Organoiridium chemistry

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Vaska's complex, an iconic organoiridium compound Vaskas.png
Vaska's complex, an iconic organoiridium compound
Crabtree's catalyst, which is an active catalyst for hydrogenation Crabtree.svg
Crabtree's catalyst, which is an active catalyst for hydrogenation
Tris(2-phenylpyridine)iridium Ir(ppy)3, useful photocatalyst and lumifore Ir(ppy) Schematic.png
Tris(2-phenylpyridine)iridium Ir(ppy)3, useful photocatalyst and lumifore
Structure of Ir4(CO)12. Ir4(CO)12.svg
Structure of Ir4(CO)12.
C-bonded isomer of "Ir(acac)3" C-bondedIr-acac.png
C-bonded isomer of "Ir(acac)3"
Oxotrimesityliridium, an example of an iridium(V) complex Ir(mes)3O.svg
Oxotrimesityliridium, an example of an iridium(V) complex

Organoiridium chemistry is the chemistry of organometallic compounds containing an iridium-carbon chemical bond. [2] Organoiridium compounds are relevant to many important processes including olefin hydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals. [3]

Contents

Classification based on principal oxidation states

Organoiridium compounds share many characteristics with those of rhodium, but less so with cobalt. Iridium can exist in oxidation states of -III to +V, but iridium(I) and iridium(III) are the more common. iridium(I) compounds (d8 configuration) usually occur with square planar or trigonal bipyramidal geometries, whereas iridium(III) compounds (d6 configuration) typically have an octahedral geometry. [3]

Iridium(0)

Iridium(0) complexes are binary carbonyls, the principal member being tetrairidium dodecacarbonyl, Ir4(CO)12. Unlike the related Rh4(CO)12, all CO ligands are terminal in Ir4(CO)12, analogous to the difference between Fe3(CO)12 and Ru3(CO)12. [4]

Iridium(I)

A well known example is Vaska's complex, bis(triphenylphosphine)iridium carbonyl chloride. Although iridium(I) complexes are often useful homogeneous catalysts, Vaska' complex is not. Rather it is iconic in the diversity of its reactions. Other common complexes include Ir2Cl2(cyclooctadiene)2, chlorobis(cyclooctene)iridium dimer, The analogue of Wilkinson's catalyst, IrCl(PPh3)3), undergoes orthometalation:

IrCl(PPh3)3 → HIrCl(PPh3)2(PPh2C6H4)

This difference between RhCl(PPh3)3 and IrCl(PPh3)3 reflects the generally greater tendency of iridium to undergo oxidative addition. A similar trend is exhibited by RhCl(CO)(PPh3)2 and IrCl(CO)(PPh3)2, only the latter oxidatively adds O2 and H2. [5] The olefin complexes chlorobis(cyclooctene)iridium dimer and cyclooctadiene iridium chloride dimer are often used as sources of "IrCl", exploiting the lability of the alkene ligands or their susceptibility to removal by hydrogenation. Crabtree's catalyst ([Ir(P(C6H11)3)(pyridine)(cyclooctadiene)]PF6) is a versatile homogeneous catalyst for hydrogenation of alkenes. [6]

(η5-Cp)Ir(CO)2 oxidatively adds C-H bonds upon photolytic dissociation of one CO ligand.

Iridium(II)

As is the case for rhodium(II), iridium(II) is rarely encountered. One example is iridocene, IrCp2. [7] As with rhodocene, iridocene dimerises at room temperature. [8]

Iridium(III)

Iridium is usually supplied commercially in the Ir(III) and Ir(IV) oxidation states. Important starting reagents being hydrated iridium trichloride and ammonium hexachloroiridate. These salts are reduced upon treatment with CO, hydrogen, and alkenes. Illustrative is the carbonylation of the trichloride: IrCl3(H2O)x + 3 CO → [Ir(CO)2Cl2]− + CO2 + 2 H+ + Cl− + (x-1) H2O

Many organoiridium(III) compounds are generated from pentamethylcyclopentadienyl iridium dichloride dimer. Many of derivatives feature kinetically inert cyclometalated ligands. [9] Related half-sandwich complexes were central in the development of C-H activation. [10] [11]

Organoiridium chemistry has been central to the development of C-H activation, two examples of which are shown here. CHactRGB+WAGimproved.png
Organoiridium chemistry has been central to the development of C-H activation, two examples of which are shown here.

Iridium(V)

Oxidation states greater than III are more common for iridium than rhodium. They typically feature strong-field ligands. One often cited example is oxotrimesityliridium(V). [12]

Uses

The dominant application of organoiridium complexes is as catalyst in the Cativa process for carbonylation of methanol to produce acetic acid. [13]

The catalytic cycle of the Cativa process. Cativa-process-catalytic-cycle.png
The catalytic cycle of the Cativa process.

Optical devices and photoredox

Iridium complexes such as cyclometallated derived from 2-phenylpyridines are used as phosphorescent organic light-emitting diodes. [14] Related complexes are photoredox catalysts.

