I
II
Robert J. Angelici
Iowa
State University
Ames,
Iowa 50010
Preparation and Characterization of
~ e i i t ~ l e nTricarbonyl e Molybdenum (0) A n experiment in orgonometallic chemistry
The large volume of interesting research in the area of organometallic chemistry has resulted in a number of recent reviews of the topic (I, 3,3, 4). Our undergraduate laboratories, however, have been notably lacking in experiments on this subject. To correct this situation we have been seeking such an experiment for our senior-level, one quarter, two credit laboratory course (35 student enrollment) which emphasizes synthetic and instrumental techniques in inorganic chemistry. Some of the desirable techniques to be illustrated were a preparation conducted under an inert atmosphere, compound purification by sublimation, and its identification by infrared and proton nuclear magnetic resonance spectroscopy and mass spectrometry. Because of the low cost of the chemicals, the reaction of MO(CO)~ ($O.lO/gm) and mesitylene ($O.lO/lOg) was chosen.
co c 0 + # 2 0 /' Mo ,/'
+
(CH&CeHa
-+
ocd-l-L;o CO
The method of preparation' is a modification of those given in the literature (5, 67,and the product has a "sandwich" structure (3,7) as shown above.
[: La
The Experiment
ML
Into a 50-ml round bottom flask with a side-arm stopcock (2 mm bore) (see the figure) are placed 2.0 g Mo(CO)&(handle carefully-metal carbonyl complexes are toxic) and 10 ml mesitylene (bp, 165°C). A simple reflux condenser and bubbler
the temperature used hood) are compounds connected to as in cool theisthe figure. are to sufficient. condenser; the sensitive flask Water Because to (in is air room not the at
The opporotvs for the preparation of rneri. tylene tricarbonyl molybdenum 101.
high temperatures, the reaction must be conducted under a nitrogen atmosphere provided by flushing the system with a moderate stream of nitrogen for about 5 min. After the nitrogen flow is turned off and the stopcock is closed, a heating mantle with a rheostat control is
'This experiment MUST be conducted in the hood.
used to heat the mixture to a moderate boil. Heating is discontinued after 30 min of reflux by removing the mantle (longer reflux produces considerable amounts of black metallic molybdenum). Then the nitrogen flush is again turned on in order to sublime any unreacted MO(CO)~, which has not already sublimed, from the solution. The flush also prevents the mineral oil in the bubbler from being sucked into the reaction vessel. After the mixture has cooled to room temperature, 15 ml hexane (or hydrocarbon fraction of 60-70°C boiling point range) is added to complete the precipitation. The mixture is filtered on a medium frit with water aspiration to give a yellow solid containing black particles of metallic molybdenum. After washing with 5 ml hexane, a crude yield of 4&50% is obtained. For purification, the compound is dissolved in a minimum amount (approximately 10 ml) of CH2C12;this solution is filtered (medium frit) and 25 ml hexane is added to aid in precipitation of the pure product. The yellow solid is filtered off on a frit and washed twice with 4 ml hexane and allowed to dry on the frit while under water aspirs, tion. (Additional product can be obtained by reducing the volume of the mother liquor under reduced pressure a t room temperature.) The compound sublimes very slowly at 100°C under high vacuum. The student then characterizes the compound by measuring its infrared spectrum to verify the presence of mesitylene and the CO groups. The C-O stretchmg absorptions at 1979 and 1905 em-' (8) are much more intense than any of the other bands in the spectrum. The nmr spectrum of the compound in CHCL solvent, which can be measured either by the student or his instructor. exhibits sinzlet resonances a t 2.26 and 5.24 ppm downfield from tetramethylsilane with relative intensities of 9 to 3, respectively. (Note that the ethanol stabilizer in CHCZ also appears in the spectrum.) These compare with 2.25 and 6.78 ppm for uncoordinated mesitylene in CDC13solvent (9). Either the instructor or a technician can determine the mass spectrum of the compound over the range of approximately 90-310 atomic mass units. I n addition to accurately verifying the molecular weight of the compound, the spectrum shows the fragmentation pattern of (CH& CfiH3Mo(CO)a. This pattern exhibits ions corresponding to the loss of one, two, and three CO groups as well as Mo+ and (CH&C6Ha+,which further breaks down into smaller organic fragments. The seven natural isotopes (present in from 9 to 24% abundance) of molybdenum allow fragments containing molybdenum to he readily identified. The student should be able to account for each fragment in the spectrum, including the +2 charged ions. Whiie this experiment has been designed for use in our advanced inorganic laboratory, it would be just as appropriate in an organic laboratory course.
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Volume 45, Number 2, February 1968
/
119
J. J.,
Literature Cited
AND
Vnl . 1-.
KING,R. B.,), Academic Press, New Ywk, 1965,
(5) FISCHER,E. O., (1) "Advance in Orgsnometdlio Chemistry," (Editms: STONE,
F. G. A,,
AND
WEST, R.), Academic Press, New York,
1964-1966, Vol. 1 4 . (2) SEYFERTH, D. AND KING,R. B., "Annual Survey of Organometallic Chemistry," Elsevier Publishing Co., Amsterdam, 1965, Vol. 1. (3) ZEISS,H.,WHEATLEY, P. J., AND WINXLER, H. J. S., "Benzenoid-Metal Complexes," Ronald Press Co., New Yark, 1966. (4) KING,R. B., "Organometdlic Syntheses," (Editom: EISCH,
120
/
Journal of Chemical Education
OPELE, AND
MORTENSEN, J. P.,
K., ESSLER,H., FROHLICH, W.,
SEMMLINGER, W., Chem Be!.., 91, 2763 (1958). (6) NICHOLLS, B., AND WHITING,M. C., J. Chem. SOC.,551 (1959). (7) BAILEY,M. F. AND DAHGL. F., Inorg. Chem., 4, 1298, 1314 (1965). (8) FISCHER, R. D., Chem. Be?., 93, 165 (1960). (9) BHACCA, N. S., JOHNSON, L. F., AND SHOOLERY, J. N., "XMR Spectra Catalog," Varian Associates, Palo Alto, 1962, p. 241.
