Three-coordinate terminal imidoiron(III) complexes: structure, spectroscopy, and mechanism of formation.

Reaction of 1-adamantyl azide with iron(I) diketiminate precursors gives metastable but isolable imidoiron(III) complexes LFe=NAd (L = bulky beta-diketiminate ligand; Ad = 1-adamantyl). This paper addresses (1) the spectroscopic and structural characterization of the Fe=N multiple bond in these interesting three-coordinate iron imido complexes, and (2) the mechanism through which the imido complexes form. The iron(III) imido complexes have been examined by (1)H NMR and electron paramagnetic resonance (EPR) spectroscopies and temperature-dependent magnetic susceptibility (SQUID), and structurally characterized by crystallography and/or extended X-ray absorption fine structure (EXAFS) measurements. These data show that the imido complexes have quartet ground states and short (1.68 +/- 0.01 A) iron-nitrogen bonds. The formation of the imido complexes proceeds through unobserved iron-N(3)R intermediates, which are indicated by QM/MM computations to be best described as iron(II) with an N(3)R radical anion. The radical character on the organoazide bends its NNN linkage to enable easy N(2) loss and imido complex formation. The product distribution between imidoiron(III) products and hexazene-bridged diiron(II) products is solvent-dependent, and the solvent dependence can be explained by coordination of certain solvents to the iron(I) precursor prior to interaction with the organoazide.

The reactivity patterns of low-coordinate iron-hydride complexes.

We report a survey of the reactivity of the first isolable iron-hydride complexes with a coordination number less than 5. The high-spin iron(II) complexes [(beta-diketiminate)Fe(mu-H)] 2 react rapidly with representative cyanide, isocyanide, alkyne, N 2, alkene, diazene, azide, CO 2, carbodiimide, and Brønsted acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across a multiple bond of the substrate, (2) reductive elimination of H 2 to form iron(I) products, and (3) protonation of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido, benzo[ c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known bond transformations at iron complexes. Additionally, they give insight into the elementary transformations that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands on high-spin, low-coordinate metal atoms.

Strong intra- and intermolecular aurophilic interactions in a new series of brilliantly luminescent dinuclear cationic and neutral au(I) benzimidazolethiolate complexes.

The structural and photophysical properties of a new series of cationic and neutral Au(I) dinuclear compounds (1 and 2, respectively) bridged by bis(diphenylphosphino)methane (dppm) and substituted benzimidazolethiolate (X-BIT) ligands, where X = H (a), Me (b), OMe (c), and Cl (d), have been studied. Monocationic complexes, [A(u2)(micro-X-BIT)(micro-dppm)](CF(3)CO(2)), were prepared by the reaction of [A(u2)(micro-dppm)](CF(3)CO(2))(2) with 1 equiv of X-BIT in excellent yields. The cations 1a-1d possess similar molecular structures, each with a linear coordination geometry around the Au(I) nuclei, as well as relatively short intramolecular Au(I)...Au(I) separations ranging between 2.88907(6) A for 1d and 2.90607(16) A for 1a indicative of strong aurophilic interactions. The cations are violet luminescent in CH(2)Cl(2) solution with a lambda(em)(max) of ca. 365 nm, assigned as ligand-based or metal-centered (MC) transitions. Three of the cationic complexes, 1a, 1b, and 1d, exhibit unusual luminescence tribochromism in the solid-state, in which the photoemission is shifted significantly to higher energy upon gentle grinding of microcrystalline samples with DeltaE = 1130 cm(-1) for 1a, 670 cm(-1) (1b), and 870 cm(-1) (1d). The neutral dinuclear complexes, [A(u2)(micro-X-BIT)(micro-dppm)] (2a-2d) were formed in good yields by the treatment of a CH(2)Cl(2) solution of cationic compounds (1) with NEt(3). 2a-2d aggregate to form dimers having substantial intra- and intermolecular aurophilic interactions with unsupported Au(I)...Au(I) intermolecular distances in the range of 2.8793(4)-2.9822(8) A, compared with intramolecular bridge-supported separations of 2.8597(3)-2.9162(3) A. 2a-2d exhibit brilliant luminescence in the solid-state and in DMSO solution with red-shifted lambda(em)(max) energies in the range of 485-545 nm that are dependent on X-BIT and assigned as ligand-to-metal-metal charge transfer (LMMCT) states based in part on the extended Au...Au...Au...Au interactions.

