Search

The catalytic relevance of Fe(IV) species in non‐heme iron catalysis has motivated synthetic advances in well‐defined five‐ and six‐coordinate Fe(IV) complexes for a better understanding of their fundamental electronic structures and reactivities. Herein, we report the syntheses of FeDipp2 and FeMes2, a pair of unusual four‐coordinate non‐heme formally Fe(IV) complexes with S=1 ground states supported by strongly donating bisamide ligands. By combining spectroscopic characterization and computational modeling, we found that small variations in ligand aryl substituents resulted in substantial changes in both structures and bonding. This work highlights the strong donor capabilities and modularity of the bisamide ligand set. More broadly, it is a critical contribution to the utilization of ligand design to modulate molecular geometries and electronic structures of low‐coordinate, high‐valent iron complexes. [ABSTRACT FROM AUTHOR]

We have synthesised and characterised the complex Ni(tpy)2 (tpy = 2,2′:6′,2′′-terpyridine). This formally Ni(0) complex is paramagnetic both in the solid state and in solution (S = 2). The crystal structure shows an octahedral geometry, with molecules arranged in independent dimers involving π-stacking between pairs of complexes. Magnetic measurementes and DFT calculations suggest the existence of temperature-dependent intermolecular antiferromagnetic coupling in the solid state. [ABSTRACT FROM AUTHOR]

The assembly of chiral molecules with multiple stereogenic elements is challenging, and, despite of indisputable advances, largely limited to toxic, costintensive and precious metal catalysts. In sharp contrast, we herein disclose a versatile C–H alkylation using a non-toxic, low-cost iron catalyst for the synthesis of substituted indoles with two chiral elements. The key for achieving excellent diastereo- and enantioselectivity was substitution on a chiral Nheterocyclic carbene ligand providing steric hindrance and extra represented by noncovalent interaction for the concomitant generation of C–N axial chirality and C-stereogenic center. Experimental and computational mechanistic studies have unraveled the origin of the catalytic efficacy and stereoselectivity. [ABSTRACT FROM AUTHOR]

The molecular-level role of alkoxide salts, used as alternative additive to N-methylpyrrolidone in iron-catalyzed alkyl–alkenyl/aryl cross-coupling reactions, is investigated. Detailed spectroscopic studies reveal that alkoxides promote the formation of homoleptic organoferrates such as [FeMe3]−, providing an alternative to toxic NMP to access these reactive intermediates. [ABSTRACT FROM AUTHOR]

Fe‐based catalysts are an active, selective, and low‐cost option for tuning Fischer‐Tropsch synthesis (FTS) selectivity toward desirable light olefins. By encapsulating Fe within ZSM‐5, the resultant core‐shell catalysts have the potential to control the product distribution via secondary reactions that occur over the acid sites of the zeolite shell. In this paper, Fe is encapsulated within ZSM‐5 via the seed‐directed growth technique and characterized with a suite of analytical techniques including Mössbauer spectroscopy and X‐ray absorption fine structure (XAFS). Characterization of the core‐shell catalysts indicates that some of the Fe‐based active phase is destabilized during seed‐directed growth, demonstrating the challenges associated with encapsulating an Fe‐based active phase within zeolites. However, comparing FTS performance of the core‐shell catalyst with the Fe‐based control synthesized via incipient wetness impregnation demonstrates improved selectivity toward the desired C2‐C4 olefins and C5+ hydrocarbons. [ABSTRACT FROM AUTHOR]

The characterization and preparation of the first sphere mutant of soybean lipoxygenase-1, N694C which allows evaluation of the [sigma]-organoiron mechanism showing a significant deuterium isotope effect for kinetic data of N694C is presented.

A combination of studies is employed to analyze the thiolate bonding to [{FeNO}.sup.2]/[{[FeO.sub.2]}.sup.8] complexes of isopencillin N synthase (IPNS). The determination of oxidase substrate versus the oxygenase activity in nonheme Fe enzymes shows that the single center reaction in IPNS allows it to avoid the oxygen bridged binding completely.

