Electron-reservoir complex

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A molecular electron-reservoir complex is one of a class of redox-active systems which can store and transfer electrons stoichiometrically or catalytically without decomposition. The concept of electron-reservoir complexes was introduced by the work of French chemist, Didier Astruc. [1] [2] From Astruc's discoveries, a whole family of thermally stable, neutral, 19-electron iron(I) organometallic complexes were isolated and characterized, and found to have applications in redox catalysis and electrocatalysis. [3] [4] The following page is a reflection of the prototypal electron-reservoir complexes discovered by Didier Astruc.

Contents

Synthesis

The parent complex, C5H5FeC6H6, is observed to undergo decomplexation and dimerization, whereas analogues containing six alkyl groups on the benzene ring exhibit stability in many solvents and remain catalytically active (written as η5-CpFe-η6-arene). [2]

Syntheses using ferrocene

In the process of ligand exchange, one of the cyclopentadienyl rings of ferrocene is replaced by an arene group. [5] This has garnered significant interest as it provides a simple means to form complexes of arenes with the CpFe+ units. The reaction of ferrocene and an aryl alkyl group is carried out at 70–90 °C for 1–16 hours in the arene as the solvent (Figure 1). Aluminum chloride, AlCl3, is a Lewis acid which prompts the reaction, and aluminum metal is added to inhibit oxidation of ferrocene to ferrocenium. Monoelectronic reduction of the FeII 18-electron monocation with Na/Hg in THF at ambient temperature yields the 19-electron complex as green-black crystals.

Figure 1. Synthesis of CpFe(arene) using ferrocene Initial scheme.svg
Figure 1. Synthesis of CpFe(arene) using ferrocene

Electron donating groups on the arene increase the yield. [6] Additionally, alkyl groups which induce steric hindrance (e.g., Me, Et, etc.) will increase the yield. [7]

Complexation of arenes to iron

A known type of complexation is the Fischer-Hafner synthesis, [8] which treats transition metal chlorides with arenes in the presence of aluminum metal and aluminum chloride. The synthesis of bis(arene) iron dications is an example of this type, where a mixture of iron dichloride, FeCl2, alkylbenzenes, and AlCl3 is refluxed in the arene as the solvent (Figure 2).

Figure 2. Synthesis of bis(arene) iron dications Bis arene Fe2.svg
Figure 2. Synthesis of bis(arene) iron dications

Although AlCl3 is almost always used to induce ligand exchange reactions, other efficient Lewis acids are AlBr3, GaCl3, ZrCl4, and HfCl4. [5]

Structure and bonding

Cyclopentadienyl iron(arene) complexes

The electronic spectra of CpFe+(arene) complexes have been compared to those of ferrocene and bis(arene) iron dications. [5] Three spin allowed transitions are observed at 22,200, 26,200 and 31,900 cm–1 and two bands at 38,200 and 41,800 cm–1 are attributed to π→π* benzene transitions. The ligand field parameters (cm–1) for the CpFe+ (arene) complexes are Δ1 = 8,500, Δ2 = 21,900, B = 320 (comparatively for ferrocene, Δ1 = 7,200, Δ2 = 22,000, B = 390). Therefore, the values of the electronic repulsion parameters B indicate large covalency between the metal–ligand bonds. In general, 19-electron complexes are thermally stable only when the arene ligand is peralkylated; steric bulk also stabilizes the complex.

The X-ray crystal structure for the CpFe(I)-C6Me6 complex exhibits alternating sandwich positions in the crystal packing. [9] Detailed EPR studies of the Fe(I) sandwiches in frozen solution, in the solid state, and in diamagnetic matrices show dynamic rhombic distortion, high degree of covalency, and spin-lattice relaxation. [9]

Bis(arene) iron dications

Figure 3. Molecular orbital diagram bis(arene) transition metal sandwiches MO Diagram for TM Sandwich.svg
Figure 3. Molecular orbital diagram bis(arene) transition metal sandwiches

Bis(arene) iron dications and their electronic configuration bear resemblance to their bis(arene) chromium counterparts. [5] These compounds exhibit a doubly degenerate bonding e2 level that is high in metal character, while the nonbonding a1 orbital is nearly a pure dz2 metal orbital (Figure 3).


