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.
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°.
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.
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.
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]
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]
The bonding mode is
η3. This compound is also reported to give access to an
η1 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.
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.
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.
^Borislav Bogdanovic (1988). "Magnesium Anthracene Systems and their Application in Synthesis and Catalysis". Accounts of Chemical Research. 21 (7): 261–267.
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^Comprehensive Organometallic Chemistry by Mike Mingos, Robert Crabtree 2007ISBN978-0-08-044590-8
^C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim.
ISBN3-527-28165-7
^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.
^
abOff 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.
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^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.
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10.1039/J19710001308.
^Fischer, Ernst Otto; Hofmann, Hermann P. (1959). "Über Aromatenkomplexe von Metallen, XXV. Di-cyclopentadienyl-beryllium". Chemische Berichte. 92 (2): 482.
doi:
10.1002/cber.19590920233.
^Nugent, KW; Beattie, JK; Hambley, TW; Snow, MR (1984). "A precise low-temperature crystal structure of Bis(cyclopentadienyl)beryllium". Australian Journal of Chemistry. 37 (8): 1601.
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^Almenningen, A; Haaland, Arne; Lusztyk, Janusz (1979). "The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction". Journal of Organometallic Chemistry. 170 (3): 271.
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10.1016/S0022-328X(00)92065-5.
^Wiegand, G.; Thiele, K.-H. (1974). "Ein Beitrag zur Existenz von Allylberyllium- und Allylaluminiumverbindungen". Zeitschrift für anorganische und allgemeine Chemie. 405: 101–108.
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^Chmely, Stephen C.; Hanusa, Timothy P.; Brennessel, William W. (2010). "Bis(1,3-trimethylsilylallyl)beryllium". Angewandte Chemie International Edition. 49 (34): 5870–4.
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^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
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^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.
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^Morosin, B; Howatson, J. (1971). "The crystal structure of dimeric methyl-1-propynyl- beryllium-trimethylamine". Journal of Organometallic Chemistry. 29: 7.
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^"Dimethylcalcium" Benjamin M. Wolf, Christoph Stuhl, Cäcilia Maichle-Mössmer, and Reiner Anwander J. Am. Chem. Soc.2018, Volume 140, Issue 6, Pages 2373–2383
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^"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
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^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.
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^"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
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^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.
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^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.
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^Miyoshi, N.; Ikehara, D.; Kohno, T.; Matsui, A.; Wada, M. (2005). "The Chemistry of Alkylstrontium Halide Analogues: Barbier-type Alkylation of Imines with Alkyl Halides". Chemistry Letters. 34 (6): 760.
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^Miyoshi, N.; Matsuo, T.; Wada, M. (2005). "The Chemistry of Alkylstrontium Halide Analogues, Part 2: Barbier-Type Dialkylation of Esters with Alkyl Halides". European Journal of Organic Chemistry. 2005 (20): 4253.
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^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.
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^Yanagisawa, A.; Habaue, S.; Yasue, K.; Yamamoto, H. (1994). "Allylbarium Reagents: Unprecedented Regio- and Stereoselective Allylation Reactions of Carbonyl Compounds". Journal of the American Chemical Society. 116 (14): 6130.
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^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.
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