Group 2 organometallic chemistry

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Magnesium anthracenide with three thf ligands. GOHRUQ.png
Magnesium anthracenide with three thf ligands.

Group 2 organometallic chemistry refers to the chemistry of compounds containing carbon bonded to any group 2 element. [2] [3] By far the most common group 2 organometallic compounds are the magnesium-containing Grignard reagents which are widely used in organic chemistry. Other organometallic group 2 compounds are rare and are typically limited to academic interests.

Contents

Characteristics

As the group 2 elements (also referred to as the alkaline earth metals) contain two valence electrons, their chemistries have similarities group 12 organometallic compounds. Both readily assume a +2 oxidation states with higher and lower states being rare, and are less electronegative than carbon. However, as the group two elements (with the exception of beryllium) have considerably low electronegativity the resulting C-M bonds are more highly polarized and ionic-like, if not entirely ionic for the heavier barium compounds. The lighter organoberyllium and organomagnesium compounds are often considered covalent, but with some ionic bond characteristics owing to the attached carbon bearing a negative dipole moment. This higher ionic character and bond polarization tends to produce high coordination numbers and many compounds (particularly dialklys) are polymeric in solid or liquid states with highly complex structures in solution, though in the gaseous state they are often monomeric.

Metallocene compounds with group 2 elements are rare, but some do exist. Bis(cyclopentadienyl)beryllium or beryllocene (Cp2Be), with a molecular dipole moment of 2.2 D, is so-called slipped 5η/1η sandwich. While magnesocene (Cp2Mg) is a regular metallocene, bis(pentamethylcyclopentadienyl)calcium (Cp*)2Ca is bent with an angle of 147°.

Dimethylmagnesium is a polymer built up from 3-center, 2-electron bonded bridging methyl groups. Dimethylberyllium adopts the same structure. Me2Mg.png
Dimethylmagnesium is a polymer built up from 3-center, 2-electron bonded bridging methyl groups. Dimethylberyllium adopts the same structure.

Synthesis

Mixed alkyl/aryl-halide compounds, which contain a single C-M bond and a C-X bond, are typically prepared by oxidative addition. Magnesium-containing compounds of this configuration are known as the Grignard reagents, though some calcium Grignard's are known and more reactive and sensitive to decomposition. Calcium grignard's must be pre-activated prior to synthesis. [6]

There are three key reaction pathways for dialkyl and diaryl group 2 metal compounds.

MX2 + R-Y → MR2 + Y-X'
M'R2 + M → MR2 + M'
2 RMX → MR2 + MX2

Compounds

Although organomagnesium compounds are widespread in the form of Grignard reagents, the other organo-group 2 compound are almost exclusively of academic interest. Organoberyllium chemistry is limited due to the cost and toxicity of beryllium. Calcium is nontoxic and cheap but organocalcium compounds are difficult to prepare, strontium and barium compounds even more so. One use for these type of compounds is in chemical vapor deposition.

Organoberyllium

Beryllium derivatives and reagents are often prepared by alkylation of beryllium chloride. [7] Examples of known organoberyllium compounds are dineopentylberyllium, [8] beryllocene (Cp2Be), [9] [10] [11] [12] diallylberyllium (by exchange reaction of diethyl beryllium with triallyl boron), [13] bis(1,3-trimethylsilylallyl)beryllium [14] and Be(mes)2. [7] [15] Ligands can also be aryls [16] and alkynyls. [17]

Organomagnesium

The distinctive feature of the Grignard reagents is their formation from the organic halide and magnesium metal. Most other group II organic compounds are generated by salt metathesis, which limits their accessibility. The formation of the Grignard reagents has received intense scrutiny. It proceeds by a SET process. For less reactive organic halides, activated forms of magnesium have been produced in the form of Rieke magnesium. Examples of Grignard reagents are phenylmagnesium bromide and ethylmagnesium bromide. These simplified formulas are deceptive: Grignard reagents generally exist as dietherates, RMgX(ether)2. As such they obey the octet rule.

