Utente:Grasso Luigi/sanbox1/Chimica organometallica gruppo 2

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Gli elementi del gruppo 2 sono noti per formare composti organometallici.[1][2] Tra questi, i composti organomagnesio, in forma di reagente di Grignard sono parecchio utilizzati in chimica organica, mentre gli altri composti di questo gruppo sono in gran parte accademici.

Proprietà[modifica | modifica wikitesto]

In many ways the chemistry of group 2 elements (the alkaline earth metals) mimics that of group 12 elements because both groups have filled s shells for valence electrons. Thus, both groups have nominal valency 2 and oxidation state +2. All group 2 elements are electropositive towards carbon and electronegativity decreases down the row. At the same time the atomic radius increases resulting in increasingly ionic character, higher coordination numbers, and increased ligand reactivity.

Many dialkyl group 2 metals are polymeric in the crystalline phase and resemble trimethylaluminium in three-center two-electron bond. In the gas-phase they are once again monomeric.

The metallocenes in this group are unusual. Bis(cyclopentadienyl)beryllium or beryllocene (Cp2Be) with a molecular dipole moment of 2.2 D rules out a classical metallocene with two hapticity 5 ligands. Instead the compound is a so-called slip 5η/1η sandwich and on top of that also fluxional up to −125 °C. While magnesocene (Cp2Mg) is a regular metallocene, bis(pentamethylcyclopentadienyl)calcium (Cp*)2Ca is actually bent with an angle of 147°. This angle increases going down the row.

Low-valent organometallics with formal oxidation state 1 having a metal to metal bond are also known.[3] A representative is LMg-MgL with L = [(Ar)NC(NPri2)N(Ar)]−.[4]

Sintesi[modifica | modifica wikitesto]

The mixed alkyl/aryl-halide compounds are typically prepared by oxidative addition. The iconic products of such reactions are the Grignard reagents. An analogou reaction proceeds with calcium but the metal must be specially activated.[5]

Three important ways to synthesize dialkyl and diaryl group 2 metal compounds are

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

See for example the formation of dimethylmagnesium.

Composti[modifica | modifica wikitesto]

Organoberillio[modifica | modifica wikitesto]

Organoberyllium chemistry is limited to academic research due to the cost and toxicity of beryllium, beryllium derivatives and reagents required for the introduction of beryllium, such as beryllium chloride. Organometallic beryllium compounds are known to be highly reactive [6]

Examples of known organoberyllium compounds are

  • dineopentylberyllium,[7]
  • beryllocene (Cp2Be),

[8][9][10][11]

  • diallylberyllium (by exchange reaction of diethyl beryllium with triallyl boron),[12]
  • bis(1,3-trimethylsilylallyl)beryllium [13]

Ligands can also be aryls[15] and alkynyls.[16]

Lo stesso argomento in dettaglio: Berylliosis.

Organomagnesio[modifica | modifica wikitesto]

Template:Main article Organomagnesium compounds are widespread. They are commonly found as Grignard reagents. The formation of alkyl or aryl magnesium halides (RMgX) from magnesium metal and an alkyl halide is attributed to a SET process. Examples of Grignards are phenylmagnesium bromide and ethylmagnesium bromide.

Relevant organic magnesium reagents outside the scope of Grignards are magnesium anthracene with magnesium forming a 1,4-bridge over the central hexagon used as a source of highly active magnesium and butadiene magnesium an adduct with butadiene and a source for the butadiene dianion.

Organocalcio[modifica | modifica wikitesto]

Further down this group calcium is nontoxic and cheap but organocalcium compounds are difficult to make. This is even more so for the remaining members strontium and barium, and for the case of radium there are none known at all. One use for this type of compounds is in chemical vapor deposition.

