Preparation of Large Perphenylbiaryls: Can Intermolecular Coupling Compete with Intramolecular Cyclization of Precursors?

The highly substituted naphthalenes 1,2,3,4,5,6,7-heptaphenylnaphthalene (13), 2,3,4,5,6,7,8-heptaphenyl-1-naphthol (12), 1-bromo-2,3,4,5,6,7,8-heptaphenylnaphthalene (4), and 1-(phenylethynyl)-2,3,4,5,6,7,8-heptaphenylnaphthalene (5) were prepared by a variety of methods, and all but 5 were crystallographically characterized. The attempted Ullmann coupling of 4 to give tetradecaphenyl-1,1'-binaphthyl (3), at both 270 °C and 350 °C, yielded instead 1,2,3,4,5,6-hexaphenylfluoranthene (17) via an intramolecular cyclization reaction. When the alkyne 5 was heated with tetracyclone (6) at 350 °C, 1-(pentaphenylphenyl)-2,3,4,5,6,7,8-heptaphenylnaphthalene (7) was formed in 3 % yield.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform.

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ-H)] complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin ( = 2) Fe hydride.

Terminal Hydride Complex of High-Spin Mn.

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe configurations, synthetically accessible hydride complexes featuring d or d electron counts are almost exclusively low-spin.

Bis[tris-(diiso-butyl-dithio-carbamato)-μ-sulfido-tri-μ-di-sulfido-trimolybdenum(IV)] sulfide tetra-hydro-furan monosolvate.

The title compound, [Mo(CHNS)(S)S]S, crystallizes on a general position in the monoclinic space group 2/ (No. 14). The cationic [MoS(SCNBu)] fragments are joined by a mono-sulfide dianion that forms close S⋯S contacts to each of the di-sulfide ligands on the side of the Mo plane opposite the μ ligand.

Four-Coordinate Co(III) Imide with an Unusually Tilted Terminal Imido Ligand.

We report herein the synthesis and characterization of a terminal Co(III) imido complex supported by an intermediate field N,N,C heteroscorpionate. This chemistry is enabled through the development of an additional member of this ligand type featuring Ph(CH)C- substituents, one of which weakly binds and stabilizes Co in the corresponding Co(I) precursor. The Co(III) imide is low-spin with no evidence for thermal population of open-shell excited states.

Characterization by ENDOR Spectroscopy of the Iron-Alkyl Bond in a Synthetic Counterpart of Organometallic Intermediates in Radical SAM Enzymes.

Members of the radical adenosyl-l-methionine (SAM) enzyme superfamily initiate a broad spectrum of radical transformations through reductive cleavage of SAM by a [4Fe-4S] cluster it coordinates to generate the reactive 5'-deoxyadenosyl radical (5'-dAdo). However, 5'-dAdo is not directly liberated for reaction and instead binds to the unique Fe of the cluster to create the catalytically competent = 1/2 organometallic intermediate Ω. An alternative mode of reductive SAM cleavage, especially seen photochemically, instead liberates CH, which forms the analogous = 1/2 organometallic intermediate with an Fe-CH bond, Ω.

Four-Coordinate Fe N and Imido Complexes Supported by a Hemilabile NNC Heteroscorpionate Ligand.

Inspired by mechanistic proposals for N reduction at the nitrogenase FeMo cofactor, we report herein a new, strongly σ-donating heteroscorpionate ligand featuring two weak-field pyrazoles and an alkyl donor. This ligand supports four-coordinate Fe(I)-N, Fe(II)-Cl, and Fe(III)-imido complexes, which we have characterized using a variety of spectroscopic and computational methods. Structural and quantum mechanical analysis reveal the nature of the Fe-C bonds to be essentially invariant between the complexes, with conversion between the (formally) low-valent Fe-N and high-valent Fe-imido complexes mediated by pyrazole hemilability.

The Pursuit of Perphenylterphenyls.

Tetradecaphenyl-p-terphenyl (2) was synthesized from 2,3,5,6-tetraphenyl-1,4-diiodobenzene (11) by two methods. Ullmann coupling of 11 with pentaphenyliodobenzene (9) gave compound 2 in 1.7 % yield, and Sonogashira coupling of 11 with phenylacetylene, followed by a double Diels-Alder reaction of the product diyne 12 with tetracyclone (6), gave 2 in 1.

Complexation and redox chemistry of neptunium, plutonium and americium with a hydroxylaminato ligand.

There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with -butyl--(pyridin-2-yl)hydroxylaminato, [2-( BuNO)py](interchangeable hereafter with [( BuNO)py]), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state.

Dinitrogen binding and activation at a molybdenum-iron-sulfur cluster.

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N to NH. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe-S cluster has been shown to form a well-defined coordination complex with N. Here we report that embedding an [MoFeS] cluster in a protective ligand environment enables N binding at Fe.

An [FeS]-Alkyl Cluster Stabilized by an Expanded Scorpionate Ligand.

