Low-valent Iron Research Articles

Stabilization of Low-Valent Iron(I) in a High-Valent Vanadium(V) Oxide Cluster.

Low-valent iron centers are critical intermediates in chemical and bio-chemical processes. Herein, we show the first example of a low-valent FeI center stabilized in a high-valent polyoxometalate framework. Electrochemical studies show that the FeIII -functionalized molecular vanadium(V) oxide (DMA)[FeIII ClVV12 O32 Cl]3- (DMA=dimethylammonium) features two well-defined, reversible, iron-based electrochemical reductions which cleanly yield the FeI species (DMA)[FeI ClVV12 O32 Cl]5- . Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the FeI species. The study presents the first example for the seemingly paradoxical embedding of low-valent metal species in high-valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts.

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO2 drive our efforts to discover ways of converting excess CO2 into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO2 requires a fundamental understanding of how to generate metal hydrides that selectively donate H- to CO2, rather than recombining with H+ to liberate H2. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO2. Of particular interest are the nitride- and carbide-containing clusters: [Fe4N(CO)12]- and its derivatives and [Fe4C(CO)12]2-. In both aqueous and mixed solvent conditions, [Fe4N(CO)12]- forms a reduced hydride intermediate, [H-Fe4N(CO)12]-, through stepwise electron and proton transfers. This hydride selectively reacts with CO2 and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe4N(CO)12]- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO2; only H2 is observed in this case. In contrast, isoelectronic [Fe4C(CO)12]2- features a concerted proton-electron transfer mechanism to form [H-Fe4C(CO)12]2-, which is selective for H2 production even in the presence of CO2, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe4N(CO)12]- and its derivative [H-Fe4N(CO)11(PPh3)]- have hydricities modest enough to avoid H2 production but strong enough to make formate. [H-Fe4C(CO)12]2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H+, only H2 is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol-1 where C-H bond formation may be favored in water or MeCN, respectively.

Iron-Catalyzed Olefin Metathesis with Low-Valent Iron Alkylidenes.

Inspired by recent reports of low-valent iron-complex-catalyzed formal [2+2] cycloaddition of olefins, we demonstrate computationally that with such low-valent iron complexes and with "strong" ligands, the olefin metathesis is also preferred over the undesired cyclopropanation side-reaction, competition already studied by Hoffmann and co-workers almost 40 years ago (J. Am. Chem. Soc. 1981, 103, 5582). The [2+2] cycloaddition step in metathesis propagation, which gives a Chauvin-type metallacyclobutane intermediate, is proposed to proceed either via a planar four-electron Craig-Möbius aromatic [π2s +π2s ] transition-state structure with a low barrier of 4.7 kcal mol-1 or, alternatively, via a twisted Zimmerman-Möbius aromatic [π2s +π2a ] transition state with a 5.5 kcal mol-1 activation-energy barrier, with respect to an "encounter" π-complex minimum obtained from an FeII alkylidene and the entering olefin, while the corresponding triplet pathways are all disfavored.

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes.

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron β-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

Structural Insights into the Nature of Fe0 and FeI Low-Valent Species Obtained upon the Reduction of Iron Salts by Aryl Grignard Reagents.

Mechanistic studies of the reduction of FeIII and FeII salts by aryl Grignard reagents in toluene/tetrahydrofuran mixtures in the absence of a supporting ligand, as well as structural insights regarding the nature of the low-valent iron species obtained at the end of this reduction process, are reported. It is shown that several reduction pathways can be followed, depending on the starting iron precursor. We demonstrate, moreover, that these pathways lead to a mixture of Fe0 and FeI complexes regardless of the nature of the precursor. Mössbauer and 1H NMR spectroscopies suggest that diamagnetic 16-electron bisarene complexes such as (η4-C6H5Me)2Fe0 can be formed as major species (85% of the overall iron quantity). The formation of a η6-arene-ligated low-spin FeI complex as a minor species (accounting for ca. 15% of the overall iron quantity) is attested by Mössbauer spectroscopy, as well as by continuous-wave electron paramagnetic resonance (EPR) and pulsed-EPR (HYSCORE) spectroscopies. The nature of the FeI coordination sphere is discussed by means of isotopic labeling experiments and density functional theory calculations. It is shown that the most likely low-spin FeI candidate obtained in these systems is a diphenylarene-stabilized species [(η6-C6H5Me)FeIPh2]- exhibiting an idealized C2v topology. This enlightens the nature of the lowest valence states accommodated by iron during the reduction of FeIII and FeII salts by aryl Grignard reagents in the absence of any additional coligand, which so far remained rather unknown. The reactivity of these low-valent FeI and Fe0 complexes in aryl-heteroaryl Kumada cross-coupling conditions has also been investigated, and it is shown that the zerovalent Fe0 species can be used efficiently as a precursor in this reaction, whereas the FeI oxidation state does not exhibit any reactivity.

