The Effect of β‐Hydrogen Atoms on Iron Speciation in Cross‐Couplings with Simple Iron Salts and Alkyl Grignard Reagents

The effects of β‐hydrogen‐containing alkyl Grignard reagents in simple ferric salt cross‐couplings have been elucidated. The reaction of FeCl₃ with EtMgBr in THF leads to the formation of the cluster species [Fe₈Et₁₂]^2?, a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross‐coupling with simple ferric salts and β‐hydrogen‐containing alkyl nucleophiles, result in the formation of [FeEt₃]^?. Reactivity studies demonstrate the effectiveness of [FeEt₃]^? in rapidly and selectively forming the cross‐coupled product upon reaction with electrophiles. The identification of iron‐ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.     

The N‐Methylpyrrolidone (NMP) Effect in Iron‐Catalyzed Cross‐Coupling with Simple Ferric Salts and MeMgBr

The use of N‐methylpyrrolidone (NMP) as a co‐solvent in ferric salt catalyzed cross‐coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross‐coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)₆][FeMe₃]₂ ( 1 ), which forms as the major iron species in situ in reactions of Fe(acac)₃ and MeMgBr under catalytically relevant conditions where NMP is employed as a co‐solvent. Importantly, combined GC analysis and ⁵⁷Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross‐coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe₈Me₁₂]^? cluster generation, which occurs in the absence of NMP.     

Stability and Unimolecular Reactivity of Palladate(II) Complexes [LnPdR3]− (L=Phosphine,R=Organyl,n=0 and 1)

The reduction of Pd^II precatalysts to catalytically active Pd⁰ species is a key step in many palladium‐mediated cross‐coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)₂ and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray‐ ionization mass spectrometry found palladate(II) complexes [L_nPdR₃]^? (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas‐phase fragmentation, the [L_nPdR₃]^? anions preferentially underwent a reductive elimination to yield Pd⁰ species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)₂ precatalyst. Other species of interest observed include the Pd^IV complex [PdBn₅]^?, which did not fragment via a reductive elimination but lost BnH instead.     

Activation of Aryl and Heteroaryl Halides by an Iron(I) Complex Generated in the Reduction of [Fe(acac)3] by PhMgBr: Electron Transfer versus Oxidative Addition

The mechanism of the reactions of aryl/heteroaryl halides with aryl Grignard reagents catalyzed by [Fe^III(acac)₃] (acac=acetylacetonate) has been investigated. It is shown that in the presence of excess PhMgBr, [Fe^III(acac)₃] affords two reduced complexes: [PhFe^II(acac)(thf)_n] (n=1 or 2) (characterized by ¹H NMR and cyclic voltammetry) and [PhFe^I(acac)(thf)]^? (characterized by cyclic voltammetry, ¹H NMR, EPR and DFT). Whereas [PhFe^II(acac)(thf)_n] does not react with any of the investigated aryl or heteroaryl halides, the Fe^I complex [PhFe^I(acac)(thf)]^? reacts with ArX (Ar=Ph, 4‐tolyl; X=I, Br) through an inner‐sphere monoelectronic reduction (promoted by halogen bonding) to afford the corresponding arene ArH together with the Grignard homocoupling product PhPh. In contrast, [PhFe^I(acac)(thf)]^? reacts with a heteroaryl chloride (2‐chloropyridine) to afford the cross‐coupling product (2‐phenylpyridine) through an oxidative addition/reductive elimination sequence. The mechanism of the reaction of [PhFe^I(acac)(thf)]^? with the aryl and heteroaryl halides has been explored on the basis of DFT calculations.     

The Electronic Ground State of [Fe(CO)3(NO)]−: A Spectroscopic and Theoretical Study

During the past 10 years iron‐catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)₃(NO)]^?, which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)₄]^2?, which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)₃(NO)]^? cannot be regarded as a Fe^?II species, but rather is predominantly a Fe⁰ species, in which the metal is covalently bonded to NO^? by two π‐bonds. A metal–N σ‐bond is not observed.     

