Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework.

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kß x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Solid-state 11B NMR studies of coinage metal complexes containing a phosphine substituted diboraanthracene ligand.

Transition metal interactions with Lewis acids (M → Z linkages) are fundamentally interesting and practically important. The most common Z-type ligands contain boron, which contains an NMR active 11B nucleus. We measured solid-state 11B{1H} NMR spectra of copper, silver, and gold complexes containing a phosphine substituted 9,10-diboraanthracene ligand (B2P2) that contain planar boron centers and weak M → BR3 linkages ([(B2P2)M][BArF4] (M = Cu (1), Ag (2), Au (3)) characterized by large quadrupolar coupling (CQ) values (4.4-4.7 MHz) and large span (Ω) values (93-139 ppm). However, the solid-state 11B{1H} NMR spectrum of K[Au(B2P2)]- (4), which contains tetrahedral borons, is narrow and characterized by small CQ and Ω values. DFT analysis of 1-4 shows that CQ and Ω are expected to be large for planar boron environments and small for tetrahedral boron, and that the presence of a M → BR3 linkage relates to the reduction in CQ and 11B NMR shielding properties. Thus solid-state 11B NMR spectroscopy contains valuable information about M → BR3 linkages in complexes containing the B2P2 ligand.

H2 evolution from H2O via O-H oxidative addition across a 9,10-diboraanthracene.

The water reactivity of the boroauride complex ([Au(B2P2)][K(18-c-6)]; (B2P2, 9,10-bis(2-(diisopropylphosphino)-phenyl)-9,10-dihydroboranthrene) and its corresponding two-electron oxidized complex, Au(B2P2)Cl, are presented. Au(B2P2)Cl is tolerant to H2O and forms the hydroxide complex Au(B2P2)OH in the presence of H2O and triethylamine. [Au(B2P2)]Cl and [Au(B2P2)]OH are poor Lewis acids as judged by the Gutmann-Becket method, with [Au(B2P2)]OH displaying facile hydroxide exchange between B atoms of the DBA ring as evidenced by variable temperature NMR spectroscopy. The reduced boroauride complex [Au(B2P2)]- reacts with 1 equivalent of H2O to produce a hydride/hydroxide product, [Au(B2P2)(H)(OH)]-, that rapidly evolves H2 upon further H2O reaction to yield the dihydroxide compound, [Au(B2P2)(OH)2]-. [Au(B2P2)]Cl can be regenerated from [Au(B2P2)(OH)2]-via HCl·Et2O, providing a synthetic cycle for H2 evolution from H2O enabled by O-H oxidative addition at a diboraanthracene unit.

Assembly and Redox-Rich Hydride Chemistry of an Asymmetric Mo2S2 Platform.

Although molybdenum sulfide materials show promise as electrocatalysts for proton reduction, the hydrido species proposed as intermediates remain poorly characterized. We report herein the synthesis, reactions and spectroscopic properties of a molybdenum-hydride complex featuring an asymmetric Mo2S2 core. This molecule displays rich redox chemistry with electrochemical couples at E½ = -0.45, -0.78 and -1.99 V vs. Fc/Fc+. The corresponding hydrido-complexes for all three redox levels were isolated and characterized crystallographically. Through an analysis of solid-state bond metrics and DFT calculations, we show that the electron-transfer processes for the two more positive couples are centered predominantly on the pyridinediimine supporting ligand, whereas for the most negative couple electron-transfer is mostly Mo-localized.

C[double bond, length as m-dash]O scission and reductive coupling of organic carbonyls by a redox-active diboraanthracene.

The boron-centered reactivity of the diboraanthracene-auride complex [Au(B2P2)][K(18-c-6)]; (B2P2, 9,10-bis(2-(diisopropylphosphino)-phenyl)-9,10-dihydroboranthrene) with a series of organic carbonyls is reported. The reaction of [Au(B2P2)]- with formaldehyde or paraformaldehyde results in a head-to-tail dimerization of two formaldehyde units across the boron centers. In contrast, the reaction of [(B2P2)Au]- with two equivalents of benzaldehyde yields the pinacol coupling product via C-C bond formation. Careful stoichiometric addition of one equivalent of benzaldehyde to [Au(B2P2)]- enabled the isolation of an adduct corresponding to the formal [4+2] cycloaddition of the C[double bond, length as m-dash]O bond of benzaldehyde across the boron centers. This adduct reacts with a second equivalent of benzaldehyde to produce the pinacol coupling product. Finally, the reaction of [Au(B2P2)]- with acetone results in a formal reductive deoxygenation with discrete hydroxo and 2-propenyl units bound to the boron centers. This reaction is proposed to proceed via an analogous [4+2] cycloadduct, highlighting the unique small molecule activation chemistry available to this platform.

CO2 reduction with protons and electrons at a boron-based reaction center.

Borohydrides are widely used reducing agents in chemical synthesis and have emerging energy applications as hydrogen storage materials and reagents for the reduction of CO2. Unfortunately, the high energy cost associated with the multistep preparation of borohydrides starting from alkali metals precludes large scale implementation of these latter uses. One potential solution to this issue is the direct synthesis of borohydrides from the protonation of reduced boron compounds. We herein report reactions of the redox series [Au(B2P2)] n (n = +1, 0, -1) (B2P2, 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) and their conversion into corresponding mono- and diborohydride complexes. Crucially, the monoborohydride can be accessed via protonation of [Au(B2P2)]-, a masked borane dianion equivalent accessible at relatively mild potentials (-2.05 V vs. Fc/Fc+). This species reduces CO2 to produce the corresponding formate complex. Cleavage of the formate complex can be achieved by reduction (ca. -1.7 V vs. Fc/Fc+) or by the addition of electrophiles including H+. Additionally, direct reaction of [Au(B2P2)]- with CO2 results in reductive disproportion to release CO and generate a carbonate complex. Together, these reactions constitute a synthetic cycle for CO2 reduction at a boron-based reaction center that proceeds through a B-H unit generated via protonation of a reduced borane with weak organic acids.

