Inclusion of Organic Cations by Calix[4]arenes Bearing Cyclohepta-2,4,6-trienyl Substituents

Organic Cations in Constrained Systems

“…The bulky tert -butyl groups used in the synthesis of calixarenes diminish the association constants (compare entries 5 and 6 of Table 12.1 ). On the other hand, substituents may enlarge the cavity, thus supporting [16] -870 a --120 a ----II -2 [16] -870 a --79 a 363 ---II -3 [16] -650 a --200 a ----II -4 [18] -----277 -360 92 II -5 [21] 210 [21] 52 --39 -19 ---II -8 [18] 52 binding. The cycloheptatrienyl substituents of host II -8 (Scheme 12.6 and Table 12.1 ) seem to support the binding of fl at iminium and aryl tropylium ions but hinder the complexation of spherical ammonium ions (see Table 12.1 , entry 7).…”

“…The tropylium moiety is closely bound to the cavity, whereas the aryl group is pointed towards the exterior (see Scheme 12.7 ). 18 A comparison of the CIS values (Hz) observed for the cycloheptatrienyl substituents with those of the protons of the aromatic rings indicates that the cationic guest is located in the upper part of the calixarene cavity. Free horizontal rotation of the aryltropylium ion is slow on the NMR time scale.…”

“…Accordingly, tetraalkoxy derivatives of the host II -8 exhibit no or very low binding properties towards ammonium and iminium ions. 18 However, a particularly high association constant of K = 3526 l mol − 1 was measured for the complex of the host II -9 (Scheme 12.6 ) with tetramethylammonium chloride. 20 The anion was proposed to act as a hydrogen bonding acceptor towards the phenolic OH -group of the substituents.…”

Anion Effects on the Recognition of Ion Pairs by Calix[4]arene-Based Heteroditopic Receptors

“…1,2 Less studied and straightforward is the situation where the coordinating power of the counterion cannot be neglected. Only (4) The interaction of the counterion with cations or anions in apolar organic solvents is mainly electrostatic in nature, but other contributions may not be neglected. In reference to the specific case of the tetraalkylammonium-anion interactions, 5 the relative evaluation of such interactions is possible only when spherical ions 6 are compared.…”

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A novel heteroditopic receptor (5) based on a rigid calix [4]arene cavity bearing at the upper rim four arylsulfonamido binding sites has been synthesized. The binding abilities of this new host have been investigated in apolar solvents toward a series of tetramethylammonium salts (tosylate, chloride, acetate, trifluoroacetate, and picrate) and compared with those of monotopic and heteroditopic calix[4]arene-bis(crown-3)-based receptors 1 and 2 in order to evaluate the role of the anion on ion-pair recognition.

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