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Hot-atom and thermal reactions of fission-product ruthenium in organic solvents
J. Inorg. Nucl. Chem.. 1966, Vol. 28. pp, 925 to 931. Pergamon Press Ltd. Printed In Northern Ireland
HOT-ATOM PRODUCT
AND THERMAL REACTIONS OF FISSIONRUTHENIUM IN ORGANIC SOLVENTS* U. ZAHNt and G. HARBOTTLE
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973
(Received 15 July 1965) AImtract--When ruthenium-103 atoms are introduced into organic solvents via fission-recoil and p-decay (of precursors of mass 103) in certain cases small yields of volatile, ruthenium-labolledorganic molecules are obtained. However, when the solvent is monomeric cyclopcntadiene, a thermal reaction transforms virtually all of the ruthenium into the "sandwich" compound, ruthenocene. When separated fission product I°eRuCIa is mixed with cyclopentadiene in a test-tube, labelled carrier-free ruthenocene is also obtained by what seems to be the same thermal reaction.
THE hot-atom chemistry of fission fragments in aqueous solutions was first investigated in the laboratories of the Manhattan Project. (1'2) In the 1950's at Harwell, WALTONand collaborators determined the radiolytic decomposition caused by fission fragments impinging upon crystalline targets such as nitrates and iodates, and also observed the hot-atom chemistry of mI growing out of its fission precursors in iodate crystals. (s'4'5) In 1961 BAUMG~mTNr~and REXCHOLDin Munich (6'7"s) and ORMONDand ROWLAND in Kansas (8) published papers dealing with the chemical interaction of fission fragments with organic systems: in the first of these the production of carrier-free ruthenocene labelled with l°~Ru was reported to occur when crystalline ferrocene [Fe(CsHr,)d was bombarded with tbe whole spectrum of fission products. BAUMG?~R~ and REICHOLD also found that iodoferrocene labelled with mI was formed: in similar fashion ORMONDand ROWLAND isolated labelled iodobenzene from fission fragments striking benzene. Since that time the "fission-recoil synthesis" of labelled compounds has found interesting practical use in the preparation of specific radiochemicals (~°'n'~) and the * This research supported by the U.S. Atomic Energy Commission. 1" Present address: Physics Department, Technische Hochschule Muenchen, Munich, Germany. (t) C. W. STANLEYand T. H. DAVIES,Radiochemical Studies, NNES IV, 9, p. 204, Book 1. McGrawHill, New York (1951). (s) W. H. BuRous and T. H. DAVITS,Radiochemical Studies, NNES IV, 9, p. 209, Book 1. McGrawHill, New York (1951). (a) G. N. W~TON and I. F. CaOALL,J. Inorg. Nucl. Chem. 1, 149 (1955). (4) D. HALL and G. N. WALTON,J. Inorg. Nucl. Chem. 1O, 215 (1959). (6) D. HALLand G. N. WALTON,jr. Inorg. Nucl. Chem. 19, 16 (1961). (e) F. BAUMO~T~ and P. R~CHOLD,Z. Naturforsch. 16 a, 374 (1961). (7) F. BAUMO~TN~ and P. RmCHOLD,Z. Naturforsch. 16 a, 945 (1961). (s) F. BAUMO~TI~mand P. RmC~OUD, Chemical Effects of Nuclear Transformations, Vol. 2, p. 319. International Atomic Energy Agency, Vienna (1961). ()) D. Om~ONDand F. S. R o w ~ , 3". Amer. Chem. Soc. 83, 1006 (1961). (1o) F. BAUMO~TNI~ and A. SciON, Proceedings of the Conference on Methods of Preparing and Storing Marked Molecules, p. 1331. EUR-1625.e. (1964). (l~) R. Hm,a~Y, D. DEsuc~iv and E. JUNOD, Radioisotopes in the Physical Sciences and Industry, Vol. 3. International Atomic Energy Agency, Vienna (1962). (1,) y. PAISSand S. AMm~,Israel. Atomic Energy Commission Report IA-906 (1963) (unpublished). 925
926
U. ZAh'N and G. tJm, nOTrL~
fast s e p a r a t i o n o f fission p r o d u c t s . (ls-1~) W h e n discussing the m e c h a n i s m o f the h o t a t o m event itself, however, the w a r n i n g s o u n d e d b y ROWLAND(ls) s h o u l d be r e m e m bered, n a m e l y t h a t in m a n y cases " t h e system u n d e r s t u d y really consists o f p r i m a r y fission-product reaction, f o l l o w e d b y h o t - a t o m /~-decay c h e m i s t r y " . I n the fission p r o d u c t decay chain o f m a s s 103 less t h a n 0.1 p e r cent o f the a°SRu a t o m s a r e directly p r o d u c e d in fission ( " i n d e p e n d e n t yield"), while m o r e t h a n 99.9 p e r cent c o m e f r o m t h e / 3 - d e c a y i n g precursors t°STc, l°*Mo a n d l°3Nb.(m BAUMG.~RTNER a n d SCHON (2°~ have recently u n d e r s c o r e d this s a m e p o i n t in their discussion o f l°SRu reactions in ferrocene catcher. W i t h this reservation in m i n d , we wish to present s o m e recent experiments o n the interaction o f fission p r o d u c t r u t h e n i u m a t o m s with v a r i o u s organic solvents. EXPERIMENTAL Materials. Cyclopentadiene was freshly prepared by depolymerization of Fisher dicyclopentadiene in boiling tetrahydrofuran and distillation of the monomer. Other solvents were also distilled and fractionated before use. Ferrocene was obtained from Chemical Procurement Laboratories Inc. and was purified by sublimation in vacua. Ruthenocene was prepared by reaedon of cyclopentadienyl sodium with ruthenium triehloride in tetratrydrofuranm~ and purified by repeated sublimation. Fission product X°6Ru, designated "Ru-Rh-106P, processed, carrier-free" was obtained from Union Carbide, Oak Ridge. It was stated to be the trichloride in 6 N hydrochloric acid: for our experiments (see Appendix) the solution (1 mc/ml) was greatly diluted with water to provide reasonable activity levels. Bombardments. A uranium metal foil (93 % "'~U) 1 cm s in area was placed in a quartz tube and covered by ca. 0.5 ml of the particular solvent. The tube was sealed in vacua and irradiated for 16 hr (unless otherwise noted) in the thermal column of the BNL Graphite Reactor. The neutron flux was 6 × I(P neutrons/era" see, Cd ratio > 5000, 7-radiation 720 r/hr and temperature 20--25°C. Separations. After bombardment the tube was stored for six days and then opened: the solvent was taken out with a syringe and added to a carrier solution of ferrocene in acetone. This mixture was dried at room temperature under reduced pressure, and the ferrocene then sublimed, together with any "recoil-synthetic" carrier-free compounds, at 140°C in vacua and collected on a watercooled finger. For further purification the sublimate was dissolved in benzene, chromatograpbed on a short alumina column (1.5 × 8 cm, Woelm basic AI~Os) and eluted with 1 : 1 benzene--hexane. Such short columns do not effectively separate many of the similar metal-organic compounds from one another. For example the separation ferrocene/ruthenoeene, which requires very long columns, c'~ does not occur here, and we may successfully employ ferrocene as an inexpensive and readily-availablecarrier for ruthenocene. The function of this chromatography step is to separate traces of polymeric and (recoil-labelled) iodinated solvent molecules from the desired ruthenium-labelled product. The coloured fraction of the eluate was collected, dried and resublimed: it still contained a small x'xI contamination. We will refer to the activity of fission ruthenium in this fraction as V. In order to determine the entire yield of fission ruthenium activity, the uranium foil, the irradiation tube and the sublimation residue were combined and treated with RuCI: carrier in 8 N nitric acid, the ruthenium oxidized with NaBiOs, distilled as RuO~, and precipitated as ruthenium metal. We may term the activity of this ruthenium metal T. When the same ruthenium analysis was applied to caa~R. HXNRY,J. BEYDONand A. BARDY,Chemical Effects of Nuclear Transformations, Vol. 2, p. 291. International Atomic Energy Agency, Vienna (1961). ¢14~p. Klm¢~, B. WECKERMANN,F. BAtn~tO~TN~R and U. ZAI-m,Naturwtssenschaflen 49, 294 (1962). ~x~jp. KIErCLE,B. WECKEn_MAm~,F. BAtrMo~'nmm and U. ZAI-m, Naturwissenschaften 49, 295 (1962). ~1'~P. Km~'LE,F. BAUMO~RTNER,B. WECKERMAk'r~and U. ZAm~, Radiochimica Acta 1, 84 (1963). cxTJy . PAISSand S. AmEL, Israel. Atomic Energy Commission Report IA-822 (1963) (unpublished). cx,~F. S. ROWLAND, Chemical Effects of Nuclear Transformations, Vol. 2, p. 325. International Atomic Energy Agency, Vienna (1961). cm A. E. NoRms. Private communication of a calculation (1965). tto~ F. BAtr~OXnTr,mRand A. ScHON, Radiochimiea Acta 3, 141 (1964). c~x~E. O. PlSCH~ and H. GRtrB~r. Unpublished.
Hot=atom and thermal reactions of fission-product ruthenium in organic solvents
927
the sub"lunationresidue alone, an activity designated R was obtained. The over-all ruthenium activity is then V + T and the active fraction which remains in the organic solvent phase and does not deposit on the walls, foil etc. (V + R). The "homogeneous yield" is defined as V/(V + R) and the over-all yield is of course V/(V + T). Activity measurements. The y-ray spectra of all fractions were measured in constant geometry in a 1 × 1~ in. sodium iodide well crystal and recorded in a 256 channel TMC pulse height analyser. The ruthenium counting and peak-integration procedure and its corrections and reliability have been recently discussed:(sl) the area under the 498 keV peak of 1°SRu(tt = 40 days) was taken as a measure of the ruthenium activity. In two weeks traces of 18xI(tt = 8 days) had decayed sufficientlyto allow measurement of X°SRu. Since the 514 keV line in the mI overlapped the 498 keV line in lURu, a correction was calculated from the observed area of the unobscured 364 keV line in ~saIand the known ratio of this area to that of the 514 keV line. This correction was needed only for activity V: activities T and R were radiochemicaUypure. The spectra due to XeeRu(tt = 1 year) in equilibrium with its daughter X°eRhwere only visible after the XOSRuhad died out. In order to measure these weak X°eR.uspectra counting periods of 1000 min in heavily shielded scintillation counters were taken. In the experiments on the thermal synthesis of XeeRu-labelled ruthenocene, larger activities were available (see Appendix). RESULTS Cyclopentadiene and its dimer as solvents As we had anticipated that yields o f recoil-synthetic ruthenium-labelled organic molecules would be quite small, it was with great surprise that we found that, under certain conditions, virtually all o f the X°aRu fission fragments caught in freshly depolymerized cyclopentadiene went to form ruthenocene (Table 1, Experiments 4, 5 and 6). It was also noted that the homogeneous yield ( V / V + R) increased with increasing bombardment time (Experiments 2, 3 and 4). When the dimer was substituted for the monomer as solvent, the yield fell to 2.5 per cent (Experiment 1) and this suggested that the high yields in the case o f monomer might not be due to a hot-atom effect at all, since to an energetic recoil ruthenium atom the monomeric and dimeric media would present nearly the same appearance. One would, however, expect that in a thermal reaction between solvent ~ind ruthenium, the monomer would be more reactive than the dimer. Therefore we undertook a series o f simple "test-tube" experiments in which aqueous solutions o f carder-free l°6Ru (Oak Ridge, fission product) were taken to dryness and the radioactive deposit covered with fresh monomeric and dimeric cyclopentadiene. We found a very substantial thermal reaction, as reported in the appendix, and feel certain that this is the fundamental explanation for the high yields reported in Table 1. It is worth mentioning that efforts were made to prove that the molecule labelled with x°~Ru activity in fraction V (see Experimental Section above) was identically ruthenocene. The samples were repeatedly chromatographed on short columns and sublimed, using ferrocene as carder, and came to constant specific activity after being purified of solvent traces. The same was true even when the catcher solvent was dicyclopentadiene. We regard this as reasonably good evidence that the recoil-synthetic compound was indeed ruthenocene. Other solvents A few experiments with other solvents were undertaken to see whether sublimable ruthenium-organic compounds similar to ruthenocene might be formed in recoil. In ctl) G. HAReOTrLEand U. Z A ~ , Chemical Effects Associated with Nuclear Reactions and Radioactive Transformations, Vol. II, p. 133. International Atomic Energy Agency, Vienna (1965).