Grignard reagents participate in the Schlenk equilibrium. Exploiting this reaction is a way to generate dimethylmagnesium. Beyond Grignard reagents, another organomagnesium compound is magnesium anthracene. This orange solid is used as a source of highly active magnesium. Butadiene-magnesium serves as a source for the butadiene dianion. Ate complexes of magnesium are also well known, e.g LiMgBu3. [18]

Organocalcium

Dimethylcalcium is obtained by metathesis reaction of calcium bis(trimethylsilyl)amide and methyllithium in diethyl ether: [19]

A well known organocalcium compound is (Cp)calcium(I).[ citation needed ] Bis(allyl)calcium was described in 2009. [20] It forms in a metathesis reaction of allylpotassium and calcium iodide as a stable non-pyrophoric off-white powder:

The bonding mode is η 3. This compound is also reported to give access to an η 1 polymeric (CaCH2CHCH2)n compound. [21]

The compound [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3] also described in 2009 [22] [23] is an inverse sandwich compound with two calcium atoms at either side of an arene.

Olefins tethered to cyclopentadienyl ligands have been shown to coordinate to calcium(II), strontium(II), and barium(II): [24]

Olefine coordination at calcium strontium barium.png

Organocalcium compounds have been investigated as catalysts. [25]

Organostrontium

Organostrontium compounds have been reported as intermediates in Barbier-type reactions. [26] [27] [28]

Structure of Ba(CH(tms)2)2(thf)3 (tms = Si(CH3)3), with H atoms omitted. Even with bulky alkyl substituents, Ba coordinates to three THF ligands. LOMBOF.png
Structure of Ba(CH(tms)2)2(thf)3 (tms = Si(CH3)3), with H atoms omitted. Even with bulky alkyl substituents, Ba coordinates to three THF ligands.

Organobarium

Organobarium compounds [29] of the type (allyl)BaCl can be prepared by reaction of activated barium (Rieke method reduction of barium iodide with lithium biphenylide) with allyl halides. [30] [31] These allylbarium compounds react with carbonyl compounds. Such reagents are more alpha-selective and more stereoselective than the related Grignards or organocalcium compounds. The metallocene (Cp*)2Ba has also been reported. [32]

Organoradium

The only known organoradium compound is the gas-phase acetylide.

See also

Related Research Articles

<span class="mw-page-title-main">Grignard reaction</span> Organometallic coupling reaction

The Grignard reaction is an organometallic chemical reaction in which, according to the classical definition, carbon alkyl, allyl, vinyl, or aryl magnesium halides are added to the carbonyl groups of either an aldehyde or ketone under anhydrous conditions. This reaction is important for the formation of carbon-carbon bonds.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

Metalation is a chemical reaction that forms a bond to a metal. This reaction usually refers to the replacement of a halogen atom in an organic molecule with a metal atom, resulting in an organometallic compound. In the laboratory, metalation is commonly used to activate organic molecules during the formation of C—X bonds, which are necessary for the synthesis of many organic molecules.

<span class="mw-page-title-main">Barbier reaction</span> Reaction in organic chemistry

The Barbier reaction is an organometallic reaction between an alkyl halide, a carbonyl group and a metal. The reaction can be performed using magnesium, aluminium, zinc, indium, tin, samarium, barium or their salts. The reaction product is a primary, secondary or tertiary alcohol. The reaction is similar to the Grignard reaction but the crucial difference is that the organometallic species in the Barbier reaction is generated in situ, whereas a Grignard reagent is prepared separately before addition of the carbonyl compound. Unlike many Grignard reagents, the organometallic species generated in a Barbier reaction are unstable and thus cannot be stored or sold commercially. Barbier reactions are nucleophilic addition reactions that involve relatively inexpensive, water insensitive metals or metal compounds. For this reason, it is possible in many cases to run the reaction in water, making the procedure part of green chemistry. In contrast, Grignard reagents and organolithium reagents are highly moisture sensitive and must be used under an inert atmosphere without the presence of water. The Barbier reaction is named after Philippe Barbier, who was Victor Grignard's teacher.

