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Source: Cialc. P. A.; Sessler. J. L.; Allen. W. E.; Tverinocs. N. A.;
Lynch. V. Chem. Commun. 1997, 665.
"Anions were added as 0.3M dichloromefhane-^A .solutions of the
respective tetrabutylammonium salts to 3 mM solutions of the receptor in
dichloromethane-˝╦ with concentration changes being accounted for by
EQNMR. In determining the stability constants, the possible effects of
ion pairing (if any) were ignored. h Estimated value. The NH-prolon
resonance broadened considerably during the titration, forcing the
frequency of the resonance to be noted manually. This value should,
therefore, be treated with caution.
N.D.: Noi delermined.
sensors for a variety of guest species. Recognizing this, it was decided
to append a redox-active ferrocene moiety to the calixpyrrole framework
and to study the solution-binding and electrochemical properties of the
resulting receptors in the absence and presence of anions.67
Toward this end, the carboxylic acids 12 and 13 were coupled to
methylaminoferrocene using the benzotriazolyl-N-oxy-
tris(dimethylamino)phosphonium hexafluorophos-phate (BOP) amide-coupling
reagent. This afforded the calixpyirole-ferrocene conjugates 14 and 15
in 67 and 84% yield, respectively (compound 5, the /?-monoester, was de-
esterified in a manner analogous to that used to obtain 12 from 11; this
generated the /^-mono-acid 13).
The stability constants of 14 and 15, elucidated via 'H NMR titration
experiments, are summarized in Table 4. Measurements were hampered in the
case of compound 14 by a broadening of the pyrrole-NH and -CH signals
during the course of the titration. Thus, unfortunately, a reliable
stability constant for fluoride anion could not be determined.
Nonetheless, at least in the case of 15, the expected affinity series F -
>C1 >H2POj could be unequivocally inferred.
The electrochemical properties of 14 and 15 were investigated using
cyclic (CV) and square wave (SWV) voltammetric techniques. When the /J-
ferrocenylamido-calixpyrrole 14 is scanned between +600 and -100 mV, a
reversible ferrocene/ferrocenium wave (E|/2 = 511 mV vs Ag/AgCl) is
observed (Figure 15). However, when the CV is scanned between + 1.8 and -
1.8 V, further oxidation features are observed (Figure 16) that are
assigned to calix[4Jpyirole-centered oxidation processes. Under these
scanning conditions the Fc/Fc 1 couple becomes less reversible. Similar
behavior is observed for the meso-ferrocenylamidocalixpyrrole 15.
The proximity of the anion-binding core of the calixpyrrole to the
electrochemically active ferrocene moiety led to the reasonable
expectation (based on electrostatic considerations) that in both
compounds 14 (Figure 17) and 15 (Figure 18) a cathodic shift in the
ferrocene/ferrocenium redox wave would be observed as an anion was bound.
However, in contrast to these expectations, anodic shifts were observed
upon the addition of certain anions to 14 and 15 (Table 4). In the case
of the /?-ferrocenylamidocalixpyn'ole 14, cathodic shifts of 14 and
207 mV were observed for the Fc ! / Fc couple upon addition of F - and Cl
anions, respectively. On the other hand, the addition of H2POj anions to
this system was found to lead to an anodic shift of 9 mV (Table 4). An
Sessler and Gale
Table 4. Stability Constants (M ') for compounds 14 and 15 with Anionic
Substrates in Dichloro-methane at 298 ╩ and Electrochemical Data in the
Absence and Presence of Anions
Anion ░ Receptor 14 Receptor 15
MM V Ei/2(mV)c ─┼ (mV)d ╠╠ V E1/2(mV)c ─┼ (mV)d
No anion N/A 511 N/A N/A 503 mV N/A
Dihydrogenphosphate 40'' 502 -9 40 534 31
Fluoride * 525 14 1496 566 63
Chloride 202 718 207 444 481 -22
Source: Data from Sessler. J. L.; Gebauer, A.; Gale. P. ╦. G"--. Chim.
hal. 1997. 127. 723.
" Used in the form of their (n-Bu4N) 1 -sails.
h Association constants for anion binding; recorded in dichloromethane-^2
errors <20%; determined from ─(<5) [ppm] N╚.
' Determined in dichloromethanc containing 0.1 mol/L (n-Bu4NPFft) as the
supporting electrolyte. Solutions of 14/15 were 5 *10 4 M and potentials
were determined with reference to Ag/AgCl. d Shifts determined by square
This value was determined using the chemical shift of the /f-Đ═ of the
pyrrole since the pyrrole-NH signal became too broad to be followed
accurately during the titration.
NMR signals became so broad in this case that an accurate determination
of this value was impossible.
E (mV) vs. Ag/AgCl
Figure 15. Cyclic voltammograms of 14 ( + 600 to - lOOmV vs Ag/AgCl)
recorded in dichloromethane at various scan rates (40, 80, 100, 200, 600,
1000 mVs- ';0.1 M n-Bu4NPF6 was used as the supporting electrolyte).
1600 1200 800 400 0 -400 -800 -1200 -1600