Reactivity of cobaloxime bound to polystyrene chain by carbon–cobalt σ bond

Reactivity of Cobaloxime Bound to Polystyrene Chain by Carbon-Cobalt t~ Bond

HIROSHI NISHIKAWA, EI-ICHI TERADA, and EISHUN TSUCHIDA, Department of Polymer Chemistry, Waseda University, Tokyo 160 Japan,

and YOSHIMI KURIMURA, Department of Chemistry, Ibaralzi University, Mito 310 Japan

Synopsis

The (alky1)-bis(dimethy1glyoximato)pyridinecobalt attached to polychloromethylstyrene by a cobalt-carbon bond was prepared by the reaction of Co(II)(DH)zPy with polychloromethylstyrene in benzene. The fraction of p-vinylbenzyl-Co(DH)zPy introduced to the polymer was 8.1 and 2.1 mole %. The photodecomposition of the polymer-bonded cobaloxime was investigated by following the change of the visible spectrum. The rate constant kdec of the polymer-bonded cobaloxime was 1.1 X sec-' in benzene; it is one-fourth of that of its monomeric analog, benzylCo(DH)zPy. The hdec values of the cobaloximes were also measured in benzene-dimethyl sulfoxide mixed solvents, and the polymer effects were discussed. The dependence of the photodecomposition on energy of the irradiation light was investigated, and it was found that the absorption band near 470 nm is important for the photodecomposition of the cobalt-carbon bond. Spectroscopic measurements of the ligand exchange reaction of polymer-bonded cobaloxime with pyridine in dimethyl sulfoxide gave a larger equilibrium constant (1.2 X lo4 litedmole) than that of benzylCo(DH)zPy (9.4 X lo2 liter/mole). The kinetic data of the ligand exchange reaction indicated that the larger equilibrium constant for the polymeric system is due to the smaller rate constant of the reverse reaction. The thermodynamic parameters were also obtained.

INTRODUCTION Organocobalt compounds have been actively researched by Schrauzer, Costa,

and other many chemists, since the coenzyme vitamin B12 was found to have a cobalt-carbon bond in uiuo. That research has been concerned not only with the syntheses of model compounds of vitamin B12 but also with the photoreac- tivities1.2 and catalytic activities3 of these compounds.

It is the first characteristic of the organocobalt compounds to have an equatorial ligand with a wide a-conjugated system. The typical ligands are bis(di- methylglyoxime) [ (DH)2], bis(salicyla1dehyde)ethylenediimine (SALEM), and bis(acety1acetonato)ethylenediimine (BAE). The cobalt complexes coordinated to these ligands are called cobaloxime, salcomine, and cobaltbaen, respectively.

Recently, polymer metal complexes consisting of polymer ligands and metal ions were studied by several investigator^.^^ The reactivity of metal complexes in solution is partially controlled by the chemical environment such as solvent. Then, if a metal complex is introduced into a polymer domain, the polymer is expected to exhibit some characteristic behaviors on the chemical reaction of metal complexes. The specific catalyses of the metalloenzymes are induced by the environmental effect of the polymer, apoprotein. Although the reaction or catalysis of polymer metal complexes have been reported by many investiga-

the detail of the effect of the polymer around the metal complex has not been clear due to the complexity of the reaction.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 16,2453-2463 (1978) 0 1978 John Wiley & Sons, Inc. 0360-6376/78/0016-2453$01.00

2454 NISHIKAWA ET AL.

In this report, an organocobalt complex, “cobaloxime,” was bonded to a polymer chain and the photoreaction and the ligand exchange reaction of the polymer-bonded cobaloxime were studied and the effects of the polymer chain on its reactivities were discussed.