Bestimmung von Gleichgewichtskonstanten für Ligandenaustauschreaktionen von [TcNX4]− (X = Cl,Br) mit Hilfe der EPR

Polynuclear Complexes with Fe? As, Fe? Sb, and Fe? Bi Frameworks The anionic iron clusters Fe₃(CO)₁₁^2? and Fe₄(CO)₁₃^2? were reacted with compounds EX₃ and with organic derivatives REX₂ and R₂EX of the elements arsenic, antimony, and bismuth. Commonly redox and cluster degradation reactions were observed. The new complexes [(CO)₄Fe? AsMe₂? Fe(CO)₄]^?, [HFe₃(CO)₉(mu;₃-SbBu^t)]^?, [Fe₃(CO)₁₀ (mu;₃-Sb)]^?, and [Fe₃(CO)₁₀(mu;₃-Bi)]^? were formed and isolated as their PPN salts. The Fe? As? Fe complex was identified by a structure determination, the other complexes were identified by their spectra.     

Molekülstruktur und thermische Stabilität des metallacyclischen Platin(II)‐Komplexes [Li(TMEDA)]2Pt(CH2CMe2CMe2CH2)2

Molecular Structure and Thermal Stability of the Metallacyclic Platinum(II) Complex [Li(TMEDA)]₂Pt(CH₂CMe₂CMe₂CH₂)₂ The X‐ray investigation at the “ate‐complex” [Li(TMEDA)]₂Pt(CH₂CMe₂CMe₂CH₂)₂ ( 1 ) revealed a new structure type of homoleptic organometallic compounds of platinum(II). Differences of the molecular structure of the “ate‐complex” [Li(TMEDA)]₂Pt(CH₂CH₂CH₂CH₂)₂ ( 2 ) as well as similarities to the structure of the homologeous “ate‐complex” of nickel(II) [Li(TMEDA)]₂Ni(CH₂CMe₂CMe₂CH₂)₂ ( 3 ) are described. A possible mechanism of the thermal decomposition of the complex 1 is discussed.     

Complexation kinetics of Fe2+ with 1,10‐phenanthroline forming ferroin in acidic solutions

The formation kinetics of ferroin is studied under varied acid conditions at 25°C and fixed ionic strength (0.48 mol dm^?3) under pseudo‐first‐order conditions with respect to Fe²⁺ by using the stopped‐flow technique. The reaction followed is first and third order with respect to Fe²⁺ and 1,10‐phenanthroline (phen)_T, respectively. Increasing the acid concentration retarded the reaction, and the reaction rate showed a positive salt effect. The rate‐limiting step involved the complexation of the phen or protonated phen with [Fe(phen)₂]²⁺ complex ion, leading to formation of [Fe(phen)₃]²⁺ ion. The observed retardation of the reaction rate with increasing [H⁺]₀ is due to the increased [phenH⁺]_eq and low reactivity of phenH⁺ with [Fe(phen)₂]²⁺ complex ion. Simulated curves for the acid variation experiments agreed well with the corresponding experimental curves and the estimated rate coefficients supporting the proposed mechanism. Relatively low energy of activation (26 kJ mol^?1) and high negative entropy of activation (?159.8 J K^?1 mol^?1) agree with the proposed mechanism and the formation of compact octahedral complex ion. © 2008 Wiley Periodicals, Inc. 40: 515–523, 2008     

Kinetic Study of the Reduction of Bromate Ion by Tris(1,10-Phenanthroline)Iron(II) and Aquoiron(II) Ions — Implication for the Bromate-Gallic Acid Oscillating Reaction

The kinetics of the bromate oxidation of tris(1,10-phenanthroline)iron(II) (Fe(phen)₃²⁺) and aquoiron(II) (Fe²⁺ (aq)) have been studied in aqueous sulfuric acid solutions at μ = 1.0M and with Fe(II) complexes in great excess. The rate laws for both reactions generally can be described as -d [Fe(II)]/6dt = d[Br^?]/dt = k[Fe(II)] [BrO^?₃] for [H⁺]₀ = 0.428–1.00M. For [BrO^?₃]₀ = 1.00 × 10^?4M. [Fe²⁺]₀ = (0.724–1.45)x 10^?2 M, and [H⁺]₀ = 1.00M, k = 3.34 ± 0.37 M^?1s^?1 at 25°. For [BrO^?₃]₀ = (1.00–1.50) × 10^?4M, [Fe²⁺]₀ = 7.24 × 10^?3M ([phen]₀ = 0.0353M), and [H⁺]₀ = 1.00M, k = (4.40 ± 0.16) × 10^?2 M^?1s^?1 at 25°. Kinetic results suggest that the BrO^?₃-Fe²⁺ reaction proceeds by an inner-sphere mechanism while the BrO^?₃-Fe(phen)₃²⁺ reaction by a dissociative mechanism. The implication of these results for the bromate-gallic acid and other bromate oscillators is also presented.     