Diastereoselective oxidative addition of dihydrogen to IrI(CO)((R)-BINAP) and [Ir(CO)2((R)-BINAP)][SbF6].

Two chiral iridium(I) (R)-BINAP complexes, IrI(CO)((R)-BINAP) (1) and [Ir(CO)2((R)-BINAP)][SbF6] (2), have been synthesized and characterized, and their reactivity with dihydrogen has been studied. Complex 1 is formed on the addition of (R)-BINAP to [Bu4N][IrI2(CO)2] in toluene, and 2 is generated by the addition of AgSbF6 to a solution of 1 in dichloromethane under CO. A structure determination of complex 2 confirms a square planar coordination geometry, while that of 1 reveals a significant tetrahedral distortion from the expected planar coordination. Additionally, the structure of 1 shows a disorder between iodide and CO ligands. The reaction of 1 with H2 proceeds under kinetic control and shows a high degree of kinetic and thermodynamic selectivity; the kinetic product is formed by H2 addition across the P-Ir-CO axis of IrI(CO)((R)-BINAP) and yields two diastereomers which then convert over time to two more stable diastereomers which correspond to oxidative addition across the P-Ir-I axis. The kinetically favored diastereomers are formed in an initial ratio of 8.6:1, corresponding to a DeltaDeltaG* of 1.27 kcal/mol. The reaction of H2 with the C2-symmetric complex 2 also leads to the formation of two diastereomers, with one favored over the other kinetically by a 9.9:1 ratio on extrapolation to t = 0. When these reactions are followed using parahydrogen NMR methods, only one of the initially formed diastereomers in each case is found to exhibit substantial parahydrogen-induced polarization in the hydride resonances at room temperature.

Quantitative geometric descriptions of the belt iron atoms of the iron-molybdenum cofactor of nitrogenase and synthetic iron(II) model complexes.

Six of the seven iron atoms in the iron-molybdenum cofactor of nitrogenase display an unusual geometry, which is distorted from the tetrahedral geometry that is most common in iron-sulfur clusters. This distortion pulls the iron along one C3 axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of the Fe atoms in the iron-molybdenum cofactor with those of the presently and previously reported model complexes, as well as with those in polynuclear iron-sulfur compounds. This understanding of the iron coordination geometry is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.

Macrocyclic Binucleating ß-Diketiminate Ligands and their Lithium, Aluminum, and Zinc Complexes.

The incorporation of rigid aromatic linkers into ß-diketiminate ligands creates a binucleating scaffold that holds two metals near each other. This paper discloses the synthesis, characterization, and reactivity of mBin(2-), which has a meta-substituted xylylene spacer, and pBin(2-), which has a para-substituted xylylene spacer. Lithium, aluminum, and zinc complexes of each ligand are isolated, and in some cases are characterized by X-ray crystallography. The lithium complexes are coordinated to solvent-derived THF ligands, while the zinc and aluminum complexes have alkyl ligands. Complexes of the mBin(2-) ligand have an anti conformation in which the metals are on opposite sides of the macrocycle, while pBin(2-) complexes prefer a syn conformation. The (1)H NMR spectra of the complexes demonstrate that the conformations rapidly interconvert in the lithium complexes, and less rapidly in the zinc and aluminum complexes.

Highly diastereoselective oxidative addition of methyl iodide to a chiral square-planar complex.

Oxidative addition of methyl iodide to the chiral square-planar complex IrI(CO)(duphos) shows a high level of diastereoselectivity. The basis for the diastereoselectivity of the reaction is best explained based on the crystal structure of IrI(CO)(duphos) in which methyl iodide approach across the two faces is differentiated by the chiral ligand.

