Synthesis, structure, and extraction behavior of 4,5′,4″,5?-tetra-tert-butyltetrabenzo-24-crown-8

Article abstract of DOI:10.1039/b008603g

Isomerically pure 4,5′,4″,5?-tetra-tert-butyltetrabenzo-24-crown-8 was synthesized by a route that eliminates any need for isomer separation. Its structure was determined using single crystal X-ray diffraction methods. Since the observed saddle-shaped con

Full text of DOI:10.1039/b008603g

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Synthesis, structure, and extraction behavior of 4,5Ј,4Љ,5ٞ-tetra-
tert-butyltetrabenzo-24-crown-8†‡
Tatiana G. Levitskaia, Richard A. Sachleben, Jeffrey C. Bryan and Bruce A. Moyer*
2
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory,
MS-6119/4500S, Oak Ridge, Tennessee 37831, USA
Received (in Cambridge, UK) 25th October 2000, Accepted 27th February 2001
First published as an Advance Article on the web 26th March 2001
Isomerically pure 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8 was synthesized by a route that eliminates any
need for isomer separation. Its structure was determined using single crystal X-ray diffraction methods. Since the
observed saddle-shaped conformation of the crown ether is the same as previously reported for the closely related
tetrabenzo-24-crown-8, the presence of the peripheral tert-butyl groups has no apparent effect on ring structure.
This structure experiences few intermolecular interactions beyond London forces, suggesting it represents a low
energy conformation. Liquid–liquid distribution behavior of CsNO3 and CsClO4 between water and 1,2-dichloro-
ethane (1,2-DCE) containing 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo)-24-crown-8 was investigated. Equilibrium
constants corresponding to the extraction of ion pairs and dissociated ions, formation of the caesium complex, and
dissociation of the ion pairs in water-saturated 1,2-DCE at 25 ЊC were obtained from equilibrium modeling using the
SXLSQI program. 4,5Ј,4Љ,5ٞ-Tetra-tert-butyltetrabenzo-24-crown-8 exhibits stronger caesium binding and a similarly
low tendency towards ion-pairing, in comparison with the previously characterized isomeric 4,4Ј- and 4,5Ј-di-tert-
octyltetrabenzo-24-crown-8 ethers.
ally, the presence of multiple positional isomers interferes with
physical and structural studies, such as single-crystal structure
Introduction
Research on crown ethers has been motivated by their remark-
determination by X-ray diffraction and solution complexation
able ability to complex and transport alkali and alkaline-earth
studies by 1H and 13C NMR. Questions regarding reproducibil-
metal cations.1 Historically, the features of the cyclic polyether
ity of results may also arise in that different batches of a com-
ligands considered to be most important in determining bind-
pound containing positional isomers may not possess identical
ing strength are: number and type of donor atoms, dimensions
isomer distributions. Consequently, it is preferable to synthesize
of the cavity, presence of substituents on the donor ring, and
and study a single isomer.
degree of ligand preorganization.1,2 Benzo-substituted crown
Caesium complexation by tetrabenzo-24-crown-8 results in
ethers have received considerable attention due to the fact that
the formation of a structure exhibiting two clefts, which provide
they are more lipophilic than the corresponding unsubstituted
unique electronic and spatial environments suitable for the
crown ethers, are readily prepared from catechol,3 and exhibit
inclusion of additional species to saturate the Csϩ coordination
selectivity for complexation of alkali metal ions.4
sphere and contribute to overall stability of the supramolecular
Unfortunately, benzo-substituted crown ethers often suffer
assembly.10,12,13 The considerable softness of the large charge-
from limited solubility in solvents of low to moderate polarity,
dispersed Csϩ ion, as well as the topography of the clefts,
limiting their practical application.5 The solubility properties of
creates a unique environment for 1,2-dichloroethane (1,2-DCE)
such macrocycles can be adjusted by the judicious addition
solvent molecules to coordinate to Csϩ.10 Liquid–liquid distri-
of substituents to the periphery of the benzo group.4–7 Addition
bution studies using bis(tert-octylbenzo)dibenzo-24-crown-8§ in
1,2-DCE revealed that solvent inclusion in the clefts likely also
of hydrocarbon groups generally increases the solubility of the
crown ether in non-polar diluents.4,5,7,8 A potential compli-
occurs in solution, influencing the speciation and ion-pairing
cation in the preparation of substituted benzo crown ethers,
behavior of the cation complex.10 Saturation of the Csϩ inner
though, is the formation of positional isomers.9 For example,
coordination sphere by the crown ether and two 1,2-DCE
alkylation of two opposing benzo groups on tetrabenzo-24-
solvent molecules leaves no open coordination sites for the
crown-8 results in a mixture of 4,4Љ and 4,5Љ positional isomers,
and elaborate procedures are required to effect their separ-
anion, which is observed to reside in the second coordination
sphere. As a result, Csϩ complexes with tetrabenzo-24-crown-8
ation.10 Random monoalkylation of all four benzo groups on
and its derivatives in 1,2-DCE exhibit unusually high stability
tetrabenzo-24-crown-8 would result in four isomers (Fig. 1). It
and a low tendency towards ion-pairing.
