Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes

Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Electronic stabilization of the negative charge by a thiophosphinoyl and pyridyl/quinolyl substituent allows for the isolation of two lithium chloride carbenoids at room temperature. Molecular structure analysis by X-ray crystallography and multinuclear NMR spectroscopy reveal no direct lithium–carbon interaction in the solid state and in solution. This leads to remarkable thermal stability but also to a reduced ambiphilic character of the compounds. Thus, properties typically observed for nonstabilized Li/Cl carbenoids are less pronounced. Nevertheless, computational studies still show that despite the charge delocalization within the compound a high negative charge remains at the carbenoid carbon atom. Preliminary reactivity studies confirm this nucleophilic character and show that the carbenoids can still be used as a “carbene” source for the formation of carbene complexes

An Exclusively Organometallic {FeNO}⁷ Complex with Tetracarbene Ligation and a Linear FeNO Unit

The iron­(II) complex <b>1</b> of a macrocyclic tetracarbene binds NO to form a low-spin (<i>S</i> = ¹/₂) {FeNO}⁷ complex (<b>2</b>) with a linear FeNO unit and a short Fe–NO bond. IR, electron paramagnetic resonance, and Mössbauer spectroscopies as well as density functional theory calculations suggest some Fe^INO⁺ character and reveal that the singly occupied molecular orbital of <b>2</b>, resulting from the σ-antibonding interaction of Fe d_<i>z</i>² and the NO lone pair, is largely iron-based. Reduction yields a quite stable {FeNO}⁸ species (<b>3</b>); both <b>2</b> and <b>3</b> feature very low Mössbauer isomer shifts (∼0.0 mm·s^–1)

Complete Series of {FeNO}⁸, {FeNO}⁷, and {FeNO}⁶ Complexes Stabilized by a Tetracarbene Macrocycle

Use of a macrocyclic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocycles though electronically distinct, has allowed for the isolation, X-ray crystallographic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}^<i>x</i> complexes (<i>x</i> = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp₂, of the {FeNO}⁷ complex <b>1</b> leads to the organometallic {FeNO}⁸ species <b>2</b>. Its crystallographic structure determination is the first for a nonheme iron nitroxyl {FeNO}⁸ and has allowed to identify structural trends among the series of {FeNO}^<i>x</i> complexes. Combined experimental data including ⁵⁷Fe Mössbauer, IR, UV–vis–NIR, NMR and Kβ X-ray emission spectroscopies in concert with DFT calculations suggest a largely metal centered reduction of <b>1</b> to form the low spin (<i>S</i> = 0) {FeNO}⁸ species <b>2</b>. The very strong σ-donor character of the tetracarbene ligand imparts unusual properties and spectroscopic signatures such as low ⁵⁷Fe Mössbauer isomer shifts and linear Fe–N–O units with high IR stretching frequencies for the NO ligand. The observed metal-centered reduction leads to distinct reactivity patterns of the {FeNO}⁸ species. In contrast to literature reported {FeNO}⁸ complexes, <b>2</b> does not undergo NO protonation under strictly anaerobic conditions. Only in the presence of both dioxygen and protons is rapid and clean oxidation to the {FeNO}⁷ complex <b>1</b> observed. While <b>1</b> is stable toward dioxygen, its reaction with dioxygen under NO atmosphere forms the {FeNO}⁶(ONO) complex <b>3</b> that features an unusual O-nitrito ligand <i>trans</i> to the NO. <b>3</b> is a rare example of a nonheme octahedral {FeNO}⁶ complex. Its electrochemical or chemical reduction triggers dissociation of the O-nitrito ligand and sequential formation of the {FeNO}⁷ and {FeNO}⁸ compounds <b>1</b> and <b>2</b>. A consistent electronic structure picture has been derived for these unique organometallic variants of the key bioinorganic {FeNO}^<i>x</i> functional units

Bis[<i>N</i>,<i>N</i>′‑diisopropylbenzamidinato(−)]silicon(II): Cycloaddition Reactions with Organic 1,3-Dienes and 1,2-Diketones