Single-Electron-Transfer Strategy for Reductive Radical Cyclization: Fe(CO)5 and Phenanthroline System.

An electron-transfer strategy using low-valent iron pentacarbonyl [Fe(CO)5] to generate radical species from alkyl iodides was achieved. A range of pyrrolidines, tetrahydrofurans, and carbocycles were synthesized via 5-exo cyclization reactions of alkyl radical intermediates generated by electron transfer from a system involving Fe(CO)5, 1,10-phenanthroline, and diisopropylamine. Moreover, tandem addition reactions with Michael acceptors were also explored. Photophysical and electrochemical studies support a mechanism that involves electron transfer from the low-valent Fe reductant to alkyl iodide.

Mechanistic Insight Into High-Spin Iron(I)-Catalyzed Butadiene Dimerization

Iron complexes are commonly used in catalysis, but the identity of the active catalyst is often unknown, which prevents a detailed understanding of structure–reactivity relationships for catalyst design. Here we report the isolation and electronic structure determination of a well-defined, low-valent iron complex that is an active catalyst in the synthesis of cis,cis-1,5-cyclooctadiene (COD) from 1,3-butadiene. Spectroscopic and magnetic characterization establishes a high-spin Fe(I) center, which is supported by DFT studies, where partial metal–ligand antibonding orbital population is proposed to allow for facile ligand exchange during catalysis.

High-Spin Iron(I) and Iron(0) Dinitrogen Complexes Supported by N-Heterocyclic Carbene Ligands.

The use of 1,3-dicyclohexylimidazol-2-ylidene (ICy) as ligand has enabled the preparation of the high-spin tetrahedral iron(I)- and iron(0)-N2 complexes, namely [(ICy)3 Fe(N2 )][BPh4 ] (1) and [(ICy)3 Fe(N2 )] (2), the electronic structures of which have been established by various spectroscopic characterization and DFT calculations. The frequency of the N-N stretching resonance of the iron(0)-N2 complex is the lowest among the reported terminal N2 complexes of iron, signifying the beneficial roles of strongly σ-donating ligands in combination with the high-spin low-valent iron center in promoting N2 -activation. The iron(0)-N2 complex can convert reversibly to the low-spin iron(II)-N2 hydride complex [(ICy)2 (ICy')Fe(N2 )(H)] (4).

A Bidentate Carbene Ligand Stabilizes a Low-Coordinate Iron(0) Carbonyl Complex

The bulky bis(carbene)borate ligand Ph2B(tBuIm)2– stabilizes low-coordinate and low-valent iron complexes. The four-coordinate iron(II) complex Ph2B(tBuIm)2FeCl(THF) is a precursor to the low-spin iron(I) complex Ph2B(tBuIm)2Fe(CO)3. Despite a reversible reduction wave in the cyclic voltammogram, access to the iron(0) complex [Ph2B(tBuIm)2Fe(CO)3]− requires reduction under a CO atmosphere. Without excess CO, KC8 reduction leads to formation of the dimer K2[Ph2B(tBuIm)2Fe(CO)2] along with loss of CO. Each iron center in this complex adopts a distorted-square-planar geometry. Spectroscopic analysis of these carbonyl complexes reveals the very high strength of the bis(carbene)borate ligand.

