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Rydel-Ciszek, K.; Pacześniak, T.; Zaborniak, I.; Błoniarz, P.; Surmacz, K.; Sobkowiak, A.; Chmielarz, P. Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands. Encyclopedia. Available online: (accessed on 20 June 2024).
Rydel-Ciszek K, Pacześniak T, Zaborniak I, Błoniarz P, Surmacz K, Sobkowiak A, et al. Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands. Encyclopedia. Available at: Accessed June 20, 2024.
Rydel-Ciszek, Katarzyna, Tomasz Pacześniak, Izabela Zaborniak, Paweł Błoniarz, Karolina Surmacz, Andrzej Sobkowiak, Paweł Chmielarz. "Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands" Encyclopedia, (accessed June 20, 2024).
Rydel-Ciszek, K., Pacześniak, T., Zaborniak, I., Błoniarz, P., Surmacz, K., Sobkowiak, A., & Chmielarz, P. (2023, December 08). Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands. In Encyclopedia.
Rydel-Ciszek, Katarzyna, et al. "Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands." Encyclopedia. Web. 08 December, 2023.
Heterogeneous Applications of Iron Complexes with Nitrogen-Containing Ligands

Iron complexes are particularly interesting as catalyst systems over the other transition metals (including noble metals) due to iron’s high natural abundance and mediation in important biological processes, therefore making them non-toxic, cost-effective, and biocompatible. Both homogeneous and heterogeneous catalysis mediated by iron as a transition metal have found applications in many industries, including oxidation, C-C bond formation, hydrocarboxylation and dehydration, hydrogenation and reduction reactions of low molecular weight molecules.

