Leveraging Non-Covalent 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 BC3plane, 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 non-covalent interactions, is favoured significantly in silico (by ca. 33.0 kcal mol-1). While 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 non-polar 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.