Zinc Catalysts for On-Demand Hydrogen Generation and Carbon Dioxide Functionalization

[Tris­(2-pyridylthio)­methyl]zinc hydride, [κ³-Tptm]­ZnH, is a multifunctional catalyst that is capable of achieving (i) rapid release of hydrogen by protolytic cleavage of silanes with either water or methanol and (ii) hydrosilylation of aldehydes, ketones, and carbon dioxide. For example, [κ³-Tptm]­ZnH catalyzes the release of 3 equivalents of H₂ by methanolysis of phenylsilane, with a turnover number of 10⁵ and a turnover frequency surpassing 10⁶ h^–1 for the first 2 equivalents. Furthermore, [κ³‑Tptm]­ZnH also catalyzes the formation of triethoxy­silyl formate by hydrosilylation of carbon dioxide with triethoxysilane. Triethoxysilyl formate may be converted into ethyl formate and <i>N</i>,<i>N</i>-dimethylformamide, thereby providing a means for utilizing carbon dioxide as a C₁ feedstock for the synthesis of useful chemicals

Synthesis, Structure, and Reactivity of a Terminal Organozinc Fluoride Compound: Hydrogen Bonding, Halogen Bonding, and Donor–Acceptor Interactions

[Tris­(2-pyridylthio)­methyl]­zinc fluoride, [κ⁴-Tptm]­ZnF, the first example of an organozinc compound that features a terminal fluoride ligand, may be obtained by the reactions of either [Tptm]­ZnX (X = H, OSiMe₃) with Me₃SnF or [κ⁴-Tptm]­ZnI with [Buⁿ₄N]­F. Not only is the fluoride ligand of [κ⁴-Tptm]­ZnF susceptible to coordination by B­(C₆F₅)₃ to give the adduct [κ⁴-Tptm]­ZnFB­(C₆F₅)₃, but it is also an effective hydrogen bond and halogen bond acceptor. For example, X-ray diffraction studies demonstrate that [κ⁴-Tptm]­ZnF forms an adduct with water in which hydrogen bonding between the fluoride ligands and water molecules serves to link pairs of [κ⁴-Tptm]­ZnF molecules with a [F···(H-O-H)₂···F] motif. Furthermore, ¹H and ¹⁹F NMR spectroscopic studies provide evidence for hydrogen bonding and halogen bonding interactions with indole and C₆F₅I, respectively

Synthesis, Structure, and Reactivity of a Terminal Organozinc Fluoride Compound: Hydrogen Bonding, Halogen Bonding, and Donor–Acceptor Interactions

[Tris­(2-pyridylthio)­methyl]­zinc fluoride, [κ⁴-Tptm]­ZnF, the first example of an organozinc compound that features a terminal fluoride ligand, may be obtained by the reactions of either [Tptm]­ZnX (X = H, OSiMe₃) with Me₃SnF or [κ⁴-Tptm]­ZnI with [Buⁿ₄N]­F. Not only is the fluoride ligand of [κ⁴-Tptm]­ZnF susceptible to coordination by B­(C₆F₅)₃ to give the adduct [κ⁴-Tptm]­ZnFB­(C₆F₅)₃, but it is also an effective hydrogen bond and halogen bond acceptor. For example, X-ray diffraction studies demonstrate that [κ⁴-Tptm]­ZnF forms an adduct with water in which hydrogen bonding between the fluoride ligands and water molecules serves to link pairs of [κ⁴-Tptm]­ZnF molecules with a [F···(H-O-H)₂···F] motif. Furthermore, ¹H and ¹⁹F NMR spectroscopic studies provide evidence for hydrogen bonding and halogen bonding interactions with indole and C₆F₅I, respectively

Synthesis, Structure, and Reactivity of a Terminal Organozinc Fluoride Compound: Hydrogen Bonding, Halogen Bonding, and Donor–Acceptor Interactions

[Tris­(2-pyridylthio)­methyl]­zinc fluoride, [κ⁴-Tptm]­ZnF, the first example of an organozinc compound that features a terminal fluoride ligand, may be obtained by the reactions of either [Tptm]­ZnX (X = H, OSiMe₃) with Me₃SnF or [κ⁴-Tptm]­ZnI with [Buⁿ₄N]­F. Not only is the fluoride ligand of [κ⁴-Tptm]­ZnF susceptible to coordination by B­(C₆F₅)₃ to give the adduct [κ⁴-Tptm]­ZnFB­(C₆F₅)₃, but it is also an effective hydrogen bond and halogen bond acceptor. For example, X-ray diffraction studies demonstrate that [κ⁴-Tptm]­ZnF forms an adduct with water in which hydrogen bonding between the fluoride ligands and water molecules serves to link pairs of [κ⁴-Tptm]­ZnF molecules with a [F···(H-O-H)₂···F] motif. Furthermore, ¹H and ¹⁹F NMR spectroscopic studies provide evidence for hydrogen bonding and halogen bonding interactions with indole and C₆F₅I, respectively

