Metallacycle
editIn organometallic chemistry a metallacycle is referred to a cyclic compound where at least one carbon center in the cycle has been replaced by a metal. The general occurrence of metallacycles is observed as intermediates during catalytic reactions. One main effect that the substitution of a metallic atom has on a cyclic carbon compound is the distortion of its geometry due to the large size of typical metals.
Metallacyclopentanes
editThe parent metallacyclopentane has the formula LnM(CH)4. Such compounds are intermediates in the metal catalysed dimerization, trimerization and tetramerization of ethylene to give 1-butene, 1-hexene and 1-octene respectively. [1] [2] These cyclopentadienyl chromium metallocycle complexes were found to be very active ethylene polymerisation catalysts. This is the reason for the involvement of these metallacyclic intermediates in the chromium-catalyzed trimerization of ethylene to 1-hexene.
Evidences suggesting use of Metallacyclopentanes as catalysts
editDue to the presence of variable oxidation states and ability to shift readily from one state to another, transition metals can be utilized as catalysts for a number of reactions. One such work has been reported for the linear and cyclodimerization of olefins.[3]
A large number of catalytic systems have been known to give linear dimers but only a certain number of catalysts can lead to cyclodimerization of strained olefins. Linear dimerization is proceeded by using metal hydrides as catalysts while cyclodimerization proceeds through metallacyclopentane intermediates. It has been proved that phosphine nickelcyclopentanes decomposed to produce cyclobutene, ethylene, or 1-butene depending on the coordination number of the complex. In the equation 2, ethylene is produced as a main product with 92% by sample of biscyclopentadienyltetramethylenetitanium. The equation 3 allows us to conclude that the metallocycles were in equilibrium with the bis-olefin complexes.[4] For every catalytic or stoichiometric reaction that is practically useful and involves a metallacyclic compound, the terminating step is the decomposition to organic products. In this metathesis mechanism below, a metallacyclopentane is considered as catalytic intermediate to undergo ring reduction. There are four types of compounds, depending on how the constituent atoms are held together.
Structure and Physical Properties
editUnsubstituted metallacycloalkanes are generally volatile and susceptible to change in their structure at high temperatures. This is also the cause for their sensitivity to surrounding atmosphere. Due to these reasons, their structural and physical properties required to be studied at low temperatures. Only a few crystallographic studies have been possible for metallacycloalkanes due to their unstable nature. Depending on steric considerations and the size of the coordinate sphere, the metallacyclopentanes exist in the form of three structural isomers – open-envelope conformation (R=H) or a twisted open-envelope structure. A third possibility is a puckered conformation (L=PPh3).[5]
Metallacycloalkanes are volatile compounds and thus, their composition can be studied by mass spectra only if they exhibit a certain amount of stability. Cyclic compounds that contain CO ligands in the coordination sphere of the metal have to undergo reactions aimed at eliminating this group first, before the rest of the ring skeleton is fragmented. A myriad of metallacycloalkanes with a wide range of properties have been reported and bring good impacts to chemical society. Nowadays, these metallacycloalkanes are attractive because of their applications as intermediates in various catalytic transformations. Based on the presence of reactive substrates, these compounds can create unexpected products that are hard to achieve by traditional methods.
References
edit- ^ Dixon, John T.; Green, Mike J. (15 November 2004). "Advances in selective ethylene trimerisation - a critical review". Journal of Organometallic Chemistry. 689 (23): 3641–3668. doi:10.1016/j.jorganchem.2004.06.008. Retrieved 15 November 2004.
- ^ Zheng, Feng; Sivaramakhrishnan, Akella (10 April 2007). "Thermal studies on metallacycloalkanes". Coordination Chemistry Reviews. 251 (15–16): 2056–2071. doi:10.1016/j.ccr.2007.04.008.
{{cite journal}}:|access-date=requires|url=(help) - ^ Grubbs, Robert H.; Miyashita, Akira (November 1978). "Metallacyclopentanes as catalysts for the linear and cyclodimerization of olefins". Journal of the American Chemical Society. 100 (23): 7416–7418. doi:10.1021/ja00491a051.
- ^ Stockis, Armel; Hoffmann, Roald (April 1980). "Metallacyclopentanes and bisolefin complexes". Journal of the American Chemical Society. 102 (9): 2952–2962. doi:10.1021/ja00529a015.
- ^ Lindner, Ekkehard (1986). Advances in Heterocyclic Chemistry doi= org/10.1016/S0065-2725(17)30059-4. Vol. 39. Germany: Scient Direct. ISBN 978-0-12-020639-1.
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