Organoplatinum chemistry

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Organoplatinum chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions. [1] [2] [3] Organoplatinum compounds exist in oxidation state 0 to IV, with oxidation state II most abundant. The general order in bond strength is Pt-C (sp) > Pt-O > Pt-N > Pt-C (sp3). Organoplatinum and organopalladium chemistry are similar, but organoplatinum compounds are more stable and therefore less useful as catalysts.

Contents

Organoplatinum(0)

Most organoplatinum(0) compounds contain alkene and alkyne ligands. Carbonyl complexes are rare, and the analogue of Ni(CO)4 is elusive. The alkene and alkyne ligands serve as two-electron donors, for example in the complexes (PPh3)2Pt(C2H4) and (PPh3)2Pt(C2Ph2). The ethylene ligand in (PPh3)2Pt(C2H4) is labile and exchanges with alkynes and electrophilic alkenes, even C60 a fullerene.

A general synthetic route to (PPh3)2Pt(un) (un = alkene, alkyne) is reduction of potassium tetrachloroplatinate with ethanolic potassium hydroxide or hydrazine in presence of a phosphine ligand such as triphenylphosphine and the alkene or alkyne. Such reactions proceed via the intermediacy of cis-dichlorobis(triphenylphosphine)platinum(II). Nitrogen-based ligands do not often support the formation of platinum complexes of alkenes and alkynes.

Zerovalent organoplatinum complexes lacking phosphine ligands are often prepared via PtCl2(COD).

Li2C8H8 + PtCl2(COD) + 3 C7H10 → [Pt(C7H10)3] + 2 LiCl + C8H8 + C8H12
Pt(C7H10)3 + 2 COD → Pt(COD)2 + 3 C7H10

where C7H10 is norbornene.

Organoplatinum(I)

Platinum(I) compounds are uncommon but generally are diamagnetic because they have Pt-Pt bonds. An example is the dication [Pt2(CO)6]2+.

Organoplatinum(II)

A historically significant organoplatinum(II) compound is Zeise's salt, which is obtained from ethylene and potassium tetrachloroplatinate:

Zeise-synth.svg

The colourless diolefin complex dichloro(cycloocta-1,5-diene)platinum(II) is a more modern relative, and is more widely used.

Cl2Ptcod.png

The stability and diversity of platinum(II) alkene complexes contrasts with the rarity of alkene complexes of nickel(II). Platinum allyl complexes are also common. In contrast to nickel chemistry, where compounds such as CpNi(L)X are common, cyclopentadienyl derivatives of Pt(II) are rare, consistent with the reluctance of Pt(II) to become pentacoordinate.

Alkyl and aryl platinum(II) complexes are often prepared by oxidative addition of an alkyl halide or aryl halide to a Pt(0) precursor such as tetrakis(triphenylphosphine)platinum(0) or Pt(C2H4)(PPh3)2. Alternatively, platinum(II) chlorides are susceptible to alkylation: [4] [5]

PtCl2(SMe2)2 + 2 MeLi → PtMe2(SMe2)2 + 2 LiCl

The dimethylsulfide ligands in PtMe2(SMe2)2 can be displaced by other ligands.

Many organoplatinum(II) complexes arise via ortho-metalation and related intramolecular C-H activation processes.

Organoplatinum(IV)

The first organoplatinum compound ever synthesised was trimethylplatinum iodide from platinum(IV) chloride and methylmagnesium iodide, reported by Pope and Peachey in 1907. [6] [7] The compound adopts a cubane-like structure with four triply bridging iodide ligands. "Tetramethylplatinum" was claimed in 1952 by Henry Gilman as a derivative of this tetramer, but this claim was later shown to be incorrect ("Tetramethylplatinum" proved to be [PtMe3OH]4). Salts of [PtMe6]2− and [PtMe4]2− have been characterized. [8]

Structure of [Me3PtI]4. FUBREA.png
Structure of [Me3PtI]4.

