Radiochemical reactions in rhenium carbonyls—I

Y. inorg,nucl. Chem.,1976.Vol.38,pp. 1103-1107. PergamonPress. Printedin GreatBritain

RADIOCHEMICAL REACTIONS IN RHENIUM CARBONYLS--I Re2(CO),o IAN WEBBER and D. R. WILES Chemistry Department, Carleton University, Ottawa, Canada (Received 12 July 1975)

Abstract--Rhenium carbonyl, Re:(COho, has been irradiated with thermal neutrons and the radioactive products analysed. Yields of ~86Reand 188Rein various product compounds were found to depend on surface condition of the crystals, 3' radiation dose and on thermal treatment. Evidence is given for the formation of the radical 'Re(CO)z, which is fairly stable in the solid up to 75°C, at which point it disappears, leaving no detected trace. A species presumed to be Re(CO)4 is observed and some properties determined. The isotope ratio for the yields of '88Reand ~rRe was 1.30 for Re2(COho, 1.08 for Re(CO)~ and 1.00 for Re(CO)4. INTRODUCTION RADIOCHEMICAL reactions which can be studied in molecular compounds are those which lead to the formation of identifiable molecular species following a nuclear transformation such as (n, 7) or/3 decay. Numerous studies have been made[l] of the radiochemical reactions of metal atoms following neutron capture in metal carbonyls. Despite this, it is not yet clear whether a single overall description can be applied to the reactions leading to the reformation of the radioactive parent molecule which will be able to cover all the metal carbonyls. These reactions can involve, in condensed states, hard-sphere collisions, "hot" or epithermal reactions, radical diffusion reactions, thermally activated reactions or even failure of bond rupture in the original molecule. In this last instance, the question is how the several million volts of nuclear excitation energy can be dissipated without affecting the molecule. Radiochemical yields are found to be high in the mononuclear carbonyls of nickel[2], chromium[3], molybdenum[3], tungsten[3] and iron[4] in which 40100% of the induced radioactivity is recoverable as the parent compound. In several of these cases it is clear that a proportion of the parent molecules are completely formed in times too short for external chemical influences to be felt. Another recognizable proportion is present [3] as M(CO),_, in the matrix and thermally induced exchange or other reactions can cause the final stage of reconstitution: M(CO),_, + CO ~ M(CO)o. The situation is obscured in the case of Ni(CO)4 by very rapid natural exchange and in Fe(CO)~ by the probable interference of radiolytic reactions. The trinuclear carbonyls Fe3(CO)I2 and Ru3(CO)~: also show high radiochemical yields[4]. This is perhaps surprising if the supposition is made that reconstitution is directly related to molecular complexity, and leads to the alternative speculations that (a) the molecules may be reconstituted from "building blocks" such as Fe(CO), (b) the molecules may remain essentially intact during the nuclear event. The former is supported by the observation that a substantial yield of radioactive Fe3(CO),2 is found following neutron irradiation of Fe(CO)5. The latter

speculation is supported by high "retentions" in other very complex molecules, notably fulvalenedimanganese hexacarbonyl[5] and dicyclopentadienyl iron tetracarbonyl [6]. The only binuclear metal carbonyl studied hitherto is Mn2(CO)~0[7] in which a modest 10% of the induced radioactivity is recoverable as the parent compound while 5% is found in the form of *Mn(CO)5 radicals together with measurable amounts of *Mn(CO),. Thus, in the absence of any theory which can predict the variety of products obtained in a nuclear reaction with an organometallic compound, the present work was undertaken in order to allow a comparison to be made with Re~(CO)~0--another binuclear metal carbonyl--to attempt to measure any possible isotopic preference between ~86Re and ~88Re and, finally, to provide a foundation for a study of ReMn(CO)~o. EXPERIMENTAL

