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Direct Photolysis of Uranium Hexafluoride as a Preparatively Utilizable Endothermic Reaction.

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2035 s, 2 026 m, 2017 w, 1992 cm-' w, br) closely resembles
that of ( 1 ) shifted to higher wave numbers by 7-20 cm-'.
Moreover, (2) shows a strong band at 1 107 cm-' (in KBr)
which can be assigned to the SO stretching vibration.
The structure of (2) was determined by X-ray methods[']
(cf. Fig. 1); this provided the first proof of the existence of an
SO Iigandf31.The S -0 bond length is 147 pm and corresponds to that in free SO (148 pm)14aland many other S=O
double bonds[4b1.The Fe~-S bonds to the SO group are significantly shorter than those to the S atom, probably due to
the radius contraction of the sulfur by the electronegative
oxygen. This shift of the SO group towards the center of the
molecule leads to an S--S contact of only 272 pm, which is
considerably shorter than the sum of the van der Waals radii
(2x 185 pm). In analogy with some complexes of the
Fe2(C0)6(p2-X)z
typef5],a partially bonding s-s interaction
must therefore be assumed in this case.
The fact that (2), like (I)[61,contains an open triangle of
iron atoms, i. e. only two metal-metal bonds, leads to the conclusion that the SO molecule is a 4-electron ligand like the S
atom. This is also supported by the close relationship observed between the structures of (1) and (2).
The extensive similarity between the two complexes also
justifies our viewing (2) as the product of addition of a Lewisacid 0 atom to ( I ) , with only minor modification of the
bonding in the skeleton of (1). The sufficient basicity of a p3bonded sulfur atom in M3S cluster units has already been detected: the 16-electron fragment Cr(CO), can be added['] to
the SF~CO,(CO)~
cluster['I, again with hardly any structural
change. Complex (2) can also be regarded as an intermediate
of reduction of sulfite to sulfide by iron carbonyl hydrides
[Fe3(CO)11H2is a less powerful reducing agent than
Fe(CO),H,!].
In contrast to its formation, the bonding and the structure
of (2) are not unusual. This suggests that further SO clusters
might be accessible.
Procedure
First H20z(16.7 mmol; 1.7 ml of a 30% aqueous solution)
and then Na,S03.7H,0 (7.5 g, 30 mmol; dissolved in water
(20 ml)) are added to a solution obtained by basic reaction of
Fe(CO)s (4.9 g, 25 mmol) and NaOH (4.0 g, 100 mmol) in
H20/CH30H (1 :2) (30 ml). After 10 min, the reaction mixture is acidified with dilute hydrochloric acid, the dark precipitate filtered off, washed with water, and chromatographed
in benzene on a silica gel column. Elution with hexane gives
(1) (24% yield) and with benzene gives (2) (yield 10%). (2)
can be recrystallized from toluene ( + 25/ - 78 "C), affording
0.28 g (0.56 mmol; 6.7%) of dark red-brown, air-stable crystals. The compound decomposes without melting at 125128 "C.
Received: August 6, 1979 [Z 416 1EI
German version: Angew. Chem. 92. 225 (1980)
[I1 W.: Hieber, J. Gmber, 2.Anorg. Allg. Chern. 296, 91 (1958).
(21 Triclinic, P1, 2=2; a=951.4(1), b=1032.0(1), c=906.5(4) pm, a=91.38(2),
8= 116.97(2), y=97.06(1)"; 2930 independent reflections, R=0.023.
[31 Cf. G. Schmid, G. Ritfer, Chem. Ber. f08,3008 (1975).
141 a) f? X.Powell, D. R. Lde, J. Chem. Phys. 41, 1413 (1964); b) L. E. Sutton:
Tables of Interatomic Distances and Configuration in Molecules and Ions,
Spec. Publ. No. I t , The Chemical Society, London 1958.
151 Cf. B. K. Teo, M. B. Hall, R. F. Fenske, L. F. Dahl, Inorg. Chem. 14, 3103
(1975).
161 C. K Wei, L. F. Dahl, Inorg. Chem. 4, 493 (1965).
171 S. A. Khatrab, L. Marko, G. Bor, B. Marko, J. Organomet. Chem. I, 373
(1964).
[81 F. Richter, H. Vahrenkamp. Angew. Chem. 90, 474 (1978); Angew. Chem.
In?. Ed. Engl. 17, 444 (1978).
Angew. Chem. Int. Ed. Engl. 19 (1980) No. 3
Direct Photolysis of Uranium Hexafluoride as a
Preparatively Utilizable Endothermic Reaction[**]
By Frank S. Becker and Eberhard Jacob"]
For the laser isotope separation of uranium the photodissociation of UF6 by an infrared multiphoton process or by a
combination of infrared and ultraviolet irradiation is considered to be one of the most promising approaches. In connection with such experiments it is important to know to what
extent the photodissociation is possible without the use of a
scavenger and what part the fluorine can play in the following reactions.
