Synthesis, Characterization, and Use of a Cobalt(l1) complex as an NMR Shift Reagent An integrated laboratory experiment

Harold M. Goff,' Jody Hines, J e a n Griesel, and Craig Mossman University of lowa, lowa City, IA 52242

A currently popular approach to advanced undergraduate laboratorv oraaoization involves inteeration of material from the variois d k p l i n e s of c h e m i ~ t r y . ~ ~ l l too often, however, the integrated laboratorv is comuosed onlv of standard ex- periment randomly selected from iht individual disciplines. A recent Inlx,ratory manual entitled "Inregrated Experimental Chemistry," in fa&, follows this route ( 1 ) r ~ n approach which is seemingly more relevant to the usual research situation involves integration of diverse chemical concepts into each experiment. The typical organization is thus: synthesis - analysis - physical measurement. A paucity of such experi- ments are available.

The following experiment was designed for and has been employed in our majors junior-level integrated laboratory. Approximately four 3-hr laboratory periods are required for completion, assuming ready access to instruments. The acetylacetonate complex of cobalt(II), CO(ACAC)~(H~O)~, is prepared and isolated following modification of literature methods (2). Spectrophotometric analysis of the cobalt con- tent serves to characterize the material. Additional charac- terization by a method(s) of the student's choice provides an open-ended dimension to the experiment. The paramagnetic complex is further employed as an NMR shift reagent to simplify the spectrum of a complex alcohol.

Synthesis A large number of metal complexes have been prepared

using B-diketone com~ounds (3). Com~lexes of acetvlacetone (2,4-ientane-dione) with first row transition metal ions are rather easilv prepared and are of considerable importance in . . . analytical and inorgmic vhemistr\.. The sodium salt of acc- tylac(.tonr is i>reunred and this adduct is allrwed t o renct with an aqueous cob&(11) chloride solution:

H,C' k.0 'CH,

The crude product precipitates from solution and is purified bv recrvstallization (2).

-~ceri~lacetont. r-1.3 ml, 0.044 moles) is added slowly with stirrin~ tu asolution of 1 . 6 ~ (r,.OI molcsl of sodium hvdruxide in 15 ml of water. The temperature during mixing should be kept below 40°C. Any white solid formed at this point must be dissolved by extensive stirring or by addition of a few mil- liliters of water. This yellow solution is added drop by drop with vigorous stirring over a period of 10 min to a solution of 4.8 g (0.02 moles) of CoClz.6H20 dissolved in 25 ml of water. The resulting orange precipitate is collected by filtration and washed with 50 ml of water. The moist solid is dissolved in a hot mixture (steam bath, in the hood) of 39 ml95% ethanol and 26 ml chloroform. Lengthy boiling must be avoided. The red solution is allowed to cool slowly to room temperature and then is chilled in ice. The orange needles are collected by fil-

tration, washed with 10 ml of cold 95% ethanol, and air dried.

Student success rate is about 90% and yields are typically 60% (3.5 g) of theoretical.

Characterization The Co(AcAC)2(H20)2 product is characterized in part by

a cobalt analysis. This is accomplished after nitric acid di- gestion hv examination of the cobalt nitroso-R comnlex ( 4 ) . . . . Lipproximarely 20 mg ot'CorAcAclrrH:,O), i.; weighed hg an- alytical Imlanre into a 30-ml hmker. The material is treated with 10-20 drops of concentrated nitric acid and gently evaporated nearly to dryness (in the hood) over a burner avoiding any loss of solution. To this moist residue 12 ml of 6 M HCl and 1.5 ml of 6 M HN03 are added and the solution is transferred quantitat~vely to a 5U-nil \.ulumrtrir tlask. The flask is made to volume u,ith water. Kxartly 2.0 ml thissw lution is transferred by pipet to a second 50-ml volumetric flask, and 8 ml of water is added. Using an asbestos gauze pad, this solution is brought to a gentle boil for 3 min. After cooling, 1.0 ml of 1% nitroso-R solution and 1 g of sodium acetate are added. The solution is heated to boiling for 1 min, 1.0 ml of 6 M HCI is added, and the solution is again brought toa boil for 1 min. After allowing the colored solution to cool to room temperature, the flask is diluted to volume with water.

A reagent blank must also be prepared. To a 50-ml volu- metric flask are added 10 drops of 6 M HCI and two drops of 6 M HN03. After adding 8 ml of water to the volumetric flask, the reagent blank solution is treated exactly as the cohalt so- lution in the previous paragraph. This reagent blank solution is used to zero the spectrophotometer, and the cobalt nitro- so-R absorbance is read at 500 nrn. Preparation of a Beer's law calibration plot is recommended, but in lieu of this a molar absorptivity value of 1.45 X 104 ~m-~. lmole- ' may be used.

Students are provided with a representative carbon and hydrogen analysis for subsequent discussion of the assumed Co(AcAc)z(HzO)z formulation. A typical student sample yielded: carbon, 40.97% calc, 41.05% found; hydrogen, 6.19% calc, 6.33% found; cobalt, 20.1% calc, 19.9% found.

Additional characterization of the compound is possible by a variety of techniques. These might include infrared spec- troscopy, magnetic moment measurement by the Evans' method (51, conductometric measurements, or redox deter- minations. The breadth of the experiment may thus be en- larged, or an open-ended approach may be taken.

NMR Shift Reagent Work Proximity of a paramagnetic metal center may induce large

chemical shifts and varying amounts of line-broadening in NMR signals. Practical advantage is taken of this fact in using paramagnetic shift reagents to simplify otherwise complex NMR swectra. A basic oxveen. nitroeen. sulfur. etc. functional . yrtjup in the mole~:~llr uiinteresr binds to the metal renter in the rapid exchange lim~t. This wndition simply mcani that

' Author to whom correspondence should be addressed.

