Syntheses and coordination of linear and macrocyclic pentapyrroles possessing a fused moiety: Confusion modulated structural diversity.

Porphyrinoids have attracted wide interest because of their vital roles in biological processes and fascinating structural and functional characters. In this work, a doubly N-confused pentapyrrane precursor N 2 C–P 5 has been synthesized with terminal pyrrole ring A and middle pyrrole C incorporated in the N-confused mode. On this basis, pentapyrrin 1 and sapphyrin 2 have been synthesized by oxidizing N 2 C–P 5 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Both 1 and 2 possess a [5.5.5]-tricyclic fused ring involving pyrrole B and the neighboring confused pyrrole C. Meanwhile, the formation of 2 involves ring closure between the terminal rings A and E. Notably, 2 exists as a diradical showing a half-field signal in the electron paramagnetic resonance (EPR) spectrum and a singlet ground state with a negative singlet-triplet energy gap of −0.50 kcal/mol. Excellent thermal stability has been observed for 2 , which may be related to its large π-conjugated framework involving the fused structure. On this basis, 1 and 2 were treated with Cu(acac) 2 to afford Cu(II) complexes 3 and 4 , respectively. As a result, an acetylacetonyl moiety is attached at the terminal pyrrole ring E to afford an NNNO coordination environment for complex 3 , and the sapphyrin macrocycle of 2 has been cleaved between rings A and B to afford a fused pentapyrrin ligand, furnishing an NNNN coordination environment for complex 4 , in which a pentafluorophenyl-ketal group generated during the ring cleavage reaction is attached to ring A, and it can be hydrolyzed to the corresponding pentafluorobenzoyl substituent to afford complex 5. Compared with free bases 1 and 2 , complexes 3 − 5 show more intense low-energy near-infrared (NIR) absorption bands at ca. 791, 926 and 907 nm, respectively. This work provides an effective approach for the syntheses of linear and macrocyclic oligopyrroles with intriguing structures and properties by oxidizing oligopyrranes with precisely designed numbers and positions of N-confused pyrroles. [Display omitted] • Facile syntheses of stable N-fused sapphyrin diradical. • Cu(II) complexes of fused pentapyrrins with an acetylacetonyl, pentafluorophenyl-ketal and pentafluorobenzoyl substituent. • Tunable low-energy near-infrared (NIR) absorption bands up to 1151 nm. [ABSTRACT FROM AUTHOR]