A Different Perspective on Tuning the Photophysical and Photochemical Properties: The Influence of Constitutional Isomers in Group 6 Carbonyl Complexes with Pyridyl-Mesoionic Carbenes.

Exploring novel and existing design principles to tune the photochemical and photophysical properties of transition-metal complexes is an important goal in contemporary research. Here, we highlight the influence of constitutional isomers of pyridyl-1,2,3-triazolylidene mesoionic carbene (MIC) ligands on the photophysical and photochemical properties of the corresponding tetracarbonyl group 6 metal complexes (M = Cr, Mo, W). All new complexes [M(C-C)] presented herein incorporate a C-C linked pyridyl-MIC ligand and were fully characterized by X-ray diffraction analysis, elemental analysis, and 1H NMR and IR spectroscopy. Detailed photophysical investigations reveal a single emission in the VIS region, which extends into the NIR with lifetimes of up to 3.5 µs in the solid state at lower temperatures. The quantum yields were determined for all three complexes, and, in particular, the W0 complex shows an unusually high quantum yield of 29% compared to the values of 0.02% obtained for the [M(C-N)] isomers investigated in earlier works. Beyond this, the investigated W0 complex also exhibits an emission at 717 nm in a fluid solution. The combination of luminescence and FTIR-step scan spectroscopy with theoretical calculations reveals an emissive 3MLCT state. Irradiation of the presented complexes leads to a clean cleavage of one axial CO ligand. A metastable 16 VE species with a vacant axial coordination site was detected in the solid state at low temperatures. In solution, the respective solvato complexes are formed. A dark reverse reaction is observed, as previously described for the [M(C-N)] analogues. The increased electron density induced by the C-C linked pyridyl-MIC ligand leads to an increased kinetic rate constant for the reformation of the starting species and is also reflected in the lower photodissociation quantum yields.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes.

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)2bpy vs mes2bpy ligands) and Lewis basic ((Ph)2bpy vs (MeOPh)2bpy vs (MeSPh)2bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO2 reduction mechanisms with [MnI(R2bpy)(CO)3(CH3CN)]+ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)2bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO2 binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes2bpy-based precatalysts. Cyclic voltammetry conducted in CO2-saturated electrolyte for the (Ph)2bpy-based precatalyst [2-CH3CN]+ confirms that CO2 binding occurs at the two-electron-reduced activated catalyst [2]- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO2 binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]0 dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO2 binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH3CN]+ maintains a lower overpotential than [2-CH3CN]+ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOFmax is observed at its reduction-first pathway with an almost identical overpotential as [2-CH3CN]+. Notably, excessive steric bulk in the mes2bpy-based precatalyst [3-CH3CN]+ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH3CN]+ and [2-CH3CN]+. In fact, [3-CH3CN]+ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO2 with 2.8 M (5%) H2O establishes [2-CH3CN]+ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]0 dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH3CN]+ possesses an increased steric presence relative to [2-CH3CN]+, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.

Fluorinated click-derived tripodal ligands drive spin crossover in both iron(II) and cobalt(II) complexes.

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently, controlling the spin state in metal complexes is challenging because a precise control of the properties of the secondary coordination sphere is often difficult. It has been shown that non-covalent interactions in the secondary coordination sphere of transition metal complexes can enable spin state control. Here we exploit this strategy for fluorinated triazole ligands and present mononuclear CoII and FeII complexes with "click"-derived tripodal ligands that contain mono-fluorinated benzyl substituents on the backbone. Structural characterization of 1 and 2 at 100 K revealed Co-N bond lengths that are typical of high spin (HS) CoII complexes. In contrast, the Fe-N bond lengths for 3 are characteristic of a low spin (LS) FeII state. All complexes show an intramolecular face-to-face non-covalent interaction between two arms of the ligand. The influence of the substituents and of their geometric structure on the spin state of the metal center was investigated through SQUID magnetometry, which revealed spin crossover occurring in compounds 1 and 3. EPR spectroscopy sheds further light on the electronic structures of 1 and 2 in their low- and high-spin states. Quantum-chemical calculations of the fluorobenzene molecule were performed to obtain insight into the influence of fluorine-specific interactions. Interestingly, this work shows that the same fluorinated tripodal ligands induce SCO behavior in both FeII and CoII complexes.

Spin-state control of cobalt(II) and iron(II) complexes with click-derived tripodal ligands through non-covalent and fluorine-specific interactions.

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF5TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF5TA)2(BF4)2·2EtOH displays spin crossover (SCO).

Copper(I) complexes bearing mesoionic carbene ligands: influencing the activity in catalytic halo-click reactions.

