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  1. Abstract

    Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth’s most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.

     
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  2. Abstract

    Two-dimensional materials have unusual properties and promise applications in nanoelectronics, spintronics, photonics, (electro)catalysis, separations, and elsewhere. Most are inorganic and their properties are difficult to tune. Here we report the preparation of Zn porphene, a member of the previously only hypothetical organic metalloporphene family. Similar to graphene, these also are fully conjugated two-dimensional polymers, but are composed of fused metalloporphyrin rings. Zn porphene is synthesized on water surface by two-dimensional oxidative polymerization of a Langmuir layer of Zn porphyrin with K2IrCl6, reminiscent of known one-dimensional polymerization of pyrroles. It is transferable to other substrates and bridges μm-sized pits. Contrary to previous theoretical predictions of metallic conductivity, it is a p-type semiconductor due to a predicted Peierls distortion of its unit cell from square to rectangular, analogous to the appearance of bond-length alternation in antiaromatic molecules. The observed reversible insertion of various metal ions, possibly carrying a fifth or sixth ligand, promises tunability and even patterning of circuits on an atomic canvas without removing any π centers from conjugation.

     
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    Free, publicly-accessible full text available December 1, 2024
  3. Abstract

    Understanding the electronic structures of high‐valent metal complexes aids the advancement of metal‐catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF3)4](1), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of1by X‐ray spectroscopies have led previous authors to contradictory conclusions, motivating the re‐examination of its X‐ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS). From analysis of DAFS measurements for a series of seven mononuclear Cu complexes including1, here it is shown that there is a systematic trifluoromethyl effect on X‐ray absorption that blue shifts the resonant Cu K‐edge energy by 2–3 eV per CF3, completely accounting for observed changes in DAFS profiles between formally Cu(III) complexes like1and formally Cu(I) complexes like (Ph3P)3CuCF3(3). Thus, in agreement with the inverted ligand field model, the data presented herein imply that1is best described as containing a Cu(I) ion with dncount approaching 10.

     
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  4. Abstract

    Metalation of the polynucleating ligandF,tbsLH6(1,3,5‐C6H9(NC6H3−4‐F−2‐NSiMe2tBu)3) with two equivalents of Zn(N(SiMe3)2)2affords the dinuclear product (F,tbsLH2)Zn2(1), which can be further deprotonated to yield (F,tbsL)Zn2Li2(OEt2)4(2). Transmetalation of2with NiCl2(py)2yields the heterometallic, trinuclear cluster (F,tbsL)Zn2Ni(py) (3). Reduction of3with KC8affords [KC222][(F,tbsL)Zn2Ni] (4) which features a monovalent Ni centre. Addition of 1‐adamantyl azide to4generates the bridging μ3‐nitrenoid adduct [K(THF)3][(F,tbsL)Zn2Ni(μ3‐NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S=). Cyclic voltammetry of5reveals two fully reversible redox events. The dianionic nitrenoid [K2(THF)9][(F,tbsL)Zn2Ni(μ3‐NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra‐ and intermolecular H‐atom abstraction processes. Ni K‐edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2Ni] nitrenoid complexes. However, DFT analysis suggests Ni‐borne oxidation for5.

     
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  5. Abstract

    We communicate a feasibility study for high‐resolution structural characterization of biomacromolecules in aqueous solution from X‐ray scattering experiments measured over a range of scattering vectors (q) that is approximately two orders of magnitude wider than used previously for such systems. Scattering data with such an extendedq‐range enables the recovery of the underlying real‐space atomic pair distribution function, which facilitates structure determination. We demonstrate the potential of this method for biomacromolecules using several types of cyclodextrins (CD) as model systems. We successfully identified deviations of the tilting angles for the glycosidic units in CDs in aqueous solutions relative to their values in the crystalline forms of these molecules. Such level of structural detail is inaccessible from standard small angle scattering measurements. Our results call for further exploration of ultra‐wide‐angle X‐ray scattering measurements for biomacromolecules.