is generally assumed that positional isomers exhibit similar
As part of our investigation into the extraction properties of
metal ion complexation properties, although variable complex-
caesium-selective tetrabenzo-24-crown-8 ligands, we developed
a synthesis of isomerically pure 4,5Ј,4Љ,5ٞ-tetra-tert-butyl-
ation and distribution behavior have been reported.11 Addition-
tetrabenzo-24-crown-8, determined its crystal structure, and
analyzed its extraction behavior toward Csϩ ion with nitrate
and perchlorate counter-anions in 1,2-DCE. As part of these
studies, we determined the caesium complex stability and the
ion-pairing tendency of this new ligand. We were curious to
† This is the work of a contractor of the US Government under con-
tract No. DE-AC05-00OR22725. Accordingly, the US Government
retains a non-exclusive, royalty-free license to publish or reproduce the
published form of this contribution, or allow others to do so, for the US
Government purposes.
‡ The parameters used in SXLSQI modeling are available as supplemen-
tary data. For direct electronic access see http://www.rsc.org/suppdata/
p2/b0/b008603g/
§ The IUPAC name for tert-octyl is 1,1,3,3-tetramethylbutyl.
808
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b008603g

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Fig. 1 Schematic representations of the four possible positional isomers of tetra-tert-butyltetrabenzo-24-crown-8.
understand how varying the alkyl substitution on the benzo
rings would affect the complexation and transport properties
of the ligand. For these purposes, we performed equilibrium
modeling of the collected distribution data. The extraction
constants thus obtained allowed calculation of the caesium
complex stability and dissociation constants in water-saturated
1,2-DCE, which were then compared with those previously
determined for the isomeric bis(tert-octylbenzo)dibenzo-24-
crown-8 ligands.10
relationship, whereas the 4,4Ј,5Љ,5ٞ isomer provides the σσ
relationship. We chose to synthesize the 4,5Ј,4Љ,5ٞ isomer since
it could be obtained in a convergent manner, as shown in
Scheme 1, from 1,2-bis(4Ј-tert-butylphenoxy)ethyl ether,
readily obtained from 4-tert-butylphenol. It may be noted that
tetrakis(tert-butylbenzo)-24-crown-8 is isomeric with the
bis(tert-octylbenzo)-24-crown-8 studied previously by us10 and
discussed herein.
Crystallographic studies
The structure of 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-
crown-8 is depicted in Fig. 2, affirming that the anticipated
isomer was obtained. Only half of the molecule makes up the
crystallographic asymmetric unit, the other half is generated
through the crystallographic inversion center. The crown
ether is collapsed in on itself, as is commonly observed
in the structures of large, uncomplexed crown ethers.9,14,15
Overall, the crown is relatively flat, and, as shown in Fig. 3,
it stacks upon itself in the crystal lattice, roughly along the
a axis, with an average inter-crown spacing of approximately
4.5 Å.
The crown conformation can be expressed in terms of the
torsion angles around the ring,16 starting with O1–C1–C2–O2,
and leaving a space before each O–C–C–O torsion angle. The
conformation observed here is: 0aa gϩagϩ 0gϩ(gϩ) gϪagϩ (Table
1), which is then repeated, albeit inverted, for the inversion
symmetry-generated portion of the crown. The value of “(gϩ)”
refers to the C10–O4–C15–C16 torsion angle of 113.3(2)Њ, and
is marked in parentheses because of its closeness to the inter-
mediate value of 120Њ.16 For binding, the pattern 0aa g aa
should be repeated four times around the ring, as observed in
Results and discussion
Synthesis
As shown in Fig. 1, tetrakis(tert-butylbenzo)-24-crown-8 can
exist as four regioisomers, depending on the relative orientation
of the tert-butyl substituents. Upon complexing caesium,
tetrabenzo-24-crown-8 adopts a saddle-shaped structure exhib-
iting two clefts formed by opposing pairs of benzo groups.
Attachment of substituents, in this case tert-butyl, onto oppos-
ing benzo groups results in two possible relative orientations
across the cleft, which can be viewed in terms of the symmetry
operations necessary to interchange them. These two orien-
tations require either reflection through a plane bisecting the
cleft, referred to henceforth as σ, or rotation about a two-fold
axis passing down the center of the cleft through the caesium
ion, denoted as C2. Since each tetrakis(tert-butylbenzo)-
24-crown-8ؒCsϩ complex contains two clefts, three possible
structures result in the caesium complexes: σσ, C2C2, and σC2.
