B-H•••π Interaction: A New Type of Nonclassical Hydrogen Bonding

For the first time, nonclassical hydrogen (H)-bonding involving a B-H center dot center dot center dot pi interaction is described utilizing both quantum chemical predictions and experimental realization. In the gas phase, a B-H center dot center dot center dot pi H-bond is observed in either B2H6 center dot center dot center dot benzene (Delta E = -5.07 kcal/mol) or carborane benzene (Delta E = -3.94 kcal/mol) complex at reduced temperatures. Ir-dimercapto-carborane complexes [Cp*Ir(S2C2B10H10)] are designed to react with phosphines PR3 (R = C6H4X, X = H, F, OMe) to give [Cp*Ir(PR3)S2C2B10H10] for an investigation of B-H center dot center dot center dot pi interactions at ambient temperatures. X-ray diffraction studies reveal that the interaction between the carborane BH bonds and the phosphine aryl substituents involves a B-H center dot center dot center dot pi H-bond (H center dot center dot center dot pi distance: 2.40-2.76 angstrom). H-1 NMR experiments reveal that B-H center dot center dot center dot pi interactions exist in solution according to measured H-1{B-11} signals at ambient temperatures in the range 0.0 <= delta <= 0.3 ppm. These are high-field shifted by more than 1.5 ppm relative to the H-1{B-11} signals obtained for the PMe3 analog without B-H center dot center dot center dot pi bonding. Quantum chemical calculations suggest that the interaction is electrostatic and the local (B)H center dot center dot center dot ring stretching force constant is as large as the H-bond stretching force constant in the water dimer.