A dinucleating ligand related to the beta-diketiminates.

A new ligand conceptually creates two sites reminiscent of beta-diketiminates, and upon deprotonation the salts exist in oligomeric forms with potassium ions linking multiple ligands.

A model iridium hydroformylation system with the large bite angle ligand xantphos: reactivity with parahydrogen and implications for hydroformylation catalysis.

Iridium complexes containing the large bite angle bisphosphine ligand xantphos have been synthesized and their reactivity studied. Several of these complexes are the first reported Ir(xantphos) systems to be characterized by X-ray diffraction. Variable-temperature NMR spectroscopic studies of IrI(CO)2(xantphos) (1-I) and Ir(COEt)(CO)2(xantphos) (8) show two separate dynamic processes in which the phosphorus donors and the backbone methyl groups of the xantphos ligand are exchanged. The addition of parahydrogen (p-H2) to 1-I leads to the formation of two dihydride isomers including one in which both hydride ligands are trans to the phosphorus donors, suggestive of an Ir(I) xantphos intermediate with the ligand chelated in a trans-spanning fashion (2b). The bromide and chloride Ir(I) analogues (1-Br and 1-Cl) also form this isomer upon reaction with parahydrogen, with 1-Cl yielding only this dihydride species. The trihydride complex IrH3(CO)(xantphos) (7) has been prepared, and its exchange with free hydrogen at elevated temperature is confirmed by reaction with p-H2. The hydride complexes IrH(CO)2(xantphos) (6) and IrH3(CO)(xantphos) (7), as well as the propionyl complex 8, are modest catalysts for the hydroformylation of 1-hexene and styrene under mild conditions. The addition of p-H2 to 8 permits direct observation of the propionyl dihydride species IrH2(COEt)(CO)(xantphos) (9) under both thermal and photolytic conditions, as well as unusual but weak polarization of the aldehydic proton of the propanal product that forms upon reductive elimination from 9.

Binding affinity of alkynes and alkenes to low-coordinate iron.

We report the synthesis, spectroscopy, and structural characterization of iron-alkyne and -alkene complexes of the type L(Me)Fe(ligand) [L(Me) = bulky beta-diketiminate, ligand = HCCPh, EtCCEt, CH2CHPh, EtCHCHEt, HCC(p-C6H4OCH3), HCC(p-C6H4CF3)]. The neutral ligand exchanges rapidly at room temperature, and the equilibrium constants have been measured or estimated. The binding affinity toward the low-coordinate Fe follows the trend HCCPh > EtCCEt > CH2CHPh > EtCHCHEt approximately PPh3 > benzene >> N2. This trend is consistent with a model in which pi back-bonding from the formally Fe(I) center is the dominant interaction in determining the relative binding affinities. In nitrogenase, alkynes are reduced while alkenes are unreactive, and this work suggests that the different binding affinities to low-coordinate Fe might explain the differential activity of the enzyme toward these two substrates.

Studies of low-coordinate iron dinitrogen complexes.

Understanding the interaction of N2 with iron is relevant to the iron catalyst used in the Haber process and to possible roles of the FeMoco active site of nitrogenase. The work reported here uses synthetic compounds to evaluate the extent of NN weakening in low-coordinate iron complexes with an FeNNFe core. The steric effects, oxidation level, presence of alkali metals, and coordination number of the iron atoms are varied, to gain insight into the factors that weaken the NN bond. Diiron complexes with a bridging N2 ligand, L(R)FeNNFeL(R) (L(R) = beta-diketiminate; R = Me, tBu), result from reduction of [L(R)FeCl]n under a dinitrogen atmosphere, and an iron(I) precursor of an N2 complex can be observed. X-ray crystallographic and resonance Raman data for L(R)FeNNFeL(R) show a reduction in the N-N bond order, and calculations (density functional and multireference) indicate that the bond weakening arises from cooperative back-bonding into the N2 pi orbitals. Increasing the coordination number of iron from three to four through binding of pyridines gives compounds with comparable N-N weakening, and both are substantially weakened relative to five-coordinate iron-N2 complexes, even those with a lower oxidation state. Treatment of L(R)FeNNFeL(R) with KC8 gives K2L(R)FeNNFeL(R), and calculations indicate that reduction of the iron and alkali metal coordination cooperatively weaken the N-N bond. The complexes L(R)FeNNFeL(R) react as iron(I) fragments, losing N2 to yield iron(I) phosphine, CO, and benzene complexes. They also reduce ketones and aldehydes to give the products of pinacol coupling. The K2L(R)FeNNFeL(R) compounds can be alkylated at iron, with loss of N2.