U. 7_~tHNand G. I-~m~Tr~
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TABt~ 1.--RuTI-mNOCENEYmLDSFROMTHE INTro,,ACTIONOF RUTHENIUMR~COmATOMS W1TH CYCLOPENTADIENE AND ITS DIMER
Experiment no.
No. of samples
Solvent
Time of bombardment
1 2 3 4 5 6
2 2 6 4 2 1
Dicyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene
16 hr 10 rain 3.5 hr 16 hr 16 hr 16 hr
Yield (~/,) Homogeneous Over-all 2.5 -4- 0.2 47.7 q- 1 79.2 q- 1.8 86.2 -t- 1.3 91 -4-4 89.3* 90"5t
* 1OSRU I" 1OeRu
TABLE 2.--YmLDS OF VOLATILE RU-LABELLED COMPOUNDS FROM RECOIL RUTHENIUM WITH VARIOUS SOLVENTS
Solvent Benzene n-Hexane Indene Cyclooctatetraene Cyclooctadiene
Homogeneous yield (~o) 3"7 -4- 0.2 < 0-4 q- 0.2 1-0 -t- 0.2 0-15 0.23
Table 2 the results of these experiments are presented: in the case of benzene there seemed to be a positive result. The compound formed by recoil into benzene could not be separated from ferrocene by immediate sublimation, but after storage in air for two months was found (presumably through oxidation or decomposition) to have become non-volatile, whereas ruthenocene under these conditions remained unaltered. These results seem to parallel those o f earlier work ~ in which a volatile rutheniumlabelled compound was found to result from the interaction o f fission fragments with dibenzene chromium crystals. This compound may have been the same one observed in the present experiments, but further work on this point will be required. CONCLUSIONS Ruthenium-103 atoms activated by the/~-decay of their precursors in the mass-103 fission chain, can form volatile, labelled metal-organic products with surrounding molecules of some solvents with a few per cent yield. In the case of monomeric eyclopentadiene, a subsequent thermal reaction eventually incorporates a very large fraction o f the ruthenium atoms into ruthenocene molecules. The extent o f this thermal reaction appears to depend on the time o f standing, the temperature (see Appendix), and the duration of bombardment in the thermal column (Table 1). The experiments reported in the appendix are of course not entirely comparable with the fission experiments since the former were heterogenous--i.e, l°eRuCl3 coated on the walls o f the test tube first had to desorb to enter the solvent, and then react, while the fission experiments were homogeneous: bare l°aRu atoms were projected bodily into the ~u~ U. ZAm~, Unpublished research.
Hot-atom and thermal reactions of fission-product ruthenium in organic solvents
929
depths o f the solvent. One would expect the latter mechanism to provide better " m i x i n g " a n d greater reactivity: however, even in the f o r m e r case (the heterogeneous experiment) yields o f ruthenocene, expressed as per cent o f those ruthenium a t o m s actually present in the solvent, were surprisingly high. As one would expect for a thermal reaction, we observed n o isotopic difference in the "fission recoil" synthesis (Table 1). Finally, it o u g h t to be mentioned that l°6Ru-labelled ruthenocene prepared by the thermal reaction o u g h t to have some practical applications as a label for h y d r o c a r b o n s because o f its solubility, long half-life, ~,-ray emission, thermal stability and general chemical inertness. The great volatility o f carrier-free ruthenocene would allow the easy preparation o f gaseous sources o f ruthenium activity. APPENDIX Thermal reaction to form carrier-free ruthenocene
Metallocenes in macroscopic quantities may in some cases be prepared by simply heating the element together with cyclopentadiene vapour. For example tu) Fe + 2CsHs
300° ~- Fe(C~Hs)2 + H2
and similarly for magnesium ~'5) at 500-600°C, and thallium and indium ('e) at 350°. Other synthetic methods involve displacement of the Grignard CsHsMgBr ~sT)or sodium cyclopentadienyl, NaCsHs, (2s) with metal salts. Ruthenocene was first prepared by WILr-aNSON(n) by reaction of tetravalent ruthenium acetylacetonate with the Grignard: yields were low. Fxsc~R and GRtmL~T(m have employed anhydrous ruthenium trichloride and sodium cyclopentadienyl in tetrahydrofuran with better results. Two syntheses of carrier-free, labelled ruthenocene are known: one, due to BAUMO~RTNERand REINHOLD,(e) has been mentioned above, while the other is the subject of a German Patent is°) and employs a vapour phase reaction and catalyst. In this Appendix we wish to present the results of our experiments on the direct reaction of carrier=free ruthenium=f06 trichloride with liquid cyclopentadiene at room temperature, Synthesis
A known amount of carrier-free ruthenium-106 solution, whose activity had been determined accurately, was pipetted into a small, soft-glass test tube, and taken to dryness. One millilitre of freshly-distilled (depolymerized) cyclopentadiene was then added. In some experiments the mixtures were simply allowed to stand, while in others they were stirred magnetically. Reaction temperatures were 0 °, room temperature and 41°C. For separation a weighed quantity of ferrocene carrier was added to the reaction mixture and dissolved. The solution was immediately chromatographed and sublimed as before (Experimental). The yield of ruthenocene was calculated from the activity of the sublimate, corrected for the percentage of ferrocene recovered, divided by the activity originally taken. This yield was termed the "over-all yield" in analogy with the fission experiments. Similarly the "homogeneous yield" was here defined as the activity of ruthenocene, corrected for ~u) S. A. MILLER,J. A. TEnBOTHand J. F. TI~MAII,m, J. Chem. Soc. 632 (1952). (15) W. A. BA~r~R, J. Inorff . NucL Chem. 4, 373 (1957). (2e) E. O. FtscrmR and H. P. HorMA~n,~, unpublished, cited in E. O. F~sc'nv~ and H. P. FRITZ, Advances in Inorganic Chemistry and Radiochemistry (Edited by H. J. E~LEUS and A. G. SrlARPI~) Vol. 1, pp. 55 et seq. Academic Press, New York (1959). (sT)T. J. K~LV and P. L. PAUSON,Nature, Lond. 168, 1039 (1951). (~s) G. WILKINSONand F. A. Co'rroN, Chem. and Industr. 307 (1954). (J)) G. WmKdNSON, J. Amer. Chem. Soc. 74, 6146 (1952). (ao) H . G0a'rE and M. WENZrZL,Ger. Pat. DBP 1049860.