     
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    Free, publicly-accessible full text available June 2, 2024
  6. Abstract

    Organofunctionalized tetranuclear clusters [(MIICl)2(VIVO)2{((HOCH2CH2)(H)N(CH2CH2O))(HN(CH2CH2O)2)}2] (1, M=Co,2: M=Zn) containing an unprecedented oxometallacyclic {M2V2Cl2N4O8} (M=Co, Zn) framework have been prepared by solvothermal reactions. The new oxo‐alkoxide compounds were fully characterized by spectroscopic methods, magnetic susceptibility measurement, DFT and ab initio computational methods, and complete single‐crystal X‐ray diffraction structure analysis. The isostructural clusters are formed of edge‐sharing octahedral {VO5N} and trigonal bipyramidal {MO3NCl} units. Diethanolamine ligates the bimetallic lacunary double cubane core of1and2in an unusual two‐mode fashion, unobserved previously. In the crystalline state, the clusters of1and2are joined by hydrogen bonds to form a three‐dimensional network structure. Magnetic susceptibility data indicate weakly antiferromagnetic interactions between the vanadium centers [Jiso(VIV−VIV)=−5.4(1); −3.9(2) cm−1], and inequivalent antiferromagnetic interactions between the cobalt and vanadium centers [Jiso(VIV−CoII)=−12.6 and −7.5 cm−1] contained in1.

     
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    Free, publicly-accessible full text available October 23, 2024
  7. Abstract

    One striking feature of enzyme is its controllable ability to trap substrates via synergistic or cooperative binding in the enzymatic pocket, which renders the shape‐selectivity of product by the confined spatial environment. The success of shape‐selective catalysis relies on the ability of enzyme to tune the thermodynamics and kinetics for chemical reactions. In emulation of enzyme's ability, we showcase herein a targeting strategy with the substrate being anchored on the internal pore wall of metal‐organic frameworks (MOFs), taking full advantage of the sterically kinetic control to achieve shape‐selectivity for the reactions. For this purpose, a series of binding site‐accessible metal metalloporphyrin‐frameworks (MMPFs) have been investigated to shed light on the nature of enzyme‐mimic catalysis. They exhibit a different density of binding sites that are well arranged into the nanospace with corresponding distances of opposite binding sites. Such a structural specificity results in a facile switch in selectivity from an exclusive formation of the thermodynamically stable product to the kinetic product. Thus, the proposed targeting strategy, based on the combination of porous materials and binding events, paves a new way to develop highly efficient heterogeneous catalysts for shifting selectivity.

     
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  8. Abstract

    We report a facile synthesis of diindeno‐fused dibenzo[a,h]anthracene derivatives (DIDBA‐2Cl,DIDBA‐2Ph, andDIDBA‐2H)with different degrees of non‐planarity using three substituents (chloro, phenyl, and hydrogen) of various sizes. The planarization of their cores, as evidenced by the decreased end‐to‐end torsional angles, was confirmed by X‐ray crystallography. Their enhanced energy gaps with twisting were investigated by a combination of spectroscopic and electrochemical methods with density functional theory, which showed a transition from singlet open‐shell to closed‐shell configuration. Moreover, their doubly reduced states,DIDBA‐2Ph2−andDIDBA‐2H2−, were achieved by chemical reduction. The structures of dianions were identified by X‐ray crystallographic analysis, which elucidated that the electron charging further distorted the backbones. The electronic structure of the dianions was demonstrated by experimental and theoretical approaches, suggesting decreased energy gaps with larger non‐planarity, different from the neutral species.

     
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  9. Abstract

    Homometallic copper complexes with alkenylidene ligands are discussed as intermediates in catalysis but the isolation of such complexes has remained elusive. Herein, we report the structural characterization of copper complexes with bridging and terminal alkenylidene ligands. The compounds were obtained by irradiation of CuIcomplexes with N‐heterocyclic diazoolefin ligands. The complex with a terminal alkenylidene ligand required isolation in a crystalline matrix, and its structural characterization was enabled byin crystallophotolysis at low temperature.

     
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  10. Abstract

    The recognition of boron compounds is well developed as boronic acids but untapped as organotrifluoroborate anions (R−BF3). We are exploring the development of these and other designer anions as anion‐recognition motifs by considering them as substituted versions of the parent inorganic ion. To this end, we demonstrate strong and reliable binding of organic trifluoroborates, R−BF3, by cyanostar macrocycles that are size‐complementary to the inorganic BF4progenitors. We find that recognition is modulated by the substituent's sterics and that the affinities are retained using the common K+salts of R−BF3anions.

     
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