Iron Pincer Complexes Research Articles

Synthesis and Catalytic Property of Iron Pincer Complexes Generated by Csp3–H Activation

When the diphosphinito PCP ligand (Ph2P(C6H4))2CH2 (1) was treated with Fe(PMe3)4 and FeMe2(PMe3)4, the Csp3–H activation products [(Ph2P(C6H4))2CH]Fe(H)(PMe3)2 (2) and [(Ph2P(C6H4))(PhP(C6H4)2)CH]Fe(PMe3)2 (3) were obtained at room temperature. The generation of product 3 underwent one Csp3–H and one Csp2–H bond activation process. The new iron hydride complex 2 showed good activity in the catalytic hydrosilylation of aldehydes and ketones by using (EtO)3SiH as the hydrogen source under mild conditions. Complexes 2 and 3 were characterized by spectroscopic methods and X-ray diffraction analysis.

Mild and selective hydrogenation of aromatic and aliphatic (di)nitriles with a well-defined iron pincer complex.

The catalytic hydrogenation of carboxylic acid derivatives represents an atom-efficient and clean reduction methodology in organic chemistry. More specifically, the selective hydrogenation of nitriles offers the possibility for a green synthesis of valuable primary amines. So far, this transformation lacks of useful, broadly applicable non-noble metal-based catalyst systems. In the present study, we describe a molecular-defined iron complex, which allows for the hydrogenation of aryl, alkyl, heterocyclic nitriles and dinitriles. By using an iron PNP pincer complex, we achieve very good functional group tolerance. Ester, ether, acetamido as well as amino substituents are not reduced in the presence of nitriles. Moreover, nitriles including an α,β-unsaturated double bond and halogenated derivatives are well tolerated in this reaction. Notably, our complex constitutes the first example of an homogeneous catalyst, which permits the selective hydrogenation of industrially important adipodinitrile to 1,6-hexamethylenediamine.

ChemInform Abstract: Iron Pincer Complex Catalyzed, Environmentally Benign, E‐Selective Semi‐Hydrogenation of Alkynes.

ChemInform Abstract: Iron Pincer Complex Catalyzed, Environmentally Benign, E‐Selective Semi‐Hydrogenation of Alkynes.

Unprecedented Iron‐Catalyzed Ester Hydrogenation. Mild, Selective, and Efficient Hydrogenation of Trifluoroacetic Esters to Alcohols Catalyzed by an Iron Pincer Complex

The synthetically important, environmentally benign hydrogenation of esters to alcohols has been accomplished in recent years only with precious-metal-based catalysts. Here we present the first iron-catalyzed hydrogenation of esters to the corresponding alcohols, proceeding selectively and efficiently in the presence of an iron pincer catalyst under remarkably mild conditions.

Unprecedented Iron‐Catalyzed Ester Hydrogenation. Mild, Selective, and Efficient Hydrogenation of Trifluoroacetic Esters to Alcohols Catalyzed by an Iron Pincer Complex

AbstractThe synthetically important, environmentally benign hydrogenation of esters to alcohols has been accomplished in recent years only with precious‐metal‐based catalysts. Here we present the first iron‐catalyzed hydrogenation of esters to the corresponding alcohols, proceeding selectively and efficiently in the presence of an iron pincer catalyst under remarkably mild conditions.

Selective Hydrogen Production from Methanol with a Defined Iron Pincer Catalyst under Mild Conditions

Molecularly well-defined iron pincer complexes promote the aqueous-phase reforming of methanol to carbon dioxide and hydrogen, which is of interest in the context of a methanol and hydrogen economy. For the first time, the use of earth-abundant iron complexes under mild conditions for efficient hydrogen generation from alcohols is demonstrated.

Selective Hydrogen Production from Methanol with a Defined Iron Pincer Catalyst under Mild Conditions

AbstractMolecularly well‐defined iron pincer complexes promote the aqueous‐phase reforming of methanol to carbon dioxide and hydrogen, which is of interest in the context of a methanol and hydrogen economy. For the first time, the use of earth‐abundant iron complexes under mild conditions for efficient hydrogen generation from alcohols is demonstrated.

