes of luminescent transition metal complexes, including rhenium(I) tricarbonyl diimine, ruthenium(II) diimine, cyclometallated iridium(III) and rhodium(III) diimine, as well as ruthenium(II) and iridium(III) terpyridine systems, tethered with rhodamine moieties, have been synthesized and characterized. The X-ray crystal structure of one cyclometallated rhodium(III) diimine (11) with a rhodamine pendant was determined. Most of the complexes were found to exhibit emission in fluid solution at room temperature. Depending on the nature of the transition metal system, the emission origin was mainly assigned to be derived from the triplet excited state of the metal-to-ligand charge transfer ((3)MLCT) or the intraligand ((3)IL) transition. The cation-binding properties of these complexes toward various cations were investigated by electronic absorption and emission spectroscopy. Some of them were found to exhibit new low-energy absorption and emission bands, attributed to the ring opening of the rhodamine moiety, with high selectivity and/or high sensitivity for various cations, in agreement with sensing and spectroscopic behaviours of the rhodamine derivative. Depending on the nature of the transition metal centres, the chelating ligands as well as the linker to the rhodamine derivative, different sensing properties in terms of selectivity, sensitivity and binding stability, could be obtained.

Ruthenium porphyrins with axial π-conjugated arylamide and arylimide ligands.

A series of ruthenium porphyrins [Ru(IV)(por)(NHY)2] and [Ru(VI)(por)(NY)2] bearing axially coordinated π-conjugated arylamide and arylimide ligands, respectively, have been synthesized. The crystal structures of [Ru(IV)(tmp)(NHY)2] (tmp = 5,10,15,20-tetramesitylporphyrinato(2-)) with Y = 4'-methoxy-biphenyl-4-yl (Ar-Ar-p-OMe), 4'-chloro-biphenyl-4-yl (Ar-Ar-p-Cl), and 9,9-dibutyl-fluoren-2-yl (Ar^Ar) show axial Ru-N(arylamide) distances of 1.978(4), 1.971(6), and 1.985(13) Å, respectively. [Ru(IV)(tmp)(NH{Ar^Ar})2] is an example of metalloporphyrins that bind an arylamide ligand featuring a co-planar biphenyl unit. The [Ru(IV)(por)(NHY)2] complexes show a quasi-reversible reduction couple or irreversible reduction wave attributed to Ru(IV)→Ru(III) with Epc from -1.06 to -1.40 V versus Cp2Fe(+/0) and an irreversible oxidation wave with Epa from -0.04 to 0.19 V versus Cp2Fe(+/0). Reaction of the [Ru(IV)(por)(NHY)2] with bromine afforded [Ru(IV)(por)(NHY)Br]. PhI(OAc)2 oxidation of the [Ru(IV)(por)(NHY)2] gave [Ru(VI)(por)(NY)2]; the latter can be prepared from reaction of [Ru(II)(por)(CO)] with aryl azides N3Y. The crystal structure of [Ru(VI)(tmp)(N{Ar-Ar-p-OMe})2] features Ru-N(arylimide) distances of 1.824(5) and 1.829(5) Å. Alkene aziridination and C-H amination catalyzed by "[Ru(II)(tmp)(CO)]+π-conjugated aryl azides", or mediated by [Ru(VI)(por)(NY)2] with Y = biphenyl-4-yl (Ar-Ar) and Ar-Ar-p-Cl, gave aziridines and amines in moderate yields. The electronic structure of [Ru(VI)(por)(NY)2] was examined by DFT calculations.

Synthesis, characterization, and luminescence studies of discrete polynuclear gold(I) sulfido and selenido complexes with intramolecular aurophilic contacts.

The synthesis, characterization, and photophysical and photochemical properties of a family of high-nuclearity gold(I) chalcogenides, specifically, the gold(I) sulfido and selenido complexes containing different bridging diphosphine ligands with nuclearities of ten ([Au10{µ-Ph2PN(R)PPh2}4(µ3-E)4](2+)) and six ([Au6{µ-Ph2PN(R)PPh2}3(µ3-E)2](2+)), are reported. The X-ray crystal structures of the complex cations of Au10 and Au6 are found to be propeller-like structures and distorted cubane structures, respectively, with the presence of short intramolecular gold···gold distances. The complexes show intense green and/or orange phosphorescence upon photoexcitation in the solid state and in solution at ambient and low temperature. The emission properties are found to be strongly dependent on the nuclearities and the chalcogenido ligands, but are rather insensitive to the substituents on the bis(diphenylphosphino)amines. The emissions are tentatively assigned to originate from the excited states derived from the phosphine-centered intraligand (IL) transition or metal-centered (ds/dp) mixed with ligand-to-metal-metal charge transfer (LMMCT) (E→Au) transition. The photochemical properties of the complexes were also studied by transient absorption spectroscopy.

Polymorphism of metal-organic frameworks: direct comparison of structures and theoretical N2-uptake of topological pto- and tbo-isomers.

The syntheses, calculated surface areas and N2 uptakes of two highly augmented {Cu}2 'paddlewheel'-based MOFs provide a direct comparison of tbo and pto framework polymorphs with identical composition.