The general method used in this type of study involves the neutron irradiation of a sample of the pure target compound, followed by chemical isolation and radioactivity measurement of any possible product compounds. Since only an extremely small amount of radioactive material is produced, non-radioactive carriers are used for the expected species. These carriers are present during dissolution of the target or are added prior to the analysis, so that their ultimate chemical recovery can be used as a reliable measure of the extent of recovery of the radioactive molecules produced. Reagents. Re2(CO)~o,used as target for the neutron irradiations, was obtained from Strem Chemicals Inc., and purified before use by either vacuum sublimation or column chromatography. The purified samples were stored under air in the dark until used. IRe(CO)5 was prepared by direct reaction of 12 with Re2(CO),o in a sealed evacuated tube at 95°C. The crude IRe(CO)5 product was purified by vacuum sublimation (2.5 torr, 80°C) to remove [IRe(CO),]2, followed by column chromatography on silica gel using 10% CHC13in petroleum ether (30--45°Cfraction) to remove Re2(CO),o and subsequently 50% CHCt3 in petroleum ether to elute the product IRe(CO)5. [IRe(CO),]2 was prepared from the Re2(CO),o-I2 reaction mixture obtained as above, by refluxingat 90-120°C in high-boiling petroleum ether for 2hr. The product was purified by recrystallization from boiling CHCI3. The identities and purity of the compounds used were determined from their IR absorption spectra. The spectra obtained were in good agreement with those reported by Kaesz et al.[8, 9]. Chemical separations. Following neutron irradiation the mix-

1103

1104

IAN WEBBERand D. R. WILES

ture containing the radioactivated target material and the carrier compounds was separated using any of several methods depending on which carrier compounds were present. All separations were done in subdued incandescent light because of the likelihood of photolytic decomposition in the solutions. Retentiont as *Re2(COho was determined without added carrier. It was found that silica gel filtration, using a 1 × 10 cm column with a variety of solvents was successful in removing all radioactive species other than Re:(CO),o. TLC on silica gel plates was also used on occasion. The two methods gave values which were in good agreement with each other, and the respective retention values determined after separation from carrier activities were also found to be well correlated. The radiochemical yield of *Re(CO)5 species was measured using I: as scavenger in petroleum ether or chloroform solution in the same way as was reported earlier with *Mn(CO)~[7]: I~+ *Re(CO)~~ I*Re(CO)~+ I.

(1)

IRe(CO)5 was added as carrier and the excess iodine removed by extraction into aqueous sodium sulphite. The use of elemental Br2 or CI: in this technique is precluded since the reaction X: + *Re2(CO),o~ 2X*Re(CO)~ occurs readily. An alternative procedure was to dissolve the radioactive target in a chloroform solution of IRe(CO)5 carrier assuming a facile exchange between the radioactive radical species and the bulk carrier. Isolation of IRe(CO)5 was effected by silica gel chromography, using a gradient elution technique, with 10% chloroform in petroleum ether (30--45°C) to remove Re2(CO)lo and 50% chloroform in petroleum ether to elute IRe(CO)~. Yield values obtained by using the exchange technique with IRe(CO)~ carrier were reproducible with either BrRe(CO)~ or C1Re(CO)~ as carrier. The separation method differed only in the final concentration of chloroform required during gradient elution. Contamination of IRe(CO)~ carrier was found to result from a conversion to the tetracarbonyl dimer, [IRe(CO)4]2. The extent of the solution decomposition was found to be sufficient to affect the radiochemical measurement seriously even though the percentage conversion was small. It was therefore found necessary to remove the [IRe(CO)4]: from the IRe(CO)~ carrier by sublimation of the latter at 2.5 torr, 80°C. The yield of the species *Re(CO)4 was determined using the exchange technique with [IRe(CO),]2 as carrier. The compound was isolated from the target material and any other carriers used by vacuum sublimation. The unsublimed pale yellow crystals were taken up in chloroform and chromatographed on silica gel. Chemical yields were determined by direct weighing of the pure, powdered crystals after evaporation of the chromatographic eluate. Neutron bombardment. Neutron irradiations were performed in the "Slowpoke" reactor at the Commercial Products Division of A.E.C.L. in Ottawa. The flux used was 2.5x 10" cm-2sec '. Irradiation times for short irradiations could be controlled to better than 5% and the reactor power to about 1%. Subsequent gamma irradiations were occasionally given using a "°Co-bomb which gave a gamma dose rate of 2.0Mr hr-~. Radioactivity measurements. The 137 keV and 155 keV y-rays of ~a6Reand 'SSRe respectively, were measured as an unresolved envelope using a 3x3in. NaI scintillation detector and a t00-channel pulse height analyser. Isotope ratios and isotopic yields were obtained by analysis of the decay curves derived by counting over a period of about 350 hr. The isotopic yields of l~Re could also be conveniently determined by counting its 478 and 633 keV y-rays which occur with sufficient abundance to give good counting statistics. Some direct measurements of isotopic yields were made using a tThe radiochemical yield of a product in a given target compound with reference to a given radionuclide is the fraction (percent) of the total activity of that nuclide in the target which appears in a given chemical form. When this form is that of the target (parent) compound, the term "retention" is used, without implication of any reaction mechanism.