Iron Pincer Complex Catalyzed, Environmentally Benign, E‐Selective Semi‐Hydrogenation of Alkynes

Iron Pincer Complex Catalyzed, Environmentally Benign, <i>E</i>‐Selective Semi‐Hydrogenation of Alkynes

Iron Pincer Complex Catalyzed, Environmentally Benign, E‐Selective Semi‐Hydrogenation of Alkynes

Iron Pincer Complex Catalyzed, Environmentally Benign, <i>E</i>‐Selective Semi‐Hydrogenation of Alkynes

Synthesis, Structures, and Dearomatization by Deprotonation of Iron Complexes Featuring Bipyridine-based PNN Pincer Ligands

The synthesis and characterization of new iron pincer complexes bearing bipyridine-based PNN ligands is reported. Three phosphine-substituted pincer ligands, namely, the known (t)Bu-PNN (6-((di-tert-butylphosphino)methyl)-2,2'-bipyridine) and the two new (i)Pr-PNN (6-((di-iso-propylphosphino)methyl)-2,2'-bipyridine) and Ph-PNN (6-((diphenylphosphino)methyl)-2,2'-bipyridine) ligands were synthesized and studied in ligation reactions with iron(II) chloride and bromide. These reactions lead to the formation of two types of complexes: mono-chelated neutral complexes of the type [(R-PNN)Fe(X)2] and bis-chelated dicationic complexes of the type [(R-PNN)2Fe](2+). The complexes [(R-PNN)Fe(X)2] (1: R = (t)Bu, X = Cl, 2: R = (t)Bu, X = Br, 3: R = (i)Pr, X = Cl, and 4: R = (i)Pr, X = Br) are readily prepared from reactions of FeX2 with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility measurements show that these complexes have a high-spin ground state (S = 2) at room temperature. Employing a 2-fold or higher excess of (i)Pr-PNN, diamagnetic hexacoordinated dicationic complexes of the type [((i)Pr-PNN)2Fe](X)2 (5: X = Cl, and 6: X = Br) are formed. The reactions of Ph-PNN with FeX2 in a 1:1 ratio lead to similar complexes of the type [(Ph-PNN)2Fe](FeX4) (7: X = Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer from the Fe(II) centers to the neutral bipyridine unit based on the determined bond lengths. Density functional theory (DFT) calculations were performed to compare the relative energies of the mono- and bis-chelated complexes. The doubly deprotonated complexes [(R-PNN*)2Fe] (9: R = (i)Pr, and 10: R = Ph) were synthesized by reactions of the dicationic complexes 6 and 8 with KO(t)Bu. The dearomatized nature of the central pyridine of the pincer ligand was established by X-ray diffraction analysis of single crystals of 10. Reactivity studies show that 9 and 10 have a slightly different behavior in protonation reactions.

ChemInform Abstract: Iron‐Catalyzed, Atom‐Economical, Chemo‐ and Regioselective Alkene Hydroboration with Pinacolborane.

AbstractThe iron pincer complex (IPC) is found to be optimal as precatalyst in the transformation of various alkenes.

Efficient Hydrogen Liberation from Formic Acid Catalyzed by a Well‐Defined Iron Pincer Complex under Mild Conditions

Hydrogen liberation: An attractive approach to reversible hydrogen storage applications is based on the decomposition of formic acid. The efficient and selective hydrogen liberation from formic acid is catalyzed by an iron pincer complex in the presence of trialkylamine. Turnover frequencies up to 836 h⁻¹ and turnover numbers up to 100,000 were achieved at 40 °C. A mechanism including well-defined intermediates is suggested on the basis of experimental and computational data.

Metal Hydride and Ligand Proton Transfer Mechanism for the Hydrogenation of Dimethyl Carbonate to Methanol Catalyzed by a Pincer Ruthenium Complex

