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 N2 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(µ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Terminal Hydride Complex of High-Spin Mn.

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N2 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 Fe2+/3+ configurations, synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn2+ and Fe3+, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.

Bis[tris-(diiso-butyl-dithio-carbamato)-µ3-sulfido-tri-µ2-di-sulfido-trimolybdenum(IV)] sulfide tetra-hydro-furan monosolvate.

The title compound, [Mo3(C9H18NS2)3(S2)3S]2S, crystallizes on a general position in the monoclinic space group P21/n (No. 14). The cationic [Mo3S7(S2CNiBu2)3]+ 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 Mo3 plane opposite the µ3 2- ligand. The two Mo3 planes are inclined at an angle of 40.637 (15)°, which gives the assembly an open clamshell-like appearance. One µ6-S2-⋯S2 2- contact, at 2.4849 (14) Å, is appreciably shorter than the remaining five, which are in the range 2.7252 (13)-2.8077 (14) Å.

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 Ph2(CH3)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. Unusually, the imido ligand in this molecule tilts markedly toward the Calkyl donor. DFT calculations suggest this structural feature to be largely a result of strong Co-C covalency, underscoring the importance of M-C bonding in determining the (electronic) structure of metal centers supported by this class of ligand.

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

Members of the radical S-adenosyl-l-methionine (SAM) enzyme superfamily initiate a broad spectrum of radical transformations through reductive cleavage of SAM by a [4Fe-4S]1+ 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 S = 1/2 organometallic intermediate Ω. An alternative mode of reductive SAM cleavage, especially seen photochemically, instead liberates CH3•, which forms the analogous S = 1/2 organometallic intermediate with an Fe-CH3 bond, ΩM. The presence of a covalent Fe-C bond in both structures was established by the ENDOR observation of 13C and 1H hyperfine couplings to the alkyl groups that show isotropic components indicative of Fe-C bond covalency. The synthetic [Fe4S4]3+-CH3 cluster, M-CH3, is a crystallographically characterized analogue to ΩM that exhibits the same [Fe4S4]3+ cluster state as Ω and ΩM, and thus an analysis of its spectroscopic properties─and comparison with those of Ω and ΩM─can be grounded in its crystal structure. We report cryogenic (2 K) EPR and 13C/1/2H ENDOR measurements on isotopically labeled M-CH3. At low temperatures, the complex exhibits EPR spectra from two distinct conformers/subpopulations. ENDOR shows that at 2 K, one contains a static methyl, but in the other, the methyl undergoes rapid tunneling/hopping rotation about the Fe-CH3 bond. This generates an averaged hyperfine coupling tensor whose analysis requires an extended treatment of rotational averaging. The methyl group 13C/1/2H hyperfine couplings are compared with the corresponding values for Ω and ΩM.

Four-Coordinate Fe N2 and Imido Complexes Supported by a Hemilabile NNC Heteroscorpionate Ligand.

Inspired by mechanistic proposals for N2 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)-N2, 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-N2 and high-valent Fe-imido complexes mediated by pyrazole hemilability. This presents a useful strategy for substrate reduction at such low-coordinate centers and suggests a mechanism by which FeMoco might accommodate the binding of both π-acidic and π-basic nitrogenous substrates.

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 1.5 % overall yield. The latter reaction also gave the monoaddition product 4-(phenylethynyl)-2,2',3,3',4',5,5',6,6'-nonaphenylbiphenyl (13) in 4 % overall yield. The X-ray structures of compounds 2 and 13 show them to possess core aromatic rings distorted into shallow boat conformations. Density functional calculations indicate that these unusual structures are not the lowest energy conformations in the gas phase and may be the result of packing forces in the crystal. In addition, while uncorrected DFT calculations indicate that the strain energy in compound 2 is approximately 50 kcal/mol, dispersion-corrected DFT calculations suggest that it is essentially unstrained, due to compensating, favorable, intramolecular interactions of its many phenyl rings. An attempted synthesis of tetradecaphenyl-o-terphenyl (4) by reaction of diphenylhexatriyne (14) with three equivalents of tetracyclone at 350 °C gave only the diadduct 2-(phenylethynyl)-2',3,3',4,4',5,5',6,6'-nonaphenylbiphenyl (15) in 17 % yield. Even higher temperatures failed to produce 4 and lowered the yield of 15, perhaps due to rapid decomposition of the starting materials. Ullmann coupling of 3,4,5,6-tetraphenyl-1,2-diiodobenzene (16) and compound 9 also failed to give compound 4.

