Xanthene-Based FLPs Featuring Perchloroarylborane Substitutents: Interplay of Noncovalent Aggregation and Dihydrogen...

The syntheses of frustrated Lewis pairs (FLPs) based on a 4,5-difunctionalised xanthene backbone, and featuring the pentachlorophenyl-containing boryl groups, are reported. At a general level, this allows for systematic comparison of these systems in H₂ activation as a function of the Lewis acid component, both internally and with respect to B(C₆F₅)₂-containing FLPs. Incorporation of C₆Cl₅ groups over C₆F₅ diminishes the thermodynamic ability of these systems to activate H₂, reflecting greater steric bulk and a disincentivized pyramidal geometry at boron. Diisopropylphosphino systems featuring -B(C₆Cl₅)₂ and -B(C₆Cl₅)(C₆F₅) groups activate dihydrogen reversibly at room temperature; solution-phase VT-NMR studies, however, uncover unprecedented behavior at lower temperatures. On cooling, the extent of H₂ uptake first increases (due to the reduced magnitude of the unfavorable TΔS° term); at even lower temperatures, phosphonium-borate formation is forced to compete with intermolecular FLP aggregation via aromatic stacking between the C₆Cl₅ rings. This leads to the observation of V-shaped van't Hoff plots with inflection points at 263 K (for xanth(PiPr₂){B(C₆Cl₅)₂}) and 268 K (for xanth(PiPr₂){B(C₆Cl₅)(C₆F₅)}). The behavior of these systems is shown to rely on the balance between the thermodynamics of H₂ activation and aggregation, and shows explicitly (but counter-intuitively) how non-covalent assembly can impede the activation of H₂ by FLPs.