Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal-Ligand Cooperation

On the Demise of PPP-Ligated Iron Catalysts in the Formic Acid Dehydrogenation Reaction

The PPP-ligated iron complexes, cis-(iPrPPRP)FeH2(CO) [iPrPPRP = (o-iPr2PC6H4)2PR (R = H or Me)], catalyze the dehydrogenation of formic acid to carbon dioxide but lose their catalytic activity over time. This study focuses on the analysis of the species formed from the degradation of cis-(iPrPPMeP)FeH2(CO) over its course of catalyzing the dehydrogenation reaction. These degradation products include species both soluble and insoluble in the reaction medium. The soluble component of the decomposed catalyst is a mixture of cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x], protonated iPrPPMeP, and oxidation products resulting from adventitious O2. The precipitate is solvated Fe(OCHO)2. Further mechanistic investigation suggests that cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x] displays diminished but measurable catalytic activity, likely through the displacement of a CO ligand by the formate ion. The formation of Fe(OCHO)2 along with the dissociation of iPrPPMeP is responsible for the eventual loss of catalytic activity.

Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Two bench-stable Fe(II) alkyl complexes [Fe(κ3PCP-PCP-iPr)(CO)2(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(κ3PCP-PCP-iPr)(CO)2(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ3PCP-PCP-iPr)(CO)2]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ3PCP-PCP-iPr)(CO)2(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.

Structural and Electronic Properties of Iron(0) PNP Pincer Complexes

In the present work we have prepared and fully characterized several Fe(0) complexes of the type [Fe(PNP)(CO)2] treating Fe(II) complexes [Fe(PNP)(Cl)2] with KC8 in the presence of carbon monoxide. While complexes [Fe(PNPNMe-iPr)(CO)2], [Fe(PNPNEt-iPr)(CO)2] adopt a trigonal bipyramidal geometry, the bulkier and more electron rich [Fe(PNPNH-tBu)(CO)2] is closer to a square pyramidal geometry. Mössbauer spectra showed isomer shifts very close to 0 and similar to those reported for Fe(I) systems. Quadrupole splitting values range between 2.2 and 2.7 mm s-1 both in experiments and DFT calculations, while those of Fe(I) complexes are much smaller (∼0.6 mm s-1).

A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols

A Cp*Ir(III) complex (1) of a newly designed ligand L¹ featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF₄ in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L¹ H)Cl]BF₄ (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by ¹ H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L² )Cl]PF₆ (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the trans-Acyl Site in H2 Heterolysis

We report syntheses and H₂ activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNS^Pre) or phosphine (CNP^Pre) donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C-H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the -PPh₂ donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H₂ activation (5 atm D_2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H₂ activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Isoelectronic Manganese and Iron Hydrogenation/Dehydrogenation Catalysts: Similarities and Divergences

Sustainable processes that utilize nontoxic, readily available, and inexpensive starting materials for organic synthesis constitute a major objective in modern chemical research. In this context, it is highly important to perform reactions under catalytic conditions and to replace precious metal catalysts by earth-abundant nonprecious metal catalysts. In particular, iron and manganese are promising candidates, as these are among the most abundant metals in the earth's crust, are inexpensive, and exhibit a low environmental impact. As far as chemical processes are concerned, hydrogenations and acceptorless alcohol dehydrogenation (AAD), sometimes in conjunction with hydrogen autotransfer reactions, are becoming important areas of research. While the first is a very important synthetic process representing a highly atom-efficient and clean methodology, AAD is an oxidant-free, environmentally benign reaction where carbonyl compounds together with dihydrogen as a valuable product and/or reactant (autotransfer) and water are formed. Carbonyl compounds, typically generated in situ, can be converted into other useful organic materials such as amines, imines, or heterocycles. In 2016 several groups, including ours, discovered for the first time the potential of hydride biscarbonyl Mn(I) complexes bearing strongly bound PNP pincer ligands or related tridentate ligands as highly effective and versatile catalysts for hydrogenation, transfer hydrogenation, and dehydrogenation reactions. These complexes are isoelectronic analogues of the respective hydride monocarbonyl Fe(II) PNP compounds and display similar reactivities but also quite divergent behavior depending on the coligands. Moreover, manganese compounds show improved long-term stability and high robustness toward harsh reaction conditions. In light of these recent achievements, this Account contrasts Mn(I) and Fe(II) PNP pincer catalysts, highlighting specific features that are connected to particular structural and electronic properties. It also addresses opportunities and restrictions in their catalytic applications. Apart from classical hydrogenations, it also covers the most recent developments of these catalysts for AAD resulting in the synthesis of complex organic molecules such as heterocycles via multicomponent reactions. The ambivalent hydrogen-based redox chemistry provides access to a variety of synthetically valuable reductive and oxidative coupling reactions. Hence, these catalysts cover a broad scope of catalytic applications and exhibit activities and productivities that are becoming competitive with those of well-established precious metal catalysts. The knowledge about the nature and characteristics of active Mn(I)- and Fe(II)-based systems paves the way for conceptually and mechanistically well-founded research, which might lead to further developments and the discovery of novel catalysts extending the current scope and limitations of reactivity. It underlines that base metal catalysts are beginning to challenge precious metal catalysts and contributes to the further advancement of waste-free sustainable base metal catalysis.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Carbon dioxide hydrogenation catalysed by well-defined Mn(i) PNP pincer hydride complexes

The catalytic reduction of carbon dioxide is of great interest for its potential as a hydrogen storage method and to use carbon dioxide as C-1 feedstock. In an effort to replace expensive noble metal-based catalysts with efficient and cheap earth-abundant counterparts, we report the first example of Mn(i)-catalysed hydrogenation of CO2 to HCOOH. The hydride Mn(i) catalyst [Mn(PNPNH-iPr)(H)(CO)2] showed higher stability and activity than its Fe(ii) analogue. TONs up to 10 000 and quantitative yields were obtained after 24 h using DBU as the base at 80 °C and 80 bar total pressure. At catalyst loadings as low as 0.002 mol%, TONs greater than 30 000 could be achieved in the presence of LiOTf as the co-catalyst, which are among the highest activities reported for base-metal catalysed CO2 hydrogenations to date.

