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

Abstract

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.

Synthesis of 10‐Methylsulfide and 10‐Alkylmethylsulfonium nido‐Carborane Derivatives: B–H···π Interactions between the B–H–B Hydrogen Atom and Alkyne Group in 10‐RC≡CCH2S(Me)‐7,8‐C2B9H11

“…This is the first example of intramolecular B-H···π(C≡C) hydrogen bonding that expands the area of weak interactions involving B-H bonds, [48] and that include unconventional homopolar dihydrogen B-H···H-B, [49] hydrogen B-H···π(Ar), [50] and hydride-halogen B-H···X [20] interactions.…”

B–H⋯π: a nonclassical hydrogen bond or dispersion contact?

“…The nature of interaction in the B-HÁ Á Áp motif observed crystallographically in Ir-dithiolene-phosphine complexes 3 has been studied here by means of advanced QM calculations including energy decomposition to physical terms. We have shown that the B-HÁ Á Áp motif is not an electrostatically attractive nonclassical hydrogen bond but rather a nonspecific weak dispersion-driven B-HÁ Á Áp contact.…”

“…14 In relation to the recent study by Cremer et al, 3 it is however counterintuitive that the terminal boron-bound hydrogens, which should bear a partial negative charge, would form an electrostatically attractive B-HÁ Á Áp hydrogen bond with a p-electron density of phenyl. This controversial conclusion may be drawn based on an inappropriate use of NBO for boron clusters as noted above.…”

“…3 However, the strength of these interactions has very recently been questioned. 4 Moreover, such an interpretation of the crystallographic data contradicts the wealth of evidence for unique noncovalent interactions of boron hydrides.…”

“…Let us note that the amphiphilicity of the BH groups of carborane will be influenced by their position on the cage, i.e. growing in the order BH (9, 12) o BH (8, 10) o BH (4,5,7,11) o BH (3,6).…”