The Schlenk equilibrium, named after its discoverer Wilhelm Schlenk, is a chemical equilibrium taking place in solutions of Grignard reagents and Hauser bases

<span class="mw-page-title-main">Grignard reagent</span> Organometallic compounds used in organic synthesis

A Grignard reagent or Grignard compound is a chemical compound with the general formula R−Mg−X, where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride Cl−Mg−CH3 and phenylmagnesium bromide (C6H5)−Mg−Br. They are a subclass of the organomagnesium compounds.

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

Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.

<span class="mw-page-title-main">Sandwich compound</span> Chemical compound made of two ring ligands bound to a metal

In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives. Because the metal is usually situated between the two rings, it is said to be "sandwiched". A special class of sandwich complexes are the metallocenes.

<span class="mw-page-title-main">Rieke metal</span> Group specially prepared, highly reactive metal powder

A Rieke metal is a highly reactive metal powder generated by reduction of a metal salt with an alkali metal. These materials are named after Reuben D. Rieke, who first described the recipes for their preparation. Among the many metals that have been generated by this method are Mg, Ca, Ti, Fe, Co, Ni, Cu, Zn, and In, which in turn are called Rieke-magnesium, Rieke-calcium, etc.

<span class="mw-page-title-main">Organonickel chemistry</span> Branch of organometallic chemistry

Organonickel chemistry is a branch of organometallic chemistry that deals with organic compounds featuring nickel-carbon bonds. They are used as a catalyst, as a building block in organic chemistry and in chemical vapor deposition. Organonickel compounds are also short-lived intermediates in organic reactions. The first organonickel compound was nickel tetracarbonyl Ni(CO)4, reported in 1890 and quickly applied in the Mond process for nickel purification. Organonickel complexes are prominent in numerous industrial processes including carbonylations, hydrocyanation, and the Shell higher olefin process.

<span class="mw-page-title-main">Organozirconium and organohafnium chemistry</span>

Organozirconium chemistry is the science of exploring the properties, structure, and reactivity of organozirconium compounds, which are organometallic compounds containing chemical bonds between carbon and zirconium. Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.

Organovanadium chemistry is the chemistry of organometallic compounds containing a carbon (C) to vanadium (V) chemical bond. Organovanadium compounds find only minor use as reagents in organic synthesis but are significant for polymer chemistry as catalysts.

In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp2TiCl2. Several reagents and much research is based on bent metallocenes.

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

Organocerium chemistry is the science of organometallic compounds that contain one or more chemical bond between carbon and cerium. These compounds comprise a subset of the organolanthanides. Most organocerium compounds feature Ce(III) but some Ce(IV) derivatives are known.

<span class="mw-page-title-main">Metal bis(trimethylsilyl)amides</span>

Metal bis(trimethylsilyl)amides are coordination complexes composed of a cationic metal M with anionic bis(trimethylsilyl)amide ligands (the −N 2 monovalent anion, or −N 2 monovalent group, and are part of a broader category of metal amides.

Hauser bases, also called magnesium amide bases, are magnesium compounds used in organic chemistry as bases for metalation reactions. These compounds were first described by Charles R. Hauser in 1947. Compared with organolithium reagents, the magnesium compounds have more covalent, and therefore less reactive, metal-ligand bonds. Consequently, they display a higher degree of functional group tolerance and a much greater chemoselectivity. Generally, Hauser bases are used at room temperature while reactions with organolithium reagents are performed at low temperatures, commonly at −78 °C.

<span class="mw-page-title-main">Transition-metal allyl complex</span>

Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. Allyl is the radical with the connectivity CH2CHCH2, although as a ligand it is usually viewed as an allyl anion CH2=CH−CH2−, which is usually described as two equivalent resonance structures.

