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Group 2 organometallic chemistry

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The group 2 elements are known to form organometallic compounds. .[1] [2] Of these, organomagnesium compounds, usually in the form of Grignard reagents are widely used in organic chemistry, while the other organometallic compounds of this group are largely academic.

Characteristics

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.

It is also important to assess the nature of the carbon-metal bond as it can just as easily be an ionic bond and not a covalent bond. For this reason metal cyanides (MCN), metal acetylides and carbides (for instance calcium carbide, an acetylene source) are excluded.

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].

Synthesis

Three important ways for synthesis of dialkyl and diaryl group 2 metal compounds is by metathesis:

MX2 + R-Y → MR2 + Y-X'

By transmetallation:

M'R2 + M → MR2 + M'

Manipulation of the Schlenk equilibrium of the organometal halides:

2 RMX → MR2 + MX2

See for example the formation of dimethylmagnesium.

Compounds

Organoberyllium

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. Examples of known organoberyllium compounds are dineopentylberyllium [5], beryllocene (Cp2Be) [6] [7] [8] [9], diallylberyllium (by exchange reaction of diethyl beryllium with triallyl boron) [10] and bis(1,3-trimethylsilylallyl)beryllium [11]. Ligands can also be aryls [12] and alkynyls [13].

See also: Berylliosis

Organomagnesium

Main article: Grignard reagent

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.

Organocalcium

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. For the case of radium, only the gas-phase acetylide. One use for this type of compounds is in chemical vapor deposition.

A well known organocalcium compound is (Cp)calcium(I). Bis(allyl)calcium was described in 2009 [14]. It forms in a metathesis reaction of allylpotassium and calcium iodide as a stable non-pyrophoric off-white powder:

2 KC3H5 + CaI2 → (C3H5)2Ca + 2 KI (THF 25 °C)

The bonding mode is η3. The compound [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3] also described in 2009 [15] [16] is an inverse sandwich compound with two calcium atoms at either side of an arene.

Organostrontium

Organostrontium compounds have been reported as intermediates in Barbier-type reactions [17] [18] [19]

Organobarium

Organobarium compounds [20] of the type (allyl)BaCl are known [21] [22] 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 [23]

See also

References

  1. ^ Comprehensive Organometallic Chemistry by Mike Mingos, Robert Crabtree 2007 ISBN 9780080445908
  2. ^ C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
  3. ^ Schulz, Stephan (2010). "Low-Valent Organometallics-Synthesis, Reactivity, and Potential Applications". Chemistry - A European Journal: n/a. doi:10.1002/chem.201000580.
  4. ^ Green, S. P.; Jones, C.; Stasch, A. (2007). "Stable Magnesium(I) Compounds with Mg-Mg Bonds". Science. 318 (5857): 1754. doi:10.1126/science.1150856. PMID 17991827.
  5. ^ Coates, G. E.; Francis, B. R. (1971). "Preparation of base-free beryllium alkyls from trialkylboranes. Dineopentylberyllium, bis(trimethylsilylmethyl)beryllium, and an ethylberyllium hydride". Journal of the Chemical Society A: Inorganic, Physical, Theoretical: 1308. doi:10.1039/J19710001308.
  6. ^ Fischer, Ernst Otto; Hofmann, Hermann P. (1959). "Über Aromatenkomplexe von Metallen, XXV. Di-cyclopentadienyl-beryllium". Chemische Berichte. 92: 482. doi:10.1002/cber.19590920233.
  7. ^ Nugent, KW; Beattie, JK; Hambley, TW; Snow, MR (1984). "A precise low-temperature crystal structure of Bis(cyclopentadienyl)beryllium". Australian Journal of Chemistry. 37: 1601. doi:10.1071/CH9841601.
  8. ^ Almenningen, A (1979). "The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction". Journal of Organometallic Chemistry. 170: 271. doi:10.1016/S0022-328X(00)92065-5.
  9. ^ Wong, C. H.; Lee, T. Y.; Chao, K. J.; Lee, S. (1972). "Crystal structure of bis(cyclopentadienyl)beryllium at –120°C". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 28: 1662. doi:10.1107/S0567740872004820.
  10. ^ Wiegand, G.; Thiele, K.-H. (1974). "Ein Beitrag zur Existenz von Allylberyllium- und Allylaluminiumverbindungen". Zeitschrift f�r anorganische und allgemeine Chemie. 405: 101. doi:10.1002/zaac.19744050111. {{cite journal}}: replacement character in |journal= at position 14 (help)
  11. ^ Chmely, Stephen C.; Hanusa, Timothy P.; Brennessel, William W. (2010). "Bis(1,3-trimethylsilylallyl)beryllium". Angewandte Chemie International Edition: n/a. doi:10.1002/anie.201001866.
  12. ^ 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.
  13. ^ Morosin, B (1971). "The crystal structure of dimeric methyl-1-propynyl- beryllium-trimethylamine". Journal of Organometallic Chemistry. 29: 7. doi:10.1016/S0022-328X(00)87485-9.
  14. ^ 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
  15. ^ 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 Grls, Lian Yu, Markus Reiher and Matthias Westerhausen J. Am. Chem. Soc., 2009, 131 (8), pp 2977–2985 doi:10.1021/ja808524y
  16. ^ 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
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  20. ^ Comprehensive organic functional group transformations Alan R. Katritzky,Otto Meth-Cohn,Charles Wayne Rees
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