Substituent effects on ion complexation of para-tert-butylcalix[4]arene esters

Article abstract of DOI:10.1002/poc.1076

Phenoxy-carboxy-methoxy-p-tert-butylcalix[4]arene esters were synthesized in order to evaluate the role of electronic parameters on the complexation of alkaline metal cations. Extraction constants of metal picrates to organic phase were determined. Plots

Full text of DOI:10.1002/poc.1076

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 765–770
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1076
Substituent effects on ion complexation
of para-tert-butylcalix[4]arene estersy
1
1
1
*
´
Marcio Lazzarotto, Marjarie Marrie Francisco, Francine Furtado Nachtigall and
Marcelo Lazzarotto2
1
´
ˆ
Departamento de Quımica Organica, UFRGS, Av. Bento Gonc¸alves 9500, Campus do Vale, Porto Alegre, Rio Grande do Sul,
CEP 91501-970 C.P. 15003, Brasil
2Embrapa Uva e Vinho, R. Livramento, 515, Bento Gonc¸alves, RS, CEP 95.700-000, C.P. 130, Brasil
Received 18 November 2005; revised 28 December 2005; accepted 8 March 2006
ABSTRACT: Phenoxy-carboxy-methoxy-p-tert-butylcalix[4]arene esters were synthesized in order to evaluate the
role of electronic parameters on the complexation of alkaline metal cations. Extraction constants of metal picrates to
organic phase were determined. Plots of log (KR/KH) against Hammett s and sþp gave good linear correlations. The best
correlations with s were obtained for Kþ and Rbþ, while the best correlations with sþp were obtained for Liþ and Naþ.
All Hammett plots gave a straight descending line, which is consistent with a dependence of the electronic density on
—
the C O. Treatment of data using the Yukawa–Tsuno equation revealed a variation in the contribution of resonance in
—
the complexation of alkaline metal ions, which is maximum for Naþ and minimum for Rbþ. Electronic parameters
were calculated for a related acyclic model structure and only the HOMO energy showed a good correlation with log
(KR/KH). Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: supramolecular chemistry; Hammett’s correlation; calixarenes; cation complexation
INTRODUCTION
complex formation occurs through the reorganization of
the host to fit the sphere around the cation, with loss of
rotational and vibrational modes of the species.8 The
structure of the sodium complex with the O-acetyl
calix[4]arene ethyl ester has been shown to have the Naþ
Investigations into free-energy linear relationships allow an
understanding of the molecular parameters involved in
dynamic chemical processes. Hammett’s plot is the most
useful relationship for the evaluation of the role of the
electronic contribution, when sterical crowding of sub-
stituents does not greatly affect the magnitude of the
constants. Host–guest chemistry usually does not fit this
requirement, because complementary structures are needed
for host–guest interactions and only a few studies dealing
with Hammett correlations in supramolecular chemistry1–4
have been reported, with fair success, despite the number of
variables that influence the degree of complexation.
The suitability of O-acetyl esters of calixarenes for the
extraction of alkaline metal cations is well known.5 It has
been found that calix[4]arene in cone conformation is
selective for sodium, whereas the selectivity of calix[4]-
arene in the 1,3 alternate conformation is shifted to
potassium,6 and calix[6]arene complexes cesium selec-
tively.7 The driving force for the formation of the metal-
calixarene complex is the electrostatic interaction of the
metal cation with the lone-pairs of the oxygen atoms. The
cation coordinated to the eight oxygen atoms of
9
OCH C O groups, which for the uncomplexed
—
—
2
metal-receptor span out of the center-symmetric axis of
the calixarene, as non-ordered chains. The association of
alkaline metals with calix[4]arene ethers brings into
proximity the four arms containing the O-acetyl ester
—
—
moiety and decreases the mobility of the OCH C
2
O
group (Scheme 1).
The hard–soft character of the groups present is
another factor that determines the metal selectivity,
shifting it from the sodium cation to the silver cation,
—
—
when the C O bond is replaced by a C S bond in
—
—
calix[4]arene esters.10 The complex formation with alkali
metal ions and substituted phenolic esters of carboxy–
methoxy calixarene is a potential process for the
application of Hammett relationships, since para-sub-
stituents can modulate the ability of carbonyl groups to
interact by ion-dipole attraction with the metal ion.
´
*Correspondence to: M. Lazzarotto, Departamento de Quımica
Organica, UFRGS, Av. Bento Gonc¸alves 9500, Campus do Vale, Porto
EXPERIMENTAL
ˆ
Alegre, Rio Grande do Sul, CEP 91501-970, C.P. 15003, Brasil.