RESULTS AND DISCUSSION

Introduction of Cobaloxime into Polymer The polymer-bonded cobaloxime, attached to a polymer chain via a Co-C

linkage, was prepared by the reaction of polychloromethylstyrene (PCMC) with the divalent cobaloxime in benzene containing a little amount of pyridine (Py), following a similar manner of the synthesis of benzyl-Co(DH)zPy reported by Halpern et a1.12 The cobaloxime could be introduced into the polymer chain a t a ratio of 8.1 or 2.1 mole % to CMS unit. The salcomine and the cobaltbaen were introduced into the polymer chain by the reaction of those cobalt(1) complexes with the chloromethylstyrene-styrene (1:l) copolymer (PCMS-St). The ratios of these complexes introduced to the CMS unit were approximately 100 mole %. These reaction schemes are shown in Figure 2. Unfortunately, the salcomine-containing polymer and the cobaltbaen-containing polymer were not soluble in most organic solvents and could not be purified thoroughly. The solubility of Co(DH)z in benzene is relatively low, and a large excess of cobaloxime could not be used. This is probably a reason of the low percentage of cobaloxime introduced into the polymer chain. On the other hand, cobalt(1) species are one of the most powerful nucleophiles capable of displacing halide and are very soluble in tetrahydrofuran (THF). Therefore, the polymer-bonded cobaltbaen and the polymer-bonded salcomine in which the CMS’unit was completely replaced with the cobalt chelate could be prepared by the use of excess cobalt(1) species.

The polymer-bonded cobaloxime is soluble in benzene and its analogs, halo- genated hydrocarbons, THF, DMF, DMSO, and pyridine, and insoluble in methanol, ethanol, saturated hydrocarbons, ether, and water. The visible spectra of these polymer-bonded complexes were quite the same as their monomeric analogs.

‘

Fig. 1. Equatorial ligands of organocobalt compounds.

COBALOXIME BOUND TO POLYSTYRENE 2455

I 4/ I x

Fig. 2. Syntheses of polymeric organocobalt compounds.

Photodecomposition of Polymer-Bonded Cobaloxime The polymer-bonded cobaloxime has a cobalt-carbon bond, in which the

carbon atom belongs to the polymer chain. Irradiation with the tungsten light caused the homolytic cleavage of the cobalt-carbon bond, and then the absorption maximum a t 370 nm was blue-shifted and the absorbance at 370 and 470 nm decreased as shown in Figure 3. The photodecomposition rate constants kdec were calculated from the changes of the absorbance at 470 nm and shown in Table I. These values were obtained under the conditions of the same light intensity. The kdec of the polymer-bonded cobaloxime in benzene was smaller than that of the benzylcobaloxime, that is, the polymer chain exerted some effect to retard the photodecomposition of alkylcobaloxime. In polymeric system, divalent cobalt complex molecules generated by the homolytic photodecomposition are surrounded by the polymer chain. Then the diffusion of the divalent cobalt complex molecules takes place less easily due to the steric hindrance of

0

Fig. 3. Change of visible spectra of the cobaloxime polymer with photodecomposition in air: (1) 0 sec; (2) 15 sec; (3) 45 sec; (14) 90 sec; (5) 1 hr. Polymer containing 8.1 mole % of cobaloxime; solvent, DMSO.


TABLE I Photodecomposition Rate Constantsa

Range of R Solvent irradiation. nm

Benzyl Benzene Whole range 4.6 X Benzyl DMSO Whole range 1.0 x 10-2 Benzyl DMSO > 350 1.1 x 10-2 Benzyl DMSO > 450 1.1 x 10-2 Benzyl DMSO > 550 <lo-3

PolymerC Benzene Whole range 1.1 x 10-2

Polymerd DMSO > 350 1.0 x 10-2 Polymerd DMSO > 450 0.9 x 10-2 Polymerd DMSO > 550 10-3

Polymerd DMSO Whole range 1.4 x

hu * RCo(DH)2L F== Re + Co(II)(DH)zL; L = Py(in benzene) or DMSO (in DMSO).

kdec = - d[RCo(DH)2L]/[RCo(DH)pL]dt was measured under irradiation from a tungsten light

Containing 8.1 mole 96 cobaloxime. Containing 2.1 mole % cobaloxime.

whose photointensity was kept constant through all runs.

the polymer chain. Also generated alkyl radicals should be fixed to the polymer chain. Thus they exist close to each other for longer time and the recombination occurs more easily than in the monomeric analog systems.

