Beyond the First Coordination Sphere─Manipulating the Excited-State Landscape in Iron(II) Chromophores with Protons.

Molecular transition metal chromophores play a central role in light harvesting and energy conversion. Recently, earth-abundant transition-metal-based chromophores have begun to challenge the dominance of platinum group metal complexes in this area. However, the development of new chromophores with optimized photophysical properties is still limited by a lack of synthetic methods, especially with respect to heteroleptic complexes with functional ligands. Here, we demonstrate a facile and efficient method for the combination of strong-field carbenes with the functional 2,2'-bibenzimidazole ligand in a heteroleptic iron(II) chromophore complex. Our approach yields two isomers that differ predominantly in their excited-state lifetimes based on the symmetry of the ligand field. Deprotonation of both isomers leads to a significant red-shift of the metal-to-ligand charge transfer (MLCT) absorption and a shortening of excited-state lifetimes. Femtosecond transient absorption spectroscopy in combination with quantum chemical simulations and resonance Raman spectroscopy reveals the complex relationship between protonation and photophysical properties. Protonation is found to tip the balance between MLCT and metal-centered (MC) excited states in favor of the former. This study showcases the first example of fine-tuning of the excited-state landscape in an iron(II) chromophore through second-sphere manipulations and provides a new perspective to the challenge of excited-state optimizations in 3d transition metal chromophores.

Silica Supported Organometallic IrI Complexes Enable Efficient Catalytic Methane Borylation.

Catalytic C-H borylation is an attractive method for the conversion of the most abundant hydrocarbon, methane (CH4), to a mild nucleophilic building block. However, existing CH4 borylation catalysts often suffer from low turnover numbers and conversions, which is hypothesized to result from inactive metal hydride agglomerates. Herein we report that the heterogenization of a bisphosphine molecular precatalyst, [(dmpe)Ir(cod)CH3], onto amorphous silica dramatically enhances its performance, yielding a catalyst that is 12-times more efficient than the current standard for CH4 borylation. The catalyst affords over 2000 turnovers at 150 °C in 16 h with a selectivity of 91.5% for mono- vs diborylation. Higher catalyst loadings improve yield and selectivity for the monoborylated product (H3CBpin) with 82.8% yield and >99% selectivity being achieved with 1255 turnovers. X-ray absorption and dynamic nuclear polarization-enhanced solid-state NMR spectroscopic studies identify the supported precatalyst as an IrI species, and indicate that upon completion of catalysis, multinuclear Ir polyhydrides are not formed. This is consistent with the hypothesis that immobilization of the organometallic Ir species on a surface prevents bimolecular decomposition pathways. Immobilization of the homogeneous IrI fragment onto amorphous silica represents a unique and simple strategy to improve the TON and longevity of a CH4 borylation catalyst.

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 -P2 W17 O61 ]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.

Red Light Absorption of [ReI(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.

Redox-Controlled and Reversible N-N Bond Forming and Splitting with an IronIV Terminal Imido Ligand.

Oxidation of the low-spin FeIV imido complex [{(tBupyrr)2py}Fe═NAd] (1) ((tBupyrr)2py2- = 2,6-bis(3,5-di-tert-butyl-pyrrolyl)pyridine, Ad = 1-adamantyl) with AgOAc or AgNO3 promotes reductive N-N bond coupling of the former imido nitrogen with a pyrrole nitrogen to form the respective ferric hydrazido-like pincer complexes [{(tBupyrrNAd)(tBupyrr)py}Fe(κ2-X)] (X = OAc-, 2OAc; NO3-, 2NO3). Reduction of 2OAc with KC8 cleaves the N-N bond to reform the FeIV imido ligand in 1, whereas acid-mediated demetalation of 2OAc or 2NO3 yields the free hydrazine ligand [(tBupyrrNHAd)(tBupyrrH)py] (3), the latter of which can be used as a direct entry to the iron imido complex when treated with [Fe{N(SiMe3)2}2]. In addition to characterizing these Fe systems, we show how this nitrene transfer strategy can be expanded to Co for the one-step synthesis of Co{(tBu-NHAdpyrr)(tBupyrr)py}] (4) ((tBu-NHAdpyrr)(tBupyrr)py2- = 2-(3-tBu-5-(1-adamantylmethyl-2-methylpropane-2-yl)-pyrrol-2-yl)-6-(3,5-tBu2-pyrrol-2-yl)-pyridine).