U. ZAns and G. I4Am~TTLV
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TABLE 3.~CARRIER-FREE RUTHENOCENE S Y ~
Reaction time 15 min 20 min 30 min 50 min 1 hr 2hr 2.5 hr 4 hr 5 hr 12 hr 13"5 hr
24 hr 48 hr 56 hr 72 hr 84 hr 96 hr 122 hr
Room temp. not stirred Over-all Homogeneous yield ( ~ ) yield (~/o)
Room temp. stirred Over-all yield (~o)
0° not stirred Over-all yield ( ~ )
41 ° not stirred Over-all yield (~o)
0.03 0.006
9.1
0.2 0.023
0-02
12.8 0.046
0"5 1.2
0.21 0.17
26-1 0.9
0-54 1.5 0.76
51
0.98 1.4"
54.7 58.6
2.1
0'15
2.7 0"10 1-9 1.3 2.1
* The average yield of all room temperature samples with standing time >24 hr was 2.0 ± 0.2 per cent (13 samples). recovery as above, divided by the total ruthenium activity (organic and inorganic) actually present in the homogeneous cyclopentadiene phase. The results of these synthetic experiments under different conditions are presented in Table 3.
Proof that the sublimable carrier-free ruthenium-106 compound is in fact ruthenocene To prove that the volatile carrier-free ruthenium compound was identical with ruthenocene, both known ruthenocene and ferrocene were taken as carriers. Repeated sublimation of the ruthenocene or ferrocene containing the synthetic compound did not change the specific activity. When a benzene solution containing the labelled molecules and rnthenocene carrier was chromatographed on a long (100 cm) alumina column, only one peak was observed in the eluate and the specific activity of the ruthenocene recovered from these fractions by sublimation was the same as that of the starting material.
Additional experiments Reaction with the dimer. It was suspected that one of the reasons for the reaction ceasing at ca. 2 per cent of completion was the dimerization of the cyclopentadiene. Two runs, each of 52 hr at room temperature, were carded out using dicyclopentadiene instead of the monomer. Over-all yields of ruthenocene were less than 0-005 and 0.02 per cent respectively. Further reaction with fresh monomer. Following a normal reaction run the glass tube, still containing virtually all the ruthenium-106 tracer on the walls was rinsed with benzene and fresh monomeric cyclopentadiene introduced. After 48 hr at room temperature, additional ruthenocene formation of 0.6 and 0.8 per cent was found in two experiments. Effect of 7-irradiation. In two experiments, samples which had reached "plateau" values of the over-all ruthenocene yield of 3"2 and 1-6 per cent respectively were exposed to external 7-radiation from a cobalt-60 source. Doses of 2 × 10~and 4 × 106 rns were given to the former and latter samples, respectively. After 7-irradiation, the observed yields were 2.4 and 2.0 per cent indicating that the radiation had little or no effect.
Hot-atom and thermal reactions of fission-product ruthenium in organic solvents
931
Effect of light. Duplicate experiments in which the reactants were kept in the dark for 48 hr gave 2.9 and 1.7 per cent over-all yields, in essential agreement with others carried out in room-light. Effect of base. A synthesis in which a base is employed,cs~ viz: 2C5H6 -k FeCll q- 2 Base -* Fe(CsH6)s q- 2 Base. HC1 has been developed for the industrial production of ferrocene. We prepared duplicate reaction mixtures containing, in addition to the l°6Ru and cyclopentadiene, a few per cent of diethyiamine. Over-all yields were 4.7 and 2.5 per cent, perhaps a little higher than in the absence of base. Behaviour of carrier-free ruthenocene. In one experiment l°6Ru(CsHs)l was prepared without any ferrocene or inert ruthenocene carrier. This carrier-free compound proved to be unmanageably volatile on drying under reduced pressure at room temperature, and distributed itself over the interior of the sublimation vessel, vacuum lines, pump etc. Non-synthesis of volatile rhodium-organic complexes in t~-decay. As mentioned above ~°eRu (1 year) decays by fl--emission to :°6Rh (30 sec) which decays by /~-,7-emission to ~°6Pd. We prepared ~°'Ru(CsH6)j (mixed with ferrocene or ruthenocene carrier) and sublimed it, starting a stopwatch at the mid-point of disappearance of the crystals. Sublimation, in the best experiment, required ca. 15 sec. The 7-radiation (~eSRh) of the residue of sublimation was then counted: a clean 30 sec half-life was obtained. This activity was extrapolated back to the time of sublimation and compared with the :°'Rh activity initially present in equilibrium with I°6Ru. These activities were found to be equal, within 4-10 per cent, indicating that ~--decay in labeled ruthenocene produces almost entirely non-volatile rhodium daughter activity.* Non-synthesis offerrocene. We obtained some S°Fe of high specific activity and attempted to form ferrocene by a similar direct reaction with monomeric cyclopentadiene. None was obtained.
AcknowledgementsmThe authors wish to thank A. E. NORRISfor calculating the independent fission yield of ~°SRu, and R. DAvis, JR. for the use of his low-level counting equipment. One of us (U. Z.) wishes to thank the Bundesminister f'~ wissensehaftliche Forschung of the Federal Republic of Germany, for financial support in the form of a stipend. * In agreement with results on f/-decay of ~°SRu(C6Hs)~.~8~ These results have recently been checked in this laboratory. csl) j. M. B~MINOI-InM,D. S E ~ T I t and G. WnJc~soN, J. Amer. Chem. Soc. 76, 4179 (1954). ~se~F. BAUMOgRT~V.R,E. O. FISO~R and U. Znm~, Chem. Ber. 91, 2336 (1958); 92, 1624 (1959).