E-mail: marcio@iq.ufrgs.br
Phenol esters 3–8 were obtained from the reaction of the
acid chloride 2 and the corresponding phenolates
(Scheme 2). The yields were only moderate (30–60%),
ySelected article presented at the Eighth Latin American Conference on
Physical Organic Chemistry (CLAFQ0–8), 9–14 October 2005, Flori-
anopolis, Brazil.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 765–770

766
M. LAZZAROTTO ET AL.
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
t-Bu
M+
+
.pic-
Heq.
Heq.
O
O
O
O
Hax.
H
axO.
-
O
O
O
M+ .pic-
O
M+
O
O
O
O
O
O
O
O
O
O
O
R
O
O
R
O
R
R
R
R
O
R
O2N
NO2
R
NO2
Scheme 1. Equilibrium of cation complexation
obtained through the partial substitution of the chloride
during the workup process, yielding some hydrolysis
products. The synthesis of 2 has been previously
described7 and the general procedure for the synthesis
of the phenol derivatives 3–8 is given below. The acid
chloride 2 was freshly prepared by the treatment of
200 mg (0.23 mmol) of tetrakis(hydroxy-carboxy-meth-
oxy)-p-tert-butylcalix[4]arene with SOCl2, and dissolved
in 5 mL of dichloromethane. This solution was added
dropwise to a solution of 1.80 mmol of the corresponding
phenol, 230 mL (1.78 mmol) of triethylamine and 200 mg
(1.78 mmol) of potassium tert-butoxide in 10 mL of dry
acetonitrile, and the mixture was left to stand overnight
under magnetic stirring. The organic phase was washed-
with 0.1 mol/L NaOH and water, dried over MgSO4,
and evaporated under reduced pressure. With addition
of methanol, the product crystallized after some
minutes. Recrystallization from dichloromethane/
methanol yielded the pure product.
NMR analyses were carried on Varian-200 MHz and
Bruker-300 MHz spectrometers, and IR spectra were
collected with
a Perkin–Elmer spectrophotometer.
Extraction experiments were carried in water/dichlor-
omethane in a closed tube, stirring magnetically for
60 min after which the samples were allowed to stand
overnight at 10 8C. The nitro ester 8 did not extract Liþ,
Rbþ, and Csþ picrates. The absorbance of picrate in the
organic phase was determined by spectrophotometry at
375–380 nm, and the association constants were deter-
mined considering the equilibrium below, as reported
by Ungaro:11
Mþaq þ Piþaq þ Lorg Ð ½LMþ:Piꢀꢁorg
where Mþaq and Piaꢀq represent the alkali cation and
picrate anion in aqueous phase and Lorg and [LMþ.Pi]org
are, respectively, the ligand and ligand-metal complex in
dichloromethane. The equation that describes the
b
a
[ ]
[ ]
4
4
[ ]
O
O
4
O
O
Cl
O
O
O
OH
2
1
;b) RPhOH, t-BuOK, Et3N, CH2Cl2
a)SOCl2
R
3) R= OMe; 4) R= C(CH3)3; 5) R= CH3; 6) R= H; 7) R= Cl; 8) R= NO2
3 - 8
Scheme 2. Synthesis of phenol esters from p-t-butyl-calix[4]arene tetraacid 1
Copyright # 2007 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2006; 19: 765–770

ION COMPLEXATION OF para-tert-BUTYLCALIX[4]ARENE ESTERS
767
equilibrium is:
(s, OCH2, 8 H), 6.70 (d, J 9 Hz arom. C—H, 4 H), 7.17 (s,
arom. CH, 8 H), 7.28 (d, J 9 Hz arom. C—H, 4 H).
½LMþ ꢂ Piꢀorg
ꢁ
p-Methyl-phenyl ester (5). NMR-1H (CDCl3) NMR
d 1.10 [s, C(CH3)3, 36H), 1.63 (s, ArCH3, 12 H), 3.26 [d, J
14 Hz, ax. PhCH2Ph, 4 H), 4.80 (d, J 14 Hz, eq. PhCH2Ph,
4 H), 5.06 (s, OCH2, 8 H), 6.82 (s, arom. CH, 8 H), 6.86
(d, J 9 Hz arom. C—H, 8 H), 7.04 (d, J 9 Hz arom. C—H,
8 H). NMR 13C (CDCl3): 169.24, 153.06, 148.14, 145.74,
135.06, 133.48, 129.73, 125.46, 121.35, 76.58, 71.27,
33.86, 31.97, 31.35, 20.85; elem. anal.: C ¼ 74.66;
H ¼ 6.99, calc. for C80H88O12.3/4CH2Cl2: C ¼ 74.31;
H ¼ 6.99.