iron-based catalysts oxidation processes “green” oxidants

1. Introduction

The previous considerations of the review were limited to homogeneous catalysts. This type of catalyst is usually characterized by high selectivity and versatility but is also susceptible to aggressive chemical agents, high temperature and problems related to separation from the reaction system. In contrast, heterogeneous catalysts (traditionally based on metals, metal oxides or zeolites) offer high thermal stability and easiness of separation, nevertheless, they are less selective then homogeneous ones. There is a hope that the route for the preservation of the best features inherent in these somewhat opposite approaches leads in the middle of them—through surface immobilization of molecular catalysts.
Since Haag and Whitehurst published their seminal paper half a century ago, reporting the first catalyst based on supported metal complexes [1], there was substantial work made to prepare the materials by anchoring homogeneous catalysts on various solid substrates. Shrinking natural resources and environmental pollution create an urgent need for rapid development of electrochemical energy converting devices, including batteries, fuel cells and water splitting systems. Traditional (and still the best) catalysts are based on precious group metals (PGMs), mainly platinum. However, Pt is scarce and very expensive. The most promising group of PGM-free electrocatalysts are metal-nitrogen-carbon (Me-N-C) materials, frequently heat-treated. Iron-based ones stand out, owing to good activity and low cost. It was shown in 1964 by Jasinski [2] that some metals together with nitrogen macrocyclic compound (phthalocyanines), attached to the electrode can effectively electrocatalyse oxygen reduction reaction (ORR). At the moment Fe complexes with N-containing ligands are exploited for the synthesis of electrocatalysts investigated for ORR, OER, HER, and CO2RR, but also in organic chemistry (Table 1). The catalysts can by non-treated thermally or subjected to thermal treatment (most frequently pyrolysis).
Table 1. A summary of iron complexes with nitrogen-containing ligands applied in homogeneous catalysis.
Ligand/Catalyst Type of Solid Substrate Type of Attachment Solvent Type of Reaction Product Ref.
5,10,15-tris(2,6-hydroxyphenyl)-20-(3-(pyren-1-yl)propyl)porphyrin CNTs, glassy carbon electrode non-covalent bonds water CO2 to CO conversion highly active catalytic carbon based materials [3]
bpy nonporous graphitic carbon nitride support non-covalent (π-π interaction) acetonitrile oxidative coupling of benzylamines hybrid visible light driven photocatalyst [4]
bpy bentonite non-covalent (π-π interaction) - limonene oxidation selective catalyst for oxidation of limonene [5]
iron(II) phthalocyanine iron(II) 1,2,3,4,8,9,10,11,15, 16,17,18,22,23,24,25-hexadeca(chloro)phthalocyanine double-walled CNTs covalent bonds isopropyl alcohol reduction of O2 in an acid medium efficient and inexpensive catalyst for the oxygen reduction reaction [6]
iron(II) tetraphenylporphyrin metal organic framework covalent bonds DMF CO2 reduction high-surface concentration catalysts for CO2 reduction [7]
(4-(3-((bis(pyridin-2-ylmethyl)amino)methyl)-4hydroxybenzamido)phenyl)phosphonic acid (a) TiO2, SrTiO3 non-covalent ethanol/water photocatalytic hydrogen generation Highly active and stable photocatalytic system for hydrogen generation [8]
PIPhen carbon powder non-covalent KOH ORR in alkaline electrolyte novel non-noble metal
ORR catalyst as alternative for Pt catalyst in fuel cell
bpy, 1,10-phenanthroline zeolite encapsulation in porous material - decomposition of hydrogen peroxide, oxidation of 2-phenyl phenol highly efficient zeolite encapsulated
metal complex for oxidation organic pollutants in the tanning
N-doped porous carbon that anchors both atomically dispersed Fe-N4 sites and Fe atomic clusters (FeAC@FeSA-N-C) (4-aminophenyl) benzene-terephthaldehyde covalent organic framework (TAPB-PDA COF) covalent KOH,
ORR alternative catalyst to noble metal-based
catalysts for highly efficient ORR
graphene encapsulated Fe/Fe3C nanocrystals- Fe-Nx configurations (Fe@C-FeNC) - - HClO4 with addition
ORR high-performance non-precious metal
catalyst for ORR
meso-tetra (4-pyridyl) porphyrin porous carbon noncovalent bonds KOH, HClO4 ORR trace-metal catalyst toward ORR in both alkaline and
acidic mediums
iron nanoparticle/hierarchical carbon framework (Fe NP/3D-C) - - KOH ORR electrocatalyst with superior ORR catalytic
activity and excellent durability with large mass activity
1,10-phenanthroline carbon, titanium dioxide, aluminum oxide noncovalent bonds water/THF hydrogenation of nitroarenes to anilines earth-abundant alternative catalysts with excellent yield under industrially viable conditions [15]
1,10-phenanthroline, bpy, 2,2′,6′,2″-terpyridine, pyridinebisbenzimidazole carbon noncovalent bonds THF chemoselective transfer hydrogenation of nitroarenes to anilines durable and reusable catalysts for transfer hydrogenation of nitroarenes to anilines with unique selectivity for the nitro group reduction [16]
1,10-phenanthroline carbon noncovalent bonds THF/water, dioxane/water, THF, dioxane,
reductive amination with hydrogen between nitroarenes and aldehydes efficient catalyst for synthesis of secondary amines [17]
1,10-phenanthroline carbon noncovalent bonds DMSO/water reductive aminations
without hydrogen
efficient catalyst for reductive aminations
without hydrogen for selective synthesis of
1,10-phenanthroline carbon noncovalent bonds THF/water hydrogenation of nitroarenes efficient catalyst for hydrogenation of nitroarenes
under water-gas shift reaction conditions
1,10-phenanthroline carbon noncovalent bonds t-amyl alcohol synthesis of nitriles from alcohols and aqueous
ammonia using molecular oxygen
efficient catalyst for synthesis of substituted and functionalized benzonitriles, heterocyclic nitriles and aliphatic nitriles [20]
1,10-phenanthroline, bpy, 2,2′,6′,2′’-terpyridine, 2,6-bis(2-benzimidazolyl)pyridine carbon noncovalent bonds t-amyl alcohol oxidation of amines in the presence of aqueous
ammonia using molecular oxygen
efficient catalyst for synthesis of substituted and functionalized benzonitriles, heterocyclic nitriles and aliphatic nitriles [21]
polyaniline-derived Fe-N-C - - KOH ORR non-precious metal catalysts for the ORR with performance in a practical anion exchange membrane fuel cell [22]
Fe-N-C nanostructures CNT, carbon black, graphene oxide SWCNT - HClO4 ORR efficient substituent of commercial C/Pt catalysts [23]
Fe-N-C nanostructures SiO2, Zn noncovalent bonds KOH ORR efficient substituent of commercial C/Pt catalysts [24]

