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

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.