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(phen- anthroline)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 oxi- dation and ligand displacement reactions. In addition, the intermediate complexes are isolated and characterized, pro- viding students with a practical opportunity to apply tech- niques 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, peri- odate, or chromic acid ( I ) . However, these reagents do not oxidize free 1.10-phenanthroline (phen), 1, to the corre- sponding 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 1 with 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

Co(quin)F, and treatment of the latter with NQEDTA to liberate 2 (see figure) (5-7).

QUIN ( 2 )

516 Journal of Chemical Education

Typlcal Student Data

Magnetic m.p. Major IR A- (Wb." 'H NMRCd ~onductivitp~ moment

Compound (OC) absorptionsLb UV vis (8, P P ~ ) (ohm~'cm2moi-'1 (B.M.)

Phenanthollne 114-117 1610,1508 224,264 - 9.02 (H2,9) - - 848.728 290.32Osh 8.69 (H4,7)

6.00 (H3.6) 7.88 (H5.6)

[Co(phenh12+ - 1625,1520 225.267 475sh - 120 4.7' 850,841 292.343sh 725

[Co(phen)d3+ - - 217.278sh. 430 sh 9.10 (H4.7) 250 Diamagnetic 270.346sh 8.51 (H5.6)

7.98 (H3.8) 7.68 (H2.9)

[Wquinhlg+ - 1 7 ~ - - - - - 1040 br"

Phenamhrolinequinone (2) 256-257' 16900 - - - - -

Our modified procedure for converting 1 into 2 is outlined below. We have isolated the cobalt complexes as tetrafluoro- borate salts, rather than the potentially hazardous perchlo- rates, and the resulting overall yield of 2 from 1 is typically between 15 and 25%. Spectroscopic, conductivity, and mag- netic da ta for the ligands and isolated complexes are given in the table.

Experimental

All materials used are readily available commercially. Caution: 1,lO-Phenanthroline-5.6-quinone attacks the na- sal and throat membranes if breathed in. Gloves should be worn when handline it. and all transfers should be made in the 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 ~ently stirred withaglass rod for several minutes until precipitation was complete. The yellow- brown product wascollected hy filtration,waahed withwater(2 X 15 ml.). and dried in air bv suction. Yield 80%. Students should mea- surethe IR and W-viiible soectra. and conductivitv of the com- plex, and determine the magnetic moment by measuring the mag- netic 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-bot- tomed 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 mstal- line 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.

1,lO-Phenanthrol/ne-5,6-quinone, 2 A mixture of [Co(q~ in )~ ] (BF~)~ (1.5 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 1 b, 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 im~ure .. .. ~.~ ~~~ ~

quinone were combined and recrystallized from'methanol yielding yellnw needles of 2 (yield fig%). Students may characterire the com- pound by its melting point and infrared spectrum.

Dlscusslon

Students should obtain da ta for the complexes and li- gands similar t o t h a t shown in the table, and they should he encouraged to explain the significance of their results by considering some of the 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. An. 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. Inorg.Nwl.Ckm. 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