Substitution reactions of iron(ii) carbamoyl-thioether complexes related to mono-iron hydrogenase

A C,N,S pincer complex has been synthesized for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD).

Synthesis and reactivity of BINEPINE-based chiral Fe(II) PNP pincer complexes

ABSTRACT

A new asymmetric chiral PNP ligand based on the 2,6-diaminopyridine scaffold featuring a R-BINEPINE moiety was prepared. Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv of PNP-iPr,BIN at room temperature afforded the coordinatively unsaturated paramagnetic complexes [Fe(PNP-iPr,BIN)X2]. The structure of [Fe(PNP-iPr,BIN)Cl2] is described. Both complexes react readily with the strong π-acceptor ligand CO in solution to afford selectively the diamagnetic complexes trans-[Fe(PNP-iPr,BIN)(CO)X2] in quantitative yield. Due the lability of the CO ligand, these complexes are only stable under a CO atmosphere and isolation in pure form was not possible. The preparation of the carbonyl hydride complex [Fe(PNP-iPr,BIN)(H)(CO)Br] was achieved albeit in low yields via a one pot procedure by treatment of [Fe(PNP-iPr,BINEP)Br2] with CO and subsequent reaction with Na[HBEt3]. This complex was obtained as an inseparable mixture of two diastereomers in a ca. 1:1 ratio and was tested as catalyst for the hydrogenation of ketones. The catalyst showed acceptable activity under mild conditions (5 bar H2, room temperature) with yields up to >99 % within 18 h.

GRAPHICAL ABSTRACT

A Functional Model of [Fe]-Hydrogenase

[Fe]-Hydrogenase catalyzes the hydrogenation of a biological substrate via the heterolytic splitting of molecular hydrogen. While many synthetic models of [Fe]-hydrogenase have been prepared, none yet are capable of activating H2 on their own. Here, we report the first Fe-based functional mimic of the active site of [Fe]-hydrogenase, which was developed based on a mechanistic understanding. The activity of this iron model complex is enabled by its unique ligand environment, consisting of biomimetic pyridinylacyl and carbonyl ligands, as well as a bioinspired diphosphine ligand with a pendant amine moiety. The model complex activates H2 and mediates hydrogenation of an aldehyde.

Recent developments of iron pincer complexes for catalytic applications

Iron pincer complexes exhibit excellent activity in homogeneous catalysis.

Reversible cyclometalation at RhI as a motif for metal–ligand bifunctional bond activation and base-free formic acid dehydrogenation

The first example of base-free catalytic dehydrogenation of formic acid using reversible cyclometalation at Rh(i) is discussed, using a combination of experimental and computational methods.

Coinage metal complexes supported by a "PN(3)P" scaffold

A series of monovalent group 11 complexes, [2,6-{Ph2PNMe}2(NC5H3)]CuBr 1, [2,6-{Ph2PNMe}2(NC5H3)]CuOTf 2, [2,6-{Ph2PNMe}2(NC5H3)]AgOTf 3, and [2,6-{Ph2PNMe}2(NC5H3)](AuCl)24, supported by a neutral PN(3)P ligand have been synthesized and characterized by multinuclear NMR and single crystal X-ray diffraction studies. The variation of the coordination properties were analyzed and electronic structure calculations have been carried out to provide insight on the bonding details in these complexes. The Cu(I) complexes displayed an unusual coordination geometry with a tridentate pincer ligand and an overall four coordinate trigonal pyramidal geometry. In contrast the Ag(I) analogue displayed a bidentate κ(2)-P,P' ligation leaving the pyridyl-N atom uncoordinated and yielding a pyramidalized trigonal planar geometry around Ag. The bimetallic Au(I) complex completed the series and displayed a monodentate P-bonded ligand and a linear coordination geometry.

Twinning of three Fe-PNP pincer complexes interpreted according to order-disorder (OD) theory

The systematic twinning of three 2,6-diaminopyridine-based Fe-PNP complexes is interpreted using order-disorder (OD) theory. The monoclinic [Fe(0)(PNP(Et)-(i)Pr)(CO)2] [P112(1)/b, Z' = 4] possesses pseudo-orthorhombic metrics and crystallizes as a reflection twin by pseudo-merohedry with the twin plane (100). The structure is made up of layers with idealized p2(1)a(b) symmetry. The a glide planes of adjacent layers do not overlap, leading to OD polytypism. trans-[Fe(II)(PNP-Et)Br2(CO)] [P2(1)/n, Z' = 1] is systematically twinned via twofold rotation about [001]. It is made up of OD layers with idealized p2(1)2(1)(2) symmetry. OD polytypism is caused by the twofold rotation axes of adjacent layers which do not overlap. [Fe(II)(κ(2)P,N-PNP-(i)Pr,TAD)Cl2]·THF [P1, Z^{\prime} = 2] is systematically twinned via a twofold rotation about [010]. It is made up of layers with idealized p121(1) symmetry. OD polytypism is caused by screw rotations relating adjacent layers with an intrinsic translation along a fourth of a primitive lattice vector. In all three structures the twin individuals are a polytype with a maximum degree of order (MDO) and at the twin interface is located a fragment of the second MDO polytype.