Investigation on Sonocatalytic Damage to Bovine Serum Albumin (BSA) Under Ultrasonic Irradiation in the Presence of Fe(III)–Citrate Complexes

The interaction of bovine serum albumin (BSA) with Fe(III)?Ccitrate complexes ([Fe^III(cit)(H₂O)₃]^? and [Fe^III(cit)₂]^5?) and the sonocatalytic damage of BSA under ultrasonic irradiation were studied. Additionally, the various factors influencing the sonocatalytic damage of BSA were also studied by means of UV?CVis and fluorescence spectra. The experimental results indicate that the probable fluorescence quenching mechanisms of BSA by Fe(III)?Ccitrate complexes ([Fe^III(cit)(H₂O)₃]^? and [Fe^III(cit)₂]^5?) are both static quenching. Under certain conditions, the degree of damage to BSA is aggravated with increases of ultrasonic irradiation time, Fe(III)?Ccitrate complex concentration, pH value and ionic strength. Moreover, all of the results demonstrate that [Fe^III(cit)₂]^5? displays higher sonocatalytic activity than [Fe^III(cit)(H₂O)₃]^? under the same experimental conditions during the damage process of BSA. Finally, the generation of ·O₂ ^? and ·OH during sonocatalytic processes was estimated using scavengers. Perhaps, the results will be significant for promoting sonodynamic treatment to treat tumors at the molecular level.     

Electroswitchable binding of [Co(dipy)<Subscript>3</Subscript>]<Superscript>3+</Superscript> and [Fe(dipy)<Subscript>3</Subscript>]<Superscript>2+</Superscript><Emphasis Type="Italic">n</Emphasis>-sulfonato(thia)calix[4]arenas

The methods of cyclic voltammetry, electrolysis, and spectrophotometry were used to study electrochemical properties of (TCAS + Fe³⁺ + dipy), (CCAS + Fe³⁺ + dipy), and (CCAS + Fe³⁺ + [Co(dipy)₃]³⁺) triple systems (where TCAS is n-sulfonatothiacalix[4]arene, CCAS is tetracarboxylate n-sulfonatocalix[4]arene, and dipy = α,α′-dipyridyl) in an aqueous solution. One-electron reduction of Fe(III) in the (TCAS + Fe³⁺ + dipy) system at pH 2.5 results in electroswitching of iron ions from the lower TCAS ring to the upper ([Fe(dipy)₃]²⁺). Reverse electrochemical switching of the system is impossible due to mediator ([Fe(dipy)₃]^2+/3+) oxidation of TCAS. Reverse electroswitching of Fe(III) ions from unbound to bound state as ([Fe(dipy)₃]²⁺) with CCAS has been revealed in the system (CCAS + Fe³⁺ + dipy) (pH 1.7) upon single-electron transfer, whereas reversible electroswitching by the upper rim of CCAS from one complex ion ([Co(dipy)₃]³⁺) to another ([Fe(dipy)₃]²⁺) has been demonstrated in the system ([Co(dipy)₃]³⁺ + CCAS + Fe³⁺ upon double-electron transfer. In all systems, electric switching was accompanied by synchronous color switching.     

Evolution of Ate-Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors

Ate-iron(II) species such as [Ar₃Fe^II]⁻ (Ar=aryl) are key intermediates in Fe-catalyzed couplings between aryl nucleophiles and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe⁰ and Fe^I oxidation states, which can themselves be catalytically active or lead to unwished organic byproducts. Analysis of the reactivity of the intermediates obtained by step-by-step displacement of the mesityl groups in high-spin [Mes₃Fe^II]⁻ by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe⁰ and Fe^I oxidation states. The formation of quaternized [Ar₄Fe^IIMgBr(THF)]⁻ intermediates allows the bielectronic reductive elimination energy required for the formation of Fe⁰ to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar₃Fe^II]⁻ enables the formation of aryl-bridged [{Fe^II(Ar)₂}₂(μ-Ar)₂]²⁻ species, which afford the Fe^I oxidation state by bimetallic reductive elimination. These results are supported by ¹H NMR, EPR, and ⁵⁷Fe Mössbauer spectroscopies, as well as by DFT calculations.     

