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(NMe 2 ) 3 . {(Ph 2 P)xanth} 3 B features a solid‐state structure in which two of the three B‐bound xanth(PPh 2 ) units are projected above the BC 3 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 C 3 ‐symmetric H‐bond donors such as NH 3 (and related guests such as H 2 O and MeNH 2 ) the binding of essentially nonpolar substrates such as CO would be expected to be weak at best. However, exposure of {(Ph 2 P)xanth} 3 B to CO under mild conditions (1 bar, 25 °C) reversibly yields {(Ph 2 P)xanth} 3 B·CO, a tractable cage‐like borane carbonyl adduct featuring a central BCO moiety shrouded by xanth(PPh 2 ) 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 .