Reaction of the donor-stabilized silylene <b>1</b> (which is three-coordinate in the solid state and four-coordinate in solution) with organic 1,3-dienes (2,3-dimethyl-1,3-butadiene, 1,3-butadiene, (<i>E</i>,<i>E</i>)-1,4-diphenyl-1,3-butadiene, 2,3-dibenzyl-1,3-butadiene, 1,3-cyclohexadiene, or cyclo­octatetraene) and 1,2-diketones (3,5-di-<i>tert</i>-butyl-1,2-benzoquinone or 1,2-diphenyl­ethane-1,2-dione) leads to the formation of the respective cycloaddition products <b>2</b>–<b>9</b>. Compounds <b>2</b>–<b>9</b> were characterized by crystal structure analyses (<b>7</b> was studied as the hemi solvate <b>7</b>·​0.5<i>n</i>-C₆H₁₄) and NMR spectroscopic studies in the solid state and in solution. As the amidinato ligands can switch between a monodentate and bidentate coordination mode, for some of the cycloaddition products studied, the silicon coordination number in the solid state and in solution is different. For example, compound <b>4</b> is four- (<b>4a</b>) and six-coordinate (<b>4b</b>) in the solid state (isolated as a 1:1 cocrystallizate of <b>4a</b> and <b>4b</b>) and five-coordinate in solution. As demonstrated for the methanolysis of <b>2</b> (formation of <b>10</b>; proof of principle), compounds <b>2</b>–<b>7</b> with their reactive Si–N bonds are starting materials for the synthesis of promising mono- and bicyclic organosilicon compounds

Model of the MitoNEET [2Fe−2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe–2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S₂} ligation which could be structurally characterized in its diferric form, <b>5</b>^<b>2–</b>. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme’s unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model <b>5</b>^<b>2–</b> and a related homoleptic {SN/SN} capped [2Fe–2S] cluster <b>4</b>^2– are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the NH bond dissociation free energy (BDFE) of <b>5H</b>^<b>2–</b> (230 ± 4 kJ mol^–1) and the free energy Δ<i><i>G</i>°</i>_PCET for the reaction with TEMPO (−48.4 kJ mol^–1) are very similar to values for the homoleptic cluster <b>4H</b>^<b>2–</b> (232 ± 4 kJ mol^–1, –46.3 kJ mol^–1) the latter is found to react significantly faster than the mitoNEET model (data for <b>5H</b>^<b>2–</b>: <i>k</i> = 135 ± 27 M^–1 s^–1, Δ<i>H</i>^‡ = 17.6 ± 3.0 kJ mol^–1, Δ<i>S</i>^‡ = −143 ± 11 J mol^–1 K^–1, and Δ<i>G</i>^‡ = 59.8 kJ mol^–1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process

Model of the MitoNEET [2Fe−2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe–2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S₂} ligation which could be structurally characterized in its diferric form, <b>5</b>^<b>2–</b>. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme’s unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model <b>5</b>^<b>2–</b> and a related homoleptic {SN/SN} capped [2Fe–2S] cluster <b>4</b>^2– are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the NH bond dissociation free energy (BDFE) of <b>5H</b>^<b>2–</b> (230 ± 4 kJ mol^–1) and the free energy Δ<i><i>G</i>°</i>_PCET for the reaction with TEMPO (−48.4 kJ mol^–1) are very similar to values for the homoleptic cluster <b>4H</b>^<b>2–</b> (232 ± 4 kJ mol^–1, –46.3 kJ mol^–1) the latter is found to react significantly faster than the mitoNEET model (data for <b>5H</b>^<b>2–</b>: <i>k</i> = 135 ± 27 M^–1 s^–1, Δ<i>H</i>^‡ = 17.6 ± 3.0 kJ mol^–1, Δ<i>S</i>^‡ = −143 ± 11 J mol^–1 K^–1, and Δ<i>G</i>^‡ = 59.8 kJ mol^–1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process