Organoplatinum(IV) hydrides are rare. [10] The first isolated representatives were prepared from organotin halides or acids with orthometalated arylplatinum(II) compounds. The compound Me(PEt3)2PtOTf reacts reversibly with triflic acid between -60 and -80 °C, forming methane and (PEt3)2Pt(OTf)2 at -20 °C. Weak acids often suffice even water and alcohol and in C-H bond activation the proton source is an alkane.

Catalysis

Heterogeneous catalysts based on platinum play a major role in the petrochemical industry, and it is assumed that these useful reactions proceed via surface-bound organoplatinum intermediates. Better defined but less commercially significant are homogeneous catalysts based on platinum.

For hydrosilylation, H2PtCl6 ("Speier's catalyst") is an important catalyst. Mechanisms for this catalytic system usually assume intermediates that contain hydride, silyl ligand (R3Si), and alkene ligands. [11] Cis-dichlorobis(diethyl sulfide)platinum(II) and Karstedt's catalyst (adduct of divinyltetramethyldisiloxane and chloroplatinic acid) also catalyse hydrosilylation. [12] Many metallodendrimers have repeating units based on organoplatinum compounds.

Idealized mechanism for metal-catalysed hydrosilylation of an alkene. HSi'n.png
Idealized mechanism for metal-catalysed hydrosilylation of an alkene.

Research themes

Organoplatinum compounds are implicated in the Shilov system for the conversion of methane into methyl chloride. Strenuous efforts have been made, thus far unsuccessfully, to extend this reactivity to practical methods for functionalizing methane. [13] For example, platinum complexes of bipyrimidine catalyze the conversion of methane, oxygen, and sulfur trioxide into methyl bisulfate. [14]

Related Research Articles

<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.

Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.

<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.

Cycloocta-1,5-diene is a cyclic hydrocarbon with the chemical formula C8H12, specifically [−(CH2)2−CH=CH−]2.

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

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

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

Zeise's salt, potassium trichloro(ethylene)platinate(II) hydrate, is the chemical compound with the formula K[PtCl3(C2H4)]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry. The salt is of historical importance in the area of organometallic chemistry as one of the first examples of a transition metal alkene complex and is named for its discoverer, William Christopher Zeise.

<span class="mw-page-title-main">Organocopper chemistry</span> Compound with carbon to copper bonds

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

Tetrakis(triphenylphosphine)platinum(0) is the chemical compound with the formula Pt(P(C6H5)3)4, often abbreviated Pt(PPh3)4. The bright yellow compound is used as a precursor to other platinum complexes.

Organophosphines are organophosphorus compounds with the formula PRnH3−n, where R is an organic substituent. These compounds can be classified according to the value of n: primary phosphines (n = 1), secondary phosphines (n = 2), tertiary phosphines (n = 3). All adopt pyramidal structures. Organophosphines are generally colorless, lipophilic liquids or solids. The parent of the organophosphines is phosphine (PH3).

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<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">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.

<span class="mw-page-title-main">Organomolybdenum chemistry</span> Chemistry of compounds with Mo-C bonds

Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.

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

Dichlorotris(triphenylphosphine)ruthenium(II) is a coordination complex of ruthenium. It is a chocolate brown solid that is soluble in organic solvents such as benzene. The compound is used as a precursor to other complexes including those used in homogeneous catalysis.

<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).

<span class="mw-page-title-main">Bis(triphenylphosphine)platinum chloride</span> Chemical compound

Bis(triphenylphosphine)platinum chloride is a metal phosphine complex with the formula PtCl2[P(C6H5)3]2. Cis- and trans isomers are known. The cis isomer is a white crystalline powder, while the trans isomer is yellow. Both isomers are square planar about the central platinum atom. The cis isomer is used primarily as a reagent for the synthesis of other platinum compounds.

<span class="mw-page-title-main">Dichlorobis(triphenylphosphine)nickel(II)</span> Chemical compound

Dichlorobis(triphenylphosphine)nickel(II) refers to a pair of metal phosphine complexes with the formula NiCl2[P(C6H5)3]2. The compound exists as two isomers, a paramagnetic dark blue solid and a diamagnetic red solid. These complexes function as catalysts for organic synthesis.

In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.