HOT-ATOM PRODUCT
AND THERMAL REACTIONS OF FISSIONRUTHENIUM IN ORGANIC SOLVENTS* U. ZAHNt and G. HARBOTTLE
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973
(Received 15 July 1965) AImtract--When ruthenium-103 atoms are introduced into organic solvents via fission-recoil and p-decay (of precursors of mass 103) in certain cases small yields of volatile, ruthenium-labolledorganic molecules are obtained. However, when the solvent is monomeric cyclopcntadiene, a thermal reaction transforms virtually all of the ruthenium into the "sandwich" compound, ruthenocene. When separated fission product I°eRuCIa is mixed with cyclopentadiene in a test-tube, labelled carrier-free ruthenocene is also obtained by what seems to be the same thermal reaction.
THE hot-atom chemistry of fission fragments in aqueous solutions was first investigated in the laboratories of the Manhattan Project. (1'2) In the 1950's at Harwell, WALTONand collaborators determined the radiolytic decomposition caused by fission fragments impinging upon crystalline targets such as nitrates and iodates, and also observed the hot-atom chemistry of mI growing out of its fission precursors in iodate crystals. (s'4'5) In 1961 BAUMG~mTNr~and REXCHOLDin Munich (6'7"s) and ORMONDand ROWLAND in Kansas (8) published papers dealing with the chemical interaction of fission fragments with organic systems: in the first of these the production of carrier-free ruthenocene labelled with l°~Ru was reported to occur when crystalline ferrocene [Fe(CsHr,)d was bombarded with tbe whole spectrum of fission products. BAUMG?~R~ and REICHOLD also found that iodoferrocene labelled with mI was formed: in similar fashion ORMONDand ROWLAND isolated labelled iodobenzene from fission fragments striking benzene. Since that time the "fission-recoil synthesis" of labelled compounds has found interesting practical use in the preparation of specific radiochemicals (~°'n'~) and the * This research supported by the U.S. Atomic Energy Commission. 1" Present address: Physics Department, Technische Hochschule Muenchen, Munich, Germany. (t) C. W. STANLEYand T. H. DAVIES,Radiochemical Studies, NNES IV, 9, p. 204, Book 1. McGrawHill, New York (1951). (s) W. H. BuRous and T. H. DAVITS,Radiochemical Studies, NNES IV, 9, p. 209, Book 1. McGrawHill, New York (1951). (a) G. N. W~TON and I. F. CaOALL,J. Inorg. Nucl. Chem. 1, 149 (1955). (4) D. HALL and G. N. WALTON,J. Inorg. Nucl. Chem. 1O, 215 (1959). (6) D. HALLand G. N. WALTON,jr. Inorg. Nucl. Chem. 19, 16 (1961). (e) F. BAUMO~T~ and P. R~CHOLD,Z. Naturforsch. 16 a, 374 (1961). (7) F. BAUMO~TN~ and P. RmCHOLD,Z. Naturforsch. 16 a, 945 (1961). (s) F. BAUMO~TI~mand P. RmC~OUD, Chemical Effects of Nuclear Transformations, Vol. 2, p. 319. International Atomic Energy Agency, Vienna (1961). ()) D. Om~ONDand F. S. R o w ~ , 3". Amer. Chem. Soc. 83, 1006 (1961). (1o) F. BAUMO~TNI~ and A. SciON, Proceedings of the Conference on Methods of Preparing and Storing Marked Molecules, p. 1331. EUR-1625.e. (1964). (l~) R. Hm,a~Y, D. DEsuc~iv and E. JUNOD, Radioisotopes in the Physical Sciences and Industry, Vol. 3. International Atomic Energy Agency, Vienna (1962). (1,) y. PAISSand S. AMm~,Israel. Atomic Energy Commission Report IA-906 (1963) (unpublished). 925
926
U. ZAh'N and G. tJm, nOTrL~
fast s e p a r a t i o n o f fission p r o d u c t s . (ls-1~) W h e n discussing the m e c h a n i s m o f the h o t a t o m event itself, however, the w a r n i n g s o u n d e d b y ROWLAND(ls) s h o u l d be r e m e m bered, n a m e l y t h a t in m a n y cases " t h e system u n d e r s t u d y really consists o f p r i m a r y fission-product reaction, f o l l o w e d b y h o t - a t o m /~-decay c h e m i s t r y " . I n the fission p r o d u c t decay chain o f m a s s 103 less t h a n 0.1 p e r cent o f the a°SRu a t o m s a r e directly p r o d u c e d in fission ( " i n d e p e n d e n t yield"), while m o r e t h a n 99.9 p e r cent c o m e f r o m t h e / 3 - d e c a y i n g precursors t°STc, l°*Mo a n d l°3Nb.(m BAUMG.~RTNER a n d SCHON (2°~ have recently u n d e r s c o r e d this s a m e p o i n t in their discussion o f l°SRu reactions in ferrocene catcher. W i t h this reservation in m i n d , we wish to present s o m e recent experiments o n the interaction o f fission p r o d u c t r u t h e n i u m a t o m s with v a r i o u s organic solvents. EXPERIMENTAL Materials. Cyclopentadiene was freshly prepared by depolymerization of Fisher dicyclopentadiene in boiling tetrahydrofuran and distillation of the monomer. Other solvents were also distilled and fractionated before use. Ferrocene was obtained from Chemical Procurement Laboratories Inc. and was purified by sublimation in vacua. Ruthenocene was prepared by reaedon of cyclopentadienyl sodium with ruthenium triehloride in tetratrydrofuranm~ and purified by repeated sublimation. Fission product X°6Ru, designated "Ru-Rh-106P, processed, carrier-free" was obtained from Union Carbide, Oak Ridge. It was stated to be the trichloride in 6 N hydrochloric acid: for our experiments (see Appendix) the solution (1 mc/ml) was greatly diluted with water to provide reasonable activity levels. Bombardments. A uranium metal foil (93 % "'~U) 1 cm s in area was placed in a quartz tube and covered by ca. 0.5 ml of the particular solvent. The tube was sealed in vacua and irradiated for 16 hr (unless otherwise noted) in the thermal column of the BNL Graphite Reactor. The neutron flux was 6 × I(P neutrons/era" see, Cd ratio > 5000, 7-radiation 720 r/hr and temperature 20--25°C. Separations. After bombardment the tube was stored for six days and then opened: the solvent was taken out with a syringe and added to a carrier solution of ferrocene in acetone. This mixture was dried at room temperature under reduced pressure, and the ferrocene then sublimed, together with any "recoil-synthetic" carrier-free compounds, at 140°C in vacua and collected on a watercooled finger. For further purification the