Phosphole-containing calixpyrroles, calixphyrins, and porphyrins: synthesis and coordination chemistry

Porphyrins are large heterocyclic macrocycles that bind metals to form complexes such as heme and chlorophyll. Porphyrinogens are related macrocycles in which the meso-carbons, the carbon atoms connecting the pyrroles or the related five-atom heterocycles, are all or partially reduced to methylene, disrupting the pi-electron conjugation. Both porphyrins and porphyrinogens are important components of natural and synthetic systems, and they have completely different coordination behavior. Consequently, the coordination chemistry of the entire porphyrin family, including regular porphyrins, porphyrinogens, and their expanded analogs, has been extensively investigated. Core modification, namely, replacing one or more pyrrole rings with other heterocyclic or carbocyclic rings, is a highly promising approach for creating unprecedented coordination properties in the porphyrin family. The size, shape, charge, and binding ability of the core-modified porphyrin platforms are readily tunable by variation of the heterole subunits. Until recently, however, the only atoms that could be incorporated into the core were carbon and chalcogens (the oxygen family). Phosphole, the phosphorus isologue of pyrrole, is considered a nonaromatic heterole because of insufficient pi-conjugation between the cis-dienic pi-system and the lone electron pair of the phosphorus atom. As a consequence, phospholes behave not only as efficient pi-conjugative frameworks but also as ordinary phosphine ligands for transition metals. With this in mind, we started a research project on core-modified porphyrins in which the phosphole subunit plays a crucial role in providing characteristic coordination environments. In this Account, we describe our efforts to explore the utility of phosphole-containing porphyrins and porphyrinogens as macrocyclic, mixed-donor ligands. We have established convenient methods for the synthesis of calixpyrroles, calixphyrins, and porphyrins with P and either O or S substitutions, that is, P,X,N(2)-hybrids, as well as the phosphatripyrrane precursors. We also have investigated the effects of varying the combination of core heteroatoms (P, N, S, and O) on the coordination properties of the hybrid macrocycles. Our recent investigations have shown that (1) the P,S,N(2)-calixpyrroles behave as monophosphine ligands while maintaining the hosting functions that originate from the pyrrole subunits, (2) the P,X,N(2)-calixphyrins behave as neutral, monoanionic, or dianionic tetradentate ligands with electronic structures that vary widely depending on the combination of heterole subunits, and (3) the P,S,N(2)-porphyrin behaves as a redox-active pi-ligand for group 10 (the Ni family) metals, affording a novel class of core-modified isophlorin complexes. As a whole, the incorporation of the phosphole subunit into the macrocyclic framework provides unprecedented coordinating properties for the porphyrin family, affording exceptional synthetic control over the behavior of these important macrocycles.