An Unorthodox Conformation of [18]porphyrin-(1.1.1.1) Heteroanalogue—21,23-Ditelluraporphyrin with a Flipped Tellurophene Ring

Abstract

Conformational flexibility of tetrapyrrole macrocycles is currently appreciated as an important factor in the fine-tuning of porphyrin and metalloporphyrin properties.[1] Thus, introduction of sterically demanding residues (including nitrogen atom protonation) in the porphyrin core or at the outer rim (alkylation, arylation), as well as coordination of metal ions, can result in a nonplanar porphyrin conformation. Coremodified porphyrins, formed by replacement of one or more nitrogen atoms by other heteroatom(s), also demonstrate severe nonplanar arrangements once the size and/or number of heteroatoms in the core increases, even though the basic framework of the porphyrin skeleton is preserved.[2] On the whole, [18]porphyrins-(1.1.1.1) and their heteroanalogues contain a similar framework of four essentially planar fivemembered rings linked by methine carbon atoms with all the nitrogen atoms or other heteroatoms pointing toward the center of the macrocycle. Even the most severe distortion of a macrocycle preserves such an TMin∫-macrocycle orientation of the five-membered rings.[1] An enlargement of the macrocycle by the addition of heterocyclic moiety(s) and/or methine carbon atoms introduces a novel structural motif. Thus a five-membered ring– axiomatic for porphyrins (heteroporphyrins)–can be oriented in such a way that one nitrogen or other heteroatom is located at the outer perimeter and two -atoms are found in the core or just over the center. An inversion of this type has been detected for the following expanded porphyrins and expanded heteroporphyrins:[3] tetrathia[22]porphyrin(2.2.2.2),[4] tetrathia[24]porphyrin-(2.2.2.2),[5] meso-substituted [22]pentaphyrin-(1.1.1.1.0) (sapphyrins)[6] and its heteroanalogues,[7, 8] [26]hexaphyrin-(1.1.1.1.1.1),[9] heteroanalogues of [26]hexaphyrin-(1.1.0.1.1.0) (rubyrin),[3, 10] and heteroanalogues of [30]heptaphyrins-(1.1.0.0.1.1.0) and [30]heptaphyrins-(1.1.0.1.0.1.0).[11] The lowest limit of the macrocycle size permitting the flipped geometry is exemplified by [18]annulene bridged by three sulfur atoms (which formally may be named trithia[18]porphyrin-(2.2.2)), but it behaves only as an array of isolated thiophene units which are totally out of plane in the solid state.[5] It was also postulated that a 180 rotation of the N-confused pyrrole ring of 2-aza-21-carbaporphyrin is necessary to produce the TMfused∫ porphyrin.[12] The conformation of 2-aza-21-carbaporphyrin with the flipped pyrrole ring is energetically accessible as demonstrated by DFT calculations.[13] Here we report on the synthesis and characterization of 5,10,15,20-tetraphenyl-21,23-ditelluraporphyrin (Te2TPP, 2) which is the first heteroanalogue of [18]porphyrin-(1.1.1.1) with the nonplanar macrocyclic conformation containing the flipped five-membered ring. The 21,23-ditelluraporphyrin 2 was synthesized by condensation of pyrrole and 2,5-bis(phenylhydroxymethyl)tellurophene[14c] (1 Scheme 1). This procedure follows the methodology previously used for the preparation of 5,10,15,20-