Ke ¼
(1)
g2ꢃ½Mþaqꢁ½Piaꢀqꢁf½Lorgꢁo ꢀ ½LMþ ꢂ Piꢀorgꢁg
where ½Lorgꢁo ¼ ½Piꢀꢁo ¼ ½Piaꢀqꢁ þ ½Pioꢀrgꢁ ¼ 1:25 ꢄ 10ꢀ4
mol ꢂ Lꢀ1 and [Mþaq] ¼ 1.0 ꢄ 10ꢀ2 mol ꢂ Lꢀ1; the activity
coefficient of the ions was determined as g ¼ 0.8, and in
order to determine [LMþ ꢂ Pi]org, ePiꢀ ¼ 1.5 ꢄ 104 was
used.
Quantum chemical calculations
Phenyl ester (6). NMR-1H (CDCl3) 1.11 (s, C(CH3)3,
36 H), 3.27 (d, J 14 Hz, ax. PhCH2Ph, 4 H), 4.88 (d, J
14 Hz, eq. PhCH2Ph, 4 H), 5.13 (s, OCH2, 8 H), 6.84 (s,
arom. CH, 8 H), 6.97 (d, J 9 Hz arom. C—H, 8 H), 7.15
(m, J 9 Hz arom. C—H, 8 H), 7.25 (d, arom. C–H, 4 H).;
NMR 13C (CDCl3): 168.98, 152.95, 150.36, 145.58,
133.49, 129.27, 125.99, 129.57, 121.69, 71.24, 33.93,
31.63, 31.41; elem. anal.: C ¼ 74.49; H ¼ 6.38 calc. for
C76H80O12.1/2CH2Cl2: C ¼ 74.83; H ¼ 6.65.
p-Chloro-phenyl ester (7). NMR-1H (CDCl3) 1.11 (s,
C(CH3)3, 36 H), 3.27 (d, J 14 Hz, ax. PhCH2Ph, 4 H), 4.90
(d, J 14 Hz, eq. PhCH2Ph, 4 H), 5.08 (s, OCH2, 8 H), 6.84
(s, arom. CH, 8 H), 6.87 (d, J 9 Hz arom. C—H, 8 H), 7.21
(d, J 9 Hz arom. C—H, 8 H). NMR 13C (CDCl3): 168.65,
152.71, 148.71, 145.84, 133.34, 131.10, 129.33, 125.57,
122.99, 71.04, 31.80, 31.34; elem. anal.: C ¼ 68.86;
H ¼ 5.79, calc. for C76H76Cl4O12: C ¼ 68.86; H ¼ 5.38.
p-Nitro-ester (8). NMR-1H (CDCl3) 1.12 (s, C(CH3)3,
36 H), 3.31 (d, J 14 Hz, ax. PhCH2Ph, 4 H), 4.87 (d, J
14 Hz, eq. PhCH2Ph, 4 H), 5.14 (s, OCH2, 8 H), 6.86 (s,
arom. CH, 8 H), 7.14 (d, J 9 Hz arom. C—H, 8 H), 8.11 (d,
J 9 Hz arom. C—H, 8 H). NMR 13C (CDCl3): 190.07,
170.05, 168.19, 162.15, 154.76, 152.47, 125.71, 125.02,
122.35, 71.02, 31.80, 31.36; elem. anal.: C ¼ 65.77;
H ¼ 5.61; N ¼ 3.75 calc. for C76H76N4O20.1/3CH2Cl2:
C ¼ 68.86; H ¼ 5.38; N ¼ 4.02.
