Group 2 organometallic chemistry

Magnesium anthracenide with three thf ligands.[1]

Group 2 organometallic chemistry refers to the organic derivativess of any group 2 element. It is a subtheme to main group organometallic chemistry.[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 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°.

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

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.

• metathesis:

MX₂ + R-Y → MR₂ + Y-X'

• transmetallation:

M'R₂ + M → MR₂ + M'

• via the Schlenk equilibrium:

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

Main article: Organoberyllium chemistry

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

See also: 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.

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

Main article: Organocalcium chemistry

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

${\displaystyle \mathrm {Ca[N\{Si(CH_{3})_{3}\}_{2}]_{2}+2\ LiCH_{3}\longrightarrow Ca(CH_{3})_{2}+2\ Li[N\{Si(CH_{3})_{3}\}_{2}]} }$

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:

${\displaystyle {\ce {{\overset {allylpotassium}{2KC3H5}}+{\overset {calcium\ iodide}{CaI2}}->[{\ce {THF}}][25^{\circ }{\ce {C}}]{(C3H5)2Ca}+2KI}}}$

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]

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.

See also

• Organometallic chemistry

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. 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
8. 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.
9. 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.
10. 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. doi:10.1071/CH9841601.
11. 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. doi:10.1016/S0022-328X(00)92065-5.
12. 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. 28 (6): 1662. doi:10.1107/S0567740872004820.
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-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.
19. "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
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
23. "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
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.
27. 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. doi:10.1246/cl.2005.760.
28. 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. doi:10.1002/ejoc.200500484.
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.
31. 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. doi:10.1021/ja00093a010.
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.