It is well known that the photodissociation of UF6 is feasible in the presence of scavengers (Hz, CO, SO2)['].Photodissociation of pure UF6 by uv light in an argon matrix at 10 K
is small[za1and at 27 K impossible[2b'.Experiments with a
rays[3a1
electron-beam~[~~]
and UV light['c~3cl
at room temperature yielded only very small quantities.
On use of a suitable experimental set-up, we were able to
dissociate by UV light about 50 g of UF6 in the gas phase at
room temperature according to
nearly quantitatively (98.8%) into P-UF5 and ultrapure F2.
This endothermic photoreaction (+ 132f 19 kJ/m01[~])is accompanied by a transformation of photon energy into chemical energy. The energy balance of usual photoreactions is
exothermic, because the always endothermic photoexcitation
is more than compensated by the exothermic character of the
following reaction~[~l.
A series of photolysis experiments showed a strong influence of the reaction cell geometry on the final attainable
dissociation yield of the UF,. Special care has to be taken to
avoid deposition of UFS on the irradiation window. In addition to a vertical window position the presence of Fz proves
to be useful, because the photofluorination of UF,['"l hinders
the formation of a deposit on the window. At constant UF6
pressure (solid state UF6) the increasing F2 pressure brings
the reaction to equilibrium. Nevertheless, a nearly complete
UF6 dissociation according to eq. (1) can be attained by
creating suitable UV-shaded regions where the produced
UF5 can be irreversibly deposited. From time to time, the F2
formed should be partially withdrawn (cf. Fig. 1). A fast production of UFS is indicated by a dense white fog, which
should be convected into the shaded regions to obtain high
yields.
A simplified kinetic consideration shows that the polymerization of the UF5 molecules should be the rate determining
step of reaction ( 1)Dc.61.These kinetic conditions make possible the thermodynamically unexpected endothermic reaction.
The direct photodissociation of UF6 is a more effective
way of preparing P-UF5 than the photoinduced UF6 reduction with CO['blwhich was described only a short time ago
(optimum amount of UFs: 5 g per experiment). The attained
yields of the direct dissociation are higher, compared with
the photoreduction['], because only in the presence of fluo1'1 Dr. E. Jacob
Abteilung Physikalische Chemie. M.A.N. - Neue Technologie
Postfach 500620, D-8000 Munchen 50 (Germany)
DipLPhys. F. S . Becker
Projektgruppe fur Laserforschung der Max-Planck-Gesellschaft
D-8046 Garching (Germany)
[**I
Chemistry of Uranium Fluorides and Oxide Fluorides. Part 3. We wish to
thank Prof. K. L. Kompa, Garching, and Prof. F. Seel, Saarbriicken, for helpful
Comments. Part 2: E. Jacob. Z . Anorg. All. Chem. 400, 45 (1973).
0 Verlag Chemre, GmbH, 6940 Weinhelm. 1980
0570-0833/80/0303-0227
$
02.50/0
227
rine is free entrance of the UV light into the reaction cell effected. In addition, the direct dissociation of UF6 is a method
for the preparation of small amounts (1-2 bar 1 per experiment) of ultrapure fl~orinef’*~].
7
Fig. 1. Photoreactor: 1, sapphire window of I. D. 44 mm in N W 100 CF flange
(Odelga-Physik D-7441 Aichj; 2, nipple NW 100 CF (Bakers Hochvakuum, D6200 Wiesbaden); 3, cold finger (0 55 x 100 mm) in NW 100 C F flange; 4, dust
filter (porous stainless steel cone); 5 , pressure transducer (&lo00 mbar, Bell and
Howell); 6, NW 16 CF valve (Balzers);, 7, to vacuum line.-All components
consisted of stainless steel and the flanges were copper sealed and He leak controlled. UV-light source: 1 OOO W mercury super high pressure lamp (Osram
HBO to00 or Philips CS 1OOO) with quartz condensor for parallell light beam,
100 mm water filter for IR elimination. The irradiation was performed in horizontal position.
In principle, the direct photodissociation of UF6 hints at
the possibility of performing the laser isotope separation
without a scavenger. On the other hand, it is of fundamental
interest to learn to what extent the unspecific reaction
UF, + F+ UF, + F2
would compensate the isotopically selective dissociation of
UF6 already accomplished.