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

Bis(cyclopentadienyl)titanium(III) chloride, also known as the Nugent–RajanBabu reagent, is the organotitanium compound which exists as a dimer with the formula [(C5H5)2TiCl]2. It is an air sensitive green solid. The complex finds specialized use in synthetic organic chemistry as a single electron reductant.

Magnesocene, also known as bis(cyclopentadienyl)magnesium(II) and sometimes abbreviated as MgCp2, is an organometallic compound with the formula Mg(η5-C5H5)2. It is an example of an s-block main group sandwich compound, structurally related to the d-block element metallocenes, and consists of a central magnesium atom sandwiched between two cyclopentadienyl rings.

<span class="mw-page-title-main">Organoberyllium chemistry</span> Organoberyllium Complex in Main Group Chemistry

Organoberyllium chemistry involves the synthesis and properties of organometallic compounds featuring the group 2 alkaline earth metal beryllium (Be). The area remains understudied, relative to the chemistry of other main-group elements, because although metallic beryllium is relatively unreactive, its dust causes berylliosis and compounds are toxic. Organoberyllium compounds are typically prepared by transmetallation or alkylation of beryllium chloride.

References

  1. ↑ Borislav Bogdanovic (1988). "Magnesium Anthracene Systems and their Application in Synthesis and Catalysis". Accounts of Chemical Research. 21 (7): 261–267. doi:10.1021/ar00151a002.
  2. ↑ Comprehensive Organometallic Chemistry by Mike Mingos, Robert Crabtree 2007 ISBN   978-0-08-044590-8
  3. ↑ C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN   3-527-28165-7
  4. ↑ Weiss, E. (1964). "Die Kristallstruktur des Dimethylmagnesiums". J. Organomet. Chem. 2 (4): 314–321. doi:10.1016/S0022-328X(00)82217-2.
  5. ↑ Snow, A.I.; Rundle, R.E. (1951). "Structure of Dimethylberyllium". Acta Crystallographica. 4 (4): 348–52. doi:10.1107/S0365110X51001100. hdl: 2027/mdp.39015095081207 .
  6. ↑ Reuben D. Rieke; Tse-Chong Wu; Loretta I. Rieke (1995). "Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol". Org. Synth. 72: 147. doi:10.15227/orgsyn.072.0147.
  7. 1 2 Off the Beaten Track—A Hitchhiker's Guide to Beryllium Chemistry D. Naglav, M. R. Buchner, G. Bendt, F. Kraus, S. Schulz, Angew. Chem. Int. Ed. 2016, 55, 10562. doi : 10.1002/anie.201601809
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  13. ↑ Wiegand, G.; Thiele, K.-H. (1974). "Ein Beitrag zur Existenz von Allylberyllium- und Allylaluminiumverbindungen". Zeitschrift für anorganische und allgemeine Chemie. 405: 101–108. doi:10.1002/zaac.19744050111.
  14. ↑ Chmely, Stephen C.; Hanusa, Timothy P.; Brennessel, William W. (2010). "Bis(1,3-trimethylsilylallyl)beryllium". Angewandte Chemie International Edition. 49 (34): 5870–4. doi:10.1002/anie.201001866. PMID   20575128.
  15. ↑ Synthesis and structural characterization of the beryllium compounds [Be(2,4,6-Me3C6H2)2(OEt2)], [Be{O(2,4,6-tert-Bu3C6H2)}2(OEt2)], and [Be{S(2,4,6-tert-Bu3C6H2)}2(THF)].cntdot.PhMe and determination of the structure of [BeCl2(OEt2)2] Karin Ruhlandt-Senge, Ruth A. Bartlett, Marilyn M. Olmstead, and Philip P. Power Inorganic Chemistry 1993 32 (9), 1724-1728 doi : 10.1021/ic00061a031
