Complexation of Porphyrin-Appended Guests by Calix[4]arene-Appended Cyclodextrins

A calix[4]arene based -cyclodextrin dimerand tetramer (1 and2, Figure 1) were synthesized by covalentattachment of amono(2-O-xylylamino)--cyclodextrinderivative to calix[4]areneplatforms, bi- or tetrafunctionalized withcarboxylic acid groups at their upperrims. The complexation of porphyrin-basedguest molecules by these hosts inwater was studied using microcalorimetry.Tetrakis(4-phenylsulfonato)porphyrin(TsPP) binds to 2 in a 1 : 2 (host : guest)fashion with enhanced bindingstrength (K₁ = 6.6 × 10⁶ M ^-1) ascompared to the monomeric TsPP–CDinteraction (K = 8.8 × 10⁵ M ^-1). Thisenhancement is attributed tothe involvement of two cyclodextrin units in theaccommodation of one TsPP guest.Increase of the number of 4-sulfonatophenyl siteson the guest by generating the-oxo-dimer of the iron(III) complex of TsPPled to further increase of thebinding strength owing to participation of three-cyclodextrin cavities of2 (K = 1.5 × 10⁷ M ^-1). The geometricincompatibility betweenhost and guest, stemming from the fact that both TsPPand its -oxo-dimer arefairly small compared to the multi-cyclodextrin hosts,probably explains why theenhancement is still moderate. A much more pronouncedincrease in complexationstrength was achieved with p- and m-pyridylporphyrinextended withp-tert-butylbenzyl guest sites. Theseguests are large enough to accommodatethree to four -cyclodextrin units. The bettermatch in size between host and guestgave association constants up 10⁸ and 10⁹ M ^-1for the -cyclodextrindimer and tetramer, respectively. In fact, the 1 : 1complex betweentetrakis(p-tert-butylbenzyl)-p-pyridylporphyrinand 2(K = 5 × 10⁹ M ^-1) is the strongestreported for cyclodextrin–porphyrininteractions.