The synthesis and properties of free-base [14]triphyrin(2.1.1) compounds and the formation of subporphyrinoid metal complexes

The synthesis of [14]triphyrin(2.1.1) compounds is described. In contrast with conventional subporphyrins, which consistently contain a central boron atom, free-base heteroaromatic compounds can be formed. A modified Lindsey method was used to prepare a range of different [14]triphyrins(2.1.1) in yields of up to 35% based on the reaction of diethylpyrrole (1a) and fused pyrroles of bicyclo[2.2.2]octadiene (BCOD) (2a-e) and dihydroethanonaphthalene (4a) with various aryl aldehydes. The concentration of BF₃OEt₂ catalyst plays the key role in determining the yield of the [14]triphyrin(2.1.1) macrocycle relative to the conventional tetrapyrrole porphyrin product. Retro-Diels-Alder reactions of 2a-e and 4a result in the formation of [14]tribenzotriphyrin (2.1.1) (3a-e) and [14]trinaphthotriphyrin(2.1.1) (5a). The effects of exocyclic ring annulation on the electronic structure are examined in detail based on optical spectroscopy, theoretical calculations, and electrochemical measurements. The availability of free-base compounds enables the formation of [Re^I(CO)₃(triphyrin)] (6a) and [Ru ^II(CO)₂Cl(triphyrin)] (7a) complexes based on a modified retro-Diels-Alder reaction. X-ray structures are reported for 4a and 6a.