Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 52 (2019)

Complex formation of Fr with crown ethers
Y. Komori,∗1 H. Haba,∗1 T. Yokokita,∗1 S. Yano,∗1 N. Sato,∗1 K. Ghosh,∗1 Y. Sakemi,∗2 and H. Kawamura∗3

∗1
∗2
∗3

RIKEN Nishina Center
Center for Nuclear Study, the University of Tokyo
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University

1018C60.5 M 18C60.01 M 18C61DFrancium (Fr) is the heaviest known alkali metal withatomic number 87. It is one of the least-studied elements among the naturally occurring elements because
all its isotopes are short-lived. The half-life of its longestlived isotope, 223 Fr, is only T1/2 = 21.8 min. Owing
to experimental diﬃculties, the chemical properties ofFr have not been studied in detail so far. We aim toclarify the chemical bonding nature of Fr, which is inﬂuenced by relativistic eﬀects, through complex formation studies of Fr. Recently, Haverlock et al. examined the complex formation of Fr+ with calix[4]arenebis(benzocrown-6) (BC6B).1) They reported that Fr+ ismore eﬀectively extracted with BC6B than Cs+ . However, the reason for the selectivity is still unclear. In
this work, we studied the complex formation of Fr using
seven types of crown ethers: 18-crown 6-ether (18C6),
dicyclohexano-18-crown 6-ether (DC18C6), dibenzo-18crown 6-ether (DB18C6), 24-crown 8-ether (24C8),
dibenzo-24-crown 8-ether (DB24C8), 21-crown 7-ether(21C7), and dibenzo-21-crown 7-ether (DB21C7). Complex formation stability and selectivity of alkali metals
with the crown ethers depend on the ring cavity size andsubstituent groups of the crown ethers. Therefore, inthis work, we will investigate the eﬀects of the cavity
size and substituent groups of the crown ethers on thestability and selectivity of the complex formation of Fr.
We produced the second longest-lived isotope of Fr,
212
Fr (T1/2 = 20 min), in the 206 Pb(11 B, 5n)212 Fr reaction. Four sets of an 864-µg/cm2 206 Pb target on a 10 µm
Be foil were placed in a 12 mm spacing in 129-kPa He.
They were irradiated with a 11 B beam at an energy of86 MeV. The beam energies on the four 206 Pb targets
are calculated to be in the range of 70–79 MeV, which
covers the peak region of the excitation function of the206
Pb(11 B, 5n)212 Fr reaction.2) The 212 Fr atoms that recoiled out of the 206 Pb target were thermalized in He
gas, attached to KCl aerosol particles, and transported
through a Teﬂon capillary to a chemistry laboratory. In
the chemistry laboratory, the aerosols were collected ona piece of Naﬂon sheet for 20 s. The aerosols were dissolved with 100 µL of 1 M nitric acid and transferred
into a polypropylene (PP) tube. Another 600 µL of 1 M
HNO3 and 700 µL of nitrobenzene containing each of thecrown ethers were then added into the PP tube. The PPtube was shaken for a certain time for extraction. Aftercentrifuging the PP tube for 2 min, 500 µL of each phase
was subjected to γ-ray spectrometry to determine thedistribution ratios (D) of Fr. The D values were measured as functions of the shaking time and the concentration of each crown ether. These results were comparedwith those of RIs of lighter alkali metals such as 24 Na,43
K, and 137 Cs.

0.10.01

100 200 300 400 500 600 700

Fig. 1. Variations of D values of Fr+ as a function of shaking
time in solvent extraction with 0.01 and 0.5 M 18C6.

Figure 1 shows the variation of the D values of Fr+ asa function of the shaking time in the solvent extraction
with 0.01 and 0.5 M 18C6. We found that the extraction
equilibrium time of Fr+ is as fast as within 30 s. Therefore, we ﬁxed the shaking time to be 3 min. The 18membered rings, with a cavity diameter of 2.6–3.2 Å,3)
show the highest aﬃnity for K+ because their cavity sizes
ﬁt the size of K+ ion (ionic diameter: 2.66 Å4) ). The cavity diameter of the 24-membered crowns is in the rangeof 4.5–5.0 Å,5) which is larger than all alkali metal ions.
Thus, they show aﬃnity for larger alkali metal ions suchas Cs+ (3.34 Å4) ) and Fr+ (3.46 Å6) ). The cavity diameter of 21-membered crowns is in the range of 3.4–4.3 Å.3)
When using DB21C7, the D values of Cs+ are slightly
larger than those of Fr+ , while the D values of Cs+ andFr+ are almost the same when using 21C7. Two benzene
rings of DB21C7 may aﬀect the selectivity between Cs+
and Fr+ .
In this work, the D values of Fr+ with several types ofcrown ethers are almost the same as those of Cs+ , which
can be qualitatively understood by the relation between
the size of the alkali metal ions and the ring cavity diameter of the crown ethers. However, in the previous
study using calixarene, the D values of Fr+ are almost
one order of magnitude higher than those of Cs+ .1) To interpret these results and elucidate the reason of complex
formation selectivity between Fr+ and Cs+ , we will perform further investigations, for example, XAFS studiesof the Cs+ -crown ether and Cs+ -calixarene complexes,and quantum chemistry calculations for Fr+ as well as
Cs+ .
References

1) T. J. Haverlock et al., J. Am. Chem. Soc. 125, 1126 (2003).
2) Y. Komori et al., RIKEN Accel. Prog. Rep. 51, 224 (2018).
3) C. J. Pedersen, J. Am. Chem. Soc. 92, 386 (1970).
4) L. H. Ahrens, Geochim. Cosmochim. Acta 2, 155 (1952).
5) M. Chamsaz, Russ. J. Inorg. Chem. 50, 413 (2005).
6) L. H. Delmau et al., J. Phys. Chem. B 117, 9258 (2013).

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