Synthesis and Structures of Bis(indolyl)-Coordinated Titanium Dichlorido Complexes and Their Catalytic Application in the Cyclotrimerization of Alkynes

The impact of the terminal ligands on the titanium center on the coordination features of deprotonated 2,2'-bis(indolyI)methanes (henceforth: bis(indolyl)s) was studied via a structural comparison between {bis(indolyl)}Ti(NEt2)(2) complexes and the corresponding dichlorido complexes. Asa result, several flexible aspects of bis(indolyl) coordination were found. For example, it was revealed that an eta(1)-coordinated indolyl moiety can change its coordination mode to coordination via the five-membered ring of indolyl when the terminal diethylamido ligands are replaced by chlorido ligands. Moreover, we found that the methoxy group in the central aromatic ring of the bis(indolyl) ligand can coordinate to the titanium center. The synthesized dichlorido complexes were applied for catalytic alkyne cyclotrimerization reactions, as Ti-based catalyst systems are less developed than Co-, Ni-, Ru-, and Ir-based systems. During this study, the cyclotrimerization of HCCSiMe3 was found to preferentially produce the 1,3,5-form (1,3,5-form:1,2,4-form = 79:21), contrary to the typical trend of transition-metal-mediated alkyne cyclotrimerization, and the isolated yield (72%) is the highest among the known 1,3,5-favoring reactions using Ti-based catalyst systems. Furthermore, the reaction mechanism was experimentally verified to proceed through a typical stepwise mechanism involving monomeric species.

Automated summary

The study revealed a significant impact of terminal ligands on the coordination features of deprotonated 2,2'-bis(indolyl)methanes, showing flexibility in coordination modes. The synthesized dichlorido complexes exhibited high catalytic activity and selectivity in alkyne cyclotrimerization reactions, with the highest isolated yield among reactions favoring the 1,3,5-form using Ti-based catalyst systems.