Anion binding: A new direction in porphyrin-related research

Abstract

A new approach to the chelation of anionic substrates, based on the use of large, pyrrole-containing macrocycles, the so-called "expanded porphyrins", is described. This anion binding, which is without precedent in simpler porphyrin-type systems, is manifest both in the solid state and in solution. In the present paper, it is illustrated in terms of solid state structural results obtained from single-crystal X-ray diffraction structural analyses of various anion complexes of three prototypic systems, namely the sapphyrins, anthraphyrins, and rubyrins.The porphyrins (e.g. octaethylporphyrin, 1) and related tetrapyrrolic compounds are among the most versatile of all macrocyclic systems (ref. 1). They play, for instance, a critical role in a large number of biological processes and are widely recognized as being metal-binding ligands par excellence.Nonetheless, the porphyrins and related systems fail with regards certain applications. They are not, for instance, capable of forming hydrolytically stable, non-labile 1: 1 complexes with many cations of the lanthanide or actinide series (ref. 2). Nor, do they work as anion complexing agents (ref. 3). Recently, however, we have found that certain larger pyrrole-containing macrocyclic analogues of the porphyrins, members of the so-called expanded porphyrin series (ref. 4), can serve not only to bind cations of the lanthanide (ref. 5) and actinide series (ref. 6), but also in certain instances to chelate anions (refs. 7-17). It is this latter finding, namely the discovery of anion binding (ref. 18) in simple porphyrin-like systems, that engenders the present report. In particular, the results of solid state structural studies, carried out using three prototypic series of expanded porphyrins, the sapphyrins (e.g. 2 and 3), anthraphyrins (e.g. 4), and rubyrins (e.g. S), will be detailed and relevant solution phase results presented.Our first realization that expanded porphyrins are capable of chelating anionic substrates arose out of an attempt to obtain X-ray diffraction quality of sapphyrin 2, a decaalkyl derivative of a class of compounds first reported by Woodward nearly 25 years ago (ref. 19). Here, an X-ray crystal analysis of what was thought to be the bis-HPF6 salt of 2, revealed the presence of only one PF6-counteranion per macrocycle as well as the presence of unexpected electron density in the center of the fully protonated pentaaza core (ref. 7). On the basis of l9F NMR and independent synthesis, this unexpected electron density was ascribed to a hydrogen-bound fluoride anion, indicating, as shown in Figure 1, that the diprotonated form of 2 acts as a fluoride anion receptor--at least in the solid state. 2. R = C H 3 3. R = CH2CH20H 393 4 5

A Time-Dependent Density Functional Study of a Non-Aromatic [1.1.1.1.1]-Pentaphyrin and Its Lutetium Complex

“…4 The enlargement of the inner cavity allows the metal coordination to larger cation ions, than in porphyrins, like lanthanides or actinides. 14 This is the case for the lutetium(III) and gadolinium(III) water-soluble texaphyrin complexes (tripyrrolic penta-aza expanded porphyrins), first synthesized by Sessler et al in 1993, with a cavity core larger by about 20% than that of porphyrins. 15 These metal complexes are in advanced clinical studies for the PDT treatment of arteriosclerotic disease and as MRI radiation enhancers, respectively.…”

Photophysical Behaviour of Corrole and its Symmetrical and Unsymmetrical Dyads

“…The interest in these new porphyrin analogues is both theoretical and practical. For example, it is possible to study aromaticity in these modified porphyrins and, based on the rich chemistry of porphyrins, to improve upon some porphyrin properties to facilitate practical applications such as photodynamic therapy [3,4] or anion binding [4][5][6][7][8].…”

Retention of Anions on Silica-based Metalloporphyrin Stationary Phases

“…1,2 This order is the same as the classical selectivity series observed for a potentiometric response of a liquid anionexchange membrane electrode. Certain anion separation, however, can also be obtained by using the stationary phases modified by macrocyclic compounds such as porphyrins, 3 phthalocyanines, 4 sapphyrins 5,6 and crown ethers. 7 Because the most macrocycles are uncharged, anion chromatographic columns can be produced using complexes of these ligands with transition-metal ions.…”