Complexation of porphyrin-appended guests by calix[4]arene-appended cyclodextrins

A calix[4]arene based beta-cyclodextrin dimer and tetramer (1 and 2, Figure 1) were synthesized by covalent attachment of a mono (2-O-xylylamino)-beta-cyclodextrin derivative to calix[4]arene platforms, bi- or tetrafunctionalized with carboxylic acid groups at their upper rims. The complexation of porphyrin-based guest molecules by these hosts in water was studied using microcalorimetry. Tetrakis(4-phenylsulfonato)porphyrin (TsPP) binds to 2 in a 1 : 2 (host: guest) fashion with enhanced binding strength (K-1 = 6.6 x 10(6) M-1) as compared to the monomeric TsPP-CD interaction (K = 8.8 x 10(5) M-1). This enhancement is attributed to the involvement of two cyclodextrin units in the accommodation of one TsPP guest. Increase of the number of 4-sulfonatophenyl sites on the guest by generating the mu-oxo-dimer of the iron(III) complex of TsPP led to further increase of the binding strength owing to participation of three beta-cyclodextrin cavities of 2 (K = 1.5 x 10(7) M-1). The geometric incompatibility between host and guest, stemming from the fact that both TsPP and its mu-oxo-dimer are fairly small compared to the multi-cyclodextrin hosts, probably explains why the enhancement is still moderate. A much more pronounced increase in complexation strength was achieved with p- and m-pyridylporphyrin extended with p-tert-butylbenzyl guest sites. These guests are large enough to accommodate three to four beta-cyclodextrin units. The better match in size between host and guest gave association constants up 10(8) and 10(9) M-1 for the beta-cyclodextrin dimer and tetramer, respectively. In fact, the 1:1 complex between tetrakis(p-tert-butylbenzyl)-p-pyridylporphyrin and 2 (K = 5 x 10(9) M-1) is the strongest reported for cyclodextrin-porphyrin interactions.