Leveraging Noncovalent Interactions for the Binding of CO by a Weakly Lewis Acidic Borane

Abstract

Known boron carbonyl complexes either exploit very high Lewis acidity or a low oxidation state boron centre in order to capture CO. By contrast, we report a carbonyl complex featuring a simple tri-coordinate borane, characterized by a Lewis acidity which is only marginally higher than B(NMe₂)₃. {(Ph₂P)xanth}₃B features a solid-state structure in which two of the three B-bound xanth(PPh₂) units are projected above the BC₃ plane, generating an up, up, down conformation. Quantum chemical methods, however, reveal that the alternative up, up, up alignment, characterized by a cage-like geometry and enhanced intramolecular noncovalent interactions, is favored significantly in silico (by ca. 33.0 kcal mol⁻¹). Although this conformation is optimal for binding polar C₃-symmetric H-bond donors such as NH₃ (and related guests such as H₂O and MeNH₂) the binding of essentially nonpolar substrates such as CO would be expected to be weak at best. However, exposure of {(Ph₂P)xanth}₃B to CO under mild conditions (1 bar, 25 °C) reversibly yields {(Ph₂P)xanth}₃B·CO, a tractable cage-like borane carbonyl adduct featuring a central BCO moiety shrouded by xanth(PPh₂) moieties. Dispersion forces are critical to substrate binding: the two binding modes in which the B-bound CO guest is located inside/outside the host cage differ in energy by 59.5 kcal mol⁻¹.