Dimethylcalcium as one of the simplest organocalcium compounds can be obtained by metathesis reaction of calcium bis(trimethylsilyl)amide and methyllithium in diethyl ether:[17]

A well known organocalcium compound is (Cp)calcium(I). Bis(allyl)calcium was described in 2009.[18] It forms in a metathesis reaction of allylpotassium and calcium iodide as a stable non-pyrophoric off-white powder:

The bonding mode is η3. This compound is also reported to give access to an η1 polymeric (CaCH2CHCH2)n compound.[19]

The compound [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3] also described in 2009[20][21] 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):[22]

Olefin complexes of calcium, strontium and barium[22]

Organocalcium compounds are investigated as catalysts.[23] [24][25] [26][27]

Organostronzio[modifica | modifica wikitesto]

Organostrontium compounds have been reported as intermediates in Barbier-type reactions.[28][29][30]

Organobario[modifica | modifica wikitesto]

Organobarium compounds[31] of the type (allyl)BaCl are known[32][33] and can be prepared by reaction of activated barium (Rieke method reduction of barium iodide with lithium biphenylide) with allyl halides at −78 °C.

Subsequent reaction of these allylbarium compounds with carbonyl compounds is reported to be more alpha-selective and more stereoselective than the related Grignards or organocalcium compounds. The metallocene (Cp*)2Ba has also been reported.[34]

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

Organoradio[modifica | modifica wikitesto]

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

Voci correlate[modifica | modifica wikitesto]

Template:ChemicalBondsToCarbon

Note[modifica | modifica wikitesto]