Unusual Dinitrogen Binding and Electron Storage in Dinuclear Iron Complexes.

A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron reduction of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on µ-N2 about the FeN2Fe core. Attempts to generate a Et2O-free version of 1 resulted instead in a dinuclear helical dimer, [(tBupyrr2py)Fe]2 (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N2 does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and 57Fe Mössbauer spectroscopy has been applied to characterize complexes 2-5, whereas DFT studies have been used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.

Metal-Ligand Cooperativity Promoting Sulfur Atom Transfer in Ferrous Complexes and Isolation of a Sulfurmethylenephosphorane Adduct.

The reaction of elemental sulfur with the cis-divacant octahedral complex [(pyrr2py)Fe(OEt2)] (1; pyrr2py2- = 3,5-tBu2-bis(pyrrolyl)pyridine) yields the iron dimer [(pyrr-1-S-pyrrpy)Fe]2 (2; pyrr-1-S-pyrrpy2- = 3,5-(tBu2-pyrrolyl)(1-S-3,5-tBu2-pyrrolyl)pyridine) resulting from a pyrr2py2- ligand based S-oxidation of one pyrrole arm. Addition of the phosphorus ylide H2CPPh3 to 1 forms the ylide adduct [(pyrr2py)Fe(CH2PPh3)] (3), which upon reaction with elemental sulfur produces a rare example of a sulfurmethylenephosphorane adduct, [(pyrr2py)Fe(SCH2PPh3)] (4). The sulfur-oxidized pyrrole group of the ligand pyrr-1-S-pyrrpy2- can be reversed, since complex 2 exhibits S atom transferability via the addition of 2 equiv of H2CPPh3 to yield a mixture of compounds 3 and 4. For all complexes reported, the ferrous ion remains S = 2. Complexes 2-4 were characterized by single-crystal X-ray diffraction as well as 1H NMR spectroscopy, solid and solution magnetic studies, and 57Fe Mössbauer spectroscopic measurements.

"Trojan Horse" Effect in Photocatalysis-How Anionic Silver Impurities Influence Apparent Catalytic Activity.

The problematic consequences of using silver carbene precursors for the synthesis of NHC-complexes is elucidated with the example of dinuclear Ru-Rh/Ir photocatalysts. The presence of silver in the products is proven and an alternative silver-free synthetic approach successfully implemented. A significant difference in performance in photocatalytic hydrogen evolution reactions of catalysts generated by the different strategies is observed.

Optical Sensing of Anions via Supramolecular Recognition with Biimidazole Complexes.

Phosphorescent metal complexes with peripheral N-H donor functionalities have attracted great attention as potential molecular sensing units for anionic species lately. In this contribution we discuss the development and potential of anion recognition and sensing features of recent examples of luminescent 2,2'-biimidazole complexes of ruthenium(II), iridium(III), osmium(II) and cobalt(III). The general dependency of photophysical features in these complexes regarding the acid-base chemistry of the peripheral N-H functionalities will be outlined as a basic requirement for optical ion recognition. Systematic strategies for the tuning and specific improvement by synthetic means will be discussed regarding recent reports. With respect to their distinct photophysical features, different transition metals are considered individually to demonstrate particular trends regarding ligand modification within the respective groups. In summary, this review elucidates the current state-of-the-art and future potential of the versatile class of 2,2'-biimidazole based sensor chromophores.

Oxygen-Dependent Photocatalytic Water Reduction with a Ruthenium(imidazolium) Chromophore and a Cobaloxime Catalyst.