When one electron is added to the dication, (C6Me6)2Fe2+, a stable 19-electron complex is formed, a compound stabilized only when the arene is endowed with multiple methyl groups. [10] Remarkably, a nearly stable complex arises when a second extra electron is added, with the case of C6Me6 being particularly noteworthy (Figure 4). Although the effective atomic number (EAN) rule states that thermodynamically stable transition metal complexes contain 18 valence electrons, there are situations found in these organoiron systems where it can be breached, given that the anti-bonding doubly degenerate e1g level harbors a high metal character and is located at relatively low energy. Three isostructural complexes (C6Me6)2Fen+, where n ranges from 0 to 2, have been isolated and their structures are shown in Figure 4.

Figure 4. Three isostructural complexes of (C6Me6)2Fe (n = 0, 1, 2) Isostructural (C6Me6)2Fe Complexes.svg
Figure 4. Three isostructural complexes of (C6Me6)2Fe (n = 0, 1, 2)

Reactions and functionalizations of the coordinated arene

In 1979, Didier et al. reported the activation of the arene group in η5-C5H5Fe-η6-C6(CH3)6 by dioxygen, O2, through an electron transfer mechanism to form the superoxide radical anion, O2–•. [11] In this paper, two unique reactions of O2 are reported: hydrogen atom abstraction by O2 from a methyl group on the arene ring (Figure 5), and the O2-induced dimerization of C5H5Fe-arene when the arene group bears less than six methyl groups.

Figure 5. Hydrogen atom abstraction by O2 from a methyl group on the arene ligand and benzylic activation by CX2 Benzylic activation by O2.svg
Figure 5. Hydrogen atom abstraction by O2 from a methyl group on the arene ligand and benzylic activation by CX2

The green η5-C5H5Fe-η6-C6(CH3)6 complex reacts readily when in contact with air (25°C) in pentane to afford complex 2 and water (Figure 5). Characterization by mass spectrometry, nuclear magnetic resonance, and X-ray crystallography revealed that the structure of 2 is best described as a cyclohexadienyl ligand coordinated in a pentahapto fashion to η5-C5H5Fe and bearing an exocyclic double bond (Figure 5). Complex 2 has shown to be a good model for intermediates in benzylic activation processes when reacting with carbon dioxide, CO2, and carbon disulfide, CS2 (Figure 5, right-side).

Dioxygen induces dimerization for complexes shown in Figure 6. The O2-induced dimerization follows a radical mechanism, whereas H-atom abstraction in Figure 5 is mainly ionic.

Figure 6. O2-induced dimerization Dimer by O2.svg
Figure 6. O2-induced dimerization

Other useful electron-transfer reactions of η5-C5H5Fe-η6-C6(CH3)6 is the deprotonation of imidazolium salts, generating N-heterocyclic (NHC) carbenes. [12] Arduengo demonstrated that deprotonating these salts produces NHCs that are isolable and relatively stable, provided that the heterocycle nitrogen atoms have appropriate substituents. [13] Reacting a stoichiometric amount of CpFe-C6(CH3)6 in THF in the presence of air with imidazolium salts, quickly results in the soluble carbenes, which are visible by the color change from deep-green to yellow (Figure 7). [12] The carbenes generated in this form were characterized by 1H and 13C NMR.

Figure 7. Deprotonation of imidazolium salts to afford N-heterocyclic carbenes NHC.svg
Figure 7. Deprotonation of imidazolium salts to afford N-heterocyclic carbenes

Further reading

In 2014, Chang et al. reported the synthesis and properties of bis(formazanate) zinc complexes. [14] These complexes exhibit reversible redox-chemistry, with the ligands serving as electron reservoirs. As a result, these complexes can be synthesized in three redox states, in which the formazanate ligands are reduced to "metallaverdazyl" radicals. The complexes were fully characterized using various methods, including single-crystal X-ray crystallography, spectroscopic techniques, and DFT calculations.

A decade prior, Venkatesan et al. investigated a series of electron-rich manganese(I) half-sandwich complexes for applications as molecular batteries. The study highlighted the possibility of using the C–C bonds in these vinylidene systems as electron reservoirs, enabling their potential as essential components in nano devices. The compounds were characterized by X-ray diffraction, NMR, IR, and cyclic voltammetry. [15]

Related Research Articles

Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and Fc+ respectively.