Oe-Li detector and a 512-channel pulse height analyser. The sample geometry was maintained constant for the measurements and decay corrections were made where applicable. Data were corrected to 100% chemical recovery of the carrier. Experimental conditions. Several experimental parameters were found to influence the results so that a set of "standard conditions" was used. The Re2(CO)~otargets were finely ground crystals, sealed in quartz under a vacuum of 2.5 x 10-~ torr. Reactor irradiations were done at a reactor temperature (above 40°C) for 50 sec at 5 kW. All operations were conducted under incandescent light. RESULTS

Retention of *Re2(CO)~o. The retention was found to be sensitive to various experimental conditions which ultimately were established arbitrarly. A comparison of the results obtained for finely-ground crystals and for larger ( - l m m edge) vacuum-grown crystals is seen in Table 1. It is seen that the lowest retentions are found for small crystals irradiated in air. The highest are for large crystals irradiated in vacuo. Such an effect of ambient atmosphere has been noted before[10, 11] and has been attributed to a scavenging or electron trapping effect. It is clearly seen in Table 1 that the effect is one of scavenger concentration and of crystal surface area. Further experiments were done using crystals finely ground under nitrogen, and sealed in vacuo (nitrogen at 2.5 × 10-3 torr). Figure 1 shows the retention for 186Re and ~88Re as a function of irradiation time at a reactor power of 2 and 5 kW. The values rise slowly to reach a gently rising plateau after about 60 sec. A similar result (Fig. 2) is found when the samples are irradiated with 6°CO y-rays following neutron irradiation. The results in these two figures show the effect to be one of accumulated dose combined with dose rate. Irradiations were normally done for 50 sec at 5 kW (2.5 x 10H cm -2 sec-~). Storage of the samples at 20° following neutron irradiation was shown not to affect the measured retention for times up to 200 hr. Thermal annealing studies for short times (10 min), following neutron irradiation, at temperatures up to 1200 showed essentially no effect except an increased scatter in the data at temperatures above about 80°. This scatter may be related to the occurrence of a phase transition[12] at 92°. It is clear from the data presented that the retention of lSSRe is greater than that of lS6Re by a factor of about 1.30. The two isotopes are similarly influenced by extraneous effects such as heat and ambient atmosphere. A recent preliminary study by Cavin, Ianovici and Table 1. Retention of RedCO)~o from neutron irradiation under various atmospheres neutron irradiation was for 50 sec at 5 kW Conditions

Retention (%) 186Re

188Re

188Re/186Re

Small Crystals air

4.7

6.8

He

6.8

g.o

1.42 1.33

Vacuum

8.2

10.7

1.30

Large Crystals air

7.4

9.6

1.30

He

8.4

10.7

1.27

11.2

15.0

1.34

Vacuum

Radiochemicalreactions in rhenium carbonyls--1

1105

Table 2. Yields of -Re(CO)5 from neutron-irradiated Re2(CO).... Each value given is the average from two independent determinations Delay before

7.0

Yield

dissolution

g (l) 6.0

2.6

n-

5.0 ~

o

•

4.c

2'0

,S Gamma

do

Dose

( x Jos rod )

Fig. 1. Radiochemical retention as Re2(CO),o, as a function of 7 dose during neutron irradiation. A, A, ~88Re;©, I , '~Re. Open

186Re

h

188Re

188Re/186Re

5.9

6.4

1.08

6.3

5.5

5.9

1.08

5.0

1.03

27.3

4.6

28.0

6.2

48.4

3.4

3.7

i .09

53.1

4.7

4.8

1.02

73,3

3.6

3,9

1,08

independently of one another. Thus, reactions such as

points 0.6 M rad hr ', filledpoints 1.5 M rad hr-'. *-Re(CO)5 + Re2(CO),0 ~ -Re(CO)5 + *ReRe(CO),o

(2) IO

can be ruled out, as was also found for the -Mn(CO)5Mm(CO)~o system. The effect of isochronal thermal annealing is shown in Fig. 3, which also shows the same information for -Mn(CO)5. It is clear that the -Re(CO)s radical in neutron irradiated Re2(CO)~o crystals is destroyed by being heated above about 75°. The relative constancy of the Re2(CO)~o retention during this treatment shows that recombination and exchange reactions to give Rm(CO)~o do not occur

9

f

8 -g

=

6

5

i

4

,

,

;

,

,

9.C I IO

2 I0 Gommo

D ose

3 ~0

40

(Mrod)

8.O

Fig. 2. Radiochemical retention as Re2(CO),o, as a function of post-neutron irradiation with 6°Co "y-raysat 1.9 M rad hr-'. A, '88Re;O, '8~Re.