However, the effect described above in the polymeric system was not found in dimethyl sulfoxide (DMSO). In DMSO, the chemical environment around the cobaloxime in the polymeric system is different from that in the monomeric system, because it is affected by the polymer chain which may form a nonpolar field. From the fact that the kdec value of benzylcobaloxime in benzene is larger than in DMSO, it seems that the nonpolar field formed by the polymer chain presumably promoted the photodecomposition. Other factors, such as the difference of an axial ligand (Py in the benzene solution and DMSO in the DMSO solution), the reactivity of the radical formed by the cleavage, and the difference in quantum yield, do not give sufficient reasons for the peculiar behavior of the photodecomposition of polymer-bonded cobaloxime. In order to clarify the polymer effect more closely, the photodecompsoition rates of the polymer-bonded cobaloxime and benzylpyridinecobaloxime were also measured in DMSO-benzene (DMSO-Bz) mixed solvents. The ratio of the rate constant of the polymer-bonded cobaloxime to that of the monomeric analog, which was employed to cancel an effect of the solvent, decreased with an increase of benzene content in the DMSO-Bz mixed solvent (Fig. 4). It was found from the viscometric data of polychloromethylstyrene, to which the cobaloxime would be bonded, that the polymer chain spread with increasing the benzene content in the solvent. The fact that the kdec of the polymer-bonded cobaloxime decreased as the polymer domain became spread appears to be inconsistent with the effect of the polymer chain discussed above. But in DMSO, cobaloximes are located in outer sphere of the polymer domain so that they do not suffer the steric hindrance of the polymer chain. The ratio kpo~ymer/&,nomer increased as the benzene content in the solvent decreased and to over unity. This suggests that the polymer chain has some effect to promote the photodecomposition. This effect may be caused by energy migration on the polymer chain, which contains an aromatic ring ca-

COBALOXIME BOUND TO POLYSTYRENE

0.1

I I I I . I ,

2457

2.0 1

Fig. 4. Change of relative rate constant of photodecomposition and ~,,,/c of polychloromethylstyrene with benzene content.

pable of absorbing the photoenergy. Thus the polymer-bonded cobaloxime partly decomposed on irradiation of the light below 350 nm in contrast with benzylcobaloxime, which required irradiation with light between 450 and 550 nm to decompose (Table I). Thus the absorption of the photoenergy by the aromatic ring on the polymer chain promotes cleavage of the Co-C bond of the polymer-bonded cobaloxime. A similar energy migration on polymer chains was reported by North et al.13

It is found from the infrared spectral analysis that the product from the photodecomposition of the cobaloxime polymer has the C=O group (1680 cm-l), and the same result was obtained in the photodecomposition of the polymers containing the salcomine and cobaltbaen.

The existence of the C=O group in the products of the photodecomposition indicates that oxygen was involved in the photodecomposition reaction. The dependence of the photodecomposition rate on the concentration of oxygen was investigated, and it was found that the rate constant determined from the change of spectra is independent of the concentration of oxygen above 0.1 mm Hg. But under high vacuum the spectrum of the divalent cobaloxime appeared, and the changes of the spectra were fairly different from those in air (Fig. 5). Oxygen may not participate in the rate-determining step under normal conditions.

Ligand Exchange Reaction of Cobaloxime A well-known reaction of cobaloxime is the ligand exchange reaction in which

one axial base is replaced by another. There is much evidence that most inorganic cobalt(II1) complexes are relatively inert to substitution. However, the presence of an organic group on the cobalt makes the five-coordinated inter- mediate relatively more stable, and allows a much faster dissociation of the axial ligand in the position trans to the organic group. Consequently, equilibria between axial ligands and organocobalt(II1) compounds are established rapidly.

The equilibrium constants K of the ligand exchange reaction at the position trans to the carbon-cobalt bond in the polymer-bonded cobaloxime and benzylpyridinecobaloxime [eq. (l)] were measured by the method of Miller and Dorough:

(1) kf

R.Co(DH)zDMSO + +RCo(CH)zL + DMSO kr


r----l

350 450 550 X (nm)

Fig. 5. Change of visible spectra of the cobaloxime polymer with photodecomposition in uucuo: (1) 0 hr; (2) 1 hr; (3) 2 hr; (4) 4 hr. Polymer containing 8.1 mole % of .cobaloxime; solvent, DMSO.

where K = kflk, and L is Py or SCN-. When the benzylpyridinecobaloxime or the polymer-bonded cobaloxime dissolves in DMSO or the mixed solvent of DMSO and benzene, coordinated pyridine is dissociated, and DMSO is associated with cobaloxime unless excess pyridine is added. This was confirmed by spectroscopic analysis. The equilibrium constant with pyridine in the monomeric complex system was 15 times larger than that with SCN-, and in the polymer- bonded cobaloxime system K value with pyridine was 50 times larger than that with SCN- (see Table 11). The larger difference of the equilibrium constants between the two ligands in the polymeric system are probably due to the polymer effect described below.