Procedure
The photodissociation is performed in the reactor depicted
in Figure 1. After evacuation down to
mbar, the cold
finger is filled with liquid N2and UF6 (47.668 g, 135.4 mmol)
i s frozen into the reactor. Afterwards the cold finger is
warmed up to 1-2 degrees below room temperature by
means of a thermostat. The UF6 pressure during the irradiation lies between 120 and 180 mbar. The progress of the photolysis can be monitored by the pressure rise caused by F2
production. After 17 h the Fz production comes to a standstill. The cold finger is filled with liquid N2 again and the
nondissociated UF6 is frozen out. The F2is collected in a 63
K cold trapCsb1,
leaving about 30-40 mbar. After warming up
the cold finger the photolysis is continued, and the fluorine
withdrawal is repeated after 6 and 16 h respectively. After a
total irradiation time of 39 h at 25 “C, 98.8% of the UF6 has
decomposed. After pumping off the remaining F1, the reactor is opened in a glovebox. It is possible to remove 43.6 g
(130.9 mmol) of a powdery light-green deposit from the
walls. X-ray analysis shows the existence of B-UF5[9al.Chemical analysis of the UF5 is accomplished by fluorination to
UF6 with IF, while monitoring the
A PVT measurement of the gas collected with the 1 5 K cryopump (“cryoToplerpump”)[2b~
gave 1641 mbar 1of mass-spectroscopically
pure fluorine. According to (1) 0.5 mole F2 is produced per
mole of dissociated UF6.
Received: August 21, 1979;
revised November 12, 1979 [ Z 417 IE]
German version: Angew. Chem. 92, 226 (1980)
111 a) J. J. Karz, E. Rabinowilch: The Chemistry of Uranium. Dover Publ., New
York 1961, p. 441; K. L. Kompa, G. C. Pimenref, 1. Chem. Phys. 47, 857
(1967);L. B. Aqrey, R. T.Paine, J. Chem. Soc., Chem. Comm. 920(1973j,U.
S. Pat. 3929601 (1974/75); G. W.Halstead, P. G. Eller, L. B. Asprey, K. V.
Salazar, Inorg. Chem. 17, 2967 (1978); b) R. T. Paine. L. B. Asprey, Inorg.
Synth. 19, 137 (1979);c) 0. Hartmannshenn, J. C. Barral, C. R . Acad. Sci. C
272, 2139 (1971);L. B. Asprey, R. T. Paine, 7th Int. Symp. Fluorine Chem.,
Santa Cruz, Calif. (1973), Abstr. 1-47,
[21 a) R. T. Paine. R. S. McDoweIl, L. B. Asprey, L. M. Jones, 1. Chem. Phys. 64,
3081 (1976);b) E. Jacob, unpublished results.
[31 a) H. A. Bernhardr, W. Dauies, Jr., C. H. Shflett, Proc. 2nd Int. Conf. Peaceful Uses At. Energy, Geneva, P/522,62 (1958);b) A. I. Migachev, A . P. Senchenkov, At. Energy 16, 510,631 (1964); c) K. C. Kim. R. Fleming, D. Seirz,
M . Reideld, Chem. Phys. Letters 62,61 (1979).
141 A H m , calculated according to data in D. L. Hildenbrand, I . Chem. Physics.
66,4788 (1979).
h
[S] For example: a) 2 [ U F J + F ,
2UF6; J. Sliunik, K. Lutar, A . Smalc,
J. Fluorine Chem. I f , 643 (1978);bj all strongly exothermic photoreductions
of UF,: compare ref. [l].
161 a) The detailed reaction mechanism is determined probably by the actual
partial pressures of UF, and F2; b) we have observed an extensively quantitative photodissociationof UF6 according to eq. (1) in the pressure range between 0.1 and 100 mbar loo.
[7]Care must be taken regarding the UF6 purity. We used 99.99+ % UFa.
[8] a) Depleted UF6 is available in large amounts, cf. G. A. Olah, J. Welch. TseLok Ho, J. Am. Chem. Soc. 98,6717(1976);b) for the purification of F2, see
E. J. Jacob, K. 0. Christie, J. Fluorine Chem. 10, 169 (1977).
191 a) W. H. Zochariosen, Acta Crystallogr. 2, 296 (1949); b) E. Jacob, DAS
2504840 (1 978).
/I
BOOK R E V I E W S
Kirk-Other: Encyclopedia of Chemical Technology. Editorial board: H. F. Mark, D. F. Othmer, C. G. Overberger,
and G. T. Seaborg. John Wiley & Sons, New York 1979,
3rd Edition. Vol. 5: Castor Oil to Chlorosulfuric Acid. xxv,
bound, f 60; Vol. 6: Chocolate and Cocoa to Copper. xxiii,
869 -pp.,
- bound, f 60.
The fifth and sixth volume of the new edition of KirkOthmer[’l are now available. The principal contributions to
Volume 5 are: Catalysis (55 pp.); Cellulose and cellulose der[‘I Angew. Chem. Int. Ed. Engl. 18,704 (1979)
228
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ivatives (90 pp.); Ceramics (80 pp.); Chemotherapeutics (100
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