Alkyl-ligated iron-sulfur clusters in the [FeS] charge state have been proposed as short-lived intermediates in a number of enzymatic reactions. To better understand the properties of these intermediates, we have prepared and characterized the first synthetic [FeS]-alkyl cluster. Isolation of this highly reactive species was made possible by the development of an expanded scorpionate ligand suited to the encapsulation of cuboidal clusters.

Assembly and Redox-Rich Hydride Chemistry of an Asymmetric MoS 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 MoS core. This molecule displays rich redox chemistry with electrochemical couples at = -0.

CO 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 CO. 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.

Multiple Bonding in Lanthanides and Actinides: Direct Comparison of Covalency in Thorium(IV)- and Cerium(IV)-Imido Complexes.

A series of thorium(IV)-imido complexes was synthesized and characterized. Extensive experimental and computational comparisons with the isostructural cerium(IV)-imido complexes revealed a notably more covalent bonding arrangement for the Ce═N bond compared with the more ionic Th═N bond. The thorium-imido moieties were observed to be 3 orders of magnitude more basic than their cerium congeners.

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 (BP, 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 BP with CuX (X = Cl, Br, I) results in the formation of bimetallic complexes of the formula (BP)CuX of two different structure types, depending on the halide.

Selective Synthesis of Site-Differentiated FeS and FeS Clusters.

Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm) that can bind [FeS] or [FeS] clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm) ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described.

Coordination Chemistry of a Strongly-Donating Hydroxylamine with Early Actinides: An Investigation of Redox Properties and Electronic Structure.

Separations of f-block elements are a critical aspect of nuclear waste processing. Redox-based separations offer promise, but challenges remain in stabilizing and differentiating actinides in high oxidation states. The investigation of new ligand types that provide thermodynamic stabilization to high-valent actinides is essential for expanding their fundamental chemistry and to elaborate new separation techniques and storage methods.

Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores.

Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1-8, Ce{(RN)C(N Pr)}, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe), G = (MeSi)NC(N Pr)], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties.

Functional Synthetic Model for the Lanthanide-Dependent Quinoid Alcohol Dehydrogenase Active Site.

The oxidation of methanol by dehydrogenase enzymes is an essential part of the bacterial methane metabolism cycle. The recent discovery of a lanthanide (Ln) cation in the active site of the XoxF dehydrogenase represents the only example of a rare-earth element in a physiological role. Herein, we report the first synthetic, functional model of Ln-dependent dehydrogenase and its stoichiometric and catalytic dehydrogenation of a benzyl alcohol.

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

The multielectron reduction of small molecules (e.g., CO) is a key aspect of fuel synthesis from renewable electricity.

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 (B P , 1), we have overcome these issues by avoiding the intermediacy of zerovalent gold and stabilizing the highly reduced gold anion through acceptor interactions.

A Terminal N2 Complex of High-Spin Iron(I) in a Weak, Trigonal Ligand Field.

The role of Fe in biological and industrial N2 fixation has inspired the intense study of small molecule analogues of Fe-(NxHy) intermediates of potential relevance to these processes. Although a number of low-coordinate Fe-(N2) featuring varying degrees of fidelity to the nitrogenase active site are now known, these complexes frequently feature strongly donating ligands that either enforce low- or intermediate-spin states or result in linear Fe-(N2)-Fe bridging motifs. Given that the nitrogenase active site uses weak-field sulfide ligands to stabilize its reactive Fe center(s), N2 binding to high-spin Fe is of great interest.

Bio-inspired transition metal-organic hydride conjugates for catalysis of transfer hydrogenation: experiment and theory.

Taking inspiration from yeast alcohol dehydrogenase (yADH), a benzimidazolium (BI(+) ) organic hydride-acceptor domain has been coupled with a 1,10-phenanthroline (phen) metal-binding domain to afford a novel multifunctional ligand (L(BI+) ) with hydride-carrier capacity (L(BI+) +H(-) ⇌L(BI) H). Complexes of the type [Cp*M(L(BI) )Cl][PF6 ]2 (M=Rh, Ir) have been made and fully characterised by cyclic voltammetry, UV/Vis spectroelectrochemistry, and, for the Ir(III) congener, X-ray crystallography. [Cp*Rh(L(BI) )Cl][PF6 ]2 catalyses the transfer hydrogenation of imines by formate ion in very goods yield under conditions where the corresponding [Cp*Ir(L(BI) )Cl][PF6 ] and [Cp*M(phen)Cl][PF6 ] (M=Rh, Ir) complexes are almost inert as catalysts.

An easy one-pot synthesis of diverse 2,5-di(2-pyridyl)pyrroles: a versatile entry point to metal complexes of functionalised, meridial and tridentate 2,5-di(2-pyridyl)pyrrolato ligands.

A wide variety of 2,5-di(2-pyridyl)pyrroles (dppHs) substituted at the C3 and C4 positions of the pyrrole core were obtained by direct condensation of a 2-pyridylcarboxaldehyde (2 equiv), an α-methylene ketone with at least one electron-withdrawing substituent and ammonium acetate. A novel 2,5-di(1,10-phenanthrolin-2-yl)pyrrole was also characterised. The dppHs provide a direct, quick entry to dipyridylpyrrolato (dpp(-) )-metal complexes.