Azide-Substituted 1,2,3-Triazolium Salts as Useful Synthetic Synthons: Access to Triazenyl Radicals and Staudinger Type Reactivity

Mesoionic carbenes (MIC) are a popular class of compound that are heavily investigated at the moment. The access to cationic MICs, and the ability of MICs to stabilize radicals are two highly attractive fields that have hardly been explored until now. Here the synthesis and characterisation of three different cationic azide-substituted 1,2,3-triazolium salts, used as building blocks for studying their reactivity towards triphenylphosphine are reported, where the reactivity is dependent on the nature of the starting triazolium salt. Furthermore, the cationic triazolium salts were used to develop a series of unsymmetrical MIC-triazene-NHC/MIC’ compounds, which can be readily converted to the radical form either by electrochemical or chemical methods. These radicals, which display NIR electrochromism, were investigated using a battery of techniques such as electrochemistry, UV/Vis/NIR and EPR spectroelectrochemistry, and theoretical calculations. Interestingly, the MIC plays an important role in the stabilization of the triazenyl radical, particularly in a competitive role vis-à-vis their NHC counterparts. These results shed new light on the ability of MICs to stabilize radicals, and possibly also on their π-accepting ability

Iridium Azocarboxamide Complexes: Variable Coordination Modes, C–H Activation, Transfer Hydrogenation Catalysis, and Mechanistic Insights

Azocarboxamides, a special class of azo ligands, display intriguing electronic properties due to their versatile binding modes and coordination flexibility. These properties may have significant implications for their use in homogeneous catalysis. In the present report, half-sandwich Ir–Cp* complexes of two different azocarboxamide ligands are presented. Different coordination motifs of the ligand were realized using base and chloride abstracting ligand to give N∧N-, N∧O-, and N∧C-chelated monomeric iridium complexes. For the azocarboxamide ligand having methoxy substituted at the phenyl ring, a mixture of N∧C-chelated mononuclear (Ir-5) and N∧N,N∧C-chelated dinuclear complexes (Ir-4) were obtained by activating the C–H bond of the aryl ring. No such C–H activation was observed for the ligand without the methoxy substituent. The molecular identity of the complexes was confirmed by spectroscopic analyses, while X-ray diffraction analyses further confirmed three-legged piano-stool structure of the complexes along with the above binding modes. All complexes were found to exhibit remarkable activity as precatalysts for the transfer hydrogenation of carbonyl groups in the presence of a base, even at low catalyst loading. Optimization of reaction conditions divulged superior catalytic activity of Ir-3 and Ir-4 complexes in transfer hydrogenation over the other catalysts. Investigation of the influence of binding modes on the catalytic activity along with wide range substrates, tolerance to functional groups, and mechanistic insights into the reaction pathway are also presented. These are the first examples of C–H activation in azocarboxamide ligands

Ruthenium Complexes of Polyfluorocarbon Substituted Terpyridine and Mesoionic Carbene Ligands: An Interplay in CO2 Reduction

In recent years terpyridines (tpy) and mesoionic carbenes (MIC) have been widely used in metal complexes. With the right combination with a metal center, both of these ligands are individually known to generate excellent catalysts for CO2 reduction. In this study, we combine the potentials of PFC (PFC=polyfluorocarbon) substituted tpy and MIC ligands within the same platform to obtain a new class of complexes, which we investigated with respect to their structural, electrochemical and UV/Vis/NIR spectroelectrochemical properties. We further show that the resulting metal complexes are potent electrocatalysts for CO2 reduction in which CO is exclusively formed with a faradaic efficiency of 92 %. A preliminary mechanistic study, including the isolation and characterization of a key intermediate is also reported

Spin Crossover and Fluorine-Specific Interactions in Metal Complexes of Terpyridines with Polyfluorocarbon Tails

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F... F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing

Electrochemistry and Spin-Crossover Behavior of Fluorinated Terpyridine-Based Co(II) and Fe(II) Complexes

Due to their ability to form stable molecular complexes that have tailor-made properties, terpyridine ligands are of great interest in chemistry and material science. In this regard, we prepared two terpyridine ligands with two different fluorinated phenyl rings on the backbone. The corresponding CoII and FeII complexes were synthesized and characterized by single-crystal X-ray structural analysis, electrochemistry and temperature-dependent SQUID magnetometry. Single crystal X-ray diffraction analyses at 100 K of these complexes revealed Co−N and Fe−N bond lengths that are typical of low spin CoII and FeII centers. The metal centers are coordinated in an octahedral fashion and the fluorinated phenyl rings on the backbone are twisted out of the plane of the terpyridine unit. The complexes were investigated with cyclic voltammetry and UV/Vis-NIR spectroelectrochemistry. All complexes show a reversible oxidation and several reduction processes. Temperature dependent SQUID magnetometry revealed a gradual thermal SCO behavior in two of the complexes, while EPR spectroscopy provided further insights on the electronic structure of the metal complexes, as well as site of reduction