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observed signals are the mole-fraction weighted average of hound and free molecules. In general. protons nearest the basic functional groupexhibit the lHgest and line-hroadening. Certain lanthnnide com~lexes have heen found t induce large shifts without excessiGe line-hroadening, and use of such lanthanide shift reazents has been well documented in the literature (6) includ:mg THIS JOURNAL (7, 8). Transition metal complexes such as Co(AcAc)2(HzO)p have been used also to simplify NMR spectra of alcohols and amines (9). Al- though somewhat less effective than the lanthanides, transi- tion metal acetylacetonate complexes have for pedagogical purposes advantages of low cost, ease of preparation, and (unlike the lanthanide reagents) nonhygroscopic proper- ties.

Student-prepared Co(AcAc)z(H20)2 may he used to dem- onstrate principles of shift reagents on a known compound, or alternately may aid in identification of an unknown com- pound with &erlapping resonances. Complex alcohols serve as appropriate unknowns. Amines are also useful, but they may react with the chloroform solvent. Chloroform (CAU- 'I'ION: use only in an adequate hood) is the best common solvent for CO(ACAC)~(H~OI?. 1)euternted chloroform is nut essential, but it serves to remove otherwise confusing down- field sidebands, Consumption of C'DC13 is minimized by providing students with a 10% ( v l v ) solution of the unknown in CDClj containing 1% TMS. The KMH spectrum of this solution is recorded. Spectra are also recorded of solutions containing varvine amounts of CO(ACACMH~OJ~. This ,'ti- tration" i s h e s i a c ~ ~ m ~ l i s h e d with minima gla%&are by first preparing the most concentrated solution followed by ap- propriate dilution with the 10% unknown solution. Enough CO(ACAC)~(H~O)~ to make 0.5 ml of 0.06 M solution is accu- rately weighed into an NMR tube. The 10% solution is added to the 0.5-ml level (pre-determined and marked on a master NMR tube bv the instructor). Gentle warmine in a water bath - ~ ~~~ ~~~-~

(not to exceed 50°C) will facilitatedissulution:'l~ut evaporation muit be avoided. Solubility of the cohnlt complex exhibits a large temperature dependence, and if pcssible the NMR probe should be o~erated ar -35OC. After this spectrum is recorded, the samplehis diluted to the 1.0-ml mar'k with the 10% un- known solution. S ~ e c t r a are recorded a t this concentration, and also a t one-haif the concentration with the tube filled to the 2.0-mark. Solutions should be examined the day they are prepared because air oxidation will slowly convert the violet cohalt(I1) complex to the cobalt(II1) form. It should he noted that the TMS reference moves upfield for more concentrated samples due to bulk susceptibility changes. This shift will he compensated largely if an internal instrument lock is em- ployed.

The figure illustrates the use of Co(AcAc)dHsO)p to sim- plify theproton NMR spectrum of 2:ethoxietianil (cello- solve). Even a t 90 MHz the three methylene signals are un- resolved in the 3.63.8 PPM region (Figure, part A). Addition of CO(ACAC)~(H~O)~ induces a significant downfield shift in the methylene protons adjacent to the hydroxyl group. Other resonances move upfield. Severe broadening and shift of the hydroxyl proton se&e to demonstrate preferred shift reagent binding a t this site versus the ether linkage. At the highest shift reagent concentrations (Figure, parts E and F ) , separa- tion of the other two methylene signals is apparent also. Line-hroadenine has nearlv wined out ~roton-proton cou- . . plings a t this point, although remnants of a quartet pattern allow assignment of the methylene signal adjacent to the

I I I I I I I 5 4 3 2 1 0

PPM Proton NMR soectra of 10% lvlvl2-ethoxvelhanol In CDCL. 1 % TMS. 33%. . . -. 90 MHz. [c&Ac)~(H~o)~]: A. 0.00: B.0.01 W C, 0.02 M 0.0.04 WE. 0.06 W F. 0.075 M.

methyl group. Plots of chemical shift versus Co(AcAc)~(Hz0)2 concentration are instructive, and extrapolation yields the resonance position for the pure compound. A linear plot is obtained for the methylene protons adjacent to the hydroxyl group. A corresponding plot for the hydroxyl proton is de- cidedlv non-linear nerhaos due to proton exchanee from the water kolecule of a d d e d ~ o ( ~ c ~ c j ~ ( ~ ~ 0 ) 2 . ~ l t h o u g h sepa- ration of the closely spaced methylene signals is incomplete a t the lower field, this experiment is performed quite well using 30 MHz or 60 MHz instruments.

Literature Cited

. . (3) Fernelius, W. C., Bryant, B. E., in '"Inorganic Synthmis,'. Moeller. T.. (Editor),

McGraw-Hill. New York, 1957,Vol. 5. p. 105. ( I ) Willard, H. H., Menitf,L.L.. Dean. J.A,"lnstrumental Methodsaf Andyd%"SthEd.,

Van Nmtrand. NawYork. 1974. o. 116, (5) L~I&;J., ~"h~ffdd,R.. J. C H E M . ' E O U C . , ~ $ , ~ ~ ~ (1972). (6) Horrocks. W. Dew., in "NMRof Paramagnetic Molecule? La Mar, G. N., Honoch ,

W. Dew., Holm, R. H., (Editors). Academic Press, New York, 1973, Ch. 12. (7) Peterwn, M. R., Wahl, G. H., J. CHEM. EDUC., 49,790 (1972). (6) Kuo,S.C., Hsrris,D. K.,Capie,R..J. CHBM. EDUC..51,280(1974). (9) Gillies,E.,Szarek, W.A.,Bsird,M.C., Con. J. Chrm.,49,211i1971).

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