Copper and Silver Complexes of a Redox-Active Diphosphine-Diboraanthracene Ligand.

Redox-active ligands and Z-type acceptor ligands have emerged as promising strategies for promoting multielectron redox chemistry at transition-metal centers. Herein, we report the synthesis and characterization of copper and silver complexes of a diphosphine ligand featuring a diboraanthracene core (B2P2, 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) that is capable of serving as both a redox reservoir and a Z-type ligand. Metalation of B2P2 with CuX (X = Cl, Br, I) results in the formation of bimetallic complexes of the formula (B2P2)Cu2X2 of two different structure types, depending on the halide. The Cu(I) cation [Cu(B2P2)]+ can be accessed by direct metalation of B2P2 with [Cu(CH3CN)4][PF6] or by halide abstraction with Na[BArF4] (ArF = 3,5-bis(trifluoromethyl)phenyl) with concomitant expulsion of CuX from the bimetallic Cu2X2 complexes. Metalation of B2P2 with AgCl results in the formation of the zwitterion Ag(B2P2)Cl featuring a diphosphine Ag cation tethered to a chloroborate anion. Metathesis of chloride for the noncoordinating [BArF4]- affords the cation [Ag(B2P2)]+. The cations [Cu(B2P2)]+ and [Ag(B2P2)]+ exhibit quasireversible reduction events at ∼ -1.6 V versus the ferrocene/ferrocenium redox couple, and the thermally sensitive radicals that result from their reduction, Cu(B2P2) and Ag(B2P2), were characterized by EPR spectroscopy and, in the case of the latter, single-crystal X-ray diffraction. Electronic structure calculations suggest these neutral radicals are best described as zwitterions with reduction centered at the diboraanthracene core.

PbS/CdS Core-Shell Quantum Dots Suppress Charge Transfer and Enhance Triplet Transfer.

A sub-monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor, 5-CT. While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the 5-CT triplet (with rates of 5.91±0.60 ns-1 and 1.03±0.09 ns-1 respectively), ultrafast hole transfer occurs only from PbS QDs to 5-CT. Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the 5-CT triplet and thus enhances TET from 5-CT to the rubrene emitter, further bolstering the upconverison QY.

Monoglobus pectinilyticus gen. nov., sp. nov., a pectinolytic bacterium isolated from human faeces.

A novel anaerobic pectinolytic bacterium (strain 14T) was isolated from human faeces. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain 14T belonged to the family Ruminococcaceae, but was located separately from known clostridial clusters within the taxon. The closest cultured relative of strain 14T was Acetivibrio cellulolyticus (89.7 % sequence similarity). Strain 14T shared ~99 % sequence similarity with cloned 16S rRNA gene sequences from uncultured bacteria derived from the human gut. Cells were Gram-stain-positive, non-motile cocci approximately 0.6 µm in diameter. Strain 14T fermented pectins from citrus peel, apple, and kiwifruit as well as carbohydrates that are constituents of pectins and hemicellulose, such as galacturonic acid, xylose, and arabinose. TEM images of strain 14T, cultured in association with plant tissues, suggested extracellular fibrolytic activity associated with the bacterial cells, forming zones of degradation in the pectin-rich regions of middle lamella. Phylogenetic and phenotypic analysis supported the differentiation of strain 14T as a novel genus in the family Ruminococcaceae. The name Monoglobus pectinilyticus gen. nov., sp. nov. is proposed; the type strain is 14T (JCM 31914T=DSM 104782T).

N-Heterocyclic Carbene-Stabilized Boranthrene as a Metal-Free Platform for the Activation of Small Molecules.

The multielectron reduction of small molecules (e.g., CO2) is a key aspect of fuel synthesis from renewable electricity. Transition metals have been researched extensively in this role due to their intrinsic redox properties and reactivity, but more recently, strategies that forego transition metal ions for p-block elements have emerged. In this vein, we report an analogue of boranthrene (9,10-diboraanthracene) stabilized by N-heterocyclic carbenes and its one- and two-electron oxidized congeners. This platform exhibits reversible, two-electron redox chemistry at mild potentials and reacts with O2, CO2, and ethylene via formal [4+2] cycloaddition to the central diborabutadiene core. In an area traditionally dominated by transition metals, these results outline an approach for the redox activation of small molecules at mild potentials based on conjugated, light element scaffolds.

A Molecular Boroauride: A Donor-Acceptor Complex of Anionic Gold.

Gold is unique among the transition metals in that it is stable as an isolated anion (auride). Despite this fact, the coordination chemistry of anionic gold is virtually nonexistent, and this unique oxidation state is not readily exploited in conventional solution chemistry owing to its high reactivity. Through the use of a new molecular scaffold based on diboraanthracene (B2 P2 , 1), we have overcome these issues by avoiding the intermediacy of zerovalent gold and stabilizing the highly reduced gold anion through acceptor interactions. We have thus synthesized a molecular boroauride [(B2 P2 )Au]- ([2]- ) and showed its reversible conversion between Au-I and AuI states. Through a combination of spectroscopic and computational studies, we show the neutral state to be a AuI complex with a ligand radical anion. Bonding analyses (NBO and QTAIM) and the isolobal relationship between gold and hydrogen provide support for the description of [2]- as a boroauride complex.