Elucidating the Fe(III) Directed 15-Step Domino Inter- and Intramolecular Progressive Coordinative Oligomerization of a Heterocycle Aggregate

Abstract

Benzo[d]thiazol-2-ylmethanol undergoes progressive oligomerization under solvothermal conditions in the presence of FeCl₃·6H₂O, yielding a heterocyclic aggregate, namely 1,2,3-tris(benzo[d]thiazol-2-yl)-2,9-dihydrobenzo[b]cyclopenta[e][1,4]thiazine. Single-crystal X-ray diffraction analysis was conducted on four distinct compounds isolated during the reaction, and electrospray ionization mass spectrometry (ESI-MS) of both solid products and intermediate reaction solutions enabled the identification of 15 consecutive reaction steps, where Fe(III) was directly involved in eight steps. These transformations comprise nine intermolecular C─C coupling events and six intramolecular ring expansion processes. The heteroatoms (N, O, and S) play distinct mechanistic roles according to their positions within the heterocyclic framework: (1) nitrogen and oxygen coordinate with Fe(III), facilitating activation of the reaction site; (2) homolytic cleavage of the C─O bond promotes C─C coupling reactions; and (3) C─S migration induces intramolecular ring expansion. Notably, theoretical calculations indicate a decrease in Gibbs free energy along the intramolecular reaction pathways, substantiating the proposed mechanism and activation mode, which underscores the essential role of Fe(III) in enabling the reaction progression. Furthermore, an investigation of the photophysical properties revealed that the resulting heterocyclic aggregates exhibit strong luminescence within the 535–610 nm wavelength range, approaching the near-infrared region. These findings highlight the significance of this reaction pathway in the controlled synthesis of functional oligomers and polymers from monomeric precursors, particularly through catalysis by cost-effective metal ions.