Catalytic, asymmetric, "interrupted" Feist-Bénary reactions.

Pyrimidine derivatives of the cinchona alkaloids function as excellent asymmetric catalysts for the "Interrupted" Feist-Bénary Reaction. This reaction produces highly substituted hydroxydihydrofurans from simple starting materials under mild conditions. The asymmetric reaction gives high enantioselectivities with unsubstituted bromoketones, and high enantio- and diastereoselectivities with substituted substrates. Mechanistic experiments suggest that the hydrobromide salt of the alkaloid derivative is the active catalyst for the reaction.

Alkane coordination selectivity in hydrocarbon activation by [Tp'Rh(CNneopentyl)]: the role of alkane complexes.

The competitive activation of C-H bonds of linear, cyclic, and branched hydrocarbons using the coordinatively unsaturated 16-electron [Tp'RhL] reactive fragment have been studied (Tp' = tris-(3,5-dimethylpyrazolyl)borate; L = CNCH2CMe3). Activation of the hydrocarbons leads to the formation of Tp'Rh(L)(R)(H) alkyl complexes, which were converted to the stable chlorides immediately following the activation of the bonds via photolysis of Tp'Rh(L)(PhN=C=NCH2CMe3) in the solvent mixture. The products were analyzed by 1H NMR spectroscopy. The experiments described provide relative rates for the coordination of primary and secondary C-H bonds to the Rh metal center, indicating a 1.5x preference for the latter.

Synthesis and reactivity of low-coordinate iron(II) fluoride complexes and their use in the catalytic hydrodefluorination of fluorocarbons.

Transition metal fluoride complexes are of interest because they are potentially useful in a multitude of catalytic applications, including C-F bond activation and fluorocarbon functionalization. We report the first crystallographically characterized examples of molecular iron(II) fluorides: [L(Me)Fe(mu-F)]2 (1(2)) and L(tBu)FeF (2) (L = bulky beta-diketiminate). These complexes react with donor molecules (L'), yielding trigonal-pyramidal complexes L(R)FeF(L'). The fluoride ligand is activated by the Lewis acid Et2O.BF3, forming L(tBu)Fe(OEt2)(eta1-BF4) (3), and is also silaphilic, reacting with silyl compounds such as Me3SiSSiMe3, Me3SiCCSiMe3, and Et3SiH to give new thiolate L(tBu)FeSSiMe3 (4), acetylide L(tBu)FeCCSiMe3 (5), and hydride [L(Me)Fe(mu-H)]2 (6(2)) complexes. The hydrodefluorination (HDF) of perfluorinated aromatic compounds (hexafluorobenzene, pentafluoropyridine, and octafluorotoluene) with a silane R3SiH (R3 = (EtO)3, Et3, Ph3, (3,5-(CF3)2C6H3)Me2) is catalyzed by addition of an iron(II) fluoride complex, giving mainly the singly hydrodefluorinated products (pentafluorobenzene, 2,3,5,6-tetrafluoropyridine, and alpha,alpha,alpha,2,3,5,6-heptafluorotoluene, respectively) in up to five turnovers. These catalytic perfluoroarene HDF reactions proceed with activation of the C-F bond para to the most electron-withdrawing group and are dependent on the degree of fluorination and solvent polarity. Kinetic studies suggest that hydride generation is the rate-limiting step in the HDF of octafluorotoluene, but the active intermediate is unknown. Mechanistic considerations argue against oxidative addition and outer-sphere electron transfer pathways for perfluoroarene HDF. Fluorinated olefins are also hydrodefluorinated (up to 10 turnovers for hexafluoropropene), most likely through a hydride insertion/beta-fluoride elimination mechanism. Complexes 1(2) and 2 thus provide a rare example of a homogeneous system that activates C-F bonds without competitive C-H activation and use an inexpensive 3d transition metal.

