The Synthesis of Cobalt(lll) Sepulcrate from ~ris(ethylenediamine)cobalt(lll) - - and Its Purification by lon-~xchangechromatography An Experiment for the Advanced Undergraduate Inorganic Laboratory Dorothy E. Hamilton Simith College, Northampton, MA 01063 The synthesis and resolut~onof trisrethylenediamine,cobalt(II1Jls dcscnbcd in manv laboratorv texts. For example, ~ n g e l i c idescribes the" synthesi' of tris(ethy1enediamine)cobalt(III) from ethylenediamine, cobalt(I1) sulfate hevtahvdrate, and air in acidic solution, resolution . . of the enantiomers wjth barium (-)-tartraw, and precipitatlon of thc Iodide salts ofthe two enantiomers ( 1 1 .Wc have used this synthesis and resolution for many years in our undergraduate laboratories. The octahedral cobalt complex can be "capped" with organic reagents to form a variety of macrobicyclic ligands that encapsulate the cobalt ion (2-6). One such example is the cobalt(II1) complex of the hexad e n t a t e ligand 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6leicosane, better known a s cobalt(II1) sepulchrate (abbreviated [Co(sep)l3+),shown in the figure. This complex is formed by reacting tris(ethylenediamine)cobalt(III) with formaldehyde and ammonia (6). The original synthesis of
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this complex involved the use of cation-exchange chromatography to purify the product (6).Recently Gahan e t al. reported a procedure for the synthesis of [Co(sep)l3+that avoided the use of chromatography for purification by forming an intermediate diethyldithiocarbamate salt that is insoluble in aqueous solution (7).Although this is an elegant method for quickly isolating cobalt sepulchrate, there is pedagogic merit in performing the synthesis with the ion-exchange chromatography. Reported herein is such a synthesis. The procedure given is a slight modification of that reported by Harrowfield et al. (6).The synthesis and chromatography can be accomplished in three 3-h lab periods. Once synthesized the compound can be characterized by its 'H NMR, '3C NMR, and UV-vis spectra and compared to data for the startingmaterial, [Co(en)3I3+.Spectral information is given for both compounds below. To make efficient use of time, students can collect spectral data for
[Co(en)313+while performing cation-exchange chromatogra~ h on v the ~ r o d u cmixture. t it is esp&ially interesting to compare ORD curves for [Co(sep)li-and [Corcn,., ifchiral startingmaterial is used in the &nthesis. Although the capping reaction occurs with retention of configuration of the [Co(en)313+starting material, the sign ofrotation a t the sodium D-line (as well as the Cotton effect in the ORD curve) is reversed. Initially students are surprised by these reversals, although they have been warned that attempts to determine absolute configuration from ORD data are very unreliable.
".
Synthesis
Caution: This reaction should be performed in a hood. Clamp a 500-mL three-necked round-bottomed flask over a magnetic stirrer. Place 125 mL pressure-equalizing addition funnels in two of the necks. Add a magnetic stir bar, 5 g lithium carbonate (to maintain basic conditions), 3.2 g (0.0050 moll (+I- or (-1[Co(en)alIs.H20, and 25 mL water to the flask via the third neck. To one of the addition funnels add 120 mL of 37% Structure of [ ~ ~ ( s e ~ u l c h r a t e ) ] ~ ~ . solution. Dilute 33 mL of concentrated ammonia to 100 mL, and add to the second addition funnel. Begin stirring the solution in the flask, then add the solutions in the two addition funnels simultaneously over a 30- min period to the contents of the flask. Stir the mixture for an additional 30 min after addition is complete. Once the reaction above is complete, filter the cobalt solution to remove the lithium carbonate. Adjust the pH of the filtrate to approximately 3 with 12 M HCI, then dilute to 1L with water. Ion Exchange Chromatography
While the reaction above is taking place, prepare an ion-exchange column using water and 4&45 g of Dowex 50W-X2 ion exchange resin, 20&400 mesh, H+form. Use a glass column that is 2.5 cm in diameter and has a solvent reservoir bulb at the top. (If this is not available, use a one-holed rubber stopper to attach a 500-mL separatory funnel to the top of the column. The seal between the column and the separatory funnel must be airtight to prevent loss of solution. However, to keep the solvent level above the top of the resin, do not attach the separatory funnel to the column until you have added a small amount of the reaction mixture to the column.) The column of prepared resin will have a height of about 10 em. Wash the column with water until the eluant is colorless. Pour the reaction solution (from the preceding section) onto the ion exchange column, and elute until the product mixture (orange) has adsorbed onto the column. This will take several hours, but, once a suitable elution rate bas been set (no faster than two drops per second), the column may be leR unattended. Discard the solution that has (Continued on page A146)
The Modern Student laboratory: Chromatography passed through the column, then stopper the column, and leave until the next lab period. Elute the column with 0.2 M sodium citrate solution (500-1000 mL) to remove a pink species, which can be discarded. Next, wash the column with 200 mL of water and 200 mL of 1M HC1 to remove Na+. Elute the orange sepulchrate from the column with 3 M HCI (300400 mL). Evaporate the orange solution to dryness using a rotary evaporator. Recrystallize the product by dissolving it in a minimum amount of hot water (about 90 "C) and adding acetone slowly while swirling or stirring. Cool in a n ice bath, t h e n filter to obtain a n orange powder, [Co(sep)ICb.HzO.
