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Isotope Effects in the Low-Pressure High-Pressure and Thin-Layer Chromatography of the Lanthanoids.

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the elongated prisms accomodate bromine atoms, the compressed neighboring prisms lead atoms (Fig. 3b).
L
h
I
I
Br3
Br3
3
TI
Br2
h2i,
t
81.3
22
BrL
t
L51.0
case of the isotopes samarium-154 and ~amarium-144[’~.
The total separation factor G, as defined by HeumannC2],
was 1.06. The detection was accomplished mass spectroscopically. In continuation of these experiments we have
now investigated the lanthanoids cerium and europium.
Both elements have proven to be especially suitable for
such studies regarding their natural isotopic composition
and their subsequent determination by neutron activation
analysis. Table 1 lists the physical properties of the isotopes used for the measurement of the isotopic enrichment.
Table 1. Physical properties of the isotopes employed. T,,*= halflife, E=yenergy, P= emission probability (absolute intensity).
Stable
nuclide
Abundance
[Yo]
Active
nuclide
‘ T e
‘“’Ce
88.48
1 1.07
47.8
52.2
“Ice
IA3Ce
Is2Eu
Is4Eu
‘”Eu
lS3Eu
Fig. 3. The sequence of tetragonal prisms formed from Br-ions parallel to
[OOll in: a) TIPb801Br9, and b) Pb90,Br,o. The five nearest Br neighbors of
Pb3 are emphasized by tilled connecting lines. Further details of the crystal
structure determination can be obtained on request from the Fachinformationszentrum Energy Pbysik Mathematik, D-7514 Eggenstein-Leopoldshafen
by quoting the number CSD 50365, the name of the author, and full citation
of the journal.
The latter atoms are shifted from the center in the direction of the square basal plane, leading to a short Pb-Brdistance of 257.7 pm. To a first approximation each such
Pb-ion shows a one-sided fivefold environment of bromine
(Fig. 3b, filled connecting lines to Br2 and Br4). This at
first sight unusual environment might be explained in
terms of the effect of the lone pair electrons (on Pb3) if
these are oriented in the opposite direction of the short
Pb-Br-distance. For the lead atom in question, in contrast
to the other, pseudotetrahedrally coordinated lead atoms,
there results in the primary neighborhood a pseudooctahedral environment. This suggests comparison with a BiC1:group in the bismuth subchloride Bi24C128L111.
Received: November 22, 1982;
revised: January 21, 1983 [Z 208 IEI
German version: Angew. Chem. 95 (1983) 318
The complete version of this communication appears in:
Angew. Chem. Suppl. 1983. 427-444
[I] H.-L. Keller, Z . Anorg. Allg. Chem. 491 (1982) 191.
[7] S. H. Hong, A. O h , Actu Chem. Scund. A 2 8 (1974) 233.
[8] H. G. von Schnering, R. Nesper, H. Pelshenke, Z . Naruforsch. 8 3 6
(1981) 1551.
191 G. Bergerhoff, Angew. Chem. 76 (1964) 697; Angew. Chem. Inr. Ed. Engl.
3 (1964) 686.
[lo] A. Smakula, J. Kalnajs, Nuouo Cimento 6, Suppl. No I (1957) 214.
