Molecular Oxygen Uptake by a Solid Co[ll) Complex Its Synthesis and Kinetic Oxygenation Daniel J. Aymes and Michel R. Paris Universite de Bourgogne. B. P. 138-21004 Dijon Cedex. France The reversible oxygen uptake by synthetic coordination comnounds mimics the hehavior of natural oxveen carriers as hknoglobin. I t may he of interest to introdice the group of these comnlexes within a course of coordination chemistw or when deaiing with heterogeneous solid-gas reaction in course of nhvsical chemist^. ~ m o n g ' t h esynthetic oxigen carriers, the Co(I1) Schiff base comnlexes nrenared from salicvlaldehvde and ethyleuediamine are we^ kLown since TS-aki's discovery of their hehavior in oxygen (I).Though intensively studied within the scope of various applications, they were very seldom selected for undergraduate experiments. They are generally referred t o as Cosalen and when dissolved in donor solvents (pyridin, DMSO, etc.) they hind with oxygen. In the crystalline state thev can he in two forms: (1) A so-called active form, an open crystalline structure that allows the progress of oxygen molecules between the loosely packed molecules planes of the complex (2). Oxygen can reach the coordination sites in the vicinitv of the cohalt atoms and hind with them. (2) An inactive fo-nn in which the molecular planes are so closelv packed that the oxygen .. molecule cannot find room to fit intothe lattice. T. G. Appleton (3)has already proposed an experiment in which he first describes one preparation of inactive Cosalen in the solid state and, second, shows the binding of oxygen when the comnlex is in solution in DMSO. As for us, wfe propose here preparation methods of both the active and inactive forms of Cosalen ( 4 ) ,whereas most of the preparation methods deriving from Calvin's work (5) systematically yield a mixture of the two forms in variable proportions. Next we describe an experiment of heterogeneous kinetics (solid-gas) that consists of following the time dependence of the oxygenation of the active solid and measuring the oxy~enationcapacity of the chelate. That last enpe;jment requires a very inenpensive setup, which allows several kinetics to be simultaneously investigated as function of temperature and pressure.
vacuum
a
Cosalen Synthesis In Organlc Medium (a) Schiff Base Preparation: N, N'bis(sa1icylaldehyde)ethylenediimine With a setup consisting of an Erlenmeyer flask topped by a dropping funnel and a Dean Stark-type receiver with a condenser, start
H C-CH2 21 \
Figure 2. Klrmtic setup. A, gWs reaction tube: B, sliding inner hlbe (allows varylng the inner volume of Uw reaction tube): C, crucible containing the complex: D, thermometer: E. water bath: F, joints made at thick-wailed rubber tubing; 0, one to four MOWSdips: H, O-ringsphericai]oint; I, glass tube. 2 mm inner diameter. 40 cm iengm: J, bead of mercury: K, ballwn; L, sliding snew cap. by mixing 25 mL of benzene with 3.66 g (3X 10W mol) of salicylaldehyde in the Erlenmeyer flask and 50 mL of benzene and 0.9 g (1.55 X 10F mol) of ethylenediamine in the dropping funnel. Slowly pour the solution of ethylenediamine, at room temperature, into the stirred reaction mixture, which then turns yellow. Then a yellow precipitate of the Schiff base quickly forms. When the addition is completed, heat to reflm, until 15 mL of benzene, plus the water, have collected in the receiver. (b) Preparation of Active Cosalen: N, N'Rinseout thedropping funnei.Then pour into it a solutionof3.85 10 'mol) of anhydrous Co(ll)acetylacetonate'in 80 mL of benzene. At room temperature and while vigorously stirring, add (fast drip) the Co(I1) solution to the reaction mixture, which then turns brown; a fine precipitate of the complex forms rapidly. Stir for 5 min further, then very quickly filter under suction (sintered class). .. .. and rinse with a small amount of benzene. From hnwn, the filtrate rapidly turns dark green, as a result of oxygenation of the solution by air. Dry the solid so obtninrd under a vacuum bell jar evacuated by suction, in the preseneenfnulfurir acid as a desiccator. After about 1h, the dried complex (5 g) is in the form of a highly electrostatic brown powder. It is made of very fine needles and can be stored in air. g (1.5 X
' .. .