Interestingly, two of the four regioisomers shown in Fig. 1
result in the C2C2 relationship; these are the 4,4Ј,4Љ,4ٞ and the
4,5Ј,4Љ,5ٞ isomers. The 4,4Ј,4Љ,5ٞ isomer produces the σC2
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814
809

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Table 1 Selected hydrogen bonding geometry (Å and Њ) and torsion angles (Њ) for 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8
D–H ؒ ؒ ؒ A
H ؒ ؒ ؒ A
D ؒ ؒ ؒ A
D–H ؒ ؒ ؒ A
C8–H8B ؒ ؒ ؒ O4
2.50
3.090(2)
2.994(2)
2.926(2)
3.70
118
108
114
130
135
C15–H15A ؒ ؒ ؒ O3
C16–H16B ؒ ؒ ؒ O2i
C8–H8A ؒ ؒ ؒ Cg2ii
C16–H16A ؒ ؒ ؒ Cg1iii
2.54
2.37
2.98
2.71
3.48
O(1)–C(1)–C(2)–O(2)
C(1)–C(2)–O(2)–C(7)
C(2)–O(2)–C(7)–C(8)
O(2)–C(7)–C(8)–O(3)
C(7)–C(8)–O(3)–C(9)
C(8)–O(3)–C(9)–C(10)
0.2(2)
O(3)–C(9)–C(10)–O(4)
C(9)–C(10)–O(4)–C(15)
C(10)–O(4)–C(15)–C(16)
O(4)–C(15)–C(16)–O(1)i
C(15)–C(16)–O(1)i–C(1)i
C(16)–O(1)i–C(1)i–C(2)i
Ϫ4.3(2)
58.9(2)
113.26(15)
Ϫ76.45(15)
165.25(12)
65.21(19)
Ϫ175.59(14)
Ϫ179.50(13)
69.93(15)
172.19(12)
67.90(18)
Cg1 represents the centroid of C1–6 and Cg2 the centroid of C9–14. Symmetry codes i 2 Ϫ x, 2 Ϫ y, Ϫz; ii x Ϫ 1, y, z; iii 1 Ϫ x, 2 Ϫ y, Ϫz.
Scheme 1 a. K2CO3, CH3CN; b. KI, K2CO3, DMF; c. Cl2CHOCH3, TiCl4, CH2Cl2; d. mCPBA, CH2Cl2; e. (C2H5)3N, CH3OH; f. TsOCH2CH2Cl,
K2CO3, acetone; g. KI, Cs2CO3, DMF.
the structures of Csϩ, Rbϩ and CH3CN complexes of tetra-
benzo-24-crown-8.10,12,13
listed in Table 1, but no edge–face arene, or π-stacking is
observed. This suggests that this crown conformation is likely a
low-energy conformation.
The conformation observed here is the same as previously
seen for the closely related tetrabenzo-24-crown-8.14 Like its
trimmed-down cousin, 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-
crown-8 experiences relatively short C–H ؒ ؒ ؒ O intramolecular
contact distances as a result of the crown ring’s collapse. Some
of these could be considered weak hydrogen bonds, although
the C–H ؒ ؒ ؒ O angles are generally quite small (Table 1).17
Unlike tetrabenzo-24-crown-8, the tetra-tert-butyl structure
presented here experiences very few intermolecular interactions
beyond London forces. Two C–H ؒ ؒ ؒ π hydrogen bonds are
Liquid–liquid distribution studies
Liquid–liquid distribution experiments using 4,5Ј,4Љ,5ٞ-tetra-
tert-butyltetrabenzo-24-crown-8 were carried out to examine
the question of the effect of alkyl substitution in comparison
with the bis(tert-octylbenzo)-24-crown-8 isomers. The effect of
varying the Csϩ ion concentration in the aqueous phase at con-
stant ligand concentration (10 mM in 1,2-DCE) on the Csϩ ion
810
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814

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Fig. 2 ORTEP representation with 50% probability ellipsoids of
4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8. Hydrogen atoms are
omitted for clarity. Symmetry code i 2 Ϫ x, 2 Ϫ y, Ϫz.
Fig. 4 Comparison of the calculated (solid line) and observed
(symbols) caesium distribution ratios as a function of equilibrium
aqueous-phase molarity of CsNO3 or CsClO4 employing a 10 mM solu-
tion of 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8 in 1,2-DCE at
25 ЊC.
4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8 into the aque-
ous phase was neglected because of the high lipophilicity of this
crown ether. To justify this assumption, the partition ratio from
water to octan-1-ol was estimated by the group contribution
method22,23 givinglogPw→oct = 13.2.Simpleionpartitioningfrom
water into 1,2-DCE was also neglected based on the results of
control distribution experiments using an organic phase con-
taining no crown ether. In the fitting, Pitzer parameters were
employed to estimate aqueous-phase activity coefficients,24
Hildebrand solubility parameters were employed to estimate
the neutral contribution to nonideality of species in the organic
phase,23,25 and the Debye–Hückel treatment was used to esti-
mate the electrostatic contribution to nonideality of ions in the
organic phase.21,24 All constants are thus corrected to infinite
dilution. Parameters used in SXLSQI modeling are summar-
ized in a table given in electronic supplementary information.
The solubility parameter and molar volume of the 4,5Ј,4Љ,5ٞ-
tetra-tert-butyltetrabenzo-24-crown-8 were estimated from the
group-contribution method.23 Excellent agreement between
measured and calculated values of DCs (Fig. 4) and statistical
agreement factors close to unity (Table 2) suggest that the
observed extraction behavior is explained well by the chosen
distribution model [based on simultaneous extraction processes
given by eqns. (1) and (2)].