sublimate was dissolved in benzene, chromatograpbed on a short alumina column (1.5 × 8 cm, Woelm basic AI~Os) and eluted with 1 : 1 benzene--hexane. Such short columns do not effectively separate many of the similar metal-organic compounds from one another. For example the separation ferrocene/ruthenoeene, which requires very long columns, c'~ does not occur here, and we may successfully employ ferrocene as an inexpensive and readily-availablecarrier for ruthenocene. The function of this chromatography step is to separate traces of polymeric and (recoil-labelled) iodinated solvent molecules from the desired ruthenium-labelled product. The coloured fraction of the eluate was collected, dried and resublimed: it still contained a small x'xI contamination. We will refer to the activity of fission ruthenium in this fraction as V. In order to determine the entire yield of fission ruthenium activity, the uranium foil, the irradiation tube and the sublimation residue were combined and treated with RuCI: carrier in 8 N nitric acid, the ruthenium oxidized with NaBiOs, distilled as RuO~, and precipitated as ruthenium metal. We may term the activity of this ruthenium metal T. When the same ruthenium analysis was applied to caa~R. HXNRY,J. BEYDONand A. BARDY,Chemical Effects of Nuclear Transformations, Vol. 2, p. 291. International Atomic Energy Agency, Vienna (1961). ¢14~p. Klm¢~, B. WECKERMANN,F. BAtn~tO~TN~R and U. ZAI-m,Naturwtssenschaflen 49, 294 (1962). ~x~jp. KIErCLE,B. WECKEn_MAm~,F. BAtrMo~'nmm and U. ZAI-m, Naturwissenschaften 49, 295 (1962). ~1'~P. Km~'LE,F. BAUMO~RTNER,B. WECKERMAk'r~and U. ZAm~, Radiochimica Acta 1, 84 (1963). cxTJy . PAISSand S. AmEL, Israel. Atomic Energy Commission Report IA-822 (1963) (unpublished). cx,~F. S. ROWLAND, Chemical Effects of Nuclear Transformations, Vol. 2, p. 325. International Atomic Energy Agency, Vienna (1961). cm A. E. NoRms. Private communication of a calculation (1965). tto~ F. BAtr~OXnTr,mRand A. ScHON, Radiochimiea Acta 3, 141 (1964). c~x~E. O. PlSCH~ and H. GRtrB~r. Unpublished.
Hot=atom and thermal reactions of fission-product ruthenium in organic solvents
927
the sub"lunationresidue alone, an activity designated R was obtained. The over-all ruthenium activity is then V + T and the active fraction which remains in the organic solvent phase and does not deposit on the walls, foil etc. (V + R). The "homogeneous yield" is defined as V/(V + R) and the over-all yield is of course V/(V + T). Activity measurements. The y-ray spectra of all fractions were measured in constant geometry in a 1 × 1~ in. sodium iodide well crystal and recorded in a 256 channel TMC pulse height analyser. The ruthenium counting and peak-integration procedure and its corrections and reliability have been recently discussed:(sl) the area under the 498 keV peak of 1°SRu(tt = 40 days) was taken as a measure of the ruthenium activity. In two weeks traces of 18xI(tt = 8 days) had decayed sufficientlyto allow measurement of X°SRu. Since the 514 keV line in the mI overlapped the 498 keV line in lURu, a correction was calculated from the observed area of the unobscured 364 keV line in ~saIand the known ratio of this area to that of the 514 keV line. This correction was needed only for activity V: activities T and R were radiochemicaUypure. The spectra due to XeeRu(tt = 1 year) in equilibrium with its daughter X°eRhwere only visible after the XOSRuhad died out. In order to measure these weak X°eR.uspectra counting periods of 1000 min in heavily shielded scintillation counters were taken. In the experiments on the thermal synthesis of XeeRu-labelled ruthenocene, larger activities were available (see Appendix). RESULTS Cyclopentadiene and its dimer as solvents As we had anticipated that yields o f recoil-synthetic ruthenium-labelled organic molecules would be quite small, it was with great surprise that we found that, under certain conditions, virtually all o f the X°aRu fission fragments caught in freshly depolymerized cyclopentadiene went to form ruthenocene (Table 1, Experiments 4, 5 and 6). It was also noted that the homogeneous yield ( V / V + R) increased with increasing bombardment time (Experiments 2, 3 and 4). When the dimer was substituted for the monomer as solvent, the yield fell to 2.5 per cent (Experiment 1) and this suggested that the high yields in the case o f monomer might not be due to a hot-atom effect at all, since to an energetic recoil ruthenium atom the monomeric and dimeric media would present nearly the same appearance. One would, however, expect that in a thermal reaction between solvent ~ind ruthenium, the monomer would be more reactive than the dimer. Therefore we undertook a series o f simple "test-tube" experiments in which aqueous solutions o f carder-free l°6Ru (Oak Ridge, fission product) were taken to dryness and the radioactive deposit covered with fresh monomeric and dimeric cyclopentadiene. We found a very substantial thermal reaction, as reported in the appendix, and feel certain that this is the fundamental explanation for the high yields reported in Table 1. It is worth mentioning that efforts were made to prove that the molecule labelled with x°~Ru activity in fraction V (see Experimental Section above) was identically ruthenocene. The samples were repeatedly chromatographed on short columns and sublimed, using ferrocene as carder, and came to constant specific activity after being purified of solvent traces. The same was true even when the catcher solvent was dicyclopentadiene. We regard this as reasonably good evidence that the recoil-synthetic compound was indeed ruthenocene. Other solvents A few experiments with other solvents were undertaken to see whether sublimable ruthenium-organic compounds similar to ruthenocene might be formed in recoil. In ctl) G. HAReOTrLEand U. Z A ~ , Chemical Effects Associated with Nuclear Reactions and Radioactive Transformations, Vol. II, p. 133. International Atomic Energy Agency, Vienna (1965).