Co(illacetvlacetonate:mix 20 a of acetvlacetone with 100 mL of 10% aoueous sodium carbonate"s0lution: and Dour in ~it -~~ ~~~~--~ , then stir. - ~ ~ v -, ~ slowly a solution of 30 g of Co(li)nitrate hexahydrate in 100 mL of ~
Figure 1. Formvie of Cosalen
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Journal of Chemical Education
~
~
7
~
~~
~
~~~
~ ~
~
~
~
-
~~~
~
water. Wash lhe pink silky precipitate, and reclystallize it in hot absolute alcohol. The dihydrated product so obtained is dehydrated by distillation of the water-benzene azeotrope using the same experimental setup as described in the synthesis section. The solution in benzene thus obtained is stored in a stoppered flask and kept away from ambient humiditv.
This form of the complex is solvated by about one molecule of benzene for one Cosalen (4). The benzene must he eliminated in order to allow the binding with oxygen (see below). (c) Preparation of Inactive Cosalen Except far the fact that the Co(I1)acetylacetonateis added at the temperatureof solvent reflux, the preparation procedure is the same as far the active form. In such conditions, the reaction mixture is red and the chelate so prepared is in the form of hexagonal platelets. The solid (4.5 g) is separated in the same way as in (h). In this form the complex is not solvated and does not hind with oxygen in the solid state. Klnetlcs of Oxygen Uptake by Active Cosalen The apparatus described here is shown in F i r e 2. The balloon K is partially filled with either pure oxygen or an oxygen-nitrogen mixture of known composition. With clips G1 and G4 tightened, flush the calibrated tube I with gas, lassening clips G2 and G3 and pressing gently the gas container K. Whatever its initial position, the mercury head J will he pushed to the open end of the tube where the gas flows out. When G3 is tightened again, the mercury head seals I and is correctly positioned for measurement. Then tighten G2.
(a)Extracting the Solvation Benzene Loosen dips G1 and G4. Evacuate the reaction tuhe A by suction, and heat the complex C to 100 OC hy boiling water in E for about three-quartersof an hour. Thus benzene is totally extracted,and the chelate can bind with oxygen at lower temperature. Without benzene or oxygen, the chelate is Light brown.
(b) Complex Oxygenation While keeping A under suction, replace the boiling water in E by water at the resoective temoerature needed for the intented exneriment. When the temperature isstable, tighten G I . Then at time r = 0, loosen GR, and trghten it again immediately, and loosen G2.The chelate is then under 1 atm prrasure of uxygen or oxygen-nitrogrn ~
~
The movement of the mercury bead in tuhe 12allows one: (1) to follow the kinetics of the oxygen uptake hy the chelate under the conditions of pressure and tem~eratureof the experiment and (2) to measure the iargest amount i f oxygen hound inder these conditions, when the bead J stops moving. The oxygenated chelate is dark brown. It can be deoxygenated and oxygenated again. (c) Complex Deoxygenation It is carried out by heating t o 100 'C in vacuum. Follow the same procedure aa in section (a). All the oxygen hound in the previuus stage is released. With this setup, the oxygenation can he performed: (1)under various oxygen partial pressures using either pure oxygen or oxygen-nitrogen mixtures in K and (2) at various temperatures if the water bath is eouinned with a thermostat. Then the deienhence of the reaction kinetics on temperature and pressure can he established (6, 7). Sample Experiment Four experiments carried out a t 18 'C and atmospheric pressure ( P = 750 torr = 0.987 atm) are described now: Experiment 1:pure oxygen in K, Experiment 2: 75%0~ 25% Nz in K, Experiment 3: 50%Oz 50%Nz in K, Experiment 4: 33%0 2 + 67% NZin K. According to all previous studies of the solid state (8-101, oxygen bridges two Cosalen species. The oxygenation equilibrium must he written
+ +
whence the stoichiometric ratio Colon = 211 when oxygenation is complete, as under the present temperature and pressure conditions. Notice that, prior to each reading of the position of the mercury bead, it is necessary to tap the tube with a pen until the bead stabilizes.