Electronic Structure and Multicatalytic Features of Redox-Active Bis(arylimino)acenaphthene (BIAN)-Derived Ruthenium Complexes

The article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru­(trpy)­(R-BIAN)­Cl]­ClO₄ ([<b>1a</b>]­ClO₄–[<b>1c</b>]­ClO₄), [Ru­(trpy)­(R-BIAN)­(H₂O)]­(ClO₄)₂ ([<b>3a</b>]­(ClO₄)₂–[<b>3c</b>]­(ClO₄)₂), and BIAO-derived [Ru­(trpy)­(BIAO)­Cl]­ClO₄ ([<b>2a</b>]­ClO₄) (trpy = 2,2′:6′,2′′-terpyridine, R-BIAN = bis­(arylimino)­acenaphthene (R = H (<b>1a</b>⁺, <b>3a</b>²⁺), 4-OMe (<b>1b</b>⁺, <b>3b</b>²⁺), 4-NO₂ (<b>1c</b>⁺, <b>3c</b>²⁺), BIAO = [<i>N</i>-(phenyl)­imino]­acenapthenone). The experimental (X-ray, ¹H NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of <b>1a</b>^<i>n</i>–<b>1c</b>^<i>n</i> or <b>2a</b>^<i>n</i> collectively establish {Ru^II–BIAN⁰} or {Ru^II–BIAO⁰} configuration in the native state, metal-based oxidation to {Ru^III–BIAN⁰} or {Ru^III–BIAO⁰}, and successive electron uptake processes by the α-diimine fragment, followed by trpy and naphthalene π-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO₂ function in the BIAN moiety in <b>1c</b>⁺ has been reflected in the five nearby reduction steps within the accessible potential limit of −2 V versus SCE, leading to a fully reduced BIAN^4– state in [<b>1c</b>]^4–. The aqua derivatives ({Ru^II–OH₂}, <b>3a</b>²⁺–<b>3c</b>²⁺) undergo simultaneous 2e^–/2H⁺ transfer to the corresponding {Ru^IVO} state and the catalytic current associated with the Ru^IV/Ru^V response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (<b>3a</b>²⁺–<b>3c</b>²⁺) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI­(OAc)₂ as an oxidant in CH₂Cl₂ at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H₂O₂ as the stoichiometric oxidant in CH₃CN at 343 K. The involvement of the {Ru^IVO} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence

Electronic Structure and Multicatalytic Features of Redox-Active Bis(arylimino)acenaphthene (BIAN)-Derived Ruthenium Complexes

The article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru­(trpy)­(R-BIAN)­Cl]­ClO₄ ([<b>1a</b>]­ClO₄–[<b>1c</b>]­ClO₄), [Ru­(trpy)­(R-BIAN)­(H₂O)]­(ClO₄)₂ ([<b>3a</b>]­(ClO₄)₂–[<b>3c</b>]­(ClO₄)₂), and BIAO-derived [Ru­(trpy)­(BIAO)­Cl]­ClO₄ ([<b>2a</b>]­ClO₄) (trpy = 2,2′:6′,2′′-terpyridine, R-BIAN = bis­(arylimino)­acenaphthene (R = H (<b>1a</b>⁺, <b>3a</b>²⁺), 4-OMe (<b>1b</b>⁺, <b>3b</b>²⁺), 4-NO₂ (<b>1c</b>⁺, <b>3c</b>²⁺), BIAO = [<i>N</i>-(phenyl)­imino]­acenapthenone). The experimental (X-ray, ¹H NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of <b>1a</b>^<i>n</i>–<b>1c</b>^<i>n</i> or <b>2a</b>^<i>n</i> collectively establish {Ru^II–BIAN⁰} or {Ru^II–BIAO⁰} configuration in the native state, metal-based oxidation to {Ru^III–BIAN⁰} or {Ru^III–BIAO⁰}, and successive electron uptake processes by the α-diimine fragment, followed by trpy and naphthalene π-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO₂ function in the BIAN moiety in <b>1c</b>⁺ has been reflected in the five nearby reduction steps within the accessible potential limit of −2 V versus SCE, leading to a fully reduced BIAN^4– state in [<b>1c</b>]^4–. The aqua derivatives ({Ru^II–OH₂}, <b>3a</b>²⁺–<b>3c</b>²⁺) undergo simultaneous 2e^–/2H⁺ transfer to the corresponding {Ru^IVO} state and the catalytic current associated with the Ru^IV/Ru^V response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (<b>3a</b>²⁺–<b>3c</b>²⁺) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI­(OAc)₂ as an oxidant in CH₂Cl₂ at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H₂O₂ as the stoichiometric oxidant in CH₃CN at 343 K. The involvement of the {Ru^IVO} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence

