Forum Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Porphyrinoid f‑Element Complexes James T. Brewster, II, Hadiqa Zafar, Harrison D. Root, Gregory D. Thiabaud, and Jonathan L. Sessler*
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Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-1224, United States ABSTRACT: Porphyrin and related pyrrole-containing macrocycles, collectively porphyrinoids, are versatile ligands that allow access to a multitude of coordination modes. Judicious modification of the porphyrin core as well as the pendant substituents has extended the coordination chemistry of porphyrinoids to include systems that are able to stabilize f-block element complexes with possible utility. This review focuses on our group’s efforts to prepare expanded porphyrin and porphyrinogen ligands that can serve as tools to study and apply f-element metal coordination chemistry: it covers the background of the topic, selected syntheses, and application of these species in the chemical and medical sciences.
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found practical use as NMR shift reagents, luminescent heme protein models, optical materials, and therapeutics, among other applications.33 However, actinide complexes based on porphyrins and other relatively small porphyrinoids remain scarce with only a limited set of thorium(IV) and uranium(IV) out-of-plane and double- or triple-decker sandwich structures being known.34−39 Attempts at transuranic complexes have been limited to neptunium and americium phthalocyanine sandwich complexes.40−42 A recent research focus in porphyrin chemistry has involved modification of the core via heterocycle replacement, addition of pendant substituents, and variations in the cavity size and shape.43−47 Expanded porphyrins, systems containing larger cavities, have received considerable attention in the context of this general paradigm.48−50 Judicious incorporation of appropriate chelating moieties and size complementarity has allowed for the stabilization of a number of f-element complexes. This new chemistry has yielded structures and functions inaccessible using smaller congeners, such as the porphyrins or related ligand species. Here, we review our group’s efforts toward the synthesis and use of expanded porphyrin and porphyrinogen species as an innovative felement chelating strategy. In providing this summary, particular focus will be placed on reviewing the relevant background, outlining synthetic developments and, when applicable, discussing potential applications. The presentation is divided according to four broad classes of porphyrin ligands with data on each ligand presented in chronological order.
he f-block elements have played a starring role in chemistry in spite of occupying a less-than-prominent position within the periodic table.1 Initial interest in these elements was driven by academic curiosity and a desire to complete the periodic table. 2 Subsequently, industrial applications and wartime efforts spurred further research.3 Early academic programs sought to expand potential applications by elucidating fundamental reactivity patterns, molecular structure, and coordination chemistry.4−9 The resultant studies revealed marked differences between the fblock elements and transition metal or main group species, allowing for useful applications across the chemical,10−15 medical,16−18 and material science fields.19−21 Recent advances, driven by the preparation of new diverse ligand sets, have continued to drive progress in f-element chemistry and revealed unique new utilities.22−24 Tetrapyrrolic ligands, notably the porphyrins and corroles as well as the ostensibly related phthalocyanines, support remarkable coordination chemistry (Figure 1).25−27 Transition
Figure 1. Tetrapyrrolic porphyrin and related congeners.
metal and main group complexes have proven invaluable as metalloprotein cofactor models,28 new materials,29 supramolecular constructs,30 pharmaceuticals,31 and catalysts.32 The inherent ability of these ligands to yield stable metal complexes stimulated f-element porphyrin research. Not surprisingly, a diverse array of lanthanide-containing systems © XXXX American Chemical Society
Special Issue: Innovative f-Element Chelating Strategies Received: March 27, 2019
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DOI: 10.1021/acs.inorgchem.9b00884 Inorg. Chem. XXXX, XXX, XXX−XXX
Forum Article
Inorganic Chemistry
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SCHIFF BASE EXPANDED PORPHYRINS Synthetic efforts within the porphyrin community initially focused on tetrapyrrolic systems. The widespread application of such constructs provided an inspiration to prepare and study more complex systems. Early expanded porphyrins demonstrated unique physical, chemical, and coordination properties.51−56 In pursuit of a new class of expanded porphyrins, Sessler and Johnson reported the preparation of a tripyrranecontaining Schiff base porphyrinogen.57 Metal insertion with concomitant oxidation yielded the so-called metallotexaphyrins.58 Similar macrocyclization strategies were then subsequently used to prepare alaskaphyrin59 and USAphyrin60 as well as tetrapyrrolic61 and calix-pyrrole derived62 Schiff base expanded porphyrins (cf. Figure 2).
plex (6) and a 7-coordinate pentagonal bipyramidal texaphyrin-Cd(py) 2 (7) complex, respectively, wherein nitrate serves as the counteranion ( Scheme 2).64,65 Scheme 2. Synthesis of Aromatic Cd(II)-Texaphyrins58
Compared to the smaller tetrapyrrolic species, these complexes contained relatively in-plane metal ions. The authors again noted on the extension of such metalation chemistry to Zn(II), Mn(II), Hg(II), and, the first attempts towards an f-element complex, Nd(III). Texaphyrin-Zn(II)Cl2 was also recharacterized as a κ4 complex with one of the pyrroles remaining in the protonated form.64 The Mn(II), Hg(II), and Nd(III) complexes were suggested to consist of 1/ 1 pentaligated species.64 Taken in aggregate, these results demonstrated that the texaphyrins could allow access to new coordination modes that were not routinely accessible using smaller porphyrins. The rich biological activity of porphyrins and phthalocyanines provided an impetus for Harriman, Sessler, and coworkers to probe transition metal and lanthanide containing metallotexaphyrins as a new class of photosensitizers for singlet oxygen generation.66 The newly reported paramagnetic lanthanide texaphyrin-Sm(III) and -Eu(III) complexes were found to be nonluminescent with an undetectable triplet excited state (
