Group 2 organometallic chemistry

The group 2 elements are known to form organometallic compounds.[2][3] Of these, organomagnesium compounds, usually in the form of Grignard reagents are widely used in organic chemistry. The other organometallic compounds of this group are only of academic interest.

Magnesium anthracenide with three thf ligands.[1]

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. As a consequence of these two factors, the ionic character of the M-C bond increases as does the coordination number. Many dialkyl group 2 metals are polymeric. Their structures feature three-center two-electron bondings, reminiscent of that in trimethylaluminium. In the gas-phase they monomeric.

The metallocenes in this group are unusual. 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°. This angle increases going down the row.

Dimethylmagnesium is a polymer built up from 3-center, 2-electron bonded bridging methyl groups.[4] Dimethylberylium adopts the same structure.[5]

Synthesis

The mixed alkyl/aryl-halide compounds are typically prepared by oxidative addition. The iconic products of such reactions are the Grignard reagents. An analogous reaction proceeds with calcium but the metal must be specially activated.[6]

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.

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. Further down this group calcium is nontoxic and cheap but organocalcium compounds are difficult to prepare as are the organic derivatives of strontium and barium. One use for this 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
Main article: Grignard reagent

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.

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.

Organocalcium

Dimethylcalcium is obtained by metathesis reaction of calcium bis(trimethylsilyl)amide and methyllithium in diethyl ether:[18]

A well known organocalcium compound is (Cp)calcium(I). Bis(allyl)calcium was described in 2009.[19] 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.[20]

The compound [(thf)3Ca{μ-C6H3-1,3,5-Ph3}Ca(thf)3] also described in 2009[21][22] 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):[23]

Organocalcium compounds have been investigated as catalysts.[24]

Organostrontium

Organostrontium compounds have been reported as intermediates in Barbier-type reactions.[25][26][27]

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[28] 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.[29][30] 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.[31]

Organoradium

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

See also

References
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