Electronic Structure and Multicatalytic Features of Redox-Active Bis(arylimino)acenaphthene (BIAN)-Derived Ruthenium Complexes

The article examines the newly designed and structurally characterized redox-active BIAN-derived [Ru­(trpy)­(R-BIAN)­Cl]­ClO₄ ([<b>1a</b>]­ClO₄–[<b>1c</b>]­ClO₄), [Ru­(trpy)­(R-BIAN)­(H₂O)]­(ClO₄)₂ ([<b>3a</b>]­(ClO₄)₂–[<b>3c</b>]­(ClO₄)₂), and BIAO-derived [Ru­(trpy)­(BIAO)­Cl]­ClO₄ ([<b>2a</b>]­ClO₄) (trpy = 2,2′:6′,2′′-terpyridine, R-BIAN = bis­(arylimino)­acenaphthene (R = H (<b>1a</b>⁺, <b>3a</b>²⁺), 4-OMe (<b>1b</b>⁺, <b>3b</b>²⁺), 4-NO₂ (<b>1c</b>⁺, <b>3c</b>²⁺), BIAO = [<i>N</i>-(phenyl)­imino]­acenapthenone). The experimental (X-ray, ¹H NMR, spectroelectrochemistry, EPR) and DFT/TD-DFT calculations of <b>1a</b>^<i>n</i>–<b>1c</b>^<i>n</i> or <b>2a</b>^<i>n</i> collectively establish {Ru^II–BIAN⁰} or {Ru^II–BIAO⁰} configuration in the native state, metal-based oxidation to {Ru^III–BIAN⁰} or {Ru^III–BIAO⁰}, and successive electron uptake processes by the α-diimine fragment, followed by trpy and naphthalene π-system of BIAN or BIAO, respectively. The impact of the electron-withdrawing NO₂ function in the BIAN moiety in <b>1c</b>⁺ has been reflected in the five nearby reduction steps within the accessible potential limit of −2 V versus SCE, leading to a fully reduced BIAN^4– state in [<b>1c</b>]^4–. The aqua derivatives ({Ru^II–OH₂}, <b>3a</b>²⁺–<b>3c</b>²⁺) undergo simultaneous 2e^–/2H⁺ transfer to the corresponding {Ru^IVO} state and the catalytic current associated with the Ru^IV/Ru^V response probably implies its involvement in the electrocatalytic water oxidation. The aqua derivatives (<b>3a</b>²⁺–<b>3c</b>²⁺) are efficient and selective precatalysts in transforming a wide variety of alkenes to corresponding epoxides in the presence of PhI­(OAc)₂ as an oxidant in CH₂Cl₂ at 298 K as well as oxidation of primary, secondary, and heterocyclic alcohols with a large substrate scope with H₂O₂ as the stoichiometric oxidant in CH₃CN at 343 K. The involvement of the {Ru^IVO} intermediate as the active catalyst in both the oxidation processes has been ascertained via a sequence of experimental evidence

Modulation of peripheral functional groups for the enhancement of electrochemical HER activity in bimetallic Ni-containing triply-fused porphyrins

We report here nickel(II) complexes (Ni-1 and Ni-2) of triply fused (covalently linked through β–β, meso–meso, β′–β′ linkages) porphyrins functionalized with tert-butyl and O-hexyl chains for the investigation of the electrocatalytic hydrogen evolution reaction (HER). The analogous monometallic porphyrins Ni-3 and Ni-4 were also synthesized for comparative analysis. Cyclic voltammetry of the bimetallic complexes functionalized with tert-butyl and O-hexyl chains showed an anodic shift in reduction potentials of Ni-1. The influence of the long alkyl chains was even more pronounced during electrochemical proton reduction with trifluoroacetic acid (TFA), as confirmed by the higher icat/ip and rate constants (kobs) of Ni-2 compared to Ni-1. Additionally, Faradaic efficiencies for the HER is more than 90 % for Ni-2, which is three times larger than Ni-1. Ni-2 also displays turnover numbers that are more than 200 times larger than Ni-1. A mechanistic investigation using UV-vis-NIR and EPR spectroscopy as well theoretical calculations indicate the involvement of the doubly reduced species as an active catalytic intermediate in the electrocatalytic process

Synthesis and Characterization of a Cobalt(III) Corrole with an Sbound DMSO Ligand

A cobalt(III) corrole complex with an apical dmso ligand is presented. Crystallographic and spectroscopic data are used to unequivocally establish the dmso(O) vs. dmso(S) coordination in this complex. Wave function based methods were used to calculate the frontier orbitals for the complexes with O-bound and S-bound dmso ligands. Electrochemical properties of the complex is presented as well.<br /