The Busy Student's Guide to the Nitration of Tris-Acetylacetonates of Cobalt(lll) and Chromium(1lI ) There are several reasons why the nitration of metal acetylocctonates using copperrll) nitrate in acetic nnhydrrde is an interesting (and expermentally simple1 example of a resctwn uf a coordinated lrgand 1) it illustrates the "aromatic" nature of the chelate ring 2) the nitrating agent is unusual 3) the parent nitrated acetylacetone ligand is unavailable The use of metal nitrates as nitrating agents was first suggested by Menke (1) and more critically examined by Bacharaeh (2) who found that copper(n) nitrate was the most efficient reagent. The method was extended to the nitration of metal aeetylacetonates, M(acac)a, by Collman and ca-worken (31 during a classic series of investigations of aromaticity in these systems in the early 1960's (4).The reliable nitration procedures reported by them have been formalized as standard methods by inclusion in "Inorganic Syntheses" (5). In all cases reported (3, 5, 6), the method involves the followingsteps 1) generation of the nitrating agent (presumably acetyl nitrate) by stirring finely ground copper nitrate with acetic anhydride at 0°C for 15 min 2) adding M ( a ~ a eand ) ~ stirring for several (i.e., 2-4) hours a t 0°C and finally for 1hr a t room temperature 3) adding the reaction mixture to ice-cold sodium acetate solution and stirring to solidify the separated oil (about 2
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4) recrystallization of the product As a result, syntheses of this type appear inconveniently long for inclusion in a laboratory course, where sessions are quanta of three or four hours, unless included in an open-ended project scheme (71. The two examples (8,91 quoted in Fieser and Fieser's "Reagents for Organic Synthesis," as well as the early references (1, 2) suggested that there was no real systematization in the use of this nitrating agent and that it might be possible to fit the synthesis into a conventional laboratory period. Consequently, the published procedure has been carefully examined and the time required for the method dramatically reduced by making the modifications set out below.
1) Fine grinding of the copper nitrate is unnecessary. Gentle crushing to eliminate lumps is sufficient. The hygroscopic nature of this reagent makes fine grinding undesirable, especially in humid conditions. 2) It is not netessan, to stir the caeDer nitrate at 0°C prior to the addition of M(acac)r. In fact, the order of mixing. the reagents in the flask seems to be irriievsnt. 3) The reaction times previously quoted are unnecessarily long. Small scale nitrations were carried out successfully with reaction times as short as 20 min at r w m temoerature 121°C). , . The use of an ice bath in these eases serves onlv to attract moisture into the acetic anhydride. 4) The oils generated in the isolation procedure can be induced to solidify quickly by the addition of ethanol to the vigorously stirred mixture. ~~~
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The complete experimental procedure is given below. The whole synthesis can be performed in l'h hr, and the time thus saved can be put to profitable use.
Experimental Procedure Crushed copper nitrate trihydrate (2.15 g) and 1 g of the metal acetylacetonate were placed in a dry flask and 40 ml of acetic anhydride added. The flask was firmly stoppered and the mixture stirred vigorously at room temperature for 30 min, using a magnetic stirrer bar. At the end of this time, the mixture was carefully poured into ice-cold sodium acetate solution (15 g sodium acetate, 120 ml water and 120 g ice) with vigoraus stirring. An oil (green for Co, red for Cr) separates which can be solidified by adding ethanol until the stirred solution assumes a cloudy appearance. The mixture is filtered, the precipitate collected and washed with small amounts (5 ml) of water and ethanol then finally recrystallized from a chloroform-ethanol mixture. Generally, 0.7 to 0.9 g of product is obtained which possesses the melting point, infrared spectrum and (for Co) the nmr spectrum (indicating absence of the 3-protons) quoted in the literature (31. Bruce D. James
Literature Cited Menke, J. B.,Rec. Two. Chim., 44.141 and270, l1!3251. Bacherach. C.. J.Amer Cham Soc. 49. 1522 (1927). Collmen, J.P.,Manhall, R . L . Young. W. L. andGaldby, S. D..lnorg. Chsm.. 1,704, (19621, See. fmerample. Collman.J.P.,Angaiu. Chom. (InternationalEdn.l.4, 132. (19651. Callmsn. J.P.andYoung, W . L . , l m r g . SyrVh.. 7.205. (1963). (6) Collman, J.P.and Yamada,M.,J. Org. Chsm., 28.3017, (1963). (7) Gray. H.B., Susnson. J. G. and Crswfmd. T. H., "Pmiect ACAC: An Experimental Inve~tigationin Synthesis and Structure," Belmonf. Cal.. 1972. (8) Anderaon. A. G.. Nelson. J.A.andTszums.J. J..JAmor. Chom S o r . 75.482Q. 119531. (91 Williams, K. I. H., Creme., S. E., Kent. F.W., Sehm, E. J. and Tarhell, D. S., J. Amer Chom. Soc. 82,3982, (19601.
(1) (2) (3) (4) (5)
University of Queensland, St. Lueia, Qld. 4067, Australia
568
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Journal of Chemical Education