The semi-empirical program package MOPAC 6.0 was
used in all calculations. For each compound, compu-
tations were carried out with both the AM112 and the
PM313 parameterizations. Input files were conveniently
generated and before being processed the output files
were visualized with RasMol 2.7.1 software. The
molecular structures obtained in this way were used in
a configuration interaction calculation to compute
electron density on O1, O2, O3, and C1, HOMO and
LUMO energies.
p-Methoxy-phenyl ester (3). NMR-1H (CDCl3) 1.10
(s, C(CH3)3, 36 H), 3.29 (d, J 14 Hz, ax. PhCH2Ph, 4 H),
3.76 (s, OCH3, 12 H), 4.96 (d, J 14 Hz, eq. PhCH2Ph,
4 H), 5.09 (s, OCH2, 8 H), 6.74 (d, J 9 Hz arom. CH, 4 H),
6.83 (s, arom. CH, 8 H), 6.90 (d, J 9 Hz arom. C–H, 8 H);
NMR 13C (CDCl3) 169.28, 157.00, 152.98, 145.50,
143.86, 133.46, 125.46, 122.49, 114.24, 77.76, 71.21,
55.49, 33.87, 31.92, 31.37, 30.97; elem. anal.: C ¼ 72.41;
H ¼ 6.60, calc. for C80H88O16.1/3CH2Cl2: C ¼ 72.34;
H ¼ 6.70.
(3)-Naþ picꢀ. NMR-1H (CDCl3) 1.25 (s, C(CH3)3,
36 H), 3.49 (d, J 12 Hz, ax. PhCH2Ph, 4 H), 3.85
(s, OCH3, 12 H), 4.26 (d, 14 Hz, eq. PhCH2Ph, 4 H),
4.67 (s, OCH2, 8 H), 6.43 (d, J 9 Hz arom. C–H, 4 H), 6.73
(s, arom. CH, 8 H), 7.17 (d, J 9 Hz arom. C–H, 8 H).
p-tert-Butyl-phenyl ester (4). NMR-1H (CDCl3)
1.10 (s, C(CH3)3, 36 H), 1.28 (s, C(CH3)3, 36 H), 3.30
(d, J 14 Hz, ax. PhCH2Ph, 4 H), 4.96 (d, 14 Hz,
eq. PhCH2Ph, 4 H), 5.12 (s, OCH2, 8 H), 6.83 (s, arom.
CH, 8 H), 6.92 (d, J 9 Hz arom. C—H, 4 H), 7.26 (d, J
9 Hz arom. C—H, 4 H), NMR 13C (CDCl3): 169.24,
153.00, 148.28, 148.01, 145.47, 133.52, 126.10, 125.46,
120.98, 71.03, 34.42, 33.90, 32.03, 31.42, elem. anal.:
C ¼ 77.28; H ¼ 7.87, calc. for C92H112O12.1/3CH2Cl2:
C ¼ 77.11; H ¼ 7.90.
RESULTS AND DISCUSSION
Extraction data show that phenolic esters 3–8 are
selective for sodium, as is known for alkyl esters of
calix[4]arenes, due to the correct fit of the ionic radii with
the size of the cavity.14 The selectivity decreases as the
electron withdrawing ability of the group at para position
increases. The best receptor is compound 3, for which the
percentage extraction (%E) of Naþ found was 85% and
the selectivity expressed in terms of KNaþ =KKþ is 150, as
reported in Table 1. Such values are not the best possible,
when compared with the ethyl ester, but fall within the
range of other alkyl esters.
(4)-Naþ picꢀ. NMR-1H (CDCl3) 1.16 (s, C(CH3)3,
36 H), 1.29 (s, C(CH3)3, 36 H), 3.52 (d, J 13 Hz, ax.
PhCH2Ph, 4 H), 4.29 (d, J 13 Hz, eq. PhCH2Ph, 4 H), 4.70
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 765–770

768
M. LAZZAROTTO ET AL.
Table 1. Percentage extraction (%E), extraction constants
(Ke) and selectivity factors (KNa/KM) in extraction equilibria of
alkali metal picrates from H2O to CH2Cl2 in the presence of
compound 3
O
O
O
O
O
M+
M
M
M
.
..
O
..
O
.
..
+
O
+
O
O
+
O
O
..
Metal ion
%E
Ke (ꢄ10ꢀ5 Mꢀ2
)
KNa/KM
Liþþ
12
85
18
11
7
1.87
497
3.26
1.80
1.07
265
R
R
R
R
I
II
III
IV
Na
Kþ
Rbþþ
Cs
152
275
463
Scheme 3. Resonance structures of the metal complex
r are reported in Table 3. The values of rYT > 1 found for
Naþ and Liþ show that even sþ underestimates the
contribution of resonance on complexation. It becomes
less important for Kþ and Csþ, while the complexation of
Rbþ correlates only to s.
Plots of log (KR/KH) with s and sþ parameters, showed
straight lines with negative slopes for all metals, and the
values of r are listed in Table 2. The plots using these
Hammett parameters are useful to gain an understanding
of the degree of electronic contribution of the para-group
to the positive charge of the metal cation (Figs. 1 and 2).
