Supramolecular aggregation control in polyoxometalates covalently functionalized with oligoaromatic groups

CLICK-chemistry has become a universal route to covalently link organic molecules functionalized with azides and alkynes, respectively. Here, we report how CLICK-chemistry can be used to attach oligoaromatic organic moieties to Dawson-type polyoxometalates. In step one, the lacunary Dawson anion [α2-P2W17O61]6− is functionalized with phosphonate anchors featuring peripheral azide groups. In step two, this organic-inorganic hybrid undergoes microwave-assisted CLICK coupling. We demonstrate the versatility of this route to access a series of Dawson anions covalently functionalized with oligoaromatic groups. The supramolecular chemistry and aggregation of these systems in solution is explored, and we report distinct changes in charge-transfer behavior depending on the size of the oligoaromatic π-system

Supramolecular activation of a molecular photocatalyst

The effects of the planar aromatic organic molecules anthracene and pyrene on the catalytic performance of the intramolecular hydrogen evolving photocatalyst [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 functioning as a photocatalytic dyad have been studied. 1H-NMR studies on [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 and [Ru(tbbpy)2(tpphz)](PF6)2 show a pronounced interaction of pyrene with the ruthenium complexes due to π–π-interactions. The solid state structure of [Ru(tbbpy)2(tpphz)PdCl2]2[Mo8O24] shows a pronounced π–π-stacking of the polyaromatic ligands. In addition, dimerization constants for the complexes and association constants between the complexes and pyrene were determined. Studies on the photocatalytic hydrogen production show a decreased induction phase and increased turn over frequencies during the initial phase of the catalysis in the presence of anthracene and pyrene utilising the catalyst [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 irrespective of the nature of the polycyclic aromatic hydrocarbon

π-Stacking attraction vs. electrostatic repulsion: competing supramolecular interactions in a tpphz-bridged Ru(II)/Au(III) complex

The synthesis and characterization of a mixed metal ruthenium(II)/gold(III) complex bridged by tetrapyridophenazine (tpphz) are described. It is isostructural and isoelectronic to the well-known photocatalysts with palladium(II) or platinum(II). Concentration dependent 1H-NMR spectroscopy and XRD studies show that the electrostatic repulsion between the gold(III) moieties exceeds the attractive π-stacking interaction. Theoretical calculations based on the new structural data confirm an increased positive charge on the bridging ligand as well as significantly altered orbital symmetry as compared to the previously investigated palladium(II) complex. This is the first example of a tpphz ruthenium(II) complex where π-stacking is completely inhibited. The detailed investigation of the solid-state structure showed for the first time in bimetallic tpphz bridged complexes no significant torsion within the bridging ligand itself. Although catalytic performance for proton reduction by gold(III) is naturally not observed, its photochemical decomposition in colloidal gold particles could be shown by TEM and DLS

Sterically confined second-sphere receptor chromophores for optical molecular recognition and photocatalysis

The investigation of molecular processes and interactions through visible light is of fundamental importance for a wide range of technological and biological applications. The chromophores developed in this work have been used as luminescent sensors and light sensitizers for photocatalysis. A particular challenge is the optical detection of biologically abundant anions such as the halides. Simple inorganic ions are ubiquitously found in the environment, but they exhibit no visible interaction with light to be readily detected. A series of new ruthenium(II) chromophores with a hydrogen bond donating bibenzimidazole ligand have been developed in order to bind small ions and to signal the binding event through changes in their visible-light absorption and emission properties. Previous studies have shown that related complexes exhibit vivid color changes in the presence of basic anions as deprotonation strongly alters the electronic properties of these coordination compounds. Interaction with weakly basic anions through hydrogen bond interactions has been observed for example by NMR spectroscopy, the corresponding optical response however remained marginal. Progress has been reported upon introduction of substituents in a manner which defines a sterically confined pocket around the hydrogen bond donor functions. Using a versatile palladium(0) mediated synthetic procedure for variably substituted bibenzimidazole ligands, some new ruthenium(II) chromophores could be synthesized with well-defined binding pockets of varying size and flexibility. In particular, two complexes with anisyl substituents differ structurally only by four methyl groups which enforce a more rigid geometry of the respective binding pocket, whereas the flexible analogue exhibits rotational freedom. Remarkably, both new complexes show unprecedented optical sensitivity towards binding of the weakly basic halide ions chloride and bromide. Comparative investigations with an unsubstituted bibenzimidazole ruthenium(II) complex reveal that the pocket formation leads to dimming of the new chromophores, which is reversed upon anion binding with remarkable sensitivity towards chloride and bromide. These complexes hence represent a promising step forward for the detection of abundant, weakly basic, inorganic anions through optical events. One potential improvement beyond the reported anion studies is the synthetic achievement of a sensor with a macrocyclic binding pocket. An alternative chromophore unit for bibenzimidazole based sensors is based on the strongly luminescent properties of cyclometallated bis(phenylpyridine) iridium(III) complexes. In contrast to its ruthenium based analogue the new iridium(III) bibenzimidazole complex presented in this thesis exhibits vivid emission also in its deprotonated forms which potentially broadens the pH range for luminescence based recognition events. A combined ruthenium(II)/iridium(III) chromophore exhibits an appealing mixture of photophysical features of its subunits. This particular chromophore has hence been probed as sensitizer for light-driven catalysis and proved to be an interesting candidate for photocatalytic hydrogen production from an aqueous solution. For the purpose of detection of metal cations, a new phenanthroline based bridging ligand has been developed which exhibits a second diimine ligand function for 3d metal ion recognition. Due to the sterically demanding substitution pattern, regioselective synthesis of the respective iridium(III) chromophore has been achieved. Binding studies with nickel(II), zinc(II), and copper(I) reveal that only the latter was suitable to access the second binding sphere. The binding event is characterized by a vivid color change and darkening due to luminescence quenching. With respect to electrochemical measurements and in agreement with literature it is suggested that the selectivity for copper(I) potentially corresponds to a redox process which allows rearrangement and binding of the reduced diimine receptor sphere. In summary, chemically and structurally versatile new second-sphere receptors are presented in this thesis. The synthetic concepts allow a modular variation of photocenters and the corresponding photophysical properties, and binding modes for either anions or cations can be altered by the choice of bridging units. The reported supramolecular recognition and sensitization studies are promising and pave the way for the further development of molecular luminescent sensors with tunable selectivity and photochemistry

