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Microwave Heating Applied to Organometallic Chemistry
© G. Whittaker, 1994 & 2007. This work, or extracts from this work may be reproduced only with the written permission of the author.
Synthesis of Organometallic and Co-ordination Compounds Organometallic and co-ordination compounds have received surprisingly little attention by microwave chemists in relation to other areas of study, despite indications that here, too, improved syntheses may result. The synthesis of a number of B-metal compounds have been accomplished in pressure vessels similar to those used in high pressure organic syntheses.50 Reactions were run using 100cm3 teflon pressure vessels containing relatively small quantities of solvent (~12cm3). Large reaction rate increases of up to 40 times were observed relative to the conventionally heated samples, and for comparable yields (Table 1). In the particular case of a reaction of Al(OPri)3 with 1,2-propandiol, a mixture of products resulted, and despite 60x rate increase the microwave synthesis was not especially advantageous. This simply underlines the fact that often, microwave heating may simply increase the rate of reaction, but not necessarily improve product selectivity despite cases where the latter occurs.
Table 1 Organo-B-metal compounds synthesised using microwaves
The synthesis of a range of transition metal co-ordination compounds at atmospheric pressure under reflux conditions has also been carried out with some success.51 Stirring was not employed in these reactions since it proved possible to maintain a steady reflux in its absence. Again, large time savings were recorded, with up to 60 times rate increases, and comparable yields to those seen by published thermal routes. Similar reactions to these, run using DMF, methanol or ethanol as a solvent in 23cm3 Teflon digestion vessels, showed even greater rate increases (see Table 2).
Table 2 Organometallic reactions carried out using microwaves51
More recently, improved syntheses of [Fe([eta]-C5H5)([eta]-arene)][PF6] and [Fe([eta]-arene)2][PF6] salts have been reported using the microwave heating properties of metal powders (Table 3).52 In a typical reaction, ferrocene, aluminium powder, and AlCl3 were ground together and formed into a slurry with the arene before being heated in an unmodified domestic oven. the bis-(arene) compounds are synthesised in an analogous manner using FeCl3 in place of ferrocene. Using solid CO2 - cooled apparatus to avoid excessive loss of solvent, yield increases up to three times that of conventional methods are reported in four minutes or less.
Table 3 Yields of [Fe([eta]-C5H5)([eta]-arene)][PF6] from microwave experiments
References
50. M. Ali, S.P. Bond, S.A. Mbogo, W.R. McWhinnie & P.M. Watts. J. Organometallic Chem.371, 11 (1989). 51. D.R. Baghurst & D.M.P. Mingos. J. Organometallic Chem.384, C57 (1990). 52. Q. Dabirmanesh & R.M.G. Roberts. J. Organomet. Chem.460, C28-C29 (1993).
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