Bulking Up the Bay-Position Substituents Enables Enhanced Selectivity of C-Symmetric Boron...

The precise synthesis of subporphyrinoid hybrids with π-expanded topologies and unique material properties plays a promising role in the design of functional macrocycles. Easy, selective, and controllable routes to boron subphthalocyanine-subnaphthalocyanine hybrids, Bsub(Pc_3--Nc)s, are desirable for this purpose yet synthetically challenging due to random mixtures of C-, C₃-, and, in some cases, C₁-symmetric compounds that form during traditional statistical mixed cyclotrimerizations. Herein, we addressed this issue by developing a sterically driven mixed cyclotrimerization with enhanced selectivity for the targeted C-symmetric hybrid and complete suppression of sterically crowded macrocyclic byproducts. This process, coupled with a rationally designed precursor bearing bulky phenyl substituents, enabled the synthesis and characterization of bay-position phenylated Ph₂-(R_p)₈Bsub(Pc₂-Nc₁) hybrids with halogens (R_p = Cl or F) in their peripheral isoindole rings. Reaction selectivity ranged between 59 and 72% with remarkable yields, significantly higher than that of conventional mixed cyclotrimerizations. These findings were augmented by theoretical calculations on precursor Lewis basicity as guiding principles into hybrid macrocycle formation. Additionally, the incorporation of unfused phenyl groups and halogen atoms into the hybrid framework resulted in fine-tuned optical, structural, electronic, and electrochemical properties. This straightforward approach achieved improved selectivity and controlled narrowing of the product distribution, affording the efficient synthesis of structurally sophisticated Bsub(Pc₂-Nc₁) hybrids. This then expands the library of 3-dimensional π-extended macrocycles for use in a range of applications, such as in optoelectronic devices with precisely tailored optical properties.