Studies on the bent thoracene with low coordinated bridging units using density functional theory

Metallocenophanes are well established as the most prominent organometallic analogs of cyclophanes. In contrast, the intriguing domain of actinocenophanes remains largely unexplored. Hence, density functional theory (DFT) calculations are employed in the present study to investigate a series of [3]thoracenophanes incorporating X-Y-X ansa moieties. These moieties feature a low-coordinated center Y (comprising C, Si, or Ge), stabilized by adjacent donor units X (NMe, O, or S). These ansa-bridged [3]thoracenophanes predominantly feature cyclooctatetraene dianion (COT²⁻) in an eclipsed configuration. Notably, a tilting of the COT²⁻rings is observed when thoracene is bridged with second-row elements, in contrast to the third-row elements. Analysis of the singlet-triplet gap ($∆E_{s-t}$) reveals that the triplet states of all investigated carbenes possess higher energy than the corresponding singlet states. Frontier molecular orbital (FMO) analysis further demonstrates that the identity of the heteroatom (X) in the ansa moiety plays a crucial role in stabilizing the low-coordinated carbon (C) of the thoracenophane complex. Furthermore, natural bond orbital (NBO) analysis was conducted to comprehend the stabilization of the resulting carbene-based thoracenophane complexes. The results reveal that, intriguingly, no direct orbital interaction occurs between the thorium center and the ansa bridge, highlighting the primary role of the carbene-based low-coordinated ansa unit in altering the properties through geometrical modifications. This study opens a new avenue for exploring the reactivity patterns of these [3]thoracenophanes toward small-molecule activation and for extending the ansa-bridge design to other actinides for advanced catalytic applications.