Mechanism of Ligand Exchange Processes for Titanocene Complexes: A Computational Study

The ligand exchange mechanisms of titanocene complexes
were investigated
by using density functional theory (DFT) to elucidate the reactivity
trends of halogenation and deprotection reactions involving dithiolate-substituted
species. Specifically, the transformations of titanocene dithiolate
into dichloride and difluoride complexes via HCl, HF, and XeF2 were examined. The study reveals that these processes follow
an associative interchange (Ia) mechanism, with significant differences in activation barriers
and intermediates depending on the halogen source. While HCl-mediated
substitution proceeds through defined multistep pathways with moderate
activation energies, HF requires higher activation energy and slower
reaction rates. In contrast, XeF2 enables a radical-mediated
pathway characterized by near-barrierless energy profiles, explaining
its experimentally observed rapid fluorination. Additionally, the
reverse reaction, formation of the dithiolate complex from titanocene
dichloride, was shown to be thermodynamically unfavorable without
base-mediated deprotonation. This comprehensive analysis underscores
the role of ligand electronics and the unique reactivity of XeF2, offering mechanistic insights that may aid future applications
of titanocenes in positron emission tomography (PET) imaging and medicinal
chemistry.