Magnesocene

Structural formula
Staggered and ecliptic conformation
General
Surname	Magnesocene
other names	Bis (η 5 -cyclopentadienyl) magnesium Di (cyclopentadienyl) magnesium (Cp) 2 mg
Molecular formula	C 10 H 10 Mg
Brief description	colorless crystalline solid
External identifiers / databases
CAS number 1284-72-6 EC number 603-275-0 ECHA InfoCard 100.110.799 PubChem 24942211 Wikidata Q1884288
properties
Molar mass	154.491 g mol −1
Physical state	firmly
Melting point	176 ° C
boiling point	290 ° C
solubility	violently decomposes in water, soluble in polar and non-polar solvents.
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 228-250-261-314 P: 210-222-231 + 232-280-305 + 351 + 338-422
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Magnesocene , or also di (cyclopentadienyl) magnesium, Mg (C ₅ H ₅ ) ₂ , abbreviated MgCp ₂ , is an organometallic compound from the family of metallocenes and forms a sandwich complex .

Manufacturing

The first production of magnesocene was reported independently by EO Fischer and G. Wilkinson as early as 1954 . These represented magnesocene by disproportionation of CpMgBr or by reaction of metallic magnesium with cyclopentadiene at 500 ° C. For the former reaction, cyclopentadienylmagnesium bromide was first obtained by reacting ethylmagnesium bromide with cyclopentadiene, which then at 220 ° C and 10 ⁻⁴  mbar to MgCp ₂ and MgBr ₂ disproportionate:

${\displaystyle \mathrm {Mg(C_{2}H_{5})Br\ {\xrightarrow[{-C_{2}H_{6}}]{+C_{5}H_{6},\ Et_{2}O}}\ Mg(C_{5}H_{5})Br{\xrightarrow[{-MgBr_{2}}]{x2,\ 220^{\circ }C,\ 10^{-4}mbar}}\ Mg(C_{5}H_{5})_{2}} }$

Et ₂ O = diethyl ether

${\displaystyle \mathrm {Mg\ +\ 2\ C_{5}H_{6}\ {\xrightarrow[{-H_{2}}]{500^{\circ }C}}\ Mg(C_{5}H_{5})_{2}\ } }$

The conversion of diorganyl magnesium compounds with cyclopentadiene is also possible on a laboratory scale .

${\displaystyle \mathrm {Mg(C_{4}H_{9})_{2}\ +\ 2\ C_{5}H_{6}\ \rightarrow \ Mg(C_{5}H_{5})_{2}\ +\ 2\ C_{4}H_{10}\uparrow \ } }$

properties

Magnesocene is a white to light yellow  powder that can be sublimated at 50 ° C / 10 ⁻³ mbar, which decomposes violently with water and can ignite spontaneously on contact with air. Of the metallocenes of the alkaline earth metal group, only magnesocene shows the classic sandwich structure. E. Weiß was able to show that magnesocene in the crystal has a staggered conformation with a Mg-C distance of 230  pm ; on the other hand, according to A. Haland, the metal-Cp bond is expanded in the gas phase and the molecule is in an ecliptic conformation. As is to be expected in a sandwich complex with 12 valence electrons, the Cp rings are arranged in parallel. It is not yet entirely clear whether the bond between the metal and the Cp ring is more of a covalent or more ionic type. The sandwich structure, which is analogous to ferrocene , does not necessarily imply a covalent bond; it could also be explained by van der Waals interactions . The electrical conductivity in liquid ammonia , the violent hydrolysis reaction and the ¹³ C-NMR shift of 108  ppm (in comparison LiCp = 103.6 ppm; NaCp = 103.4 ppm, FeCp ₂ = 68.2 ppm) speak for the rather ionic character ). On the other hand, ²⁵ Mg NMR data suggest a largely covalent bond.

Like the heavier homologues , magnesocene also forms corresponding adducts with oxygen-, nitrogen- and phosphorus-containing Lewis bases. It forms 1: 1 or 1: 2 complexes with ammonia or amines , which in the case of primary or secondary amines can also be isolated and analyzed crystallographically . In these adducts, the hapticity of a Cp ring changes from η ⁵ to η ² . With THF , however, an η ⁵ , η ¹ complex is formed.

Spectroscopic data

In the IR spectrum, magnesocene shows bands at 2926 cm ⁻¹ (aromatic CH stretching vibrations), 1630 cm ⁻¹ (aromatic ring vibrations) and 770 cm ⁻¹ (CH deformation vibrations). The ¹ H-NMR spectrum shows a singlet signal at 6.00 ppm, the ¹³ C-NMR spectrum a signal at 107.8 ppm.

use

Because of the low stability of the magnesocene, it is often used for cyclopentadienylation, i.e. H. used for the transfer of Cp units to other metals, with the formation of the more stable magnesium dihalide.

${\displaystyle \mathrm {Mg(C_{5}H_{5})_{2}\ +\ MCl_{2}\rightarrow \ M(C_{5}H_{5})_{2}\ +\ MgCl_{2}\ } }$

Magnesocene and its adducts are said to be easily sublimable raw materials for gas separation processes such as MOCVD , with which z. B. green or blue LEDs are used.

Individual evidence

1. ↑ ^{a b c d e f g} Anja Jaenschke: Base adducts of magnesocene: Representation and structural analysis of cyclopentadienyl, pentamethylcyclopentadienyl and indenyl metallocenes of magnesium . Dissertation, University of Hamburg, 2006 urn : nbn: de: gbv: 18-30786 .
2. ↑ WA Barber: Magnesium cyclopentadienide . In: Eugene G. Rochow (Ed.): Inorganic Syntheses . tape 6 . McGraw-Hill Book Company, Inc., 1960, pp. 11-15 (English).
3. ↑ Entry on bis (cyclopentadienyl) magnesium at ChemicalBook , accessed December 30, 2011.
4. ↑ ^{a b} Data sheet bis (cyclopentadienyl) magnesium (II), ≥95%, ≥99.99% trace metals basis (excludes ~ 300ppm Al) from Sigma-Aldrich , accessed on January 2, 2012 ( PDF ).
5. ↑ EO Fischer, W. Hafner: Cyclopentadienyl-vanadine-tetracarbonyl. In: Journal of Nature Research B . 9, 1954, pp. 503-504 ( online ).
6. ↑ G. Wilkenson, FA Cotton, Chem. Ind. (London) 1954, 307-308
7. ↑ ^{a b c d e} Christoph Elschenbroich: Organometallchemie. 6th edition, Teubner Wiesbaden, 2008, ISBN 978-3-8351-0167-8 , p. 65 ( limited preview in the Google book search).
8. ↑ Erwin Riedel: Modern Inorganic Chemistry. de Gruyter, 2007, ISBN 978-3-11-019060-1 , p. 623 ( limited preview in the Google book search).
9. ^ ^{A b} Zvi Rappoport, Ilan Marek: The Chemistry of Organomagnesium Compounds, Volume 2. John Wiley & Sons, 2008, ISBN 978-0-470-05719-3 , pp. 122–129 ( limited preview in Google book search) .