The equilibrium constant of pyridine with the polymer-bonded cobaloxime was about 10 times that with the monomeric analog, though the constant of the polymeric system with SCN- was about three times that of the monomeric system. This difference between the increase of K value in the polymeric system with pyridine and that with SCN- is probably due to the difference of stability of the ligands in the polymer domain in which nonpolar field may be formed.

The equilibrium constants of Py in both systems increased with the addition of benzene. This fact suggests that the high equilibrium constant of polymer- bonded cobaloxime is partially due to the environmental effect of the styrene

TABLE I1 Equilibrium Constants of Ligand Exchange Reactions on Cobaloxime

R Ligand Solvent K , litedmole

Polymer(l)a SCN- DMSO 210

P o l ~ m e r ( 2 ) ~ PY DMSO 1.2 x 104 P o l ~ m e r ( 2 ) ~ PY DMSO-Bz(31) 2.0 x 104 Benzyl PY DMSO 9.4 x 102 Benzyl PY DMSO-Bz(3: 1) 2.0 x 103

Benzyl SCN- DMSO ' 68

a Polychloromethylstyrene containing 8.1 mole 96 cobaloxime, 25°C. kf

R-Co(DH)zDMSO + L F=+ R-Co(DH)zL + DMSO L = Py or SCN-, K = k$k,. kr

!I Polychloromethylstyrene containing 2.1 mole % cobaloxime, 25OC.


unit in the polymer chain. However, in benzene (Bz)-DMSO(l:l) solvent, the equilibrium constant of polymer-bonded cobaloxime was 7.5 times that of the monomeric analog. Therefore there must be another polymer effect to enhance the complexation with Py.

In order to clarify the cause of the larger equilibrium constants in the polymeric system, the rate constants of the ligand exchange reactions in the cobaloxime were investigated in DMSO (Table 111). The rate constant of the coordination (kf ) of pyridine or SCN- to the cobaloxime polymer was the same as that to benzylcobaloxime, respectively. However, in the reverse reaction in eq. (l), namely, in the dissociation of ligand, the rate constant (k,) of pyridine in the monomeric system was about nine times that of the polymeric system and k, of SCN- in the monomeric system was about three times that of the polymeric system. It is obvious from these results that the larger equilibrium constants of the ligand exchange reaction in the polymeric system were due to the smaller rate constants of the reverse reaction.

This smaller k, value could be explained by the polymer effect as follows: (1) the local concentration of the cobaloxime in the polymer domain is higher than in bulk solvent; (2) the polymer chain acts as a steric hindrance for diffusion. Because of these two facts, a dissociated ligand can not easily escape from the polymer domain resulting in the recombination with the cobaloxime on the polymer chain.

Thus the apparent rate constants of the reverse reaction in the polymeric system should be smaller than those in the monomeric system. Under the higher local concentration of the cobaloxime in the polymer domain, the kf value in the polymeric system is expected to increase, but was almost the same value as that in the monomeric system. This is probably because the kf value was obtained with the stopped-flow spectrophotometer following spectral change after mixing the ligand-containing solution and the cobaloxime-containing solution.

