Cobalt(l1) and Cobalt(III) Coordination Compounds Niiholas C. Thomas andKatrlna Pringle Auburn University at Montgomery, Montgomery. AL 36193 Glen 6. Deacon Monash University. Clayton. VIC. Australia 3168 This experiment illustrates the formation of tris(phenanthroline)cobalt complexes in the 2+ and 3+ oxidation states, the effect of coordination on reactions of the ligand, and the use of a Kcand displacement reaction in recoverinn the transformed ligand. Asa laboratory teaching exercise we have used this experiment to introduce students to several important reactions including complex formation, and oxidation and ligand displacement reactions. In addition, the intermediate complexes are isolated and characterized, providing students with a practical opportunity to apply techniques of infrared, UV-visible and NMR spectrometry, as well as conductivity and magnetic moment measurements, to study the properties of coordination compounds. Oxidation of phenanthrene to phenanthrenequinone is readily achieved with reagents such as acidic vanadate, periodate, or chromic acid ( I ) . However, these reagents do not oxidize free 1.10-phenanthroline (phen), 1, to the corresponding lJ0-phenanthroline-5,6-quinone(quin), 2, (2). The latter has been isolated in very small yield (-1%) by oxidation of 1, with a nitric-sulfuric acid mixture, but this reaction yields predominantly 5-nitrophenanthroline (3). Oxidation of 1with permanganate gives 2,2'-bipyridyl-3,3'dicarboxylic acid (4). Gillard et al. (5) have reported the conversion of 1 into 2 by first coordinating 1 to cobalt(II), followed by oxidation of both metal and ligand to yield
516
Journal of Chemical Education
Co(quin)F, and treatment of the latter with NQEDTA to liberate 2 (see figure) (5-7).
QUIN ( 2 )
Typlcal Student Data m.p. Compound
(OC)
Major IR
absorptionsLb
A-
(Wb."
UV
vis
Phenanthollne
114-117
1610,1508 848.728
224,264 290.32Osh
-
[Co(phenh12+
-
1625,1520 850,841 725
225.267 292.343sh
475sh
[Co(phen)d3+
-
217.278sh. 270.346sh
430 sh
[Wquinhlg+
-
-
-
-
-
Phenamhrolinequinone (2)
256-257'
-
1 7 ~ 1040 br" 16900
Our modified procedure for converting 1into 2 is outlined below. We have isolated t h e cobalt complexes as tetrafluoroborate salts, rather than t h e potentially hazardous perchlorates, and t h e resulting overall yield of 2 from 1is typically between 15 and 25%. Spectroscopic, conductivity, and magnetic d a t a for t h e ligands and isolated complexes are given in the table. Experimental All materials used are readily available commercially. Caution: 1,lO-Phenanthroline-5.6-quinoneattacks t h e nasal and throat membranes if breathed in. Gloves should be worn when handline it. and all transfers should be made in t h e fume hood.
Tris( 1,lO-phenanthroIine)cobalt(ll)Bromide Cobalt(l1) sulfate (3.0 g) and 1,lO-phenanthroline (6.0 g) were dissolved in wster (45 mL) in a 100-mI, Erlenmeyer flask and KHr (2.5g) wasadded. The aolution wsa ~entlystirred withaglass rod for several minutes until precipitation was complete. The yellowbrown product wascollected hy filtration,waahed withwater(2 X 15 ml.). and dried in air bv suction. Yield 80%. Students should measurethe IR and W-viiible soectra. and conductivitv of the complex, and determine the magnetic moment by measuring the magnetic susceptibility. Trls( 1,lO-phenanthroline)cobalt(lll)Tetrafluoroborate A mixture of [Co(phen)a]Br?(4.0 g), saturated bromine water (80 mL), and water (40 mL) was placed into a 250-mL round-bottomed flask. The solution was stirred and gently heated under reflux for 25 min. To the hot solution was added HBFl (8 mL, 48% aqueous solution), and the flask was cooled to 0 'C. The yellow product was collected by filtration and dried by suction. Yield 75%. Students should measure the UV-visible and NMR spectra, and conductivity and magnetic moment of the complex. Tris( 1,lO-phenanthroline-5,6quinone)cobalt(lli) Tetrafluoroborate A mixture of [Co(phen)a](BF& (3.0 g) and NaBr (1.5 g) were dissolved in concentrated H2SO4(20 mL) in a 100-mL raund-bottomed flask immersed in an ice bath. Concentrated HN03 (20 mL) was added slowly, and the mixture was heated under reflux for 40 min. The hot reaction mixture was slowly added to a solution of NaBFl (4.0 g) in water (200 mL). The solution was cooled to 0 Y! until cmtallization was comolete (-12-24 hl. The vellow mstalline prdduct was collected b;filtraiion, washid withwater ( j x 25 ml.).and dried by suction. Yield 65%.Studrntsshould record the IR spectrum of the complex.