<span class="mw-page-title-main">Chromium hexacarbonyl</span> Chemical compound

Chromium hexacarbonyl is a chromium(0) organometallic compound with the formula Cr(CO)6. It is a homoleptic complex, which means that all the ligands are identical. It is a colorless crystalline air-stable solid, with a high vapor pressure.

<span class="mw-page-title-main">Pentamethylcyclopentadiene</span> Chemical compound

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diene with the formula C5(CH3)5H, often written C5Me5H, where Me is CH3. It is a colorless liquid.

<span class="mw-page-title-main">Hapticity</span> Number of contiguous atoms in a ligand that bond to the central atom in a coordination complex

In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated. In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity, and the κ-notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.

<span class="mw-page-title-main">Bis(benzene)chromium</span> Chemical compound

Bis(benzene)chromium is the organometallic compound with the formula Cr(η6-C6H6)2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containing two arene ligands.

In organometallic chemistry, a transition metal indenyl complex is a coordination compound that contains one or more indenyl ligands. The indenyl ligand is formally the anion derived from deprotonation of indene. The η5-indenyl ligand is related to the η5cyclopentadienyl anion (Cp), thus indenyl analogues of many cyclopentadienyl complexes are known. Indenyl ligands lack the 5-fold symmetry of Cp, so they exhibit more complicated geometries. Furthermore, some indenyl complexes also exist with only η3-bonding mode. The η5- and η3-bonding modes sometimes interconvert.

Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. While iron adopts oxidation states from Fe(−II) through to Fe(VII), Fe(IV) is the highest established oxidation state for organoiron species. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

<span class="mw-page-title-main">Organoruthenium chemistry</span>

Organoruthenium chemistry is the chemistry of organometallic compounds containing a carbon to ruthenium chemical bond. Several organoruthenium catalysts are of commercial interest and organoruthenium compounds have been considered for cancer therapy. The chemistry has some stoichiometric similarities with organoiron chemistry, as iron is directly above ruthenium in group 8 of the periodic table. The most important reagents for the introduction of ruthenium are ruthenium(III) chloride and triruthenium dodecacarbonyl.

<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

<span class="mw-page-title-main">Rhodocene</span> Organometallic chemical compound

Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.

Transition metal carbyne complexes are organometallic compounds with a triple bond between carbon and the transition metal. This triple bond consists of a σ-bond and two π-bonds. The HOMO of the carbyne ligand interacts with the LUMO of the metal to create the σ-bond. The two π-bonds are formed when the two HOMO orbitals of the metal back-donate to the LUMO of the carbyne. They are also called metal alkylidynes—the carbon is a carbyne ligand. Such compounds are useful in organic synthesis of alkynes and nitriles. They have been the focus on much fundamental research.

<span class="mw-page-title-main">Cyclopentadienyliron dicarbonyl dimer</span> Chemical compound

Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).

Ligand bond number (LBN) represents the effective total number of ligands or ligand attachment points surrounding a metal center, labeled M. More simply, it represents the number of coordination sites occupied on the metal. Based on the covalent bond classification method, the equation for determining ligand bond number is as follows:

<span class="mw-page-title-main">Half sandwich compound</span> Class of coordination compounds

Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples include cyclobutadieneiron tricarbonyl and (C5H5)TiCl3. Commercially useful examples include (C5H5)Co(CO)2, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.

Russell P. Hughes an American/British chemist, is the Frank R. Mori Professor Emeritus and Research Professor in the Department of Chemistry at Dartmouth College. His research interests are in organometallic chemistry, with emphasis on the chemistry of transition metal complexes interacting with fluorocarbons. His research group’s work in this area led to several creative syntheses of complexes of transition metal and perfluorinated hydrocarbon fragments.

<span class="mw-page-title-main">Phosphenium</span> Divalent cations of phosphorus

Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates.