The quality of the fittiþngs, meaþsured by the correlation
factor r, for Liþ and Na and Cs were better using sþp ,
whereas for Kþ and Rbþ the correlations were better
using the parameter s. The best correlations with sþ for
Liþ and Naþ indicate that the para-group acts through
resonance with a positive charge in these cases. This
suggests a significant contribution of the mesomeric form
IV (Scheme 3) for Liþ and Naþ complexes, where there is
a significant electronic interaction between the ion and
the n and p electrons of the ligand, with a partial
development of positive charge located on the oxygen
directly linked to the aromatic ring.
K
log
¼ r½s þ rYTðsþ ꢀ sÞꢁ
(2)
KH
Semi-empirical calculations of parameters that could
be related to the complexation, for example, electron
density on O1, O2, O3 and C1, HOMO and LUMO
energies, were obtained using AM1 and PM3 Hamilto-
nians, using the structure shown in Scheme 4 as a model,
in syn-like and anti-like conformations.
Initially, plots of log (KR/KH) with the electronic
parameters were constructed for the Naþ ion, which
showed the highest variation among the values of log (KR/
KH) and, thus, is more sensitive to the variation of the
structure, and the results of the correlations are listed in
The Yukawa–Tsuno equation15 allows a numeric
evaluation of the contribution of resonance of the para
group, expressed in terms of factor rYT. Values of rYT and
—
Table 4. The HOMO (n C O) energies exhibit fair to
—
good correlations for both AM1 and PM3 Hamiltonians,
the best result being obtained using AM1 on the anti
conformer. The HOMO parameter was therefore chosen
for the plots of log (KR/KH) for all metals, and the
Table 2. Values of the slopes (r) and correlation factors (r)
for the plots of log (KR/KH) versus sþ and s for the extraction
experiments
Table 3. Values of rYT and r obtained through the Yukawa-
Tsuno equation
Liþ
Naþ
Kþ
Rbþ
Csþ
rþ(sþp )
rþ(sþp )
Liþ
Naþ
Kþ
Rbþ
Csþ
ꢀ0.60
0.97
ꢀ1.63
0.98
ꢀ1.09
ꢀ0.40
ꢀ0.39
0.98
0.95
0.84
rYT
r
1.3
ꢀ0.52
1.9
ꢀ1.31
0.2
ꢀ1.06
0.0
ꢀ0.81
0.7
ꢀ0.46
r (s)
r (s)
ꢀ0.90
ꢀ1.96
ꢀ1.49
ꢀ0.81
ꢀ0.64
0.84
0.89
0.99
0.97
0.92
O1
O3
C1
O1
O3
C1
R
O
O
2
2
R
syn
anti
Scheme 4. Model structures for the calculation of electronic parameters
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 765–770

ION COMPLEXATION OF para-tert-BUTYLCALIX[4]ARENE ESTERS
769
0,6
Table 4. Angular coefficients, standard deviations (SD) and
correlation factors (r) for the plot of log (KR/KH)Naþ versus
OMe
t-Bu
0,4
0,2
different electronic parameters
r= 0.98
H
Parameter
Angular coefficients
SD
r
0,0
Me
-0,2
-0,4
-0,6
-0,8
-1,0
-1,2
-1,4
-1,6
Cl
Electron density O1
Electron density O2
Electron density O3
Electron density C1
HOMO
13.45
56.55
ꢀ41.71
7.13
1.17
1.09
1.11
1.16
0.18
0.97
0.05
0.36
0.30
0.07
0.98
0.56
4.71
1.28
LUMO
NO2
correlation factors are listed in Table 5. It is worth noting
that sþ is linearly correlated to EHOMO. Thus, the
substituent effect by resonance may account for the
increase in HOMO energy, which allows a more effective
interaction with the electrophile.
There is a clear inversion of the best correlation for the
plot of log (KR/KH) against HOMO-energy going from the
small (Liþ, Naþ) to the larger (Rbþ, Csþ) cations. An
example is the plot of log (KR/KH) versus EHOMO for the
Csþ ion shown in Fig. 3. For Liþ and Naþ the best
correlation was obtained for AM1-HOMO-anti, while for
Rbþ and Csþ the best correlation was obtained for AM1-
HOMO-syn. This is indicative of the change in the
conformation of the host to accommodate larger cations
by an opening of the way for the binding site formed by
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
+
σp
Figure 1. Hammett’s plot (log KR/KHvs. sþp ) for extraction
data of sodium picrate
0,6
OMe
0,4
Me
r= 0.99
0,2
H
0,0
t-Bu
Cl
-0,2
-0,4
-0,6
-0,8
-1,0
-1,2
—
the OCH C O group.
