An Integrated First-Year Laboratory Experiment Involving Synthesis, Spectroscopy, and Chromatography of Metal Acetylacetonates Christopher Glidewell and James S. McKechnie University of St. Andrews, St. Andrews, Fife, KY16 $ST, Scotland. U.K. T h e preparation of metal acetylacetonato (pentane-2,4dionato) complexes by reaction of metal salts with acetylacetone in weakly basic solution,
provides a n effective a n d colorful procedure on which elementary students can develop manipulative skills in synthesis and purification. We have developed this procedure into a comprehensive experiment, involving synthesis, thin-layer chromatography (TLC), and interpretation of both 'H and I3C NMR and mass spectra, for use in our integrated first-year laboratory class. It has long been part of this department's approach t h a t the training of students in diagnostic and interpretative spectroscopy should begin a t thevery outset of their undergraduate courses, so that, for example, the treatment of isomerism in alkanes is developed in terms of the number of distinct chemical shifts in their '3C N M R spectra. This experiment was designed t o be a contribution t o t h a t early training and has been used with great success for a number of years. Each student is required toprepare and purify asample of one complex M(CH3COCHCOCH3),, selected by the instruc= A13+ (colorless), V02+ (hlue), Cr3+ tor from a choice of Mn+ (deep maroon), Mn3+ (green-black), FeJ+ (red), Co3+ (dark green), and Cu2+ (blue-gray); after pooling samples, the students determine by T L C the Ri characteristics of sample of all except t h e colorless A1(CH3COCHCOCH3)3and hence identify, b y comparison of Ri values, t h e three components of a n unknown mixture of complexes provided by the instructor. The students are each provided with copies of 'H and I3C (both fully coupled and proton-decoupled) NMR spectra of the diamagnetic Al(CH3COCHCOCH3)a and of CH3COCH2COCH3:they are asked first to assign fully the spectra of the aluminum complex, then by comparison t o identify the resonance of both the keto and en01 forms of CH3COCH2COCH3, and finally t o determine from the 'H spectrum the keto:enol ratio. Likewise a listing is provided of i o a l. oeaks in the mass snectra of the same two the ~ r i n c. compounds, and the students are asked t o assign these, in this case startine with the simpler CHaCOCH&OCH?. en01 T h e parent CH~COCH~COCH~, whkther in keto
R, Values for M(CHsCOCHCOCH3),,* M*
R
V02+
Cr3+
Mn3+
Fe3+
Co3+
CUZ+
0.1
0.7
0.0
0.3
0.6
0.6
"(Silica suppat: 1% methanol In CH2CI,).
form, and the aluminum complex A1(CH3COCHCOCH3)3 are ideal NMR subjects for beginning students since all the resonances in the 'H spectra are well separated sinelets. .. . while the well separaterl~resonancesin rhe ?c. spectra are 311 readily assigned from a combination of chemiral shift data and H-coupled multiplicity.
Preparation of MCH3COCHCOCH3). Complexes are prepared, on a scale of 5-25 mmol, using methods based on published procedures (1-6). Full experimental details for preparations and purifications are available upon request from the authors. Thin-Layer Chromatography Small samples of each of the colored complexes and of the unknown mixture are placed in small sample tubes and dissolved in the minimum volume of CH2C12.Melting-point tubes are drawn out for spotting the TLC plate. While conventional glass TLC plates (or microscope slides) can be used as the support for the SiOz layer, we prefer touse for larger classes 4- X 1-in. strips cut from commercially available aluminized TLC sheeting: (these strips can, if desired, he conveniently incorporated into the students' reports). Each solution is applied in very small (1-mm-diameter) spots-2 cm from one end of the TLC strip: solutions of the VOZ+,Cr3+,and Cuz+ eomplexes can be respotted up to six times. When the strip is dry, it is placed in a tank containing a 1-cm depth of a 1%solution of methanol in CH2C12,and the chromatograph is run until the solvent front isabout three-fourths of the way up the strip. The position of the solvent front is marked, the strip dried, and the Rt values measured. Typical results obtained under these chromatography conditions are given in the table. On the basis of these values, an ideal unknown mixture far identification under and Colt complexes, these conditions consists of the Mn3+,Fewt, which are respectively,green-black (brown on TLC plate), red (orange on TLC plate), and dark green. NMR Spectra