A transition metal phosphido complex is a coordination complex containing a phosphido ligand (R2P, where R = H, organic substituent). With two lone pairs on phosphorus, the phosphido anion (R2P−) is comparable to an amido anion (R2N−), except that the M-P distances are longer and the phosphorus atom is more sterically accessible. For these reasons, phosphido is often a bridging ligand. The -PH2 ion or ligand is also called phosphanide or phosphido ligand.

<span class="mw-page-title-main">Hexaphosphabenzene</span> Chemical compound

Hexaphosphabenzene is a valence isoelectronic analogue of benzene and is expected to have a similar planar structure due to resonance stabilization. Although several other allotropes of phosphorus are stable, no evidence for the existence of P6 has been reported. Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1, and would therefore not be observed in the uncomplexed state under normal experimental conditions. The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6. The moderate barrier suggests that hexaphosphabenzene could be synthesized from a [2+2+2] cycloaddition of three P2 molecules. Currently, this is a synthetic endeavour which remains to be conquered.

Metal-ligand cooperativity (MLC) is a mode of reactivity in which a metal and ligand of a complex are both involved in the bond breaking or bond formation of a substrate during the course of a reaction. This ligand is an actor ligand rather than a spectator, and the reaction is generally only deemed to contain MLC if the actor ligand is doing more than leaving to provide an open coordination site. MLC is also referred to as "metal-ligand bifunctional catalysis." Note that MLC is not to be confused with cooperative binding.

<span class="mw-page-title-main">Paula Diaconescu</span> Inorganic chemist

Paula L. Diaconescu is a Romanian-American chemistry professor at the University of California, Los Angeles. She is known for her research on the synthesis of redox active transition metal complexes, the synthesis of lanthanide complexes, metal-induced small molecule activation, and polymerization reactions. She is a fellow of the American Association for the Advancement of Science.

