Characterization of a Borane σ Complex of a Diiron Dithiolate: Model for an Elusive Dihydrogen Adduct

The azadithiolate complex Fe₂[(SCH₂)₂NMe]­(CO)₆ reacts with borane to give an initial 1:1 adduct, which spontaneously decarbonylates to give Fe₂[(SCH₂)₂NMeBH₃]­(CO)₅. Featuring a Fe–H–B three-center, two-electron interaction, the pentacarbonyl complex is a structural model for H₂ complexes invoked in the [FeFe]-hydrogenases. The pentacarbonyl compound is a “σ complex”, where a B–H σ bond serves as a ligand for iron. The structure of this σ complex was characterized by variable-temperature NMR spectroscopy and X-ray crystallography. Complementary to the 1:1 borane adduct is the quaternary ammonium complex [Fe₂[(SCH₂)₂NMe₂]­(CO)₆]⁺, which was also characterized. It represents a kinetically robust analogue of the N-protonated amine cofactor, as indicated by its mild reduction potential

Osmium(II) Complexes Bearing Chelating N‑Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium­(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of Os^II-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[Os^II-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [Os^II-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an Os^II-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the Os^II-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from <i>Myrothecium verrucaria</i> while achieving high potential electroenzymatic oxygen reduction at <i>E</i> = +0.45 V vs Ag/AgCl at pH 6.5

Osmium(II) Complexes Bearing Chelating N‑Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium­(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of Os^II-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[Os^II-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [Os^II-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an Os^II-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the Os^II-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from <i>Myrothecium verrucaria</i> while achieving high potential electroenzymatic oxygen reduction at <i>E</i> = +0.45 V vs Ag/AgCl at pH 6.5

Hosting Adamantane in the Substrate Pocket of Laccase: Direct Bioelectrocatalytic Reduction of O₂ on Functionalized Carbon Nanotubes

We report the efficient immobilization and orientation of laccase from <i>Trametes versicolor</i> on MWCNT electrodes using 1-pyrenebutyric acid adamantyl amide as a supramolecular linker. We demonstrate the ability of adamantane to specifically interact with the hydrophobic cavity of laccase, while pyrene interacts with MWCNT sidewalls by π–π interactions. Adamantane allows the oriented immobilization of laccases on MWCNT electrodes. Using an anthraquinone-modified pyrene derivative for comparison, adamantane-modified MWCNTs achieve the stable immobilization and orientation of a higher number of enzymes per surface units, as confirmed by electrochemistry, theoretical calculations, and quartz crystal microbalance experiments. Furthermore, the efficient direct electron transfer ensures bioelectrocatalytic oxygen reduction at high half-wave potential of 0.55 V vs SCE accompanied by no kinetic limitation by the heterogeneous electron transfer and maximum current densities of 2.4 mA cm^–2

Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C–S Bond Homolysis in a Mixed Valence Diiron Dithiolate

The synthesis and redox properties are presented for the electron-rich bis­(monothiolate)­s Fe₂(SR)₂­(CO)₂­(dppv)₂ for R = Me ([<b>1</b>]⁰), Ph ([<b>2</b>]⁰), CH₂Ph ([<b>3</b>]⁰). Whereas related derivatives adopt <i>C</i>₂-symmetric Fe₂(CO)₂P₄ cores, [<b>1</b>]⁰–[<b>3</b>]⁰ have <i>C</i>_s symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [<b>1</b>]⁰–[<b>3</b>]⁰ undergo 1e^– oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe₂(SR)₂­(CO)₂­(dppv)₂]⁺. As established crystallographically, [<b>3</b>]⁺ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [<b>1</b>]⁺ and [<b>2</b>]⁺ and many other [Fe₂­(SR)₂L₆]⁺ derivatives, [<b>3</b>]⁺ undergoes C–S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂­(SCH₂Ph)­(S)­(CO)₂­(dppv)₂]⁺ ([<b>4</b>]⁺). According to X-ray crystallography, the first coordination spheres of [<b>3</b>]⁺ and [<b>4</b>]⁺ are similar, but the Fe–sulfido bonds are short in [<b>4</b>]⁺. The conversion of [<b>3</b>]⁺ to [<b>4</b>]⁺ follows first-order kinetics, with <i>k</i> = 2.3 × 10^–6 s^–1 (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [<b>3</b>]⁺ to [<b>4</b>]⁺ is accelerated about 10×, the main organic product being TEMPO-CH₂Ph. DFT calculations predict that the homolysis of a C–S bond is exergonic for [Fe₂­(SCH₂Ph)₂­(CO)₂­(PR₃)₄]⁺ but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [<b>4</b>]⁺ is indicated by its double carbonylation to give [Fe₂­(SCH₂Ph)­(S)­(CO)₄­(dppv)₂]⁺ ([<b>5</b>]⁺), which adopts a bioctahedral structure