Brandon Q. Mercado

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.

Sc2(μ2-O) Trapped in a Fullerene Cage: The Isolation and Structural Characterization of Sc2(μ2-O)@Cs(6)-C82 and the Relevance of the Thermal and Entropic Effects in Fullerene Isomer Selection

The new endohedral fullerene, Sc(2)(mu(2)-O)@C(s)(6)-C(82), has been isolated from the carbon soot obtained by electric arc generation of fullerenes utilizing graphite rods doped with 90% Sc(2)O(3) and 10% Cu (w/w). Sc(2)(mu(2)-O)@C(s)(6)-C(82) has been characterized by single crystal X-ray diffraction, mass spectrometry, and UV/vis spectroscopy. Computational studies have shown that, among the nine isomers that follow the isolated pentagon rule (IPR) for C(82), cage 6 with C(s) symmetry is the most favorable to encapsulate the cluster at T > 1200 K. Sc(2)(mu(2)-O)@C(s)(6)-C(82) is the first example in which the relevance of the thermal and entropic contributions to the stability of the fullerene isomer has been clearly confirmed through the characterization of the X-ray crystal structure.

Is the Isolated Pentagon Rule Merely a Suggestion for Endohedral Fullerenes? The Structure of a Second Egg-Shaped Endohedral Fullerene—Gd3N@Cs(39663)-C82

The structure of Gd3N@Cs(39663)-C82 has been determined through single crystal X-ray diffraction on Gd3N@Cs(39663)-C82.NiII(OEP).2(C6H6) The carbon cage has a distinct egg shape because of the presence of a single pair of fused pentagons at one apex of the molecule. Although 9 IPR structures are available to the C82 cage, one of the 39709 isomeric structures that do not conform to the IPR was found in Gd3N@Cs(39663)-C82. The egg-shaped structure of Gd3N@Cs(39663)-C82 is similar to that observed previously for M3N@Cs(51365)-C84 (M = Gd, Tm, Tb). As noted for other non-IPR endohedral fullerenes, one Gd atom in Gd3N@Cs(39663)-C82 is nestled within the fold of the fused pentagons.

The Shape of the Sc2(μ2-S) Unit Trapped in C82: Crystallographic, Computational and Electrochemical Studies of the Isomers, Sc2(μ2-S)@Cs(6)-C82 and Sc2(μ2-S)@C3v(8)-C82

Single-crystal X-ray diffraction studies of Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82)·Ni(II)(OEP)·2C(6)H(6) reveal that both contain fully ordered fullerene cages. The crystallographic data for Sc(2)(μ(2)-S)@C(s)(6)-C(82)·Ni(II)(OEP)·2C(6)H(6) show two remarkable features: the presence of two slightly different cage sites and a fully ordered molecule Sc(2)(μ(2)-S)@C(s)(6)-C(82) in one of these sites. The Sc-S-Sc angles in Sc(2)(μ(2)-S)@C(s)(6)-C(82) (113.84(3)°) and Sc(2)(μ(2)-S)@C(3v)(8)-C(82) differ (97.34(13)°). This is the first case where the nature and structure of the fullerene cage isomer exerts a demonstrable effect on the geometry of the cluster contained within. Computational studies have shown that, among the nine isomers that follow the isolated pentagon rule for C(82), the cage stability changes markedly between 0 and 250 K, but the C(s)(6)-C(82) cage is preferred at temperatures ≥250 °C when using the energies obtained with the free encapsulated model (FEM). However, the C(3v)(8)-C(82) cage is preferred at temperatures ≥250 °C using the energies obtained by rigid rotor-harmonic oscillator (RRHO) approximation. These results corroborate the fact that both cages are observed and likely to trap the Sc(2)(μ(2)-S) cluster, whereas earlier FEM and RRHO calculations predicted only the C(s)(6)-C(82) cage is likely to trap the Sc(2)(μ(2)-O) cluster. We also compare the recently published electrochemistry of the sulfide-containing Sc(2)(μ(2)-S)@C(s)(6)-C(82) to that of corresponding oxide-containing Sc(2)(μ(2)-O)@C(s)(6)-C(82).

Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst

Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. We report a Co catalyst for anti-Markovnikov alkene hydrosilylation that can be used without added solvent at low temperatures with low loadings, and can be generated in situ from an air-stable precursor that is simple to synthesize from low-cost, commercially available materials. In addition, a mixture of Co catalysts performs a tandem catalytic alkene isomerization/hydrosilylation reaction that converts multiple isomers of hexene to the same terminal product. This regioconvergent reaction uses isomerization as a benefit rather than a hindrance.

Isolation of a Small Carbon Nanotube: The Surprising Appearance of D5h(1)‐C90

have become well-known; however, thecarbon soot from arc generators contains small amounts(generally less than 1%) of higher fullerenes. The isolation ofthese higher fullerenes in isomerically pure form is challeng-ing, especially since the number of isomers that follow theisolated-pentagon rule (IPR) increases as the size of thefullerene cage expands.

Binding of dinitrogen to an iron–sulfur–carbon site

Nitrogenases are the enzymes by which certain microorganisms convert atmospheric dinitrogen (N2) to ammonia, thereby providing essential nitrogen atoms for higher organisms. The most common nitrogenases reduce atmospheric N2 at the FeMo cofactor, a sulfur-rich iron–molybdenum cluster (FeMoco). The central iron sites that are coordinated to sulfur and carbon atoms in FeMoco have been proposed to be the substrate binding sites, on the basis of kinetic and spectroscopic studies. In the resting state, the central iron sites each have bonds to three sulfur atoms and one carbon atom. Addition of electrons to the resting state causes the FeMoco to react with N2, but the geometry and bonding environment of N2-bound species remain unknown. Here we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe–S bond and binds N2. The product is the first synthetic Fe–N2 complex in which iron has bonds to sulfur and carbon atoms, providing a model for N2 coordination in the FeMoco. Our results demonstrate that breaking an Fe–S bond is a chemically reasonable route to N2 binding in the FeMoco, and show structural and spectroscopic details for weakened N2 on a sulfur-rich iron site.

Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution.

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) ions, each with two water molecules bound trans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=2.2±1.4 cm(3) mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co(II) species (such as the [Co(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.

[2 + 2] Cycloaddition Reaction to Sc3N@Ih-C80. The Formation of Very Stable [5,6]- and [6,6]-Adducts

The [2 + 2] cycloaddition reaction of Sc(3)N@I(h)-C(80) with benzyne was successfully conducted for the first time. The reaction affords both the [5,6]- and [6,6]-monoadducts with a four-membered ring attached to the cage surface on 5,6- and 6,6-ring fusions, respectively. The compounds were characterized by MALDI-TOF, NMR, UV-vis-NIR spectroscopy and single-crystal X-ray structure determination. The electrochemical behavior of both monoadducts was investigated. The [5,6]-regioisomer displays reversible cathodic behavior similar to that observed for the fulleropyrrolidines with a 5,6-addition pattern. Surprisingly, the [6,6]-regioisomer also exhibits reversible cathodic behavior. The interconversion reaction of the isomers was also explored, and the results showed that both monoadducts are thermally very stable.

Very Large, Soluble Endohedral Fullerenes in the Series La2C90 to La2C138: Isolation and Crystallographic Characterization of La2@D5(450)-C100

An extensive series of soluble dilanthanum endohedral fullerenes that extends from La(2)C(90) to La(2)C(138) has been discovered. The most abundant of these, the nanotubular La(2)@D(5)(450)-C(100), has been isolated in pure form and characterized by single-crystal X-ray diffraction.