—
2
The 1H-NMR spectra of Naþ-complexes with methoxy
3 and t-butyl 4 derivatives are consistent with an oxygen-
bonded complex in the lower-rim. Some selected values
of d are reported in Table 6. The complexation leads to a
freezing of the motion of the pendant groups, with an
increase in symmetry to C4v.16 The complexation opens
the t-butyl groups (upper-rim) and brings the phenolic
oxygen atoms closer (lower-rim). The most evident
change is the proximity of the doublets of the equatorial
and axial methylenic hydrogen atoms. The d values
change from 5.00 and 3.27 ppm for the uncomplexed
calixarene to 4.27 and 3.52 ppm for the Naþ complex of
t-butyl derivative 4, as a result of the motion of the oxygen
atoms linked to the calix cavity. The proximity of the
axial hydrogens to the oxygen atoms in the free host is
responsible for their low-field absorption and the
interaction with the metal increases the distance between
these oxygen atoms and the axial hydrogen atoms, so the
-1,4
NO2
-1,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
σ
Figure 2. Hammett’s plot (log KR/KHvs. s) for extraction
data of potassium picrate
net result is a high-field shift. The same pattern for the
chemical shifts of the methylenic hydrogen atoms is
observed for the complexation by the methoxy derivative
3, indicating that the nature of the complexation is the
same for both. The signals related to OCH2 hydrogens
were also shifted to a higher field: from 5.00 to 4.67 ppm
Table 5. Correlation factors for the plots of log (KR/KH) with HOMO energies of anti and syn conformations of the model
structures for calixarene receptors
AM1-HOMO-anti
PM3-HOMO-anti
AM1-HOMO-syn
PM3-HOMO-syn
Liþþ
Na
0.92
0.98
0.92
0.68
0.93
0.11
0.63
0.64
0.33
0.02
0.90
0.92
0.81
0.79
0.98
0.81
0.95
0.88
0.61
0.89
Kþ
Rbþþ
Cs
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 765–770

770
M. LAZZAROTTO ET AL.
complexes with 3 and 4, there were shifts of the peaks
0,30
0,25
0,20
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
corresponding to C—H at ortho position to the phenol
esters to higher fields, when compared with the free host,
while we would expect a lower field shift due to the
draining of electronic density by the phenolic oxygen. We
attributed this effect to a shielding of these aromatic
hydrogen atoms by the pendant aromatic rings, which are
brought nearer by the conformational change promoted
by the metal cation complexation. This is a further
indication of the anti conformation around the C—O
bond of the ester for the Naþ complex, when the
proximity between the aromatic rings places the hydrogen
atoms in the shielding region of the aromatic p-clouds.
OMe
r= 0.98
t-Bu
H
Me
Cl
-9,5
-9,4
-9,3
-9,2
-9,1
EHOMO
CONCLUSIONS
Figure 3. Plot of log (KR/KH) versus EHOMO of syn confor-
mation for extraction constants of cesium picrate
Hammett plots for constants of alkaline metal
extraction by para-substituted-phenoxy-ester-calixarenes
3–8 showed good correlations and revealed that electronic
parameters can play a major role in the magnitude of the
constants and improve the selectivity of metal extraction.
The good correlations with HOMO energies of the host
suggest that the frontier orbital interactions contribute to
the complex formation.
Table 6. Selected d of free calixarene esters 3 and 4 and
complexed with Naþ
OMe.Naþ;-
t-Bu.Naþ-
OMe-3
3.NaR
t-Bu-4
4.NaR
Hax. (d)
Heq. (d)
OCH2 (s)
3.29
4.96
5.09
6.74
6.90
6.83
3.49
4.26
4.67
7.17
6.43
6.73
3.30
4.96
5.12
6.83
6.92
7.26
3.52
4.29
4.70
7.17
6.70
7.28
H
arom.(s)
Hortho-R (d)
Hmeta-R (d)
Acknowledgements
PROSET and CNPq for financial support.
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for OMe derivative-3 and from 5.12 to 4.70 ppm for t-
butyl derivative 4. We conclude that the conformational
change again had a considerable influence on the
chemical shifts of the hosts.
However, we noticed some unexpected trends in the
chemical shifts with the complexation. In the Naþ
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J. Phys. Org. Chem. 2006; 19: 765–770