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 N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-( t BuNO)py]-(interchangeable hereafter with [( t BuNO)py]-), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[( t BuNO)py]4 (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, 1H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [( t BuNO)py]- ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E 1/2 = -2.26 V vs. Fc+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of UIV[( t BuNO)py]4 that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [( t BuNO)py]- ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [AmIII(µ2-( t BuNO)py)(( t BuNO)py)2]2 (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-( t BuNO)py]-, in good agreement with experimental observations.

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

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N2 to NH3. 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 N2. Here we report that embedding an [MoFe3S4] cluster in a protective ligand environment enables N2 binding at Fe. The bridging [MoFe3S4]2(µ-η1:η1-N2) complex thus prepared features a substantially weakened N-N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe3S4] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N2 ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N2 activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N2. These findings indicate that covalent interactions within the cluster play a critical role in N2 binding and activation.

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.

An [Fe4S4]3+-Alkyl Cluster Stabilized by an Expanded Scorpionate Ligand.

Alkyl-ligated iron-sulfur clusters in the [Fe4S4]3+ 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 [Fe4S4]3+-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. Like the proposed enzymatic intermediates, this synthetic [Fe4S4]3+-alkyl cluster adopts an S = 1/2 ground state with giso > 2. Mössbauer spectroscopic studies reveal that the alkylated Fe has an unusually low isomer shift, which reflects the highly covalent Fe-C bond and the localization of Fe3+ at the alkylated site in the solid state. Paramagnetic 1H NMR studies establish that this valence localization persists in solution at physiologically relevant temperatures, an effect that has not been observed for [Fe4S4]3+ clusters outside of a protein. These findings establish the unusual electronic-structure effects imparted by the strong-field alkyl ligand and lay the foundation for understanding the electronic structures of [Fe4S4]3+-alkyl intermediates in biology.

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.

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. More generally, these results provide unique experimental evidence for the larger covalent character of 4f05d0 Ce(IV) multiple bonds compared to its 5f06d0 Th(IV) actinide congener.

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.

Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 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(NImR)3 that can bind [Fe4S4]2+ or [Fe6S6]3+ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NImR)3 ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NImMe)3 (tris(1,3,5-(3-( N, N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph4P]2[Fe4S4Cl4] and 3 equiv of NaBPh4 furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph4P]2[Fe4S4Cl4] provides (L(NImMe)3)Fe6S6Cl6 in high yield. Formation of an [Fe6S6]3+ cluster using L(NImTol)3 (tris(1,3,5-(3-( N, N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe4S4]2+ cluster [(L(NImTol)3)(Fe4S4Cl)][BPh4] is cleanly generated when 1 equiv of [Ph4P]2[Fe4S4Cl4] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe6S6]3+ clusters but not [Fe4S4]2+ clusters. Thus, the structure and nuclearity of L(NImR)3-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.

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. We report herein the preparation and characterization of Th and U complexes of the pyridyl-hydroxylamine ligand, N- tert-butyl- N-(pyridin-2-yl)hydroxylamine (pyNO-). Electrochemical studies performed on the homoleptic complexes [M(pyNO)4] (M = Th, U) revealed significant stabilization of the U complex upon one-electron oxidation. The salt [U(pyNO)4]+ was isolated by chemical oxidation of [U(pyNO)4]; spectroscopic and computational data support assignment as a UV cation.

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{(R2N)C(N iPr)2}3, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe3)2, G = (Me3Si)2NC(N iPr)2], and guanidinate-aryloxide (GOAr, OAr = 2,6-di- tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1-8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure-luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.

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. Density functional theory calculations implicate a hydride transfer mechanism for these reactions.

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.