Fig. 3 Stacking diagram for 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-
crown-8. Atoms are represented by circles and hydrogen atoms are
omitted for clarity.
distribution ratio between organic and aqueous phases is shown
in Fig. 4. In general, the values of the Csϩ ion distribution ratio
(DCs = [Cs]org/[Cs]aq) observed with ClO4Ϫ are much greater than
those obtained with NO3Ϫ, as expected since the more charge-
Ϫ
dispersed ClO4 has a less positive standard Gibbs energy of
partitioning between water and 1,2-DCE.18 As seen in Fig. 4,
the value of DCs remains relatively unchanged at lower concen-
trations of Csϩ. In view of the known behavior of similar
systems,7,10,19,20 it may be readily concluded that both CsNO3
and CsClO4 are extracted as dissociated ion pairs over a reason-
ably wide concentration range of complexed Csϩ ion in 1,2-
DCE. The onset of significant ion-pairing is indicated by the
observed rise in slope as the aqueous caesium salt concentration
increases. When significant loading of the extractant is
achieved, values of DCs decrease. This effect is especially
pronounced for CsClO4, which loads the crown ether at lower
aqueous concentrations of caesium salt due to its greater
extractability into the organic phase.
Examination of log Kex and log Kex values (Table 2) indicates
that perchlorate gives approximately three orders of magnitude
greater extractability of caesium than nitrate whether the anion
is ion paired or dissociated. This implies that the discrimination
of the anion by the caesium–crown complex cation is independ-
ent of the identity and relative location of the anion. In other
words, the cleft formed by the crown ether as it envelops Csϩ
has no ability to recognize nitrate versus perchlorate. The same
trend was observed with the isomeric bis(tert-octylbenzo)-
dibenzo-24-crown-8 ligands studied previously (Table 2). From
the tabulated values of log Kex and log Kex , constants corre-
sponding to the homogeneous processes of ion-pair dissoci-
ation (Kdiss) and complex formation (Kf) in 1,2-DCE may be
Distribution data were fitted using the program SXLSQI21
assuming simultaneous extraction of ion pairs and dissociated
ions to obtain the values of log Kex and log Kex corresponding
to the expected extractive processes given in eqns. (1) and (2).
Csϩ(aq) ϩ XϪ(aq) ϩ crown(org)
derived as described previously.7,10,20 on the basis of eqns. (3)
Ϫ
and (4). Taking the differences log Kex(NO3Ϫ) Ϫ log Kex(ClO4
)
Cs(crown)X(org) Kex (1)
Csϩ(aq) ϩ XϪ(aq) ϩ crown(org)
Cs(crown)X(org)
Cs(crown)ϩ(org) ϩ XϪ
Kdiss (3)
Kf (4)
(org)
Cs(crown)ϩ(org) ϩ XϪ
Kex (2)
(org)
Csϩ(org) ϩ crown(org)
Cs(crown)ϩ
(org)
Based on our previous studies of caesium complexation by
tetrabenzo-24-crown-8 ligands both in solution and in the solid
state,10,12 formation of only a 1 : 1 caesium–crown ether com-
plex was considered in the extraction model. Partitioning of
and log Kex (NO3Ϫ) Ϫ log Kex (ClO4Ϫ) yields the constants
corresponding to the exchange processes represented in
eqns. (5) and (6).10 Table 2 summarizes the log K values so
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814
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Table 2 Equilibrium constants for extractive reactions involving caesium complexes of tetrabenzo-24-crown-8 ligands in 1,2-dichloroethane at
25 ЊCa
log Kex
[eqn. (1)]
log Kex
[eqn. (2)]
log Kdiss
[eqn. (3)]
log Kf
[eqn. (4)]
log Kexch
[eqn. (5)]
log Kexch
[eqn. (6)]
Anion
σb
4,5Ј,4Љ,5Љ-Tetra-tert-butyltetrabenzo-24-crown-8c
Ϫ
NO3
0.54 0.03
Ϫ2.39 0.01
Ϫ2.93 0.03
Ϫ2.92 0.07
10.9 0.2
11.1 0.2
1.02
1.28
3.13 0.08
3.14 0.02
Ϫ
ClO4
3.67 0.07
0.75 0.02
4,4Љ-Di-tert-octyltetrabenzo-24-crown-8d
Ϫ
NO3
0.14
Ϫ2.76
Ϫ2.88
Ϫ2.98
10.5
10.6
3.09
3.00
Ϫ
ClO4
3.22
0.254
4,5Љ-Di-tert-octyltetrabenzo-24-crown-8d
Ϫ
NO3
0.30
Ϫ2.88
3.18
10.4
10.4
2.86
2.98
Ϫ
ClO4
3.16
0.104
Ϫ3.26
a In the experiments, 1,2-DCE solutions of crown ether were equilibrated with equal volumes of aqueous CsNO3 or CsClO4 solutions at 25 0.2 ЊC.