The visible spectrum of an aqueous solution of [Co(sep)13+ has peaks a t 472 (E = 109)and 340 nm (E = 116)(3,8), while that of [Co(en)3I3+has peaks a t 466 (E = 98) and 339 nm (E = 87) (9). ORD curves were determined using aqueous solutions containing 0.05 g of the appropriate complex diluted to 10 mL. APerkin-Elmer 241 MC Polanmeter was used to measure mtationas afunction ofwavelength. These rotations were wnverted to molecular rotations and plotted versus wavelength to obtain the ORD m e . The ORD curve for A(+)-[Co(en)#+is given in Angelici ( I ) ;it shows a minimum at 460 nm and a maximum at 520 nm. [MIDis +570°. The ORD curve for A(-1-[Co(sep)13+has been reported by Sargeson et al. (2, 8) and exhcbits the following: [Mlrso = -3790°, [MI428 = +6540°, [Mlm = +5880° (8). Literature Cited
The 'H NMR spectrum of [Co(sep)13+(in DzO with DSS as an internal reference) exhibits an AB doublet pair a t 4.0 pprn ( J = 12 Hz) for the protons in the cap and a complex AA'BB' pattern centered near 3.2 pprn for the ethylene protons (6).[Co(en)3I3+exhibits a broad singlet a t 2.85 ppm. The I3C NMR spectrum of [Co(sep)13+(in DzO with 1,4-dioxane as an internal reference) exhibits two peaks, one 13.25 pprn upfield from l,4dioxane for the ethylene carbons and the other 0.39 pprn dowdieldfmm 1,ldioxane for the carbons of the cap (6). [Co(en)d3+exhibits one peak, 21.81 pprn upfield f?om 1,Cdioxane.
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1. See for exsrnple Angelin, R. J. Synthesis a d Technipw in Inoqonic Chemistry: Saunders: Philadelphia, 1977; pp 71-78. 2. Sargeson,A. M. Chsm B ~ 1379,15,23-27. L 3. Creaser, I. I.;Geue,R. J.: Harrowfield. J. MacB.; Her1t.A. J.: Sarges0n.A. M.: Snow, M.R.;Springborg, J. JAm. Chem. S a . 1982.104.6015-8025. 4. Gene, R. J.; Harnble~,T. W.; Harrowiield, J. M.; Sargeson, A. M.; Snow, M. R.J.Am. Cham. S a . 1984,106,5478-5488. 5 . Harrowtieid, J. MacB.; Lawranee. G . A,: Sargesan, A. M. J . Chem. Educ 19%. 62,
8"bR"li ... .... 6.Harrowfield, J. MacB.:Her1t.A.J.:Sargeson, A. M. In InorgonicSynfhese#;Buseh,D. H.,Ed.; Wiley: New York, 1980:Vol. XX,pp 85-86. 7. Gahan, L.R.: Healy, P. C.: Patch. G.J. J. Chem. Educ 1969.66,445446. 6.Creaaer,I.I.;Hamwfield,J.MaeB.:Herlt,AJ.:Sargeaon,A.M.;Springborg,J.;Geue, R. J.: Snow. M. R . J h Chem. Sa.1971,89,3181-3182. 9. Linhand, M 2. Ebktrahem. ISM, 50,224