2. Nitrogen-Containing Iron Complexes Anchored on a Solid Support (Non-Treated Thermally)

In order to combine intrinsic activity and selectivity of the molecular catalyst with chemical and thermal robustness as well as ease of separation of heterogeneous systems, some Fe complexes with N-containing ligands (including macrocycles, e.g., porphyrins, phtalocyanines, and non-macrocycles, e.g., bipyridine, phenanthroline, polypyrrole) were anchored to the porous inorganic support (including zeolites, clays, alumina, titania, silica, carbon) or organic polymers (including metal organic frameworks, MOFs). The researches were frequently inspired by biological systems [25].
The undeniable merit of the adsorption method is its simplicity. The method frequently exploits π–π interactions or electrostatic ones. Maurin and Robert utilized this method for preparation of a CO2 reduction electrocatalyst [3]. They modified the known molecular catalyst of CO2 to CO reduction, tetraphenyl iron porphyrin, removing one phenyl group and appending a pyrene unit as immobilizing linkage. The compound was attached to carbon nanotubes (CNTs) and then to a glassy carbon electrode. Thus synthesized catalyst was selective, stable, fast, and afforded low CO2 reduction potential in unbuffered water at a neutral pH.
Another example exploiting the same principle of immobilization was anchoring iron(II) trisbipyridine complex to the nanoporous graphitic carbon nitride support [4]. Heterogenization enhances photocatalytic activity in oxidative coupling of benzylamines under mild conditions when O2 was used as an oxidant, and a household white LED as a light source. Using a similar principle, 2,2′-bipyridine (bpy) complexes of Mn(II) and Fe(II) were adsorbed on bentonite. The prepared material was used for limonene oxidation by O2 [5]. The reaction was more selective than the homogeneous one.
The most important feature of catalyst covalent immobilization is a strong attachment. There are many studies following this strategy. Recio et al. [6] reasoned that the active site of Fe phthalocyanine (FePc) and Fe hexadecachloro-phthalocyanine [16(Cl)FePc] complexes, chemically anchored to carbon nanotubes by pyridine, axial, fifth ligand, is harder compared to a physisorbed catalyst. Therefore, according to the hard-soft acid base principle, it should promote high activity for the ORR in an acidic environment because dioxygen is a hard base. They found that electron-withdrawing groups (like Cl and axial, anchoring pyridine ligand) increase activity of the catalyst in ORR. The coordination allows for decoupling of the metal from the electrode, hindering the production of undesired H2O2 by-product.
Functionalized Fe-porphyrines were immobilized into metal organic framework MOF-525 by electrophoresis, creating redox-active linkers [7]. This strategy increased surface availability of the corresponding homogeneous catalyst 600 times, resulting in 2–3 fold increase in CO2 reduction current, when the obtained porous material was immobilized on an electrode surface.
In the recent study, an iron(III) complex with polypyridyl, phosphonic acid functionalized ligand was assembled on TiO2 and SrTiO3 [8]. When irradiated in a solution of fluorescein and trimethylamine (sacrificial donor) the system produced hydrogen, with TON > 7800 in 71 h.
Multiple N active sites ligand, 2-(2-(4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)pyridin-2-yl)pyridin-4-yl)-1-H-imidazo[4,5-f][1,10]phenanthroline (PiPhen) was synthesized and its iron(II) complex, was loaded on carbon powder in the form of coordination polymer [9]. The material, when deposited on platinum carbon electrode showed high electrocatalytic activity in ORR in an alkaline electrolyte with low yield for H2O2.
Encapsulation in porous material allows for engineering of electronic features of active sites via structural adjustment of the framework. Due to high porosity, the entrapped catalyst molecules are easily available for the substrate. Mixed ligand complexes of bpy and 1,10-phenanthroline with iron(III) and nickel(II) were encapsulated into a NaY zeolite cage by ship in bottle method, i.e., utilizing the size of complex formed in the zeolite cage [10]. Based on density functional theory (DFT) calculations they predicted high reactivity of such immobilized species and showed that the material catalyzes oxidation of 2-phenyl phenol by H2O2. They also found that the catalyst can be used more than three times without losing catalytic efficiency.

