Di(hydroperoxy)cycloalkane Adducts of Triarylphosphine Oxides: A Comprehensive Study Including Solid-State Structures and Association in Solution

Abstract

Four new di(hydroperoxy)cycloalkane adducts (Ahn adducts) of p-Tol₃PO (1) and o-Tol₃PO (2), namely, p-Tol₃PO·(HOO)₂C(CH₂)₅ (3), o-Tol₃PO·(HOO)₂C(CH₂)₅ (4), p-Tol₃PO·(HOO)₂C(CH₂)₆ (5), and o-Tol₃PO·(HOO)₂C(CH₂)₆ (6), have been synthesized and fully characterized. Their single crystal X-ray structures have been determined and analyzed. The ³¹P NMR data are in accordance with hydrogen bonding of the di(hydroperoxy)alkanes to the P═O groups of the phosphine oxides. Due to their high solubility in organic solvents, natural abundance ¹⁷O NMR spectra of 1-6 could be recorded, providing the signals for the P═O groups and additionally the two different oxygen nuclei in the O-OH groups in the adducts 3-6. The association and mobility of 3-6 were explored by ¹H DOSY (diffusion ordered spectroscopy) NMR, which indicated persistent hydrogen bonding of the adducts in solution. Competition experiments with phosphine oxides allowed ranking of the affinities of the di(hydroperoxy)cycloalkanes for the different phosphine oxide carriers. On the basis of variable temperature ³¹P NMR investigations, the Gibbs energies of activation ΔG^‡ for the adduct dissociation processes of 3-6 at different temperatures, as well as the enthalpy ΔH^‡ and entropy ΔS^‡ of activation, have been determined. IR spectroscopy of 3-6 corroborated the hydrogen bonding, and in the Raman spectra, the ν(O-O) stretching bands have been identified, confirming the presence of peroxy groups in the solid materials. The high solubilities in selected organic solvents have been quantified.