See text for definitions of each constant according to the indicated equation. Values of log Kex and log Kex were determined from the computer
modeling. All others were derived from them. Uncertainties listed represent the standard error of fitting obtained directly (log Kex and log Kex ) or as
propagated. The uncertainties for the values given for log Kf also include the uncertainties in the standard Gibbs energies of ion partitioning
obtained from the literature. b The goodness of fit for a particular model to the given data set is quantified by the agreement factor σ defined
¹
2

²
according to the least-squares criterion as σ = [Σwi(Yi Ϫ Yc,i) /(No Ϫ Np)] where Yi is the i-th experimentally observed quantity (i.e., DCs), Yc,i is the
corresponding quantity calculated from the model being tested, wi is the weighting factor defined as the reciprocal of the square of the estimated
uncertainty of Yi, No is the number of observations, and Np is the number of adjustable parameters (i.e., log K values). The value of σ will approach
unity when the error of fitting is equal to the estimated experimental error; values less than unity are interpreted as experimental precision being
better than the estimated precision. c This work. d Taken from ref. 10.
Ϫ
Cs(crown)NO3(org) ϩ ClO4
are slightly higher than those (10.5) obtained for caesium
complexes with 4,5Љ- and 4,4Љ-bis(tert-octylbenzo)dibenzo-24-
crown-8 (Table 2). This increase of the complex stability can be
readily attributed to higher binding strength of the tetra-tert-
butyltetrabenzo-24-crown-8 due to the increased number of
electron-donating alkyl substituents on the benzo groups.
4,5Ј,4Љ,5ٞ-Tetra-tert-butyltetrabenzo-24-crown-8 possesses four
tert-butylbenzo groups attached to the crown ether moiety,
whereas bis(tert-octylbenzo)dibenzo-24-crown-8 has two alkyl-
ated and two unsubstituted benzo groups. Subsequently,
electron density on the donor oxygen atoms of tetrakis(tert-
butylbenzo)-24-crown-8 is expected to be slightly higher and
results in stronger interaction with the cationic guest.
(aq)
Ϫ
Cs(crown)ClO4(org) ϩ NO3
Kexch (5)
(aq)
Ϫ
Ϫ
NO3 (org) ϩ ClO4
(aq)
Ϫ
Ϫ
ClO4 (org) ϩ NO3
Kexch (6)
(aq)
derived. All the constants in Table 2 refer to water-saturated
1,2-DCE as the solvent.
The results support the conclusion that the tendency towards
ion-pairing is low and that there is relative insensitivity to the
nature of the anion. Namely, values of log Kdiss are high and
nearly equal (Table 2), within observed standard deviations, for
both nitrate and perchlorate. Further, the observed values of
log Kdiss compare well with values calculated from ion-pair
theory.10 Such dissociation behavior can be explained by form-
ation of ligand-separated ion pairs in 1,2-DCE with large
interionic distance determined mainly by the bulky caesium–
crown complex. This dissociation behavior is consistent with
that previously observed for bis(tert-octylbenzo)dibenzo-24-
crown-8 ligands in an analogous distribution study (Table 2),
and with X-ray crystal structure results in which 1,2-DCE
coordinates to caesium(tetrabenzo-24-crown-8) in preference to
nitrate.10 A similar solution structure is proposed for the
4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-8–caesium com-
plex in which the anion is excluded from the inner coordination
sphere.
It may be seen from Table 2 that the observed values of log
Kexch and log Kexch for ion pairs and dissociated ions are
nearly equal within observed standard deviations. This result
confirms that the caesium–crown complex does not exhibit
any specific interaction with the counterion in 1,2-DCE and
is unable to affect the natural discrimination between anions
by the solvent. The same effect was demonstrated for the
distribution studies of CsNO3 and CsClO4 using 4,5Љ- and
4,4Љ-di-tert-octyltetrabenzo-24-crown-8 extractants in 1,2-
DCE (Table 2).
Conclusions
4,5Ј,4Љ,5ٞ-Tetra-tert-butyltetrabenzo-24-crown-8 was syn-
thesized by a route that ensures an isomerically pure product
without the necessity for isomer separation. The structure of
the crown ether was characterized by single crystal X-ray dif-
fraction, showing that the conformation of the crown ether
is unaffected by addition of four tert-butyl groups. Liquid–
liquid distribution studies using 1,2-DCE as a diluent revealed
that this ligand is a significantly stronger caesium extractant
compared with the closely related 4,5Љ- and 4,4Љ-di-tert-octyl-
tetrabenzo-24-crown-8 ethers, but the speciation behavior is
otherwise quite similar. Improved solubility, strong caesium
binding, low tendency towards ion-pairing, and a structure
that excludes direct ion-pairing make this ligand an attractive
candidate for caesium-selective complexation and transport in
biphasic liquid systems.
Experimental
General
All reagents were obtained from commercial suppliers and used
as received. The 3-chloroperoxybenzoic acid obtained from
Aldrich Chemical Co. was stated to be of 57–86% purity. The
caesium nitrate and caesium perchlorate (Johnson Matthey,
99.9%) salts were dried at 110 ЊC for at least 48 hours before use.
