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). 1H NMR experiments
reveal that B–H···π interactions exist
in solution according to measured 1H­{11B} 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 1H­{11B} 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.