Metal complexes of mesoionic carbene (MIC) ligands are known to catalyze a variety of chemical transformations. In this contribution, we report on the synthesis of a dicationic dicopper(i) complex containing a di-MIC ligand. Two routes are presented for the synthesis of the dicopper complex: Ag-mediated transmetalation and direct deprotonation. For the Ag-mediated transmetalation route, the detection and isolation of several Ag-containing intermediates that are relevant for the final formation of the aforementioned dicopper complex are reported. We then investigate a series of copper(i) complexes based on MIC ligands as precatalysts for the azide halo-alkyne (Click) cycloaddition reaction. In a comparative study, three different halide-containing (I, Br, Cl) substrates have been investigated with different catalysts to survey the behaviour for mono/di-copper-MIC complexes as well as neutral, mono- and di-cationic complexes. The cationic complexes proved to have superior activities compared to the neutral species. These are the first reports on the use of Cu-MIC complexes as precatalysts for the halo-Click reaction.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction.

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three ReI complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The ReI compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO2 to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive ReI complexes and for electrocatalytic CO2 reduction.

Tuning PtII -Based Donor-Acceptor Systems through Ligand Design: Effects on Frontier Orbitals, Redox Potentials, UV/Vis/NIR Absorptions, Electrochromism, and Photocatalysis.

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

Influence of Mesoionic Carbenes on Electro- and Photoactive Ru and Os Complexes: A Combined (Spectro-)Electrochemical, Photochemical, and Computational Study.

In recent years, mesoionic carbenes (MICs) are finding increasing use as building blocks of electro- and photoactive metal complexes. We present here a series of RuII and OsII polypyridine complexes where one or two pyridyl moieties of the well-known tris(bipyridine) analogues are replaced by MICs. We probe the structural, electrochemical, UV-vis-NIR/electron paramagnetic resonance spectroelectrochemical, and photophysical properties of these complexes as a function of the number of MICs in them. Insights from theoretical studies are used to describe the electronic structures of the various redox states. Additionally, electron flux density calculations provide an idea of the flow of electron densities in the excited states of these molecules. This is the first time that such electron flux density calculations are used to probe the excited state properties of transition metal complexes. Our results conclusively prove that the incorporation of MICs into Ru/Os-polypyridyl complexes has a profound influence on the ground and the excited state redox potentials, the position of the emission bands, as well as on the lifetimes of the excited states. These observations might thus be useful for the generation of novel photocatalysts and photosensitizers for dye-sensitized-solar-cells based on MICs.

Serendipitous discoveries of new coordination modes of the 1,5-regioisomer of 1,2,3-triazoles enroute to the attempted synthesis of a carbon-anchored tri-mesoionic carbene.

We report herein the synthesis and solid state structures of a protonated 1,5-diphenyl-1,2,3-triazole and a hydroxyl-alkyl-substituted 1,5-diphenyl-1,2,3-triazolium salt. Furthermore, we present a simple silver(i) triflate-triazole adduct, which forms di-, tri- and tetranuclear complexes depending on the crystallization conditions. The 1,5-regioisomer of the triazole acts as a bridging as well as a terminal ligand in these structures, coordinating via N2 as well as N3 nitrogen donors. These observations underline the versatility in coordination chemistry of this underused ligand class. All observations were made while attempting the synthesis of a carbon-anchored tripodal mesoionic carbene ligand. Apart from showcasing the coordination flexibility and the versatility of this underused regioisomer, we also present sound evidence for the organic products obtained during the attempted synthesis of the tri-mesoionic carbene. The latter results might be insightful in future synthesis of this elusive molecule.

An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities.

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)˙+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF˙+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole π-surface in comparison to pristine tetrathiafulvalene.

Illuminating Iron: Mesoionic Carbenes as Privileged Ligands in Photochemistry.

The new iron age: The stabilization of an iron(III) complex with mesoionic carbenes has been shown to lead to record-breaking excited-state lifetimes and an unprecedented spin-allowed radiative decay from a 2 LMCT state. These results set the stage for a sustainable future in the new "iron age".

Ruthenium Complexes with Strongly Electron-Donating Terpyridine Ligands: Effect of the Working Electrode on Electrochemical and Spectroelectrochemical Properties.

The combination of 2,2':6',2''-terpyridines (tpy) and RuII is known to deliver molecular and supramolecular assemblies with remarkable properties. Here new RuII complexes, with modified tpy ligands substituted with varying numbers of dimethlyamino groups, are presented. Electrochemistry shows that the incorporation of the strongly electron-donating groups on the tpy ligands leads to a negative shift of the RuII oxidation potential by close to 1 V. The reductive electrochemical responses are strongly dependent on the nature of the working electrode, with glassy carbon and gold working electrodes showing the best results. These observations led to the development of a modified Optically Transparent Thin Layer Electrochemical (OTTLE) cell, based on a gold working electrode. The use of UV/Vis/NIR spectroelectrochemical methods with that OTTLE cell, together with simulations of the cyclic voltammograms, allowed the characterization of four reduction steps in these complexes, the final two of which lead to bond activations at the ruthenium center. This observation is to the best of our knowledge unprecedented in coordinatively saturated complexes of type [Ru(tpy)2 ]2+ . The various redox states of the complexes were characterized by EPR spectroelectrochemistry and through DFT calculations. The results presented here establish these substituted tpy ligands as highly attractive ligands in coordination chemistry, and display the utility of a gold-based OTTLE cell for spectroelectrochemical measurements.

Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes.

Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other N-heterocyclic carbene counterparts. We present here seven rhenium(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through 1H and 13C NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV-vis-near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their fac-Re(CO)3-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where fac-Re(CO)3 complexes of triazoles/triazolylidenes are likely to find use.

Heteromultimetallic Complexes with Redox-Active Mesoionic Carbenes: Control of Donor Properties and Redox-Induced Catalysis.

Mesoionic carbenes (MICs) are currently hugely popular as ligands, and triazolylidenes are arguably the most prominent classes of such MICs. Mesoionic carbenes with ferrocenyl substituents are presented that can act as metalloligands for the generation of heteromultimetallic iridium(I) and gold(I) complexes. The ferrocenyl substituents allow for reversible oxidation of these heteromultimetallic complexes, and these oxidation steps have a strong influence on the donor properties of the MICs. Tolman electronic parameters (TEP) determined from analysis of the iridium-carbonyl complexes show that the neutral ferrocenyl-MIC ligands are stronger donors than the imidazolylidene based carbenes, the one-electron oxidized ferrocenyl MICs are in the range of the tricyclohexyl phosphines and the two-electron oxidized forms, which are electron-poor, lie in the range of triphenyl phosphines. Taking advantage of the generation of these electron-poor MICs, we show their gold(I) complexes are potent catalysts for the synthesis of oxazolines, with complexes of the oxidized MIC ligands, without any additional additive, outperforming their neutral counterparts by almost a factor of ten. These results thus present the first examples of MIC ligands that are reversibly electronically tunable, and show the potential of the oxidized MIC ligands in types of catalysis where electron-poor ligands are necessary. The potential of MICs for molecular electroactive materials is also shown.

Expanding the Scope of Chelating Triazolylidenes: Mesoionic Carbenes from the 1,5-"Click"-Regioisomer and Catalytic Synthesis of Secondary Amines from Nitroarenes.

Chelating 1,2,3-triazolylidenes have been established as privileged ligands in homogeneous catalysis. We present herein a new approach towards chelating 1,2,3-triazolylidene ligands based on the 1,5-regioisomer of the corresponding triazole, which can be obtained through simple click chemistry. The new ligands are compared to their 1,4-regioisomeric counterparts through coordination to the ruthenium p-cymene fragment. The complexes are characterized structurally and spectroscopically and are employed as (pre)catalysts in the reductive condensation of nitroarenes and primary alcohols to yield secondary amines. The activity of chelating mesoionic carbene ligands obtained from the two different regioisomers of the triazoles are compared and contrasted in catalysis. The performance of the ruthenium complexes with mesoionic carbenes could be improved through the choice of the employed base and reaction conditions, giving rise to the most effective systems thus far. The results presented here prove the utility of chelating mesoionic carbenes as an extremely potent class of ligands for the synthesis of secondary amines from nitroarenes.

Structural snapshots in the copper(ii) induced azide-nitrile cycloaddition: effects of peripheral ligand substituents on the formation of unsupported µ1,1-azido vs.µ1,4-tetrazolato bridged complexes.

The azido ligand is widely used in coordination chemistry both as a ligand and as a metal-bound reactant. Its role as a bridge for magnetic exchange coupling has attracted a lot of attention in polynuclear metal complexes. However, only a very limited number of complexes are known in which a single azide anion, particularly in the µ1,1-mode, is the only unsupported connection between two metal centers. We present here a series of copper(ii)-azido complexes with amine anchored, triazole-based tripodal ligands containing varying substituents. In the mononuclear copper-azido complexes there is only a negligible effect of these substituents on the structure of the metal complexes. However, the substituents seem to play a decisive role in the type and formation of the dinuclear complexes. Using the tripodal ligand TBTA with flexible benzyl substituents resulted in a rare example of an unsupported and solely µ1,1-azido-bridged dinuclear complex. The use of the TDTA ligand with 2,6-diisopropylphenyl moieties as rigid and sterically demanding substituents resulted in the formation of a scarce example of a solely µ1,4-tetrazolato-bridged dinuclear complex by in situ cycloaddition between the azide and solvent nitrile. This observation of a reaction of unactivated aliphatic nitrile with the azide anion at room temperature is very unusual. The isolation and characterization (by means of X-ray diffraction) of intermediates allows for mechanistic insights into the cycloaddition reaction. The isolated bridges in both dinuclear complexes render them ideal model compounds for the investigation of the magnetic exchange mediated by these ligands usually employed in polynuclear complexes and frameworks together with additional bridging ligands. Magnetic measurements and broken-symmetry DFT calculations were used to shed light on the magnetic exchange revealing weak and moderate antiferromagnetic exchange for the azide and tetrazolate, respectively.