The values of log Kf derived from the extraction con-
stants obtained using CsNO3 and CsClO4 agree, reflecting the
expected independence of the complex stability constant on the
nature of the counterion. The values of log Kf observed here
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HPLC grade 1,2-dichloroethane (Sigma–Aldrich) was purified
by passage through activated alumina followed by distillation.
The radiotracer 137Cs was received as 137CsCl in 1 M HCl
(Amersham) and converted to a neutral aqueous solution of
137CsNO3 by ion exchange as described previously.20 Aqueous
solutions were prepared using distilled deionized water.
bis(2Ј-hydroxy-4Ј-tert-butylphenoxy)ethane (12 g, 67%). Mp
129–130 ЊC; δH 7.03 (4 H, m, HAr), 6.87 (2 H, s, HAr), 5.77 (2 H,
s, OH), 4.38 (4 H, s, CH2), 1.30 (18 H, s, CH3); δC 146.3, 145.8,
143.3, 116.9, 113.1, 112.9, 68.5, 34.5, 31.6.
1,2-Bis[2Ј-(2Љ-chloroethoxy)-4Ј-tert-butylphenoxy]ethane.
A
solution of 1,2-bis(2Ј-hydroxy-4Ј-tert-butylphenoxy)ethane (8.0
g, 20 mmol), 2-chloroethyl toluene-p-sulfonate (14 g, 60 mmol),
and K2CO3 (15 g, 100 mmol) in acetone (150 mL) was heated at
reflux overnight (~18 h), filtered, and evaporated in vacuo. The
residue was dissolved in dichloromethane (50 mL), washed with
water (50 mL), and passed through a silica gel (100 g) column.
Elution with 5% ethyl acetate in hexanes gave 1,2-bis[2Ј-(2Ј-
chloroethoxy)-4Ј-tert-butylphenoxy]ethane (4 g, 43%). Mp
173–174 ЊC; δH 7.01–6.92 (6 H, m, HAr), 4.37 (4 H, s, CH2), 4.29
(4 H, t, J 6.0, CH2), 3.79 (4 H, t, J 6.0, CH2), 1.31 (18 H, s, CH3);
δC 147.9, 147.3, 145.4, 119.6, 115.1, 114.9, 70.6, 68.4, 42.5, 34.5,
31.7.
Synthesis
1,2-Bis(p-tert-butylphenoxy)ethane. A suspension of p-tert-
butylphenol (50 g, 0.33 mol), 2-chloroethyl toluene-p-sulfonate
(40 g, 0.17 mol), and K2CO3 (50 g, 0.36 mol) in acetonitrile (150
mL) was heated at reflux overnight (~18 h). Following addition
of KI (1 g), K2CO3 (50 g, 0.36 mol) and DMF (300 mL), the
mixture was heated at 80 ЊC for 24 hours, and then poured into
water (500 mL) and allowed to cool to room temperature. The
solid product was filtered, washed twice with 1 M NaOH (200
mL each), and recrystallized from methanol to obtain 37 g (0.11
mol, 67%) of 1,2-bis(p-tert-butylphenoxy)ethane. Mp 87–88 ЊC;
δH 7.42 (4 H, d, J 8.9, HAr), 7.00 (4 H, d, J 8.9, HAr), 4.39 (4 H, s,
CH2), 1.41 (18 H, s, CH3); δC 156.5, 143.8, 126.4, 114.3, 66.6,
34.2, 31.7.
4,5Ј,4Љ,5ٞ-Tetra-tert-butyltetrabenzo-24-crown-8.|| A mixture
of 1,2-bis(2Ј-hydroxy-4Ј-tert-butylphenoxy)ethane (3.5 g,
9.8 mmol), 1,2-bis[2Ј-(2Ј-chloroethoxy)-4Ј-tert-butylphenoxy]-
ethane (4.0 g, 8.7 mmol), KI (~0.5 g), and Cs2CO3 (15 g, 46
mmol) in N,N-dimethylformamide (150 mL) was stirred at
65 ЊC overnight (~18 h). The mixture was evaporated in vacuo,
dissolved in dichloromethane (100 mL) and washed once with
1 M HCl (100 mL) and three times with distilled water. The
organic phase was dried over Na2SO4, passed through dry silica
gel to remove phenolic impurities, and evaporated in vacuo to
obtain 4.8 g (72%) of crude product. Recrystallization from
hexanes gave pure 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-
crown-8 (3.5 g, 52%). Mp 173–174 ЊC; δH 7.05 (4 H, s, HAr), 6.95
(4 H, d, J 8.3, HAr), 6.85 (4 H, d, J 8.3, HAr), 4.35 (8 H, s, CH2),
4.30 (8 H, s, CH2), 1.29 (36 H, s, CH3); δC 148.4, 147.6, 145.1,
119.1, 115.6, 114.6, 69.7, 68.1, 34.4, 31.6.