Red Light Absorption of [Re^I(CO)₃(α-diimine)Cl] Complexes through Extension of the 4,4′-Bipyrimidine Ligand’s π-System

Rhenium(I) complexes of type [Re(CO)3(NN)Cl] (NN = α-diimine) with MLCT absorption in the orange-red region of the visible spectrum have been synthesized and fully characterized, including single crystal X-ray diffraction on two complexes. The strong bathochromic shift of MLCT absorption was achieved through extension of the π-system of the electron-poor bidiazine ligand 4,4′-bipyrimidine by the addition of fused phenyl rings, resulting in 4,4′-biquinazoline. Furthermore, upon anionic cyclization of the twisted bidiazine, a new 4N-doped perylene ligand, namely, 1,3,10,12-tetraazaperylene, was obtained. Electrochemical characterization revealed a significant stabilization of the LUMO in this series, with the first reduction of the azaperylene found at E1/2(0/−) = −1.131 V vs. Fc+/Fc, which is the most anodic half-wave potential observed for N-doped perylene derivatives so far. The low LUMO energies were directly correlated to the photophysical properties of the respective complexes, resulting in a strongly red-shifted MLCT absorption band in chloroform with a λmax = 586 nm and high extinction coefficients (ε586nm > 5000 M−1 cm−1) ranging above 700 nm in the case of the tetraazaperylene complex. Such low-energy MLCT absorption is highly unusual for Re(I) α-diimine complexes, for which these bands are typically found in the near UV. The reported 1,3,10,12-tetraazaperylene complex displayed the [Re(CO)3(α-diimine)Cl] complex with the strongest MLCT red shift ever reported. UV–Vis NIR spectroelectrochemical investigations gave further insights into the nature and stability of the reduced states. The electron-poor ligands explored herein open up a new path for designing metal complexes with strongly red-shifted absorption, thus enabling photocatalysis and photomedical applications with low-energy, tissue-penetrating red light in future

A Triad Photoanode for Visible Light‐Driven Water Oxidation via Immobilization of Molecular Polyoxometalate on Polymeric Carbon Nitride

Due to their availability, low cost, nontoxicity, and tunability, polymeric carbon nitrides (CNx) represent one of the most attractive materials classes for the development of fully sustainable photo(electro)catalytic systems for solar‐driven water splitting. However, the development of CNx‐based photoanodes for visible light‐driven water oxidation to dioxygen is rather challenging, particularly due to issues related to photoelectrode stability and effective coupling of the light absorber with water oxidation catalysts. Herein, a triadic photoanode comprising a porous TiO2 electron collector scaffold sensitized by CNx coupled to a molecular cobalt polyoxometalate (CoPOM = [Co4(H2O)2(PW9O34)2]10) catalyst is reported. Complete water oxidation to dioxygen under visible (λ > 420 nm) light irradiation is demonstrated, with photocurrents down to relatively low bias potentials (0.2 V vs RHE). Furthermore, polyethyleneimine (PEI), a cationic polymer is shown to act as an effective and non‐sacrificial electrostatic linker for immobilization of the anionic CoPOM onto the negatively charged surface of CNx. The optimized deposition of CoPOM using the PEI linker translates directly into improved efficiency of the transfer of photogenerated holes to water molecules and to enhanced oxygen evolution. This work thus provides important design rules for effective immobilization of POM‐based catalysts into soft‐matter photoelectrocatalytic architectures for light‐driven water oxidation.A triadic photoanode comprised of a porous TiO2 electron collector scaffold sensitized by polymeric carbon nitride and coupled to a molecular cobalt polyoxometalate (CoPOM) catalyst exhibits visible (λ > 420 nm) light‐driven water oxidation to dioxygen. The beneficial role of the cationic polyethyleneimine polymer as an effective electrostatic linker for immobilization of CoPOM onto carbon nitride is highlighted.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Vector Stiftung http://dx.doi.org/10.13039/50110001391

