B–H···π Interaction: A New Type of Nonclassical Hydrogen Bonding

For the first time, nonclassical hydrogen (H) -bonding involving a B-H···π interaction is described utilizing both quantum chemical predictions and experimental realization. In the gas phase, a B-H···π H-bond is observed in either B2H6···benzene (ΔE = -5. 07 kcal/mol) or carborane···benzene (ΔE = -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···π interactions at ambient temperatures. X-ray diffraction studies reveal that the interaction between the carborane BH bonds and the phosphine aryl substituents involves a BH···π H-bond (H···π distance: 2. 40-2. 76 Å). (1) H NMR experiments reveal that B-H···π interactions exist in solution according to measured (1) H (11) B signals at ambient temperatures in the range 0. 0 ≤ δ ≤ 0. 3 ppm. These are high-field shifted by more than 1. 5 ppm relative to the (1) H (11) B signals obtained for the PMe3 analog without B-H···π bonding. Quantum chemical calculations suggest that the interaction is electrostatic and the local (B) H···ring stretching force constant is as large as the H-bond stretching force constant in the water dimer.