Detailed investigations of a photocatalytic system capable of producing hydrogen under pre-catalytic aerobic conditions are reported. This system consists of the NHC precursor chromophore [Ru(tbbpy)2 (RR'ip)][PF6 ]3 (abbreviated as Ru(RR'ip)[PF6 ]3 ; tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, RR'ip=1,3-disubstituted-1H-imidazo[4,5-f][1,10]phenanthrolinium), the reduction catalyst Co(dmgH)2 (dmgH=dimethylglyoximato), and the electron donor ascorbic acid (AA). Screening studies with respect to solvent, cobaloxime catalyst, electron donor, pH, and concentrations of the individual components yielded optimized photocatalytic conditions. The system shows high activity based on Ru, with turnover numbers up to 2000 under oxygen-free and pre-catalytic aerobic conditions. The turnover frequency in the latter case was even higher than that for the oxygen-free catalyst system. The Ru complexes show high photostability and their first excited state is primarily located on the RR'ip ligand. X-ray crystallographic analysis of the rigid cyclophane-type ligand dd(ip)2 (Br)2 (dd(ip)2 =1,1',3,3'-bis(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(1H-imidazo[4,5-f][1,10]phenanthrolinium)) and the catalytic activity of its Ru complex [{(tbbpy)2 Ru}2 (µ-dd(ip)2 )][PF6 ]6 (abbreviated as Ru2 (dd(ip)2 )[PF6 ]6 ) suggest an intermolecular catalytic cycle.

Synthesis and characterization of a trisheteroleptic Ru(II) -based molecular switch.

The synthesis of a trisheteroleptic ruthenium complex [Ru(tb)(dppz)(tmbiH2 )][PF6 ]2 (tb=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazin, tmbiH2 =5,6,5',6'-tetramethyl-2,2'-bibenzimidazole) is described. In addition, the structural characterisation by means of 1D, 2D (1) H NMR spectroscopy, and mass spectrometry, along with determination of the solid-state structure of the important precursor Ru(tb)(dppz)Cl2 , supports the proposed octahedral coordination geometry. The capability of tmbiH2 to form hydrogen bonds is corroborated by the solid-state structure. The photochemical characteristics of this complex can be described as a combination of the "light switch" effects, which are either attributed to the dppz or to the tmbiH2 ligand. To illustrate the molecule's double switchable features, steady-state absorption and emission measurements were performed, which include the determination of the quantum yield and the pKa values of the acidic protons of the tmbiH2 ligand. Notably, the emission lifetimes are sensitive to the solvents used. This phenomenon is due to a proton-coupled deactivation of the excited metal-to-ligand charge transfer (MLCT) state of the complex.

Stabilizing Decavanadate Cluster as Electrode Material in Sodium and Lithium-ion Batteries.

Decavanadate ([V10 O28 ]6- , {V10 }) clusters are a potential electrode material for lithium and post-lithium batteries; however, their low stability due to the solubility in liquid organic electrolytes has been challenging. These molecular clusters are also prone to transform into solid-state oxides at a moderate temperature needed in the typical electrode fabrication process. Hence, controlling the solubility and improving the thermal stability of compounds are essential to make them more viable options for use as battery electrodes. This study shows a crystal engineering approach to stabilize the cluster with organic guanidinium (Gdm+ ) cation through the hydrogen-bonding interactions between the amino groups of the cation and the anion. The comparison of solubility and thermal stability of the Gdm{V10 } with another cluster bearing tetrabutylammonium (Tba+ ) cation reveals the better stability of cation-anion assembly in the former than the latter. As a result, the Gdm{V10 } delivers better rate capability and cycling stability than Tba{V10 } when tested as anode material in a half-cell configuration of a sodium-ion battery. Finally, the performance of the Gdm{V10 } anode is also investigated in a lithium-ion battery full cell with LiFePO4 cathode.

Mechanistic insights into template-driven polyoxovanadate self-assembly: the role of internal and external templates.

The self-assembly of molecular metal oxides, polyoxometalates (POMs), can be controlled using internal or, more rarely, external templates. Here, we explore how the interplay between internal templates (halides, oxoanions) and organic external templates (protonated cyclene species) affect the self-assembly of a model polyoxovanadate cluster, [V12O32X]n- (X = Cl-, Br-, NO3-). A combination of crystallographic analyses, spectroscopic studies and in situ as well as solid-state 51V NMR spectroscopy provide critical insights into the initial formation of an intermediate vanadate species formed during the process. Structural and spectroscopic studies suggest that a direct interaction between internal and external templates allows tuning of the internal template position within the cluster cavity. These insights form the basis for further developing the template-driven synthetic chemistry of polyoxovanadates.