+
Flgure 3. (a) Kinetic cuves: 1. pure oxygen: 2. 75% oxygen 25% nitrogen; 3.50% oxygen 50% nitrogen: 4.33% oxygen 67% nitrogen. Dirplacement (mm) of mercury bead versus time. (b) Plot of the reaction rate versus oxygen pressure
+
+
T h e theoretical weight capacity is defined as maximum weight percent of oxygen bound by Cosalen: Capacity =
M~21boundi
lM) 32 X 100 =-= 2 X 324.9
4,92%
2 X Mcale,
One may also define the fractional oxygenation, a:a = 1if the complex is fully oxygenated, a = 0 if it is fully deoxygenated. T h e weight of solvated Cosalen used for the experiments mol. was 32.23 mg, i.e., about 8 X T h e kinetic curves (displacement of mercury bead versus time) are shown in Figure 3. In experiment 1,the reaction is completed within about 1 h. The chelate is then in the oxygenated form (Cosalen)rOa. Then the apparatus is opened to air and the weight of chelate determined to be 29.00 mg. This corresponds to 4.25 X mol of ( C 0 s a 1 e n ) ~ Oor ~ to 8.5 X lo-= mol of initial Cosalen. The number of bound oxygen molecules can he determined: X v ,,= PRXT
where u is the glass tube volume between the initial and the final position of bead; i.e., r=lmm
CoIO,
= 8.5 X
10@/4.35 X
= 1.95 (theoretical = 2)
experimental capacity - 32 X 4.35 X (theor. = 4.92%) 8.5 X
lo-' X
X 100 = 5.04% 324.9
I n experiments 2,3, and 4, the reaction was not complete after 1h. From the curves in Figure 3a, the reaction rate a t the given fractional oxygenation (i.e., a = 0.5) can he compared and the pressure dependence u = f(P0,) can be established as shown in Figure 3b. From curves obtained in isobaric conditions and different temperatures (5 OC < T < 30 "C), one may determine the temperature dependence u = f(T). All the kinetic curves plotted here are for the first oxygenation of the chelate. Following this oxygenation and after deoxygenation, if we perform another oxygen uptake, the curve obtained under the same conditions as the first one is not s u ~ e r i m ~ o s a bon l e the previous curve, and so on for about five ti six suc~cssive~xy~enationldeoxygenatiun cycles: it is here a matter of a progressive evolution of the solid structure. Then we can observe for the next cycles a reproducibility of the curves. Under the same conditions, if i t is possible to do a large Volume 66
Number 10
October 1989
855
number of cycles (>loo), we can observe a loss of oxygenc a v i n g capacity due to a damaging of the chelate (11): oxidation of the organic ligand, oxidation of the Co(I1) into co(xI1), and breaking of the crystalline structure are occurring.
Literature Clted 1. Tsmnaki,T.Bull. Chem.Soe. Jop. L938,13,2522MI. 2. Aymos, D. J.; Mutin. J. C.; Paris, M. R A n n Ch. Fr. 1583,8,261-'87.
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Journal of Chemical Education
;:~$2;~~;;$$2;;,7;;;~1,7$;~r7~i6,11 1717~1721, sac. R. H.: W. K. J. A ~ .
I N W , 2254-2256. 3. c.1,in, M.:RGIB, wilmarth, 6. Barkdew, C. H.:Calvin, M. J. Am. Cham. Soc. 1946.68.2257-2262. 7. Rampa"o. L.; Silvestroni, P.; Trazrs, A. Ric.Sci. 1967.37.64a654. 8. Jones, R. D.; Summerville, U. A.: BmIo, F. Chem. Rou. 1979, 2, 139-118 and ref .;thin. 9. Aymw, D. J.: Paris. M.R.; Mutin, J. C. J Mol. C = t d 1983.18.31b328. 10. Aymes, D. J.; Paria, M. R.: Soustelle, M. J. Mol. Cotol. 1983.18.329349. 11. Rollotte,R.:Aymes,D. J.;Paris, M.R.Bul1. Sac. Chim.Fr. 1979,1,141-144.