The rates of the ligand exchange reactions of pyridine with the polymer- bonded cobaloxime and the monomeric analog were measured also in the DMSO-benzene mixed solvent. The variations of the relative rate constants of the forward reaction, kf(polymer)/kf(monomer) and that of the reverse reaction, k,(polymer)/k,(monomer) with benzene content are shown in Figure 6, along with the change of the reduced viscosity of the polychloromethylstyrene, which is the backbone of the polymer-bonded cobaloxime. With an increase of the benzene content in the mixed solvent, the relative rate constant of the forward

TABLE I11 Rate Constants of Ligand Exchange Reactions on Cobaloximea

k f, k rl

R Ligand M-l sec-I M-' sec-1 b

PolymerC SCN- 1.0 x 103 4.8 Benzyl SCN- 1.2 x 103 15 Polymerd PY 23.6 2.0 x 10-3 Benzvl Pv 16.5 1.7 x

kf a R-Co(DH)zDMSO + L F R*CO(DH)~L + DMSO.

kr

b Calculated by the equation, k , = kf/K. Polychloromethylstyrene containing 8.1 mole % cobaloxime, 25°C. Polychloromethylstyrene containing 2.1 mole % cobaloxime, 25°C.


Bz Content in CMSO tk solvent 1%)

Fig. 6. Relative rate constants of coordination of pyridine with cobaloxime polymer and q3Jc of polychloromethylstyrene in DMSO-benzene solvent: (0) kf; (0 ) k,; (a) qsp/c.

reaction decreased and that of the reverse reaction increased. These phenomena could be explained by the expansion of the polymer domain with increase of benzene content in the mixed solvent. This expansion results in a decrease of the local concentration of cobaloxime in the polymer domain and a decrease of steric hindrance of the polymer chain. The decrease of these polymer effects results in the decrease of the relative rate constant of the forward reactions and the increase of that of the reverse reactions.

The thermodynamic parameters of the ligand exchange reaction of pyridine with the benzylpyridinecobaloxime and the polymer-bonded cobaloxime were calculated and shown in Table IV. It is found that the larger equilibrium constant of the ligand exchange reaction of pyridine with the polymer-bonded cobaloxime than with the benzylcobaloxime is due to the larger change of the entropy.

The differences in AH and A S between the monomeric system and the polymeric system could be explained by the steric effect of the polymer chain. The coordination bond of DMSO with the polymer-bonded cobaloxime in the initial state and that of pyridine in the final state were weakened by steric hindrance of the polymer chain. This results in the increase of enthalpy and entropy of the polymeric system in both states. But in the initial state, the increases of enthalpy and entropy by the polymer effect are not so large because the coordi-

TABLE IV Thermodynamic Parameter of Coordination Reaction of Pyridine with Cobaloxime (25%)"

m, AH, A s , R Solvent kcal/mole kcal/mole e.u.

Thermodynamic parameterb

Polymerc DMSO -4.2 -9.2 -16 Benzyl DMSO -3.9 -16 -40 PolymerC DMSO-Bz(3:l) -5.9 -3.5 7.8 Benzyl DMSO-Bz(31) -4.5 -3.5 3.2

a R*Co(DH)2DMSO + Py s R-CO(DH)~PY + DMSO b Calculated from the value of K,, at several temperatures (15,20,25,30, and 35OC).

Polychloromethylstyrene containing 8.1 mole % cobaloxime.


nation bond of DMSO is not strong originally. The degree of freedom of the ligands, especially of pyridine, in the polymeric system is larger than that in the monomeric system because of weakening of the coordination bond by the steric hindrance. Thus the A S and AH in the polymeric system become larger than those in the monomeric system.

EXPERIMENTAL

Preparation of Polychloromethylstyrene and Chloromethylstyrene- Styrene Copolymer

Chloromethylstyrene was obtained from Seimi Chemical Co., Ltd. Styrene and chloromethylstyrene were purified by distillation under reduced pressure. The polymerization and the copolymerization of these monomers were carried in benzene a t 60°C with azobisisobutyronitrile as initiator. The reaction mix- tures were poured into methanol and the precipitated polymers were purified by reprecipitation twice. The molecular weight of the polychloromethylstyrene was 4000 and that of copolymer was 7000.

Preparation of the Polymer-Bonded Cobaloxime (Cobaloxime Attached to Polymer by a Cobalt-Carbon Bond)

Co(DH)2 was prepared from Co(DH)2(H20)2 by the procedure of Schrauzer.14 The following procedure closely follows that of Halpern.12 Polychloromethyl- styrene (1.04 g) was dissolved in 100 ml of benzene containing 2.5 ml of pyridine. After bubbling through nitrogen for 30 min, 4.34 g of Co(DH)z was added to the polymer solution under nitrogen in the dark, and the reaction vessel was closed. The reaction mixture was stirred for 20 hr and then concentrated and filtered. The filtrate was poured into methanol and the precipitated polymer was purified by reprecipitation. The structure of the polymeric organocobalt compound was determined by the spectroscopic analysis (infrared, visible spectra).