Iron-Catalyzed C-H Functionalization Processes.

Iron-catalyzed C-H activation has recently emerged as an increasingly powerful tool for the step-economical transformation of unreactive C-H bonds. Particularly, the recent development of low-valent iron catalysis has set the stage for novel C-H activation strategies via chelation assistance. The low-cost, natural abundance, and low toxicity of iron prompted its very recent application in organometallic C-H activation catalysis. An overview of the use of iron catalysis in C-H activation processes is summarized herein up to May 2016.

A Tertiary Carbon–Iron Bond as an FeICl Synthon and the Reductive Alkylation of Diphosphine-Supported Iron(II) Chloride Complexes to Low-Valent Iron

Ligand-induced reduction of ferrous alkyl complexes via homolytic cleavage of the alkyl fragment was explored with simple chelating diphosphines. The reactivities of the sodium salts of diphenylmethane, phenyl(trimethylsilyl)methane, or diphenyl(trimethylsilyl)methane were explored in their reactivity with (py)4FeCl2. A series of monoalkylated salts of the type (py)2FeRCl were prepared and characterized from the addition of 1 equiv of the corresponding alkyl sodium species. These complexes are isostructural and have similar magnetic properties. The double alkylation of (py)4FeCl2 resulted in the formation of tetrahedral high-spin iron complexes with the sodium salts of diphenylmethane and phenyl(trimethylsilyl)methane that readily decomposed. A bis(cyclohexadienyl) sandwich complex was formed with the addition of 2 equiv of the tertiary alkyl species sodium diphenyl(trimethylsilyl)methane. The addition of chelating phosphines to (py)2FeRCl resulted in the overall transfer of Fe(I) chloride concurrent with...

Electron-donating capabilities and evidence for redox activity in low oxidation state iron complexes bearing bis(amidine)pyrimidylidene ligands

Low-valent iron complexes bearing bis(amidine)pyrimidylidene (or carbenodiamidine, CDA) ligands have been synthesized and characterized by X-ray crystallography, Mössbauer spectroscopy, infrared spectroscopy, EPR spectroscopy, and DFT calculations. One electron reduction of (CDA)FeCl2 (1) resulted in the formation of (CDA)FeCl (2), which demonstrated an unusual metal−N-heterocyclic carbene (NHC) interaction where a redox non-innocent NHC participates in a one electron π-interaction with the metal center. This interaction renders the complex best described as a low-spin iron(II) complex that is antiferromagnetically coupled to a singly reduced CDA ligand. Two electron reduction of 1 under an atmosphere of carbon monoxide resulted in the formation of the diamagnetic complex (CDA)Fe(CO)2 (3). Complex 3 demonstrated significantly lower CO stretching frequencies compared to previously reported bis(imino)pyridine and even bis(N-heterocyclic carbene)pyridine iron complexes, which reflects the significant σ-donating capabilities of the CDA ligands.

Two-State Reactivity in Low-Valent Iron-Mediated C-H Activation and the Implications for Other First-Row Transition Metals.

C-H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C-H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C-H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C-C coupling and finalizing the C-H functionalization. Regeneration of the Fe(II) catalyst for the next round of C-H activation involves SET oxidation of the Fe(I) species generated after the C-C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C-H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C-H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C-H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C-H activation reactions.

Theoretical investigation on the mechanism of iron catalyzed cross coupling reactions via ferrate intermediates

Iron is a versatile catalyst for cross coupling reactions. These reactions may proceed either via classical redox cycles involving low-valent iron species or via highly alkylated organoferrate complexes. Experimentally, it is difficult to trap reactive intermediates, but it has been possible to prepare iron complexes similar to the supposed active catalyst that are able to methylate activated electrophiles (J. Am. Chem. Soc. 130 (2008) 8773–8787). Motivated by these experiments we studied the methylation of 4-chlorobenzoyl chloride by the organoferrate complex [(Me)4 Fe(MeLi)][Li(OEt2)]2 employing density functional theory at the OPBE/6-311+G** level, as well as B3LYP/6-311+G** calculations with explicit inclusion of dispersion and solvent effects (describing iron with the QZVP basis and SDD pseudopotential). In the preferred mechanism, methyl transfer takes place via substitution at the organoferrate complex, with the leaving methyl group being replaced by chloride. In line with the experimental findings, up to four methyl groups can be transferred in this manner. By locating all conceivable transition states and intermediates, the calculations shed light on the relative ease of substitution at the various positions of the organoferrate complex, both in the first and subsequent methyl tranfers. Transition states for an alternative redox mechanism could not be located.