1,2-Bis(2Ј-formyl-4Ј-tert-butylphenoxy)ethane. A solution of
TiCl4 (25 mL, 0.23 mol) in dichloromethane (100 mL) was
added dropwise over 30 min to an ice-cold solution of 1,2-bis-
(p-tert-butylphenoxy)ethane (33 g, 0.10 mol) and α,α-dichloro-
methyl methyl ether (25 mL, 0.27 mol) in dichloromethane (350
mL) under argon. Additional aliquots of TiCl4 solution (3 × 5
mL, 0.14 mol) and α,α-dichloromethyl methyl ether (2 × 5 mL,
0.11 mol) were added alternately at 5 minute intervals. Ice-
cold 1 M HCl (500 mL) was then added (cautiously), the phases
were separated, and the organic phase washed sequentially with
1 M HCl (100 mL), saturated NaCl (200 mL), and saturated
NaHCO3 (200 mL) solutions. The organic phase was dried over
Na2SO4 and evaporated in vacuo¶ to obtain 39 g (0.10 mol,
100%) of 1,2-bis(2Ј-formyl-4Ј-tert-butylphenoxy)ethane as an
oil of sufficient purity for the next reaction. Recrystallization
from cyclohexane gave pure 1,2-bis(2Ј-formyl-4Ј-tert-butyl-
phenoxy)ethane (27 g, 71%). δH 10.35 (2 H, s, COH), 7.79 (2 H,
d, J 2.6, HAr), 7.53 (2 H, dd, J 2.6, 8.7, HAr), 6.92 (2 H, d, J 8.7,
HAr), 4.43 (4 H, s, CH2), 1.25 (18 H, s, CH3); δC 189.9, 159.0,
144.6, 133.3, 125.3, 124.8, 112.8, 67.4, 34.5, 31.5.
Crystal structure determination of 4,5Ј,4Љ,5ٞ-tetra-tert-butyl-
tetrabenzo-24-crown-8**
The crystal structure reported here was determined by single
crystal X-ray diffraction methods. Data collection and treat-
ment of data were the same as previously described.26
Crystal data. C48H64O8, M = 769.0, triclinic, a = 5.8214(8),
b = 10.7769(19), c = 17.258(3) Å, α = 76.083(14), β = 82.120(12),
1,2-Bis(2Ј-formyloxy-4Ј-tert-butylphenoxy)ethane. Solid 3-
chloroperoxybenzoic acid (25 g of 57–86% purity, ~82–124
mmol) was added in 5 g portions at 5 min intervals to a
solution of 1,2-bis(2Ј-formyl-4Ј-tert-butylphenoxy)ethane (20
g, 52 mmol) in dichloromethane (150 mL). The resulting
mixture was cooled in an ice bath and filtered. The filtrate
was washed sequentially with 10% Na2SO3 (100 mL), 1 M
NaOH (100 mL), and saturated NaHCO3 (100 mL) solutions,
dried over Na2SO4 and evaporated in vacuo to obtain 21 g
(98%) of crude 1,2-bis(2Ј-formyloxy-4Ј-tert-butylphenoxy)-
ethane, which was used without further purification or
characterization.
3
¯
γ = 89.600(13)Њ, U = 1040.6(3) Å , T = 100 K, space group P1
(no. 2), Z = 1, µ(Mo-Kα) = 0.082 mmϪ1, 5827 reflections
collected, 4533 independent (Rint = 0.023) which were used in all
calculations. The final wR(F2) was 0.137 (all data).
Liquid–liquid distribution experiments
Procedures. Equal volumes (0.5 mL each) of organic phase
(10 mM solution of 4,5Ј,4Љ,5ٞ-tetra-tert-butyltetrabenzo-24-
crown-8 in 1,2-DCE) and aqueous phase (aqueous solution of
CsNO3 or CsClO4 spiked with 137Cs tracer) were equilibrated in
2 mL vials by repeated inversion on a Glass-Col laboratory
rotator in a thermostated air box at 25 0.2 ЊC for 2 h. Sub-
sequently, the samples were centrifuged at 4000 rpm for 3 min.
To determine the Csϩ ion distribution ratio, a fraction of each
phase was removed and the activity of 137Cs measured by
gamma-radiometric techniques. The initial caesium salt con-
1,2-Bis(2Ј-hydroxy-4Ј-tert-butylphenoxy)ethane. A solution
of crude 1,2-bis(2Ј-formyloxy-4Ј-tert-butylphenoxy)ethane (21
g, 51 mmol) in methanol (80 mL) containing triethylamine
(1 mL) was heated at 55 ЊC for 20 minutes and then evaporated
in vacuo. The residue was dissolved in dichloromethane (100
mL), washed sequentially with 1 M HCl (50 mL), saturated
NaCl (50 mL), and saturated NaHCO3 (50 mL) solutions, dried
over Na2SO4 and evaporated in vacuo to obtain 18 g (99%) of
a white solid. Recrystallization from methanol gave pure 1,2-
centration was varied in the ranges of 3 × 10Ϫ4 to 9 × 10Ϫ1
M
and 3 × 10Ϫ5 to 6.8 × 10Ϫ2 M for CsNO3 and CsClO4 systems,
respectively. Duplicate experiments were performed for every
|| The IUPAC name for 4,5Ј,5Љ,5ٞ-tetra-tert-butyltetrabenzo-24-crown-
8
is 14,64,115,164-tetra-tert-butyl-2,5,7,10,12,15,17,20-octaoxa-1,6,11,
¶ Note: the dichloromethane solution of 1,2-bis(2Ј-formyl-4Ј-tert-
butylphenoxy)ethane may be used directly for the next reaction without
evaporation or isolation.
