D. W. Johnson
and G. w. Rawer-canham Simon Fraser University Burnaby 2, B. C. Canada
I
I
Ferricinium Picrate Preparation
The synthesis of ferrocene is a standard part of many advanced inorganic chemistry laboratory courses. In the recently published textbook by W. J . Jolly (I), the preparation of the derivative ferricinium tungstosilicate is described. Although the preliminary preparation of tungstosilicic acid is useful to demonstrate polyhedral systems at an intermediate level, we consider it to be a tedious and time-consuming method of preparing a bulky anionic system used merely for precipitation of the ferricinium ion. Utilization of readily available picric acid instead saves valuable laboratory time, and relates the fields of organic and organometallic chemistry, as does the use of benzoquinone as an oxidizing agent. Precipitation of the ferricinium cation by means of the picrate ion was first described by G. Wilkinson, et al. (8). Preparation. Dissolve 0.4 g picric acid and 0.3 g benzoquinone in 10 ml beneene. Add with stirring, a, benzene solution of 0.4 g ferrocene. Filter off the dark green product (-80% yield) and wash with benzene (twice)and diethyl ether (twice). Dry under vacuum.
A wide range of physical techniques may be utilized to study ferrocene and the ferricinium ion. The infrared spectrum of ferrocene has been fully assigned (S), and the spectrum of ferricinium picrate can be similarly assigned, noting the additional absorptions due to the C-Oe (1625K), and C-N02, (-1500K and -1300K) stretches of the picrate ion. The Raman spectrum may be obtained from ferrocene itself (4), and the strong bands a t 1105K and 301K have been assigned as the al symmetric ring breathing and symmetric rinemetal stretch, respectively. The visible spectrum of ferrocene shows d-d bands at 22.7 kK (6 = 102) and 30.8 kK ( r = 58) in cyclohexane (6) [N.B. in chlorinated solvents an additional hand at 32.6 kK is observed (6)l. The ferricinium ion shows a characteristic band at 16.2 kK which has been recently re-assigned as a ligand to metal charge
of
a ferrocene derivative
transfer transition (7). The edge of the picrate ion absorption in the ultraviolet is also observed. Both ferrocene and ferricinium picrate are low spin systems, the former ( 8 ) being diamagnetic and the latter (dS) having a magnetic moment of 1.79 B.M., correspondingto one unpaired electron. The mass spectrum of ferrocene is of considerable educational value, for as well as showing peaks due to Fe(CsH5)z+,Fe(C5H5)+,Fe+ and progressive fragmentation of the cyclopentadienyl rings, the double ion Fe(CsH5)zZ+(8) and the ion-molecule Fe2(CsH&+ (9) may also be observed. The ferricinium picrate spectrum shows, in addition, peaks due to the fragmentation of the picrate ion. Ferrocene has a characteristic single nmr peak at 4.3 ppm, whereas with the ferricinium ion, the paramagnetic shift causes the cyclopentadienyl protons to be shifted 24.8 ppm downfield with respect to the ring protons in 1,11-dimethyl ferrocene (10). Integration with respect to the two picrate ion protons may also be performed. At this level, we feel it is worthwhile to point out to students the usefulness of nmr to distinguish between ?r bonded and fluxional u bonded cyclo~entadienylions (11). We thank Dr. F. W. B. Einstein and Dr. D. Sutton for helpful discussion. Literature Cited (1) Jomr.W. L., "The Synthesis and Characterisation of Inorganio Com~ o u n d s , "PrentioeHsll, h a . , N. J., 1970, p. 487. (2) WmmNson, G., &at., J. Amer. Chem. Soc., 74,2125 (1952). (3) LIPPENCOTT, E. R., AND NBLBON,R. D.. Spectrochin. Ado. 10, 307 (1958). LONG,JR.. T. V.. AND HUEDE,F . R.. Chem. Cornm.. 1239 (1968). A n ~ s ~ n o n A. a . T., SMITH,F., Emen. E.. A N D MOGLTNN.S. P.. 3. Cham. Phvs.. 46,4321 (1967). BBAND,J. C. D., A N D SNBDDEN. W., T m n ~Farnd. . Soe., 53,894 (1957). PRLNB. R.. J. Chem. Soc. (Dl, 280 (19701. MOLGEB, J.. A N D D'On. L., 3. O~oonomelollicCh'hsm.,10,313 (1967). cnAMeEBs. D. B.. G ~ C X L ~ NF.. ( I . A~. LIGHT.J. R. c., a a a ~ t ~ . m . (London), 22,317 (1968). KELLER,H. J.. AND SCRWARZHANB, K. E., AWW. Cham. Iniilavnot. E d L . 9 , I96 (1970). COTTON.F. A,. Chcm.m Evil., 4,345 (1988).
Volume 49, Number 3, March 1972
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