1111 A. Hershaft, J. D. Corbett, Inorg. Chem. 2 (1963) 979.
Isotope Effects in the Cow-Pressure, High-pressure,
and Thin-Layer Chromatography of the Lanthanoids
By Alfons Hufnagel and Hermann Specker*
During experiments on the HPLC separation of the lanthanoids we were able to detect an isotope effect in the
[*] Prof. Dr. H. Specker, Dr. A. Hufnagel
Lehrstuhl Anorganische Chemie I der Universitat
Postfach 1021 38, D-4630 Bochum 1 (Germany)
Angew. Chem. Int. Ed. Erigl. 22 (1983) No. 4
32.5 d
33 h
12.7 y
8.5 y
P
E
Ikevl
[04
145.45
293.3
344.3
123.1
41
43.3
27
40.46
In the first series of experiments, Ce(NO,), was subjected to multiple repurification and then chromatographed on two low-pressure columns coupled in cascade
[Lobar, Merck, size A (240-lo), Li Chroprep Si 60 (40-64
pm)]. 50 pL of a 1 M Ce3+-solution ( e 7 mg Ce3+) in methanol/HN03 (3 :1) was introduced onto the column via a
three-way tap. Elution was carried out with a mixture of
diisopropyl ether/tetrahydrofuran/HNO, (65%) in the ratio 100 :20 :5. The consistently reproducible retention time
for cerium was 187 min, the peak width 39 min. After fractionation of the cerium peak into three twelve-minute fractions the respective “head” and the “tail” fractions were
very carefully worked-up and then irradiated (neutron flux
l O I 4 n cm-’ s-I). The absolute content of cerium in each
of the fractions was not measured; for measurement of the
isotope effect according to the aforementioned definition
of the total separation factor G, we calculated the quotients of the net count rates of the characteristic y-emission
lines of the isotopes Ce-143 to Ce-141 in the “head” fraction and in the “tail” fraction. From ten single measurements an enrichment of the heavier isotope was found in
the “head” fraction of the cerium peak with a mean total
separation factor of 0.65 :0.59 = 1.102 at the 99.9% confidence level.
In the case of HPLC- and TLC-enrichment we used previously activated cerium and europium samples as starting
material. The eluent used in the HPLC separation [HPLC
“Constametric” pump, Knauer columns (250 x 4.6 mm),
silica gel 60 Li Chrosorb, particle size 7 pm, pore size 60 A]
was diisopropyl ether/tetrahydrofuran/HNO, (100 :35 :4),
the amount of sample 150 pg Ce3+ and 0.38 pg Eu3+, respectively. The monitoring was carried out with a continuous flow NaI(TI1) scintillator. On the basis of the chromatograms obtained, five 1-minute fractions were collected in each case and the individual fractions measured
y-spectroscopically. For the cerium isotopes a total separation factor of 1.085 was obtained; this result was also measured at the 99.9% confidence level, from 18 measurements. The corresponding factor in the case of europium
amounted to 1.03, at the same confidence level. According
to our investigations so far, the different average total separation factors measured for the elements cerium and europium are not in the first instance the result of different
fractionation but rather of different retention times. We
0 Verlag Chemie GmbH, 6940 Weinheim. 1983
0570-0833/83/0404-0325 $ 02.50/0
325
are at present investigating the step heights in our sample
carrier-eluent systems, so as to enable calculation of the
element separation factors.
So far, TLC has proven to be the most efficient chromatographic method for the separation of the lanthan~ids‘~]
[Merck plates, silanized silica gel (0.25 mm), size 20 x 20
cm, without fluorescence indicator]. We therefore resorted
to TLC for the enrichment of the europium isotopes. The
zones in conventional chromatography cannot, of course,
be reproducibly defined as “head” and “tail” zones and so
separated. It is known, however, that the zones can be concentrated by constricting the flow of eluentI4]. In this way
the zones could indeed be separated into “head”, “tail”
and “middle” zones. Diisopropyl ether/tetrahydrofuran/HN03 (100 :50 :5 ) was used as eluent. In the case of
the isotopes europium-154 :europium-152 we obtained a
total separation factor of 1.06 from 21 single measurements on the “head” and “tail” zones. The corresponding
depletion in the “tail” zones was also confirmed from 23
single measurements at the 99.9% significance level. To our
knowledge an isotope effect has hitherto never been detected in inorganic TLC. In the case of the isotope effects
so far detected, an enrichment of the heavier isotopes in
the “head” fraction or “head” zone was found in all cases
during column and thin-layer chromatography, respectively. This is consistent with the chromatographic methodologies developed by us for the separation of lanthanoids
using the same eluent system.