0 7.0

" Milman[13] is in substantial agreement with the present results, although the objective of their work was more to study the ionic forms of Rhenium. Yield of -Re(CO)5. The radical 'Mn(CO)5 was shown[7, 14] to be formed in the (n, 7) reaction in Mn2(CO),0. This radical has a lifetime of some hours in the solid, but much less in solution. It is destroyed in the solid by being heated to temperatures in excess of 600, and does not recombine in this case to form the parent compound. By analogous experiments, the radical -Re(CO)5 is found in neutron irradiated Re2(COh0. The effects of crystal size and ambient atmosphere of the yield of "Re(CO)5 were found to be similar to that observed for the Re2(CO)~o retention. The yield of 'Re(CO)~ varies from 3.5% for finely powdered target material to 9.4% for 1 mm crystals, this great difference reflects the expected sensitivity of the radical to atmospheric or surface scavenging. The yield is also sensitive to room temperature annealing, following neutron irradiation, and shows a chemical half-life of about 50--60hr in the solid state at 23° -+2°. The data are given in Table 2. No dependence of yield on post-neutron 7 irradiation was detected. The isotope ratio for "Re(CO)5 is found to be 1.08+. The difference between this value and that found for the Re2(CO)~o indicates that the two products are formed

o-,

o

o

o

~ 6.0

- 5.0 4.O 3.0 2.( I.C

0

0

40

60

80

I00

120

Temperature , *C

Fig. 3. Effect of 10-minthermal treatment on the radiochemical yield of '*Re(CO)~in neutron irradiated Re2(CO),o.The results for "*Mn(CO), in Mn2(CO),o[7]are given for comparison, as are the retentions of *Re2(CO),o(upper5f,two curves). A, '"~Re;O, '86Re; [i], Mn.

1106

IAN WEBBERand D. R. WILES

thermally in the solid state. The similarity with the corresponding manganese carbonyl system is quite striking. It is possible that the lattice is loosening up just prior to the phase transition reported[12] to occur at 92°, and that this permits otherwise improbable reactions to occur.

Yield a s Re(CO)4. Note. There is no direct evidence to show that we are dealing with Re(CO)a, other than that the radioactive species is apparently converted to [IRe(CO)4]2 by an exchange-like process. We will use this designation recognizing that it may ultimately prove to be incorrect. As was done for -Mn(CO)4, the yield of -Re(CO)4 was determined following exchange of the radioactive species with carrier [IRe(CO)4]:. *Re(CO), + [IRe(CO)4]2~- [IRe(CO),'I*Re(CO)4] + Re(CO)4.

(3)

The results are given in Table 3. Aside from the rather high yield, two things are noteworthy in Table 4. Firstly, there is no apparent isotopic difference. This suggests that this fraction may have arisen from a series of chemical reactions and from a variety of primary species so that any initial differences will have been obliterated. It is of course true that the scavenging reaction may be much more complex than is implied in eqns (2) and (3), which would have the same effect. Secondly, there is no thermal annealing effect. This suggests that we are here dealing with species which are already quite stable. Moreover, it is interesting to note that the yield of Re(CO)4 does not increase as that of Re(CO)5 decreases at 75°C. This means that the reaction (4)

Re(CO)5 ~ Re(CO)4 + CO

shown in Fig. 4. This strong adsorption was used in the separation scheme when the yields of Re2(CO)~o and Re(CO)5 were sought. When carrier is used during the dissolution the adsorption is insignificant and the active species seems to have exchanged totally with the carrier. A comparison of these results with those obtained for Mn2(CO),o targets is given in Table 4. Striking parallels are seen in the products formed and in their thermal behaviour. It appears from this that the greatest influences in determining the radiochemical behaviour are chemical rather than nuclear, although the isotope effect shows a nuclear effect as well. More interesting than comparison with 56Mn is the comparison of the two Rhenium isotopes with each other. Such isotopic differences as we see here are well known in hot atom chemistry[l] and stem, not from mass differences but, it is claimed, from differences in the deexcitation spectrum. From these can arise different degrees of Auger ionization of electronic excitation. The mechanism whereby this affects the radiochemical yields is not known. A similar effect was observed[15] by Henrich, Ianovici, Milman and Wolf in a study of (n, xn) reactions on Re2(CO)lo. Their data showed a strong correlation of retention with the spin of the ground state and, they argued, with the spin change between the compound nucleus and the ground state. Our isotope ratios agree qualitatively with theirs for the two isotopes studied in the present work. Our numbers cannot be too closely compared with theirs, however, both because they used higher energies for their reactions and because they used a less selective chemical separation method. The pertinent data are given in Table 5. The much greater isotope ratio observed by Henrich et al. is not surprising in view of the higher energy of their