Benzylpyridinecabaloxime was prepared according to the procedure of Shrauzer.15

Preparation of Polymer-Bonded Salcomine or Cobaltbaen

The tetrahydrofuran solution of Na[Co'(SALEM)] or Na[CoI(BAE)] (10 mmole) was prepared by the procedure of Costa.16J7 This solution was treated with tetrahydrofuran solution of the chloromethylstyrene-styrene (1:l) copolymer (1.34 g) a t -80°C under a nitrogen atmosphere in the dark. The reaction mixture was poured into methanol, and the precipitated polymer was filtered and washed with methanol. The polymer-bonded salcomine or cobaltbaen was slightly soluble in most solvents.

Photodecomposition of Organocobalt Compounds

A tungsten lamp (100 V, 100 W, Kondou Co., Ltd.) was used as a source of irradiation light. Some filters were used to limit the range of irradiation light. The distance between the lamp and the samples was kept constant to keep the light intensity constant. Photodecomposition rates were proportional to the


concentration of alkylcobaloxime and were determined by observing the change of absorbance at 470 nm in the initial period.

Determination of Equilibrium Constants and Kinetic Measurement of the Ligand Exchange Reaction

Equilibrium constants were calculated by the method of Miller and Dorough from the change of spectra with the addition of ligand which were recorded with Shimazu MPS-50L spectrophotometer. Kinetic constants were determined by observing the change of absorbance at 470 nm followed by the stopped-flow spectrophotometer (Union Giken RA-1300) under the pseudo-first-order conditions.

CONCLUSION

The role of the polymer chain in the reactivity of cobaloxime is concluded to lie in the decrease in mobilities of compounds in the polymer domain by the steric hindrance of the polymer chain. In the photodecomposition reaction the polymer chain decreases the mobility of CoI1(DH)2Py and increases the proba- bility of recombination of Co11(DH)2Py and a radical fixed on the polymer chain. In the ligand exchange reaction, escape of a ligand dissociated from cobaloxime '

from a polymer domain was obstructed by the steric hindrance of the polymer chain; in retarding the dissociation, this resulted in a larger equilibrium constant for the polymeric system than that for the monomeric analog system.

The authors thank Mr. M. Kasai, one of their colleagues, for assistance with the stopped-flow spectroscopic measurement and helpful discussion.

References

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Marinsky and Harcus, Eds., Marcel Dekker, New York, 1973, Chap. 5. 6. K. Liu and H. P. Gregor, J. Phys. Chem., 69,1252 (1965). 7. Y. E. Kirsh, V. Y. Kovner, A. I. Kokorin, K. I. Zamaraev, V. Y. Chernyak, and V. A. Kabanov,

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9. M. H. Hughes, The Inorganic Chemistry of Biological Processes, Wiley, New York, 1972. 10. N. A. Vengerova, Yu. E. Kirsh, V. A. Kabanov, and V. A. Kargin, Dokl. Akad. Nauk SSSR,

11. E. Tsuchida, H. Nishikawa, and E. Terada, J. Polym. Sci. Polym. Chem. Ed., 14, 532

12. P. W. Schneider, P. F. Phelan, and J. Halpern, J. Am. Chem. SOC., 91,77 (1969). 13. A. M. North, Br. Polym. J., 7,119 (1975). 14. G. N. Schrauzer, Inorganic Syntheses, Vol. XI, W. L. Jolly, Ed., Wiley, New York, 1968, pp.

Eur. Polym. J., 10,671 (1974).

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15. G. N. Schrauzer and R. J. Windgassen, J. Am. Chem. Soc., 88,3738 (1966). 16. G. Costa and G. Mestroni, J. Organornet. Chem., 11,325 (1968). 17. G. Costa and G. Mestroni, J. Organornet. Chem., 11,333 (1968).

Received March 1,1977 Revised June 29,1977

Documents

Reactivity of cobaloxime bound to polystyrene chain by carbon–cobalt σ bond