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

Trimethylplatinum iodide is the organoplatinum complex with the formula [(CH3)3PtI]4. It is a white, air-stable solid that was one of the first σ-alkyl metal complexes reported. It arises from the reaction of potassium hexachloroplatinate with methylmagnesium iodide. The complex exists as a tetramer: a cubane-type cluster with four octahedral Pt(IV) centers linked by four iodides as triply bridging ligands. Due to its stability, it is often utilized as a precursor en route to the synthesis of other organoplatinum compound, such as hydrosilylation catalysts. It is also used as a precursor for forming platinum layers for electronics.

References

  1. ↑ Nickel, Palladium and Platinum (Comprehensive Organometallic Chemistry II) R.J. Puddephatt (Editor) 2002 0080423167
  2. ↑ Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. S. Komiya, M. Hurano 1997
  3. ↑ Jwu-Ting Chen "Platinum: Organometallic Chemistry" in Encyclopedia of Inorganic Chemistry 2006, John Wiley & Sons, New York. doi : 10.1002/0470862106.ia195
  4. ↑ Costa, E.; Pringle, P. G.; Ravetz, M. (1997). [(1,2,5,6-N)-1,5-Cyclooctadiene]Dimethyl-Platinum(II). Inorganic Syntheses. Vol. 31. p. 284. doi:10.1002/9780470132623.ch49. ISBN   978-0-470-13262-3.
  5. ↑ Hill, Geoffrey S.; Irwin, Michael J.; Levy, Christopher J.; Rendina, Louis M.; Puddephatt, Richard J. (1998). Platinum(II) Complexes of Dimethyl Sulfide. Inorganic Syntheses. Vol. 32. p. 149. doi:10.1002/9780470132630.ch25. ISBN   978-0-470-13263-0.
  6. ↑ "Proceedings of the Chemical Society, Vol. 23, No. 323". Proceedings of the Chemical Society (London). 23 (323): 81–94. 1907. doi:10.1039/PL9072300081.
  7. ↑ Pope, W. J.; Peachey, S. J. (1909). "LXXIII.?The alkyl compounds of platinum". Journal of the Chemical Society, Transactions. 95: 571–576. doi:10.1039/CT9099500571.
  8. ↑ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford: Butterworth-Heinemann. ISBN   0-7506-3365-4.
  9. ↑ Ebert, K. H.; Massa, W.; Donath, H.; Lorberth, J.; Seo, B. S.; Herdtweck, E. (1998). "Organoplatinum Compounds: VI. Trimethylplatinum Thiomethylate and Trimethylplatinum Iodide. The Crystal Structures of [(CH3)3PtS(CH3)]4 and [(CH3)3PtI]4·0.5CH3I". J. Organomet. Chem. 559 (1–2): 203–207. doi:10.1016/S0022-328X(98)00414-8.
  10. ↑ Puddephatt, R. (2001). "Platinum(IV) hydride chemistry". Coordination Chemistry Reviews. 219–221: 157–185. doi:10.1016/S0010-8545(01)00325-3.
  11. ↑ C. Elschenbroich, Organometallics (2006) Wiley and Sons-VCH: Weinheim. ISBN   978-3-527-29390-2
  12. ↑ Kettler, P. B. (2003). "Platinum Group Metals in Catalysis: Fabrication of Catalysts and Catalyst Precursors". Organic Process Research & Development. 7 (3): 342–354. doi:10.1021/op034017o.
  13. ↑ Stahl, Shannon S.; Labinger, Jay A.; Bercaw, John E. (1998). "Homogeneous Oxidation of Alkanes by Electrophilic Late Transition Metals". Angewandte Chemie International Edition. 37 (16): 2180–2192. doi:10.1002/(SICI)1521-3773(19980904)37:16<2180::AID-ANIE2180>3.0.CO;2-A. PMID   29711451.
  14. ↑ Brian G. Hashiguchi; Steven M. Bischof; Michael M. Konnick; Roy A. Periana (2012). "Designing Catalysts for Functionalization of Unactivated C–H Bonds Based on the CH Activation Reaction". Acc. Chem. Res. 45 (6): 885–898. doi:10.1021/ar200250r. PMID   22482496.
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