Low‐Temperature N2 Binding to Two‐Coordinate L2Fe0 Enables Reductive Trapping of L2FeN2− and NH3 Generation

The two‐coordinate [(CAAC)₂Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T2 complex, [(CAAC)₂Fe(N₂)], was trapped by one‐electron reduction to its corresponding anion [(CAAC)₂FeN₂]^? at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β‐nitrogen atom. The redox‐linked complexes [(CAAC)₂Fe^I][BAr^F₄], [(CAAC)₂Fe⁰], and [(CAAC)₂Fe^?IN₂]^? were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC₈ and HBAr^F₄?2 Et₂O at ?95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N₂ reduction activity is highly temperature dependent, with significant N₂ reduction to NH₃ only occurring below ?78 °C. This reactivity profile tracks with the low temperatures needed for N₂ binding and an otherwise unavailable electron‐transfer step to generate reactive [(CAAC)₂FeN₂]^?.     

Reactions of bis-μ-iodo-bis(dinitrosyliron) with halide ions and with thiols: An electron paramagnetic resonance study

Solutions of Fe₂(NO)₄I₂ in DMF exhibit EPR spectra characteristic of [Fe(NO)₂]⁺ at concentrations of 2 x 10^?4 mol dm^?3, and of an equilibrium mixture of [Fe(NO)₂⁺, Fe(NO)₂I, and [Fe(NO)₂I₂]^? at higher concentrations: in THF solutions only Fe(NO)₂I is observed, regardless of concentration. Addition of excess halide ions X^? (X=Cl, Br, I) to the DMF solution yields [Fe(NO)₂X₂]^?, but addition of excess I^? or Br^? to the THF solution yields [Fe(NO)₂I₂^? or Fe₂(NO)₄Br₂ respectively. In mixed THF/Et₃N solutions, mixtures of [Fe(NO)₂]⁺, Fe(NO)₂I, and [Fe(NO)₂I₂]^? are again formed, and subsequent addition of a thiol RSH causes formation of [Fe(NO)₂(SR)₂]^?, a precursor of Fe₂(NO)₄(SR)₂. A scheme is suggested to describe the steps in the preparatively useful conversion of Fe₂(NO)₄I₂ into Fe₂(NO)₄(SR)₂.     

Correlations and Contrasts in Homo‐ and Heteroleptic Cyclic (Alkyl)(amino)carbene‐Containing Pt0 Complexes

An improved synthetic route to homoleptic complex [Pt(CAAC^Me)₂] (CAAC=cyclic (alkyl)(amino)carbenes) and convenient routes to new heteroleptic complexes of the form [Pt(CAAC^Me)(PR₃)] are presented. Although the homoleptic complex was found to be inert to many reagents, oxidative addition and metal‐only Lewis pair (MOLP) formation was observed from one of the heteroleptic complexes. The spectroscopic, structural, and electrochemical properties of the zero‐valent complexes were explored in concert with density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) calculations. The homoleptic [Pt(CAAC)₂] and heteroleptic [Pt(CAAC)(PR₃)] complexes were found to be similar in their spectroscopic and structural properties, but their electrochemical behavior and reactivity differ greatly. The unusually strong color of the CAAC‐containing Pt⁰ complexes was investigated by TD‐DFT calculations and attributed to excitations into the LUMOs of the complexes, which are predominantly composed of bonding π interactions between Pt and the CAAC carbon atoms.     

The Role of Ate Complexes in the Copper‐Mediated Trifluoromethylation of Alkynes

Trifluoromethylation reactions have recently received increased attention because of the beneficial effect of the trifluoromethyl group on the pharmacological properties of numerous substances. A common method to introduce the trifluoromethyl group employs the Ruppert–Prakash reagent, that is, Si(CH₃)₃CF₃, together with a copper(I) halide. We have applied this method to the trifluoromethylation of aromatic alkynes and used electrospray‐ionization mass spectrometry to investigate the mechanism of these reactions in tetrahydrofuran, dichloromethane, and acetonitrile as well as with and without added 1,10‐phenanthroline. In the absence of the alkyne component, the homoleptic ate complexes [Cu(CF₃)₂]^? and [Cu(CF₃)₄]^? were observed. In the presence of the alkynes RH, the heteroleptic complexes [Cu(CF₃)₃R]^? were detected as well. Upon gas‐phase fragmentation, these key intermediates released the cross‐coupling products R?CF₃ with perfect selectivity. Apparently, the [Cu(CF₃)₃R]^? complexes did not originate from homoleptic cuprate anions, but from unobservable neutral precursors. The present results moreover point to the involvement of oxygen as the oxidizing agent.     