U. 7_~tHNand G. I-~m~Tr~
928
TABt~ 1.--RuTI-mNOCENEYmLDSFROMTHE INTro,,ACTIONOF RUTHENIUMR~COmATOMS W1TH CYCLOPENTADIENE AND ITS DIMER
Experiment no.
No. of samples
Solvent
Time of bombardment
1 2 3 4 5 6
2 2 6 4 2 1
Dicyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene Cyclopentadiene
16 hr 10 rain 3.5 hr 16 hr 16 hr 16 hr
Yield (~/,) Homogeneous Over-all 2.5 -4- 0.2 47.7 q- 1 79.2 q- 1.8 86.2 -t- 1.3 91 -4-4 89.3* 90"5t
* 1OSRU I" 1OeRu
TABLE 2.--YmLDS OF VOLATILE RU-LABELLED COMPOUNDS FROM RECOIL RUTHENIUM WITH VARIOUS SOLVENTS
Solvent Benzene n-Hexane Indene Cyclooctatetraene Cyclooctadiene
Homogeneous yield (~o) 3"7 -4- 0.2 < 0-4 q- 0.2 1-0 -t- 0.2 0-15 0.23
Table 2 the results of these experiments are presented: in the case of benzene there seemed to be a positive result. The compound formed by recoil into benzene could not be separated from ferrocene by immediate sublimation, but after storage in air for two months was found (presumably through oxidation or decomposition) to have become non-volatile, whereas ruthenocene under these conditions remained unaltered. These results seem to parallel those o f earlier work ~ in which a volatile rutheniumlabelled compound was found to result from the interaction o f fission fragments with dibenzene chromium crystals. This compound may have been the same one observed in the present experiments, but further work on this point will be required. CONCLUSIONS Ruthenium-103 atoms activated by the/~-decay of their precursors in the mass-103 fission chain, can form volatile, labelled metal-organic products with surrounding molecules of some solvents with a few per cent yield. In the case of monomeric eyclopentadiene, a subsequent thermal reaction eventually incorporates a very large fraction o f the ruthenium atoms into ruthenocene molecules. The extent o f this thermal reaction appears to depend on the time o f standing, the temperature (see Appendix), and the duration of bombardment in the thermal column (Table 1). The experiments reported in the appendix are of course not entirely comparable with the fission experiments since the former were heterogenous--i.e, l°eRuCl3 coated on the walls o f the test tube first had to desorb to enter the solvent, and then react, while the fission experiments were homogeneous: bare l°aRu atoms were projected bodily into the ~u~ U. ZAm~, Unpublished research.
Hot-atom and thermal reactions of fission-product ruthenium in organic solvents
929
depths o f the solvent. One would expect the latter mechanism to provide better " m i x i n g " a n d greater reactivity: however, even in the f o r m e r case (the heterogeneous experiment) yields o f ruthenocene, expressed as per cent o f those ruthenium a t o m s actually present in the solvent, were surprisingly high. As one would expect for a thermal reaction, we observed n o isotopic difference in the "fission recoil" synthesis (Table 1). Finally, it o u g h t to be mentioned that l°6Ru-labelled ruthenocene prepared by the thermal reaction o u g h t to have some practical applications as a label for h y d r o c a r b o n s because o f its solubility, long half-life, ~,-ray emission, thermal stability and general chemical inertness. The great volatility o f carrier-free ruthenocene would allow the easy preparation o f gaseous sources o f ruthenium activity. APPENDIX Thermal reaction to form carrier-free ruthenocene
Metallocenes in macroscopic quantities may in some cases be prepared by simply heating the element together with cyclopentadiene vapour. For example tu) Fe + 2CsHs
300° ~- Fe(C~Hs)2 + H2
and similarly for magnesium ~'5) at 500-600°C, and thallium and indium ('e) at 350°. Other synthetic methods involve displacement of the Grignard CsHsMgBr ~sT)or sodium cyclopentadienyl, NaCsHs, (2s) with metal salts. Ruthenocene was first prepared by WILr-aNSON(n) by reaction of tetravalent ruthenium acetylacetonate with the Grignard: yields were low. Fxsc~R and GRtmL~T(m have employed anhydrous ruthenium trichloride and sodium cyclopentadienyl in tetrahydrofuran with better results. Two syntheses of carrier-free, labelled ruthenocene are known: one, due to BAUMO~RTNERand REINHOLD,(e) has been mentioned above, while the other is the subject of a German Patent is°) and employs a vapour phase reaction and catalyst. In this Appendix we wish to present the results of our experiments on the direct reaction of carrier=free ruthenium=f06 trichloride with liquid cyclopentadiene at room temperature, Synthesis
A known amount of carrier-free ruthenium-106 solution, whose activity had been determined accurately, was pipetted into a small, soft-glass test tube, and taken to dryness. One millilitre of freshly-distilled (depolymerized) cyclopentadiene was then added. In some experiments the mixtures were simply allowed to stand, while in others they were stirred magnetically. Reaction temperatures were 0 °, room temperature and 41°C. For separation a weighed quantity of ferrocene carrier was added to the reaction mixture and dissolved. The solution was immediately chromatographed and sublimed as before (Experimental). The yield of ruthenocene was calculated from the activity of the sublimate, corrected for the percentage of ferrocene recovered, divided by the activity originally taken. This yield was termed the "over-all yield" in analogy with the fission experiments. Similarly the "homogeneous yield" was here defined as the activity of ruthenocene, corrected for ~u) S. A. MILLER,J. A. TEnBOTHand J. F. TI~MAII,m, J. Chem. Soc. 632 (1952). (15) W. A. BA~r~R, J. Inorff . NucL Chem. 4, 373 (1957). (2e) E. O. FtscrmR and H. P. HorMA~n,~, unpublished, cited in E. O. F~sc'nv~ and H. P. FRITZ, Advances in Inorganic Chemistry and Radiochemistry (Edited by H. J. E~LEUS and A. G. SrlARPI~) Vol. 1, pp. 55 et seq. Academic Press, New York (1959). (sT)T. J. K~LV and P. L. PAUSON,Nature, Lond. 168, 1039 (1951). (~s) G. WILKINSONand F. A. Co'rroN, Chem. and Industr. 307 (1954). (J)) G. WmKdNSON, J. Amer. Chem. Soc. 74, 6146 (1952). (ao) H . G0a'rE and M. WENZrZL,Ger. Pat. DBP 1049860.