3. Iron-Nitrogen Functionalities on a Solid Support (Obtained by Thermal Treatment of Precursors)

The discovery of Jasinski [2] triggered a new research trend. After a period of time it was demonstrated that thermal treatment increases activity and stability of the M-N-C catalyst [26][27].
Since that time many pyrolyzed Me-N-C systems were investigated, including Fe-N-C ones. It was also found that apart from Fe complexes with N-containing macrocycles, other available sources of Fe (e.g., salts), C (any organic compound), and N can be precursors of the catalyst. The sources of N can be organics, e.g., bipyridine, phenanthroline, terpyridine, pyrrole, imidazole melamine, polymers (e.g., pylyaniline, polydopamine); and inorganics, like sodium azide or ammonia. The typical substrates are carbonaceous materials (e.g., carbon black, carbon nanotubes, carbon nanofibers (CNFs), reduced graphite oxide (rGO), graphene), which frequently display good electrical conductivity. These nanomaterials offer also a highly developed surface, porous structure, or are rich in structural defects. Reviews of carbon-supported Fe-N-C catalysts and/or electrocatalysts have recently been published [28][29][30].
There are two basic synthetic methods for carbon-based iron-nitrogen catalysts: pyrolysis and hydrothermal carbonization (HTC). In the pyrolysis method the precursors (composed of carbon, nitrogen and iron, and sometimes other elements, e.g., sulfur or zinc), usually immobilized on solid substrates, are heated in inert gas atmosphere at a temperature usually in the range of 600–1100 °C. Hydrothermal carbonization (HTC) consists in heating a precursor in water solvent at moderate temperatures (200 °C) in an autoclave.
Thermal decomposition results in a formation of FeNx active sites and usually other structures, including Fe, iron oxide, or carbide phases. The active sites of most materials have not been clearly identified so far [31]. Probably, the majority of the early Fe-N-C catalysts obtained by thermal treatment are dozens of nanometers in size, corresponding to the definition of a large nanoparticle (LNP). Owing to the development of synthetic methods and analytic instruments, single-site materials have recently been synthesized [30][32][33][34][35][36][37][38][39]. Such ideal catalysts are characterized by the highest exposure of active sites, good availability for substrate and minimized metal utilization. However, some results show enhanced ORR after incorporating nanoclusters of Fe to single-atom Fe-N-C catalyst [11], others emphasize the importance of Fe-Fe3C structures [12]. Another group of catalysts comprises subnano ones. Subnano materials are usually defined as the ones of atomic-level size and showing unique, size-related properties compared to their larger nano-counterparts, or just ones of dimensions smaller than LNPs [40][41]. Some subnano Fe-N-C catalytic structures have been recently obtained [13][14].
Recently a review concerning M-N-C catalysts for ORR was published by Osmieri [42]. The clear division of synthetic strategies that was recognized for these materials perfectly covers most Fe-N-C catalysts, not only the ones used for ORR.
The first synthetic method consists of thermal treatment of carbon support and nitrogen-containing molecules. This strategy resulted in spectacular examples of catalysis in the field of organic chemistry. Beller’s group’s [15] seminal discovery of a new heterogeneous catalyst for important organic reactions opened new vistas of potential applications. The catalyst, obtained by pyrolysis of nitrogen precursor and iron salt, affords selective hydrogenation (with H2) of nitroarenes to anilines, at mild conditions with high yields. N-containing ligand and pyrolysis temperature have been optimized. The best catalytic material was obtained by pyrolysis at 800 °C of iron-phenanthroline complex adsorbed on carbon black. Later it was demonstrated that the catalyst exhibited remarkable activity for the activation of formic acid in the transfer hydrogenation of nitroarenes to anilines [16]. Next, an environmentally friendly method for the selective synthesis of secondary amines, by hydrogenation of nitroarenes as well as imines was proposed [17] and later for the synthesis of tertiary amines [18]. Recently the utilization of this catalyst under water–gas shift conditions [19] and also for the oxidation of primary alcohols or primary amines to nitriles with dioxygen has been demonstrated [16][20].
The second synthetic strategy utilizes a nitrogen-containing polymer. PANI-derived Fe-N-C catalyst was developed by sonication of ferric chloride with aniline and carbon black, instead of using traditional, expensive ammonium persulfate for polymerization [22]. After pyrolysis, carbonization of aniline by heat treatment (at various temperatures in the range 300–900 °C), removal of impurities and iron oxide by acid leaching, and graphitization of the remaining carbon by a second heat treatment at 900 °C, the best catalyst (pyrolyzed at 700 °C) displayed half-wave potential for ORR only 10 mV less than commercial Pt/C catalyst and showed outstanding performance in anion exchange membrane fuel cell.
