Christopher D. Incarvito

Highly active and robust Cp* iridium complexes for catalytic water oxidation.

A series of Cp*Ir catalysts are the most active known by over an order of magnitude for water oxidation with Ce(IV). DFT calculations support a Cp*Ir=O complex as an active species.

Half-Sandwich Iridium Complexes for Homogeneous Water-Oxidation Catalysis

Iridium half-sandwich complexes of the types Cp*Ir(N-C)X, [Cp*Ir(N-N)X]X, and [CpIr(N-N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*Ir(H(2)O)(3)]SO(4) and [(Cp*Ir)(2)(μ-OH)(3)]OH can show even higher turnover frequencies (up to 20 min(-1) at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H(2)(18)O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide.

Molecular Recognition in the Selective Oxygenation of Saturated C-H Bonds by a Dimanganese Catalyst

Although enzymes often incorporate molecular recognition elements to orient substrates selectively, such strategies are rarely achieved by synthetic catalysts. We combined molecular recognition through hydrogen bonding with C-H activation to obtain high-turnover catalytic regioselective functionalization of sp3 C-H bonds remote from the –COOH recognition group. The catalyst contains a Mn(μ-O)2Mn reactive center and a ligand based on Kemps triacid that directs a –COOH group to anchor the carboxylic acid group of the substrate and thus modify the usual selectivity for oxidation. Control experiments supported the role of hydrogen bonding in orienting the substrate to achieve high selectivity.

Distinguishing homogeneous from heterogeneous catalysis in electrode-driven water oxidation with molecular iridium complexes.

Molecular water-oxidation catalysts can deactivate by side reactions or decompose to secondary materials over time due to the harsh, oxidizing conditions required to drive oxygen evolution. Distinguishing electrode surface-bound heterogeneous catalysts (such as iridium oxide) from homogeneous molecular catalysts is often difficult. Using an electrochemical quartz crystal nanobalance (EQCN), we report a method for probing electrodeposition of metal oxide materials from molecular precursors. Using the previously reported [Cp*Ir(H(2)O)(3)](2+) complex, we monitor deposition of a heterogeneous water oxidation catalyst by measuring the electrode mass in real time with piezoelectric gravimetry. Conversely, we do not observe deposition for homogeneous catalysts, such as the water-soluble complex Cp*Ir(pyr-CMe(2)O)X reported in this work. Rotating ring-disk electrode electrochemistry and Clark-type electrode studies show that this complex is a catalyst for water oxidation with oxygen produced as the product. For the heterogeneous, surface-attached material generated from [Cp*Ir(H(2)O)(3)](2+), we can estimate the percentage of electroactive metal centers in the surface layer. We monitor electrode composition dynamically during catalytic turnover, providing new information on catalytic performance. Together, these data suggest that EQCN can directly probe the homogeneity of molecular water-oxidation catalysts over short times.

Copper complexes of anionic nitrogen ligands in the amidation and imidation of aryl halides.

Copper(I) imidate and amidate complexes of chelating N,N-donor ligands, which are proposed intermediates in copper-catalyzed amidations of aryl halides, have been synthesized and characterized by X-ray diffraction and detailed solution-phase methods. In some cases, the complexes adopt neutral, three-coordinate trigonal planar structures in the solid state, but in other cases they adopt an ionic form consisting of an L 2Cu (+) cation and a CuX 2 (-) anion. A tetraalkylammonium salt of the CuX 2 (-) anion in which X = phthalimidate was also isolated. Conductivity measurements and (1)H NMR spectra of mixtures of two complexes all indicate that the complexes exist predominantly in the ionic form in DMSO and DMF solutions. One complex was sufficiently soluble for conductance measurements in less polar solvents and was shown to adopt some degree of the ionic form in THF and predominantly the neutral form in benzene. The complexes containing dative nitrogen ligands reacted with iodoarenes and bromoarenes to form products from C-N coupling, but the ammonium salt of [Cu(phth) 2] (-) did not. Similar selectivities for stoichiometric and catalytic reactions with two different iodoarenes and faster rates for the stoichiometric reactions implied that the isolated amidate and imidate complexes are intermediates in the reactions of amides and imides with haloarenes catalyzed by copper complexes containing dative N,N ligands. These amidates and imidates reacted much more slowly with chloroarenes, including chloroarenes that possess more favorable reduction potentials than some bromoarenes and that are known to undergo fast dissociation of chloride from the chloroarene radical anion. The reaction of o-(allyloxy)iodobenzene with [(phen) 2Cu][Cu(pyrr) 2] results in formation of the C-N coupled product in high yield and no detectable amount of the 3-methyl-2,3-dihydrobenzofuran or 3-methylene-2,3-dihydrobenzofuran products that would be expected from a reaction that generated free radicals. These data and computed reaction barriers argue against mechanisms in which the haloarene reacts with a two-coordinate anionic copper species and mechanisms that start with electron transfer to generate a free iodoarene radical anion. Instead, these data are more consistent with mechanisms involving cleavage of the carbon-halogen bond within the coordination sphere of the metal.

