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.