does not account for the thermal effect on Re(CO)5, as had been conjectured for Mn(CO)~. The Re(CO)4 fragments as they exist in the solid are rapidly (5-10 min) adsorbed on the glass vessel at the time of dissolution if no carrier is present. This adsorption is reversible if carrier is added later, but only slowly, as is

i_~ ~ zo

Table 3. Yields of *Re(CO), in neutron irradiated Re2(CO)lo, with 10min isochronalannealing

"~ ~8 Io

i

i

1

i

,

~ 30

"o

Temperature

Yield (%)

oC

186Re

188Re

188Re/186Re

23.8

18.7

17.9

54,2

20.0

19.9

1,00

81.0

16.3

17.9

I.I0

97.0

18.7

18.7

1.00

0

Time (Min)

1.01

i

Table 4. Radiochemical yields in Re2(CO),o and in Mn2(CO),o. Since the actual values depend somewhat on experimental conditions, the data given are only representative of the ranges, but are comparable for the two target compounds Product M2(CO)Io annealing

-M(CO) 5

annealing -M(CO) 4 annealing

Re2(CO)Io

Mn2(CO)Io

10%

11%

no

no 5%

5% d. 750 18% no

d.

60 °

30 A

o O

"~20

~

10

0

LI.I 0

I0

I 30

I

I 5=0

Time (hr)

Fig. 4. Adsorption of Rhenium activity on glass walls (a)

adsorptionfrom carrier-freesolutionin chloroform(b) desorption by exchangewith [IRe(CO)4]2in Chloroformsolution.A, '88Re;O, ,86Re"

Radiochemical reactions in rhenium carbonyls--I

1107

Table 5. Ratios of the yields of '88Reto those of '~Re Product

This work

Henrich, et. al. (15)

Re2(CO)Io

1,30

"~

-Re(CO)5

1,08

)

-Re(CO) 4

1.00

reaction, and consequently the higher the possible angular momentum of their initial excited nucleus. This would have the effect, as they describe the phenomenon, of accentuating the isotopic deexcitation differences. One cannot draw sound conclusions as to reaction mechanisms from these data. The similarities with the Manganese Carbonyl system are, however, significant and will be further discussed in a subsequent publication.

Acknowledgements--We wish to express our thanks to Mrs. I. G. deJong for assistance and advice on separation methods, and to the National Research Council of Canada for financial support. REFERENCES

1. D. R. Wiles, Adv. Organometal. Chem. 11, 202 (1973). 2. O. H. Wheeler, J. E. Trabal and D. R. Wiles, Can. J. Chem. 48, 3609 (1970). 3. U. Zahn, K. E. Collins and C. H. Collins, Radiochim. Acta 11, 33 (1969).

2.36 (total sublimate)

4. S. R. Narayan and D. R. Wiles, Can. J. Chenl. 47, 1019 (1969). 5. I.G. deJong and D. R. Wiles, Can. J. Chem. 48,1614 (1970). 6. W. Kanellokopulos-Drossopulos and D. R. Wiles, Can. J. Chem. 49, 2977 (1971). 7. I. G. deJong, S. C. Srinivasan and D. R. Wiles, 3. Organometal. Chem. 26, 119 (1971). 8. J. C. Hileman, D. K. Huggins and H. D. Kaesz, Inorg. Chem. 1,933 (1962). 9. N. Flitcroft, D. K. Huggins and H. D. Kaesz, Inorg. Chem. 3, 1123 (1964). 10. A. Nath, K. A. Rao and V. G. Thomas, Radiochim. Acta 3, 134 (1%4). 11. G. Grossmann, Isotopenpraxis 5, 262, 283, 370 (1%9). 12. P. Lemoine, M. Gross and J. Boissier, J. Chem. Soc. (Dalton) 15, 1626 (1972). 13. P. A. Cavin, E. Ianovici and M. Milman, Radiochim. Radioanal. Lett. 14. I. G. deJong and D. R. Wiles, Chem. Communs. 519 (1968). 15. E. Henrich, E. lanovici, M. Milman and G. K. Wolf, Paper presented to the 7th International Hot Atom Symposium, Jiilich, Germany (1973); personal communication (1974).