A Stacking Interaction between a Bridging Hydrogen Atom and Aromatic π Density in the n-B18H22-Benzene System

The structures of n‐B18H22 and of n‐B18H22⋅C6H6 were determined by single‐crystal X‐ray analysis at −60 °C. The geometry of the boron cluster itself does not seem to be appreciably affected by solvation. There does, however, appear to be an unusual interaction of a polyborane bridging hydrogen atom with the benzene π system, giving rise to an extended stacked structure. The 1H{11B} spectrum of n‐B18H22 in [D6]benzene differs from that in [D12]cyclohexane most noticeably in the bridging proton region. Upon moving from the aliphatic to the aromatic solvent, the greatest increase in shielding was for the signal corresponding to the bridge hydrogen atom that interacts with the π system of benzene; the signal was shifted upfield by 0.49 ppm. Density functional theory calculations were performed on 1:1 and 2:1 complexes of the n‐B18H22 unit with benzene. An unusual interaction of a bridging hydrogen atom on n‐B18H22 with π‐electron density on benzene (illustrated here) has been characterized by X‐ray diffraction. The existence of the interaction is supported by NMR observations and density functional theory calculations.