Magnetic circular dichroism (MCD) spectroscopy is a powerful experiment used to probe the electronic structure and bonding in paramagnetic metal-based complexes. While C-term MCD spectroscopy has been utilized in many areas of chemistry, it has been underutilized in studying paramagnetic organometallic transition metal and f-element complexes. From the analysis of isolated organometallic complexes to the study of in situ generated species, MCD can provide information regarding ligand interactions, oxidation and spin state, and geometry and coordination environment of paramagnetic species. The pratical aspects of this technique, such as air-free sample preparation and cryogenic experimental temperatures, allow for the study of highly unstable species, something that is often difficult with other spectroscopic techniques. This perspective highlights MCD studies of both transition metal and f-element organometallic complexes, including in situ generated reactive intermediates, to demonstrate the utility of this technique in probing electronic structure, bonding and mechanism in paramagnetic organometallic chemistry. [ABSTRACT FROM AUTHOR]

Herein, we expand the current molecular‐level understanding of one of the most important and effective additives in iron‐catalyzed cross‐coupling reactions, N,N,N′,N′‐tetramethylethylenediamine (TMEDA). Focusing on relevant phenyl and ethyl Grignard reagents and slow nucleophile addition protocols commonly used in effective catalytic systems, TMEDA‐iron(II)‐aryl intermediates are identified via in situ spectroscopy, X‐ray crystallography, and detailed reaction studies to be a part of an iron(II)/(III)/(I) reaction cycle where radical recombination with FePhBr(TMEDA) (2Ph) results in selective product formation in high yield. These results differ from prior studies with mesityl Grignard reagent, where poor product selectivity and low catalytic performance can be attributed to homoleptic iron–ate species. Overall, this study represents a critical advance in how amine additives such as TMEDA can modulate selectivity and reactivity of organoiron species in cross‐coupling. [ABSTRACT FROM AUTHOR]

A mechanistic study is performed on the reaction method for iron-catalyzed C–H methylation with AlMe3 reagent, previously proposed to involve cyclometalated iron(III) intermediates and an iron(III)/(I) reaction cycle. Detailed spectroscopic studies (57Fe Mössbauer, EPR) during catalysis and in stoichiometric reactions identify iron(II) complexes, including cyclometalated iron(II) intermediates, as the major iron species formed in situ under catalytic reaction conditions. Reaction studies identify a cyclometalated iron(II)-methyl species as the key intermediate leading to C–H methylated product upon reaction with oxidant, consistent with a previously proposed iron(II)/iron(III)/iron(I) reaction manifold for C–H arylation. [ABSTRACT FROM AUTHOR]

The high abundance, low toxicity and rich redox chemistry of iron has resulted in a surge of iron‐catalyzed organic transformations over the last two decades. Within this area, N‐heterocyclic carbene (NHC) ligands have been widely utilized to achieve high yields across reactions including cross‐coupling and C−H alkylation, amongst others. Central to the development of iron‐NHC catalytic methods is the understanding of iron speciation and the propensity of these species to undergo reduction events, as low‐valent iron species can be advantageous or undesirable from one system to the next. This study highlights the importance of the identity of the NHC on iron speciation upon reaction with EtMgBr, where reactions with SIMes and IMes NHCs were shown to undergo β‐hydride elimination more readily than those with SIPr and IPr NHCs. This insight is vital to developing new iron‐NHC catalyzed transformations as understanding how to control this reduction by simply changing the NHC is central to improving the reactivity in iron‐NHC catalysis. [ABSTRACT FROM AUTHOR]

C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d–d and f–f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(V) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f–f transitions of [UCl6]− and [UF6]− are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]−. The experimental NIR MCD studies were further extended to [U(OC6F5)6]−, [U(CH2SiMe3)6]−, and [U(NC(tBu)(Ph))6]− to evaluate the effects of ligand-type on the f–f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f–f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(V) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(V) complexes and identify areas for continued theoretical development. [ABSTRACT FROM AUTHOR]

A new synthesis of Th(II) complexes has been identified involving addition of simple MX salts (M = Li, Na, K; X = H, Cl, Me, N3) to Cp′′3ThIII [Cp′′ = [C5H3(SiMe3)2] in the presence of 18-crown-6 or 2.2.2-cryptand, forming [M(chelate)][Cp′′3ThII] and Cp′′3ThIVX. Cptet3ThIII (Cptet = C5Me4H) reacts with KH to form Cptet3ThIVH and the C–H bond activation product, [K(crypt)]{[Cptet2ThIVH[η1:η5-C5Me3H(CH2)]}. [ABSTRACT FROM AUTHOR]