Anodic deposition of a robust iridium-based water-oxidation catalyst from organometallic precursors

Artificial photosynthesis, modeled on natural light-driven oxidation of water in Photosystem II, holds promise as a sustainable source of reducing equivalents for producing fuels. Few robust water-oxidation catalysts capable of mediating this difficult four-electron, four-proton reaction have yet been described. We report a new method for generating an amorphous electrodeposited material, principally consisting of iridium and oxygen, which is a robust and long-lived catalyst for water oxidation, when driven electrochemically. The catalyst material is generated by a simple anodic deposition from Cp*Ir aqua or hydroxo complexes in aqueous solution. This work suggests that organometallic precursors may be useful in electrodeposition of inorganic heterogeneous catalysts.

Thermal decomposition of energetic materials 79 thermal, vibrational, and X-ray structural characterization of metal salts of mono- and di-anionic 5-nitraminotetrazole

Abstract The energetic alkali metal salts (Li+, Na+, K+, Rb+, Cs+) of the 5-nitraminotetrazole mono-anion(II) and di-anion(III) were synthesized and their properties compared with neutral 5-nitraminotetrazole(I). The decomposition temperatures by DSC follow the order (III)>(II)>(I), and for the salts of (III) Li+>Na+>K+>Rb+>Cs+. Both thermal stability trends are consistent with a major role played by the crystal lattice energy. Loss of H2O is detected by TGA in cases where hydrates also form. The crystal and molecular structures of Cs(II) and Rb3(III)(NCNNO2)·H2O were determined by X-ray diffraction. The latter compound is the first structural characterization of (III) and also contains the nitrocyanamide anion, NCNNO2−. The compound NCNNO2− appears to have formed by loss of HN3 from (II) or N3− from (III) during one of the synthesis attempts of Rb(II). The major atom motions in the IR spectra of (I), (II), and (III) were assigned on the basis of density functional theory (DFT) using the B3LYP method. Both the IR spectral and crystal structure trends show that increasing the negative charge on the anion primarily shifts the partial double bond character from the CNNO2 bond to the CNNO2 bond.

Mechanism of the Rhodium(III)-Catalyzed Arylation of Imines via C–H Bond Functionalization: Inhibition by Substrate

Rh(III)-catalyzed arylation of imines provides a new method for C-C bond formation while simultaneously introducing an α-branched amine as a functional group. This detailed mechanistic study provides insights for the rational future development of this new reaction. Relevant intermediate Rh(III) complexes have been isolated and characterized, and their reactivities in stoichiometric reactions with relevant substrates have been monitored. The reaction was found to be first order in the catalyst resting state and inverse first order in the C-H activation substrate.

Palladium(I)-bridging allyl dimers for the catalytic functionalization of CO2.

In general, the chemistry of both η(1)-allyl and η(3)-allyl Pd complexes is extremely well understood; η(1)-allyls are nucleophilic and react with electrophiles, whereas η(3)-allyls are electrophilic and react with nucleophiles. In contrast, relatively little is known about the chemistry of metal complexes with bridging allyl ligands. In this work, we describe a more efficient synthetic methodology for the preparation of Pd(I)-bridging allyl dimers and report the first studies of their stoichiometric reactivity. Furthermore, we show that these compounds can activate CO(2) and that an N-heterocyclic carbene-supported dimer is one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO(2).

Group 10 metal compounds of 1,1'-bis(diphenylphosphino)ferrocene (dppf) and 1,1'-bis(diphenylphosphino)ruthenocene: a structural and electrochemical investigation. X-ray structures of [MCl2(dppr)] (M = Ni, Pd)

Abstract The oxidative electrochemistry of 1,1′-bis(diphenylphosphino)ferrocene (dppf) and 1,1′-bis(diphenylphosphino)ruthenocene (dppr) was investigated at a variety of temperatures and concentrations. In addition, the oxidative electrochemistry of [NiCl2(dppf)] and [MCl2(dppr)] (M=Ni, Pd or Pt) compounds was studied. During the preparation of the dppr compounds, crystals of [NiCl2(dppr)] and [(PdCl2(dppr)]·CH2Cl2 were obtained and the structures were determined. With the previously determined structures of [MCl2(dppf)] (M=Ni, Pd or Pt) and [PtCl2(dppr)], a thorough examination of the binding of dppf and dppr to Group 10 metals was performed.