  1. ^ Comprehensive Organometallic Chemistry by Mike Mingos, Robert Crabtree 2007 ISBN 978-0-08-044590-8
  2. ^ C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
  3. ^ Low-Valent Organometallics-Synthesis, Reactivity, and Potential Applications, in Chemistry: A European Journal, vol. 16, n. 22, 2010, pp. 6416–28, DOI:10.1002/chem.201000580.
  4. ^ Stable Magnesium(I) Compounds with Mg-Mg Bonds, in Science, vol. 318, n. 5857, 2007, pp. 1754–7, DOI:10.1126/science.1150856.
  5. ^ Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol, in Org. Synth., vol. 72, 1995, DOI:10.15227/orgsyn.072.0147.
  6. ^ a b 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
  7. ^ Preparation of base-free beryllium alkyls from trialkylboranes. Dineopentylberyllium, bis(trimethylsilylmethyl)beryllium, and an ethylberyllium hydride, in Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 1971, p. 1308, DOI:10.1039/J19710001308.
  8. ^ Über Aromatenkomplexe von Metallen, XXV. Di-cyclopentadienyl-beryllium, in Chemische Berichte, vol. 92, n. 2, 1959, p. 482, DOI:10.1002/cber.19590920233.
  9. ^ A precise low-temperature crystal structure of Bis(cyclopentadienyl)Beryllium, in Australian Journal of Chemistry, vol. 37, n. 8, 1984, p. 1601, DOI:10.1071/CH9841601.
  10. ^ The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction, in Journal of Organometallic Chemistry, vol. 170, n. 3, 1979, p. 271, DOI:10.1016/S0022-328X(00)92065-5.
  11. ^ Crystal structure of bis(cyclopentadienyl)beryllium at −120 °C, in Acta Crystallographica Section B, vol. 28, n. 6, 1972, p. 1662, DOI:10.1107/S0567740872004820.
  12. ^ Ein Beitrag zur Existenz von Allylberyllium- und Allylaluminiumverbindungen, in Zeitschrift für anorganische und allgemeine Chemie, vol. 405, 1974, p. 101, DOI:10.1002/zaac.19744050111.
  13. ^ Bis(1,3-trimethylsilylallyl)beryllium, in Angewandte Chemie International Edition, vol. 49, n. 34, 2010, pp. 5870–4, DOI:10.1002/anie.201001866.
  14. ^ 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
  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], in Inorganic Chemistry, vol. 32, 1993, p. 1724, DOI:10.1021/ic00061a031.
  16. ^ The crystal structure of dimeric methyl-1-propynyl- beryllium-كس امك trimethylamine, in Journal of Organometallic Chemistry, vol. 29, 1971, p. 7, DOI:10.1016/S0022-328X(00)87485-9.
  17. ^ "Dimethylcalcium" Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, and Reiner Anwander J. Am. Chem. Soc. 2018, Volume 140, Issue 6, Pages 2373–2383 DOI: 10.1021/jacs.7b12984
  18. ^ "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
  19. ^ 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
  20. ^ "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
  21. ^ "Organometallic Compounds of the Heavier s-Block Elements—What Next?" J. David Smith Angew. Chem. Int. Ed. 2009, 48, 6597–6599 DOI: 10.1002/anie.200901506
  22. ^ a b Butenyl-Substituted Alkaline-Earth Metallocenes: A First Step towards Olefin Complexes of the Alkaline-Earth Metals, in Angew. Chem. Int. Ed., vol. 43, 2004, pp. 6208–6211, DOI:10.1002/anie.200460927.
  23. ^ Harder, S., Feil, F. and Knoll, K. (2001), Novel Calcium Half-Sandwich Complexes for the Living and Stereoselective Polymerization of Styrene . Angew. Chem. Int. Ed., 40: 4261–4264. DOI: 10.1002/1521-3773(20011119)40
  24. ^ Calcium-Mediated Intramolecular Hydroamination Catalysis Mark R. Crimmin, Ian J. Casely, and Michael S. Hill Journal of the American Chemical Society 2005 127 (7), 2042-2043 DOI: 10.1021/ja043576n
  25. ^ 2,5-Bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl Complexes of the Heavy Alkaline Earth Metals: Synthesis, Structures, and Hydroamination Catalysis Jelena Jenter, Ralf Köppe, and Peter W. Roesky Organometallics 2011 30 (6), 1404-1413 DOI: 10.1021/om100937c
  26. ^ Cation Charge Density and Precatalyst Selection in Group 2-Catalyzed Aminoalkene Hydroamination Merle Arrowsmith, Mark R. Crimmin, Anthony G. M. Barrett, Michael S. Hill, Gabriele Kociok-Köhn, and Panayiotis A. Procopiou Organometallics 2011 30 (6), 1493-1506 DOI: 10.1021/om101063m
  27. ^ Penafiel, J., Maron, L. and Harder, S. (2014), Early Main Group Metal Catalysis: How Important is the Metal?. Angew. Chem. Int. Ed. DOI: 10.1002/anie.201408814
  28. ^ The Barbier-Type Alkylation of Aldehydes with Alkyl Halides in the Presence of Metallic Strontium, in Bulletin of the Chemical Society of Japan, vol. 77, n. 2, 2004, p. 341, DOI:10.1246/bcsj.77.341.
  29. ^ The Chemistry of Alkylstrontium Halide Analogues: Barbier-type Alkylation of Imines with Alkyl Halides, in Chemistry Letters, vol. 34, n. 6, 2005, p. 760, DOI:10.1246/cl.2005.760.
  30. ^ The Chemistry of Alkylstrontium Halide Analogues, Part 2: Barbier-Type Dialkylation of Esters with Alkyl Halides, in European Journal of Organic Chemistry, vol. 2005, n. 20, 2005, p. 4253, DOI:10.1002/ejoc.200500484.
  31. ^ Comprehensive organic functional group transformations Alan R. Katritzky, Otto Meth-Cohn, Charles Wayne Rees
  32. ^ Allylbarium in organic synthesis: unprecedented .alpha.-selective and stereospecific allylation of carbonyl compounds, in Journal of the American Chemical Society, vol. 113, n. 23, 1991, p. 8955, DOI:10.1021/ja00023a058.
  33. ^ Allylbarium Reagents: Unprecedented Regio- and Stereoselective Allylation Reactions of Carbonyl Compounds, in Journal of the American Chemical Society, vol. 116, n. 14, 1994, p. 6130, DOI:10.1021/ja00093a010.
  34. ^ Solid state structure of bis(pentamethylcyclopentadienyl)barium, (Me5C5)2Ba; the first X-ray crystal structure of an organobarium complex, in Journal of the Chemical Society, Chemical Communications, n. 15, 1988, p. 1045, DOI:10.1039/C39880001045.
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