Potential applications

Iridium complexes are highly active for hydrogenation both directly and via transfer hydrogenation. The asymmetric versions of these reactions are widely studied.

Many half-sandwich complexes have been investigated as possible anti-cancer drugs. Related complexes are electrocatalysts for the conversion of carbon dioxide to formate. [9] [15] In academic laboratories, iridium complexes are widely studied because its complexes promote C-H activation, but such reactions are not employed in any commercial process.

See also

Related Research Articles

Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidative addition is often a step in catalytic cycles, in conjunction with its reverse reaction, reductive elimination.

<span class="mw-page-title-main">Wilkinson's catalyst</span> Chemical compound

Wilkinson's catalyst is the common name for chlorido­tris(triphenylphosphine)­rhodium(I), a coordination complex of rhodium with the formula [RhCl(PPh3)3], where 'Ph' denotes a phenyl group). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who first popularized its use.

<span class="mw-page-title-main">Vaska's complex</span> Chemical compound

Vaska's complex is the trivial name for the chemical compound trans-carbonylchlorobis(triphenylphosphine)iridium(I), which has the formula IrCl(CO)[P(C6H5)3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide and a chloride ion. The complex was first reported by J. W. DiLuzio and Lauri Vaska in 1961. Vaska's complex can undergo oxidative addition and is notable for its ability to bind to O2 reversibly. It is a bright yellow crystalline solid.

<span class="mw-page-title-main">Rhodium(III) chloride</span> Chemical compound

Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble trihydrated (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.

<span class="mw-page-title-main">Iridium(III) chloride</span> Chemical compound

Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare, but the related hydrate is much more commonly encountered. The anhydrous salt has two polymorphs, α and β, which are brown and red colored respectively. More commonly encountered is the hygroscopic dark green trihydrate IrCl3(H2O)3 which is a common starting point for iridium chemistry.

<span class="mw-page-title-main">Metal carbonyl</span> Coordination complexes of transition metals with carbon monoxide ligands

Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysis, such as hydroformylation and Reppe chemistry. In the Mond process, nickel tetracarbonyl is used to produce pure nickel. In organometallic chemistry, metal carbonyls serve as precursors for the preparation of other organometallic complexes.

<span class="mw-page-title-main">Dicarbonyltris(triphenylphosphine)ruthenium(0)</span> Chemical compound

Dicarbonyltris(triphenylphosphine)ruthenium(0) or Roper's complex is a ruthenium metal carbonyl. In it, two carbon monoxide ligands and three triphenylphosphine ligands are coordinated to a central ruthenium(0) center.

<span class="mw-page-title-main">Crabtree's catalyst</span> Chemical compound

Crabtree's catalyst is an organoiridium compound with the formula [C8H12IrP(C6H11)3C5H5N]PF6. It is a homogeneous catalyst for hydrogenation and hydrogen-transfer reactions, developed by Robert H. Crabtree. This air stable orange solid is commercially available and known for its directed hydrogenation to give trans stereoselectivity with respective of directing group.

Martin Arthur Bennett FRS is an Australian inorganic chemist. He gained recognition for studies on the co-ordination chemistry of tertiary phosphines, olefins, and acetylenes, and the relationship of their behaviour to homogeneous catalysis.

In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">Organocobalt chemistry</span> Chemistry of compounds with a carbon to cobalt bond

Organocobalt chemistry is the chemistry of organometallic compounds containing a carbon to cobalt chemical bond. Organocobalt compounds are involved in several organic reactions and the important biomolecule vitamin B12 has a cobalt-carbon bond. Many organocobalt compounds exhibit useful catalytic properties, the preeminent example being dicobalt octacarbonyl.

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">Organorhodium chemistry</span> Field of study

Organorhodium chemistry is the chemistry of organometallic compounds containing a rhodium-carbon chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions.

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">Metal-phosphine complex</span>

A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).

In organometallic chemistry, a transition metal alkene complex is a coordination compound containing one or more alkene ligands. The inventory is large. Such compounds are intermediates in many catalytic reactions that convert alkenes to other organic products.

<span class="mw-page-title-main">Cyclooctadiene iridium chloride dimer</span> Chemical compound

Cyclooctadiene iridium chloride dimer is an organoiridium compound with the formula [Ir(μ2-Cl)(COD)]2, where COD is the diene 1,5-cyclooctadiene (C8H12). It is an orange-red solid that is soluble in organic solvents. The complex is used as a precursor to other iridium complexes, some of which are used in homogeneous catalysis. The solid is air-stable but its solutions degrade in air.

<span class="mw-page-title-main">Chlorobis(cyclooctene)iridium dimer</span> Chemical compound

Chlorobis(cyclooctene)iridium dimer is an organoiridium compound with the formula Ir2Cl2(C8H14)4, where C8H14 is cis-cyclooctene. Sometimes abbreviated Ir2Cl2(coe)4, it is a yellow, air-sensitive solid that is used as a precursor to many other organoiridium compounds and catalysts.

