Titanium Cyclooctatetraene Sandwich Complexes: Synthesis, Crystal Structures, and Prospects for Spin Coherence Studies

In this work, we present two new synthetic routes to [Ti(cot)₂], as well as its complete crystallographic description in a new crystal system and space group. The coordination modes of the two cot rings are not equivalent, showing η⁸ and η⁴  hapticities. Consequently, two distinct ​¹H-NMR signals are observed at 298 K, which coalesce into one broad resonance as the temperature is increased due to the dynamic interconversion of the rings. We estimate an activation energy barrier of 15–20 kcal mol⁻¹ for this process based on DFT calculations. A byproduct of the [Ti(cot)₂] preparation is the paramagnetic, triple-decker [Ti₂(cot)₃] complex, which was characterized by infrared and EPR spectroscopies, as well as by single-crystal X-ray diffraction. The intriguing electronic structure of this binuclear compound, for which the early literature is largely contradictory, is the focus of DFT studies in our laboratory to elucidate the composition of the molecular frontier orbitals.