U. ZAns and G. I4Am~TTLV
930
TABLE 3.~CARRIER-FREE RUTHENOCENE S Y ~
Reaction time 15 min 20 min 30 min 50 min 1 hr 2hr 2.5 hr 4 hr 5 hr 12 hr 13"5 hr
24 hr 48 hr 56 hr 72 hr 84 hr 96 hr 122 hr
Room temp. not stirred Over-all Homogeneous yield ( ~ ) yield (~/o)
Room temp. stirred Over-all yield (~o)
0° not stirred Over-all yield ( ~ )
41 ° not stirred Over-all yield (~o)
0.03 0.006
9.1
0.2 0.023
0-02
12.8 0.046
0"5 1.2
0.21 0.17
26-1 0.9
0-54 1.5 0.76
51
0.98 1.4"
54.7 58.6
2.1
0'15
2.7 0"10 1-9 1.3 2.1
* The average yield of all room temperature samples with standing time >24 hr was 2.0 ± 0.2 per cent (13 samples). recovery as above, divided by the total ruthenium activity (organic and inorganic) actually present in the homogeneous cyclopentadiene phase. The results of these synthetic experiments under different conditions are presented in Table 3.
Proof that the sublimable carrier-free ruthenium-106 compound is in fact ruthenocene To prove that the volatile carrier-free ruthenium compound was identical with ruthenocene, both known ruthenocene and ferrocene were taken as carriers. Repeated sublimation of the ruthenocene or ferrocene containing the synthetic compound did not change the specific activity. When a benzene solution containing the labelled molecules and rnthenocene carrier was chromatographed on a long (100 cm) alumina column, only one peak was observed in the eluate and the specific activity of the ruthenocene recovered from these fractions by sublimation was the same as that of the starting material.
Additional experiments Reaction with the dimer. It was suspected that one of the reasons for the reaction ceasing at ca. 2 per cent of completion was the dimerization of the cyclopentadiene. Two runs, each of 52 hr at room temperature, were carded out using dicyclopentadiene instead of the monomer. Over-all yields of ruthenocene were less than 0-005 and 0.02 per cent respectively. Further reaction with fresh monomer. Following a normal reaction run the glass tube, still containing virtually all the ruthenium-106 tracer on the walls was rinsed with benzene and fresh monomeric cyclopentadiene introduced. After 48 hr at room temperature, additional ruthenocene formation of 0.6 and 0.8 per cent was found in two experiments. Effect of 7-irradiation. In two experiments, samples which had reached "plateau" values of the over-all ruthenocene yield of 3"2 and 1-6 per cent respectively were exposed to external 7-radiation from a cobalt-60 source. Doses of 2 × 10~and 4 × 106 rns were given to the former and latter samples, respectively. After 7-irradiation, the observed yields were 2.4 and 2.0 per cent indicating that the radiation had little or no effect.
Hot-atom and thermal reactions of fission-product ruthenium in organic solvents
931
Effect of light. Duplicate experiments in which the reactants were kept in the dark for 48 hr gave 2.9 and 1.7 per cent over-all yields, in essential agreement with others carried out in room-light. Effect of base. A synthesis in which a base is employed,cs~ viz: 2C5H6 -k FeCll q- 2 Base -* Fe(CsH6)s q- 2 Base. HC1 has been developed for the industrial production of ferrocene. We prepared duplicate reaction mixtures containing, in addition to the l°6Ru and cyclopentadiene, a few per cent of diethyiamine. Over-all yields were 4.7 and 2.5 per cent, perhaps a little higher than in the absence of base. Behaviour of carrier-free ruthenocene. In one experiment l°6Ru(CsHs)l was prepared without any ferrocene or inert ruthenocene carrier. This carrier-free compound proved to be unmanageably volatile on drying under reduced pressure at room temperature, and distributed itself over the interior of the sublimation vessel, vacuum lines, pump etc. Non-synthesis of volatile rhodium-organic complexes in t~-decay. As mentioned above ~°eRu (1 year) decays by fl--emission to :°6Rh (30 sec) which decays by /~-,7-emission to ~°6Pd. We prepared ~°'Ru(CsH6)j (mixed with ferrocene or ruthenocene carrier) and sublimed it, starting a stopwatch at the mid-point of disappearance of the crystals. Sublimation, in the best experiment, required ca. 15 sec. The 7-radiation (~eSRh) of the residue of sublimation was then counted: a clean 30 sec half-life was obtained. This activity was extrapolated back to the time of sublimation and compared with the :°'Rh activity initially present in equilibrium with I°6Ru. These activities were found to be equal, within 4-10 per cent, indicating that ~--decay in labeled ruthenocene produces almost entirely non-volatile rhodium daughter activity.* Non-synthesis offerrocene. We obtained some S°Fe of high specific activity and attempted to form ferrocene by a similar direct reaction with monomeric cyclopentadiene. None was obtained.
AcknowledgementsmThe authors wish to thank A. E. NORRISfor calculating the independent fission yield of ~°SRu, and R. DAvis, JR. for the use of his low-level counting equipment. One of us (U. Z.) wishes to thank the Bundesminister f'~ wissensehaftliche Forschung of the Federal Republic of Germany, for financial support in the form of a stipend. * In agreement with results on f/-decay of ~°SRu(C6Hs)~.~8~ These results have recently been checked in this laboratory. csl) j. M. B~MINOI-InM,D. S E ~ T I t and G. WnJc~soN, J. Amer. Chem. Soc. 76, 4179 (1954). ~se~F. BAUMOgRT~V.R,E. O. FISO~R and U. Znm~, Chem. Ber. 91, 2336 (1958); 92, 1624 (1959).