  16. ↑ Ruhlandt-Senge, Karin; Bartlett, Ruth A.; Olmstead, Marilyn M.; Power, Philip P. (1993). "Synthesis and structural characterization of the beryllium compounds [Be(2,4,6-Me3C6H2)2(OEt2)], [Be{O(2,4,6-tert-Bu3C6H2)}2(OEt2)], and [Be{S(2,4,6-tert-Bu3C6H2)}2(THF)].cntdot.PhMe and determination of the structure of [BeCl2(OEt2)2]". Inorganic Chemistry. 32: 1724. doi:10.1021/ic00061a031.
  17. ↑ Morosin, B; Howatson, J. (1971). "The crystal structure of dimeric methyl-1-propynyl- beryllium-كس امك trimethylamine". Journal of Organometallic Chemistry. 29: 7. doi:10.1016/S0022-328X(00)87485-9.
  18. ↑ Arredondo, Juan D.; Li, Hongmei; Balsells, Jaume (2012). "Preparation of t-Butyl-3-Bromo-5-Formylbenzoate Through Selective Metal-Halogen Exchange Reactions". Organic Syntheses. 89: 460. doi: 10.15227/orgsyn.089.0460 .
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  20. ↑ "Bis(allyl)calcium" Phillip Jochmann, Thomas S. Dols, Thomas P. Spaniol, Lionel Perrin, Laurent Maron, Jun Okuda Angewandte Chemie International Edition Volume 48 Issue 31, Pages 5715–5719 2009 doi : 10.1002/anie.200901743
  21. ↑ Lichtenberg, C., Jochmann, P., Spaniol, T. P. and Okuda, J. (2011), "The Allylcalcium Monocation: A Bridging Allyl Ligand with a Non-Bent Coordination Geometry". Angewandte Chemie International Edition, 50: 5753–5756. doi : 10.1002/anie.201100073
  22. ↑ "Stable 'Inverse' Sandwich Complex with Unprecedented Organocalcium(I): Crystal Structures of [(thf)2Mg(Br)-C6H2-2,4,6-Ph3] and [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3]" Sven Krieck, Helmar Görls, Lian Yu, Markus Reiher and Matthias Westerhausen J. Am. Chem. Soc. , 2009, 131 (8), pp 2977–2985 doi : 10.1021/ja808524y
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  24. ↑ H. Schumann; S. Schutte; H.-J. Kroth; D. Lentz (2004). "Butenyl-Substituted Alkaline-Earth Metallocenes: A First Step towards Olefin Complexes of the Alkaline-Earth Metals". Angew. Chem. Int. Ed. 43 (45): 6208–6211. doi: 10.1002/anie.200460927 . PMID   15549740.
  25. ↑ Arrowsmith, Merle; Crimmin, Mark R.; Barrett, Anthony G. M.; Hill, Michael S.; Kociok-KöHn, Gabriele; Procopiou, Panayiotis A. (2011). "Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination". Organometallics. 30 (6): 1493–1506. doi:10.1021/om101063m.
  26. ↑ Miyoshi, N.; Kamiura, K.; Oka, H.; Kita, A.; Kuwata, R.; Ikehara, D.; Wada, M. (2004). "The Barbier-Type Alkylation of Aldehydes with Alkyl Halides in the Presence of Metallic Strontium". Bulletin of the Chemical Society of Japan. 77 (2): 341. doi:10.1246/bcsj.77.341.
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  29. ↑ Comprehensive organic functional group transformations Alan R. Katritzky, Otto Meth-Cohn, Charles Wayne Rees
  30. ↑ Yanagisawa, A.; Habaue, S.; Yamamoto, H. (1991). "Allylbarium in organic synthesis: unprecedented .alpha.-selective and stereospecific allylation of carbonyl compounds". Journal of the American Chemical Society. 113 (23): 8955. doi:10.1021/ja00023a058.
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  32. ↑ Williams, R. A.; Hanusa, T. P.; Huffman, J. C. (1988). "Solid state structure of bis(pentamethylcyclopentadienyl)barium, (Me5C5)2Ba; the first X-ray crystal structure of an organobarium complex". Journal of the Chemical Society, Chemical Communications (15): 1045. doi:10.1039/C39880001045.
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