CH (enol)), 15.3 (5H, s, hr, 0.. H ..0 (enol)):'WDCId. 6 26 ( 0 . CHS(enol)),31 (q,CH3 (keto)),58 (t, CH2 (keto);, 99 (d, I% (enoljj; 188 (5, CO (enol)),198 (s, CO (keto)). For A1(CH3COCHCOCH3)3both the 'H and I3C spectra ((a) and (h) in figure) are readily interpreted in terms of a symmetrical trischelate structure of overall Da symmetry, with three equivalent ligands each possessing twofold symmetry. The very simplicity of these spectra, with only two'H and three13Cshifts for a complex of composition C I S H ~ ~ A Ipoints O ~ , to very high symmetry. For each chemical shift in the spectra of AI(CH&OCHCOCH& there is an almost exactly corresponding shift in the spectra bf CH&OCHzCOCHs ((4 and (dl in figure), which are assignable to the end form: in addition there is a broad IH resonance s t 6 15.3, which must beassigned toa hydrogen-handed 0 . . .H ...0 system. There remain only two 'H and threeT3Csignals, readily assigned to the keto form. s,
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The existence of sharp signals for both keto and enol forms shows that the equilibrium 1+ 2 is slow
while the occurrence of only three 'H and three "C signals for the en01 form shows either that the equilibrium 2 4 2'is fast or that the enol form exists as a symmetrical cyclic species 3 analogous to the metal complexes 4.
The present NMR data cannot distinguish hetween these possibilities. The integrated 'H spectrum of CH3COCH2COCH3shows that in CDC13 solution at amhient temperature the molar ratio [enol]:[keto] is -55. Mass Spectra In the mass spectrum of CH&OCH&OCHz, the principal peaks occur at mlz (RIl%): 100(67),85(89), 72(8), and 43(100): these are readily assigned by students as M+, (M-CH#, (M-COP, and (CH3CO)+.The mass spectrum of AI(CH3COCHCOCH3)3is more complex,containingmajorpeeks:324(4), 226(17), 225(100),143(10), 141(17), 127(5), 126(6), and 43(18). The students are first asked to assign the ions with mlz 324,225,126, and 43: these are, respectively, [A1(CH3COCHCOCH3),lt for n = 3, 2, and 1, and (CH&O)+. They are then asked to suggest compositions for the ions of mlz 226, 141, and 127: these are the rearrangement ions [HA~(CHJCOCHCOCH3)2]+, [CH3AI(CH8COCHCOCH3)li and [HAI(CHICOCHCOCHdlt, respectively. Finally they are asked to suggest a structure for the ion at mlz 143, having a composition established by accurate mass measurement as (CsHsAlOd+; [HOAl(CH&OCHCOCH3)]+is a plausible constitution for this ion. I t should be emphasized that we have found that the mass spectral part of the exercise generally requires considerable input from the instructor: if this is provided, the students rapidly gain insight into, and confidence in, the process of spectral assignment. Concluslon W e have used this experiment for a n u m b e r of years in t h e first semester of freshman chemistry classes where few if any of t h e students have a n y pre-university experience of either
(a)A1(CH3COCffiOCH3)3.'H NMR spectrum.
Ib) AI(CH3COCHCOCHds,proton-coupled '3C NMR spectrum.
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Journal of Chemical Education
(c)CH3COCHZCOCHI(keto-end
mixture), 'H NMR spectrum.
(d) CH3COCH2COCHs(keto-en01 mixture), proton-coupled l3CNMR spectrum.
practical chromatography or spectral interpretation: many indeed have very little experience of any hands-on experimental chemistry of any kind prior to taking this class, Nevertheless, we have found that the vast majority undertake this experiment with confidence and success: on the average it requires two to three %hour laboratory sessions.
Llterature Clted 1. R . c . I ~ ~ ~ ~ . 1s 9Y~ ,~2~. 2~5 .. 2. Man. G.; Roekatt, B. W. Prnclicol Inorgonie Chamislry; Van Nootrand Reinhold: London, 1972: p243. 3. w,,,li,, W.c . ; B I . . ~ ~ .J. E.~ ~ ~ ~1957.6,130. ~ . ~ ~ ~ t h . 4. Charles. R . G . l n o v Synth. 1983.7.183. 5. Chades. R.G.;Pswlikowski. M. A.J. P h y s Chem. 1953,62.440. 6 . Bryant.B. E.;Forneiius, W. C. Inorg.Smth. 1957,5. 183.
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