'H NMRCd (8, P P ~ )
~onductivitp~ (ohm~'cm2moi-'1
Magnetic moment (B.M.)
-
-
9.02 (H2,9) 8.69 (H4,7) 6.00 (H3.6) 7.88 (H5.6) -
9.10 (H4.7) 8.51 (H5.6) 7.98 (H3.8) 7.68 (H2.9)
120
4.7'
250
Diamagnetic
-
-
-
-
1,lO-Phenanthrol/ne-5,6-quinone,2 A mixture of [ C o ( q ~ i n ) ~ ] ( B(1.5 F ~ )g) ~ and NazHzEDTA (1.5 g) was dissolved in water (20 mL) (warming if necessary) in a 50-mL round-bottomed flask.The pH of the solution was adjusted to 5.5 by addition of NaHCOz. The solution was heated under reflux for 1b, during which time the color of the solution changed from yellow to red. Cooling the solution to 0 OC produced a yellow solid (crude 2). which was filtered off. The aqueous filtrate yielded afurther portion of 2 after extraction with chloroform (3 X 20 mL). The combined chloroform extracts were dried over anhydrous MgSOa, filtered and evaporated to drvness .. eivine .. crude 2. Both samoles of the i m ~ u r e quinone were combined and recrystallized from'methanol yielding yellnw needles of 2 (yield fig%). Students may characterire the compound by its melting point and infrared spectrum.
~.~ ~~~
~
Dlscusslon Students should obtain d a t a for t h e complexes and ligands similar t o t h a t shown i n t h e table, and they should h e encouraged to explain t h e significance of their results b y considering some of t h e following observations (literature references are provided t o assist with explanations): 1. The coordination of phenanthroline enhances its rate of oxida-
tion (9). 2. Changes occur in the infrared (10,11), W (12),andlH NMR (13) spectra of phenanthroline on coordination. 3. The difference in A, of [Co(phen)3I2+and [Co(phen)d3+(12). 4. The significance of magnetic (14,15) and conductivity (16) data in establishing the proposed [Co(pben)s]Brz and [Co(pben)~](BF&formulations. 5. The reaction wcuning when 2 is displaced by EDTA (5). Literature CHed 1. Richter, F. J. A n . Chen. Soe. IY4.66.398. 2. Karmsek. W. 0.; MeKd, J. W. Hetemeyclis
Compounds: Rodd, E. H..Ed.; Wiley:
New York. 1961: Val. 7. D 344.
3. Smith 0. F.: Cagla,F. W J. 0%.Chem. 1Y7,12,781. 4. Eekhard. I. F.; Summers, L. A. Arraf. J. Chem. l97J,T6,2721. 5. Gillard. R. D.: Hill. R. E. E.; Mmkit1.R. J.Chem. Soe. ( A ) IWD, 1447. 6. Pfcifk.P.: W e d e b , Br. 2.Anarg. Chem. 1950.T61.197. 7. Baker. B. R.; Bmto.F.: Neanaann, H.M.J. Phys. Chem. 1959.03. 371. 8. Dickeson. J. E.:Summer8.L.A. Awl. J. Chem. 1970,W.1023. 9. Rieharda,A. F.;Ridd, J. H.; Tobe, M. L.C k m . Ind. 1963,1727. 10. I n s k p , R. G. J. 1narg.Nucl. Chsm. 1962.%,763. 11. Sehil(,A.A.:Taylor,R.C. J . I n o r g . N w l . C k m . 1969,9,211. 12. McWhinnis, W. R.: Miller, J. D. Ad". Ioorg. Chem. h d i o c k m . 1989, 12, 135, md .erereneea therein. 18. Miller, J. D.; Prince, R H. J. Chen. Sac. 1W5.3185 and4708 14. Figgis. B. N.; Nyholm. R. S.J. Chem. Sor. 1959.338. 15. Lindo~,L.F.;Livingstons.S.E.Caord.Chem.Reu. 1%7.4174andmfffenc~lth~in. 16. Gesry. W. J. Coord. Chen.Reu. 1971.7,Sl.
Volume 66
Number 6
June 1989
517