Rational Design of a Catalyst for the Selective Monoborylation of Methane

Combined computational and experimental studies elucidate the mechanism and suggest rational design and optimization strategies of a bis(phosphine)-supported iridium-catalyst for methane monoborylation. The activation of the C-H bond in methane via oxidative addition using tris(boryl) iridium(III) complexes bearing bis-chelating supporting ligands is modeled computationally. This model shows that the use of the soft Lewis base ligand such as 1,2-bis(dimethylphosphino)ethane (dmpe) lowers the activation barrier of the rate-determining step as it facilitates polarization of the metal-center, lowering the barrier of the oxidative addition to afford a seven-coordinate iridium(V) intermediate. The experimental optimization of this reaction using high-throughput methods shows that up to 170 turnovers can be achieved at 150 degrees C (500 psi) within 16 h using bis(pinacolato)diboron, a well-defined homogeneous and monomeric catalyst (dmpe)Ir(COD)Cl that is readily available from commercial precursors, with selectivity for the monoborylation product. High-boiling cyclic aliphatic solvents decalin and cyclooctane also prove suitable for this reaction, while being inert toward borylation. In accordance with the lower calculated activation barrier, catalytic turnover is also observed at 120 degrees C with up to 50 turnovers over the course of 4 days in cyclohexane solvent. It was found that the borylation of methane is only achieved via one catalytic cycle, and buildup of pinacolborane, a side-product from methane borylation with bis(pinacolato)diboron, inhibits catalytic activity.11Nsciescopu

Coupled reaction equilibria enable the light-driven formation of metal-functionalized molecular vanadium oxides

The introduction of metal sites into molecular metal oxides, so-called polyoxometalates, is a key approach to tune their structure and reactivity. To-date, the complex solution mechanisms which govern metal-functionalizatio of polyoxometalates is still poorly understood. Here, we reveal the existence of a coupled set of light-dependent and light-independent reaction equilibria control the mono- and di-metal-functionalization of a prototype molecular vanadium oxide cluster. Comprehensive mechanistic analyses show that coordination of a single Mg2+ ion to the native species (NMe2H2)2[V12O32Cl]3- results in formation of the mono-functionalized system (NMe2H2)[(MgCl)V12O32Cl]3-. This species is photoactive, and irradiation with visible light triggers a second, light-dependent reaction equilibrium which drives the formation of the di-metal-functionalized species [(MgCl)2V12O32Cl]3-. The use cations which compete with Mg2+ can effectively inhibit the formation of the metal functionalized clusters. The study therefore demonstrates how external and internal stimuli can be used to control supramolecular polyoxometalate assembly

A Triad Photoanode for Visible Light-Driven Water Oxidation via Immobilization of Molecular Polyoxometalate on Polymeric Carbon Nitride

Due to their availability, low cost, non-toxicity and tunability, polymeric carbon nitrides (CNx) represent one of the most attractive materials classes for the development of fully sustainable photo(electro)catalytic systems for solar-driven water splitting. However, the development of CNx-based photoanodes for visible light-driven water oxidation to dioxygen is rather challenging, particularly due to issues related to photoelectrode stability and effective coupling of the light absorber with water oxidation catalysts. Herein, a triadic photoanode comprising a porous TiO2 electron collector scaffold sensitized by CNx coupled to a molecular cobalt polyoxometalate (CoPOM = [Co4(H2O)2(PW9O34)2]10─) catalyst is reported. Complete water oxidation to dioxygen under visible (λ > 420 nm) light irradiation is demonstrated, with photocurrents down to relatively low bias potentials (0.2 V vs. RHE). Furthermore, polyethyleneimine (PEI), a cationic polymer is shown to act as an effective and non-sacrificial electrostatic linker for immobilization of the anionic CoPOM onto the negatively charged surface of CNx. The optimized deposition of CoPOM using the PEI linker translates directly into improved efficiency of the transfer of photogenerated holes to water molecules and to enhanced oxygen evolution. This work thus provides important design rules for effective immobilization of POM-based catalysts into soft-matter photoelectrocatalytic architectures for light-driven water oxidation