ChemInform Abstract: 1,4‐Functionalization of 1,3‐Dienes With Low‐Valent Iron Catalysts

AbstractReview: 65 refs.

Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

A bulky tris(carbene)borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron(II) complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) ([PhB(MesIm3)]− = phenyltris(1-mesitylimidazol-2-ylidene)borate) yields the low-spin (S = 1/2) iron(I) complex PhB(MesIm)3Fe(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB(MesIm)3Fe(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) with CO. The strong donor ability of the tris(carbene)borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.

Iron(II) Active Species in Iron-Bisphosphine Catalyzed Kumada and Suzuki-Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides.

While previous studies have identified FeMes2(SciOPP) as the active catalyst species in iron–SciOPP catalyzed Kumada cross-coupling of mesitylmagnesium bromide and primary alkyl halides, the active catalyst species in cross-couplings with phenyl nucleophiles, where low valent iron species might be prevalent due to accessible reductive elimination pathways, remains undefined. In the present study, in situ Mössbauer and magnetic circular dichroism spectroscopic studies combined with inorganic syntheses and reaction studies are employed to evaluate the in situ formed iron species and identify the active catalytic species in iron–SciOPP catalyzed Suzuki–Miyaura and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl halides. While reductive elimination to form Fe(η6-biphenyl)(SciOPP) occurs upon reaction of FeCl2(SciOPP) with phenyl nucleophiles, this iron(0) species is not found to be kinetically competent for catalysis. Importantly, mono- and bis-phenylated iron(II)–SciOPP species that form prior to reductive elimination are identified, where both species are found to be reactive toward electrophile at catalytically relevant rates. The higher selectivity toward the formation of cross-coupled product observed for the monophenylated species combined with the undertransmetalated nature of the in situ iron species in both Kumada and Suzuki–Miyaura reactions indicates that Fe(Ph)X(SciOPP) (X = Br, Cl) is the predominant reactive species in cross-coupling. Overall, these studies demonstrate that low-valent iron is not required for the generation of highly reactive species for effective aryl-alkyl cross-couplings.

1,4-Functionalization of 1,3-Dienes With Low-Valent Iron Catalysts

Iron-catalyzed or -mediated transformations of organic substrates have been important throughout the development of organic chemistry due to iron's abundance, low cost, and favorable toxicity profile. Highly reduced iron species, although difficult to isolate and characterize, have proven valuable as catalysts for a variety of C-C and C-heteroatom bond forming processes, as well as cyclization and cycloisomerization reactions. We have developed iminopyridine-ligated low-valent iron catalysts that facilitate selective 1,4-hydrovinylation, hydoboration, hydrosilylation, and polymerization of 1,3-dienes. The catalysts are generated in situ from iron(II) precursors in the presence of activated magnesium metal or trialkylaluminum reductants. The 1,4-addition processes provide access to valuable products such as 1,4-dienes, allylboronic esters, allylsilanes, and highly regioregular polyisoprene. In these transformations, addition is stereoselective, providing (E)-alkene isomers selectively, and (1,2)-addition products are generally not observed. Moreover, modification of steric bulk on the iminopyridine ligand can be used to change selectivity for (1,4)- versus (4,1)-addition to dienes with nonsymmetric substitution. Access to low-valent iron precursor complexes is limited, and we have developed a diaryliron(II) precursor that undergoes smooth reductive elimination in the presence of iminopyridine ligands to provide easy access to low-valent iron catalysts without the use of heterogeneous reductants, which complicate the isolation and study of low-valent iron complexes. We obtained crystal structures of our iron(II) catalyst precursor and an iminopyridine-ligated reduced iron species generated from it. Spectroscopic analysis suggests that although this species is formally iron(0), the redox-active iminopyridine ligands accept electron density from the metal and the complex is more properly formulated as iron(II) coordinated by two radical-anion ligands. We believe that a closely-related set of reaction manifolds is responsible for the 1,4-functionalization reactivity displayed by the iron(iminopyridine) complexes (see text). Kinetics experiments and deuterium-labeling studies provide evidence for the proposed catalytic cycle. The geometry of the double bond remaining after 1,4-addition is set by the requirement that the diene bind to the iron center in an s-cis geometry, and the regioselectivity of addition can be rationalized by the location of steric bulk on the iminopyridine ligand. The transformations presented in this Account utilize iron catalysts to provide access to valuable diene 1,4-addition products such as 1,4-dienes, allylboronate esters, and allylsilanes, as well as highly regioregular polyisoprene. The development of a stable diaryliron(II) precatalyst, structural characterization of an iminopyridine-ligated iron(0) complex, and mechanistic insights into the selective nature of this transformation provide a window into the reactivity profile of low-valent iron.