The hydrogenation of dimethyl carbonate to methanol catalyzed by a PNN-ligated ruthenium complex (PNN)Ru(CO)(H) was studied computationally using the density functional theory at the range-separated and dispersion-corrected ωB97X-D functional level in conjunction with an all-electron 6-31++G(d,p) basis set (Stuttgart ECP28MWB basis set for Ru). A direct metal hydride and ligand proton transfer mechanism with three cascade catalytic cycles for the hydrogenation of dimethyl carbonate, methyl formate, and formaldehyde to methanol is proposed. The resting state in the catalytic reaction is the trans dihydride complex trans-(PNN)Ru(H)2(CO). Calculation results indicate that the rate-determining step in the whole reaction is the formation of the second methanol molecule through simultaneous breaking of a C–OCH3 bond and transferring a ligand methylene proton to the dissociated CH3O– in the catalytic cycle for hydrogenation of methyl formate. The essential role of the noninnocent PNN pincer ligand is to split H2 and assist methanol formation through the aromatization and dearomatization of the pyridine ring in the ligand. A new iron pincer complex, trans-(PNN)Fe(H)2(CO), is proposed and evaluated as a promising low-cost and high efficiency catalyst for this reaction.

Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base‐Free Conditions: Synthesis and Mechanistic Insight

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η(1)-BH(4))] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH(3) scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)(2)(CO)] (4a) and cis-[(iPr-PNP)Fe(H)(2)(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe(0) complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

ChemInform Abstract: Synthesis and Catalytic Application of Iron Pincer Complexes

AbstractReview: 109 refs.

Unexpected Direct Reduction Mechanism for Hydrogenation of Ketones Catalyzed by Iron PNP Pincer Complexes

The hydrogenation of ketones catalyzed by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated iron pincer complexes was studied using the range-separated and dispersion-corrected ωB97X-D functional in conjunction with the all-electron 6-31++G(d,p) basis set. A validated structural model in which the experimental isopropyl groups were replaced with methyl groups was employed for the computational study. Using this simplified model, the calculated total free energy barrier of a previously postulated mechanism with the insertion of ketone into the Fe-H bond is far too high to account for the observed catalytic reaction. Calculation results reveal that the solvent alcohol is not only a stabilizer of the dearomatized intermediate but also more importantly an assistant catalyst for the formation of trans-(PNP)Fe(H)(2)(CO), the actual catalyst for hydrogenation of ketones. A direct reduction mechanism, which features the solvent-assisted formation of a trans dihydride complex trans-(PNP)Fe(H)(2)(CO), direct transfer of hydride to acetophenone from trans-(PNP)Fe(H)(2)(CO) for the formation of a hydrido alkoxo complex, and direct H(2) cleavage by hydrido alkoxo without the participation of the pincer ligand for the regeneration of trans-(PNP)Fe(H)(2)(CO), was predicted.

Low‐Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity

Ein hoch aktiver Eisenkatalysator wurde synthetisiert, der die Hydrierung von Kohlendioxid und Hydrogencarbonaten unter bemerkenswert niedrigen Drücken und mit ähnlicher Effizienz wie einige der besten Edelmetallkatalysatoren katalysiert. Als Mechanismus wird der direkte Angriff eines trans-Dihydrid-Eisenkomplexes an Kohlendioxid mit anschließendem Ligandenaustausch und Diwasserstoff-Addition vorgeschlagen.

Low‐Pressure Hydrogenation of Carbon Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal Activity

A highly active iron catalyst for the hydrogenation of carbon dioxide and bicarbonates works under remarkably low pressures and achieves activities similar to some of the best noble metal catalysts. A mechanism is proposed involving the direct attack of an iron trans-dihydride on carbon dioxide, followed by ligand exchange and dihydrogen coordination.

ChemInform Abstract: Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer Complex.

ChemInform Abstract: Efficient Hydrogenation of Ketones Catalyzed by an Iron Pincer Complex.

SYNTHESIS AND CATALYTIC APPLICATIONS OF IRON PINCER COMPLEXES

Transition metal complexes bearing pincer ligands have been extensively studied during the past decade. One of the less-explored or perhaps even neglected areas of pincer chemistry is on the reactivity of iron complexes that incorporate these enormously popular ligand sets. Meanwhile, recent emphasis on homogeneous catalysis has been placed on the developments of catalytic reactions promoted by inexpensive and environmentally benign metals, for which iron is particularly attractive. In that regard, investigating the reactivity patterns of well-defined iron pincer complexes can potentially provide valuable guidelines for the rational design of iron catalysis. This review analyzes the challenges and successes of synthesizing iron complexes with different pincer-type ligands, and discusses the established or potential utility of these complexes for catalytic reactions.