The trivalent oxidation state of uranium has been shown to undergo unique reactivity, from its ability to activate a variety of small molecules to its role in the catalytic reduction of ethene to ethane amongst others. Central to this unique reactivity and ability to rationally design ligands for isotope separation is the underlying uranium electronic structure. While electronic structure studies of U(IV), U(V), and U(VI) have been extensive, by comparison, analogous studies of more reduced oxidation states such as U(III) remains underdeveloped. Herein we report a combined MCD and EPR spectroscopic approach along with density functional theory and multireference wavefunction calculations to elucidate the effects of ligand perturbation in three uranium(III) Tp* complexes. Overall, the experimental and computational insight suggests that the change in ligand environment across this series of U(III) complexes resulted in only minor perturbations in the uranium electronic structure. This combined approach was also used to redefine the electronic ground state of a U(III) complex with a redox non-innocent Bipy− ligand. Overall, these studies demonstrate the efficacy of the combined experimental and theoretical approach towards evaluating electronic structure and bonding in U(III) complexes and provide important insight into the challenges in altering ligand environments to modify bonding and reactivity in uranium coordination chemistry. [ABSTRACT FROM AUTHOR]

N,N,N′,N′‐Tetramethylethylenediamine (TMEDA) has been one of the most prevalent and successful additives used in iron catalysis, finding application in reactions as diverse as cross‐coupling, C−H activation, and borylation. However, the role that TMEDA plays in these reactions remains largely undefined. Herein, studying the iron‐catalyzed hydromagnesiation of styrene derivatives using TMEDA has provided molecular‐level insight into the role of TMEDA in achieving effective catalysis. The key is the initial formation of TMEDA–iron(II)–alkyl species which undergo a controlled reduction to selectively form catalytically active styrene‐stabilized iron(0)–alkyl complexes. While TMEDA is not bound to the catalytically active species, these active iron(0) complexes cannot be accessed in the absence of TMEDA. This mode of action, allowing for controlled reduction and access to iron(0) species, represents a new paradigm for the role of this important reaction additive in iron catalysis. [ABSTRACT FROM AUTHOR]

We report the syntheses of a family of tetrahedral iron complexes bearing a bulky redox active o-phenylenediamide ligand. The electronic structures of these complexes have been investigated by Mössbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. [ABSTRACT FROM AUTHOR]

Homoleptic σ‐bonded uranium–alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium–methyl complexes. Amongst these complexes, the first example of a homoleptic uranium–alkyl dimer, [Li(THF)4]2[U2(CH3)10], as well as a seven‐coordinate uranium–methyl monomer, {Li(OEt2)Li(OEt2)2UMe7Li}n were both crystallographically identified. The diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium–alkyl chemistry. [ABSTRACT FROM AUTHOR]

The effect of the three-His motif in Dke1 on the geometric and electronic structure of the [Fe.sup.II] centers is analyzed by a combination of absorption, CD, MCD and VTVH MCD spectroscopies and DFT calculations. The studies have helped in understanding the effects of perturbation of the facial triad ligation of the non-heme ferrous enzymes on their geometric and electronic structure and also their contributions to reactivity.

The article describes a new subclass of [alpha]-ketoglutarate ([alpha]-KG)-dependent oxygenases, which do not contain the conventional facial triad. These compounds are found to exhibit novel reactivity and excellent catalytic activity.

A study was conducted on N694G sLO-1 to probe its contribution to Asn694's weak sixth ligand that is thought to be critical for enzymatic catalysis in wild-type soybean LO-1 (WT sLO-1). It as proposed on the basis of kinetic and structural factors that the Asn694 water-derived ligand could likely be a hydroxide and the active site, water-derived ligand a water in the ferric state, hence lowering the reaction rate of N694G more than would be expected from the lowered reduction potential alone.

A study was conducted, with circular dichroism (CD) and magnetic circular dichroism (MCD), to examine the resting Fe site of (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) and its interaction with (4-hydroxyphenyl)pyrvate (HPP) substrate. The result provides the insight into the mechanism of oxygen binding and activation by HPPD, and its active-site structural relationship to both alpha-KG-dependent dioxygenases and extradiol class of mononuclear non-heme iron enzymes.