Modulation of Zn–C Bond Lengths Induced by Ligand Architecture in Zinc Carbatrane Compounds

Bond lengths between pairs of atoms in covalent molecules are generally predicted well by the sum of their respective covalent radii, such that there are usually only small variations in related compounds. It is, therefore, significant that we have demonstrated that the incorporation of appropriately sized linkers between carbon and a metal center provides a means to modulate the length and nature of a metal–carbon interaction. Specifically, X-ray diffraction studies on a series of tris­(1-methyl­imidazol-2-ylthio)­methyl zinc complexes, [Titm^Me]­ZnX, demonstrate how the Zn–C bond lengths are highly variable (2.17–2.68 Å) and are up to 0.67 Å longer than the average value listed in the Cambridge Structural Database (2.01 Å). Furthermore, density functional theory calculations on [Titm^Me]­ZnCl demonstrate that the interaction is very flexible, such that either increasing or decreasing the Zn–C length from that in the equilibrium structure is associated with little energy change in comparison to that for other compounds with Zn–C bonds

Mechanisms by which Alkynes React with CpCr(CO)₃H. Application to Radical Cyclization

The reaction of CpCr­(CO)₃H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr­(CO)₃H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr­(CO)₃H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (<b>8</b>) gave cyclized products in 78% yield under catalytic conditions (35 psi H₂), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (<b>12</b>) hydrogenation was completely eliminated to yield 100% cyclized products

Mechanisms by which Alkynes React with CpCr(CO)₃H. Application to Radical Cyclization

The reaction of CpCr­(CO)₃H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr­(CO)₃H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr­(CO)₃H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (<b>8</b>) gave cyclized products in 78% yield under catalytic conditions (35 psi H₂), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (<b>12</b>) hydrogenation was completely eliminated to yield 100% cyclized products

Dihydrogen Activation by Cobaloximes with Various Axial Ligands

We have investigated the effect of axial ligands on the ability of cobaloximes to catalyze the generation of transferable hydrogen atoms from hydrogen gas and have learned that the active catalyst contains one and only one axial ligand. We have, for example, shown that Co­(dmgBF₂)₂ coordinates only one Ph₃P and that the addition of additional Ph₃P (beyond 1 equiv) to solvated Co­(dmgBF₂)₂ does not affect its catalytic turnover for H• transfer from H₂

Electron Transfer from Hexameric Copper Hydrides

The octahedral core of 84-electron LCuH hexamers does not dissociate appreciably in solution, although their hydride ligands undergo rapid intramolecular rearrangement. The single-electron transfer proposed as an initial step in the reaction of these hexamers with certain substrates has been observed by stopped-flow techniques when [(Ph₃P)­CuH]₆ is treated with a pyridinium cation. The same radical cation has been prepared by the oxidation of [(Ph₃P)­CuH]₆ with Cp*₂Fe⁺ and its reversible formation observed by cyclic voltammetry; its UV–vis spectrum has been confirmed by spectroelectrochemistry. The 48-electron trimer [(dppbz)­CuH]₃ has been prepared by use of the chelating ligand 1,2-bis­(diphenylphosphino)­benzene (dppbz)

Synthesis, Structural Characterization, and Reactivity of Cp₂- and (CpMe)₂‑Ligated Titanaaziridines and Titanaoxiranes with Fast Enantiomer Interconversion Rates

A new synthetic route to (CpR)₂-ligated titanaaziridines and titanaoxiranes from stable Ti­(II) precursors has been developed, and the enantiomer interconversion rate constants for chiral titanaaziridines and titanaoxiranes have been measured for the first time. The titanaaziridines (CpR)₂Ti­(η²-N­(R¹)­CHPh)­(L) (R = H (<b>10</b>), Me (<b>12</b>); R¹ = Ph (<b>a</b>), <i>o</i>-anisyl (<b>b</b>), SiMe₃ (<b>c</b>); L = PMe₃ (<b>a</b>, <b>c</b>), −OMe (<b>b</b>)) and titanaoxiranes Cp₂Ti­(η²-Ph­(R)­CO)­(L) (R = Ph (<b>14</b>), H (<b>15</b>); L = PMe₃) have been synthesized and characterized spectroscopically; titanaaziridine <b>10a</b> and titanaoxirane <b>14</b> have been characterized by X-ray crystallography. The enantiomer interconversion rate constants for the chiral titanaaziridines and titanaoxiranes have been measured by variable-temperature NMR; <i>k</i>_inv for <b>10b</b> is the fastest enantiomer interconversion rate constant reported for any metallaaziridine or metallaoxirane to date. Titanaaziridines <b>10</b> and <b>12</b> undergo exchange reactions with CC and CX bonds, whereas the titanaoxiranes <b>14</b> and <b>15</b> undergo insertions