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.

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.

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.

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.

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.

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 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).