<span class="mw-page-title-main">Transition metal acyl complexes</span>

Transition metal acyl complexes describes organometallic complexes containing one or more acyl (RCO) ligands. Such compounds occur as transient intermediates in many industrially useful reactions, especially carbonylations.

Iridium compounds are compounds containing the element iridium (Ir). Iridium forms compounds in oxidation states between −3 and +9, but the most common oxidation states are +1, +3, and +4. Well-characterized compounds containing iridium in the +6 oxidation state include IrF6 and the oxides Sr2MgIrO6 and Sr2CaIrO6. iridium(VIII) oxide was generated under matrix isolation conditions at 6 K in argon. The highest oxidation state (+9), which is also the highest recorded for any element, is found in gaseous [IrO4]+.

References

  1. ↑ S. M. Bischoff; R. A. Periana (2010). Oxygen and Carbon Bound Acetylacetonato Iridium(III) Complexes. Inorganic Syntheses. Vol. 35. p. 173. doi: 10.1002/9780470651568 . ISBN   978-0-470-65156-8.
  2. ↑ Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997
  3. 1 2 Crabtree, Robert H. (2005). The Organometallic Chemistry of the Transition Metals (4th ed.). USA: Wiley-Interscience. ISBN   0-471-66256-9.
  4. ↑ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford:Butterworth-Heinemann. pp. 1113–1143, 1294. ISBN   0-7506-3365-4.
  5. ↑ Vaska, Lauri; DiLuzio, J.W. (1961). "Carbonyl and Hydrido-Carbonyl Complexes of Iridium by Reaction with Alcohols. Hydrido Complexes by Reaction with Acid". Journal of the American Chemical Society . 83 (12): 2784–2785. doi:10.1021/ja01473a054.
  6. ↑ Crabtree, Robert H. (1979). "Iridium Compounds in Catalysis". Acc. Chem. Res. 12 (9): 331–337. doi:10.1021/ar50141a005.
  7. ↑ Keller, H. J.; Wawersik, H. (1967). "Spektroskopische Untersuchungen an Komplexverbindungen. VI. EPR-spektren von (C5H5)2Rh und (C5H5)2Ir". J. Organomet. Chem. (in German). 8 (1): 185–188. doi:10.1016/S0022-328X(00)84718-X.
  8. ↑ Fischer, E. O.; Wawersik, H. (1966). "Über Aromatenkomplexe von Metallen. LXXXVIII. Über Monomeres und Dimeres Dicyclopentadienylrhodium und Dicyclopentadienyliridium und Über Ein Neues Verfahren Zur Darstellung Ungeladener Metall-Aromaten-Komplexe". J. Organomet. Chem. (in German). 5 (6): 559–567. doi:10.1016/S0022-328X(00)85160-8.
  9. 1 2 Liu, Zhe; Sadler, Peter J. (2014). "Organoiridium Complexes: Anticancer Agents and Catalysts". Accounts of Chemical Research. 47 (4): 1174–1185. doi:10.1021/ar400266c. PMC   3994614 . PMID   24555658.
  10. ↑ Andrew H. Janowicz; Robert G. Bergman (1982). "Carbon–hydrogen activation in saturated hydrocarbons: direct observation of M + R−H → M(R)(H)". J. Am. Chem. Soc. 104: 352–354. doi:10.1021/ja00365a091.
  11. ↑ Graham, William A.G. (1982). "Oxidative addition of the carbon–hydrogen bonds of neopentane and cyclohexane to a photochemically generated iridium(I) complex". Journal of the American Chemical Society. 104 (13): 3723–3725. doi:10.1021/ja00377a032.
  12. ↑ Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. (1993). "Synthesis and X-ray Crystal Structure of Oxotrimesityl-Iridium(V)". Polyhedron . 12 (16): 2009–2012. doi:10.1016/S0277-5387(00)81474-6.
  13. ↑ Cheung, Hosea; Tanke, Robin S.; Torrence, G. Paul (2000). "Acetic acid". Ullmann's Encyclopedia of Industrial Chemistry. Wiley. doi:10.1002/14356007.a01_045. ISBN   3-527-30673-0.
  14. ↑ Jaesang Lee; Hsiao-Fan Chen; Thilini Batagoda; Caleb Coburn; Peter I. Djurovich; Mark E. Thompson; Stephen R. Forrest (2016). "Deep Blue Phosphorescent Organic Light-Emitting Diodes with Very High Brightness and Efficiency". Nature Materials. 15 (1–2): 92–98. Bibcode:2016NatMa..15...92L. doi:10.1038/nmat4446. PMID   26480228.
  15. ↑ Maenaka, Yuta; Suenobu, Tomoyoshi; Fukuzumi, Shunichi (2012). "Catalytic interconversion between hydrogen and formic acid at ambient temperature and Pressure". Energy & Environmental Science. 5 (6): 7360–7367. doi:10.1039/c2ee03315a.
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