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 key for tuning their structure and reactivity. The complex mechanisms which govern metal-functionalization of polyoxometalates are still poorly understood. Here, we report a coupled set of light-dependent and light-independent reaction equilibria controlling the mono- and di-metal-functionalization of a prototype molecular vanadium oxide cluster. Comprehensive mechanistic analyses show that coordination of a Mg2+ ion to the species {(NMe2H2)2[VV12O32Cl]}3- results in formation of the mono-functionalized {(NMe2H2)[(MgCl)VV12O32Cl]}3- with simultaneous release of a NMe2H2+ placeholder cation. Irradiation of this species with visible light results in one-electron reduction of the vanadate, exchange of the second NMe2H2+ with Mg2+, and formation/crystallization of the di-metal-functionalized [(MgCl)2VIVVV11O32Cl]4-. Mechanistic studies show how stimuli such as light or competing cations affect the coupled equilibria. Transfer of this synthetic concept to other metal cations is also demonstrated, highlighting the versatility of the approach.

Cation-controlled capture of polyoxovanadate-based organic-inorganic 1D architectures.

Metal cations are used to control the selective crystallization of organic-inorganic supramolecular polymers. Two complementary monomers, a dodecanuclear vanadate [V12O32(NO3)]5- and the organic macrocycle cyclen assemble into hybrid host-guest aggregates. In the presence of Ba2+ or La3+, supramolecular polymerization and crystallization is driven by a complex interplay of cyclene protonation, hydrogen-bonding and electrostatics.

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 (1)H-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.

Synthesis and characterization of ruthenium and rhenium dyes with phosphonate anchoring groups.

, a series of rhenium(i) tricarbonyl chloride complexes with bpy-R2 derivatives (bpy = 2,2'-bipyridine, R represents the substitution at the 4- and 4'-positions), and their corresponding trishomoleptic as well as heteroleptic ruthenium(ii) complexes and have been synthesized and characterized. Their applicability as immobilizable metal-organic chromophores in solar and photosynthesis cells is enabled by R, since it includes phosphonic ester groups as precursors for potent phosphonate anchoring groups. Conjugated linkers (phenylene and triazole moieties) serve as distance control between bpy and the anchor. Photophysical and electrochemical studies reveal pronounced effects of the aryl substitution. These effects were further investigated using resonance Raman experiments and supported by theoretical calculations. After hydrolysis the triazole containing was successfully immobilized on NiO, suggesting that its application in photovoltaic cells is feasible. The solid state structures of , , and are reported in this paper, enabling the determination of the distances and intermolecular interactions.

Novel phenanthroline-diaryldiazadiene ligands with heteroditopic coordination spheres.

1,10-Phenanthroline-5,6-diaryldiazadienes are key structures for the development of novel heterodinuclear photocatalysts and for the construction of extended heterocycles of potential biological use. Herein, the first examples of this compound family are presented together with a wide range of initial reactivity studies. Synthetic strategies are presented to access the two first derivatives of the ligand and to accomplish subsequent metal coordination to the phenanthroline sphere.

Synthesis and characterization of an immobilizable photochemical molecular device for H2-generation.

With [Ru(II)(bpyMeP)2tpphzPtCl2](2+) (4) a molecular photocatalyst has been synthesized for visible-light-driven H2-evolution. It contains the ligand bpyMeP (4,4'-bis(diethyl-(methylene)-phosphonate)-2,2'-bipyridine) with phosphate ester groups as precursors for the highly potent phosphonate anchoring groups, which can be utilized for immobilization of the catalyst on metal-oxide semiconductor surfaces. The synthesis was optimized with regard to high yields, bpyMeP was fully characterized and a solid-state structure could be obtained. Photophysical studies showed that the photophysical properties and the localization of the excited states are not altered compared to similar Ru-complexes without anchoring group precursors. (4) was even more active in homogenous catalysis experiments than [Ru(II)(tbbpy)2tpphzPtCl2](2+) (6) with tbbpy (4,4'-bis(tbutyl)-2,2'-bipyridine) as peripheral ligands. After hydrolysis (4) was successfully immobilized on NiO, suggesting that an application in photoelectrosynthesis cells is feasible.

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. (1)H-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.