References

  1. ↑ Astruc, Didier; Roman E., Enrique; Hamon, Jean Rene; Batail, Patrick (April 1979). "Novel reactions of dioxygen in organometallic chemistry. Hydrogen atom abstraction vs. dimerization of the 19-electron complexes .eta.5-cyclopentadienyliron(I) .eta.6-arene". Journal of the American Chemical Society. 101 (8): 2240–2242. doi:10.1021/ja00502a070. ISSN   0002-7863.
  2. 1 2 Astruc, Didier; Hamon, Jean Rene; Althoff, Gisela; Roman, Enrique; Batail, Patrick; Michaud, Pascal; Mariot, Jean Pierre; Varret, Francois; Cozak, Daniel (August 1979). "Design, stabilization, and efficiency of organometallic "electron reservoirs". 19-Electron sandwiches .eta.5-C5R5FeI-.eta.6-C6R'6, a key class active in redox catalysis". Journal of the American Chemical Society. 101 (18): 5445–5447. doi:10.1021/ja00512a071. ISSN   0002-7863.
  3. ↑ Astruc, Didier (April 2012). "Electron-transfer processes in dendrimers and their implication in biology, catalysis, sensing and nanotechnology". Nature Chemistry. 4 (4): 255–267. Bibcode:2012NatCh...4..255A. doi:10.1038/nchem.1304. ISSN   1755-4349. PMID   22437709.
  4. ↑ Green, Jennifer C.; Kelly, M. Ruth; Payne, Martin P.; Seddon, Elaine A.; Astruc, Didier; Hamon, Jean Rene; Michaud, Pascal (February 1983). "Photoelectron study of electron-rich iron(I) cyclopentadienyl arene complexes and of the related iron(II) cyclopentadienyl cyclohexadienyl complexes". Organometallics. 2 (2): 211–218. doi:10.1021/om00074a002. ISSN   0276-7333.
  5. 1 2 3 4 Astruc, Didier (1983-01-01). "Organo-iron complexes of aromatic compounds. Applications in synthesis". Tetrahedron. 39 (24): 4027–4095. doi:10.1016/S0040-4020(01)88627-0. ISSN   0040-4020.
  6. ↑ Astruc, Didier; Dabard, Rene (26 August 1975). "Elsevier Enhanced Reader". Journal of Organometallic Chemistry. 96 (2): 283–287. doi: 10.1016/S0022-328X(00)83560-3 . Retrieved 2023-03-17.
  7. ↑ Hamon, Jean Rene; Astruc, Didier; Michaud, Pascal (February 1981). "Syntheses, characterizations, and stereoelectronic stabilization of organometallic electron reservoirs: the 19-electron d7 redox catalysts .eta.5-C5R5Fe-.eta.6-C6R'6". Journal of the American Chemical Society. 103 (4): 758–766. doi:10.1021/ja00394a006. ISSN   0002-7863.
  8. ↑ Seyferth, Dietmar (2002-07-01). "Bis(benzene)chromium. 2. Its Discovery by E. O. Fischer and W. Hafner and Subsequent Work by the Research Groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and Others". Organometallics. 21 (14): 2800–2820. doi: 10.1021/om020362a . ISSN   0276-7333.
  9. 1 2 Delville-Desbois, Marie-Hélène; Mross, Stefan; Astruc, Didier; Linares, Jorge; Varret, François; Le Beuze, Albert; Saillard, Jean-Yves; Culp, Robert D.; Atwood, David A.; Cowley, Alan H. (1996-01-01). "17- and 19-Electron Complexes [Fe III (η 5 -C 5 R 5 )(S 2 CNMe 2 )L] n + ( n = 1, 0): Electronic Structure and Substitution and Redox Chemistry. Formation of [Fe IV (η 5 -C 5 R 5 )(dtc) 2 ] and Characterization of both 17e and 19e States of a Transition-Metal Complex". Journal of the American Chemical Society. 118 (17): 4133–4147. doi:10.1021/ja953603x. ISSN   0002-7863.
  10. ↑ Delville-Desbois, Marie-Hélène; Mross, Stefan; Astruc, Didier; Linares, Jorge; Varret, François; Le Beuze, Albert; Saillard, Jean-Yves; Culp, Robert D.; Atwood, David A.; Cowley, Alan H. (1996-01-01). "17- and 19-Electron Complexes [Fe III (η 5 -C 5 R 5 )(S 2 CNMe 2 )L] n + ( n = 1, 0): Electronic Structure and Substitution and Redox Chemistry. Formation of [Fe IV (η 5 -C 5 R 5 )(dtc) 2 ] and Characterization of both 17e and 19e States of a Transition-Metal Complex". Journal of the American Chemical Society. 118 (17): 4133–4147. doi:10.1021/ja953603x. ISSN   0002-7863.
  11. ↑ Astruc, Didier; Roman E., Enrique; Hamon, Jean Rene; Batail, Patrick (April 1979). "Novel reactions of dioxygen in organometallic chemistry. Hydrogen atom abstraction vs. dimerization of the 19-electron complexes .eta.5-cyclopentadienyliron(I) .eta.6-arene". Journal of the American Chemical Society. 101 (8): 2240–2242. doi:10.1021/ja00502a070. ISSN   0002-7863.
  12. 1 2 Méry, Denise; Aranzaes, Jaime Ruiz; Astruc, Didier (2006-05-01). "Use of an Electron-Reservoir Complex Together with Air to Generate N-Heterocyclic Carbenes". Journal of the American Chemical Society. 128 (17): 5602–5603. doi:10.1021/ja058421+. ISSN   0002-7863. PMID   16637604.
  13. ↑ Arduengo, Anthony J.; Harlow, Richard L.; Kline, Michael (January 1991). "A stable crystalline carbene". Journal of the American Chemical Society. 113 (1): 361–363. doi:10.1021/ja00001a054. ISSN   0002-7863.
  14. ↑ Chang, Mu-Chieh; Dann, Thomas; Day, David P.; Lutz, Martin; Wildgoose, Gregory G.; Otten, Edwin (2014-04-14). "The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States". Angewandte Chemie International Edition. 53 (16): 4118–4122. doi:10.1002/anie.201309948. PMID   24615928.
  15. ↑ Venkatesan, Koushik; Blacque, Olivier; Fox, Thomas; Alfonso, Montserrat; Schmalle, Helmut W.; Kheradmandan, Sohrab; Berke, Heinz (2005-02-01). "μ-Carbon−Carbon Bonds of Dinuclear Manganese Half-Sandwich Complexes as Electron Reservoirs". Organometallics. 24 (5): 920–932. doi:10.1021/om049284c. ISSN   0276-7333.
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