Poly­[iron(II)‐di‐μ‐imidazole‐4,5‐di­carboxyl­ato‐κ3N3,O4:O5]

In the title compound, [Fe(C₅H₃N₂O₄)₂]_n, each Fe atom lies on a centre of symmetry, in an octahedral coordination environment consisting of two chelate rings [Fe—N = 2.154 (3) Å and Fe—O = 2.180 (3) Å] and two carboxyl­ate O atoms [Fe—O = 2.111 (2) Å] from imidazole‐4,5‐di­carboxyl­ate ligands. Extensive hydrogen‐bonding interactions exist between layers constructed of Fe₄ squares, forming tunnels along the a axis with large voids.     

C‐Selective and Diastereoselective Alkyl Addition to β,γ‐Alkynyl‐α‐imino Esters with Zinc(II)ate Complexes

Since umpolung α‐imino esters contain three electrophilic centers, regioselective alkyl addition with traditional organometallic reagents has been a serious problem in the practical synthesis of versatile chiral α‐amino acid derivatives. An unusual C‐alkyl addition to α‐imino esters using a Grignard reagent (RMgX)‐derived zinc(II)ate was developed. Zinc(II)ate complexes consist of a Lewis acidic [MgX]⁺ moiety, a nucleophilic [R₃Zn]^? moiety, and 2 [MgX₂]. Therefore, the ionically separated [R₃Zn]^? selectively attacks the imino carbon atom ,which is most strongly activated by chelation of [MgX]⁺. In particular, chiral β,γ‐alkynyl‐α‐imino esters can strongly promote highly regio‐ and diastereoselective C‐alkylation because of structural considerations, and the corresponding optically active α‐quaternary amino acid derivatives are obtained within 5 minutes in high to excellent yields.     

Synthesis and reactivity of the five-coordinate {Fe(NO)2} [(TMEDA)Fe(NO)2I]

The reaction of [Fe(μ-I)(NO)₂]₂ and TMEDA in a 1:2 molar ratio in THF affords the neutral five-coordinate DNIC [(TMEDA)Fe(NO)₂I] (1). The single-crystal X-ray structure shows that the geometry of iron center of complex 1 is best described as a distorted trigonal bipyramidal with two nitrosyl groups positioned in the equatorial plane. The EPR spectrum of complex 1 displays the six-line signal with g = 2.031 (a_I = 37.6 G) at 298 K. The coincident g values of EPR among complex 1, protein-bound DNICs and low-molecular-weight DNICs implicate that the five-coordinate DNICs may exist in biological system. The interconversion between complex 1 and [(TMEDA)Fe(NO)₂] (2) reveals that the {Fe(NO)₂}⁹ DNICs containing [amine, amine] ligation mode could be stabilized by the five-coordinated geometry while the {Fe(NO)₂}¹⁰ DNICs containing [amine, amine] ligation mode favors the four coordination sphere. In addition, the transformation from complex 1 to [Fe(NO)₂(C₃H₃N₂)]₄ (3), [Fe(μ-SPh)(NO)₂]₂ (4), [PPh₄][(PhS)₂Fe(NO)₂] (5) and [Na-18-crown-6-ether][(C₃H₃N₂)₂Fe(NO)₂] (6), respectively, in the presence of thiolates or imidazolates indicates that complex 1 could be employed as the precursor for the syntheses of the DNICs containing the [N,N]/[N,S]/[S,S] different ligations.     

Solvent‐Free Ball‐Milling Subcomponent Synthesis of Metallosupramolecular Complexes

Subcomponent self‐assembly from components A , B , C , D , and Fe²⁺ under solvent‐free conditions by self‐sorting leads to the construction of three structurally different metallosupramolecular iron(II) complexes. Under carefully selected ball‐milling conditions, tetranuclear [Fe₄( AD ₂)₆]^4? 22‐component cage 1 , dinuclear [Fe₂( BD ₂)₃]^2? 11‐component helicate 2 , and 5‐component mononuclear [Fe( CD ₃)]²⁺ complex 3 were prepared simultaneously in a one‐pot reaction from 38 components. Through subcomponent substitution reaction by adding subcomponent B , the [Fe₄( AD ₂)₆]^4? cage converts quantitatively to the [Fe₂( BD ₂)₃]^2? helicate, which, in turn, upon addition of subcomponent C , transforms to [Fe( CD ₃)]²⁺, following the hierarchical preference based on the thermodynamic stability of the complexes.