Pyrolyzed Fe-N-C materials are also investigated for potential application in zero-carbon fuels generation. Recently polyaniline derived Fe-N-C catalysts were synthesized [43] and the ratio of different N functionalities was tuned according to the pyrolysis temperature (in the range 750–1050 °C). A larger amount of formed FeNx sites, formed at higher temperatures was responsible for higher selectivity towards CO2RR.
Another approach exploits carbon nanotubes. Polypyrrole coated CNTs, subsequently enriched in highly concentrated Fe atoms prepared by simultaneous adsorption of Fe3+ and Zn2+ cations, and pyrolyzed at 900 °C in gas ammonia were developed [23]. The material exhibited a half-wave potential of ORR comparable to commercial Pt/C catalyst in acidic water solution. Consequently, they used this synthetic method for carbon cloth (CC) and also for flexible films of single-wall carbon nanotubes (SWCNT), which both displayed even better ORR performance than Pt/C.
The third group of catalysts derives from application of Hard Template Method and utilizes silica template and organic precursors. The method takes advantage of the high porosity of the silica template. The presence of silica obviates the need for carbon support. After the carbonization of the precursors, the silica scaffolding is removed by leaching with HF or a strong base, leaving its excellent porosity in the structure of a formed catalyst. A dual-template method of synthesizing porous Fe-N-C catalyst has been developed [24]. The dual-template was prepared by mixing glutaraldehyde and chitosan (the constituents of the first template) with SiO2, porous nanoparticles (second template). Finally, the iron precursor (FeCl3) was added to the mixture and zinc nitrate to increase porosity due to evaporation of Zn during heat treatment. The resulting hydrogel was freeze-died, pyrolyzed, and leached with HF. The material with multiscale porosity displayed better performance in ORR than commercial C/Pt catalysts, as measured by rotating disc electrode (RDE) method.
Another synthetic method utilizes metal-organic frameworks (MOFs). MOFs are porous, highly ordered, crystalline structures, composed of metal ions and organic linkers. The positive charge of this net is balanced by negative counter ions, located in the pores, together with solvent molecules. The metal ions and the linkers can be adjusted in a way which allows for the desired structure and composition of MOF. This structure can partially remain in the material after heat treatment. Recently Wang et al. developed a “MOF-protective-pyrolysis” strategy [44]. The iron- and nitrogen-containing metal-organic framework NH2-MIL-101-Fe of excellent, regular porosity was embedded in zinc-based ZIF-8 (another MOF, an additional source of N and C). The material they obtained after pyrolysis, containing basically the structure of MOF and some carbon nanotubes, displayed good performance in ORR. The authors proposed a mechanism assuming Fenton reaction, whereby formed FeN4 centres react with hydrogen peroxide affording active carbon radicals of strong adsorption to O2, and consequently accelerating ORR.
Additional possibilities can be created by utilizing additional precursors during the synthesis of Fe-N-C catalyst with MOFs. A highly porous structure of MOF can contain, e.g., iron complexes with nitrogen-containing ligands. For example, Fe-phenanthroline has been exploited by Li et al. [45] ZIF-8 was calcined at 400 °C and such prepared material was a substrate for Fe-phenanthroline complex. After two consecutive heat treatments at 1050 °C (at Ar and NH3 atmosphere) the material was leached with hydrochloric acid. The catalyst not only showed high activity in ORR but also intrinsic immunity to poisoning by chlorides.
Two-dimensional hierarchical Fe-N-C materials were demonstrated to be highly effective electrocatalysts for ORR in Zn-air batteries [46], enabling ultrahigh specific capacity. Such configuration was obtained by confined-space pyrolysis of assembled precursors, using GO, ZIF-8 and iron salt.
The other method used for the synthesis of Fe-N-C materials exploits vapour deposition (CVD). Iron is usually used as a catalyst in the CVD method of carbon nanotubes production and frequently considered contamination, responsible for its toxicity [47]. Deng et al. showed earlier that encapsulated Fe or FeCo NPs in CNTs can activate O2 on the outer surface of the nanotube, while the mentioned nanoparticles are preserved against leaching in the acidic medium [48][49]. Inspired by this discovery they designed the catalytic material, encapsulating Fe, Co, and FeCo alloy NPs via the CVD method in N-doped (using pyridine vapours at 700 °C) CNTs [50]. The catalyst displayed long-time durability and high activity in HER.


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