Received: November 19, 1982 [ Z 205 IEJ
German version: Angew. Chem. 95 (1983) 327
react smoothly with Ph3PAuCl to give the pentanuclear
cluster 1 (47%, black crystals, m. p. = 150 “C) or the hexanuclear cluster 2 (38%, black crystals, m.p.=133 “C). 1
and 2 were identified by crystal structure analysis (see
Figs. 1 and 2, as well as Table 1).
(Ph, PAu)2Fe3(CO)9S
1
m
Fig. 1. Molecular framework of 1 in the crystal (small circles represent CO ligands; phenyl groups are not shown).
CAS Registry numbers:
I4’Ce, 14191-73-2: I4’Ce, 14119-20-1; ‘”Eu, 14378-48-4: IS3Eu, 13982-02-0;
I4‘Ce, 13967-74-3; ’43Ce, 14119-19-8; l5*Eu, 14683-23-9; ‘14Eu, 15585-10-1;
dirsopropyl ether, 108-20-3; tetrahydrofuran, 109-99-9
[I]
[2]
I31
[4]
(P~~PAU),RII~CO,(CO)~~
2
0
\p
v
H. Specker, W. Weuster, Fresenrus 2. Anal Chem. 302 (1980) 205.
K. G. Heumann, 2. Narurlorsch. 8 2 7 (1972) 492.
K. Jung, H. Specker, Fresenius 2. Anal. Chem 300 (1980) 15.
E. Stahl: Dunnschichtchromatographie, 2nd edition, Springer, Berlin
1967.
Capping of Metal Atom Triangles in Clusters by
R,PAu Moieties**
By Eckehart Roland, Klaus Fischer, and
Heinrich Vahrenkamp*
The growth of metallic microcrystallites or metal-rich
clusters requires the incorporation of the monometal species at energetically favored face positions. Sporadic examples suggest that the additional species is bonded preferentially over triangles or quadrangles of metal atoms[’].
The recently‘” discovered exchange of hydride ligands for
R3PAu moieties in clusters offers a method of studying
these types of growth reactions systematically. Previously,
a R,PAu moiety could be incorporated as a p2-bridge
across two metal atoms[31or as a p3-bridge across three metal atom^[^,'^. We have now found that incorporation of
two R3PAu moieties leads to a repetition of the p3-bridging
principle.
Deprotonation of H2Fe3(C0)$ or H2R~2C02(C0)12
by
KH in tetrahydrofuran (THF) leads to dianions which
I*] Prof. Dr. H. Vahrenkamp, E. Roland, K. Fischer
[**I
326
lnstitut fur Anorganische und Analytische Chemie der Universitat
Albertstrasse 21, D-7800 Freiburg (Germany)
Basic Cluster Reactions, Part 4. This work was supported by Heraeus
GmbH (Hanau) and by the Rechenzentrum, Universitat Freiburg.-Part
3: E. Roland, H. Vahrenkamp, Organomerallics 2 (1983) 183.
0 Verlag Chemie GmbH. 6940 Weinheim, 1983
Fig. 2. Molecular framework of 2 in the crystal (small circles represent CO ligands: phenyi groups are not shown).
The stepwise growth of the clusters is apparent from
their structures. One of the two gold atoms is located over
a triangular face of the starting material, and the other
over a triangular face formed by addition of the first gold
atom. Of the metal-metal bonds, the inner (Fel-Fe3 and
Col -Ru2, respectively) are extraordinarily long. This indicates a tendency of these bonds to rupture; the simultaneous formation of the Au2-S and Rul-Aul bonds, respectively, would lead to an octahedral cluster framework. By
analogy, the growth of polymetal species can be comprehended as a sequence of p,-cappings and reorganizations.
Recently described triply gold-bridged ruthenium clusters‘”’ exhibit the same structural principle, whereas the
obtained in a
carbido cluster [CFe4(CO)12(AuPR3)211121,
0570-0833/83/0404-0326 $02.50/0
Angew. Chem. Inr. Ed. Engl. 22 (1983) No. 4
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