When phosphorus takes over: hyperconjugation-driven intramolecular O-H⋯O[double bond, length as m-dash]P hydrogen bonds beyond classical RAHB.

Abstract

Intramolecular hydrogen bonds embedded in conjugated frameworks are commonly rationalized within the resonance-assisted hydrogen bonding (RAHB) paradigm, in which π-electron delocalization cooperatively strengthens the O-H⋯O interaction. Here, we demonstrate that RAHB-like geometries can arise from a fundamentally different electronic mechanism. A family of phosphorylated hydroquinolinols featuring rare intramolecular O-H⋯O[double bond, length as m-dash]P hydrogen bonds was investigated using single-crystal X-ray and neutron diffraction, NBO analysis, and solution-state NMR and IR spectroscopy. Although the hydrogen-bond-closed quasi-rings display structural signatures reminiscent of classical RAHB systems, bond-length equalization is limited and exhibits only minor variation across the investigated substitution series. Electronic structure analysis reveals that the phosphoryl group acts as a σ-resonance and negative-hyperconjugation-dominated hydrogen-bond acceptor, indicating that classical cooperative π-resonance is not the dominant factor governing hydrogen-bond strength within the investigated series. As a result, these O-H⋯O[double bond, length as m-dash]P interactions are strong, persistent in solution, and exhibit only limited sensitivity to the structural modifications examined in the present series. These findings establish phosphoryl-centered intramolecular hydrogen bonds as a distinct electronic regime beyond classical RAHB and extend the concept of resonance assistance to heteronuclear, hyperconjugation-dominated frameworks.

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