16(1,2)-tetrabenzenacycloicosaphane.
** CCDCreferencenumber152308.Seehttp://www.rsc.org/suppdata/p2/
b008603g/ for crystallographic data in CIF or other electronic format.
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814
813

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ing (see below), experimental precision for each data point was
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ations, the precision was taken to be constant at 3% for all
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5 W. J. McDowell, Sep. Sci. Technol., 1988, 23, 1251; L. Tusek,
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type detector (Packard Cobra Quantum Model 5003). Caesium
distribution ratios DCs, defined as the ratio of total organic- and
aqueous-phase caesium concentrations [Cs]org/[Cs]aq, were
obtained directly as the ratio of background-corrected count
rates per mL. In control experiments, no extraction within
background was observed for 1,2-DCE (without crown ether)
equilibrated with aqueous CsNO3 or CsClO4 solutions under
the experimental conditions employed.
9 R. A. Sachleben, J. C. Bryan, J. M. Lavis, C. M. Starks and
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10 T. G. Levitskaia, J. C. Bryan, R. A. Sachleben, J. D. Lamb and
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of Pitzer parameters for aqueous ions, the Hildebrand–Scott
treatment for non-ionic effects in the organic phase, and the
Debye–Hückel treatment for electrostatic effects in the organic
phase.21,23–25,27,28 All constants are thus corrected to infinite dilu-
tion. The program converts the molar concentration to the
molality scale employed in the Pitzer treatment using Masson
coefficients.29 The solubility parameters and molar volumes of
the ligand were estimated from group contributions.23 Param-
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electronic supplementary information.
11 A. H. Bond, R. Chiarizia, V. J. Huber, M. L. Dietz, A. W. Herlinger
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12 J. C. Bryan, K. Kavallieratos and R. A. Sachleben, Inorg. Chem.,
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13 G. G. Talanova, N. S. A. Elkarim, V. S. Talanov, R. E. Hanes, Jr.,
H.-S. Hwang, R. A. Bartsch and R. D. Rogers, J. Am. Chem. Soc.,
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14 J. C. Bryan, G. J. Bunick and R. A. Sachleben, Acta Crystallogr.,
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15 J. C. Bryan, J. M. Lavis, B. P. Hay and R. A. Sachleben, Acta
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16 J. Dale, Isr. J. Chem., 1980, 20, 3.
17 T. Steiner, Crystallogr. Rev., 1996, 6, 1.
18 J. Czapkiewicz and B. Czapkiewicz-Tutaj, J. Chem. Soc., Faraday
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19 I. M. Kolthoff, M. K. Chantooni and W.-J. Wang, J. Chem. Eng.
Data, 1993, 38, 556.
20 T. J. Haverlock, P. V. Bonnesen, R. A. Sachleben and B. A. Moyer,
J. Inclusion Phenom. Macrocyclic Chem., 2000, 36, 21.
21 C. F. Baes, Jr., B. A. Moyer, G. N. Case and F. I. Case, Sep. Sci.
Technol., 1990, 25, 1675; C. F. Baes, Jr., SXLSQI, A Program for
Modeling Solvent Extraction Systems, Report ORNL/TM-13604,
Oak Ridge National Laboratory, 1998; C. F. Baes, Jr., Solvent Extr.
Ion Exch., in the press.
22 B. A. Moyer, in Comprehensive Supramolecular Chemistry, ed. G. W.
Gokel, Pergamon, Oxford, 1996, vol. 1, p. 377.
23 A. F. M. Barton, CRC Handbook of Solubility Parameters and Other
Cohesion Parameters, CRC Press, Boca Raton, Florida, 1991.
24 K. S. Pitzer, in Activity Coefficients in Electrolyte Solutions, ed. K. S.
Pitzer, CRC Press, Boca Raton, Florida, 1991, ch. 3.
25 E. J. Henley and J. D. Seader, Equilibrium-Stage Separation
Operation in Chemical Engineering; John Wiley & Sons, New York,
1981.
26 J. C. Bryan, R. A. Sachleben, J. M. Lavis, M. C. Davis, J. H. Burns
and B. P. Hay, Inorg. Chem., 1998, 37, 2749.
Acknowledgements
This research was sponsored by the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, US Department of Energy, under contract DE-AC05-
00OR22725 with Oak Ridge National Laboratory, managed
and operated by UT-Battelle, LLC.
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814
J. Chem. Soc., Perkin Trans. 2, 2001, 808–814