Nickel and iron pincer complexes as catalysts for the reduction of carbonyl compounds.

The reductions of aldehydes, ketones, and esters to alcohols are important processes for the synthesis of chemicals that are vital to our daily life, and the reduction of CO2 to methanol is expected to provide key technology for carbon management and energy storage in our future. Catalysts that affect the reduction of carbonyl compounds often contain ruthenium, osmium, or other precious metals. The high and fluctuating price, and the limited availability of these metals, calls for efforts to develop catalysts based on more abundant and less expensive first-row transition metals, such as nickel and iron. The challenge, however, is to identify ligand systems that can increase the thermal stability of the catalysts, enhance their reactivity, and bypass the one-electron pathways that are commonly observed for first-row transition metal complexes. Although many other strategies exist, this Account describes how we have utilized pincer ligands along with other ancillary ligands to accomplish these goals. The bis(phosphinite)-based pincer ligands (also known as POCOP-pincer ligands) create well-defined nickel hydride complexes as efficient catalysts for the hydrosilylation of aldehydes and ketones and the hydroboration of CO2 to methanol derivatives. The hydride ligands in these complexes are substantially nucleophilic, largely due to the enhancement by the strongly trans-influencing aryl groups. Under the same principle, the pincer-ligated nickel cyanomethyl complexes exhibit remarkably high activity (turnover numbers up to 82,000) for catalytically activating acetonitrile and the addition of H-CH2CN across the C═O bonds of aldehydes without requiring a base additive. Cyclometalation of bis(phosphinite)-based pincer ligands with low-valent iron species "Fe(PR3)4" results in diamagnetic Fe(II) hydride complexes, which are active catalysts for the hydrosilylation of aldehydes and ketones. Mechanistic investigation suggests that the hydride ligand is not delivered to the carbonyl substrates but is important to facilitate ligand dissociation prior to substrate activation. In the presence of CO, the amine-bis(phosphine)-based pincer ligands are also able to stabilize low-spin Fe(II) species. Iron dihydride complexes supported by these ligands are bifunctional as both the FeH and NH moieties participate in the reduction of C═O bonds. These iron pincer complexes are among the first iron-based catalysts for the hydrogenation of esters, including fatty acid methyl esters, which find broad applications in industry. Our studies demonstrate that pincer ligands are promising candidates for promoting the first-row transition metal-catalyzed reduction of carbonyl compounds with high efficiency. Further efforts in this research area are likely to lead to more efficient and practical catalysts.

Low-valent iron: an Fe(I) ate compound as a building block for a linear trinuclear Fe cluster.

A low-valent trinuclear iron complex with an unusual linear Fe(I)-Fe(II)-Fe(I) unit is presented. It is accessed in a rational approach using a salt metathesis reaction between a new anionic Fe(I) containing heterocycle and FeCl2. Its electronic structure was studied by single crystal XRD analysis, EPR and Mössbauer spectroscopy, and magnetic susceptibility measurements.