Formation of disubstituted beta-lactones using bifunctional catalysis.

[reaction: see text] Acid chlorides and aromatic aldehydes react in the presence of a stoichiometric amount of a tertiary amine and catalytic amounts of a cinchona alkaloid derivative and a Lewis acid to produce beta-lactones in high diastereo- and enantioselectivity. The sense of the diastereoselectivity depends on the substitution of the acid chloride, with the reactions of aliphatic acid chlorides giving predominantly the trans-isomer and those of alkoxyacetyl chlorides favoring formation of the cis-isomer.

A sulfido-bridged diiron(II) compound and its reactions with nitrogenase-relevant substrates.

The active site iron-molybdenum cofactor of nitrogenase has sulfide-bridged pairs of redox-active, trigonal pyramidal iron atoms that are postulated to be the site of N2 transformation. A synthetic compound is described in which two three-coordinate iron(II) ions are bridged similarly by sulfide. The compound binds nitrogen donors to become trigonal pyramidal and cleaves the N-N bond of phenylhydrazine with oxidation of iron(II) to iron(III).

Synthesis, structure, electrical properties, and band structure calculations of TiAsTe.

The new compound TiAsTe has been synthesized by the reaction of the elements in a LiCl/KCl flux at 923 K. The compound crystallizes with four formula units in space group Immm of the orthorhombic system in a cell at 153 K of a = 3.5730(8) A, b = 5.249(1) A, c = 12.794(3) A, V = 240.0(1) A(3). The structure, which is of the NbPS structure type, is a three-dimensional extended framework built from bicapped TiAs(4)Te(4) trigonal prisms. It may be considered to comprise infinity (2) [TiTe] slabs perpendicular to [001] that are interspersed with linear infinity (1)[As] chains running along [010]. The As-As distances alternate at 2.554(2) and 2.695(2) A. Electrical and thermopower measurements indicate that TiAsTe is an n-type metallic compound. Density functional theory calculations help rationalize the chemical bonding and physical properties.

Synthesis and characterization of volatile, fluorine-free beta-ketoiminate lanthanide MOCVD precursors and their implementation in low-temperature growth of epitaxial CeO(2) buffer layers for superconducting electronics.

A new class of volatile, low-melting, fluorine-free lanthanide metal-organic chemical vapor deposition (MOCVD) precursors has been developed. The neutral, monomeric Ce, Nd, Gd, and Er complexes are coordinatively saturated by a versatile, multidentate ether-functionalized beta-ketoiminato ligand series, the melting point and volatility characteristics of which can be tuned by altering the alkyl substituents on the keto, imino, and ether sites of the ligand. Direct comparison with conventional lanthanide beta-diketonate complexes reveals that the present precursor class is a superior choice for lanthanide oxide MOCVD. Epitaxial CeO(2) buffer layer films can be grown on (001) YSZ substrates by MOCVD at significantly lower temperatures (450-650 degrees C) than previously possible by using one of the newly developed cerium beta-ketoiminate precursors. Films deposited at 540 degrees C have good out-of-plane (Deltaomega = 0.85 degrees ) and in-plane (Deltaphi = 1.65 degrees ) alignment and smooth surfaces (rms roughness approximately 4.3 A). The film growth rate decreases and the films tend to be smoother as the deposition temperature is increased. High-quality yttrium barium copper oxide (YBCO) films grown on these CeO(2) buffer layers by pulsed organometallic molecular beam epitaxy exhibit very good electrical transport properties (T(c) = 86.5 K, J(c) = 1.08 x 10(6) A/cm(2) at 77.4 K).